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.
An asymmetric ring current belt consisting of a symmetric ring current and a superimposed partial ring current system is proposed as the explanation for the low‐latitude disturbance daily variation. The magnetic effects of the partial ring current system are derived using a scale model together with a small magnetometer as an analog computer. The magnetic field of an asymmetric ring current belt is derived by assigning an amplitude A to the partial ring current function and an amplitude S to the (constant) symmetric ring current function. The span in longitude, the initial position in local time, and the local‐time drift rate of the partial ring current are also adjustable parameters in the asymmetric ring current model. Recovery phases for stations with various longitudes are derived by assuming exponential decays for the partial ring current and symmetric ring current. Measured and computed recovery phases are compared for a few magnetic storms. The comparisons show that even this very simple model of the asymmetric ring current can account for most of the low‐latitude disturbance daily variation in the recovery phase.
The behavior of the magnetic field at the synchronous orbit during magnetospheric substorms is discussed for several events during December 1966 and January 1967. The vector measurements of the field were made with magnetometers on board the geostationary satellite ATS 1. The field was observed to be depressed and inclined radially outward in the dusk‐to‐midnight quadrant while substorms were in progress. Similar distortions of the magnetosphere were not observed in other local‐time sectors. When the satellite was near local midnight, the onset of the expansive phase of an auroral substorm was coincident with the recovery of the field at ATS 1. When the satellite was further toward the dusk meridian similar recoveries were observed, but they were followed by a renewed depression in the field. An interpretation of the data in terms of partial ring currents in the dusk‐to‐midnight quadrant is discussed.
Certain satellite and terrestrial observations of transient magnetic fluctuations show a high degree of localization, while other observations are of a definite worldwide character. The worldwide fluctuations in the magnetic field are probably well explained in terms of hydromagnetic waves propagating through the magnetosphere in modes with the wave polarization current flowing perpendicular to the geomagnetic field. However, for the localized fluctuations such an interpretation is inconsistent with present theory of hydromagnetic wave propagation in the magnetosphere. We suggest that the observations of localized magnetic fluctuations might be better interpreted in terms of field‐aligned currents in the magnetosphere.
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