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The main driver of the ionospheric dayside dynamics is the interaction between the interplanetary magnetic field (IMF) and the Earth’s magnetic field near the dayside magnetopause, while magnetotail activities control the nightside ionospheric dynamics. In spite of that, our knowledge about the influence of magnetotail activity on the dayside ionospheric dynamics and vice versa is limited. We investigate the nightside influence on the dayside ionospheric current by performing superposed epoch analyses of ground magnetic field data for northward IMF substorms. Such substorms encounter minimal influence of the dayside reconnection, granting an opportunity to isolate the effects of magnetotail activity on the dayside current system. Our analyses indicate that as nightside activity elevates, the dayside ionospheric current changes. We also find that lobe reconnection is weaker before substorm onset than what is expected for northward IMF conditions and then increases after onset, possibly due to reconfiguration of the magnetosphere. We present three possible mechanisms that can explain our observations.

In many experimental studies of ionosphere-thermosphere (IT) coupling and ionospheric electrodynamics, the limitations of existing observational data sets require one to represent the three-dimensional IT system as an infinitely thin, two-dimensional sheet. This 2D representation of the coupled IT system cannot however represent some of its most basic properties, such as the existence of different ionospheric layers. On the other hand, observing systems that are capable of probing the altitudinal structure of relevant quantities, such as plasma density and drift, often can only provide information within a very narrow overhead volume. This limitation typically requires one to assume that these quantities have no horizontal gradients. Measurements from the upcoming EISCAT_3D incoherent scatter radar therefore present an unprecedented opportunity to probe the 3D IT system in three dimensions and on relatively short time scales (of order minutes). Here we present a data assimilation technique, EISCAT_3D-based reconstruction of ionosphere-thermosphere electrodynamics (E3D-BRITE), for routine estimation of all three components of the ionospheric current density and their uncertainties. We illustrate the technique using synthetic EISCAT_3D measurements of the plasma density and ion drift. We describe how the E3D-BRITE technique can also be used to simultaneously estimate the neutral wind and the perpendicular electric field. The technique relies on a 3D generalization of curl-free and divergence-free Cartesian elementary current systems. We also discuss the limitations imposed on this technique by the geometry of the three EISCAT_3D sites in Skibotn, Karesuvanto, and Kaiseniemi.

Sunlight makes it difficult to measure the aurora. We use data obtained by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program’s (DMSP) F16-19 satellites to quantify the auroral intensities during sunlit conditions. We use Environmental Disk Radiance (EDR) Aurora data product from the SSUSI-team, where the dayglow is already subtracted. The error in the dayglow-subtraction is proportional to the strength of the dayglow and represents an uncertainty in the observations. We characterize the auroral intensity and the dayglow part of the signal by combining multiple observations during similar solar illumination, and magnetic local time/latitude. By fitting convolutions of symmetric (dayglow-error) and fat-tailed distributions (aurora) on data from many years, it might be possible to separate the signal from the aurora. By comparing this to the regular methods of estimating auroral intensity (mean/median) we will assess the usefulness and importance of this method. We examine the validity of our method by evaluating the fits of the convoluted distributions.

The aurora is a visible manifestation of Earth’s coupling to near-Earth space. The emitted light is produced by charged particles that precipitate into the upper atmosphere. These particles are usually located on closed magnetic field lines that connect directly between the northern and southern hemisphere. As a result, Earth’s aurora often appears in an oval shape surrounding the magnetic pole. Inside the oval, at high magnetic latitudes, is a region with open magnetic field lines that extend into the solar wind. This region of open magnetic flux is the polar cap and is a consequence of the Dungey cycle: Reconnection between the solar wind magnetic field and closed terrestrial field lines at the dayside magnetopause produces open field lines which are transported to the nightside where they are again closed by reconnection. A fundamental property of the magnetosphere-ionosphere system is that changes in the amount of open magnetic flux is equal to the net difference between the dayside and nightside reconnection rates. That is, the polar cap expands when dayside reconnection dominates and contracts when nightside reconnection dominates. This is known as the expanding/contracting polar cap paradigm, and has been studied extensively in the last few decades. The expansion and contraction of the aurora itself has received less attention. In this work, we use global auroral images to study the spatiotemporal evolution of the auroral oval. We investigate how the solar wind, open flux and auroral flux covary. Furthermore, we attempt to determine how well a pure fluid description of the auroral zone can explain the observed evolution.

