Auroras have been observed at unusual latitudes of China over the past couple of years, which may be a direct result of the north magnetic pole’s drift and intense solar activity. However, the specific impact on the Asian space environment remains unknown. Here, we present auroral activities recorded in southern Inner Mongolia (~37.2° N in magnetic latitude) and the resulting ionospheric environmental changes detected by the Chinese Dual Auroral Radar Network (CN-DARN) during a recent severe geomagnetic storm. Leveraging the wide spatial coverage and continuous high time resolution monitoring capabilities of the CN-DARN, comprehensive analysis of ground-based and space-based multi-source data reveals that CN-DARN has captured the spatiotemporal evolution characteristics of dawnside subauroral polarization streams (SAPS). The study identifies a direct link between auroral intensification and dawnside SAPS acceleration for the first time, establishing a mechanistic connection between auroral activity and ionospheric convection dynamics in subauroral region. Moreover, the observations show that the ionospheric irregularities with high velocity of 1,000 m/s induced by the dawnside SAPS have propagated to Mohe (~ 48.6° N in magnetic latitude), the northernmost region of China. The research also reveals that intense auroral particle precipitation caused severe degradation of high-frequency (HF) communications in the Asian region. This study represents the first comprehensive investigation of auroral activity observed at unusual latitudes of China, unraveling the impact of auroral activities on the ionospheric environment of Asian mid-to-high latitudes. It also showcases the critical capabilities of the Chinese Meridian Project in addressing space environmental challenges of Asia.

Oblique high-frequency (HF) ionospheric measurements contain valuable information about electron density structure, but remain difficult to assimilate operationally due to the computational cost of ray tracing. PyRayHF is an open-source, fully Python-based framework that addresses such difficulties through the implementation of efficient forward operators for HF propagation and parameter retrieval tailored to dataassimilation (DA) applications. PyRayHF implements magneto-ionic vertical tracing and a hierarchy of twodimensional oblique ray tracing methods, which are verified against one another for internal consistency and validated through comparative analysis with a full three-dimensional Jones-Stephenson ray tracer. Validation results show that all solvers reproduce key observables, including group path and apex location, with typical errors below 10% when compared to the reference solution. For group path, the primary observable in oblique ionogram interpretation, simple stratified Snell's-law formulations perform comparably to gradient-based solvers while being several orders of magnitude faster, making them suitable for DA workflows. To enable efficient assimilation, PyRayHF combines a fast vertical forward operator with a midpoint-based oblique-to-vertical transformation. For transmitter-receiver separations of 1,000–2,000 km, midpoint-derivedfoF2 errors remain below 10%. PyRayHF further provides a parameter-minimization framework that retrieves F2-layer parameters directly from midpoint-converted observations, producing anchor-point measurements suitable for direct assimilation into parametrized systems such as ANCHOR.

In this investigation, we present and discuss the ionospheric F region observations in the equatorial, low-, mid-, and near high-latitude regions in the South American and Antarctic sectors during the intense geomagnetic storm that occurred on 25-27 August 2018. The geomagnetic storm reached a minimum Dst of -175 nT at similar to 0700 UT on 26 August. We present the variations of vertical total electron content (VTEC) from a chain of almost 200 GPS stations, covering the South American and Antarctic sectors. A comparison with model simulations from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIE-GCM) is realized. The results obtained show that during the main phase of the storm, a southward Bz component of the interplanetary magnetic field (IMF) and an eastward prompt penetration electric field (PPEF) can be observed, but they had no significant impact on the ionospheric plasma. A long recovery phase a predominance of positive phase is observed during daytime. The observations show the effects of an unusual case of multiple PPEF, occurred on 26 August, and effects of thermospheric winds disturbances, occurred on 27 August, resulting in increased VTEC values on both days. The TIE-GCM model reproduces the VTEC increases during the main and recovery phases from mid-latitudes to the equatorial region, but it underestimates the observed values near high-latitudes.

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 2 π 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.

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 similar to ${\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.

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 similar to ${\sim} $30 min. Changes in the line-of-sight Doppler shifts are better aligned with the time derivative of eclipse obscuration.

