Large-scale electric fields resulting from a combination of viscous interactions and magnetic reconnection processes occurring at the magnetopause and in the magnetotail map along magnetic field lines with little attenuation into the high-latitude ionosphere. The total variation in the resulting ionospheric electric potential, referred to as the cross polar cap potential, or , is therefore an indicator of the amount of energy flowing into and through the magnetosphere-ionosphere (M-I) system. In addition to being an important parameter for describing the state of the magnetosphere, is useful for comparison with and validation of real-time and predictive space weather models.
Several techniques have been used to measure and to study its correlation with solar wind drivers and other geophysical parameters. They include high-latitude, low-altitude spacecraft measurements of the convecting plasma velocity; Ogo 6 [Heppner, 1972], AE and S3 [Reiff et al., 1981; Reiff and Luhmann, 1986; Doyle and Burke, 1983], DE 2 [Weimer, 1995, 1996, 2001], and Defense Meteorological Satellite Program (DMSP) [Rich and Hairston, 1994; Boyle et al., 1997; Burke et al., 1999]; assimilation and mapping of ground magnetometer and radar measurements such as the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique [Richmond and Kamide, 1988]; linear regression relationships between solar wind parameters, ground-based magnetometers, and DMSP data such as the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN) Electrodynamic Model (IZMEM) [Papitashvili et al., 1994] or the Linear Modeling of Ionospheric Electrodynamics (LiMIE) [Papitashvili et al., 1999]; fitting backscattered ionospheric line-of-sight (LOS) convection velocities from ground-based radars to functional forms of the electrostatic potential [Ruohoniemi and Baker, 1998]; and global magnetospheric modeling such as the Lyon-Fedder-Moybarry (LFM) global magnetohydrodynamic (MHD) code [Fedder and Lyon, 1987; Lyons, 1998; Slinker et al., 2001].
Each of these techniques has limitations on the degree and accuracy to which it can determine or predict . Satellite measurements are spatially and temporally limited to the spacecraft orbit path, magnetometer data are spatially limited and must be inverted using ionospheric conductivity models, differences exist between global MHD models and observations possibly due to the lack of some necessary ionospheric physics in these models, radar measurements can be spatially limited, and parameterization techniques provide only typical or average values. The consequence is that comprehensive and definitive determinations of the ionospheric electric potential and the associated have yet to be made.
The technique developed by Ruohoniemi and Baker, , however, has some benefits over other techniques. This method involves fitting an expansion of spherical harmonic functions to Doppler measurements of the drifting ionospheric plasma provided by the Super Dual Auroral Radar Network (SuperDARN) coherent backscatter radars [Ruohoniemi and Baker, 1998], heretofore referred to as the Johns Hopkins University (JHU)/Applied Physics Laboratory (APL) fitting technique, or simply APL FIT. While SuperDARN is not exempt from spatial and temporal limitations, and sparse data from a statistical model [e.g., Ruohoniemi and Greenwald, 1998] are used to prevent nonphysical solutions in areas lacking measurements, the coverage provided by these radars is often a significant portion of the high-latitude ionosphere. Indeed, Shepherd:00 show that at times the coverage is sufficient to effectively determine a global solution of in the high-latitude ionosphere based on the radar measurements. During such periods, and even during periods with less stringent data coverage requirements than shown by Shepherd and Ruohoniemi, 2000, is well-defined by the APL FIT technique.
In this study we use APL FIT to determine for 9464 10-min averaged periods between 1 February 1998 and 31 December 2000. Solar wind conditions are provided by the Advanced Composition Explorer (ACE) satellite, orbiting around the so-called L1 Lagrangian point, for comparisons of with the solar wind conditions driving the ionospheric convection. The periods were chosen to minimize uncertainty in determining the geoeffective solar wind and interplanetary magnetic field (IMF) conditions and to occur during times when APL FIT provided a suitable determination of . The results presented in this study comprise the most comprehensive comparison of SuperDARN-determined and solar wind conditions to date.