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A N E W S U NThe Solar Results From SkylabBy John A . EddyEdi ted by Rein Is e

Prepa red byGeorge C . M a r s h a l lSpace F l igh t Cen te r

Scientific and Technical Informa t ion B ra nchNATIONAL A E R O N A U T I C S AND S P A C E ADMINISTRATION

Washington, D.C.1979


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A ne w sun.( N A S A S P ;402)Inc l ude s i nde x .S u p t . o f D o c s . n o . : N A S 1.21:4021 . Sun. 2. S k y l a b Pro je c t . I. Ise,R e i n .II . Uni ted States . George C. Marshal l Space R i g h t C e n t e r , H u n t s -ville, Ala. III . Ti t le . IV . Series : Uni t e d S ta te s . Na t i ona l A e r o n a u t i c sa nd S pac e A d m i n i s t r a t i o n . N A S A S P ;402.Q B 5 2 1 . E 3 3 523.7 78-606052

F or sale by the S u p e r i n t e n d e n t o f D o c u m e n t sU.S. G o v e r n m e n t P r i n t i n g Office, W a s h i n g t o n , D.C. 20402S toc k Numbe r 033-000-00742-6


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Foreword

'h e success of th e Sk y l a b mi s s io n a nd i t s A po l lo- L T e le s c ope M o u n t (A T M ) h a s b e en a t h r i l l i n ge v e n t t o t h o s e o f u s w h o h a v e b e e n e n ga ge d w i thN A S A s in c e 195 8 in p l a n n in g a n d c o n s u m m a t in gso la r r esea rch miss ions in space . Con ce ive d in 1965as a r e la t ive ly s imp le exerc ise to tes t m an ' s capa b i l -i ty fo r do ing usefu l work in space , the ATM con-cept was subjec ted to a con t inuous process o f c r i t i -c a l r e v i e w , r e v i s io n , a n d u pg r a d in g b y N A SA ,w o rk i n g i n c o n c e r t w i th i n du s t r y a n d t h e sc ien t i f icc o m m u n i t y . T h e A T M t h a t f inally e me r ge d b e c a meone of the mo s t impo r t a n t m i l e s t o n e s i n the his-to ry o f so la r a s t rophy s ics . The miss ion in fac t r ep-resen ted the f ina l s tep in the in i t ia l long- te rm s t r a t -egy fo r solar a s t r o ph y s i c s t h a t w a s s e t b y t h e Spa c eScience B o a r d o f t h e N a t io n a l A c a de m y o fSc iences in 1960, wh ich ca l led fo r the ins ta l la t ionin space of a very la rge o rb i t ing so la r observa tory ,weighing s e v e r a l t h o u s a n d me ga g r a ms a n d c o m p a r -in g f a v o r a b ly w i th g r o u n d - b a s e d o b s e r v a to r i e s a sr e ga r ds l i gh tga th e r in g po w e r a n d a n g u l a r a n d spec -t r o s c o p ic r e s o lu t i o n .In t e r m s o f v e r s a t i l i t y , s e n s i t i v i t y , a n d r e l i a b i l i t y ,t h e pe r f o r m a n c e o f t h e A T M t e l e s c o pe s a n d in s t r u -m e n t s exceeded the h ighes t a spi r a t ions o f a s t ron-o m e r s d u r i n g th e 1960's. Y e t , th e mission a p -

pe a r e d h e a de d f o r t o t a l a n d c o mple t e d i s a s t e r ,u n t i l r escued b y the hero ic a nd sk i l l fu l explo i t s o ft h e a s t r o n a u t s i n r e pa i r i n g t h e da m a ge in c u r r e d b yth e s pa c e c r a f t du r in g l a u n c h . In this a n d o t h e rways , the ro le o f man in the opera t ion of spaceobserva tor ies was c lea r ly , even br i l l i an t ly , de l ine-a t e d .T h e s t u dy o f A T M o b s e r v a t i o n s h a s a l r e a dy l e dto m a n y n e w discover ies about th e n a t u r e of th eSu n a n d a b o u t t h e f a s c in a t i n g e v e n t s t h a t o c c u r o neven a very o rdina ry s tar . E s pe c i a l l y i l l u min a t in gh a s been th e r ecogni t ion of th e e x t e n t to w h ic h th eSu n ' s ma gn e t i c field is respon sible for the st ruc-t u r e , d y n a m i c s , a n d h e a t i n g of the Su n ' s o u t e r l ay -e r s . So mass ive was the ha rves t o f in forma t ion ,h o w e v e r , tha t i t will b e ma n y y e a r s b e f o r e t h e po s -s ib i l i t ies f o r p r o du c t iv e a n a ly s e s a r e e x h a u s t e d . Ia m c o n f ide n t t h a t w e l l b e f o r e t h a t t ime w e s h a l lhave ga ined such en t i r e ly new per spec t ives on theS u n t h a t w e w i l l b e c o n f r o n t in g a b r a n d n e w s e t o fq u e s t i o n s a n d a dd r e s s in g t h e m w i th a second gener -a t ion o f so la r space exper iments .

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G o r d o n A . N e w k i r k , Jr . , Cha i rman Jam es H . U n d e r w o o dR o b e r t M . M a c Q u e e n E i n a r T a n d b e rg -Ha n s s e nE d m o n d M . Reeves Edward G . GibsonR i c h a r d T o u s e y Ad r i e n n e F . T i m o t h yG i u s e p p e S . V a i a n a Ja m e s B . D o z i e rAllen S . Krieger William C. S n o d d y


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Contents

PageI N T R O D U C T I O N xiii1. TH E N E A R E S T S T A R 12 . T H E M A S K S O F T H E S U N 73 . TH E S U N F R O M S P A C E 394 . T H E SO L A R T E L E SC O P E S ON S K Y L A B 475 . T H E S O L A R R E S U L T S F R O M S K Y L A B : A N I N T R O D U C T I O N . . 6 16 . T H E Q U I E T S U N 6 7

In t h e U l t r a v io l e t : T h e Invis ib le Su n 6 8A t th e Poles of th e S un 80X - R a y s F r o m t h e S un 88H o le s i n the C o r o n a : T he Open S un 9 8So la r P r o m in e n c e s : T h e R u f f l e d Su n 10 6O u te r C o r o n a : T h e E th e r e a l Su n 1177 . T H E A C T I V E SU N 12 7B r i gh t P o in t s : T h e G l i t t e r i n g Su n 12 8Act i ve Re g io n s : T he V i o l e n t S un 1 34Fla r e s : T h e E x p lo s iv e Su n 14 4A c t i v e P r o m i n e n c e s 154C o r o n a l T r a n s i e n t s : T h e E x t r a v a ge n t Su n 16 8

8. A L OO K A H E A D 179E D I T O R ' S N O T E 1 83P H O T O C RE DI T S 185I N D E X . 195

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O F F T O T H E S U N ! A s t r o n a u t s C a r r , G i b s o n , a n d P o g u ebla s t o f f f r o m C a p e K e n n e d y o n N o v e m b e r 1 6, 1 9 7 3 , tor e n d e z v o u s w i t h S k y l a b , 4 3 5 k m above Ea r th . They wereth e t h i rd a n d f ina l c rew t o m a n t h e sc ien t i f i c space s t a t iona n d t h e i r s t a y o f 8 4 d a y s s e t a n e n d u ra n c e r e c o rd f o r m a nin space .

X l l A N E W S U N


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I n t r o d u c t i o nby E dw a rd G . Gibson*W h a t y o u c a n d o o r d e a m y o u can,b e g i n i t: B o l d n e s s h a s g e n i u s ,p o w e r , an d m a g i c i n i t .

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F t i s N o v e m b e r 1 6 , 1973. W e a r e 4 3 5 k m a b o v e_ L E a r t h , t r a v e l i n g o v e r it at 8 k m / s e c . S l o w l y ,o u r c o m m a n d m o d u l e m o v e s i n a n d d o c k s w i t hS k y l a b . W e h a v e ar r i v e d a t o u r c o u n t r y ' s f i r s t s p a c es t a t i o n , a 9 1 - M g l a b o r a t o r y i n o r b i t a r o u n d o u rp l a n e t E a r t h .T w o c r e w s h a v e b e e n h e r e b e f o r e u s t h e y r e -paired th e d a m a g e d l a b o r a t o r y , p e r f o r m e d t h e i rp r o g r a m e d o b s e r v a ti o n s s u c c e s s f u l ly , a n d r e t u r n e dh o m e . N o w i t i s o u r t u r n . P e e r i n g t h r o u g h t h e c o m -m a n d m o d u l e w i n d o w s , w e c a n s e e t h e i m m e n s ew o r k s h o p , i t s s o l a r - p a n e l w i n g s s t a n d i n g o u t s t a r k -ly a g a i n s t t h e b l a c k sky.W e a n t i c i p a te w i t h e x c i t e -m e n t (and t o s o m e d e g r e e , awe) o u r p l a n n e d 5 6 -d ay s o j o u r n h e r e . I t t u r n s o u t t o b e 8 4 days-n e a r l y 3 m o n t h s .L o o k i n g b a c k , I can again f e e l th e e x c i t e m e n t ,th e s e n s e o f c h a l l e n g e an d a d v e n t u r e , o f t h o s e f i r s tm o m e n t s . S k y l a b o f f e r e d th e f i r s t tw o c r e w s an du s a n o p p o r t u n i t y t o d o w h a t n o n e h ad d o n e b e -fore-to s t u d y t h e S u n f r o m space. F o r o n e o f t h em o s t e x c i t i n g p a r t s o f t h e S k y l a b c h a l l e n g e w a s t h eo p e r a t i o n o f t h e g r e a t b a t t e r y o f s o l a r t e l e s c o p e so n th e A p o l l o T e l e s c o p e M o u n t (A T M ) , p ar t o f t h eo r b i t i n g s p a c e s t a t i o n . F o r o v e r 7 y e a r s I h a d b e e nc l o s e ly i n v o l v e d i n th e d e s i g n , te s t i n g , a n d p l a n n i n gf o r t h e s e t e l e s c o p e s , a n d n o w I w a s t o u s e t h e man d t o s e e t h e S u n a s i t c o u l d n o t b e s e e n f r o mE a r t h . J u d g i n g f r o m th e s u c c e s s e s o f t h e f i r s t tw om a n n e d m i s s i o n s , w e c o u l d a n t i c i p a t e d i s c o v e r i e s

*Scientist Pilot of Sk y lab' s t h i r d m a nne d m i s s i o n .

S K Y L A B l o o k e d l i k e t h i s t o a s t r o n a u t s C a r r , G i b s o n , a n dP o g u e a s t h e y a p p r o a c h e d i t o n N o v e m b e r 1 6 . 1973.r e a d yt o b e g i n t h e i r r e c o r d 8 4 d a y s i n s p a c e . A c l o u d - c o v e r e dE a r t h l ies b e l o w . O n t h e d i s t a n t h o r i z o n c u r v e s t h e t h i nb a n d o f a t m o s p h e r e t h a t i s b l u e s k y f o r t h o s e b e l o w . T h ec o l o r c o m e s f r o m s u n l i g h t s c a t t e r e d b y m o l e c u l e s o f air.A b o v e i t i s t h e e n d l e s s r e a c h o f S k y l a b ' s b l a c k sky.


