Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 461

with two different scale heights. Since the gravitational binding energy of O + is
about 9 eV, heating either the electron or ion gas to even a few eV will not eject
O + into the magnetosphere. In fact, downflow due to a transition to a smaller
ionospheric scale height is also observed.
Surveys of ion outflow below 1000 km have been performed using DE-2
(Loranc et al., 1991) and Hi-Lat (Tsunoda et al., 1989). The Loranc et al.
study focused on the altitude range of 200-800 km altitude. They confirmed
the prenoon cleft/cusp as the largest source of ion fluxes and found upward
fluxes capable of supplying the cleft ion fountain. They also found upward drift
velocities exceeding 250m/s at altitudes above 600 km and smaller upward drift
velocities at lower altitudes. The Tsunoda et al. study examined 76 orbits of
Hi-Lat observations at 800 km altitude and found the mean ion outflow velocity
to be 700m/s. Furthermore, their events were characterized by upward field-
aligned currents; intense, soft electron precipitation; and convection velocity
shears. The DE-2 data were re-examined by Seo et al. (1997) in the altitude
range of 850-950 km to investigate the details of ionospheric plasma param-
eters and soft electron precipitation. They found that the upward ion velocity
correlated best with electron temperature ( r = 0 . 97) and less well with ion tem-

perature ( r 91), where r is the correlation coefficients. They concluded that

soft electron precipitation and the associated electron heating, expansion, and
ambipolar electric field were the most common sources for ion outflow, but that
frictional heating contributes as well.
The third source of hot O + ions is wave-induced ion acceleration associated
with field-aligned currents. This mechanism is easily identified by the existence
of transversely accelerated ions, sometimes referred to as ion conics. The full
analysis involves plasma physics beyond the scope of this text but a few comments
are in order. Figure 9.22 shows rocket data from a flight over the dayside auroral
zone (P. M. Kintner, personal communication, 2002). The rocket was launched
fromNorway, overflew the island of Spitzbergen, and landed in the Arctic Ocean.
Initially the rocket was located on field lines that threaded the plasma sheet and
which are most likely closed deep in the tail of the magnetosphere. This region
is characterized by the 1-5 keV electron fluxes in the upper panel. Abruptly
the payload enters the region of highly structured, intense 0.2-1 keV electrons
and energetic ions that originated in the magnetosheath, the shocked dayside
solar wind that connects directly to the ionosphere in the cusp. In the expanded
plots below, we see intense O + ,H + , and low frequency plasma waves in this
region. Even the O + ions in this region are energetic enough to escape earth's
gravitational pull. Furthermore, these field lines are swept over the polar cap and
the oxygen ions certainly become part of the magnetosphere. This region is very
likely a significant source of magnetospheric ions, many of which are oxygen.
The clear correlation of energized ions and low frequency waves shown in the
lower panel suggests ion energization by waves. Current thinking is that a com-
bination of ion acoustic, lower hybrid, and ion cyclotron waves are responsible
for this energization. The latter two waves, in particular, preferentially accelerate

= 0 .