T H E X U V S T R U C T U R E O F S O L A R A C T I V E R E G I O N S

KENNETH P. DERE

E. O. Hulburt Center[or Space Research, Naval Research Laboratory, Washington, D.C. 20375, U.S.A.

(Received 17 November, 1980; in revised form 9 March, 1981)

Abstract. XUV spectroheliograms of 2 active regions are studied. The images are due to lines emitted at temperatures between 8 x 10 4 K and 2 • 10 6 K and thus are indicative of transition region and coronal structures. The hot coronal lines are formed solely in loop structures which connect regions of opposite photospheric magnetic polarity but are not observed over sunspots. Transition region lines are en~itted in plages overlying regions of intense photospheric magnetic field and in loops or loop-segments connecting such regions. The hot coronal loops are supported hydrostatically while only some of the transition zone loops are. The coronal and transition zone loops are distinctly separated and are not coaxial. A comparison of direct measurements of electron densities using density sensitive line ratios with indirect measurements using emission measures and path lengths shows the existence of fine structures of less than a second of arc in transition region loops. From a similar analysis, hot coronal loops do not have any fine structure below about 2 seconds of arc.

1. Introduct ion

In this p a p e r X U V s p e c t r o h e l i o g r a m s of two act ive reg ions will be used to i l lus t ra te

the s t ruc ture of t rans i t ion zone and co rona l emiss ions . The obse rva t ions were m a d e

with the N R L X U V s p e c t r o h e l i o g r a p h a b o a r d Skylab which has been desc r ibed in

de ta i l by T o u s e y et al. (1977). Basical ly , the in s t rumen t consis ts of a s ingle concave

gra t ing which focuses and d i sperses images of the Sun on to p h o t o g r a p h i c film. The

film is loca ted nea r the gra t ing n o r m a l in o r d e r to min imize aber ra t ions . The

d i spe r s ion is l inear to a high degree of accuracy nea r the gra t ing normal . By m e a n s

of two gra t ing pos i t ions , the c o m p l e t e wave leng th range 1 7 0 - 6 3 0 / ~ is cove red at

high r e so lu t ion (2 -3 arc sec, 0.03 ~ ) . The shor t wave leng th m o d e covers the r ange

170 to 340 ~ with the gra t ing n o r m a l at 255 ~ and the long wave leng th m o d e

covers the r ange 300 to 6 3 0 / ~ with the in s t rumen t n o r m a l at 400 ~ . On ly for

images at wave leng ths g rea te r than 550 ~ does the r e so lu t ion b e c o m e signif icant ly

d e g r a d e d .

Because all images are o b t a i n e d s imul t aneous ly on only one or two p ieces of

film, it is poss ib le to accura te ly s u p e r p o s e images in d i f ferent l ines in o r d e r to s tudy

the r e l a t ionsh ip of the s t ruc tures at d i f ferent t e mpe ra tu r e s . The d i spe rs ion of the

i n s t rumen t is well k n o w n and the wave leng ths of the o b s e r v e d spec t ra l l ines a re

all k n o w n to wi thin + 0 . 0 3 / ~ (Dere , 1978). W a v e l e n g t h s of m a n y of the l ines have

been m e a s u r e d to even g rea t e r p rec i s ion by Behr ing et al. (1976) and when ava i lab le

the i r va lues have been used. A n accuracy of + 0 . 0 3 ~ c o r r e s p o n d s to the f i lm- l imi ted

2" spa t ia l r e so lu t ion of the spec t rohe l i og raph . T h e ac tua l supe rpos i t i on is p e r f o r m e d

t h rough the use of m i c r o d e n s i t o m e t e r ras te r scans. A pa r t i cu l a r l ine wi th its image

a rea is chosen to be r a s t e r ed and co r r e spond ing image a reas for o t h e r l ines a re

Solar Physics 75 (1982) 189-203. 0038-0938/82/0752-0189 $02.25. Copyright (~) 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

190 K E N N E T H P. D E R E

determined by shifting the raster limits by the known separation along the axis of

dispersion. When using images on different plates, a reference point is obtained

by superposing raster contours of the same monochromatic image or of images

formed at the same temperature. Since the human eye can in some ways better

interpret the available information in a photographic image than in a contour map,

the final presentations and comparisons of the data rely on both. The photographic

images, when magnified to the scale of the contour drawings, can be superposed

on their corresponding contour map to the accuracy of an emulsion grain.

