Correlation and asymmetry between solar flare hard X-ray footpoints: a statistical study

Department of Astronomy, Nanjing University, Nanjing 210093, PR China e-mail: dmd@nju.edu.cn

Received: 31 January 2007
Accepted: 23 April 2007

Abstract

Aims.We study the correlation and asymmetry between solar flare hard X-ray footpoint time profiles that can be used to constrain the energy release and particle acceleration processes in solar flares.

Methods.We perform a statistical analysis of correlations and asymmetries of hard X-ray footpoints for 29 solar flares that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in 2002–2005. Using the CLEAN method, we derive the hard X-ray images and then extract light curves for each footpoint. We decompose the hard X-ray light curves into slowly-varying components and fast-varying components using the Fourier method and check their correlations separately.

Results.Our analysis indicates that the asymmetry between hard X-ray footpoints is a ubiquitous feature in solar flares. However, in most asymmetric cases, the correlations between the light curves of the two footpoints remain high. In particular, the correlation coefficients between the slowly-varying components are significantly higher than those between the fast-varying components. We discuss the causes and implications of the asymmetries and correlations between hard X-ray footpoints.