The Case for Combining a Large Low-Band Very High Frequency Transmitter With Multiple Receiving Arrays for Geospace Research: A Geospace Radar

We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements

Comparison of flow angle variations of E-region echo characteristics at VHF and HF

In this study, characteristics of the auroral E-region echoes at two significantly different radar frequencies of 12 and 50 MHz are compared. Considered observations were performed at the Syowa Antarctic station in March of 1997 using two HF and one VHF radars at various angles with respect to the magnetic L shells. The diurnal variation of echo occurrence was found to be similar at two frequencies and consistent with previous studies. On the other hand, variation of echo occurrence with L-shell angle φ was shown to be significantly different at two frequencies. 50-MHz echoes were detected preferentially along the L shell (dominating direction of the electrojet flow) while 12-MHz echoes were detected in a broad range of azimuths with the maximum in echo occurrence at φ=40-50°. By plotting the Doppler velocity versus L-shell angle, we demonstrate that 12-MHz echoes can be divided into two populations, the high- and low-velocity echoes. The high-velocity echoes were observed mostly along the L shells while the low-velocity echoes were observed at all directions. We also show that the echo populations exhibit different variation of the Doppler velocity with the L-shell angle. We argue that while the 50-MHz echoes are related to the Farley-Buneman and gradient drift plasma instabilities, the 12-MHz echoes can have additional sources, such as the thermo-diffusion instability and/or neutral wind-related plasma instabilities

GPS scintillations associated with cusp dynamics and polar cap patches

This paper investigates the relative scintillation level associated with cusp dynamics (including precipitation, flow shears, etc.) with and without the formation of polar cap patches around the cusp inflow region by the EISCAT Svalbard radar (ESR) and two GPS scintillation receivers. A series of polar cap patches were observed by the ESR between 8:40 and 10:20 UT on December 3, 2011. The polar cap patches combined with the auroral dynamics were associated with a significantly higher GPS phase scintillation level (up to 0.6 rad) than those observed for the other two alternatives, i.e., cusp dynamics without polar cap patches, and polar cap patches without cusp aurora. The cusp auroral dynamics without plasma patches were indeed related to GPS phase scintillations at a moderate level (up to 0.3 rad). The polar cap patches away from the active cusp were associated with sporadic and moderate GPS phase scintillations (up to 0.2 rad). The main conclusion is that the worst global navigation satellite system space weather events on the dayside occur when polar cap patches enter the polar cap and are subject to particle precipitation and flow shears, which is analogous to the nightside when polar cap patches exit the polar cap and enter the auroral oval

Wind-Wave induced velocity in ATI SAR Ocean Surface Currents: First experimental evidence from an airborne campaign

Conventional and along-track interferometric (ATI) Synthetic Aperture Radar (SAR) sense the motion of the ocean surface by measuring the Doppler shift of reflected signals. Measurements are affected by a Wind-wave induced Artefact Surface Velocity (WASV) which was modelled theoretically in past studies and has been estimated empirically only once before with Envisat ASAR by Mouche et al., (2012). An airborne campaign in the tidally dominated Irish Sea served to evaluate this effect and the current retrieval capabilities of a dual-beam SAR interferometer known as Wavemill. A comprehensive collection of Wavemill airborne data acquired in a star pattern over a well-instrumented validation site made it possible for the first time to estimate the magnitude of the WASV, and its dependence on azimuth and incidence angle from data alone. In light wind (5.5 m/s) and moderate current (0.7 m/s) conditions, the wind-wave induced contribution to the measured ocean surface motion reaches up to 1.6 m/s upwind, with a well-defined 2nd order harmonic dependence on direction to the wind. The magnitude of the WASV is found to be larger at lower incidence angles. The airborne WASV results show excellent consistency with the empirical WASV estimated from Envisat ASAR. These results confirm that SAR and ATI surface velocity estimates are strongly affected by WASV and that the WASV can be well characterized with knowledge of the wind knowledge and of the geometry. These airborne results provide the first independent validation of Mouche et al., 2012, and confirm that the empirical model they propose provides the means to correct airborne and spaceborne SAR and ATI SAR data for WASV to obtain accurate ocean surface current measurements. After removing the WASV, the airborne Wavemill retrieved currents show very good agreement against ADCP measurements with a root mean square error (RMSE) typically around 0.1 m/s in velocity and 10° in direction

Seasonal Propagation Characteristics of MSTIDs Observed at High Latitudes Over Central Alaska Using the Poker Flat Incoherent Scatter Radar

