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  • 1
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    The processing of electron density profiles from the Mars Express MARSIS topside sounder (2013)
    Wiley
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    Publication Date: 2013-03-02
    Description: [1]  We here present a manual for the reduction of data from ionograms obtained from the Mars Express MARSIS Active Ionospheric Sounding topside radar sounder. Sample data are presented with the procedure for processing them explained as simply as possible. We discuss the uncertainties inherent in the measurements as well as systematic problems with the data. A sample code is included to facilitate the inversion process. We also include a comparison with an electron density profile taken from the Mars Express Radio Science occultation experiment, showing agreement between the two methods, although the data are not simultaneous.
    Print ISSN: 0048-6604
    Electronic ISSN: 1944-799X
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Antarctic Radio Frequency Albedo and Implications for Cosmic Ray Reconstruction (2014)
    Wiley
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    Publication Date: 2014-10-08
    Description: We describe herein a measurement of the Antarctic surface “roughness” performed by the balloon-borne ANITA (ANtarctic Impulsive Transient Antenna) experiment. Originally purposed for cosmic-ray astrophysics, the radio-frequency (RF) receiver ANITA gondola, from its 38 km altitude vantage point, can scan a disk of snow surface 600 km in radius. The primary purpose of ANITA is to detect RF emissions from cosmic rays incident on Antarctica, such as neutrinos which penetrate through the atmosphere and interact within the ice, resulting in signal directed upwards which then refracts at the ice-air interface and up and out to ANITA, or high-energy nuclei (most likely irons or protons), which interact in the upper atmosphere (at altitudes below ANITA) and produce a spray of down-coming RF which reflects off the snow surface and back up to the gondola. The energy of such high-energy nuclei can be inferred from the observed reflectedsignal only if the surface reflectivity is known. We describe herein an attempt to quantify the Antarctic surface reflectivity, using the Sun as a constant, unpolarized RF source. We find that the reflectivity of the surface generally follows the expectations from the Fresnel equations, lending support to the use of those equations to give an overall correction factor to calculate cosmic ray energies for all locations in Antarctica. The analysis described below is based on ANITA-II data. After launching from McMurdo Station in December, 2008, ANITA-II was aloft for a period of 31 days with a typical instantaneous duty cycle exceeding 95%.
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    Topics: Geosciences , Physics
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  • 3
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    Characterizing the Pre-Space-Age Ionosphere over Washington, D.C. (2014)
    Wiley
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    Publication Date: 2014-07-13
    Description: The study of long-term changes requires researchers to identify relevant historical measurements; quantifying and quality-controlling these measurements becomes central to their utilization by broader communities. This pilot study reports on the evaluation of ionograms and their inversion from the decade prior to the Space Age. The ionograms were recorded on 35 mm film reels and archived at the National Geophysical Data Center (NGDC) World Data Center A in Boulder, Colorado. For this study, ionogram film from one of the earliest operational sites in North America, Fort Belvoir (near Washington D.C.) is analyzed. This analysis generates two distinct products: a digital ionogram with virtual height and frequency coordinate registration, and scaled values including an electron density profile (EDP). Validation of the ionogram registration and its resolution are described. The scaling and inversion processing, including uncertainties for the parameters and quality assessment, are explained. This study demonstrates how the archived film ionograms can have extensive value as historic measurements of the ionosphere once they are digitized and coordinate-registered. Furthermore, modern analysis of the ionograms shows that a complete bottomside ionospheric specification may be obtained at better time resolution than is typically available from original scaled data. This research has set the stage for an effective recovery of ionospheric information for almost a full solar cycle prior to IGY. In addition, between Fort Belvoir and Wallops Island archives, an East Coast mid-latitude ionospheric data set can be generated from the late 1940s through the present.
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  • 4
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    Implications of a heuristic model of auroral Farley Buneman waves and heating (2013)
    Wiley
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    Publication Date: 2013-09-07
    Description: [1]  The global implications, particularly with respect to altitude dependence, of the heuristic model of Farley Buneman waves put forward initially by Milikh and Dimant (2002) (see also Dimant and Milikh (2003); Milikh and Dimant (2003)) are studied. This model prescribes a relationship between the background convection electric field that excites the waves and the transverse electric fields of the waves that grow in response. It also prescribes the magnetic aspect angle of the waves, which is related to their ability to heat the auroral E region. The prescription is based on the condition of marginal stability. We reformulate the basic model, which is local, and embed it in the SAMI2 ionospheric model, which includes wave and Joule heating, heat transport, cooling, temperature-dependent collisions, and related chemistry. Within the limits of its underlying assumptions, the combined model can be used to predict the phase speeds and magnetic aspect widths of Farley Buneman waves in the auroral zone and the heating they can cause, all as functions of altitude. Model predictions are compared with experimental results, and the efficacy of the model assessed. This modeling exercise highlights the importance of the thickness of the layer in which Farley Buneman waves exist, the strong variations in wave characteristics across the layer, and the consequences this has for coherent scatter radar measurements of the phenomenon.
