Waves and Instabilities at Midlatitudes - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 289

disturbance. A single vertically pointed ionosonde would obtain echoes from
several ranges due to these tilts—hence the name range spread F. Such plasma
observations of TIDs were part of the data set that led Hines to develop the
neutral wave theory described in Section 6.2. One must remember, though,
that the neutral gas is not measured directly in these and in other TID obser-
vations. Figure 6.13b shows that longer wavelength features do not create the
same sort of spread in the trace. However, Fig. 6.13c shows that the associ-
ated long-period features can be detected by ionosondes using time variation of
plasma contours. Here, 1.5-hour periodicity is found, which has the characteris-
tic downward phase progression associated with gravity waves. Bowman (1981,
1985) used the term “large” to include the 200 km scale feature in Fig. 6.13c.
Here we reserve that term for structures greater than 500 km and use the term
medium scale or mesoscale for features in the range of 50 to 500 km.
Large-scale TIDs are usually very fast and come equatorward out of the auroral
zone. A computer simulation of a neutral atmospheric disturbance launched by
a high-latitude heating event was presented in Section 3.5. It is clear that such
a pulse would drive the midlatitude ionosphere upward due to the southward
wind and that this uplift would seem to travel equatorward. Less obvious is the
fact that a temperature enhancement also changes the composition of the lower
thermosphere, decreasing the O/N 2 ratio. Such a composition effect changes
the production and recombination rates in the F region in such a way that a
negative midlatitude ionospheric storm often accompanies a geomagnetic storm
(see Fig. 5.1).
An example of the negative phase of an ionospheric storm is shown in
Fig. 6.14a, in which five consecutive nights of Arecibo plasma density profiles
are shown during a modest magnetic storm in September 1999. Nights 1, 2, and
4 in this sequence had depressed nighttime densities relative to the climatologi-
cal prediction shown in the final panel. These nights are consistent with the low
values of f 0 F 2 in the storm-related time series presented in Fig. 5.1. Examples of
descending (intermediate) layers and sporadic E layers are also evident, as are
height variations of the F layer with several-hour periodicities. For gravity waves
with k vectors near the meridian, the alternating poleward and equatorward per-
turbation wind will create corresponding downward and upward F-layer plasma
motions. These will be detected as TIDs. In addition, when the plasma is pushed
downward, it will recombine more quickly, enhancing the ionospheric storm
effect.
An event of this type is presented in Fig. 6.14b, in which extreme oscillations
of the F layer occurred over Arecibo due to a large-scale TID. When the layer
reached its highest altitude just before sunrise, the bottomside of the layer clearly
went unstable, most likely up to the Rayleigh-Taylor instability (Nicolls and
Kelley, 2005). Analysis of the corrugation of the layer all evening showed that at
an earth-fixed frequency of 30minutes the corrugations grew exponentially and
broke into the striations seen in Fig. 6.14b. This is the only example we know of