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An annular solar eclipse was visible on 14 October 2023 from 15:00–21:00 UT as its path traveled across North, Central, and South America. In this letter, we present the first multi‐frequency Super Dual Auroral Radar Network (SuperDARN) observations of the bottomside ionospheric response to a solar eclipse using a novel experimental mode designed for the October 2023 annular eclipse. We compare our results from the mid‐latitude Christmas Valley East radar with measurements of the vertical electron density profile from the nearby Boulder Digisonde, finding the changes in 1‐ and 2‐hop ground scatter skip distance are well correlated with the F2 ${F}_{2}$‐layer density response, which lags the peak obscuration by ∼ ${\sim} $30 min. Changes in the line‐of‐sight Doppler shifts are better aligned with the time derivative of eclipse obscuration. Plain Language Summary: During a solar eclipse when the moon passes between our planet and the sun, the amount of incoming solar extreme ultraviolet radiation is reduced over the shadowed region of Earth. As a result, temperatures can become cooler and the rate of plasma production in the ionized layers of the Earth's upper atmosphere, known as the ionosphere, can also decrease. Users of high‐frequency radio systems, such as over‐the‐horizon radar and amateur radio, depend on knowledge of the ionospheric plasma density, and are therefore susceptible to abrupt changes due to space weather phenomena such as a solar eclipse. While not attracting as much media attention as the total solar eclipses over North America in August 2017 and April 2024, the annular solar eclipse of October 2023 nevertheless presented a unique opportunity to study the response of Earth's upper atmosphere using a diverse array of ground‐based instruments in a similar region but at a different time of day. For this event, we designed a new experimental mode for a pair of space weather radars located in central Oregon to transmit signals at a range of frequencies in the HF band to monitor their evolution as the eclipse shadow passed nearby. Key Points: A novel multi‐frequency experiment was conducted with the Christmas Valley East SuperDARN radar during the 14 October 2023 annular eclipseOur comparison with the nearby Boulder Digisonde shows a strong correlation in 1‐hop skip distance with F2‐layer critical frequencyThe transition from negative to positive Doppler shifts is clearly aligned with the time derivative of eclipse shadow [ABSTRACT FROM AUTHOR]

The path of totality of the 8 April 2024 solar eclipse traversed the fields‐of‐view of four US SuperDARN radars. This rare scenario provided an excellent opportunity to monitor the large‐scale ionospheric response to the eclipse. In this study, we present observations made by the Blackstone (BKS) SuperDARN radar and a Digisonde during the eclipse. Two striking effects were observed by the BKS radar: (a) the Doppler velocities associated with ground scatter coalesced into a pattern clearly organized by the line of totality, with a reversal in sign across this line, and, (b) a delay of ∼ ${\sim} $45 min between time of maximum obscuration and maximum effect on the skip distance. The skip distance estimated using a SAMI3 simulation of the eclipse did not however capture the asymmetric time‐delay. These observations suggest that the neutral atmosphere plays an important role in controlling ionospheric plasma dynamics, which were missing in SAMI3 simulations. Plain Language Summary: The total solar eclipse on 8 April 2024 was the last one to be observed over the continental United States until 2045. In addition to blocking the visible light from the Sun over a swath ranging a few hundred kilometers, a solar eclipse will also partially obscure the Sun's extreme ultraviolet radiation. As a result, an eclipse can drive significant changes in the ionosphere, a charged region in the Earth's upper atmosphere that is predominantly ionized by solar radiation. A unique feature of this eclipse was that the path of totality traversed the fields‐of‐view of three Super Dual Auroral Radar Network (SuperDARN) radars–Fort Hays East, Blackstone, and Wallops Island. SuperDARN radars operate in the High Frequency (HF) range, and radio waves in this frequency range are particularly sensitive to changes in the ionosphere. In this study, we use observations provided by SuperDARN radars alongside a Digisonde to determine the temporal response of the different ionospheric layers to the eclipse and compare these observations with modeled behavior. Key Points: We present observations made by the Blackstone SuperDARN radar, a Digisonde, and SAMI3 simulations to analyze the eclipsed ionosphereSuperDARN Doppler velocities cohered into a pattern clearly organized by the line of totality with a reversal in sign across the lineSuperDARN skip distance and Digisonde derived foF2 show a delayed response to the shadow, suggesting slower response at higher altitudes [ABSTRACT FROM AUTHOR]

