The interplanetary plasma is collisionless. The plasma particles interact only by the large scale electric and magnetic fields. Interesting structures which may be excited in such a plasma are collisionless shocks. They are generated near the sun and propagate through interplanetary space. Further on they may be built up in front of obstacles, e.g. forming bow shocks at planets and comets. The structure of these collisionless shocks is strongly influenced by the orientation of the interplanetary magnetic field.
Perpendicular shock wave (mpg, 900kB)
Parallel shock wave (mpg, 750kB)
Perpendicular and parallel shocks are presented in the animations. One-dimensional hybrid codesimulations were used to produce these results.
The interplanetary plasma does not consist of one ion species alone. The protons are the primary ion component, but there are minor ion components as He2+, He+, O6+ and other heavy ions. Heavy ions are generated in the sun as a result of nuclear fusion. At a collisionless shock the heavy ions show a rather different behaviour than protons. The figure demonstrates a multi-ion shock wave consisting of protons, He2+ and He+. Because of their higher mass the velocity of He2+ and He+ is reduced less than for protons. Thus in the downstream region a differential velocity with respect to the protons is present. This velocity drives the pickup of He2+and He+ to be seen in the oscillations of the He2+and He+ components.
References
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