Frank D. Lind

Multiradar observations of the polar tongue of ionization

[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

Mobile crowd sensing in space weather monitoring: the mahali project

Space weather refers to the conditions and evolution of Earths near space environment including electron density variations in the ionosphere. This environment is influenced by both the Sun and terrestrial processes, and has an impact on communications, navigation, and terrestrial power systems. The recent discovery of clear signatures in the ionosphere related to tsunamis and earthquakes suggests that the ionosphere itself may serve as a valuable and versatile sensor, registering many types of Earth- and space-based phenomena. To realize this potential, ionospheric electron density must be monitored through a dense wide-area sensor mesh that is expensive to realize with traditional deployments and observation techniques. Crowdsourcing can help pursue this novel direction by providing new capabilities, including an increase in the number of sensors as well as expanding data transport capabilities through participating devices that act as relays. This article describes the Mahali project, which is currently at the beginning of exploring these promising techniques. Mahali uses GPS signals that penetrate the ionosphere for science rather than positioning. A large number of ground-based sensors will be able to feed data through mobile devices into a cloud-based processing environment, enabling a tomographic analysis of the global ionosphere at unprecedented resolution and coverage. This novel approach brings the exploitation of the ionosphere as a global earth system sensor technologically and economically within reach.

Computer-Aided Discovery: Toward Scientific Insight Generation with Machine Support

The process of scientific discovery is traditionally assumed to be entirely executed by humans. This article highlights how increasing data volumes and human cognitive limits are challenging this traditional assumption. Relevant examples are found in observational astronomy and geoscience, disciplines that are undergoing transformation due to growing networks of space-based and ground-based sensors. The authors outline how intelligent systems for computer-aided discovery can routinely complement and integrate human scientists in the insight generation loop in scalable ways for next-generation science. The pragmatics of model-based computer-aided discovery systems go beyond feature detection in empirical data to answer fundamental questions, such as how empirical detections fit into hypothesized models and model variants to ease the scientists work of placing large ensembles of detections into a theoretical context. The authors demonstrate successful applications of this paradigm in several areas, including ionospheric studies, volcanics, astronomy, and planetary landing site identification for spacecraft and robotic missions.

Aircraft-protection radar for use with atmospheric lidars

A modified X-band radar system designed to detect aircraft during atmospheric lidar operations is described and characterized. The capability of the radar to identify aircraft approaching from a variety of directions was tested, and first detections were found to occur between the -10 and -3 dB perimeters of the gain horns antenna pattern. A model based on the radar equation projects the performance of the radar for different sizes of aircraft and at different altitude levels. Risk analysis indicates that the probability of accidently illuminating an aircraft with the laser beam during joint lidar-radar operations is low.

Detection of traveling ionospheric disturbances by medium-frequency Doppler sounding using AM radio transmissions

Nighttime traveling ionosphere disturbances (TIDs) propagating in the lower F region of the ionosphere have been detected by measuring time variations in the Doppler shifts of commercial AM radio broadcast signals. Three receivers, components of the Intercepted Signals for Ionospheric Science (ISIS) Array software radio instrumentation network in the northeastern United States, recorded signals from two radio stations during 11 nights in March–April, 2012. By combining these measurements, TIDs were detected as approximately 40min periodic variations in the frequencies of the received signals resulting from Doppler shifts produced by the ionosphere. The variations had amplitudes of up to a few tenths of a hertz and were correlated across the array. For one study interval, 0000–0400 UT on 13 April 2012, simultaneous GPS total electron content, Digisonde®, and Super Dual-Auroral Radar Network coherent backscatter radar measurements confirmed the detection of TIDs with the same characteristics. Besides TIDs, the receiver network often detected large (nearly 1 Hz) upward (downward) Doppler shifts of the AM broadcast signals at the dawn (dusk) terminator. These results demonstrate that AM radio signals can be used for detection and monitoring of nighttime TIDs and related effects.

Radio Array of Portable Interferometric Detectors (RAPID): Development of a deployable multiple application radio array

The Radio Array of Portable Interferometric Detectors (RAPID) is an advanced radio designed for multi-role applications. The system implements a spatially diverse sparse array technology and can be deployed and reconfigured easily. Data are captured at the raw voltage level using the system in the field and processed post-experiment. Signal processing for the system is software defined and uses a scalable Cloud computing architecture. The system builds upon the Square Kilometer Array Low Frequency Aperture antenna (SKALA) in combination with custom hardware for data acquisition on a per antenna basis. The instrument uses physically disconnected elements, a high performance direct digitization receiver, hot swap solid state storage, solar and battery power, and wireless control for interconnection. Schedule based operation can also be used in radio quiet locations or to enable minimally attended operation. RAPID is intended for application as both an Astronomical radio telescope and a Geospace imaging radar system. The high degree of mobility a orded by the system enables a wide variety of interferometric configurations and allows deployment of the instrument at locations which are optimal for specific scientific goals.

