Direct observations of the formation of the solar wind halo from the strahl

C Gurgiolo,A N Fazakerley,M L Goldstein,A F Viñas

doi:10.5194/angeo-30-163-2012

C Gurgiolo, A N Fazakerley + Show 2 more

Open Access

https://doi.org/10.5194/angeo-30-163-2012

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Abstract

Abstract. Observations of a continual erosion of the strahl and build up of the halo with distance from the sun suggests that, at least in part, the halo may be formed as a result of scattering of the strahl. This hypothesis is supported in this paper by observation of intense scattering of strahl electrons, which gives rise to a proto-halo electron population. This population eventually merges into, or becomes the halo. The fact that observations of intense scattering of the strahl are not common implies that the formation of the halo may not be a continuous process, but one that occurs, in part, in bursts in regions where the conditions responsible for the scattering are optimum.

Highlights

The solar wind has been studied for more than 40 years and while most of the major processes involved in the formation and expansion of the wind are reasonably understood there are still details that need to be worked out
The electron component of the solar wind consists of four distinct populations; a thermal core and a superthermal halo (Feldman et al, 1975), a superhalo which is a halo-like population beginning about 2 keV and extending upwards to about 100 keV Lin (1998), and the field-aligned strahl (Rosenbauer et al, 1976, 1977)
Models and simulations indicate that the electron solar wind is formed within the solar corona through a combination of wave-particle interactions and Coulomb collisions

Summary

Introduction

The solar wind has been studied for more than 40 years and while most of the major processes involved in the formation and expansion of the wind are reasonably understood there are still details that need to be worked out. (Ideally the projections should be computed from the magnetic field averaged over the time during which the analyzer spends scanning the field aligned population and not over the entire spin, but we have not done that in this paper.) At lower energies (less than about 6 eV in potential corrected energy) some displacement is expected due to the drift velocity introduced by the interplanetary electric field. The set of PT plots show the three typical solar wind electron components (core, halo, and strahl) plus a backscattered or return electron population common to the foreshock. Over the two plots, the peak in strahl electrons begins to shift off the magnetic field in the direction of the ecliptic plane This is caused by the overlap of the strahl electrons with the high energy tail of the halo. Unless the potential is wildly varying over the time frame being shown, this should be a reasonable value for the other columns of plots

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Discussion

Conclusions