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The radar signal is Doppler-shifted due to any irregularity drift in the line-of-
sight direction. The central panel in Fig. 4.2 is coded to show this with green to
red, denoting ascent. Indeed, the central plume region is moving rapidly upward.
An interferometric data analysis method has been developed at Jicamarca to
reveal east-west motion. The lower panel in Fig. 4.2 is coded in such a way
that eastward and westward velocities can be distinguished. As discussed in
Chapter 3, by sunset the zonal F-region drift has long since reversed from west-
ward to eastward, easily explaining the westward irregularity motions. But
the initial thin echoing region is traveling westward, creating a shear in the
plasma flow (Hysell and Burcham, 1998; see Chapter 3). These westward drifting
irregularities are termed “bottom-type” irregularities. The thickening of the
layer after 2040 (discounting plumes) occurs in the region of F-region dynamo
control, since the drift is eastward. Such extended regions are called bottom-
side irregularities. Plumes usually form just once on a given night, if at all,
and are sometimes called “apogee” plumes when they rise out of a bottom-
side layer that has reached its apex height (e.g., at 1935 LT and 2125 LT in
Fig. 4.1; 2110 LT in Fig. 4.2). Detecting two apogee plumes (as in Fig. 4.1) at
a limited field-of-view ground site is very unusual and is discussed again in Sec-
tion 4.2.3. However, satellite and all-sky imagers often seemultiple plumes, prob-
ably because the terminator, which sets up the unstable condition, moves rapidly
westward, whereas the plumes move eastward. Thus, a narrow field-of-view
ground site may only see one plume on a typical night but, on occasion, can detect
several.
Several rockets and satellites have now penetrated both the irregularity lay-
ers and the plume structures. For such studies, it has proven useful to cor-
relate the in situ probing with Jicamarca or with the Altair scanning radar
located on the Kwajalein Atoll in the South Pacific. An example of a radar
(RTI) map made at 0
96m wavelength with the Altair radar on Kwajalein is
presented in Fig. 4.3a, along with the plasma density profile made simultane-
ously onboard the PLUMEX I rocket (Rino et al., 1981). The profile is highly
irregular throughout the rocket trajectory, but the most crucial observation is
that the intensely scattering top, or “head,” of the radar plume is collocated
with a region of depleted plasma, a bubble. Another example that correlates
AE-E satellite plasma density measurements with Altair data is presented in
Fig. 4.3b. The plot shows that the radar plume is associated with a very struc-
tured region of plasma in which the mean density is much less than the value
outside the volume of backscatter (Tsunoda et al., 1982). An Altair radar scan
in the north-south plane is represented in Fig. 4.4 and shows that the depleted
regions are elongated along the direction of the magnetic field for hundreds of
kilometers.
Plasma density wave number spectra from several rocket flights through the
unstable bottomside layers are presented in Fig. 4.5a. The spectra are similar but
.
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