Abstract
EISCAT measurements of the solar wind
Highlights
EISCAT observations of interplanetary scintillation have been used to measure the velocity of the solar wind at distances between 15 and 130 RW from the Sun
In earlier IPS work using EISCAT (e.g. Bourgois et al, 1985) the apparent velocity was determined by fitting the observed correlation functions with a theoretical model in which the solar wind velocity had a compact distribution about a single mean speed
EISCAT allows the fast and slow streams to be resolved, something which is not possible with single-site observations or with two-site observations on shorter baselines (Grall et al, 1996), and the smooth range of velocities reported in earlier work resulted from differing proportions of fast and slow streams in the line of sight through the solar atmosphere
Summary
Measurements of IPS (interplanetary scintillation) have been used for many years to study electron density fluctuations in the solar wind (e.g. Cohen et al, 1967) and to measure the solar wind velocity (Dennison and Hewish, 1967; Armstrong and Coles, 1972; Coles 1995). IPS has the advantage that it can provide measurements from close to the Sun to beyond 1 AU and at all heliographic latitudes (Coles, 1995). The solar wind velocity is determined from the correlation of scintillations observed at separate sites and the three EISCAT receiving stations provide an ideal system, offering baselines up to 390 km. IPS observations were first made at EISCAT in 1981 (Bourgois et al, 1985), and since 1990 a regular series of measurements has been made every summer (Coles et al, 1991; Breen et al, 1996; Grall et al, 1996). As the distance between the two antennas is known, the time lag for maximum cross-correlation can be used to estimate the component of solar wind velocity perpendicular to the line of sight (Coles et al, 1991; Breen et al, 1996). The long baselines available between EISCAT sites mean that these velocity measurements can be made with high resolution so that streams of different velocity can be distinguished (Grall et al, 1996)
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