Near‐continuous electron density measurements obtained over a ∼3 year period, 2010–2013, using the Poker Flat Incoherent Scatter Radar (PFISR) in central Alaska (69°N, 147°W) have been analyzed to quantify the properties of over 650 high‐latitude medium‐scale traveling ionospheric disturbances (MSTIDs). Our analysis focused on the altitude range 100–300 km encompassing the lower ionosphere/thermosphere and yielded first full seasonal day/night distributions of MSTIDs at high northern latitudes with mean values: horizontal wavelength 446 km, horizontal phase speed 187 m/s, and period 41 min. These year‐round measurements fill an important summertime gap in existing MSTID measurements revealing predominantly eastward wave propagation during the summer, while continued winter season observations agree well with previous reports of near southward propagating MSTIDs. Our 3 years of results suggest a cyclic change in the seasonal horizontal propagation directions that was found to be quantitatively consistent with critical level wind and dissipative filtering. Concurrent measurements of the vertical wavelength spectrum as a function of altitude also compared favorably in shape with that calculated using a theoretical dispersion relation (Vadas & Fritts, 2005, https://doi.org/10.1029/2004JD005574) for the thermosphere, but with a higher mean value. Evidence supporting the systematic broadening and shrinking in the azimuthal distributions of the MSTIDs during the course of the year was also found, as well as an unexpected correlation between the MSTID propagation directions and the AE index, both of which are under further investigation

On the Origin of Close-Range E Region Echoes Observed by SuperDARN HF Radars in the Mid- and High Latitudes

The Super Dual Auroral Radar Network (SuperDARN) is a global network of coherent high frequency (HF) radars located in the polar, high- and mid-latitudes of both the Northern and Southern hemispheres. This thesis deals with close-range SuperDARN echoes (oblique HF backscatter from the lower part of the ionosphere). The aim of this thesis is to shed light on the origin of these echoes. Previous studies have been content to propose explanations for the origin of these echoes without thorough checking of the proposed mechanisms against constraints available from various radars and other important information. For the purpose of clarifying the situation, a chain of SuperDARN radars in the Northern and Southern hemispheres and several years of daily statistics have been used. This has allowed for several findings. Notably, the close-range SuperDARN echoes show diurnal and seasonal variations and their properties with respect to signal-to-noise-ratio, Doppler velocity and Doppler width vary. Three distinct populations of close-range HF backscatter have been established: (1) a morning population (0400-0700 LT), (2) a midday summer population (0800-1300 LT) and (3) a pre-midnight (2100-2300 LT) population. The morning population is associated with meteor trails which are observed to be peaking near local dawn as expected, and already suggested by previous research. High latitude SuperDARN radars also had echoes (pre-midnight population) with higher Doppler velocities than the others yet the Doppler velocities are smaller than that expected from auroral E region echoes. Given the time and location of this population of echoes, it has been concluded that they are a special class of high latitude E region echoes at high aspect angle which have been termed ``high aspect irregularity region" echoes in the past. Lastly, the midday summer population was found to be too high for polar mesosphere summer echoes and too early for plasma instabilities. It is proposed that these SuperDARN echoes are produced either from contribution from meteors trails or by neutral turbulence which is suspected (from other work) to be present near 100 km. The properties of the midday summer population resembles those of meteor trails as they have the same power, and the same altitude and have high summer occurrence as expected for meteors. Their late morning occurrence could be due to particular look direction of individual radars which may change the occurrence statistics in the presence of meteor showers. With respect to neutral turbulence, the drift of the midday summer population is similar to that of neutral wind

Sequential 90° Rotation of Dual-Polarized Antenna Elements in Linear Phased Arrays with Improved Cross-Polarization Level for Airborne Synthetic Aperture Radar Applications

[EN] In this work, a novel rotation approach for the antenna elements of a linear phased array is presented. The proposed method improves by up to 14 dB the cross-polarization level within the main beam by performing a sequential 90° rotation of the identical array elements, and achieving measured cross-polarization suppressions of 40 dB. This configuration is validated by means of simulation and measurements of a manufactured linear array of five dual-polarized cavity-box aperture coupled stacked patch antennas operating in L-Band, and considering both uniform amplitude and phase distribution and beamforming with amplitude tapering. The analysis is further extended by applying and comparing the proposed design with the 180° rotation and non-rotation topologies. This technique is expected to be used for the next generation L-Band Airborne Synthetic Aperture Radar Sensor of the German Aerospace Center (DLR).Lorente-Catalán, D.; Limbach, M.; Gabler, B.; Esteban González, H.; Boria Esbert, VE. (2021). Sequential 90° Rotation of Dual-Polarized Antenna Elements in Linear Phased Arrays with Improved Cross-Polarization Level for Airborne Synthetic Aperture Radar Applications. Remote Sensing. 13(8). https://doi.org/10.3390/rs1308143013

Broadband Meter-Wavelength Observations of Ionospheric Scintillation

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally these observations are relatively narrow band. With Low Frequency Array (LOFAR) technology at the Kilpisj\"arvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a 3 octave bandwidth. ``Parabolic arcs'', which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broad-band observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus-A and covered the entire 30-250\,MHz band of KAIRA. Well-defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus-A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments, and indicate that scattering is most likely to be associated more with the topside ionosphere than the F-region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.Comment: 11 pages, 17 figure

Measuring ocean surface velocities with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation of the SKIM mission, Submitted to Ocean SCience, July 2019

Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterom-eters (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at 5 near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration , with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam, with a circular footprint only 45 m in diameter. 10 The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C 0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little 15 variations ∆ C associated to changes in sea state. The Ka-band measurements at an incidence of 12 • are 10% lower than the theoretical estimate C 0 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 1 https://doi. 30% lower than the theoretical estimate 2.8 m/s. ∆ C is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18 • incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements