fl214.sersen01 Posted: 17-Jul-95 Updated: 08-Apr-96, 27-Jan-97 Events specified: N/A (many)
Collaboration: M. SERSEN (Ondrejov Obs., Czech Republic & Comenius U., Bratislava, Slovak Republic; sersen@fmph.uniba.sk) F. FARNIK (Ondrejov Obs., Czech Republic) J. KLACKA (Comenius U., Bratislava, Slovak Republic) - and any other scientists who would like to participate
Events specified: many
Motivation and scientific goals: Since the beginning of the Yohkoh mission many spotless flares occurred, ranging from small flare-like events and subflares to relatively large, high-importance LDE flares. It is our intent to make a systematic study of morphological, spectral, and temporal behavior of the spotless flares using mostly Yohkoh SXT images and BCS spectra. We should like to derive physical environment of flares occurring in relatively magnetically simple solar regions to elucidate the physics of this type of events. The low-energy BCS channels (SXV and Ca XIX) should allow us to study small and flare-like events, whereas information about the super-hot flare plasma we expect to obtain from the Fe XXIV spectra. We also try to collect a relevant database of ground-based data such as magnetograms, H-alpha and He 10830 A spectroheliograms, CaK images, and radio observations when these are available, to get a complex physical view on both, thermal and non-thermal characteristics of studied flares. Numerical simulations of processes that might occur in spotless circumstances should be done after we obtain initial results. Investigation of spotless flares may also be a good chance to test popular "elementary burst" or microflare hypothesis. The microflaring may be well studied just in the case of events occurring in regions which are out of magnetic complexity.
Data required: SXT, BCS
This project is a part of Ph.D. thesis of Michal Sersen
Update 27-Jan-97
I have been analysing all SXT FFIs of 11-June-1992 to see soft X-ray (SXR) evolution of the "spotless" region in which the C2/1F 19:11 UT flare occurred. From the all-day analysis it has been found that the dark filament mass was probably present in the bright SXR structure without flying away into the corona. This is similar to the initial results of McAllister et al. (1992, PASJ 44, L205). However, first FFIs of the day indicate that there were several cusp-shaped structures in the vicinity, with a faint plasma blob high in the corona above the east limb. It is suggested that an eruption of a large-scale structure, possibly plasmoid or big loop, preceded the "spotless" flare. Subsequent reconnection may have induced the observed enhanced activity in the region. Two distinct SXR arch systems can be seen at the east limb, separated by a dark vertical lane, suggesting interaction (and possibly slow magnetic field reconnection) of two bipolar regions. Looking at Kitt Peak full-disk magnetograms, it can be concluded that three side-by-side large-scale bipoles (SXR arch systems) were present in the south-eastern part of the Sun. The dark filament seems to lay just between two of the arch systems. This seems to be consistent with the quadrupole photospheric magnetic source model (Uchida et al, 1979, 1980). In the northern part of the entire complex of activity, a SXR structure connecting the top of the candle-flame structure back to the lower part of the corona, i.e., structure like that seen in the arcade type flare of 21-February-1992, may be seen on the FFIs, together with a dark tunnel (visible on low-exposure FFIs). Despite the H-alpha "spotless" flare occurred in the filament channel, the filament itself was inactive throughout the flare, suggesting passive role of the dark filament matter in the flare process. Some preliminary results were published at the Bath YOHKOH Conf. in March 1996 and the Montana CME Conf. in August 1996. From the analysis of the situation on 11-June-1992 at the eastern limb it is concluded that the "spotless" flare itself was triggered by global restructuring of large-scale magnetic structures rather than by any internal physical mechanism. It is suggested that the initial reconnection took place high in the corona. The agents originating in the reconnection region then triggered activity around the dark filament without disrupting the filament. The above analysis is a part of my Ph.D. thesis that I submitted recently. The results are discussed in detail in the thesis. Since I was forced to submitt the thesis in slovak language, I have to re-write the thesis to English in order to publish results in a journal. Several other "spotless" events are to be studied in detail shortly, and I would like to ask DUCs kindly to let this project active since I expect to get some new results on triggering "spotless" events. Analysis is rather slow as it is necessary to use all-day SXT data, but several events have already been selected and partly studied (e.g., 19/20-June-1993 "spotless" flares).
Update 08-Apr-96
Preliminary results of analysis of a June 11, 1992 (start near 19:10 UT) flare were presented at the astrophysics conference "The Sun and Beyond" that was held on October 21-28, 1995 in Saigon, Vietnam. The abstract follows below. Work is now proceeding to derive basic physical parameters of the emission regions and, particularly, of the jet-like structures related to this flare, using SXT and BCS(?). Thanks D. Gary of Caltech we obtained OVRO microwave data related to this flare; the data will be included in the subsequent study that is in preparation. Michal Sersen recently visited YDAC/MSSL and selected a number of "spotless" flares from the Yohkoh database. The events now analyzed are: 5-Dec-91 at 07:34 UT, 30-Jan-92 at 06:03 UT, and 31-Oct-93 at 10:56 UT. There are also another events which will be analyzed shortly. The question is whether physical conditions in "spotless" flares are different from those in "spotted" events. We are also doing some numerical simulations on small-scale energy release in the solar corona, which may have an application in a theory of flares occurring in magnetically simple active regions. We will send an another update with new findings in a few months.
JUNE 11, 1992 "SPOTLESS" FLARE: PRELIMINARY RESULTS
M i c h a l S e r s e n
Astronomical Institute, Ondrejov, The Czech Republic
and
Comenius University, Bratislava, The Slovak Republic
ABSTRACT: Preliminary results are presented of study of a small solar flare that occurred on June 11, 1992 near 19:10 UT outside a complex of two active regions NOAA 7198 and 7194. The flare occurred close to a dark filament in a non-spotted region. H-alpha images show an enhanced activity in the form of numerous bright regions and loops in the vicinity of the flare. There is no remarkable change in the filament shape during the flare. Yohkoh "Soft X-Ray Telescope" images reveal a peculiar morphology of this event. X-ray plasmoid ejection seems to be associated with this flare. It is speculated that the emergence of new magnetic flux might trigger this flare.