Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm33a1889x&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM33A-1889
Physics
[2435] Ionosphere / Ionospheric Disturbances, [2730] Magnetospheric Physics / Magnetosphere: Inner, [2753] Magnetospheric Physics / Numerical Modeling, [2776] Magnetospheric Physics / Polar Cap Phenomena
Scientific paper
In global modeling, magnetosphere-ionosphere (MI) coupling physically connects a global magnetospheric (GM) model and a global ionospheric-thermospheric (GIT) model. The field-aligned current from the GM model and the conductance distributions from the GIT model are used in a Poisson equation derived from the ionospheric Ohm's law combined with current continuity to determine the electrostatic potential in the ionosphere. In current GM models, this electrostatic potential is mapped to the inner boundary of the GM simulation to determine electrostatic boundary conditions on the electric field and MHD velocity there. Inductive effects and the finite Alfven transit time between the low-altitude GM boundary and the high-altitude GIT boundary (MI gap region) are neglected in this formulation of MI coupling. Using fields and currents derived from Lyon-Fedder-Mobarry GM simulations, and conductance distributions derived from its standalone empirical conductance model in the MI coupling Poisson equation, we have computed the fast Fourier transform of the electrostatic field at the low-altitude LFM simulation boundary as described above, and the FFT of the inductive electric field at the boundary under the assumption that μ 0 Σ P vA ≤ 1, where Σ P is the ionospheric Pedersen conductance and vA is the smallest value of the Alfven speed in the MI gap region. In this regime, the complete electric field at the low-altitude simulation boundary includes the usual mapped electrostatic field with an inductive addition for which the finite Alfven transit time and the diversion of field-aligned into polarization currents in the gap region are negligible (Lotko, 2004). By comparing the boundary-averaged spectra of the electrostatic and so-determined inductive fields, we confirm that the purely electrostatic formulation of MI coupling is valid when the MHD state varies on times scales exceeding about 200 s. For faster MHD time variations, the inductive electric field is shown to be larger than the electrostatic field at the low-altitude boundary and is thus non-negligible. For example, inductive corrections are expected to be important for sudden impulse events and substorm-related Pi2 fluctuations. We are currently implementing the inductive MI coupling algorithm of Lotko (2004) in the LFM global simulation. Lotko, W. (2004), Inductive magnetosphere-ionosphere coupling, JASTP 66, 1443-1456.
Brambles O.
Lotko William
Lyon John
Merkin V. G.
Wiltberger Michael J.
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