Production of Energy-dependent Time Delays in Impulsive Solar Flare Hard X-Ray Emission by Short-Duration Spectral Index Variations

Details Production of Energy-dependent Time Delays in Impulsive Solar Flare Hard X-Ray Emission by Short-Duration Spectral Index Variations Production of Energy-dependent Time Delays in Impulsive Solar Flare Hard X-Ray Emission by Short-Duration Spectral Index Variations

Aug 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...503..429l&link_type=abstract

Astrophysical Journal v.503, p.429

Astronomy and Astrophysics

Astronomy

5

Sun: Flares, Sun: X-Rays, Gamma Rays, Sun: Particle Emission

Scientific paper

Cross-correlation techniques have been used recently to study the relative timing of solar flare hard X-ray emission at different energies. These studies find that for the majority of the impulsive flares observed with BATSE there is a systematic time delay of a few tens of milliseconds between low (~50 keV) and higher energy emission (~100 keV). These time delays have been interpreted as energy-dependent time-of-flight differences for electron propagation from the corona, where they are accelerated, to the chromosphere, where the bulk of the hard X-rays are emitted. We show in this paper that cross-correlation methods fail if the spectral index of the flare is not constant. BATSE channel ratios typically display variations of factors of 2 to 5 over time intervals as short as a few seconds. Using simulated and observed data, we demonstrate that cross-correlating energy channels with identical timing characteristics, but with variations in the amplitudes of one or a small number of relatively strong emission spikes, produces asymmetric time delays of either sign. The reported time delays are therefore largely due to spectral index variations and are not signatures of time-of-flight effects.

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