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Abstract. We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable. The evolution of the unstable layer is controlled by the history of the vertical drift velocity. When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present. The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux. The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent. These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset. The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour. These irregularities occur most often at -0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux.

Abstract. We use Jicamarca radar observations of F region vertical plasma drifts and auroral electrojet indices during 1968-1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields. These electric fields result from the dynamo action of storm time winds and/or thermospheric composition changes driven by enhanced energy deposition into the high-latitude ionosphere during geomagnetically active conditions. The equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high-latitude currents. On the average, this process can be described by two basic components with time delays of about 1-12 hours and 22-28 hours between the high-latitude current enhancements and the equatorial velocity perturbations. Our data indicate strong coupling between dynamo processes with different timescales. The short-term disturbance dynamo drives upward equatorial drifts (eastward electric fields) at night with largest amplitudes near sunrise and small downward drifts during the day. These perturbation drifts are in good agreement with results from the BlancRichmond disturbance dynamo theory. The dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise-noon sector. In this case, the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions. We also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts.

Abstract. Ionospheric plasma drifts often show highly complex and variable signatures during geomagnetically active periods due to the effects of different disturbance processes. We describe initially a methodology for the study of storm time dependent ionospheric electric fields. We present empirical models of equatorial disturbance zonal electric fields obtained using extensive F region vertical plasma drift measurements from the Jicamarca Observatory and auroral electrojet indices. These models determine the plasma drift perturbations due to the combined effects of short-lived prompt penetration and longer lasting disturbance dynamo electric fields. We show that the prompt penetration drifts obtained from a high time resolution empirical model are in excellent agreement with results from the Rice Convection Model for comparable changes in the polar cap potential drop. We also present several case studies comparing observations with results obtained by adding model disturbance drifts and season and solar cycle dependent average quiet time drift patterns. When the disturbance drifts are largely due to changes in magnetospheric convection and to disturbance dynamo effects, the measured and modeled drift velocities are generally in good agreement. However, our results indicate that the equatorial disturbance electric field pattern can be strongly affected by variations in the shielding efficiency, and in the high-latitude potential and energy deposition patterns which are not accounted for in the model. These case studies and earlier results also suggest the possible importance of additional sources of plasmaspheric disturbance electric fields. !. IntroductionThe ionospheric electric field plays a dominant role on the dynamics, distribution of ionization, and generation of plasma waves in the low-latitude and equatorial thermosphere. In the last 2 decades, significant progress has been achieved in the study of equatorial electrodynamic plasma drifts and electric

We use extensive radar measurements of F region vertical plasma drifts and auroral electrojet indices to determine the storm time dependence of equatorial zonal electric fields. These disturbance drifts result from the prompt penetration of high latitude electric fields and from the dynamo action of storm time winds which produce largest perturbations a few hours after the onset of magnetic activity. The signatures of the equatorial disturbance electric fields change significantly depending on the relative contributions of these two components. The prompt electric field responses, with lifetimes of about one hour, are in excellent agreement with results from global convection models. The electric fields generated by storm time winds have longer lifetimes, amplitudes proportional to the energy input into the high latitude ionosphere, and a daily variation which follows closely the disturbance dynamo pattern of Blanc and Richmond [1980]. The storm wind driven electric fields are responsible for the larger amplitudes and longer lifetimes of the drift perturbations following sudden decreases in convection compared to those associated with sudden convection enhancements.

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] Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low-latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low-latitude ionosphere. We have used more than 5 million low-latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well-defined longitudinal enhancements in the low-latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well-defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low-latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low-latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.

Our primary goal is to construct a real-time data assimilation model for the ionosphere-plasmasphere system that will provide reliable specifications and forecasts. A secondary goal is to validate the model for a wide range of geophysical conditions, including different solar cycle, seasonal, storm, and substorm conditions. OBJECTIVES We propose to develop a software program that will provide for a Global Assimilation of Ionospheric Measurements (GAIM). GAIM will use a physics-based ionosphere-plasmasphere model as a basis for assimilating a diverse set of real-time (or near real-time) measurements. The program will provide specifications and forecasts on a specified spatial grid that can be global, regional, or local (50 km x 50 km). The specifications/forecasts will be in the form of 3-dimensional electron density distributions from 90 km to geosynchronous altitudes (35,000 km). Auxiliary plasma parameters will also be provided, such as N m F 2 , h m F 2 , N m E, h m E, and slant and vertical TEC. In its specification mode, GAIM will provide quantitative estimates for the accuracy of the reconstructed ionospheric densities. The measurements GAIM will assimilate include: (1) Slant path TECs between 80-90 ground receivers and the Global Positioning System (GPS) satellites; (2) Occultation data from a satellite constellation such as COSMIC; (3) TECs associated with the CIT network; (4) Bottomside N e profiles from digisondes associated with the Air Force DISS network; (5) In situ plasma parameters from the SSIES instrument package on the DMSP satellites; and (6) Line-of-sight UV emissions and deduced plasma parameters from the Naval Research Laboratory's SSUSI and SSULI instruments. GAIM will have a modular construction, so that new data types can be readily assimilated when they become available. APPROACH Our approach is to use a two-step process to obtain a 3-D ionospheric reconstruction. First, certain data sets will be assimilated so that the inputs (neutral parameters, electric fields, precipitation) to the physics-based ionosphere-plasmasphere model can be adjusted, within expected errors, to match observations, and then the physics-based model will be run in order to obtain a 3-D N e distribution that is consistent with the measured inputs. This first step should result in realistic ionospheric density features. Next, this simulated ionosphere-plasmasphere system will be used as a starting point for an electron density reconstruction using all of the different data types that pertain to N e and a Kalman filter assimilation technique. The use of a simulated ionosphere-plasmasphere system will provide