Cluster Observations of Plasma in the High Latitude Magnetotail Associated With Cusp-Aligned Arcs

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 under northward IMF the magnetotail becomes dominated by closed magnetic flux and associated trapped particle populations. In this study, we analyze 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 observed by all 4 Cluster spacecraft near simultaneously and has some substructure observed on length scales of 0.5–1.5 $R_E$. The plasma characteristics in each of the three intervals studied are similar. The ion and electron densities are on the order of ${10}^{-1}$–${10}^0$ ${\text{cm}}^{-3}$. The electron energies typically vary between ${10}^2$ and ${10}^3$ eV. The ion energies are higher compared to the electron energies and range between ${10}^2$ and ${10}^4$ eV. The ion temperatures are on the order of 5–18 MK. The speed of the plasma crossing the Cluster spacecraft is between 1 and 10 ${\text{kms}}^{-1}$. In one interval Cluster observes distinctly different but adjacent plasma populations. We suggest these plasma populations are on interleaving flux tubes and may present supporting evidence for dual lobe reconnection acting to trap plasma within the magnetosphere and resulting cusp-aligned arc formation.