On the Correlation of HXR and WL Emission in Solar Flares

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Nugget
Number:	265
1st Author:	Matej Kuhar
2nd Author:
Published:	23 November 2015
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Introduction (Or, what do we know so far?)

In a solar flare, the energy is emitted over a whole electromagnetic spectrum; particularly interesting is the radiation in the "white light" (WL) range, because it contains a significant fraction of the total flare energy. Even though the first detection of a solar flare was done by Carrington and by Hodgson (as long ago as 1859) in WL, observations in this wavelength range are difficult due to the relatively faint enhancement of WL emission due to the flare, compared to the non-flaring, pre-flare level. It was due to this reason that WL flares were long considered to be rare and exotic phenomena; however, with the development of new observatories, WL emission has been detected in weaker flares (even GOES C-class flares). Numerous studies have explored the correlation of hard X-ray (HXR) and WL emission in solar flares, observed to be both in space and in time. There are two mechanisms proposed for explaining this correlation: 1) direct heating, in which the precipitating electrons from the corona locally heat and ionize the middle chromosphere, after which a WL continuum emission is produced directly by the recombination of hydrogen, and 2) radiative back-warming, in which electrons are stopped in the upper chromosphere. In the latter scenario, the recombination produces UV continuum radiation that irradiates the photosphere below, resulting in enhanced continuum emission from the photosphere itself. The two models predict different heights for WL/HXR sources and, equally important, their formation from the same (direct-heating) or different volumes (radiative back-warming) in the solar atmosphere. In this Nugget, we will present the results of a statistical study on the correlation of HXR and WL emission in solar flares. We identified 43 white-light flares (WLFs) for this study, spanning GOES classes M and X, as observed by the HMI instrument on the SDO observatory spacecraft.

Observations and data analysis

Observations were made using RHESSI and HMI for the HXR and WL range, respectively. From RHESSI data, we had the information about the timing and positions of the flares. This was used to guide which were used to guide the HMI search. For each flare, we overlaid RHESSI and HMI images and calculated the WL flux of the part of the active region that lay inside the 30% contour of RHESSI emission. We calculated the time evolution of WL emission for a long interval around the time of the flare. This gave us lightcurves such as those presented in Figure 1. In this figure we can see the WL lightcurves for two extreme examples from the sample, an X2.1 flare and an M2.4 flare. When we plot the WL lightcurves on the same scale, we can see how faint the WL signal in the latter event really is. From these lightcurves, we chose the peak- and pre-flare frames, made background-subtracted images in WL, and calculated the WL flux inside the 30% RHESSI contour. The HXR fluxes were calculated by fitting a composite spectrum with thermal (soft X-ray) and power-law (hard X-ray tail) to the RHESSI data. The energy deposition by non-thermal electrons was derived from the HXR spectral parameters, using the thick-target approximation.