Dynamics and Electrodynamics of the Mesosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 349

Neutral temperature (deg K)
Height 5

85.0 Zonal means

80

2 30

60

220

40

20

0

220

240

22 0

2 60 230 230

240

2 80

Feb Mar May Jun

Month (Mtimes 010 00:00 to 360 00:00)

Apr Jul Aug Sep Oct Nov Dec

Figure 7.6 TIME-GCM variation of zonal average neutral gas temperature in Kelvin,
over a year at 85 km. (Figure courtesy of R. G. Roble.)

shown in Fig. 7.6. A huge difference exists between the hemispheres for solstice
conditions at high latitudes. The polar summer hemisphere is indeed colder than
the polar winter hemisphere by 100K, as is observed.

7.4 New Observations of NLC and Related Phenomena

New ways to study NLC have been developed in the latter part of the twentieth
century. For example, in the early 1980s satellite measurements detected NLC,
but due to the different method of detection, they were called Polar Mesospheric
Clouds, or PMC (Thomas, 1991). They are believed to be the same as NLC with
the only difference being that, from orbit, they could be observed for 24 hours
and at a much less restrictive latitude range. Rocket observations of the electron
density have been available as early as those of Pedersen et al. (1970), although
such flights are fairly rare. Another observation method that is not hampered
by lighting conditions takes advantages of the unexpectedly high radar cross
section for VHF radars in the polar summer mesosphere. This radar scattering