Polarization and propagation characteristics of ultra‐low frequency (ULF, ≃1−1000 $\simeq 1-1000$ mHz) waves are conventionally studied using arrays of ground‐based magnetometers. However, the ground magnetometer observations are subject to distortions due to polarization rotation and spatial integration effects caused by the transition of the magnetohydrodynamic wave into an electromagnetic wave at the lower ionospheric boundary. In contrast, high‐frequency (3–30 MHz) radars, like those comprising the Super Dual Auroral Radar Network (SuperDARN), are capable of direct observations of the ULF wave characteristics at ionospheric altitudes via measuring plasma drift velocity variations caused by the wave's electric field. In this work, we use multi‐beam data from SuperDARN Hokkaido East, Hokkaido West, and Christmas Valley West radars to identify the dominant polarization modes as well as azimuthal wave numbers of evening‐night‐side‐morning ULF waves in the Pc5 frequency band (1.67–6.67 mHz) propagating over sub‐auroral and mid‐latitude regions. The observed statistical characteristics of these waves point at the solar wind dynamic pressure variations and Kelvin‐Helmholtz instability at the magnetopause as their potential principal sources, although the drift‐bounce resonance with trapped energetic ions may contribute to the small‐scale part of the observed Pc5 wave population. Plain Language Summary: Ultra‐low frequency waves (ULF, ≃1−1000 $\simeq 1-1000$ mHz) are large‐scale hydromagnetic waves, which are generated by different mechanisms and propagate in the near‐Earth's plasma environment. They contain important information about Space Weather phenomena and have been intensively studied since the late 1950s. ULF waves are routinely studied using ground‐based magnetometers, which provide data with relatively low spatial resolution and geographic coverage restricted to landmasses. In this work, we mitigate these problems by using high‐frequency (3–30 MHz) ground‐based radars comprising the Super Dual Auroral Radar Network, which provides superior spatial resolution and covers inaccessible geographic regions like the sea or mountains. Using more than 10 years of mid‐latitude data from the Hokkaido East and Hokkaido West radars (Japan) and the Christmas Valley West radar (US), we performed detailed statistical studies of ULF wave propagation characteristics in the Pc5 frequency range (1.67–6.67 mHz). These studies provide important information about the possible generation and propagation of Pc5 ULF waves in the near‐Earth's plasma environment. Key Points: Statistical characteristics of Pc5 ultra‐low frequency (ULF) waves at mid‐latitude and sub‐auroral ionosphere was studied using Super Dual Auroral Radar Network radarsObserved occurrence rate, frequency, polarization, and azimuthal m $m$‐number point at a common source of these wavesSolar wind dynamic pressure variations and Kelvin‐Helmholtz instability at the magnetopause may be the principal generation mechanisms [ABSTRACT FROM AUTHOR]

The phenomenon known as strong thermal emission velocity enhancement (STEVE) is a narrow optical structure that may extend longitudinally for thousands of kilometers. Initially observed by amateur photographers, it has recently garnered researchers' attention. STEVE has been associated with a rapid westward flow of ions in the ionosphere, known as subauroral ion drift (SAID). In this work, we investigate three occurrences of STEVE, using data from one of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) ground-based all-sky imagers (ASIs) located at Pinawa, Manitoba, and from the Super Dual Auroral Radar Network (SuperDARN). This approach allows us to verify the correlation between STEVE and SAID, as well as analyze the temporal variation of SAID observed during STEVE events. Our results suggest that the SAID activity starts before the STEVE, and the magnitude of the westward flow decreases as the STEVE progresses toward the end of its optical manifestation. [ABSTRACT FROM AUTHOR]

The ground‐based, high‐frequency radars of the Super Dual Auroral Radar Network (SuperDARN) observe backscatter from ionospheric field‐aligned plasma irregularities and features on the Earth's surface out to ranges of several thousand kilometers via over‐the‐horizon propagation of transmitted radio waves. Interferometric techniques can be applied to the received signals at the primary and secondary antenna arrays to measure the vertical angle of arrival, or elevation angle, for more accurate geolocation of SuperDARN observations. However, the calibration of SuperDARN interferometer measurements remains challenging for several reasons, including a 2π phase ambiguity when solving for the time delay correction factor needed to account for differences in the electrical path lengths between signals received at the two antenna arrays. We present a new technique using multi‐frequency ionospheric and ground backscatter observations for the calibration of SuperDARN interferometer data, and demonstrate its application to both historical and recent data. Key Points: Calibration of Super Dual Auroral Radar Network (SuperDARN) interferometer angle of arrival data remains an outstanding challengeAnalysis of data at multiple frequencies can overcome the inherent 2π measurement ambiguity in the time delay correction factor (tdiff)Example applications are shown for historical and contemporary multi‐frequency SuperDARN observations [ABSTRACT FROM AUTHOR]

