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Journal of Geophysical Research: Space Physics

Electric field measurements at subcritical, oblique bow shock crossings

Authors

  • J. R. Wygant,

  • M. Bensadoun,

  • F. S. Mozer


Abstract

Electric field measurements at oblique, subcritical bow shock crossings are presented from the ISEE 1 University of California, Berkeley, double-probe electric field experiment. The measurements averaged over the 3-s spin period of the spacecraft provide the first observations of the large-scale (100 km) laminar oscillations in the longitudinal component of the electric field associated with the whistler precursor which is characteristic of these dispersive shocks. The amplitude of the oscillations increases from ∼0.5 mV/m to a maximum of 6 mV/m across the magnetic ramp of the shock (directed along the shock normal). The calculated electric potential drops across the shocks varied from 340 to 550 volts, which is 40–60% of the observed loss of kinetic energy associated with the bulk flow of the ions. These measurements suggest that at these shocks the additional deceleration of incident ions is due to the Lorentz force. The contributions to the normal component of the large-scale electric field at the shock due to the parallel and perpendicular components (relative to the magnetic field) of the electric field are evaluated. It is shown that the perpendicular component of the electric field dominates, accounting for most of the cross-shock potential, but that there is a nonnegligible parallel component. This large-scale parallel component has a magnitude of 1–2 mV/m which sometimes results in a potential well for electrons with a depth of ≈150 eV. It is experimentally demonstrated that the dominance of the perpendicular over the parallel component of the electric field resulted in a correlation between the longitudinal component of the large-scale electric field and the fluctuations in the magnetic field component perpendicular to the coplanarity plane. This paper also presents, for the first time, high time resolution (dc to 32 Hz) measurements of the electric field waveform in the current layer of a collisionless shock. The measurements in this region reveal the presence of intense electric field spikes with amplitudes ranging up to 100 mV/m lasting about 0.1 s superimposed on the large-scale laminar electric field wave train described earlier. The period of these spikes is consistent with lower hybrid or ion acoustic waves which have been strongly Doppler shifted as a consequence of the solar wind motion. These electric field structures can have significant components along the shock normal and may play a variety of roles in the microphysics of shock dissipation.

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