The sub-auroral region is located immediately equatorward from the auroral oval, where important magnetosphere-ionosphere-thermosphere dynamical processes take place.Historically, low-Earth orbit satellites as well as ground-based imagers and radars have provided important information about the region.However, in recent years it has become increasingly clear that there are several unexplored aspects of the region that remain elusive in part due to instrumental and observational limitations.Furthermore, the most commonly used large-scale "state of the art" models and theoretical frameworks of the region rely on a quasi steady-state approach which does not accurately represent the rich sub-auroral electrodynamics underway.Recent ground-based and in-situ measurements have revealed new and compelling dynamics underway in the sub-auroral region and reinvigorated the community's interest there.Optical signatures with unusual spectrographic properties, such as SAR arcs, STEVE, and the picket fence, have been associated with extreme and unusual sub-auroral plasma conditions, such as large ion flow velocities (~5-10 km/s) and extreme electron temperatures (>6000 K).Additionally, recent observations have revealed that the transformation of the sub-auroral region into these extreme conditions occurs within minutes.These new measurements pose a significant challenge to our existing theories and available instrumentation, and demonstrate the necessity of new development and deployment of in-situ and remote measurements of the sub-auroral ionosphere.This white paper highlights some of the open questions in the sub-auroral region that have arisen since the previous decadal survey.It provides recommendations on how science advances can be achieved to help close these open questions, and how new discoveries can be made possible.In short: 1.Establishing new and long-term support for unifying citizen and "traditional" scientists is required to sustain observations and discovery in sub-auroral science.2.NASA's Geospace Dynamics Constellation (GDC) mission is required to address and close many outstanding sub-auroral science questions and should be carried-out without delay. 3.New and long-term support for ground-based observations infrastructure is needed to better understand the nature and dynamics of the sub-auroral ionosphere.Existing infrastructure is too sparse in geographic distribution in the sub-auroral region within the United States. 4.In situ measurements of the lower-thermosphere in the sub-auroral region by rockets, lowaltitude satellites, and/or some other techniques are required to conclusively identify the source, energetic pathways, and mechanism of the extreme conditions reported at sub-auroral latitudes.

During minor to moderate geomagnetic storms, caused by corotating interaction regions (CIRs) at the leading edge of high-speed streams (HSSs), solar wind Alfvén waves modulated the magnetic reconnection at the dayside magnetopause. The Resolute Bay Incoherent Scatter Radars (RISR-C and RISR-N), measuring plasma parameters in the cusp and polar cap, observed ionospheric signatures of flux transfer events (FTEs) that resulted in the formation of polar cap patches. The patches were observed as they moved over the RISR, and the Canadian High-Arctic Ionospheric Network (CHAIN) ionosondes and GPS receivers. The coupling process modulated the ionospheric convection and the intensity of ionospheric currents, including the auroral electrojets. The horizontal equivalent ionospheric currents (EICs) are estimated from ground-based magnetometer data using an inversion technique. Pulses of ionospheric currents that are a source of Joule heating in the lower thermosphere launched atmospheric gravity waves, causing traveling ionospheric disturbances (TIDs) that propagated equatorward. The TIDs were observed in the SuperDual Auroral Radar Network (SuperDARN) high-frequency (HF) radar ground scatter and the detrended total electron content (TEC) measured by globally distributed Global Navigation Satellite System (GNSS) receivers.

Solar wind Alfvén waves [1] coupling to the magnetosphere-ionosphere-thermosphere (MIT) have been associated with high-intensity long-duration continuous auroral electrojet activity [2] and shown to modulate ionospheric convection in the cusp generating polar cap patches and atmospheric gravity waves [3,4]. The Resolute Bay Incoherent Scatter Radars (RISR-C and RISR-N) [5] are well suited for observing the ionospheric signatures of flux transfer events and subsequent polar patch formation in the cusp. During minor to moderate geomagnetic storms caused by corotating interaction regions at the leading edge of solar wind high speed streams polar patches were observed as they convected over the RISR, and the Canadian High-Arctic Ionospheric Network (CHAIN) ionosondes and GPS receivers [6]. The patches were generated by the MIT coupling of Alfvén waves in the upstream solar wind. The coupling process modulated the ionospheric convection and the intensity of ionospheric currents, including auroral electrojets. The horizontal equivalent ionospheric currents and vertical current amplitudes are estimated from the ground-based magnetometer data using an inversion technique [7]. Pulses of ionospheric currents that are a source of Joule heating in the lower thermosphere launched atmospheric gravity waves causing traveling ionospheric disturbances (TIDs) propagating equatorward. TIDs were observed in the SuperDARN HF radar ground scatter [8], in the detrended GPS TEC maps, and in one case, in the altitude profiles of ionospheric electron densities observed by the Poker Flat ISR [9]. [1] Belcher, JW, Davis, L, Jr. 1971. J. Geophys. Res. 76, 3534–3563. [2] Tsurutani, BT, Gonzalez, WD. 1987. Planet. Space Sci. 35(4), 405–412. [3] Prikryl, P, et al., 1999. Ann. Geophys. 17, 463–489. [4] Prikryl, P, et al., 2005. Ann. Geophys. 23, 401–417. [5] Gillies RG, et al., 2016. Radio Sci., 51(10):1645-1659. [6] Jayachandran, PT, et al., 2009. Radio Sci., 44, RS0A03. [7] Weygand, JM, et al., 2011. J. Geophys. Res. 116, A03305. [8] Chisham, G., et al., 2007. Surv. Geophys. 28, 33–109. [9] Heinselman, CJ, Nicolls, MJ, 2008. Radio Sci., 43, RS5013.

