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We present a new technique for the upcoming tri‐static incoherent scatter radar system EISCAT 3D (E3D) to perform a volumetric reconstruction of the 3D ionospheric electric current density vector field, focusing on the feasibility of the E3D system. The input to our volumetric reconstruction technique are estimates of the 3D current density perpendicular to the main magnetic field, j⊥, and its covariance, to be obtained from E3D observations based on two main assumptions: (a) Ions fully magnetized above the E region, set to 200 km here. (b) Electrons fully magnetized above the base of our domain, set to 90 km. In this way, j⊥ estimates are obtained without assumptions about the neutral wind field, allowing it to be subsequently determined. The volumetric reconstruction of the full 3D current density is implemented as vertically coupled horizontal layers represented by Spherical Elementary Current Systems with a built‐in current continuity constraint. We demonstrate that our technique is able to retrieve the three dimensional nature of the currents in our idealized setup, taken from a simulation of an active auroral ionosphere using the Geospace Environment Model of Ion‐Neutral Interactions (GEMINI). The vertical current is typically less constrained than the horizontal, but we outline strategies for improvement by utilizing additional data sources in the inversion. The ability to reconstruct the neutral wind field perpendicular to the magnetic field in the E region is demonstrated to mostly be within ±50 m/s in a limited region above the radar system in our setup. Plain Language Summary: We introduce a novel method for the upcoming EISCAT 3D (E3D) radar system to reconstruct the 3D electric current density vector in Earth's ionosphere. Here we present the new technique and assess its feasibility for the E3D system. The input to the 3D reconstruction technique relies on estimates of the current density perpendicular to the Earth's magnetic field, obtained from the E3D observations. We include estimates of uncertainties originating from the observations of the 3D ion velocity vectors and electron density in our reconstruction. Comparisons with simulations of an active auroral ionosphere exemplify that our technique provides reasonably accurate estimates of current density, especially in the 90–150 km altitude range. Our results demonstrate success in retrieving the horizontal part of the electric current system in the E region, while the vertical part has more uncertainty. Our method offers insight into how electric currents flow in a specific region of the Earth's atmosphere. The results can be further improved with additional data sources; this flexibility is a significant advantage of our approach. Overall, our study facilitates the advanced knowledge of Earth's upper atmosphere using innovative radar observations in companion with advanced analysis techniques. Key Points: A technique for volumetric reconstruction of 3D electric current density from tri‐static incoherent scatter radar observations is presentedConsidering the anticipated noise levels, the radar system is likely to produce good current density estimates in a limited regionThe reconstruction technique is particularly well suited for inclusion of additional data sources that improve overall performance [ABSTRACT FROM AUTHOR]

We investigate the role of auroral particle precipitation in small‐scale (below hundreds of meters) plasma structuring in the auroral ionosphere over the Arctic. In this scope, we analyze together data recorded by an Ionospheric Scintillation Monitor Receiver (ISMR) of Global Navigation Satellite System (GNSS) signals and by an All‐Sky Imager located in Longyearbyen, Svalbard (Norway). We leverage on the raw GNSS samples provided at 50 Hz by the ISMR to evaluate amplitude and phase scintillation indices at 1 s time resolution and the Ionosphere‐Free Linear Combination at 20 ms time resolution. The simultaneous use of the 1 s GNSS‐based scintillation indices allows identifying the scale size of the irregularities involved in plasma structuring in the range of small (up to few hundreds of meters) and medium‐scale size ranges (up to few kilometers) for GNSS frequencies and observational geometry. Additionally, they allow identifying the diffractive and refractive nature of fluctuations on the recorded GNSS signals. Six strong auroral events and their effects on plasma structuring are studied. Plasma structuring down to scales of hundreds of meters is seen when strong gradients in auroral emissions at 557.7 nm cross the line of sight between the GNSS satellite and receiver. Local magnetic field measurements confirm small‐scale structuring processes coinciding with intensification of ionospheric currents. Since 557.7 nm emissions primarily originate from the ionospheric E‐region, plasma instabilities from particle precipitation at E‐region altitudes are considered to be responsible for the signatures of small‐scale plasma structuring highlighted in the GNSS scintillation data. Plain Language Summary: We investigate the role of auroral particle precipitation in plasma structuring in the auroral ionosphere over the Arctic. For this we use Ionospheric Scintillation Monitor Receiver of Global Navigation Satellite System (GNSS) signals to detect plasma structuring and an All‐Sky‐ Camera to monitor the Aurora. Six strong auroral events are studied. Plasma structuring down to scales of hundreds of meters are seen when strong gradients in green auroral emissions cross the line of sight between the GNSS satellite and receiver. Intensification of ionospheric currents are observed to coincide with small‐scale structuring processes. Green auroral emissions originate from the ionospheric E‐region, which is considered the source of plasma instabilities causing the signatures of small‐scale plasma structuring as observed in the GNSS scintillation data. Key Points: Enhanced values of the GNSS‐based 1–s amplitude scintillation index are observed at auroral intensity gradientsThe IFLC is often elevated at the same time as the S4 index, confirming the diffractive nature of observed scintillation eventsSignificant increases in the auroral particle precipitation result in joint signatures of increased ionospheric currents and plasma structuring below the Fresnel's scale [ABSTRACT FROM AUTHOR]