When IMF By is dominant, which is the typical situation, a highly vortical convection pattern is seen inside the dayside polar cap in the summer hemisphere. In the winter hemisphere, however, the convection is mainly from noon to midnight with little vorticity inside the polar cap. Combined with the vastly different ionospheric conductance between summer and winter due to solar EUV irradiance, these differences in convection cause large summer/winter differences in the Birkeland currents in the dayside polar cap. Hence, the joule heating rates will be very different in the dayside polar cap between summer and winter during the IMF By dominant periods, which is typically associated with weak geomagnetic activity inside the auroral oval. This presentation will focus on the hemispheric differences in e.g. joule heating during such conditions, which will be quantified using the newly developed LOcal Mapping of Polar ionospheric Electrodynamics (Lompe) data assimilation technique. The Lompe technique is similar to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique, but allows the electrodynamics to be described only in a limited region to reflect the observational coverage. The prescribed conductance will be provided from UV imaging of the aurora (in addition to EUV), allowing also the energy from precipitation to be estimated. While existing empirical models [e.g. Weimer 2005, doi:10.1029/2004JA010884] capture some aspects of the hemispheric asymmetries, this presentation will focus on how recent advances in data assimilation techniques allows us to quantify these asymmetries on an event basis, showing how these typical conditions can lead to vastly different energy input into the two hemispheres.

We use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy

In this study we use high‐quality convection data from the Electron Drift Instrument on board Cluster to investigate how near‐Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east‐west (By) component. Two different proxies have been used to represent different levels of reconnection in the near‐Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly YGSM direction, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north‐south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north‐south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near‐Earth tail. publishedVersion

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 unambi...

North-south asymmetries arise in the magnetosphere-ionosphere system when a significant east-west (By) component is present in the interplanetary magnetic field (IMF). During such conditions, a By component with the same sign as the IMF By component is induced in the magnetosphere, and the locations of conjugate magnetic footpoints are displaced between the two hemispheres. It has been suggested that these asymmetries are introduced into the closed magnetosphere by tail reconnection. However, recent studies instead suggest that asymmetric lobe pressure induces the asymmetries, which are then reduced during periods of enhanced tail reconnection. To address this, we use the Lyon-Fedder-Mobarry (LFM) model and initiate a loading-unloading cycle in multiple runs by changing the IMF. Asymmetries are induced during the loading phase and reduced during the unloading phase. The model results thus suggest that asymmetries arise during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection. publishedVersion

Lobe reconnection is usually considered to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common 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 in a dawn-dusk direction, plasma flows initiated by dayside as well as lobe reconnection map to high latitude ionospheric locations in close proximity to each other. This has been emphasized in the literature earlier, mainly on a conceptual level, but quantifying the relative importance of lobe reconnection to the observed ionospheric convection is highly challenging during these IMF By dominated conditions, since one has to identify and distinguish these regions. By normalizing the ionospheric convection (observed by SuperDARN) to the polar cap boundary (inferred from simultaneous AMPERE observations), we are able to do this separation, allowing us to quantify the relative contribution of both lobe reconnection and dayside/nightisde reconnection to the ionospheric convection pattern. Using this segmentation technique we can get new quantitative insights into the importance of the various mechanisms that affect the lobe reconnection rate. In this presentation we will describe the technique and show results of analysis of periods when the IMF is mainly in the dawn-dusk direction. Our quantification of the average lobe reconnection rate during various conditions yields quantitative knowledge of the importance of the lobe reconnection process, which can act independently in the two hemispheres. We will specifically constrain the influence from parameters such as the dipole tilt angle and the product of IMF transverse component and solar wind velocity.

According to the expanding-contracting polar cap paradigm, dayside and nightside reconnection control magnetosphere-ionosphere dynamics at high latitudes by increasing or decreasing the open flux respectively. The dayside reconnection rate can be estimated using parameters measured in the solar wind, but there is no reliable and available proxy for the nightside reconnection rate. We want to remedy this by using AMPERE to estimate a time series of open flux content. The AMPERE data set originates from the global Iridium satellite system, enabling continuous measurements of the field-aligned Birkeland currents, from which the open magnetic flux of the polar caps can be derived. These estimates will be used to derive empirical relationships with available measurements on the ground and in the solar wind. This work can also help improve estimates of dayside reconnection rates.

By utilising measurements from twenty ground magnetometer stations in Fennoscandia, divergence-free ionospheric currents above this region are modelled using spherical elementary currents (SECS). New modelling techniques are implemented that coerce the model to find a solution that resembles the resolvable ionospheric currents. The divergence-free currents are evaluated along the 105o magnetic meridian covering a period of almost 20 years with a resolution of 1 minute, as a result of the magnetometers chosen. From these sheet current density latitude profiles, the boundaries of the auroral electrojet are identified. After performing a large statistical analysis it is found that there is a significant IMF By effect on the poleward boundary of the electrojets during the Summer but not during the Winter. We suggest that this seasonal effect can be attributed to the effects of lobe reconnection on the extent of currents in the auroral electrojets. Further work is done to compare the SECS derived electrojet boundaries with particle precipitation data from low orbit satellites.

The geospace response to rapid changes in solar wind pressure results in a perturbation of the magnetospheric-ionospheric system. Ground magnetometer stations located at polar latitudes have long been known to measure a sudden impulse only minutes after a solar wind structure reaches the magnetopause.Here a list of events associated with a step-like feature in the solar wind dynamic pressure between 1994 and 2020 is compiled based on in situ observations from ACE and Wind. Arrival time estimates are calculated using a simple propagation method and validated with a correlation analysis using SYM-H from low/mid latitude stations. A superposed epoch analysis is carried out to investigate the impact of season, interplanetary magnetic field orientation and other attributes pertaining to the interplanetary shock. All available ground magnetometer stations in SuperMAG, during each event, are used allowing for global coverage. Global data coverage is important for this kind of comparative analysis as it is needed to determine changes in the systems response due to e.g. season, which might lead to an improved understanding of the magnetospheric-ionospheric-thermospheric coupling.

