[1] During magnetic storms the ionospheric total electron content (TEC) at low-and midlatitudes often shows great enhancements, which may be associated with mechanisms producing midlatitude storm-enhanced density (SED). The TEC enhancements may result from different ionospheric drivers such as electric fields, neutral winds, and neutral composition effects. To study the importance of the ionospheric drivers in producing the TEC enhancement, we perform numerical simulations for the 29-30 October 2003 superstorm period in the American longitude sector ($ À70°W) using the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with values for the neutral wind, temperature, and composition provided by the National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere General Circulation Model (TIEGCM). Various numerical experiments were run to identify the relative importance of the storm-time ionospheric drivers. For carrying out the storm-time SUPIM simulation, the storm-time upward/poleward E Â B drifts are derived from ROCSAT-1 satellite measurements at low and equatorial latitudes and input to SUPIM, while the storm-time neutral wind and composition disturbances are obtained from TIEGCM run. The simulation results presented in this paper, mainly during the evening period, show that the enhanced upward E Â B drifts due to storm-time eastward penetration electric field can expand the low-latitude equatorial ionization anomaly (EIA) to higher latitudes and produce the TEC enhancement. However, by the effect of penetration electric fields alone, the TEC enhancement is less than by combining the storm-generated equatorward neutral winds and the penetration electric fields. Disturbance neutral composition effects decrease the plasma density at higher latitudes and increase it at low and equatorial latitudes. However, the composition effects do not produce a density increase as large as that produced by the neutral-wind and electric-field effects. Our simulations suggest that the storm-generated equatorward neutral winds play an important role in producing the TEC enhancement at low-and midlatitudes, in addition to the eastward penetration electric field.

[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 GPS-derived total electron content (TEC), ion drift measurements from the ROCSAT-1 spacecraft at around 600 km altitude, and far-ultraviolet airglow measured by the Global Ultraviolet Imager (GUVI) carried on board the NASA TIMED satellite are utilized for studying large disturbances of the low-latitude ionosphere during the October-November 2003 superstorm period. Two chains of GPS receivers, one in the American sector ($70°W) and the other in the Asian/Australian sector ($120°E), are used to simultaneously observe the daytime equatorial ionization anomaly (EIA) during the entire storm period. It is found from the GPS-TEC measurements that the EIA expanded to very high latitudes with large increases of TEC right after the storm started. The large expansion of the EIA was associated with strong upward E Â B drifts measured from the Ionospheric Plasma and Electrodynamics Instrument (IPEI) on board the ROCSAT-1, providing evidence of a penetration electric field and a strong plasma fountain effect. Suppression of the EIA was observed during the storm recovery, associated with downward E Â B drifts that were observed by the ROCSAT-1. Significant negative storm effects in the southern hemisphere were also observed in the GPS-TEC during the first day of the recovery phase. The areas of negative storm effects are in good agreement with reductions in the [O]/[N 2 ] density ratio inferred from the ratio of OI (135.6 nm) to LBH emissions measured from GUVI. An enhancement of the EIA was observed on the day, 1 November, that the storm was about to fully recover.

No abstract

No abstract

Abstract. The low-latitude ionospheric tomography network (LITN) consists of a chain of six Naval Navigation Satellite System (NNSS) receiving stations established along 121øE longitude from a geographic latitude of 14.6øN to 3 IøN. It is specifically designed to observe large-scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques. Recently, the network LITN was applied to observations of the October 24, 1995, solar eclipse. Two-dimensional images of ionospheric electron density during the eclipse period were reconstructed. These images and the corresponding results from a nearby ionosonde were compared with those for a reference day. It is shown that during the eclipse day the ionosphere experienced some large-scale changes. In particular, four episodes of electron density enhancement or depression have been identified. ionosphere. More detailed study suggests that the two enhancements have their origins in the ionospheric day-to-day variations, the first depression is related to the combined photochemical and the equatorial fountain effects, and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres. These observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region.

No abstract

No abstract