The active region images obtained by the Skylab XUV spectroheliograph span

the temperature range 8 x 104 K (He II A 304) to 3 x 10 6 K (Fe xvI h 335). The

temperatures of formation Tm of the lines that will be used in this study are given in Table I.

TABLE I

Temperatures of formation (T,,)

Ion Temperature (K)

Fe xv 2 • 10 6

Mg IX 9 x 105 Mg viii 8 x 105 Ne vii 5 x l0 s Mg vI 4 • 10 s Ne vI 4 x 105 Ne v 2.5 x 105 He II 8 x 104

2. Active Region McMath 12651

The first active region discussed is McMath 12651 for which observations are

available of its passage across the solar disk. These plates show that in coronal lines such as Fe x v A284 (2 x 1 0 6 K ) the active region evolved slowly from day

to day but did not exhibit any sudden, drastic changes. Observations made on

December 14 1973 near 13 : 37 UT when the active region was near Sun-center

have been selected. Consecutive long- and short-wavelength exposures were made

at that time. In Figure 1, several monochromatic images are presented as well as the Kitt Peak magnetogram taken on the same day at 19 : 57 UT. The rectangular area shown in each image corresponds to the same area on the solar disk. The

short- and long-wavelength plates were coaligned through Mg v u images present on both plates. Coalignment of the Kitt Peak magnetogram was determined by comparison with the He u A 304 spectroheliogram over an area considerably larger

than is shown. This is possible because of the strong correlation of network and

plage emission with regions of enhanced photospheric field. However, the accuracy with which the magnetogram can be superposed is on the order of 10 9 c m or 10".

Figure 2 overlays raster contours of several images on the photographic images of

r~

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Fig.

1.

Mo

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651)

.

192 K E N N E T H P. D E R E

Fig. 2. Isophote contours superposed over monochromatic images of an active region (McMath 12651).

Figure 1 for comparison. Isophotes of Ne vii A465 are superposed on the Fe x v h 284 image, isophotes of He n it 304 on the Ne vi i it 465 image, and isophotes of

Fe x v it284 on the He ii it304 image. In Figures 1 and 2 the lowest tempera ture line, He ii it 304, appears to outline

essentially two dimensional structures. He iI it 304 is intense where the photospheric magnetic field is intense. The most noticeable exception to this seems to be over the sunspot where the He n emission is generally weaker than over plage regions. On the other hand, the Fe x v it 284 emission originates solely in loops. The Fe x v

loops generally end in or near regions of intense He ii it304 emission and appear to follow the magnetic field lines connecting regions of opposite polarity. This characteristic of hot active region loops has been pointed out previously by Tousey et al. (1973) and Vaiana et al. (1976). This is further illustrated in Figure 3 where isophotes of Fe x v it 284 and He ir it 304 are superposed. As with He u, the Fe x v loops seen here do not originate over the sunspot. Ne vii A465, formed midway in tempera ture between these two lines, shows both the loop structures characteristic of Fe x v and the plage emission characteristic of He u. The correlation between the diffuse Ne vi i network emission and the He Ii emission can be seen in Figure 2 and again in Figure 4 where isophote contours of Ne vi i are superposed on

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194 KENNETH P. DERE

contours of He iI. In the quiet Sun the greatest contrast between network and cell interior emission occurs around 103 K. From Figure 1 it is clear that the plage network has disappeared by 2 x 106 K and the greatest contrast occurs in the He II image. Observat ions of the network in M g v t h403 are probably degraded by blending with images of other nearby lines. Otherwise the data is consistent with

a plage contrast that behaves similarly to the quiet Sun network to cell center contrast. Also indicated in Figure 4 is a filament channel that can be clearly seen in H a pictures of the same day.

