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Anomalous Electron Temperatures in the evening equatorial ionosphere. These are outputs of simulations from the semi-empirical SAMI2-PE (Varney et al. 2012) for the night of the 02 and 05 August 2011.

[1] Using the equatorial electrojet (EEJ) peak current intensity as deduced from CHAMP magnetic observations from the years 2001 through 2009, we investigated the relationship between sudden stratospheric warming (SSW) and lunitidal signatures in the tropical ionosphere. There is a practically one-to-one correspondence between midwinter SSW periods and the strongest 13 day modulation of the EEJ strength as observed by CHAMP. That is, all the midwinter SSW periods from December 2001 to August 2009 were accompanied by an enhanced 13 day modulation of the EEJ strength. No other geophysical phenomenon brought about as strong a 13 day modulation as those of the midwinter SSW periods. During each midwinter SSW period the amplified 13 day modulation of the EEJ strengths starts roughly within ±1 week around the first peak in stratospheric temperature difference. An oscillation with a period of 13.26 days is predicted by the lunitidal equation when considering the precession of the CHAMP orbit. When fitting the lunitidal equation to the EEJ modulations during the midwinter SSW periods, consistent phase delays of 4.4 ± 0.3 days of the tidal signal emerge for all the cases. The results suggest that the pronounced 13 day modulation of the EEJ strength is related to an enhancement of the lunar tide in the ionosphere by the SSW effect.

[1] The extended solar minimum conditions in 2008 and 2009 presented an opportunity to investigate the ionosphere at lower solar activity levels than previously observed. The Coupled Ion Neutral Dynamics Investigation (CINDI) Ion Velocity Meter (IVM) instrument onboard the Communication/Navigation Outage Forecasting System is used to construct the median meridional (vertical) ion drifts, ion densities, and O+ concentrations during periods of low geomagnetic activity for four characteristic seasons each year spanning late 2008 to 2010. The presence of a large semidiurnal component in the ion drift variation at the equator produced significant differences from typical ionospheric conditions. Instead of upward drifts during the day and downward drifts at night, downward drifts in the afternoon and upward drifts near midnight are observed. This semidiurnal component is present in all seasons though it is strongest during the solstice seasons. It is shown that upward drifts at night correspond to regions with a high occurrence of postmidnight irregularities during the December 2008 and June 2009 solstices. A comparison with vertical ion drifts observed by the Jicamarca Radio Observatory supports the methodology used to extract meridional drifts from the IVM.

[1] Large scale electrodynamic and plasma density variations in the low latitude ionosphere have recently been associated with sudden stratospheric warming (SSW) events. We present average patterns of largely enhanced lunar semidiurnal equatorial vertical plasma drift perturbations during arctic winter low and high solar flux SSW events. These perturbations play a dominant role in the electrodynamic response of the low latitude ionosphere to SSWs. Our models indicate that the amplitudes of the enhanced lunar semidiurnal drifts are strongly local time and solar flux dependent, with largest values during early morning low solar flux SSW periods. These results suggest that ionospheric conductance strongly modulate low latitude ionospheric changes during SSWs. They also indicate that lunar semidiurnal effects need to be taken into account by global ionospheric models for their improved forecasting of the low latitude ionospheric response to SSW events, especially for low solar flux conditions.

[1] We have used plasma drift and magnetic field measurements during the 2001–2009 December solstices to study, for the first time, the longitudinal dependence of equatorial ionospheric electrodynamic perturbations during sudden stratospheric warmings. Jicamarca radar measurements during these events show large dayside downward drift (westward electric field) perturbations followed by large morning upward and afternoon downward drifts that systematically shift to later local times. Ground-based magnetometer measurements in the American, Indian, and Pacific equatorial regions show strongly enhanced electrojet currents in the morning sector and large reversed currents (i.e., counterelectrojets) in the afternoon sector with onsets near new and full moons during northern winter warming periods. CHAMP satellite and ground-based magnetic field observations indicate that the onset of these equatorial afternoon counterelectrojets is longitude dependent. Our results indicate that these large electrodynamic perturbations during stratospheric warming periods are due to strongly enhanced semidiurnal lunar wave effects. The results of our study can be used for forecasting the occurrence and evolution of these electrodynamic perturbations during arctic winter warmings.

[1] We present strong evidence that during the January 2008 minor sudden stratospheric warming (SSW) event, the equatorial vertical E × B drifts exhibit a unique and distinctive daytime pattern. We do not think one event causes the other, however both events might be related through the global effects of planetary waves. The drifts were measured by the Jicamarca Incoherent scatter radar located under the magnetic equator. We have observed an anomalous temporal variation of the vertical E × B drifts during the minor SSW event, showing a semidiurnal variation with very large amplitudes lasting for several days. Large differences in the E × B drifts were observed during a period of large increase of temperature and a large decrease of mean zonal wind, in the high latitude stratosphere (60°–90°N). This high correlation is an unexpected finding which might shed new light on sources and mechanisms of quiet-time ionospheric variability.

We provide here an overview of, and a summary of results arising from, an extensive experimental campaign (the Spread F Experiment, or SpreadFEx) performed from September to November 2005, with primary measurements in Brazil. The motivation was to define the potential role of neutral atmosphere dynamics, specifically gravity wave motions propagating upward from the lower atmosphere, in seeding Rayleigh-Taylor instability (RTI) and plasma bubbles extending to higher altitudes. Campaign measurements focused on the Brazilian sector and included ground-based optical, radar, digisonde, and GPS measurements at a number of fixed and temporary sites. Related data on convection and plasma bubble structures were also collected by GOES 12, and the GUVI instrument aboard the TIMED satellite. Initial results of our SpreadFEx analyses are described separately by Fritts et al. (2009). Further analyses of these data provide additional evidence of 1) gravity wave (GW) activity near the mesopause apparently linked to deep convection predominantly to the west of our measurement sites, 2) small-scale GWs largely confined to lower altitudes, 3) larger-scale GWs apparently penetrating to much higher altitudes, 4) substantial GW amplitudes implied by digisonde electron densities, and 5) apparent influences of these perturbations in the lower F-region on the formation of equatorial spread F, RTI, and plasma bubbles extending to much higher altitudes. Other efforts with SpreadFEx data have also yielded 6) the occurrence, locations, and scales of deep convection, 7) the spatial and temporal evolutions of plasma bubbles, 8) 2-D (height-resolved) structures in electron density fluctuations and equatorial spread F at lower altitudes and plasma bubbles above, and 9) the occurrence of substantial tidal perturbations to the large-scale wind and temperature fields extending to bottomside F-layer and higher altitudes. Collectively, our various SpreadFEx analyses suggest direct links between deep tropical convection and large GW perturbations at large spatial scales at the bottomside F-layer and their likely contributions to the excitation of RTI and plasma bubbles extending to much higher altitudes.

