The RHESSI Microflare Height Distribution

TL;DR

This study provides a detailed statistical analysis of the heights of solar microflares observed by RHESSI, revealing an exponential height distribution with implications for flare energization processes.

Contribution

First comprehensive statistical survey of RHESSI microflare heights, introducing a forward-fit model and comparing observed distributions with potential field models.

Findings

Flare heights follow an exponential distribution with a scale height of 6.1 Mm.

Power-law distribution with index 3.1 also fits the data.

Observed heights are steeper than potential field loop heights, indicating flare energization effects.

Abstract

We present the first in-depth statistical survey of flare source heights observed by RHESSI. Flares were found using a flare-finding algorithm designed to search the 6-10 keV count-rate when RHESSI's full sensitivity was available in order to find the smallest events (Christe et al., 2008). Between March 2002 and March 2007, a total of 25,006 events were found. Source locations were determined in the 4-10 keV, 10-15 keV, and 15-30 keV energy ranges for each event. In order to extract the height distribution from the observed projected source positions, a forward-fit model was developed with an assumed source height distribution where height is measured from the photosphere. We find that the best flare height distribution is given by g(h) \propto exp(-h/{\lambda}) where {\lambda} = 6.1\pm0.3 Mm is the scale height. A power-law height distribution with a negative power-law index, {\gamma} = 3.1 \pm 0.1 is also consistent with the data. Interpreted as thermal loop top sources, these heights are compared to loops generated by a potential field model (PFSS). The measured flare heights distribution are found to be much steeper than the potential field loop height distribution which may be a signature of the flare energization process.