Solar flare hard X-ray spikes observed by RHESSI: a statistical study

¹ School of Astronomy & Space Science, Nanjing University, 210093 Nanjing, PR China
e-mail: chengjx@shao.ac.cn
² Shanghai Astronomical Observatory, Chinese Academy of Sciences, 200030 Shanghai, PR China
³ Department of Physics, Montana State University, Bozeman MT 59717-3840, USA
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Ministry of Education), Nanjing University, 210093 Nanjing, PR China
⁵ New Jersey Institute of Technology, 323 Martin Luther Kind Blvd., Newark, NJ 07102, USA

Received: 8 December 2011
Accepted: 14 September 2012

Abstract

Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions.

Aims. We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes.

Methods. Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV.

Results. The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times. (3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions.