J. Providakes

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.

Large amplitude quasi-periodic fluctuations associated with a mid-latitude sporadic E layer

Abstract For the first time a sounding rocket has been launched into a mid-latitude sporadic E event which was shown to be the source of VHF radar echoes. The layer had a very high peak electron density (∼10 6 cm −3 ) and was thicker (∼5 km) than most events previously studied by rockets and incoherent scatter radars. The layer was modulated in a remarkable quasi-periodic manner which has not been reported earlier. Twenty cycles of these structures were detected and they seem to be oriented horizontally rather than vertically with periods in the rocket frame in the rage 6–10 s. There is also some evidence that the modulation was detected below as well as above the peak in the electron density, although the bulk of the flight was above the peak. Although the VHF radar echoes were decaying at the time and place where the rocket traversed the E layer, one burst of high amplitude short wavelength fluctuations was detected by the space-borne instruments and had a power spectrum similar to that of a secondary gradient drift mode. This burst occurred at the peak of one of the periodic electron density fluctuations. We discuss two possible sources for the dominant fluctuations: large-scale gradient drift waves and atmospheric acoustic waves. The latter seem most consistent with the data.

Interpretation of the electric fields measured in an ionospheric critical ionization velocity experiment

This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV⋕ mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.

The E-region rocket/radar instability study (ERRRIS): scientific objectives and campaign overview

Abstract The E -region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude ( E -region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn / n ) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons), ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

The altitude of type 3 auroral irregularities: Radar interferometer observations and implications

VHF coherent scatter radars at auroral latitudes have observed scatterers with narrow power spectra at sub-ion acoustic mean Doppler shifts. These spectra have been designated type 3. The mean Doppler shift of these waves is often near the atomic (O{sup +}) or, less frequently, the molecular (O{sub 2}{sup +} and NO{sup +}) gyro frequencies. These type 3 echoes have been attributed to an electrostatic ion-cyclotron (EIC) instability in the upper E region (h > 140 km), where the ion collision frequency becomes low enough to permit ion gyromotion. Interferometric analysis of recent coherent radar observations with the CUPRI (Cornell University portable radar interferometer) shows that type 3 events occur at typical electrojet altitudes (100-120 km), however. At these altitudes the ion collision frequency is greater than the ion gyrofrequency and there can be no cyclotron motion. The cause of the observed type 3 echoes hence remains a mystery.

Particle and fluid simulations of resistive current‐driven electrostatic ion cyclotron waves

The results from 1‐D numerical simulations of electrostatic ion cyclotron waves (EIC) are presented for a model in which the electrons are a resistive (collisional) fluid. Simulations of both the kinetic and fluid descriptions are performed and compared in order to assess the fundamental limitations of fluid theory for EIC waves. The effect of ion–neutral collisions is also included using a simple Monte Carlo technique. It is found that a small ion–neutral collision frequency destroys the frequency harmonic coupling of kinetic EIC waves and tends to validate the fluid description. The saturation amplitude of the current driven EIC instability is in agreement with recent laboratory experiments. The coherent nature (extremely narrow spectral width) and phase velocity agree with ground based (coherent backscatter radars) and in situ observations of current‐driven EIC waves in the high latitude ionosphere.

Observations of 3-m auroral irregularities during the ERRRIS campaigns

Abstract In the late winter of 1988 and 1989, three NASA sounding rockets were flown through the auroral electrojet from ESRANGE (Sweden) as part of the E -region Rocket-Radar Instability Study (ERRIS). Many ground-based instruments supported these flights, including the EISCAT, STARE, and CUPRI radars, as well as all-sky cameras, riometers, and magnetometers. In this paper we summarize the observations of the Cornell University Portable Radar Interferometer (CUPRI), which detected coherent backscatter from 3-m irregularities in the auroral E -region. Twenty hours of power spectra and interferometry data are available, and, during the 1989 campaign, three weeks of nearly continuous Range-Time-Intensity (RTI) and first moment data were recorded.

HF radar probing of the lower magnetosphere

The first observations of ion acoustic waves at altitudes of about 4000 km in the auroral zone were carried out using the powerful SURA HF radar in 1991. Here we report additional observations made with the Russian SURA radar and the Ukrainian UTR 2 receiving system in December 1992. Although echoes similar to the earlier observations were detected, we performed a number of diagnostics and have concluded that no unambiguous magnetospheric echoes were seen at SURA during the 1992 campaign. (There is some indication that the more sensitive UTR 2 system received magnetospheric echoes.) Some aspects of the coding scheme used in the experiments that might lead to spectral artifacts are discussed. A new antenna under construction at SURA will allow transmission at twice the frequency used to date and should permit more definitive observations in the future.

Ion cyclotron harmonics in auroral radar echoes: Real effect or analysis artifact?

Certain analyses of Canadian radar studies of auroral electrojet echoes have apparently shown spectra with multiple ion cyclotron harmonic peaks. On the basis of these, it has been argued that ion cyclotron effects must be important for all echo types seen by VHF radars, not just type 3. We reject this conclusion for two reasons: (1) at the altitude of auroral backscatter (∼ 100–120 km) the ion-neutral collision frequency is much higher than the gyro frequency—ions are essentially unmagnetized; (2) we show that the “high resolution” signal processing algorithm employed does not distinguish between echoes with and without harmonic structure. With a simulation, we show that the algorithm itself can impose apparent quasi harmonic structure onto the spectrum of random noise.