Browsing by Author "Baker, Joseph B. H."

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    Analysis and Synthesis of a New Class of Low Side Lobe Planar Arrays
    Khan, Iqtidar Ahmad (Virginia Tech, 2018-02-28)
    Numerical techniques for designing planar arrays with low side lobe level often require memory intensive optimization algorithms and also initialization in the form of some known values of radiation pattern parameters beforehand - information that may not be available when designing arrays. A few analytical methods exist in the literature that can be used to design rectangular lattices of isotropic elements for desired half-power beamwidth and side lobe level, but the number of elements of the array often should be known before the design process. Some array designs based on analytical techniques may suffer from severe performance limitations, an example is the uniformly excited array which cannot produce side lobe levels below ̶13.3 dB. The goal of this study is to present an analytical technique for synthesis of planar arrays that, for specified radiation pattern requirements, not only provides quick solutions for the required number of elements and its distribution along the length and width of the array rectangular lattice, but also produces low side lobes without any limitation. A new class of non-uniformly excited equally spaced planar arrays is introduced and investigated in this study. The new array uses the patterns of uniformly excited linear arrays as its building blocks and has a separable element current distribution, hence making it mathematically convenient to analyze its radiation properties in terms of those of its constituent linear arrays. The proposed planar array does not suffer from the side lobe level limitation of uniformly excited planar arrays, and its synthesis, due to the analytical nature of description of its radiation properties, does not require iterative procedures that are inherent to numerical techniques. Radiation characteristics of the proposed planar array, including directivity, side lobe level, half-power beamwidths, far-field three dimensional radiation patterns, and element excitation currents, are examined and simulation results for several example cases are presented. The analysis culminates with successfully mapping a continuous radiation pattern to discrete element currents in a rectangular lattice geometry. The synthesis procedure is validated by successfully designing various planar arrays with desired requirements in terms of side lobe level and half-power beamwidths in the principal planes. Several design examples are presented. Radiation characteristics of the synthesized arrays are compared with the desired design requirements which were used as input information in the synthesis process. For the cases studied, the achieved performance characteristics are close to the desired ones.
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    Analysis of Refractive Effects on Mid-Latitude SuperDARN Velocity Measurements
    Dixon, Kristoffer Charles (Virginia Tech, 2014-10-27)
    First time ionospheric refractive index values have been determined at mid latitudes using frequency switched SuperDARN plasma convection velocity estimates. Previous works have found a disparity between high latitude SuperDARN plasma convection velocities and those made by other devices. It was noted that the scattering volume’s refractive index was being neglected when estimating plasma convection velocities, meaning a correction factor was needed in order to more accurately reflect other measurements. Later work proposed a solution which implemented frequency switching in SuperDARN radars and determined a single correction factor based off of many years of data. We present case study driven research which applies the principles of these previous works to mid latitudes in an attempt to determine the refractive effect in mid latitude SuperDARN plasma convection velocity data by examining frequency switched quiet time ionospheric scatter. It was found that the 1/2 hop ionospheric scatter exhibited little to no measurable refractive effect (n ∼ 1), while the 11/2 hop ionospheric scatter tended to exhibit measurable refractive effects (n ∼ 0.7). This is then expanded to a storm-time 1/2 hop ionospheric scatter case study. It was again found that the refractive effects were nearly negligible (n ∼ 1), indicating that the 1/2 hop plasma convection velocities reported by mid latitude SuperDARN radars only require a very small correction factor, if any at all.
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    Ancillary Scientific Instrument Attachment (ASIA):  A Distributed Hitchhiker Payload Carrier
    Scheid, Robert Jon (Virginia Tech, 2018-05-18)
    As more spacecraft reach more locations in the solar system, and with increasing residual capability of those spacecraft, an opportunity exists to improve scientific return at low cost to the satellite operator and minimal effect on its primary mission, regardless of the nature of that mission. The practicality of permanently attaching a small, mass-produced, non-deployable hitchhiker payload to modern spacecraft buses is investigated, and a case study of one such payload is presented. The Ancillary Scientific Instrument Attachment (ASIA) is a modified CubeSat bus that can be mass produced, independently tested, and delivered to spacecraft manufacturers with the design, analysis, integration, test, and software development already complete. All it requires are single-string power and data connections, and a location to mount the bus structure. The unit includes power regulation; data collection and storage; command processing; thermal control; and structural support. As many as five small scientific instruments can be included, all of which increase scientific value of the host spacecraft's primary mission without significantly interfering with that mission. Generally, ASIA would operate independently, with a minimum of interaction from the host spacecraft operations team; only routine data dumps for scientific return need be executed. Scientific data processing, distribution, and bus subsystem troubleshooting are offloaded to an independent facility. One possible scientific instrument loadout is described, designed to collect data about the space environment at any location in the solar system. Recommended forward steps for designing, testing, demonstrating, and implementing such a space-based system and its ground elements are presented.
