Transmission of geomagnetic micropulsations through the ionosphere and lower exosphere

Abstract

In this paper we consider the propagation of hydromagnetic waves at altitudes below about 2000 km. This work is an extension of a previous paper in which heights below about 500 km were considered. We treat the case of plane wave propagation in the vertical direction, and assume the geomagnetic field to be vertical (polar propagation). Currently used models indicate that the ionosphere (the height region between about 80 and 500 km) may be reasonably represented by a constant Alfvén speed and locally exponential ion-neutral collision frequency, while the lower exosphere (the region between about 500 and 2000 km) can be adequately described by an Alfvén speed which increases exponentially with height. By using these approximate forms, we can express the solutions of the relevant forms of the electromagnetic wave equation in terms of known functions. Analytic expressions for the magnetic transmission and reflection coefficients are derived and analyzed, and numerical results are obtained. These results, although strictly applicable only to high magnetic latitudes, compare favorably with many geographically widespread experimental data. Of particular interest is the prediction of a prominent double transmission resonance in the daytime and a single strong resonance at night. This agrees with the measurements of various workers. Many lesser resonances are also found. The advantage of the analytic representation is the ease of interpretation of the physical results. For example, simple expressions describing the transmission resonances found by Jacobs and Watanabe for a grounded exosphere are found to be a limiting case of the equations derived here.