The spectral evolution of impulsive solar X-ray flares

By P. Grigis, A. O. Benz. 2004

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The time evolution of the spectral index and the non-thermal flux in 24 impulsive solar hard X-ray flares of GOES class M was studied in RHESSI observations. The high spectral resolution allows for a clean separation of thermal and non- thermal components in the 10-30 keV range, where most of the non-thermal photons are emitted. Spectral index and flux can thus be determined with much better accuracy than before. The spectral soft-hard-soft behavior in rise-peak-decay phases is discovered not only in the general flare development, but even more pronounced in subpeaks. An empirically found power-law dependence between the spectral index and the normalization of the non-thermal flux holds during the rise and decay phases of the emission peaks. It is still present in the combined set of all flares. We find an asymmetry in this dependence between rise and decay phases of the non-thermal emission. There is no delay between flux peak and spectral index minimum. The soft-hard-soft behavior appears to be an intrinsic signature of the elementary electron acceleration process.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/bbf00ff762953b816529c95c127965dadc316277

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Citations in the corpus

Hard X-ray emission from the solar corona (2008, background): “2005b), and the soft-hard-soft spectral pattern2 (Parks and Winckler 1969; Hudson and Fárník 2002; Grigis and Benz 2004).”
Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares (2011, methodology): “As a compromise, Grigis and Benz (2004) fitted the data to a single power-law function at all times.”
Solar Flare Electron Spectra at the Sun and near the Earth (2007, background): “(so-called soft-hard-soft effect; e.g., Grigis & Benz 2004), the choice of shorter or longer time interval around the peak gives slightly harder or softer spectra.”
An Observational Overview of Solar Flares (2011, background): “This pattern differs from the otherwise ubiquitous “soft-hard-soft” pattern associated with the impulsive phase (Parks & Winckler 1969; Hudson & Fárnı́k 2002; Grigis & Benz 2004).”
Quasi-Periodic Pulsations in Solar Flares (2009, background): “Grigis and Benz (2004) investigated spectral evolution during individual peaks of the flaring hard X-ray emission and found each such subpeak to display a soft-hard-soft (SHS) evolution of the hard X-ray spectra.”
Global Properties of Solar Flares (2011, background): “The time variations during this phase exhibit the “softhard-soft” pattern, with a close negative correlation between the hard X-ray flux and its power-law spectral index γ as determined by a fit to j(ε) ∝ ε−γ (Kane and Anderson 1970; Grigis and Benz 2004).”
Recent Advances in Understanding Particle Acceleration Processes in Solar Flares (2011, methodology): “Indeed, Grigis & Benz (2004) reportRHESSI observations suggesting the existence of a point in the spectrum at which the hard X-ray flux does not change with time during an event.”