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    Characteristics and Sources of Intense Geoelectric Fields in the United States: Comparative Analysis of Multiple Geomagnetic Storms
    Shi, Xueling; Hartinger, Michael D.; Baker, Joseph B. H.; Murphy, Benjamin S.; Bedrosian, Paul A.; Kelbert, Anna; Rigler, Erin Joshua (American Geophysical Union, 2022-04)
    Intense geoelectric fields during geomagnetic storms drive geomagnetically induced currents in power grids and other infrastructure, yet there are limited direct measurements of these storm-time geoelectric fields. Moreover, most previous studies examining storm-time geoelectric fields focused on single events or small geographic regions, making it difficult to determine the typical source(s) of intense geoelectric fields. We perform the first comparative analysis of (a) the sources of intense geoelectric fields over multiple geomagnetic storms, (b) using 1-s cadence geoelectric field measurements made at (c) magnetotelluric survey sites distributed widely across the United States. Temporally localized intense perturbations in measured geoelectric fields with prominences (a measure of the relative amplitude of geoelectric field enhancement above the surrounding signal) of at least 500 mV/km were detected during geomagnetic storms with Dst minima (Dst(min)) of less than -100 nT from 2006 to 2019. Most of the intense geoelectric fields were observed in resistive regions with magnetic latitudes greater than 55 degrees even though we have 167 sites located at lower latitudes during geomagnetic storms of -200 nT <= Dst(min) < -100 nT. Our study indicates intense short-lived (<1 min) and geoelectric field perturbations with periods on the order of 1-2 min are common. Most of these perturbations cannot be resolved with 1-min data because they correspond to higher frequency or impulsive phenomena that vary on timescales shorter than that sampling interval. The sources of geomagnetic perturbations inducing these intense geoelectric fields include interplanetary shocks, interplanetary magnetic field turnings, substorms, and ultralow frequency waves.
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    Characterization and Modeling of Solar Flare Effects in the Ionosphere Observed by HF Instruments
    Chakraborty, Shibaji (Virginia Tech, 2021-06-08)
    The ionosphere is the conducting part of the upper atmosphere that plays a significant role in trans-ionospheric high frequency (HF, 3-30 MHz) radiowave propagation. Solar activities, such as solar flares, radiation storms, coronal mass ejections (CMEs), alter the state of the ionosphere, a phenomenon known as Sudden Ionospheric Disturbance (SID), that can severely disrupt HF radio communication links by enhancing radiowave absorption and altering signal frequency and phase. The Super Dual Auroral Radar Network (SuperDARN) is an international network of low-power HF coherent scatter radars distributed across the globe to probe the ionosphere and its relation to solar activities. In this study, we used SuperDARN HF radar measurements with coordinated spacecraft and riometer observations to investigate statistical characteristics and the driving mechanisms of various manifestations of solar flare-driven SIDs in HF observations. We begin in Chapter 2 with a statistical characterization of various effects of solar flares on SuperDARN observations. Simultaneous observations from GOES spacecraft and SuperDARN radars confirmed flare-driven HF absorption depends on solar zenith angle, operating frequency, and intensity of the solar flare. The study found flare-driven SID also affects the SuperDARN backscatter signal frequency, which produces a sudden rise in Doppler velocity observation, referred to as the ``Doppler flash'', which occurs before the HF absorption effect. In Chapter 3, we further investigate the HF absorption effect during successive solar flares and those co-occurring with other geomagnetic disturbances during the 2017 solar storm. We found successive solar flares can extend the ionospheric relaxation time and the variation of HF absorption with latitude is different depending on the type of disturbance. In Chapter 4, we looked into an inertial property of the ionosphere, sluggishness, its variations with solar flare intensity, and made some inferences about D-region ion-chemistry using a simulation study. Specifically, we found solar flares alter the D-region chemistry by enhancing the electron detachment rate due to a sudden rise in molecular vibrational and rotational energy under the influence of enhanced solar radiation. In Chapter 5, we describe a model framework that reproduces HF absorption observed by riometers. This chapter compares different model formulations for estimating HF absorption and discusses different driving influences of HF absorption. In Chapter 6, we have investigated different driving mechanisms of the Doppler flash observed by SuperDARN radars. We note two particular findings: (i) the Doppler flash is predominantly driven by a change in the F-region refractive index and (ii) a combination of solar flare-driven enhancement in photoionization, and changes in the zonal electric field and(or) ionospheric conductivity reduces upward ion-drift, which produces a lowering effect in the F-region HF radiowave reflection height. Collectively, these research findings provide a statistical characterization of various solar flare effects on the ionosphere seen in the HF observations, and insights into their driving mechanisms and impacts on ionospheric dynamics.