Radio array of portable interferometric detectors (RAPID)

The Radio Array of Portable Interferometric Detectors (RAPID) is a new radio array with a flexible architecture. The instrument uses per element software defined radios and a software signal processing architecture to enable the flexible study of a wide range of natural phenomena using radio imaging techniques. The array will be used for investigations of ionospheric phenomena, solar radio emission, the Galactic synchrotron background, and ultra-high energy cosmic rays via air-shower emission. The array will consist of ~100 small, low gain antennas operating over a frequency range of 48 to 450 MHz. RAPID is designed to make flexible and coherent radio observations, capturing the amplitude and phase of the electric field across a user-defined aperture with easily reconfigurable spatial sampling. Key technical elements include a novel absolute broadband antenna calibration method, elimination of a clock distribution network with a compact, low power chip scale atomic clock in each unit, state-of-the-art high performance voltage data recording, and low power consumption, via use of the latest low-power A/D converters and digital processing chips. By minimizing the power per element the RAPID system will be able to use compact, portable solar panels and batteries. Unlike existing arrays, RAPID will be operated without any cabling between the antennas and a central location, and can be shipped, deployed and physically reconfigured quickly and easily with zero site infrastructure. This creates a unique capability to locate and configure an imaging radio interferometer array, highly customized to the specific science goal of any given field campaign, thereby supporting science investigations that have not before been feasible. When used in conjunction with existing incoherent scatter radar transmitters or other transmitters of opportunity the array will provide a flexible capability for radar imaging of coherent and enhanced backscatter (e.g. E and F-region irregularities; naturally or artificially enhanced ion acoustic lines). The RAPID system architecture is based on voltage data capture with all processing performed in software, simplifying field operations and reducing equipment complexity. Data and work-flow management for the system will exploit distributed messaging, cloud technologies for scalable processing, and be implemented using open source Object Oriented Data Technology (OODT) software.

Vector antenna and maximum likelihood imaging for radio astronomy

Radio astronomy using frequencies less than ~100 MHz provides a window into non-thermal processes in objects ranging from planets to galaxies. Observations in this frequency range are also used to map the very early history of star and galaxy formation in the universe. Much effort in recent years has been devoted to highly capable low frequency ground-based interferometric arrays such as LOFAR, LWA, and MWA. Ground-based arrays, however, cannot observe astronomical sources below the ionospheric cut-off frequency of ~10 MHz, so the sky has not been mapped with high angular resolution below that frequency. The only space mission to observe the sky below the ionospheric cut-off was RAE-2, which achieved an angular resolution of ~60 degrees in 1973. This work presents alternative sensor and algorithm designs for mapping the sky both above and below the ionospheric cutoff. The use of a vector sensor, which measures the full electric and magnetic field vectors of incoming radiation, enables reasonable angular resolution (~5 degrees) from a compact sensor (~4 m) with a single phase center. A deployable version of the vector sensor has been developed to be compatible with the CubeSat form factor. Results from simulation as well as ground testing of the vector sensor are presented. A variety of imaging algorithms, including expectation-maximization (EM), space-alternating generalized expectation-maximization (SAGE), projected gradient ascent maximum likelihood (PGAML), and non-negative least squares (NNLS), have been applied to the data. The results indicate that the vector sensor can map the astronomical sky even in the presence of strong interfering signals. A conceptual design for a spacecraft to map the sky at frequencies below the ionospheric cut-off is presented. Finally, the possibility of using multiple vector sensors to form an interferometer is discussed.

GNSS‐ISR data fusion: General framework with application to the high‐latitude ionosphere

A mathematical framework is presented for the fusion of electron density measured by incoherent scatter radar (ISR) and total electron content (TEC) measured using global navigation satellite systems (GNSS). Both measurements are treated as projections of an unknown density field (for GNSS-TEC the projection is tomographic; for ISR the projection is a weighted average over a local spatial region) and discrete inverse theory is applied to obtain a higher fidelity representation of the field than could be obtained from either modality individually. The specific implementation explored herein uses the interpolated ISR density field as initial guess to the combined inverse problem, which is subsequently solved using maximum entropy regularization. Simulations involving a dense meridional network of GNSS receivers near the Poker Flat ISR demonstrate the potential of this approach to resolve sub-beam structure in ISR measurements. Several future directions are outlined, including (1) data fusion using lower level (lag product) ISR data, (2) consideration of the different temporal sampling rates, (3) application of physics-based regularization, (4) consideration of nonoptimal observing geometries, and (5) use of an ISR simulation framework for optimal experiment design.

Intercepted Signals for Ionospheric Science

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.