Ion and electron pressure distributions from the tail plasma sheet to the inner magnetosphere: THEMIS and Geotail observations and comparisons with the RCM simulations

Details Ion and electron pressure distributions from the tail plasma sheet to the inner magnetosphere: THEMIS and Geotail observations and comparisons with the RCM simulations Ion and electron pressure distributions from the tail plasma sheet to the inner magnetosphere: THEMIS and Geotail observations and comparisons with the RCM simulations

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41c1883w&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM41C-1883

Physics

[2730] Magnetospheric Physics / Magnetosphere: Inner, [2740] Magnetospheric Physics / Magnetospheric Configuration And Dynamics, [2753] Magnetospheric Physics / Numerical Modeling, [2764] Magnetospheric Physics / Plasma Sheet

Scientific paper

Plasma pressure is one of the most important controlling factors in magnetospheric dynamics. Using 3 years of THEMIS data and 11 years of Geotail measurements, we have determined statistically how equatorial ion and electron pressures change from the tail plasma sheet (r~30 Re) to the inner magnetosphere (r~5 Re), and how they vary with the strength of the cross polar-cap potential (CPCP). Both ion and electron pressures increase by at least an order of magnitude from r=30 to 5 Re. With decreasing r, ion pressure becomes slightly larger (a factor of <~1.5) in the pre-midnight than the post-midnight sector, while an opposite and much stronger MLT asymmetry (up to a factor of 3) is seen in electron pressure. As CPCP increases, number densities decrease while temperatures increase in the region outside r~10 Re, resulting in no substantial changes in pressures. Inside r~10 Re, ion pressure increases slightly with increasing CPCP mainly in the pre-midnight sector, while electron pressure increases significantly in the post-midnight sector. In most of the nightside region (except very close to the Earth) and under different CPCP, total plasma pressure is fairly isotropic. The electron to proton pressure ratio is relatively constant (~0.15) at r > ~15 Re and does not change with CPCP. Inside r~15 Re, the ratio increases with decreasing r and becomes larger with increasing CPCP with larger ratio (up to > 0.5) in the post-midnight sector. To understand the above distributions in the near-Earth plasma sheet and inner magnetosphere, we have simulated ion and electron pressures resulting from particle drift transport using the Rice Convection Model (RCM) and the Tsyganenko 96 magnetic field model (T96) with outer MLT-dependent plasma boundary conditions (at r~20 Re) established from Geotail observations. Despite the simulations are not in force balance, there is qualitative agreement in pressure distributions between simulations and observations. The RCM shows that as particles drift toward smaller r, magnetic drift becomes stronger due to adiabatic energization, while electric drift becomes increasingly affected by eastward corotation. Since electric and magnetic drifts are in the same direction for electrons but in the opposite directions for ions, the combined total drift results in more pronounced MLT asymmetry in electron pressures than ion pressures. As CPCP increases, decrease of cold particle source in the tail results in the density decrease and temperature increase, while enhanced convection moves plasma sheet particles closer to the Earth, resulting in the pressure increase seen at smaller r. We are currently running the RCM with force-balance magnetic fields so that quantitative comparisons can be made. In addition, we are evaluating magnetic field configuration, field aligned currents, and ionospheric mapping of the equatorial structures from 3D force-balanced magnetospheric configurations that we are now able to establish using the observed pressures and a force balance magnetic field code.

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