[1] We use radar observations from 1996 to 2006 to study the climatology of postsunset equatorial 3-m spread F irregularities over Jicamarca during all seasons. We show that the spread F onset times do not change with solar flux and that their onset heights, which occur near the altitude of the evening F region velocity vortex, increase linearly from about 260 to 400 km from solar minimum to solar maximum. Higher onset heights generally lead to stronger radar echoes. During the equinox, spread F onset occurs near vertical drift evening reversal times, while during the December solstice, they occur near the drift reversal times close to solar minimum and near the time of the prereversal velocity peak for high solar flux conditions. On average, radar plume onset occurs earlier with increasing solar flux in all seasons. Plume onset and peak altitudes increase with solar activity, and the peak heights are generally highest during the equinox. The F region upward drift velocities that precede spread F onset increase from solar minimum to solar maximum and are approximately proportional to the maximum prereversal drift peak velocities.

[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] 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] The occurrence of an additional layer, called F3 layer, in the equatorial ionosphere at American, Indian, and Australian longitudes during the super double geomagnetic storm of 7–11 November 2004 is presented using observations and modeling. The observations show the occurrence, reoccurrence, and quick ascent to the topside ionosphere of unusually strong F3 layer in Australian longitude during the first super storm (8 November) and in Indian longitude during the second super storm (10 November), all with large reductions in peak electron density (Nmax) and total electron content (GPS-TEC). The unusual F3 layers can arise mainly from unusually strong fluctuations in the daytime vertical E × B drift as indicated by the observations and modeling in American longitude. The strongest upward E × B drift (or eastward prompt penetration electric field, PPEF) ever recorded (at Jicamarca) produces unusually strong F3 layer in the afternoon hours (≈1400–1600 LT) of PPEF, with large reductions in Nmax and TEC; the layer also reappears in the following evening (≈1700–1800 LT) owing to an unusually large downward drift. At night, when the drift is unusually upward and strong, the F region splits into two layers.

The Spread F Experiment, or SpreadFEx, was performed from September to November 2005 to define the potential role of neutral atmosphere dynamics, primarily gravity waves propagating upward from the lower atmosphere, in seeding equatorial spread F (ESF) and plasma bubbles extending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of neutral atmosphere and ionosphere dynamics and structure described in this special issue provide enticing evidence of gravity waves arising from deep convection in plasma bubble seeding at the bottomside F layer. Our purpose here is to employ these results to estimate gravity wave characteristics at the bottomside F layer, and to assess their possible contributions to optimal seeding conditions for ESF and plasma instability growth rates. We also assess expected tidal influences on the environment in which plasma bubble seeding occurs, given their apparent large wind and temperature amplitudes at these altitudes. We conclude 1) that gravity waves can achieve large amplitudes at the bottomside F layer, 2) that tidal winds likely control the orientations of the gravity waves that attain the highest altitudes and have the greatest effects, 3) that the favored gravity wave orientations enhance most or all of the parameters influencing plasma instability growth rates, and 4) that gravity wave and tidal structures acting together have an even greater potential impact on plasma instability growth rates and plasma bubble seeding.

In this study, we investigate two global climatological data sets; the occurrence rate of Equatorial Spread- F (ESF), associated with equatorial plasma irregularities, at 400 km altitude obtained from CHAMP observations, and the evening equatorial vertical plasma drift, vz, from ROCSAT-1 measurements. First, as retrieved for a solar flux level of F10.7=150, the longitudinal variation of the two independently derived quantities correlates between 84% and 93% in the seasons December solstice, equinox and June solstice. The highest correlation is found for the solstice seasons when vz is integrated over local time around the prereversal enhancement (PRE) and displaced 6 towards east. The integrated vz is a suitable estimate of the ionospheric height at the time just after the PRE and the 6 displacement is consistent with ESF eastward drift during 2 h which is assumed between creation and detection at satellite altitudes. Second, our analyses reveal a global threshold vz which is required to observe ESF at satellite altitudes. This threshold depends linearly on solar flux with correlations of 97%. Both results bring global evidence on the linear relations between ESF and the vertical plasma drift which have been proven only by local observations so far. This paper includes the first global map of the seasonal/longitudinal variation of the ESF occurrence rate over local time being valid for high solar flux years 2001–2004. The map reveals, e.g. a longitudinal dependence of the persistence of the plasma irregularities indicating that longitude dependent mechanisms other than the PRE determine the ESF lifetime.

[1] We use radar measurements from the Jicamarca Radio Observatory, magnetometer observations from the Pacific sector and ionosonde data from Brazil to study equatorial ionospheric electric fields during the November 2004 geomagnetic storm. Our data show very large eastward and westward daytime electrojet current perturbations with lifetimes of about an hour (indicative of undershielding and overshielding prompt penetration electric fields) in the Pacific equatorial region during the November 7 main phase of the storm, when the southward IMF, the solar wind and reconnection electric fields, and the polar cap potential drops had very large and nearly steady values. This result is inconsistent with the recent suggestion that solar wind electric fields penetrate without attenuation into the equatorial ionosphere for several hours during storm main phase. The largest daytime prompt penetration electric fields (about 3 mV/m) ever observed over Jicamarca occurred during the November 9 storm main phase, when large equatorial electrojet current and drift perturbations were also present in the Pacific and Brazilian equatorial regions. The rise and decay times of these equatorial electric fields were about 20 min longer than of the corresponding solar wind electric fields. The ratios of prompt penetration electric fields and corresponding solar wind electric field changes were highly variable even during the day, and had largest values near dawn. Also, the prompt penetration electric fields did not show polar cap potential drop saturation effects. Our results clearly highlight that the relationships of prompt penetration and solar wind electric fields, and polar cap potentials are far more complex than implied by simple proportionality factors.