Medium‐scale Traveling Ionospheric Disturbances (MSTIDs) are prominent and ubiquitous features of the mid‐latitude ionosphere, and are observed in Super Dual Auroral Radar Network (SuperDARN) and high‐resolution Global Navigational Satellite Service (GNSS) Total Electron Content (TEC) data. The mechanisms driving these MSTIDs are an open area of research, especially during geomagnetic storms. Previous studies have demonstrated that nightside MSTIDs are associated with an electrodynamic instability mechanism like Perkins, especially during geomagnetically quiet conditions. However, dayside MSTIDs are often associated with atmospheric gravity waves. Very few studies have analyzed the mechanisms driving MSTIDs during strong geomagnetic storms at mid‐latitudes. In this study, we present mid‐latitude MSTIDs observed in de‐trended GNSS TEC data and SuperDARN radars over the North American sector, during a geomagnetic storm (peak Kp reaching 9) on 7–8 September 2017. In SuperDARN, MSTIDs were observed in ionospheric backscatter with Line of Sight (LOS) velocities exceeding 800 m/s. Additionally, radar LOS velocities oscillated with amplitudes reaching ±500 m/s as the MSTIDs passed through the fields‐of‐view. In detrended TEC, these MSTIDs produced perturbations reaching ∼50 percent of background TEC magnitude. The MSTIDs were observed to propagate in the westward/south‐westward direction with a time period of ∼15 min. Projecting de‐trended GNSS TEC data along SuperDARN beams showed that enhancements in TEC were correlated with enhancements in SuperDARN SNR and positive LOS velocities. Finally, SuperDARN LOS velocities systematically switched polarities between the crests and the troughs of the MSTIDs, indicating the presence of polarization electric fields and an electrodynamic instability process during these MSTIDs. Key Points: Medium‐scale Traveling Ionospheric Disturbance (MSTID) signatures were observed in Global Navigational Satellite Service Total Electron Content and Super Dual Auroral Radar Network (SuperDARN) ionospheric backscatter during a strong geomagnetic stormMSTID characteristics were broadly consistent between the data sets, with periods of 10–20 min and phase speeds of ∼800 m/sSuperDARN Line of Sight velocities systematically switched polarities between MSTID crests and troughs, indicating polarization electric fields [ABSTRACT FROM AUTHOR]

A new technique for estimating the global‐scale pattern of magnetospheric convection in the ionosphere is presented. The technique uses the Super Dual Auroral Radar Network line‐of‐sight velocity observations combined with an empirical convection model and the assumption that the resulting velocity field is divergence free. In contrast to other techniques for convection estimation, it does not express the velocity field in terms of known basis vectors and it does not assume that the velocity can be determined from a static potential. The velocity is estimated by applying Bayesian inverse theory to the input data, model, and constraints. Linear equations for the plasma velocity at every point in the domain are solved simultaneously in a least squares sense. Application of the technique results in convection patterns with spatial resolution equal to the calculation grid spacing. The resulting patterns conform with expectations based on the observed interplanetary magnetic field conditions and display features that show close correspondence to simultaneously observed features in the auroral luminosity. Plain Language Summary: A new technique for estimating the global‐scale pattern of plasma flow in the ionosphere is presented. The technique uses radar observations from a network of radars called the Super Dual Radar Network or Super Dual Auroral Radar Network. The observations are combined with a model that gives average patterns for the conditions at the time of the observations and some assumptions about the nature of the flow velocity to produce global‐scale patterns. The results will be useful for other studies of the magnetosphere, ionosphere, and upper atmosphere where understanding how the plasma moves is important. Key Points: Development of a new technique for estimating flow in the ionosphereThe new technique provides high‐resolution estimates of the flowsThe flows show good correlation with the auroral luminosity [ABSTRACT FROM AUTHOR]

The dependencies of global positioning system (GPS) scintillation indices on ionospheric plasma flow and the rate of change of total electron content (TEC) around the dawn sector for the first time of the polar ionosphere are investigated. The phase scintillation index (σφ) derived from GPS measurements of the Canadian High Arctic Ionospheric Network (CHAIN) shows linear dependencies on both the plasma drift speed measured by the SuperDARN radar and on the rate of change of TEC estimated from the GPS receivers of CHAIN. However, the amplitude scintillation index (S4) does not show any dependence on the plasma flow or the rate of change of TEC. These results further support Wang et al. (2018), https://doi.org/10.1002/2017JA024805 at the noon sector. The dependence of the phase scintillation index on the plasma flow further evidences that the standard phase scintillation index is dominated by refractive variations due to the use of a fixed cut‐off frequency of 0.1 Hz while detrending the phase observable. The dependence of the phase scintillation index on the rate of change of TEC consolidates the dominance of refractive variations inside. Plain Language Summary: In decades, the standard scintillation indices are widely used to represent the strength of scintillations in the ionosphere, which were usually calculated from the sixth order Butterworth filter with a fixed cut‐off frequency of 0.1 Hz by the ground‐based Global Navigation Satellite System (GNSS) receiver automatically. At middle‐to‐low latitudes, the applications of these indices are working very well. However, over the polar ionosphere, the completely different scintillation phenomenon "Phase without Amplitude" occurred when confronting the hazard conditions. Then, in order to explaining this weird, many researchers have carried out a lot of valuable approaches, fundamentally challenging the direct adopt of standard phase scintillation index. Here, for the first time, we present an experimental evidence on the dawn sector to prove the clear positive dependence of phase scintillation index on the convection flow speed and also the TEC variations. It reminds us to be careful when using the standard phase scintillation index over the polar region, in particular with high‐speed flows. Key Points: The dependencies of global positioning system scintillation indices on plasma flow and total electron content (TEC) variation were evaluated around the dawn sector of polar ionosphereThe phase scintillation index depends linearly only on the plasma flow speed and the rate of change of TECHowever, the amplitude scintillation index does not rely on the plasma flow and also the rate of change of TEC [ABSTRACT FROM AUTHOR]

