Heat conduction waves in the upper atmosphere

Abstract

In the thermosphere a temperature and pressure disturbance of the neutral air can be transported vertically by a wave mode that mainly depends on the heat conductivity of the air. In the low-frequency region (ω < 10−2 sec−1 or periods of τ > 10 min) these heat conduction waves propagate with phase and group velocity proportional to the reciprocal density of the air. The mean velocity is of the order 10–100 m/sec within the ionospheric F region. The traveling time of a disturbance from the exosphere down into the lower F region is about 10 hours and about 2 days down into the E region. These waves may be excited by hydromagnetic waves in heights above 1000 km. Their large propagation time can explain the time delay of ionospheric storms and of the geomagnetic activity effect of the neutral air, as well as the after-storm effect of VLF phase heights in the nighttime E layer related to the time of impact of an enhanced solar wind as the beginning of a geomagnetic storm. Moreover, heat conduction waves may be the cause of ionospheric traveling disturbances excited within the exosphere and propagating downward with a phase velocity equally directed to their group velocity.