We used ion drift observations from the DE-2 satellite to study for the first time the longitudinal variations of middle and low latitude F region zonal plasma drifts during quiet and disturbed conditions. The quiet-time middle latitude drifts are predominantly westward; the low latitude drifts are westward during the day and eastward at night. The daytime quiet-time drifts do not change much with longitude; the nighttime drifts have strong season dependent longitudinal variations. In the dusk-premidnight period, the equinoctial middle latitude westward drifts are smallest in the European sector and the low latitude eastward drifts are largest in the American-Pacific sector. The longitudinal variations of the late night-early morning drifts during June and December solstice are anti-correlated. During geomagnetically active times, there are large westward perturbation drifts in the late afternoon-early night sector at upper middle latitudes, and in the midnight sector at low latitudes. The largest westward disturbed drifts during equinox occur in European sector, and the smallest in the Pacific region. These results suggest that during equinox SAPS events occur most often at European longitudes. The low latitude perturbation drifts do not show significant longitudinal dependence.

[1] We present a model and observations of the evolution of equatorial ionospheric bubbles during a large auroral electrojet (AE) index increase in the recovery phase of a geomagnetic storm. Using a three-dimensional time-dependent numerical simulation model, we find, for the 19–21 October 1998 storm, that the equatorial bubble evolution is different during storm time as compared to quiet time conditions. We have found that the storm time vertical drift in conjunction with reduced off-equatorial E region shorting is the primary mechanism that distinguishes the large AE increase recovery phase storm time evolution from the quiet time case. Comparison of the simulation model with ground-based storm time radar observations is made.

[1] We use extensive incoherent scatter radar observations made at the Jicamarca Radio Observatory between 1970 and 2003 to study and model empirically the equatorial zonal plasma drifts near the F region peak using Bernstein polynomials as base functions. Our quiet-time model results confirm that the daytime drifts are westward and are nearly season and solar cycle independent. The nighttime drifts are eastward, have larger magnitudes, and increase strongly with solar flux, particularly near equinox and December solstice. Enhanced geomagnetic activity drives small eastward perturbation drifts during the day and much larger westward disturbance drifts at night. The nighttime drift perturbations are largest near midnight and increase strongly with solar flux near equinox and December solstice but are essentially absent near June solstice. The Jicamarca zonal disturbance drifts can be largely accounted for by disturbance dynamo electric fields with a dominant time delay of about 3–15 hours following enhanced geomagnetic activity. In the postmidnight sector, there are also smaller westward disturbance drifts associated with time delays of about 15–24 hours and perhaps even longer. Our results strongly suggest that the longitudinal dependence of both the quiet and disturbed equatorial nighttime zonal drifts varies with season.

[1] We use low- and mid-latitude wind data from the Wind Imaging Interferometer (WINDII) on board the Upper Atmosphere Research Satellite (UARS) to study the average response of nighttime upper thermospheric winds to geomagnetic activity. We calculate perturbation winds in magnetic coordinates and analyze them as a function of magnetic local time, latitude, geomagnetic activity, and solar EUV flux. The nighttime zonal disturbance winds are predominately westward, with the strongest effects extending from dusk at 70° to midnight at 45°. Westward disturbance winds are also observed throughout most of the night at low latitudes, where they change to eastward at dawn. Eastward perturbations occur in the post-midnight sector above 50°. The meridional disturbance winds are primarily equatorward above 40° and after 0300 MLT. In the midnight sector during low and moderate solar flux conditions, poleward winds are observed below 40°; during solar maximum, the perturbations are largely equatorward throughout the night.

[1] The interplanetary shock/electric field event of 5–6 November 2001 is analyzed using ACE interplanetary data. The consequential ionospheric effects are studied using GPS receiver data from the CHAMP and SAC-C satellites and altimeter data from the TOPEX/Poseidon satellite. Data from ∼100 ground-based GPS receivers as well as Brazilian Digisonde and Pacific sector magnetometer data are also used. The dawn-to-dusk interplanetary electric field was initially ∼33 mV/m just after the forward shock (IMF BZ = −48 nT) and later reached a peak value of ∼54 mV/m 1 hour and 40 min later (BZ = −78 nT). The electric field was ∼45 mV/m (BZ = −65 nT) 2 hours after the shock. This electric field generated a magnetic storm of intensity DST = −275 nT. The dayside satellite GPS receiver data plus ground-based GPS data indicate that the entire equatorial and midlatitude (up to ±50° magnetic latitude (MLAT)) dayside ionosphere was uplifted, significantly increasing the electron content (and densities) at altitudes greater than 430 km (CHAMP orbital altitude). This uplift peaked ∼2 1/2 hours after the shock passage. The effect of the uplift on the ionospheric total electron content (TEC) lasted for 4 to 5 hours. Our hypothesis is that the interplanetary electric field “promptly penetrated” to the ionosphere, and the dayside plasma was convected (by E × B) to higher altitudes. Plasma upward transport/convergence led to a ∼55–60% increase in equatorial ionospheric TEC to values above ∼430 km (at 1930 LT). This transport/convergence plus photoionization of atmospheric neutrals at lower altitudes caused a 21% TEC increase in equatorial ionospheric TEC at ∼1400 LT (from ground-based measurements). During the intense electric field interval, there was a sharp plasma “shoulder” detected at midlatitudes by the GPS receiver and altimeter satellites. This shoulder moves equatorward from −54° to −37° MLAT during the development of the main phase of the magnetic storm. We presume this to be an ionospheric signature of the plasmapause and its motion. The total TEC increase of this shoulder is ∼80%. Part of this increase may be due to a “superfountain effect.” The dayside ionospheric TEC above ∼430 km decreased to values ∼45% lower than quiet day values 7 to 9 hours after the beginning of the electric field event. The total equatorial ionospheric TEC decrease was ∼16%. This decrease occurred both at midlatitudes and at the equator. We presume that thermospheric winds and neutral composition changes produced by the storm-time Joule heating, disturbance dynamo electric fields, and electric fields at auroral and subauroral latitudes are responsible for these decreases.

