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[1] We have performed a statistical study of a substantial amount of electron data acquired in the solar wind to understand the constraints on electron temperature anisotropy by plasma instabilities and Coulomb collisions. We use a large data set of electron measurements from three different spacecraft (Helios I, Cluster II, and Ulysses) collected in the low ecliptic latitudes covering the radial distance from the Sun from 0.3 up to 4 AU. We estimate the electron temperature anisotropy using fits of the measured electron velocity distribution functions acquired in situ. We use a two population (core and halo) analytical model and properties of both populations are studied separately. We examine all the acquired data in terms of temperature anisotropy versus parallel electron plasma beta, and we relate the measurements to the growth rates of unstable modes. The effect of Coulomb collisions is expressed by the electron collisional age A e defined as the number of collisions suffered by an electron during the expansion of the solar wind. We show that both instabilities and collisions are strongly related to the isotropisation process of the electron core population. In addition we examine the radial evolution of these effects during the expansion of the solar wind. We show that the bulk of the solar wind electrons are constrained by Coulomb collisions, while the large departures from isotropy are constrained by instabilities.

Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most I-t•As have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

[1] Large-amplitude (up to $50 mV/m) solitary waves, identified as electron holes, have been observed during waveform captures on two of the four Cluster satellites during several plasma sheet encounters that have been identified as the passage of a magnetotail reconnection x line. The electron holes were seen near the outer edge of the plasma sheet, within and on the edge of a density cavity, at distances on the order of a few ion inertial lengths from the center of the current sheet. The electron holes occur during intervals when there were narrow electron beams but not when the distributions were more isotropic or contained beams that were broad in pitch angle. The region containing the narrow beams (and therefore the electron holes) can extend over thousands of kilometers in the x and y directions, but is very narrow in the z direction. The association with electron beams and the density cavity and the location along the separatrices are consistent with simulations shown herein. The velocities and scale sizes of the electron holes are consistent with the predictions of Drake et al. [2003]. Particle simulations of magnetic reconnection reproduce the observed Cluster data only with the addition of a small (0.2 of the reversed field) ambient guide field. The results suggest that electron holes may sometimes be an intrinsic feature of magnetotail reconnection and that in such cases the traditional neglect of the guide field may not be justified. Very large amplitude lower hybrid waves (hundreds of millivolts per meter), as well as waves at frequencies up to the electron plasma frequency, were also observed during this interval.

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