Soft X-ray Fluxes of Major Flares Far Behind the Limb as Estimated Using STEREO EUV Images

TL;DR

This study develops a method to estimate the X-ray peak fluxes of solar flares occurring behind the limb using EUV images from STEREO, enabling analysis of otherwise unobservable solar activity.

Contribution

It introduces a relation between EUVI 195 A fluxes and GOES X-ray fluxes to estimate X-ray peaks of behind-the-limb flares, accounting for multithermal effects and uncertainties.

Findings

EUVI 195 A fluxes can estimate X-ray peaks with uncertainties of a factor of a few.

The method is effective for intense flares above M4 class.

Examples include eruptions from far-side regions with eruptive signatures.

Abstract

With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe xxiv line at 192 A, the response of the EUVI 195 A channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncer- tainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 A data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) with uncertainties of a factor of a few. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images.