Comparison of solar wind velocity measurements with a theoretical acceleration model

Abstract

Interplanetary radio scintillation (IPS) measurements of the solar wind velocity were made using the receiving antennas of the European Incoherent Scatter Facility (EISCAT) radar system in northern Scandinavia from June through October 1990. The observations, which cover the distance range from. 11 to 90 Rs from Sun center, were taken with sufficient density to measure the same stream at two (or more) different distances. The deduced velocities are in the range 100 ≲ u ≲ 540 km s−1. We selected from 192 observations, 16 examples of streams observed with good radial alignment, of which 12 were observed unchanged for several days. The measured velocities are compared with calculations based on a two‐fluid solar wind model with Alfvén waves. In eight cases the measurements are in good agreement with the model when a moderate amount of wave energy is added to the flow. In four cases the observed streams show low or moderate velocities below, say, 20 Rs but then accelerate fast at larger distances from the Sun. This delayed acceleration is much steeper than the acceleration in the model at these distances. In the remaining four cases the streams seem to reach their final velocities much closer to the base than in other cases, and they are not observed to accelerate much between 10 and 90 Rs. At these distances all related solar wind models we have seen give the same results; they all fit half the data, and none can fit the other half. We do not see an obvious modification to these models which will allow us to fit either the delayed acceleration or the early acceleration cases.