Electron Time-of-Flight Measurements

Summary

High-precision time delay measurements of hard X-ray emission in solar flares can be used to infer the energy-dependent time scales of particle acceleration, propagation, and energy loss. A systematic time delay of T=t _25keV —t _50keV ≈20 ms between the 25 keV and 50 keV HXR-producing electrons was discovered in a statistical study of 640 flares observed by BATSE/CGRO (Aschwanden, Schwartz, & Alt 1995). The magnitude and sign of the observed delay provides a diagnostic of the predominant timing mechanism (Aschwanden & Schwartz 1995), i.e. propagation time-of-flight differences in the thick-target model (0 < T ≲ 100 ms), energy loss time differences in the thin-target model (−1 s ≲ T < 0), trapping time differences (−10 ≲ T ≲ −1 s), or convolution delays with thermal effects (+1 s ≲ T ≲ + 10 s). In the flare of 1992 Jan 13 (Fig.l), which became famous for the discovery of a hard X-ray looptop source (Masuda 1994), the delays of the spiky HXR pulses (Fig.2) are characteristic for time-of-flight differences. The inferred electron time-of-flight distance was found to exceed the path difference between the HXR looptop and footpoint sources (Aschwanden et al. 1995).