VLF Phenomena Research Articles

Plasmaspheric hiss observed in the topside ionosphere at mid- and low-latitudes

Latitudinal characteristics of ELF hiss in mid- and low-latitudes have been statistically studied by using ELF/VLF electric field spectra (50 Hz-30 kHz) from ISIS-1 and -2 received at Kashima station, Japan from 1973 to 1977. Most ISIS ELF/VLF data observed in mid- and low-latitude include ELF hiss at frequencies below a few kHz. The ELF hiss has the strongest intensity among VLF phenomena observed by the ISIS electric dipole antenna in mid- and low-latitudes, but the ELF hiss has no rising structure like the chorus in the detailed frequency-time spectrum. The ELF hiss is classified into the steady ELF hiss whose upper frequency limit is approximately constant with latitude and the ELF hiss whose upper frequency limit increases with latitude. These two types of ELF hiss occur often in medium or quiet geomagnetic activities. Sometimes there occurs a partial or complete lack of ELF hiss along an ISIS pass. Spectral shape and bandwidth of ELF hiss in the topside ionosphere are very similar to those of plasmaspheric hiss and of inner zone hiss. The occurrence rate of steady ELF hiss is about 0.3 near the geomagnetic equator and decreases rapidly with latitude around L = 3. Hence it seems likely that ELF hiss is generated by cyclotron resonant instability with electrons of several tens of keV in the equatorial outer plasmasphere beyond L = 3. Thirty-seven per cent of ELF hiss events received at Kashima station occurred during storm times and 63% of them occurred in non-storm or quiet periods. Sixty-seven per cent of 82 ELF hiss events during storm times were observed in the recovery phase of geomagnetic storms. This agrees with the previous satellite observations of ELF hiss by search coil magnetometers. The electric field of ELF hiss becomes very weak every 10 s, which is the satellite spin period, in mid- and low-latitudes, but not near the geomagnetic equator. Ray tracing results suggest that waves of ELF hiss generated in the equatorial outer plasmasphere propagate down in the electrostatic whistler mode towards the equatorial ionosphere, bouncing between the LHR reflection points in both the plasmaspheric hemispheres.

VLF wave observation by satellite JIKIKEN (EXOS-B).

Instrumentation, data processing and preliminary results of EXOS-B VLF wave receiver are presented. The VLF receiver is connected, by command, either to a long dipole antenna or to a loop antenna, and observes both electromagnetic and electrostatic waves in a frequency range between 150Hz and 9.5kHz. Up to the present time after the launch, various VLF phenomena including whistler echo trains, whistler triggered emissions, hiss, chorus, and possible power line harmonic radiation have been observed. In addition to these natural phenomena, artificial VLF waves excited by electron beam injection have also been observed.

40 years of whistlers

The history and present state of research on whistlers are reviewed. Systematic research in this field began in the 1920s, and led in the 1950s to the discovery of the magnetosphere. Since the early 1960s, satellite data have contributed immensely to the understanding of whistlers and related VLF phenomena, notably those involving interactions between whistler-mode waves and energetic particles. Artificial as well as natural whistler-mode waves have been used to study the distribution and dynamics of the magnetospheric plasma, as well as in the study of wave-particle interactions. Recently the latter phenomena have also been studied by direct injection of energetic particles into the ionosphere and magnetosphere from sounding-rockets. Nevertheless, many problems remain to be solved in years to come.

A semi-automated whistler analyser

A system to facilitate the analysis of wide-band VLF data from goniometer receivers has been designed and built and is now in use. Whistlers and other types of VLF phenomena are selected from a background of noise and from these signals are obtained their frequency-time dependence and direction of arrival, which can then be used to study magnetospheric morphology and dynamics. The system, which uses a spectrum analyser and programmable tracking filter, combines the human skill in pattern recognition, discrimination and selection with the speed, efficiency and accuracy of modern electronic digital techniques.

Ion gyrofrequency phenomena observed on Whistlers, auroral hiss and ELF hiss

Modulation of auroral hiss at harmonics of the proton gyrofrequency has been observed by the VLF receiver on the Canadian ISIS 2 satellite; the observed value of the proton gyrofrequency is less than the local proton gyrofrequency in the immediate vicinity of the satellite. In addition, modulation at harmonics of the helium gyrofrequency has been observed on ELF hiss inside and outside the plasmasphere. These observations and a number of other related VLF phenomena observed by others (electrostatic noise seen on the Javelin 8.46 rocket, V-shaped VLF noise and saucers) are discussed in terms of the Landau instability process excited by non-thermal electrons streaming along the magnetic field of the Earth. Whistler modulation is also discussed.

