A Statistical Study of the Distant Magnetotail Under Northward Interplanetary Magnetic Field

Abstract We perform a statistical analysis of the plasma and magnetic field parameters in the magnetotail at −60 observed by ARTEMIS under both northward and southward interplanetary magnetic field (IMF) conditions. We examine changes in the observed plasma content and magnetic structure of the magnetotail under northward IMF and analyze how the parameters vary across the magnetotail in the y ‐direction. We find that in the distant magnetotail the plasma density and temperature are higher under northward IMF compared to southward IMF. The increased plasma pressure results in a 50% decrease in the magnetic pressure under northward IMF compared to southward IMF conditions. Under northward IMF, the median plasma and magnetic pressures are equal suggesting that both the plasma and magnetic pressure make an equal contribution to the outward pressure balance of the magnetotail against the solar wind pressure acting on the tail. We suggest that the increased plasma pressure in the tail acts to maintain the tail‐like structure under northward IMF. We interpret our results as evidence for the plasma sheet expanding to a greater extent in the magnetotail under northward IMF conditions such that a large portion of the magnetotail becomes dominated by trapped plasma on closed flux, supporting the idea that the magnetosphere can become closed or almost entirely closed under northward IMF.