Solar Wind Properties from IPS Observations

Abstract

Since Hewish et al. (1964) discovered the interplanetary scintillation (IPS) phenomena, the IPS method has been used as one of the few devices which can be used to observe solar wind in three-dimensional space. However because of the line-of-sight integration effect of IPS, solar wind had to be studied with blurred images. In the late 1990s new methods of IPS observation and analysis which can deconvolve the line-of-sight integration effect were developed independently by a group at University California at San Diego (Grall et al., 1996) and a group at the Solar-Terrestrial Environment Laboratory, Nagoya University (Asai et al., 1998; Kojima et al., 1998; Jackson et al., 1998) . Today we can obtain unbiased solar wind images with high spatial resolution from IPS observations. The Ulysses spacecraft has been observing detailed structures of solar wind in three dimensions since its launch in 1990. However, Ulysses takes ten months even to make a rapid latitudinal scan from the south to north poles. IPS measurements have several advantages in comparison with spacecraft measurements. It can observe three-dimensional solar wind in a short time, and the observations can be carried out consistently over a solar cycle. Making use of these advantages of IPS, we have been studying several interesting solar wind features observed by Ulysses; namely, whether they are stable structures and how they depend on the solar cycle. We introduce these studies and propose a model to determine the solar wind velocity structure.