Although simultaneous satellite results from VLF, LF, and MF receivers and low‐energy electron detectors have in the past few years demonstrated that electrons with energies of a few hundred electron volts are the source of the auroral hiss band in the day side cleft region, the source of the hiss in the night side auroral zone has not been clearly identified. To deduce the origin of this hiss, we have compared the records of a VLF experiment (0.3–18 kHz) of the Ogo 4 spacecraft with simultaneous data obtained by the same satellite on precipitating electrons at 0.7, 2.3, and 7.3 keV. As a general observation, at these energies the correlation with VLF auroral hiss is best at 0.7 keV and worst at 7.3 keV. From a detailed inspection of the records we have concluded that while auroral electrons in the keV range may enhance the intensity of VLF auroral hiss on the night side, electrons of energies below about 0.7 keV are the predominant source of the night side hiss as well. The observation that whenever auroral hiss is observed, it tends to occur simultaneously over a broad range of frequencies strongly suggests that hiss of all frequencies is generated by electrons of essentially the same energy range, i.e., by those with energies below about 1 keV. This conclusion is supported by the results of a study based on Ogo 6 data which revealed a lack of correlation between keV electrons and LF auroral hiss. We have also concluded that in the day side cleft the excellent correlation between auroral hiss and 0.7‐keV electrons is maintained in situations in which the region of very soft electron precipitation is in motion. In the absence of soft electron data, auroral hiss records can thus be used in studies relating to the cleft location and motions.

The payload of the polar‐orbiting Ogo 6 spacecraft includes an experiment with four broadband receivers (0.02–15, 15–30, 92.5–107.5, and 280–295 kHz), two narrowband receivers at 200 and 540 kHz, and a broadband intensity detector. The receivers are connected to an electric dipole antenna and thus respond not only to electromagnetic but also to electrostatic waves, such as lower hybrid resonance (LHR) noise. LHR noise bands, observed below auroral latitudes in the range from a few to about 20 kHz are, in fact, among the most intense phenomena observed by the experiment. The broadband intensities of the signals often exceed the full‐scale reading of about 3 mv/m of the broadband detector. The intense signals are usually contained in a bandwidth of only a few kHz. Full‐scale readings are also exceeded by auroral hiss, but its bandwidth often extends from a lower cutoff near the LHR frequency to the highest frequency monitored by the experiment (540 kHz). The intensity and spectral shape of auroral hiss vary both within an event and from event to event, but a typical E field intensity is about 1 μv/m/Hz1/2 at audio frequencies, decreasing to 0.15 μv/m/Hz1/2 at 200 kHz and 0.025 μv/m/Hz1/2 at 540 kHz. For longitudinal propagation in the whistler mode with an index of refraction near unity, this would yield a flux density of about 6×10−17 w/m2/Hz at 200 kHz and 1.7×10−18 w/m2/Hz at 540 kHz, indicating a rapid decrease in the intensity of auroral hiss with increasing frequency. Auroral hiss has been observed with an intensity as high at 4.5 μv/m/Hz1/2 (≃5.5×10−14 w/m2/Hz) at 200 kHz. Under geomagnetically quiet conditions the center of the ‘auroral hiss zone’ extends from about 70° invariant at magnetic midnight, through 75° invariant at 0600 and 1800 MLT, to about 78° invariant at magnetic noon. The zone moves on the average about 5° toward the equator under disturbed conditions. In subpolar and polar latitudes very intense signals are observed in a narrow band near the low‐frequency cutoff of the experiment, suggesting the presence of dc electric fields. Intense VLF hiss has been observed near the equator with the dipole antenna nearly parallel to the geomagnetic field. These signals may result from some type of interaction between the spacecraft and the local plasma, but they could also be related to the recently detected fluxes of trapped low‐energy electrons at very low L values.

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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...

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