fl119.canfield03 Posted: 23-Apr-93 Updated: 26-Nov-93 Events specified: flare on 24-Oct-91 at 22:36
Collaboration: R. C. Canfield, J.-F. de La Beaujardiere, J.-P. Wuelser, T.R. Metcalf, L.W. Acton, K.T. Strong, T. Kosugi, S. Masuda, J.L. Culhane, R.D. Bentley
Primary objective: To compare H-alpha source morphology with soft and hard X-ray observations, to determine whether this flare indeed showed no observed preflare or flare mass motions and eruptions and, if so, what "triggered" it.
Secondary Objectives: (1) To investigate preflare brightenings and mass motions in the 1991 October 24 flare; (2) to compare H-alpha signatures of nonthermal electron precipitation with hard X-ray observations; (3) to compare chromospheric condensation phenomena with soft X-ray blueshifts.
Motivation: This flare was well observed by Mees; we have H-alpha imaging spectroscopy, H-alpha filtergrams, and magnetograms. The impulsive phase is preceded by H-alpha brightenings at three sites and by a surge-like motion of absorbing gas (no filament eruption occurs). To determine the overall geometry of the flaring region and impose constraints on the flare model, we wish to compare H-alpha morphology, magnetic geometry, and soft X-ray source structure. In order to determine whether mass motion is a consequence or a cause of the flare, we want to know whether preflare brightening precedes or follows mass motion. To test the prediction that nonthermal electron precipitation produces an unambiguous signature in H-alpha, we wish to compare sites showing this signature with the locations of hard X-ray emission. Finally, in order to investigate the presumed connection between chromospheric condensation and evaporation, we want to compare the timing of H-alpha redshifts and soft X-ray blueshifts.
Update 26-Nov-93
The following paper has been submitted to the Astrophysical Journal.
Multi-Wavelength Observations of a Confined Two-Ribbon Flare
J.-F. de La Beaujardiere, R. C. Canfield, H. S. Hudson, & J.-P. Wuelser {Institute for Astronomy, University of Hawaii, Honolulu HI 96822 USA}
L. Acton {Department of Physics, Montana State University, Bozeman MT 59717 USA}}
T. Kosugi & S. Masuda {National Astronomical Observatory, Osawa, Mitaka, Tokyo 181, Japan}
ABSTRACT
The large two-ribbon flare of 1991 October 24 22:30 UT was observed by the YOHKOH satellite in soft and hard X-rays and by ground-based observatories at optical and microwave wavelengths. The combination of optical and soft X-ray data permit us to search confidently for plasma at both coronal and chromospheric temperatures. The H-alpha ribbons in this event are long and well-separated from the magnetic inversion line. However, the ribbons do not move apart during the course of the flare. Also, no filament is observed or ejected, and no evidence of eruptive behavior is discerned. Explaining this flare therefore presents a significant challenge: the erupting-filament model, typically invoked in the case of large two-ribbon flares, is clearly not appropriate here. In this paper we describe the salient characteristics of this flare, including the evidence for its confinement and the presence of a surge-like motion of chromospheric material along closed field lines. We argue that this surge, which is the only mass motion observed besides chromospheric condensation, does not drive the flare but is merely a low-energy by-product. We discuss several alternate explanations to the erupting-filament model, including a confined filament eruption, current interruption, interacting loops, and the so-called "avalanche" model, and conclude that none of the models unequivocally explains this flare.