Instabilities and Structure in the High-Latitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 474

In the storm-time trough the electric field-enhanced recombination effect
discussed in Chapter 9 then acts to help create a deep decrease in the electron
density in this sector. An example of such an event is shown in the upper panel of
Fig. 10.3b, where the plasma density and the meridional electric field are plotted
from data taken during an azimuth scan of the Millstone Hill radar (Providakes
et al., 1989). This event occurred during a large solar minimum magnetic storm
in which auroras were seen in the Washington, D.C. area of the United States.
There is a clear anticorrelation between the observed plasma density and the flow
velocity that is at least in part due to the velocity-dependent recombination rate.
The ion and electron temperatures plotted in the lower panel are also elevated
in the trough region. This deep trough is in some sense the ionospheric image of
the interface region that bounds two quite different plasma populations in the
magnetosphere: the cool, dense, corotating plasmasphere and the hot, tenuous,
rapidly moving plasma sheet. It is not surprising, then, that a number of inter-
esting phenomena occur at this location. The extremely large localized electric
fields mentioned previously are one such phenomenon, and it is interesting to
note that the potential difference across the region is the order of the temperature
difference between the plasmasphere and the plasma sheet. A complete discus-
sion of this interesting region is beyond the scope of this text, but we can touch
on a few topics of particular interest.
10.1.3 Longitudinal Structures Due to Localized Sub-Auroral
Electric Fields
The ionospheric region just poleward of the trough is in contact with the plasma
sheet and is rendered visible by the widespread particle precipitation that causes
the atmospheric emissions referred to as the diffuse aurora. At times the equa-
torward edge of the diffuse aurora is structured in very interesting patterns
(Lui et al., 1982; Kelley, 1986). Two photographs taken in visible light from
the DMSP satellite (800 km altitude) that illustrate this effect are shown in
Fig. 10.4a. Note that the equatorial edge of the diffuse aurora is scalloped
in a highly nonlinear fashion. Typical observed wavelengths of this feature
range from 200 to 800 km. This seems to be a clear example of ionospheric
F-region structuring due to a magnetospheric process. The argument is as fol-
lows. Since the plasma sheet is the source of diffuse auroral precipitation, if the
inner edge of the plasma sheet is distorted the light emissions and the produc-
tion of plasma (see Section 10.1.3) in the ionosphere will mirror that distortion.
Consequently, the F-region plasma will take on a horizontal structure with the
same scale size. These relationships have been verified by simultaneous observa-
tions of the undulations by radar and optical means (Providakes et al., 1989).
This longitudinal structure will be superimposed on the latitudinal structure of
the trough itself. There is evidence that the undulations on the equatorward
edge of the auroral oval seen in high-altitude photographs (e.g., Fig. 10.4) occur
at the time when the large localized electric fields exist and that the latter drive