No abstract

We present a comprehensive compilation of the average distribution of the electrostatic potential across the high‐latitude ionosphere. The averages are compiled from potential along the satellite path calculated from thermal ion drift data from instrumentation on the Defense Meteorological Satellite Program (DMSP) flights 8 and 9 satellites. Data were collected from the DMSP F8 satellite during the period September 1987 to December 1990 and from the DMSP F9 satellite during the period March 1988 to December 1990. The potential distributions are separated by geomagnetic position, season, and orientation of the interplanetary magnetic field (IMF), and then averages of the distributions are calculated. The average potential distributions clearly show the displacement of polar cap convection contours to the dusk or dawn flanks under the influence of the IMF By component. The cross‐cap potential decreases as IMF Bz changes from southward to northward. The average distributions indicate that the development of more than two convection cells for northward IMF is either uncommon or nonexistent. For IMF Bz > 0 and Bz > |By|, a distorted pattern is observed in the average potential distribution, not a four‐cell pattern as some previous studies suggest it should be. For all orientations of the IMF, the convection reversal boundary at the poleward edge of the auroral zone is observed in the average distributions to be a rotational boundary. It is not a shear boundary as suggested by some previous investigations. On average, the Harang discontinuity (convection reversal in the auroral zone near midnight) is observed to exist weakly or not at all. When examining individual passes, a strong eastward flow is present sometimes in the region of the Harang discontinuity, especially on the poleward boundary, but not at all times as implied by the Heppner‐Maynard model.

The ionosphere is a highly dynamic medium that exhibits weather disturbances at all latitudes, longitudes, and altitudes, and these disturbances can have detrimental effects on both military and civilian systems. In an effort to mitigate the adverse effects, we are developing a physics‐based data assimilation model of the ionosphere and neutral atmosphere called the Global Assimilation of Ionospheric Measurements (GAIM). GAIM will use a physics‐based ionosphere‐plasmasphere model and a Kalman filter as a basis for assimilating a diverse set of real‐time (or near real‐time) measurements. Some of the data to be assimilated include in situ density measurements from satellites, ionosonde electron density profiles, occultation data, ground‐based GPS total electron contents (TECs), two‐dimensional ionospheric density distributions from tomography chains, and line‐of‐sight UV emissions from selected satellites. When completed, GAIM will provide specifications and forecasts on a spatial grid that can be global, regional, or local. The primary output of GAIM will be a continuous reconstruction of the three‐dimensional electron density distribution from 90 km to geosynchronous altitude (35,000 km). GAIM also outputs auxiliary parameters, including NmF2, hmF2, NmE, hmE, and slant and vertical TEC. Furthermore, GAIM provides global distributions for the ionospheric drivers (neutral winds and densities, magnetospheric and equatorial electric fields, and electron precipitation patterns). In its specification mode, GAIM yields quantitative estimates for the accuracy of the reconstructed ionospheric densities.

[1] Large-scale plasma density depletions are typically associated with equatorial spread F (ESF) plasma irregularities in the nightside F region, especially in the postsunset sector. Data gathered on the ROCSAT-1 spacecraft reveal numerous cases of localized, discrete plasma density enhancements in the nightside low-latitude region at $600 km altitude. In some cases, nearly simultaneous DMSP observations at $800 km reveal similar density enhancements in the same local time sector. These density enhancement structures occur in association with ESF plasma depletions, i.e., the density enhancements are observed in the same local time where ESF plasma depletions are also present simultaneously. Within these discrete structures, the plasma density may be enhanced by $2-3 times above the background density. The density enhancement regions have sharp, distinct edges with embedded irregularities that appear to have similar scale sizes and density fluctuation spectra as those typically found in plasma depletions. Examples studied here occur at local times about 3 hours after sunset near the equatorial anomaly region, $10°to 20°from the magnetic equator. The ion velocity data within the density enhancement regions show upward plasma drifts perpendicular to the magnetic field, similar to those within adjacent plasma depletion regions. The magnetic field-aligned plasma flows are generally poleward within the density enhancement regions. The observations suggest that density enhancement structures are caused by the polarization electric field which is generated within the equatorial plasma depletions and then maps to the higher latitudes along the magnetic field lines.

.[1] The long-standing hypothesis that plasmaspheric dynamics are described by superposition of corotation and solar-wind-driven sunward convection is tested via direct comparison between plasmasphere observations and simulation output. The observations consist of global plasmasphere images produced by the IMAGE extreme ultraviolet (EUV) imager during plasmasphere erosion on 2 June 2001. The simulation is a plasmapause evolution model driven by a time-varying Volland-Stern (VS) electric potential distribution. On the dawnside and much of the nightside the model matches the EUV plasmapause position to within 0.2-0.5 earth radii (R E ). Near dusk the model plasmapause is about 0.7-1.2 R E farther out than the EUV plasmapause, suggesting that an improved model should include the duskside flow enhancement known as the sub-auroral polarization stream (SAPS). We demonstrate that including a simplified ad-hoc SAPS potential can correct the model on the duskside.