H I G H R E S O L U T I O N S P E C T R O S C O P Y ,OF

S O L A R A C T I V I T Y

I. Observing Procedures

L. E. C R A M , R. D. R O B I N S O N , H. A. M A U T E R , G. R. MANN, and G. L. P H I L L I S

Sacramento Peak Observatory,* Sunspot, Nh4 88349, U.S.A.

IReceived 30 July, 1980)

Abstract. We describe an observing program designed to obtain high spatial resolution photographic spectra of solar active region phenomena, with time resolution as short as 6 s. The Vacuum Tower Telescope and Echelle Spectrograph at Sacramento Peak Observatory are used to make observations simultaneously in Ha, He D3, Ca I! K, Mg bl, the CN bandhead at A3883, and the magnetically-sensitive line Fe r A6302. Images reflected from the slit jaw are exposed simultaneously in white-light and Ha. Observations of chromospheric heating, following a high-velocity infall along an Hc~ superpenumbral filament, are presented to illustrate the capabilities of the program.

1. Introduction

The spectroscopic study of fine structures in solar active regions is an important supplement to the large number of observations that have been made with narrow- band filters. While the latter observations are of tremendous v~,lue in studying morphological properties of solar activity, and of some value in providing coarse line profile data by using a rapidly-tunable filter, at present only spectroscopy can provide the kind of detailed information required to diagnose reliably the physical conditions in the radiating source volume. Compared with filter observations, there have been relatively few spectroscopic studies of active region phenomena. For example, there are only a very few published Ha line profiles in solar flares, yet these profiles are of crucial importance in understanding the processes involved in flare-induced chromospheric heating.

In an attempt to provide critical spectroscopic data during the current solar maximum period we have begun a project with the Vacuum Tower Telescope and Echelle Spectrograph at Sacramento Peak Observatory. The program exploits the possibility of obtaining simultaneous observations in widely separated spectral lines. We have chosen lines which span the height range accessible witlh ground-based observations, and which are suitable for diagnosing the physical conditions in active region sources. The present program is similar in concept to the successful HIRK- H A D project (Beckers et aL, 1972), the main difference being a selection of lines more appropriate for work on solar activity. As with the H I R K H A D program, we propose to make observational material resulting from the project available to interested astronomers. We will endeavor to obtain high quality spectra of a variety

* Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78- 17292 with the National Science Foundation.

Solar Physics 71 (1981) 237-245. 0038-0938 /81 /0712-0237 $01.35. Copyright �9 1981 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U,S.A.

2 3 8 L . E . C R A M ET AL.

of active region phenomena , including flares, plages, surges, sunspots, filaments and El lerman bombs. The acronym for the project is SOAP, for Spectroscopic Obser- vations of Active Phenomena.

2. Instrumental Details and Observing Procedures

A . I N S T R U M E N T A T I O N

The Vacuum Tower Telescope and Echelle Spectrograph at Sacramento Peak Observatory have been described by Dunn (1964, 1969). The 76 cm aperture

telescope produces a 51 cm solar image on the slit of the spectrograph. Light reflected f rom the slit-jaws is observed by photography (white light and H a filter- grams) and by TV cameras to monitor the alignment and guiding of the instrument with respect to white light features and H a fine structure. Light entering the

spectrograph is predispersed by a prism monochromator . A set of slits in the

monochromator focal plane then selects narrow spectral bands around the lines of interest. This light reflects f rom an echelle grating to form a set of partially-

overlapping, high dispersion spectra at the focal plane. Glass filters are used to isolate the desired spectral lines which are recorded photographically.

