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, $\mathbf{j}_\perp$, and its co-variance, to be obtained from E3D observations based on two main assumptions: 1) Ions fully magnetised above the $E$ region, set to 200 km here. 2) Electrons fully magnetised above the base of our domain, set to 90 km. In this way, $\mathbf{j}_\perp$ 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 idealised 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 utilising 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 $\pm 50$ m/s in a limited region above the radar system in our setup.

The aurora often appears as an approximately oval shape surrounding the magnetic poles, and is a visible manifestation of the intricate coupling between the Earth’s upper atmosphere and the near-Earth space environment. While the average size of the auroral oval increases with geomagnetic activity, the instantaneous shape and size of the aurora is highly dynamic. The identification of auroral boundaries holds significant value in space physics, as it serves to define and differentiate regions within the magnetosphere connected to the aurora by magnetic field lines. In this work, we demonstrate a method to detect and model the poleward and equatorward boundaries in global UV images. Our methodology enables analysis of the spatiotemporal variation in auroral boundaries from 2.5 years of UV imagery from the IMAGE satellite. The resulting dataset reveals a root mean square boundary normal velocity of 149 m/s for the poleward boundary and 96 m/s for the equatorward boundary and the velocities are shown to be stronger on the nightside than on the dayside. Interestingly, our findings demonstrate an absence of correlation between the amount of open magnetic flux and the amount of flux enclosed within the auroral oval. Furthermore, we highlight the inadequacy of a simplistic generalization of the expanding-contracting polar cap paradigm in explaining temporal variations in the auroral oval area, underscoring the imperative for an enhanced understanding of equatorward boundary fluctuations.

Using a substorm onset list from the Imager for Magnetopause to Aurora Global Exploration (IMAGE) and Polar satellite images, we have constructed a new dataset of 63 substorm onsets observed in both hemispheres and their locations. We find that incorporating the history of the interplanetary magnetic field (IMF) improves the predictive power of IMF By in explaining conjugate displacement in Magnetic Local Time (MLT). Our dataset, approximately five times larger than in previous studies, enhances statistical power and focuses on the substorm onset feature. This allows for better correlation of IMF By with MLT conjugate displacement (ΔMLT), since it has been shown that IMF-induced By tends to diminish as substorms progress. In line with previous work, we find a linear relationship between IMF By and ΔMLT, though with a shallower gradient. Additionally, we analyse predictors of conjugate displacement in Magnetic Latitude (ΔMLat), showing that dipole tilt (ψ) is the strongest predictor. However, we find that this relationship is not strictly linear, and that a larger dataset and a better understanding of the physical mechanisms are needed to more accurately capture the exact functional relationship between ψ and ΔMLat.

In most large-scale MHD models of Earth’s space environment, coupling between the magnetosphere and the ionosphere-thermosphere (IT) is addressed by representing the latter as a two-dimensional spherical shell. Similarly, most empirical models of IT electrodynamics are based on solving the height-integrated ionospheric Ohm’s law on a spherical shell. We show that there is in general no single suitable definition of the neutral wind term in high-latitude, height-integrated IT electrodynamics. Instead, two neutral wind terms weighted by Hall and Pedersen conductivities appear. We show that a commonly used expression for Joule heating in terms of height-integrated quantities is a lower bound of the actual Joule heating. Using neutral wind profiles derived from sounding rocket chemical release experiments near Poker Flat, Alaska, and Poker Flat Incoherent Scatter Radar (PFISR) measurements, we find differences of order 10–100 m/s between the two neutral wind terms.

This paper introduces a modernized application of the falling spheres technique for measurement of neutral winds and densities in the mesosphere and lower thermosphere (MLT) region regardless of time of day or tropospheric conditions, using an Observing System Simulation Experiment (OSSE) of falling spheres equipped with commercial-off-the-shelf (COTS) Global Navigation Satellite Systems (GNSS) receivers. The modernization of this technique is crucial since current techniques to measure the neutral winds in the MLT are often tied to the location of a certain instrument or heavily dependent on clear nighttime skies. We show how state-of-the-art COTS inertial measurement units (IMUs) and GNSS receivers enable precise retrieval of neutral wind and density profiles under various atmospheric conditions represented both by wind profiles measured during the Super Soaker and Auroral Jets sounding rocket missions and by wind profiles simulated via the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X). Even under highly conservative conditions (e.g., relatively extreme position uncertainty of $\sigma =\sim$100 m, a low sampling rate of 100~Hz, and strong vertical winds of 25 m/s) the estimated neutral densities exhibit errors of less than 1\%, while estimated neutral wind errors typically do not exceed 2 m/s. The latter errors are largest where the shears maximize in the lower thermosphere. The significance of this work lies in its potential to enhance our understanding of the dynamics within the MLT region, including in situ processes and the interaction with the lower atmosphere.

