Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...484..455n&link_type=abstract
Astrophysical Journal v.484, p.455
Astronomy and Astrophysics
Astronomy
6
Acceleration Of Particles, Sun: Corona, Sun: Flares, Sun: Particle Emission, Sun: X-Rays, Gamma Rays
Scientific paper
Despite some measure of theoretical and predictive success of the electron-heated model of solar flares, observations of low levels of soft X-ray emission prior to the detection of hard X-rays in most flares present a problem for the paradigm, since it suggests some sort of "preheating" of the chromospheric plasma before electron acceleration. Particularly troublesome are assertions that plasma flows have been observed prior to hard X-ray emission, because they imply significant and rapid energy input by some agent without leaving a (currently) detectable hard X-ray signature. Given this challenge to a prevailing paradigm, it is imperative to investigate more comprehensively the existence of "precursor flows" manifested in the asymmetry of X-ray spectral lines. In this paper, we examine a sample of 50 disk-center flares observed by the Yohkoh Bragg Crystal Spectrometer (BCS) between 1991 and 1993, and determine with the velocity differential emission measure the extent to which "precursor flows" occur. We explore specifically the possibility that these "precursor flows" are only apparent, arising from the existence of more than one emission region on the solar disk. The spatial separation between regions could cause the emission to be registered by the detector in a wavelength bin that is shifted relative to the line's nominal location, giving rise to line asymmetries. In our sample, we find that only five flares manifest "precursor flows," three of which can be explained definitively by the presence of other active regions. It also appears plausible that the remaining two events can be explained in terms of such spatial effects, although the evidence is more circumstantial. We conclude, therefore, that one can reasonably question the existence of "precursor flows" in our sample and that the observations to date cannot contribute decisively to the electron versus proton debate. These results are contrasted with those of Plunkett & Simnett, who identified 14 precursor events in a sample of 35 flares and hence concluded, without further analysis, that protons, rather than electrons, must be the primary energy carriers in flares. Our paper's sample included eight of Plunkett & Simnett's 14 precursor events, and yet our analysis, which employed substantially different methods, identified only one of these eight as a "precursor" event. The remaining seven were either troubled by BCS data dropout or failed to exceed the 10% threshold required to classify a flare as a"precursor" event.
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