Physical Parameters of Substorms from Automated Forward Modeling (AFM)

Details Physical Parameters of Substorms from Automated Forward Modeling (AFM) Physical Parameters of Substorms from Automated Forward Modeling (AFM)

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm51b1409c&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM51B-1409

Other

2409 Current Systems (2721), 2721 Field-Aligned Currents And Current Systems (2409), 2788 Magnetic Storms And Substorms (7954), 2790 Substorms, 2794 Instruments And Techniques

Scientific paper

Automated Forward Modeling (AFM) inverts magnetic data to give physical parameters for electric current flow in near-Earth space. On a meridian, it gives the total electric current crossing it, and latitudinal boundaries. AFM uses nonlinear optimization of parameters of a forward model. Characteristic behaviors of substorms are seen: the current strengthens rapidly at an onset, with electrojet boundary motion. The current rises for approximately 30 minutes, but poleward border expansion progresses slightly faster. Recovery is accompanied by a current decrease, but not poleward retreat of the auroral oval on up to a two- hour timescale. Average characteristics of the current closely follow those of the AL index, with large variation in individual events. Boundary motion is similar to that deduced for the electron aurora from satellite studies. AFM allows both the current strength and the borders to be determined from ground magnetic data alone, generally available on a continuous basis. In this study, 63 separate onsets in 1997 were characterized using AFM on the CANOPUS Churchill meridian. The provisional AL index was also obtained for the same events. The parametrization of Weimer (1993), JGR 99, 11005 was found to be extremely accurate for both AL and meridian current, which is I(MA) = c0 + c_1 te^{pt}, with c0 0.151 MA, c_1 1.63 MA/h, and p -1.98/h. This permits a current/AL relation of I(MA) = -0.0322 - 0.00165 * AL, where we stress that I and AL are averages. Further, on average the equatorward border of the electrojet does not change much at onset, while the poleward border's latitude in central dipole coordinates is well represented by 67.5+4.21*(1.0-e^{-5.47*t}), with t the postonset time in hours. These results agree very well with those of Frey et al. (2004), JGR 109, doi:10.1029/2004JA010607 for electron auroras observed using Image WIC near the onset meridian. AFM permits quantification of electrojet parameters, facilitating their interpretation and comparison to other quantities measured during substorms.

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