Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 301

which takes into account the shorting effect of a conducting background E or F
region and the finite size of the cloud, since k 0 = L . Considerable effort has

gone into the barium cloud striation problem, and a vast literature exists. Much
of this research is applicable to naturally occurring spread F phenomena as well.
Some subtleties that arise involve the effect of velocity shear on the instability,
which is a stabilizing factor (Perkins and Doles, 1975), and the production of
image striations in the background ionosphere (Goldman et al., 1976). Both of
these processes introduce additional scale size factors into the linear growth rate
expression. These processes are discussed further in Chapter 10.

2 π/

6.6.2 Local Midlatitude F-Region Plasma Instabilities: A New Process
A few early experimental results indicated that something new was happening
at midlatitudes. Most notable were data reported by Behnke (1979) using the
Arecibo radar. He found what he called height-layer bands aligned from NW
to SE and traveling in the southwest direction. Adjacent bands sometimes were
displaced from each other by 80 km in altitude. One structure was found to
have an internal electric field five times the average background electric field and
corresponding to drifts exceeding 400m/s.
The next breakthrough was from the MU radar, which reported regular
patches of 3m irregularities moving across the various beams (Fukao et al., 1990;
Kelley and Fukao, 1991). An example of such data is presented in Fig. 6.22.
Three patches of high-echo strength are shown in the top panels, and the corre-
sponding line-of-sight Doppler shifts are plotted in the lower panels. Large away
drifts accompany the strong echoes, and at the edges, as the signals disappear,
they weaken and even downward irregularity motions occur. The patches moved
westward from beam to beam.
Airglow observations at 630 nm have greatly clarified the situation (Mendillo
et al., 1997; Garcia et al., 2000a; Saito et al., 2001). Figure 6.23 shows five
midlatitude examples over Puerto Rico and Hawaii and contrasts them with
an image taken from Christmas Island during equatorial CEIS conditions. The
latter features are very elongated and aligned with the magnetic meridian. The
midlatitude structures are more localized and make a large angle with the mag-
netic meridian. Another key difference is the motion of these structures; their
phase fronts almost invariably move in the southwest direction, as illustrated
in Fig. 6.24. Since the process is not yet understood for mesoscale structures,
next we will consider the only existing theoretical development before present-
ing more observations to provide a context for interpretation.
Figure 6.25 shows the plasma density and plasma drift/electric field observa-
tions associated with two uplift events over Arecibo. From 2215 to 2345 LT and
again from 0100 to 0215 the ionosphere was elevated by 100 km altitude rela-
tive to adjacent time periods, and the associated electric field was highly unusual.
A perpendicular southward perturbation drift was accompanied by a perpendic-
ular east drift (e.g., at 2340 AST). The former was related to the decrease in