Plasma particles drifting in the equatorial plane of quantitative magnetospheric model and related magnetospheric phenomena

Using the quantitative magnetospheric model derived by MEAD and FAIRFIELD (J. Geophys. Res., 80,523,1975) from satellite observations of the magnetic field, we have computed equatorial profiles of the total geomagnetic field, electric equipotential lines and drift paths of plasma particles with pitch angle of 90° in the magnetosphere under the superquiet (SQ) and superdisturbed (SD) conditions in case of the magnetospheric tilt angle of 0°. The uniform dawn to dusk electric field used is 0.1mV/m for a quiet time and 0.4mV/m for a disturbed time. All particles start from geocentric circles of 17 earth\u27s radii (R_E) in the equatorial plane of the tail region. Electric equipotential lines in the equatorial plane of the Mead-Fairfield (MF) model are concave on the dawn and dusk sides since the geomagnetic field lines of the MF model in the equatorial plane are curved greatly tailwards on the dawn and dusk sides compared with the radial field lines of a dipole model. The Alfven layer or the boundary of the forbidden region for the zero-energy particles computed in the MF magnetospheric model is compared with the average location of the plasmapause in the equatorial plane. The computed drift paths of zero-energy particles for the uniform dawn to dusk electric field of 0.1mV/m show a stagnation region in the late evening sector which agrees well with the plasmapause bulge observed by ground whistlers. The trapped particle region for zero-energy particles and energetic electrons in the late-evening outer magnetosphere seems to be produced by the particle drift motion in the late-evening outer magnetosphere, where the geomagnetic field lines are greatly curved tailwards under a weak dawn to dusk electric field of 0.1mV/m. The dayside extent of equatorial drift paths for electrons with 0.5keV/nT in the MF-SQ and MF-SD geomagnetic fields for the dawn to dusk electric field of 0.4mV/m corresponds well with the hard electron precipitation region associated with the active mantle aurora

Some characteristics and propagation of whistler-mode LF/MF emissions as observed at low altitudes above the Antarctic region with ISIS-1

The broadband whistler-mode (w-mode) emissions in LF/MF bands observed at low altitudes in the nighttime auroral zone are examined in detail, especially from the viewpoint of the spectral characteristics and directions of wave vectors of the emissions using the ISIS-1 data including the fixed and swept frequency ionograms and AGC voltage traces of the sounder receiver. The emissions are characterized by spectra with relatively sharp upper cutoff ranging 0.75 to 0.4f_H (f_H : electron gyrofrequency) and with lower frequency cutoff less than 0.1 MHz (the lowest frequency of the sounder) for many cases. The directions of wave vectors at the satellite are determined from the antenna orientation at the instant when the emissions exhibit intensity minima on AGC voltage traces in the fixed frequency mode. The resultant directions of wave vectors are used as initial conditions for ray tracing to locate the source regions and to know the directions of wave vectors at the source region. It is shown that the cyclotron maser instability driven by auroral electron with upward loss cone at low altitudes is the most probable source mechanism which explains some characteristics of the observed emissions, particularly, of their high frequency portion

Efficient coding and resonance spike identification for topside ionogram processing

This paper describes an effective coding method to eliminate the redundancy contained in digital ionograms and an algorithm to identify the resonance spikes appearing on topside ionograms. This work is a first step toward automatic profile reduction to obtain ionospheric electron density profiles. Topside sounder data recorded by ISIS-2 satellite are digitized and converted to digital ionograms. A quantitative comparison of data compression techniques, based on the run-length and predictive coding method, is made and leads the conclusion that the modified run-length coding method is most effective and useful from the practical view point. This simulation experiment results in self-consistent determination of characteristic frequencies with good accuracy except for the ionograms with obscure resonance spikes

Spectral characteristics of radio noise at low and medium frequencies in the Antarctic topside ionosphere

The ISIS topside sounder data obtained at Syowa Sation, Antarctica, for the period from April 1976 to November 1977 are examined with emphasis on the noise spectra appearing on the Automatic Gain Control (AGC) data and on the ionograms. The noise events were observed on 16 out of 88 ISIS-1 passes and on 8 out of 138 ISIS-2 passes. At high altitudes near ISIS-1 apogee, almost all of the noise events are due to auroral kilometric radiations (AKR). A special event of AKR observed in the dayside ionosphere is investigated in detail. The result shows that this cusp-associated AKR occurred in a large scale region of electron density depletion where the ratio of electron plasma frequency, f_N to electron gyrofrequency, f_H ranges from 0.1 to 0.05. At altitudes below 2900km, two types of noise were observed; the whistler mode noise and the noise band appearing between the local f_N and f_T (upper hybrid resonance frequency). These noises are examined in connection with the local characteristic frequencies. The dependence of these noise intensities on the relationship between f_N and f_H is found to be in a qualitative agreement to Maggs\u27 power flux calculation of the electrostatic noise using the plausible auroral electron beam models