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We have used total electron content (TEC) values from low, middle, and high latitudes recorded over the American continent and density and ion temperature measured in situ by the DMSP-F15 and F17 satellites during the geomagnetic storms of 3–4 August 2010 and 5–6 August 2011 to study the formation and dynamics of plasma density enhancements that developed during these two storms. Common to both storms are the timing of the main phase that extends between 20 and 24 UT and their seasonality with both storms occurring near the end of the Northern Hemisphere summer solstice. During both storms, TEC data show incipient equatorial anomalies lacking a poleward expansion beyond 20° magnetic latitude. Two large-scale TEC enhancements were observed at middle latitudes showing a complicated pattern of structuring and merging. The first TEC enhancement corresponds to a storm-enhanced density (SED) seen between 21 and 01 UT on the following day. The second TEC enhancement was observed over Central America, located equatorward of the SED and apparently moving northward. However, careful analysis of the TEC values indicates that this second TEC enhancement is not transported from lower latitudes through a superfountain effect. Instead, the enhanced plasma has a local origin and is driven by a southward directed meridional wind that moves plasma up the tilted magnetic field lines. DMSP flights passing over the second TEC enhancement show a reduction of the ion temperature, confirming an adiabatic expansion of the plasma as it moves up the field lines. It is concluded that the midlatitude TEC enhancements do not arise from a low-latitude ionospheric fountain effect.

The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and GPS navigation systems. The effects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar flares and coronal mass ejections. The development of a global specification and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation specification systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation specification and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being analyzed to study the effects of magnetic storms on communication and navigation systems.

The effects of the great magnetic storm of July 15, 2000 on the equatorial ionosphere have been studied by ground-based and satellite in-situ measurements. A large westward plasma drift in the evening equatorial ionosphere was observed as a result of the ionospheric disturbance dynamo. In that environment, the IMF Bz turned southward and presumably caused penetration of E-fields to low latitudes. This E-field initiated the onset of 250 MHz and L-band scintillations at Ascension Island (15°W) and precipitous TEC decrease at Fortaleza, Brazil (38°W), bounding the narrow longitude region in the South Atlantic. These impulsive ionospheric effects were extremely well correlated with abrupt decreases of SYM-H (1-min resolution Dst). The DMSP in-situ measurements showed the presence of severe ion density bite-outs extending over 30° latitude in the South Atlantic Magnetic Anomaly region. The ROCSAT-1 satellite measured upward and large southward ion drifts in the same sector.

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day-to-day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi-Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced-antenna satellite scintillation, digisonde, all-sky imager, and Fabry-Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all-sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

A case study of the temporal behavior of ionospheric scintillations and their frequency spectra in the cusp/cleft and polar cap regions is presented. These measurements were made at Sondrestrom and Thule, Greenland, using the 243-MHz transmissions from quasi-stationary satellites during a coupling energetic and dynamics of atmospheric regions (CEDAR) high-latitude plasma structure (HLPS)/solar terrestrial energy program (STEP) global aspects of plasma structures (GAPS) campaign. During this campaign, the incoherent scatter radar (ISR) observations were also performed at Sondrestrom, which defined the dynamic ionospheric environment in the cusp/cleft region. The availability of the radar results has enhanced this case study. It is found that scintillations at Sondrestrom are abruptly enhanced about an hour before magnetic noon when the propagation path to the satellite entered the cusp/cleft region. Subsequently, a series of enhanced and reduced scintillation activity was detected. The enhanced scintillation structures were found to be asymmetric, with sharp leading edges and diffuse trailing edges. Spaced-antenna scintillation measurements at Sondrestrom detected considerable velocity shear, and the frequency spectra showed flat low-frequency portions, implying the presence of turbulent plasma flows. A comparison with the ISR observations indicates that the temporal variation of scintillation was caused by the poleward convection of alternate regions with high- and low-ionization density, the density depletions being caused by channels of high zonal flows associated with velocity shear. The level of scintillation observed in the low-density regions imply the presence of small-scale irregularities with considerable irregularity amplitude. In contrast to the above behavior, the polar cap scintillations exhibit deep minima between the transit of successive “patches” of ionization, and their frequency spectra imply the absence of turbulent plasma flows. It is postulated that in the cusp/cleft and polar cap regions, the gradient-drift instability mechanism generates the observed small-scale irregularities associated with discrete density enhancements, whereas a shear-driven instability, such as the nonlinear collisional Kelvin-Helmholtz (K-H) instability mechanism, may generate the irregularities in the intervening low-density regions.