The most intense Ne vii feature is located near the sunspot. The emission there seems to come from a small diffuse loop which bridges the edge of the sunspot with its foot-points located on either side of the spot. Brueckner and Bartoe (1974) have also pointed out that O IV, O v, and Ne vi i emission is strong over sunspots where He i and He ii emission is weak. The remaining intense Ne vi i areas (all indicated as shaded regions in Figure 4) appear to be associated with loops or loop segments. When seen in somewhat hotter lines such as Mg vii i and Mg IX, the

Complete loop is often visible. In Figures 2 and 5 the emissions in Ne vii and Fe x v are compared. The dashed lines indicate the longer loop-like structures visible in

I .ss.~

F e ~ 2 8 4 1 McMATH 12651

. . . . Ne ~ 465 .~

~SxlO9crn -,,4

Fig. 5. Isophote contours of Fe x v A 284 superposed on contours of Ne v i i )1465 in an active region (McMath 12651). The dashed lines indicate loop or loop segments evident in Ne v i i ;1465 emission.

T H E X U V S T R U C T U R E O F S O L A R A C T I V E R E G I O N S 195

Ne vn. The Ne vi i loops are located in the general vicinity of Fe x v loops but clearly the two are not coaxial. This result is typical and is confirmed by visual

inspection of active region structures on several hundred plates. Foukal (1975) has

reported similar observations showing a general displacement between cool loops and hot loops. However , his suggestion that the hot loops are the coaxial outer shell of cool sunspot loops does not appear to be consistent with the present

observations. From the line of sight emission measures and the widths of the Fe x v loops,

electron densities of 3 • 10 9 cm -3 are calculated. Values about twice this high have been found in small intense Fe x v loops in another active region. The electron pressures of 6 - 1 0 x 10 I5 c m - 3 K are consistent with the active region pressures

obtained f rom density sensitive line ratios reported by a number of observers

summarized by Dere and Mason (1981).

3. Active Region McMath 12702

In contrast to McMath 12651, McMath 12702 presents a somewhat different pattern. The X U V emission here is dominated by three loop structures at transition

Fig. 6. Monochromatic XUV images of an active region (McMath 12702). The dashed lines indicate the center of emission of the three Ne vii A465 loop segments.

196 K E N N E T H P. D E R E

zone temperatures. Observat ions of this region were obtained on January 19 1974. The long wavelength plate was exposed at 14:56 U T and the short wavelength plate at 16 : 29 UT. Representat ive images are shown in Figure 6 along with the

Kitt Peak magnetogram taken at 17 : 29 UT. The alignment procedure is the same as previously discussed except that the common reference system for the long and

short wave plates was established by comparing Si vii1 X 319 with Mg v m 436. The dashed lines in Figure 6 indicate the centroid of emission in the 3 Ne vtl loops.

The Fe x v image illustrates a larger area in order to show the complete loop system but shares a common northeast corner with the other images. All images are at

the same spatial scale. First it should be noted that none of the loops seen in Figure 6 are directly

connected with a sunspot. The loops do originate in areas of strong vertical magnetic fields as shown by the He II A304 network emission and the magnetogram. The

coronal Fe x v loops do not appear to be wrapped around the cooler loops. Rather, the hot and cool loops are separate structures. If a cool loop exists in the same general region as hot loops, the hot loops are missing at the location of the cooler

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I

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r

I

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I I I I I I I I I I I I

0 -- 0

0

D Z~

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--O

M c M A T H 12702 LOOP N o �9 Mg VIII 436A o N e V l l 465/~,

O

Mg Vl + Ne VI 403A

O0

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0 o

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o 0

A 0 A

A A A

O 0 0 0 0

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I 1 I I I LX I I I I I I I 1 2 3 4 5 6 7 8 9 10 11 12

HEIGHT (10 4 km) Intensity versus height in the northernmost loop (loop N) in McMath 12702.

13

T H E X U V S T R U C T U R E O F S O L A R A C T I V E R E G I O N S 1.97

loop. The crossover temperature between the hot and cool loops occurs somewhere between the temperature of Fe x v (Tin = 2 x 10 6 K) and Mg Ix (Tin = 9 x 10 5 K).

