A tiny fraction of the atoms and molecules of the Earth's upper atmosphere is ionized. Fig 1: Cluster satellites - their polar orbit makes them ideal to address NS asymmetry.This altitude range is what we know as the ionosphere. Ions in the ionosphere can be heated and accelerated by electromagnetic fields. Some of the ions reach escape velocities and escape the Earth's gravitational field and fills the magnetosphere (fig 1). Every year, the Earth's atmosphere looses an estimated 50'000 - 100'000 tons of material - mainly protons and Oxygen - due to this ion outflow.
Solar illumination, in particular in the extreme ultraviolet range is the most significant driver of ionization. Around equinox, the average daily solar illumination in the Northern and Southern hemisphere is equal. One would thus expect that the outflow from the two hemisphere's to be identical. However, a comprehensive study conducted by the Birkeland Center and highlighted in the January issue of Journal of Geophysical Research suggest that this is not the case. In words of the editor: Fig 2: Overview of Cluster coverage (from Haaland et al., 2017)
This paper addresses an interesting physical phenomenon, namely the asymmetry of the ionospheric polar caps in supplying the magnetosphere with ions. The authors were able to isolate other (diurnal and seasonal) effects and also eliminate measurement errors and biases. They were able to perform these tasks due to the large volume of data that covered more than one solar cycle. They suggested (with a large level of certainty) that the difference is due to the different sizes and positions with respect to the geographic poles of the two polar caps, and strength of magnetic field. This should open a new avenue of theoretical and observational investigations.
The investigation used 16 years of data from the Cluster satellites and measured the plasma density in the magnetospheric lobes (see Fig 2) - a large volume of the magnetosphere treaded by magnetic field lines connected to the polar cap ionosphere. The resulting density values were surprising. Around equinox, plasma density in the northern hemisphere lobe was persistently larger than in the southern hemisphere.
The reason for this asymmetry is puzzling, but properties of the ionosphere and the Earth's magnetic field may give a clue to the mystery. In particular, the difference in offset between the magnetic and rotational axis of the Earth leads to differences in properties of the Northern and Southern hemisphere (click this link for pdf page showing NSdifferences - explanation).