Electron temperature anisotropy constraints in the solar wind

• Published in: Journal of Geophysical Research: Space Physics 2008-03, Vol.113 (A3), p.n/a
• Main Authors: Štverák, Štěpán, Trávníček, Pavel, Maksimovic, Milan, Marsch, Eckart, Fazakerley, Andrew N., Scime, Earl E.
• Format: Article
• Language: English
• Subjects: Astrophysics; Earth sciences; Earth, ocean, space; electrons; Exact sciences and technology; Physics; solar wind; temperature anisotropy
• ISSN: 0148-0227; 2169-9380; 2156-2202; 2169-9402
• DOI: 10.1029/2007JA012733

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 Ae 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.