Whistler-mode hiss at low and medium frequencies in the dayside-cusp ionosphere

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.