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Using the 47 MHz Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (10.36°S dip latitude), it is shown that postmidnight irregularities during solar minimum are morphologically different from those detected during solar maximum and are quite similar to those observed with the middle and upper atmosphere (MU) radar in midlatitudes (29.3°N dip latitude). Utilizing the rapid beam‐steering capability of the EAR, the spatial structure of the postmidnight irregularities is clearly presented for the first time. It is found that they usually propagate westward and can be categorized into two types. One shows sharp upwelling plumes near local midnight, which should not be a mere passage of fossil plasma bubbles. The other has successive tilted structures which have the same orientation as medium‐scale traveling ionospheric disturbances typically observed at midlatitudes. We suggest that the convergence of the equatorward thermospheric wind which is believed to be responsible for the midnight temperature maximum may be an important factor to produce a preferable condition for the upwelling plumes in the postmidnight sector. The displacement between geographic and magnetic equators may also be important for seasonal/longitudinal variation of the postmidnight irregularities. Key Points Detailed observations of postmidnight irregularities in low‐latitude regions Similarity between low‐ and midlatitude irregularities during solar minimum Proposed importance of neutral ionosphere coupling

Accurate estimation of global vertical distribution of ionospheric and plasmaspheric density as a function of local time, season, and magnetic activity is required to improve the operation of space‐based navigation and communication systems. The vertical density distribution, especially at low and equatorial latitudes, is governed by the equatorial electrodynamics that produces a vertical driving force. The vertical structure of the equatorial density distribution can be observed by using tomographic reconstruction techniques on ground‐based global positioning system (GPS) total electron content (TEC). Similarly, the vertical drift, which is one of the driving mechanisms that govern equatorial electrodynamics and strongly affect the structure and dynamics of the ionosphere in the low/midlatitude region, can be estimated using ground magnetometer observations. We present tomographically reconstructed density distribution and the corresponding vertical drifts at two different longitudes: the East African and west South American sectors. Chains of GPS stations in the east African and west South American longitudinal sectors, covering the equatorial anomaly region of meridian ∼37°E and 290°E, respectively, are used to reconstruct the vertical density distribution. Similarly, magnetometer sites of African Meridian B‐field Education and Research (AMBER) and INTERMAGNET for the east African sector and South American Meridional B‐field Array (SAMBA) and Low Latitude Ionospheric Sensor Network (LISN) are used to estimate the vertical drift velocity at two distinct longitudes. The comparison between the reconstructed and Jicamarca Incoherent Scatter Radar (ISR) measured density profiles shows excellent agreement, demonstrating the usefulness of tomographic reconstruction technique in providing the vertical density distribution at different longitudes. Similarly, the comparison between magnetometer estimated vertical drift and other independent drift observation, such as from VEFI onboard Communication/Navigation Outage Forecasting System (C/NOFS) satellite and JULIA radar, is equally promising. The observations at different longitudes suggest that the vertical drift velocities and the vertical density distribution have significant longitudinal differences; especially the equatorial anomaly peaks expand to higher latitudes more in American sector than the African sector, indicating that the vertical drift in the American sector is stronger than the African sector. Key Points Longitudinal vertical ionospheric density distributions difference Simultaneous observation of vertical drift and density distribution Validation of in situ density using tomographically imaged density

It has been observed that the zonal drift velocity of equatorial plasma bubbles is generally eastward. However, it has not been well understood whether the zonal drift of plasma bubbles is the same as the ambient plasma drift and what process causes differences in the drift velocities of the ambient plasma and bubbles. In this study we analyze the ion drift velocities measured by the Defense Meteorological Satellites Program and ROCSAT‐1 satellites and the electric fields measured by the Communications/Navigation Outage Forecasting System (C/NOFS) satellite in the presence of equatorial spread F. We find that the zonal drift velocity of the plasma particles inside plasma bubbles is significantly different from the ambient plasma drift. The relative zonal velocity of the ions inside the depletion region with respect to the ambient plasma is generally westward. In most cases it can be as high as several hundreds of meters per second. The plasma bubbles detected by the C/NOFS satellite in the midnight‐dawn sector are still growing, and the polarization electric field inside the postmidnight bubbles is much stronger than the electric field in the ambient plasma. We suggest that the zonal drift velocity of the plasma particles inside the depletion region is driven by polarization electric field. When a plasma bubble is tilted, the E × B drift velocity caused by the polarization electric field has an upward component and a zonal component. Because of the zonal motion of the plasma particles inside the bubble, the eastward drift velocity of the bubble structure is faster than the ambient plasma drift for a west‐tilted bubble and slower than the ambient plasma drift for an east‐tilted bubble.