Substorm onset location varies over a range of magnetic local time (MLT) and magnetic latitudes (MLat). It is well known that about 5% of the variation in onset MLT can be explained by variations in interplanetary magnetic field orientation and dipole tilt angle. Both parameters introduce an azimuthal component in the magnetic field in the magnetosphere such that the projection of the onset MLT in the ionosphere is shifted. The MLT of the onset near the magnetopsheric equatorial plane is even less predictable. Recent studies have suggested that gradients in the ionospheric Hall conductance lead to a duskward shift of tail dynamics, which could also influence the location of substorm onset. Our goal is to test these ideas by quantifying the dependence of the spatial variation of the onset location on external and internal conditions. We focus on the correlation between the substorm onset location with conditions prior to the onset, such as the interplanetary magnetic field By component, dipole tilt angle, and estimates of the Hall conductance. Linear regression analysis is used to determine the substorm onset location dependence on the proposed variables.

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't 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., 2021) onboard the Electrojet Zeeman Imaging Explorer 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. publishedVersion

One of the primary mechanisms of loss of Earth's atmosphere is the persistent “cold” ( urn:x-wiley:jgra:media:jgra56068:jgra56068-math-0001 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. publishedVersion

We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) $\mathit{B}_{y}$ component on the coupling between the solar wind and magnetosphere-ionosphere system when IMF $\mathit{B}_{z}$ $>$0. We describe the evolution of how a magnetospheric $\mathit{B}_{y}$ component is induced on closed field lines during these conditions. Starting from dayside lobe reconnection, the magnetic tension on newly reconnected field lines redistribute the open flux asymmetrically between the two hemispheres. This results in asymmetric magnetic energy density in the lobes. Shear flows are induced to restore equilibrium, and these flows are what effectively induces a local $\mathit{B}_{y}$ component. We show the radial dependence of the induced $\mathit{B}_{y}$ and compare the results to the induced $\mathit{B}_{y}$ during southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF $\mathit{B}_{y}$ reversals during northward IMF $\mathit{B}_{z}$. A superposed epoch analysis of magnetic field measurements from seven Geostationary Operational Environmental Satellite spacecraft at different local times both for negative-to-positive and positive-to-negative IMF $\mathit{B}_{y}$ reversals is presented. We find that the induced $\mathit{B}_{y}$ responds within 16 min of the arrival of IMF $\mathit{B}_{y}$ at the bow shock, and it completely reconfigures within 47 min.

Presently, all empirical coupling functions quantifying the solar wind - magnetosphere energy- or magnetic flux conversion, assume that the coupling is independent of the sign of the dawn-dusk component (By) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF By effect on the solar wind - magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF By is found to on average lead to a larger polar cap than when IMF By is negative during otherwise similar conditions. This explicit IMF By effect is found to reverse when the Earth's dipole is inclined in the opposite direction (northern summer), and is consistently observed from both hemispheres using the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to infer the size of the region 1/2 current system. Two interpretations are presented: 1) The dayside reconnection rate is affected by the combination of dipole tilt and IMF By sign in a manner explaining the observations 2) The combination of dipole tilt and IMF By sign affect the global conditions for maintaining a given nightside reconnection rate. The observations as well as idealized magnetohydrodynamic (MHD) model runs are analyzed and discussed in light of the two different interpretations in order to enhance our understanding of this explicit IMF By effect.

The auroral electrojet is traditionally measured remotely with magnetometers on ground or in low Earth orbit. The long distance, more than 100 km, means that smaller scale sizes are not detected. Because of this, the spatiotemporal characteristics of the electrojet are not known. Recent advances in measurement technology give hope of 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 a constellation of small satellites carrying the Microwave Electrojet Magnetogram (MEM) instrument (Yee et al., 2020), could be used to essentially image the equivalent current at unprecedented spatial resolution.

