GOES class estimation for behind-the-limb solar flares using MESSENGER SAX

TL;DR

This study develops an empirical method to estimate GOES X-ray classes for behind-the-limb solar flares using MESSENGER SAX data, enabling better flare characterization when direct observations are blocked.

Contribution

It introduces a new empirical relationship between SAX and GOES X-ray fluxes, allowing GOES class estimation from SAX measurements for occulted flares.

Findings

Predictions of GOES class from SAX data are accurate within a factor of two.

SAX can serve as a proxy for occulted flares not visible from Earth.

Empirical relationship applies to flares above C2 class.

Abstract

Mercury mission MESSENGER's %(MErcury Surface, Space ENvironment, GEochemistry, and Ranging Solar Assembly for X-rays (SAX) observed almost 700 solar flares between May 28, 2007 and August 19, 2013, as cataloged by \citet{dennisSOLARFLAREELEMENT2015}. The SAX instrument, part of the X-ray Spectrometer (XRS), operated at 1 -- 10 keV, partially overlapping the energy range of the GOES X-ray spectrometers. SAX provides viewing angles different from the Earth-Sun line and can therefore be used as a GOES proxy for partially or fully occulted flares as seen from Earth. For flares with GOES classes above C2 seen on-disk for both instruments, we found an empirical relationship between the soft X-ray (SXR) fluxes measured by both SAX and GOES. Due to the different energy response of the two SXR instruments, individual events can deviate on average by about a factor of two from the empirical relationship, implying that predictions of the GOES class of occulted flares from SAX data are therefore accurate to within the same factor. The distinctive GOES energy response in combination with the multithermal nature of flares makes it difficult for any instrument, even other soft X-ray spectrometers, to provide a GOES proxy more accurate than a factor of two.