LARGE-SCALE CHAINS ON THE SOLAR DISK: BRIEF DESCRIPTION

1. Introducing remarks

It is a working material only, prepared to outline and to illustrate a novel phenomenon of the large-scale solar activity, which have been found analyzing of modified images of the Nobeyama Radioheliograph (NRH) (Nakajima et al., 1994), Yohkoh/SXT (Tsuneta et al., 1991), SOHO/EIT (Delaboudiere et al., 1995), and TRACE EUV. I mean large-scale (i.e. comparable with the solar disk diameter) shining chains consisting from numerous bright blobs and points.

!!! An important point !!! These relatively weak features have been found due to the following modification of the original images: I formed modified images with a restricted range of the brightness so that the most intense sources are suppressed and the relatively weak features become the most pronounced.

I used the NRH images collected during my stay in the Nobeyama Radio Observatory in January-February 1997. In the further study, I analyzed also the NRH, Yohkoh/SXT, SOHO/EIT, TRACE and other relevant satellite and ground-based solar images and data available on the corresponding INTERNET www pages.

It should be emphasized that the large-scale chains under consideration are relatively weak features as a rule. Perhaps, it is one of reasons why these structures have been not discovered so far. To distinguish them on the solar images, including many other features, one should to examine the images with attention (see the October 30-31, 1992 event).

2. Microwave chains

At first the large-scale chains were found on the NRH images at 17 GHz (see a brief description in Chertok and Shibasaki 1997,1998; a comprehensive paper is in preparation now). The analysis of the modified radioheliograms with a restricted range of the brightness temperature (for example, for Tb~(7-20)x10^{3} K) showed presence of these unexpected phenomena at microwaves. The blobs consisting the microwave chains are characterized by the angular sizes of 30-60 arcseconds and the brightness temperature of about (11-13)x10^{3} K (at 17 GHz the brightness temperature of the undisturbed background Sun is (10.3-10.8)x10^{3} K).

At least two types of the large-scale chains should be distinguished. Firstly, there are so-called long-living radio chains which exist and keep their general form during many days, have a global character (their extent may exceed 180 heliodegrees), stretch from one active region to another removed active region/plage, and rotate together with these formations (see Figure 1 of the November 1-4, 1993 event). Sometimes these long-living chains form closed structures or cells surrounding some active regions, plages, or filaments.

The preliminary comparison with other solar images and maps shows that there are clear examples in which the form and location of such long-living radio chains coincide with sharp boundaries of coronal holes (see Figures 1of the May 10-15, 1993 and November 1-4, 1993 events), observed with Yohkoh/SXT, as well as with footpoint lines of some large X-ray arcades. They appear to correspond also to the general picture of large-scale magnetic fields and filaments. In the latter case, the microwave chains repeat the form of the corresponding radio filaments but are located at considerable distance from the filaments.

According to the preliminary conclusion of Obridko and Shilova (1998), some long-living microwave chains are located above hills of the radial component of the large-scale magnetic fields.

The second category of the relatively short-living (or transient) chains at microwaves with a characteristic time scales of several hours is associated very closely with coronal mass ejections (CMEs) and post-CME energy release in the corona that are revealed, in particular, by long-duration events (LDEs) in the soft X-ray and microwave ranges. In some cases, such LDEs are accompanied by strong changes of the form and the location of the pre-existing long-living chains, especially near the LDE source. In other cases, new shining radio cells, extending of several tens of heliodegrees, arise also in a huge region near the LDE source. (See Figures 2 of the May 10-15, 1993 and November 1-4, 1993 events).

3. Chains of bright points in EUV and soft X-ray ranges

(See some examples of the SOHO/EIT and Yohkoh/SXT , TRACE EUV , and combined images of chains)

The outlined analysis of the modified Nobeyama radio heliograms stimulated the corresponding processing and search of the similar large-scale structures at the soft X-ray and, especially, at EUV images gathered with Yohkoh/SXT and SOHO/EIT (see Chertok, 1997,1998). It turned out that in these ranges the large-scale chains are still more typical and pronounced than in at microwaves. Here the chains consist of bright points of smaller sizes in comparison with the blobs forming the microwave chains. Their typical sizes are tens of arcseconds, and life-time is several hours.

!!! Recently this remarkable feature of the large-scale solar activity has been confirmed also by consideration of the modified mosaic full-disk TRACE EUV images (see July 19,1998 , August 3, 1998 and August 6, 1998 event).

The analysis of the modified Yohkoh/SXT, SOHO/EIT, and TRACE EUV images shows that rather often the bright points are not scattered chaotically through the solar disc, but are aligned at definite lines forming large-scale chains and treads of various (sometimes very puzzled) configurations(see, e.g., the May 22, 1996 , December 16, 1997 , January 3, 1998 , March 29, 1998 , April 7, 1998 , April 28, 1998 , June 1, 1998 , June 3, 1998 , August 3, 1998 events and others). Usually the spatial interval between the neighbor bright points is of several times larger than their sizes.

