fl141.nakajima02 Posted: 04-Oct-93 Updated: 03-May-94, 13-Feb-95 Events specified: M4.0 flare of 18-Feb-93 02:58, M2.3 flare of 23-Mar-93 01:17 and M4.0 flare of 14-May-93 21:54
H. Nakajima, T. Sakao, Radioheliograph Group, Yohkoh Group, a nd Solar Flare Telescope Group
Motivation: Gradual,nonthermal microwave flares were observed with the Nobeyama Radioheliograph on the following days.
February 18,1993, 0250 - 0400 UT
March 23, 1993, 0100 - 0400 UT
May 14,1993 ; 2258 - 2400 UT
Such flares usually have large source size and so can be easily
resolved by rather low angulasr resolution of the Heliograph. I am
particularly interested in how and where electrons are accelerated
and also magnetic field configulation of flare regions. Relation
between acceleration and heating is also interested.
4) Data and Method to be Used: Heliograph, SXT, HXT, and H-alppha,
Magnetogram data.
Update 13-Feb-95
We have studied a gradual microwave/hard X-ray flare on 1993 March 23 by using data from the Nobeyama Radioheliograph at 17 GHz, the Yohkoh/Soft X-ray Telescope, and BATZE aboard Compton Gamma Ray Observatory. We find that: (1) A soft X-ray loop-top source is seen from the beginning of the pre-flash phase, and it shows a temperature distribution typical of a cusp-shaped structure which is a distinctive feature of long-duration soft X-ary flares, though the typical cusp-shaped structure can be seen in the decay phase but not in the pre-flash phase. This suggests that large-scale magnetic reconnection works as a primary energy release mechanism during the whole duration of the event. (2) The time profiles of 17 GHz brightness temperature for various locations in the flare loop show larger decay times and peak-delay times at the loop top than at the footpoint. It is inferred from this observation that electrons are accelerated to energies of at least several MeV and further that these electrons are efficiently trapped in a magnetic loop with the mirror ratio of 10, where pitch angle scattering of the electrons is only due to Coulomb collisions but not due to wave-particle interaction. (3) There is evidence that acceleration of higher-energy electrons are accelerated more slowly than lower-energy electrons.
Update 03-May-94
Present Status of Analysis and Some Results: The three events are still extensively being analysed. Up to now, we have the following results. (1). These flare consists of disk and limb flares. These events have large structures (in microwave and soft X-ray) reaching 1 arcmin. Microwave spectral analyses suggest that magnetic field strength in microwave emitting region is about 100 Gauss and that electrons with energies around MeV contribute to microwave emissions at 17 GHz. (2) The 1993 March 23 event (N19,W76): Major microwave emissions are originated rather from within a large loop-like structure observed with SXT in the pre-flash and decay phases. An impulsive flare is observed around a footpoint of the SXT large loop-like structure, about 1 hour before the microwave gradual phase. After the impulsive phase, upward motion of SXT and microwave sources, associated with some reconfiguration of SXT sources, are observed. (3) The 1993 February 18 event (N14,W02): This flare rather abruptly occurs without any prominent preflare emissions in microwave and soft X-rays. The flare initially begins in form of a slendar structure which is possibly associated with magnetic shear, and then develops to trigger of another larger structure, and finally results in a large relaxed strucure in the decay phase. Such development of the microwave source seems to be observed in the March 23 event. (4) The 1993 May 14 event (N19, W48): This event is associated with soft X-ray eruptions (H-alpha prominence eruption). Microwave observations begins after the peak time but still in the flash phase. Major nonthermal emissions in microwaves come from rather small limitted region around the top of the post-flare loops seen in microwave and soft X-rays. Overlay among microwave, hard X-ray and soft X-ray images is important.