Abstract
Abstract Strahl is the strongly field-aligned, beam-like population of electrons in the solar wind. Strahl width is observed to increase with distance from the Sun, and hence strahl electrons must be subject to in-transit scattering effects. Different energy relations have been both observed and modeled for both strahl width and the width increase with radial distance. Thus, there is much debate regarding what mechanism(s) scatter strahl. In this study, we use a novel method to investigate strahl evolution within 1 au by estimating the distance traveled by the strahl along the interplanetary magnetic field (IMF). We do this by implementing methods developed in previous studies, which make use of the onset of solar energetic particles at ∼1 au. Thus, we are able to obtain average strahl broadening in relation to electron energy and distance, while also taking into account the general effect of IMF topology and adiabatic focusing experienced by strahl. We find that average strahl width broadens with distance traveled along the IMF, which suggests that strahl width is related to the path length taken by the strahl from the Sun to 1 au. We also find that strahl pitch-angle width broadening per au along the IMF length increased with strahl energy, which suggests that the dominant strahl pitch-angle scattering mechanism likely has an inherent energy relation. Our pitch-angle broadening results provide a testable energy relation for the upcoming Parker Solar Probe and Solar Orbiter missions, which are both set to provide unprecedented new observations within 1 au.
Highlights
The solar wind is generally considered to have three constituent electron populations: a thermal “core,” a suprathermal “halo,” and a suprathermal “strahl” (e.g., Feldman et al 1975; Maksimovic et al 2005)
We observed events where the average strahl width broadened with energy, events where it decreased with energy, and events with unclear or no energy relation
In this investigation we focus our discussion on the relationship between average strahl width and interplanetary magnetic field (IMF) length, as well as the energy relation for beam broadening with heliospheric distance
Summary
The solar wind is generally considered to have three constituent electron populations: a thermal “core,” a suprathermal “halo,” and a suprathermal “strahl” (e.g., Feldman et al 1975; Maksimovic et al 2005). Studies, such as those referenced above, find that core and halo populations are typically relatively isotropic compared to strahl, which is a strongly field-aligned beam of electrons. Certain IMF topologies, such as a closed loop with both IMF footpoints connected to the solar surface, can result in bidirectional strahl, in which electron beams are observed in both the parallel and anti-parallel magnetic field direction simultaneously (e.g., Gosling et al 1987). It should be noted that there are times when no clear strahl population is observed in the solar wind (e.g., Anderson et al 2012), during observations in the slow solar wind (e.g., Gurgiolo & Goldstein 2017)
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