A unique example of a polar cap arc producing clear amplitude and phase scintillations in GPS L-band signals is presented using observations from an all-sky imager and a GPS receiver and a digital ionosonde at Resolute Bay and the SuperDARN Inuvik radar. During the southward interplanetary magnetic field (IMF) condition, the polar cap arc moved quickly from the dusk-side to the midnight auroral oval at a speed of ∼700 m/s, as revealed by all-sky 557.7 and 630.0 nm images. When it intersected the raypath of GPS signals, both amplitude and phase scintillations appeared, which is very different from previous results. Moreover, the scintillations were precisely determined through power spectral analysis. We propose that the strong total electron content (TEC) enhancement (∼6 TECU) and flow shears in association with the polar cap arc under the southward IMF condition were creating the scintillations. It provides evidence for the existence of polar cap arc scintillations that may be harmful for satellite applications even through L-band signals. [ABSTRACT FROM AUTHOR]

Auroral whistler‐mode radio emissions called saucers are of fundamental interest because they require an unusually stationary emission process in the dynamic auroral environment, and it is a mystery how that can happen in this or similar conditions elsewhere in geospace. The Cusp Alfvén and Plasma Electrodynamics Rocket (CAPER‐2), launched into the cusp, obtained the first rocket measurements of a large‐scale, multiple‐armed dayside saucer, similar to those observed by the DEMETER satellite, with the addition of particle measurements and ground‐based measurements. Analysis of saucer shapes, directional measurements using waveforms, and ground‐based data show that, accounting for estimated uncertainties, these originate at altitudes ∼4,000 km within the cusp, the eastern side of which is penetrated by the rocket ∼100 s after the saucers are encountered. On‐board particle instruments show dispersed electron bursts in the cusp, Alfvénically accelerated at altitudes at or above the saucer sources. Plain Language Summary: Electrons, precipitating down Earth's high‐latitude magnetic field lines, emit radio waves at angles to the background magnetic field, depending on frequency. When spacecrafts traverse through a region close to the source, they observe descending and ascending frequency signatures referred to as saucers. This research focused on rocket measurements of large‐scale, multiarmed saucers on the dayside. Using ray‐tracing software and hodogram analysis of the electric field waveforms combined with ground‐based measurements, we were able to determine the source location of the saucers to be in the magnetic field's cusp at altitudes near 4,000 km. Additionally, particle measurements on‐board the rocket showed time dispersed bursts of electrons typically associated with Alfvénic acceleration, which can be traced back to a source height that is equal to or above the source heights of the observed saucers. This is the first time that dayside large‐scale saucers have been associated with the cusp. Key Points: First observation of large‐scale dayside saucers from a sounding rocketThe saucers are estimated to originate at ∼4,000 km on cusp field lines to within estimated uncertaintiesDispersed electrons observed in the cusp were accelerated at altitudes at or above the saucer signal source region [ABSTRACT FROM AUTHOR]

