Production of energy-dependent time delays in impulsive solar flare hard X-ray emission by short-duration spectral index variations

Cross-correlation techniques have been used recently to study the relative timing of solar flare hard X-ray emission at different energies. These stud ies 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 (approximate to 50 keV) and higher energy emission (approximate to 100 keV) . These time delays have been interpreted as energy-dependent time-of-fligh t differences for electron propagation from the corona, where they are acce lerated, to the chromosphere, where the bulk of the hard X-rays are emitted . We show in this paper that crosscorrelation 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 secon ds. Using simulated and observed data, we demonstrate that cross-correlatin g energy channels with identical timing characteristics, but with variation s in the amplitudes of one or a small number of relatively strong emission spikes, produces asymmetric time delays of either sign. The reported time d elays are therefore largely due to spectral index variations and are not si gnatures of time-of-flight effects.