Inner-Heliosphere Observations of Sunward Strahl Electrons on Magnetic Switchbacks

The 'strahl' component of the solar wind electron population takes the form of a field-aligned, typically anti-sunward, suprathermal (>100 eV) beam. Solar wind intervals where the strahl instead travels sunward along the field indicate local heliospheric magnetic field (HMF) inversions (also known as 'switchbacks') in which the field 'bends-back' upon itself. These inversions in the past have been suggested to be remnant structures from interchange reconnection processes. Such processes are candidate mechanisms for the production of slow solar wind; a key topic to be explored by Solar Orbiter. Magnetic inversions are also emerging as crucial near-Sun targets for study by Parker Solar Probe. Using Helios observations, we radially profile the properties of the sunward strahl electrons which are present during these magnetic inversions, and compare them to anti-sunward strahl in a statistical study spanning ~0.3 to 1 AU. These observations give insight into the origins and evolution of strahl electrons, and these HMF structures. Comparing pitch angle distributions (PADs) of sunward and anti-sunward strahl, we find the sunward strahl distributions to be the broader and less intense of the two; particularly at distances 0.3 to 0.75 AU. Further, sunward and anti-sunward strahl both become increasingly broad and less intense with distance from the Sun. Combined, these results suggest that both are subject to continuous scattering during propagation through the heliosphere, and that sunward strahl is more strongly scattered at a given heliocentric distance. These observations lead us to conclude that the longer and more variable path from the Sun to the spacecraft, along inverted magnetic field, leads to the additional scattering of the sunward strahl. The results also suggest that the relative importance of scattering along this inverted field drops off with heliocentric distance. We discuss the insight which will be gained by extending these observations with Solar Orbiter.