Čerenkov radiation within the Earth's upper atmosphere
TL;DR: A brief review of Cerenkov radiation within the upper atmospheric plasma has been presented in this article, where the results of analysis about the nature and characteristics of VLF hisses in terms of incoherent Cererkov radiation are given in a concise manner.
Abstract: A brief review of Cerenkov radiation within the upper atmospheric plasma has been presented. Different attempts in this context are systematically discussed. The results of analysis about the nature and characteristics of VLF hisses in terms of incoherent Cerenkov radiation are given in a concise manner. The occurrence of resonance cone has also been reported.
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26 Dec 1962
TL;DR: In this article, the authors investigated the radiation from a point charge moving uniformly in a plasma, when the charge is moving in the direction of an external magnetic field, and obtained two sets of values of these parameters, the frequency and the angular spectrum of the emitted radiation.
Abstract: The radiation from a point charge moving uniformly in a plasma is investigated when the charge is moving in the direction of an external magnetic field. In general, there are two modes, for each of which all the components of the electric and magnetic field are present. The two parameters of interest in this problem are the ratio of the velocity of the charges to the free-space velocity of electromagnetic waves, and the ratio of the gyromagnetic frequency to the plasma frequency of the electrons. For two sets of values of these parameters, the frequency and the angular spectrum of the emitted radiation are obtained. In certain cases, as many as three Cherenkov rays are found to propagate in the same direction; these multiple-rays, however, correspond to different frequency components and to different modes of propagation. The motivation for this investigation is indicated briefly. (auth)
13 citations
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TL;DR: In this paper, the diurnal and latitudinal occurrence and intensity patterns of ELF, VLF, and LF whistler-mode noise emissions were observed with the Alouette 2 satellite.
Abstract: Statistical studies have been made of the diurnal and latitudinal occurrence and intensity patterns of ELF, VLF, and LF whistler-mode noise emissions observed with the Alouette 2 satellite. The ELF emissions occur characteristically below 1 kHz, are peculiar to the daylight hours, and show a peak in average intensity in the invariant latitude range of 50°–70°. In the VLF range, broadband emissions extending upward from the LHR frequency show a maximum average intensity near local noon and a Λ of about 77°. Along the auroral oval toward earlier and later hours, they show a decreasing intensity. The apparent high-frequency end of this emission band extends upward from 100 kHz to the local fN or fH frequency in the LF range. The LF emissions were studied at a fixed frequency of 200 kHz, and maximum intensities were found along the day and evening portions of the auroral oval. This type of noise also shows a latitude dependence on Kp similar to that of the auroral phenomena. A second region of peak intensity, at about the position of the nighttime plasmapause, appears in the LF statistical results, but a complete study has not been made of that region.
70 citations
TL;DR: In this paper, the relative variations of the LF and MF spectra can be roughly accounted for by a simple ray-tracing technique assuming electric waves, and absolute intensities of noise have been computed by using observed electron flux densities and assuming an incoherent Cerenkov mechanism.
Abstract: Simultaneous measurements from the Isis 1 satellite of soft-electron fluxes and of auroral hiss at low and medium frequencies indicate that such noise emissions can propagate to the north or south of the L shell containing the precipitating electrons through distances up to 1000 km or about 3° invariant latitude at 2000 km altitude. This latitudinal spreading suggests that the noise propagation corresponds to those parts of the whistler-mode dispersion curves where the waves are electric, that is, near the resonance cone. The relative variations of the LF and MF spectra can be roughly accounted for by a simple ray-tracing technique assuming electric waves. Ionograms recorded at the same time as the noise measurements were reduced to real height profiles of electron density, thus giving a two-dimensional ambient electron distribution for ray tracing. Absolute intensities of noise have been computed by using observed electron flux densities and assuming an incoherent Cerenkov mechanism. The theoretical power levels are at least two orders of magnitude lower than those observed. It is concluded that the incoherent theory for the wave generation mechanism is inapplicable, assuming that the antenna impedance theory that was used is suitable.
