Microstructure of the interplanetary magnetic field near 4 and 5 AU

Abstract

Seventy‐two days of vector magnetic field measurements from Pioneer 10 and 11 are analyzed for information about magnetic field fluctuations in the quiet solar wind near 4 and 5 AU. Calculated as functions of frequency over the range 4 × 10−5 to 9 × 10−3 Hz, directional properties of magnetic field fluctuations are presented and are discussed with reference to theoretical predictions for MHD plane waves. The distributions of maximum variance directions computed from ≃500 spectral matrices for each spacecraft are peaked along the normal to the plane of the minimum variance direction and the mean magnetic field. This orientation is expected for a planar, small‐amplitude Alfvén wave with a minimum variance direction close to the wave vector. Noting also that magnetic fluctuations tend to conserve field magnitude, as is required for Alfvénic fluctuations, we infer that single small‐amplitude Alfvén waves are occasionally present in the temporal and frequency bands analyzed. However, statistics of the relative spatial orientations of the mean field and the minimum and maximum variance directions permit that waves with wave vectors not well approximated by the minimum variance direction, Alfvén wave ensembles, finite‐amplitude waves, and nonplanar fluctuations may also be present. Assuming that the minimum variance direction approximates the wave vector when a small‐amplitude wave is present, we note that (1) among those data in which the field fluctuations behave in accord with theoretical predictions for single, planar, small‐amplitude MHD waves, observations suggest the predominance of the Alfvén mode over the fast and slow modes and (2) there is a general lack of observational support for theoretical models of outward propagating plane Alfvén waves of solar origin.