Physics – Plasma Physics
Scientific paper
Apr 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012jgra..11704203c&link_type=abstract
Journal of Geophysical Research, Volume 117, Issue A4, CiteID A04203
Physics
Plasma Physics
Magnetospheric Physics: Cusp, Magnetospheric Physics: Numerical Modeling, Space Plasma Physics: Charged Particle Motion And Acceleration, Space Plasma Physics: Magnetic Reconnection (2723, 7526)
Scientific paper
Dispersed ion structures observed near the magnetosphere cusps have long been used to infer locations and properties of reconnection at the Earth's magnetopause. However, observations are often difficult to interpret since spacecraft move relative to a cusp ion structure, creating temporal and spatial ambiguity in the observations. Models of cusp ion structures are also limited to the cases during stable solar wind and interplanetary magnetic field (IMF) because empirical models are used to obtain the Earth's electromagnetic fields. We introduce a new model of a cusp ion structure by using the Liouville theorem particle tracer (LTPT) with the OpenGGCM 3D global MHD model. The OpenGGCM produces time-dependent magnetospheric electromagnetic fields under various solar wind and IMF conditions, while the LTPT traces test particles backward from an observation point to the magnetosheath to map the phase space density and construct an energy-time spectrogram. This allows our model to study cusp ion structures under dynamic solar wind and IMF conditions. In this paper, we first test our model's capability by reconstructing the cusp ion structures observed from three cusp-crossing events of Cluster and Polar satellites. We show that the model reproduces various observed ion structures, such as normal dispersion, reverse dispersion, double dispersions, and stepped dispersion. We then show that the cusp structures observed by Cluster on 23 September 2004 and on 23 August 2003 are temporal structures caused by a sudden increase of solar wind dynamic pressure and various reconnection rates, respectively. We also show that the stepped dispersion observed by Polar on 25 August 1998 is not only spatial but also temporal, caused by two different subsolar reconnection sites during a change of the IMF clock angle. In addition, we find that the ions entering the cusp often cross the magnetopause far away from the reconnection site, even though the reconnection is the cause of precipitation, and that the magnetic configuration of the magnetosheath is also sometimes a cause of energy dispersion in a cusp structure.
Connor H. J.
Raeder Joachim
Trattner K. J.
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