Traveling solar-wind bulk-velocity fluctuations and their effects on electron heating in the heliosphere

Abstract

Ambient plasma electrons undergo strong heating in regions associated with compressive bulk-velocity jumps U that travel through the interplanetary solar wind. The heating is generated by their specific interactions with the jump-inherent electric fields. After this energy gain is thermalized by the shock passage through the operation of the Buneman instability, strong electron heating occurs that substantially influences the radial electron temperature profile. We previously studied the resulting electron temperature assuming that the amplitude of the traveling velocity jump remains constant with increasing solar distance. Now we aim at a more consistent view, describing the change in jump amplitude with distance that is caused by the heated electrons. We describe the reduction of the jump amplitude as a result of the energy expended by the traveling jump structure. We consider three effects: energy loss due to heating of electrons, energy loss due to work done against the pressure gradient of the pick-up ions, and an energy gain due to nonlinear jump steepening. Taking these effects into account, we show that the decrease in jump amplitude with solar distance is more pronounced when the initial jump amplitude is higher in the inner solar system. Independent of the initial jump amplitude, it eventually decreases with increasing distance to a value of about U = 0.1 at the position of the heliospheric termination shock, where U is the jump amplitude, and U is the average solar-wind bulk velocity.The electron temperature, on the other hand, is strongly correlated with the initial jump amplitude and leads to electron temperatures between 6000 K and 20000 K at distances beyond 50 AU. We compare our results with in situ measurements of the electron-core temperature from the Ulysses spacecraft in the plane of the ecliptic for 1.5 AU = r = 5 AU, where r is the distance from the Sun. Our results agree very well with these observations, which corroborates our extrapolated predictions beyond r = 5 AU.