The Super Dual Auroral Radar Network (SuperDARN) is a network of High Frequency (HF) radars that are typically used for monitoring plasma convection in the Earth's ionosphere. A majority of SuperDARN backscatter can broadly be divided into three categories: (a) ionospheric scatter due to reflections from plasma irregularities in the E and F regions of the ionosphere, (b) ground scatter caused by reflections from the ground/sea surface following reflection in the ionosphere, and (c) backscatter from meteor trails left by meteoroids as they enter the Earth's atmosphere. Due to the complex nature of HF propagation and mid‐latitude electrodynamics, it is often not straightforward to distinguish between different modes of backscatter observed by SuperDARN. In this study, we present a new two‐stage machine learning algorithm for identifying different backscatter modes in SuperDARN data. In the first stage, a neural network that "mimics" ray‐tracing is used to predict the probability of ionospheric and ground scatter occurring at a given location along with parameters like the elevation angles, reflection heights etc. The inputs to the network include parameters that control HF propagation, such as signal frequency, season, UT time, and geomagnetic activity levels. In the second stage, the output probabilities from the neural network and actual SuperDARN data are clustered together to determine the category of the backscatter. Our model can distinguish between meteor scatter, 1/2 hop E‐/F‐region ionospheric as well as ground/sea scatter. We validate our model by comparing predicted elevation angles with those measured at a SuperDARN radar. Plain Language Summary: The Super Dual Auroral Radar Network (SuperDARN) is an international network of radars that operate in the High Frequency (HF) range (between 8 and 18 MHz). The radars are primarily used for monitoring the motion of plasma in the charged portion of Earth's upper atmosphere, called the ionosphere. A key feature of HF radio waves is that they are refracted or "bent" by the ionosphere. As a result, these frequencies are usually reflected back by decameter‐scale density structures in the ionosphere or by the ground, depending on the density of the ionosphere and operational characteristics of the radar. SuperDARN data is therefore mostly composed of measurements from the ionosphere, ground/sea, and meteor trails. At high latitudes, it is usually straightforward to distinguish between fast‐moving ionospheric and the slow‐moving ground observations. However, at mid‐latitudes, due to the similarities between ionospheric and ground measurements it is usually difficult to distinguish between these two types of measurements. In this study, we present a new methodology that uses a physics‐based approach to encode certain HF propagation characteristics into a machine learning model that can: (a) provide the ability to classify between different types of measurements, and (b) determine the uncertainties in the model ionosphere used to characterize HF propagation. Key Points: We developed a new machine learning model that is guided by ray‐tracing and the International Reference Ionosphere (IRI) to examine SuperDARN backscatter modesOur model can be used to classify SuperDARN backscatter into different categories such as meteor, E‐/F‐region ionosphere and ground/seaThe model's error rate is lowest in winter and highest in summer, suggesting uncertainties in the IRI ionosphere are larger in summer [ABSTRACT FROM AUTHOR]

Propagation of high‐frequency (HF) radio signals is strongly dependent on the ionospheric electron density structure along a communications link. The ground‐based, HF space weather radars of the Super Dual Auroral Radar Network (SuperDARN) utilize the ionospheric refraction of transmitted signals to monitor the global circulation of E‐ and F‐region plasma irregularities. Previous studies have assessed the propagation characteristics of backscatter echoes from ionospheric irregularities in the auroral and polar regions of the Earth's ionosphere. By default, the geographic location of these echoes are found using empirical models which estimate the virtual backscattering height from the measured range along the radar signal path. However, the performance of these virtual height models has not yet been evaluated for mid‐latitude SuperDARN radar observations or for ground scatter (GS) propagation modes. In this study, we derive a virtual height model suitable for mid‐latitude SuperDARN observations using 5 years of data from the Christmas Valley East and West radars. This empirical model can be applied to both ionospheric and GS observations and provides an improved estimate of the ground range to the backscatter location compared to existing high‐latitude virtual height models. We also identify a region of overlapping half‐hop F‐region ionospheric scatter and one‐hop E‐region GS where the measured radar parameters (e.g., velocity, spectral width, elevation angle) are insufficient to discriminate between the two scatter types. Further studies are required to determine whether these backscatter echoes of ambiguous origin are observed by other mid‐latitude SuperDARN radars and their potential impact on scatter classification schemes. Key Points: We derive a new empirical virtual height model suitable for improved geolocation of mid‐latitude Super Dual Auroral Radar Network high‐frequency radar observationsThe new model provides the first characterization of ground scatter (GS) propagation modesCharacteristics of half‐hop F‐region ionospheric scatter and one‐hop E‐region GS are examined [ABSTRACT FROM AUTHOR]

