Electron density profiles in the auroral ionosphere were measured by the impedance probe on board the S-310JA-11 and -12 rockets. The S-310JA-11 rocket was launched from Syowa Station into quiet auroral arcs (-2kR) on May 29, 1985, while the S-310JA-12 rocket was launched into active arcs (-14kR) on July 12, 1985. The S-310JA-11 data showed a typical altitude profile of the electron number density in an auroral arc under geomagnetically quiet condition, with a peak number density of 2×10
5cm
-3 at 115km altitude. The S-310JA-12 data, which was obtained in bright auroras in the expansion phase of a substorm, showed a peak density of 1.2×10
6cm
-3 at 101km altitude. On the downleg of the S-310JA-12 rocket, strong irregularities in the electron density distribution were observed at
F region height in the active auroral arc region. It is pointed out that the ionospheric structure in the
E region is produced mainly by precipitating particles while that in the
F region seems to be largely affected by dynamical processes.
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