Microwave enhancement and variability in the elephant's trunk coronal hole' Comparison

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A5, PAGES , MAY 1, 1999 Microwave enhancement and variability in the elephant's trunk coronal hole' Comparison with SOHO observations N. Gopalswamy, 1'2 K. Shibasaki, 3 B. J. Thompson, 2,4 j. Gurman,2 and C. DeForest s Abstract. We report on an investigation of the microwave enhancement and its variability in the elephant's trunk coronal hole observed during the Whole Sun Month campaign (August 10 to September 9, 1996). The microwave images from the Nobeyama radioheliograph were compared with magnetograms and EUV images obtained simultaneously by the Michelson Doppler imager and the extreme ultraviolet imaging telescope (EIT) on board the SOHO spacecraft. The combined data set allowed us to understand the detailed structure of the microwave enhancement in the spatial and temporal domains. We find that the radio enhancement is closely associated with the enhanced unipolar magnetic regions underlying the coronal hole. The radio enhancement consists of a smooth component originating from network cell interiors and a compact component associated with network magnetic elements. When a minority polarity is present near a majority polarity element, within the coronal hole, the resulting mixed polarity region is associated with a bright-point-like emission in coronal EUV lines such as the Fe XII 195. These coronal bright points are also observedistinctly in the EIT 304 band, but not in microwaves. On the other hand, the lower-temperature line emission (304/ ) and the microwave enhancement are associated with the unipolar magnetic flux elements in the network. We found strong time variability of the radio enhancement over multiple timescales, consistent with the initial results obtained by SOHO instruments. The microwave enhancement is most probably due to temperature enhancement in the chromosphere and may be related to the origin of solar wind. 1. Introduction An accurate definition of coronal holes has existed since the Skylab era [Bohlin, 1977, p. 29]: "Fairly large scale, cool, low density areas at both low latitude and the polar caps, encompassing weak, predominantly unipolar magnetic fields which extend away from the Sun as diverging, open lines of force, and which give rise to high speed solar wind streams that cause geomagnetic storms." This definition of the large-scale properties of coronal holes has changed very little, but we have accumulated a wealth of information on the X Physics Department, Catholic University of America, Washington, D.C. 2NASA Goddard Space Flight Center, Greenbelt, Maryland. 3Nobeyama Radio Observatory, Nobeyama, Japan. 4Space Applications Corporation, Greenbelt, Maryland. SCenter for Space Science and Astrophysics, Stanford University, Palo Alto, California. Copyright 1999 by the American Geophysical Union. Paper number 1998JA /99 / 1998 JA small scale structure and evolution of the coronal holes since then. Since the advent of the Solar and Helio- spheric Observatory (SOHO) mission, we have had an excellent opportunity to make significant progress in understanding the details of the small-scale structure in coronal holes [Inslcy ½t al., 1997]. The small-scale structures and their dynamics are closely connected to the magnetic nature of coronal holes with different spatial distribution of magnetic fields at different heights. Polar plumes and coronal bright points are known fine structures in coronal holes. In addition, the radio data and photospheric magnetograms indicate that the enhanced network elements may play an essential role in determining the physical properties of the coronal hole. In this paper, we are concerned with one particular aspect of coronal holes: enhanced radio emission with respect to the quiet Sun. It has been a puzzling result that the coronal hole emits microwave radiation in ex- cess of the quiet Sun level, in direct contradiction to what is observed in most other wavelengths [Wcfer and Bl½iw½iss, 1976; Kosugi ½t al., 1986; Gopalswamy ½t al., 1998]. The radio enhancement is confined to a narrow range of wavelengths (0.3 to 3 cm). At these wavelengths, most of the radio emission originates from the

2 9768 GOPALSWAMY ET AL.: MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES chromosphere, and proper identification of the mechanism of radio enhancement can provide us with an invaluable tool for temperature diagnostics in the upper chromosphere. The purpose of this paper is to study liospheric Observatory (SOHO) spacecraft, images the solar disc and inner corona in up to four selected wavelength regions corresponding to strong EUV spectral lines, each of which is characteristic of a narrow temand characterize the microwave enhancement in coronal perature regime in the solar atmosphere: (1) He II holes in light of new information provided by SOHO im- (8 x 104 K) and Si XI (1.6 MK); (2) Fe IX ages at other wavelengths. To this end, we analyze the A (0.8 MK) and Fe X (1 MK); (3) Fe extensive multiwavelength data acquired during Whole XII 192.3, 193.5, and (1.6 MK); and (4) Fe Sun Month (August 10 to September 9, 1996 [Gibson