A comprehensive, time-dependent, high-latitude, one-species F region model has been developed to study the various physical processes which are believed to affect the polar cap plasma density distributions as a function of altitude, latitude, longitude, and local time. These processes include production of ionization by solar extreme ultraviolet radiation and particle precipitation; loss through charge exchange with N 2 and O 2 ; and transport by diffusion, neutral winds, and convection E×B drifts. In our initial calculations we have modeled highly structured plasma densities characterized by digisonde observations at Sondrestrom using both a time-dependent global convection pattern and spatially localized regions of transient high-speed flow

[1] In early October 2008, the C/NOFS satellite orbited near the magnetic equator at its perigee altitude of ∼400 km at dusk in the Peruvian sector. This provided an ideal opportunity for a comparison, under the current very low solar flux condition, of equatorial ionospheric disturbances observed with the Communication/Navigation Outage Forecasting System (C/NOFS) in situ measurements and ground-based observations available near Jicamarca Observatory. The primary objective was the comparison of plasma density disturbances measured by a Planar Langmuir Probe (PLP) instrument on the C/NOFS satellite with VHF scintillation activity at Ancon near Jicamarca for this period. Here we discuss in detail two extreme cases: one in which severe in situ disturbances were accompanied by mild scintillation on a particular day, namely, 10 October while there was little in situ disturbance with strong scintillation on 5 October. This apparent contradiction was diagnosed further by a latitudinal ground-based GPS network at Peruvian longitudes, a Digisonde, and the incoherent scatter radar (ISR) at Jicamarca. The crucial distinction was provided by the behavior of the equatorial ionization anomaly (EIA). The EIA was well-developed on the day having severe in situ disturbances (10 Oct). This led to lower equatorial plasma density and total electron content (TEC) at the equator and consequently reduced the scintillations detected at Ancon. On the other hand, on the day with severe scintillations (5 Oct), the EIA was not so well developed as on 10 October, leading to relatively higher equatorial plasma density and TEC. Consequently the severe scintillations at Ancon were likely caused by ionospheric structure located below the altitude of C/NOFS. The NRL SAMI2 model was utilized to gain a greater understanding of the role of neutral winds and electric fields in reproducing the TEC as a function of latitude for both classes of irregularities. Spectral studies with high resolution in situ PLP data were also performed. The power law spectra within the plasma bubbles showed two slopes: the low frequency slope being ∼−5/3 and the high frequency ∼−5 with a break around λ = 70 m. This particular type of two-slope spectra may be related to the extremely low solar activity and its impact on ion composition and temperature.

The objective of the study is to utilize the high resolution PLP and the beacon on the C/NOFS satellite to determine the impact on the equatorial ionosphere of two moderate magnetic storms during solar cycle 24. These two storms perturb various SCINDA sites at dusk. The in-situ C/NOFS data allows the tracking of the plasma bubbles on a global scale. The spectral analysis of the PLP data shows interesting variation in the spectral shapes depending on its location with respect to the bubbles. Current analysis is ongoing to determine the impact of the spectral shapes on UHF scintillations from C/NOFS.

[1] Satellite communication and navigation systems operating at low latitudes suffer outages due to ionospheric scintillations during large magnetic storms that are not currently specified by any model. This paper describes and demonstrates how in the framework of an eastward electric field penetration from high to low latitudes at dusk during the main phase of a large storm, for which the rate of change of Dst ≤ −50 nT/h and the Dst minimum index ≤ −100 nT, it is possible to specify the longitude interval within the low-latitude ionosphere where scintillations and plasma bubbles are most likely to occur. It is known that the eastward prompt penetration electric field becomes enhanced near sunset due to the day-to-night conductivity gradient. Such enhanced eastward electric fields generally set off the Rayleigh-Taylor plasma instability at F region heights and cause the formation of plasma bubbles and irregularities of electron density that give rise to scintillations of satellite signals. We first discuss two individual magnetic storms that satisfy the criterion of large magnetic storms mentioned above and for which the onsets of the main phase are about 15 h apart. We show that the dusk sectors corresponding to these two storms are such that irregularities and scintillations were observed in the Atlantic-Peruvian longitude sector for one storm and in the Pacific sector for the other. We then present a statistical study with 30 large magnetic storms during solar cycle 23 which satisfy the two criteria of large magnetic storms and we attempt to specify the longitude interval of irregularity and scintillation occurrence during the main phase of such storms. We have tracked globally the occurrence of equatorial scintillations during magnetic storms by the use of scintillation observations made by the Air Force Research Laboratory's Scintillation Network Decision Aid (SCINDA) network and the DMSP satellite in situ measurements of plasma bubbles at 840 km. The statistical study reveals that during large magnetic storms, scintillations and plasma bubbles occur over a specific longitude sector for which the local dusk corresponds to the time interval of the main phase of storms. The magnetic storm induced scintillations may enhance the general seasonal/longitudinal pattern of quiet time scintillations at the station but may also occur where it is least expected in accordance with climatology. The storm time response of the equatorial ionosphere discussed in this paper will be implemented in the SCINDA algorithm to enhance its capability to specify scintillations during large magnetic storms.

