Draping of the local interstellar magnetic field over the heliopause

Abstract

A recent theory for radio events at 2-3 kHz observed by the Voyager spacecraft suggests that the emission is generated when shocks associated with global merged interaction regions (GMIRs) enter a region just beyond the heliopause nose that is primed with an enhanced level of superthermal electrons. In this GMIR/priming theory the superthermal electrons are accelerated by lower hybrid waves generated by pick-up ions. For this acceleration to be efficient the pick-up ion ring speed vr and the Alfvên speed v A must satisfy the inequality vr/vA ≲ 5, implying that the local magnetic field B must be sufficiently large. Here this constraint is used to predict which regions generate radio emission by calculating the draping of the interstellar magnetic field B∞ over the heliopause using the convected field equations and a gas-dynamic simulation of the solar wind-VLISM interaction. The size and shape of the regions with large |B| are predicted to depend on the orientation of B ∞ relative to the interstellar flow velocity. For sufficiently perpendicular orientations the high |B| region is a linear band parallel to B∞ in the plane of the sky, centered near where the surface is closely parallel to B∞, but the band shape is only a ∼10% effect compared with a circular surrounding region. The magnetic amplification factor increases with decreasing distance to the heliopause nose and increasingly perpendicular orientation of B∞, with factors ≳ 5 typical within axial and transverse distances to the nose of 5 and 35 AU, respectively. Combining the magnetic amplification with plausible neutral and plasma parameters, the constraint vr/vA ≲ 5 requires B∞ S: 0.06 nT for the GMIR/priming theory to operate within the draping region. A recently proposed constraint, that B be nearly perpendicular to the normal vector n̂ to the GMIR surface for effective electron acceleration by the GMIR shock, is also considered. A supporting argument is provided for the previous claim that this constraint predicts strong emission in a band perpendicular to B∞: Calculations show that the shockaccelerated electrons produce significant emission only for distances parallel to B that are small (≈1 AU) compared with the macroscopic regions on the shock where B ·≈ 0. This predicted source orientation agrees well with observations of the source and an independent estimate of the direction of B∞ based on Lyman-a observations. It is argued that the B n̂ ≈ 0 constraint is a natural component of the GMIR/priming theory. The large, relatively circular nature of the draping region where v r/vA≲ 5 will plausibly lead to the constraint B n̂ ≈0 determining the intrinsic source shape in the plane of the sky. Copyright 2008 by the American Geophysical Union.