The polar cap ionosphere (here defined as the region above 80° magnetic latitude) is the primary source region of cold plasma outflows observed in the magnetosphere. The two factors controlling cold plasma outflows are the availability of plasma in the polar cap ionosphere, and transport from the ionosphere to the magnetosphere. Some statistical studies have indicated that the former of these two factors, availability of cold plasma, is the limiting factor. We use 15 years of electron density measurements made by Swarm and CHAMP spacecraft, corrected for variations in observation altitude and solar activity, to investigate how variations in solar wind driving and local hemispheric season affect the polar cap ionosphere electron density Ne. We show that the dependence of Ne on the By component of the interplanetary magnetic field is apparently antisymmetric in the two hemispheres, that Nestatistically decreases with decreasing Dst index (i.e., increasing geomagnetic activity) and that Ne is apparently insensitive to the AE index. We also show that Ne distributions around March and September equinoxes display weak evidence of hemispheric asymmetry. We show that during local summer, observed Ne distributions under high solar wind driving conditions are relatively lower than Ne distributions under low solar wind driving conditions. During local winter the reverse is true, with Ne distributions under low solar wind driving conditions being relatively lower than Ne distributions under high solar wind driving conditions. Thus solar wind driving and seasonal effects may apparently both constructively and destructively interfere. Altitude variation in Swarm and CHAMP Nemeasurements is accounted for via an empirical scale height derived from 1687 conjunctions between Swarm B and either Swarm A or Swarm C during 2013–2019. The approximately linear dependence of Ne on F10.7 measurements is also accounted for. Swarm Ne measurements are additionally corrected using the Lomidze et al. (2018) calibrations.

The IMF By component is a source of numerous asymmetric features in our magnetospheric system, e.g. north-south asymmetries in the aurora, the magnetospheric and ionospheric current systems and the plasma convection. Several recent studies have shown that asymmetries in the lobe pressure play a major role in inducing these asymmetries. It has also been reported that enhanced tail reconnection affects the dynamics of the system by reducing the north-south asymmetries imposed by the IMF. A possible interpretation of these observations is that enhanced reconnection in the near-Earth tail reduces the pressure in the lobes and thus suppresses the cause of the initial asymmetry. In this study, we present the results from global MHD simulations using the LFM model to further investigate how enhanced tail reconnection affects the asymmetric state of the system. A relaxation of the asymmetry in the closed magnetosphere is seen in the model when the reconnection rate in the tail increases, consistent with observations, and we use the simulation output to gain further insight into the physical mechanism(s) responsible for the return to a more symmetric state.

The Zeeman effect of the O2 118 GHz spectral radiance measurements can be utilized to remotely measure the magnetic field perturbations at altitudes close to the auroral electrojets. The technique has been demonstrated using the measurements provided by the Microwave Limb Souncer onboard the Aura spacecraft. The derived current-induced magnetic field perturbations were found to be highly correlated with those coincidently obtained by ground magnetometers and to be consistent with the well-known auroral electrojet current distribution thereby providing a strong argument for the validity of the technique. With today's technology, a 118 GHz instrument, can be miniaturized allowing it to fly on small satellites such as CubeSats. A constellation of small satellites with each one carrying a number of these identical mini-radiometers would have the ability to provide simultaneous multipoint measurement of the magnetic field perturbations at altitudes close to the electrojet, thereby greatly advancing our understanding of the ionospheric current system. In this paper, we present the Zeeman magnetic field sensing technique, the requirements and specifications of the instrument, and an example of a cost effectively cubesat mission that provides unprecedented measurements of the evolution and structure of the auroral electrojet system.

Presently, all empirical coupling functions quantifying the solar wind—magnetosphere energy—or magnetic flux conversion assume that the coupling is independent of the sign of the dawn-dusk component (B𝑦) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF B𝑦 effect on the solar wind-magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF B𝑦 is found to on average lead to a larger polar cap than when IMF B𝑦 is negative during otherwise similar conditions. This explicit IMF B𝑦 effect is found to reverse when the Earth's dipole is inclined in the opposite direction (northern summer) and is consistently observed from both hemispheres. We interpret the different responses of the polar cap size due to the sign of IMF B𝑦 to likely be a result of differences in the dayside reconnection rate. publishedVersion

The solar wind influence on geospace can be described as the sum of a directly driven component, or dayside reconnection, and an unloading component, associated with the release of magnetic energy via nightside reconnection. The two processes are poorly correlated on short time scales, but exactly equal when averaged over long time windows. Because of this peculiar property, regression models of ionospheric electrodynamics that are based on solar wind data are time scale specific: Models derived from 1 min resolution data will be different from models derived from hourly, daily, or monthly data. We explain and quantify this effect on simple linear regression models of various geomagnetic indices. We also derive a time scale-dependent correction factor that can be used with the Average Magnetic field and Polar current System model. Finally, we show how absolute estimates of the nightside reconnection rate can be calculated from solar wind measurements and geomagnetic indices.

A number of interdependent conditions and processes contribute to ionospheric-origin energetic ( ∼ 10 eV to several keV) 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 ( ΔB EW ) and energetic outflows in the magnetosphere-ionosphere transition region. We use dayside cusp region FAST satellite observations made near apogee ( ∼ 4,180-km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upflow and ΔB 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 ΔB EW power spectral density. Derived relationships are of the γ form J ||,i = J 0,i P EW for upward ion flux J ||,i at 130-km altitude, with J 0,i the mapped upward ion flux for a nominal spectral power P EW = 1 nT 2 . The highest correlation coefficients are obtained for spacecraft frame frequencies ∼ 0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices γ ≃ 0.9–1.3 and correlation coefficients r ≥ 0.92, while winter solstice observations yield γ ≃ 0.4–0.8 with r ≳ 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 reinforce the importance of ion composition in outflow models. If observed ΔB EW perturbations result from Doppler-shifted wave structures with near-zero frequencies, we show that spacecraft frame frequencies ∼ 0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers. publishedVersion

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...

