On the causes of hard x-ray asymmetry in solar flares

¹ Astronomical Institute of Wrocław University, 51-622 Wrocław, ul. Kopernika 11, Poland e-mail: falewicz@astro.uni.wroc.pl
² Space Research Centre, Polish Academy of Sciences, 51-622 Wrocław, ul. Kopernika 11, Poland e-mail: ms@cbk.pan.wroc.pl

Received: 23 May 2006
Accepted: 17 August 2006

Abstract

Aims.Hard X-ray (HXR) emission in the footpoints of flaring loops often indicates an asymmetry, where brighter X-ray flux usually appears in the footpoint with a weaker magnetic field. This is explained by the fact that more electrons can reach the chromosphere in the weaker magnetic field. However, there are numerous exceptions to above rule, where the stronger HXR source is located in the loop's foot that is rooted in the stronger magnetic field. In our paper we analyse three such exceptional events from the paper by Goff et al. (2004, A&A, 423, 363). For all three events, we found evidence of the magnetic loops interactions near a brighter footpoint. Such loop interactions suggest that for these flares the energy release is located closer to one end of the loop.

Methods.We analysed the process of electron transport in detail in the dense flaring loop as a possible cause of the observed asymmetry. Using the flare parameters derived from observations, we calculated how the collisional energy losses in the loop reduce the number of electrons impacting the chromosphere at the other footpoint.

Results.We show that these calculations reproduce the observed flux ratios within the error limits in both footpoints for each of the analysed events. Thus we conclude that the injection of electrons near one of the footpoints, together with enhanced density in the flaring loop, can explain observed HXR footpoints asymmetry.