J. LaBelle

Plasma waves at the dayside magnetopause

Experimental investigations of plasma waves at the magnetopause, including recent results from the AMPTE/IRM satellite, show that both δE and δB fluctuations typically have a featureless spectrum which monotonically decreases with frequency; integrated rms amplitudes are typically a few mV m-1 for δE and 10 nT for δB, though in particular δE can be as much as an order of magnitude larger in exceptional cases. Surveys show a lack of correlation between wave parameters and the magnetopause parameters. Under the assumption that crossing the diffusion region would give a pronounced signature in the waves, the survey data allow an upper limit to be placed on the latitudinal extent of the diffusion region, which is about 1000 km — implying that it is not surprising that the wave data surveys have so far failed to detect it. The observed wave turbulence levels have been used to estimate diffusion coefficients under different assumptions for the wave mode, but the resulting diffusion coefficient is always too small to explain either reconnection or boundary layer formation. Recent work of Galeev et al. (1986) indicates that the dominant diffusion process may be ‘magnetic field migration’, which is a macroscopic process involving the interaction of tearing mode islands. Assuming this mode to be present at the observed level of δB, a particle diffusion coefficient of nearly 109 m2 s-1 is obtained. Another macroscopic diffusive process which could occur at the magnetopause is stochastic E × B scattering, which also implies a diffusion coefficient the order of 109 m2 s-1 if the observed δE spectrum is assumed to be a turbulent cascade consisting of convective cells.

Auroral Radio Emissions, 1. Hisses, Roars, and Bursts

The Earths auroral electrons produce copious non-thermal radio emissions of various types, including auroral kilometric radiation (AKR), whistler mode auroral hiss, mode conversion radiation such as auroral roar and MF-burst, and possibly HF/VHF emissions. In some cases, mechanisms have been identified and quantitatively described, whereby the energy of the auroral electrons is converted into electromagnetic radiation. In many other cases, the radiation mechanism, or the relative significance of several possible mechanisms, remains uncertain. This review covers fairly comprehensively experimental and theoretical research on types of auroral radiation other than AKR, concentrating on emissions with frequency higher than about 1 kHz and treating only emissions which are unique to the auroral zone. The review covers both ground-based and in-situ observations. It covers a wide range of theoretical approaches, emphasizing those which at present appear most important for producing non-AKR auroral radiations.

Low-frequency fluctuations in the magnetosheath near the magnetopause

There are four low-frequency modes which may propagate in a high-beta nearly bi-Maxwellian plasma. These are the magnetosonic, Alfven, ion acoustic, and mirror modes. This manuscript defines a procedure based on linear Vlasov theory for the unique identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode deviation, is defined which characterizes the difference between the theoretical transport ratios of a particular mode and the observed ratios. The mode deviation is calculated using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module spacecraft to identify the modes observed in the terrestrial magnetosheath near the magnetopause. As well as determining the mode which best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique. Using 17 time periods temporally close to a magnetopause crossing, and confining our study to the frequency range from 0.01 to 0.04 Hz, we find that the only clearly identified mode in this frequency range is the mirror mode. Most commonly, the quasi-perpendicular mirror mode (with wave vector k roughly perpendicular to the background magnetic field B0) is observed. In two events the quasi-parallel mirror mode (k ∥ B0) was identified.

High resolution OI (630 nm) image measurements of F‐region depletion drifts during the Guará Campaign

A high performance, all-sky, imaging system has provided data on the evolution and drift motions of F-region depletions above the magnetic dip equator at Alcântara, Brazil, (2.3°S, 44.5°W). Monochromatic images of depletions in the OI(630 nm) nightglow were recorded on eight nights during 1-16 October, 1994, as part of the Guara campaign. The drift motions of the depletions were typically 80–100 m/s eastward prior to local midnight and reduced to a minimum of ∼30–50 m/s in the morning hours, in accord with previous observations. However, on October 2–3 and 12–13 the depletions were observed to reverse direction for ∼60–90 min, achieving westward speeds of ∼30 m/s before the motion reverted to eastward around 0100 LT and accelerated to 35–45 m/s near dawn. Magnetic activity and other evidence suggests that these reversals in the motion of the airglow depletions probably result from reversals in the F-region dynamo rather than from shifts in the altitude of the shear in the nighttime F-region plasma drift.

The polarization of auroral radio emissions

Ground level observations using two vertical loop antennas oriented at 90° to each other reveal the sense of polarization of several types of auroral radio emissions in the frequency range 30-5000 kHz. Auroral hiss is observed to be right elliptically polarized (RP) with respect to the local magnetic field, consistent with theoretical expectation for the whistler mode and with earlier measurements. Two less well-understood auroral emissions, are found to be left elliptically polarized (LP). This polarization is inconsistent with their generation in the X-mode as suggested by some theories.

