Solar wind electron suprathermal strength and temperature gradients: Ulysses observations

We use observations from the Ulysses electron spectrometer to examine globa l trends of the electron suprathermal population and to study, for the fi r st time, the electron core, halo, and total temperature gradients in the fa st solar wind over the poles. We use a data set covering the period from th e beginning of the mission ( 1990, day of year 322) to approximately the fi rst completion of Ulysses out of-ecliptic orbit around the Sun (1998, day 3 2). This allows us to characterize very welll the two states of the solar w ind: the high-speed wind, emanating from polar coronal holes , and the low- speed streams, emanating from equatorial regions. From a classical bi-Maxwe llian (core and halo) model of electron velocity distribution functions, we define the electron suprathermal strength S as the ratio of the halo to co re kinetic pressures: S = n(h)T(h)/n(c)T(c). The fast wind has larger avera ge values of S than the slow wind. This global correlation between S and th e solar wind hulk speed is also observed on a smaller scale in the polar re gions. We find that the small-scale variations of the solar wind bulk speed in the polar regions, typically +/-50 kin/s around an average value, of 75 0 km/s, are correlated with small-scale variations of S. We present also th e first observations of the electron core, halo, and total temperature grad ients in time-stationary fast solar wind periods over the poles, We examine all the previous observations in the context of simple solar wind exospher ic models. We find, for instance, that the total electron temperature can b e well fitted by a law of the form T-c = T-0 + T-1 r(-4/3), as predicted by the exospheric approximation.