Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975apj...202..823m&link_type=abstract
Astrophysical Journal, vol. 202, Dec. 15, 1975, pt. 1, p. 823-831.
Astronomy and Astrophysics
Astrophysics
2
Comets, Optical Pumping, Polarized Light, Solar Radiation, Comet Nuclei, Fluorescence, Hyperfine Structure, Line Spectra, Optical Resonance, Quantum Mechanics, Sodium
Scientific paper
At some distance from the nucleus of a comet, the rate of fluorescent scattering of solar radiation by an atom or molecular will be comparable to, or exceed, the rate of collisional relaxation. This is the condition for optical pumping. If the ground state has fine structure or hyperfine structure, then significant nonequilibrium distributions of these levels can result, and the fluorescence spectrum can deviate markedly from the laboratory spectrum. In particular, since the incident solar radiation is unidirectional, the pumping causes an alignment of the hyperfine magnetic sublevels along the incident axis, and observations from the earth should exhibit an enhanced linear polarization in the sun-comet-earth plane. Therefore, polarization measurements can be used as a diagnostic for the presence or absence of collisional relaxation. Conditions are especially favorable for the enhanced polarization of the Na D2 resonance line, and, at a scattering angle of 90 deg, detailed calculations are presented which predict a polarization of 16 percent, and under favorable circumstances as much as 21 percent, as compared with 10.5 percent for a collisionally equilibrated Na gas.
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