SURVEY OF TRANSIENT MAGNETIC-FIELD EVENTS IN THE DAYSIDE MAGNETOSPHERE

Transient magnetic variations in the dayside magnetosphere are examined by using data acquired with Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE). These events have a time scale of 1 to 15 min, and their signature is similar to that of the flux transfer events (FTEs) reported in the literature. From inspection of data covering the entire dayside at L = 6.0 approximately 9.4 during 1984 and 1986, 305 events were found, mostly near the apogee of the spacecraft (8.8 R(E)). The events are distributed in the 0700-1600 magnetic local time (MLT) sector with an occurrence maximum at 1000-1100 MLT. The statistical properties of these events include the following: (1) at northern latitudes the radial magnetic field perturbation is first positive and then becomes negative, while at southern latitudes the reversed sequence is seen; this radial field polarity is the same as that of FTEs; (2) the rotation of the magnetic field perturbation in the plane perpendicular to the ambient field is predominantly left handed (right handed) in the prenoon (afternoon) sector; (3) events with larger total magnetic field perturbations are observed closer to the apogee; (4) short-duration events are localized near the magnetopause and tend to occur in association with high geomagnetic activities, while long-duration events are observed at a wide range of distance from the magnetopause and independent of geomagnetic activity; (5) simultaneously obtained IMP 8 magnetic field data indicate that the transient events, especially the short-duration events, occur in association with southward orientation of the interplanetary magnetic field. Regardless of the generation mechanism, the MLT dependence of the rotational polarity suggests that the source of the magnetic field perturbation, presumably located near the magnetopause, moves azimuthally tailward (dawnward or duskward) from the subsolar magnetopause. We suggest that the short-duration events are caused by transient and patchy reconnection, while the long-duration events are caused by changes in solar wind pressure.