The distant geomagnetic field: 4. Microstructure of a disordered hydromagnetic medium in the collisionless limit

Abstract

The fine-scale structure of the magnetic field data given in part 3 is examined. The front slopes of the large pulses generally rise to peak value in about 1 second. The rear elopes apparently decay exponentially, often with a satellite structure having a period about one-sixth that of a medium fixed with respect to the magnetometer, suggesting strong Doppler shifting. Thus, the assumption of a Larmorlike periodicity for these satellite pulses indicates an Alfven Mach 6 velocity away from the spacecraft. The large pulses (hyperwaves) also appear to be consistent with collisionless, finite-amplitude hydromagnetic pulses. The slope asymmetry is in qualitative accord with theoretical views, i.e., an anisotropic plasma pressure and/or first-order dissipation and relaxation. The power spectrums for neither H⊥ nor ψ* are Kolmogoroffian. The H⊥ spectrums are generally compatible with a two-step process; ψ* (vorticity) spectrums have increasing power density with wave number. Velocity bounds provide a scale size for both the hyperwaves and the eddy distribution. The condition ∂H/∂t«ωH, as discussed at length in part 2 of this series, necessarily forms the basis for certain of the conclusions drawn, especially those just enumerated. The steeply peaked pulse profile implies large crest electric fields due to the Hall effect. Acceleration of charged particles is suggested and is in consonance with a cosmic-ray view of this region of space on Pioneer 4 and the theory of Morawetz. The conclusion that the pulses are primarily magnetoacoustic with a preponderance of propagation vectors directed earthward is in keeping with the conclusion in part 1 that the hydromagnetic excitation mechanism was a source function of large scale with respect to the magnetosphere. The existence of a copious supply of field reversals always followed by a pulse lends support to a model in which hypersonic gas bubbles are propagating into the vestigial field, inertial reaction creating the pulse and a condition of minimum field potential energy being achieved by twisting of tubes of force on the upstream boundary (magnetopause model). An alternative model implies a large bowlike shock wave, the shock microstructure being fine-scale collisionless pulse phenomena like those suggested in the type of wave studied by Davis, Lust, and Schluter. This last model, however, requires the generation of an exotic ad hoc mechanism to provide plasma instability leading to the X-type nulls often associated with the waves. The termination of activity at about 14 Re would suggest magnetic termination on the leading edge of the macroscopic shock. The identical termination distance on Pioneer 4 with cosmic-ray pulses seen as far into the field as 10 Re where the dipole magnitude becomes appreciable favors the magnetopause model over any simple structure such as a detached bow shock.