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Spectral Hardening of Large Solar Flares  

Paolo C. Grigis   Submitted: 2007-08-21 03:08

Context: Bremsstrahlung hard X-rays emitted by electrons accelerated in solar flares are an important diagnostic for understanding the acceleration mechanism. Strong spectral variability is observed, usually following a soft-hard-soft pattern during impulsive emission spikes. In addition to this behavior, larger events occasionally show gradual hardening, usually in the late phase of the flares. Aims: We study quantitatively the hard X-ray spectral evolution of large solar flares featuring hardening trends. In particular, we investigate whether two different acceleration mechanisms are responsible for the impulsive and gradual phases. Methods: Spectral fitting of the non-thermal emission at high (4 s) and medium (32 s) cadence are obtained from RHESSI data, yielding time profiles of the non-thermal fit parameters (flux, spectral index, spectral curvature) for five X-class solar flares. The temporal evolution of the spectra is compared with the configuration and motion of the hard X-ray sources in RHESSI images. Results: Both soft-hard-soft (impulsive) phases and hardening (gradual) phases are observed during the events and are well described by piecewise linear dependence of the spectral index on the logarithm of the flux. The transition between the impulsive and gradual phases is smooth and progressive rather than abrupt, both in spectra and images. Comparison with a pure trapping model in the late phase leads to good agreement with the observation for the spectral index vs. flux relation, but poor predictions for the spectral curvature. Conclusions: The evidence we find points toward a single acceleration mechanism acting in the two phases, rather than two different separated mechanisms, because the impulsive and gradual phases are closely interconnected in time and space.

Authors: Paolo C. Grigis & Arnold O. Benz
Projects: RHESSI

Publication Status: A&A (submitted)
Last Modified: 2007-08-21 08:38
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Spectral Hardening of Large Solar Flares

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