An evaluation of the intensity of cerenkov radiation from auroral electrons with energies down to 100 ev

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.