Implications of the observed anticorrelation between solar wind speed and coronal electron temperature

Abstract

Electron temperatures at the source of the solar wind are now obtained routinely and continuously from measurements of charge states of solar wind ions with modern solar wind composition spectrometers over a wide range of solar wind speeds. While the general anticorrelation between solar wind speed and electron temperature was previously noted, the physical processes responsible for this anticorrelation were not well understood, nor were mechanisms proposed that would produce this observed anticorrelation. We present a detailed analysis of solar wind measurements made with the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses over nearly an entire solar cycle and the full latitude range of Ulysses. We show that the electron temperature, T, derived from the O7+/O6+ density ratios, is not only well anticorrelated with measured solar wind speed, Vsw, in general, but also that the dependence of Vsw on 1/T is well represented by the solar wind equation derived by Fisk [2003]. This equation is based on the simple model in which the plasma of both the fast and slow wind is released from magnetic loops, which are opened by reconnection with open field lines. Fitting these SWICS data to the Fisk equation, we infer the dependence on loop height of the ratio of the magnetic field strength to mass density near the base of the loops (at the altitude of reconnection) and infer the solar cycle and latitude dependence of the size of the loops and of the strength of the average open field. We suggest that the same simple mechanism can account for both the fast and slow solar wind and that the final speed of the solar wind is determined primarily by the electron temperature in magnetic loops on the Sun, from which the solar wind originates.