Electrons in the low‐density solar wind

Abstract

The recent occurrence of an interval (May 9–12, 1999) of abnormally low density solar wind has drawn attention to such events. The Solar Wind Experiment (SWE) instrument on the Wind spacecraft observed nine similar events between launch (November 1994) and August 1999: one in 1997, three in 1998, and five in January–August 1999. No such events were observed in 1996, the year of solar minimum. This already suggests a strong dependence upon solar activity. In this paper we discuss observations of the electron strahl, a strong anisotropy in the solar wind electrons above 60 eV directed along the magnetic field and observed continuously during the periods of low density in 1998 and 1999. When the solar wind density was less than 2 cm−3, the angular width of the strahl was below 3.5° and the temperature deduced from the slope of the electron strahl phase density (as a function of energy) in the energy range 200–800 eV was 100–150 eV, equivalent to a typical coronal electron temperature. Three examples of this phenomenon, observed on February 20–22, April 26–27, and May 9–12, 1999, are discussed to show their similarity to one another. These electron observations are interpreted to show that the strahl occurs as a result of the conservation of the first adiabatic invariant, combined with the lack of coulomb collisions as suggested by Fairfield and Scudder [1985].