A determination of the outflow speeds in the lower solar wind

Abstract

The acceleration of the low-latitude solar wind, which comes from regions of the corona above large, closed magnetic loops, is a theoretical challenge. Until recently we have lacked direct measurements close to the Sun with which to constrain the theory. When a series of high-resolution coronagraph images is viewed in the form of a movie the continuous outflow of matter from the Sun is very obvious. We report here the results of a correlation analysis of the LASCO coronagraph images from the SOHO spacecraft which we have used to estimate the low-latitude solar wind speed from 3-27 R_\odot. Acceleration occurs to the outer parts of this radial range, by which time the solar wind has approached its measured speed at 1AU, implying little acceleration beyond ~30 R_\odot. A very significant result is the detection of a component flowing out at speeds up to ~1500 km s^{-1}, which is energetically very important and possibly dominant.