2.1 General considerations

The solar wind is the classical paradigm of a collisionless plasma. Detailed in situ measurements have been carried out over about four decades in the solar wind, and therefore its state is well known over a wide range of distances from the Sun, essentially from the corona to the heliopause, and over the entire range of heliospheric latitudes and longitudes. For comprehensive reviews of its microstate see the introduction section, in particular the reviews of Marsch (1991aJump To The Next Citation Point) and more recently Feldman and Marsch (1997Jump To The Next Citation Point). We refer the reader to these works and the many references therein, to avoid unnecessary repetitions. For a recent reviews of solar wind theory see, e.g., the book article of Marsch et al. (2003Jump To The Next Citation Point) or the concise paper of Hansteen et al. (1999Jump To The Next Citation Point).

The solar wind is known to originate in three basic types from the solar corona, whose open fields yield steady fast flows, the transiently open fields unsteady slow flows, and active fields sometimes very fast ejections of mass and magnetic flux. The solar wind microstate still carries remnants from its origin and information about the plasma state in the coronal source regions, which thus become remotely accessible through interplanetary solar wind measurements.

Kinetic processes prevail in the solar corona and solar wind, because the plasma is tenuous, multi-component and non-uniform. As a result of this, and owing to the macroscopic forces, significant deviations from LTE are bound to arise, and complexity is caused in particle phase space. These effects are signaled by strong distortions of the VDFs in the thermal regime, as well as by the occurrence of suprathermal particles, e.g., the electron strahl or non-thermal ion beams and heavy ion differential streaming. Because of the weak collisionality, there is a lasting influence of the boundary conditions in the corona on the interplanetary characteristics of the solar wind.

The Sun’s magnetic field varies over the solar cycle, and the solar wind varies correspondingly in response to solar activity (see, e.g., the review of Marsch, 2005). The global coronal field always consists of three major components: long-term open coronal holes (CHs), closed streamers and transiently open or closed loops. These components are respectively associated with uniform fast solar wind, filamentary slow wind, and transient variable-speed wind that is related with coronal mass ejections. The three basic types of wind differ substantially in their kinetic properties, because of different solar boundary conditions and interplanetary plasma dynamics. The radial evolution of the internal state of the wind thus resembles a complicated relaxation process, in which the particles’ free energy (as compared to energy bound by kinetic and magnetohydrodynamic equilibrium conditions) is converted to thermal and wave energy distributed over a range of scales.

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