Spectral studies of the sources of ionospheric electric fields

Abstract

Spectral analyses have been performed upon a number of incoherent scatter radar data sets obtained at Jicamarca, Peru; Chatanika, Alaska; and Arecibo, Puerto Rico, with the goal of understanding the sources of electric fields with periods in the range of 1–10 hours. Two distinct sources are identified and studied in some detail. In quiet times, atmospheric gravity waves seem the most likely source of the ionospheric electric field. In fact, both in an average sense and in the single case study available the mesospheric winds measured at Poker Flat, Alaska, in this frequency range are remarkably similar in magnitude to the quiet time thermospheric plasma drifts measured overhead by the nearby Chatanika radar. Such drifts are driven by electric fields which, we argue, could easily be generated by the observed wind fields. Comparison with the spectra of electric field measurements at other latitudes suggests that such a source is worldwide and determines the geophysical noise level of low‐ and mid‐latitude electric field measurements. Turning to active times, we present a measure of the transfer function for electric field penetration between high‐ and low‐altitude L shells. At the very lowest frequencies (periods of ≥10 hours) the low‐altitude sites are well shielded, presumably by an Alfvén layer at the inner edge of the ring current. Higher frequency fluctuations penetrate very easily to low latitudes. A response peak seems to occur in the 3‐ to 5‐hour range of periods, with a lower response occurring at 1 cycle/hour, although this result must be viewed as preliminary for now. Between L = 5.5 and L = 1.4 the zonal electric field component as projected to the equatorial plane of the magnetosphere penetrates with little or no attenuation. At Jicamarca the field is reduced to about 75% of the high‐latitude source. Even so, the very high sensitivity of the Jicamarca measurement of zonal electric fields makes this component very easy to detect. We find that for fluctuation periods less than 5 hours the magnetospheric electric field source dominates atmospheric sources for Kp ≥ 3 at Jicamarca. In one remarkable event, six cycles of a 1‐ cycle/hour signal were recorded with nearly identical spectral features in the interplanetary medium, at auroral zone latitudes, and on both sides of the earth at equatorial ionospheric heights. The ratio of the relative magnitude of this oscillation at L = 5.5 to its value at L = 1.04 is in good agreement with the value of the average transfer function at 1 cycle/hour found in the 24‐hour runs.