Cerenkov relations between r.f. noise and energetic particles on Ariel 4

Intense noise bands have been observed by the Ariel 4 r.f. experiment operating at frequencies between 0.25 and 4 MHz in the topside ionosphere. An examination of the frequency coverage of the bands shows that they are mainly confined to those domains which correspond to a high refractive index of the medium. The 0 mode whistler band appears in a range which extends from the lowest frequency of measurement mainly up to the gyrofrequency. The X mode upper-hybrid band extends from a lower limit which depends on the local plasma frequency, to a frequency slightly beyond the upper-hybrid resonance frequency. Both bands attain their highest levels in the auroral zone at latitudes where electrons in the hundreds of eV range are most numerous. On the dayside the latitude of these highest levels depends on the K p value. By carrying out a calculation of the Cerenkov radiation from the energetic electrons present, a determination of the frequency spectrum of the whistler noise at m.f. is made and found to fit well with the measured data.