Ulysses/GRB Measurements of Hard X-Ray Flares on the Far Side of the Sun

Astronomy and Astrophysics – Astrophysics

Scientific paper

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[7519] Solar Physics, Astrophysics, And Astronomy / Flares, [7554] Solar Physics, Astrophysics, And Astronomy / X-Rays, Gamma Rays, And Neutrinos, [7999] Space Weather / General Or Miscellaneous

Scientific paper

The Solar X-ray/Cosmic Gamma-Ray Burst Experiment (GRB) on Ulysses provided continuous measurements of energetic (25-150 keV) solar X-ray activity from launch in October 1990 to November 2003, when it was temporarily switched off due to instrument power sharing requirements. The unique high latitude orbit of the spacecraft made it possible to monitor solar X-ray flare activity on the far side of the Sun during extended periods of the mission. We correlate all X-class flares measured by the GOES fleet of satellites listed in the NOAA reports, with count rate increases measured by GRB to confirm that energetic flares have hard non-thermal components which extend into the energy range above 25 keV. By comparing peak GRB count rates with the GOES flare magnitudes, we establish a scaling law between the two sets of measurements, characterized by a power-law fit. Having accounted for all the X-class flares seen simultaneously by both GRB and GOES, we are able to identify signatures of intense X-ray activity in the GRB data set which must originate from flares on the hidden face of the Sun. In total, we list 82 such events during the 13 years of GRB operation. We provide timing information for each event and also coarse flare site locations based on the geometry of the Ulysses orbit. Estimates of the flare intensity are made using the scaling law derived from the correlated GRB and GOES measurements. Global monitoring of flare activity on the complete surface of the Sun can provide useful information to validate and refine models and numerical simulations of heliospheric and space weather processes.

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