fl158.farnik01 Posted: 06-Nov-93 Updated: 27-Jul-94, 03-Apr-95 Events specified: See Text
Collaboration : F.Farnik, T.Watanabe, H.Hudson, T.Sakao, J.Lemen, L.van Driel
Motivation:
GOES-PHOBOS stereoscopic observations made during 1988-89 opened some questions (as well as many other studies did) about the vertical structure of flares. Two of the observed events enable us to conclude that their precursor emission comes from the high corona while the main emission from the low corona. Because YOHKOH give us the best opportunity to study vertical structure of flares as observed in soft X-rays, we would like to show where the precursor emission comes from relatively to the main phase source and find the spectral differences (if they do exist) between the precursors and the main phase emission.
Required observations: SXT, HXT
Analysis techniques:
Approximately 470 flares closer than 30 deg. to the limb were observed by YOHKOH between 24-Sep-91 and 9-Oct-93. Out of these appr. 307 events were observed without re-pointing the instruments after onset of the flare. But only a few flares with a precursor-like feature were observed during the precursor as well as the main phases (due to satellite nights and passages through SAA). For these selected events we are going to derive their temperature characteristics and the positions (including temporal changes) of the bright kernels as observed by both SXT and HXT (if possible) instruments.
Update 03-Apr-95
I hope to be able to send the paper whose abstract is shown below to Solar Physics within a few weeks.
Spatial Relations between Preflares and Flares as Derived from Yohkoh Soft X-ray Images.
F.Farnik, H.Hudson, T.Watanabe
We have conducted an initial search for pre-flare brightenings as observed in soft X-radiation by Yohkoh. The Yohkoh images allow us to identify, to within a few arc seconds, the location of a preflare event relative to the succeeding flare. Our initial motivation in this study was to search for early coronal brightenings leading to flare effects, as had been suggested by earlier studies; thus we concen- trated on Yohkoh limb events. We find no evidence for such early coronal brightenings. Between 15% and 41% of the 131 suitable events matched our criteria for preflare brightening: the same ac- tive region; brightening within one hour of the flare peak; preflare brightness less than 30% of the flare peak. In the great majority of the preflare cases, we found that physically separate nearby struc- tures brightened initially. Often these structures appeared to share a common footpoint location with the flare brightening itself. In a few cases the preflare could have occurred in exactly the same structure as the flare.
Update 27-Jul-94
I have completed the first part of the study and the preliminary results can be sumarized: 1. Starting from the list of 484 flares we selected 55 flares with some
preflare activity. The data show that 15% of flares have preflares without any doubts and this number can be as high as 42% because in many cases we have no YOHKOH coverage due to satellite nights. 2. All the data show preflares originating at the same altitude as themain flares. Thus, the original concept of high-altitude precursors was proved to be false.
I am going to talk about these preliminary results at the SOHO meeting in Estes Park in more detail and together with other colleagues we are going to prepare final publication before the end of this year. Just now we are discussing the first draft of the paper.
With my best regards -- Frantisek Farnik