This paper presents and discusses electron density depletion regions observed with the incoherent scatter EISCAT Svalbard Radar (ESR) located at 75.43°N geomagnetic latitude. The data include several decades of measurements, which make them suitable for studying statistical features and characteristics of the ionospheric parameters. Here we focus on the electron density depletions and their dependence on diurnal and seasonal variations and solar activity. An electron density depletion region is identified in the ESR data in the early morning sector. This depletion region seems to be clearest during equinox and winter and moderate/high solar activity. An enhancement in the ion temperature is often colocated with the electron density depletion region. The ion temperature enhancement could indicate that ion frictional heating is related to the electron density depletion region. However, during summer when the solar activity is low, the electron density depletion is not observed although the ion temperature is enhanced, suggesting that formation of the electron density depletion regions due to ion frictional heating may depend on the background effective temperature and O/N2 ratio. In addition, seasonal changes in the solar zenith angle could also contribute to the formation of the depletion region. [ABSTRACT FROM AUTHOR]

One of the primary mechanisms of loss of Earth's atmosphere is the persistent "cold" (T≲ 20 eV) ion outflow that has been observed in the magnetospheric lobes over large volumes with dimensions of order several Earth radii. As the main source of this cold ion outflow, the polar cap F region ionosphere and conditions within it have a disproportionate influence on these magnetospheric regions. Using 15 years of measurements of plasma density Ne made by the Swarm spacecraft constellation and the Challenging Mini Satellite Payload (CHAMP) spacecraft within the F region of the polar cap above 80° Apex magnetic latitude, we report evidence of several types of seasonal asymmetries in polar cap Ne. Among these, the transition between "winter‐like" and "summer‐like" median polar cap Ne occurs 1 week prior to local spring equinox in the Northern Hemisphere (NH) and 1 week after local spring equinox in the Southern Hemisphere (SH). Thus, the median SH polar cap Ne lags the median NH polar cap Ne by approximately 2 weeks with respect to hemispherically local spring and fall equinox. From interhemispheric comparison of statistical distributions of polar cap plasma density around each equinox and solstice, we find that distributions in the SH are often flatter (i.e., less skewed and kurtotic) than those in the NH. Perhaps of most significance to cold ion outflow, we find no evidence of an F region plasma density counterpart to a previously reported hemispheric asymmetry whereby cold plasma density is higher in the NH magnetospheric lobe than in the SH lobe. Plain Language Summary: The Earth's magnetic poles are not perfectly aligned with the Earth's geographic poles, and the degree of misalignment is greater in the Southern Hemisphere. Furthermore, as a result of the Earth's elliptical orbit around the Sun, summer and fall in the Northern Hemisphere together are approximately 1 week longer than summer and fall in the Southern Hemisphere, because the Earth is very slightly closer to the Sun around December solstice (summer in the Southern Hemisphere). These seasonal asymmetries, together with the asymmetric displacement of the Earth's magnetic poles relative to the geographic poles, suggest that the plasma density in the topside ionosphere's geomagnetic polar regions may also be subject to seasonal and hemispheric asymmetries. The polar regions are the primary site of loss of the Earth's atmosphere via so‐called ion outflow processes that, over geological time scales, are believed to lead to loss of the Earth's atmosphere. Using 15 years of plasma density measurements made by four different satellites to statistically study the plasma density of each hemisphere's geomagnetic polar cap ionosphere in the altitude range 350–520 km, we find that the polar cap ionosphere at these altitudes exhibits a variety of seasonal and hemispheric asymmetries. Key Points: Statistics of F region polar cap plasma density derived from 15 years of measurements exhibit several types of seasonal asymmetriesStatistics do not support the conjecture that limited plasma availability is the cause of observed hemispheric asymmetries in lobe densitySouthern Hemisphere polar cap plasma densities lag those in Northern Hemisphere by at least 2 weeks around local spring and fall equinox [ABSTRACT FROM AUTHOR]

