|Number of pages||18|
|Journal||Journal of Geophysical Research: Space Physics|
|Early online date||31 Aug 2015|
|Publication status||Published - 29 Sep 2015|
The twin Van Allen Probes spacecraft witnessed a series of lobe encounters between 0200 and 0515 UT on 14 November 2012. Although lobe entry had been observed previously by other spacecraft, the two Van Allen Probe spacecraft allow us to observe the motion of the boundary for the first time. Moreover, this event is unique in that it consists of a series of six quasi-periodic lobe entries. The events occurred on the dawn flank between 4 and 6.6 local time and at altitudes between 5.6 and 6.2 RE. During the events Dst dropped to less than −100nT with the IMF being strongly southward (Bz = −15nT) and eastward (By = 20 nT). Observations by LANL-GEO spacecraft at geosynchronous orbit also show lobe encounters on the dawn and dusk flanks. The two spacecraft configuration provides strong evidence that these periodic entries into the lobe are the result of local expansions of the OCB propagating from the tail and passing over the Van Allen Probes. Examination of pitch angle binned data from the HOPE instrument shows spatially large, accelerated ion structures occurring near simultaneously at both spacecraft, with the presence of oxygen indicating that they have an ionospheric source. The outflows are dispersed in energy and are detected when the spacecraft are on both open and closed field lines. These events provide a chance to examine the global magnetic field topology in detail, as well as smaller-scale spatial and temporal characteristics of the OCB, allowing us to constrain the position of the open/closed field line boundary and compare it to a global MHD model using a novel method. This technique shows that the model can reproduce a periodic approach and retreat of the OCB from the spacecraft but can overestimate its distance by as much as 3 RE. The model appears to simulate the dynamic processes that cause the spacecraft to encounter the lobe but incorrectly maps the overall topology of the magnetosphere during these extreme conditions.
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- Multipoint observations of the open-closed field line boundary as observed by the Van Allen Probes and geostationary satellites during the 14 November 2012 geomagnetic storm
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