The heliospheric boundary regions

Abstract

Estimates of heliosphere size based on analysis of VLF emissions measured by Voyager spacecraft are discussed. Observed spectral frequency drift of ∼ 1 kHz/year, if interpreted in terms of a 1st order Fermi effect for radiations trapped in a cavity, leads to average heliopause radius L ≤ 100 AU and local interstellar plasma density of 0.15/cm3. The distance to the termination shock, found from the flight time across the heliosphere of three solar wind transients whose interaction with the shock is thought to be the cause of emissions, gives average shock distance R = 90 AU. Discussion of charge exchange processes in the shocked solar wind plasma (inner heliosheath) indicates that observable fluxes of suprathermal neutral hydrogen atoms should be emitted. Atoms with energies corresponding to anomalous cosmic rays energy range should come preferentially from the tail side of the heliosphere (effect of tailward convection of the cosmic ray particles). Plasma energy loss by energetic neutral atoms emission may cause a thermal collapse of the frontal heliosheath. Analysis of possible oscillations of the termination shock caused by varying solar wind is developed for small amplitude perturbations. Resulting shock displacements are < 1 AU for variations associated with typical solar rotation and may attain ∼ 10 AU for solar cycle time scale effects.