An algorithm to detect high‐speed ionospheric flow channels (FCs) in the polar cap was applied to data from the Longyearbyen radar of the Super Dual Auroral Radar Network. The Longyearbyen radar is at high latitude (78.2°N, 16.0°E geographic coordinates) and points northeast; therefore, it is in an ideal position for measuring zonal flows in the polar cap. The algorithm detected 998 events in the dayside polar cap region over 2 years of observations. The detected FCs typically were between 200 and 300 km latitudinal width, 1.1–1.3 km s−1 peak velocity, and 3 min in duration. The FC location shows an interplanetary magnetic field (IMF) By dependency, moving dawnward/duskward for a +By/−By. The FC monthly occurrence shows a bimodal distribution with peaks around the spring and autumn equinoxes, likely due to increased coupling between the solar wind‐magnetosphere‐ionosphere system at these times. The highest peak velocities show an absence of broad FC widths, suggesting that as the flow speed increases in the polar cap, the channels become more localized and narrow. Key Points: We present the statistics of flow channels in the dayside polar cap area including duration, width, peak velocity, and monthly occurrenceTheir formation is intimately related to IMF By, and the flow channels shift dawnward/duskward for +By/−ByHigher velocity flows in the polar cap concentrate into localized, narrower channels [ABSTRACT FROM AUTHOR]

This paper presents a new algorithm for detecting high‐speed flow channels in the polar cap. The algorithm was applied to Super Dual Auroral Radar Network data, specifically to data from the new Longyearbyen radar. This radar is located at 78.2°N, 16.0°E geographical coordinates looking north‐east, and is therefore at an ideal location to measure flow channels in the high‐latitude polar cap. The algorithm detected >500 events over 1 year of observations, and within this paper two case studies are considered in more detail. A flow channel on "old‐open field lines" located on the dawn flank was directly driven under quiet conditions over 13 min. This flow channel contributed to a significant fraction (60%) of the cross polar cap potential and was located on the edge of a polar cap arc. Another case study follows the development of a flow channel on newly opened field lines within the cusp. This flow channel is a spontaneously driven event forming under strong solar wind driving and is intermittently excited over the course of almost an hour. As they provide a high fraction of the cross polar cap potential, these small‐scale structures are vital for understanding the transport of magnetic flux over the polar cap. Key Points: Polar cap flow channels can account for a substantial amount (40–60%) of the cross polar cap potentialFlow channels can form due to dayside reconnection or appear on the edge of polar cap arcsMagnetic field lines that opened 25 min ago can still cause fast flow channels deep inside the polar cap [ABSTRACT FROM AUTHOR]

Throat auroras have been suggested to be related to indentations on the subsolar magnetopause. However, the indentation generation process and the resulting ionospheric responses have remained unknown. An EISCAT Svalbard Radar experiment was designed to run with all‐sky cameras, which enabled us for the first time to observe the temporal and spatial evolution of flow reversals, Joule heating, and ion upflows associated with throat aurora. The high‐resolution data enabled us to discriminate that the flow bursts and Joule heating were concurrent and co‐located, but were always observed on the west side of the associated throat auroras, reflecting that the upward/downward field‐aligned currents associated with throat aurora are always to the east/west, respectively. These results are consistent with the geometry of Southwood (1987) flux transfer event model and provide strong evidence for throat aurora being associated with magnetopause reconnection events. The results also support a conceptual model of the throat aurora. Key Points: An EISCAT radar experiment was designed to investigate the ionospheric characteristics of throat auroraMesoscale twin flow cells, Joule heating effects, and ion upflows were associated with the throat auroraThe observations support the idea that throat auroras are associated with magnetopause reconnection [ABSTRACT FROM AUTHOR]

This paper investigates the influence of Earth's dipole tilt angle on the reverse convection cells (sometimes referred to as lobe cells) in the Northern Hemisphere ionosphere during northward IMF, which we relate to high‐latitude reconnection. Super Dual Auroral Radar Network plasma drift observations in 2010–2016 are used to quantify the ionospheric convection. A novel technique based on Spherical Elementary Convection Systems (SECS) that was presented in our companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026634) is used to isolate and quantify the reverse convection cells. We find that the dipole tilt angle has a linear influence on the reverse cell potential. In the Northern Hemisphere the reverse cell potential is typically two times higher in summer than in winter. This change is interpreted as the change in interplanetary magnetic field‐lobe reconnection rate due to the orientation of the dipole tilt. Hence, the dipole tilt influence on reverse ionospheric convection can be a significant modification of the more known influence from vswBz. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions. Key Points: For purely northward IMF the reverse convection potential difference is typically two times higher in summer than in winterThe reverse convection potential difference has a linear dependence on the Earth's dipole tilt angleThe Earth's dipole tilt angle is a secondary important controlling parameter of the lobe reconnection rate [ABSTRACT FROM AUTHOR]