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o f h i t h e r t o u n k n o w n a n d i n t e r e s t i n g f e a t u r e s o fth e Su n .On ce s e t tl e d o n b o a rd S k y l a b , o u r dai ly o p e ra -t ion o f the ATM began . In b o t h a v i sua l and i n t e l -l e c t u a l sense , i t was very s imi la r to the e x t e n s i vet r a i n i n g w e h a d o n o u r AT M s i m u l a t o r in H o u s t o n .In a phys ica l sense , however , we were in the wor ldo f z e ro g r a v i t y , a c o n s t a n t r e m i n d e r of the r e a l i t yof our s i t u a t i o n . M a s s w i t h o u t w e i g h t w a s g r a d u -a l ly accep ted as a no rmal way o f l i fe . N e w h a b i t s inl iv ing a n d n e w m e t h o d s o f w o rk d e ve l o p e d : f l o a t -in g r e p l a ce d w a l k i n g , l e v i t a t i o n r e p l a ce d f a l l ing ,a n d u p w a s a n y d i r e c t i o n t h a t y o u c o u ld c o n v i n c ey o u r m i n d w a s u p .While w a i t i n g on the d a rk s i d e o f E a r t h fo r ourfirst look of the day a t the c o m p l e x a n d e ve rch a n g -ing Sun , we rece ived a b r ie f ing f rom Scien t i s t -A s t r o n a u t Bil l Le n o i r in M i s s i o n C o n t ro l in H o u s -t o n :A c o u p l e o f w o r d s h e r e o n t h e S u n . T h e f i r s t i s , I ' ve jus tg o t t e n w o r d f r o m th e ba ck r o o m t h a t w e p r e s e n t l y h a v e a nact ive p r o m i n e n c e o n t h e s o u t h w e s t l i m b . I t i s ou t t o 0.03solar radii n o w a n d i s g r o w i n g . Y o u m a y w a n t t o be t a k inga look a t t h a t as t he Su n c o m e s over th e hil l . In a c t iveregion 3 3 w e show a few b r i g h t p o i n t s c o m i n g o u t t o d a y inth e p la ge . T h a t m a y i n d i c a t e t h a t w e ' r e going t o g e t so m er e surgence o f a c t iv i ty the re . B a sed o n w h a t w e h a v e seenhere over th e las t fe w d a y s a n d s o m e s p e c u l a t i o n , t h e r e a reo b v i o u s c o n n e c t i o n s b e t w e e n a c t i v e reg ions 3 1 a n d 3 2 ,which are up in t he n o r t h e r n h e m i sp h e re , a n d 3 3 , w h i c h isin th e s o u t h e r n h e m i s p h e r e .T h e s e w o rd s w e re t y p i ca l of the m a n y i n t e r c h a n g e sb e t w e e n g ro u n d a n d c r e w . T h e y were mos t we l -co m e b e ca u s e t h e y e s t a b l i s h e d a r a p p o r t t h a t m a d eth e o b s e r v a t o ry ' s o p e r a t i o n m u c h m o r e p r o d u c t i v ea n d e n j o y a b l e .T h e S k y l a b s o l a r o b s e rva t o ry co m b i n e d th e b e s to f m any wor lds . F i rs t , in t e rm s o f re so lu t io n inspace , w a v e l e n g t h , a n d t i m e , t h e i n s t r u m e n t s of theA T M w e re th e m o s t a d va n c e d e ve r p u t i n t o o p e r a -t ion . They cou ld re so lve th e f ine d e t a i l of the solars u r f a ce m u ch b e t t e r t h a n those used in previousu n m a n n e d s a t e l l i t e s ; b e i n g a b o ve E a r t h ' s a t m o s -p h e re , t h e y co u l d o b s erve t h e S u n i n w a ve l e n g t h st h a t a r e n o t v i s i b le f r o m E a r t h ; t h e y w a t ch e d th eS u n , p a r t i c u l a r l y t h e o u t e r co ro n a ( t h e S u n ' s ve ryf a i n t o u t e r a t m o s p h e r e , o n l y v is ib le f rom t h eg r o u n d d u r i n g f lee t i ng m o m e n t s o f t o t a l e c l i p s e )n e a r l y c o n t i n u o u s l y f o r 9 m o n t h s .S e co n d , t h e c o n t ro l s a n d d i s p l a y s f o r t h e i n s t r u -m e n t s w e r e a r r a n g e d so t h a t w e co u l d u s e o u r o n -x i v A N E W S U N

t h e - s p o t s c i e n t i f i c j u d g m e n t i n m a n a g i n g t h e e x p e r -imen ts . W e could w a t ch T V displays of the Sunand the c o ro n a a s t h e y a p p e a re d in u l t r a v i o l e tl igh t , in X- r a y s , w h i t e l i g h t , and in H a (visible l i gh tf r o m h y d ro g e n a t o m s ) . W e w e re a b l e to m a k e agood m a n y of th e d e c i s i o n s a b o u t w h e re to p o i n tt h e i n s t r u m e n t s a n d w h e n a n d h o w t o o p e r a t et h e m . T h e c o m p l e x a n d d y n a m i c n a t u r e of theSu n , the m any an d o f ten d ive rse sc ien t i f ic ob jec-t ives o f e a c h i n s t r u m e n t , th e l i m i t e d a m o u n t o ff i lm a va i l a b l e , a n d e ve n o u r g ro w i n g e x p e r i e n cem e a n t t h a t a good dea l o f j u d g m e n t c ou ld b e exer -c i sed in t ak ing the da ta . We rea l ize d tha t th i s p ro -v i de d grea t l eve rage fo r m u l t i p l y i n g t h e e f f o r t s o fall of the p e o p l e w h o h a d w o r k e d o n t h e A T M f o rm a n y y e a r s . W e a ls o r e a l i z e d , h o w e v e r , t h a t t h i se x c i t i n g o p p o r t u n i t y w a s n o t w i t h o u t r i s k . T h e rew e re a l s o n u m e ro u s i n co r r e c t w a y s o f o p e r a t i n gt h e co m p l e x o b s e rva t o ry t h a t o f t e n r e q u i r e d g r e a tcare to a v o i d .T h i rd , o u r o p e ra t i o n of the so la r obse rva to rycen te red a ro u n d coopera t ive observing p r o g r a m st h a t s p e c i f i e d j u s t h o w w e s h o u l d o p e ra t e e a ch in -s t r u m e n t to y i e l d th e b e s t d a t a . T h e se p ro g ra m sT H R O U G H T H E D O O R , h e a d f i r s t , c o m e s A s t r o n a u t G i b -s o n , f l o a t i n g f r e e l y f r o m o n e o f S k y l a b ' s rooms to a n o t h e r .R o o m - t o - r o o m t r a v e l i n S k y l a b 's w e i g h tl e ss e n v i r o n m e n tw a s m o r e l i k e s w i m m i n g t h a n w a l k i n g , a n d m u c h m o r e f u n .


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w e r e p l a n n e d in great d e t a i l m a n y m o n t h s b e f o reSk y l a b w a s l a u n c h e d , a n d w e h a d p r a c t ic e d t h e mo n a s im u la to r a t o u r t r a in in g fa c i li t ie s in H o u s to n .W e s ta r ted ou t wi th 26 program s , and o ther s werea dde d du r in g th e miss ions . They proved to be ane x ce l l e n t m e a n s o f o r ga n i z in g o u r w o r k so t h a t th eobserva tory was used a s e f f i c i e n t l y as poss ib le .T h e f o u r th f a c to r t h a t h e lpe d m a k e Sk y l a b apa r t ic u la r ly po we r fu l observa tory was our c losec o n t a c t w i th a w e l l - i n f o r m e d g r o u n d - s u p p o r tteam-we were n o t j u s t a l o n e ly i s l a n d i n the s k y .Each d a y sc ien t i s t s r espons ib le fo r e a c h i n s t r u m e n td e ve l o p e d a n d s e n t t o u s on ou r t e l e p r i n t e r a n o p -e r a t i n g p l a n f o r t h e A T M - a m i n u t e - b y - m i n u t e p r o -gram for the da y l igh t pa r t s o f each orbit . They a lsos e n t ge n e r a l i n f o r ma t io n o n t h e c h a n g in g c h a r a c t e r -i s ti c s o f t h e Su n , ga th e r e d b y t h e N a t io n a l O c e a n ica n d A t m o s p h e r i c A d m i n i st r a ti o n ( N O A A ) f a c i li t yin H o u s to n . T h i s p l a n p r o v ide d u s w i th a goods t a r t i n g p o i n t , a n d w e u s u a l l y f o l l o w e d i t u n l e s s w ej u d g e d t h a t t h e c h a n g in g c o n d i t i o n s o n t h e Su nma de o th e r p l a n s mo r e p r o du c t iv e . H o u s to n r o u -t i n e l y r e c e iv e d da t a f r o m ma n y o b s e r v a to r i e s a swel l a s f r o m o t h e r ope ra t ing sa te l l i tes . Som e of theda t a f r o m t h e A T M t e lev is ion d isp lays and the t e -l e me t r y f rom a n o n p h o t o g r a p h i c i n s t r u m e n t w a sassembled in H o u s t o n fo r "qu i c k l o o k s " so t h a tso la r sc ien t i s t s there cou ld upda te th e o b s e r v in gplan every d a y a n d s e n d u s mo r e spec i f i c i n f o r m a -t i o n f o r e a c h o f t h e i n s t r u m e n t s .F i f t h , in the same w a y t h a t we in ou r o r b i t i n go b s e r v a to r y w e r e i n c o n s t a n t c o n t a c t w i th t h eg r o u n d , th e w h o l e S k y l a b t e a m w a s a l w a y s int o u c h w i th o th e r w o r k in g s o l a r a s t r o n o me r st h r o u g h o u t th e w o r l d . N e a r l y 1 5 0 s c i e n t i s t s in 17c o u n t r i e s w e r e da i l y ma de a w a r e o f o u r p l a n n e do b s e r v a t i o n s . So me w e r e n o t i f i e d o n a n h o u r l ybas is o f changes in the schedule . These s c i e n t i s t st h e n s o me t ime s c o o r d in a t e d t h e i r o w n observa -t i o n s w i th o u r s s o th a t mo r e c o mple t e s e t s o f d a t aw e r e o b t a in e d o n ma n y sola r f e a tu r e s .In th e p l a n n i n g stages w e w o n d e r e d s o m e t i m e sw h e t h e r s u c h a n i n t r i c a t e w o r l d w i d e n e t w o r k c o u l dr e s po n d q u ic k ly t o s u dde n c h a n ge s o n t h e Su n .Be ca u s e of i t s s ize , would th is ne twork be s low toreac t? Le t ' s t ake a look a t s o m e s pe c i f ic e v e n t s .On J a n u a r y 17 , 1974, whi le I was l o o k in g a tsome very ho t sp o t s in an ac t ive r egion , a call c a m ef rom H o u s to n s a y in g t h a t t h e o b s e r v a to ry a tM a u n a Loa , Hawa i i , h a d just seen a la rge promi-n e n ce , a s pe c t a c u l a r c l o u d o f dense gas , l i f t i ng o ff

IN S H I R T S L E E V E S , G i b s o n o p e r a t e s S k y l a b ' s so l ar t e l e -scope a r r a y f r o m t h e c o n t r o l c o n s o l e i n t h e m u l t i p l e d o c k -in g a d a p t e r . O n e h a n d g r i p s t h e t a b l e to k e e p h im f r o mf l o a t i n g a w a y ; h e needs n o c h a i r . A t l o w e r l e f t a n u n u s e de l ec t r i ca l c o n n e c t o r f loa t s f r ee ly in S k y l a b ' s w e i g h t l e s s e n -v i r o n m e n t .

th e sur face of the S u n . H o u s t o n r e c o m m e n d e d o n eo f o u r p l a n n e d p r o g r a ms o f o b s e r v in g " c o r o n a lt r a n s i e n t s . " I s t a r t e d i n t o t h i s n ew o b s e r v in g p r o -g ra m a n d b e c a me e x c i t e d as I got my f i rs t look a tth e p r o m i n e n c e s p e e d i n g o u t w a r d t h r o u g h th e cor-ona . I t w a s c o mpa c t , e x c e p t io n a l l y b r i l l i a n t , a n dq u i t e u n l ik e a n y c o r o n a l s t r u c tu r e w e h a d s e e n b e -fore . Fa s c in a t e d , a n d w i th a n o v e r w h e lmin g s e n s eof awe, I wa tched th is new fea tu re grow. In lesst h a n a n h o u r i t was s e v e r a l h u n d r e d t ime s a s la rgea s E a r t h , a n d s t i l l m o v i n g o u t w a r d t o w a r d th e p l a n -e ts . N o t u n t i l w e h a d s l i ppe d i n t o E a r th ' s s h a do w ,a n d could n o longer see i t , d id I rea l i ze h o w q u i c k -ly t h e t o t a l A T M t e a m h a d re a c t e d : M a u n a L o aobserva tory h a d r a p i d l y c o n ta c t e d N O A A in H o u s -t o n , a n d t h e y h a d i m m e d i a t e l y r e l a y e d t h e n e w s toth e s c i e n t i s t o n du ty i n t h e A T M s u ppo r t r o o m . H eh a d r e q u e s t e d t h r o u gh t h e A T M r e p r e s e n t a t i v e i nMiss ion C o n t r o l t h a t w e o b se r v e th e p r o m in e n c e .T h e F l igh t D i r e c to r h a d a pp r o v e d th e r e q u e s t , a n dit w a s r a d io e d to us as soon a s we c a m e i n t o c o n -t a c t w i th t h e t r a c k in g f ac i l i ty a t Go lds to n e , Cal i f .I t wa s only a m a t t e r of 9 or 10 min . b e tw e e n th eM a u n a Loa s igh t in g and the s ta r t o f our observa -t i o n s .