The grating has 79 lines per ram, and is blazed at 63.~ To conveniently position the chosen lines across the focal plane, the grating is used at 64~ This produces only

a slight decrease in efficiency. The image scale is approximately 290 ix: 1 arc sec, and the projected length of the slit is about 220 arc sec. For most observations the slit width will be 200~ - about 0.7 arc sec. With this slit width, the exposure time at disk center is about 3 s. The exposure time is automatically corrected for limb darkening and variations in image brightness (due to changes in sky transparency and air mass) to produce uniform exposures. The film magazines can be advanced in about 2 s, so

that an exposure rate of 6 s is possible. Since about 400 frames can be contained in

the magazines, uninterrupted observing sequences of 40 min are available at this rate. The solar image can be rapidly reposit ioned between exposures, under flexible and reproducible computer control. A recent ly- implemented laser alignment system guides the image on the slit-jaws with an accuracy of a few arc seconds over several hours.

B. OBSERVED LINES

Table I lists the important parameters of the lines observed in this program. Working within the constraints of the overlapping echelle orders and the space needed for each film magazine at the focal plane, we have endeavored to select a set of lines suited to activity studies. We chose H a because it is mos t -commonly observed in filters, and because it is a useful chromospheric diagnostic in the fine structure. The Ca II K line is a very useful chromospheric and upper photospheric diagnostic. While studies of the formation of He D3 are difficult because of compe t i t i onbe tween collisional and photo-excitation, the line provides information on the hotter parts of

H I G H R E S O L U T I O N S P E C T R O S C O P Y O F S O L A R A C T I V I T Y 239

TABLE I

Parameters of observed lines

Wavelength Dispersion Line (,~) Order (mm ~-1; Film

Ha 6563 35 8.047 S0-392 a FeI 6302 36 7.841 S0-392 He D3 5876 39 8.861 2498 Mg bl 5183 44 9.777 2498 CaII K 3933 58 12.864 5375 a CN 3884 59 13.329 5375

a It would be advisable to use a lower contrast film in studies of solar flares.

active regions. Mg b~ is a useful diagnostic of the low chromosphere, while the CN bandhead is very sensitive to structural changes in the upper photosphere. There is a

useful magnetically-insensitive line just redward of the bandhead. Other spectral lines near K, Mg b, and CN fall on the film and can be used for photospheric studies.

Magnetic fields play a key role in solar activity, and it is important to measure field properties in a project such as this. We exploit the Zeeman effect in the magnetically

sensitive lines F~ I A6301.515 (ge~ = 1.667) and the simple triplet Fex A6302.5 (gen = 2.5) of multiplet 816 to infer some properties of the photospheric magnetic field. These lines are rather insensitive to temperature changes, allowing reliable

field measurement in both hot plages and cold umbrae. Moreover, the lines are

conveniently placed relative to strong atmospheric 02 lines, which greatly facilitates registration of images made in opposite polarizations. We do not a t tempt to measure the full Stokes vector profiles in the lines, but restrict ourselves to an analysis of both

senses of circular polarization. Precise Stokes vector profiles (albeit with poorer spatial and temporal resolution) may be obtained by using the High Altitude Observatory 's Stokes Polarimeter (Baur et al., 1980) in the Big Dome simul- taneously with the Tower Telescope.

The polarization analysis is effected by means of a quarter-wave plate and a

polarizing Wollaston beamspli t ter placed 45 cm above the spectrograph focal plane. The deviation of the beamsplit ter is designed to produce simultaneous images in opposite polarizations on two halves of a frame of 70 m m film. Because the two

images are equally influenced by atmospheric and instrumental effects, the difference polarization signal (ideally the Stokes V-profile) is easily measured.

The optical path through the instrument encompasses several optical elements that can modify the polarization state of the incident light, including the stressed entrance window, two 45 ~ mirrors, and the grating. Accurate quantitative polari- metry requires that the effects of these elements on incident light be measured. But studies of the polarization properties of the Tower Telescope (Evans, private communication) reveal the presence of complex, t ime-dependent birefringence, depolarization and retardation which pose a presently insurmountable obstacle to

240 L . E . CRAM ET AL.

satisfactory compensation. While these problems preclude accurate polarimetry, it is nevertheless possible to make useful measurements. For example, in strong fields the splitting of the o'-components is very clear and readily yields a field strength estimate. The greatest caution is required when interpreting properties near the center of the Stokes V-profile, where the optical system can mix originally linearly or unpolarized light into the circular polarization signal.

