Micropulsations and hydromagnetic waves in the exosphere

Recent evidence indicates that the steady-state hydromagnetic wave spectrum in the lower exosphere is sharply cut off for frequencies above 1.5–3 cps. It is argued that the spectrum has this form essentially everywhere below R≃5Re (geocentric) and that no model without a loss mechanism can reproduce the cutoff, because the ordinary and extraordinary circularly polarized waves are strongly coupled. Thermal damping is investigated by means of the collisionless Boltzmann equation; it is found that this effect may explain the observed cutoff if the proton temperature near 5Re is of the order of 105 deg K, with a density N≃102–103 cm−3. It is suggested that above 5Re the exosphere is not in thermal equilibrium, so that long-range damping cannot occur.