AAS 200th meeting, Albuquerque, NM, June 2002
Session 29. Eruptive Flares/CMEs
Oral, Monday, June 3, 2002, 2:00-3:30pm, Mesilla

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[29.01] Impulsive and gradual energetic emissions in a flare

J. Qiu, J. Lee, D. E. Gary (New Jersey Institute of Technology)

We present a comprehensive case study of an X-class flare observed on April 6, 2001. The flare consists of two peaks, the first characterized by an impulsive spiky emission and the second by a gradual smooth emission in the same magnetic environment. We compare the temporal and spectral evolution of the two components in hard X-rays and microwaves. Images at both wavelengths are obtained to locate the emission source as it evolves during the flare. We find the following results: (1) The impulsive hard X-ray and microwave emissions are from complex loop structures, primarily in low-lying loops, while the gradual energetic emissions occur in a simple and over-lying loop. Both the impulsive and gradual hard X-rays at > 50keV are thick-target emission. We also find a microwave source that evolves from the loop top toward the foot-point during the gradual phase. (2) A time lag analysis reveals energy-dependent time delays in the impulsive hard X-rays, with the 200~keV hard X-rays lagging the 40keV emission by about 5s. (3) The gradual component is a microwave-rich event. Large delays are seen in the gradual hard X-ray emissions, and the gradual microwaves further lag the hard X-rays by tens of seconds. The correlation study suggests that the gradual microwaves are emitted by > 200keV hard X-ray electrons, possibly MeV electrons. The gradual hard X-rays exhibit a harder and hardening spectrum than the impulsive component, while the electron spectrum derived from the gradual microwaves is still harder.

Based on the observations, we suggest that magnetic energy release primarily occurs during the impulsive phase in the complex loop system involving both low-lying and over-lying magnetic loops. Electrons injected into the low-lying loops rapidly escape the coronal trap and precipitate, giving rise to the impulsive energetic bursts at both wavelengths. Electrons that are injected into the over-lying loop are well trapped, which soon undergo a new acceleration, most probably in the form of a stochastic mechanism. It is likely that the secondary acceleration produces a pitch angle distribution in favor of electron trapping, and an evolving trapping condition may result in the moving microwave source along the magnetic loop. This work is supported by NASA grants NAG5-10212 and NAG5-10891. OVSA/NJIT is supported by NSF under grant AST-9987366.

The author(s) of this abstract have provided an email address for comments about the abstract: qiuj@plage.njit.edu

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.