fl075.nitta06 Posted: 05-Sep-92 Updated: 31-Jul-93, 8-Feb-94, 8-Oct-94, 29-Jun-95, 26-Apr-96 Events specified: N/A
N. Nitta, H. Hudson
With the advent of a high spatial and temporal resolution telescope in soft X-rays such as the SXT, we may have to redraw the flare classification scheme that was built during the last solar cycle based chiefly on hard X-ray temporal as well as imaging observations. On the other hand, one should not ignore time profiles, since they still have a potential to give one an intuitive insight on what is going on in the flare. It appears, however, that SXT flare images have so far been analyzed without much regard to time profiles due partially to difficulty in accessing data from the spectrometers.
In order to avoid having biased pictures established as a result of quick-look analyses of SXT images alone, we are in the process of making an extensive flare catalog based on Yohkoh WBS (SXS/HXS) observations. More than 400 > C3 flares have been carefully identified to be observed by Yohkoh at least partially. We are in the position to compare the SXT flare images with time profiles on a STATISTICAL basis. We are interested in how a variety of hard/soft X-ray time profiles reflect different flare mechanisms although there is still a possibility that a single flare model can explain all the varieties. We will also aim at assessing the importance of particle acceleration in flares, by finding out if there is a systematic difference between hard X-ray rich and hard X-ray deficient flares in terms of the environment of magnetic field as well as plasma parameters. Although we plan to use WBS data for spectral information, we are also interested in analyzing HXT and GRO data wherever necessary.
Update 26-Apr-96
We have analyzed about 50 "energetic" events that have significant signals in the HXT H-band. Some of them are more impulsive than others. To our surprise, we have been able to identify signatures of eruptions from 50-60% of flares. The central part of the SXT images has a wide variety of morphology, but it is generally difficult to identify an isolated flare loop clearly in the sense that hard X-ray emission often does not come from footpoints of what looks like a loop. Cusp structures or high temperature (~20 MK from SXT Be/Al.12) regions are often seen, and even those flares without them it is likely that either saturation or scatter from the most intense pixels prevents them from being observed. It appears that hardness in the 15-100 keV range is not a key factor to give systematic difference of SXT image morphology
Update 29-Jun-95
I will retain only real contributors and let others go. So the only co-worker is H. Hudson.
We are in the process of writing a paper on an unusual flare which would have been classified as type A based on previous observations.
Working abstract:
A Super-hot Flare observed by Yohkoh
We present an analysis of X-ray data for an unusual solar flare that was observed in a flare-productive region. The X-ray light curves of the flare consist of two parts whose peaks are separated by 4-5 min. The first part appears to correspond to a usual impulsive flare, since the soft X-ray images reveal a compact loop structure being filled from the footpoints and the nonthermal velocity is quite high (300 km/s) around the time of the hard X-ray spike. During the second part, while the compact loop structure still dominates soft X-ray (> 3 A) images, low-energy hard X-ray (14-23 keV) emission comes primarily from a much larger strucutre mear-by. Thermal fits to the hard X-ray data yield temperatures ~3x10^7 K, similar to that of the super-hot component. The origin of this component should be different from that of the 1-2x10^7 component.
Update 08-Oct-94
It is well known that the ratio of the peak soft and hard X-ray fluxed has about an order of magnitude scatter from event to event. We have confirmed that this is true for not only AR7260 but also for AR7270, both emerging flux regions. However, within this scatter, it appears that those events whose soft X-ray emission is concentrated tend to have stronger hard X-rays. Using the very useful flare catlogue compiled by S. Masuda and the HXT team, we have found that the Yohkoh flare that has the largest hard to soft X-ray ratio is the one on 3-Oct-93, which Franta Farnik is analyzing. This flare again shows fairly concentrated soft X-ray smission at the beinning. We will collect other circumstantial information which may have to do with this hard and soft X-ray ratio.
Update 8-Feb-94
We have performed further study of flares in AR 7260. It has turned out that they are in relatively narrow parameter ranges as far as duration and hardness are concerned. Instead of using WBS/HXS data, we have concentrated on HXT data because of its higher sensitivity, examining the imaging data also. It seems that footpoints that footpoints are emphasized from flares of hard spectra. We are trying to make this view more quantitative.
Update 31-Jul-93
We have concentrated on flares in active region NOAA 7260, and tried to characterize them. Some of them appear to be triggered by flux emergence. We initially expected that they have higher temperature and/or harder spectra than others which have nothing to do flux emergence. However, such a tendency has not been confirmed. Single peak vs double or more peaks appear to simply reflect the number of loops that are involved.