Bela G. Fejer

The equatorial ionospheric electric fields: A review

Abstract The equatorial electric fields have been extensively studied over the last decade using radio, radar and rocket observations. Radar measurements of vertical and east-west F -region drift velocities at Jicamarca have determined the seasonal and solar cycle variations of the F -region east-west and vertical electric fields, respectively. Rocket observations during twilight indicate the existence of a shear in the east-west drifts with eastward velocities near and above the F -region density peak and westward velocities in the lower F -region where incoherent scatter radar observations are not effective. The vertical drift (i.e. east-west electric field) changes considerably with magnetic activity, but the F -region east-west drift (vertical electric field) is not affected. During magnetically disturbed periods the zonal equatorial electric field shows frequent and large fluctuations indicating the penetration of magnetospheric electric fields into the low-latitude ionosphere. The coupling processes are not yet totally clear. Considerable progress has been made recently in the understanding of these quiet-time and disturbed equatorial electric fields.

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere. During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromso, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (< 1 min) electron heating events during which the temperature suddenly increased by a factor of 2–4 at altitudes near 108 km and the electron densities were less than 7 × 104 cm−3. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.

High Latitude E-Region Irregularities: New Results

Radar observations have shown that the phase velocity of small scale (a few meters and smaller) auroral two stream waves remains close to the ion acoustic speed. However, during periods of very large plasma drifts the electron temperature, and consequently, the ion acoustic speed in the auroral electrojet region is enhanced possibly by strongly driven two stream waves. Recent nonlinear theories can explain the large anomalous electron heating as well as the variation of the two stream phase velocity. However, the observations of plasma waves at angles larger than a few degrees from the perpendicular to the magnetic field remain unexplained. Radar interferometer measurements show that during very active periods strong echoes are often observed from highly localized and dynamic scattering regions. On these occasions, the two stream waves have phase velocities which can exceed 900 m/s, and the gradient drift waves have very broad spectra with mean phase velocities sometimes larger than 600 m/s. Furthermore, resonant (i.e., sharply peaked) auroral radar echoes, with Doppler shifts independent of the ambient cross field plasma drift are often present near the edges of auroral arcs. These waves have been associated with ion cyclotron waves driven by very large field aligned drifts in the upper E-region. This paper will review recent experimental results on the study of meter scale plasma waves in the auroral E-region, particularly during highly disturbed periods, and will also discuss the physical processes responsible for their generation and saturation.

Comparative in situ studies of the unstable day-time equatorial E-region

Abstract Plasma waves measured by probes on sounding rockets are used to characterize the unstable equatorial E-region and to reflect the changing irregularity morphology with respect to altitude within the layer. We present measurements from three sounding rockets launched at the geomagnetic equator from Punta Lobos. Peru, including both a strong and a mild electrojet experiment conducted at midday and a weak electrojet experiment conducted in the late afternoon. The observed irregularities are analyzed in relation to simultaneous measurements of the electron number density and to either the measured or inferred profiles of the electron current density. The linear growth rate for the combined two-stream and gradient drift instabilities is computed using these profiles and the changing unstable wavenumber regimes are then compared to the power spectra of the wave observations as a function of altitude. We have allowed for long wavelength waves in the growth rate and have included the effect of recombination. In each case, the waves are observed only in the altitude regions, which, on the basis of the growth rate, are predicted to be unstable for horizontally propagating waves. Further, although the conversion of observed frequency to wavenumber is not definitive, the theoretical range of wavenumbers that will be unstable agrees at least qualitatively with the corresponding frequencies which have associated fluctuations displayed in spectrograms of the in situ time series measurements. In the case of the strong electrojet experiment, both the growth rate calculations and the wave observations show a region of high frequency (short wavelength) oscillations in the upper portion of the layer, where the medium was unstable to the primary two-stream instability. In the mild electrojet experiment, current measurements show that the two-stream threshold was not met, which is supported by the absence of observed high frequency oscillations for this flight. The medium may still have been unstable to this process as a secondary mechanism, as suggested in the 3 m backscatter radar data, implying that the large scale wave electric fields were on the order of or greater than, the polarization electric field. Where the payloads encountered a positive (upwards) gradient in electron density, all three rockets show a strong low frequency component which we attribute, in general, to the gradient drift instability.

Evidence of internal gravity waves in the lower ionosphere

Abstract Temperature and wind measurements extending up to 95 km have been made with rocket-grenade experiments at Natal, Brasil. On many occasions the temperature and wind speed above 60 km show uniformly spaced maxima and minima. In one series of these experiments four rockets were launched during a period of 18 hr. A comparison of these spaced observations gives an indication of the propagating character of the maxima and minima and also suggests a downward phase propagation. The perturbations in the temperature and the wind speed are similar in form but they appear to differ in phase. The wavelike appearance of the temperature profiles is believed to be caused by the adiabatic heating and cooling associated with propagating gravity waves. The wavelength obtained from these observations is 10–12 km which coincides with the expected vertical wavelength of 12 km for the dominant gravity wave in this altitude range. The observed temperature variations are also in agreement with the computed values for the prevailing conditions.