Spectral Hardening in Large Solar Flares
| Autor: | Paolo C. Grigis, Arnold O. Benz |
|---|---|
| Rok vydání: | 2008 |
| Předmět: |
Physics
Spectral index Photon 010504 meteorology & atmospheric sciences Solar flare Astrophysics::High Energy Astrophysical Phenomena Astrophysics (astro-ph) Bremsstrahlung FOS: Physical sciences Astronomy and Astrophysics Astrophysics Electron 01 natural sciences Spectral line Particle acceleration Space and Planetary Science 0103 physical sciences Hardening (metallurgy) 010303 astronomy & astrophysics 0105 earth and related environmental sciences |
| Zdroj: | The Astrophysical Journal. 683:1180-1191 |
| ISSN: | 1538-4357 0004-637X |
| Popis: | RHESSI observations are used to quantitatively study the hard X-ray evolution in 5 large solar flares selected for spectral hardening in the course of the event. The X-ray bremsstrahlung emission from non-thermal electrons is characterized by two spectroscopically distinct phases: impulsive and gradual. The impulsive phase usually consists of several emission spikes following a soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as spectral hardening while the flux slowly decreases. Both the soft-hard-soft (impulsive) phase and the hardening (gradual) phase are well described by piecewise linear dependence of the photon spectral index on the logarithm of the hard X-ray flux. The different linear parts of this relation correspond to different rise and decay phases of emission spikes. The temporal evolution of the spectra is compared with the configuration and motion of the hard X-ray sources in RHESSI images. These observations reveal that the two stages of electron acceleration causing these two different behaviors are closely related in space and time. The transition between the impulsive and gradual phase is found to be smooth and progressive rather than abrupt. This suggests that they arise because of a slow change in a common accelerator rather than being caused by two independent and distinct acceleration processes. We propose that the hardening during the decay phase is caused by continuing particle acceleration with longer trapping in the accelerator before escape. accepted by ApJ |
| Databáze: | OpenAIRE |
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