F o u r da y s l a t e r , o n J a n u a r y 2 1 - l e s s t h a n 3w e e k s b e f o r e o u r m i s s io n w o u ld b e o v e r I w a s a tt h e A T M c o n s o l e w a t c h in g a mo de r a t e l y a c t i v eSun . One ac t ive r egion in pa r t icu la r was very h o t ,a n d s po r a d i c a l l y r e l e a s in g s ma l l b u t b r i l l i a n t b u r s t so f r a d i a t i o n ca l l ed s u b f l a r e s . B y n o w c o n t i n u o u sm o n i t o r i n g o f o u r c o n s o l e d i s p l a y s h a d a l l o w e d u sINTRODU CTION X V


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S K Y L A B w a s t h e m o d e rn i s t i c h o m e fo r t h r e e a s t ro n a u tc r e w s . A m u l t i p l e d o c k i n g a d a p t e r w a s a t t a c h e d to the endo f t h e w o rk sh o p , a n d t o i t w a s f ix e d the solar te lescopea r r a y called the Apollo Telescope M o u n t . F i n a l ly assembled,i t m a d e u p a n o rb i t a l la b o ra t o ry 3 6 m l o n g a n d w e i g h i n g ,o n E a r t h , a b o u t 9 1 M g .


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S o m e Facts A b o u t Sky l a bS k y l a b 1

L a u n c h d a t eLaunch s i t eTotal m a n n e d o p e r at io n a l t i m eT o t a l opera t ional t ime( m a n n e d a n d u n m a n n e d )Date to reenter Earth'sa t m o sp h e re ( e s t i m a t e d )Overal l lengthOveral l weightL e n g t h o f o rb i t a l w o rk sh o pD i a m e t e r o f o rb i t a l w o rk sh o pOrbi t a l t i t ude (ave r age)Orbi t a l ve loc i t yPeriod to circ le EarthEar th orb i t s comple t ed dur ingopera t ional time

A s t r o n a u t c r e w sS k y l a b 2-first m a n n e d m i s s i o n( M a y 25 to June 21 , 1973) :

M ay 14, 1973C a p e K e n n e d y1 71 d a y s

25 1 days1 9 8 0 / 1 9 8 1

36ma b o u t 9 1 000 k g8 . 2 m6.7 m4 3 5 k m8 k m / s93 min3900

2 8 d a y sC h a r l e s C o n ra d , U S NJ o se p h K e rw i n , M . D . ,P a u l J . W e i tz , U S N U S N

S k y l a b 3 s e c o n d m a n n e d m i s s i o n( J u l y 28 to Sept . 24 , 1973) :A l a n L . B e a n , U S NOwen K. Garr io t t , Ph . D.Jack R . L o u s m a , U S M C

S k y l a b 4 t h i r d m a n n e d m i s s i o n( N o v . 1 6, 1973, to F e b . 8, 1 9 7 4 ) :Gera ld P . Carr, U S M CE d w a rd G . G i b so n , Ph . D .William R . Po g u e , U S A F

First m a n n e d m i s s io n b a c k u p c r e w :Russe l L . Schweickar tB ru c e M c C a n d l e s s II , USNStory M u s g r a v e , M . D .

S e c o n d a n d t h i rd m a n n e d m i s s io nb a c k u p c re w :V a n c e D . B r a n dD o n L . L i n d , Ph . D .Will iam B . L e n o i r , Ph. D .

5 9 d a y s

84 d a y s

T H E N I N E A S T R O N A U T S o f t h e t h r e e S k y l a b c r e w sp o sed f o r t h i s p o s t fl i g h t p o r t r a i t i n a t r a i n i n g s i m u l a t o r a tth e N A S A J o h n so n S p a c e C e n t e r n e a r H o u s t o n , T e x . T hi su n i q u e 360 v i e w , m a d e b y p h o t o g r a p h e r M i c h a e l L a w t o nfo r th e Nat ion al Geograph ic Magaz ine , shows th e b o t t o mf loo r o f the f o u r - s t o r y l i v ing sp a c e w h e re t h e se m e n l iveda n d w o rk e d f o r a t o t a l o f 1 7 1 d a y s d u r i n g t h e S k y l a b m i s -

s i o n . B e a n , L o u sm a , C o n ra d , W e i t z , K e rw i n , a n d C a r r s t a n di n t h e e x p e r i m e n t c o m p a r t m e n t w h e r e m e d i c a l t e st s a n dp h y s i c a l c o n d i t i o n i n g w e re p e r f o rm e d : G i b so n s t a n d s i n th ew a r d r o o m ( k i t c h e n - d i n i n g r o o m ) ; P o g u e at t he d o o r o f t h ew a s t e m a n a g e m e n t c o m p a r t m e n t ( b a t h ) ; a n d G a r r i o t t i n t h es le ep c o m p a r t m e n t ( b e d r o o m ) .


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to de v e lo p a fee l for the Sun ' s leve l o f a c t i v i t y i n asense , to t a k e it s pu l s e . W e h a d b e e n d i s a p p o i n t e dth a t n e i t h e r o u r crew n o r e i ther of th e pr e v io u sc r e w s h a d m a n a g e d to observe th e v e ry b e g i n n i n gof a f lare, a shor t- l ived expl osio n on the Su n's sur-face .We h o pe d t h a t b y observ ing a flare 's bir th w ec o u ld f ind k e y s to the m e c h a n i s m b y w h ic h th eS un r e lea ses grea t q ua n t i t ies o f energy f rom a verysmal l v o l u m e in a f r a c t i o n of a m i n u t e . B ut e v e r yt i m e w e r ea l ized tha t a f lare w a s t a k i n g p l a c e , a n ds t a r t e d th e i n s t r u m e n t s , w e were to o l a te t o o b -serve th e in i t ia l energy re lease .Owen G ar r io t t , Sc ien t i s t P i lo t o f the prev io usS k y l a b mis s io n , h a d s u gge s t ed t h a t w h e n a t i n yb r i gh t p o i n t a p p e a r s o n t h e e x t r e m e u l t r a v i o l e ti m a g e of the S un , i t ma y be an ea r ly s igna l tha t af lare is beginn ing. So, as the active region shown onm y T V d i s p l a y s e e th e d w i th s ma l l e n e r gy f l u c tu a -t ions , I prac t iced the exac t con t ro l pane l proce-du r e s I w o u ld use if a f lare e r u p t e d . (I had d o n eth i s ma n y t ime s b e f o r e , b u t t o n o a v a i l . )Su dde n ly , a s ma l l b r igh t po in t a p pe a r e d r i gh t i nthe c en te r o f my sc reen . As I s ta r ted the ins t ru -m e n t s t a k i n g d a t a r a p i d ly in the i r f la re modes , Iwas e la ted to see the br igh t po in t b lossom in to af u l l - b l o w n f l a r e .M y mo s t i n t e r e s t i n g a n d c h a l l e n g in g da y o n Sk y -la b w a s J a n u a r y 26, 1974, when Hous ton a sked m eto d e v e l o p a n d i m p l e m e n t m o s t of the p l a n s fo ro n e f u l l d a y ' s o b s e r v a t i o n s . T he y did th is a s ane x p e r i m e n t , t o s e e h o w a n o r b i t a l o b s e r v a to r yc o u ld b e r u n o n f u tu r e N A S A Spa c e Sh u t t l e m is -s ions w h e n e x pe r ime n t e r s t h e ms e lv e s wil l f l y .Using the te lepr in te r , they sen t up a genera l s ta te -m e n t a b o u t t h e s c i e n t i f ic go a ls a n d l im i t a t i o n s o fe a c h e x pe r im e n t e r a n d t h e i r k n o w le dge o f t h e s t a t eo f th e S u n . T h e res t w a s lef t t o m e .

I had a f a s c i n a t i n g d a y , c o m p l e t e l y i n v o l v e d , en -t i rely c o m m i t t e d . L a y in g o u t t h e s c h e du l e w a s r e l a -t ive ly s t r a i g h t f o r w a r d . R o u t i n e d a t a - g a t h e r i n gf o r m e d th e f r a m e w o r k t h a t fi lled in w i th o b s e r v a -t i o n s o f t h e Su n ' s mo s t i n t e r e s t i n g f e a tu r e s . K n o w -in g th e e x pe r ime n t s ' o b j e c t i v e s , th e degree tow h i c h t h e y h a d a l r e a d y b e e n m e t , a n d t h e c u r r e n ts t a t e of the S u n , I c o u ld do a l l the s h o r t - t e rm p l a n -n i n g a n d m a k e a l l the r e a l - t ime c h a n ge s in the ob -serving pr o g r a m th a t w e r e r e q u i r e d . B y n o w th em a t c h i n g o f sc ien t i f i c o b j e c t i v e s w i t h th e o p e r a t i o no f th e conso le h a d b e c o m e s e c o n d n a t u r e a n d t h e

x v i i i A N E W S U N

A T M could be p layed l ike a p iano , by ea r , w i thc u mb e r s o me s h e e t mu s i c p u t a w a y . O n th e o t h e rh a n d , th i s da y ' s w o r k a l s o de mo n s t r a t e d th e i m p o r -t a n c e o f a n e v e n c l o s e r l i n k t h a n w e h a d b e tw e e nground sc ien t i s t s an d in-f l ight observers. I f o u n dth a t I s o m e t ime s m i s s ed t h e w i s do m o f th e g r o u n dpe r s o n n e l i n f o r min g my de c i s io n s . T w o h e a ds a r ecer ta in ly b e t t e r than one , espec ia l ly i f they cane x c ha n ge k n o w le dge f re e ly .T h e A T M c o n t r i b u t e d a s t a gge r in g q u a n t i t y o fn e w d a t a o n t h e S u n . I t a l so showed u s h o w to u s ema n t o g r e a t e s t a dv a n t a ge o n l o n g - du r a t i o n s pa c emis s io n s .W e r ea l ized th is dur ing our 84 d a y s in f l ight , asw e a s s u me d ma n y ro les : observers, doctors , scien-t i s t s , space p i lo t s , c o o k s , h o u s e k e e p e r s , s pa ce w a lk -ers, a n d t e c h n i c i a n s . In f i l l ing these roles w e recog-n i ze d t h a t ma n ma k e s h i s b e s t c o n t r i b u t io n s a n d i sin h is bes t psychologica l hea l th when h is ta sks le th i m b e crea t ive an d i n t e l l e c tu a l l y c h a l l e n g e d . A so u r m i s s io n e n de d , w e w e r e s t i l l b e c o m i n g m o r ea n d mo r e p r o f i c i e n t , a n d w e w e r e s t i l l i n c r e a s in g lyin t e r e s t e d i n w h a t w e w e r e do in g . Sk y l a b de mo n -s t r a t e d t h e v a lu e , t o u s a n d t o t h e s c i e n t i f i c w o r ld ,o f g iv ing th e onboa rd observer a high degree o ff r e e d o m , l e t t i n g h im l e n d h i s m in d a n d h i s c r e a t i v eab i l i t i e s to the t a sks a t h a n d . W e lef t S k y l a b w i t hth e f i r s t - h a n d k n o w le dge of h ow to p l a n th e o pe r a -tion of the space sta t ion of the f u t u r e , p e r h a p s ap e r m a n e n t n a t i o n a l o r i n t e r n a t i o n a l fac i l i t y .S i g n i f i c a n t d i v i d e n d s will r esu l t f rom heavy re l i -

H O M E A T L A S T , a f t e r a r e c o rd s t a y in o r b i t , G i b s o n s t e p st o t he d e c k of t he U.S. a i r c ra f t c a r r ie r W a sp . S t i l l ins ide a reC a r r an d P o g u e and a h e a v y c a r g o o f p r e c i o u s f i l m . T h e i rt r u s t y c o m m a n d m o d u l e , s c o r c h ed b y a i r f r i c t i o n o n r e e nt ry i n t o t h e a t m o s p h e r e , w a s t h e s a m e t h a t h a d ca r r iedt h e m to S k y l a b n e a r l y 3 m o n t h s b e f o r e . F o r 8 4 d a y s i t wasp a r k e d o u t s i d e S k y l a b ' s door, w a i t i n g f o r t h e t r i p b a c kh o m e .