Evolution of large‐scale and fine‐scale plasmaspheric plume density structures was examined using space‐ground coordinated observations of a plume during the 7–8 September 2015 storm. The large‐scale plasmaspheric plume density at Van Allen Probes A was roughly proportional to the total electron content (TEC) along the satellite footprint, indicating that TEC distribution represents the large‐scale plume density distribution in the magnetosphere. The plasmaspheric plume contained fine‐scale density structures and subauroral polarization streams (SAPS) velocity fluctuations. High‐resolution TEC data support the interpretation that the fine‐scale plume structures were blobs with ∼300 km size and ∼500–800 m/s in the ionosphere (∼3,000 km size and ∼5–8 km/s speed in the magnetosphere), emerging at the plume base and drifting to the plume. The short‐baseline Global Navigation Satellite System receivers detected smaller‐scale (∼10 km in the ionosphere, ∼100 km in the magnetosphere) TEC gradients and their sunward drift. Fine‐scale density structures were associated with enhanced phase scintillation index. Velocity fluctuations were found to be spatial structures of fine‐scale SAPS flows that drifted sunward with density irregularities down to ∼10 s of meter‐scale. Fine‐scale density structures followed a power law with a slope of ∼−5/3, and smaller‐scale density structures developed slower than the larger‐scale structures. We suggest that turbulent SAPS flows created fine‐scale density structures and their cascading to smaller scales. We also found that the plume fine‐scale density structures were associated with whistler‐mode intensity modulation, and localized electron precipitation in the plume. Structured precipitation in the plume may contribute to ionospheric heating, SAPS velocity reduction, and conductance enhancements. Plain Language Summary: A plume of cold plasma circulates in the system during geomagnetic storms, and it is important for global positioning system (GPS) signal scintillation at mid‐latitudes of the Earth. We show that the plume contains fine‐scale density structures across multiple‐scales, and reveal their 2‐d structure and motion using GPS remote sensing and multiple satellites. The network of GPS receivers showed that the fine‐scale plume structures were blobs with hundreds of km size in the ionosphere, and GPS signal scintillation was highly correlated with the fine‐scale density structures. Radar observations identified a close relation between the fine‐scale density and fast plasma streams. The plume density structures formed first at large‐scales and then small‐scale density structures emerged near the peak of the geomagnetic storm. We suggest that fast plasma streams created fine‐scale density structures and their turbulent cascading to smaller scales. Key Points: Two dimensional structure and motion of fine‐scale plume density has been revealed by Van Allen Probes‐defense meteorological satellite program‐Swarm‐global positioning system conjunctionFine‐scale subauroral polarization streams flows are related to fine‐scale density and are suggested to create turbulent density cascadingFine‐scale density contributes to waves, precipitation, heating, and scintillation [ABSTRACT FROM AUTHOR]

The sub‐auroral polarization stream (SAPS) is a region of westward high velocity plasma convection equatorward of the auroral oval that plays an important role in mid‐latitude space weather dynamics. In this study, we present observations of SAPS flows extending across the North American sector observed during the recovery phase of a minor geomagnetic storm. A resurgence in substorm activity drove a new set of field‐aligned currents (FACs) into the ionosphere, initiating the SAPS. An upward FAC system is the most prominent feature spreading across most SAPS local times, except near dusk, where a downward current system is pronounced. The location of SAPS flows remained relatively constant, firmly inside the trough, independent of the variability in the location and intensity of the FACs. The SAPS flows were sustained even after the FACs weakened and retreated polewards with a decline in geomagnetic activity. The observations indicate that the mid‐latitude trough plays a crucial role in determining the location of the SAPS and that SAPS flows can be sustained even after the magnetospheric driver has weakened. Plain Language Summary: The sub‐auroral polarization stream (SAPS) is an important phenomenon that controls the dynamics of the sub‐auroral region. While SAPS has been studied for several decades, our understanding of the drivers of this phenomenon is still limited. In this study, we analyze an event to examine the validity of different SAPS mechanisms, and to determine the importance of the ionosphere‐thermosphere system in driving and sustaining SAPS flows. We find that the ionosphere can play an important role in determining SAPS location and that SAPS can be sustained even after the magnetospheric driver has weakened. Key Points: During the recovery phase of a small geomagnetic storm, substorm activity drove a new set of FACs into the ionosphere, initiating a sub‐auroral polarization stream (SAPS)The SAPS location remained relatively constant, firmly inside the trough, independent of the variability in the field‐aligned currents (FACs)The SAPS flows were sustained for almost an hour after the FACs weakened and retreated polewards [ABSTRACT FROM AUTHOR]

