HR: 1340h
AN: SM13A-1196    [Abstracts]
TI: Data-based Modeling of the Dynamical Inner Magnetosphere During Strong Geomagnetic Storms
AU: * Tsyganenko, N
EM: Nikolai.Tsyganenko@gsfc.nasa.gov
AF: USRA/NASA GSFC, Code 695.1. NASA GSFC, Greenbelt, MD 20771 United States
AU: Sitnov, M
EM: sitnov@umd.edu
AF: University of Maryland, College Park, Institute for Research in Electronics and Applied Physics, UMCP, College Park, MD 20742 United States
AB: This work builds on and extends our previous effort [{\it Tsyganenko et al.,} 2003] to develop a dynamical model of the storm-time geomagnetic field in the inner magnetosphere, using space magnetometer data taken during 37 major events in 1996--2000 and concurrent observations of the solar wind and IMF. The essence of the approach is to derive from the data the temporal variation of all major current systems contributing to the geomagnetic field during the entire storm cycle, using a simple model of their growth and decay. Each principal source of the external magnetic field (magnetopause, cross-tail current sheet, axisymmetric and partial ring currents, Birkeland currents) is controlled by a separate driving variable that includes a combination of geoeffective parameters in the form $N^\lambda V^\beta B_s^\gamma$, where $N$, $V$, and $B_s$ are the solar wind density, speed, and the magnitude of the southward component of the IMF, respectively. Each source was also assumed to have an individual relaxation timescale and residual quiet-time strength, so that its partial contribution to the total field was calculated for any moment as a time integral, taking into account the entire history of the external driving of the magnetosphere during each storm. In addition, the magnitudes of the principal field sources were assumed to saturate during extremely large storms with abnormally strong external driving. All the parameters of the model field sources, including their magnitudes, geometrical characteristics, solar wind/IMF driving functions, decay timescales, and saturation thresholds were treated as free variables, to be derived from the data by the least squares. The relaxation timescales of the individual magnetospheric field sources were found to largely differ between each other, from as large as $\sim$30 hours for the symmetrical ring current to only $\sim$50 min for the region~1 Birkeland current. The total magnitudes of the currents were also found to dramatically vary in the course of major storms, with the peak values as large as 5--8 MA for the symmetric ring current and region 1 field-aligned current. At the peak of the main phase, the total partial ring current can largely exceed the symmetric one, reaching $\sim$10 MA and even more, but it quickly subsides as the external solar wind driving disappears, with the relaxation time $\le$2 hours. The tail current dramatically increases during the main phase and shifts earthward, so that the peak current concentrates at unusually close distances $\sim$4-6$R_E$. This is accompanied by a significant thinning of the current sheet and enormous tailward stretching of the inner geomagnetic field lines. As an independent consistency test, we calculated the expected Dst-variation based on the model output at Earth's surface and compared it with the actual observed Dst. A good agreement (cumulative correlation coefficient R=0.92) was found, in spite of that $\sim$90% of the spacecraft data used in the fitting were taken at synchronous orbit and beyond, while only 3.7% of those data came from distances $2.5\le R\le4\,R_E$. The obtained results demonstrate the possibility to develop a dynamical model of the magnetic field, based on magnetospheric and interplanetary data and allowing one to reproduce and forecast the entire process of a geomagnetic storm, as it unfolds in time and space. Reference: N. A. Tsyganenko, H. J. Singer, J. C. Kasper, Storm-time distortion of the inner magnetosphere: How severe can it get ? J. Geophys. Res., v. 108(A5), 1209, 2003.
DE: 2708 Current systems (2409)
DE: 2730 Magnetosphere--inner
DE: 2740 Magnetospheric configuration and dynamics
DE: 2753 Numerical modeling
DE: 2778 Ring current
SC: SPA-Magnetospheric Physics [SM]
MN: 2004 AGU Fall Meeting