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    A comparison of SuperDARN ACF fitting methods
    Ribeiro, A. J.; Ruohoniemi, J. Michael; Ponomarenko, Pavlo V.; Clausen, Lasse B. N.; Baker, Joseph B. H.; Greenwald, R. A.; Oksavik, Kjellmar; de Larquier, S. (American Geophysical Union, 2013-05-01)
    The Super Dual Auroral Radar Network (SuperDARN) is a worldwide chain of HF radars which monitor plasma dynamics in the ionosphere. Autocorrelation functions are routinely calculated from the radar returns and applied to estimate Doppler velocity, spectral width, and backscatter power. This fitting has traditionally been performed by a routine called FITACF. This routine initiates a fitting by selecting a subset of valid phase measurements and then empirically adjusting for 2 phase ambiguities. The slope of the phase variation with lag time then provides Doppler velocity. Doppler spectral width is found by an independent fitting of the decay of power to an assumed exponential or Gaussian function. In this paper, we use simulated data to assess the performance of FITACF, as well as two other newer fitting techniques, named FITEX2 and LMFIT. The key new feature of FITEX2 is that phase models are compared in a least-squares fitting sense with the actual data phases to determine the best fit, eliminating some ambiguities which are present in FITACF. The key new feature of LMFIT is that the complex autocorrelation function (ACF) itself is fit, and Doppler velocity, spectral width, and backscatter power are solved simultaneously. We discuss some of the issues that negatively impact FITACF and find that of the algorithms tested, LMFIT provides the best overall performance in fitting the SuperDARN ACFs. The techniques and the data simulator are applicable to other radar systems that utilize multipulse sequences to make simultaneous range and velocity determinations under aliasing conditions.
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    Computational Challenges in Space Research (CS Seminar Lecture Series)
    Baker, Joseph B. H. (2012-04-20)
    The Center for Space Science and Engineering Research (Space@VT) in the College of Engineering is a relatively new center having been initiated in summer 2007. At the present time Space@VT comprises twelve Faculty in the ECE and AOE departments and approximately 30 graduate students and postdoctoral associates. Space@VT research and education activities are focused on developing improved scientific understanding of the near-Earth space environment and expanding its technological exploitation for societal needs. In this presentation I will provide an overview of Space@VT research activities with a particular emphasis on those aspects that touch on computational issues. The intent is to expand the conversation with CS faculty beyond the ad-hoc collaborations that are currently ongoing and, hopefully, generate new collaborations. Some of the themes that will be covered in the presentation include: (1) data mining the archive of space physics datasets for enhanced scientific productivity, (2) the necessity for development of new compression algorithms for data downlinks and attitude control on small university-built satellites (i.e. CubeSats), and (3) high performance numerical simulations of the near-Earth space plasma environment. Bio: Joseph Baker is an Assistant Professor in the Bradley Department of Electrical and Computer Engineering at Virginia Tech, and a member of the Center for Space Science and Engineering Research (Space@VT). Dr. Baker's current research uses data from the Super Dual Auroral Radar Network (SuperDARN) in conjunction with other ground- and space-based datasets to investigate electromagnetic coupling in the near-Earth space environment between the solar wind, the magnetosphere, and the ionosphere (or "space weather"). Prior to joining Virginia Tech in 2008, Dr. Baker was a Senior Staff Scientist at the Johns Hopkins University Applied Physics Laboratory. He received his Ph.D. in Atmospheric and Space Sciences from the University of Michigan in 2001, and his B.Sc. in Physics from the University of New England (Australia) in 1994. In 2011, Dr. Baker was named the Steven O. Lane Junior Faculty Fellow by the Virginia Tech Board of Visitors and an Outstanding New Assistant Professor in the College of Engineering. Dr. Baker is a member of the American Geophysical Union (AGU) and serves on its Education Award Committee. He is also a 2012 NSF CAREER award recipient. The Computer Science Seminar Lecture Series is a collection of weekly lectures about topics at the forefront of contemporary computer science research, given by speakers knowledgeable in their field of study. These speakers come from a variety of different technical and geographic backgrounds, with many of them traveling from other universities across the globe to come here and share their knowledge. These weekly lectures were recorded with an HD video camera, edited with Apple Final Cut Pro X, and outputted in such a way that the resulting .mp4 video files were economical to store and stream utilizing the university's limited bandwidth and disk space resources.