XV (2.0 MK). Wavelength regimes (1-4)are and Bi½s½cker, 1997]). For the first time, we are in a position to determine the spatial and temporal evolution of the microwave enhancement, thanks to the high temporal and spatial resolution images of the Sun obtained by the Nobeyama radioheliograph. Preliminary investigations show that the microwave enhancement consists of a smooth component with a size corresponding to that seen in coronal emission lines and a set of compact sources superposed on the smooth component [Gopalswamy ½! al., 1998]. In this paper, we attempt to find the magnetic nature of these two components by comparing the radio images with longitudinal magnetograms. We also compare the radio enhancement with fine structures observed in EUV images to obtain commonly referred to as 304, 171, 195, and 284 t,, respectively. Images are recorded on a 1024 x 1024 pixel (44.2 x 44.2 arc min) EUV-sensitive CCD camera, with a spatial resolution of 2.59 arc sec. The nominal daily observing plan includes one full-resolution, full-disc image in each of the EIT's wavelengths. For the Whole Sun Month period, the full-disc synoptic images were obtained in the Nobeyama time window so we could make radio images precisely matching the time of the EIT images. In this paper, we use primarily EIT 195 and 304 images, corresponding to the coronal and transition region temperatures. The Michelson Doppler imager (MDI) on board the SOHO mission obtains longitudinal photospheric maginformation on the vertical structure. netograms in the Ni I 6768 t, absorption line [Scherrer e! al., 1995]. Full-disc magnetograms used in this paper 2. Observational Results were obtained every 96 min, and at least four of them fall in the Nobeyama time window. Microwave images were made to match the times of these magnetograms for effective comparison. The spatial resolution of the magnetogram is 2 arc sec. The elephant's trunk coronal hole (ECH) was ob- The microwave data were acquired by the Nobeyama radioheliograph, a dedicated solar radio imaging instrument that acquires visibility information at 17 and 34 GHz with a time resolution of 100 ms [Nakajima ½t al., 1994; Takano ½t al., 1997]. Since the field of view of the interferometric array is 40 arc min at 17 GHz, fulldisc solar images can be made from these visibilities with desired time resolution starting from 100 ms. The best spatial resolution the instrument can provide at 17 GHz is 10 arc sec, and the actual value depends on the way the images are synthesized from the visibility data. For the present purpose, we made snapshot images of the Sun every 5 min, with an integration time of 10 s. The resulting images have a spatial resolution of 18 arc sec. The sensitivity of the instrument is very high (4.4 x 10 -a solar flux units (sfu)). To determine the noise level on the Nobeyama images, we measured the fluctuation in the sky where no emission is expected and obtained an rms noise of 150 K. The typical quiet Sun brightness temperature is 104 K, and we are interested in enhancements above this level. In addition to served on the disc during August 19, to September 3, We use the daily best image of the Nobeyama radioheliograph made around local noon (0300 UT) to present an overview of the large-scale structure of the radio Sun. When the ECH was closer to the disc center (August 26-30), we made radio images every 5 min to study the short-timescale variation of the ECH. On August 27, the ECH was closesto the disc center, and we made a detailed spatial comparison between the radio and SOHO images because there is minimal geometrical effect and almost no contamination from the coronal material in the foreground. We present an overview of the ECH first and then concentrate on the fine struc- tures. 2olo Hole Overview of Elephant" Trunk Coronal the noise fluctuations, the Nobeyama images also contain regular fluctuation patterns with several hundred Figure 1 is a series of microwave images of the Sun, kelvins peak to peak. When these patterns are super- one per day between August 20 and September 3, 1996, posed on real bright features, they cause an apparent which shows various large- and small-scale structures fluctuation in the latter. However, the weak spurious present at various levels of the solar atmosphere. The features change from image to image and hence can be bright features (active regions and ephemeral regions) easily distinguished. More details on the image quality are enhancements above the quiet Sun level, and the are given by Koshiishi [1996]. dark features (filaments and filament channels) are de- The extreme ultraviolet imaging telescope (EIT) [D½- pressions. The striking feature on each radio image is laboudini r½ et al., 1995] on board the Solar and He- the brightening at both the poles, called the polar cap

