Simulated Radial and Azimuthal Structure of Proton Ring-Current Field

Details Simulated Radial and Azimuthal Structure of Proton Ring-Current Field Simulated Radial and Azimuthal Structure of Proton Ring-Current Field

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh51a..07c&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SH51A-07

Physics

2708 Current Systems (2409), 2730 Magnetosphere: Inner, 2753 Numerical Modeling, 2778 Ring Current, 2788 Storms And Substorms

Scientific paper

We simulate the radial and azimuthal structure of the proton ring-current magnetic field by tracing the guiding centers of representative ions from the plasma sheet as they drift within a model magnetosphere. The ambient magnetic field model we use for this study is the Dungey model, which consists of a dipole field plus a uniform southward ``tail'' field. We map a spatially analytical expansion of the AMIE ionospheric electric potential, expressed as a function of magnetic latitude and magnetic local time, along magnetic field lines (for L >= 2) throughout this model magnetosphere. We trace the bounce-averaged drift for ions conserving the first two adiabatic invariants μ and J (with values that correspond to energies of ˜10 -300 keV at L = 3). Using these simulation results, we map proton phase space densities according to Liouville's theorem but taking into account losses due to charge exchange. We specify an ``initial'' proton ring current distribution by solving the steady-state transport equation that balances quiescent radial diffusion against charge exchange. To obtain MLT-dependent and UT-dependent boundary values for our phase space density distribution, we map geosynchronous LANL ion data to the boundary of our model magnetosphere as boundary conditions. From the simulated phase-space density, we calculate the proton pressure and energy density distributions. From the pressure distributions, we compute the separate contributions of the magnetic field perturbation from the gradient-curvature drift currents, magnetization currents, and field-aligned currents needed to satisfy Ampere's law. We add these contributions together to obtain the total ring current magnetic field. We perform simulations for a few storm events including the 19 October 1998 storm. We find that during the 19 October 1998 storm the large AMIE electric field in the evening sector would have led to rapid ( ˜ 20 minutes) inward transport of ions from the plasma sheet to the dusk meridian at L ˜ 3. We can thus account for the observed rapid formation of the partial ring current there and its subsequent symmetrization to a wider range of MLT. In regions where the ring current is especially intense, the ring current magnetic field can be a significant fraction of the Earth's magnetic field. This suggests a need for eventually calculating the particle transport in a magnetically self-consistent model in the future.

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