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The accuracy of the Global Positioning System (GPS) as an instrument for measuring the integrated water vapor content of the atmosphere has been evaluated by comparison with concurrent observations made over a 14-day period by radiosonde, microwave water vapor radiometer (WVR), and Very Long Baseline Interferometry (VLBI). The Vaisala RS-80 A-HUMICAP radiosondes required a correction to the relative humidity readings (provided by Vaisala) to account for packaging contamination; the WVR data required a correction in order to be consistent with the wet refractivity formulation of the VLBI, GPS, and radiosondes. The best agreement of zenith wet delay (ZWD) among the collocated WVR, radiosondes, VLBI, and GPS was for minimum elevations of the GPS measurements below 10°. After corrections were applied to the WVR and radiosonde measurements, WVR, GPS, and VLBI (with 5° minimum elevation angle cutoff) agreed within ∼6 mm of ZWD [1 mm of precipitable water vapor (PWV)] when the differences were averaged,...

A strong positive storm phase was observed by both the Millstone Hill and Arecibo incoherent scatter radars during a moderate geomagnetic storm on 10 September 2005. The positive storm phase featured an interesting UT-altitude profile of the F region electron density enhancement that closely resembles the Greek letter A. The radar measurements showed that the uplift of the electron density peak height corresponded to a strong upward ion drift, whereas the subsequent falling of the peak height coincided with a downward ion drift. Using realistic, time-dependent ionospheric convection and auroral precipitation as input, the thermosphere-ionosphere electrodynamics general circulation model (TIEGCM) is able to reproduce the same A-like structure in the electron density profile, along with many large-scale features in electron temperature and vertical ion drift as observed by the radars. Over the 3-day period of 8―10 September, our simulation results show an error of 1%-4% for h m F 2 , electron, and ion temperatures at both radar locations. The estimated error for N m F 2 is about 9% at Millstone Hill and 19% at Arecibo. However, the simulated vertical ion drifts are less accurate, with the normalized root-mean-square errors of 72% at Millstone Hill and 52% at Arecibo, due largely to model's inability to capture the large temporal fluctuations measured by the radars. However, it reproduces reasonably well the overall large-scale variations during the 3-day period, including the storm-time-enhanced upward ion drift that is responsible for the interesting F region density profile. The model is also able to reproduce the temporal and spatial total electron content variations as shown in the global GPS maps. The comparison with the GUVI O/N 2 is less satisfactory, although there is a general agreement in terms of relative O/N 2 changes during the storm in the longitudinal sector between 60°W and 80°W where the radars are located. The detailed data-model comparison carried out in this study is helpful not only to validate the model but also to interpret the complex observations. The TIEGCM simulations reveal that it is the enhanced meridional neutral wind, not the penetration electric field, that is the primary cause of the A structure of the F region electron density profile.

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives., National Science Foundation (U.S.) (Grant AST-0457585), National Science Foundation (U.S.) (Grant PHY-0835713), National Science Foundation (U.S.) (Grant CAREER-0847753), National Science Foundation (U.S.) (Grant AST-0908884), United States. Air Force Office of Scientific Research (Grant FA9550-0510247), Smithsonian Astrophysical Observatory, MIT School of Science

[1] Midlatitude space weather events are being monitored at a location in Western Australia (−26.7° latitude, 116.7° longitude, and −38° magnetic latitude). This location is that of the Murchison Widefield Array (MWA), a new low radio frequency array (80 to 300 MHz) being built with the objective of developing powerful new capabilities for radio astronomy, solar, heliospheric, and ionospheric science. The MWA is a radio interferometer that images large fields of view with unprecedented fidelity and dynamic range at these frequencies. With respect to the ionosphere, it will be able to make important new measurements of ionospheric scintillation and improve our understanding of midlatitude space weather events in the Southern Hemisphere. For example, by studying midlatitude space weather events in the Australian sector, the seasonal and geomagnetic dependencies of storm enhanced density (SED) can be characterized and separated from other forces that may impact its formation elsewhere, such as in the longitudinal sector of the South Atlantic Anomaly (SAA). After full commissioning, the MWA will consist of 2048 dipole antennas arranged as 128 “tiles,” each being a 4 × 4 array of dual-polarization dipoles. Currently, an engineering prototype comprising 32 tiles has been installed and has been used to observe the Faraday rotation from a linearly polarized satellite beacon on the DMSP-15 satellite. We will report on these initial observations of ionospheric Faraday rotation and on results from GPS receivers located at the site and elsewhere in the Australian continent. We will also discuss future space weather products enabled by the MWA.