The Super Dual Auroral Radar Network (SuperDARN) is a collection of radars built to study ionospheric convection. We use a 7-year archive of SuperDARN convection maps, processed in 3 different ways, to build a statistical understanding of dusk-dawn asymmetries in the convection patterns. We find that the data set processing alone can introduce a bias which manifests itself in dusk-dawn asymmetries. We find that the solar wind clock angle affects the balance in the strength of the convection cells. We further find that the location of the positive potential foci is most likely observed at latitudes of 78 degrees for long periods (>300 min) of southward interplanetary magnetic field (IMF), as opposed to 74 degrees for short periods (<20 min) of steady IMF. For long steady dawnward IMF the median is also at 78 degrees. For long steady periods of duskward IMF, the positive potential foci tends to be at lower latitudes than the negative potential and vice versa during dawnward IMF. For long periods of steady Northward IMF, the positive and negative cells can swap sides in the convection pattern. We find that they move from similar to 0-9 MLT to 15 MLT or similar to 15-23 MLT to 10 MLT, which reduces asymmetry in the average convection cell locations for Northward IMF. We also investigate the width of the region in which the convection returns to the dayside, the return flow width. Asymmetries in this are not obvious, until we select by solar wind conditions, when the return flow region is widest for the negative convection cell during Southward IMF.

This study provides first storm time observations of the westward‐propagating medium‐scale traveling ionospheric disturbances (MSTIDs), particularly, associated with characteristic subauroral storm time features, storm‐enhanced density (SED), subauroral polarization stream (SAPS), and enhanced thermospheric westward winds over the continental US. In the four recent (2017–2019) geomagnetic storm cases examined in this study (i.e., 2018‐08‐25/26, 2017‐09‐07/08, 2017‐05‐27/28, and 2016‐02‐02/03 with minimum SYM‐H index −206, −146, −142, and −58 nT, respectively), MSTIDs were observed from dusk‐to‐midnight local times predominately during the intervals of interplanetary magnetic field (IMF) Bz stably southward. Multiple wavefronts of the TIDs were elongated NW‐SE, 2°–3° longitude apart, and southwestward propagated at a range of zonal phase speeds between 100 and 300 m/s. These TIDs initiated in the northeastern US and intensified or developed in the central US with either the coincident SED structure (especially the SED basis region) or concurrent small electron density patches adjacent to the SED. Observations also indicate coincident intense storm time electric fields associated with the magnetosphere–ionosphere–thermosphere coupling electrodynamics at subauroral latitudes (such as SAPS) as well as enhanced thermospheric westward winds. We speculate that these electric fields trigger plasma instability (with large growth rates) and MSTIDs. These electrified MSTIDs propagated westward along with the background westward ion flow which resulted from the disturbance westward wind dynamo and/or SAPS.

Abstract 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.

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.

The Super Dual Auroral Radar Network (SuperDARN) was built to study ionospheric convection and has in recent years been expanded geographically. Alongside software developments, this has resulted in many different versions of the convection maps data set being available. Using data from 2012 to 2018, we produce five different versions of the widely used convection maps, using limited backscatter ranges, background models and the exclusion/inclusion of data from specific radar groups such as the StormDARN radars. This enables us to simulate how much information was missing from older SuperDARN research. We study changes in the Heppner‐Maynard boundary (HMB), the cross polar cap potential (CPCP), the number of backscatter echoes (n) and the χ2/n statistic which is a measure of the global agreement between the measured and fitted velocities. We find that the CPCP is reduced when the PolarDARN radars are introduced, but then increases again when the StormDARN radars are added. When the background model is changed from the RG96 model, to the most recent TS18 model, the CPCP tends to decrease for lower values, but tends to increase for higher values. When comparing to geomagnetic indices, we find that there is on average a linear relationship between the HMB and the geomagnetic indices, as well as n, which breaks when the HMB is located at latitudes below ∼50° due to the low observational density. Whilst n is important in constraining the maps (maps with n > 400 data points are unlikely to differ), it is insufficient as the sole measure of quality.