We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF By+ in NH (By− in SH) in most local seasons under both signs of IMF Bz. For By− in NH (By+ in SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During By+ and Bz+ in NH (By− and Bz+ in SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.18 ± 0.09 in winter and 1.17 ± 0.09 in autumn. During By+ and Bz− in NH (By− and Bz− in SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.16 ± 0.07 for FAC. We also find an explicit By effect on auroral currents in a given hemisphere: on average By+ in NH and By− in SH causes larger currents than vice versa. The explicit By effect on divergence‐free current during IMF Bz+ is in very good agreement with the By effect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer. Key Points: Hemispheric asymmetry in auroral currents is larger for interplanetary magnetic field (IMF) By+ in the Northern Hemisphere (NH) (By− in the Southern Hemisphere [SH]) than vice versa during both signs of IMF BzStrongest asymmetry occurs in local winter and autumn for By+ in the NH (By− in the SH) and IMF Bz+ with NH/SH field aligned current ratio of about 1.18IMF By+ in the NH and By− in the SH causes larger auroral currents than vice versa. Effect is stronger for IMF Bz+ than for IMF Bz− [ABSTRACT FROM AUTHOR]

The total solar eclipse over the continental United States on 21 August 2017 offered a unique opportunity to study the dependence of the ionospheric density and morphology on incident solar radiation at different local times. The Super Dual Auroral Radar Network (SuperDARN) radars in Christmas Valley, Oregon, and Fort Hays, Kansas, are located slightly southward of the line of totality; they both made measurements of the eclipsed ionosphere. The received power of backscattered signal decreases during the eclipse, and the slant ranges from the westward looking radar beams initially increase and then decrease after totality. The time scales over which these changes occur at each site differ significantly from one another. For Christmas Valley the propagation changes are fairly symmetric in time, with the largest slant ranges and smallest power return occurring coincident with the closest approach of totality to the radar. The Fort Hays signature is less symmetric. In order to investigate the underlying processes governing the ionospheric eclipse response, we use a ray-tracing code to simulate SuperDARN data in conjunction with different eclipsed ionosphere models. In particular, we quantify the effect of the neutral wind velocity on the simulated data by testing the effect of adding/removing various neutral wind vector components. The results indicate that variations in meridional winds have a greater impact on the modeled ionospheric eclipse response than do variations in zonal winds. The geomagnetic field geometry and the line-of-sight angle from each site to the Sun appear to be important factors that influence the ionospheric eclipse response. [ABSTRACT FROM AUTHOR]

Super Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high‐frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3‐day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great‐circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN. Key Points: Bistatic capability has been developed and demonstrated using two midlatitude SuperDARN radarsThree distinct propagation modes were commonly observed during bistatic campaignsGood agreement between observations and ray tracing using the IRI model is obtained [ABSTRACT FROM AUTHOR]

The Sun was remarkably active during the first week of September 2017 producing numerous solar flares, solar radiation storms, and coronal mass ejections. This activity caused disruption to terrestrial high‐frequency (HF, 3–30 MHz) radio communication channels including observations with the Super Dual Auroral Radar Network (SuperDARN) HF radars. In this paper, we analyze the response of SuperDARN groundscatter observations and decreases in background sky noise level in response to multiple solar flares occurring in quick succession and co‐occurring with solar energetic protons and auroral activity. We estimate the attenuation in HF signal strength using an approach similar to riometry and find that the radars exhibit a nonlinear response to compound solar flare events. Additionally, we find that the three different space weather drivers have varying degrees of influence on the HF signal properties at different latitudes. Our study demonstrates that in addition to monitoring high‐latitude convection, SuperDARN observations can be used to study the spatiotemporal evolution of disruption to HF communication during extreme space weather conditions. Plain Language Summary: High‐frequency (HF, 3–30 MHz) communication system plays an essential role in emergency communications such as amateur radio, missile defense, and air traffic control. Most of these systems solely depend on HF communication that can travel beyond the horizon (over the horizon) without any relay or repeater network. This bending of the HF signal is feasible because of the presence of an electrically charged upper atmosphere, also known as ionosphere which can bend the HF signal back to the Earth. This electrically conducting upper atmosphere (ionosphere) can be influenced by the Sun and the outer space, commonly known as space weather. Extreme space weather events such as solar flares, radiation storms, and geomagnetic storms produced by the Sun can alter the state of the ionosphere and disrupt HF communication. During the first week of September 2017, the Sun produced numerous solar flares, radiation storms, and geomagnetic storms. This paper compares the impacts of isolated versus co‐occurring space weather disturbances on HF communications as observed by the Super Dual Auroral Radar Network HF radar network distributed across the North American sector. Key Points: SuperDARN HF radars observed a drop in signal strength in response to extreme space weather phenomena during September 2017Multiple solar flares occurring in quick succession produce nonlinear effects on radio wave absorptionCo‐occurring space weather phenomena have varying degrees of impact at different latitudes [ABSTRACT FROM AUTHOR]