[1] This paper presents a comprehensive modeling investigation of ionospheric and thermospheric variations during a prompt penetration electric field (PPEF) event that took place on 9 November 2004, using the Thermosphere-Ionosphere-Mesosphere Electrodynamic General Circulation Model (TIMEGCM). The simulation results reveal complex latitudinal and longitudinal/local-time variations in vertical ion drift in the middle- and low-latitude regions owing to the competing influences of electric fields and neutral winds. It is found that electric fields are the dominant driver of vertical ion drift at the magnetic equator; at midlatitudes, however, vertical ion drift driven by disturbance meridional winds exceeds that driven by electric fields. The temporal evolution of the UT-latitude electron density profile from the simulation depicts clearly a super-fountain effect caused by the PPEF, including the initial slow-rise of the equatorial F-layer peak height, the split of the F-layer peak density, and the subsequent downward diffusion of the density peaks along magnetic field lines. Correspondingly, low-latitude total electron content (TEC) becomes bifurcated around the magnetic equator. The O/N2column density ratio, on the other hand, shows very little variations during this PPEF event, excluding composition change as a potential mechanism for the TEC variations. By using realistic, time-dependent, high-latitude electric potential and auroral precipitation patterns to drive the TIMEGCM, the model is able to successfully reproduce the large vertical ion drift of ∼120 m/s over the Jicamarca incoherent radar (IS) in Peru, which is the largest daytime ion drift ever recorded by the radar. The simulation results are validated with several key observations from IS radars, ground GPS-TEC network, and the TIMED-GUVI O/N2column density ratio. The model-data intercomparison also reveals some deficiencies in the TIMEGCM, particularly the limitations imposed by its upper boundary height as well as the prescribed O+ flux.

[1] A prominent ionospheric longitudinal variation at midlatitudes, in particular, over the continental US, was found recently. This variation is characterized as a higher east-side electron density in the evening and a higher west-side electron density in the morning, and with clear seasonal and solar activity dependencies. A combined effect of geomagnetic declination and changing zonal winds was proposed to explain it. This paper represents a comprehensive investigation of this effect by examining climatology for both electron density longitudinal differences and the nighttime zonal winds in the eastern US. Electron density is from incoherent scatter radar extra-wide coverage experiments during 1978–2011 over Millstone Hill for which the spatial separation of the data can be up to 50° in longitude. The thermospheric zonal wind is from the on-site Fabry-Perot interferometer measurements during 1989–2001. The observed zonal wind climatology is found to be perfectly consistent with the expectation based on the east-west electron density differences in terms of local time, seasonal, and solar cycle dependencies. The correlation between the zonal wind and the east-west differential ratio is extremely high with an overall correlation coefficient of 0.93. The observed time delay of ∼3 hours in the response of electron density differences to zonal winds is a marked feature. Thus these results confirm positively the declination-zonal wind mechanism and provide new insight into longitudinal variations at midlatitudes for other geographic sectors.

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

The recent discovery of large ionospheric disturbances associated with sudden stratospheric warmings (SSW) has challenged the current understanding of mechanisms coupling the stratosphere and ionosphere. Non-linear interaction of planetary waves and tides has been invoked as a primary mechanism for such coupling. Here we show that planetary waves may play a more complex role than previously thought. Planetary wave forcing induces a global circulation that leads to the build-up of ozone density in the tropics at 30–50 km altitude, the primary region responsible for the generation of the migrating semidiurnal tide. The increase in the ozone density reaches 25% and lasts for ∼35 days following the SSW, long after the collapse of the planetary waves. Ozone enhancements are not only associated with SSW but are also observed after other amplifications in planetary waves. In addition, the longitudinal distribution of the ozone becomes strongly asymmetric, potentially leading to the generation of non-migrating semidiurnal tides. We report a persistent increase in the variability of ionospheric total electron content that coincides with the increase in stratospheric ozone and we suggest that the ozone fluctuations affect the ionosphere through the modified tidal forcing.

[1] Ionospheric storms represent large global disturbances of the ionospheric F region electron density in response to geomagnetic storms. In this study, we use a combination of ionospheric total electron content (TEC) global maps and data from in-situ satellite measurements, such as solar wind data from the Advanced Composition Explorer (ACE), the Defense Meteorological Satellite Program (DMSP), and TOPographic EXplorer (TOPEX) and JASON-1 satellites, to investigate the ionospheric response during the geomagnetic storm event on 24–27 July 2004. A chain of ground-based Global Positioning System (GPS) stations and ionosonde measurements across South Africa have been used to give a comprehensive coverage over this midlatitude location. The most pronounced ionospheric effects of the storm occurred at low- and midlatitudes in the Southern hemisphere, with the most significant enhancements, observed on 25 and 27 July, presented here. The DMSP F15 satellite observed a sharp density enhancement over the midlatitudes. Over South Africa, the enhancement on 25 July was about twice as large as that observed on 27 July. The positive storm enhancements on 25 and 27 July both lasted over 7 hours, and can be classified as long-duration positive storm effects. Also, IMF Bz had southward orientation for an extended number of hours (exceeding 9 hours) and could have been the means by which energy was continuously fed into the magnetosphere and ionosphere. In addition, the F region critical frequency (foF2) values observed at two ionosonde stations showed marked positive responses that were associated with an increase in the ionospheric peak height (hmF2).

[1] With the storm of 7–8 September 2002 as a study case, we demonstrate that an ionospheric model driven by a suitable storm time convection electric field can reproduce the F region dayside density enhancements associated with the ionospheric storm positive phase. The ionospheric model in this case is the Utah State University Time Dependent Ionospheric Model (TDIM); the electric field model is the University of Michigan's Hot Electron and Ion Drift Integrator (HEIDI). Extensive ground truth is available throughout the study period from two independent sources: ground-based vertical TEC and ionosonde stations; our simulation results are in good agreement with these observations. We address the question of what is the source of the high-density plasma that is seen during the positive storm phase and show that in this case a magnetospheric electric field with an eastward component that penetrates to midlatitudes increases local production on the dayside to a degree that is sufficient to account for the storm time density increases that have been observed.

