Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 344

detection in the polar daylight and sometimes at night when auroral precip-
itation creates a plasma at mesopause heights (Nicolls et al., 2008). Satellite
instruments can detect the scattered photons in any lighting conditions, and
thus the term polar mesospheric clouds (PMC) is a more general term for these
clouds.
An important related discovery was made during the International Geo-
physical Year (IGY) of 1957-1958. Rocket grenades launched from many loca-
tions and detonated at high altitudes revealed that, against all expectations, the
temperature in the polar summer mesosphere is colder than the temperature in
the winter polar zone (Nordberg et al., 1965). In fact, a temperature difference
of about 100K is found between summer and winter. More recent data are
presented in Fig. 7.2, which not only reveal these temperature differences quite
well but also show that the level of temperature fluctuation is higher in winter
(Lübken and vonZahn, 1991). This result supports speculations that theNLC are
composed of ice, which forms around dust particles (or large ions) at extremely
low temperatures, even at the low water vapor pressure of the mesopause zone
(1-2 parts per million).
Gravity waves, which create the interesting visible structures in Fig. 7.1 and
the wintertime variability in Fig. 7.2, are of more than passing importance to
understanding the low temperature of the polar summer mesosphere. Typical

100 100

90 90

80 80

70 70

60 60

50 50

Summer 1987 Winter 1983/84

40 40

30 30

100 150 200 250 300 100 150 200 250 300

Temperature (K) Temperature (K)

Figure 7.2 Temperature profiles measurements taken using radar tracking of falling
spheres during the summer 1987 (left) and winter 1983-1984 (right) over Andøya, Nor-
way. [After Lübken and von Zahn (1991). Reproduced with permission of the American
Geophysical Union.]