The main results of the study of the spectra of noctilucent clouds (NLC) are presented. It is discussed that the formation of their spectra involves not only the processes of aerosol scattering by MSO particles, but also the effects of fluorescence. The role of water vapor in the formation of fields of silvery clouds is considered.
Observations of the spectra of noctilucent clouds
Summarize the results of our study of spectra of noctilucent clouds (NLC).
In NLC spectrum, there are two significant features.
— First, for all the spectra there are peaks at the range 460 — 470 nm.
— Second, there is a characteristic drop of intensity towards the longer wavelengths, approximately described by 1 / λn, where n> 4.
— Third, the half-width of each NLC spectrum is about 150 nm.
All the three facts, in our opinion, indicate that, not only the processes of aerosol scattering, but also, possibly, the effects of fluorescence are involved in the formation of NLC spectra.
Analysis of the radiation regime at the level of NLC formation points to the fulfillment of all the conditions that are necessary for fluorescence excitation.
We can assume that the hydrocarbon compounds and dissolved organic matter (DOM), in particular, humic acids, can be main claimants to the fluorescence. We cannot also exclude a possibility of the comets’ decay products.
We want to draw the attention of the readers to the following interesting facts, taken from good works of [15, 18, 30, 31], where the properties of the water in all its phase states are considered, including its role in the form of atmospheric aerosol.
First, as we have said, the pure water is virtually absent in nature. The liquid water, even well cleaned, is not a homogeneous mixture of single molecules of H2O. 80-85% of water are associates (clusters) having tetrahedral liquid-crystal structure, and 15-20% of it are “free” molecules. In addition, it includes (in low concentrations) hydrate formations (clathrates). In the center of each clathrate a gas molecule (O2, N2, Ar, CO2) is located, but a shell is made up of water molecules. In the water there are also stable micro bubbles of sizes 10-100 Ǻ («babstons»), surrounded by the ion shell (the hydrated ions of OH—, H+ and various salts), as well as dissolved gases O2, N2, Ar, CO2, radicals, atoms and molecules (OH, H, O, HO2, H2O2), the solid micro particles and organic molecules.
The bulk of liquid water is clusters (of the sizes from 6 to 400-600 Ǻ), in which there are from 6 to 3×107 molecules. The clathrates, containing within themselves a gaseous molecule (N2, Ar, CO2), are surrounded by water molecules forming the pentahedral and hexagonal flat figures due to the three hydrogen bonds, but the fourth bond of each H2O molecule stands out. Highly polarized water molecules surround the micro bubbles (babstons). In such a way, the liquid water has a complex liquid-crystal structure.
Calculations show that the water molecules in the liquid form and in the other phase states (ice, film, hydrated shell of ions, in capillary channels, in aerosol droplets, in the pores of hydrophilic substances) are compressed and are in a nonequilibrium vibrational-excited state. From this fundamental conclusion, another important fact immediately follows: the vibrational-excited H2O molecules in the liquid water must exchange vibrational-rotational quantums of energy (photons of light) with each other. A large number of experimental spectroscopic data (sonoluminescence, heluminescence, fluorescence of the water and aqueous solutions) confirm that fact.
The solid phase of water (the ice) turns out to have also not the only form of existence. As a function of external parameters (temperature and pressure), there is implemented one of the eleven ice modifications: Ih, Ic, II, III, IV, V, VI, VII, VIII, IX or the amorphous ice. They are different from each other by a mutual arrangement of the water molecules in the crystal lattice.
The hexagonal ice Ih is the most common in nature, and therefore it is best studied. It is generated at atmospheric pressure and a smooth decreasing temperature below 0°C.
When cooled to -130° C the cubic ice Ic is generated with another arrangement of molecules in the crystal lattice, but, nevertheless, with a completely identical absorption spectrum.
In a high vacuum at a temperature below -150°C, an amorphous (or glassy) ice forms, devoid of crystalline structure. Individual molecules of the frozen water are not ordered, as those in crystalline ice under normal conditions. However, the structure of such ice is more compact and its density reaches 2.3 g/cm3. These forms of ice or similar forms may be components of the composition of comets or form on surfaces of other planets or moons.
Rocket researches showed that the particles at the altitude of NLCs are often the cores consisting of a large central particle surrounded by a large number of smaller particles.
A generation of electromagnetic radiation by the growing crystals of ice is another unusual property of the ice.
Water aerosol particles, as well as free water, have an ability of polarization in external electric and magnetic fields, the presence of which is a characteristic feature of the mesosphere and ionosphere. They experience the phase transitions, including the non-equilibrium transitions. Under influence of these fields and ionizing radiations, there happens electrical and chemical activation of atmospheric aerosols and instability of associates arises and, therefore, their decay happens. The most active distortions, being the resonators of electromagnetic waves, are in the atmosphere at the borders of the ionosphere.
Processes, occurring in these cases, are much like the processes at cavitation and sonoluminescence.
