North-south asymmetries in cold plasma density in the magnetotail lobes: Cluster observations

TL;DR

This study analyzes 16 years of Cluster spacecraft data revealing a persistent north-south asymmetry in cold plasma density in Earth's magnetotail lobes, primarily driven by magnetic field differences in hemispheres.

Contribution

It provides the first long-term statistical evidence of persistent plasma density asymmetry in magnetotail lobes and links it to magnetic field configuration differences.

Findings

Higher cold plasma densities in the northern lobe.

External factors like Earth's tilt and magnetic field influence asymmetry.

Persistent asymmetry mainly caused by magnetic field differences.

Abstract

In this paper, we present observations of cold (0-70 eV) plasma density in the magnetotail lobes. The observations and results are based on 16 years of Cluster observation of spacecraft potential measurements converted into local plasma densities. Measurements from all four Cluster spacecraft have been used, and the survey indicates a persistent asymmetry in lobe density, with consistently higher cold plasma densities in the northern lobe. External influences, such as daily and seasonal variations in the Earth's tilt angle, can introduce temporary north-south asymmetries through asymmetric ionization of the two hemispheres. Likewise, external drivers, such as the orientation of the interplanetary magnetic field can set up additional spatial asymmetries in outflow and lobe filling. The persistent asymmetry reported in this paper is also influenced by these external factors but is mainly caused by differences in magnetic field configuration in the Northern and Southern Hemisphere ionospheres.