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T H O U S A N D S U N S d o t t he sky b e t w e e n u s a n d th i sa n t g a l a x y , w h e r e u n c o u n t e d m o r e b l u r t o g e t h e r i n ai s m u c h l i k e o u r o w n g a l a x y , th e M i l k yT w o s ma l le r s a te l l i t e g a la x i es a ppea r a s c i rcu la r a ndin t h i s l o n g e x p o s u re p h o t o g r a p h . T h e nea r -s ta r s seen h e r e as dots are a p a r t of our ow n s p i r a lT h e A n d r o m e d a G a l a x y , o u r closest e x t r a g a l a c t i cis a b o u t 2 m i l l io n l i g h t y e a r s a w a y an d con ta i n s1 00 bi l l ion suns .


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a n c e on th e j u d g m e n t of the observer ins ide ascience o r t e c hn o l o gy l ab o ra t o ry , an o t he r sola r ob-s e r v a to r y , or a n E a r t h o b s e r v a t o ry .T h i s v o l u m e p r e s e n t s th e h igh l igh ts o f t h e n e wa n d e x c i t i n g p i c t u r e o f th e Su n t h a t h a s c o me f r o mS k y l a b . T u r n i n g t h e v a s t a m o u n t o f d a t a i n t ok n o w l e d g e h a s r e a l l y j u s t b e gu n , a n d wil l c o n t i n u ef o r m a n y y e a r s . B e h i n d t h e d a t a l i e t h e m e c h a n i c a l

a n d sc ien t i f i c c o m p l e x i t y o f S k y l a b , t h e p e o p l eand resources t h a t ma de i t a success , a n d t h e m a n yy e a r s o f p l a n n in g .A l t h o u g h S k y l a b w a s n o t a s v is ib le a n d d r a m a t -ic , perhaps , a s our f i rst l u n a r l a n d i n g , I h o p e t h a t a syou read you wi l l sha re wi th m e the fee l ing t h a t i tw a s a n e x c i t i n g , me a n in g f u l , a n d mo s t p r o du c t iv es tep i n ou r e x p l o r a t i o n of the univer se .

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The Neares t StarI t is t rue t h a t f r o m th e h ig h e s t p o in t o f v iew the Sun i s on lyo n e o f a m u l t i t u d e a s in g le star a m o n g m i l l i o n s t h o u s a n d s o fwhich , mos t l ike ly , exceed h i m i n br ightness , magn i tude , a n dp o w e r . He i s on ly a pr iva te i n t h e h o s t o f h e a v e n . Bu t h e a l o n e ,a m o n g th e c o u n t l e s s m y r i a d s, is near enough to a f fec t terrestrialaffairs i n any sens ib le degree; and h i s in f luence u p o n t h e m is s u c h

t h a t i t i s ha rd to f ind the w ord to n a m e it; it is m o r e t h a n m e r ec o n t r o l a n d d o m i n a n c e .-Charles A . Y o u n g , 1 89 6

|| ur u n iv e r s e is u n b o u n d e d and l imi t less , cons is t -\J ing of u n c o u n t e d b i l l i o n s of s t a r s b u r n in g inth e d a r k , u n m e a s u r e d s p a n o f space . On and on i tg oe sa r e a c h o f space a n d t ime t h a t s e e ms mo r eth a n min d c a n g r a s p . A m o n g t h e s t a r s a n d n e b u l a s ,a m o n g t h e ma n y ga l a x ie s a n d c l u s t e r s o f ga l a x ie swhir l s the sp i r a l ga laxy of w h ich w e a re a pa r t . Wesee i t f rom wi th in a s a f a i n t blur o f d is tan t s ta r stha t a rches in a milky s t r eak ac ross th e n i g h t t i m es k y .To us th is s ingle ga laxy , our M i l k y W a y , s e e msi n c o m p r e h e n s i b l y vast-a h u n d r e d bi l l ion s u n ssp r ink led in a s p i r a l pa t t e r n to form a f l a t , r o t a t i n gdisk . T he sp i r a l i s so la rge tha t a f la sh of l igh t f romo n e e dge mu s t r a c e a h u n d r e d t h o u s a n d y e a r s tor each th e other s ide .F a r o u t a long o n e spira l a rm t h e r e g l e a ms th es ta r we ca l l our Sun . It i s , we th in k , an average s ta ro f m idd l e a ge a n d m idd l e s iz e , m u c h l i k e h u n d r e d so f t h o u s a n d s of i ts d i s t a n t n e i g h b o r s . It b e l o n g s toa com m on c la ss o f s ta r s ca l led dw ar f s , to d is t in -gu ish t h e m f r o m g i a n t r e d s t a r s l i k e A n t a r e s a n dBete lgeuse , w h o s e d i a m e te r s w o u ld s t re t c h a c ro s s500 s u n s . T h e d i a m e t e r of our Sun i s a b o u t 1 .4mil l ion km, nea r ly 10 t imes tha t o f the la rges tp l a n e t , J u p i t e r , a nd 1 00 t ime s t h a t o f Ear th .Ou r s ma l l S u n m a y b e u n i q u e in o n e r e s pe c t : i ts

fami ly o f p l a n e t s ; h o w e v e r , t h e y t o o , m a y b e c o m -mo n f e a tu r e s o f t h e u n iv e r s e . W e a r e m u c h t o o f a rf rom other s ta r s t o de t e r min e d i r e c t l y w h e th e r a n yh a s E a r th s o r m o o n s o r J u p i t e r s .Like a l l p lane ts , Ea r th der ives i t s l igh t and hea tf rom th e S u n . It is somet imes surpr is ing to r ea l izeh o w f a r w e a r e f r o m th e Su n ; i n i ts w a r m r a y s w efeel much c loser t o ou r s t a r t h a n f a c t s o f d i s t a n c esa y w e are. T h e E a r th t r a v e l s a r o u n d the Sun in an e a r l y c i rcu la r o rb i t , never ge t t ing c loser thana b o u t 1 40 m i l l i o n k m , m o r e t h a n 4 00 t ime s th ed i s t a n c e b e tw e e n E a r th a n d M o o n - m a n ' s f u r t h e s te x c u r s io n i n t o s pa c e . A spacecra f t sen t to the Sunw o u l d spend severa l yea r s en rou te . Were i t pos-s ib le to f ly to the S un in a je t a i r l ine r , i t wou ld tak en e a r l y 18 y e a r s to m a k e t h e t r i p a t 10 0 0 k m / h r .T h e Su n h a s n o pe r m a n e n t f e a tu r e s a n d i s e n t i r e -ly g a s e o u s a g l o w i n g ba l l of ch ief ly i n c a n d e s c e n th y d r o ge n . O th e r e l e me n t s , a l l i n ga s e o u s s t a t e , a r ep r e s e n t i n a m o u n t s t h a t seem to f i t a un iv er sa l ,c o s mic r e c ipe : a b o u t 10 pa r t s h y d r o ge n t o 1 pa r th e l i u m , w i t h a p i n c h o f e a c h o th e r e l e m e n t f r o mth e pe r io d i c t a b l e , b u t mo s t l y o x y ge n , c a r b o n , n i -t r o ge n , m a gn e s iu m , a n d i r o n . A l th o u g h w e s pe a ko f th e "sur face" of the S u n , a n d o f s pe c i f i c l a y e r sin i t s a tmo s p h e r e , t h e Su n i s r e a l l y a l l a tm o s ph e r e .It h a s no e a s il y de f in e d b o u n da r i e s o r sharp discon-


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T E M P E R A T U R E A N D D E N S I T Y v ar y w i t h h e i g h t i n t h eSun's a t m o s p h e r e a c c o r d i n g t o t h e s e c u r v e s . H e i g h t i n k i l o -meters i s s h o w n i n c r e a s i n g u p w a r d o n t h e scale a t l e f t ,m e a s u r e d f r o m t h e t o p o f t h e p h o t o s p h e r e w h e r e s u n s p o t sa r e seen. Y e l l o w a n d o r a n g e peaks a r e c h r o m o s p h e r i c s p i c -u l e s t h a t j u t u p i n t o t h e c o r o n a ; t h e t r a n s i t i o n r e g i o n b e -t w e e n c h r o m o s p h e r e an d c o r o n a i s s h o w n as a d a r k y e l l o wb a n d , o n l y a f e w h u n d r e d k i l o m e t e r s t h i c k , w h i c h f o l l o w st h e s p i c u l e o u t l i n e s .A t t h e t o p o f t h e p h o t o s p h e r e ( z e r o h e i g h t ) t h e s o l a rt e m p e r a t u r e i s a b o u t 6000 K : b e l o w t h i s , i n u n s e e n l a y e r s o ft h e s o l a r i n t e r i o r , t h e t e m p e r a t u r e i n c r e a s e s a s t h e c e n t e r o ft h e S u n i s a p p r o a c h e d . T e m p e r a t u r e c o n t i n u e s to fa l l a b o v eth e p h o t o s p h e r e u n t i l a s h a r p m i n i m u m o c c u r s i n t h e l o wc h r o m o s p h e r e . T h e t e m p e r a t u r e o f t h e s o l a r a t m o s p h e r et h e n b e g i n s t o r i s e , s l o w l y i n t h e u p p e r c h r o m o s p h e r e , a n dt h e n r a p i d l y , i n s t e p s , t h r o u g h t h e t h i n t r a n s i t i o n r e g i o n . A ta h e i g h t o f a b o u t 5000 k m a b o v e t h e p h o t o s p h e r e , i n t h ec o r o n a , a t e m p e r a t u r e o f 1 0* > a n d m o r e i s r e a c h e d . N u m -b e r e d t e m p e r a t u r e l i n e s a t l o w e r l e f t s h o w f a m i l i a r l a b o r a -t o r y t e m p e r a t u r e s s u ch a s ( 1 ) t e m p e r a t u r e a t w h i c h g o l dm e l t s , 1337 K ; ( 2 ) m e l t i n g p o i n t o f i r o n , 1808 K ; ( 3 ) b o i l -i n g p o i n t o f s i l v e r , 2485 K ; ( 4 ) t e m p e r a t u r e o f a c e t y l e n ew e l d i n g f l a m e ; a n d ( 5 ) i r o n w e l d i n g arc. H i g h e r t e m p e r a -t u r e s t o r i g h t o f (5), w h i c h c h a r a c t e r i z e m o s t o f t h e s o l a ra t m o s p h e r e , ar e s e l d o m a c h i e v e d i n o u r t e r r e s t r i a l e x p e r i -e n c e . D e n s i t y o f t h e g a s e o u s s o l a r a t m o s p h e r e f a l l s r a p i d l yw i t h h e i g h t a b o v e t h e p h o t o s p h e r e . (See t h e s c a l e a t top,e x p r e s s e d i n g r a m s p e r c u b i c c e n t i m e t e r . ) B e t w e e n th e pho-tosphere a n d t h e t o p o f t h e t r a n s i t i o n r e g i o n , i n a range o fless t h a n 3000 k m i n h e i g h t , d e n s i t y fa l l s t h r o u g h 1 0 orderso f m a g n i t u d e . E v e n i n t h e r e l a t i v e l y d e n s e p h o t o s p h e r e , th es o l a r g a s i s s o t h i n t h a t i t w o u l d b e c o n s i d e r e d a v a c u u m o nE a r t h . L e t t e r e d l i n e s a t t o p g i v e t e r r e s t r i a l d e n s i t i e s s u c h a s( A ) d e n s i t y o f o u r a t m o s p h e r e at an a l t i t u d e o f 5 0 k m ; ( B )E a r t h a t m o s p h e r e a t 9 0 k m ; ( C , D , E ) r a n g e s o f v a c u u md e n s i t i e s a c h i e v e d b y l a b o r a t o r y v a c u u m p u m p s : ( C ) m e -c h a n i ca l v a c u u m p u m p , ( D ) d i f f u s i o n p u m p , a n d ( E ) i o np u m p .

