COLD-PLASMA SOURCE OF UPFLOWING IONOSPHERIC IONS IN THE NIGHTSIDE AURORAL IONOSPHERE

The frequent occurrence of energetic O+ upflowing ionospheric ions (UF I) at high altitude during auroral substorms raises the question of th e cold plasma source and acceleration altitude for the O+ ions, and th eir possible effects on the substorm-time plasma sheet. Ion compositio n observations on Akebono in the nightside auroral ionosphere reveal t he significant presence of thermal-energy (a few eV) O+ ions in the 60 00-10,000 km altitude region both during and between auroral substorms . Their upward flux normalized to 2000 km altitude is about 2 x 10(8) cm(-2) s(-1). They are believed to be a significant source of cold pla sma for the energetic UFI. During auroral substorms, Akebono occasiona lly observes molecular (N-2(+) and NO+) upflowing ions up to similar t o 60 eV, in or near regions of auroral electron precipitation up to 10 ,000 km altitude. This suggests the occurrence of a fast ion accelerat ion process in the F-region or topside ionosphere, where freshly creat ed molecular N-2(+) and NO+ ions are accelerated to several eV or grea ter within their dissociative recombination lifetime (similar to a few minutes). Ground photometric observations and simultaneous particle m easurements on sounding rockets confirm the presence of transversely a ccelerated ions (TAI) up to similar to 200 eV in the topside ionospher e (near 600 km altitude) within tens of seconds of substorm expansion onset. Such hundred-eV TAI are frequently observed on Akebono in latit udinally confined regions of the nightside auroral ionosphere down to similar to 2000 km altitude. They can reach the so-called ''parallel a cceleration region'' at similar to 1-2 R(E) altitude within a few minu tes, where they are often accelerated further to keV energy and can ty pically reach the plasma sheet during a substorm. Their flux is an ord er of magnitude smaller than the thermal ion flux. They are believed t o be a minor source of plasma for the energetic UFI. In contrast, the lower-energy (less than or equal to 10-eV) O+ TAI are typically too sl ow to reach the parallel acceleration region during the substorm, as t hey are decelerated by gravitation or trapped by it and traverse repea tedly along the magnetic field line. Hence they constitute a possible source of quiet-time thermal ions in the parallel acceleration region. Another possible source of thermal ions in the region is the polar wi nd O+ ions convected anti-sunward along the auroral oval from the dawn and dusk sectors. The thermal O+ ions in the parallel acceleration re gion at similar to 1-2 R(E) altitude of the nightside auroral ionosphe re are believed to be the dominant source of cold plasma for energetic UFI at high altitude; they can reach the plasma sheet during a substo rm, thereby modifying its composition significantly.