This paper presents updated methods for locating the Poleward and Equatorward Auroral Luminosity Boundaries (PALB and EALB) directly from IMAGE Far UltraViolet (FUV) images of the Northern Hemisphere auroral oval. Separate boundaries are determined from images measured at different FUV wavelengths. In addition, new methods for indirectly estimating the Open‐Closed magnetic field line Boundary (OCB) and the Equatorward Precipitation Boundary (EPB) locations are presented; these new boundaries are derived from a combination of the auroral luminosity boundary estimates with statistical latitudinal offsets derived from comparisons with low‐altitude spacecraft Particle Precipitation Boundaries (PPBs). Subsequently, we derive new circle model fits for all these boundary data sets, as well as new quality control criteria for these model fits. The suitability of circle fits for each of the data sets is discussed, and the OCB and PALB circle fits are validated against the Convection Reversal Boundary (CRB), as measured by low‐altitude in situ spacecraft. All the new boundary data sets, covering the epoch May 2000 to October 2002, are freely available online.

This Polar Science special issue, "SuperDARN/Studies of Geospace Dynamics - Today and Future," originated from an international SuperDARN (Super Dual Auroral Radar Network) annual workshop held in Japan in June 2019, and is focused on studies of geospace dynamics particularly related to SuperDARN. Its purpose is to overview recent wide and active research, new scientific results and future perspectives mainly through, but not limited to, the scientific papers presented at the workshop. This special issue is an opportunity to commemorate a quarter century since the establishment of SuperDARN in 1995 and to contribute to the further development of geospace sciences and relevant technology. Thirteen valuable papers have been published covering a wide variety of scientific and technical topics.

Lobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn‐dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high‐latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMF By. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMF By is dominating and IMF Bz ≥ 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.

The high frequency radars in the Super Dual Auroral Radar Network (SuperDARN) estimate the elevation angles of returned backscatter using interferometric techniques. These elevation angles allow the ground range to the scattering point to be estimated, which is crucial for the accurate geolocation of ionospheric measurements. For elevation angles to be accurately estimated, it is important to calibrate the interferometer measurements by determining the difference in the signal time delays caused by the difference in the electrical path lengths from the main array and the interferometer array to the point at which the signals are correlated. This time delay is known as tdiff. Several methods have been proposed to estimate tdiff using historical observations; these methods are summarised in this paper. Comparisons of the tdiff estimates from the different calibration methods are presented and sources of uncertainty discussed. The effect of errors in the estimated tdiff value on the accuracy of geolocation is evaluated and discussed. The paper concludes with a series of recommendations for both scientific SuperDARN data users and SuperDARN radar operators.

Abstract The occurrence of St. Patrick's Day (17 March) geomagnetic storms during two different years (2013 and 2015) with similar solar flux levels but varying storm intensity provided an opportunity to compare and contrast the responses of the ionosphere‐thermosphere (IT) system to different levels of geomagnetic activity. The evolution of positive ionospheric storms at the southern polar stations Bharati (76.6°S MLAT) and Davis (76.2°S MLAT) and its causative connection to the solar wind driving mechanisms during these storms has been investigated in this paper. During the main phase of both the storms, significant enhancements in TEC and phase scintillation were observed in the magnetic noon/ midnight period at Bharati and Davis. The TEC in the midnight sector on 17 March 2015 was significantly higher compared to that on 17 March 2013, in line with the storm intensity. The TEC enhancements during both the storm events are associated with the formation of the storm‐enhanced densities (SEDs)/tongue of ionization (TOI). The strong and sustained magnetopause erosion led to the prevalence of stronger storm time electric fields (prompt penetration electric field (PPEF)/subauroral polarization streams (SAPS)) for long duration on 17 March 2015. This combined with the action of neutral winds at midlatitudes favored the formation of higher plasma densities in the regions of SED formation on this day. The same was weaker during the 17 March 2013 storm due to the fast fluctuating nature of interplanetary magnetic field (IMF) Bz. This study shows that the duration and extent of magnetopause erosion play an important role in the spatiotemporal evolution of the plasma density distribution in the high‐midlatitude ionosphere.