Among the cool loops, the hotter ones extend to greater heights. So whether or not the loops are in hydrostatic equilibrium, they are to a certain extent self supporting. The intensity as a function o f height has been determined for two of the loops seen in Figure 6: The northernmost loop 'N' and the southernmost loop 'S'. In Figures 7 and 8 the intensity as a function of height is plotted for the various images of loop 'N' and l o o p ' S ' respectively. The height scale takes into account the effect of perspective by assuming that the loops rise vertically from the solar disk in the plane perpendicular to the line of sight. From the intensity decline, scale heights for the various emissions are determined and are compared in Table II with the scale heights of an isothermal gas at temperature T~. For the collisionally excited lines observed, the hydrostatic intensity scale height is half the density scale

10 4

I

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E O

, - 10 3

>- 1-- O3 Z LLI I-- Z

10

Fig. 8.

I

O

O O O o

I I I I I I I I

McMATH 12702 LOOP S o �9 Mg VIII 436oA o N e V l l 465A

O

/ X M g V i + N e V I 4 0 3 A

A iX

0 A 0

0 A e Z~eA �9 �9

0 0 A

A 0 A

Z~

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0

0 A A

0

A 0 0

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I I I I I~ I 13 I 1 1 2 3 4 5 6 7 8 9

HEIGHT (104 km) Intensity versus height in the southernmost loop (loop S) of McMath 12702.

10

198 KENNETH P. DERE

T A B L E II

Intensity scale heights (104 km)

Ion A (,~) Tm Hydrostatic Observed

Loop N Loop S

Mg v i and Ne v t 403 4 • 1.2 1.5-2.6 1.1-1.6 Ne v i i 465 5 • 105 1.5 2.8 1.7 Mg v i i i 436 8 • l0 s 2.4 5.3 1.6-2.9

height. Because of the high thermal conductivity parallel to field lines a constant temperature atmosphere should be appropriate. Loop 'N' which is the longest of the three yields scale heights around a factor of 2 higher than the hydrostatic values. Conversely, l o o p ' S ' is apparently supported hydrostatically. From the similarity in height structure of the middle loop to loop 'S', it is estimated that it too is in hydrostatic equilibrium.

In Figure 9, the variation of specific intensity as a function of position along the axis of dispersion is plotted for images in several transition zone lines from linear scans obtained near the base of the loop system. Figure 10 shows similar data obtained at a height of 1.9 • 104 km (corrected for perspective) above the scans in Figure 9. To within the accuracy of the data, the loops are generally coaxial although for the northernmost loop (Figure 6), there is a tendency for the hotter emission to be shifted more to the north at the higher altitude. Foukal (1975) suggested that such structures are consistent with a loop whose temperature increases with radius, thereby creating cylindrical shells of emission. This interpretation is supported by the fact that the loops, when seen in hotter lines such as Mg v m and Mg IX appear to be thicker. An alternative description is that the loops are made up of a number of unresolved isothermal strands with more of the cooler strands near the core of the ensemble. This is compatible with evidence that photospheric magnetic fields (aside from sunspots and pores) exist in sub arc sec discrete elements with field strengths between 1000 and 2000 G (Livingston and Stenflo, 1979) if this fine structure can persist to higher altitudes. This suggestion can be checked by compar- ing densities derived from the emission measure-volume method with those derived from density sensitive line ratios. Emission measures and volumes can be obtained from the intensities and dimensions of the loops shown in Figures 9 and 10. The loops are assumed cylindrically symmetric so that the path length (Al) is equal to the observed width of the loops corrected for perspective. The densities obtained are listed in Table III. These values are comparable to the values obtained by Foukal (1978) with the emission measure method.

In Figure 11, active region pressures (NET) derived from density diagnostics collected in Dere and Mason (1981) have been plotted. As they found, active region pressures are roughly constant (to within a factor of 2). A quiet Sun pressure of 1.3 • 1015 c m -3 K has also been indicated as a dashed line. This quiet Sun pressure

T H E X U V S T R U C T U R E O F S O L A R A C T I V E R E G I O N S 199

I

T C",4

I

E

> - I - -

Z i i i I..- Z

2 X 103

103

102 -

2 x 1 0 3 - -

103 -

102

Fig. 9.

