Substorm Associated Magnetotail Field and Plasma Fluctuations: a Case Study

A sudden dipolarization followed by long period ( 25 min) magnetic compression-like fluctuations with shorter period waves superimposed on them was observed by the WIND spacecraft in the magnetotail near midnight at 15RE from the earth on Sep 17, 1995. The spacecraft was near the boundary between the northern tail lobe and the plasmasheet. Observations of anticorrelation between the variations of the magnetic field and of the energetic electrons are consistent with the signature of alternating plasma sheet thinning and expanding. Ground data from stations located near the footprints of the magnetic field lines crossed by the spacecraft during the event showed negative bays and Pi2 pulsations occurring near local midnight, which indicate substorm activity. Simultaneous observations from Geotail, which was near local noon at 20RE upstream from the Earth, showed similar fluctuations in IMF Bz . It is suggested that the periodic southward-northward turning of the IMF may have caused the alternating plasma sheet thinning and expanding and the compressional fluctuations, which provide the energy released during the substorm.