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    Design of Software Defined Radio for SuperDARN Radar
    Kennedy, Paul (Virginia Tech, 2019)
    Software defined radio (SDR) is a rapidly developing field enabled by continuing improvements in digital electronics. Software defined radio has been used extensively in communication systems due to its flexibility and cost effectiveness. Recently, SDR has been incorporated into radar systems, particularly for ionospheric research. This study investigated the benefits and design of a high frequency (HF) SDR receiver for the next generation of Super Dual Auroral Network (SuperDARN) radars. This work analyzed digital beamforming and waveform design approaches that would be enabled by the adoption of a SDR based radar design and found that these techniques could improve the performance of SuperDARN radars. This work also developed a prototype receiver to demonstrate the feasibility of a SDR based SuperDARN radar. The hardware selection for this receiver leveraged low-cost commercial off-the-shelf software defined radios and amplifier designs supplemented by custom filters. The software implementation utilized GNU Radio, an open source SDR and signal processing platform, to process and record receiver data. A prototype was successfully designed and constructed using the Red Pitaya software defined radio. This prototype included a 4 channel receiver which was evaluated in the laboratory setting and tested at the Blackstone, Virginia radar site. A comparison of results from the prototype receiver and the existing hardware showed promise for the use of this platform in future ionospheric research.
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    Direct observations of the role of convection electric field in the formation of a polar tongue of ionization from storm enhanced density
    Thomas, E. G.; Baker, Joseph B. H.; Ruohoniemi, J. Michael; Clausen, Lasse B. N.; Coster, A. J.; Foster, J. C.; Erickson, P. J. (American Geophysical Union, 2013-03-01)
    We examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from midlatitude plumes of storm enhanced density (SED). Observations from the geomagnetic storm on 26-27 September 2011 are presented for two distinct SED events. During an hour-long period of geomagnetic activity driven by a coronal mass ejection, a channel of high-density F region plasma was transported from the dayside subauroral ionosphere and into the polar cap by enhanced convection electric fields extending to middle latitudes. This TOI feature was associated with enhanced HF backscatter, indicating that it was the seat of active formation of small-scale irregularities. After the solar wind interplanetary magnetic field conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED plume was observed extending to, but not entering, the dayside cusp region. This prominent feature in the distribution of total electron content (TEC) persisted for several hours and elongated in magnetic local time with the rotation of the Earth. No ionospheric scatter from SuperDARN radars was observed within this SED region. The source mechanism (enhanced electric fields) previously drawing the plasma from midlatitudes and into the polar cap as a TOI was no longer active, resulting in a fossil feature. We thus demonstrate the controlling role exercised by the convection electric field in generating a TOI from midlatitude SED.
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    Driving Influences of Ionospheric Electrodynamics at Mid- and High-Latitudes
    Maimaiti, Maimaitirebike (Virginia Tech, 2020-01-15)
    The ionosphere carries a substantial portion of the electrical current flowing in Earth's space environment. Currents and electric fields in the ionosphere are generated through (1) the interaction of the solar wind with the magnetosphere, i.e. magnetic reconnection and (2) the collision of neutral molecules with ions leading to charged particle motions across the geomagnetic field, i.e. neutral wind dynamo. In this study we applied statistical and deep learning techniques to various datasets to investigate the driving influences of ionospheric electrodynamics at mid- and high-latitudes. In Chapter 2, we analyzed an interval on 12 September 2014 which provided a rare opportunity to examine dynamic variations in the dayside convection throat measured by the RISR-N radar as the IMF transitioned from strong By+ to strong Bz+. We found that the high-latitude plasma convection can have dual flow responses with different lag times to strong dynamic IMF conditions that involve IMF By rotation. We proposed a dual reconnection scenario, one poleward of the cusp and the other at the magnetopause nose, to explain the observed flow behavior. In Chapters 3 and 4, we investigated the driving influences of nightside subauroral convection. We developed new statistical models of nightside subauroral (52 - 60 degree) convection under quiet (Kp <= 2+) to moderately disturbed (Kp = 3) conditions using data from six mid-latitude SuperDARN radars across the continential United States. Our analysis suggests that the quiet-time subauroral flows are due to the combined effects of solar wind-magnetosphere coupling leading to penetration electric field and neutral wind dynamo with the ionospheric conductivity modulating their relative dominance. In Chapter 5, we examined the external drivers of magnetic substorms using machine learning. We presented the first deep learning based approach to directly predict the onset of a magnetic substorm. The model has been trained and tested on a comprehensive list of onsets compiled between 1997 and 2017 and achieves 72 +/- 2% precision and 77 +/- 4% recall rates. Our analysis revealed that the external factors, such as the solar wind and IMF, alone are not sufficient to forecast all substorms, and preconditioning of the magnetotail may be an important factor.
