Modulation of Magnetospheric Substorm Frequency: Dipole Tilt and IMF By Effects

Substorm activity is heavily influenced by the Interplanetary Magnetic Field (IMF) Bz component and magnetospheric substorms occur most frequently when Bz is strongly negative. The substorm occurrence rate is also affected by the magnitude of the By component, but it is usually presumed that this contribution is independent of the sign of By. Using five independent substorm onset lists, we show that substorm activity does depend on the sign of By near the solstices. Specifically, we show that substorms occur more frequently when By and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that the magnetosphere exhibits an explicit dependence on the polarity of By for nonzero Ψ, 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 By regardless of Ψ. Last, for the onset list based on identifying negative bays at auroral latitudes, we observe an overall trend of more frequent onsets for positive By, regardless of season. However, substorm frequency in the other four substorm lists does not exhibit an overall preference for positive By. We show that this phenomenon is very likely a consequence of the particular substorm identification method (i.e., identification of negative bays), which is affected by local ionospheric conditions that depend on By and Ψ.publishedVersio

Magnetic Effects of Plasma Pressure Gradients in the Upper F Region

The Swarm satellites fly at altitudes that at polar latitudes are generally assumed to only contain currents that are aligned with the local magnetic field. Therefore, disturbances along the main field direction are mainly signatures of auroral electrojet currents, with a relatively smooth structure due to the distance from the currents. Here we show that superimposed on this smooth signal is an irregular pattern of small perturbations, which are anticorrelated with the plasma density measured by the Langmuir probe. We show that the perturbations can be remarkably well reproduced by assuming they represent a j  × B force, which balances the plasma pressure gradient implied by the density variations. The associated diamagnetic current, previously reported to be most important near the equator, appears to be a ubiquitous phenomenon also at polar latitudes. A spectral analysis indicates that this effect dominates magnetic field intensity variations at small‐scale sizes of a few tens of kilometers.publishedVersio

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

It is often assumed that on average, polar ionospheric electrodynamics in the Northern and Southern Hemispheres are mirror symmetric or antisymmetric with respect to the interplanetary magnetic field By component and the dipole tilt angle ψ. For example, one might assume that the average Birkeland current density j at magnetic latitude λ is equal to the current density at magnetic latitude −λ if the signs of By and ψ are reversed and all other parameters are equal: j(λ, By, ψ, … ) = j(−λ, −By, −ψ, … ). This is a convenient assumption for empirical models, since it effectively doubles the amount of information that a measurement made in one hemisphere contains. In this study we use the Average Magnetic field and Polar current System (AMPS) model to quantify to what extent the assumption holds for Birkeland and ionospheric currents. The AMPS model is an empirical model based on Swarm and CHAMP magnetic field measurements, with no constraints on hemispheric symmetries, and with differences in main magnetic field geometry as well as biases in data point distributions in magnetic coordinates accounted for. We show that when averaged over IMF clock angle orientation, the total ionospheric divergence-free current in each hemisphere largely satisfies the mirror symmetry assumption. The same is true for the total Birkeland current in each hemisphere except during local winter, during which the Northern Hemisphere tends to dominate. We show that this local winter asymmetry is consistent with the average winter hemispheric asymmetry in total precipitating electron current derived from Fast Auroral SnapshoT (FAST) satellite observations. We attribute this and other more subtle deviations from symmetry to differences in sunlight distribution in magnetic coordinates, as well as magnetic field strength and its influence on ionospheric conductivity. Important departures from mirror symmetry also arise for some IMF clock angle orientations, particularly those for which IMF Bz > 0, as suggested by other recent studies.publishedVersio

Auroral oval morphology: Dawn-dusk asymmetry partially induced by Earth’s rotation

Under embargo until: 2023-12-05The auroral oval morphology has been investigated in previous studies presenting maps of average auroral precipitation. However, such distributions tend to emphasize auroral intensity rather than the actual extent of the auroral oval. We develop a statistical method to characterize the auroral oval morphology by using 20 years of electron energy flux measurements from the Defense Meteorological Satellite Program/Special Sensor J (DMSP/SSJ); instead of relying on auroral oval boundaries, we derive the probability of observing aurora from a threshold of 2.109 eV/cm2/s/sr above which the total energy flux of electrons (in the energy range 1–30 keV) is defined as aurora. We then investigate the auroral occurrence probability (AOP) in the magnetic latitude-magnetic local time (MLat-MLT) sectors covered by DMSP for various conditions related to geomagnetic activity. Regardless of those conditions, the AOP distributions reveal a width asymmetry with a wider dawn-to-noon sector (06–12 MLT) compared to the dusk-to-midnight sector (18–24 MLT), the dawn preference getting even more pronounced as the geomagnetic activity decreases. In the context of an open magnetosphere, we investigate the relation between the observed extent asymmetry in the auroral oval and the magnetospheric plasma convection. Representing the plasma sheet magnetic flux as a one-dimensional fluid subject to production on the nightside (closing of flux via reconnection) and loss on the dayside (opening of flux), we highlight similarities with the AOP in terms of MLT asymmetries. Finally, making use of this fluid model, we demonstrate that the corotation influence on the plasma convection pattern is consistent with the dawn-dusk asymmetry observed in the AOP distributions.publishedVersio

Transient high latitude geomagnetic response to rapid increases in solar wind dynamic pressure

