[1] We have used five years of measurements on board the ROCSAT-1 satellite to develop a detailed quiet time global empirical model for equatorial F region vertical plasma drifts. This model describes the local time, seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions. The model results are in excellent agreement with measurements from the Jicamarca radar and also from other ground-based and in situ probes. We show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier, especially during December and June solstice. The late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons, the morning and afternoon December solstice drifts have significantly different longitudinal dependence, and the daytime upward drifts have strong wave number-four signatures during equinox and June solstice. The largest evening upward drifts occur during equinox and December solstice near the American sector. The longitudinal variations of the evening prereversal velocity peaks during December and June solstice are anti-correlated, which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere.

[1] We compare observations of equatorial plasma bubbles (EPBs) by polar-orbiting satellites of the Defense Meteorological Satellite Program (DMSP) with plasma density measurements from the Republic of China Satellite (ROCSAT-1) in a low-inclination orbit. DMSP data were acquired in the evening sector at low magnetic latitudes between 1989 and 2002. ROCSAT-1 plasma densities were measured in March and April of 2000 and 2002. Observations of individual EPBs detected by both ROCSAT-1 and DMSP were well correlated when satellite orbital paths crossed the same longitude within approximately ±15 min. We compiled a statistical database of ROCSAT-1 EPB occurrence rates sorted by magnetic local time (MLT), magnetic latitude, and geographic longitude. The rate of ROCSAT-1 EPB encounters at topside altitudes rose rapidly after 1930 MLT and peaked between 2000 and 2200 MLT, close to the orbital planes of DMSP F12, F14, and F15. EPB encounter rates have Gaussian distributions centered on the magnetic equator with half widths of $8°. Longitudinal distributions observed by ROCSAT-1 and DMSP are qualitatively similar, with both showing significantly fewer occurrences than expected near the west coast of South America. A chain of GPS receivers extending from Colombia to Chile measured a west-to-east gradient in S4 indices that independently confirms the existence of a steep longitudinal gradient in EPB occurrence rates. We suggest that precipitation of energetic particles from the inner radiation belt causes the dearth of EPBs. Enhancements in the postsunset ionospheric conductance near the South Atlantic Anomaly cause a decrease in growth rate for the generalized Rayleigh-Taylor instability. Results indicate substantial agreement between ROCSAT-1 and DMSP observations and provide new insights on EPB phenomenology.

[1] We used equatorial measurements from the ROCSAT-1 satellite to determine the seasonal and longitudinal dependent equatorial F region disturbance vertical plasma drifts. Following sudden increases in geomagnetic activity, the prompt penetration vertical drifts are upward during the day and downward at night, and have strong local time dependence at all seasons. The largest prompt penetration drifts near dusk and dawn occur during June solstice. The daytime disturbance dynamo drifts are small at all seasons. They are downward near dusk with largest (smallest) values during equinox (June solstice); the nighttime drifts are upward with the largest magnitudes in the postmidnight sector during December solstice. During equinox, the downward disturbance dynamo drifts near sunset are largest in the eastern hemisphere, while the late night upward drifts are largest in the western hemisphere. The longitudinal dependence of the disturbance dynamo drifts is in good agreement with results from simulation studies.

[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.

The global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between ±35° geographic latitudes has been studied with the ROCSAT data during moderate to high solar activity years of 1999 to 2004. The result indicates that the global occurrence distribution of the intermediate‐scale (0.1 to 50 km) density irregularities can be grouped into two different populations, one in the equatorial region and the other in the middle‐to‐subauroral latitude region. The global seasonal/longitudinal (s/l) distribution of equatorial irregularities in the current report reproduces the result of McClure et al. (1998) obtained with the AE‐E observations of the mesoscale (50 to 1000 km) plasma bubble structures during high solar activity years of 1978 to 1980, two solar cycles ago. This implies that the density irregularities of different scales from multistage cascading process of the large‐scale (>1000 km) gravitational Rayleigh‐Taylor instability have manifested in same global s/l distribution pattern. Furthermore, global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. In addition, the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about ±30°. A different kind of midlatitude irregularity distribution starts to fill in from this dip latitude. In other words, the equatorial density irregularity inside a depleted flux tube can only rise, on statistical average, to an apex height of ∼2000 km. Different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes. The occurrence frequency of equatorial density irregularities increases with solar flux intensity; whereas the midlatitude density irregularity is more likely to occur during low solar activity period. The equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times. On the other hand, the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity. The local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight. Such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate‐scale density irregularities.