Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 278

major axis to minor axis being equal to | ω i /ω | , where it must be emphasized

that is the intrinsic frequency. Due to the lidar data, this event is impor-

tant because it is the most complete data set ever obtained. Other examples
of rotating winds exist from TMA rocket releases but simultaneous lidar data
did not exist. This event shows that an 8-hour wave in the earth frame, which
might be considered a gravity wave or a higher-order tide, is actually an inertial
gravity wave.
Turning to the tidal case, complications arise in the analysis because of the
requirement that the solutions satisfy boundary conditions on the spherical earth.
The solutions must exhibit certain altitudinal, latitudinal, and longitudinal forms
referred to as Hough functions. Which propagating tidal normal modes actually
are generated depends on how well the forcing function—for instance, solar
or lunar forcing—matches the radial form of the mode structure. As alluded
to earlier in the Chapter 3 discussion of electrodynamics, the diurnal tide due
to atmospheric heating is important at low latitudes, but the response is small
above 30 ◦ latitude. The semidiurnal forcing is smaller, but the altitude pro-
files of ozone and water vapor content fit the so-called (2,2) semidiurnal tidal
mode quite well. The local heating due to these minor constituents thus couples
well to the semidiurnal tide, explaining its importance at the higher latitudes.
The solar heating function is similar to a half-wave rectifier, since it is on for
only half of a day. This means higher-order Fourier components exist, and thus
higher-order tides are expected. In the layer motions of Fig. 5.23c, some are
separated regularly by about 6 hours. This may be a higher-order tide, but the
analysis of Drummond et al. (2001) shows that an 8-hour period in the earth
frame was actually a 20-hour inertio-gravity wave, not a third-order tide. Thus,
great care must be taken in interpreting long-period waves. Recently a number
of studies have revealed that a wave number three, nonmigrating diurnal tide
(Hagan et al., 2003) creates a four-model perturbation in many ionospheric
parameters, including the equatorial electric (Kil et al., 2007) and magnetic
(England et al., 2006) fields and the intensity of the anomaly plasma density
(Immel et al., 2006).

ω

6.3 Role of Gravity Waves and Tides in Creating Vertical
Ionospheric Structure

Given that gravity waves, inertio-gravity waves, and tides can be generated by a
variety of sources and that they grow to respectable amplitudes by the time they
reach ionospheric heights, we can now investigate their effect on the ionization.
For λ z , the vertical wavelength, much less than the

scale height, the first row in (6.2e) corresponds to

ω much less than ω b and

k y v + k z w = 0 (6.11)