Abstract
The Fraunhofer line filling-in effect associated with lunar spectra has been investigated by spectropolarimetry. From considerations that the effect is attributable to luminescence, simple calculations suggest that changes of the order of 0.001 in the degree of polarization,
p, are anticipated in the spectral line relative to its nearby continua. Observations have been made of several lunar areas at H
α and H
β with spectral resolution ∼2 Å and with polarimetric uncertainties in
p ∼ ±0.0002 (
i.
e., 2/100
of 1%). In general, the measurements reveal no significant changes of
p between the core of the lines and their nearby continua. For the few data sets considered as displaying marginal changes, no consistent pattern of behavior emerges between them. Although the measurements neither confirm not negate the filling-in effect, they clearly indicate that any luminescence at these wavelengths cannot be unpolarized. For the Hα measurements, one lunar region displayed an anomalous polarization position angle, θ, relative to the local scattering plane with the core of the line being less affected than the continuum. Other observed lunar regions suggest that a rotation of
θ ∼ 0°.25 occurs within the linear core. No explanation is immediately available for this phenomenon although it may be related to Rayleigh-Broullin scattering. A similar effect was not observed in Hβ.