Noctilucent clouds

Fogle and Haurwitz (1966) have given an excellent review of noctilucent clouds, to which we refer the reader for further details about their appearance as well as when, where, and for how long they may be observed. 

Two characteristics of the light from noctilucent clouds may be observed with no more than one s eyes and a polarizing filter its color and whether or not it is strongly polarized. This enabled Ludlum (1957) to estimate the size range of noctilucent cloud particles. Because of the observed strong polarization he set 0.16 jum as their upper size limit on the basis of the observed color—white, silvery, sometimes bluish, but not sufficiently so as to indicate very small particles—he set 0.008 jam as their lower size limit. From other than optical evidence he also concluded that the particles were not ice, but were more likely to be volcanic, meteoric, or interplanetary dust. 

Light from noctilucent clouds carries with it more than just information about how it was scattered by cloud particles, however. On its journey from the sun to the clouds, and thence to an observer, it must travel long atmospheric paths along which it suffers selective absorption and scattering by various gases and particles of uncertain kind and amount. This selective extinction... 

The strongest evidence supporting small (< 0.1 jum) particles in noctilucent clouds is the high degree of measured linear polarization it increases monotonically with scattering angle to almost unity near 90°. It is difficult to reconcile these observations with any conclusion other than that the particles are small. 

Noctilucent cloud particles are now generally believed to be ice, although more by default—no serious competitor is still in the running—than because of direct evidence. The degree of linear polarization of visible light scattered by Rayleigh ellipsoids of ice is nearly independent of shape. This follows from (5.52) and (5.54) if the refractive index is 1.305, then P(90°) is 1.0 for spheres, 0.97 for prolate spheroids, and 0.94 for oblate spheroids. 

A necessary condition for the correctness of the multiple-scattering explanation of the observed circular polarization is that scattering by noctilucent cloud particles does not appreciably reduce the degree of circular polarization of the incident light. That this is so for randomly oriented Rayleigh ellipsoids is readily shown. M in (5.52) is nearly unity for ice ellipsoids, so to good approximation... 

If the particles are highly absorbing, it may be difficult to reconcile this with both the observed color and linear polarization of the light from noctilucent clouds. 

Deirmendjian, D., and E. H. Vestine, 1959. Some remarks on the nature and origin of noctilucent cloud particles, Planet. Space Sci., 1, 146-153. 

Donahue, T. M., B. Guenther, and J. E. Blamont, 1972. Noctilucent clouds in daytime circumpolar particulate layers near the summer mesopause, J. Atmos. Sci., 29, 1205-1209. 

Gadsden, M., 1975. Observations of the colour and polarization of noctilucent clouds, Ann. Geophys., 31, 507-516. 

Gadsden, M., 1978. The sizes of particles in noctilucent clouds implications for mesospheric water vapor,/. Geophys. Res., 83, 1155-1156. 

Gadsden, M., P. Rothwell, and M. J. Taylor, 1979. Detection of circularly polarised light from noctilucent clouds, Nature, 278, 628-629. 

Grishin, N. I., 1956. Research on the continuous spectrum of noctilucent clouds, All Union Ast. Geod. Soc. Bull. USSR, 19, 3-16. 

Tozer, W. F., and D. E. Beeson, 1974. Optical model of noctilucent cloud based on polarimetric measurements from two sounding rocket campaigns, J. Geophys. Res., 79, 5607-5612. 

A particular, and unusual, atmospheric application of such data involves the formation of noctilucent clouds (NLC s) in the vicinity of the mesopause (at 82 km, in the summer hemisphere, where temperatures can fall as low as 130 K, and ice can exist even at the miniscule ambient water vapor concentrations found there). The presence of laige water-aggregated hydronium ions led to the suggestion [e.g., 63-65] that these provide condensation sites for ice particles. Detailed simulation studies bore out the likely relationship between positive ion nucleation and the behavior of some NLC s [66], notwithstanding a strong possibility that meteoritic dust and smoke also had a dominant role [67], ITie contribution to NLC formation of hydronium-ion/electron... 

Sugiyama, T Ion-recombination nucleation and growth of ice particles in noctilucent clouds, J. Geopkys. 

On the other hand, the effect of the wet removal can be practically neglected here.3 It is thus understandable that the residence time of trace constituents is greater in the stratosphere than in the troposphere. Above the tropopause the horizontal wind speed first decreases then increases with height. Consequently, a secondary maximum in the wind speed can be observed in this atmospheric layer. The increase of the temperature ends approximately at an altitude of SO km (stratopause), where the temperature is around 0 °C (see Fig. 1). Above this level, in the mesosphere, the temperature again decreases (third layer in the homosphere). For this reason the stratopause can be considered as an active heat-supplying surface similar to the Earth s surface. In this atmospheric region the distribution of the temperature makes possible the convection which, in favourable cases, results in a formation of so-called noctilucent clouds at an altitude of about 80 km (mesopause) where the temperature is only around — 80 °C. This is the coldest level of our atmosphere. 

Swenson, G.R., and C.S. Gardner, Special section The 1993 airborne lidar and observations of Hawaiian airglow airborne noctilucent cloud campaigns-preface. J Geophys Res 103, 6249, 1998. 

The most abundant ions generally display hydration orders from 2 to 4, but at cold temperatures near the mesopause hydration orders of 8 or 9 are not uncommon. Bjorn and Arnold (1981) observed ions as large as H+(H20)2o near a very cold summer mesopause at high latitude. These authors also suggest that such ions may be important in the formation of noctilucent clouds. 

Plate 13. Noctilucent clouds observed over Ktthlungsbom, Germany (54°N) on 10 July 1997 at 01 00 GMT. These polar mesospheric clouds are found at approximately 90 km altitude and are illuminated by the Sun, which at that time is located approximately 11 degrees below the horizon. Courtesy ofF.-J. LUbken, Leibniz Institute for Atmospheric Physics, Kiihlungsbom, Germany. 

Box 1. The nature of noctilucent clouds and the Earth s ozone layer (Nikolaev and Fomin, 1997)... 

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