3 GOPALSWAMY ET AL.- MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 9769 Pigure 1. Pull-disc 17 GHz microwave images obtained by the Nobeyama radioheliograph for August 1 to September 3, 1. North is to the top, and east is to the left. Bright polar coronal holes, weak limb brightening, brighi active region, dark filament, and filament channels can be seen on the images. The elephant's trunk coronal hole (ECH) also appears as bright, but much weaker than the polar hole brightening. The arrows point to the two bright patches of enhancemen in he Augus 26 image. brightening [Kosugi et al., 198, and references therein] gions that can be seen as bright compact features. The and corresponding to the polar coronal holes. The radio primary feature we are interested in is the ECH, which enhancement in the north polar coronal hole is of larger is barely discernible on these images. We have overextent, reflecting the relative extent of the coronal hole laid the approximate boundary of the ECH as observed during this period and the large positive solar B0 angle. by he SOHO EIT 195 A o identify he ECH in ra- The linear dark feature at high southern latitudes dio. We notice immediately that the coronal hole is not (Augus 1-2 ) is a dark filamen ha was ra her s a- dark in microwaves but is enhanced similarly to the poble for several days and showed some evolution as it lar coronal hole. The enhancement is not uniform as rotated behind the west limb. The filaments are dark observed in X ray images or EUV coronal spectroheli- in microwaves because they are cooler ( K) structures and hence appear as depressions below the 10,000 K quiet Sun. A large filament channel rotated onto the disc from the east limb on August 23 and can be best seen on August 29 as a westward convex dark feature near the disc center. The southern end of the filament channel contained a dark filament and extended along the neutral line of the large bright active region, NOAA AR 7 8 (see the image on August 31). This active region rotated on to the limb on August 22. Apart from AlL 798, there were only a few compact ephemeral re- ograms. The inhomogeneity may have something to do with the magnetic nature of the coronal hole, as will be shown shortly. In Figure 2, we show a series of partial images of the Sun around the ECH, which clearly demonstrates the radio enhancemen and its day-to-day variation. We have also overlaid he ECH boundary from EIT 195 A observations. By visual examination, we traced the boundary of the ECH in EIT 195 A images on all the days except Augus 29. On this day, we used the EIT 171 A image because the EIT 195 A image was incom-

4 9770 GOPALSWAMY ET AL.- MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES Figure 2. Nobeyama subimages (15.7 arc minx 23.5 arc min) showing microwav enhancement in the ECH for August 24 to 29, when it was closer to the central meridian. The boundary of the ECH as traced from the extreme ultraviolet imaging telescope (EIT) 195 A image is overlaid. Note that the north polar coronal hole is the brightest, with patchy enhancements at lower latitudes. At the location where the ECH shows a break on August 29, there was no enhanced radio emission on any of the days. plete. The shape of the ECH changed considerably from day to day, as can be seen from the boundary trace. The boundary line drawn is approximate and is used only to show the general location of the ECH. On August 29, the ECH looked fragmented, with a break just south of the north polar coronal hole. In fact, the ECH was never a "complete" hole, especially in lines that correspond to lower layers of solar atmosphere. In fact, there were some weak emission features at the location of the break on all of the days. The radio images clearly show enhancements consistently in three places: the north polar coronal hole, the lower end of the trunk, and in the middle portion of the trunk. In coronal images, the two locations on the trunk appeared the darkest, apart from the polar holes. The radio images and EIT images were nearly simultaneous, and the widths in the two wavelength domains were comparable Magnetic Nature of the Microwave Enhancement It is somewhat puzzling why the radio enhancement is patchy in the ECH. There are three patches of radio enhancements, one corresponding to the north polar coronal hole and the other two located on the low- latitude section of the ECtt. Since the higher coronal layers show a relatively uniform coronal hole, we mayget some clue to the microwave structure from the photospheric magnetogram. Figure 3 shows a full disc longitudinal magnetogram obtained by SOHO MDI on August 27, It becomes immediately clear that the two radio enhancements on disc are associated with two bright patches (SP and NP) in the magnetogram with dominant positive polarity. SP is the southern patch bounded by the dashed lines; NP is the northern patch bounded by the smaller box; the large rectangular box encloses the trunk portion of the ECH. Kosugi et al. [1986] were the first to point out the relation between enhanced unipolar magnetic field within coronal holes and the radio enhancement at millimeter wavelengths. We shall explore this relation in greater detail. The pattern of enhanced unipolar magnetic fields was persistent throughout the observing period. In Figure 4 we show the evolution of the region enclosed by the large rectangular box in Figure 3, which includes the trunk of the ECH, for August For each day, the microwave enhancement is overlaid on the MDI magnetogram. The magnetograms and the radio