Evidence of Low-Frequency Hydromagnetic Waves in the Exosphere

Damped waves of a period of several minutes with amplitude up to several hundred gammas are found in the geomagnetic field in the auroral zones. These damped waves occur simultaneously in magnetically conjugate regions in the northern and southern hemispheres. The waves are very nearly elliptically polarized in the plane approximately perpendicular to the magnetic field. Hence the damped waves are interpreted as low-frequency hydromagnetic waves generated in the exosphere at an altitude of several earth radii and transmitted to the earth along the lines of magnetic force in the longitudinal mode of propagation. The number of occurrences of these damped waves has a distinct local time variation that appears to vary with season.