Dynamic magnetic structure of large amplitude Alfvénic variations in the solar wind

Abstract

The dynamic structure of large amplitude Alfvénic disturbances of the interplanetary magnetic field (IMF) is examined by transforming one‐hour intervals of Explorers 33 and 35 magnetometer data from the solar ecliptic coordinate system to a coordinate system defined by the principal axes of the variance matrix. Graphic displays of the rotated magnetometer data, in the form of linear time series’ for the field component in the direction of minimum variance and hodographs for the magnetic field vector component in the plane of maximum variance, reveal ordered structure to field components which appear almost random when displayed as time series’ in the solar ecliptic coordinate system. When there is a well defined minimum variance direction, the component of the magnetic field in the plane normal to the direction of minimum variance tends to oscillate with constant magnitude so that the tip of the vector lies on a circular arc. However, the angular motion of the vector appears to be unconstrained. This type of behavior for the IMF satisfies the necessary local conditions for plane large amplitude Alfvén waves. Residual variability of the magnetic field in the direction of minimum variance may be due to the fully three‐dimensional oscillations of the nearly plane waves or a small compressional component associated with fluctuations in the magnetic field magnitude. The different types of angular motion of the magnetic field component normal to the direction of minimum variance may be indicative of the detailed conditions of the solar coronal plasma in the regions generating the Alfvén waves, or some aspect of local generation.