Observations of ULF electric field fluctuations in the dayside auroral oval

Electric field oscillations at frequencies centered below 10 Hz were measured by a payload equipped with double-probe electric field detectors which was launched into the dayside auroral oval during the Greenland rocket campaign on January 11, 1975. These oscillations exceeded an amplitude of 3 mV/m broadband and occurred over a period of about 90 s. These oscillations occurred during a proton injection event when the quasi-static electric field was less than 20 mV/m on the average. However, during a brief 2-s interval the electric field exceeded 50 mV/m. Analysis of the amplitude as a function of the spin frequency showed that the electric field was confined to a plane perpendicular to the magnetic field. The power spectrum was of the form f to the -6.5 + or - 1.5 power, where f is the frequency over the range from 8 to 18 Hz. The existence of a large electric field fluctuation in a spatially or temporally confined region of low-amplitude turbulence suggests that this may be an observation of the low-altitude projection of a turbulent region associated with electrostatic shocks which have recently been observed at higher altitudes.