C. A T M O S P H E R I C D I S P E R S I O N

When spectrographic observations are made in widely separated parts of the spectrum, care must be taken to correct for the effects of atmospheric dispersion. Using Simon's (1966) tables, we find that our redmost (Ha) and bluemost (CN) images are separated by about 2 arc sec in altitude when the Sun is 30 ~ above the horizon (i.e. during the good-seeing interval of the morning). This displacement is intolerable for high quality spectra in which the resolution and entrance slit width may be smaller than 1 arc sec. While the atmospheric dispersion problem could be solved by a pair of prisms at the expense of placing such an optical system before the slit (and so interfering with the slit-jaw viewing system) we use the simpler procedure of rotating the telescope until the slit is perpendicular to the horizon. This alignment can be made precise only at one instant if the observing sequence requires the slit to be fixed on the solar image, but the rotation of the slit away from the optimal orientation in a 2 hr observing run leads to insignificant differential image motion (~<0.2 arc sec). The major drawback of this method is that it dictates a definite slit orientation which will often not be optimal for the configuration of the active region, particularly near the solar limb.

D . C A L I B R A T I O N

Photometric calibration of the spectrograms is effected by means of a step-wedge and calibrated neutral-density filters placed near the entrance slit. As Beckers et al.

(1972) pointed out, the use of a single calibrated step-wedge has two drawbacks: (1) Scattered light in the spectrograph will preferentially contaminate exposures cor- responding to the densest steps, and (2) the slit width may not be uniform. These problems are solved by making calibration exposures through a step-wedge a n d a

series of calibrated neutral-density filters. Each step in the wedge then provides an independent H-D curve. To cover the full density range, one calibration exposure is made with a longer exposure time and no neutral-density filter. A lengthened exposure is preferred to widening the slit, since the latter changes the illumination pattern in the spectrograph. An extensive discussion of this calibration procedure is presented by Brown (1975).

The sources of stray light may be identified and quantitatively estimated in the following way. Light scattered in the telescope before reaching the spectrograph can be detected in exposures made with the slit crossing the solar limb, provided the sky is clear. It is found that the intensity of this stray light amounts to 1-2% of the disk center intensity. A comparison of the spectrum of this scattered light with the

H I G H R E S O L U T I O N S P E C T R O S C O P ~ " O F S O L A R A C T I V I T Y 241

rotationally-displaced spectrum of the solar limb shows that the scattered light

originates fairly uniformly from all parts of the solar disk. Scattered light in the

spectrograph can be measured by placing a mask across the entrance slit. This

scattered light appears to be white, and it has an intensity of about 1% of the continuum level in most spectral bands. To ensure that the monochromator - glass filter-film sensitivity combination is adequately separating the grating orders, it is prudent to make exposures with masks over single monochromator slits. When the

spectrograph is set up correctly, stray light from overlapping orders is small compared with the white light produced by general scattering in the spectrograph optics. Although the scattered light amounts to only a few percent of the photo- spheric continuum intensity, its presence makes it very difficult to do quantitative

photometry in the cores of strong lines (such as Ca II K) and in sunspots and other dark structures.

3. Results

In the first observing run with this program we obtained time-series observations of an event involving localized chromospheric heating as a consequence of high-

velocity infall along a superpenumbral filament (Loughhead, 1974). Because the spectroscopic appearance of the heated region is not that of a subflare or an Ellerman bomb, we do not think that we have identified the precursor of these phenomena. However , after having observed this particular event, we have 'visually observed

several sunspots with superpenumbral filament systems and found that such infall events are common. Several may be occurring at any instant around a moderately-

sized sunspot. The superpenumbral filaments visible in H a can be regarded as

relatively cool channels embedded in coronal loops, and since there is presently considerable interest in the structure and stability of such loops (e.g. Rosner et al., 1978; Antiochos, 1979; Hinata, 1981) we thought that a description of this event would be of interest since it appears to be a characteristic dynamical property of the loops.