[1] We present complementary methods for calibrating the dual-polarization feed of a ground-based tracking antenna used for ionospheric measurements. Several calibration parameters are defined which describe the state of the measurement equipment. These key parameters then form a transformational Mueller matrix which may be used to remove system bias in data received from known beacons. Several strategies are developed to quantify these time-dependent distortions for the Green Bank 140′ diameter antenna and receiver, although these methods could be applied to other similar systems. One approach quantifies the receiver biases in terms of amplitude and gain differentials. This is accomplished by measuring polarimetric differences between channels using a test signal with a known amplitude and phase. Due to variations over time periods on the scale of a beacon track, this procedure is most effective when the calibration signal is injected concurrently with the beacon measurements. To determine antenna feed distortion, including cross-polarization and ellipticity, data is recorded from an external celestial source. An example shows the effectiveness of this calibration technique by comparing calibrated data to data without correction. Error estimates of the calibration parameters in the example establish an upper bound of 0.22 TEC units for measurements at 150 MHz along track.

[1] Total electron content (TEC) measurements made by a network of dense GPS receivers over the continental US are used to investigate ionospheric longitudinal differences. We find that the evening TEC is substantially higher on the US east coast than on the west, and vice versa for the morning TEC; the longitudinal difference displays a clear diurnal variation. Through an analysis of morning-evening variability in the east-west TEC difference, minimum variability is found to coincide with the longitudes of zero magnetic declination over the continental US. We suggest that these new findings of longitudinal differences in ionospheric TEC at midlatitudes are caused by the longitudinal difference in magnetic declination combined with the effects of thermospheric zonal winds which are subject to directional reversal over the course of a day. This study indicates that longitudinal variations in TEC measurements contain critical information on thermospheric zonal winds. The proposed declination-zonal wind mechanism may also provide a new insight into longitude/UT changes at midlatitudes on a global scale, as well as into some geospace disturbances.

Significant changes in the ionosphere associated with stratospheric warming events have been clearly demonstrated in the analysis of global total electron content (TEC) maps that utilize measurements from networks of ground-based GPS receivers. In the American longitude sector, studies of the equatorial region show increases in the TEC of 50–150% over the background during the morning sector and decreases of approximately 50% in the afternoon sector. We have extended this study to include all longitude sectors with the addition of COSMIC and JASON TEC to allow for better global TEC coverage, especially over the oceans.

The unique and powerful diagnostic capabilities of low radio frequencies for solar, heliospheric, and ionospheric science have long been recognized, but the challenges associated with high-fidelity low frequency radio imaging have limited their exploitation. The Murchison Widefield Array (MWA) is a pioneering new interferometer, currently under construction in the radio-quiet Western Australian outback, which exploits the recent advances in digital signal processing to rise to this challenge. We present an overview of the exciting new solar, heliospheric, and ionospheric science which this instrument will enable, along with early results from a prototype array.

Low frequency arrays such as the Murchison Widefield Array (MWA), which use fixed dipoles operating at meter wavelengths and longer, have very large fields of view and exhibit strong cross-polarization response due to geometric effects, as well as the characteristics of the electronics. The linearly polarized sky at these frequencies is bright and complex in structure, demanding accurate polarimetric calibration for high fidelity imaging. This calibration is made more challenging by high-amplitude time- and direction-dependent ionospheric and plasmaspheric Faraday rotation. In this contribution we describe the relevant ionospheric behaviors, and discuss approaches for dealing with the resulting effects.

We will discuss the application of coherent software radio technology to remote sensing of the ionosphere. Using networks of advanced software radio systems it is possible to make observations of the ionosphere with both wide spatial coverage and simultaneous high resolution in space and time. Such observations can be made using a variety of techniques such as active and passive multi-static radar imaging, satellite beacon observations of TEC and scintillation, spectral monitoring, and signal time difference of arrival. Using software radio techniques it is possible to build instrumentation networks which can accomplish these observations using a unified set of hardware and software. While different antennas are often necessary or useful for particular applications the underlying analog and digital hardware systems remain unchanged from application to application. Such multi-role instruments can be highly integrated and can dynamically change their modes of operation and receive antennas precisely relative to a global time reference (e.g. GPS). Networks of such instruments scale rapidly in capability and spatial coverage with increasing number of nodes. The cost of this scaling is primarily in the complexity of control and operations as well as the required computational power needed to analyze data from the software radio array. The Intercepted Signals for Ionospheric Science (ISIS) Array is a coherent software radio network that has recently been deployed. Nodes of this array are installed along the northern United States, primarily in the Northeast and Northwest regions. The ISIS Array is well positioned for observation of the mid-latitude Geospace environment and the plasmasphere boundary layer during active geomagnetic conditions. The array is capable of making coherent observations over a wide range of frequencies with operations focusing on active radar, passive radar, and satellite beacon observations. For coherent scatter radar applications the array has a field of view that extends over southern Canada. This field of view is well instrumented with complementary data available from the Millstone Hill Incoherent Scatter Radar, GPS total electron content maps, passive optical systems, and the new mid-latitude SuperDARN radars. We will describe the design of the array, the status of its deployment, and give examples of observations from sites in the field. As part of this discussion we will present coherent scatter observations of E-region irregularities made with the array during the Geomagnetic storm of August 3–5, 2010. We will discuss the characteristics of this event and relate them to previous coherent scatter observations. To conclude we will provide an overview of future capabilities and directions for distributed software radio sensor networks that can be used for studies of the Geospace environment.

