High time resolution observations of solar Hα flares

II. Search for signatures of electron beam heating

¹ Astronomical Institute of Wrocław University, 51-622 Wrocław, ul. Kopernika 11, Poland
e-mail: [radziszewski;rudawy]@astro.uni.wroc.pl
² Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
e-mail: kjhp@mssl.ucl.ac.uk

Received: 23 February 2009
Accepted: 13 September 2011

Abstract

Aims. The Hα emission of solar flare kernels and associated hard X-ray (HXR) emission often show similar time variations but their light curves are shifted in time by energy transfer mechanisms. We searched for fast radiative response of the chromosphere in the Hα line as a signature of electron beam heating.

Methods. We investigate the time differences with sub-second resolution between the Hα line emission observed with a Multi-channel Subtractive Double Pass (MSDP) spectrograph on the Large Coronagraph and Horizontal Telescope at Białków Observatory, Poland, and HXR emission recorded by the RHESSI spacecraft during several flares, greatly extending our earlier analysis (Paper I) to flares between 2003 and 2005.

Results. For 16 Hα flaring kernels, observed in 12 solar flares, we made 72 measurements of time delays between local maxima of the RHESSI X-ray and Hα emissions. For most kernels, there is an excellent correlation between time variations in the Hα line emission (at line centre and in the line wings) and HXR (20–50 keV) flux, with the Hα emission following features in the HXR light curves generally by a short time lapse Δt = 1–2 s, sometimes significantly longer (10–18 s). We also found a strong spatial correlation.

Conclusions. Owing to our larger number of time measurements than in previous studies, the distribution of Δt values shows a much clearer pattern, with many examples of short (1–2 s) delays of the Hα emission, but with some flares showing longer (10–18 s) delays. The former are consistent with energy transfer along the flaring loop legs by non-thermal electron beams, the latter to the passage of conduction fronts.