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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A01206, doi:10.1029/2006JA012015, 2007

A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables

P. T. Newell

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


T. Sotirelis

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


K. Liou

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


C.-I. Meng

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA


F. J. Rich

Space Vehicles Directorate, Air Force Research Laboratory, Hanscom Air Force Base, Bedford, Massachusetts, USA


Abstract

We investigated whether one or a few coupling functions can represent best the interaction between the solar wind and the magnetosphere over a wide variety of magnetospheric activity. Ten variables which characterize the state of the magnetosphere were studied. Five indices from ground-based magnetometers were selected, namely Dst, Kp, AE, AU, and AL, and five from other sources, namely auroral power (Polar UVI), cusp latitude (sin(Λc)), b2i (both DMSP), geosynchronous magnetic inclination angle (GOES), and polar cap size (SuperDARN). These indices were correlated with more than 20 candidate solar wind coupling functions. One function, representing the rate magnetic flux is opened at the magnetopause, correlated best with 9 out of 10 indices of magnetospheric activity. This is dΦ MP /dt = v 4/3 B T 2/3sin8/3(θ c/2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin8/3(θ c/2))(interplanetary field magnitude, B T )(merging line length, ∼(B MP /B T )1/3). The merging line length is based on flux matching between the solar wind and a dipole field and agrees with a superposed IMF on a vacuum dipole. The IMF clock angle dependence matches the merging rate reported (albeit with limited statistics) at high altitude. The nonlinearities of the magnetospheric response to B T and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dΦ MP /dt. Our results show that a wide variety of magnetospheric phenomena can be predicted with reasonable accuracy (r > 0.80 in several cases) ab initio, that is without the time history of the target index, by a single function, estimating the dayside merging rate. Across all state variables studied (including AL, which is hard to predict, and polar cap size, which is hard to measure), dΦ MP /dt accounts for about 57.2% of the variance, compared to 50.9% for E KL and 48.8% for vBs. All data sets included at least thousands of points over many years, up to two solar cycles, with just two parameter fits, and the correlations are thus robust. The sole index which does not correlate best with dΦ MP /dt is Dst, which correlates best (r = 0.87) with p 1/2 dΦ MP /dt. If dΦ MP /dt were credited with this success, its average score would be even higher.

Received 10 August 2006; accepted 25 October 2006; published 12 January 2007.

Keywords: coupling; function; prediction.

Index Terms: 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions; 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions (2431); 2722 Magnetospheric Physics: Forecasting (7924, 7964); 2407 Ionosphere: Auroral ionosphere (2704).

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Citation: Newell, P. T., T. Sotirelis, K. Liou, C.-I. Meng, and F. J. Rich (2007), A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables, J. Geophys. Res., 112, A01206, doi:10.1029/2006JA012015.