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

¹ Department of Physics, Montana State University, Bozeman, MT 59717-3840, USA
² School of Astronomy & Space Science, Nanjing University, 210093 Nanjing, PR China
e-mail: chengjx@shao.ac.cn
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 200030 Shanghai, PR China
⁴ Space Sciences Lab, University of California, Berkeley, CA 94720-7450, USA
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. Fast-varying hard X-ray spikes of subsecond time scales were discovered by space telescopes in the 70s and 80s, and are also observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These events indicate that the flare energy release is fragmented.

Aims. In this paper, we analyze hard X-ray spikes observed by RHESSI to understand their temporal, spectral, and spatial properties.

Methods. A recently developed demodulation code was applied to hard X-ray light curves in several energy bands observed by RHESSI. Hard X-ray spikes were selected from the demodulated flare light curves. We measured the spike duration, the energy-dependent time delay, and count spectral index of these spikes. We also located the hard X-ray source emitting these spikes from RHESSI mapping that was coordinated with imaging observations in visible and UV wavelengths.

Results. We identify quickly varying structures of  ≤ 1 s during the rise of hard X-rays in five flares. These hard X-ray spikes can be observed at photon energies over 100 keV. They exhibit sharp rise and decay with a duration (FWHM) of less than 1 s. Energy-dependent time lags are present in some spikes. It is seen that the spikes exhibit harder spectra than underlying components, typically by 0.5 in the spectral index when they are fitted to power-law distributions.

RHESSI clean maps at 25–100 keV with an integration of 2 s centered on the peak of the spikes suggest that hard X-ray spikes are primarily emitted by double foot-point sources in magnetic fields of opposite polarities. With the RHESSI mapping resolution of  ~4′′, the hard X-ray spike maps do not exhibit detectable difference in the spatial structure from sources emitting underlying components. Coordinated high-resolution imaging UV and infrared observations confirm that hard X-ray spikes are produced in magnetic structures embedded in the same magnetic environment of the underlying components. The coordinated high-cadence TRACE UV observations of one event possibly reveal new structures on spatial scales  ≤1–2′′ at the time of the spike superposed on the underlying component. They are probably sources of hard X-ray spikes.