Utilising magnetic field measurements made by the Iridium satellites and by ground magnetometers in North America we calculate the full ionospheric current system and investigate the substorm current wedge. The current estimates are independent of ionospheric conductance, and are based on estimates of the divergence-free (DF) ionospheric current from ground magnetometers and curl-free (CF) ionospheric currents from Iridium. The DF and CF currents are represented using spherical elementary current systems (SECS), derived using a new inversion scheme that ensures the current systems’ spatial scales are consistent. We present 18 substorm events and find a typical substorm current wedge (SCW) in 12 events. Our investigation of these substorms shows that during substorm expansion, equivalent field-aligned currents (EFACs) derived with ground magnetometers are a poor proxy of the actual FAC. We also find that the intensification of the westward electrojet can occur without an intensification of the FACs. We present theoretical investigations that show that the observed deviation between FACs estimated with satellite measurements and ground-based EFACs are consistent with the presence of a strong local enhancement of the ionospheric conductance, similar to the substorm bulge. Such enhancements of the auroral conductance can also change the ionospheric closure of pre-existing FACs such that the ground magnetic field, and in particular the westward electrojet, changes significantly. These results demonstrate that attributing intensification of the westward electrojet to SCW current closure can yield false understanding of the ionospheric and magnetospheric state.

EISCAT3D, which is under its final stage of construction, will be the first incoherent scatter radar (ISR) system to provide the three-dimensional ion velocity across hundreds of kms in vertical and horizontal directions. This presents a tremendous opportunity to study the three-dimensional nature of ionospheric electrodynamics. Here we present a data-driven regional model of the electric field based on the EISCAT3D observations, where the measured F-region ion velocity data are fitted to a regional electric potential produced by a grid of spherical elementary systems. Performance of the model is demonstrated using simulated ionospheric parameters obtained from the three-dimensional GEMINI model. To simulate realistic radar measurement of the ion velocity, error estimates obtained from the e3doubt package are added to the ground truth GEMINI data. Our model can be used either with multistatic or monostatic measurements of the ion velocity, and it can also integrate ion velocity data from other platforms, such as satellite sensors, into existing ISR measurements. The model captures the ground truth electric field including its complex spatial structure with average percentile differences of about 1.5%. Most accurate results are achieved with the multistatic data, but the general spatial structure of the electric field can be captured also with monostatic data, if optimal beam patterns and regularization are used. The modeling method is also applied using real monostatic line-of-sight ion velocity data measured by the Poker Flat ISR. The modeled electric field shows reasonably well-behaved variations in latitude and longitude within the radar’s field of view.

The boundaries of the auroral oval and auroral electrojets are an important source of information for understanding the coupling between the solar wind and the near-earth plasma environment. Of these two types of boundaries the auroral electrojet boundaries have received comparatively little attention, and even less attention has been given to the connection between the two. Here we introduce a technique for estimating the electrojet boundaries, and other properties such as total current and peak current, from 1-D latitudinal profiles of the eastward component of equivalent current sheet density. We apply this technique to a preexisting database of such currents along the 105◦ magnetic meridian producing a total of eleven years of 1 minute resolution electrojet boundaries during the period 2000–2020. Using statistics and conjunction events we compare our electrojet boundary dataset with an existing electrojet boundary dataset, based on Swarm satellite measurements, and auroral oval proxies based on particle precipitation and field aligned currents. This allows us to validate our dataset and investigate the feasibility of an auroral oval proxy based on electrojet boundaries. Through this investigation we find the proton precipitation auroral oval is a closer match with the electrojet boundaries. However, the bimodal nature of the electrojet boundaries as we approach the noon and midnight discontinuities makes an average electrojet oval poorly defined. With this and the direct comparisons differing from the statistics, defining the proton auroral oval from electrojet boundaries across all local and universal times is challenging.

