Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm32c..08w&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM32C-08
Physics
2731 Magnetosphere: Outer, 2740 Magnetospheric Configuration And Dynamics, 2753 Numerical Modeling, 2760 Plasma Convection, 2764 Plasma Sheet
Scientific paper
In order to understand the evolution of protons and magnetic fields in the inner plasma sheet as increasing convection electric field brings protons earthward to regions of strong dipole field, we use a modified version of the Magnetospheric Specification Model to simulate proton electric and magnetic drifts and obtain proton distributions. The magnetic field is provided by a modified version of the Tsyganenko 96 model with two-dimensional force balance maintained along the midnight meridian. The convection electric field is determined mainly by the cross polar-cap potential drop (Δ Φ ) and the equatorward edge of the convection electric field (shielding latitude θ ). The local-time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer. We previously used this model to simulate the inner plasma sheet under weak convection corresponding to Δ Φ and θ equal to 26 kV and 66° , and obtained two-dimensional quiet time equilibrium that agrees well with observations. In the current simulation, we start with the quiet time equilibrium and boundary particle sources and enhance convection by increasing Δ Φ steadily from 26 to 146 kV and lowering θ from 66° to 52° in 5 hours while keeping the boundary sources time independent. The inner edge of the plasma sheet at midnight, mapped to the equatorial plane using the self-consistent magnetic field, moves from ~ 10 RE for Δ Φ = 26 kV to ~ 4 RE for 146 kV. This earthward penetration to regions of dipole field results in strong plasma pressure increase, which significantly stretches the original dipole field lines. Proton pressure at r = 6.6 RE increases from ~1 to 6 nPa as Δ Φ increases from 26 to 146 kV, while the magnetic field strength decreases from ~80 to 10 nT. The simulated pressure and magnetic field strength, their radial variations at midnight, and their changes with convection strength in the inner plasma sheet in general agree with observations. Protons undergo enhanced earthward electric drift but are significantly diverted toward dusk by azimuthal magnetic drift around midnight. The enhanced magnetic drift, responding to enhanced particle energy, changes particles' trajectories and magnetic field configuration in a way that restrains particle energization, thus plays an important role in plasma sheet dynamics. Simulations are also run to a steady state under constant Δ Φ = 98 kV and θ = 57.6° , which indicates the inner plasma sheet can remain steady under moderate and constant convection. A scale analysis of our results suggests that using the frozen-in condition E = -vxB in MHD can lead to an inconsistency with Faraday's law in the inner plasma sheet because of neglect of the Hall term in the generalized term.
Chen Margaret W.
Lyons Larry R.
Toffoletto Frank R.
Wang Chenjie
Wolf Richard A.
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