Standing Alfvn Waves in the Magnetosphere

Abstract

Transverse, low-frequency oscillations in the magnetic field have been recorded in the equatorial plane at 6.6 Rr (earth radii) with the UCLA magnetometer on board ATS 1. The oscillations have peak-to-peak amplitudes of 2 to 20 y and have been observed predominantly on geomagnetically quiet days in the morning and noon quadrants. The fluctuations are very nearly monochromatic, and those with periods ranging from 50 to 300 sec have been studied. This paper reports on observations made during January 1967, when 25 separate events were recorded with durations ranging from 10 to 400 min. The oscillations could be grouped into two period ranges, one centered about T _-- 190 sec and the other about T _-- 102 sec. The oscillations were confined to a plane that was approximately perpendicular to the main magnetic field vector. They were generally elliptically polarized in this plane, with the major axis of the polarization ellipse typically inclined eastward at an angle of _30 o to the radially outward direction. An MHD analysis is given for an idealized model in which the earth is considered a perfect conductor, the background magnetic field is that of a dipole, and the plasma