Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 514

well between 90 and 95 km. The lower panel is the sodium atom density mea-
sured using sodium lidar and is clearly related to the plasma layer. The most
likely explanation for such effects is that ionized sodium recombines to yield the
atom layer (Cox and Plane, 1998). There seems to be little evidence that auroral
particles release sodium from a dust layer (Heinselman et al., 1998), as has been
suggested (von Zahn et al., 1987). For example, no precipitation occurred but
a strong Na layer developed. The magnetic field is almost vertical at high lati-
tudes, so the midlatitude wind-shear mechanism is not very effective in gathering
the plasma. The small off-vertical angle, however, is very important because of
large-magnitude perpendicular electric fields. For example, a southward electric
field will cause a small Pedersen drift downward, which is a strong function of
altitude. As the particles drift down perpendicular to B , their velocity decreases
due to the increasing collision frequency. As they slow down, a layer develops
with time. The abrupt onset in Fig. 10.23 could be due to advection of the plasma
cloud into the view of the radar.
In the polar cap neither of the mechanisms involving horizontal electric fields
and/or neutral winds will work because the vertical velocity is proportional to
cosine of a dip angle, which becomes 0.034 for the dip of 88 ◦ compared to
0.17 for the dip angle of 80 ◦ . Based on observations made with the Canadian
Advanced Digital Ionosonde (CADI) at three stations in the north polar cap
(Alert, Eureka, and Resolute Bay), MacDougall et al. (2000a, b) distinguish two
types of ionization layers: height-spread (maximum occurrence during 20:00
LT in winter) and thin (maximum occurrence near 12:00 LT in summer). They
proposed that the height-spread Es is caused by transport of metallic ions from
the dayside E region to the central polar cap lower F region. The metallic ions
then sediment into an exponentially increasing atmosphere and form a layer.
They may also be converged by gravity waves. The gravity wave signature is
clearly seen for many high-latitude observations, including the one in Fig. 10.23
for Sondrestrom (dip is 80 ◦ ). MacDougall et al. (2000a, b) observed similar Es
undulations at Eureka and Resolute Bay on January 1, 1995. Enhancements in
E-region ionization were accompanied by plasma depletions in the F region (see
their Fig. 1), again suggesting some sedimentation. The thin Es that is observed
only in summer is produced by photoionization followed by charge exchange
with neutral metallic atoms. MacDougall et al. (2000a, b) found that both winter
and summer Es types were associated with positive IMF By. Why this is so is
yet not clear. One explanation could be in the enhanced gravity wave activity
during positive IMF By.
10.5 Plasma Waves and Irregularities in the High-Latitude
E Region: Observations
Due to the high neutral density at E-region heights, plasma production, dif-
fusion, and recombination all proceed very rapidly. The large-scale horizontal