[1] The major magnetic storm of 4–5 June 1991 was well observed with the Combined Release and Radiation Experiment (CRRES) satellite in the duskside inner magnetosphere and with three Defense Meteorological Satellite Program (DMSP) spacecraft in the polar ionosphere. These observations are compared to results from the Rice Convection Model (RCM), which calculates the inner magnetospheric electric field and particle distribution self-consistently. This case study, which uses the most complete RCM runs to date, demonstrates two significant features of magnetospheric storms, the development of subauroral polarization streams (SAPS) and plasma-sheet particle injection deep into the inner magnetosphere. In particular, the RCM predicts the electric field peak near L = 4 that is observed by the CRRES satellite during the second injection. The RCM calculations and DMSP data both show SAPS events with similar general characteristics, though there is no detailed point-by-point agreement. In the simulation, SAPS are generated by the deep penetration of plasma sheet protons to L < 4 and Earthward of the plasma sheet electrons. Similarly, the vast majority of the ions that make up the storm-time ring current came from the plasma sheet; most of the particles that made up the prestorm quiet-time ring current escaped through the dayside magnetopause during ring current injection. The RCM demonstrates the capability of plasma sheet ions to reach all ring current orbits and predicts the location of the injected particles (both ions and electrons) reasonably well. However, it overpredicts the ion flux in the inner magnetosphere.

[1] Low-latitude ionospheric electric fields and currents are often strongly disturbed during periods of enhanced geomagnetic activity. These perturbations can last for several hours after geomagnetic quieting. We use incoherent scatter radar measurements from Jicamarca and Arecibo during 19–21 October 1998 to study, for the first time, the low-latitude disturbance electric fields during the recovery phase of a large magnetic storm. On 19 October the Jicamarca data showed initially large and short-lived (time scale of about 10–20 min) upward and westward drift perturbations in the early afternoon sector, due to the penetration of strong magnetospheric electric fields probably driven by an increase in the solar wind dynamic pressure. Following the decrease of auroral activity, very strong afternoon and nighttime zonal and vertical disturbance dynamo drifts were observed over Jicamarca but, surprisingly, only small and sporadic perturbation drifts were present over Arecibo. The latitudinal variation of the daytime zonal disturbance dynamo drifts during this day is in good agreement with that previously observed for thermospheric disturbance winds. The disturbance dynamo drifts also indicate significantly different local, storm time, and possibly latitudinal dependences for their vertical/perpendicular and zonal drift components, suggesting large variations in both the amplitude and direction of the disturbance dynamo electric field vector. In the next day, during moderately disturbed conditions, the daytime perturbation drifts were small, but following transient increases in geomagnetic activity, large upward/perpendicular drifts were observed near dusk over Jicamarca and Arecibo, where simultaneous westward perturbation drifts were also seen. These perturbations are consistent with the occurrence of strong prompt penetration electric fields reaching the magnetic equator. The measured prompt penetration drift patterns are generally in good agreement with predictions from global convection models. Later at night, under moderately disturbed conditions, relatively large zonal and meridional disturbance dynamo electric fields were observed by the two radars. Our results illustrate the large variability of the low-latitude perturbation electric fields relative to their climatological values after large storms, probably due to the importance of additional disturbance processes. They also indicate that a much deeper understanding of solar-wind/magnetospheric/ionospheric processes is required for accurate predictions of these electric fields.

[1] Equatorial counterelectrojet (CEJ) events are analyzed in association with changes in the interplanetary magnetic field (IMF), polar cap potential (PCP), and electric field measured in the equatorial ionosphere. In one event on 16 July 1995, the equatorial CEJ was observed at the afternoon dip equator during the recovery phase of the substorm when the IMF turned northward. Rapid decreases in the PCP and in the auroral electrojet occurred simultaneously with the equatorial CEJ, suggesting instantaneous equatorward penetration of the rapid decrease in the electric field associated with the region 1 field-aligned currents (R1 FACs) under the condition of a well-developed shielding electric field due to the R2 FACs. In the other event on 8 April 1993, the equatorial CEJ associated with the northward turning of the IMF was directly related to a rapid decrease in the equatorial electric field measured by the Jicamarca incoherent scatter radar as well as to a decrease in the PCP. We confirm the scenario for the substorm-associated equatorial CEJ as caused by the dominant R2 FACs when the R1 FACs decrease abruptly because of the northward turning of the IMF. We also suggest that the DP 1 current system is composed of the Hall currents surrounding the R2 FACs and the equatorial CEJ closing with the R2 FACs, which are superposed on the DP 2 currents caused by the R1 FACs, being dominant when the IMF turns northward. The coherent occurrence of the electric field in the F region with the electric current in the E region at the equator is explained by applying the Earth-ionosphere waveguide model of Kikuchi and Araki [1979b] as a most promising transmission mechanism. All the conditions for the equatorial CEJ most likely occur during the substorm, but the northward turning of the IMF and the resultant decrease in the PCP play a crucial role under the condition of well-developed R2 FACs.

[1] Using numerical simulation techniques, we present the first study of the three-dimensional nonlinear evolution of an equatorial spread-F bubble. The background ionosphere used to initialize the bubble evolution is computed using a time-dependent first-principles equatorial plasma fountain model together with a prereversal enhancement vertical drift model. We find that finite parallel conductivity effects slow down both the linear and nonlinear bubble evolution compared to the two-dimensional evolution. In addition we find that bubble-like structures with extremely sharp density gradients can be generated off the equator at equatorial anomaly latitudes in agreement with recent observations.

