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:
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(1)
The overall magnetic field configuration in which the flare occurred was a fairly simple, closed arch containing nonpotential substructure.
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(2)
The flare occurred spontaneously within the arch; it was not triggered by emerging magnetic flux.
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(3)
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.
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(4)
The impulsive energy release most likely occurred in the upper part of the arch; it had three immediate products:
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(a)
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.
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(b)
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.
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(c)
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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Tandberg-Hanssen, E., Kaufmann, P., Reichmann, E.J. et al. Observation of the impulsive phase of a simple flare. Sol Phys 90, 41–62 (1984). https://doi.org/10.1007/BF00153784
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DOI: https://doi.org/10.1007/BF00153784