  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.
