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A multi-day (6–8 May 2013), apparently medium-scale traveling ionospheric disturbance (MSTID) event with an ∼1 hour period was recorded using both the vertical-looking and steerable Millstone Hill incoherent scatter radar (ISR) systems during quiet geomagnetic activity. The dense network of Global Positioning System (GPS) total electron content (TEC) receivers has allowed the horizontal imaging of this event across a far larger scale than is possible with the ISR(s) alone. This MSTID event was identified in the GPS-TEC dataset on the mesoscale centered at Haystack. However, GPS-TEC imaging also revealed that this event was coherent across the United States. The GPS-TEC imaging also revealed that similar events occurred globally. We conclude that this MSTID event was actually a coherent ionospheric pulsing structure (CIPS) that, when viewed locally, masqueraded as an MSTID event. The GPS-TEC images reveal that, on a global scale, the CIPS were stationary in longitude, but moved southward in the Northern Hemisphere—imaging of the southern hemisphere was not possible. The CIPS originated in high latitudes and at one instance were coherent across 23,000 km. The large-scale coherence, ubiquity, and source location of the CIPS points to an unidentified auroral zone source with sufficient energy to continually force the observed MSTID-like structures.

Low-frequency, wide-field radio telescopes such as the Murchison Widefield Array (MWA) enable the dense spatial sampling of the ionosphere and plasmasphere on regional scales. For a physically compact array such as the MWA, the refractive shifts in the positions of celestial sources in the synthesised radio images are proportional to spatial gradients in the total electron content (TEC) transverse to the line of sight. By measuring the angular position shifts of celestial radio sources, one can probe waves and disturbances in the intervening plasma. Radio telescopes differ fundamentally from other techniques for measuring plasma fluctuations in that they are sensitive to TEC gradients/differences rather than absolute TEC. This makes them sensitive specifically to fluctuations about the ambient density, and therefore powerful probes of plasma density waves and irregularities.

To address challenges of assessment of modeling capabilities, the CCMC (Community Coordinated Modeling Center) initiated a series of community-wide model validation projects, such as the GEM, CEDAR and GEM-CEDAR Modeling Challenges. The CEDAR ETI (Electrodynamics Thermosphere Ionosphere) Challenge focused on the ability of ionosphere-thermosphere (IT) models to reproduce basic IT system parameters, such as electron and neutral densities, NmF2, hmF2, and TEC. Model-data time series comparisons were performed for a set of selected events with different levels of geomagnetic activity (quiet, moderate, storms). The follow-on CEDAR-GEM Challenge aims to quantify geomagnetic storm impacts on the IT system. On-going studies include quantifying the storm energy input, such as increase in auroral precipitation and Joule heating, and quantifying the storm-time variations of neutral density and TEC. The community-wide model validation activities involve international collaborations (e.g., between the CCMC and UK Met Office) to enhance the studies. In this paper, we focus on results of validation of IT models for reproducing storm impacts on TEC. In order to quantify storm impacts on TEC, we considered several parameters: TEC changes compared to quiet time (the day before storm), TEC difference between 24-hour intervals, and maximum increase/decrease during the storm. We investigated the spatial and temporal variations of the parameters during storm events (e.g., 2006 AGU storm) using ground-based GPS TEC measurements in several longitude sectors where data coverage is relatively better. The latitudinal variations were also studied. We obtained modeled TEC from various IT models. The parameters from the models were compared with each other and with the observed values. We quantified performance of the models in reproducing the TEC variations during the storm using skill scores. Model output and observational data used for the challenge will be permanently posted at the CCMC website (http://ccmc.gsfc.nasa.gov) as a resource for the space science communities to use.

Ionospheric electron density and the total electron content (TEC) are representative and highly variable electromagnetic parameters in the coupled magnetosphere-ionosphere system. Because our modern society increasingly relies on ground-to-ground and ground-to-space communications and navigation, understanding the sources of the ionospheric density variability and monitoring its dynamics during space weather events have great practical importance. During geomagnetically disturbed times, ground-based instruments, such as incoherent scatter radars (ISRs) and GPS receivers, have observed great electron density and TEC enhancements and depressions over a wide range of magnetic latitudes and local times. We present observations of Storm-Enhanced Density (SED) [1] from the Poker Flat Advanced Modular ISR and GPS TEC during multiple geomagnetic storms. In particular, we show in detail the temporal evolution of SED observed during the Oct. 24–25, 2011 geomagnetic storm and discuss the mechanisms for the formation of SED [2]. Other key ionospheric properties, such as the convection flows and plasma temperature, within and surrounding these density structures are also presented and their relationship to the electron density and TEC changes are discussed.

