Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry

Abstract

[1] The nondipolar portions of the Earth's main magnetic field constitute substantial differences between the two hemispheres. Beside the magnetic flux densities and patterns being different in the Northern Hemisphere (NH) and Southern Hemisphere (SH), also the offset between the invariant magnetic and the geographic poles is larger in the SH than in the NH. We investigated the effects of this magnetic field asymmetry on the high-latitude thermosphere and ionosphere using global numerical simulations and compared our results with recent observations. While the effects on the high-latitude plasma convection are small, the consequences for the neutral wind circulation are substantial. The cross-polar neutral wind and ion drift velocities are generally larger in the NH than the SH, and the hemispheric difference shows a semidiurnal variation. The neutral wind vorticity is likewise larger in the NH than in the SH, with the difference probably becoming larger for higher solar activity. In contrast, the spatial variance of the neutral wind is considerably larger in the SH polar region, with the hemispheric difference showing a strong semidiurnal variation. Its phase is similar to the phase of the semidiurnal variation of the hemispheric magnitude differences. Hemispheric differences in ion drift and neutral wind magnitude are most likely caused partly by the larger magnetic flux densities in the near-polar regions of the SH and partly by the larger offset between the invariant and geographic pole in the SH, while differences in spatial variance are probably just caused by the latter. We conclude that the asymmetry of the magnetic field, both in strength and in orientation, establishes substantial hemispheric differences in the neutral wind and plasma drift in the high-latitude upper atmosphere, which can help to explain observed hemispheric differences found with the Cluster/Electron Drift Instrument (EDI) and the Challenging Minisatellite Payload (CHAMP).Citation: Förster, M., and I. Cnossen (2013), Upper atmosphere differences between northern and southern high latitudes: The role of magnetic field asymmetry,

North–South Asymmetries in Earth’s Magnetic Field

“…The interhemispheric difference in convection speed is smallest just after 00 and 12 UT, in good agreement with the modeling results by Förster and Cnossen (2013). These calculations show that even if the flux transport is the same in the two hemispheres and at all UTs, there will be a diurnal variation and a hemispheric asymmetry in convection velocities as seen in geographic coordinates.…”

Universal time effect in the response of the thermosphere to electric field changes

“…This aligns the same universal times with the other plots for better comparison. Other studies have shown that the neutral wind response follows a similar response in each hemisphere during these times [Förster and Cnossen, 2013]. To investigate this, the pure dipole case was rerun with MSIS tides removed as well as a 0 ∘ tilt specified for the potential model.…”

“…Longitudinal variations in the strength of Earth's magnetic field strength can also contribute to UT variations [Förster and Cnossen, 2013;. The magnetic field magnitudes are shown for each hemisphere in Figure 1.…”

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

“…We do this by centring the circular polar caps around the invariant magnetic poles (e.g. Emmert et al, 2010;Förster and Cnossen, 2013) instead of the geomagnetic poles. These are located at 82…”