t i n u i t i e s l i k e t h o s e o n E a r t h t h a t s e p a r a t e a i r a n dw a t e r a n d s o l i d g r o u n d . T h e s o l a r c o r o n a , c h r o m o -s p h e r e , an d p h o t o s p h e r e ar e d i s t e n d e d , r a r i f i e d r e -g i o n s t h a t b l u r i n t o e a c h o t h e r a n d h a v e d e f i n i t ef o r m o n l y w h e n s e e n a t g r e a t d i s t a n c e , a s f r o mE a r t h . T h e a v e r a g e d e n s i t y o f t h e Sun,a b o u t 1 . 4g / c m 3 , i s o n l y a b o u t a q u a r t e r o f t h a t o f E a r t h .T h e s o la r p h o t o s p h e r e w h i c h w e s e e w i t h t h en a k e d e y e i s s o d i f f u s e a n d t h i n t h a t w e w o u l d calli t v a c u u m h e r e o n E a r t h , a n d th e l a y e r s a b o v e i t ,th e c h r o m o s p h e r e an d c o r o n a , a r e f a r le s s dense .T h e S u n b e c o m e s g r a d u a l l y d e n s e r a s w e g o i n -w a r d , b u t i t i s n o t u n t i l w e h a v e g o n e a t e n t h o ft h e w a y t o t h e c e n t e r t h a t w e f i n d m a t e r i a l a sd e n s e a s t h e a i r w e b r e a t h e o n E a r t h , a n d n o t u n t i lw e a r e h a l f w a y t o t h e c e n t e r t h a t t h e S u n i s a sd e n s e as w a t e r .N o o n e h a s s e e n w i t h i n t h e Sun. W e pr e s u m e t h ec e n t r a l t e m p e r a t u r e i s a b o u t 1 5 m i l l i o n k e l v i n s an d

t h e c e n t r a l d e n s i t y i s a b o u t 1 6 0 g / c m 3 , o r 1 0 t i m e st h a t o f o r d i n a r y m e t a l s . T h e s e e s t i m a t e s a r e b as e do n c a l c u l a t i o n s t h a t d e p e n d u p o n o u r k n o w l e d g eo f t h e s u r f a c e t e m p e r a t u r e a n d p r e s s u r e , t h e m a s sa n d s i z e o f t h e Sun, a n d t h e p h y s i c a l l a w s t h a tg o v e r n s t e l l a r i n t e r i o r s .T h e e x c e e d i n g l y h i g h t e m p e r a t u r e s a n d d e n s i t i e so f t h e s o l ar i n t e r i o r tr i g g er n u c l e a r r e a c t i o n s t h a tp r o d u c e t h e e n e r g y t h a t f u e l s t h e s o l a r f u r n a c e .T h e e x a c t n u c l e ar p r oc es s i s n o t k n o w n , b u t t h em o s t l i k e l y i s o n e b y w h i c h h y d r o g e n , t h e S u n ' sm o s t a b u n d a n t f u e l , i s c o n v e r t e d t o h e l i u m . I n t h i sp r o c e s s o f n u c l e a r f u s i o n , e n e rg y i s r e l e a s e d . I n t h eh i g h l y c o m p r e s s e d s o l a r i n t e r i o r , t h e d i s r u p t i v ef o r c e s o f n u c l e a r r e a c t i o n s a r e c o n t a i n e d a n d m u f -f le d b y a p r e s s o f g r a v i t y t h a t i s 2 5 0 b i l l i o n t i m e sas g r e a t as t h a t o n E a r t h .F o r each h e l i u m a t o m c r e a t e d i n t h i s way, as m a l l a m o u n t o f e n e r g y i s e m i t t e d i n t h e f o r m o f ag a m m a r a y a t t h e i n v i s i b l e , s h o r t - w a v e le n g t h en d o fth e e l e c t r o m a g n e t i c s p e c t r u m . T h e n o r m a l l y p i e r c -i n g g a m m a r a y s t r a v e l b u t a s h o r t d i s t a n c e t h r o u g ht h e d e n s e l y p a c k e d m a t e r i a l a t t h e c e n t e r o f t h eS u n b e f o r e t h e y a r e a b s o r b e d a n d t h e n r e e m i t t e dt o b e a b s o r b e d a g a i n . T h e n u c l e a r e n e r g y c a r r i e d i ng a m m a r a y s f i n d s i ts w a y o u t w a r d t h r o u g h t h e S u nb y a lo n g a n d t o r t u o u s s e r i e s o f r e p e a t e d a b s o r p -t i o n s a n d r e e m i s s i o n s , g r a d u a l l y l o s i n g e n e r g y a n dc h a n g i n g t o l o n g e r a n d l o n g e r w a v e l e n g t h s i n t h ep r o c e s s .W h e n t h e e n e r g y f i n a l l y r e a c h e s t h e s u r f a c e o fth e Sun, m o s t o f t h e g a m m a r a y s h a v e b e e n r e -p l a c e d , f i r s t b y X - r a y s an d t h e n b y u l t r a v i o l e t r a y s ,s o t h a t w h e n t h e y a t l a s t e m e r g e , t h e y a r e i n t h ef o r m o f t h e v i s i b l e a n d i n f r a r e d r a d i a t i o n t h a tm a k e s u p t h e s u n s h i n e t h at w e f e e l o n E a r t h . I t i s am e a s u r e o f t h e s i z e a n d d e n s i t y o f th e s o la r co r et h a t e n e r g y c r e a t e d b y n u c l e ar f u s i o n a t t h e c e n t e ro f t h e S u n t a k e s n e a r l y 5 0 m i l l i o n y e a r s t o j o s t l ei t s w a y t o t h e s u r f a c e . T h e r e t h e s o l a r e n e r g y e a s i l yescapes t o s p a c e , t r a v e l i n g a t t h e s p e e d o f l i g h t tor e a c h t h e o r b i t o f t h e E a r t h i n b u t 8 m i n u t e s .T h e t e m p e r a t u r e o f t h e S u n d e c r e a s e s o u t w a r df r o m t h e c o r e , a n d a t t h e v i s i b l e s u r f a c e o f t h ep h o t o s p h e r e i t h a s f a l l e n t o a b o u t 6000 K . T h i sw h i t e - h o t s u r f a c e r a d i a t e s e n e r g y a t a r a t e o f a b o u t3.8 X 1023 k W , w h i c h f l o w s o u t e q u a l l y i n a l ld i r e c t i o n s . E a r t h a t i n y t a r g e t at a g r e a t dis-t a n c e - i n t e r c e p t s b u t a b i l l i o n t h p a r t o f t h e pro-d i g i o u s s o l a r o u t p u t o f h e a t a n d l i g h t . B u t e v e n th i ss m a l l s h a r e d e p o s i t s o n E a r t h a c o n t i n u o u s s u p p ly

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o f a b o u t 8 X 1 0 1 4 k W , w h i c h i s v a s t ly m o r e t h a nt h e f u l l c a p a c i t y o f a l l t h e p o w e r p l a n t s e v e r e n v i s -i o n e d o n E a r t h . E a c h s q u a r e m e t e r o f E a r t h r e -c e i v e s a b o u t 1 . 5 k W o f s o l a r e n e r g y , w h i c h , i f h a r -n e s s e d e f f i c i e n t l y , c o u l d h e a t an d l i g h t a s m a l lr o o m .I n p r o d u c i n g t h i s e n e r g y , t h e S u n ' s n u c l e a r f u r -

c o n s u m e s a b o u t 5 X 1 0 9 k g o f h y d r o g e n e a c hp a s s i n g s ec o n d, b u r n i n g o u t w a r d i n s u c c e s s i v es h e l l s a n d g r a d u a l l y t r a n s f o r m i n g t h e s o l a r i n t e r i ot o a p r e d o m i n a n t l y h e l i u m c o r e . W e e s t i m a t e t h ah y d r o g e n w i t h i n t h e S u n h a s b ee n c o n s u m e d at h i s f a n t a s t i c r a t e f o r t h e p a s t 4 o r 5 b i l l i o n y e a r sY e t t h e S u n i s s o l a r g e t h a t t h e r e i s s t i l l h y d r o g e nw i t h i n i t t o c o n t i n u e t h e p r o c e s s a t t h e p r e s e n t r a t ef o r a n o t h e r 1 0 0 b i l l i o n y e a r s , o r 2 0 t i m e s a s l o n g asi t h as b u r n e d i n t h e p a s t . L o n g b e f o r e t h e h y d r og e n i s d e p l e t e d , h o w e v e r , h e l i u m w i l l p r o b a b l y b e -g i n t o s e r v e a s f u e l , i n o t h e r f u s i o n r e a c t i o n s t oc r e a t e o t h e r e l e m e n t s , w h i c h w i l l l a t e r t a k e t h e it u r n a s s o l a r f u e l .T h e S u n : R u l e r , Fire, L i g h t an d L i f e o f t h e eta r y S y s t e m w a s R i c h a r d P r o c t o r ' s t i t l e f o r h ib o o k o n s o l a r p h y s i c s w r i t t e n i n 1 8 7 1 . I n d e e d , all i f e o n E a r t h d e p e n d s u p o n t h e S u n . "Solar e n e rg y , " w h i c h w e t a l k a b o u t t o d a y a s n e w , h a s s e r v e dE a r t h w i t h c o n s t a n c y f o r a b o u t 5 b i l l i o n y e a r s .T h e S u n i s t h e u l t i m a t e s o u r c e o f a l m o s t a ll t h ee n e r g y w e u s e , b e c a u s e w o o d , c o a l , a n d o i l a r e foss i l f u e l s t h a t s t o r e t h e e n e r g y o f s u n l i g h t f r o m e o np a s t , a n d i n h a r n e s s i n g w i n d a n d r i v e r w e a r e b uh a r v e s t i n g t h e s o l a r h e a t t h a t d r i v e s t h e c i r c u l a t i o no f t h e a i r a n d l i f t s t h e m o i s t u r e f r o m t h e s e a . O uf o o d s u p p l y i s t o t a l l y d e p e n d e n t o n t h e S u n , d ir e c t l y o r i n d i r e c t l y , t h r o u g h s o la r p h o t o s y n t h e s i si n g r e e n p l a n t s a n d a q u a t i c f o r m s . P h o t o s y n t h e s i salso r e l e a s e s t o t h e a t m o s p h e r e t h e o x y g e n e s s e nt i a l to a l l a n i m a l l i f e .

T h e s a m e s u n l i g h t th a t f e e d s a n d g i v e s u s f u ean d o x y g e n w a r m s u s a s w e l l . W i t h o u t i t t h e s a l t ysea s w o u l d f r e e z e an d th e ve r y a t m o s p h e r e w o u l dc o n d e n s e an d s o l i d i f y . W i t h o u t t h e S u n t h e r ew o u l d b e n o r a i n , n o s n o w , n o r i v e r s o r l a k e s , n ow i n d s o r c l o u d s o r b l u e o f t h e s k y , n o r a i n b o w , n om o o n l i g h t , a n d n o v e s t i g e o f l i f e u p o n o u r p l a n e tL i t t l e w o n d e r t h a t e a r l y m a n o n e v e r y c o n t i n e n tr e c o g n i z e d h i s t o t a l d e p e n d e n c e o n t h e s o l a r s t a ra n d f e l l t o w o r s h i p i t .G r a d u a l l y t h e e a r l y w o r s h i p o f t h e S u n g r e wi n t o a m o r e p r a c t i c a l d e s i r e t o k n o w a n d u n d e r -s t a n d i t . W a s i t , a s t h e G r e e k A n a x a g o r a s s a i d , am a s s o f f i e r y s t o n e , 5 6 k m a c r o s s ? O r , a s d i c t a t e db y m e d i e v a l d o g m a , a p e r f e c t f i r e ? H o w f a r a w a yw a s i t ? H o w l ar g e ? H o w h o t ? H o w c o n s t a n t ? H o wl o n g w o u l d i t l a s t ? W h a t w e r e t h e s p o t s s e e n r ep e a t e d l y o n i t s s u r f a c e b y C h i n e s e a s t r o n o m e r s 2 0y e a r s b e f o r e C h r i s t , a n d b y G a l i l e o w i t h a te l e s c o p e

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in 16 11 ? W e r e th e r e c h a n ge s o n t h e S u n t h a t m i g h te x p l a i n the cyc l ic change of w e a t h e r a n d c l i m a t eo n E a r th ? The i ce ages? T h e r e c u r r e n t d r o u gh t sa n d t h e i r im p l i c a t i o n s f o r w o rld c o n c e r n s of the1970 's?T o d a y , a b o u t 20 p e r c e n t of th e wor ld ' s a s t rono-me r s , i n c lu d in g o v e r 2 0 0 in th e U n i t e d S t a t e sa lone , a re engaged in the fu l l - t im e s tud y of theSu n . T h e y w o r k in univer s i t ies , observa tor ies , inGo v e r n m e n t l a b o r a to r i e s , a n d i n p riv a t e i n d u s t r y .A l a rg e n u m b e r o f a m a t e u r s m a k e so la r a s t r o n o m ythe i r spec ia l ty .The S un is s tud ied by a s t rop hys ic is t s a s a s t a r th e o n ly o n e t h a t can be seen in de ta i l . It is s t u d i e db y p h y s i c i s t s a s a l a b o r a to r y w h e r e u n iq u e c o n d i -t i o n s o f t e m p e r a t u r e , d e n s i t y , f lu id m o t i o n s , a n dm a g n e t i c f ie lds ex is t . It is s t u d i e d b y a t m o s p h e r i cphys ic is t s , ae ronomers , and c l ima to logis t s fo r i t s

impo r t a n t t e r r e s t r i a l e f fec ts .Sola r r esea rch is p u r s u e d in a l m o s t al l the na-t i o n s o f t h e w o r ld , a n d , b y n e e d a s mu c h a s b yt r a d i t i o n , i t ha s en joyed a v igorous sp i r i t o f in te r -n a t i o n a l coopera t ion . In the la s t decades , th roughthe co ldes t w in te r s o f po l i t ica l and idea lo gica l war ,so la r da ta a n d resea rch resu l t s f lowed f ree ly b e-tween so la r observa tor ies o f Eas t and West .