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 v_sw B_z. These results could be adopted by the scientific community as key input parameters for lobe reconnection coupling functions. publishedVersion

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. publishedVersion

In the day-side sunlit polar ionosphere the varying and IMF dependent convection creates strong ionospheric currents even during quiet geomagnetic conditions. Observations during such times are often excluded when using satellite data to model the internal geomagnetic main field. Observations from the night-side or local winter during quiet conditions are, however, also influenced by variations in the IMF. In this paper we briefly review the large scale features of the ionospheric currents in the polar regions with emphasis on the current distribution during undisturbed conditions. We examine the distribution of scalar measurements of the magnetic field intensity minus predictions from a geomagnetic field model. These ‘residuals’ fall into two main categories. One category is consistently distributed according to the well-known ionospheric plasma convection and its associated Birkeland currents. The other category represent contributions caused by geomagnetic activity related to the substorm current wedge around local magnetic midnight. A new observation is a strong IMF $B_{y}$ control of the residuals in the midnight sector indicating larger ionospheric currents in the substorm current wedge in the northern polar region for $B_{y} > 0$ and correspondingly in the southern hemisphere for $B_{y} < 0$ .

In this paper, we present observations of cold (0–70 eV) plasma density in the magnetotail lobes. The observations and results are based on 16 years of Cluster observation of spacecraft potential measurements converted into local plasma densities. Measurements from all four Cluster spacecraft have been used, and the survey indicates a persistent asymmetry in lobe density, with consistently higher cold plasma densities in the northern lobe. External influences, such as daily and seasonal variations in the Earth's tilt angle, can introduce temporary north‐south asymmetries through asymmetric ionization of the two hemispheres. Likewise, external drivers, such as the orientation of the interplanetary magnetic field can set up additional spatial asymmetries in outflow and lobe filling. The persistent asymmetry reported in this paper is also influenced by these external factors but is mainly caused by differences in magnetic field configuration in the Northern and Southern Hemisphere ionospheres. publishedVersion

Computational analysis has become ubiquitous within the heliophysics community. However, community standards for peer-review of codes and analysis have lagged behind these developments. This absence has contributed to the reproducibility crisis, where inadequate analysis descriptions and loss of scientific data have made scientific studies difficult or impossible to replicate. The heliophysics community has responded to this challenge by expressing a desire for a more open, collaborative set of analysis tools. This article summarizes the current state of these efforts and presents an overview of many of the existing Python heliophysics tools. It also outlines the challenges facing community members who are working towards the goal of an open, collaborative, Python heliophysics toolkit and presents guidelines that can ease the transition from individualistic data analysis practices to an accountable, communalistic environment., Comment: Published in JGR

Previous studies have shown that conjugate auroral features are displaced in the two hemispheres when the interplanetary magnetic field (IMF) has a transverse (Y) component. It has also been shown that a BY component is induced in the closed magnetosphere due to the asymmetric loading of magnetic flux in the lobes following asymmetric dayside reconnection when the IMF has a Y component. The magnetic field lines with azimuthally displaced footpoints map into a “banana”-shaped convection cell in one hemisphere and an “orange”-shaped cell in the other. Due to the Parker spiral our system is most often exposed to a BY-dominated IMF. The dipole tilt angle, varying between ±34∘, leads to warping of the plasma sheet and oppositely directed BY components in dawn and dusk in the closed magnetosphere. As a result of the Parker spiral and dipole tilt, geospace is asymmetric most of the time. The magnetic storm on 17 August 2001 offers a unique opportunity to study the dynamics of the asymmetric geospace. IMF BY was 20–30 nT and tilt angle was 23∘. Auroral imaging revealed conjugate features displaced by 3–4 h magnetic local time. The latitudinal width of the dawnside aurora was quite different (up to 6∘) in the two hemispheres. The auroral observations together with convection patterns derived entirely from measurements indicate dayside, lobe and tail reconnection in the north, but most likely only dayside and tail reconnection in the Southern Hemisphere. Increased tail reconnection during the substorm expansion phase reduces the asymmetry.

