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Jicamarca vertical backscatter at 3 meters
March 21, 1979
1000
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4 4 36
3 24
1 12
6 db
19:0 0
21:00
22:00
23:00
20:00
Local tim e
Figure 4.1 Range-time-intensity (RTI) map displaying the backscatter power at 3m
wavelengths measured at Jicamarca, Peru obtained on March 21, 1979. The gray scale
is decibels above the thermal noise level. [After Kelley et al. (1981). Reproduced with
permission of the American Geophysical Union.]
reproduced in Fig. 4.1, and a more typical map is reproduced in the top panel of
Fig. 4.2. The Jicamarca radar transmits nearly vertically, and at its 50MHz fre-
quency it is sensitive to backscatter from waves that satisfy the Bragg matching
condition. This condition requires that k r =
k s +
k m , where k r is the radar wave
vector, k s the scattered wave vector, and k m the wave vector in the medium.
(See Appendix A for more details.) Since k s
=−
k r for backscatter, it follows
that k m =
6m) detects only waves
with vertical wave vectors corresponding to a 3m wavelength. In the F region
of the ionosphere (above 200 km), both figures display intense echoes over a
100 km altitude extent for much of the time. The gray scale shows the echo
strength to be up to 50 dB higher than that caused by thermal fluctuations at the
3m wavelength. This “thermal backscatter level” is the source of the incoherent
scatter echoes often used to determine ionospheric parameters (see Appendix A).
The thick irregularity layer moves up or down with time rather periodically in
Fig. 4.1 and twice is interrupted by a period of intense backscatter signal that
extends to very high altitudes. In Fig. 4.2 this happens only once at about 2110
local time. These towering echoing features have been termed “plumes.” As we
shall see, this name is quite apt for structures that are probably the result of a
convective instability driven by buoyancy forces, much like tropospheric thun-
derstorms. The horizontal time scale is such that the mean horizontal plasma
drift speed (125m
2 k r . Thus, the Jicamarca 50MHz radar (
λ =
s toward the east) multiplied by time yields the same hori-
zontal distance scale as given in the vertical altitude scale. Thus, the picture yields
an accurate description of the ionospheric scatterers (as viewed looking south
/
 
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