Other
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21924002n&link_type=abstract
American Astronomical Society, AAS Meeting #219, #240.02
Other
Scientific paper
Solar and stellar flares result from the dynamical rearrangement of magnetic fields, which heats plasma and accelerates particles. Solar flare sizes can be directly measured from space-based observatories, giving insight into the behavior and morphology of the solar magnetic field. With current technology, however, we cannot spatially resolve flares that occur on other stars. Consequently, models have been developed that infer flare sizes using X-ray emission produced by the flare-related plasma heating. One model, the "hydrodynamic method” (HM), considers the potential presence of heating during the decay phase of a flare event, along with cooling of the hot material by conduction and radiation. HM assumes that the flare is a single semi-circular loop and that the X-ray emission from the cooling material in the flare declines as a simple exponential. However, several solar and stellar flare observations suggest that this flux decay occurs in two stages, each described by a separate exponential decay timescale. We applied HM to 257 highly energetic, X-class solar flares to determine whether a one or two-stage exponential decay was a better fit for the decay phase. We compared the two fits using a chi-squared minimization technique and F-tests. Through this analysis we determined that 98% of the solar flares in our sample exhibited a two-stage decay phase. Furthermore, of the flares exhibiting a two-stage decay, the flare sizes inferred from stage two were typically much larger than those from stage one, and we found no correlation between them. Given that we do not normally observe significant increases in solar flare sizes during the decay phase _as implied by our results_ the two-stage decay phase in X-class flares may indicate a change in the geometry of the flare region (like the appearance of an "arcade") thus invalidating the results of HM in such instances.
Nunez Alejandro
Osten Rachel
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