In these t r a d i t i o n s S k y l a b g a v e m u c h of i ts b u d -get of t i m e a n d resources to an i n t e n s iv e , c o o pe ra -t ive s t u dy o f o u r S u n , to a c h ie v e w h a t n o w s e e msto be the grea tes t step e v e r ma de i n the long his-tory o f m a n ' s s t u d y of the S u n .O b s e r v a t i o n s of the Sun f r o m Sk y l a b h a v echanged th e course o f modern so la r phys ics a n dh a ve b u i l t a l a s t in g b a n k o f so la r da ta t ha t w i l l longserve a s t r o n o m e r s o f a l l th e w o r ld . T h e po w e r o fS k y l a b was i t s a b i l i t y t o k e e p a c o n t in u o u s w a t c hon the Sun s i m u l t a n e o u s l y in al l w a v e l e n g t h s , in -c l u d i n g t h e c r u c i a l u l t r a v io l e t a n d X - r a y r a d i a t i o nn o r ma l l y b lo c k e d b y o u r a tmo s ph e r e . T h i s po w e rw as b r o u g h t to bea r by th e force of the mo s t so -p h i s t i c a t e d a r r a y o f t e lescopes ever tu rned on th eSun , the a dv an t ag es o f t r a ined observers in space ,a n d t h e s u ppo r t o f t h e mo s t e x t e n s iv e w o r ld e f fo r tever o rgan ized to wres t th e secrets f rom th e S u n .T h e r e s u l t s e x c e e de d a l l e x pe c t a t i o n s .Th e a i m o f th is b o o k is to e x p l a i n th e m a j o rf in d in gs o f S k y l a b ' s sola r e f f o r t , a n d t o s h o w th en e w a n d c o l o r f u l S u n t h a t S k y l a b b ro u g h t to v i e w .T o a n s w e r th e i n e v i t a b l e " W h a t d o w e k n o w n o wth a t w e d id n o t k n o w b e f o r e ? " w e m u s t fi rs t e x -p l a i n , i n b r o a d t e r ms , w h a t w e k n e w o f t h e Su nb e f o r e Sk y l a b , a n d h o w w e c a me t o k n o w i t .

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T h e M as ks of the SunW h e n a n o b s e r v e r , p r o v i d e d w i t h s u i t a b l e t e l e s c o p i c appl i-a n c e s , e x a m i n e s t h e s u r f a c e o f t h e s u n , h e f i n d s a m o s t i n t e r e s t i n g

field before h i m . At first v i e w , indeed, it is less impressive t h a nth e m o o n ; t h e r e i s n o t s o m u c h to a t t r a c t t h e i m m e d i a t ea t t e n t i o n - n o m o u n t a i n - r a n g e s a n d craters, n o s h a d o w s , rills, o rrays. B u t , if the telescope is a good one and the a t m o s p h e r i cc o n d i t i o n s f a v o r a b l e , th e details s o o n b e g i n t o c o m e o u t .-Charles A . Y o u n g , 1 8 9 6

' h e s t o r y o f w h a t w e k n o w o f t h e S u n i s also_ L th e h i s t o r y o f s o l a r i n s t r u m e n t s : th e t e l e -s c o p e , s p e c t r o s c o p e , a n d m a n y o t h e r s . W i t h o u tt h e m w e w o u l d k n o w l i t t l e m o r e o f t h e S u n t h a ni t s p o s i t i o n i n t h e s k y . I n d e e d , m a n ' s u n d e r s t a n d -i n g o f t h e S u n h a s n e v e r p r o c ee d e d r e g u l a r l y , b u ta l w a y s i n s u r g e s , f o l l o w i n g n e w i n s t r u m e n t a l b r e a k -t h r o u g h s . E a c h n e w lo o k , fr o m G a l i le o ' s s i m p l et e l e s c o p e t o S k y l a b ' s p o w e r f u l a r r a y , h a s b r o u g h t an e w a n d o f t e n s u r p r i s i n g v i e w o f w h a t w a s o n c et h o u g h t t o b e a s i m p l e s p h e r e o f f i r e . E a c h n e wl o o k h a s s h o w n t h e S u n t o b e m o r e a c t i v e a n dm o r e c o m p l e x . I n m a n y c a s e s , n e w w a y s o f l o o k i n gh a v e r e v e a l e d n e w a n d p r e v i o u s l y h i d d e n f a c e s o ft h e S u n , f o r t h e S u n ' s a p p e a r a n c e c h a n g e s d r a m a t i -cally f r o m l a y e r t o l a y e r , a s t h o u g h i t w o r e m a s ko n m a s k o n m a s k .T H E S U N I N W H I T E L IG H TT h e P h o t o s p h e r e

T h e m a s k m o s t c o m m o n l y s e e n , a n d t h e f i r s t t ob e s t u d i e d i n d e t a i l , i s t h e S u n ' s w h i t e l i g h t f a c e :t h e l a y e r i n w h i c h s u n s p o t s a r e s e e n , a n d w h i c ha s t r o n o m y ca l l s t h e s p h e r e o f l i g h t o r p h o t o s p h e r e .It i s t h e w h i t e , b r i g h t s u r f a c e w e s e e w i t h t h en a k e d e y e t h e l u m i n o u s , 6000 s u r f a c e t h a t

r a d i a t e s m o s t o f t h e S u n ' s l i g h t a n d h e a t to s p a c e .T o m o s t o f u s i t i s t h e S u n .T h e p h o t o s p h e r e i s a v e r y t h i n l a y e r o f t h eSun-no m o r e t h a n a b o u t 5 0 0 k m t h i c k , o r l e s s t h a n0 . 1 p e r c e n t o f t h e s o la r r a d i u s . W e c a n s e e n od e e p e r t h a n t h i s t h i n s h e l l ; a l t h o u g h g a s e o u s a n dd i f f u s e , i t b e c o m e s c o m p l e t e l y o p a q u e i n a r e la -t i v e l y s h o r t d i s t a n c e , a n d t h a t i s w h y t h e e d g e o ft h e S u n a p p e ar s s h a r p l y d e f i n e d , a n d w h y e a r l ya s t r o n o m e r s w e r e led to s u p p o s e i t c o u l d b e so l ido r l i q u i d . T h e p h o t o s p h e r e w as l o ng t h o u g h t t o b ep e r f e c t l y s p h e r i c a l a n d w i t h o u t b l e m i s h , b u t i n t h ee a r l y y e a r s o f t h e 1 7 t h c e n t u r y , w i t h t h e i n v e n t i o no f t h e a s t r o n o m i c a l t e l e s c o p e , m a n ' s e y e s w e r es h o w n t h e f i r s t d e t a i l s o f i t s i n t r i c a t e a n d c h a n g i n gs u r f a c e .S u n s p o t s

T h e f i r s t a n d m o s t r e m a r k a b l e f e a t u r e s f o u n dw e r e s u n s p o t s . D a r k f e a t u r e s o n t h e S u n h a v e b e e nseen an d r e c o r d e d f o r a t l e a s t 2000 y e a r s . R e c o r d sf r o m C h i n a a n d t h e O r i e n t r e p o r t , o n t h e a v e r a g e ,a b o u t 5 o r 1 0 s i g h t i n g s p e r c e n t u r y , w h i c h ar et a k e n n o w a s e v i d e n c e o f u n u s u a l l y l a r g e s p o t s o rs u n s p o t g r o u p s , b e c a u s e s m a l l e r f e a t u r e s c o u l d n o tb e s e e n . I t i s n o t d i f f i c u l t t o s e e l a r g e s u n s p o t s w i t ht h e n a k e d e y e i f o n e l o o k s q u i c k l y w h e n t h e


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T r a n s i t ion region Cor on a

I M A G I N A R Y S L I C E c u t f r o m t h e b a l l o f t h e S u n l e t s u ss e e a l l i t s k n o w n a n d s u s p e c t e d l a y e r s . I n t r u t h , n o o n e h a se v e r s e e n b e n e a t h t h e p h o t o s p h e r e . T h e S u n ' s p r o d i g i o u se n e r g y i s b e l i e v e d t o b e g e n e r a t e d b y n u c l e a r processes i nt h e d e e p i n t e r i o r o f t h e Sun, i n a dense, h o t c e n t r a l c o r e ,a b o u t t h e s i z e o f J u p i t e r , t h e l a r g e s t p l a n e t . C o r e t e m p e r a -t u r e i s e s t i m a t e d t o b e a b o u t 1 5 X 1 0 ^ K . : h o t e n o u g h t ot r i g g e r t h e p r o c es s o f a t o m i c f u s i o n b y w h i c h h y d r o g e n , t h eS u n ' s a b u n d a n t f u e l , i s c o n v e r t e d t o h e l i u m . E n e r g y r e -leased f r o m t h i s c o n t i n u i n g process seeps o u t w a r d t h r o u g ht h e de n s e i n t e r i o r b y r a d i a t i o n t h e s a m e m e t h o d t h a t car-r i e s s u n l i g h t t h r o u g h t h e s o l a r s y s t e m . N e a r t h e s o l a r sur-f a c e , t h e o u t w a r d f l o w o f e n e r g y b e c o m e s t u r b u l e n t a n du n s t a b l e , c r e a t i n g g i a n t , r i s i n g b u b b l e s o f g a s t h a t m a k e u pt h e c o n v e c t i v e zo n e . F i n a l l y , i n a t h i n l a y e r c a l l e d t h e p h o -

t o s p h e r e , t h e s o l a r e n e r g y t h a t h a s w o r k e d i t s w a y t o t h es u r f a c e a t l a s t b e c o m e s v i s i b l e a s t h e w h i t e b a l l o f l i g h t w es e e e a c h d a y w i t h t h e n a k e d eye. T o escape t h e S u n t h el i g h t m u s t s t i l l pass t h r o u g h t h r e e m o r e l a y e r s , e ac h lessd e n s e t h a n t h e o n e s b e f o r e : t h e c h r o m o s p h e r e , h e r e s h o w no v e r t h e f u l l d i s k o f t h e Sun; t h e t h i n t r a n s i t i o n region; a n dth e e t h e r e a l , e x t e n d e d c o r o n a , la r g e s t o f t h e m a n y m a s k s o ft h e Sun.T e m p e r a t u r e i n t h e S u n i s h o t t e s t i n t h e c o r e , a n dd e c r e a s e s s l o w l y t o t h e s u r f a c e . A t t h e p h o t o s p h e r e t h es o l a r t e m p e r a t u r e h as d r o p p e d to a b o u t 6000 T h e t e m -p e r a t u r e r e a c h e s a m i n i m u m o f a b o u t 4300 i n t h e l o wc h r o m o s p h e r e , t h e n rises r a p i d l y to m o r e t h a n 1 0 ^ i n t h el o w e r c o r o n a . W a v e s c a r r y i n g en e r g y u p w a r d f r o m t h e con-v e c t i v e z o n e p r o b a b l y r e l e a s e t h e i r e n e r g y i n t h e c h r o m o -s p h e r e , c a u s i n g t h e a b r u p t t e m p e r a t u r e r i s e .

