Observation of the impulsive phase of a simple flare |
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| Authors: | E Tandberg-Hanssen P Kaufmann E J Reichmann D L Teuber R L Moore L E Orwig H Zirin |
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| Institution: | 1. NASA, Marshall Space Flight Center, 35812, AL, U.S.A.
2. INPE, Institute de Pesquisas Espaciais, 12200, S. José dos Campos, SP Brazil
3. NASA, Goddard Space Flight Center, 20771, MD, U.S.A.
4. Big Bear Solar Observatory, 91125, Caltech, Pasadena, CA, U.S.A.
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| Abstract: | We present a broad range of complementary observations of the onset and impulsive phase of a fairly large (1B, M1.2) but simple two-ribbon flare. The observations consist of hard X-ray flux measured by the SMM HXRBS, high-sensitivity measurements of microwave flux at 22 GHz from Itapetinga Radio Observatory, sequences of spectroheliograms in UV emission lines from Ov (T ≈ 2 × 105 K) and Fexxi (T ≈ 1 × 107 K) from the SMM UVSP, Hα and Hei D3 cine-filtergrams from Big Bear Solar Observatory, and a magnetogram of the flare region from the MSFC Solar Observatory. From these data we conclude: - The overall magnetic field configuration in which the flare occurred was a fairly simple, closed arch containing nonpotential substructure.
- The flare occurred spontaneously within the arch; it was not triggered by emerging magnetic flux.
- The impulsive energy release occurred in two major spikes. The second spike took place within the flare arch heated in the first spike, but was concentrated on a different subset of field lines. The ratio of Ov emission to hard X-ray emission decreased by at least a factor of 2 from the first spike to the second, probably because the plasma density in the flare arch had increased by chromospheric evaporation.
- The impulsive energy release most likely occurred in the upper part of the arch; it had three immediate products:
- An increase in the plasma pressure throughout the flare arch of at least a factor of 10. This is required because the Fexxi emission was confined to the feet of the flare arch for at least the first minute of the impulsive phase.
- Nonthermal energetic (～ 25 keV) electrons which impacted the feet of the arch to produce the hard X-ray burst and impulsive brightening in Ov and D3. The evidence for this is the simultaneity, within ± 2 s, of the peak Ov and hard X-ray emissions.
- Another population of high-energy (～100keV) electrons (decoupled from the population that produced the hard X-rays) that produced the impulsive microwave emission at 22 GHz. This conclusion is drawn because the microwave peak was 6 ± 3 s later than the hard X-ray peak.
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