Probing the local interstellar medium

Abstract

Kurth et al.¹, using Voyager radio data and a simple model of solar-wind expansion, have inferred the location of the heliospheric shock to be 46 AU. Additional consideration of their data and the use of a modern model of the distant solar wind allow us to reduce this estimate to as little as 30 AU. The shock occurs where the solar wind pressure falls to that of the local interstellar medium (LISM). This value for the distance to the shock is less than half that usually expected, indicating that the parameters thought to describe the LISM differ from those commonly assumed. Provisionally accepting this interpretation, we applied a sophisticated model of solar-wind expansion to deduce a range of parameters for the LISM that predicts a comparable shock location. Specifically, we find that either the interstellar magnetic field is more than double the presently accepted value of 0.3 nT, or the pressure due to galactic cosmic rays with energies near 0.1 MeV is that obtained by simple extrapolation of the observed flux at higher energies inside the heliosphere, or some combination of these two external effects to yield an effective interstellar pressure approximately quadruple present estimates.