Kinetic models of the solar wind

A kinetic model of the solar wind with Kappa distributions in the corona

Maksimovic, M., V. Pierrard and J. Lemaire, Astronomy and Astrophysics, 324, 725-734, 1997.

A kinetic model of the solar wind based on Kappa velocity distribution functions for the electrons and protons escaping out of the corona is presented. The high velocity particles forming the tail of these distribution functions have been introduced in the pioneering work of Scudder (1992a,b) to explain the high temperature of the coronal plasma. The first results obtained with this new kinetic model of the solar wind are very encouraging, indeed they fit better many major features observed in the solar wind than earlier models: e.g. the large bulk velocities observed in high speed streams emitted out from the coronal regions where the plasma temperature is smaller, and the low speed solar wind originating in the hotter equatorial regions of the solar corona. This new kinetic model is also able to predict the high speed solar wind streams without additional heating of the outer region of the corona, as it is needed in hydrodynamic models to achieve the same solar wind speed.

Ulysses electron distributions fitted with Kappa functions

M. Maksimovic, V. Pierrard and P. Riley Geophys Res. Let., 24, 9, 1151-1154, 1997.

We fit Kappa functions to 16,000 velocity distribution functions measured in the solar wind by the electron plasma instrument on board Ulysses. Statistically, the electron distributions are observed to have important high velocity tails in the fast solar wind but are closer to a Maxwellian in the slow wind. We also discuss how this result could support a recent kinetic model of the solar wind proposed by Maksimovic, Pierrard and Lemaire [1997].

Electron velocity distribution functions from the solar wind to the corona

V. Pierrard, M. Maksimovic, and J. Lemaire Journ. Geophys. Res., 104, 17021-17032, 1999.

Typical electron velocity distribution functions observed at 1 AU from the Sun by the instrument 3DP aboard of WIND are used as boundary conditions to determine the electron velocity distribution function at 4 solar radii in the corona. The velocity distribution functions (VDF) at low altitude are obtained by solving the Fokker-Planck equation, using two different sets of boundary conditions. The first set typically corresponds to a VDF observed in a low speed solar wind flow (i.e.\ characterized by "core" and "halo" electrons); the second one corresponds to high speed solar wind (i.e. characterized by "core", "halo" and "strahl" populations). We use the observed electron VDFs as test particles which are submitted to external forces and Coulomb collisions with a background plasma. Closer to the Sun, the relative density of the core electrons is found to increase compared to the density of the halo population. Nevertheless, we find that in order to match the observed distributions at 1 AU, suprathermal tails have to be present in the VDF of the test electrons at low altitudes in the corona.

Author: V. Pierrard   Curator: V. Pierrard    V.Pierrard@bira-iasb.oma.be