Energetic particle data from the 1977-007 and 1979-053 satellites and magnetic field data from the GOES 2 and 3 satellites have been used to study eight compressional Pc 5 wave events observed at geostationary orbit during 1979. All the events occurred on the dayside, and most of them were observed during the recovery phase of a geomagnetic storm. By using the data from two of the satellites which were close to each other, we measured the azimuthal phase velocity V/sub phi/ and azimuthal wave number m for selected intervals. For all these intervals the waves propagated westward in the spacecraft frame, and we obtained Vertical Bar V/sub phi/ Vertical Bar = 4--14 km/s and Vertical Bar m Vertical Bar = 40--120. In addition, harmonics of a local standing Alfven wave were often present simultaneously with a compressional Pc 5 wave. The frequency of the compressional wave was typically 25% of that of the second harmonic of the Alfven wave. These observed features are discussed in the light of existing theories of instabilities in the ring current plasma.

Azimuthal propagation and frequency characteristic of compressional Pc 5 waves observed at geostationary orbit

Magnetic field and particle flux observations on board ATS 6 at synchronous altitude are used to examine the dawn-dusk asymmetry of characteristics of Pc 4–5 waves and associated particle flux modulation. Most waves at synchronous orbit having ground correlations are polarized in the azimuthal direction (A class) and are usually detected in the dawn sector. Waves with a radially oriented polarization ellipse (R-class) are almost never observed near the subsatellite point on the ground, except for the regular pulsations known as giant pulsation Pg, observed in the early morning. R class Pc 4 waves occur at all local times and have an occurrence peak in the afternoon. It is found that A class Pc 4–5 waves are observed during moderately disturbed conditions, associated with a solar wind velocity of about 600 km/s or more. R class Pc 4 waves tend to predominate in the afternoon sector under quiet magnetic condition. The modulation of proton fluxes in the energy range 25–500 keV depends on the mode of the associated magnetic waves. The phase relationship between magnetic field and proton flux oscillations is simpler for the A-class waves as compared with the R class waves. The oscillation of the azimuthal component is out of phase with proton flux oscillations in the energy below about 120 keV. Proton flux modulation is larger at pitch angle near 90° than it is at smaller angles. In contrast, the phase relationship is not so simple for R class Pc 4 waves which are the dominant waves in the afternoon sector. For these waves the phase depends on both energy and pitch angle. Two thirds of the R class Pc 4 events examined show a large modulation at small pitch angle of 30°–40°. A large-amplitude pulsation of R class, storm-time Pc 5, observed in the afternoon is associated with proton flux oscillations around 90° pitch angle. For these Pc 5 waves the proton flux variations are out of phase with the compressional component of magnetic field, and their phase is almost independent of energy. Implications of the observed dawn-dusk asymmetry in characteristics of the azimuthal and radial wave modes and their associated proton flux modulations are discussed.

Local time asymmetry of Pc 4‐5 pulsations and associated particle modulations at synchronous orbit

The study of hydromagnetic waves in the magnetosphere has seen some notable advances over the last four years. We owe a large debt to the IMS observational programs both in space and on the ground. These programs have produced extremely valuable data sets which should be worked on for many years to come.

The improvement in instrumentation has been a major factor in observational work. The new generation of spacecraft are exemplified by the ISEE and GEOS missions, both of which have novel aspects. Both missions have played a large part in helping us understand how important heavy ions are in the propagation of hydromagnetic waves of all frequencies. The dual spacecraft, ISEE 1 and ISEE 2, are helping to unravel the spatial structure of these waves in space, while both missions are providing the first measurements of electric field oscillations associated with hydromagnetic waves. The ionospheric electric field measurements made by the STARE radar system have introduced a new dimension to the field, and help provide a link between ground based and spacecraft measurements. Several ground based magnetometer networks were either introduced or expanded, largely as a result of the IMS.

Hydromagnetic waves in the magnetosphere

In October 1981, ELF/VLF waves were produced in the ionosphere in an altering sequence of approximately 1, 2, 3, and 5 kHz by modulated HF heating of the ionosphere near Tromsoe, Norway, during a 32-hour period of high geomagnetic disturbance. The apparent source heights, which are derived from the wave phases, show a diurnal variation from about 55 km during the day to about 74 km at night. Other wave parameters such as amplitude, direction, and ellipticity of the ELF polarization ellipse also show a diurnal variation as well as amplitude, direction, and ellipticity of the ELF polarization ellipse also show a diurnal variation as well as modulation by Pc 5 hydromagnetic waves. They compare the variation of the ELF wave parameters with electric fields measured by the Scandinavian Twin Auroral Radar Experiment (STARE) and with riometer and magnetometer data in an attempt to understand the factors controlling the ELF wave generation process. They are able to successfully model many of the measured wave characteristics. High electron densities at low altitudes were found necessary to explain the daytime measurements.

The characteristics of ionospheric heating-produced ELF/VLF waves over 32 hours

One hundred eighty-six days of electric field data from the GEOS 2 electron beam experiment have been used to study magnetospheric fluctuations at geostationary orbit with periods between 150 and 600 s. While fluctuations are nearly always present in the electric field data from the dayside magnetosphere with typical amplitudes between 0.2 and 0.5 mV/m, it is often hard to find well-defined concurrent pulsations in the GEOS 2 magnetic field data. Most events occur near noon and have the same characteristics: They are toroidal and nearly linearly polarized, the sense of polarization and the orientation angles of the polarization ellipses change sign near noon, the instantaneous frequency of the fluctuations is correlated with the instantaneous electron density, and in a given sector of the magnetosphere the sense of polarization depends on the frequency. There is strong evidence that these fluctuations are fundamental mode eigenoscillations of field lines in the vicinity of the spacecraft which are generated in the inhomogeneous plasma of the magnetosphere by some kind of solar wind-driven surface waves at the magnetopause or at the low-latitude boundary layer.

A statistical study of dayside magnetospheric electric field fluctuations with periods between 150 and 600 s

Ground-based magnetometer observations are used to confirm the properties of a large azimuthal wave number Pc 5 pulsation observed previously with the STARE radar. The magnetometer and STARE results allow an estimate to be made of the ionospheric height-integrated Hall conductivity, as well as supporting theories that predict 90/sup 0/ rotation of polarization between ionosphere and ground. The properties of the observed vertical magnetic component at the ground are also found to be consistent with recent theory.

Ground magnetometer detection of a large‐M Pc 5 pulsation observed with the STARE radar

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Ground magnetometer detection of a large‐M Pc 5 pulsation observed with the STARE radar