The effect of magnetic topography on high‐latitude radio emission at Neptune

Abstract

Occultation by a local elevation on the surface of constant magnetic field is proposed as a new interpretation for the unusual properties of Neptune high‐latitude emission. Abrupt changes in intensity and polarization of this broadband smooth radio emission were observed as the Voyager 2 spacecraft passed near the north magnetic pole before closest approach. The observed sequence of cutoffs with polarization reversal would not occur during descent of the spacecraft through regular surfaces of increasing magnetic field. The sequence can be understood in terms of constant‐frequency (constant‐field) surfaces that are not only offset from the planet center but are locally highly distorted by an elevation that occults the outgoing extraordinary‐mode beam. The required occulter is similar to the field enhancement observed directly by the magnetometer team when Voyager reached lower altitude farther to the west. We present evidence that the sources of the high‐latitude emission are located near the longitude of the minimum‐B anomaly associated with the dipole offset and that the local elevation of constant‐B surfaces extends eastward from the longitude where it is directly measured by the magnetometer to the longitude where occultation of the remote radio source is observed. Together, the radio and magnetometer experiments indicate that the constant‐frequency surfaces are distorted by an elevation that extends 0.3 rad in the longitudinal direction. On the 462 kHz surface, about 1.1 RN from the dipole center, the local elevation must be at least 0.45 RN above the undisturbed (dipole model) surface.