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Introduction

Plasma in the high latitude ionosphere responds to magnetospheric electric fields resulting from a combination of viscous interactions and magnetic reconnection processes occurring at the magnetopause and in the magnetotail. The resulting large-scale electric fields map along geomagnetic field lines with little attenuation to the ionosphere where they drive plasma convection according to ${\bf v}={\bf E}\!\times\!{\bf B}/{{\rm B}^2}$. Measuring the velocity of the convecting ionospheric plasma allows the convection electric field, ${\bf E}=-\nabla\Phi$, to be locally determined, where $\Phi$ is the electrostatic potential. Such measurements provide insight into solar wind-magnetosphere-ionosphere (SW-M-I) and ionosphere-thermosphere (I-T) coupling processes as well as being valuable for comparison and validation of real-time and predictive space weather models.

While knowledge of $\Phi$ over localized regions is useful in some specific studies, a solution of $\Phi$ over the entire high-latitude convection region ($>\sim$50$^\circ\Lambda$) is often desirable, and sometimes required. For example, recent comparisons of global magnetospheric magnetohydrodynamic (MHD) models with indirect measurements of ionospheric electric fields have been made [Slinker et al., 1999]. Differences in derived ionospheric quantities, such as $\Phi$, were observed and used to adjust parameters in the MHD model in order to make the outputs more realistic. Further improvements are possible from comparisons of MHD models with direct measurements of the ionospheric electric fields over the entire convection zone.

The total potential variation across the polar cap, $\Phi_{\sf PC}$, is an important measure of the coupling between the solar wind and the magnetosphere. By accurately determining $\Phi_{\sf PC}$ with direct measurements it is possible to validate several empirical relations between $\Phi_{\sf PC}$ and the interplanetary magnetic field (IMF) [Reiff et al., 1981,Boyle et al., 1997].

With the recent addition of new radars to the SuperDARN system, an important threshold has been reached, namely, definition of the global characteristics of the high-latitude convection pattern based solely on direct measurements of convection. The purpose of this paper is to demonstrate that definitive global solutions of $\Phi$ are now possible with direct measurements. Towards this end, we show that, given sufficient coverage, the solution for $\Phi$ is insensitive to the selection of statistical model data. The measurements during such periods largely constrain the global solution of $\Phi$ and $\Phi_{\sf PC}$, minimizing the impact of the statistical model.

Maps of $\Phi$ determined using this technique are suitable for SW-M-I and I-T coupling studies and as validation measures for MHD models.


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Simon Shepherd 2000-07-13