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    Dynamics of the geomagnetically disturbed ionosphere as measured by GPS receivers and SuperDARN HF radars
    Thomas, Evan Grier (Virginia Tech, 2012-12-07)
    Total electron content (TEC) data measured from ground-based GPS receivers is compared to HF backscatter from ionospheric irregularities obtained by Super Dual Auroral Radar Network (SuperDARN) radars.  We present the first observations of a recurrent region of anomalous enhanced TEC at mid-latitudes over North America and attempt to characterize its frequency of occurrence.  Next, we examine the relationship of convection electric fields to the formation of a polar cap tongue of ionization (TOI) from mid-latitude plumes of storm enhanced density (SED) during a geomagnetic storm on 26 September 2011.  A channel of high density F region plasma was transported from the dayside ionosphere and into the polar cap by enhanced convection electric fields extending to mid-latitudes.  After the solar wind IMF conditions quieted and the dayside convection electric fields retreated to higher latitudes, an SED was observed extending to, but not entering, the dayside cusp region.  The source mechanism (enhanced electric fields) previously drawing the plasma from mid-latitudes and into the polar cap was no longer active, resulting in a fossil feature which persisted for several hours as it elongated in magnetic local time. Finally, we discuss ground surface effects on the HF backscatter observed by four SuperDARN radars. Monthly ground scatter occurrence rates are calculated for comparison with Arctic sea ice boundaries derived from satellite observations, showing reduced backscatter from regions covered by ice.
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    Estimating along-track plasma drift speed from electron density measurements by the three Swarm satellites
    Park, J.; Luehr, H.; Stolle, Claudia; Malhotra, G.; Baker, Joseph B. H.; Buchert, S.; Gill, R. (European Geosciences Union, 2015)
    Plasma convection in the high-latitude ionosphere provides important information about magnetosphere-ionosphere-thermosphere coupling. In this study we estimate the along-track component of plasma convection within and around the polar cap, using electron density profiles measured by the three Swarm satellites. The velocity values estimated from the two different satellite pairs agree with each other. In both hemispheres the estimated velocity is generally anti-sunward, especially for higher speeds. The obtained velocity is in qualitative agreement with Super Dual Auroral Radar Network data. Our method can supplement currently available instruments for ionospheric plasma velocity measurements, especially in cases where these traditional instruments suffer from their inherent limitations. Also, the method can be generalized to other satellite constellations carrying electron density probes.
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    An examination of ionospheric plasma irregularities detected by the mid-latitude SuperDARN radars
    Ribeiro, Alvaro John (Virginia Tech, 2011-04-08)
    The data from the new mid-latitude radars of the Super Dual Auroral Radar Network (SuperDARN) provide new types of challenges and observations. We have developed a method for identifying periods of ionospheric backscatter that increase the number of data and reduce the average velocity in agreement with previous incoherent scatter radar (ISR) studies. Analysis of the data identified by this method clearly shows that different types of ionospheric irregularities are being observed in the mid-latitude region. One type of irregularity is clearly subauroral and equatorward of the plasmapause. Fitting a convection pattern to the Doppler velocities associated with subauroral ionospheric scatter reveals some interesting features. Subauroral convection is shown to be westward thought most of the night, with an eastward turning near dawn. The rotation factor of the ionosphere relative to the rotation of the earth is shown to be ~0.95, which is in good agreement with previous studies of plasmaspheric corotation.
