Light scattering by nonspherical ice grains: An application to noctilucent cloud particles

The light scattered by noctilucent cloud particles is nearly fully polarized at scattering angles in the vicinity of 90 . This was one of the reasons to conclude that the upper limit of their sizes is not larger than about 0.12 m. Nevertheless, this estimate was made on the basis of the Mie scattering theory for spherical particles, whereas many investigators noted usefulness of highly aspherical shapes of noctilucent cloud particles. In this paper, we used rigorous light scattering theory for randomly oriented nonspherical particles to calculate the degree of linear polarization of the scattered light for ice grains of different shape. By comparing these calculations with rocket polarization measurements of noctilucent clouds, we show that, as for spherical particles, the upper limit of particle equal-volume radii for slightly flattened and elongated grains is of about 0.12 m, while for highly aspherical plate-like and needle-like particles this upper limit is substantially larger and is of about 0.18–0.20 m. We also report calculations of the volumetric scattering cross-section for particles of different shape and show that randomly oriented spheroids have (slightly) smaller scattering cross section per unit particle mass than equal-volume spherical grains. Nevertheless, if in noctilucent clouds plate-like and needle-like grains grow to much larger sizes than spherical particles, their scattering efficiency may be much greater.     

The Effect of the Shape of Dust Aerosol Particles in the Martian Atmosphere on the Particle Parameters

The influence of the shape of dust aerosol particles in the Martian atmosphere on the imaginary part of the refractive index n _i as derived from photometric observations during the period of the highest activity of the dust storm in 1971 was studied and exemplified for particles of spherical and oblate spheroidal shape. A similar analysis was performed for mean particle radii r ₀ and optical thicknesses ₀ of the dust layer estimated from polarization observations for periods of high atmospheric transparency. It was demonstrated that the values obtained for these optical parameters are affected by the adopted aerosol shape. Namely, the values of n _i, r ₀, and ₀ found for spheroidal particles proved to be nearly twice as large as those for spheres. However, they are still much less than the available estimates of these parameters inferred, in particular, from interpreting space experiments. The reason for this difference needs further investigation.     

Low-cost long-term monitoring of global climate forcings and feedbacks

We describe the rationale for long-term monitoring of global climate forcings and radiative feedbacks as a contribution to interpretation of long-term global temperature change. Our discussion is based on a more detailed study and workshop report (Hansenet al., 1993b). We focus on the potential contribution of a proposed series of inexpensive small satellites, but we discuss also the need for complementary climate process studies and ground-based measurements. Some of these measurements could be made inexpensively by students, providing both valuable climate data and science educational experience.     

Relationship of high-frequency geoacoustic emission and electric field in the atmosphere in seismotectonic process

In July–October 2006 and 2007, combined measurements of geoacoustic emission in the range of 2.0–6.5 kHz, the electric field in the atmosphere near the ground, and meteorological values were carried out in Kamchatka. Using the nonparametric method of Spearman’s correlation analysis, the relationship between their average hourly values was examined. After excluding results of bad weather (rain, strong and moderate wind, low atmospheric pressure), a highly important negative relation between disturbances in geoacoustic emission and the electric field were detected. Most probably, it was caused by amplification of the strain of near-surface sedimentary rocks at the observation point during a seismotectonic process. The revealed relation is evidence for another manifestation of the lithosphere’s influence on surface atmosphere in a seismoactive region.     

On the relation between high frequency acoustic emissions in near-surface rocks and the electric field in the near-ground atmosphere

A field instrument package was installed for synchronous measurements of acoustic emission in rocks at frequencies of 0.1–10000 Hz and the vertical gradient of electric potential in near-ground atmosphere. These investigations for the first time revealed a relationship between emission disturbances in the kilohertz frequency range due to deformation of near-surface rocks and the electric field. The relationship may be observed both during seismically quiet periods and at the final phase of earthquake precursory periods.     

Simultaneous response of high-frequency geoacoustic emission and atmospheric electric field to strain of near-surface sedimentary rocks

During the period of October 1–18, 2009, 41 km southwest of Petropavlovsk-Kamchatsky, in the intersection zone of tectonic faults of various orders, simultaneous recording of the geoacoustic emission, gradient of the atmospheric electric field’s potential, strains of the Earth’s surface, atmospheric pressure, wind speed, and rain intensity was made. It was found for the first time that anomalous disturbances of high-frequency geoacoustic emission and atmospheric electric field near the Earth’s surface originate as a simultaneous response to extension of near-surface sedimentary rocks. In the case of compression, only disturbances of geoacoustic emission occur. Anomalies were recorded under quiet weather conditions and with rocks strains being two orders greater than those of tidal ones.     

The angular width of the coherent back-scatter opposition effect: An application to icy outer planet satellites

It has been suggested recently that coherent back-scattering of light from powder-like regolithic surfaces can explain remarkable opposition brightening of some atmosphereless solar system bodies. In this paper, a dense-medium light-scattering theory is used to calculate the half-width at half-maximum (HWHM) of the coherent back-scattering peak for a number of scattering models. We demonstrate that HWHM strongly depends on the optical properties of the scattering medium and can serve as a critical test in comparing alternative models. It is shown that coherent back-scaterring may be a likely explanation of the opposition effect exhibited by icy outer planet satellites.     

Polarization characteristics of the coherent backscatter opposition effect

It has been suggested recently that coherent backscattering of photons from discrete disordered media, which has been recently discovered in laboratory-controlled experiments, may play a role in peculiar radar and light scattering from some atmosphereless solar system bodies. In this paper, a rigorous vector theory recently developed by Mishchenko (1991b, 1992a) is used to study polarization characteristics of the coherent backscatter opposition effect. Backscattering enhancement in different polarization components is studied and results of computer calculations for a representative selection of scattering models are presented. It is pointed out that these calculations support recent Hapke's (1990) explanation of unusual radar characteristics of icy outer planet satellites.     

Extinction of light by randomly-oriented non-spherical grains

Waterman'sT-matrix approach is used to derive a simple analytical expression for the extinction cross-section for randomly-oriented non-spherical grains. Numerical results are presented for randomly-oriented oblate and prolate spheroids and Chebyshev particles composed of astronomical silicate. These results are compared with those for spherical grains, and possible influence of the shape of dust grains on the value of interstellar extinction is considered. The range of validity of the Rayleigh approximation for computing extinction efficiency factors for randomly-oriented non-spherical grains is discussed.