Abstract We review theoretical descriptions of low Mach number collisionless shock waves from an historical perspective in order to place recent observations in context. Naturally occurring shocks such as the earth's bow shock are typically strong shocks where dissipation is provided by the reflection of a fraction of the incoming ions. We discuss here weaker shocks where shock steepening is limited by dispersion and/or anomalous resistivity. The character of such shocks is determined in the main by the dispersive properties of the ambient plasma. For oblique shock normal angles the shock itself in such cases is simply the largest amplitude cycle of a whistler mode wave which extends upstream in the form of a damped precursor wavetrain. Such dispersive shocks were studied in detail in early laboratory work, but searches for their counterparts in spacecraft data brought mixed results. Studies of low Mach number bow shocks found cases in which the predicted oscillatory structure appeared, but found many sharp monotonic transitions as well. These observations lead to a reconsideration of the nature of the resistive processes in weak shocks. Attention focused on nearly perpendicular shocks where dispersive scale lengths become small and the associated cross field currents become strong enough to generate significant plasma wave turbulence. The anomalous resistivity produced by this turbulence then limits shock steepening and the structure of such resistive shocks is dependent on the properties of the associated turbulence. A number of oblique low Mach number bow shocks have now been studied using data from the ISEE dual spacecraft pair, which has for the first time enabled us to accurately determine shock scale lengths. These studies confirm that we understand the dispersive contributions to shock structure rather well. There are still, however, insufficient detailed data on the more perpendicular resistive shocks.


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    Title :

    The physical mechanisms of subcritical collisionless shock-wave formation


    Contributors:

    Published in:

    Advances in Space Research ; 4 , 2-3 ; 245-253


    Publication date :

    1984-01-01


    Size :

    9 pages




    Type of media :

    Article (Journal)


    Type of material :

    Electronic Resource


    Language :

    English