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An approximate wave solution for the VLF modes near cut-off frequencies in the high-latitude Earth-ionosphere waveguide is obtained. The vertical inhomogeneity of the lower D-region of the ionosphere is included in the form of a simple exponential profile of the parallel conductivity. The E-region is represented as a magnetoactive plasma with a sharp bottom, at which the electron density increases strongly and then varies slowly at the scale of the local wavenumber. Approximate expressions are derived for the phase velocities and attenuation rates of the VLF modes near cut-off frequencies (⨍ c 1 ≈ 1.5–1.7 kHz, ⨍ cm ≈ m⨍ c 1 ,m = 1,2,… , in the night-time ionosphere conditions). They describe a strong dispersion of the QTE modes (left-hand polarized near corresponding cut-off frequencies). Taking into account the D-region vertical inhomogeneity results in a small strengthening of the QTE modes' dispersion, but it is in principle necessary for correct description of the VLF signal amplitude (in particular, tweek-atmospherics and whistlers with additional traces in the Earth-ionosphere waveguide).

The statistical dependence of the occurrence rate of polarization (left-handed, right-handed and linear) of Pc1 micropulsations on local time and wave frequency has been deduced on the basis of the data observed in Iceland and in Antarctica ( L = 6 ≈ 7) during the whole year of 1985. It is found that left-handed polarized pulsations tend to be most numerous until L.T. = 14:00 and at wave frequency of 0.4–0.6 Hz in Iceland, while the corresponding peak occurrence rate of right-handed polarization is found to take place at later L.T.s and at a frequency of 0.2–0.4 Hz in Iceland. The joint consideration of these statistical characteristics with the previous classification of Pcl micropulsations, enables us to study the mechanism of generation and propagation of different types of Pcl pulsations, and also simultaneity and conjugacy of polarization at different stations are discussed.

An analysis of the excitation of a night-time Earth-ionosphere waveguide near the cut-off frequencies of the waveguide modes (fc1 ∼ 1.5–1.7 kHz, fcm ∼ mc1) is presented as applicable to very unusual whistlers with additional dispersion. Additional dispersion of a whistler in a night-time Earth-ionosphere waveguide is caused by weakly-attenuated QTEm waveguide modes (left-hand polarized near respective cut-off frequencies), excited by the whistler inside the waveguide as a result of anisotropy of the ionosphere. Far from the corresponding cut-off frequencies fcm (separated by several hundred Hz), the attenuation of the first QTEm modes in mean and low latitudes is appreciably higher upon West-East propagation compared with East-West propagation. Close to the cut-off frequencies (f−fcm ⩽ 100 Hz), these modes have attenuation coefficient minima which exhibit a relatively weak dependence upon azimuth.

The polarization of irregular pulsations of diminishing period (IPDPs) observed in Iceland ( L = 6–7) and at Syowa has been investigated and some new findings have emerged: (1) the IPDP event which has so far been considered to be one single event, normally consists of a few Subelements (or slopes); and (2) in general the first major element occupying the lowest frequency range is left-handed circularly polarized and the second major element in the higher frequency range is right-handed polarized. A hypothesis is presented to account for these observational facts. The first fact is indicative of the nature of successive particle injections during a substorm. The protons with higher energy during the first injection seem to encounter the bulge of the plasmasphere or plasmatail in the local time (L.T.) sector of 15–18 h and at L = 6–8, which would result in the generation of ion cyclotron waves. This expectation is supported by observational fact, especially the left-handed circular polarization of the first element observed in Iceland. Furthermore, the delay time of drift of protons for the first injection in a time-dependent convection electric field is calculated, and is compared with the observed time delay and the slope of the fine structures. Then, if we suppose that protons with lower energy are injected during a subsequent burst, they meet the plasmapause body at L at L . T . > 18 h and, due to the ionospheric duct propagation over the great distance between the generation region and observing stations, the polarization in Iceland would be right-handed, which is consistent with observations.

The purpose of the present paper is to discuss the propagation mechanism of very unusual whistlers with additional traces of the Earth-ionosphere waveguide propagation observed simultaneously at Sakushima and Kagoshima on the basis of the simultaneous location of their causative atmospherics and their ionospheric exit regions in the Northern Hemisphere. This is the first simultaneous location of exit regions of whistlers and their causative atmospherics. The study has great potential in whistler propagation studies. It is found that the causative atmospherics of very unusual whistlers are located exactly at the duct entrance and are trapped in a magnetospheric duct in the local sunrise meridian. Hence the additional traces of the Earth-ionosphere waveguide propagation are concluded to be due to the subionospheric propagation after leaving the ionosphere. The longitudinal gradient of the ionosphere at the duct exit enables the whistler wave energy to be concentrated in a restricted sector in azimuth (in this case East to West) and to be received at our stations. Wave emergence from the ionosphere over a wide exit angle, together with the extremely small divergence loss, results in an efficient wave interference. This leads to the formation of the consplcuous traces of the first and second order mode cut-offs of very unusual whistlers. On the other hand, atmospherics not accompanying any whistlers are estimated to be located far from the duct entrance.

The GEOS l satellite wave data have been used to determine the wave normal directions of chorus emissions in the off-equatorial region (geomagnetic latitude ∼ 17) of the outer magnetosphere based on the direction finding of the wave distribution function method. The local time and L shell of the data analysed are L.T. ≅ 10 h and L ∼ 7.60, respectively, and only one event is extensively studied. There are two frequency bands of emissions; one above and the other below one half the electron gyrofrequency, but the latter is the dominant emission and is investigated here. Two types of chorus are dealt with ; rising tones and impulsive ones. About 90% of chorus emissions analysed are found to be composed of a single plane wave, which is in contrast to plasmasphcric hiss. There is no conspicuous relationship between θ (the angle of wave normal direction with respect to the Earth's magnetic field) and frequency for rising tones, and their θs take larger values in a range of 30°–55, whereas a tendency seems to appear, for an impulsive charus. for waves at higher frequencies to travel at a larger angle to the field. The wave normal directions observed have been discussed as functions of wave frequency normalized by the local electron gyrofrequency and the slope of an emission, d ƒ d t . A comparison of the present results on the off-equatorial wave normal directions with the previous equatorial direction finding results have made it possible to discuss the generation and propagation mechanism of chorus emissions with different structures including rising tones, falling tones, constant frequency tones and impulsive (burstlike) ones, and finally we have pointed out future work for investigation.

Simultaneous direction finding measurements of equatorial latitude whistlers have been performed in January 1988 at three stations in China; Zhanjiang (geomagnetic latitude 10.0°N), Guilin (14.1°) and Wuchang (19.4°). The preliminary analyses for a particular day have yielded experimentally that there is a single-valued dispersion value (D = 10.5 s12) for night-time whistlers at the three stations and that their ionospheric exit region is located just in the zenith of Zhanjiang, with approximately a right-handed circular polarization. This finding lends strong evidence to our previous implication inferred from the simultaneous multi-station network that there is a preferred propagation channel at a geomagnetic latitude around 10°.

The purpose of the paper is to present the statistical characterictics of mid-latitude VLF emissions (both unstructured hiss and structured emissions) based on the VLF data obtained at Moshiri in Japan (geomag. lat. 35°; L = 1.6) during the period January 1974–March 1984. Local time dependence of occurrence rate and the association with geomagnetic disturbances have been studied for both types of emissions. Both types (unstructured and structured) of mid-latitude VLF emissions are found to have definite correlations with geomagnetic disturbances. Then, the time delay of the emission event behind the associated geomagnetic disturbance has enabled us to estimate the resonant electron energy for VLF hiss to be ∼ 5 keV at L = 3–4 and that for structured VLF emissions to be considerably larger, such as ∼ 20 keV at L ∼ 4. Combined considerations of these estimated resonant energies, theoretical electron drift orbits and the local time dependences, allow us to construct the following model to explain the experimental results in a reasonable way. Electrons in a wide energy range are injected during disturbances around the midnight sector, followed by the eastward drift. Lower energy (∼ 5 keV) electrons tend to drift closer to the Earth, resulting in the dawnside enhancement of VLF hiss within the plasmasphere. Further, these lower energy electrons are allowed to enter the duskside asymmetric plasmaspheric bulge and to generate VLF hiss there. On the other hand, higher energy (∼20 keV) electrons tend to drift at L shells farther away from the Earth and those substorm electrons are responsible for the generation of structured VLF emissions around dawn due to an increase of plasma density from the sunlit ionosphere. However, such higher energy electrons are forbidden from entering the duskside of the magnetosphere and so we cannot expect a duskside peak in the occurrence of structured VLF emissions, which is in agreement with the experimental result.