Concurrent Observations of Equatorial Bubbles by an Imager as well as Networks of GPS Receivers and Ionosondes along the Taiwan Longitude
On the night of 4 March of 2000, a network of twenty ground-based receivers of the global positioning system (GPS) and a 6300A airglow imager monitored bubble events in the Taiwan area. Movies of the total electron content (TEC) have been derived from the GPS measurements to monitor ionospheric plasma bubble drifts. Two events have been observed. The first event was simultaneously registered by the imager and GPS receiver network, which reveals bubbles drifting about 100-200 m/s in the eastward direction, during 1212-1330 UT. For the second event, bubble bands are relatively faint and narrow, which drifting approximately 120 m/s in the eastward direction can be observed by the imager but not by the GPS receiver network during 1400-1515 UT. To further understand dynamics and mechanisms of the bubbles, ionograms recorded by five ionosondes scattered from geomagnetic equator to mid-latitude along the Taiwan longitude are examined and discussed.
A Prototype Operational Ionospheric Forecasting System
This paper describes an operational system sponsored by the US Air Force for generating and distributing near real-time three-dimensional ionospheric electron densities and corresponding GPS propagation delays. The core ionospheric model solves plasma dynamics and composition equations governing evolution of density, velocity and temperature for ion species on a fixed global three-dimensional grid in magnetic coordinates. It uses a realistic model of the Earth's magnetic field and solar indices obtained in real time from NOAA Space Environment Center. At the present time the model computes real-time ion and electron densities at a grid of more than a million points. Higher resolutions are anticipated in the future. While the core model is capable of delivering realistic results, its accuracy can be significantly improved by employing a special set of numerical techniques known as data assimilation. These techniques originated and are currenly used for numerical weather forecasting. The core ionospheric model is constantly fed real-time observational data from a network of reference GPS ground stations. This improves both the nowcast and the forecast. The system also computes uncertainties of the forecast and nowcast via a large-scale suboptimal Kalman filter. The combination of the global numerical model and the Kalman filter provides a unique framework for robust receiver and satellite differential code bias determination. Each receiver that supplies data to the system has its bias estimated in near real time. For longer term forecasting we have explored the use of Support Vector Machine -based classifier that analyzes a time sequence of SOHO EIT solar images and generates a prediction of probabilities of disturbed magnetic conditions at the L1 point. Preliminary results of this work will also be presented. We provide web-based access to the system to early users for validation and exploration purposes at http://www.fusionnumerics.com/ionosphere
Variability of Equatorial Electrodynamic Processes
Equatorial electrodynamic processes play fundamental roles in the distribution of ionization in the low latitude ionosphere and also in the generation of plasma irregularities, which affect communication and navigation systems. These processes exhibit a high degree of temporal and spatial variability during both geomagnetically quiet and disturbed conditions. We use incoherent scatter radar and satellite measurements to examine the short-term variability of equatorial zonal electric fields and their effects on the generation and evolution of equatorial spread-F. We describe the general characteristics of small-scale plasma irregularities during geomagnetically quiet and disturbed conditions. We also discuss new and more powerful data analysis techniques which are being developed for improved for improved forecasting of the occurrence of equatorial spread-F and scintillations.
Saturation of the Transpolar Potential and the Direct Penetration of Electric Fields to the Equatorial Ionosphere
The interplanetary electric field has been observed to penetrate directly to the equatorial latitude in the ionosphere. region where it affects the generation of plasma bubbles which, in turn, cause the scintillation of transionospheric electromagnetic signals. We use the approach of Nopper and Carovillano  to model effects of the Region I and Region II currents on the global electric field. Starting with just Region I currents we reproduce the usual two-cell convection pattern in the polar region, with the Hall currents driving a westward electrojet on the dawnside and an eastward electrojet on the duskside. Near the equatorial region a Pedersen-type relation is found between the ionospheric electric field and current. The global Pedersen conductivity used in the Nopper-Carovillano model is modified so that the associated transpolar potential is in agreement with that predicted by dayside magnetic reconnection (Siscoe et al., 2002). In this way, we find a direct relation between the solar wind electric field and that expected near the equatorial dusk terminator. Nopper R.W. and R. L. Carovillano, Geophys. Res. Ltrs., 5, 699, 1978. Siscoe, G. et al., J. Geophys. Res., 107, A6, SMP 8, 2002.
RF Link Degradations by Spread-F Observed on Cluster Spacecraft
The Cluster spacecraft suffer occasionally from RF link degradations up to complete signal loss. The duration of these disturbances ranged typically from 10 minutes to 4.5 hours. The anomaly coincides with the local evening hours and depends on the latitude of the ground station. The worst station is located in Maspalomas at approximately +18 degree magnetic latitude. The perturbations happen when the receiver is directed southward and the RF signal crosses the equatorial ionosphere. The source of the anomalies are identified to be Spread-F that typically occurs at the geomagnetic equator in the pre-midnight sector but also occasionally in the post-midnight sector. This presentation gives an overview of the events and studies geomagnetic conditions during the events.
Comparison of GPS Phase Fluctuation and Ionosonde Spread F near the Crest of Equatorial Ionization Anomaly in 1996 and 2000
The global positioning system (GPS) has become a useful tool as well as the ionospheric sounder (ionosonde) to investigate ionospheric irregularities. During solar minimum year, 1996, and solar maximum year, 2000, this work studied long-term F-region irregularities using GPS phase fluctuations recorded by a GPS receiver and spread F echoes recorded by an ionosonde. The GPS receiver and the ionosonde operate at two very co-located sites in the north equatorial ionization anomaly (EIA) region. The temporal variations of GPS phase fluctuations and two types (range and frequency types) of spread-F echoes simultaneously observed by the two instruments are analyzed and compared. It is the first time to compare GPS phase fluctuations with ionosonde spread F in the EIA region. The findings demonstrate that when range spread F occurs the corresponding phase fluctuations enhance. In addition, the statistical results exhibit that GPS phase fluctuations are pronouncedly more associated with range spread F than frequency spread F. Furthermore, the solar activity variations in both GPS and ionosonde observations are obvious.