Abstract
Abstract. On 26 February 2001, the Cluster spacecraft were outbound over the Northern Hemisphere, at approximately 12:00 MLT, approaching the magnetosheath through the high-altitude (and exterior) cusp region. Due to macroscopic motions of the cusp, the spacecraft made multiple entries into the exterior cusp region before exiting into the magnetosheath, presenting an excellent opportunity to utilize the four spacecraft techniques available to the Cluster mission. We present and compare 2 methods of 4-spacecraft boundary analysis, one using PEACE data and one using FGM data. The comparison shows reasonable agreement between the techniques, as well as the expected "single spacecraft" plasma and magnetic signatures when associated with propagated IMF conditions. However, during periods of highly radial IMF (predominantly negative BX GSM), the 4-spacecraft boundary analysis reveals a dynamic and deformed cusp morphology.
Highlights
The Earth’s magnetospheric cusps represent topological boundaries, separating the dayside field lines from those extending into the lobes, along the magnetotail
The magnetic field orientation at this time suggests Cluster is on the duskward edge of the cusp at this time
Over a period from ∼04:00–06:20 UT Cluster made 4 separate observations of the cusp, which were identified in data from the Plasma Electron And Current Experiment (PEACE) and fluxgate magnetometer (FGM) instruments on board the Cluster spacecraft
Summary
The Earth’s magnetospheric cusps represent topological boundaries, separating the dayside field lines from those extending into the lobes, along the magnetotail. The magnetosheath-cusp interface is still a region of difficulty with respect to definition (Lavraud et al, 2002; Savin et al, 2004), possibly due to ambiguity in similar signatures being interpreted differently (Eastman, 2000; Dubinin et al, 2002). First results from the initial high-altitude cusp phase of the mission (January–April 2001) have demonstrated some of the new science that is available, from multipoint measurement techniques to state-of-the-art instrument capabilities The present study focuses on an event during this first cusp phase, where we utilise the four-point measurement capabilities of Cluster to examine the dynamics of the high-altitude cusp. Using a combination of electron, magnetic field, and ion measurements from the Plasma Electron And Current Experiment (PEACE), fluxgate magnetometer (FGM), and Cluster Ion Spectrometer (CIS) instruments, we investigate the morphology and dynamic nature of the cusp. As in the work by Fung et al (1997), we encompass the various terms related to the cusp region, such as “cleft”, “entry layer”, etc., and refer to a general “exterior cusp” as the region observed at high altitudes, unless otherwise stated
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