Observation of mixed ion populations deep inside earth magnetosphere as evidence for reconnection during northward IMF with substantial By component Original Research Article

We present and discuss observations of a region in the high-latitude near-Earth outer magnetosphere when over a distance of 9RE – from GSM [0.83RE, 1.6RE, 8.5RE] to [−0.957RE, 10.175RE, 12.738RE] – Interball-1 observed multiple alternative changes in plasma characteristics: hot plasma sheet population is replaced by a region of mixed magnetosheath – plasma sheet population with the presence of ionospheric ions and vice versa. In the ‘mixed’ region, both ion populations were nearly stagnant, their velocity rarely exceeded 50 km/s and plasma-sheet electrons were absent. Most of the time interplanetary magnetic field (IMF) clock angle was less than 90°, rotating to more than 270° at the end of the interval, IMF Bx being positive and dominating. A flux transfer event was registered near the magnetopause. Plasma characteristics suggest that observations took place at the high-latitude boundary of the near-Earth plasma sheet on closed field lines. We suggest that the mixed regions are formed on filed lines first reconnected to the magnetosheath magnetic field, then draped and convected duskward and tailward and secondary reconnected and closed.