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Abstract We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF Bin NH (Bin SH) in most local seasons under both signs of IMF B. For Bin NH (Bin SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During Band Bin NH (Band Bin SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.180.09 in winter and 1.170.09 in autumn. During Band Bin NH (Band Bin SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.160.07 for FAC. We also find an explicit Beffect on auroral currents in a given hemisphere: on average Bin NH and Bin SH causes larger currents than vice versa. The explicit Beffect on divergence‐free current during IMF Bis in very good agreement with the Beffect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer.
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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 B y 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 B y and ψ are reversed and all other parameters are equal: j ( λ , B y , ψ , … ) = j (− λ , − B y , − 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 B z > 0, as suggested by other recent studies.
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- Award ID(s):
- 2002574
- PAR ID:
- 10423056
- Date Published:
- Journal Name:
- Frontiers in Astronomy and Space Sciences
- Volume:
- 9
- ISSN:
- 2296-987X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fspas.2022.958977
