The structure of the distant geomagnetic tail during long periods of northward IMF

We have used a newly developed, parallelized, global MHD magnetosphere‐ionosphere simulation model with a 400 R E long tail to study the evolution, structure, and dynamics of the distant magnetotail during extended periods of northward IMF. We find that the tail evolves to a nearly time stationary structure about one solar wind transit time after the IMF turns northward. Four regions of different magnetic topology can be distinguished which extend at least to the end of the simulation box at 400 R E . Besides lobe field lines and open solar wind field lines tailward of an X‐line, there is a broad boundary layer of closed field lines which we call the tail flank boundary layer (TFBL). Just inside the TFBL there is a region of closed field loops. Besides the X‐line we find two O‐lines which are enclosed by the closed field loops and are roughly aligned with the tail axis. Together they form a U shaped separator between the northward and the southward plasma sheet fields.