[1] We investigate the ionospheric response to several stratospheric sudden warming events which occurred in Northern Hemisphere winters of 2008 and 2009 during solar minimum conditions. We use GPS total electron content data in a broad latitudinal region at ±40° geographic latitude and a single longitude, 75°W. In all cases, we find a strong daytime ionospheric response to stratospheric sudden warmings. This response is characterized by a semidiurnal character, large amplitude, and persistence of perturbations for up to 3 weeks after the peak in high-latitude stratospheric temperatures. The ionospheric perturbations at the lower latitudes usually begin a few days after the peak in stratospheric temperature and are observed as an enhancement of the equatorial ionization anomaly (EIA) in the morning sector and a suppression of the EIA in the afternoon sector. There is also evidence of a secondary enhancement in the postsunset hours. Once observed in the low latitudes, the phase of semidiurnal perturbations progressively shifts to later local times in subsequent days. This progressive shift occurs at a different rate for different stratospheric warming events. The large magnitude and persistence of ionospheric perturbations, together with the predictability of stratospheric sudden warmings several days in advance, present an opportunity to investigate these phenomena in a systematic manner which may eventually lead to a multiday forecast of low-latitude ionosphere conditions.

[1] On 28 October 2008, small-scale rayed artificial airglow was observed at the High frequency Active Auroral Research Program (HAARP) heating facility by the HAARP telescopic imager. This airglow occurred during an experiment at twilight from 0255–1600 UT (1855–2000 LT) and with estimated scale sizes of 100 m (at assumed 225 km altitude) constitutes the smallest structure observed in artificial airglow to date. The rays appeared to be oriented along the geomagnetic field lines. During this period, other instruments, SuperDARN, GPS receivers, stimulated electromagnetic emissions receivers, also recorded unusual data sets with the general characteristic of time scales longer than anticipated for features to form. The experiment took place at the commencement of a small geomagnetic disturbance (Kp of 4.3). This unique observation is as yet unexplained. The airglow features start as large scale structures and then become smaller as heating continues in apparent contradiction to current theories on irregularity development. A thermal gradient instability at boundary of the ionospheric footprint of the plasmapause may be responsible for causing the small-scale structuring. Observations of 427.8 nm N2+ (first negative group) emissions indicate the presence of ionization.

[1] Ionospheric responses to the initial phases of three geomagnetic storms: 2–5 April 2004, 7–9 November 2004, and 13–16 December 2006, were compared using both ground-based GPS total electron content (TEC) data and coupled magnetosphere ionosphere thermosphere (CMIT) model simulations. The onset times for these storms all occurred at local daytime in the North American sector. This similarity of onset times and other factors resulted in some common features in their ionospheric response. These common features include (1) enhanced TEC (positive response) at low and middle latitudes in the daytime, (2) depleted TEC (negative response) around the geomagnetic equator in the daytime, (3) a north-south asymmetry in the positive response as the northern hemispheric response appeared to be more pronounced, and (4) negative response at high latitudes as the storms progressed. The CMIT model captured most of these features. Analysis of model results showed that storm-time enhancements in the daytime eastward electric field were the primary cause of the observed positive storm effects at low and middle latitudes as well as the negative response around the geomagnetic equator in the daytime. These eastward electric field enhancements were caused by the penetration of high latitude electric fields to low latitudes during southward interplanetary magnetic field (IMF) periods, when IMF Bz oscillated between southward and northward direction in the initial, shock phase of the storms. Consequently, the ionosphere was lifted up at low and middle latitudes to heights where recombination was weak allowing the plasma to exist for a long period resulting in higher densities. In addition, the CMIT model showed that high-latitude negative storm responses were related to the enhancements of molecular nitrogen seen in TIMED/Global Ultraviolet Imager observations, whereas the negative storm effects around the geomagnetic equator were not associated with thermospheric composition changes; they were the result of plasma transport processes.

[1] The coupling of the ionosphere to processes from below remains an elusive and difficult problem, as rapidly changing external drivers from above mask variations related to lower atmospheric sources. Here we use superposition of unique circumstances, current deep solar minimum and a record-breaking stratospheric warming event, to gain new insights into causes of ionospheric perturbations. We show large (50–150%) persistent variations in the low-latitude ionosphere (200–1000 km) that occur several days after a sudden warming event in the high-latitude winter stratosphere (∼30 km). We rule out solar irradiance and geomagnetic activity as explanations of the observed variation. Using a general circulation model, we interpret these observations in terms of large changes in atmospheric tides from their nonlinear interaction with planetary waves that are strengthened during sudden warmings. We anticipate that further understanding of the coupling processes with planetary waves, accentuated during the stratospheric sudden warming events, has the potential of enabling the forecast of low-latitude ionospheric weather up to several days in advance.

