fl108.canfield02 Posted: 21-Feb-93 Updated: 06-Nov-93, 27-Jul-94 Events specified: the 15-Nov-91 flare
Collaboration: R. Canfield, J.-P. Wuelser, K.P. Reardon, L.W. Acton, H. Kurokawa (other interested Yohkoh scientists are welcome)
Primary objective: To determine whether filament eruption or heating began first during the pre-impulsive phase of the 1991 Nov 15 X-flare.
Secondary Objectives: (1) To understand the dynamics of the filament eruption; (2) To evaluate the role of flux emergence before this flare.
Motivation: This famous Yohkoh flare is well observed at Mees -- H-alpha observations from the MCCD Imaging Spectrograph, magnetograms from the Stokes Polarimeter. From the MCCD data, we know that the filament started to erupt at least ten minutes before the impulsive phase. From SXT and the MCCD, we know that brightening started more than 5 minutes before the impulsive phase. We want to know when heating started, to learn whether the filament motion was triggered by reconnection, on the assumption that it is accompanied by heating. We already know that the H-alpha filament showed very interesting motions, and we want to characterise the growth of the various modes as the eruption progressed. Finally, we want to know the role of emerging flux in the preflare period, by studying the behavior of the arch filaments that are apparent a few hours before the flare.
Update 27-Jul-94
This research has been finished, and is waiting for me to write the paper.
The paper is in preparation -- here is an abstract.
The X1.5/3B flare on 1991 November 15, 22:33 UT was well observed by the Mees Solar Observatory CCD Imaging Spectrograph and Stokes Polarimeter and by the Yohkoh Soft X-ray Telescope. Five hours of preflare observations in H-alpha show a variety of dynamic phenomena prior to and during the filament eruption. Longer spatial scales characterize the eruption's early phase; smaller scales dominate later. The initial motions are not eruptive: rotation precedes disruption, and disruption precedes eruption. Both arch filaments and photospheric magnetograms indicate magnetic flux emergence. Ca II K-line wing images to provide unambiguous evidence of flare heating; we find that detectable flare heating starts after filament motion. Detectable large-scale filament Doppler shifts start about 20 minutes before the impulsive phase, whereas K-line wing brightening starts about 10 minutes before.
A new phenomenon has been discovered, which we associate with the subsequent eruption of the filament and occurrence of the flare. We have observed short-lived H-alpha blueshift events (duration about 1 minute, velocity about 10 km/s) near the neutral line, adjacent to a region of flux emergence. These blueshift events are followed by linear structures that are redshifted, and then longer (static) fibrils that cross the magnetic neutral line, linking the emerging flux region with the active region filament. These events are concentrated in the two-hour period immediately before the flare, although flux was emerging for at least five hours before. Some of these events show a rolling motion; some are cotemporal with filament velocity episodes. We associate these phenomena with what has been termed ``tether cutting'' by Sturrock and others.
We conclude that this flare resulted from active region destabilization, not the other way around; preflare motions preceded significant heating. We conclude that emerging flux played a dual role; it globally destabilized the active region, through tether cutting, and it reconnected with other flux systems during the flare.
Update 06-Nov-93
We have now analysed both the SXT data and the Mees data. We have found more than 10 examples of transient events, which appear to be the manifestation of the activity that leads to the preflare eruption of the filament. All transient events that have cotemporal SXT and Mees coverage appear as both SXR brightenings and dynamic H-alpha episodes. We believe that these are the manifestations of what is called tether-cutting, leading to filament destabilization.
We are presently in what we expect to be the last month or so of research, and expect to begin writing up the results. Preliminary talks hav been given at the Flares 22 meeting in Ottawa, at the Solar Physics Division of the American Astronomical Society in Stanford, and at the NSO/Sacramento Peak Summer workshop, and a short conference-proceeding paper for the latter is in preparation.