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    Examining Plasma Instabilities as Ionospheric Turbulence Generation Mechanisms Using Pseudo-Spectral Methods
    Rathod, Chirag (Virginia Tech, 2021-03-30)
    Turbulence in the ionosphere is important to understand because it can negatively affect communication signals. This work examines different scenarios in the ionosphere in which turbulence may develop. The two main causes of turbulence considered in this work are the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI). The likelihood of the development of the GDI during the August 17, 2017 total solar eclipse is studied numerically. This analysis uses the ``Sami3 is Also a Model of the Ionosphere" (SAMI3) model to study the effect of the eclipse on the plasma density. The calculated GDI growth rates are small compared to how quickly the eclipse moves over the Earth. Therefore, the GDI is not expected to occur during the solar eclipse. A novel 2D electrostatic pseudo-spectral fluid model is developed to study the growth of these two instabilities and the problem of ionospheric turbulence in general. To focus on the ionospheric turbulence, a set of perturbed governing equations are derived. The model accurately captures the GDI growth rate in different limits; it is also benchmarked to the evolution of instability development in different collisional regimes of a plasma cloud. The newly developed model is used to study if the GDI is the cause of density irregularities observed in subauroral polarization streams (SAPS). Data from Global Positioning System (GPS) scintillations and the Super Dual Auroral Radar Network (SuperDARN) are used to examine the latitudinal density and velocity profiles of SAPS. It is found that the GDI is stabilized by velocity shear and therefore will only generate density irregularities in regions of low velocity shear. Furthermore, the density irregularities cannot extend through regions of large velocity shear. In certain cases, the turbulence cascade power laws match observation and theory. The transition between the KHI and the GDI is studied by understanding the effect of collisions. In low collisionality regimes, the KHI is the dominant instability. In high collisionality regimes, the GDI is the dominant instability. Using nominal ionospheric parameters, a prediction is provided that suggests that there exists an altitude in the upper textit{F} region ionosphere above which the turbulence is dominated by the KHI.
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    Field-aligned current reconfiguration and magnetospheric response to an impulse in the interplanetary magnetic field B-Y component
    Wilder, F. D.; Eriksson, S.; Korth, H.; Baker, Joseph B. H.; Hairston, M. R.; Heinselman, C.; Anderson, B. J. (American Geophysical Union, 2013-06-16)
    When the interplanetary magnetic field (IMF) is dawnward or duskward, magnetic merging between the IMF and the geomagnetic field occurs near the cusp on the dayside flanks of the magnetosphere. During these intervals, flow channels in the ionosphere with velocities in excess of 2km/s have been observed, which can deposit large amounts of energy into the high-latitude thermosphere. In this study, we analyze an interval on 5 April 2010 where there was a strong dawnward impulse in the IMF, followed by a gradual decay in IMF magnitude at constant clock angle. Data from the Sondrestrom incoherent scatter radar and the Defense Meteorological Satellite Program spacecraft were used to investigate ionospheric convection during this interval, and data from the Active Magnetospheric and Planetary Electrodynamics Response Experiment (AMPERE) were used to investigate the associated Field-Aligned Current (FAC) system. Additionally, data from AMPERE were used to investigate the time response of the dawnside FAC pair. We find there is a delay of approximately 1.25h between the arrival of the dawnward IMF impulse at the magnetopause and strength of the dawnward FAC pair, which is comparable to substorm growth and expansion time scales under southward IMF. Additionally, we find at the time of the peak FAC, there is evidence of a reconfiguring four-sheet FAC system in the morning local time sector of the ionosphere. Additionally, we find an inverse correlation between the dawn FAC strength and both the solar wind Alfvenic Mach number and the SYM-H index. No statistically significant correlation between the FAC strength and the solar wind dynamic pressure was found.
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    First Observations of Large Scale Traveling Ionospheric Disturbances Using Automated Amateur Radio Receiving Networks
    Frissell, Nathaniel A.; Kaeppler, Stephen R.; Sanchez, Diego F.; Perry, Gareth W.; Engelke, William D.; Erickson, Philip J.; Coster, Anthea J.; Ruohoniemi, J. Michael; Baker, Joseph B. H.; West, Mary Lou (American Geophysical Union, 2022-03-16)
    We demonstrate a novel method for observing Large Scale Traveling Ionospheric Disturbances (LSTIDs) using high frequency (HF) amateur radio reporting networks, including the Reverse Beacon Network (RBN), Weak Signal Propagation Reporter Network (WSPRNet), and PSKReporter. LSTIDs are quasi-periodic variations in ionospheric densities with horizontal wavelengths >1,000 km and periods between 30 and 180 min. On Nov 3, 2017, LSTID signatures were observed simultaneously over the continental United States in amateur radio, SuperDARN HF radar, and GNSS Total Electron Content with a period of similar to 2.5 hr, propagation azimuth of similar to 163 degrees, horizontal wavelength of similar to 1680 km, and phase speed of similar to 1,200 km hr(-1). SuperMAG SME index enhancements and Poker Flat Incoherent Scatter Radar measurements suggest the LSTIDs were driven by auroral electrojet intensifications and Joule heating. This novel measurement technique has applications in future scientific studies and for assessing the impact of LSTIDs on HF communications.
