Configuration of the outer cusp after an IMF rotation Original Research Article

We present the results of a two-point study of the topology and dynamics of high-altitude cusp regions under changing interplanetary magnetic field (IMF) direction. The study uses INTERBALL-1 and MAGION-4 measurements on April 2, 1996. An analysis of the MAGION-4 ion energy spectra indicates the presence of magneto sheath-like plasma well below the expected magnetopause position and a large region occupied by low-temperature dense plasma with the very low bulk velocity near the magnetopause. The examinations of two-point observations of the cusp-magnetosheath transition have confirmed our knowledge of the structure of a plasma entry region during stable IMF. Our study reveals a complicated topology of the magnetic field during cusp reformation from an initial to a final quasisteady state. We have found that such a reformation can take 10 – 15 minutes.