S. E. M. and J. A. C. were supported by the Science and Technology Facilities Council (STFC), UK, grant ST/N000749/1; H. S. was supported by a STFC studentship. The work at the Birkeland Centre for Space Centre, University of Bergen, Norway, was supported by the Research Council of Norway/CoE under contract 223252/F50. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium‐derived data products; AMPERE products are available at http://ampere.jhuapl.edu. The OMNI data, including solar wind parameters and geomagnetic indices, were obtained from the GSFC/SPDF OMNIWeb interface at http://omniweb.gsfc.nasa.gov. The SuperMAG substorm list was downloaded from http://supermag.jhuapl.edu. For the ground magnetometer data from which this list was derived, we gratefully acknowledge the following: Intermagnet; USGS, Jeffrey J. Love; CARISMA, PI Ian Mann; CANMOS; The S‐RAMP Database, PI K. Yumoto and K. Shiokawa; The SPIDR database; AARI, PI Oleg Troshichev; The MACCS program, PI M. Engebretson, Geomagnetism Unit of the Geological Survey of Canada; GIMA; MEASURE, UCLA IGPP and Florida Institute of Technology; SAMBA, PI Eftyhia Zesta; 210 Chain, PI K. Yumoto; SAMNET, PI Farideh Honary; the institutes who maintain the IMAGE magnetometer array, PI Eija Tanskanen; PENGUIN; AUTUMN, PI Martin Connors; DTU Space, PI Juergen Matzka; South Pole and McMurdo Magnetometer, PIs Louis J. Lanzarotti and Alan T. Weatherwax; ICESTAR; RAPIDMAG; PENGUIn; British Artarctic Survey; McMac, PI Peter Chi; BGS, PI Susan Macmillan; Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN); GFZ, PI Juergen Matzka; MFGI, PI B. Heilig; IGFPAS, PI J. Reda; University of L'Aquila, PI M. Vellante; SuperMAG, PI Jesper W. Gjerloev.

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. publishedVersion

Statistical analyses have shown that the sunward component of the interplanetary magnetic field, Bx (Geocentric Solar Magnetospheric), moderately but significantly affects the auroral intensity. These observations have been interpreted as signatures of a similar interplanetary magnetic field Bx control on Birkeland currents yet to be observed directly. Such a control, attributed to differences in magnetic tension on newly opened magnetic field lines, would lead to stronger region 1 (R1) Birkeland currents for Bx negative (positive) conditions in the Northern (Southern) Hemispheres than when Bx is positive (negative). In this paper we perform a detailed investigation of three different sets of magnetic field measurements, from the Challenging Minisatellite Payload and Swarm low Earth orbit satellites, from the Active Magnetosphere and Planetary Electrodynamics Response Experiment products derived from the Iridium satellite constellation, and from the SuperMAG ground magnetometer network, each analyzed using different techniques, to test these predictions. The results show that a change in sign of Bx changes the Birkeland currents by no more than ≈10%. The current patterns show little support for an interhemispheric asymmetry of the kind proposed to explain auroral observations. Instead, we propose an alternative interpretation, which is consistent with most of the auroral observations and with the current observations in the present paper, except for those based on Active Magnetosphere and Planetary Electrodynamics Response Experiment: The solar wind‐magnetosphere coupling is more efficient when the dipole tilt angle and Bx have the same sign than when they are different. We suggest that the higher coupling is because the dayside reconnection region is closer to the subsolar point when the dipole tilt angle and Bx have the same sign. publishedVersion

Ground magnetic field measurements can be mathematically related to an overhead ionospheric equivalent current. In this study we look in detail at how the global equivalent current, calculated using more than 30 years of SuperMAG magnetometer data, changes with sunlight conditions. The calculations are done using spherical harmonic analysis in quasi-dipole coordinates, a technique which leads to improved accuracy compared to previous studies. Sorting the data according to the location of the sunlight terminator and orientation of the interplanetary magnetic field (IMF), we find that the equivalent current resembles ionospheric convection patterns on the sunlit side of the terminator but not on the dark side. On the dark side, with southward IMF, the current is strongly dominated by a dawn cell and the current across the polar cap has a strong dawnward component. The contrast between the sunlit and dark side increases with increasing values of the F10.7 index, showing that increasing solar EUV flux changes not only the magnitude but also the morphology of the equivalent current system. The results are consistent with a recent study showing that Birkeland currents indirectly determine the equivalent current in darkness and that Hall currents dominate in sunlight. This has implication for the interpretation of ground magnetic field measurements and suggests that the magnetic disturbances at conjugate points will be asymmetrical when the solar illumination is different. publishedVersion

We acknowledge the use of NASA/GSFC’s Space Physics Data Facility for OMNI data. Simulation results have been provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their public Runs on Request system (http://ccmc.gsfc.nasa.gov). The CCMC is a multiagency partnership between NASA, AFMC, AFOSR, AFRL, AFWA, NOAA, NSF, and ONR (Paul-Tenfjord-032514-1). We thank the AMPERE team and the AMPERE Science Center for providing the Iridium-derived data products. This study was supported by the Research Council of Norway/CoE under contract 223252/F50.

Magnetic perturbations on ground at high latitudes are directly associated only with the divergence-free component of the height-integrated horizontal ionospheric current, $\textbf{J}_{\perp,df}$. Here we show how $\textbf{J}_{\perp,df}$ can be expressed as the total horizontal current $\textbf{J}_\perp$ minus its curl-free component, the latter being completely determined by the global Birkeland current pattern. Thus in regions where $\textbf{J}_\perp = 0$, the global Birkeland current distribution alone determines the local magnetic perturbation. We show with observations from ground and space that in the polar cap, the ground magnetic field perturbations tend to align with the Birkeland current contribution in darkness but not in sunlight. We also show that in sunlight, the magnetic perturbations are typically such that the equivalent overhead current is anti-parallel to the convection, indicating that the Hall current system dominates. Thus the ground magnetic field in the polar cap relates to different current systems in sunlight and in darkness.

