A satellite survey of vector electric fields in the ionosphere at frequencies of 10 to 500 Hertz: 1. Isotropic, high-latitude electrostatic emissions

Abstract

Vector electric fields in the frequency range from 10 to 500 Hz have been measured on the 400-km altitude polar orbiting OV1-17 satellite. The detector had a broad band threshold level measured in flight to be 20 μv/m; it was more sensitive to ionospheric electromagnetic waves below 80 Hz than presently available search coil magnetometers and was free from in-orbit interference. Two distinct fluctuation modes have been observed on every orbit to be characteristic of the ionosphere at 400 km. The first of these modes, discussed in detail in this paper, has an electric field vector in the satellite frame of reference that is isotropic, that has a power law spectrum with an exponent of −0.73 ± 0.1, and that has a peak intensity at 12 Hz as great as 1 mv/m Hz1/2. This mode is observed throughout the high-latitude region between two sharp boundaries located at about 55° invariant latitude on the nightside and 75° on the dayside. The observed boundary for low-frequency electric field fluctuations is thus not coincident with the auroral oval at all local times, as had been reported earlier. These properties and additional information on the wavelength spectrum of the fluctuations are consistent with interpretation as electrostatic disturbances whose spatial variations along the trajectory were measured. The observation of strong electric field fluctuations parallel to the earth's magnetic field B0 implies that ionospheric plasma physics cannot be described by first-order magnetohydrodynamic theory in which the parallel electric field is identically equal to zero. If the fluctuations are due to waves, then under certain assumptions on the wave frequency, the signal is intense enough to cause wave-particle interactions which reduce the plasma conductivity parallel to B0 sufficiently to support parallel dc electric fields of the order of 10 mv/m. The region of occurrence of the turbulent electric field is in good agreement with that of ionospheric plasma turbulence as deduced from the scintillations of radio stars and measurements of spread F.