On the frequency of Langmuir waves in the ionosphere

Abstract

Diverse geophysical applications exploit the existence of Langmuir waves in the natural ionosphere and magnetosphere. The frequency of these waves is often used as an absolute measure of plasma concentration in both remote sensing and in‐situ diagnostics. It is often wrongly assumed, however, that the frequency of Langmuir waves is directly interpretable in terms of the natural oscillation frequency of the thermal electron gas. This is only approximately true. Studies requiring more precision include the effects of the plasma temperature on the dispersion relation for these waves. Because this temperature dependence is also a function of the k vector of the waves, care must be taken when comparing measurements at different probing wavelengths (e.g., ionosondes and incoherent scatter radars). Herein, we point out that even further considerations are important in determining the exact frequency of Langmuir waves. The velocity distribution function of the entire electron gas must be accounted for properly. In particular, the presence of photoelectrons or auroral secondary electrons can affect the frequency of these waves. Failure to account properly for these effects may explain the unphysically large currents previously estimated by incoherent scatter measurements of the frequency asymmetry in upshifted and downshifted plasma line echoes.