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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Particle and magnetic field data from the ATS 6 spacecraft have been combined to study a Pc 4 pulsation event. The event occurred as the satellite passed the dawn meridian during the recovery phase of a moderate magnetic storm, Plasma drifts associated with the wave are determined from the measured modulation of low-energy protons. Using the observed ambient and perturbation magnetic fields in conjunction with the plasma drift velocity, we calculate the Poynting vector of the wave, assuming that the electric field is given by the frozen field approximation. The Poynting vector gives convincing evidence that the wave is a standing hydromagnetic wave along the magnetic field. In addition, it shows that the wave is propagating predominantly azimuthally from noon toward the dawn meridian. The phase relationships between the magnetic and plasma drift velocity oscillations are consistent with the occurrence of an odd harmonic of the standing wave. Comparison of observed particle fluxes with model predictions demonstrates that the fundamental mode is the most reasonable interpretation of the data. Ambient ion number densit.es, predicted by the wave data on the basis of a simple model of standing hydromagnetic waves, are tbund to be somewhat higher than observed values, on the basis of the assumption that the ambient plasma consists of protons and electrons only. Predicted and observed values of the ion number densities can be reconciled if it is assumed that a significant fraction of the ambient plasma consisted of He + and/or O +. By using the fact that the wave is a standing wave the particle flow velocity is predicted from the observed magnetic variation and a model-dependent theory. These predictions are found to agree very well with the flow values determined from the data. [Barfield et al., 1971, 1972; Cummings and Coleman, 1968; Cummings et al., 1969, 1972; Dwarkin et al., 1971; Judge and Coleman, 1962; Patel, 1965; Sugiura and Wilson, 1964; Troitskaya and Gul'elmi, 1967], and these observations have led to several theories and reviews [Chen and Hasegawa, 1974a, b;However, all of these previous observations have been hampered by the lack of simultaneous field and particle measurements. Ideally, one would like to have complete electric and magnetic field measurements along with complete particle spectra. On ATS 6 we approximate the ideal situation with data'from the University of California at Los Angeles (UCLA) magnetometer and the University of'California at San Diego (UCSD) plasma instrument, which makes a complete energy scan every 16 s. This is sufficient resolution to allow the inference of electric fields associated with waves of periods greater than about 100 s. (No other electric field measuring devices are carried on the A TS 6.) This article discusses the observation of a train of hydromagnetic waves with a period of about 150 s seen at synchronous orbit by ATS 6 on June 27, 1974 (day of year 178). The critical observation is a phase shift of 90 ø between east-west oscillations of the particle flow and ...

At 0007 UT on January 14, 1967, the magnetopause was detected at 6.6 RE with the synchronous equatorial satellite ATS 1. Several boundary crossings were recorded during the succeeding hour as the spacecraft local time changed from 1400 to 1500. The magnetic fields in the magnetopause and on both sides of this boundary were measured during this 1‐hour interval. The normal to the boundary at the first crossing was 5° from the equatorial plane projection of the earth‐sun line, indicating that the magnetopause was flattened appreciably. In general, the boundary normal was nearly radial from the earth during most of the interval. The field strength in the magnetosheath was initially 165 γ. The mean value of the magnetosheath field strength for the period was 123 γ. The magnetospheric field strength varied from 165 γ just before the first boundary crossing to 210 γ just after the last crossing; the mean value for the interval was 176 γ. The measured quiet‐day field strength at this location is about 124 γ. The ratio of the particle kinetic energy density to the magnetic energy density just inside the boundary at the time of the first crossing is estimated to have been greater than 0.8. Most of the particles apparently left this region during the interval between the first and last magnetopause crossings. The fluctuations in the magnetosheath in the range 0–0.1 Hz were predominantly compressional, while those in the range 0.2–0.8 Hz were predominantly transverse to the mean field. In the magnetosphere transverse fluctuations were predominant at all but the lowest frequencies. The power below 0.8 Hz in the magnetosheath fluctuations was nearly 50 times greater than that in the magnetosphere just before the first crossing. The power in the fluctuations in the magnetospheric field also increased after this event by a factor of about 10. No evidence was found for rotational discontinuities at the magnetopause, whereas a number of relatively clear tangential discontinuities were identified. Thus, despite the fact that the prerequisite solar‐wind conditions for field‐line merging were satisfied, including the southward orientation of the interplanetary field, the observed field behavior at the magnetopause was not consistent with an open magnetosphere. Thus, despite the fact that the prerequisite solar‐wind conditions for field‐line merging were satisfied, including the southward orientation of the interplanetary field, our analysis failed to yield evidence for the rotational discontinuities that would presumably be required at the magnetopause in this case.

We calculate the equilibrium latitude and the period of oscillations about this equilibrium latitude for plasma in a centrifugally dominated tilted dipole magnetic field representing Jupiter's inner magnetosphere. For a hot plasma the equilibrium latitude is the magnetic equator; for a cold plasma it is the centrifugal equator; for a warm plasma it is somewhere in between. In the small-tilt-angle approximation, the latitudinal oscillation period of a cold particle in the centrifugal potential field is Po = Pj/31/2, where Pj is the rotation period of Jupiter. For a warm plasma the magnetic mirror force causes the bounce period to be decreased slightly. These results are applicable to the plasma injected by Io to form the torus of plasma surrounding Jupiter. We adopt an illustrative model in which atoms are sputtered from the Jupiter-facing hemisphere of Io and escape Io's gravity to be subsequently ionized some distance from Io. Ionization generally does not take place at the equilibrium latitude, and the resulting latitudinal oscillations provide an explanation for the irregularities in electron concentration within the torus, as reported by the radio astronomy experiment aboard Voyager 1.