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

Abstract

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.

Auroral Oval Morphology: Dawn‐Dusk Asymmetry Partially Induced by Earth's Rotation

“…We now compare the distributions of AOP under different polarities of IMF B y . Since we combine both hemispheres in our analysis, and since the y component of the IMF is known to affect differently the two hemispheres in terms of ionospheric electrodynamics depending on its sign (Hatch et al, 2022), here we have flipped the sign of B y in the Southern Hemisphere such that the comparison between the two IMF polarities is valid. From left to right the threshold values are respectively 10 8 eV/cm 2 /s/sr (light gray), 10 9 eV/cm 2 /s/sr (gray), and 10 10 eV/cm 2 /s/sr (black).…”

Magnetic Field Observations on Interhemispheric Conjugate Chains

“…The validity of this assumption of mirror symmetry under these conditions has been verified by Hatch et al. (2022). Shown superposed on these maps are the locations of the Greenland and Antarctic magnetometer chains using blue and red lines respectively.…”

“…The Southern Hemisphere maps use IMF B Y values in the model inputs that have their signs flipped from the values used in north, and the dipole tilt angle is also reversed. The validity of this assumption of mirror symmetry under these conditions has been verified by Hatch et al (2022). Shown superposed on these maps are the locations of the Greenland and Antarctic magnetometer chains using blue and red lines respectively.…”

Occurrence Probability of Magnetic Field Disturbances Measured With Swarm: Mapping the Dynamic Magnetosphere‐Ionosphere Coupling

“… MLat‐MLT distributions of low‐frequency [0.1–1 Hz] distributed magnetic field occurrence probability (dBOP) and magnetic local time profiles of 1D dBOP for different interplanetary magnetic field B y orientations, for (a) Northern and (b) Southern Hemispheres. For inter‐hemispheric comparison, B y positive (negative) in the Northern Hemisphere is often assumed to correspond to B y negative (positive) in the Southern Hemisphere (Hatch et al., 2022). …”