A long-period Pi 2 associated with a mini-substorm observed by the Japanese satellite JIKIKEN /EXOS-B/

A very long period Pi2 event, associated with a typical minisubstorm, was observed simultaneously on the ground at Onagawa, Japan, and by the Jikiken satellite at 14-15 h UT on December 11, 1978. The fundamental period of the Pi2 pulsation attained 220 sec, and the range of the substorm recorded at Onagawa on the 23 h LT meridian was only 2 nT. The plasmapause, observed by the satellite on the 21 h LT meridian, was at L greater than 6.3. In the extreme case, the Pi2 event is well explained by a model in which a temporal odd-mode torsional oscillation of the dipole-like field lines anchored in the auroral ovals is observed as Pi2. It is found that the initiation region for the minisubstorm must be X(absolute value) greater than 16 earth radii in the magnetotail, and the tail-like configuration of the field line must be changed to the dipole-like configuration within several minutes.