Using data acquired from a GPS receiver, a real-time synoptic ionospheric monitoring system has been developed. The system is used at the Millstone Hill satellite tracking radar. Each GPS satellite transmits signals at two different L-band frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz). The total electron content (TEC) along the path to each satellite can be determined by combining both frequencies using the pseudorange and the phase data. The GPS data is input into a Kalman filter, which is used to predict the coefficients of a simple TEC model with azimuth and elevation dependence. This model takes advantage of the real-time knowledge of the variations in TEC around the radar site provided by the GPS data. The coefficients for this model are then sent to the satellite tracking computer, and the model is applied in real time to account for the ionospheric path delay to whatever satellite is currently in track. The preliminary results of using this ionospheric monitoring system are discussed.

[1] Solar radio bursts during December 2006 were sufficiently intense to be measurable with GPS receivers. The strongest event occurred on 6 December 2006 and affected the operation of many GPS receivers. This event exceeded 1,000,000 solar flux unit and was about 10 times larger than any previously reported event. The strength of the event was especially surprising since the solar radio bursts occurred near solar minimum. The strongest periods of solar radio burst activity lasted a few minutes to a few tens of minutes and, in some cases, exhibited large intensity differences between L1 (1575.42 MHz) and L2 (1227.60 MHz). Civilian dual frequency GPS receivers were the most severely affected, and these events suggest that continuous, precise positioning services should account for solar radio bursts in their operational plans. This investigation raises the possibility of even more intense solar radio bursts during the next solar maximum that will significantly impact the operation of GPS receivers.

[1] Under normal operating conditions marine DGPS horizontal positioning accuracies on the order of several meters are achieved in North America. Degradations in positioning accuracy can occur during enhanced ionospheric activity. An ionospheric phenomenon known as storm enhanced density (SED) is observed to develop in the middle to high latitudes during ionospheric storm events. Very large gradients in total electron content are observed in the vicinity of this feature with DGPS positioning errors increased by a factor of 10–30 versus quiet conditions. The specific evolution of a given SED event and the magnitude of expected impact are not generally predictable. A method to monitor development of SED is to compute ionospheric maps in real time. Local gradients can then be computed for various geographic regions from North American maps of ionospheric delay. Sources of real-time ionospheric information include the Wide Area Augmentation System (WAAS) and the Canadian GPS•C service. These are wide area differential GPS systems. In this paper, a real-time ionospheric warning system is investigated for North American (primarily Canadian) DGPS users based on available real-time data. The WAAS and GPS•C ionospheric models are inadequate to resolve ionospheric gradients for 100–200 km scale sizes. Raw GPS data from GPS•C reference sites can be used, however, to observe large ionospheric gradients and interpret the expected impact on DGPS users. Potential exists to issue marine user warnings based on this method. Results of this work can readily be extended to land DGPS applications, such as the NDGPS service in the United States.

[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 have investigated the thermospheric and ionospheric response to the 14–15 December 2006 geomagnetic storm using a Coupled Magnetosphere Ionosphere Thermosphere (CMIT) 2.0 model simulation. In this paper we focus on observations and simulations during the initial phase of the storm (about 8 h), when the shock was driving changes in geospace. The global ionospheric maps of total electron content (TEC), ionosonde data at four stations and Millstone Hill incoherent scatter radar (ISR) observations are compared with the corresponding simulation results from the CMIT model. The observations showed significant positive storm effects occurred in the Atlantic sector after the onset of this storm. The CMIT model is able to capture the temporal and spatial variations of the ionospheric storm effects seen in the GPS TEC observations, although the model slightly underestimates the daytime positive ionospheric storm in the South American sector. The simulations are also in agreement with the ionosonde and ISR ionospheric measurements. Term analysis of the ion continuity equation demonstrates that changes in the electric fields play a dominant role in generating the observed ionospheric positive storm effect in the American sector during the initial phase, although neutral winds and composition changes also contribute. The difference in the strength of the enhancements over North and South America can be explained by the slope of the topside electron density profiles in the two hemispheres. In the southern hemisphere electron densities decrease slowly with altitude, whereas the decrease is much more rapid in the northern (winter) hemisphere. The electric fields, therefore, cannot cause large increases in electron density by uplifting the plasma, so positive storm effects are small in the southern hemisphere compared with the northern hemisphere, even though the increase in hmF2 is greater in the southern hemisphere. Nighttime changes in electron density in other longitude sectors are small, because the topside electron densities also decrease slowly with altitude at night.

[1] We report on measurements of ionospheric plasma dynamics conducted at the Arecibo Observatory between 20:00 and 24:00 local time (LT) on December 25 and 26, 2004 using the 430 MHz incoherent scatter radar (ISR). For interpretive purposes these measurements are supported by data from two nearby ionosondes and Global Positioning System (GPS) satellites. The ISR detected different ionospheric behaviors during the vertical-transmission periods on the consecutive, magnetically quiet nights. On the night of December 25 the ionosphere descended smoothly and spread F signatures faded. For about two hours on the following evening the bottomside ionosphere rose by ∼50 km, inducing plasma irregularities and intense spread F. Alternating cycles of bottom-side plasma rising and falling persisted through the remainder of the experiments. We postulate that this sinusoidal behavior is a response to gravity waves propagating above Puerto Rico. Nearly simultaneous data from two nearby stations show that GPS signals were modified by variations in total electron content (TEC) indicating the presence of traveling ionospheric disturbances (TIDs). The December 26 experiments were conducted about a day after an MW = 9.2 earthquake launched tsunami waves first across the Indian, then into the Atlantic and Pacific Oceans. We suggest that coupling at the tsunami sea-air interface launched gravity waves that propagated for great distances beneath the mesopause. GPS data recorded TEC variation in Asia, Europe, and the Caribbean, suggesting that TIDs were induced on a global scale at the wake of tsunami-launched gravity waves. Energy from imperfectly ducted gravity waves leaked into the ionosphere, partially over Puerto Rico. The wind-velocity field of these gravity waves caused local ionospheric plasma to rise, seeding bottomside irregularities via the generalized Rayleigh-Taylor instability.

In a series of experiments at Millstone Hill the total electron content along the path to different GPS satellites has been compared using simultaneous integrated phase and incoherent scatter measurements. An analysis of these data allows verification of our technique for determining the ionospheric and tropospheric path delay from the integrated phase data. It also provides the opportunity for evaluation of the IRI model in an operational sense.

Precision radar tracking data on three spherical satellites have been used to evaluate atmospheric density models: Jacchia 1971 /1/, Jacchia 1977 /2/, Barlier et al 1977 /3/, Hedin 1983/4/, and Hedin 1987 /5/. The satellites have perigee heights (q) of 270 km, 780 km, and 1500 km. The Precipitation Index (PI) quantifies the intensity and spatial extent of high-latitude particle precipitation from observations made by the NOAA/TIROS weather satellites. None of these models now use this index, whereas we have previously shown as much as 0.50 correlation of the density model error and the PI. We will report on extension of our previous work in three ways: 1) The data set has been extended to more than one year; 2) A satellite at 1500 km altitude has been added, and is important since it provides information about the atmosphere at altitudes where little or no data is input into the models; and 3) The recent CIRA87 Model and a thermosphere wind model are evaluated.

[1] Nighttime and daytime medium-scale traveling ionospheric disturbances (MSTIDs) are detected with dense and wide detrended total electron content (TEC) maps over North America using multiple GPS receiver networks. The TEC maps cover a wide region of 60–130°W and 24–54°N (30–65°N in geomagnetic latitude), and have a spatial resolution of 1.05° × 1.05° in latitude and longitude (0.15° × 0.15° with 7 × 7 pixel smoothing) and a temporal resolution of 30 seconds. The TEC maps reveal, for the first time, that the nighttime MSTIDs propagate southwestward with 200–500 km wavelengths over North America and have wavefronts longer than ∼2,000 km. We also observe that daytime MSTIDs with 300–1,000 km wavelengths propagate southeastward until mid-afternoon and southwestward in the late afternoon. In the mid-to-late afternoon, these MSTIDs propagating in the different directions are superimposed. The TEC maps can be a new powerful tool to investigate the MSTIDs.

[1] Examples of storm enhanced density (SED) formation over Russia and Northern Europe are presented. These events, which persisted 15–20 hours, were fixed in local time near noon over Europe and then later observed over the American sector. The amount of total electron content (TEC) at the base of the SED erosion plume is found to be greatest in the American sector. A persistent, repeatable pattern is apparent in the time evolution of the latitude location of the SED plume base, although the latitudinal rate of change differs between the two sectors. In the European sector the invariant latitude (Λ) of the SED plume base is observed to be between 61°–63° Λ and at a time close to local noon. In the American sector, the position of the base of the plume shifts from local noon towards dusk, and moves to a lower latitude at a nearly fixed longitude.

[1] The solar activity dependence of the summer-winter hemispheric asymmetry (SWHA) of the sudden increase in total electron content (SITEC) due to solar flares and of the O/N2 ratio is statistically analyzed using global GPS-total electron content data and TIMED Global Ultraviolet Imager column O/N2 ratio data. We focus on observations with nonnegligible residuals of the solar zenith angle (SZA) dependency of SITEC. We examined 109 SITEC events associated with flares larger than M5 X-ray class flare from 2000 through 2006 and compared the residual SITEC (δ) to the O/N2 ratio. We observed that the latitude gradient of δ has not only an annual variation but also a year-to-year variation that is similar to those of the O/N2 ratio. The SWHA magnitude (defined as the annual maximum of latitude gradient) of both δ and O/N2 decreases as the solar activity declines toward its minimum. The correlation coefficient between the annual SWHA magnitudes of δ and those of O/N2 is 0.92, indicating strongly that the SWHA of O/N2 is responsible for that of SITEC in both the annual and year-to-year variations. The X-ray classes of the solar flares have no clear correlation with the solar activity, F10.7 index. We observe that the SWHA magnitude of δ does not depend on the magnitude of solar flare but rather on the background solar activity through the SWHA magnitude of the O/N2.