Rapid changes in solar wind dynamic pressure can produce a transient geomagnetic response in the high latitude ionosphere. In this study we carry out a superposed epoch analysis of the geomagnetic response based on 2,058 events. The events are divided into 12 groups based on interplanetary magnetic field clock angle and dipole tilt and the magnetic perturbation field is modeled using spherical harmonics. We find that the high latitude transient current vortices associated with a sudden commencement are most clearly observed when the interplanetary magnetic field is northward during equinox and winter in the northern hemisphere. The high latitude geomagnetic response during northward interplanetary magnetic field is decomposed into a preliminary and main impulse. The preliminary impulse onset is 1–2 min prior to the onset of the low/mid latitude geomagnetic response and its rise time is 4–6 min. The main impulse onset is around 2 min after the low/mid latitude geomagnetic response and has a rise time of 6–11 min. When examining the change relative to pre-onset conditions a coherent transient geomagnetic response emerges for all IMF clock and dipole tilt angles. The current vortex associated with the main impulse on the dawnside appears at (9.3 ± 0.5 mlt, 64.8° ± 1.5° mlat) and moves westward with a velocity of 5 ± 1.4 km/s. The vortex on the duskside appears at (15.3 ± 0.9 mlt, 65.8° ± 2.5° mlat) and does not move significantly. In addition, the models were used to recreate the SMR index showing a significant mlt dependence on the magnetic perturbation above 40° mlat and below 10° mlat. The former is thought to be caused by high latitude ionospheric currents. The latter is potentially a combination of the event occurrence probability being skewed toward certain UT ranges for large dipole tilt angles and a UT dependence of the equatorial electrojet magnitude caused by the south atlantic magnetic anomaly.publishedVersio

The relationship between interhemispheric asymmetries in polar ionospheric convection and the magnetic field line footpoint displacement field

Polar electrodynamics is largely controlled by solar wind and magnetospheric forcing. Different conditions can make plasma convection and magnetic field disturbances asymmetric between hemispheres. So far, these asymmetries have been studied in isolation. We present an explanation of how they are linked via displacements of magnetic field line footpoints between hemispheres, under the assumption of ideal magnetohydrodynamics. This displacement has so far been studied only on a point by point basis; here we generalize the concept to a 2D displacement vector field. We estimate displacement fields from average patterns of ionospheric convection using the Weimer et al. (J. Geophys. Res., 2005a, 110, A05306) model. These estimates confirm that the influence of the interplanetary magnetic field extends deep into the magnetosphere, as predicted by models and in-situ observations. Contrary to predictions, the displacement associated with dipole tilt appears uniform across the nightside, and it exceeds the effect of IMF By. While more research is needed to confirm these specific findings, our results demonstrate how ionospheric observations can be used to infer magnetospheric morphology, and that the displacement field is a critical component for understanding geospace as a coupled two-hemisphere system.publishedVersio

Possible Ionospheric Influence on Substorm Onset Location

Auroral substorm onset locations are highly unpredictable. Previous studies have shown that the By component of the interplanetary magnetic field (IMF) explains ∼5% of the variation in onset magnetic local time (MLT), while solar wind conditions and the other IMF components have even less explanatory power. In this study, we show that the level of geomagnetic activity before substorm onset, as indicated by the AL index, explains an additional ∼5% of the variation in onset MLT. We discuss our results with regard to recent modeling studies, which show that gradients in the ionospheric Hall conductance can lead to a duskward shift of the magnetotail dynamics. Our findings suggest that this magnetosphere-ionosphere coupling effect may also influence the location of substorm onsets.publishedVersio

Geomagnetic Response to Rapid Increases in Solar Wind Dynamic Pressure: Event Detection and Large Scale Response

Discontinuities in the solar wind trigger a variety of processes in the magnetosphere-ionosphere system. A rapid increase in solar wind dynamic pressure causes compression of the magnetosphere. This manifests itself as a positive perturbation of the horizontal ground magnetic field at low/mid latitudes. In this study we present a method for detecting these discontinuities in situ solar wind data by using the random forest machine learning algorithm. Each detected event is propagated to Earth and its arrival time is aligned with a corresponding response in the low latitude ground magnetic field. A list of 3,867 events, detected between 1994 and 2019, is presented. We use the list in a superposed epoch analysis of the low/mid latitude response in the ground magnetic field at different local times, and of the high latitude response using the Polar Cap index. A dawn-dusk asymmetry is found at low/mid latitudes with weaker positive perturbations at dawn compared to any other local time sector. This suggests a stronger ring current contribution at dawn assuming the magnetopause contribution to be uniform. During northward IMF the initial response is asymmetric, but returns to symmetry after 30 min. During southward IMF the low/mid latitude response decays rapidly in all local sectors except dawn. After around 30 min the asymmetry has flipped such that the strongest positive perturbation is at dawn. This suggests an amplification of the partial ring current. In addition, a noon-midnight asymmetry is observed during southward IMF with the strongest positive perturbation on the night side suggesting a significant contribution from dipolarization of the geomagnetic field in the near tail. The complex geomagnetic response to rapid increases in solar wind dynamic pressure demonstrates a need for further statistical analyses. Event lists, such as the one presented here, are critical components in such studies.publishedVersio

Evolution of IMF By Induced Asymmetries: The Role of Tail Reconnection

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.publishedVersio

Volumetric Reconstruction of Ionospheric Electric Currents From Tri-Static Incoherent Scatter Radar Measurements

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