A satellite study of low latitude electron and proton whistlers

Whistler dispersion, measured on the ISIS 2 satellite, at an altitude of 1400 km, showed a variation with latitude from about 4.5s1/(2) at a magnetic latitude (λ) of 30° to about 1 12+/-2s1/(2) at the equator. Frequently, fractional hop whistlers were observed from both hemispheres with dispersions which converged to the same value at the equator. Ray tracing calculations revealed that whistler mode signals of different frequencies take different paths from a source on the ground to the satellite and yet their dispersion is indistinguishable from the Eckersley form. The extent of the entry region to the ionosphere depends on the satellite location and may be more than 300km for different frequencies. The fractional hop whistlers frequently excite proton whistlers and their crossover frequencies have been used to determine fractional hydrogen ion concentrations. The measurements show that the average fractional H+ concentrations at 1400km increase from about 0.80 at λ = 25° to 0.88 at the equator.