[1] Midlatitude nighttime thermospheric neutral winds are strongly dependent on season, solar activity, and latitude. We use an extensive database of wind measurements made during 1989–2001 by the Millstone Hill Fabry-Perot interferometer to study the detailed climatology of quiet time neutral winds near an altitude of 250 km. To facilitate the analysis of these data, we develop a local time, day-of-year, solar flux, and latitude-dependent empirical model, with the latitude dependence obtained by considering north looking and south looking observations separately. Our results show that the zonal winds are predominantly eastward after dusk and westward before dawn, with the strongest eastward winds occurring in the winter and with an east-to-west transition that occurs earliest in the summer. The zonal winds exhibit weak-to-moderate latitudinal gradients, with more westward values to the north. The zonal wind magnitudes decrease with increasing solar flux; the strongest trends occur during winter. The meridional winds are predominantly equatorward in all cases and exhibit strong latitudinal gradients, with larger values to the north. The maximum nighttime equatorward winds decrease with increasing solar flux, except during summer, when there is no significant solar activity variation. They are largest during the summer, except at solar minimum when a semiannual variation is observed and the peak winds occur during the equinoxes. Earlier studies of midlatitude wind measurements are generally consistent with our data, with our results providing a considerably more detailed description of the nighttime wind climatology at midlatitudes.

[1] Large-scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10°S–20°S, longitudes 50°W–10°E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450-km altitude show vertically downward ion drift velocities exceeding 120 m s−1 and depleted regions where the ion density is around 2 × 104 cm−3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE-E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.

[1] Thermospheric neutral winds exhibit strong altitudinal and latitudinal variation during geomagnetically quiet and active times. We use daytime middle and low-latitude neutral winds measured by the Wind Imaging Interferometer (WINDII) instrument on board the Upper Atmosphere Research Satellite (UARS) over the 90–275 km height range to study the altitude and season dependent climatology of disturbance winds (i.e., with quiet time patterns removed) in magnetic coordinates. The daytime perturbations winds are generally equatorward and westward and decrease toward the magnetic equator. Both the zonal and meridional components decrease sharply below 120 km and are essentially insignificant below 100 km. The seasonal dependence of the disturbance winds is strongest in the early morning sector. The zonal disturbance winds are predominantly westward with largest magnitudes near 150 km at late afternoon upper midlatitudes (magnetic latitudes 50°–60°) and near 120 km in the early morning winter between 25° and 45°. At upper midlatitudes in the early morning sector, the zonal perturbation winds are eastward above ∼120 km in the summer hemisphere and between ∼120 and 180 km in the winter hemisphere. Eastward perturbations are also observed near the magnetic equator with largest values near noon at 250 km. The meridional perturbation winds are mostly equatorward and nearly height-independent above ∼150 km, with the largest magnitudes occurring in the early morning winter hemisphere. In the late afternoon sector the upper midlatitude meridional disturbance winds are poleward, and there is a small winter-to-summer flow at lower latitudes. In general, the storm-time dependence of the disturbance winds is quite complex. The perturbation winds begin to develop after 0–9 hours and tend to saturate 12–24 hours after storm onset. The development does not change much with altitude, and the seasonal dependence is generally important only at storm times greater than 6–12 hours.

[1] We use 630.0 nm nightglow Fabry-Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late-night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at ∼2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM-93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time-dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in ∼6 hours; the late-night eastward winds are smaller and reach their peak values ∼3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward ∼6–10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.

International audience; We use incoherent scatter radar measurements from Millstone Hill and Saint Santin to study the midlatitude F region electrodynamic plasma drifts during geomagnetically quiet and active periods. We present initially a local time, season, and solar flux dependent analytical model of the quiet time zonal and meridional ExB drifts over these stations. We discuss, for the first time, the Saint Santin drift patterns during solar maximum. We have used these quiet time models to extract the geomagnetic perturbation drifts which were modeled *s * function of the time history of the auroral electrojet indices. Our results illustrate the evolution of the disturbance drifts driven by the combined effects of prompt penetration *nd longer lasting perturbation electric fields. The meridion*l electrodynamic disturbance drifts have largest amplitudes in the midnight-noon sector. The zonal drifts *re predominantly westward, with largest amplitudes in the dusk-midnight sector *nd, following a decrease in the high-latitude convection, they dec*y more slowly th*n the meridional drifts. The prompt penetration *nd steady state zonal disturbance drifts derived from radar measurements *re in good agreement with results obtained from both the ion drift meter data on board the Dynamics Explorer 2 (DE 2) satellite *nd f?om the Rice Convection Model.

Vertical ion drift velocity data from Jicamarca have been analyzed for a lunar semidiurnal tide using a least squares fitting method. Amplitudes of up to 6 m s−1 are obtained with phases in agreement with lunar tidal determinations of other associated physical parameters. Variations between season, solar activity, and day to night are also examined. Generally, amplitudes are larger in the southern summer. Much of the phase variation with season is very similar for solar maximum and minimum years. There is a summer to winter phase change that is most distinct at solar maximum nighttime. A day-to-night phase reversal can also been seen in some seasons. Hints of this are also found in the lunar tide in the F region height and in the magnetic variations at Huancayo.

We investigate the seasonal, local solar time, and geomagnetic activity variations of the average Doppler velocity measured by an HF digital ionosonde deployed at Bundoora, Australia. The Doppler velocities were heavily averaged to suppress the short-term effects (hours) of atmospheric gravity waves, and thereby obtain the diurnal variations attributed to the tidally-driven ionospheric dynamo and electric fields generated by magnetic disturbances. The observed seasonal variations in Doppler velocity were probably controlled by variations in the lower thermospheric winds and ionospheric conductivity above Bundoora and in the magnetically conjugate location. The diurnal variations of the meridional (fieldperpendicular) drifts and their perturbations exhibited a complex structure, and were generally smaller than the variations in the zonal drifts. The latter were basically strongly westward during the evening to early morning, and weakly eastward during the late morning to just past noon. The zonal perturbations were strongly enhanced by increasing geomagnetic activity, and closely resembled the perturbation drifts measured by the incoherent scatter radar (ISR) at Millstone Hill. There was also some resemblance between the diurnal variations in the meridional drifts. Overall, the comparisons suggest that with sufficient averaging, Doppler velocities measured with digital ionosondes at mid-latitudes correspond to true ion motions driven by ionospheric electric fields. This is a useful result because apart from the ISRs located in the American-European sector, there are no ground-based instruments capable of measuring electric fields in the mid-latitude ionosphere.