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    GNSS Hardware-In-The-Loop Formation and Tracking Control
    Harris, Frederick Bernard Jr. (Virginia Tech, 2016-06-20)
    Formation and tracking control are critical for of today's vehicle applications in and this will be true for future vehicle technologies as well. Although the general function of these controls is for data collection and military applications, formation and tracking control may be applied to automobiles, drones, submarines, and spacecraft. The primary application here is the investigation of formation keeping and tracking solutions for realistic, real-time, and multi-vehicle simulations. This research explores the creation of a predictive navigation and control algorithm for formation keeping and tracking, raw measurement data collection, and building a real-time GNSS closed HWIL testbed for simulations of different vehicles. The L1 frequency band of the Global Positioning System (GPS) constellation is used to observe and generate raw measurement data that encompasses range, pseudo-range, and Doppler frequency. The closed HWIL simulations are implemented using Spirent's Communication Global Navigation Satellite system (GNSS) 6560 and 8000 hardware simulators along with Ashtech, G-12 and DG-14, and Novetel OEM 628 receivers. The predictive navigation control is similar to other vision-based tracking techniques, but relies mainly on vector projections that are controlled by acceleration, velocity magnitude, and direction constraints to generate realistic motion. The current state of the testbed is capable of handling one or more vehicle applications. The testbed can model simulations up to 24 hours. The vehicle performance during simulations can be customized for any required precision by setting a variety of vehicle parameters. The testbed is built from basic principles and is easily upgradable for future expansions or upgrades.
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    HF Radar Observations of Inter-Annual variations in Mid-Latitude Mesospheric Winds
    Malhotra, Garima (Virginia Tech, 2016-04-09)
    The equatorial Quasi Biennial Oscillation (QBO) is known to be an important source of inter-annual variability at mid and high latitudes in both hemispheres. Coupling between QBO and the polar vortex has been extensively studied over the past few decades, however, less is known about QBO influences in the mid-latitude mesosphere. One reason for this is the relative lack of instrumentation available to study mesospheric dynamics at mid-latitudes. In this study, we have used the mid-latitude SuperDARN HF radar at Saskatoon (52.16 N, -106.53 E) to study inter-annual variation in mesospheric winds. The specific aim was to determine whether or not a Quasi Biennial signature could be identified in the Saskatoon mesosphere, and if so, to understand its relationship with the equatorial stratospheric QBO. To achieve this goal, a technique has been developed which extracts meteor echoes from SuperDARN near-range gates and then applies least-squares fitting across all radar beam directions to calculate hourly averages of the zonal and meridional components of the mesospheric neutral wind. Subsequent analysis of 13 years (2002-2014) of zonal wind data produced using this technique indicates that there is indeed a significant QBO signature present in Saskatoon mesospheric winds during late winter (Jan-Feb). This mesospheric QBO signature is in opposite phase with the equatorial stratospheric QBO, such that when QBO (at 50 hPa) is in its easterly (westerly) phase, the late winter winds in Saskatoon mesosphere become more (less) westerly. To further examine the source of the signature, we also analyzed winds in the Saskatoon stratosphere between 5 hPa and 70 hPa using the ECMWF ERA-Interim reanalysis data set, and found that the late winter stratospheric winds become less (more) westerly when QBO is easterly (westerly). This QBO signature in the mid-latitude stratospheric winds is essentially the same as that observed for the polar vortex in previous studies but it is opposite in phase to the mid-latitude mesospheric QBO. We therefore conclude that filtering of gravity waves through QBO-modulated stratospheric winds plays a major role in generating the mesospheric QBO signature we have identified in the Saskatoon HF radar data. When the Saskatoon stratospheric winds are anomalously westward during easterly QBO, the gravity waves having westward momentum might be filtered out, depositing a net eastward momentum in the mesosphere as they propagate upwards. This would result in increased westerly mesospheric winds at Saskatoon. The opposite would happen when the equatorial QBO is westerly.
