Geoscience Reference
In-Depth Information
10
4
10
3
10
2
10
1
10
0
10
1
1
10
Radius (nm)
Figure 7.16
Concentration of meteoric smoke and dust particles at 90 km for the
assumed initial particle radii indicated on the bottom axis. [From data in Hunten et al.
(1980). Reproduced with permission of Elsevier.]
communication, 2008) has used the PFISR radar to utilize this method for ice
particle detection. This is a very exciting new area of radar research!
7.7 On the Possible Relationship Between PMSE, NLC,
and Atmospheric Change
As time has passed, more observations of NLChave been reported and an increas-
ing trend has been detected (see Fig. 7.17). This change in NLCmay be due to the
observed increment in atmospheric methane and carbon dioxide due to human
activity. A doubling increment in any of these components will produce a cool-
ing of the thermosphere and mesosphere by about 50K and 10K, respectively
(Roble and Dickinson, 1989). Remember that cold temperatures are necessary
conditions for NLC generation, but not the only one: water is also needed. About
half of the mesospheric water vapor is believed to come from the photodissocia-
tion and oxidation of upwardly transported methane with the chemically active
hydroxal radical (OH). Colder temperatures and more water vapor can produce
more NLC events, so they can be used as indicators of changes in the atmosphere.
The necessary conditions for the occurrence of PMSE and NLC (or PMC)
appear to be similar: they require low temperatures and are apparently
related to water vapor (Gadsden and Schröder, 1989). The seasonal PMSE
occurrence corresponds well with the high-latitude seasonal occurrence of NLC.
Recent studies have shown a correlation of PMC with PMSE in the Northern
Hemisphere where the mean long-term PMC occurrence ratio curve fits symmet-
rically inside the PMSE occurrence ratio. A close correlation between NLC and
PMSE has also been observed using lidar and radar data (von Zahn and Bremer,
1999), respectively. These studies were made using a common volume and they
Search WWH ::
Custom Search