Geoscience Reference
In-Depth Information
two-dimensional, since electric currents can easily flow down the magnetic field
lines if there is a conducting path through the E region. The physics is identical
to that discussed in Chapter 3, which showed that the F-region dynamo was sup-
pressed during the day, when the low-altitude conductivity dominates. During
the daytime, any perturbation electric fields due to the instability are shorted out
in the E region, which explains why spread F does not begin until well after sun-
set, even though
g
and
n
are antiparallel on the bottomside during the day as
well as at night. A full explanation of the morphology of CEIS thus must include
the diurnal, seasonal, and solar cycle effects on the electric field, on the neutral
density, temperature, and wind patterns, and on conductivities of the E and F
regions. Developments in the ability to create realistic models of the equatorial
ionosphere are occurring rapidly and many of the effects mentioned above now
can be included (de La Beaujardiére et al., 2006).
Longitudinal differences occur due to the offset of the magnetic and geographic
equators, the possible influences of orographic features, and the varying declina-
tion of the magnetic field. Fejer et al. (1999) suggest that all of these factors influ-
ence the prereversal enhancement electric field. This quantity, in turn, controls
the height and tilt of the ionosphere and, thus, CEIS. We showed in Chapter 3
that the zonal electric field measured over Peru as a function of local time cannot
be extrapolated to represent the zonal electric field as a function of longitude for
a fixed local time. Thus, to ever predict CEIS, we need data-assimilative models
that include information on the worldwide zonal electric field, the thermospheric
wind patterns, and the conductivity of the E region.
The zonal electric field and neutral winds also influence the equatorial ioni-
zation anomaly (discussed in Chapter 5), which develops well before the sunset
and thus possibly can be used to predict the onset of CEIS. Based on ionospheric
sounding studies over the Indian zone, an empirical correlation between the
occurrence of CEIS and the strength (ratio) between the electron densities at
the crest and trough region of the equatorial ionization anomaly was found by
Raghavarao et al. (1988). This same concept was followed by Sridharan et al.
(1994) using a daytime optical technique to get a real time prediction. Along
similar lines, Alex et al. (1989) reported a correlation between the occurrence
of CEIS and the latitudinal gradient in electron densities at a fixed altitude over
American longitudes during postsunset hours. Recent multi-instrumented studies
(Mendillo et al., 2001) also suggested that a good precursor for premidnight
CEIS is the late-afternoon strength of the equatorial ionization anomaly. Further
investigations over various regions and seasons are required to consolidate this
notion.
∇
4.2.3 The Seeding of Convective Ionospheric Storms by Gravity Waves
In addition to the factors just mentioned, there is very likely an additional random
factor in the occurrence of CEIS that may hinder attempts to develop predictive
Search WWH ::
Custom Search