This paper describes a novel technique that allows separation and quantification of different sources of convection in the high‐latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high‐latitude reconnection associated with lobe cells from the low‐latitude reconnection associated with Dungey two‐cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models. Key Points: Spherical Elementary Convection Systems (SECS) are used to represent the high‐latitude convection electric fieldA novel technique is presented that allows separation and quantification of different sources of ionospheric convectionThe convection pattern can be separated to quantify the magnetic flux transport associated with different reconnection sites [ABSTRACT FROM AUTHOR]

Plasma convection in the coupled inner magnetosphere‐ionosphere is influenced by different factors such as neutral winds, penetration electric fields, and polarization electric fields. Several crucial insights about the dynamics in the region have been derived by interpreting observations in conjunction with numerical simulations, and recent expansion in ground‐ and space‐based measurements in the region along with improvements in theoretical modeling has fueled renewed interest in the subject. In this paper we present a comprehensive review of the literature with an emphasis on studies since 2012 relevant to the National Science Foundation Geospace Environment Modeling program. We cover four specific areas: (1) the subauroral polarization stream, (2) penetration electric fields, (3) the disturbance dynamo, and (4) quiet time subauroral convection. We summarize new observations and resulting insights relevant to each of these topics and discuss various outstanding issues and unanswered questions. Key Points: Subauroral convection is driven by complex interactions involving neutral winds, penetration electric fields, and polarization electric fieldsKey results on subauroral convection are summarized, with an emphasis on studies related to the SIMIC focus group of the NSF GEM programModeling quiet geomagnetic conditions in conjunction with observations is necessary to understand the physics behind subauroral convection [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]

It is known that the magnetic field of the Earth's closed magnetosphere can be highly displaced from the quiet‐day configuration when interacting with the interplanetary magnetic field (IMF), an asymmetry largely controlled by the dawn‐dusk component of the IMF. The corresponding ionospheric convection has revealed that footprints in one hemisphere tend to move faster to reduce the displacement, a process we refer to as the restoring of symmetry. Although the influence on the return flow convection from the process of restoring symmetry has been shown to be strongly controlled by the IMF, the influence from internal magnetospheric processes has been less investigated. We use 14 years of line‐of‐sight measurements of the ionospheric plasma convection from the Super Dual Auroral Radar Network to produce high‐latitude convection maps sorted by season, IMF, and geomagnetic activity. We find that the restoring symmetry flows dominate the average convection pattern in the nightside ionosphere during low levels of magnetotail activity. For increasing magnetotail activity, signatures of the restoring symmetry process become less and less pronounced in the global average convection maps. We suggest that tail reconnection acts to reduce the asymmetric state of the closed magnetosphere by removing the asymmetric pressure distribution in the tail set up by the IMF By interaction. During active periods the nightside magnetosphere will therefore reach a more symmetric configuration on a global scale. These results are relevant for better understanding the dynamics of flux tubes in the asymmetric geospace, which is the most common state of the system. Plain Language Summary: In this study we use observations of plasma drift from the Earth's ionosphere to study the symmetry of the Earth's magnetosphere on a large scale. On this global scale we say that the magnetic field is asymmetric when the field lines connecting the two hemispheres are displaced from their usual location. This can happen when the magnetosphere interact with the interplanetary magnetic field, especially when the latter has a significant magnitude in the east‐west direction. The major discovery of this study is that geomagnetic activity related to processes within the magnetosphere (tail reconnection) also seem to influence the degree of global asymmetry in the system. We find that the magnetosphere can become very asymmetric during periods of low geomagnetic activity, while it is more symmetric during times with higher activity. These results give us a better understanding of the processes leading to an asymmetric magnetosphere, which is needed to better understand the complex near‐Earth space system that is becoming increasingly important for our society. Key Points: Tail activity reduces asymmetries in convection speed toward the dayside between the dawn and dusk cellsTail reconnection reduces any asymmetric pressure distribution in the tail, leading to a more symmetric magnetosphereNightside convection pattern becomes more symmetric for increasing levels of tail activity [ABSTRACT FROM AUTHOR]