[1] We investigate the variations in the thermosphere and ionosphere using multi-instrument observations during the April 2002 period, with a particular focus on periods during small geomagnetic disturbances. Large and long-lasting reductions in the daytime electron density were observed at midlatitudes by incoherent scatter radars, ionosondes, and GPS receivers. These reductions reached 30–50% and were observed over an extended longitudinal area. They propagated to middle latitudes (35–40°N) in the case of a weak geomagnetic disturbance (Kp = 3−) and to low latitudes (0–10°N) in the case of a stronger disturbance (Kp = 5−). Data from the GUVI instrument aboard the TIMED satellite reveal a reduction in the daytime O/N2 ratio in the coincident area. Similar decreases are also predicted by the TIMEGCM/ASPEN model in both O/N2 ratio and electron density, though the magnitude of the decrease from the model is smaller than observed. We suggest that these ionospheric and thermospheric disturbances result from high-latitude energy input and efficient transport of regions with reduced O/N2 to lower latitudes. We discuss the possible role of a strong positive By component of the interplanetary magnetic field in the transport of regions with reduced O/N2.

[1] We present a global view of large-scale ionospheric disturbances during the main phase of a major geomagnetic storm. We find that the low-latitude, auroral, and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system. For the large geomagnetic storm on 20 November 2003, we examine data from the high-latitude incoherent scatter radars at Millstone Hill, Sondrestrom, and EISCAT Tromso, with SuperDARN HF radar observations of the high-latitude convection pattern and DMSP observations of in situ plasma parameters in the topside ionosphere. We combine these with north polar maps of stormtime plumes of enhanced total electron content (TEC) derived from a network of GPS receivers. The polar tongue of ionization (TOI) is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern. The dayside source of the TOI is the plume of storm enhanced density (SED) transported from low latitudes in the postnoon sector by the subauroral disturbance electric field. Convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral F region is significantly enhanced by the SED material. The three incoherent scatter radars provided full altitude profiles of plasma density, temperatures, and vertical velocity as the TOI plume crossed their different positions, under the cusp, in the center of the polar cap, and at the midnight oval/polar cap boundary. Greatly elevated F peak density (>1.5E12 m 3 ) and low electron and ion temperatures (2500 K at the F peak altitude) characterize the SED/TOI plasma observed at all points along its high-latitude trajectory. For this event, SED/TOI F region TEC (150–1000 km) was 50 TECu both in the cusp and in the center of the polar cap. Large, upward directed fluxes of O+ (>1.E14 m 2 s 1 ) were observed in the topside ionosphere

[1] During geomagnetic storms, the ionospheric F region electron density may be greatly increased or decreased, which are termed positive storms or negative storms. It is generally accepted that negative storm phases are caused by neutral composition changes. In contrast, different mechanisms have been proposed to explain the generation of positive storm phases. In this paper, we present observations of a strong positive storm at middle latitudes. The Millstone Hill incoherent scatter radar detected significant increases of the midlatitude ionospheric F region electron density during a magnetic storm on 3 April 2004. The positive phase of the ionospheric storm started to occur in the morning sector (0912 LT at Millstone Hill) and lasted for more than 10 hours. Compared with the quiet-time ionosphere, the daytime F peak altitude over Millstone Hill during the storm was ∼80 km higher, the F region electron density was increased by a factor of 2–4, and the F region electron temperature was decreased by ∼1000 K (or ∼40%). The radar also measured an enhanced eastward electric field and a slightly more equatorward neutral wind. Global GPS measurements show that total electron content (TEC) was increased at middle latitudes and decreased at lower latitudes, and the large TEC enhancements occurred primarily in the Atlantic sector. We suggest that electric fields may play an important role in the generation of the observed positive storm phase. The eastward electric field will cause increases in the midlatitude ionospheric electron density by moving the plasma particles upward and decreases in the equatorial ionospheric electron density by strengthening the fountain effect. The daytime poleward wind was reduced or even reversed during the storm, which may also contribute to the occurrence of the positive storm.

[1] The bulk redistribution of plasma in the near-Earth space environment during the onset and main phase of a magnetic storm has been observed on a global scale with 2-minute resolution using Far-Ultraviolet images from the NASA-IMAGE satellite and ground-based total electron content (TEC) maps using Global Positioning Satellite (GPS) receivers. The dynamic variations under the storm conditions show a redistribution of plasma away from the nominal low-latitude location of the equatorial ionospheric anomalies, resulting in peak nightside ionospheric densities at middle latitudes. A zonal redistribution of plasma is also evident, as the plasma is depleted across the Atlantic sector, while TEC values grow in the Caribbean sector to the largest values on the nightside. The ground-based GPS measurements provide 24-hour observations of TEC, while the FUV measurements provide nightside observation with greater spatial resolution, continuous over land and sea. The complimentary observations are shown to have a very good correspondence, promising new analyses of storm time plasma redistribution with high temporal and spatial resolution and greatly improved global coverage.

