Comparative Studies of Hard X-Ray Spectral Evolution in Solar Flares with High-Energy Proton Events Observed at Earth
Abstract
This paper presents the results of two extensive studies of hard X-ray spectral evolution in solar flares and their associations with energetic interplanetary proton events. The focus of this work is to establish the degree to which events that display progressively hardening hard X-ray spectra, at any time and over all observable timescales, are associated with high-energy interplanetary proton events. The first study examined a sample of 152 hard X-ray flares well observed with the HXRBS instrument on the Solar Maximum Mission (SMM). The study showed that 22 events revealed a progressive spectral hardening either over flux peaks (i.e., a soft-hard- harder spectral evolution) or during flux decays and that 18 of these 22 events (82%) had associated 10 MeV proton events or enhancements. Conversely, the absence of spectral hardening is associated with the absence of interplanetary protons with 124 of the 130 remaining flares (95.4%). Since the hard X-ray counting rate threshold of the first study was sufficiently high (5000 counts s-1) to exclude many flares (more than 36%) associated with the largest interplanetary proton events, a second study was conducted using 193 less intense HXRBS events (a one out of three sample) and their associations with only large proton events. This study also identifies events with progressive spectral hardening. It also employs selection criteria suggested by the results of the first study to "predict" which flares would or would not have associated large proton events. This prescription for "predicting" proton events did so correctly for four large (SESC qualified) proton events, missed none, and produced only one "false alarm" in which the criteria were met but only a small proton event was seen at earth. Thus, a correct "prediction" was made for all but one of the 193 events. The results of the first study are then combined with the weighted results of the one out of three study, using the same selection criteria, to project correctly predicted associations of 22 out of 23 SESC events, for a 96% success rate, while 700 out of 708 flares were projected to be correct rejections with no associated proton events. The data suggest that progressive hardening is a diagnostic of high-energy particle acceleration of electrons and of protons and that it is not a manifestation of the "big flare syndrome" which asserts that the largest flares are associated with many or most known phenomena. There also appears to be an approximate relationship between the timescales (FWHM) of progressively hardening X-ray peaks and the cube of the interplanetary peak proton fluxes. The strong associations of particular hard X-ray characteristics and interplanetary proton events are of interest both on physical grounds and because the techniques employed can be directly adapted into a practical means of predicting which events are most likely to be associated with large interplanetary proton events that pose threats to humans in space and to spacecraft.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1995
- DOI:
- 10.1086/176457
- Bibcode:
- 1995ApJ...453..973K
- Keywords:
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- SUN: FLARES;
- SUN: X-RAYS;
- GAMMA RAYS;
- SUN: SOLAR-TERRESTRIAL RELATIONS