Analysis of waves in Saturn's dayside magnetosphere: Voyager 1 observations

Abstract

Magnetic field fluctuations were observed in the data of Voyager 1 on its inbound leg in Saturn's magnetosphere from about 19 to 8.4 Saturn radii, during the interval 7–17 hours before closest approach. These low‐amplitude oscillations had the appearance of irregular micropulsations. The wave periods in the spacecraft frame varied between ≈1 and 20 min with a tendency to be inversely correlated with the field strength. An eigenfunction analysis yielding wave propagation direction as a function of frequency showed that at and near peak frequency two distinct types of waves were present, one propagating along field lines (within 30°) usually at the higher frequencies considered, (4–14) × l0−3 Hz, and the other propagating along (the azimuthal direction in a standard spherical coordinate system) at lower frequencies, (0.8–4) × l0−3 Hz. The power spectral density (in (nT)2/Hz) at the peaks tended to be inversely related to frequency over the full set of frequencies. An interpretation of these results is that Alfven waves are propagating along field lines at the higher frequencies with wavelengths between 1/4 and 1/2 RS. These occur in both the mantle and plasma sheet. The waves traveling along , which are observed to occur in the low‐latitude plasma mantle, are apparently manifestations of rapidly corotating MHD waves with large compressional components and are probably due to the operation of the centrifugal flute instability at the plasma sheet‐mantle boundary. For both types, wave absorption occurs with a high‐frequency cutoff at or near the gyrofrequency of O+ (and/or N+). Our results establish the framework for MHD waves within Saturn's outer magnetosphere, and it is expected that the Cassini mission, for which orbit insertion occurred on 1 July 2004, will add considerably to that presented here.