We find that slow magnetoacoustic waves produce the magnetic field perturbations of the largest amplitude in the 0.01‐ to 0.1‐Hz frequency range within the ‘highly disturbed’ magnetosheath. The frequencies quoted are frequencies seen by the satellite. Rotational Alfvén waves with periods of several minutes and longer are also detected on most orbits. The power spectrum of rotational waves usually rises much more steeply below 0.01 Hz than the power spectrum of magnetoacoustic waves. We conclude that the magnetoacoustic waves are produced or strongly amplified at the earth's bow shock or in the outer magnetosheath. The observed rotational waves may be produced beyond the bow shock and carried into the magnetosheath by the solar wind.
The high‐latitude trapping boundary for 20‐kev electrons and 100‐kev protons became very thin in the early morning hours during two intense substorms. The gradients were too steep to be maintained by drifting particles, so they must have been produced locally over the nightside of the earth. The flux gradient is seen to move at speeds in excess of 100 km/sec. Plasma appears to move away from the tail and around the earth at these high speeds during the sudden expansion phases of the substorms. The rapid plasma motion requires the presence of fluctuating electric fields that sometimes exceed 50 to 100 mv/m at a geomagnetic latitude of 30° on the L = 5 field line. Our observations fit best into a model that contains two field‐aligned sheet currents. Current flows down toward the ionosphere at or beyond the poleward edge of the disturbance and away from the earth at lower latitudes. The high‐latitude trapping boundary appears to be distorted by waves. As these waves propagate around the earth, the satellite alternately enters and leaves the trapped‐particle region. Electrons that have been newly accelerated during the substorm arrive at the satellite at about the same time that ground activity commences at the satellite's local time. The high electric fields that accompany the rapid plasma flow can produce nonadiabatic acceleration of 0.1‐ to 1‐Mev electrons and protons.
We describe a physical model of the magnetic‐field structure that exists in the inner magnetosheath, near the earth‐sun line, and during highly disturbed periods. Magnetometer data are presented in a new format that uses the ordinary inclination‐declination coordinate system. Measurements of the inclination angle show how much the field lines are distorted from being tangent to the magnetopause. Distortions in the inclination angle sometimes exceed 45° in the low‐field regions that are believed to be associated with plasma condensations or clouds. The magnetic field is usually within 15° of being tangent to the magnetopause in high‐field regions, but some larger distortions have also been seen here. The clouds usually appear to be extended along the average magnetic‐field direction. A characteristic dimension for the observed structures is between several hundred and several thousand kilometers. Some large changes in the declination angle are not directly associated with clouds. These distortions imply that the field structure can rotate and still remain roughly tangent to the magnetopause. The large rotations are probably produced by changes in the solar wind and in the interplanetary magnetic field. Other declination angle changes are associated with clouds and provide evidence that field‐aligned currents can be present in the magnetosheath. Several sets of measurements were made very near the earth‐sun line. Some changes in the wave amplitude and frequency are noted in this region, but these changes can simply be the result of a reduction in the plasma‐flow speed. No unexpected effects that require new types of waves or instabilities have been found near the subsolar point.
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