fl100.nitta07 Posted: 17-Jan-93 Updated: 31-Jul-93, 8-Feb-94, 8-Oct-94, 29-Jun-95, 26-Apr-96 Events specified: flares in NOAA regions 6906, 6994, 7260 and 7270 and other regions abounding in new flux emergence
N. Nitta, L. van Driel-Gesztelyi, K.D. Leka, K. Shibata
It has been argued for long time that reconnection of an emerging flux with pre-existing field plays a major role in the primary energy release in solar flares. Indeed, an emerging flux region tends to be more flare productive than a region that is dominated by a well- established spot. A major attraction of the emerging flux models for solar flares is that in principle we do not have to worry about moving field lines to reconnect. The new flux has to emerge.
However, change in the magnetic field has seldom been observed with certainty directly associated with a flare. It is possible that only the azimuthal component of magnetic field of a loop is involved in the reconnection processes, leaving the longitudinal component almost unchanged. It is also possible that reconnection is just part of the on-going change that forms the evolution of the region. In other words, a flare may be produced when certain parameters reach some threshold values in the course of the eveolution of the active region. The change in the magnetic field that is attributable to the flare itself may be not large enough to be observed.
One of us (NN) closely looked at the SXT aspect telescope data for the region NOAA 7260 to accomplish a goal of the proposal fl075 (to statistically correlate soft X-ray emission with the magnetic field environment of the flare location). In at least a couple of flares, he noticed that a magnetic field element, possibly a pore, became faint or more pronounced a couple of hours prior to the flare and that the brightest soft X-ray emission came from the same location.
We propose to see if it is a common phenomenon, by systematically looking at aspect telescope images with our eyes wide open. While the narrow-band images from the aspect telescope do not have good spatial resoltion and they are not as sensitive to flux emergence as magnetograms, their high cadence as well as exemption from seeing effects are major advantages. We will try to relate changes seen in aspect telescope images with flares for the above regions that enjoyed remarkable growth at some phase of their evolution. Of course, wherever available, we will reference our findings with magnetograms and H alpha images.
Update 26-Apr-96
We have been able to understand the occurrence of a sequence of flares in the emerging flux region of AR 7260 in terms of new and old bipoles. For example, the flare of 19-Aug-92 2009 UT was probably due to the interactions of relatively new bipoles P40-F40 and P41-F41 (nomenclature of Leka et al. 1994), resulting in a long loop P40-F41. In the two subseqent flares (20-Aug-92 0350 UT and 20-Aug-92 0904 UT), another new bipole P48-F50 brigthend with the previously formed long loop brigtening probably as a secondary effect. Then the second delta spot, which grew to the south of the first one, was involved as it approached the first, and the next flare (20-Aug-92 1430 UT) established a loop connection between the two deltas (P38-F50), which was observed to brighten later. Despite all these scenarios, we could not find positive evidence that flux emergence is a key factor for flares. It is likely that the resolution to detect energence is not sufficient, but it is equally likely that only flux reaching the coronal level causes instability in the existing structures. This study will contine with other regions (such as AR 7270) to confirm the above hypothesis.
Update 29-Jun-95
In a process of relating flares and active region evolution, which is characterized by flux emergence, the following paper has been submitted to Solar Physics.
Flares in Active Region NOAA 7260 (I): Overview
N. Nitta, L. van Driel-Gesztelyi, K. D. Leka, K. T. Strong, J. R. Lemen, S. L. Freeland, T. Kosugi, T. Sakurai and K. Ichimoto
Active Region NOAA 7260 (AR 7260), which produced many flares up to the GOES M4 level, was well observed by instruments on board the Yohkoh spacecraft as well as various ground-based observatories. Yohkoh did not only observe a total of about two-dozen flares in the flare observing mode but also took data of the region during non-flaring periods. This whole dataset provides a rare opportunity to study flares in the context of active region evolution. We make detailed analysis of X-ray and optical data, whose pixel resolution is typically 2"-5", and present the results in a few papers with different points of emphasis.
In this paper we overview the evolution of AR 7260 and flares therein. The region appeared on the east limb as a large matured sunspot accompanied by a plage region in the following polarity area. In this area, new flux started to emerge systematically, resulting in double delta spots. As expected, many flares occurred in this emerging flux region (EFR) within a short interval. Both the X-ray intensity and temperature of these flares peaked at about the time the EFR developed fully. This was shortly followed by a long-duration event (LDE) in the large-scale loops connecting the preceding spot and the EFR. The large preceding spot, which appeared to consist of two umbrae divided by a light bridge, also had several flares around it. Their occurrence was roughly clockwise with time, starting in the SE sector. These flares were presumably associated with disintegration of the preceding spot.
Based on a detailed analysis of the X-ray data for all the flares in AR~7260, we have found the following, irrespective of whether the flare took place in the EFR or around the large preceding spot. X-ray emission from the flare comes from a wider area than previously believed. A high temperature area seems to exist already in the impulsive phase, being not cospatial with the brightest area and presumably located at higher altitudes. The soft X-ray impulsive brightening is not always observed even from the footpoints that emit hard X-rays. No evidence has been found for eruptions in the early phase with one exception; this event took place at a location whose magnetic connectivity seemed to be different from those of other areas. We conclude that major flares in this region generally had a closed nature.
Update 08-Oct-94
Two proceedings papers either have come out or are in press.
1. Proceedings of Kofu Symposium (1994), p. 385. Comparing the fluxes in SXT X-ray and white-light images, we suggest that major flares do not occur during the period of vigorous flux emergence.
2. COSPAR, July 1994, in press. We have shown that emerging-flux-associated major flares are preceded by microflare activity which appears to start around the time of flux emergence as recorded in white-light images. This may explain the time difference between the major flares and vigorous flux emergence. It is also suggested that even during the slow reconnection stage fast reconnection on a smaller scale can occur intermittently.
A full paper addressing the above findings is in preparation, and they are also addressed in part in a series of the AR 7260 flare papers.
Update 8-Feb-94
We have not found clear evidence for flux emergence actively participating in triggering flares. However, we have found that flares in AR 7260 occurred where magnetic field was changing greatly, which was seen in both photospheric and chromospheric levels.
Update 31-Jul-93
We have made quite a detailed analysis for the flare at 2358 UT on 17 August 1992, which took place along a sheared neutral line to the northeast of the preceding spot. Narrow-band images suggest that the small sunspot, apparently underneath the brightest core (both in X-ray and H-alpha), weakened over several hours before and after the flare, which is also seen in Mees MCCD H-alpha off-band images. The small sunspot appear to split around 2100 UT. We initially thought this was a signature of flux emergence, but we no longer believe it, since the two spots seem to be of the same polarity. Rather, we regard this splitting as part of the sunspot decaying process. In conclusion, at least one flare shows evidence aginst flux emergence. We will write a paper on this flare, probably in contrast with another one at 1325 on 18 August, which happened to take place in nearly the same area but showed more loop emission.