The observations were made on March 25, 1979 in McMath region 15904, which

at that time was located near N 7 ~ E 16 ~ The slit was positioned close to the outer edge of a regular penumbra and stepped between two positions separated by

5 arc sec. Exposures were made at a 10 s rate so that each position was observed every 20 s. The observing sequence started at 16:16:40 UT; for the first 30 rain the

seeing was fairly good (1-2 arc sec), then it progressively degraded until the end of the sequence at 17:20:00 UT.

Figures l(a) and l(b) display selected frames from the time series at each position. The H a +0.3 .~ slit-jaw picture at the start of the series show that the slit intersected several H a superpenumbral filaments. Each filament is associated with red- shifted features in the H a spectrum: the line-of-sight velocities are in the range 5-10 k m s -1. Around 16:26:00 U T one of the filaments begins to break up, and strong red-shifted absorption appears in Ho~. By 16:42:00 U T the red-shift of the

242 L . E . C R A M ET AL.

16:58:10

16:50:10

16:42:10

16:36:30

16:27:10

16:17:30

Fig. ]a.

Fig. la-b. Time series of Ha and Ca I K spectra through the impact heating event, at two slit positions separated by 5 arc sec. The slit height is 70 arc sec. The simultaneous slit-jaw exposures are made at

Ha +0.3 A. UT times are indicated.

a b s o r p t i o n ex tends to 120 k m s -1, and emiss ion a p p e a r s in H a and Ca II K nea r l ine

center . By 16 :50 :00 U T the r ed - sh i f t ed H a has a lmos t d i s a p p e a r e d , whi le t he re is

in tense c h r o m o s p h e r i c emiss ion in H a and Ca II K. A f t e r a fu r the r 8 min, the even t

has f inished. F igu re 2 is a m o n t a g e of all of the s imu l t aneous ly o b s e r v e d spec t ra l reg ions at

16 :50 :00 UT. T h e r e is p r o m i n e n t c h r o m o s p h e r i c emiss ion in H a and Ca II K; the K

emiss ion is d i sp laced r edwards . T h e cen te r of Mg b l is also fi l led in by emiss ion.

H I G H R E S O L U T I O N S P E C T R O S C O P Y O F S O L A R A C T I V I T Y 243

16:58:00

16:50:00

16:42:20

16:36:40

16:26:40

16:17:20

Fig. lb.

There is no trace of emission at the CN bandhead, and none of the photospheric lines is noticeably weakened or Doppler shifted. The magnetically-sensitive lines reveal the presence of a line-of-sight penumbral field, but no prominent field structure is specifically associated with the infall event. At the time of maximum heating there is

a weak He D3 absorption feature, although no phase of the event is prominent in this line.

In Figure l(b) the red-shifted H a absorption is clearly bifurcated at 16:36:30 UT. One interpretation of this spectral structure is that the infalling material has velocity components both along and around the filament. Assuming that the fibril is inclined at 45 ~ to the line of sight, we deduce a velocity of ~150 km s -1 along the filament, and --~25 km s -1 tangential to the filament. If this interpretation is correct, it provides

244 L E . C R A M E T A L .

Fig. 2. A montage of the observed spectral regions at 16:50:00 UT. Some important spectral features are indicated. Note that granulation is clearly visible in the slit-jaw picture, and in the threads and line wiggles in the Mgbl line spectrum, indicating that the resolution is about 1 arc sec in this set of

simultaneous exposures.

some evidence for the existence of twist in the magnetic field. The H a slit-jaw images

show that the filament exhibits complex structure during the event, although there

are severe difficulties in the interpretation of filtergrams made with off-band Hale

filters in the presence of large line shifts. For this reason we cannot study what

appears to be a small surge excited by the infall (this feature extends towards the

upper right from the infall, and is most clearly seen at 16:36:30 UT).