images used are nearly simultaneous, and the error in alignment is less than a magnetogram pixel ( arc see). We see that on all 4 days, the radio enhancement is consistently associated with the enhanced unipolar magnetic region, although there are many differences in detail. For instance, the compact component of the radio enhancement is different in number and location on dif- ferent days. The ECH roughly runs through the middle of the enhanced unipolar magnetic region, with its area slightly smaller than that of the magnetic region. We now discuss the detailed comparison between microwave and EUV observations at the two patches of enhanced unipolar magnetic flux in Figure Fine Structures The longitudinal magnetic field structure under the coronal hole can be simply described as a collection of small magnetic elements, mostly of positive (majority) polarity with a very small number of negative (minority) polarity elements. The minority polarity element, when located near a majority polarity element, forms a mixed polarity, though predominantly unipolar region; this type of region is considered to be the basic configuration for the base of most polar plumes [DeForest et al., 1997]. Most of the other magnetic flux elements seem to be associated with the network structure. Figure 5 is an overlay of the MDI magnetogram with radio contours 'corresponding to the microwave enhancement in the southern patch (defined in Figure 3). There are two small, strong bipolar regions in the subimage (one marked ER near the southern edge and the other can be seen near the middle of the left edge of the subimage) outside the ECH, which are bright in microwaves. Note that the radio contours essentially cover the region of positive polarity. Wherever there are mixed polarities, there is hardly any radio enhancement, except for the

5 .. GOPALSWAMY ET AL' MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 9771 :;,%.?.',-'.E Figure 3. Michelson Doppler imager (MDI) longitudinal magnetogram obtained at 0452 UT on August 27, The magnetogram is displayed with a scaling of-50 to +50 G. White (black) represents positive (negative) magnetic polarity here and in all subsequent figures. SP (box bounded by dashed lines) and NP (smaller box) are the southern and northern patches of unipolar magnetic flux discussed in the text. The solid rectangular box corresponds to the trunk of the ECH where enhanced radio emission was observed. The bright bipolar region to the left of the rectangular box is AR The size of SP is 11 arc minx 11 arc min, and that of NP is 4.8 arc rain x 4.8 arc min. strong regions like ER, where the coronal contribution is significant. A close correspondence between the network elements and the radio emission can be seen for the circular array of magnetic elements marked "S". We come back to this latere What is the basic difference between the mixed polarity regions and the unipolar network elements in their association with the radio enhancement? To answer this question, Figure 6 compares the microwave enhancement and the magnetic field structure associated with the northern patch. The radio and MDI data were obtained simultaneously. We note again that the radio emission is confined to the region of the magnetogram where the magnetic flux elements are concentrated. The 200 K contour completely encloses the enhanced magnetic region. At contour levels less than 600 K we see mostly the smooth component; at higher levels, we see the compact component. The compact bright points have a peak brightness temperature of, 2000 K. The half-power size of the compact components is, 30 arc sec, much larger than the synthesized radio beam size. The three major compact sources and some weaker ones are well associated with unipolar flux elements. The bipolar region B at the northern periphery of the patch has only weak radio enhancement. Similarly, the large minority polarity element C at the northwest edge of the patch has only a marginal radio enhancement. To explore further, we have compared microwave en- hancement in the northern patch with the EIT 195 spectroheliogram obtained at 0024 UT in Figures 7a- 7c. This is a few hours before the image shown in Figure 6, so there is some evolution in the radio enhancement. Note that the microwave enhancement is confined to the darkest portion of the coronal hole (see Figure 7a). There is some excess microwave emission from the boundary of the coronal hole (marked A), which probably is the foot point of structure overlying the filament channel to the east. The bipolar re-