The exchange of kinetic and electromagnetic energy by precipitation and/or outflow, and through field-aligned currents are two aspects of the ionosphere-magnetosphere coupling. A thorough investigation of these processes is required to better understand magnetospheric dynamics. Building on our previous study using DMSP spectrometer data, here we use Swarm vector field magnetometer data to describe the auroral oval morphology in terms of east-west magnetic field perturbations. We define a threshold for detecting magnetic fluctuations based on the power spectral density of ΔBEW and derive the disturbed magnetic field occurrence probability (dBOP) at low [0.1–1Hz] and high [2.5–5Hz] frequencies. High-frequency distributions of dBOP reveal a dayside-nightside asymmetry, whereas low-frequency dBOP exhibits a persistent morphological asymmetry between the dawn-to-noon and the dusk-to-midnight sectors, peaking at dawn. Notably, weak solar wind conditions are associated with an increase in the dBOP asymmetric patterns. At low frequency in particular, while the dBOP seems to be primarily constant at dawn, the dusk dBOP decreases during quiet times, inducing a relatively larger dawn-dusk asymmetry in such conditions. We find that the dBOP distributions at low frequencies exhibit features similar to those present in distributions of the auroral electron precipitation occurrence probability, suggesting that the low-frequency dBOP constitutes a reasonable proxy for the large-scale auroral oval. Our interpretation is that the dBOP at low frequencies reflects a quasi-steady state circulation of energy, while the high-frequency dBOP reflects the regions of rapid changes in the magnetosphere. The dBOP is therefore a crucial source of information regarding the magnetosphere-ionosphere coupling.

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 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 one week prior to local spring equinox in the Northern Hemisphere (NH), and one 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 two 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 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.

Using five independent substorm onset lists, we show that substorms occur more frequently when the Interplanetary Magnetic Field (IMF) By component and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that for Ψ ≠ 0 the magnetosphere exhibits an explicit dependence on the polarity of By, as other recent studies have suggested, and imply variation in the dayside reconnection rate and/or the magnetotail response. On the other hand, we find no clear relationship between substorm intensity and this explicit By effect. We additionally observe more frequent onsets for positive By in an onset list based on identifying negative bays in the auroral electrojet, regardless of season. Taking into account all five onset lists, we conclude that this phenomenon is not real, but is rather a consequence of the particular substorm identification method, which is affected by local ionospheric conditions that depend on By and Ψ.

A number of interdependent conditions and processes contribute to ionospheric-origin energetic ion outflows. Due to these interdependences and the associated observational challenges, energetic ion outflows remain a poorly understood facet of atmosphere-ionosphere-magnetosphere coupling. Here we demonstrate the relationship between east-west magnetic field fluctuations ($\Delta B_{\textrm{EW}}$) and energetic outflows in the magnetosphere-ionosphere transition region. We use dayside cusp-region FAST satellite observations made at apogee ($\sim$4200-km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upflow and ($\Delta B_{\textrm{EW}}$) spectral power as a function of spacecraft-frame frequency bands between 0 and 4 Hz. Identification of ionospheric-origin energetic ion upflows is automated, and the spectral power $P_{EW}$ in each frequency band is obtained via integration of $\Delta B_{\textrm{EW}}$ power spectral density. Derived relationships are of the form $J_{\parallel,i} = J_{0,i} P_{EW}^\gamma$ for upward ion flux $J_{\parallel,i}$ at 130-km altitude. The highest correlation coefficients are obtained for spacecraft-frame frequencies $\sim$0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices $\gamma \simeq$ 0.9–1.3 and correlation coefficients $r \geq 0.92$, while winter solstice observations yield $\gamma \simeq$ 0.4–0.8 with $r \gtrsim 0.8$. Mass spectrometer observations reveal that the oxygen/hydrogen ion composition ratio near summer solstice is much greater than the corresponding ratio near winter. These results thus reinforce the importance of ion composition in any outflow model. If observed $\Delta B_{\textrm{EW}}$ variations are purely spatial and not temporal, we show that spacecraft-frame frequencies $\sim$0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers.