SOLAR-CYCLE EVOLUTION OF THE STRUCTURE OF MAGNETIC CLOUDS IN THE INNER HELIOSPHERE

Nearly ten years of continuous magnetic field observations by the Pion eer Venus spacecraft allows us to study the correlation between the st ructure of magnetic clouds in the inner heliosphere and the phase of t he solar cycle. Fifty-six magnetic clouds have been identified in the PVO data at .7AU during 1979-1988. As this period spans nearly two sol ar maxima and one solar minimum we can study the evolution of the stru cture of these magnetic clouds through varying solar activity and unde r various orientations of the coronal streamer belt. Until shortly aft er the 1979 solar maximum the majority of clouds had an initially sout hward magnetic field which turned northward as the cloud was traversed , while in the period leading up to the 1988 solar maximum the majorit y had a northward field that turned southward. In the declining phase of solar activity magnetic clouds continued to occur, but only a minor ity can be classified as having south-to-north and north-to-south rota tions. The majority of these clouds occurred with the field remaining entirely north or south relative to the solar equator. These results c onfirm observations using Hellos and ISEE data indicating that the str ucture of magnetic clouds varies in response to changes in the magneti c structure of the source region. By interpreting these observations t o imply that the leading magnetic field in magnetic clouds is controll ed 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 clou ds, we can predict preferred magnetic cloud structure and orientation during varying phases of the solar cycle. The helicity of the observat ions does not seem to be ordered by the solar cycle.