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Evidence for double injections in scatter-free solar impulsive electron events

  • Author(s): Wang, Linghua
  • Lin, Robert P.
  • Krucker, Sam
  • Gosling, John T.
  • et al.
Abstract

We investigate the injection at the Sun for three scatter-free impulsive electron events (7 August 1999, 28 June and 22 August 2000) observed from ~0.4 to 300 keV by the WIND 3DP instrument. Taking into account the interplanetary scatter-free propagation and instrumental effects, we find that the observed time profiles of electron fluxes at all energies fit well to triangular injections at the Sun with equal rise and fall times. We find two distinct injections: that of ~0.4 to 6–9 keV electrons begins 9.1 ± 4.7 min before the start of the type III radio burst and lasts for 50–300 min, while that of ~3 to 300 keV electrons starts 7.6 ± 1.3 min after the start of the type III burst and lasts for a factor of 5–10 times shorter. Electrons of the low-energy injection are likely to be the source of the type III radio bursts, and the delayed high-energy injection occurs when the associated CME passes altitudes of ~1–6 Rs. The observed electron energy spectra fit to double power-law with a downward break at ~40–50 keV, but exhibit a smooth power-law across the transition (~6–13 keV) between the two injections, suggesting the low-energy injection may provide the seed electrons for the high-energy injection.

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