MULTI-POINT ELECTRIC FIELD OBSERVATIONS IN THE HIGH-LATITUDE MAGNETOSPHERE

摘要

We use multi-point electric field observations from the four Cluster satellites to study the dynamical behavior of the high-latitude magnetosphere on February 13-14, 2001, 20-02 UT. At 20:00 UT the vehicles enter the cusp where three satellites observe a 500-volt potential drop. It implies that at lower altitudes there likely exist some parallel electric fields that accelerate electrons downward and ions upward. In the following 2-3 hours the satellites move over the southern polar cap where all four satellites pass through a number of stationary, large-scale density enhancements that are associated with 200-volt potential drops. The observed events are possibly ionospheric ion outflows, triggered by geomagnetic activity. At 23:20 UT, the satellites move in the distant plasma sheet, and an hour later they have a brief encounter with the auroral region where a density cavity of a few degrees wide is observed. At the equatorward edge of the cavity, large electric fields of 100 mV/m are observed, which are likely related to an auroral arc. Similar observations are collected from all four satellites within a few minutes, but a detailed comparison reveals plenty of differences, apparently due to small spatial and temporal scale sizes. Near the perigee pass, the vehicles traverse the plasma trough near local midnight, where they all detect a ULF wave event. A preliminary analysis of the event shows that it is a resonant mode of a 120-sec period. Surprisingly the observations from four satellites are not well correlated, which suggests a short spatial and temporal scale for the event. A possible source mechanism for ULF waves at this local time sector is drifting ring current protons.