Polarization of OH Radiation

Abstract

The ground ²π_3/2 state of OH consists of 2 A-doubled levels which are separated by about 1666 MHz. The upper (parity = +1) and lower (parity = −) levels each have eight hyperfine subleveis which consist of a three-fold degenerate F=1 and five-fold degenerate F=2 energy state, and transitions between these levels give rise to the OH-18-cm radiowave spectrum. Of the four possible transitions the F=2 → 2 and F= 1 → 1 transitions are most intense and are the source of the 1667 MHz and 1665 MHz signals observed from comet Kohoutek (Biraud, et al (1973), Turner (1973)). The peak antenna temperature ΔT_b/T_b for these lines are approximately proportional to the ratio i = (N⁺-N^-)/(N⁺+N^-) where N^± are the total concentrations of ² π_3/2, J = 3/2 molecules in the indicated parity state. In the optically thin, collisionless atmosphere of a comet these populations are determined predominantly by the fluorescent scattering of solar u.v. radiation by 12 absorption lines of the OH(A² Σ⁺ ← X² π) transition. The steady state distribution is only a function of the relative solar flux at these 12 absorption wavelengths. The molecules are pumped into a large set of ²π states, which then rapidly cascade by infrared transitions back to either the + or − levels of the ground state. Because of the Doppler shift of the absorption spectrum relative to the solar Franhaufer spectrum, the ratio i is a sensitive function of the heliocentric velocity V_h of the comet, and the radio signals can be seen either in absorption or stimulated emission relative to the galactic background temperature T_b, depending on whether the levels are anti-inverted, i < o, or inverted, i > o.