There are examples, in which almost continuous thin bright threads rather than the bright point chains are observed. These large-scale threads appear to differ from the relatively small filament-associated soft X-ray threads described by Solberg and McAllister (1997) which appear to correspond to the arcade tops on the solar disk.

The reality of these chains and threads is confirmed by the fact that in many cases their form and location coincide in the EUV and soft X-ray ranges. Sometimes the coinciding large-scale features are seen also on some ground-based images, for example, on the Kitt Peak He I (10830 A) heliograms. Comparison with various magnetograms seems to be promising as well. There are examples where similar large-scale chains are present on the EUV/soft X-ray images and the Nobeyama radioheliograms (see the August 22, 1996 event), although, as one could expect, there is not one-to-one correspondence between them due to different conditions (temperature, emission measure) of the emission origin in these ranges.

Similar to the microwave chains, the EUV and soft X-ray chains and threads seem to be related to CMEs. However, the question on the character of the relationship between the large-scale chains and CMEs (post-CME energy release) appears to be opened.

One more strange feature of the analyzed chains and threads (which, however, should be verified carefully in the course of the further studies as well) is that they seem to cross such essential large-scale structures as the reverse polarity lines of the magnetic fields, filaments, coronal holes and so on.

My preliminary impression is that sometimes the dark (i.e. absorption) thin large-scale threads are observed at the disk. Perhaps some of them transform later into the bright threads or chains.

4. Concluding remarks

Thus we appear to deal with a novel phenomenon of the large-scale solar activity. The found microwave and EUV/X-ray chains mean that some large-scale structures shine as a result of energy release associated either with the long-term evolution of the large-scale magnetic fields or with disturbances of extended magnetic fields identified with CMEs.

After seeing the chains on the modified images, in many cases one can see these structures also on original images and movies under attentive consideration.

At present, it is not clear to what extent the microwave blob chains and the EUV/X-ray chains of bright points are the same phenomenon.

The chains and threads under consideration do not coincide with radio and optical filaments, i.e. they do not correspond to the neutral lines of the large-scale magnetic fields.

The chains in all analyzed ranges display very strong evolution at the time scale of hours, especially in the CME-associated events.

It is obvious that the discovered chains reflect some unknown peculiarities of the organization and evolution of just the large-scale magnetic fields, which should be studied in the course of the further analysis. Apparently, the prolonged energy release, occurring under such an evolution (including the process of CMEs and the post-eruption relaxation of the magnetic fields), initiates a plasma heating in the wide temperature range (from tens of thousands to millions of degrees), which takes place, on the one hand, in sufficiently compact local elements corresponding to the individual bright points, and, on the other hand, in some selected, globally organized magnetic structures corresponding to the observed large-scale chains.

The overall analysis of these novel interesting phenomena and especially detailed comparison with other images and magnetograms are in progress. These studies are carrying out in the frames of the Proposa l accepted for the SOHO Guest Investigator Programme, as well as of the Existing EIT Analysis Proposal No.163 and TRACE Proposal . The work is supported by the Russian Foundation of Basic Research (RFBR), by the Russian Federal Programme on Astronomy, and by the INTAS/RFBR grant for the Joint Research Project "Support for the SOHO Solar Maximum Science Mission".

Some References:

Chertok, I.M.: 1997, Some Features of the Post-CME Energy Release in the Solar Corona. Proc. of the Fifth SOHO Workshop, ESA SP-404. P. 269.

Chertok, I.M. and Shibasaki, K.: 1997, Solar Large-Scale Bright Structures in the Microwave Range by Observations with the Nobeyama Radioheliograph. Problems of the Contemporary Radio Astronomy, XXVII Radio Astronomical Conference, IAA, S.-Petersburg, V. 2. P. 76, (in Russian).

Chertok, I.M.: 1998, Solar Large-Scale Chains of Bright Points After a Coronal Mass Ejection of 22 August 1996. Bull. of the Russian Academy of Sciences. V. 62. No. 9, in press.

Chertok, I.M. and Shibasaki, K.: 1998, Solar Microwave Large-Scale Bright Structures by Observations with Nobeyama Radioheliograph. 32nd Scientific Assembly of COSPAR, Abstracts. P. 351; Adv. Space Res., in press.

Delaboudiere, J.-P., Artzner, G.E., Brunaud, J. et al.: 1995, Solar Phys., V. 162. P. 291.

Nakajima, H., Enome, S., Shibasaki, K. et al.: 1994, Proc. IEEE, V. 82. P. 703.

Obridko, V.N. and Shilova, N.S.: 1998, personal communication.

Solberg, F.C.R. and McAllister, A.: 1997, Axial Soft X-ray Brightening Associated with H-alpha Filaments. Proc. of the Fifth SOHO Workshop, ESA SP-404. P. 675.

Tsuneta S., Acton L., Bruner M. et al.: 1991, Solar Phys., V. 136. P. 37.

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Last update: 9 August 1998