Comparison of very-low-frequency auroral hiss with precipitating low-energy electrons by the use of simultaneous data from two Ogo 4 experiments

Abstract

Hano. ver, New Hampshire 03755Recent satellite observations have shown that 'auroral hiss,' covering the frequency range from a few kilohertz to several hundred kilohertz, is a common phenomenon in polar regions. To deduce the origin of this hiss, we have compared the records of a VLF experiment (0.3-18 kHz) with simultaneous data obtained by an auroral-particle experiment having detectors for precipitating electrons at 0.7, 2.3, and 7.3 kev. We have found that, on the dayside of the earth, the occurrence of VLF hiss correlates well with precipitation events at 0.7 kev, but in general very poorly with activity in the higher-energy channels. Exact correlation between variations in VLF hiss intensity and in electron fluxes is rare even at 0.7 kev.In addition, VLF hiss tends to be observed over a somewhat larger spatial region than precipitating 0.7-kev electrons. We conclude that, on the dayside, auroral hiss is generated by soft (E • I key) 'cusp region' electrons and that the lack of detailed correlation between the two phenomena is caused by propagation effects as the hiss travels downward and spreads from the generation region. Further study is required of observations made on the nightside, where VLF hiss may correlate with fluxes of harder electrons.To explain some observations of radio noise at 520 kHz that did not appear to be of cosmic origin, Ellis [1957] proposed that 'auroral particles approaching the earth' may emit radiation by the Cerenkov process 'throughout a frequency band extending from hundreds of kilocycles per second to low audiofrequencies' and that the intensity of the radiation may be sufficient to make it observable at the ground. This suggestion was indeed soon followed by anumber of reports directly linking auroras with radio noise at VLF frequencies [Duncan and Ellis, 1959; Martin et al., 1960; J•rgensen and Ungstrup, 1962]. The broadband nature of the phenomenon was demonstrated by Dowden, who reported the simultaneous occurrence of the noise at 4.6, 9.6, 27, 70, and 180 kHz [Dowden, 1959], and also at 9 and 230 kHz [Dowden, 1960]. The term 'auroral hiss' was apparently first used by Martin et al. [1960] to distinguish hiss associated with auroral phenomena from other types of VLF hiss. With satellite-borne VLF receivers it became possible to study the morphology of auroral hiss above the absorbing lower ionosphere. On the basis of Injun 3 data, Gurnett [1966] reported that VLF hiss below 8.8 kHz occurred mostly between noon and midnight magnetic local time (MLT) and that the region of occurrence was typically about 7 ø wide, centered on 77 ø invariant latitude (inv) at 1200 MLT, and decreased to 72 ø inv at 2300 MLT. Using simultaneous data from electron detectors at 10 and 40 key, Gurnett also showed that VLF hiss was related only to the softer (10-key) electrons in the evening hours. The results further suggested that during local afternoon VLF hiss was probably related to electrons whose energies were not great enough to cause a response from the 10key detector. Hartz [1970] subse...

High latitude minor ion enhancements: A clue for studies of magnetosphere-atmosphere coupling

“…While direct precipitating particle data has not yet been possible, the extensive correlation already established between soft electron precipitation and auroral disturbances (e.g. Hoffman and Laaspere, 1972) supports the premise that the ion anomalies may be associated with particle events.…”

Whistler‐mode radiation from the Spacelab 2 electron beam

“…Very similar funnel-shaped emissions have been observed on a variety of polar orbiting spacecraft and have been called V-shape hiss, saucers and funnels [Gurnett, 1966;Smith, 1969;Mosier and Gurnett, 1969;Gurnett and Frank, 1972;James, 1976;Gurnett et al, 19831. These emissions are a special case of a general class of emissions known as auroral hiss [Helliwell, 19651. It is widely believed that this noise is whistler-mode radiation produced by electron beams associated with the aurora [Gurnett, 1966;Hartz, 1971;Hoffman and Laaspere, 1972;Gurnett and Frank, 1972].…”

High-latitude day side electric field and particle measurements

“…It is most intense in the top two channels reaching saturation for much of the time over the first half of the flight in the 10 -100 kH z channel. This indicates auroral hiss which has been shown by Hoffman and Laaspere (1972) to be present in regions where low energy particle precipitation exists.…”