SA32C-01 10:20h
Equatorial F-Region Zonal Plasma Drifts Over Jicamarca During Quiet and Disturbed Conditions
F-region zonal plasma drifts are important low latitude ionospheric parameters. We use extensive incoherent scatter observations obtained at the Jicamarca Radio Observatory between 1970 and 2003 to study the characteristics of equatorial zonal plasma drifts near the F-region peak. We present initially the results of a local time, season, and solar flux dependent quiet zonal drift model which uses Bernstein polynomials as base functions. These results indicate that over Jicamarca the dependence of the nighttime eastward drift is stronger during equinox and December solstice than during June solstice. We also present the results of the first detailed study of the effect of geomagnetic activity on the zonal drifts. Our results indicate that the zonal disturbance drifts are predominantly westward with the largest values near midnight. In the post-midnight sector, these perturbation drifts are strongly solar flux dependent. The Jicamarca zonal disturbance drifts are largely accounted for by disturbance dynamo electric fields with a dominant time delay of 3-15 hours. The effects of disturbance dynamo electric field with longer time constant are restricted to the late night sector and are largest near solar maximum
SA32C-02 10:35h
Modeling Low Latitude Ion Densities During Magnetic Storms
The impact of magnetic storms in the low-latitude ionosphere is examined. The April 17-18, 2002 and October 29-31, 2003 storms have been simulated using two models: TIME-GCM and the Communication/Navigation Outage Forecasting System (C/NOFS) ionospheric model (PBMod). TIME-GCM was driven with high-latitude AMIE data, providing energy input from Joule heating and particle precipitation. PBMod was driven by ion drift measured at the magnetic equator (at Jicamarca, Peru) and empirical neutral winds. The ground truth data include in situ density from the Challenging Minisatellite Payload (CHAMP) and the Defense Meteorological Satellite Program (DMSP) satellites, as well as ground-based data from ionosondes, GPS receivers and scintillation receivers. Close to Jicamarca, given limitations inherent in the input data, both models show significant agreement with the ground truth data before the storms, and at the beginning of the storms. However, at the end of the storms, the ionospheric densities derived from the C/NOFS model are too high and the densities derived from TIME-GCM are too low. The variations as a function of latitude are not well represented by either model. Several factors contribute to these mediocre results, including, for the C/NOFS model, lack of knowledge of the neutral wind, neutral density composition, and ions production terms. In particular, we illustrate with a few examples how the neutral wind model influences the electron density profiles and total electron content. These magnetic storm simulations clearly demonstrate the need for improved measurements of ionospheric and thermospheric parameters in the equatorial regions, such as will become available from C/NOFS and other satellites.
http://www.vs.afrl.af.mil/factsheets/cnofs.html
SA32C-03 10:50h
Near Synchronous Plasma Structuring in the Middle and Equatorial Ionosphere During Intense Magnetic Storms
Near simultaneous formation of plasma density structures in the ionosphere at middle and equatorial latitudes during intense magnetic storms is investigated. The magnetic storms of July 15, 2000, March 30-31, 2001 and October 29 - 31, 2003 are studied. These storms are characterized by minimum SYM-H (1-min Dst) values of -350 nT, -420 nT and -400 nT respectively. The evolution of plasma density structures in the midlatitude and the equatorial ionosphere is investigated by measuring amplitude scintillation of satellite signals at 250 MHz and L-band, phase fluctuations of GPS signals and by detecting equatorial plasma bubbles with DMSP satellites. It is shown that at the time of the fast rate of change of SYM-H, an impulsive onset of scintillation occurs at Hanscom AFB, a sub-auroral station, and the associated plasma structures in the equatorial ionosphere are observed in the specific longitude sector for which the early evening period corresponds to the time of rapid SYM-H variation, as shown earlier for moderate storms (Basu et al., JGR, 2001). From continuous measurements of scintillation of signals from geostationary satellites and phase fluctuations of GPS signals, it is found that the onset of equatorial plasma structures is delayed by about 20 minutes from the onset of midlatitude scintillation. This delay is discussed in the framework of instantaneous storm-time electric field penetration from high latitudes to middle to equatorial latitudes and the instability growth time of sub-km scale irregularities. It is also shown that during intense storms, the equatorward neutral wind can cause the post-sunset plasma drift in the equatorial region to be as large as 200 m/sec in the westward direction in contrast to the quiet time drift of 100 m/sec in the eastward direction.
SA32C-04 11:05h
Satellite observations of penetration electric fields during a severe magnetic storm
During the severe magnetic storm of July 15, 2000, the ROCSAT-1 low earth orbiting satellite detected an unusually large region of density depletion at low latitudes (<35o) in the nightside ionosphere in co-rotation with the Southern Atlantic Anomaly (SAA). Adjacent to the region of density dropout was a region of density enhancement with irregularities. We deduced convective electric fields during the storm from measurements of drift meters and retarding potential analyzer (RPA) on board ROCSAT-1. Large eastward zonal and outward radial electric fields were detected at apex height below 5000 km. The magnitude of the radial electric field was about 10 mV/m in the beginning of storm main phase and decayed to about 1-2 mV/m during the recovery phase. The zonal electric field followed the similar variation with a smaller magnitude. Electric fields were perturbed with distinct localized features in association with density depletion and enhancement structures in the SAA region. The mean magnitude of electric fields increased as density decreased in the density depletion region. The zonal and radial electric fields were generally reduced in the density enhancement region, suggesting an appreciable enhancement of ionospheric conductivities. Interestingly electric fields in the density enhancement region contained distinct wavy structures with wave length about 100 km. Both electric field components peaked at the apex height of discontinuity separating the density depletion and enhancement. These results suggest that feedback effects in the SAA region are important for the electric field penetration to the low latitude nightside ionosphere.