Phenomenological study of LHR hiss

The 1963 and 1964 Alouette 1 records from the Ottawa station were studied to determine the statistical characteristics of VLF hiss exhibiting a sharp lower-cutoff frequency associated with the lower-hybrid resonance (LHR). A particular class of midlatitude hiss events were emphasized and designated ‘S-type hiss.’ These events have lower-cutoff frequencies that vary smoothly with time, have limited bandwidths, and do not appear to be associated with other VLF phenomena. Their diurnal time of peak occurrence provides evidence that their sources may be connected with the type of magnetospheric activity usually associated with auroral-zone phenomena; yet, they are definitely midlatitude events. A likely means for transporting the electromagnetic energy is a horizontal duct that occurs as a result of the presence of more than one ion species in the topside ionosphere. Nonducted whistler-mode signals may be trapped in the duct, and satellites within the region observe signals with characteristics determined by this guiding structure. Other forms of LHR hiss, such as whistler-triggered events, may also be associated with these ducts. The characteristics of noise at altitudes above such ducts, or in the absence of ducts, may be different from that of S-type hiss. Some of the observed Alouette 1 events may be such nontrapped noises.

Correlation between convection electric fields in the nightside magnetosphere and several wave and particle phenomena during two isolated substorms

Correlation of several magnetoionospheric wave and particle phenomena previously linked observationally to magnetospheric substorms and inferred to involve convection electric fields with whistler measurements of convection activity during two relatively isolated substorms. The events occurred at about 0600 UT on July 15, 1965, and about 0500 UT on Oct. 13, 1965. The correlated phenomena include cross-L inward plasma drifts near midnight within the plasmaphere, diffuse auroral radar echoes observed near the dusk meridian, IPDP micropulsations (intervals of pulsations of diminishing period) in the premidnight sector, apparent contractions and expansions of the plasma sheet at about 20 earth radii in the magnetotail, and Pc 1/Pi 1 micropulsation events near or before midnight. Two new vlf phenomena occurred during the October 13 event - a noise band within the plasmasphere associated with a convecting whistler path, and ?hisslers,' falling-tone auroral-hiss forms repeated at intervals of about 2 sec.

Observations of chorus below 1500 cycles per second at Godhavn, Greenland, from July 1957 to December 1961

Whistlers and VLF phenomena were observed at Godhavn, Greenland (79.9°N geomagnetic latitude), during a period of 4 1/2 years. Of the phenomena observed, only chorus will be treated in this paper. The chorus observed is usually below 1500 cps, and the diurnal variation of the occurrence shows a peak at 1235 UT (0901 LMT) independent of the time of year. The seasonal variation of occurrence shows a maximum in the summer months. The correlation between the daily sum of the local K index and the number of hours per day with chorus is negative. Since the features of chorus observed at Godhavn are different from those at middle-latitude stations, we propose to introduce the term ‘polar chorus’ as a name for this type of emission.

Whistler and other very low frequency phenomena associated with the high-altitude nuclear explosion on July 9,1962

VLF phenomena observed near Wellington, New Zealand, after the nuclear explosion of July 9, 1962, include the occurrence of an explosion-excited whistler, evidence of high ionospheric absorption around New Zealand, and a remarkably transient effect on the propagation of VLF radio waves near the explosion area. A reduction in electron content of the exosphere may also have occurred.

Whistler and other VLF phenomena associated with the high-altitude nuclear explosion on 9 July 1962

Abstract VLF phenomena observed near Wellington after the nuclear explosion included: the occurrence of an explosion-excited whistler; a possible reduction of about 30 per cent in the electron content of the exosphere; and a marked reduction in amplitude of VLF signals passing near the explosion area, which lasted for less than 1 sec.

Further comments on the role of secondary scattering in the Lyman-α problem

Modulation of auroral hiss at harmonics of the proton gyrofrequency has been observed by the VLF receiver on the Canadian ISIS 2 satellite; the observed value of the proton gyrofrequency is less than the local proton gyrofrequency in the immediate vicinity of the satellite. In addition, modulation at harmonics of the helium gyrofrequency has been observed on ELF hiss inside and outside the plasmasphere. These observations and a number of other related VLF phenomena observed by others (electrostatic noise seen on the Javelin 8.46 rocket, V-shaped VLF noise and saucers) are discussed in terms of the Landau instability process excited by non-thermal electrons streaming along the magnetic field of the Earth. Whistler modulation is also discussed.