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.

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.

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.

The auroral electrojet is traditionally measured remotely with magnetometers on ground or in low Earth orbit (LEO). The sparse distribution of measurements, combined with a vertical distance of some 100 km to ground and typically >300 km to LEO satellites, means that smaller scale sizes can not be detected. Because of this, our understanding of the spatiotemporal characteristics of the electrojet is incomplete. Recent advances in measurement technology give hope of overcoming these limitations by multi-point remote detections of the magnetic field in the mesosphere, very close to the electrojet. We present a prediction of the magnitude of these disturbances, inferred from the spatiotemporal characteristics of magnetic field-aligned currents. We also discuss how Zeeman magnetic field sensors (Yee et al., 2020) onboard the Electrojet Zeeman Imaging Explorer (EZIE) satellites will be used to essentially image the equivalent current at unprecedented spatial resolution. The electrojet imaging is demonstrated by combining carefully simulated measurements with a spherical elementary current representation using a novel inversion scheme.

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.

Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The CHAMP satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists on statistical timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine-scale Poynting fluxes are associated with the density perturbations on a case-by-case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping SuperDARN and AMPERE datasets to generate global-scale patterns of the high-latitude and height-integrated Poynting flux into the ionosphere, with a time resolution of two minutes. From these, average patterns are generated based on the IMF orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Mesoscale height-integrated Poynting fluxes thus cannot fully account for the cusp neutral mass density enhancement, meaning energy deposition in the F-region or on fine-scales, which is not captured by our analysis, could be the primary driver.

Substorm onset location varies over a range of magnetic local times (MLTs) and magnetic latitudes (MLats). Different studies have shown that about 5${\%}$ of the variation in onset MLT can be explained by variations in interplanetary magnetic field orientation and seasonal variations. Both parameters introduce an azimuthal component to the magnetic field in the magnetosphere such that the projection of the onset MLT in the ionosphere is shifted. Recent studies have suggested that gradients in the ionospheric Hall conductance lead to a duskward shift of the magnetotail dynamics, which could also influence the location of substorm onset. In this paper, we quantify the dependence of the spatial variation of the onset location on the geomagnetic activity level prior to onset. We find that the dependence of onset location on prior conditions is as strong as the dependence on IMF By.

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.

We present the implementation of an improved technique to coherently model the high-latitude ionospheric equivalent current. By using a favourable and fixed selection of 20 ground magnetometers in Fennoscandia, we present a method based on Spherical Elementary Current Systems (SECS) to model the currents coherently during 2000–2020. Due to the north-south extent of the ground stations used, we focus on the model output along the 105◦ magnetic meridian. In addition to the fixed data locations and SECS analysis grid, our improvements involve taking into account a priori knowledge of the large-scale current systems to improve the robustness of solving the underdetermined inverse problem. We account for contributions from ground induced currents assuming so-called mirror currents. An advantage of this data set over existing empirical models of ionospheric currents is the 1-min output resolution. High temporal resolution enables investigation of temporal changes in the magnetic field. We present an analysis of statistical properties of where (in magnetic latitude and local time) and at what rate (∂Br /∂t) the radial magnetic field component fluctuates. We show that ∂Br /∂t, which is equivalent to the radial component of the curl of the induced electric field, is dependent on latitude, local time, and solar cycle. Other applications of the presented data set are also highlighted, including investigations of how Ultra Low Frequency oscillations in ground magnetic perturbations vary in space and time.

An accurate description of the state of the ionosphere is crucial for understanding the physics of Earth’s coupling to space, including many potentially hazardous space weather phenomena. To support this effort, ground networks of magnetometer stations, optical instruments, and radars have been deployed. However, the spatial coverage of such networks is naturally restricted by the distribution of land mass and access to necessary infrastructure. We present a new technique for local mapping of polar ionospheric electrodynamics, for use in regions with high data density, such as Fennoscandia and North America. The technique is based on spherical elementary current systems (SECS), which were originally developed to map ionospheric currents. We expand their use by linking magnetic field perturbations in space and on ground, convection measurements from space and ground, and conductance measurements, via the ionospheric Ohm’s law. The result is a technique that is similar to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique, but tailored for regional analyses of arbitrary spatial extent and resolution. We demonstrate our technique on synthetic data, and with real data from three different regions. We also discuss limitations of the technique, and potential areas for improvement.