Jinsuke Ohtsu

The morphologies of low-latitude and auroral VLF ‘hiss’

Abstract The purpose of this paper is to make the extensive comparison of features between the auroral and low-latitude VLF hiss from the morphological standpoint. Then it is found that the low-latitude hiss is not considered to be the consequence of the Earth-ionosphere waveguide mode propagation of the auroral hiss and it is totally different from the auroral hiss. Further, the low-latitude VLF hiss is found to be divided into two types. One is the quiet-time hiss and the other the storm-time one. The diurnal variation of the occurrence rate of quiet-time hiss shows a quite similar shape with that of the occurrence rate of low-latitude whistlers, which may be resulted from the diurnal variation of the ionospheric absorption of plasmaspheric VLF hiss. Then the diurnal variation of the occurrence rate of the storm-time hiss consists of two peaks in the evening and in the morning. The characteristics of the evening and morning hiss are different from each other, which suggests that the asymmetric structure of the plasmasphere has an essential influence on the generation of the storm-time VLF hiss. Lastly, we discuss the association of auroral and storm-associated low-latitude hiss with the magnetospheric substorms and storms.

Ducted propagation of low-latitude whistlers deduced from simultaneous observations at multi-stations

Abstract The purpose of this paper is to make clear the ducted propagation of low-latitude whistlers by making use of the simultaneous observations at several stations in Japan, i.e. Kagoshima (geomag. lat. 20°N), Sakushima (24°N) and Moshiri (34°N). The analyses are based on 5 days of highest whistler activity in February 1970, and the observed characteristics are given separately for sunset and nighttime whistlers. The peculiar features of sunset whistlers are the concurrence of the peak in the occurrence histogram at a certain dispersion value at Sakushima and Moshiri, the greater occurrence number at Sakushima than at Moshiri, and the non-detection at Kagoshima. Next the characteristics of nighttime whistlers are summarized: 1. (1) Nighttime whistlers are only observed at Moshiri, not at Kagoshima and Sakushima; and 2. (2) The dispersion values consisted of a few values, suggesting the isolated and persistent ducts. The observed results are found not to be explained in terms of non-ducted propagation by using a ray tracing technique, and the observations are satisfactorily explained by ducted propagation. Also the path latitudes of sunset and nighttime whistlers are roughly estimated.

Simultaneous spaced direction-finding measurements of medium-latitude VLF/ELF emissions

Abstract Simultaneous spaced measurements of medium-latitude VLF/ELF emissions were carried out during the three northern winters from 1976 to 1979. The experiment was making use of two different kinds of direction-finding systems (a field-analysis method and a goniometer network) at two stations in Europe, namely Brorfelde in Denmark ( L = 2.9) and Chambron-la-Foret in France ( L = 1.9); this enabled us to locate the ionospheric exit regions of emissions over a wide range of L -values up to and beyond 4.0, the average plasmapause location. In order to study the time delay in the temporal evolution of VLF emissions or the longitudinal drift of the emissions, observations from the Moshiri Observatory in Japan, widely separated in longitude, are also used. The overall system of the VLF equipment installed at the three stations is described. Then we present the VLF/ELF data of good quality obtained during the final years campaign (Nov. 1978–Feb. 1979). By making use of the direction-finding data, we were able to classify the observed emissions into several categories, and some early results for some of the emissions are presented.

On the propagation of ionospheric whistlers at low latitude

Abstract The K-9M-26 rocket was launched from Kagoshima Space Center in Japan (geomag. lat. 20°N) to study the propagation characteristics of ionospheric or short fractional-hop whistlers coming from the local Earth-ionosphere waveguide. Their dispersion and amplitude characteristics are investigated in detail, and they are checked against the propagation theory. An analysis of their dispersions shows that the quasi-longitudinal (QL) approximation is pertinent above the E-region even at such low latitudes as 20°. Then it is found from the study of wave field intensities that the observed ionospheric whistlers are identified as having propagated up to the rocket after propagation in the Earth-ionosphere waveguide over considerable distances.

Transmission and reflection of magnetospheric whistlers in the ionosphere and lower exosphere at high latitudes

Abstract The propagation characteristics of downgoing magnetospheric whistlers through models of the lower exosphere are theoretically treated taking into account the Earth-ionosphere cavity. The situation is that the whistler mode propagates downward in the presence of the homogeneous magnetic field of vertical direction, and the lower exosphere is tentatively divided into several characteristic regions. The analytical expressions for the reflection and transmission coefficients, for these models, are derived and their numerical values are shown for nighttime and daytime conditions. It is found that the most striking effects of the introduction of the Earth-ionosphere cavity are summarized as follows: (1) the reflection coefficients of magnetospheric whistlers become nearly unity for the usually encountered collisionfrequency in the lower ionosphere, and (2) there appear several characteristic minima in the transmission coefficients at certain frequencies, which are closely related with the spacing of the Earth-ionosphere cavity. These results will be useful in the study of the loss mechanism of VLF emissions as well as the whistler penetration problem.

Annual and semi-annual variations in the electron density of the inner magnetosphere deduced from whistler dispersion

Abstract The annually and semi-annually varying components in the electron density of the inner magnetosphere are studied by using the one solar cycle dispersion data of whistlers observed at low latitude stations in Japan, Wakkanai (35.3°N geomagnetic lat.) and Moshiri (34…0°N). The amplitude and phase of varying components are investigated by the method of harmonic analysis. It is found that the time of occurrence of maximum in the variation of annual component is early February at night, and early January at day. The maxima in the variation of semi-annual one take place in late January and July at night, and in late March and September at day. These are a new finding peculiar to the density variation of the inner magnetosphere, which is significantly different from the high latitude results. The superposition of such annual and semi-annual variations, and annual mean in the electron density of the inner magnetosphere results in the observed annual curve of dispersion value, which shows maxima in spring and autumn, and minima in summer and winter.

Tunneling transmission through the equatorial lower ionosphere of ELF and VLF electromagnetic waves

Abstract The purpose of this paper is to discuss the transmission of ELF and VLF electromagnetic waves through the lower ionosphere from below at equatorial latitude. We treat the case in which the electromagnetic waves are incident on the lower ionosphere whose electron density increases linearly wih height, and the geomagnetic field is taken to be horizontal. Analytical expressions for the reflection and transmission coefficients are derived and numerical results are given. The most outstanding result is as follows. The wave transmission is, generally, extremely small due to the presence of an evanescent region. However, there exists a narrow band of possible tunneling penetration around 1 kHz. The frequency of maximum transmission is found to depend on the density gradient. Lastly some discussion is given on the relation with equatorial ELF and VLF radio phenomena.

On the Polarization of Half-Gyrofrequency Whistler-Mode Waves in the Two-Component Magnetospheric Plasma

Quasi-electrostatic half-gyrofrequency whistler-mode VLF emissions with wave normals close to the oblique resonance cone (θ res ) have been found to be unstable for the two-component (cold and anisotropic lower-energy hot electrons) magnetospheric plasma. The present paper deals theoretically with the electric field polarization of half-gyrofrequency whistler-mode waves in such an unstable plasma, by means of numerically solving the full dispersion relation. At wave normal angles relatively away from θ res , there exists no effect of hot electrons on the polarization. When the wave normal angle further increases above a specific value (smaller by 5–10° from θ res ) where we have wave growth, we expect the deformation of electric field polarization from that predicted by cold plasma theory; the wave becomes more transverse. The deformation becomes more pronounced for more populated hot electrons with fixed cold plasma density.