Particle characteristics from the spectra of polar mesospheric clouds

Abstract

[1] Spectrographic imagers on the Midcourse Space Experiment (MSX) satellite have obtained the first spectra of polar mesospheric clouds (PMCs) from the far ultraviolet to the visible (164-585 nm). Several hundred PMC spectra were extracted over a 3 week period during the summer of 1999. To improve statistics, the spectra were normalized and averaged within 32 groups of clouds having common latitudes and scattering angles. The subsequent spectra, from three spectrographic imagers, produced a PMC signal that fell into the noise at wavelengths short of 200 nm and showed Earth albedo effects longward of $315 nm. The spectra between these limiting wavelengths were subjected to a least squares analysis to determine characteristics of the scattering particles assuming a lognormal distribution of Mie scatterers made of ice. Averaged over all PMCs, the distribution mode (size) was 65.2 ± 2.2 nm and the distribution dispersion (width) was 1.15 ± 0.04. Over the period of observation, neither the mode nor the dispersion exhibited systematic variations in time or latitude, although less intense clouds at lower latitudes may have been undersampled.

Spectral properties of mesospheric ice clouds: Evidence for nonspherical particles

“…We have converted this spectrum to Angstrøm exponents yielding an average value of 1.6 over the full wavelength range between 200 nm and 310 nm. This is consistent with the values quoted from Carbary et al [2002] above. However, if smaller wavelength intervals are considered the hump in the spectrum actually leads to negative Angstrøm exponents (which means increasing scattered intensity with increasing wavelength instead of the usual decreasing behavior) with maximum values of −2 in the narrow wavelength range between 245 nm and 260 nm.…”

“…As far as the MSX observations are concerned, the situation becomes even more difficult. In Table 1 we have listed values between 1 and 2, following the statement by Carbary et al [2002, paragraph 32] that “the index was generally between −1.0 and −2.0 and in all cases differed significantly from −4.0, which would be expected for pure Rayleigh scattering.” However, 2 years later, Carbary et al [2004, paragraph 1] published a reanalysis of their data now claiming that “the scattering spectrum always exhibits a peculiar ‘hump’ at ∼260 nm….”…”

Spectral measurements of PMCs from SBUV/2 instruments

“…The extra moisture injected from the lower thermosphere could have caused much larger ice particles to form. However, STS-93 was launched on 27 July 1999, while approximately half of the MSX PMC observations in 1999 were made before this date (Carbary et al, 2002), including the specific case presented in Carbary et al (2004). Thus, we feel that a lognormal particle distribution continues to be the most appropriate model to explain observed PMC characteristics.…”

“…Measurements in the 200-315 nm region were found to be most useful for PMC analysis because of the absence of earth albedo effects. Further studies using subsets of the MSX data have investigated PMC altitudes, transpolar structure, latitude variations, and particle size distributions (Carbary et al, 1999(Carbary et al, , 2000(Carbary et al, , 2001(Carbary et al, , 2002.…”

“…We have investigated this issue for scattering angle regimes corresponding to both SBUV/2 and MSX measurements, using Mie scattering theory based on microphysical PMC models by Jensen (1989). The Northern Hemisphere 1999 MSX observations were taken at scattering angles between 1251 and 1451 (Carbary et al, 2002), giving an average angle of approximately y ¼ 1351. Fig.…”