6 9772 GOPALSWAMY ET AL.: MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES Figure 4. Superposition of microwave contours on the longitudinal magnetogram (left) corresponding to the trunk of the ECH (see Figure 3) for August (a) 26, (b) 27, (c) 28, and (d) 29, and (right) the magnetogram alone, shown for comparison. The radio contours are 300,600,900, 1200, 1500, and 2000 K above the quiet Sun. Smooth as well as compact microwave enhancements are obvious. The radio images and magnetogram were simultaneous to within 2 min. gion B discussed in Figure 6 can be seen as bright in The striking feature of this figure is that the EIT 304 fi the EIT 195 image. Similarly, region C is also very and microwave emissions rarely overlap in the coronal bright and forms an emission structure connecting the hole. In fact, the only place they overlap is region A, nearby positive polarity. Even the fainter EIT feature just outside the coronal hole. In the coronal hole itlocated between B and C has a weak bipolar magnetic self, the EIT 304 fi is associated with bipolar or mixed structure (see Figure 6). This suggests that the contri- polarity magnetic regions, while the radio enhancement bution to the microwave emission from the mixed po- is associated with the rest of the coronal hole associlarity region is much smaller than that from the coronal ated with predominantly unipolar elements. In regions hole. In Figure 7b, we have compared the EIT 304 where there is no enhanced magnetic flux, neither the image at 0043 UT, overlaid with the corresponding ra- microwave nor the EIT 304 fi shows any emission. This dio enhancement. The coronal hole boundary is clearly is a very important advancement compared with what marked by enhanced emission in EIT 304 line. The was known before about radio enhancement in coronal darkest portions of the coronal hole again correspond to holes. the enhanced radio emission. Regions B and C also have The magnetic nature of the coronal hole is further reexcellent correspondence in the EIT 304 line. The vealed in Figure 8a, which shows a small section of the magnetic features associated with radio and EIT 304 solar surface centered around the feature S (see Figure enhancements are shown in Figure 7c. Here we have 5). Most of the flux elements in the subimage are of plotted the EIT 304 (black contours) and the radio positive polarity, except for a few weak minority polar- (white contours) emissions on the MDI magnetogram. ity elements. The circular arrangement of the magnetic

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8 9774 GOPALSWAMY ET AL.- MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES :ll Figure 7a. Comparison between the microwave enhancement (contours) in the northern patch with the emission in the EIT 195 ]t line (gray scale) on August 27 at 0024 UT. The contours are at-200, 200, 400, 600, 800, 1000, 1300, and 1600 K above the quiet Sun. Labels B and C denote the same bipolar regions as in Figure 6 and correspond to bright emission in the EIT 195 line. A is a region located at the boundary of the coronal hole, which probably is one leg of an arcade overlying the filament channel. Note that the brightest part of the radio enhancement coincides with the darkest region of the EIT 195 A coronal hole. Figure 7c. A comparison between the magnetogram (gray scale), EIT 304 ]t emission (black contours), and the microwave enhancement (white contours). The EIT 304 ]t intensity contours are at 2000, 2500, and 3000 intensity units (data numbers). The microwave contours are at 200, 300, 400, 600, 800, 1000, 1300, 1600, and 1900 K above the quiet Sun. This figure emphasizes the "preferred" association between the bipolar mag- netic regions and the EIT 304 emission on one hand and between unipolar network elements and microwave enhancement on the other. Fi. gure 7b. Same as Figure 7a, but with EIT 304 image on August 27 at 0043 UT. The microwave contour levels are at-200,200, 400,600,800, 1000, 1300, and 1600 K above the quiet Sun. Note the enhanced emission features in the 304 line at the edges of the coronal hole. There is very weak 304 line emission from the hole. The brightest part of the radio enhancement again coincides with the darkest region of the EIT 304 image.

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" ":: :%:? ;... :;'*: :': :.?:;::;:'::?: '""' : :'": :t:::, :: ß :'...:.;:...:;;:"'"':':/::::'"?;; t :../:":' "' :?':::;..:"'" " :x:" :½:?:'"":: ;' ; ':'" :. :... :s:?:,: ;.:?:....::g:'"*.:;....:?'*- - :: :; : :..::;.;,:;::: :,;; : ;- ':'... *%?::½::;:: :j:? ::: :; : :,s::.? :? :,:,: ':-'..:.. : ' x,:":". :::: : ;;:'; :;';, :?:::::'d":?.: -'g :½...:... :: '"'.-.-.:.-..:... ;:.iw<:::;":':.?:::: ': :::,.:r...::;::.....::.:.. : :'"%:;?:::;? :; E:' '?,-' '.:? ::": :::. ::. ::-":: : :. : :::' ':".:... '"' : ' ::::,,... :.:x:.'"' ':;,'...?;. -4 { :::::. :::::::.::::½:.:? E... -:-...,..;::: :.:--..-:: ' : :;! ;:'"... ' ::;:%..., '... : ½;:½:;:::;<:.:.. ' ::: ::: 5.:;:' Figure 8b. Comparison between the magnetogram (gray scale) and EIT 195 X image (contours) corresponding to the S feature at 002 UT on August 27. The solid contours represent the boundary of he ECH within the subimage. The dashed contours represent depressed emission. The con our levels are 980, 1000, 1020, 10 0, 1100, 1150, and 1200 intensity units (data numbers). Figure 8a. Comparison between the magnetogram and the microwave emission associated with the S feature noted in Figure 5. The radio contours are 300,400,500, 600,700,900, 1100, 1300, 1500, and 1800 K above the quiet Sun. Note [he over all correspondence in the two images. The peaks of the two compac[ radio sources are slightly shifted with respect to the network magnetic elements. This is probably due to the fanning out of the field lines toward the cell interior between the photospheric and chromospheric levels. The size of the magnetogram is 2.3 arc minx 2.3 arc min. :"-::'" '.!;; i:i::;;".....,:-?... '... '-.;;... ::.. '.i-.:'.": %.:::.. :::... : 6. :--.