We present first results from a study of the plasma instability mechanism responsible for the small-scale (∼10 m) ionospheric density irregularities commonly observed by the Super Dual Auroral Radar Network (SuperDARN) HF radars in the vicinity of Sub Auroral Polarization Streams (SAPS) during periods of geomagnetic disturbance. A focus is placed on the mid-latitude region of the ionosphere over North America where recent expansion of the SuperDARN network allows for extensive direct comparisons with total electron content (TEC) measurements from a dense network of ground-based GPS receivers. The TEC observations indicate that high-speed SAPS channels and the associated small-scale irregularities are typically located within the mid-latitude ionospheric trough. The Millstone Hill Incoherent Scatter Radar (ISR), operating in campaign mode in support of the NASA Van Allen Probes mission, provided measurements of F region ion/electron density, velocity, and temperature suitable for identifying potential mechanisms of plasma instability during a SAPS event that extended over 12 hours of magnetic local time (MLT) on 2 February 2013. Previous work has indicated that the density gradients associated with the poleward wall of the mid-latitude trough can produce small-scale irregularities due to the gradient drift instability during quiet periods by cascade from larger-scale structures. In this study we demonstrate that the gradient drift instability is a viable source for the direct generation of the small-scale irregularities observed by SuperDARN radars in the mid-latitude ionosphere during geomagnetically disturbed conditions.

The era of Global Navigation Satellite Systems (GNSS) and burgeoning worldwide receiver networks have increased the fidelity of ionospheric models and capacity of their applications. Precise slant total electron content (STEC) measurements are critical for a plethora of modern applications including precision navigation and geolocation, radio communication, surveillance, and weather modeling. The precision of STEC estimates from dual-frequency Global Positioning System (GPS) data is bounded by carrier phase noise, multipath effects, and systematic errors of estimating satellite and receiver biases (C. Brunini and F. Azpilicueta, Geodesy, 84:293-304). Through a collaborative effort, this presentation investigates precision of STEC estimates by analyzing disparities in STEC derived from International GNSS Service (IGS) GPS data among three independent processing chains: an open source algorithm developed at the Applied Research Laboratories at the University of Texas at Austin (ARL:UT) and algorithms developed at the Jet Propulsion Laboratory (JPL) and Haystack Observatory at the Massachusetts Institute of Technology (MIT). A common data set consisting of RINEX files from nearly 150 IGS stations in early March 2012 was processed by all three algorithms. After removing satellite and receiver biases from the STEC estimates, three ΔSTEC comparisons were analyzed over a period of quiescent ionospheric conditions on March 4th, 2012 and a period following a coronal mass ejection on March 7th, 2012. The debiased ΔSTEC values represent the inherent processing noise due to the Kalman filtering technique of obtaining STEC from dual-frequency GPS observations. Results of the ΔSTEC distributions and impact of the processing error will be presented.

We present first results from a study of the relationship between small-scale ionospheric irregularities observed by Super Dual Auroral Radar Network (SuperDARN) radars and large-scale density structures, such as the mid-latitude trough associated with subauroral polarization stream (SAPS) events. A focus is placed on mid-latitudes over North America where recent expansion of the SuperDARN network allows for comparison with total electron content (TEC) measurements from a dense network of ground-based GPS receivers. Numerical models show that gradients associated with the mid-latitude trough walls can produce small scale structures due to both the ion temperature gradient instability and gradient drift instability. However, previous studies of the gradient drift instability using high-latitude SuperDARN radars suggest that only sunward directed plasma density gradients at the duskside and dawnside ends of the trough present favorable conditions for the growth of decameter-scale field-aligned irregularities (FAIs). Recent campaigns by the Millstone Hill Incoherent Scatter Radar in support of the NASA Van Allen Probes mission provide crucial ion/electron velocity, temperature, and density measurements for the study of these instability processes. Specifically, we re-examine the gradient drift instability as a source of the small-scale FAIs regularly observed by SuperDARN radars in the mid-latitude ionosphere during geomagnetically disturbed conditions.

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.