Solar cycle and seasonal variations have been found in the occurrence of strong thermally excited 630.0 nm emissions in the polar ionosphere. Measurements from the European Incoherent Scatter Svalbard Radar have been used to derive the thermal emission intensity. Thermally excited emissions have been found to maximize at solar maximum with peak occurrence rate of similar to 40% compared to similar to 2% at solar minimum. These emissions also have the highest occurrence in equinox and the lowest occurrence rate in summer and winter. There is an equinoctial asymmetry in the occurrence rate which reverses with the solar cycle. This equinoctial asymmetry is attributed to variations of the solar wind-magnetosphere coupling arising from the Russell-McPherron effect. The occurrence rate of thermal excitation emission on the dayside, at Svalbard, has been found to be higher in autumn than spring at solar maximum and the reverse at solar minimum. Enhanced electron temperatures characterize the strong thermal component for solar minimum and winter, whereas enhanced electron densities characterize the thermal component for solar maximum. The results point to solar wind-magnetosphere-ionosphere coupling as the dominant controlling process. China (CRIRP); Finland (SA); Japan (NIPR); Japan (STEL); Norway (NFR); Sweden (VR); United Kingdom (NERC); Norwegian Research Council [223252]; U.S.-Norway Fulbright Foundation The EISCAT data were accessed from https://www.eiscat.se and processed using GUISDAP. EISCAT is an international association supported by research organizations in China (CRIRP), Finland (SA), Japan (NIPR and STEL), Norway (NFR), Sweden (VR), and the United Kingdom (NERC). The interplanetary magnetic field and solar wind data were provided by the NASA OMNIWeb service (https: omniweb.gsfc.nasa.gov/). The NRLMSISE-00 Atmospheric model was accessed from https://ccmc.gsfc.nasa.gov/modelweb/models/nrlmsise00. php. Simulation results have been provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their public Runs on Request system (http://ccmc.gsfc.nasa.gov). The TIE-GCM Model was developed by the R.G. Roble et al. at the High Altitude Observatory, National Center for Atmospheric Research (HAO NCAR). The TIE-GCM is an open-source community model available at the HAO/NCAR web site. This project has been funded by the Norwegian Research Council under the contract 223252. Kjellmar Oksavik was also grateful for being selected as the 2017-2018 Fulbright Arctic Chair, and his sabbatical at Virginia Tech was sponsored by the U.S.-Norway Fulbright Foundation for Educational Exchange.

During southward interplanetary magnetic field, dayside reconnection will drive the Dungey cycle in the magnetosphere, which is manifested as a two‐cell convection pattern in the ionosphere. We address the response of the ionospheric convection to changes in the dayside reconnection rate by examining magnetic field perturbations at 780 km altitude. The Active Magnetosphere and Planetary Electrodynamics Response Experiment data products derived from the Iridium constellation provide global maps of the magnetic field perturbations. Cluster data just upstream of the Earth's bow shock have been used to estimate the dayside reconnection rate. By using a statistical model where the magnetic field can respond on several time scales, we confirm previous reports of an almost immediate response both near noon and near midnight combined with a 10–20 min reconfiguration time of the two‐cell convection pattern. The response of the ionospheric convection has been associated with the expansion of the polar cap boundary in the Cowley‐Lockwood paradigm. In the original formulation of this paradigm the expansion spreads from noon to midnight in 15–20 min. However, also an immediate global response has been shown to be consistent with the paradigm when the previous dayside reconnection history is considered. In this paper we present a new explanation for how the immediate response can be accommodated in the Cowley‐Lockwood paradigm. The new explanation is based on how MHD waves propagate in the magnetospheric lobes when newly reconnected open flux tubes are added to the lobes, and the magnetopause flaring angle increases. publishedVersion

The Earth's magnetic field provides a unique natural source of orientation information that is particularly useful for subsurface magnetic measurement-while-drilling (MWD) navigation. In order to utilize the MWD magnetic field measurements for calculating the orientation of the bottom hole assembly (BHA), an accurate geomagnetic reference model is needed for comparison. In this paper we present the CHAOS-X model, a new geomagnetic reference model that provides global vector field estimates of Earth's magnetic field, with high resolution in both space and time, for precision magnetic directional surveying applications. The model is derived from more than one million satellite and ground-based observatory magnetic measurements and consists of modules representing internal sources (in the Earth's core and crust), mag-netospheric sources, and ionospheric sources. Compared with previous reference models, the CHAOS-X model is particularly designed for better characterization of the time variations in the geomagnetic field. In this paper, we describe the model and present benchmark comparisons with magnetic observatory data to establish the uncertainty values required in models of wellbore positional errors in magnetic directional surveying applications. The discrepancy between geomagnetic measurements and reference models are typically dominated by spatial variations caused by local geology. In applications requiring high accuracy, these variations can be taken into account by using a dedicated local model. In such cases, when the errors associated with local geology is small, our results show that the CHAOS-X reference model may yield a significant improvement compared with existing reference models. This result holds both when the model is used predictively and retrospectively. We also argue that using a model with an accurate description of the time variations improves recent magnetic surveys while drilling, since the description of the rapid time variations can be updated near real-time.

