Standing Alfvén waves in the magnetosphere

Abstract

Transverse, low‐frequency oscillations in the magnetic field have been recorded in the equatorial plane at 6.6 RE (earth radii) with the UCLA magnetometer on board ATS 1. The oscillations have peak‐to‐peak amplitudes of 2 to 20 γ 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° 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 density varies as a power law. For the case of a standing Alfvén wave the poloidal and toroidal wave equations uncouple. These equations are solved numerically, and the eigenfrequencies appropriate to the synchronous orbit are tabulated for the first six harmonics for seven density models. From the results of the analysis it is argued that the observed transverse oscillations are the second harmonic of a standing Alfvén wave. Under this interpretation the data are consistent with the hypothesis that the plasmapause is beyond 6.6 RE only during very quiet periods.

Ionospheric conductance distribution and MHD wave structure: observation and model

“…Then the observed quarter wave coincides with the ®rst quarter wave harmonic and would explain the low frequency. The occurrence of such a low frequency under quiet conditions agrees well with a statistical study performed by Cummings et al (1969). It is di cult to decide on the basis of our data which mode has been observed exactly, because the eigenfrequencies depend on the density distribution along the ®eld lines and in the equatorial plane (Cummings et al, 1969) and are very di erent under night-time and day-time conditions (Allan and Poulter, 1992).…”

“…Furthermore it a ects the mode structure of the waves, because with changing boundary conditions the 3D wave equation has di erent eigenvalues and hence di erent eigensolutions for a particular ®eld line. Cummings et al (1969) derived eigenperiods of the guided toroidal and poloidal wave mode resonances for a range of v-values and plasma density distributions assuming a perfectly re¯ecting ionosphere. Allan and Knox (1979a, b) ®rst suggested the existence of quarter waves in the magnetosphere in which the electric perturbation has a node in one ionosphere and an antinode in the conjugate ionosphere.…”

Quiettime observation of a coherent compressional Pc-4 micropulsation at synchronous altitude

“…The oscillations observed at this time were quite different from any magnetic oscillations previously reported at synchronous altitude; e. g. , storm-associated Pc-5 wave events [Barfield and Coleman, 1970], substorm-associated irregular pulsations [McPherron and Coleman, 1970], quiet-time transverse magnetic oscillations [Cummings et al, 1969], and band-limited pulsations [McPherron and Coleman, 1971]. Of the previously reported oscillations, only the transverse oscillations described by Cummings et al [1969] occur during magnetically quiet periods.…”

“…Of the previously reported oscillations, only the transverse oscillations described by Cummings et al [1969] occur during magnetically quiet periods. The storm-associated Pc-5 wave events are distinct from the event of February 14, 1967. in that they have a large transverse component.…”

Pi2 pulsations in a small and strongly asymmetric plasmasphere

“…[19] For the fundamental frequency of the FLR we use a toroidal mode equation [Cummings et al, 1969] assuming dipole field geometry and the plasma mass density from Denton et al (submitted manuscript, 2005). The radial profile of the calculated frequency for the fundamental FLR mode at the magnetic equator at 0100 MLT is plotted in Figure 9.…”