Ion Composition of Geomagnetic Storm-Time Events

  • Patrick Dixon

Student thesis: Master's ThesisMaster of Philosophy


This work focuses on the study of geomagnetic lobe encounters, where a spacecraft orbiting within the Earth’s magnetosphere crosses the open/closed field line boundary into the tail lobes and back again. A case study is performed on a series of flank lobe encounters observed by the twin Van Allen Probes between 0200 and 0515 on November 14th 2012. Observations by LANL GEO spacecraft at geosynchronous orbit also show lobe encounters in the northern hemisphere and on the dusk flank, providing evidence for a global phenomenon driving the events. Using the twenty-one total events observed by the Van Allen Probes and LANL GEO spacecraft, the global magnetic field topology is examined, as well as smaller scale spatial and temporal characteristics of the open/closed field line boundary, allowing the position and motion of the boundary to be constrained. A novel method is used to compare the observed behaviour of the OCB to that predicted by the BATS-R-US global MHD model. The model appears to simulate the dynamic processes that cause the spacecraft to repeatedly cross the OCB but incorrectly maps the overall topology of the boundary during these extreme conditions, overestimating the distance to it by as much as 3 RE. A survey of Van Allen Probe data was performed over a two year period from October 2012 – October 2014, where the spacecraft sampled the magnetosphere at near geosynchronous orbit for all MLTs and in both the northern and southern hemispheres. This allows a survey to examine the distribution of magnetospheric lobe encounters spatially, in MLT and magnetic latitude, as well as comparing their frequency to factors such as IMF conditions and substorm occurrences. This survey highlighted how unusual it was to see a strong series of flank events - like that seen for November 14th 2012 - at a relatively low apogee (5.8 RE) compared to previous studies. The predominance of substorm associated near-midnight encounters implies that these events are able to bring the lobe closer in to the Earth, making it more likely for spacecraft with lower orbits to encounter them.
Analysis of the Interplanetary Magnetic Field drivers of these events shows strong IMF BY and BZ in the day preceding the event. Negative IMF By, was shown to prefer the south/dawn region of the magnetosphere, in agreement with Moldwin et al. [1995]. In-situ magnetometer data was analyzed and showed the spacecraft encountering a highly stretched field typical of the lobe, as well as a strong dipolarization of the field prior to exiting the lobe, which can be associated with a substorm injection.
Date of Award2018
Original languageEnglish
Awarding Institution
  • Aberystwyth University
SponsorsScience and Technology Facilities Council
SupervisorEleri Pryse (Supervisor) & Manuel Grande (Supervisor)


  • geomagnetic lobe encounters
  • Van Allen Probe
  • LANL GEO spacecraf

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