Perpendicular scattering for electron beams by the electron/electron instability in solar electron bursts

Abstract

[1] Energy dependence of electron pitch angle distributions in solar electron bursts is studied using both particle-in-cell simulations of the electron/electron instability and a time-of-flight model for streaming electrons. The simulations show that the electron/ electron instability leads to a maximum of electron pitch angle width in electron kinetic energy. The energy where the maximum appears varies with electron beam parameters. The simulations further show that the maximum electron pitch angle width increases with increasing electron beam density. The time-of-flight effect which leads to positive slopes in electron velocity distributions implies that the probability of electron/electron instability growth in the solar wind depends on electron kinetic energy because of the presence of an electron halo distribution. It is concluded that the electron/electron instability induced by the time-of-flight effect is likely to yield a maximum electron pitch angle width near 1 keV.Citation: Saito, S., and S. P. Gary (2008), Perpendicular scattering for electron beams by the electron/electron instability in solar electron bursts,

Electron distributions during the solar electron burst of 22 March 2002

“…The protuberance in the PAD width‐energy profiles observed during the three time periods discussed above is qualitatively similar to the protuberance reported by Gary and Saito [2007] and Saito and Gary [2008] in their simulations, which they call a characteristic signature of electron scattering by the electron/electron instability. Further evidence supporting the conclusion that the observed protuberance is caused by scattering due to the electron/electron instability comes from observations of concurrent type III radio events.…”

“…Recent simulations by Gary and Saito [2007] and Saito and Gary [2008], pertaining to electron scattering by the well‐known electron/electron instability, may provide an explanation for the protuberance observed in the electron PADs of 22 March 2002. It has long been understood [see, e.g., Lin , 1990 and references therein] that as suprathermal electrons stream away from the Sun along the background magnetic field in the solar wind, the time‐of‐flight effect will gradually generate a positive slope in the electron velocity distribution; this is the classic bump‐on‐tail distribution.…”

“…When the positive slope reaches the threshold of the electron/electron instability, fluctuating electric fields are generated by the instability and scatter the electrons near velocities where the positive slope arises. According to Gary and Saito [2007] and Saito and Gary [2008], the fluctuating parallel electric field, δE ∥ , decreases the average beam speed in the parallel direction and provides the energy that leads to the fluctuating field growth, whereas the fluctuating perpendicular electric field, δE ⊥ , perpendicularly scatters the electrons. Scattering by these enhanced electric fluctuations corresponds to a general broadening of strahl pitch angles, leading to a characteristic protuberance in the electron PAD width‐energy profile with a peak at 1.0–1.5 keV; note that the energy at which the peak of the protuberance occurs lies considerably below the initial average energy of the strahl [ Saito and Gary , 2008].…”

Variability of the solar wind suprathermal electron strahl

“…Although the scattering that broadens the strahl is presumably due to wave‐particle interactions, the exact scattering mechanism is not currently understood. Various models have demonstrated strahl broadening with various characteristics [e.g., Vocks et al , 2005; Saito and Gary , 2007; Gary and Saito , 2007; Saito and Gary , 2008]. Whistler scattering can produce strahl widths that either increase or decrease with energy, depending on whether the whistlers are generated by wave‐wave interactions or temperature anisotropy of the core electrons [ Saito and Gary , 2007].…”

“…Whistler scattering can produce strahl widths that either increase or decrease with energy, depending on whether the whistlers are generated by wave‐wave interactions or temperature anisotropy of the core electrons [ Saito and Gary , 2007]. Strahl scattering may also result from electron‐electron instabilities, particularly within solar electron bursts [ Gary and Saito , 2007; Saito and Gary , 2008]. Our results showing variable scattering with energy indicate that more than one scattering mechanism likely operates in the solar wind, and that different scattering mechanisms may operate with roughly equal probability.…”