Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 508

density is neither too high nor too low. Vickrey et al. (1984) provided evidence
that images can be formed on off-equatorial field lines in contact with a structural
F region during a convective ionospheric storm.

10.3 Small-Scale Waves in the High-Latitude F Region

For this purpose we define small-scale wavelengths to be less than 100m.
Research in this area has not progressed as rapidly as it has in the equatorial
zone. One reason is that the observational data base using radars is not very
large. In the equatorial case the Jicamarca and Altair radars can easily scatter
from irregularities with wave numbers perpendicular to B , since the geometry
is favorable. Since these waves have much larger growth rates than waves with
finite k , they dominate the spectrum. At the high-latitude radar sites, it is geo-

metrically impossible to obtain the appropriate backscatter angle in the F region
using VHF and UHF systems.
F-region radar observations are possible at HF frequencies, since ionospheric
refraction can be used to bend the radar signals and attain a scattering geom-
etry nearly perpendicular to B . Successful measurements of this type were first
made systematically in Scandinavia (Villain et al., 1985) and from a transmitting
station in Goose Bay, Labrador, Canada (Greenwald et al., 1983). The obser-
vation volume for the latter site is located over the Sondre Stromfjord, Green-
land, incoherent scatter radar station. The results show that echoes are indeed
received in the tens of meters wavelength range but a detailed explanation does
not yet exist for the scattering structures. The Super Darn HF radar system
uses these structures and it is very surprising that we do not understand them
better.
Clues concerning the origin of these waves come from the Doppler shift of the
returned HF signal. During one event simultaneous measurements were made
with the incoherent scatter radar at Sondre Stromfjord. The latter can be used
independently to determine the plasma drift velocity projected along the HF
radar line of sight. The two data sets are superimposed in Fig. 10.19 and show
quite good agreement. The implication is that the irregularities are frozen into
the plasma flow—that is, that the phase velocity of the structures is small in the
plasma reference frame.
The long-wavelength instability processes discussed in Section 10.2 do have
this low-phase velocity property. However, it is easy to show that at a wavelength
of 10m, the E

||

× B instability is stable. For example, we may let E 0 ⊥ = 20mV/m,

1m 2 /s, which is on the small side, and solve the equation

L = 10 km, and D

⊥ =

k m D γ = mE 0 ⊥ / LB −

⊥ =

0

for the marginally stable wave number k m . This yields a wavelength λ m = 30m.

A larger diffusion coefficient will only make the value of λ m larger.