The JOULE‐II sounding rocket salvo was launched from Poker Flat Rocket Range into weak pulsating aurora following a moderate substorm at 0345 LT on 19 January 2007. We present in situ measurements of ion flow velocity and electric and magnetic fields combined with neutral wind observations derived from ground observations of in situ chemical tracers. Measured ion drifts in the 150–198 km and 92–105 km altitude ranges are consistent with × motion to within 16 m s−1 rms and with neutral wind velocity to within 20 m s−1, respectively. From these measurements we have calculated the ratio κ of the ion cyclotron and ion collision frequencies, finding κ = 1 at an altitude of 118 ± 0.3 km. Using direct measurements of ion current, we calculate the Joule heating rate and Pedersen and Hall conductivity profiles for this moderately active event and find height‐integrated values of 390 W km−2 and 0.59 and 2.22 S, respectively. We also find that these values would have errors of up to tens of percent without coincident neutral wind measurements, and presumably more so during more active conditions. Ion flow vectors were measured at a rate of 125 s−1; however, no significant fluctuations were observed at spatial/temporal scales below ∼350 m and 0.5 s. Observational limits were 5.5 m and 0.016 s.

We report and discuss interesting observations of the variability of electric fields and ionospheric densities near sunrise in the equatorial ionosphere made by instruments onboard the Communications/Navigation Outage Forecasting System (C/NOFS) satellite over six consecutive orbits. Electric field measurements were made by the Vector Electric Field Instrument (VEFI), and ionospheric plasma densities were measured by Planar Langmuir Probe (PLP). The data were obtained on 17 June 2008, a period of solar minimum conditions. Deep depletions in the equatorial plasma density were observed just before sunrise on three orbits, for which one of these depletions was accompanied by a very large eastward electric field associated with the density depletion, as previously described by de La Beaujardière et al. (2009), Su et al. (2009) and Burke et al. (2009). The origin of this large eastward field (positive upward/meridional drift), which occurred when that component of the field is usually small and westward, is thought to be due to a large-scale Rayleigh–Taylor process. On three subsequent orbits, however, a distinctly different, second type of relationship between the electric field and plasma density near dawn was observed. Enhancements of the eastward electric field were also detected, one of them peaking around 3 mV m−1, but they were found to the east (later local time) of pre-dawn density perturbations. These observations represent sunrise enhancements of vertical drifts accompanied by eastward drifts such as those observed by the San Marco satellite (Aggson et al., 1995). Like the San Marco measurements, the enhancements occurred during winter solstice and low solar flux conditions in the Pacific longitude sector. While the evening equatorial ionosphere is believed to present the most dramatic examples of variability, our observations exemplify that the dawn sector can be highly variable as well.

Broad plasma depletions in the equatorial ionosphere near dawn are region in which the plasma density is reduced by 1–3 orders of magnitude over thousands of kilometers in longitude. This phenomenon is observed repeatedly by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite during deep solar minimum. The plasma flow inside the depletion region can be strongly upward. The possible causal mechanism for the formation of broad plasma depletions is that the broad depletions result from merging of multiple equatorial plasma bubbles. The purpose of this study is to demonstrate the feasibility of the merging mechanism with new observations and simulations. We present C/NOFS observations for two cases. A series of plasma bubbles is first detected by C/NOFS over a longitudinal range of 3300–3800 km around midnight. Each of the individual bubbles has a typical width of ∼100 km in longitude, and the upward ion drift velocity inside the bubbles is 200–400 m s−1. The plasma bubbles rotate with the Earth to the dawn sector and become broad plasma depletions. The observations clearly show the evolution from multiple plasma bubbles to broad depletions. Large upward plasma flow occurs inside the depletion region over 3800 km in longitude and exists for ∼5 h. We also present the numerical simulations of bubble merging with the physics‐based low‐latitude ionospheric model. It is found that two separate plasma bubbles join together and form a single, wider bubble. The simulations show that the merging process of plasma bubbles can indeed occur in incompressible ionospheric plasma. The simulation results support the merging mechanism for the formation of broad plasma depletions. Key Points New observations of broad plasma depletions New simulations of bubble merging Verification of bubble merging mechanism