[1] We have investigated a data set of 19 h of simultaneous global conjugate auroral imaging from space. The data set consists of 10 sequences with durations from 1 to 5 h during active geomagnetic conditions (average AE ∼ 400 nT). We have identified 15 features (including two presented earlier) of auroral forms that appear mainly in one hemisphere, and we define this as non-conjugate aurora. Three generator mechanisms has been suggested for producing interhemispheric currents and non-conjugate aurora: (1) Hemispherical differences in solar wind dynamo efficiency due to interplanetary magnetic field (IMF) Bx and dipole tilt angle leading to asymmetric region 1 currents in the two polar hemispheres, (2) interhemispheric currents induced by the penetration of IMF By into the closed nightside magnetosphere, and (3) hemispheric differences in ionospheric conductivity controlled by the dipole tilt angle inducing interhemispheric currents on closed field-lines. We want to find out if our observations are consistent with these mechanisms. Our analysis shows that five features were consistent with the IMF By penetration mechanism, seven features consistent with the solar wind dynamo mechanism, three features consistent with the conductivity mechanism, and two features could not be explained by any of the three suggested mechanisms. Because two features were consistent with two different mechanisms, the numbers add up to 17 although the total number of features is 15. The analysis also shows the expected correlation between the magnitude of the longitudinal shift of conjugate points, ΔMLT, and the occurrence of non-conjugate aurora consistent with the By mechanism.

[1] We report two instances of nightside subauroral proton precipitation observed during a geomagnetic storm: (1) An arc at 蝶 50° magnetic latitude (MLAT), which extended roughly from midnight to 6 magnetic local time (MLT), and (2) A corotating spot at 51.5° MLAT, postmidnight. The proton precipitation was observed by both the IMAGE SI-12 proton aurora (Doppler-shifted Lyman- α) imager, and low-altitude Polar-orbiting Operational Environmental Satellites (POESs) that serendipitously traversed both the arc and the spot, providing in situ particle measurements. The Lyman-alpha emission and the particle observations match closely. The particle measurements showed that the energies of the precipitating protons extended to several hundred keV, but no enhancements in the protons below 20 keV were observed. Cluster observations showed that the ring current contained protons with energies extending from tens of eV to several hundred keV; thus, the particles are scattered into the atmosphere from an existing reservoir. Outside the luminous regions, but at the same magnetic latitude, the POES satellites observed localized regions with enhanced proton fluxes outside, but close to, the loss cone. This shows that the protons along a large region in MLT are subjected to pitch angle scattering. Model calculations of the plasmasphere show that the arc and the proton precipitation are located just inside the plasmapause. The intensity and longitudinal extension of the arc are modulated by the dynamical pressure in the solar wind. In the dusk sector, ground stations recorded electromagnetic ion cyclotron waves associated with localized proton precipitation just inside the plasmapause. This suggests that the proton precipitation is caused by wave/particle interaction.

Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive y component of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large-scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside. publishedVersion

The position, velocity, and shape of the solid–liquid interface are key parameters for ensuring high crystal quality and good mechanical properties of solidifying metals, alloys, and semiconductors. The traditional techniques for measuring this interface online involve immersing sensors in the molten material, which directly interferes with the solidification process. Significant efforts have thus been made to design noninvasive measurement techniques to locate the growth interface. Their industrial implementation, however, has been challenging. In this paper, we review and discuss techniques that can overcome the encountered problems and hence enable monitoring of the solid–liquid boundary without disturbing the solidification process. These techniques are based on optics, x-ray radiography, ultrasound, and induced eddy currents.

The solar-wind magnetosphere interaction primarily occurs at altitudes where the dipole component of Earth’s magnetic field is dominating. The disturbances that are created in this interaction propagate along magnetic field lines and interact with the ionosphere–thermosphere system. At ionospheric altitudes, the Earth’s field deviates significantly from a dipole. North–South asymmetries in the magnetic field imply that the magnetosphere–ionosphere–thermosphere (M–I–T) coupling is different in the two hemispheres. In this paper we review the primary differences in the magnetic field at polar latitudes, and the consequences that these have for the M–I–T coupling. We focus on two interhemispheric differences which are thought to have the strongest effects: 1) A difference in the offset between magnetic and geographic poles in the Northern and Southern Hemispheres, and 2) differences in the magnetic field strength at magnetically conjugate regions. These asymmetries lead to differences in plasma convection, neutral winds, total electron content, ion outflow, ionospheric currents and auroral precipitation. publishedVersion