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  • 5
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    Typhoon Melor and ionospheric weather in the Asian sector: A case study (2012)
    Wiley
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    Publication Date: 2012-04-13
    Description: The Space Environment Group of the National Institute of Information and Communications Technology, Tokyo, Japan, operates four ionosondes at Okinawa, Yamagawa, Kokubunjii, and Wakkanai in the Asian sector. Okinawa is located at the lowest latitude and lies at the northern edge of the northern equatorial anomaly, while Wakkanai is located at the higher latitudes of the midlatitude ionosphere. For this study, ionograms obtained from the Internet are analyzed using the automated Expert System for Ionogram Reduction (ESIR). An anomalous 3 day foF2 enhancement observed by the Wakkanai ionosonde from 9 to 11 October 2009 forms the basis for this study. The scientific question being addressed pertains to the remarkably quiescent geomagnetic activity experienced during the extended solar minimum between cycles 23 and 24 that enables a search for the ionospheric response to weather in the lower atmosphere. The analysis of the ionograms from these four stations using the proprietary ESIR technique provides an extended database of electron density profiles that describes the ionospheric variability as a function of latitude, local time, and season. In addition, independent observations of the ionospheric TEC used by the USU Global Assimilation of Ionospheric Measurements (GAIM) model verify the anomalous ionospheric behavior as well as establishing its extent. Typical solar minimum conditions were seen during this study, with geomagnetic activity restricted to well-characterized corotating interaction region (CIR) events. After eliminating geomagnetic and solar disturbances as drivers of the October 2009 anomaly, the presence of Typhoon Melor is suggested as a possible source mechanism for the ionospheric anomaly.
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  • 6
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    Performance evaluation of selected ionospheric delay models during geomagnetic storm conditions in low-latitude region (2011)
    Wiley
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    Publication Date: 2011-06-29
    Description: Investigation of space weather effects on GPS satellite navigation systems is very crucial in high-precision positional applications such as aircraft landings and missile guidance, etc. The geomagnetic storms can drastically affect the total electron content (TEC) of the ionosphere even in low latitudes, especially for Indian region as it comes under low-latitude region. Hence, the performance of three prominent ionospheric models is investigated for adverse ionospheric conditions using 17 GPS TEC stations data. The models characterized the ionospheric disturbances due to two magnetic storms well.
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  • 7
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    Accurate and efficient algorithms for boundary element methods in electromagnetic scattering: A tribute to the work of F. Olyslager (2011)
    Wiley
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    Publication Date: 2011-12-17
    Description: Boundary element methods (BEMs) are an increasingly popular approach to model electromagnetic scattering both by perfect conductors and dielectric objects. Several mathematical, numerical, and computational techniques pullulated from the research into BEMs, enhancing its efficiency and applicability. In designing a viable implementation of the BEM, both theoretical and practical aspects need to be taken into account. Theoretical aspects include the choice of an integral equation for the sought after current densities on the geometry's boundaries and the choice of a discretization strategy (i.e. a finite element space) for this equation. Practical aspects include efficient algorithms to execute the multiplication of the system matrix by a test vector (such as a fast multipole method) and the parallelization of this multiplication algorithm that allows the distribution of the computation and communication requirements between multiple computational nodes. In honor of our former colleague and mentor, F. Olyslager, an overview of the BEMs for large and complex EM problems developed within the Electromagnetics Group at Ghent University is presented. Recent results that ramified from F. Olyslager's scientific endeavors are included in the survey.
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  • 8
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    Evaluation of time domain electromagnetic fields radiated by constant velocity moving particles traveling along an arbitrarily shaped cross-section waveguide using frequency domain Green's functions (2012)
    Wiley
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    Publication Date: 2012-09-21
    Description: A technique for the accurate computation of the time domain electromagnetic fields radiated by a charged distribution traveling along an arbitrarily shaped waveguide region is presented. Based on the transformation (by means of the standard Fourier analysis) of the time-varying current density of the analyzed problem to the frequency domain, the resulting equivalent current is further convolved with the dyadic electric and magnetic Green's functions. Moreover, we show that only the evaluation of the transverse magnetic modes of the structure is required for the calculation of fields radiated by particles traveling in the axial direction. Finally, frequency domain electric and magnetic fields are transformed back to the time domain, just obtaining the total fields radiated by the charged distribution. Furthermore, we present a method for the computation of the wakefields of arbitrary cross-section uniform waveguides from the resulting field expressions. Several examples of charged particles moving in the axial direction of such waveguides are included.
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  • 9
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    Intercepted Signals for Ionospheric Science (2013)
    Wiley
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    Publication Date: 2013-04-28
    Description: [1]  The ISIS Array (Intercepted Signals for Ionospheric Science) is a distributed, coherent software radio array designed for study of geospace phenomena by observing the scatter of ambient radio frequency (RF) signals. ISIS data acquisition and analysis is performed using the MIDAS-M platform (Millstone Data Acquisition System - Mobile). Observations of RF signals can be performed between HF and L-band using the Array nodes and appropriate antennas. The deployment of the Array focuses on observations of the plasmasphere boundary layer. We discuss the concept of the coherent software radio array, describe the ISIS hardware, and give examples of data from the system for selected applications. In particular we include the first observations of E-region irregularities using the Array. We also present single-site passive radar observations of both meteor trails and E-region irregularities using adaptive filtering techniques.
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  • 10
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    Reexamining X-mode suppression and fine structure in artificial E-region field aligned plasma density irregularities (2013)
    Wiley
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    Publication Date: 2013-08-09
    Description: [1]  Artificial field-aligned plasma density irregularities (FAIs) were generated in the E region of the ionosphere above the High Frequency Active Auroral Research Program (HAARP) facility during campaigns in May and August of 2012 and observed using a 30 MHz coherent scatter radar imager in Homer, Alaska. The purpose of this ionospheric modification experiment was to measure the threshold pump power required to excite thermal parametric instabilities by O-mode heating and to investigate the suppression of the FAIs by simultaneous X-mode heating. We find that the threshold pump power for irregularity excitation was consistent with theoretical predictions and increased by approximately a factor of two when X-mode heating was present. A modified version of the SAMI2 ionospheric model was used to simulate the threshold experiments and suggested that the increase was entirely due to enhanced D region absorption associated with X-mode heating. Additionally, a remarkable degree of fine structure possibly caused by natural gradient drift instability in the heater-modified volume was observed in experiments performed during geomagnetically active conditions.
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