Statistical and case studies of radio emissions observed near 2ƒce and 3ƒce in the auroral zone

Receivers located at Two Rivers, Alaska (64.9°N, 146.9°W), and Circle Hot Springs, Alaska (65.5°N, 144.7°W), have been used to monitor the spectrum between 0.05-4.8 MHz for extended periods. Seasonal and diurnal effects of auroral roar, a weak narrow band radio emission near 2 and 3 times the ionospheric electron cyclotron frequency (ƒce), have been determined. Many individual auroral roar events correlate with magnetic activity, and superposed epoch analysis using planetary K indices shows a correlation between magnetic activity and auroral roar commencement. For a 3ƒce roar event on March 23, 1992, riometer, magnetometer, photometer, and all-sky camera data are available and show that individual bursts of 3ƒce auroral roar are associated with intensifications of the aurora, as was known previously for 2ƒce roar. Finally, two possible generation mechanisms are investigated in detail: nonlinear interaction between lower hybrid and electrostatic upper hybrid waves producing electromagnetic waves, and the feasibility of a lower ionospheric decameter maser for direct generation of X mode waves at harmonics of ƒce.

Plasma diffusion at the magnetopause? The case of lower hybrid drift waves

We have recalculated the diffusion expected from the quasi-linear theory of the lower hybrid drift instability at the Earths magnetopause. The resulting diffusion coefficient is marginally large enough to explain the thickness of the boundary layer under quiet conditions, based on observational upper limits for the wave intensities. Thus one possible model for the boundary layer could involve equilibrium between the diffusion arising from lower hybrid waves and various loss processes.

Propagation of medium frequency (1–4 MHz) auroral radio waves to the ground via the Z‐mode radio window

Recent ground-based observations of auroral radio waves have identified narrowband emissions near 2 and 3 times the lower ionospheric electron cyclotron frequency (fce) known as auroral roars. In this paper the propagation of these waves in the auroral ionosphere is investigated by means of a ray-tracing technique. We model one particular scenario in which a large-scale (tens of kilometers) horizontal density structure, based on density structures observed with the Sondrestrom radar at times of auroral roar emissions, plays a crucial role in both guiding the waves to the ground and enabling mode conversion. The location and the mode characteristics of the initial waves are determined on the basis of local stability properties, which suggests that Z-mode wave excitation is favored near 2fce. However, since Z-mode cannot propagate to the ground they must first undergo a mode conversion to one of the free-space modes (X and O). It is found that for a narrow range of frequencies and initial wave phase angles the trapped Z mode can be converted to O mode via the Ellis radio window. This finding is consistent with the fact that auroral roar emissions are nearly 100% O-mode polarized. However, it is important to note that the evaluation of the damping of the Z-mode waves along the ray path is not considered within the context of this preliminary study and will be critical for eventually determining the exact physical scenario of the auroral roar generation mechanism.

Lower ionospheric cyclotron maser theory: A possible source of 2ƒ ce and 3ƒ ce auroral radio emissions

A lower ionospheric electron cyclotron maser has recently been suggested as a possible source of natural radio emissions that have been observed with ground-based instruments in the auroral zone. According to this theory, the observed emissions are interpreted as X mode electromagnetic waves at 2 and/or 3 times the ionospheric electron cyclotron frequency (ƒce). In the present study we carry out a more thorough analysis of the cyclotron maser instability, using a more complete growth rate expression and more realistic energetic electron distribution functions. Growth rates of various electromagnetic modes and harmonics are calculated. It is shown that growth rates for 2ƒce and 3ƒce X modes can exceed collisional ionospheric damping rates, confirming the original estimations. However, the present analysis also reveals that O mode waves at the fundamental cyclotron frequency, ƒce, should also be excited at the source. The propagation of excited electromagnetic waves from the source region to the ground inside a model horizontal density cavity structure is also examined by means of ray tracing technique. It is found that the O mode wave is unable to reach the ground, an explanation of why the emissions at ƒce are not detected on the ground.

Rocket observations of banded structure in waves near the Langmuir frequency in the auroral ionosphere

Using data from the PHAZE II sounding rocket, launched from Poker Flat, Alaska, we present high-resolution observations of structure in auroral HF waves at and below the local plasma frequency. These observations were made in the altitude range of 390–945 km where the local plasma frequency is below the electron cyclotron frequency. We observe monochromatic, long-lived, narrowband emissions occuring below the local plasma frequency during times of intense HF wave emission. We have termed these emissions “HF bands” due to their appearance in spectrogram images. These emissions are probably identical to the “spike” emissions identified by previous observers using lower time resolution data from the AUREOL/ARGAD3 satellite which showed a narrow peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of intense Langmuir wave generation. We investigate the hypothesis that the HF bands are created when a Langmuir wave propagates from a low-density region into a higher density region. The wave moves onto the whistler mode branch and propagates as an HF band. Theoretical calculations of propagation times of whistler mode waves support this hypothesis.