Interhemispheric Asymmetries in Large Magnetic Field Residuals Between Swarm Observations and Earth Magnetic Field Models during Non-storm Times

We investigate large magnetic field residuals between Swarm observations and the IGRF-12 Earth magnetic field model during non-geomagnetic storm times. Large residuals show up in both hemispheres most frequently in the auroral zone but also commonly in the polar cap of the two hemispheres. Continuous large residual occurrences during quiet times mostly concentrate around the geographic poles for the two hemispheres respectively, thus resulting in a significant asymmetry in the large residual locations in magnetic longitude and local time.

Magnetic field residuals are calculated as the difference between Swarm level 1b magnetic field vector data and IGRF-12 modeled magnetic field at the Swarm satellite locations. Residuals of larger than 300 nT between 2014 and 2020 for all Dst index values larger than -50 nT are used for this study. Magnetic field residuals at Swarm satellite positions are transferred from geocentric coordinates to geomagnetic coordinates and divided into geomagnetic northern and southern hemispheres.

Continuous large residuals lasting longer than 5 minutes between Swarm observation and the model are mapped and studied statistically in this study. For these residuals, there is a clear concentration around the geographic poles, which are located around 85° and -75° magnetic latitude for the two hemispheres respectively. This concentration results in additional large residuals in the polar cap region for the two hemispheres with another peak occurrence around 80° and -80° latitude. It results in a significant interhemispheric asymmetry for quiet time large residuals where the northern hemisphere residuals center around 180° magnetic latitude and the southern hemisphere around 0° magnetic latitude with significantly more residuals in the southern hemisphere.