-: :-:::: :- :::,,... '},'!' '.* ::. :-- :'.7%... ':4 --: ':::i. "'-½'.: :::.:.... ": ' :-:.' ::"..,,,..'.... ;'i:;. 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':: -'::';*:- :: :'... :'.'."-.?'..:::;::-' ':::.'1 -.'.:..... ;..:...,... ::;.::::...,..:; ,- :::... -:: ,:::.' ;: ,_...:..:,-,:. :.: '"'::':: ' : : " :' -.:. :.:,:, ::-.... :.*.;: -": '"" : ; elements clearly has correspondence in microwaves. Enhanced radio emission can be seen associated with the two extended magnetic elements of the structure, with a peak brightness temperature of, K. The diffuse component has a brightness temperature of,- 600 K. For the compact radio sources, the radio peak is slightly shifted from the photospheric magnetic element, probably owing to the fanning out of field lines toward cell center because the radio emission originates from the upper chromosphere, where the temperature is around 104 K. This effect was also noticed when the magnetic elements were compared between chromospheric and We have used dotted contours to denote decrease in in- :,::...;.:'?' :::-.:...:'i;;;g:.'½...*'f%;:.:.'... ::...;:,..,::**.: %,X;:, ' :*: " ':.: /"-:a * ;::'::.:.' * ': ':;:..",- ::' :; :;::::... :::::::::::::::::::::::::... '* :::.:5:,.. 'C.,. :: 5 l' ' *:" ':.:.*:'*:--.:-.. ":' ß. '". ;:-. :: :*::: -:-: ;::. ;. :*'::;* :. %: :::, :4-....:' '-: ;:; :.:-..'::'::... :; '"':.::::':::::'::::::':::"' ':' : L'::::":'"'":*:** ß :':":'.-.½-:'":**-.:... C: ; ':? "*.:;... '". :00:-;'4:***...-,-,: Figure 8c. Same as Figure 8b, but for the EIT 304 3, image. The MDI image was taken at 0000 UT, while the EIT 304 3, image was taken at 0043 UT. The EIT 304 contours are at 1200, 1300, 1400, 1550 (dashed) photospheric magnetograms[wang al., 1997]. In Figand 2500, 3000, 3500, and 4000 (solid). The intensity ures 8b and 8c, we show the same section of the solar decreases as one goes from the outermost to the innersurface as in Figure 8a, but with the EIT 195 and 304 most dashed contours. For the solid contours, the in- intensity contours overlaid on the MDI magnetogram. tensity decreases from the innermost to the outermost contours.

10 9776 GOPALSWAMY ET AL.: MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 96 Aug Figure 9. Full-disc microwave image on August 28, 1996, showing the southern patch where we tracked the variability in the microwave enhancement. The size of the box is 6.9 arc minx 6.9 arc min. tensity. The outermost contour in Figure 8b roughly corresponds to the boundary of the coronal hole as obsee Figure 9) containing the southern radio enhancement every 10 min in Figure 10 for August 28, served in EIT 195 ]t. Note that the coronal hole passes We see that there is significant brightness variation over most of the area of the S structure. The only place in the smooth and compact components. The numthe EIT 195 ]t intensity is higher is at the magnetic ber of compact sources and their locations change with element located on the northern boundary of the ECH, where it bends to the north. In Figure 8c the EIT 304 intensity contours show a similar trend, except that the time. Sometimes we see elongated brightness enhancements, very similar to the arrangement of magnetic elements in the magnetogram. We tracked roughly the ECH is somewhat narrower and is confined to the mid- same patch of the solar surface on each day and found dle portion of the S feature. There are no mixed polarity similar variability (not shown). The average brightregions, and hence there are no bright, compact 304 ]t ness temperature within the box is plotted in Figure structures, except for the bright point structure marked BP. In a later magnetogram (see Figure 8a, top), emergence of a new flux can be seen at this location. There may be low level of flux emergence at 0043 UT, which seems to be responsible for the bright emission in for August over a period of 8 hours each day. On the time axis, "0" corresponds to 0000 UT on each day. The brightness enhancement shows variability over multiple timescales. It must be pointed out that much faster changes were observed in individual o There is a weak enhancement corresponding to the network bright points that do not show up in this plot. network elements. Note that the coronal hole as seen For example, the brightness temperature variation in in the EIT 304 ]t image corresponds to the inner part a small area (25x25 arc sec) around the three network of the EIT 195 ]t hole. BPs in the S structure (Figure 8a) is shown in Figure 12. The brightness temperature values were obtained every 2.4. Time Variability of Microwave Enhancement Microwave enhancement in the ECH shows significant time variability. In order to show this, we have displayed a small region (6.9 arc minx 6.9 arc min; 5 min. We see brightness enhancements up to,, 300% over a timescale of 30 min. The effective time resolu- tion of the plots is 15 min because we have smoothed the curves over three points. The large increases may be similar to the explosiv events [see, e.g., Innes et al., 1997].

