Solar cycle evolution of the structure of magnetic clouds in the inner heliosphere

Abstract

Nearly ten years of continuous magnetic field observations by the Pioneer Venus spacecraft allows us to study the correlation between the structure of magnetic clouds in the inner heliosphere and the phase of the solar cycle. Fifty‐six magnetic clouds have been identified in the PVO data at .7AU during 1979–1988. As this period spans nearly two solar maxima and one solar minimum we can study the evolution of the structure of these magnetic clouds through varying solar activity and under various orientations of the coronal streamer belt. Until shortly after the 1979 solar maximum the majority of clouds had an initially southward magnetic field which turned northward as the cloud was traversed, while in the period leading up to the 1988 solar maximum the majority had a northward field that turned southward. In the declining phase of solar activity magnetic clouds continued to occur, but only a minority can be classified as having south‐to‐north and north‐to‐south rotations. The majority of these clouds occurred with the field remaining entirely north or south relative to the solar equator. These results confirm observations using Helios and ISEE data indicating that the structure of magnetic clouds varies in response to changes in the magnetic structure of the source region. By interpreting these observations to imply that the leading magnetic field in magnetic clouds is controlled by the polarity of the sun's global field and that the inclination of the coronal streamer belt controls the axis of symmetry of the clouds, we can predict preferred magnetic cloud structure and orientation during varying phases of the solar cycle. The helicity of the observations does not seem to be ordered by the solar cycle.