Equatorial Plasma Instabilities and Mesospheric Turbulence - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 174

3. The generalized Rayleigh-Taylor instability (0 20 km)

4. Diffusive damping via wave-wave coupling to damped waves (1m

. 1km <λ<

<λ< 100m)

As an event progresses, the free energy released by the RT process is dissipated
via wave-wave coupling, leaving features with 1 km intact in the postmid-

night period. Classical diffusion and plasma production via sunlight eventually
smooth out the ionosphere. Questions such as “Why do storms occur some
nights and not on others?” and “Why are certain seasons preferred at certain
locations?” are still open. The E-region conductivity, the plasma uplift by electric
fields, and the neutral atmospheric density and dynamics all influence the prob-
ability of this phenomenon's occurrence and must enter any predictive theory.
One of the goals of the National Space Weather Program is just such a predictive
capability (Kelley et al., 2006; de La Beaujardiere et al., 2006).
Progress on these topics requires attacking the following issues:

λ>

1. Due to the variable declination of the magnetic field with longitude, the terminator
aligns with the magnetic meridian at some locations depending on the season. In this
case the two E regions conjugate to the equatorial plasma become dark at the same
time, reducing the shorting effect (Tsunoda, 1985).
2. The South Atlantic Anomaly in the earth's magnetic field causes energetic particle
precipitation in that sector, which affects the E-region conductivity and electric field
in that sector. This may explain behavior that is not consistent with item 1 above
(Burke et al., 2004).
3. The four-wave number behavior of the diurnal tide (Hagan and Forbes, 2002) modu-
lates the daytime SQ current system as a function of longitude. In turn, this affects
the development of the equatorial arcs and possibly the RT growth rate (Immel et al.,
2006; Kil et al., 2007).
4. Knowledge of the global electric field and neutral wind is crucial to predicting CEIS
and must come from satellite measurements.

4.6 E-Region Plasma Instabilities: The Observational
Data Base

As discussed in Chapter 3, the equatorial electrojet is part of the worldwide sys-
tem of electric fields and currents driven by the dynamo action of the neutral
wind. The dynamo currents primarily flow in the E region where the conduc-
tivity is greatest, and the current is essentially horizontal except perhaps at high
altitudes and high latitudes. The basic reason for the existence of the equatorial
electrojet is the large value of the Cowling conductivity close to the dip equator.
In the simplest electrojet model the east-west dynamo electric field E x sets up a
vertical polarization field E z , which completely inhibits the vertical Hall current
everywhere. The polarization electric field points upward during the day and has
about the same magnitude but points downward at night. Thus, the drift velocity
of E-region electrons at night is of the same order of magnitude as the daytime
drifts, but the electrical current is much smaller due to the low electron density.