An analysis of noise attenuation during 80 solar flares between 2013 and 2017 was carried out at frequencies 8–20 MHz using 34 Super Dual Auroral Radar Network radars and the EKB ISTP SB RAS radar. The attenuation was determined on the basis of noise measurements performed by the radars during the intervals between transmitting periods. The location of the primary contributing ground sources of noise was found by consideration of the propagation paths of radar backscatter from the ground. The elevation angle for the ground echoes was determined through a new empirical model. It was used to determine the paths of the noise and the location of its source. The method was particularly well suited for daytime situations, which had to be limited for the most part to only two crossings through the D region. Knowing the radio path was used to determine an equivalent vertical propagation attenuation factor. The change in the noise during solar flares was correlated with solar radiation lines measured by GOES/XRS, GOES/EUVS, SDO/AIA, SDO/EVE, SOHO/SEM, and PROBA2/LYRA instruments. Radiation in the 1 to 8 Å and near 100 Å are shown to be primarily responsible for the increase in the radionoise absorption, and by inference, for an increase in the D and E region density. The data are also shown to be consistent with a radar frequency dependence having a power law with an exponent of −1.6. This study shows that a new data set can be made available to study D and E regions. Key Points: HF noise attenuation in the ionosphere was investigated with 35 radars during 80 X‐ray solar flaresThe evolution of HF noise attenuation following an X‐ray solar flare is explained by a linear combination of 1‐ to 8‐ and 94‐Å solar radiationThe experimental frequency dependence of the absorption at 8–20 MHz is A[dB]∼f−1.6 [ABSTRACT FROM AUTHOR]

Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of millihertz or lower‐frequency range with horizontal velocities of a few hundred meters per second appeared in the subauroral to midlatitude region during a magnetic storm on 27 March 2017. A set of the radars have provided the first ever observations that the fluctuations propagate azimuthally both westward and eastward simultaneously, showing bifurcated phase propagation associated with substorm expansion. Concurrent observations near the conjugate site in the inner magnetosphere made by the Arase satellite provide evidence that multiple drifting clouds of electrons in the near‐Earth equatorial plane were associated with the electric field fluctuations propagating eastward in the ionosphere. We interpret this event in terms of mesoscale pressure gradients carried by drifting ring current electrons that distort field lines one after another as they drift through the inner magnetosphere, causing eastward propagating ionospheric electric field fluctuations. Plain Language Summary: Midlatitude ionospheric radars called SuperDARN observed azimuthally propagating fluctuations of ionospheric plasma flow in association with substorm expansion during a magnetic storm on 27 March 2017. The radar observations have shown for the first time that the flow fluctuations bifurcate toward eastward and westward simultaneously. The Arase satellite in the near‐conjugate site of the eastward propagating ionospheric flow fluctuations in the inner magnetosphere saw repeated flux enhancement of energetic electrons. The good satellite‐radar conjunction gives observational evidence that the eastward propagating ionospheric flow fluctuations are driven by eastward drifting pressure bumps of electrons injected into the inner magnetosphere upon substorm expansion. Key Points: This study shows the first ever comprehensive observation of azimuthal bifurcation of substorm‐associated ionospheric flow fluctuationsThe eastward propagating portion of the flow fluctuations can be mapped to electron pressure enhancements in the inner magnetosphereWe propose that multiple pressure bumps of drifting energetic electrons directly cause the observed ionospheric flow fluctuations [ABSTRACT FROM AUTHOR]

Increased electron density in the ionosphere due to photoionization by radiation emitted during a solar X‐ray flare impacts high‐frequency (HF) radio wave propagation. Shortwave fadeout (SWF) due to the enhanced D region absorption that results is characterized by the level of cosmic radio noise attenuation derived from riometer measurements. SWF impacts HF radio propagation and has been identified in the Super Dual Auroral Radar Network (SuperDARN) data. An X2.1 solar X‐ray flare that erupted on 11 March 2015 is examined to determine its effects on HF radio propagation. Riometer data indicate a sharp enhancement in absorption, which falls off with increasing solar zenith angle. SuperDARN radars observed a suppression of both ground scatter and ionospheric echoes. Ground scatter data indicated a rapid weakening of signal from far to near ranges followed by a ~20‐min interval of complete signal loss. Recovery lasted ~30 min and proceeded from near to far ranges. Prior to the complete signal loss, an apparent sharp velocity impulse (Doppler flash) lasting 1–2 min was observed in the ground scatter data. The peak of this flash preceded the onset of enhanced absorption. The onset of signal loss by SuperDARN preceded the onset of enhanced absorption observed by riometers. Both data sets observed a positive correlation between increasing delay in onset and increasing solar zenith angle with onset progressing at an average rate of 16.7°/min (0.060 min/°). Agreement between riometer and SuperDARN indicates the possibility of using a joint data set for improved monitoring of the space weather impact of solar X‐ray flares. Key Points: Solar X‐ray flares may impact HF radio wave propagation on the dayside, to which riometers and SuperDARN radars are sensitiveRiometer and SuperDARN instruments demonstrate a solar zenith angle dependence on the onset time of the observed response to the solar X‐ray flareA velocity impulse observed by SuperDARN radars prior to complete signal loss may serve as a warning that shortwave fadeout is imminent [ABSTRACT FROM AUTHOR]