1 I I I I I I I

�9 �9 �9 �9 e

~ soo o e

�9 �9 � 9 �9 �9 �9

�9 O

Mg VIII 436A

�9 e l �9 �9 �9 �9 �9 � 9 1 4 9 l i e

l

I l l �9 �9 �9

�9 e e � 9 �9 �9

Ne V 4 1 6 ' "." I I I I I I I I I

0 1 2 3 4 5 6 7

DISTANCE (10 4 km)

Intensity across the base of the three loops of McMath 12702.

4 x 103

103

102

5 • 102

10 2 8

has been found over a wide temperature range: 3.5 • 104 K (Nicolas et al., 1979), 6 • 104 K (Doschek et al., 1978), 1.3 • 105 K (Dufton et al., 1979) and 1.3 • 106 K (Feldman et al., 1978). This is about a factor of two higher than the pressure found by Vernazza et al. (1981). Also plotted are the pressures (NeTm) for the Fe x v

200 K E N N E T H P . D E R E

I

I

o4 I

E o

>- I - O 0 Z LLI I - z

2 x 103 I I I I I I 1 I

i 03 - �9 .

�9 I ~

- - �9 = eeeo

L . -- 2 • 103 436~ M g VII I

102 - - 103

2 x 103 . Ne VII 465~,

103 ! 102

- M g VI + Ne VI 403,~ "

102 I I I I I I I I 0 1 2 3 4 5 6 7 8

D I S T A N C E (104 km)

Fig. 10. Intensity across the three loops of McMath 12702 at a height of 1.9 • 104 km above the base,

THE XUV STRUCTURE OF SOLAR ACTIVE REGIONS 201

TABLE III Densities in cool loops from emission measures and path lengths

Ion (s Loop Base Base + 1.9 x 10 4 km

I Al(km) Ne(cm 3) I Al(km) Ne(cm -3)

N e v 416 N 2 .7+2 1.4+4 1.2+9 C 3 .7+2 7 .5+3 2 .0+9 S 3 .7+2 6 .0+3 2 .2+9

Mg vI and 403 N 1.1+3 1.6+4 1.5+9 6 .4+2 1.2+4 1.3+9 N e v I C 1.4+3 9 .5+3 2 .2+9 4 .7+2

S 1.8+3 8 .0+3 2 .8+9 9 .8+2 8 .0+3 2 .0+9

N e v l I 465 N 2 .0+3 8 .5+3 2 .5+9 1.1+3 1.2+4 1.6+9 C 1.8+3 1.0+4 2 .2+9 8 .4+2 1.1+4 1.4+9 S 3 .0+3 6 .0+3 3 .7+9 1.5+3 8 .5+3 2 .2+9

Mg vii i 436 N 1.1+3 2 .2+4 1.7+9 1.0+3 1.4+4 2 .0+9 C 9 .2+2 - (1.1+3) - - S 1.6+3 - 1.3+3 - -

loops of McMath 12651 and the cool loops as listed in Table III. In addition the results of Foukal (1978) for O IV and O vI for sunspot loops are plotted. It is clear that the emission measure values are not consistent with the line ratio values except at coronal temperatures. This can either be taken as evidence that cool loops have low pressures (contradicting the line ratio results) or that they consist of unresolved filamentary structures. This latter conclusion will be argued.

It is not immediately clear whether the densities derived from transition zone density sensitive lines actually pertain to cool loops. This is because none of the relevant observations were made with the necessary two dimensional imaging capability to separate plage from loop emissions. It is thus possible that the plages are regions of high density and the loops are regions of low density as suggested by their emission measure derived densities. However, Feldman and Doschek (1978) find pressure of 1016 c m -3 K from C III and O IV at 8" above the solar limb

in an active region. Clearly this reflects the high pressure in transition zone loops at this height. Also, the stigmatic HRTS spectra of Si iii (Nicolas et al., 1979) and N IV (Dere, 1980) do not show pressures less than 2 x 1015 cm-3K whether in

sunspots or active region loops or plage regions. Consequently these transition zone loops must be at pressures higher than the quiet Sun by at least a factor of 2 which is contrary to the low pressures derived from the emission measures. The two density results can be reconciled only if the fill factor for the transition zone loops is less than unity.