Three algorithms for the estimation of the parameters of the cross-correlation analysis of spacedreceiver data are described. These algorithms use different representations of the turbulence effects on the model for the space-time correlation function of the diffraction pattern defined on the ground by the propagation of transionospheric satellite signals. These algorithms are then applied to spaced-receiver data recorded at Thule, Greenland. The comparison of the corresponding results shows that some of the parameters estimated by the spaced-receiver analysis are very sensitive to the change in the basic model. It also indicates that it may be necessary to extend this systematic comparison to the several other spaced-receiver algorithms proposed in the literature.

[1] The day-to-day variability in ionospheric irregularity generation giving rise to equatorial scintillation has remained an unresolved issue over many decades. We take a fresh look at the problem utilizing the global imagery provided by the Global Ultraviolet Imager (GUVI) instrument on NASA's Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite. GUVI has been acquiring images of 135.6-nm emission in the Earth's ionosphere-thermosphere system since 2001. These GUVI disk images at dusk have been used to identify cases where the equatorial ionization anomaly (EIA) crests lie near the magnetic equator over a relatively narrow longitude range, so that the anomaly looks collapsed. A 16-month period of GUVI data collected during evening at solar maximum is used to study the morphology of these so-called collapses, since the EIA collapse is shown to be linked to the suppression of equatorial plasma bubbles and scintillations. In particular, we look at the June solstice, during which the Atlantic and Pacific show very different climatology and EIA collapses are most frequent in the GUVI data. On the other hand, EIA collapses are a relatively rare occurrence during the equinox period when scintillations are most prevalent globally. We obtained a few dramatic examples of day-to-day variability in EIA behavior and scintillations over India. The Sami3 is Also a Model of the Ionosphere (SAMI3) model was used to investigate the conditions during the evening collapse of the anomaly in the Indian longitude sector, where measurements of total electron content (TEC) and scintillations and estimates of the daytime vertical drifts and those at dusk were available. Results from SAMI3 show that the observed collapse of the anomaly at dusk can be simulated by a reversal of the upward vertical drift in midafternoon in agreement with the drift estimates from magnetometer observations. Such reversed vertical drifts at this time of the day are generally seen during counterelectrojet events. Introduction of neutral winds into SAMI3 better approximates the dusk behavior of TEC at low-latitude stations in India. This study reveals that the longitudinally confined EIA collapse may explain some of the differences in day-to-day variability of scintillations at different locations around the globe.

[1] Analysis of GPS phase fluctuations in conjunction with regional total electron content (TEC) maps, in situ measurements of subauroral polarization streams (SAPS) and auroral convection from several Defense Meteorological Satellite Program spacecraft, and dynasonde measurements at the Bear Lake Observatory obtained during the intense magnetic storm of 7–8 November 2004 have indicated the serious impact of large ionospheric velocities on GPS-based navigation systems within the midlatitude region in the North American sector. The major difference between this superstorm and the others observed during the earlier October-November 2003 events is the absence of appreciable storm-enhanced density gradients, with the midlatitude region being enveloped by either the auroral oval or the ionospheric trough within which the SAPS were confined during the local dusk to nighttime hours. This shows that it is possible to disable GPS-based navigation systems for many hours even in the absence of appreciable TEC gradients, provided an intense flow channel is present in the ionosphere during nighttime hours. The competing effects of irregularity amplitude ΔN/N, the background F region density, and the magnitude of SAPS or auroral convection are discussed in establishing the extent of the region of impact on such systems.

[1] We investigate the response of the equatorial ionosphere at dusk to the intense magnetic storms of 30 October 2003 and 20 November 2003, different aspects of which have been widely studied by the community. We present here a very complete set of space and ground-based diagnostics that provide the vertical and latitudinal structures of the ionosphere within the South Atlantic magnetic anomaly (SAMA) region and the contiguous parts of South America and Africa. We show that for both storms, the dusk sector corresponding to the universal time (UT) interval between the fast decrease of the SYM-H index and minimum SYM-H value determines uniquely the longitude interval populated by equatorial plasma bubbles and depletions. Further, we find that the UT of these storms is such that the ionospheric density perturbations occur in the SAMA region, which are most extended in latitude and altitude compared with other regions of the globe. In the dusk sector, the eastward penetration electric field, associated with rapid SYM-H decrease, adds to the postsunset eastward E-field because of the F region dynamo, which may be specially enhanced in this longitude interval because of the increased zonal conductivity gradient caused by energetic particle precipitation. This enhanced E-field at dusk causes a rapid uplift of the ionosphere and sets off plasma instabilities to form bubbles or bite-outs. The decreased ion density seen in the Defense Meteorological Satellite Program (DMSP) in situ data at 840 km indicates that the ionospheric plasma has been lifted above the DMSP altitude and transported away from the region by diffusion along magnetic field lines. Plasma bubbles and bite-outs impact satellite communication and navigation systems by introducing scintillations and steep density gradients. This paper corroborates that intense magnetic storms follow the framework, developed by Su. Basu et al. (2001) for moderate storms, that specifies the longitude interval in which such disturbances are most likely to occur.