The occurrence of equatorial density depletions in the nightside F region ionosphere has been investigated by using observations gathered by the polar-orbiting Dynamics Explorer 2 satellite from August 1981 to February 1983. A variety of electric field/plasma drift patterns were observed within these depletions, including updrafting, downdrafting, bifurcating, converging, subsonic, and supersonic flows. The depletions, 116 events in total, are distributed over two groups: group I (eveningside depletions) consists of the events in the 1900–2300 MLT sector, and group II (morningside depletions) are the events in the 2300–0600 MLT sector. A statistical analysis reveals clear differences in the density depletion occurrence rates between the two groups. Magnetic activity appears to suppress the generation of eveningside depletions with a delay of 2–3 hours. In the morningside the probability of observing depletions increases instantly with increasing magnetic activity; yet the best correlation is found with a 2-hour delay. This indicates that in the premidnight sector the substorm-induced dynamo and prompt penetration electric fields induce westward electric fields in the equatorial ionosphere 2 hours after the onset in the auroral region. In the postmidnight sector, high-latitude ionospheric disturbances induce instantly equatorial eastward electric fields that move the F layer to higher altitudes, where it can become unstable to the Rayleigh-Taylor instability. Eveningside depletions were observed at all longitudes except those over the Pacific Ocean, while the morningside depletions occurred mostly over the Pacific and Atlantic-African sectors.

We present a technique which combines time series of line-of-sight (LOS) velocity and electron density measurements from the Sondrestrom incoherent scatter radar (74.5° invariant latitude) to reconstruct the large-scale horizontal structure of the F region ionosphere during polar cap patch events. This reconstruction technique provides a new density-based means of examining patch morphology. Its wide region of coverage also facilitates comparison of radar measurements with other observational data sets. For two periods when patches were present and convection conditions in the nightside polar cap could be adequately approximated by the simple velocity model used in this initial implementation of the technique, we compare reconstructed radar data montages with in situ ion density data from DMSP satellites and 630.0 nm all-sky images taken at the radar site. The satellite data agree well with the radar reconstructions near the observation site, and show general agreement well beyond the radar field of view (FOV). Data from satellite passes to the east and west of the radar coverage suggest high densities were present over many hours of magnetic local time in the nightside polar cap. Many of the patches observed in the radar data were elongated perpendicular to the convective flow and may have extended well beyond the radar FOV. The characteristics of density enhancements observed by the satellites in the nightside auroral zone were found to be consistent with the creation of auroral zone blobs by distortion of patches seen exiting the polar cap in the radar data. Radar data reconstructions also show reasonable qualitative agreement with instantaneous 630.0 nm all-sky images regarding the location, size, and shape of patch features. In addition to validating the streamline-mapping technique, this result also lends support to previous optical studies of patch occurrence and morphology.

Optical emissions excited by high-power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful high-frequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large-scale (≈10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E × B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 and 1995 at the ionospheric modification facility near Nizhny Novgorod, Russia. Analysis of HPAA clouds yields zonal plasma drifts of 70 m s−1 eastward at night. On the basis of artificial airglow from energetic electrons generated at 260 km the zonal neutral wind speed was estimated to be 96 m s−1 and the O(1D) diffusion coefficient was determined to be between 0.8 and 1.4 × 1011 cm2 s−1. The quenched lifetime of the O(1D) was determined to be 29.4 s. The diffusion and quenching rates are directly related to the atomic and molecular concentrations in the thermosphere. Improvements in the remote-sensing technique may be obtained if the intensity of the artificial airglow emissions is increased. High-power radio transmissions employing pulse sequences and tuning near electron cyclotron harmonics were attempted to increase the optical emissions. Both of these, however, produced reduced intensity, and consequently, continuous transmission at frequencies away from electron gyro harmonics is the preferred heating regime.

Thermospheric neutral winds play important roles on the dynamics of the upper atmosphere. We use extensive F region zonal and meridional wind data measured by the Wind Imaging Interferometer (WINDII) instrument on board the Upper Atmosphere Research Satellite (UARS) to study the mid-and low-latitude daytime average perturbation winds during magnetically disturbed periods. We have determined the perturbation winds by subtracting the quiet time wind values along the satellite orbits. Our data indicate large changes in the latitudinal profiles of both the zonal and meridional disturbance winds from morning to afternoon hours. The early morning zonal disturbance winds are eastward near the equator and at upper mid-latitudes (above 50°), and westward at lower mid-latitudes. The mid-morning and afternoon zonal disturbance wind patterns show very small amplitudes at low latitudes and increasing westward winds at mid-latitudes. These latitudinal profiles are in excellent agreement with the patterns derived from zonal wind measurements on board the DE-2 satellite. The meridional daytime perturbation winds are equatorward and have magnitudes increasing linearly with latitude, with largest values in the early morning sector. The zonal and meridional disturbance winds show an approximately linear increase with the Kp index during geomagnetically active conditions.

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.

