Energetic Electron Distribution Function Research Articles

A study of the interaction of VLF waves with equatorial electrons and its relationship to auroral X-rays in the morning sector

Precipitation of electrons in the morning sector of the auroral zone related to magnetospheric substorm activity has been observed via X-ray measurements from balloons. Coordinated observations within the IMS period both aboard balloons and on the geostationary satellite GEOS-2, provided detailed information for the study of the relation between the distribution function of energetic electrons at GEOS 2 and the precipitation at and near the magnetically conjugate point in the auroral zone. The anisotropy of the electron distribution function is determined by the injection processes and by further modifications, e.g. convection and drift. It is large enough to allow the growth of VLF waves responsible for scattering of electrons into the loss cone.

Low‐altitude ISIS 1 observations of auroral radio emissions and their significance to the cyclotron maser instability

Low‐altitude ISIS 1 observations are presented which support the cyclotron maser instability as the generation mechanism of most, but not all, auroral radio emissions. The observations confirm the ability of the mechanism to generate different wave modes and harmonics as the ratio of the plasma frequency to gyrofrequency ƒN/ƒH increases depending on the form of the energetic electron distribution function. The present observations correspond to conditions where ƒN/ƒH is always larger than 0.2 and is typically ∼1.0. The observations and instability growth rate calculations indicate that second harmonic radiation in the extraordinary and the Z modes is directly generated at low altitudes. By far the most common signals observed near ISIS 1 perigee, however, are in the fundamental Z and whistler modes. The observations and calculations in the case of the Z mode suggest that the pertinent process involves a maser that does not saturate, i.e., that group velocity effects are important. The whistler mode is the most intense signal observed at low altitudes. Since the cyclotron maser generated whistler mode waves have peak growth rates for downward field‐aligned propagation, they should be able to penetrate the ionosphere. This process may provide the explanation of occasional reports, dating back to the International Geophysical Year time period, of ground level detection of radio noise (∼100 kHz) associated with the aurora.

A theory of diffuse emission bursts triggered by coherent VLF signals propagating parallel to the geomagnetic field

Satellite and ground‐based studies have demonstrated that coherent VLF signals propagating in the terrestrial magnetosphere can trigger diffuse and structured emission bursts. A triggering mechanism for diffuse emissions which may operate when the coherent signal propagates parallel to the geomagnetic field is investigated in this note. The theory is based on the concept that trapping of energetic particles in the potential wells of the coherent signal alters the gradient of the particle distribution function with respect to parallel velocity. If, as a result of trapping, the gradient becomes sufficiently steep, broad band wave growth takes place as predicted by established theory. The resulting pitch angle diffusion of particles produces a more stable configuration in which the steep gradient is reduced towards that corresponding to the marginally stable distribution. For initially positive gradients in the energetic electron distribution function with respect to parallel velocity, wave growth takes place above the frequency of the causative coherent signal. The theory predicts that the bandwidth of emissions triggered in this way should rapidly increase with time and may reach maximum widths of several hundred hertz.

Auroral kilometric radiation/aurora correlation

Auroral kilometric radiation (AKR) observations from the ISIS 1 topside sounder receiver were compared with visual auroral observations from the network of Alaskan all‐sky camera stations. The goal was to relate AKR source region encounters to specific auroral forms on the same magnetic field line. Thirty‐eight simultaneous data sets were identified and analyzed. In general, intense AKR was associated with bright auroral arcs, and conditions of weak or no AKR corresponded to times when either no aurora or only a faint arc or weak diffuse aurora was observed. Five cases, when both intense AKR and bright visual aurora were present, were analyzed in detail. Complete electron density Ne contours, from the satellite altitude down to the F region ionization peak, were obtained along N‐S traversals of the AKR source region. In addition, the ISIS 1 orbital tracks were projected down the magnetic field lines to the auroral altitude and compared to auroral features on a map derived from the all‐sky camera images. Density cavities (regions where Ne < 100 cm−3) were encountered on each of these passes. In one case the cavity extended in altitude down to just below 2000 km. The latitudinal extent of these cavities ranged from 3° to almost 30°. A variety of situations relating AKR to the visual aurora were identified, and both spatial (within about 100 km at the satellite altitude) and temporal (within about l min) correlations were identified. In addition to confirming that a density cavity is a necessary but not sufficient condition for the generation of AKR, the present results indicate that even the combination of a density cavity on an auroral‐arc field line is not always sufficient for AKR generation. Thus additional constraints must be satisfied in order to generate intense AKR from density cavities. These constraints most likely involve the characteristics of the energetic electron distribution function and possibly also latitudinal Ne gradients.

Amplification of radiation near cyclotron frequency due to electron population inversion

Amplification of electromagnetic waves via the cyclotron maser mechanism by a population of weakly relativistic electrons is studied. The effect of a tenuous population of low energy background plasma is included. It is found that both the ordinary and extraordinary modes can be excited by the weakly relativistic electrons with a loss-cone distribution. The growth rate for the extraordinary mode is much higher than that for the ordinary mode. Velocity spread in the energetic electron distribution function may reduce the growth rate by a factor of approximately 10 from that in the monoenergetic case. The maximum growth rate for the fast extraordinary mode (X mode) occurs near the upper hybrid cutoff frequency. Numerical results are obtained and discussed.

Negative Absorption and Energy Distribution of Hot Electrons in Relativistic Magnetized Plasma

Negative absorption for extraordinary waves by the weakly relativistic electrons with the energy less than 20 keV gyrating in a tenuous plasma confined in a mirror field is studied by the microwave transmission method. Typical amplification factor of approximately 30 dB for the input power of a few milliwatts is observed during both heating period and after-glow. The experimental results are best explained by the relativistic mass effect obtained by Bekefi et al. for the energetic electron plasmas. Further the formation and relaxation of the distribution function of the energetic electrons are studied through the analysis of the temporal variation of the negative absorption. New method of microwave diagnostics for the determination of energy distribution function of energetic electrons in the tenuous magnetized plasma is proposed.