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

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

Evolution of IMF By induced asymmetries during substorms: Superposed epoch analysis at geosynchronous orbit

The By component of the magnetic field inside the magnetosphere is positively correlated with the By component of the Interplanetary Magnetic Field (IMF). This leads to asymmetries in aurora, plasma convection and electric currents between the northern and southern hemispheres It has been demonstrated that magnetic conjugate locations in the northern and southern ionosphere become less displaced during magnetospheric substorms, which are associated with enhanced reconnection in the near-Earth tail. Here we directly address how the average By component in the magnetotail evolves relative to substorm onset by performing a superposed epoch analysis of the magnetic field observed at nightside geosynchronous orbit during periods with dominant IMF By. The observations demonstrate that the average |By| in the magnetotail increases during the loading phase prior to onset. |By| maximizes in the expansion phase and is subsequently reduced during the remaining unloading phase. The observed trends become more pronounced using substorm onset lists that on average identify stronger substorms. Since dayside reconnection dominates over tail reconnection during the loading phase, whereas tail reconnection dominates during the unloading phase, the results demonstrate how asymmetries build up during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection in agreement with previous case studies of conjugate auroral substorm features.publishedVersio

The Magnitude of IMF By Influences the Magnetotail Response to Solar Wind Forcing

Under embargo until: 2022-04-21The dynamics of substorms are known to be dominated by the North-South (Bz) component of the Interplanetary Magnetic Field (IMF), which is the most important driver of the dayside reconnection. Even though the dawn-dusk (By) component is also known to play a role in substorm dynamics, its effects are not yet fully understood. In this paper we study how IMF By modulates the onset latitude, strength and occurrence frequency of substorms as well as the isotropic boundary (IB) latitude of energetic protons. We show that the substorm onset latitude and the IB latitude are about one degree lower for large magnitude By (>|By>|>3 nT) than for small By. In contrast, the substorm occurrence frequency is larger for small >|By>|. We suggest that the magnetotail is more stable during large >|By>|, requiring the magnetotail lobes (and hence the polar cap) to contain more flux to initiate a substorm compared to the situation when By is small.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

An explicit IMF By dependence on solar wind ‐ magnetosphere coupling

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 (B) of the Interplanetary Magnetic Field (IMF). In this paper we present observations strongly suggesting an explicit IMF B effect on the solar wind-magnetosphere coupling. When the Earth's dipole is tilted in the direction corresponding to northern winter, positive IMF B is found to on average lead to a larger polar cap than when IMF B is negative during otherwise similar conditions. This explicit IMF B 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 B to likely be a result of differences in the dayside reconnection rate.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

Substorm Impact on Dayside Ionospheric Currents

Ionospheric dayside dynamics is strongly controlled by the interaction between the Interplanetary Magnetic Field (IMF) and the Earth's magnetic field near the dayside magnetopause, while nightside ionospheric dynamics depends mainly on magnetotail activity. However, we know little about the influence of magnetotail activity on the dayside ionospheric dynamics. We investigate this by performing superposed epoch analyses of ground magnetic field data for substorms occurring during northward IMF. In such substorms, dayside reconnection is minimized, allowing us to separate the effects of the magnetotail activity on the dayside current system. We find that as nightside activity elevates, the dayside ionospheric current elevates. Our analyses indicate that the lobe cells are less distinct before onset than during non-substorm northward IMF conditions. They become more pronounced after onset, possibly due to magnetospheric reconfiguration or a remote effect of the nightside current. We discuss possible mechanisms that may explain our observations.publishedVersio