Thermospheric winds have been systematically determined at Arequipa, Peru, and Arecibo, Puerto Rico, from Fabry-Perot interferometer measurements of Doppler shifts in the nightglow 630 nm line. The wind databases (1983 – 1990 at Arequipa and 1980 – 1990 at Arecibo) have been edited to eliminate measurements during geomagnetically disturbed conditions, then sorted by season and solar flux level. Following this, they were averaged to obtain the climatological behavior of the nighttime wind variations at the two locations. A new averaging technique, multivariate regression analysis, has been applied to the data, and the results compared to our prior binning averages. The observed wind behaviors at the Arequipa and Arecibo Observatories, which are at equal geographic latitudes on opposite sides of the equator, are contrasted to establish the seasonal flow patterns. The regression analysis results have then been compared with the predicted behavior provided by the National Center for Atmospheric Research's Thermosphere-Ionosphere-Electrodynamics General Circulation Model. In many cases, qualitative agreement between measurements and predictions is found as to wind directions and temporal variations, with differences in magnitude of ∼0–50 m/s. However, some striking differences are found that may arise from ionosphere-thermosphere coupling effects. The overall results provide an important step in establishing the climatology of the thermospheric winds at equatorial and low-latitude sites.

We present the first global empirical model for the quiet time F region equatorial vertical drifts based on combined incoherent scatter radar observations at Jicamarca and Ion Drift Meter observations on board the Atmospheric Explorer E satellite. This analytical model, based on products of cubic-B splines and with nearly conservative electric fields, describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values. Our results indicate that during solar minimum, the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15–20 m/s, is observed only in the American sector during December solstice with amplitudes of about 5–10 m/s, and is absent at all longitudes during June solstice. The solar minimum evening reversal times are fairly independent of longitude except during December solstice. During solar maximum, the evening upward vertical drifts and reversal times exhibit large longitudinal variations, particularly during the solstices. In this case, for a solar flux index of 180, the June solstice evening peak drifts maximize in the Pacific region with drift amplitudes of up to 35 m/s, whereas the December solstice velocities maximize in the American sector with comparable magnitudes. The equinoctial peak velocities vary between about 35 and 45 m/s. The morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. We also show that our model results are in good agreement with other equatorial ground-based observations over India, Brazil, and Kwajalein.

We have used ion drift observations from the DE-2 satellite to determine the latitudinal variation and the temporal evolution of mid- and low-latitude prompt penetration zonal plasma drifts driven by magnetospheric electric fields. Our results indicate that sudden increases in convection lead to predominantly westward perturbation drifts which decrease equartorwards and have largest amplitudes in the dusk-midnight sector. The diurnal perturbation drift patterns shift to later local times with increasing storm time and decay to new quasi-equilibrium values in about 2 hours, as the ring current readjusts to the new polar cap potential. The daily and latitudinal variations and temporal evolution of the DE-2 prompt penetration drifts are generally in good agreement with predictions from the Rice Convection Model, although the experimental results show larger amplitudes and longer shielding time constants.

We use low- and mid-latitude zonal ion drift observations from the DE-2 satellite and auroral electrojet indices to study the temporal and latitudinal variations of F-region perturbation drifts during magnetically disturbed conditions. These perturbation drifts are driven by magnetospheric and ionospheric disturbance dynamo electric fields with time constants from less than one to several hours. We determine, initially, the drift patterns due to the prompt penetration of magnetospheric electric fields and of longer lasting disturbances. In this study, we concentrate on the properties of the longer lasting perturbations which occur with latitude-dependent time delays after enhancements in the high-latitude ionospheric currents. These perturbation drifts are predominantly westward at all latitudes with largest amplitudes in the midnight sector and smallest near noon. The daily variation of these disturbance drifts derived from the satellite data is in good agreement with results from incoherent scatter radar observations. Our results indicate that these longer lasting perturbations are due to the combined effects of ionospheric disturbance dynamo electric fields and leakage of high-latitude steady-state electric fields to lower latitudes.

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.

We review the experimental observations of meter scale plasma irregularities in the auroral E region and the status of their theoretical understanding. Most of the experimental data is derived from VHF radar scatter experiments, but sounding rockets also provide crucial information not obtainable from radars. Linear theories correctly predict the altitude of occurence, strong magnetic aspect sensitivity, marginal instability, and typical phase velocities. Subsequent nonlinear theories have been developed to account for other observed features but with less satisfying application. Further understanding of auroral electrojet irregularities is impeded by precision limitations of existing instruments, by radar data which may seem incompatible, by the usually poor knowledge of the ambient conditions during these experiments, and by some confusion in the nomenclature (e.g. “type 2”) used to describe the irregularities. We hope to clarify some of these experimental and theoretical issues. We will discuss observational “facts” that need to be refined and point out weaknesses of existing theories or their common interpretations. Finally, we will outline some avenues for future experimental and theoretical pursuit.

A radio-interferometer array illuminated by 136-MHz beacons of several geosynchronous satellites has been used to study small (≥ 1013 m−2) transient disturbances in the total electron content along the lines of sight to the satellites. High-frequency (ƒ> 3 mHz) electron content oscillations are persistently observed, particularly during night and particularly during geomagnetically disturbed periods. The oscillations move across the array plane at speeds in the range 200–2000 m/s, with propagation azimuths that are strongly peaked in lobes toward the western half-plane. Detailed analysis of this azimuth behavior, involving comparison between observations on various satellite positions, indicates compellingly that the phase oscillations originate in radio refraction due to geomagnetically aligned plasma density perturbations in the inner plasmasphere. The motion of the phase perturbations across the array plane is caused by E × B drift of the plasma medium in which the irregularities are embedded. We review the statistics of 2.5 years of around-the-clock data on the local time, magnetic disturbance, seasonal, and line-of-sight variations of these observed irregularities. We compare the irregularities' inferred electrodynamic drifts to what is known about midlatitude plasma drift from incoherent scatter. Finally, we show in detail how the observation of these irregularities provides a unique and complementary monitor of inner plasmasphere irregularity incidence and zonal drift.