64 citations
TL;DR: In this paper, it is suggested that very intense electron fluxes are responsible for, or at least closely connected with, the generation of hiss, and that high-latitude electron flux is correlated with the intensity variations in the outer radiation belt.
Abstract: Observations of wide-band hiss in the frequency range 4–9 kc/s made at 13 stations in both hemispheres since last sunspot maximum show that hiss is generated in certain zones. Apparently these zones are the same as the zones of auroral precipitation and so are approximately circular, with their centers close to the magnetic midnight meridian some few degrees from the geomagnetic poles. Hiss is not observed equally often everywhere in these zones; there is a maximum of the occurrence at about 70° magnetic latitude shortly before magnetic midnight. Hiss seems to be a rare phenomenon at the very highest latitudes close to the geomagnetic poles. Observations by the Alouette 1 satellite of very intense electron fluxes at latitudes higher than the boundary of the outer radiation belt show that these fluxes are very well correlated to ground-based hiss observations both in space and in time. It is therefore suggested that these very intense electron fluxes are responsible for, or at least closely connected with, the generation of hiss. Owing to the positive correlation between hiss, activity and high-latitude electron fluxes, which again are correlated with the intensity variations in the outer radiation belt, it would appear to be possible to monitor, from the ground, high-intensity electron fluxes in the polar regions and intensity variations in the outer radiation belt.
55 citations
TL;DR: In this article, it was shown that, for an electron flux spectrum of the approximate form dJ/dE ∝ E−2, electrons in the range 100 ev to 1 kev contribute about 2 orders of magnitude more to the intensity of Cerenkov hiss than those in the ranges 1-10 kev.
Abstract: The intensity of incoherent Cerenkov radiation in the dayside polar-cusp region has been estimated by including the very soft (100 ev to 1 kev) energy range, which recent experimental results have shown to be the source of broad-band auroral hiss at least on the dayside of the earth. Our results show that, for an electron flux spectrum of the approximate form dJ/dE ∝ E−2, electrons in the range 100 ev to 1 kev contribute about 2 orders of magnitude more to the intensity of Cerenkov hiss than those in the range 1–10 kev. If the electron flux is assumed to have the value of 5 × 105 el cm−2 sec−1 ster−1 ev−1 at 700 ev, a peak Cerenkov hiss intensity of ≃10−13 w m−2 Hz−1 results at ionospheric heights at ≃70 kHz and decreases in intensity toward both lower and higher frequencies. A limitation of the present theories is pointed out, and the possible role of anomalous cyclotron radiation in the auroral hiss generation is discussed. It is concluded that, even if the generation mechanism is not totally incoherent, as has been assumed in the calculations, it almost certainly involves electrons that are moving faster than the wave. If the calculations are repeated at a lower latitude, appropriate to the nightside auroral hiss zone, essentially identical results are obtained if the dayside model ionosphere and the dayside electron flux spectrum are used. These results indicate that, if incoherent Cerenkov radiation is the source of auroral hiss, the generally weaker hiss on the nightside of the earth must be due to lower electron fluxes, harder spectra, lower magnetospheric electron densities, or a combination of these factors.
50 citations
TL;DR: In this article, a series of simultaneous observations of very low-frequency radio noise by the Injun 5 satellite and of visual aurora along the same geomagnetic field line by the Fort Churchill Auroral Observatory are discussed.
Abstract: This paper presents a series of simultaneous observations of very-low-frequency radio noise by the Injun 5 satellite and of visual aurora along the same geomagnetic-field line by the Fort Churchill Auroral Observatory. Seven observations from the period of August 29, 1968, to March 4, 1969, are discussed. In 5 of the 7 events studied VLF hiss is observed in association with auroral-light emissions. These observations typically show the occurrence of VLF hiss in the general region of the auroral arc, with significant changes in the VLF spectrum sometimes observed in the immediate vicinity of the auroral arc. One event for which the associated charged-particle fluxes have been analyzed is investigated in detail. The VLF radio noise intensity for this event is among the largest observed with Injun 5 and is much greater than can presently be explained by an incoherent Cerenkov radiation mechanism.
41 citations