Energy dependence of electron pitch angle distribution widths in solar bursts

Solar activity regularly produces suprathermal electron bursts at energies below 1.4 keV. At 1 AU, these bursts have been detected at energies as low as 73 eV. The characteristics of such bursts vary considerably from event to event due in part to the physical processes involved in their propagation to Earth. In 2002, Advanced Composition Explorer/Solar Wind Electron Proton Alpha Monitor (ACE/SWEPAM) observed 101 solar electron bursts at energies below 1.4 keV, 40 of which exhibited broader pitch angle distributions than the preburst strahl. In general, the width of the pitch angle distribution from 73 to 1370 eV is a function of energy. In ∼65% of the events the pitch angle distribution width increased with energy during the burst, in ∼10% of the events it decreased with energy, and ∼25% of the cases demonstrated no clear energy dependence. The delay time between the onset of the burst and the onset of pitch angle distribution broadening also showed energy dependence. At 987 eV broadening almost always occurred after onset of the burst; however, at energies less than 519 eV broadening of the pitch angle distribution sometimes preceded the burst onset. Although energy dispersion was observed in the burst onset times, several events revealed nearly simultaneous broadening over an extended range of energies, in one case from 197 to 1370 eV. Broadening of the pitch angle distribution function during solar electron bursts almost certainly results from wave‐particle scattering. Scattering, in turn, should depend on solar wind plasma and magnetic field conditions. Nevertheless, we were not able to find any general statistical correlations between ACE plasma and magnetic field parameters and the burst pitch angle distribution width. Because ACE measurements are inherently local, whereas scattering can occur anywhere between the Sun and Earth, we also examined five bursts that we infer underwent relatively local scattering. These bursts exhibited maximum pitch angle distribution widths greater than 75°. Even these special cases failed to manifest any correlation with local plasma or magnetic field measurements.