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    Ion-neutral coupling in the geomagnetically disturbed mid-latitude ionosphere as observed by SuperDARN HF radars and NATION Fabry-Perot Interferometers
    Joshi, Pratik Prasad (Virginia Tech, 2015-09-17)
    The earth's ionosphere-thermosphere region is a coupled environment which is governed by interactions between the overlapping neutral constituents and ionospheric plasma. The mid-latitude thermosphere-ionosphere system is very complex owing to its sensitivity to both the polar and equatorial processes. The mid-latitudes is also a relatively unexplored and less understood region primarily due to the paucity of observing instruments that have traditionally been available. However, the past 9 years of mid-latitude expansion of the Super Dual Auroral Radar Network (SuperDARN) has provided new access to continuous large-scale observations of the sub-auroral ionosphere. On the other hand, the past 3 years of mid-latitude expansion of the North American Thermosphere Ionosphere Observation Network (NATION) Fabry-Perot interferometer array, has created a critical resource for measuring the thermospheric neutral winds. The overlap of these two observing networks in the mid-east North American sector has resulted in a strong ground-based large-scale platform for co-located study of mid-latitude thermosphere-ionosphere dynamics for the first time. The coupling between ions and neutrals is a very important process for controlling the thermospheric dynamics. Ion-neutral coupling at high latitudes has been studied in many previous papers, but there have been very few studies focused on the mid-latitude region. Hence, in this work we have studied the ion-neutral coupling mechanisms and timescales at mid-latitudes during disturbed geomagnetic conditions by using the co-located observations from the SuperDARN-NATION array. The study has focused on the main phase as well as the late recovery phase of a geomagnetic storm which occurred on October 2-3, 2013. Ion drag is known to drive the neutral circulation during the main phase of storm at auroral latitudes, while the neutral wind disturbance dynamo mechanism is known to generate ionospheric electric fields and currents during the recovery phase. By using the methods of ion-neutral momentum exchange theory and time lagged correlation analysis, we analyzed the timescales at which the ion-neutral coupling operates. The ions are observed to drive the neutral winds on a timescale of ~ 84 minutes in the storm main phase which is significantly faster than expected from the driving due to local ion-drag alone (~ 124 minutes). This suggests that along with ion-drag, other local and non-local storm-time influences like Joule heating are also playing an important part in driving the neutral winds. On the other hand, in the late recovery phase, the neutral winds are found to be strongly coupled with the ions and maintain the ion convection without any significant time delay which is consistent with effect of the 'disturbance dynamo' or 'neutral-flywheel' persisting well into the late recovery phase. The timescales and underlying physics understood through this work serve as an important contribution to our knowledge of ion-neutral coupling processes at the middle latitudes. Looking forward, the expansion of co-located SuperDARN-NATION coverage at mid-latitudes, and developments in the tools of large-scale visualization through FPI wind field mapping and SuperDARN convection maps, has created a very strong basis for using the results and analysis tools developed in this work for large-scale ion-neutral coupling characterization in future.
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    Ionospheric Disturbances: Midlatitude Pi2 Magnetospheric ULF Pulsations and Medium Scale Traveling Ionospheric Disturbances
    Frissell, Nathaniel A. (Virginia Tech, 2016-06-01)
    The ionosphere is an electrically charged atmospheric region which is coupled to the sun, the magnetosphere, and the neutral atmosphere. The ionospheric state can significantly impact technological systems, especially those which utilize radio frequency energy. By studying ionospheric disturbances, it is possible to gain a deeper understanding of not only the ionosphere itself, but also the natural and technological systems it is coupled to. This dissertation research utilizes high frequency (HF) radio remote sensing techniques to study three distinct types of ionospheric disturbances. First, ground magnetometers and a new mid latitude SuperDARN HF radar at Blackstone, Virginia are used to observe magnetospheric Pi2 ultra low frequency (ULF) pulsations in the vicinity of the plasmapause. Prior to these pulsations, two Earthward moving fast plasma flows were detected by spacecraft in the magnetotail. Signatures of inner magnetospheric compression observed by the Blackstone radar provide conclusive evidence that the plasma flow bursts directly generated the ground Pi2 signature via a compressional wave. This mechanism had previously been hypothesized, but never confirmed. Next, ten SuperDARN radars in the North American Sector are used to investigate the sources and characteristics of atmospheric gravity waves (AGW) associated medium scale traveling ionospheric disturbances (MSTIDs) at both midlatitudes and high latitudes. Consistent with prior studies, the climatological MSTID population in both latitudinal regions was found to peak in the fall and winter and have a dominant equatorward propagation direction. Prior studies suggested these MSTIDs were caused by mechanisms associated with auroral and space weather activity; however, it is shown here that the AE and Sym-H indices are poorly correlated with MSTID observations. A new, multi-week timescale of MSTID activity is reported. This leads to the finding that MSTID occurrence is highly correlated with an index representative of polar vortex activity, possibly controlled by a filtering mechanism that is a function of stratospheric neutral wind direction. Finally, a case study of a radio blackout of transionospheric HF communications caused by an X2.9 class solar flare is presented. This study demonstrates the potential of a novel technique employing signals of opportunity and automated receiving networks voluntarily created by an international community of amateur radio operators.