IPS MEASUREMENTS AND AZIMUTHAL VARIATION OF SOLAR-WIND VELOCITY ON THE HELIOSPHERIC CURRENT SHEET

The solar-wind velocity tends to be a minimum along the heliospheric c urrent sheet and increases with heliomagnetic latitude. In addition, a n azimuthal variation of solar-wind velocity is found to exist on the current sheet. In this study of the solar-wind velocity variations on the current sheet, we have made use of interplanetary scintillation (I PS) measurements. This method provides solar-wind data over a larger r ange of heliolatitudes: +/- 80-degrees or greater. We found that there exists a clear longitudinal variation of solar-wind velocity on the c urrent sheet during 1973-1985. It is also noticeable that the pattern of variation changes with the phase of the solar cycle. On average, th e solar-wind velocity varied from 300 to 550 km s-1 on the current she et during the period of study. It is also found that the longitudinal variation of the solar-wind velocity on the current sheet obtained usi ng near-Earth satellite data agrees well with that obtained using IPS data.