[1] With the advent of the Global Positioning System (GPS) measurements (from both ground-based and satellite-based receivers), the number of available ionospheric measurements has dramatically increased. Total electron content (TEC) measurements from GPS instruments augment observations from more traditional ionospheric instruments like ionospheric sounders and Langmuir probes. This volume of data creates both an opportunity and a need for the observations to be collected into coherent synoptic scale maps. This paper describes the Ionospheric Data Assimilation Three-Dimensional (IDA3D), an ionospheric objective analysis algorithm. IDA3D uses a three-dimensional variational data assimilation technique (3DVAR), similar to those used in meteorology. IDA3D incorporates available data, the associated data error covariances, a reasonable background specification, and the expected background error covariance into a coherent specification on a global grid. It is capable of incorporating most electron density related measurements including GPS-TEC measurements, low-Earth-orbiting ''beacon'' TEC, and electron density measurements from radars and satellites. At present, the background specification is based upon empirical ionospheric models, but IDA3D is capable of using any global ionospheric specification as a background. In its basic form, IDA3D produces a spatial analysis of the electron density distribution at a specified time. A time series of these specifications can be created using past specifications to determine the background for the current analysis. IDA3D specifications are able to reproduce dynamic features of electron density, including the movement of the auroral boundary and the strength of the trough region.

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1] Recent developments in tomographic imaging allow the use of GPS satellite data to image the Earth's ionosphere. Ground-based GPS receivers monitor the Earth's ionosphere continuously, and a comprehensive database of ionospheric measurements suitable for tomographic processing now exists. The tomographic inversion of these GPS data in a three-dimensional time-dependent inversion algorithm can reveal the spatial and temporal distribution of ionospheric electron density. This new technique is unique for studying ionospheric physics because it gives a time-continuous near-global view of the ionosphere. The tomographic algorithms have been under continuous development for several years and are now yielding new geophysical results. Two fundamentally different algorithms (Multi-instrument Data Analysis System and Ionospheric Data Assimilation Three-Dimensional) are presented. They show the ionospheric impact of two major space weather events during the recent solar maximum. Results obtained from these two algorithms are similar, which provides additional confidence in the accuracy of the images.

.[1] The major magnetic storm of 4-5 June 1991 was well observed with the Combined Release and Radiation Experiment (CRRES) satellite in the duskside inner magnetosphere and with three Defense Meteorological Satellite Program (DMSP) spacecraft in the polar ionosphere. These observations are compared to results from the Rice Convection Model (RCM), which calculates the inner magnetospheric electric field and particle distribution self-consistently. This case study, which uses the most complete RCM runs to date, demonstrates two significant features of magnetospheric storms, the development of subauroral polarization streams (SAPS) and plasma-sheet particle injection deep into the inner magnetosphere. In particular, the RCM predicts the electric field peak near L = 4 that is observed by the CRRES satellite during the second injection. The RCM calculations and DMSP data both show SAPS events with similar general characteristics, though there is no detailed point-by-point agreement. In the simulation, SAPS are generated by the deep penetration of plasma sheet protons to L < 4 and Earthward of the plasma sheet electrons. Similarly, the vast majority of the ions that make up the storm-time ring current came from the plasma sheet; most of the particles that made up the prestorm quiet-time ring current escaped through the dayside magnetopause during ring current injection. The RCM demonstrates the capability of plasma sheet ions to reach all ring current orbits and predicts the location of the injected particles (both ions and electrons) reasonably well. However, it overpredicts the ion flux in the inner magnetosphere.

[1] While quantitative theories of plasma flow from the magnetotail to the inner magnetosphere typically assume adiabatic convection, it has long been understood that these convection models tend to overestimate the plasma pressure in the inner magnetosphere. This phenomenon is called the pressure crisis or the pressure balance inconsistency. In order to analyze it in a new and more detailed manner we utilize an empirical model of the proton and electron distribution functions in the near-Earth plasma sheet (À50 R E < X < À10 R E ), which uses the Tsyganenko [1989] magnetic field model and a plasma sheet representation based upon several previously published statistical studies. We compare our results to a statistically derived particle distribution function at geosynchronous orbit. In this analysis the particle distribution function is characterized by the isotropic energy invariant l = EV 2/3 , where E is the particle's kinetic energy and V is the magnetic flux tube volume. The energy invariant is conserved in guiding center drift under the assumption of strong, elastic pitch angle scattering. If, in addition, loss is negligible, the phase space density f (l) is also conserved along the same path. The statistical model indicates that f (l,x) is approximately independent of X for X À35 R E but decreases with increasing X for X ! À35 R E . The tailward gradient of f (l,x) might be attributed to gradient/curvature drift for large isotropic energy invariants but not for small invariants. The tailward gradient of the distribution function indicates a violation of the adiabatic drift condition in the plasma sheet. It also confirms the existence of a ''number crisis'' in addition to the pressure crisis. In addition, plasma sheet pressure gradients, when crossed with the gradient of flux tube volume computed from the Tsyganenko [1989] magnetic field model, indicate Region 1 currents on the dawn and dusk sides of the outer plasma sheet.