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7.5 Polar Mesosphere Summer Echoes (PMSE)
Early studies of VHF radar echoes from the high-latitude summer mesosphere
and lower thermosphere using the Poker Flat MST radar in Alaska
65
◦
N
)
showed a relatively narrow and surprisingly intense echoing layer centered at
about 86 km (Ecklund and Balsley, 1981). The echoes were characterized by their
strong VHF (50MHz) radar backscattering cross section, with backscattered
powers 3-5 orders of magnitude greater than typical values observed at low or
middle latitudes (in any season) or at high latitudes (in nonsummer periods). The
echoes are now referred to as PolarMesosphere Summer Echoes, or PMSE. PMSE
are both intriguing and surprising, because the 3-m irregularities responsible for
Bragg backscatter at 6m radar wavelengths (i.e., the irregularities responsible
for VHF backscattering) should lie within the viscous subrange of turbulence at
86 km, and as a consequence, should be strongly damped.
As observed at VHF (
(
∼
50MHz), northern-hemispheric PMSE exhibit the
following characteristics:
They comprise a thin but intense echoing region near and above the summer
mesopause (
●
85 km), with a typical thickness of 5 km.
●
Although some relatively strong, albeit sporadic, echoes have been reported
at latitudes as low as 52
◦
, the strongest, most continuous echoes are observed
at latitudes poleward of about 65
◦
.
∼
The echoes appear around mid-May, last until mid-August, and are relatively
continuous.
●
Both the height range and seasonal variations of PMSE correlate reasonably
well with those of the cold mesosphere.
●
Joint observations using VHF radar and sounding rockets show that intense
PMSE can be often associated with sharp “bite-outs” in the ambient electron
density.
●
The scatter is aspect sensitive, with the largest signals from the zenith.
●
Subsequent to their discovery in 1981, many observations related to PMSE
and NLC have been made using radar, lidar, and rockets. These observa-
tions have helped formulate a number of theories proposed to explain the
generation of the intense radar echoes and the remarkable physical condi-
tions associated with them. Subfields in research as disparate as dusty (icy)
plasma physics, interplanetary dust cloud studies, meteor ablation, and recoagu-
lation science all have something in common with the polar summer mesopause
region.
Many mechanisms have been proposed as being responsible for, or at least
partly responsible for, PMSE generation. An example showing 6 hours of
radar echo data from this region is presented in Fig. 7.9. The downward echo
progression is similar to that of the higher altitude ionospheric layers discussed
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