Cluster observations of plasma in the high latitude magnetotail
associated with cusp-aligned arcs
Abstract
During periods of northward interplanetary magnetic field (IMF), the
magnetospheric structure and dynamics are dramatically different
compared to the southward IMF case. Previous studies using both
observations and simulations have shown that the magnetotail becomes
dominated by closed magnetic flux and associated trapped particle
populations. In this study, we analyse three intervals of plasma
observed in the high latitude magnetotail during a period of prolonged
northward IMF, coinciding with observations of cusp-aligned arcs in the
polar region. We observe that the plasma is typically continuous with
some substructure observed on length scales of 0.5 - 1.5 R$_E$ which
may be linked to the substructure observed in cusp-aligned arcs. The
plasma characteristics in each of the three intervals studied are
similar. The ion and electron densities are on the order of 0.2 - 1
cm$^{-3}$. The electron energies typically vary between
10$^2$ - 10$^3$ eV. The ion energies are higher compared to
the electron energies and range between 10$^2$ - 10$^4$ eV.
The ion temperatures are on the order of 5 - 18 MK. The speed of the
plasma crossing the Cluster spacecraft is between 4 - 21 km/s which is
on the order of magnitude of convection speeds in the magnetotail plasma
sheet. We tentatively suggest that this may provide supporting evidence
for dual lobe reconnection acting to trap plasma within the
magneotsphere and resulting cusp-aligned arc formation.