Digital ionosondes, of the type known as the Canadian Advanced Digital Ionosonde (CADI), have been in routine operation at Sao Jose dos Campos (low latitude region) and Palmas (equatorial region), Brazil, since August 2000 and April 2002, respectively. These CADIs are part of a new network established in a collaborative program between UNIVAP and CEULP/ULBRA. The main objective of the network is to study the equatorial and lowlatitude ionospheric regions in the Brazilian sector during both quiet and geo-magnetically disturbed conditions. In this work we present the results of the ionospheric observations related to the superstorm (|Dst|max > 250 nT) on 20 November 2003 in the Brazilian sector, as evidenced by the ionospheric parameter changes from simultaneous ionospheric sounding observations carried out at Sao Jose dos Campos and Palmas. A comparison of the observed ionospheric response at the two stations, separated only by about 1460 km, shows considerable latitudinal differences associated with the geomagnetic disturbances. Both the stations show copious storm-time changes.

[1] We consider VHF amplitude scintillations, GPS phase fluctuations, ionosonde measurements, maps of GPS total electron content (TEC), observations of daytime aurora and TIMED GUVI images during the large magnetic storms of October 29–31, 2003, and find two distinct classes of plasma processes that produce midlatitude ionospheric irregularities. One is associated with auroral plasma processes; the other, with storm enhanced density (SED) gradients, a part of which occur in close proximity to sub-auroral polarization stream (SAPS) electric fields as discussed by J. C. Foster et al. (2002). We analyze in detail the storm event of October 30, 2003. The SAPS-associated plasma structures may occur by an ion temperature gradient convective instability (M. J. Keskinen et al., 2004), but structuring by auroral processes requires elucidation.

On February 9, 1988 a multi-instrument observation of the dayside polar ionosphere was made at Sondre Stromfjord, Greenland as part of a CEDAR high latitude campaign. Our objective was to observe the low altitude signature of the cusp during southward IMF but under fairly quiet ionospheric conditions (Kp=2). The HiLat satellite measured in-situ particle precipitation, field-aligned currents and velocity shears in the dayside cusp/cleft, while the VHF beacon on board showed large phase scintillation and saturated intensity scintillation. The phase spectral index was fairly shallow indicating that large power spectral density was present up to scales on the order of 100m. During the spacecraft overflight, the radar was scanning close to the F-region projection of the satellite track. It is the object of this paper to compare the radar measured thermal density, Ti and Te features in the F-region with topside density and velocity structures and particle precipitation characteristics measured in-situ by the satellite at 830 km. The precipitating electron flux measured by HiLat was also input to an ionospheric chemistry model that calculates electron density and temperature profiles for different exospheric temperatures. The modeling results show that most of the features of the radar data may be explained in terms of cleft precipitation, consequent higher exospheric temperatures and enhanced convection.

During the Condor campaign a number of instruments were set up in Peru to support the rocket experiments. In this series of papers we report on the results of the experiments designed to study the equatorial F region. In this overview paper we summarize the main results as well as report upon the macroscopic developments of spread F as evidenced by data from backscatter radars, from scintillation observations, and from digital ionosonde meaurements. In this latter regard, we argue here that at least two factors other than the classical gravitational Rayleigh-Taylor plasma instability process must operate to yield the longest-scale horizontal organization of spread F structures. The horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow, although detailed comparison with theory seems to require shear frequencies a bit higher than observations indicate. On the other hand, the largest-scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions. Using simultaneous rocket and radar data, we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over Jicamarca are primarily spatial structures advecting overhead. The detailed rocket-radar comparison verified several other earlier results and speculations, particularly those made in the PLUMEX experiments. In particular, companion papers discuss and extend some of the PLUMEX results to include the role of anomalous diffusion (LaBelle et al., this issue) in the theory of equatorial spread F and to shed light upon the shallow spectral form often observed in the intermediate-scale regime (LaBelle and Kelley, this issue). Por pares