Yasuhide Hobara

Propagation characteristics of whistler waves in the Jovian ionosphere and magnetosphere

The purpose of this paper is to carry out extensive reexamination of the propagation characteristics of unducted whistlers in the Jovian magnetosphere. In addition to the previous work on the dispersion of whistlers, our emphasis is placed on the detailed study of their amplitude variations in order to investigate the plasma structure of the magnetosphere and characteristics of causative lightning discharges. The two-dimensional ray-tracing computations are used as before to investigate magnetospheric whistler propagation paths, but with the first attempt of estimating the whole attenuation factors (polarization effect, focusing/defocusing and interactive damping (Landau and cyclotron)). The total attenuation due to three effects is estimated, which is compared with the amplitude of whistlers observed by the Voyager spacecraft. The following conclusions are reached. (1) Some bias in magnetospheric plasma density is required to explain the high-frequency cutoff of whistlers, (2) the magnetospheric temperature is about 20 eV for the interpretation of low-frequency cutoff of whistlers, and (3) estimated total radiated power per flash is about 4.5×106 W(0∼20 kHz) on the assumption of unducted propagation and nearly the same even in the case of ducted propagation.

On estimating the amplitude of Jovian whistlers observed by Voyager 1 and implications concerning lightning

In this paper we extensively reexamine the amplitude of many whistlers detected by the Voyager 1 and try to deduce information about the causative lightning discharges with the use of our ray-tracing computations taking into account the amplitude. As a result, we have derived the frequency spectra and mean radiation power of the causative lightning discharges and have also applied statistical method to the analysis. We can summarize our findings in the following. The average power flux spectral density of the whistlers falls in a range from 10−12.7V2m−2Hz−1 to 10−11.0V2m−2Hz−1. We calculated the total decrease of the whistler amplitude from the bottom of the ionosphere toward the spacecraft, as a range from about 30 to 40 dB. One of the strongest estimated lightning events exhibits a frequency dependence comparable to the terrestrial one, but its peak frequency seems to be similar to the upward current strokes on the Earth. Moreover, the rather smooth profile obtained implies a small possibility of the presence of stratified layers in the Jovian ionosphere. Other events possibly have features similar to those of the terrestrial return strokes. We calculated the mean radiation power per flash of the lightning in the Jovian atmosphere for a 1-kHz bandwidth over 60 ms, as a range from the order of 102 to 105 W. The probability distribution of the radiation power in Jupiter is found to follow a lognormal distribution, just as in the terrestrial case.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over seven years from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion and nighttime fluctuation) and earthquakes with magnitude greater than 6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40km). Finally some comments on the mechanism of seismo-ionospheric perturbations are discussed.