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b r i l l i a n c e o f t h e S u n i s d u l l e d i n s e t t i n g o r i n r i s i n g ,o r w h e n i t i s r e d d e n e d b y h e a v y s m o k e o r h a z e .B u t f e w , i f a n y , o f t h e e a r l y r e c o r d s s e e m to h a v ea s s o c i a t e d t h e s p o t s w i t h t h e S u n i t s e l f , p r o b a b l yb e c a u s e th e d a r k s p o t s w e r e s o p o o r l y an d i n t e r -m i t t e n t l y s e e n . T h e y c o u l d b e m i s t a k e n f o r d a r kf e a t u r e s f l o a t i n g i n o u r o w n a t m o s p h e r e , o r p l an e t sb e t w e e n E a r t h a n d t h e S u n .T h e f i r s t t e le s c o p e s , i n t r o d u c e d b y G a li l e o a n ds e v e r al o t h e r E u r o p e an a s t r o n o m e r s i n a b o u t 1 6 1 0 ,d i d n o t p r o p e r l y d i s c o v e r s u n s p o t s , b u t r a t h e r g a vet h e f i r s t c le a r l o o k s a t t h e m . T h e y r e v e a l e d t h a t t h espots w e r e m o r e t h a n d o t s . G a l i l e o a n d o t h e r sd e m o n s t r a t e d i n c a r e f u l , p u b l i s h e d d r a w i n g s t h a tth e s p o t s c o n s i s t o f a d a r k , c e n t r a l r e g i o n c a l l e d th e" u m b r a , " m e a n i n g "shadow," s u r r o u n d e d b y al i g h t e r r i m called th e " p e n u m b r a , " o r " p a r t i a ls h a d o w . "S U N S P O T O B S E R V A T I O N S i n t h e 1 7 t h c e n tu r y w e r emade m u c h as t o d a y . T h e s o l a r i m a g e i s p r o j e c t e d b y al o n g - f o c u s t e l e s c o p e onto a card, w h e r e o b s e r v e r s t r a c e t h ed e t a i l s a n d p o s i t i o n s o f i n d i v i d u a l spots, n o t i n g c h a n g e s a n dm o t i o n s . H e r e C h r i s t o p h e r S c h e m e r a n d a f e l lo w J e s u i ts c i e n t i s t trace s u n s p o t s i n I t a l y i n a b o u t 1 6 2 5 .

d? w ., fA N E A R L Y D R A W I N G o f t h e S u n an d s u n s p o t s b yJ o h a n n e s H e v e l iu s , r e n o w n e d P o l is h a s t r o n o m e r o f t h e 1 7 t hc e n t u r y . H e r e H e v e l i u s shows th e path a n d c h a n g e s i n s u n -spots t h a t crossed th e d i s k o f t h e S u n b e t w e e n M a y 2 2 a n dM ay 3 1 i n 1643 a s t h e y w e r e s e e n i n D a n z i g .

**cui*e* u a r i i s o b f e r u a n d j m o d i s . f l d b i t i u n t u r .T H E M A S K S O F T H E S U N 9


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B e t t e r t e l e s c o p e s s h o w e d s u n s p o t s i n b e t t e rd e t a i l . T h e s u n s p o t s ar e u s u a l l y i r r e g u l a r i n s h a p e ,s o m e l a r g e a n d s o m e s m a l l , a n d t h e y m o s t o f t e na p p e a r i n g r o u p s , f r e q u e n t l y w i t h t w o p r i n c i p a ls p o t s pe r g r o u p . P e n u m b r a s a r e m a d e u p o ff i l a m e n t s t h a t s t r e a m o u t w a r d f r o m t h e c e n t r a lu m b r a i n r a d i a l f a s h i o n . I n d i v i d u a l s u n s p o t s p e r s i s tf r o m s e v e r a l d a y s to as l o n g a s s e v e r a l m o n t h s ,g e n e r a l ly i n c r e a s i n g i n s i z e a n d c o m p l e x i t y b e f o r ef i n a l l y f a d i n g a w a y .O n c e t h e s p o t s w e r e s h o w n t o b e a p a r t o f t h eS u n , s c i e n t i s t s u s e d t h e m t o d e m o n s t r a t e t h a t t h eS u n r o t a t e d , b y w a t c h i n g a n d m e a s u r i n g t h e i ra p p a r e n t d r i f t a c r o s s t h e f a c e o f t h e S u n f r o m d a yt o d a y .T h e S u n r o t a t e s i n a b o u t 27 d a y s . I t d o e s n o tr o t a t e l i k e a r i g i d b a l l ; t h e S u n ' s a t m o s p h e r e s p i n sf a s t e r a t t h e e q u a t o r t h a n n e a r i t s p o l e s , i n t r o d u c -i n g a k i n d o f a t m o s p h e r i c s l i p p a g e o r s h e a r k n o w na s " d i f f e r e n t i a l r o t a t i o n . "B u t w h a t w e r e t h e s e d a r k s p o t s i n t h e p h o t o -s p h e r e ?M o s t e a r l y a s t r o n o m e r s b e l i e v e d t h e m t o b ec l o u d s d r i f t i n g o v e r a n u n d i s t u r b e d a n d s t i l l c o m -f o r t a b l y p e r f e c t s o l a r s u r f a c e . I n 1 8 0 1 , S i r W i l l i a mH e r s c h e l , d i s c o v e r e r o f t h e p l a n e t U r a n u s a n d t h el e a d i n g a s t r o n o m e r o f h i s d a y , p u b l i s h e d a d i f f e r -e n t v i e w . S u n s p o t s , H e r s c h e l s a i d , w e r e n o t c l o u d sb u t h o l e s i n c l o u d s . T h e w h i t e p h o t o s p h e r e , i n h i so p i n i o n , w a s a n i n c a n d e s c e n t c l o u d l a y e r t h a tc o v e r e d a d a r k e r , c o o l e r l a y e r b e l o w ; s u n s p o t so f f e r e d o c c a s i o n a l v i e w s o f t h i s l o w er s u r f a c e ,w h i c h , H e r s c h e l f e l t ,. . . i s m o s t p r o b a b l y a l s o i n h a b i t e d , l i k e t h e r e s t o f t h ep l a n e t s , b y b e i n g s w h o s e o r g a n s a r e a d a p t e d t o t h e p e c u l i a rc i r c u m s t a n c e s o f t h a t v a s t g l o b e .

O t h e r e x p l a n a t i o n s o f t h e 1 9 t h ce n t u r y a t t r i b u t e ds u n s p o t s t o s t o r m s , t o b u b b l e s o n a l i q u i d S u n .s p l a s h m a r k s o f m e t e o r i m p a c t s o r o f d e b r i st h r o w n o u t b y s o l ar e r u p t i o n s , m o u n t a i n s p o k i n gt h r o u g h t h e c l o u d s , o r s o l i d s l a g o n a w h i t e - h o t ,m o l t e n s u r f a c e .T h e s e f a n c i f u l t h e o r i e s o f b u t a c e n t u r y a g o a r eo f i n t e r e s t b e c a u s e t h e y r e v e a l t h e l i m i t a t i o n s o fa s t r o n o m y w i t h o u t p h y s i c s : o f p i c t u r e s a n d v i s u a lo b s e r v a t i o n s w i t h o u t t h e b e n e f i t o f a c c o m p a n y i n gm e a s u r e m e n t s a n d d e d u c t i o n s o f p h y s i c a l q u a n t i -t i es , s u c h a s t e m p e r a t u r e , d e n s i t y , v e l o c i t y , a n dc h e m i c a l c o m p o s i t i o n .T h e u s e o f p h y s i c a l m e t h o d s i n a s t r o n o m y , n o wc a l l e d a s t r o p h y s i c s , b e g a n i n e a r n e s t i n t h e l a t t e r

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T H IS R E M A R K A B L Y D E T A I L E D D R A W I N G o f a s i n g l es u n s p o t w a s m a d e b y t h e e m i n e n t A m e r i c a n s c i e n t is tS a m u e l L a n g l e y , i n t h e c o l d , c lear a i r o f P i t t s b u r g h , j u s tb e f o r e C h r i s t m a s i n 1 8 7 3 . L a n g l e y l a t e r b e c a m e th e S e c r e -t a r y ( D i r e c t o r ) o f t h e S m i t h s o n i a n I n s t i t u t i o n i n W a s h i n g -t o n , D . C . , a n d i n t h e e a r l y 1900's w a s a p i o n e e r i n a v i a t i o n .P h o t o g r a p h s m a d e i n m o d e r n t i m e s c o n f i r m m u c h o f t h ed e t a i l c a p t u r e d b y e a r l y v i s u a l o b s e r v e r s s u c h a s L a n g l e y ,w h o s p e n t l o n g h o u r s a t t h e i r t e l e s c o p e s t o c a t c h m o m e n t so f u n u s u a l s k y c l a r i t y .1 0 A N E W S U N


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pa r t of th e 19 th c e n tu r y , w i th the use of thes pe c t r o s c o pe . This dev ice , wh ich w e wi l l soonde s c r ib e , w a s t h e s e c o n d o f t h e i n s t r u m e n t a lb reak th roughs tha t r e lea sed a n a v a l a n c h e o f n e wknowledge of the Sun.T he f i rs t s tudies o f the spec t rum of l igh t f roms u n s po t s , u s in g a spec t roscope , showed tha t theywere cooler b y a b o u t 3 0 pe r c e n t t h a n th e sur-r o u n d in g ph o to s ph e r e . A t y p i c a l t e mpe r a tu r e f o rthe darkes t and cooles t p a r t o f a s u n s po t is a b o u t4300 K, which i s s t i l l h o t t e r t h a n a n a c e ty l e n ew e l d i n g f l a me . T h e ph o to s ph e r e , i n c o n t r a s t , isa b o u t th e t e m p e r a t u r e of a n e lec t r ic welding a rc .The sp ec t roscope a l so r evea led tha t ma ter ia l insunsp o t s f lowed genera l ly upward a n d o u t w a r df r o m t h e u m b r a , a l o ng t h e p e n u m b r a l f i l a m e n t s ,sugges t ing tha t sunspo ts were sea ts o f s t r o n g ,c i r c u l a t o r y mo t io n , pe r h a ps l i k e t e r r e s t r i a l t o r n a -

does. If sunspots were so la r s to rms , as m a n y t h e nsup p osed , they were gigantic an d persis tent b yt e r r e s t r i a l s t a n d a r d s . M a n y of the spo ts observedw e re as la rge a s Ea r th , and some were 10 t im esl a r g e r t h e s ize o f Jupi te r . They a l so were longerl a s t ing than te r res t r ia l s to rms .A n a da p t a t i o n of the spect roscope by GeorgeHa l e of the Mt . Wilson Observa tory in the ea r lyy e a r s o f t h i s c e n tu r y p r o v ide d t h e r e a l k e y t o t h eu n d e r s t a n d i n g o f s u n s po ts . H a l e f o u n d t h a t w i th ins u n s p o t s w e r e m a g n e t i c f i e lds o f s u c h e n o r mo u ss t r engths t h a t they d o m i n a t e d a ll ma te r i a l in t he i rv i c i n i t y . T h i s p r o v ide d a n e x p l a n a t i o n f o r a lmo s tall the observed fea tu res o f sunsp o ts : the i r coo le rt e m p e r a t u r e s (ma gn e t i c f i e l d s i n h ib i t e d t h e n o r ma lf low of hea t toward the sur face) , the a rchedpe n u mb r a l f i l a me n t s (ma t t e r a r r a n ge d o n ma gn e t i cl ines of fo rce) , the f r equen t occur rence of s po t s inpa i r s (o f o ppo s i t e m a gn e t i c po l a r i t y ) , a n d t h e i rlong per s is tence .T he S u n s p o t Cycle

Su n s p o t s , t o ma n y , a r e be s t k n o w n in c on n e c -t i o n w i th th e sunspot cyc le : th e r e gu l a r pa t t e r n int h e n u m b e r o f sp o t s t h a t a ppe a r o n t h e Su n f r o my e a r to y e a r .C o n t in u o u s o b s e r v a t i o n fo r mo r e t h a n tw o cen-tu r ies has es tab l i she d tha t s unspo ts r each a m axi-mu m n u m b e r e v er y 1 1 .2 y e a r s, o n t h e a v er a ge .B e t w e e n t i m e s o f m a x i m a , t h e i r n u m b e r f a l l s to awe l l -de f ined m i n i m u m . A t t h e t i m e o f m a x i m u m inth e sunspot cyc le we m a y f ind 1 00 o r mo r e s po t so n t h e visible h e mis ph e r e o f t h e Su n a t o n e t ime ;

A M O D E R N P H O T O G R A P H o f sunspots t a k e n a t theS a c r a m e n t o P e a k S o l a r O b s e r v a t o r y i n N e w M exico r evea l st h e i r i n t r i c a t e p a t t e r n s a n d t h e f i l a m e n t a r y n a t u r e o f s u n -spot p e n u m b r a s . Only a small portion of the solar surf ace iss h o w n in t h i s f i n e p h o t o g r a p h . T h e c e n t e r ( u m b r a ) o f t h el a r g e s u n s p o t a t b o t t o m is ha l f a ga in a s l a rge a s o u r E a r t h .T H E MAS KS O F T H E S U N 1 1


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140120100

604 020 A -

1610 1620 1630 1640 1650 1660 1670 1680 1690 1700 1710 1 720 1 730160140

1730

160140

1740 1750 1760 1 7 7 0 1780 1790 1800 1810 1820 1830 1840 1850200180

1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960T H E S U N S P O T C Y C L E , 1 6 1 0 - 1 9 7 6 . S u n s p o t s c o m e a n d g oo n t h e S u n i n a n i r r e g u l a r c y c l e o f a b o u t 1 1 y e a r s , w i t hs o m e o b v i o u s l o n g - t e r m c h a n g e s . A n n u a l m e a n s u n s p o tn u m b e r s ( a n a v e r a g e m e a s u r e o f s p o t t e d n e s s o f t h e S u n ) ar es h o w n h e r e f o r e a c h y e a r s i n c e t h e d i s c o v e r y o f t h e tele-s c o p e . D a t a o n t h e e a r l i e r p a r t s o f t h e g r a p h a r e l e s s r e l i a b l et h a n i n l a t e r p e r i o d s .