We investigate the origin of low‐energy (Ek < 10 eV) ion upflows in Earth's low‐altitude dayside cusp region. The Cusp‐2002 sounding rocket flew from Ny Ålesund, Svalbard, on 14 December 2002, carrying plasma and field instrumentation to an altitude of 768 km. The Suprathermal Ion Imager, a two‐dimensional energy/arrival angle spectrograph, observed large (>500 m s−1) ion upflows within the cusp at altitudes between 640 km and 768 km. We report a significant correlation between ion upflow and precipitating magnetosheath electron energy flux in this altitude range. There is only very weak correlation between upflow and wave power in the VLF band. We find a small negative correlation between upflow and the magnitude of the DC electric field for fields less than about 70 mV m−1. The apparent relation between upflow and electron energy flux suggests a mechanism whereby ions are accelerated by parallel electric fields that are established by the soft electrons. Significant ion upflows are not observed for electron energy fluxes less than about 1010 eV cm−2 s−1. The lack of correspondence between ∣∣ and upflow on the one hand, and wave power and upflow on the other, does not rule out these processes but implies that, if operating, they are not local to the measurement region. We also observe narrow regions of large ion downflow that imply either a rebalancing of the ionosphere toward a low‐Te equilibrium during which gravity dominates over the pressure gradients or a convection of the upflowing ions away from the precipitation region, outside of which the ions must fall back into equilibrium at lower altitudes.

A three‐dimensional numerical simulation of plasma density irregularities in the postsunset equatorial F region ionosphere leading to equatorial spread F (ESF) is described. The simulation evolves under realistic background conditions including bottomside plasma shear flow and vertical current. It also incorporates C/NOFS satellite data which partially specify the forcing. A combination of generalized Rayleigh‐Taylor instability (GRT) and collisional shear instability (CSI) produces growing waveforms with key features that agree with C/NOFS satellite and ALTAIR radar observations in the Pacific sector, including features such as gross morphology and rates of development. The transient response of CSI is consistent with the observation of bottomside waves with wavelengths close to 30 km, whereas the steady state behavior of the combined instability can account for the 100+ km wavelength waves that predominate in the F region. Key Points CSI‐gRT produce waves that agree with satellite and radar observations The transient response of CSI is consistent with the observation of 30 km waves Steady state behavior of the combined CSI‐gRT can account for the 100+ km waves

We report on plasma densities and electric fields measured by the C/NOFS satellite between 10 and 20 June 2008. Midway through the interval, geomagnetic conditions changed from quiescent to disturbed as a high speed stream (HSS) in the solar wind passed Earth. During the HSS passage C/NOFS encountered post‐midnight irregularities that ranged from strong equatorial plasma bubbles to longitudinally broad depletions. At the leading edge of the HSS the interplanetary magnetic field rapidly intensified and rotated causing auroral electrojet currents to rise and fall within a few hours. As the electrojet relaxed, C/NOFS witnessed a rapid transition from a weakly to a strongly disturbed equatorial ionosphere that lasted ∼10 hours. Eastward polarization electric fields intensified within locally depleted flux tubes. We discuss relative contributions of gravity‐driven currents, overshielding electric fields and disturbance dynamos as drivers of post‐midnight depletions.

We use the C/NOFS satellite's Vector Electric Field Instrument (VEFI) to study the relationship of impulsive electron whistlers in the low-latitude ionosphere to lightning strokes located by the World-Wide Lightning Location Network (WWLLN). In order to systematize this work, we develop an automated algorithm for recognizing and selecting the signatures of electron whistlers amongst many Very Low Frequency (VLF) recordings provided by VEFI. We demonstrate the application of this whistler-detection algorithm to data mining of a ~ two-year archive of VEFI recordings. It is shown that the relatively simple oblique electron whistler adequately accounts of the great majority of low-latitude oscillatory VLF waves seen in this study.