We interpret the event as follows. Material initially too hot to be visible in Ha, and

probably at coronal temperatures, undergoes a change of phase along a part of a

magnetic structure, one end of which terminates near the sunspot. Instabilities that lead to such phase changes have been discussed by Antiochos (1979), Hinata (1981), and others. The phase change involves a rapid decrease in temperature and increase

in density. The dense material falls under the influence of gravity along the magnetic field lines, until it collides with the chromosphere whereupon compressive heating is

eventually dissipated by radiation. In terms of this model we can crudely estimate a few quantitative parameters

which may be of interest to theoreticians working on coronal loop stability. The cross-sectional area A of the filament is about 6 {arc sec)2-~2 x 1017 cm 2. The

H I G H R E S O L U T I O N S P E C T R O S C O P Y OF S O L A R A C T I V I T Y 245

m a x i m u m ver t ica l veloci ty is V ~ 1 x 107 cm s -1, which is p r o d u c e d by an unres i s ted

f ree fall over a d i s tance h = 2 x 109 cm u n d e r solar gravity. The du ra t i on of such a fall

is t ~ 4 0 0 s. The mass involved in the even t is m ~ A p h , and t ak ing a dens i ty

p ~ 2.5 x 10 -14 g cm 3 (Nu = 101~ cm -3) we find m ~ 1013 g. The: po t en t i a l ene rgy

ga ined by this ma te r i a l in fai l ing a d is tance h is Ep = mgh ~ 5 ~,: 1026 ergs. This is

c o m p a r a b l e to the c h r o m o s p h e r i c r ad ia t ion loss ER = F R A t if we t ake the excess flux

FR ~ 5 • 106 erg cm -2 s -1, a r ea sonab le va lue given the s imi lar i ty of the spec t rum of

the hea t ed reg ion to a p lage .

These es t imates a re of course uncer ta in , but they m a y be useful in assessing the

role of such condensa t ion - in fa l l events in the in te rna l ene rgy ba lance of co rona l

loops. In pa r t i cu l a r the energy loss ra te , t), f rom the loop involved in this even t is

~ 1024 erg s -1. This m a y be c o m p a r e d with the s t eady co rona l loop r ad ia t ion loss,

which R o s n e r et al. (1978, E q u a t i o n (3.13)) es t imate to be

~--fR = 7 x 10 -17 T - 1 N 2 r r R 2 L erg s -1 .

T a k i n g T = 2 x 106 K, N = 10 l~ cm 3, R = 2.5 x 108 cm, and L = 4 x 10 9 cm, we find

/-)'g ~ 3 X 1024 erg S -1. Thus, the infall even t is an i m p o r t a n t ene rgy sink dur ing its

l i fe t ime; however , s ince these kinds of events a re t rans ien t and do not fully occupy

the loops e m a n a t i n g f rom a sunspot , we suspect tha t they are: not a d o m i n a n t

c o m p o n e n t of the g loba l energy ba lance .

References

Antiochos, S. K.: 1979, Astrophys. Jr. 232, L125. Baur, T., House, L. L., and Hull, H. K.: 1980, SolarPhys. 65, 111. Beckers, J. M., Mauter, H. A., Mann, G. R., and Brown, D. R.: 1972, SolarPhys. 25, 81. Brown, D. R.: 1975, Ph.D. Thesis, Department of Physics and Astrophysics, University of Colorado. Dunn, R. B.: 1964, Appl. Opt. 3, 1353. Dunn, R. B.: 1969, Sky Telesc. 38, 368. Hinata, S.: 1981, Astrophys. J. (in press). Loughhead, R. E.: 1974, Solar Phys. 38, 77. Rosner, R., Golub, L., Coppi, B., and Vaina, G. S.: 1978, Astrophys. J. 222, 317. Simon, G. W.: 1966, Astron. J. 71, 190.