1970

1 2 A NEW SUN


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a t the t i m e o f s u n s p o t m i n i m u m v e ry few are seen,a nd fo r p e r i od s as l ong a s a m o n th non e ca n bef o u n d .Th e l a s t m a x i m um of th e sunsp o t cyc le wa s i n1968-69 , a nd th e one be fo r e th a t , wh i ch w a sp a r t i cu l a r l y s t rong , was in 1957-58. The nextshould come in ea r ly 1980. The las t minimum ofsunsp o t s wa s in 1 9 7 6 . S ky la b t o o k i t s observa t ionso f the Su n in the dec l in ing phase of the sunspo tcyc le , a s t h e num ber o f sp o t s d ec r ea sed towa r dt h e 1 9 7 6 m i n i m u m .To th e as t ronomer , th i s long , c lea r record o fs u n s p o t n u m b e r s i s ev i d ence th a t th e m a g n e t i cd i s t u r b a n c e s r e sp ons i b l e fo r spots a re d r i ven b ysom e cyc l i c m ech a n i sm , p r oba b ly i nvo lv i ng th eforces of the d i f feren t i a l ro ta t ion of the Su n, a t andbe low th e level of the p h o tosp h e r e . T h e search fo ra com p le te d esc r i p t i on o f t h e ca use o f sunsp o t sa nd th e sola r cyc le h a s been one o f th e l o n g a n dc o n t i n u i n g e f f o r t s o f t h eo r e t i c a l a s t r op h ys i c s .O th e r i n t e r e s t in the sunsp o t cyc le sp r ing s fr omi ts f r eq uen t a s soc i a t i on , in fac t a n d f i c t i o n , w i t hcyclic changes o n E a r th .Soon a f te r the sunsp ot cyc le was d i scovered ,m o r e t h a n a cen tu r y a g o , a s t ro n g r e l a t i o n s h i p w a sn o t e d b e t w e e n t h e n u m b e r o f s p o ts o n t h e S u n a n dt h e n u m b e r o f a u r o r a s t ha t were seen in thenor th e r n ( a nd l a t e r t h e sou th e r n ) sk ie s . A s i m i l a rr e l a t i o n s h ip w a s soon es t a b l i sh ed be tween th en u m b e r o f s u n s p o t s a n d t h e s t a t e o f E a r th ' sm a g n e t i c f i e ld : W h e n t h e Su n sh owed m or e sp o t s ,E a r th ' s m a g n e t i c f i eld w a s m or e f r eq uen t ly d i s -t u r b e d by v i o l en t " m a g ne t i c s t o r m s . " I n a d d i t i o n ,i t h a s been known fo r m a ny yea r s th a t t h e s t a t e o fE a r t h 's i o n o s p h e r e t h e i o n iz e d l a y e r of the higha tm osp h er e th a t m a k es p ossi b le l on g -d i s t a nce r a d i oc o m m u n i c a t i o n - i s a f f e c t e d v e ry c r i t ic a l l y b y sola ra c t i v i t y , in s t ep w i th th e n u m b e r o f sunsp o t s . Ineach o f these cases , however , i t i s not the sp o t st h e ms e lv e s tha t br ing a b o u t t h e t e r r e s t r i a l ch a ng es ,b u t o t h e r , l es s f r e q u e n t a n d m o r e d y n a m i c e v e n t ssuch a s so la r f l a r e s a nd e r up t i ons i n th e ch r o m o -sphere an d c o r o n a . These events o c c u r m o r e of tenw h e n the Sun i s m or e sp o t t ed a n d ac t ive. Becauses u n s p o t s a re the most eas i ly observed of a l l solarf e a t u r e s , t h ey h a ve t r a d i t i ona l ly se r ved as thes to r m wa r n i ng s o f t h e so l a r sys t em .C an these warnings also serve to p r ed i c t m or ei m m e d i a t e e f f ec t s on E a r th , such a s ch a ng es i n ou rw e a t h e r a n d c l i m a t e ?

Th i s i m p o r t a n t q ues t i on , so f r eq ue n t ly a sked , i snot eas i ly a nsw ered . Change in sola r ou tpu t i s onlyone of a number of poss ib le causes of change in thelower a tm osp h er e o f E a r th . Th e S un i s a l i ke ly ca useof long- term c l imate change-as m ea sur ed i n t i m escales of mi l lenia , centur i es , and poss ib ly dec-ades. I t i s a lso a possible cause o f s ho r t e r termc l i m a t e ch a ng e . It is p r o b a b l y not a d o m i n a n tcause of d a y - to -d a y o r l oca l wea th e r p a t t e r ns .These a re a t p resent on ly theor ies , because meas-u r e m e n t s a n d p h y s ic a l u n d e r s t a n d i n g in th i s a rea o fso l ar a nd a tm osp h er i c p h ys i c s a r e ve r y l i m i t ed .Befo re m or e sa t i s f a c to r y a nswer s c a n b e g i ven ,w e need fac ts in two speci f i c a reas of research . Thefirst is the s t i l l una nswer ed q ues t i on o f w h a t a n dh o w m uch ch a ng e occu r s in the v a r i o u s o u t p u t s o ft h e S u n a s t h e s u n s p o t n u m b e r w a x e s a n d w a n e s .T h is i n c l u d e s n o t o n l y i t s v a r io u s c o m p o n e n t s o fh e a t a n d l i g h t b u t a lso th e f low o f sola r par t i c les inth e sp a ce be tween t h e S u n a n d Ear th . T h e secondn e e d is for a m o r e c o m p l e t e u n d e r s t a n d i n g o f w o r l d -w ide c i r c u l a t i o n , w e a t h e r , r a d i a t i o n , a n d c l i m a te .A tm osp h er i c p h ys i c i s t s a re n o w able to m o d e l th ec o mple x i t i e s o f g l o b a l w e a t h e r a n d c l i m a te w i tha dv a n c e d c o m p u t e r s . On the solar side, f u t u r em e a s u r e m e n t s f r om sp a ce sh ou ld a d d m uch to th esca t t e r ed d a t a o n so la r ou tp u t t h a t n o w exis t ,espec ia l ly f r o m i n s t r u m e n t s o r programs tha t con-t i n u e t h r o u g h a com p le te so l ar cyc le of 11 yea r s o rm o r e .G r a n u l a t i o n

Th e p h o tos p h e r e , i n sp it e o f i t s wh i t e -h o tt e m p e r a t u r e an d i n t e n s e m a g n e t i c f ields, is are l a t ive ly q u i e t l a ye r of the S u n , a s c o m p a r e d w i t hth e m o r e t u r b u l e n t l a y e rs a b o v e i t . B u t c loser looksa t the wh i te l i g h t su r f a ce , w i th g ood t e l e scop esu n d e r g ood obse r v i ng cond i t i ons , r evea l a n o v e r a l lp a t t e r n o f c h a n g e a n d m o t i o n . T h e p h o t o s p h e r e ,u n d e r h ig h m a g n i f i c a t i o n , a p p ea r s com p le te lycove r ed w i th a n i r r eg u la r p a t t e r n o f cel ls ca l ledgr a n u l e s , wh i ch l ook som eth i ng l i ke th e p o l y g o n a lcells o f a h o n e y c o m b .T o sc i en t i s t s w h o s t u d y f l u i d a n d gaseousmot ions , these cel ls bet ray the p resence of convec-t i on , as in the case o f t u r b u l e n t , b i l l o w i n g c l o u d so r b ubb les i n a bo i l i ng p o t . Convec t i on i n th epho tosp here ca r r i es hot ga s upw ard in g lobs orcells ; t h ese g r a n u les r a d i a t e som e o f th e i r h ea t

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e n e rg y a n d th e n s in k b a c k do w n . I n d i v i d u a l cellsmo v e u p a n d do w n a t r e l a t i v e ly s l o w s pe e ds ( a b o u t1 8 k m / m i n a n d l a s t abou t 8 m i n , on th e a v e r a ge ,b e f o r e b r e a k in g up; the s o la r g r a n u l a t i o n pa t t e r n ist h u s c o n s t a n t l y c h a n g in g . A l t h o u g h i n d i v i du a lg ra n u l e s a r e h a r d t o de t e c t a t o u r d i s t a n c e f r o m th eSu n , t h e y a r e a b o u t 180 0 k m in d i a m e te r , n e a r l yh a l f th e size of the c o n t i n e n t a l U n i t e d S t a te s . A swe sha l l see , t h e up a nd d o w n m o t i o n s of thep h o t o s p h e r i c g r a n u l a t i o n b e c o m e m o r e e v i d e n t a n da m p l i f i e d in the so la r layer s above the p h o t o -s ph e r e .S k y l a b observed the pho tosphe re in whi te l igh tw i t h s e v e ra l i n s t r u m e n t s a n d k e p t i t s o w n p h o t o -gr a ph ic r e c o r d o f s u n s po t s , c h i e f l y f o r c o mpa r i s o nw i t h o b s e r v a t i o n s i n o the r w a v e l e n g t h s a n d w i t hs o l a r o b s e r v a t i o n s ma de on th e g r o u n d d u r i n g th eS k y l a b mis s io n .T h r o u gh th e Sp ec t roscop e : th e C h r o mo s ph e r e ,C o r o n a , a n d Prominences

A b o v e t h e ph o to s ph e r e a n d s u n s po t s l i e tw oe x t e n s i ve r egions of the Sun t h a t w e c a n ca l l i tso u t e r a t m o s p h e r e - t h e c h r o mo s ph e r e a n d c o r o n a .T he c h r o mo s ph e r e , or "color s phe re , " is a l a y e rsevera l t imes th icker than th e ph o to s ph e r e . It isstill a t h in l a y e r , e x t e n d in g a b o v e t h e ph o to s ph e r eto a m i n i m u m h e i g h t o f a b o u t 2000 km . I t e n d s ina r a gge d , i r r e gu l a r b o u n da r y w i th o c c a s io n a l s p ik e dextens ion s , ca l led sp icu les, tha t s h o o t u p a na d d i t i o n a l 5000 t o 1 5 000 km i n t o th e v a p o r o u s" c r o w n of the S u n " o r corona . Sp icu les r ise a n df a l l l i k e c h o p p y w a v e s o n a s t o r my s e a , in pe r io dsof 5 to 10 m i n . The top of the c h r o m o s p h e r e , l i k eth e t o p o f t h e o c e a n , i s t h u s i n c o n s t a n t a n dd y n a m i c c h a n g e .The dens i ty in these h igh layer s o f the Sun i s bu ta f r a c t i o n o f t h a t i n t h e ph o to s ph e r e , w h ic h i si t se l f n e a r l y a v a c u u m l i k e t h a t of the u p p e r m o s ta tmo s ph e r e o f E a r th . B e c a u s e t h e y a r e s o d i f fu sea n d s o m u c h f a i n t e r t h a n t h e ph o to s ph e r e , t h ec h r o mo s ph e r e a n d c o r o n a c a n n o t b e s e e n w i th t h en a k e d e y e o r s imp le t e l e s c o pe . T h e y can bed e t e c t e d , m o m e n t a r i l y , a t e c l ip s e , w h e n t h e S u nb lo t s o u t t h e p h o to s ph e r e , o r se e n i n mo r e l i m i t e df a sh i o n b y c a r e f u l l y e x c l u d i n g th e g l a r e of the so la rd i sk a n d l o o k i n g a t t h e e x t r e m e l i m b o f t h e S u nw i t h i n s t r u m e n t s d e s ig n e d fo r the p u r p o s e . A m o s tu s e f u l too l fo r observ ing these ou te r r e g i o n s - a n dt h e i r r e a l d i s c o v e r e r - i s t h e s pe c t r o s c o pe , w h ic h

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