[1] Here we present North American observations of the 3-dimensional structure of large-scale traveling ionospheric disturbances (TIDs) that affected the low- and mid-latitude ionosphere during the stormtime period of October 1–2, 2002. Using TEC data from the American network of dual-frequency GPS receivers, we are able to examine the effect of the TIDs on the background TEC and thus image their horizontal scales. Incoherent scatter radar data from the Arecibo Observatory give information on the vertical structure of the disturbances. The meridional and zonal structure of the TIDs are explored by computing the TEC perturbation, a method which has a resolution of a fraction of a TEC unit. The technique is effective in mapping the global structure of large-scale disturbances, as the lower latitude perturbations can be examined at the same time as the higher latitude sources. The method may prove valuable in studying the formation and propagation of TIDs.

[1] For the large geomagnetic disturbance on April 11, 2001, we combine data from the Millstone Hill radar, a network of GPS TEC receivers, and the DMSP F-12 and IMAGE satellites, to estimate the sunward flux of thermal plasma associated with erosion of the outer plasmasphere/ionosphere in the dusk sector. Direct radar observations of the E×B advection of SED plasma determine a flux of >10E26 ions/s to the noontime cusp at F-region heights. DMSP in situ observations provide a similar estimate. Assuming a dipolar magnetic field, we project our low-altitude observations into the outer plasmasphere, obtaining a total sunward flux of >10E27 ions/s. High-altitude IMAGE EUV observations of the plasmasphere drainage plume provides an estimate of 1.5 × 10E27 ions/s for the sunward flux. Such rates of sunward plasma transport are sufficient to deplete a 1-Re shell of the outer plasmasphere in ∼1 hour.

[1] This paper presents a quantitative explanation of storm-enhanced density (SED). The plasma's root origin lies in the fully sunlit equatorial ionosphere where a penetrating zonal electric field drives plasma upward so fast that it cannot recombine. This plasma spills over into the anomaly and then is driven poleward by a penetrating zonal electric field. However, the poleward field cannot extend into the dayside due to high conductivity and the flow stagnates, causing plasma to build up in a narrow channel along the dusk terminator and flow into the convection pattern. It is remarkable that plasma finds its way into the polar cap from the daytime equator. We believe that this plasma structure is connected to the plasmaspheric tails reported in the literature.

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(deg) 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 x 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(deg) to -37(deg) 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] Space weather events on the Sun, such as coronal mass ejections and solar flares, can lead to a worldwide disturbance of the geomagnetic field and associated ionospheric and thermospheric disturbances. These events can, and do, impact the performance and reliability of space-based and ground-based operational systems. This paper presents specific examples of the operational impact of space weather events on space surveillance systems. The paper concentrates on the 14 July 2000 event during which the solar-terrestrial environment experienced its largest disturbance in the past 11 years. We report on effects that were detected during the July storm with the Space-Based Visible (SBV) sensor, a visible-band electro-optical system that operates as a contributing sensor for the U. S. Space Surveillance Network. We also discuss the impact of this space weather event on the Global Positioning System (GPS) and on satellite tracking observations by ground-based radars.

[1] We make direct comparisons between GPS maps of total electron content (TEC) over the North American continent, Millstone Hill radar observations of storm enhanced density, and low and high-altitude satellite measurements of the perturbation of the outer plasmasphere during the March 31, 2001 geomagnetic storm. We find that storm enhanced density (SED) and plumes of greatly-elevated TEC are associated with the erosion of the outer plasmasphere by strong sub-auroral polarization electric fields. The SED/TEC plumes identified at low altitude map closely onto the magnetospheric determination of the boundaries of the plasmapause and plasmaspheric tail determined by EUV imaging from the IMAGE spacecraft. Characteristics of the SED/TEC plumes/tails for the March 31, 2001 event are: TEC ∼ 100 TECu; F-region sunward velocity ∼1000 m/s; sunward flux ∼5*1024 ions s−1; total transport to dayside magnetopause/merging region (3-hr event) ∼5*1028 ions.

Radio signals from Naval (NAU) transmitter in Puerto Rico can interact effectively with naturally occurring or HF heater wave-induced large-scale ionospheric irregularities, allowing them to propagate as whistler-modes in the ionosphere and to the inner radiation belts. NAU-generated whistler-modes have intensities sufficient to parametrically excite lower hybrid waves and ten-meter and meter-scale ionospheric irregularities over Arecibo. Subsequent heating of electrons and ions by the lower hybrid waves yield a sequence of ionospheric plasma effects such as airglow, short-scale density depletion and plasma line enhancements in a range of altitudes which far exceed that caused by the HF heater. Furthermore, they can interact with trapped energetic electrons in inner radiation belts at L=1.35 and trigger precipitation of electrons into the lower ionosphere. We suggest that disturbances in the ionosphere above NAU caused by whistler-mode signals can significantly affect heater-induced perturbations and partially explain unique results obtained at other heater sites.

Solar flares and coronal mass ejections erupting from the roiling Sun can smash into the Earth's magnetosphere causing geomagnetic storms that penetrate deep into the atmosphere, which can short out satellites, upset radio communications, disrupt navigation, and even damage terrestrial electrical power grids. Though effects on other regions of the atmosphere have been analyzed, the mechanism by which geomagnetic storms influence the ionosphere's middle latitudes remains poorly understood. This brief report provides an overview of current knowledge in midlatitude ionospheric dynamics and disturbances, from the historic record to recent discoveries presented at a January AGU Chapman Conference.