The emission measure densities have been derived from the formula

= ( N 2 Al~ 1/2 N, \ ~ ] , (1)

202 KENNETH P. DERE

1017

A

03 i

E 1016 O

v

LM t r

09 UJ t r Q .

z O t r -

1015

' ' ' ' ' ' " 1 ' ' ' ' " I

A C T I V E R E G I O N P R E S S U R E S

Si l l l IO

I I

III�9

OV

I T 0 I V

C III ~ V

�9 LINE RATIO oEMISSlON MEASURE-

VOLUME �9 O VII

Fe Xll �9 ~Fe XV

Fe XlII �9 Xll

Si X

Mg VIII 0

O

- - N e VII O

0 Vl Mg VI O

o + - N e V N e Vl

Fe XIV

I I I 1 1 1 _

u

m

m

O IV O

1014 J I I I 1 1 1 1 1 J i l i l t i , i , , r e , m ,

10 4 10 5 10 6 10 7

TEMPERATURE (OK)

Fig. 11. Active region pressure versus temperature. The line ratio results have been obtained in a variety of active regions. The emission measure results from MgvII l , Ne vii, M g v I and Ne vI and Ne v are for McMath 12702 and from Fe x v for McMath 12651. The O IV and O vz results

are from Foukal (1978).

where Al was taken as Alobs, the observed cross-sectional diameter of the loops. If an average line-of-sight fill factor a is defined such that a A/obs = Al where a --< 1, then a can be determined by requiring the formula to yield pressures consistent

with density sensitive line ratios. In the case of Ne v this yields a fill factor of 5 • 10 -3 and total path length of 30 km or 0.04". The width of any individual

filament is considerably smaller. Such fine structure is considerably below the resolution of present instrumentation.

One dimensional static energy balance models (such as Rosner et al., 1978) have

been fairly successful in reproducing the properties of hot (T ~ 106 K) active region

T H E X U V S T R U C T U R E O F S O L A R A C T I V E R E G I O N S 203

loops . H o w e v e r , these m o d e l s all p red ic t shor t (103 km) t rans i t ion zone loops which

are cons ide rab ly smal le r than the o b s e r v e d heights . O n e hypo thes i s is tha t cross

field t r anspo r t m a y p lay a signif icant ro le in the mass and ene rgy ba lance of these

thin loops s ince the g rad ien t s p e r p e n d i c u l a r to the field m a y be qui te high.

4. Summary

In conclus ion, X U V emiss ion in act ive regions is p r o d u c e d main ly in e i the r loops

or ne twork . L ines f o r m e d a b o v e 106 K are f o r m e d sole ly in loop s t ruc tures whi le

l ines be low tha t t e m p e r a t u r e are f o r m e d in the ne tw ork and in loops or l oop

segments . The hot loops (T > 1 x 106 K) are not cha rac t e r i zed as having cool cores

( T < 1 • 106 K) bu t r a the r the hot and cool loops are f o r m e d in dis t inct ly di f ferent

locat ions . The hot loops genera l ly end in regions of e n h a n c e d t rans i t ion zone

emiss ions (ne twork) which are well co r r e l a t ed with p h o t o s p h e r i c magne t i c fields.

T h e coo le r loops also end in n e t w o r k regions and are not a lways assoc ia ted with

sunspots . This agrees wi th the conclus ions of Cheng (1980) based on an i n d e p e n d e n t

analysis of s imi lar da ta . Some of these cool loops are in hydros t a t i c equ i l ib r ium

and o the rs not. The hot loops are r e so lved with p r e se n t i n s t rumen ta l r e so lu t ions

(2") bu t the coo le r loops mus t be c o m p o s e d of e x t r e m e l y smal l s t ruc tures with, in

s o m e cases, widths of less than 30 km (0.04").

References

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