Graham, Georgina Alessandra; (2018) The Evolution of Solar Wind Strahl. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Suprathermal electrons are an important kinetic feature of the solar wind; however, their origin and evolution is yet to be fully understood. Suprathermal electrons are divided into two component populations: the field-aligned, beam-like `strahl' and the approximately isotropic `halo'. Observations have revealed that the strahl component cannot be subject solely to expansion effects as it travels away from the Sun along the interplanetary magnetic field (IMF). Thus, strahl scattering processes must occur within the solar wind and may be at least partly responsible for the existence of the isotopic halo. In this thesis, strahl evolution is examined, in order to constrain the possible strahl scattering mechanisms and investigate strahl utility as an IMF probe. Novel analysis methods are applied to strahl observations to obtain the largest radial range of strahl width observations to date; examine the effect of distance travelled along the IMF on strahl width; and provide estimates of IMF orientation. It is found that strahl scattering likely occurs throughout the solar wind, to at least to 5.5 AU, and strahl electrons are most likely scattered to form part of the halo population by 9 AU. It is also concluded that strahl scattering occurs quasi-continuously and results indicate that scattering may be more intense closer to the Sun, particularly for higher energy electrons. The increase in strahl beam-width with distance is also greater for higher energy strahl, suggesting that there is a dominant scattering mechanism with an inherent energy relation, most likely explained by resonant wave-particle interactions. Finally, observing relatively narrow strahl beams at 1 AU using high resolution Cluster data allowed estimations of strahl width that generally lie within 20° of the magnetometer observations

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