Magnetohydrodynamic shock waves in the solar corona, with applications to bursts of radio-frequency radiation

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Abstract
It is pointed out that the Type II burst velocities which ere usually associated with the velocities of streams of ‘auroral’ particles should rather be associated with the velocities of the shock fronts ahead of such stream. Likewise, the far greater Type III burst velocities can be associated with the shock velocities of corpuscular streams having similar velocities, if it is possible to raise the local value of the sound velocity to the magnitude of the Type III estimates. This can be effected if local magnetic fields of some hundreds of gauss are present in the solar corona, resulting in a predominant magnetohydrodynamic component in the sound velocity. Starting from a set of transport equations previously investigated for an ionized gas, it is shown that in quasi-static electromagnetic fields the coronal medium satisfies an adiabatic equation of state and that the magnetic field lines ere effectively ‘frozen’ into the medium. The details of the transition across the shock front ahead of a stream in which all physical quantities depend only on the coordinate in the direction of flow is investigated for the case where the stream velocity is small compared with the sound velocity, and the results am compared with those for the hydrodynamic shock associated with a Type II burst. The coronal conditions and the excitation of radiation near the shock front are briefly discussed.