fl152.nitta10 Posted: 25-Oct-93 Updated: 24-Jul-94, 25-Apr-95, 17-Jan-96, 12-Oct-96 Events specified: microflares in AR 7260
Collaboration: Nariaki Nitta and anyone interested
While I was overplotting HXT light curves on top of GOES light curves for the 16 major (> C1) flares that SXT observed in AR 7260, I occasionally noticed weak signals in the low-energy channel of HXT at times of smaller flares in soft X-rays. This suggests the existence of non-thermal electrons even in such small events. It is not clear at the moment how sensitive the low-energy channel of HXT is, in comparison with BATSE on GRO, but the preliminary survey suggests that these HXT data can be used to identify microflares. One advantage of the HXT data is the availability of simultaneous soft X-ray images.
I am currently studying the major flares in AR 7260 on a semi- statistical basis, and I would like to extend it to smaller flares, some of which may be called transient brightenings, microflares, jets, bright point flares, etc. My basic question is whether they are really different from flares or they are just a small version of flares. As mentioned in several places, the region AR 7260 consisted of a slowly decaying leader spot and a rapidly growing region, and flare activity was found in and around both places. I hope to find out the relative strength of hard and soft X-ray emissions in the expanded sample of flares and its dependency on (1) the morphology of soft X-ray emission, (2) the location and (3) the evolution of the active region.
Although I cannot say for sure, I am also interested in using SXS data to study soft and hard X-ray emissions in the small flares in AR 7260 that have hard X-ray signatures.
Update 12-Oct-96
Using the HXT L-band (14 -- 23 keV) data, a systematic search has been conducted to identify the hard X-ray signals from microflares in AR 7260 with reference to GOES and SXT data and Solar Geophysical Data. We have extracted about 50 clear examples. We have also searched bright soft X-ray microflares without hard X-ray signals. It is found that almost all the strong microflares, say > B2 or 2e6 DN/s in SXT images in the thin aluminum filter, accompany significant signals in the HXT L band. Some microflares
corresponding to GOES A class also show hard X-ray emission. In some cases, microflares occur at different areas of the active region one after the other, making the identification of the associated soft X-ray loop non-trivial. The usual relation between hard and soft X-rays (the Neuperteffect as called by some people) is always seen. For major flares, say > C class, there is some known correlation between the hard X-ray peak counts and the soft X-ray peak flux. A better correlation is seen between the time integrated hard X-ray counts and the peak soft X-ray flux. The seme relation seems to hold for microflares also, given the uncertainty in both quantities. Extrapolating this relation, it is argued that nonthermal process is present even in weaker microflares that can be observed with a better sensitivity hard X-ray detector.
Update 17-Jan-96
The microflares that are associated with hard X-rays are generally tiny in size. This may be similar to those compact flares that have high hard to soft X-ray ratio. Conclusions of this study depend on how we wrap up our study of flares from the stand point of temporal variations.
Update 25-Apr-95
I am still identifying the location of those microflares which had some signals in the HXT L-band, making use of the well-known temporal relation between soft and hard X-rays. One of the things that makes this procedure difficult is that in the later evolution of AR 7260 there were many examples of two or three microflares that occurred nearly at the same time at different locations. This itself suggests that various parts of the active region were somehow connected.
Despite this difficulty, I will probably be able to report shortly on characteristics of the microflares that accompany hard X-ray emission.
Update 24-Jul-94
We have found more hard X-ray events associated with microflares in AR7260 as the region became old than when it was vigorously growing. One interesting place is the western side of the large sunspot, most probably close to the light bridge. Several flares took place in association with MMF, forming loop like structure toward south. The northern footpoint produced hard X-ray emission repeatedly even though the soft X-ray counterpart was not so intense. We will study the relationship between these hard X-ray events with H-alpha surges.