Studies of equatorial F region evening vertical plasma drifts using different measurement techniques have produced conflicting results. We examine the relationship of incoherent scatter radar and ionosonde drift observations over the Peruvian equatorial region, and AE-E satellite drifts for different geophysical conditions. Our data show that there is large day-to-day variability on the ratios of radar and ionosonde drifts, but on the average the measurements from these two techniques are in fair agreement during low and moderate solar flux conditions. For high solar activity, however, the Jicamarca evening drifts during equinox and December solstice are significantly larger than the ionosonde drifts. These results can be explained by the different height ranges of the radar and ionosonde measurements, and the increase of the upward drift velocity with height below the F region peak. This altitudinal variation is related to the longitudinal gradient of the zonal plasma drifts as a result of the curl-free electric field condition. Our results also indicate that during equinox the increase of the vertical prereversal velocity enhancement with solar activity is largely longitude independent.

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.

Ion drift meter observations from the Atmosphere Explorer E satellite during the period of January 1977 to December 1979 are used to study the dependence of equatorial (dip latitudes ≤ 7.5°) F region vertical plasma drifts (east-west electric fields) on solar activity, season, and longitude. The satellite-observed ion drifts show large day-to-day and seasonal variations. Solar cycle effects are most pronounced near the dusk sector with a large increase of the prereversal velocity enhancement from solar minimum to maximum. The diurnal, seasonal, and solar cycle dependence of the longitudinally averaged drifts are consistent with results from the Jicamarca radar except near the June solstice when the AE-E nighttime downward velocities are significantly smaller than those observed by the radar. Pronounced presunrise downward drift enhancements are often observed over a large longitudinal range but not in the Peruvian equatorial region. The satellite data indicate that longitudinal variations are largest near the June solstice, particularly near dawn and dusk but are virtually absent during equinox. The longitudinal dependence of the AE-E vertical drifts is consistent with results from ionosonde data. These measurements were also used to develop a description of equatorial F region vertical drifts in four longitudinal sectors.

Electron density profiles and isodensity contours derived from Jicamarca incoherent scatter radar observations in Peru for October 1–2, 1970, are compared in detail with results from the Phillips Laboratory global theoretical ionospheric model. This model solves the ion continuity equation for O+ concentration through production, loss, and transport of ionization. The primary factor controlling the peak plasma density at Jicamarca is the vertical E×B drift, which drives the ionization upward during the day and downward at night. When we use the measured drift in the model, we achieve excellent results with the measured electron density profiles. We illustrate the sensitivity of the low-latitude plasma density calculations to changes in the vertical E×B drift and changes in the neutral winds. We also compare the calculated profiles and peak parameters with an empirical model, the International Reference Ionosphere (IRI). We illustrate several limitations associated with the IRI that contribute to its limited capability at the magnetic equator.

Incoherent scatter radar observations of ion drifts taken at Millstone Hill (42.6°N, 288.5°E) during 73 experiments in the period February 1984 to February 1992 are used to construct, for the first time at this station, average quiet-time E×B drift patterns for both solar cycle maximum and minimum, for the summer, winter, and equinox seasons. The daily variation of V⊥N shows a reversal from northward to southward drifts near noon, and a return to northward drifts in the premidnight hours. The weaker southward drift in the afternoon in summer noted by Wand and Evans (1981) is shown to occur only at sunspot minimum. The daily variation of V⊥E shows daytime eastward drifts and nighttime westward drifts, except in summer when the usual daytime eastward maximum near 1200 LT is suppressed. The daily mean drift is westward for all seasons, and is largest in summer. The daytime eastward drift and nighttime westward drift tend to be stronger at solar maximum than at solar minimum. Average drift patterns are also constructed for equinox for both extremely quiet and geomagnetically disturbed periods. V⊥N is appreciably more northward under extremely quiet than under disturbed conditions in the postmidnight and morning periods. During extremely quiet periods, V⊥E turns slightly eastward in the evening hours, while it is strongly westward for disturbed conditions. This result contrasts with the strong eastward drifts in the evening in summer reported for extremely quiet conditions at Millstone Hill by Gonzales et al. (1978). A strong anticorrelation is seen at Millstone Hill between V⊥N and V∥, as is found at lower latitude stations. The quiet-time patterns are discussed in terms of the causative E and F region dynamo mechanisms. At Millstone Hill, conjugate point electric fields are also important in winter when the conjugate ionosphere is sunlit for much of the night.

We have used Arecibo incoherent scatter measurements from 1981 to 1990 to determine the characteristics of low-latitude F region plasma drifts. The measurements show large day-to-day variability even during magnetically quiet periods. The average poleward/perpendicular plasma drifts do not change significantly with season and solar cycle except in the midnight-morning sector. The zonal drifts show clear solar cycle and seasonal effects. The afternoon-nighttime eastward drifts increase with solar flux; the westward drifts in the early morning-afternoon sector show a large increase from summer to winter but are independent of solar activity. The two perpendicular velocity components also respond differently to magnetic activity. The average northward/perpendicular drifts decrease with magnetic activity during the day but do not exhibit a systematic response at night. The zonal component shows increased westward drifts occurring predominantly at night. The plasma drifts along the magnetic field lines exhibit large altitudinal and seasonal variations, particularly near solar minimum, and are generally anticorrelated with the perpendicular/north drifts. The drift patterns observed by the Arecibo and the middle and upper atmosphere radars have significantly different seasonal dependence. This can be explained by electrodynamic effects in the corresponding local and conjugate ionospheres. The large longitudinal variation of the quiet time F region plasma drifts results from the displacement between the geographic and dip equators and from magnetic field declination effects. In general, the longitudinal variation should also depend on magnetic activity.

We have used observations from both the Bennett ion mass spectrometer and the retarding potential analyzer on board the Atmosphere Explorer E satellite to study the longitudinally averaged O+, H+, and He+ concentrations from 150 to 1100 km in the equatorial ionosphere during the 1975–1976 solar minimum. Our results suggest that the ion mass spectrometer measurements need to be increased by a factor of 2.15 to agree with the densities from the retarding potential analyzer and with ground-based measurements. The peak H+ concentrations are about 2.5×104 cm−3 during the day and 104 cm−3 at night and vary little with season. The O+/H+ transition altitude lies between 750 and 825 km during the day and between 550 and 600 km at night. He+ is a minor species at all altitudes; its concentration is highly variable with a maximum value of about 103 cm−3 during equinox daytime.