The relative timing of supra-arcade downflows in solar flares

Abstract

Context. Supra-arcade downflows (generally dark, sunward-propagating features located above the bright arcade of loops in some solar flares) have been reported mostly during the decay phase, although some have also been reported during the rise phase of solar flares. Aims. We investigate, from a statistical point of view, the timing of supra-arcade downflows during the solar flare process, and thus determine the possible relation of supra-arcade downflows to the primary or secondary energy release in a flare. Methods. Yohkoh Soft X-ray Telescope (SXT) imaging data are examined to produce a list of supra-arcade downflow candidates. In many of our events supra-arcade downflows are not directly observed. However, the events do show laterally moving (or "waving") bright rays in the supra-arcade fan of coronal rays which we interpret as due to dark supra-arcade downflows. The events are analysed in detail to determine whether the supra-arcade downflows (or the proxy waving coronal rays) occur during a) the rise and/or decay phases of the soft X-ray flare and b) the flare hard X-ray bursts. It is also investigated whether the supra-arcade downflows events show prior eruptive signatures as seen in SXT, other space-based coronal data, or reported in ground-based Hα images. Results. A substantial majority of supra-arcade downflow events show downflows which start during the soft X-ray flare rise phase (73%), occur during hard X-ray bursts (90%), and have prior eruptive signatures associated with them (73%). However, we find a single event (2% of the total) which clearly and unambiguously showed supra-arcade downflows starting during the soft X-ray flare decay phase. Conclusions. Since the majority of supra-arcade downflows occur during the rise phase of the soft X-ray flare and the time of hard X-ray bursts, and have prior eruptive signatures this suggests that they are related to the main flare energy release process. Furthermore, the suggested association of supra-arcade downflows with recently reconnected magnetic field lines means they may indeed be considered as evidence for a magnetic reconnection process. The single supra-arcade downflow event which unambiguously started during the decay phase of the flare occurred during hard X-ray bursts and thus appears to be related to late energy release.

Quantitative Examination of a Large Sample of Supra-Arcade Downflows in Eruptive Solar Flares

“…Long-duration flaring events are often associated with downflowing voids and/or loops in the supra-arcade region (see Figure 1 for example images) whose theoretical origin as newly reconnected flux tubes has been supported by observations (McKenzie & Hudson 1999;McKenzie 2000;Innes et al 2003;Asai et al 2004;Sheeley et al 2004;Khan et al 2007;Reeves et al 2008;McKenzie & Savage 2009;Savage et al 2010).…”

“…Therefore, in order to identify any trends in the data, analysis of several flows from each of many flares must necessarily be performed. Table 1 contains a list of flares which have been noted to display downflow signatures (i.e., observable SADs, SADLs, or swaying fan above the arcade as suggested by Khan et al 2007). Several of these flares were selected from (Khan et al 2007, see Table 1 therein) and were supplemented from McKenzie (2000) as well as by personal flare data investigation.…”

“…Table 1 contains a list of flares which have been noted to display downflow signatures (i.e., observable SADs, SADLs, or swaying fan above the arcade as suggested by Khan et al 2007). Several of these flares were selected from (Khan et al 2007, see Table 1 therein) and were supplemented from McKenzie (2000) as well as by personal flare data investigation. Some flows from the Khan et al (2007) list were excluded from this study if the presence of flows was not confirmed by visual inspection.…”

“…Several of these flares were selected from (Khan et al 2007, see Table 1 therein) and were supplemented from McKenzie (2000) as well as by personal flare data investigation. Some flows from the Khan et al (2007) list were excluded from this study if the presence of flows was not confirmed by visual inspection.…”

An Observational Overview of Solar Flares

“…The impulsivephase reconnection of the coronal magnetic field is not visible in coronal signatures with current instruments, though its effects certainly may be. These effects include the EUV and SXR flare loops and cusp-shaped structures that appear in the gradual phase (see Section 1.3) and impulsive phase supra-arcade downflows (Asai et al 2004b;Khan et al 2007), Fig. 1.3 SOL2002-10-04T05:38 (M4.0), with TRACE UV (1700Å) ribbons in color overlaid on TRACE continuum in black and white.…”