11 ... GOPALSWAMY ET AL.- MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 9777 AUG 28 Figure 10. Variability of the microwave enhancement in the southern patch marked in Figure 9 for August 28, 1996, over a period of 8 hours. Successive images are 10 min apart. 8. Discussion We have presented a large volume of observational material on the ECH during its disc passage during Whole Sun Month. Because of the extensive data cov- erage, we were able to obtain detailed information on radio enhancement in the coronal hole. We considered global as well as local variability and structure of ECH in microwaves and compared them with SOHO observations. We were able to study the fine structures in the ECH and their manifestations in the chromospheric, transition region and coronal layers. Microwave emission from the coronal holes typically originates from the upper chromosphere owing to freefree emission from a plasma at a temperature of by SOHO CDS. Note that the microwave enhancement K. The coronal contribution to the observed brightness corresponds to a temperature above 10,000 K, which is temperature was found to be negligible [Gopalswamy very close to the lower end of the temperature range et al., 1998], based on the density estimates by Del Zanna and Bromage [1997] for the ECH. Any contriconsidered by Inslcy ½! al. [1997]. In coronal holes the field lines originating from the bution from the transition region also seems to be neg- network elements fan out toward the cell interior and ligible owing to possible unresolved structure [see, e.g., Feldman, 1983; Grebinskii, 1987]. This leads to the conclusion that almost all of the microwave emission in coronal holes must originate in the chromospheric layer, where the temperature is K. Thus microwav enhancement is an important signature of chromospheric dynamics. The variability in radio emission probably reflects the temperature variability. The radio signature is also very important from another point of view; there is a paucity of spectral lines in the temperature range around 104 K, and the radio telescope serves as a thermometer of the chromosphere. The variability in microwave emission is consistent with the results of Inslcy ½! al. [1997], who found inten- sity enhancements of % at the junctions of chromospheric network cells in several spectral lines formed in the temperature range 20,000 K to 1 MK, as observed then become completely open. This fanning out does not seem to be observed in the quiet Sun [Bastian ½! al., 1996]. One would expect this effect to be somewhat

12 9778 GOPALSWAMY ET AL.' MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 8C0 6OO 400 2OO - 1 O0 0 1 O Time (rain) Figure 11. Time variability of the average brightness temperature within the boxes in Figure 10 and average brightness temperatures for August 26, 27, and 29 (marked by curves). The brightness temperature was measured every 5 min and smoothed over three points, resulting in an effective time resolution of 15 mino On the time axis, 0 corresponds to 0000 UT on each day. There are variabilities over tens of minutes to hours. There is also day-to-day variability. The baseline of each curve is shifted to show them distinctly. portant signature of the small-scale episodes of energy release that heat and accelerate the solar wind in the coronal holes. 4. Summary and Conclusions The microwave enhancement in the elephant's trunk coronal hole is associated with the enhanced unipolar photospheric magnetic field pattern observed beneath the coronal hole. The microwave enhancement consists of a smooth component and many compact sources. The smooth component probably comes from network cell interiors. The typical size of the compact sources is 30 arc sec. The smooth component is typically enhanced by 500 K with respect to the quiet Sun. The compact sources show enhancements up to 2000 K. These correspond to an enhancement of 5% for the smooth component and 20% for the compact component. Both the smooth and compact components in the microwave images show time variability over multiple timescales. This time variability is indicative of the dynamic nature of the coronal hole where heating and pronounced in the quiet Sun because of the closed nature of the field lines. Bastian et al. [1996] used 1.3 and 2 cm and did not see any fanning. However, a closer examination of their figure does indicate fanning out between the photospheric magnetogram and radio images. The close association between bipolar or mixed polarity regions (SOHO MDI) within the coronal hole and the coronal (PIT 195 ) and transition region (PIT 304 ) emissions was also noted by Wan9 et al. [1997]. Combining this fact with the information on microwave enhancement helps us follow these features from the chromosphere to the corona. The close association between PIT 304 and PIT 195 may be indicative of the enhanced contribution due to the Si XI component in the PIT 304 emission as in active regions, because the Si XI lines forms at the same temperature as the Fe XII line. The lack of pronounced radio emission may be due to the smaller free-free opacity of the coronal plasma above the bright points. However, this has to be explored further to arrive at a firm conclusion. Therefore the compact sources observed in the polar coronal hole may not be the bases of polar plumes but the enhanced network elements. As concluded by Wang et al. [1997], the polar plumes may not contribute significantly to the solar wind. Since the cell interiors mostly contain horizontal fields, network elements are the most likely origin of the solar wind. It must be pointed out that the SOHO images used in this study had a cadence too low for us to compare all the time variability observed in the radio data with other wavelengths. The time variability may be an im :00 02:00 04:00 06:00 Stort Time (26-Aug-96 22'56'40) Figure 12. Time variability of the compact radio sources associated with the network elements of the S feature and brightness temperature averaged over a 25 arc sec x 25 arc sec area around the compact sources. The brightness temperature was measured every 5 min and smoothed over three points, resulting in an effective time resolution of 15 min. Note a flare-like brightening around 0400 UT on August 27. The three curves correspond to three different network elements of the S feature.