Regions of persistent westward directed flows are often observed equatorward of the auroral oval in the dusk‐midnight sector. In general, the midnight narrow flows are termed as subauroral ion drifts and the duskside broader flows are termed subauroral polarization streams (SAPS). SAPS/subauroral ion drift electric fields play an important role in controlling the dynamics of the midlatitude ionosphere. In this paper we analyze longitudinally extended observations of SAPS measured by midlatitude Super Dual Auroral Radar Network (SuperDARN) radars under varied geomagnetic conditions. We find that SAPS speeds exhibit a strong dependence on geomagnetic activity, with flows exceeding 1,500 m/s during geomagnetic storms and dropping to 100 m/s during periods of geomagnetic quiet. Moreover, SAPS flows turn increasingly poleward when moving from the midnight sector toward dusk and this effect is more pronounced during disturbed geomagnetic conditions. The variations in SAPS speeds with magnetic local time (MLT) are also found to be strongly dependent on geomagnetic conditions. Specifically, SAPS speeds increase quasilinearly with MLT during disturbed geomagnetic conditions, whereas during relatively quiet geomagnetic conditions there is no discernible trend. This behavior suggests the possibility of different mechanisms influencing SAPS during geomagnetically quiet conditions. Average cross‐SAPS potentials increase with geomagnetic activity and typically vary between 15 and 45 kV. Finally, a new empirical model of SAPS potentials has been developed parameterized by Asy‐H index, MLT, and magnetic latitude. Key Points: We present a new empirical model of SAPS potentials parameterized by Asy‐H, MLT, and magnetic latitudeSAPS speeds exhibit considerable differences during storm time and non‐storm time conditionsSAPS speeds increase quasilinearly with MLT during disturbed geomagnetic conditions; no trend is seen for quiet times [ABSTRACT FROM AUTHOR]

Abstract: Over the last decade, the Super Dual Auroral Radar Network (SuperDARN) has undergone a dramatic expansion in the Northern Hemisphere with the addition of more than a dozen radars offering improved coverage at mid‐latitudes (50°–60° magnetic latitude) and in the polar cap (80°–90° magnetic latitude). In this study, we derive a statistical model of ionospheric convection (TS18) using line‐of‐sight velocity measurements from the complete network of mid‐latitude, high‐latitude, and polar radars for the years 2010–2016. These climatological patterns are organized by solar wind, interplanetary magnetic field (IMF), and dipole tilt angle conditions. We find that for weak solar wind driving conditions the TS18 model patterns are largely similar to the average patterns obtained using high‐latitude radar data only. For stronger solar wind driving the inclusion of mid‐latitude radar data at the equatorward extent of the ionospheric convection can increase the measured cross‐polar cap potential (ΦPC) by as much as 40%. We also derive an alternative model organized by the Kp index to better characterize the statistical convection under a range of magnetic activity conditions. These Kp patterns exhibit similar IMF By dependencies as the TS18 model results and demonstrate a linear increase in ΦPC with increasing Kp for a given IMF orientation. Overall, the mid‐latitude radars provide a better specification of the flows within the nightside Harang reversal region for moderate to strong solar wind driving or geomagnetic activity, while the polar radars improve the quality of velocity measurements in the deep polar cap under all conditions. [ABSTRACT FROM AUTHOR]

The international scientific radars known as the Super Dual Auroral Radar Network (SuperDARN) are designed to primarily measure plasma convection at ionospheric altitudes over a large region of the Northern and Southern Hemispheres. SuperDARN radars are equipped with a secondary interferometry array that is used to measure the elevation angle ( α) of signals. These values of α have been used in relatively few studies; however, they are important for estimating ionospheric quantities and for accurate geolocation of the ionospheric source region of backscattered signals. The majority of SuperDARN radars are constructed with interferometers that are separated from their main array in one or two dimensions, and a relatively straightforward algorithm gives reasonably accurate results. A solution to the more general case, where offsets in all three dimensions are present, is desirable for future designs and necessary for at least one operational radar. Details of such an algorithm are described here and applied to phase measurements from several radars. For radars with interferometers offset only along the radar boresight and vertical directions, small differences of up to ∼1.5° in α are observed and negative values of α, which are deemed unphysical, are no longer produced. For the radar interferometer that is offset in all three dimensions the resulting values of α are consistent with expected behavior. The algorithm presented here provides a technique for accurately determining α from SuperDARN radars with offsets in all three dimensions and significantly reduces constraints placed on the positioning of interferometers for future SuperDARN radars. [ABSTRACT FROM AUTHOR]

The interval of geomagnetic storms of 7- 17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere-ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap. [ABSTRACT FROM AUTHOR]