On Collisionless Electron‐Ion Temperature Equilibration in the Fast Solar Wind

Abstract

We explore a mechanism, entirely new to the fast solar wind, of electron heating by lower hybrid waves to explain the shift to higher charge states observed in various elements in the fast wind at 1 AU relative to the original coronal hole plasma. This process is a variation on that previously discussed for two-temperature accretion flows by Begelman {&} Chiueh. Lower hybrid waves are generated by gyrating minor ions (mainly $α$-particles) and become significant once strong ion cyclotron heating sets in beyond 1.5 R☉. In this way the model avoids conflict with SUMER electron temperature diagnostic measurements between 1 and 1.5 R☉. The principal requirement for such a process to work is the existence of density gradients in the fast solar wind, with scale length of similar order to the proton inertial length. Similar size structures have previously been inferred by other authors from radio scintillation observations and considerations of ion cyclotron wave generation by global resonant MHD waves.