13 GOPALSWAMY ET AL.: MICROWAVE ENHANCEMENT AND VARIABILITY IN CORONAL HOLES 9779 acceleration of the solar wind plasma take place. The compact microwave sources associated with network elements sometimes show impulsive brightening with a brightness temperature increase more than 3 times the prebrightening level. Bipolar magnetic features within the coronal hole are associated with coronal and transition region emission. These may be similar to the polar plumes observed in polar coronal holes. Since the number of such bipolar regions is very small compared to the network elements, we think the latter may be the origin of the solar wind, ra[her than the plumes. Acknowledgments. N.G. is supported by NASA contract NAG and the 1997 SOHO Guest Investigator Program (NAG5-7238). C.D. is supported by NASA grant NAG The Nobeyama radioheliograph was supported by the Ministry of Education, Science, Sports and Culture, Japan. SOHO is a project of international cooperation between ESA and NASA. Janet G. Luhmann thanks Takeo Kosugi and Dale E. Gary for their assistance in evaluating this paper. References Bastian, T. S., G. A. Dulk, and Y. Leblanc, High resolution microwave observations of the quiet solar chromosphere, Astrophys. J., {75, 539, Bohlin, J. D., An observational definition of coronal holes, in Coronal Holes and High-Speed Wind Streams, edited by J. Zirkcr, p. 27 Colo. Univ. Press, Boulder, DeForest, C. E., J. T. Hoekscma, J. B. Gutman, B. J. Thompson, S. P Plunkett, R. Howard, R. A. Harrison, and D. M. Hassler, Polar plume anatomy: Results of a co-ordinated observation, Sol. Phys., 175, 393, Delaboudini re, J.P., et al., EIT: Extreme-Ultraviolet Imaging Telescope for the SOHO Mission, Sol. Phys., 162, 291, Del Zanna, G., and B. J. I. Bromagc, Spectroscopic diagnostics applied to the August 1996 equatorial Coronal hole, in Proceedings. of the Fifth SOHO Workshop, Eur. Space Agency Spec. Publ. ESA SP-JOJ, 323, Feldman, U., On the unresolved fine structures of the solar atmosphere in the 30, ,000 K temperature region, Astrophys. J., 275, 367, Gibson, S., and D. Biesecker, Results from the "Whole Sun Month" campaign, Bull. Am. Astron. Soc., 29, 4.01, Gopalswamy, N., K. Shibasaki, C. E. DeForest, B. J. I. Bromage, and G. Del Zanna, Multiwavelength observations of a coronal hole, in Solar Synoptic Solar Physics Conf. Set., vol 140 edited by K. S. Balasubramaniam, J. W. Harvey, and D. M. Rabin, p. 363, Astron. Soc. Pac., San Francisco Calif., Grebinskii, A. S., Suppression of radio emission from fine structures in the transition region in the solar atmosphere, Soy. Astron., Engl. Transl. 13, 299, Innes, D. E., P. Brekke, D. Germerott, and K. Wilhelm, Bursts of explosive events in the solar network, Sol. Phys., 175, 341, Insley, J. E., V. Moore, and R. A. Harrison, First observations of coronal hole structure and evolution using SOHO- CDS, Sol. Phys., 175, 437, Koshiishi, H., Deep CLEAN imaging method applied to the Nobeyama radioheliograph and observations of polarcap brightenings and their association with coronal holes, Doctoral thesis, Dept. of Astron., School of Sci., Univ. of Tokyo, Tokyo, Kosugi, T. M. Ishiguro, and K. Shibasaki, Polar cap and coronal hole associated brightenings of the Sun at millimeter wavelengths, Publ. Astron. Soc., 38, 1, Nakajima, Hoet al., The Nobeyama Radioheliograph, Proco IEEE, 82, 705, Scherrer, P., et al., The solar oscillations investigation - Michelson Doppler Imager, Sol. Phys., 162, 129, Takano, T., et al., An upgrade of Nobeyama radioheliograph to a dual frequency (17 and 34 GHz) system, in Coronal Physics from Radio and Space Observations, Lect. Notes Phys., Springer-Verlag, New York, Wang, Y.-M., N. R. Sheeley Jr., K. P. Dere, R. T. Duffin, R. A. Howard, D. J. Michels, and J. D. Moses, Association of Extreme Ultraviolet Imaging Telescope (EIT) polar plumes with mixed polarity magnetic network, A s- trophys. J., 4{84{, L75, Wefer, F. L., and M. P Bleiweiss, Observations of coronal hole associated features at wavelengths of 2.0 cm and 8.6 mm, Bull. Am. Astron. Soc., 8, 338, C. DeForest, Center for Space Science and Astrophysics, Stanford University, Palo Alto, CA N. Gopalswamy, J. B. Gurman, and B. J. Thompson, NASA Goddard Space Flight Center, Code 682.3, Bldg 26, Room G-l, Greenbelt, MD (gop gsfc. n as a. gov ) K. Shibasaki, Nobeyama Radio Observatory, Minamisaku, Nagano , Japan. (Received April 6, 1998; revised July 27, 1998; accepted August 27, 1998.)