Solar coronal dust scattering in the infrared

Abstract

Observations of the total and polarized brightness of the solar corona at wavelength 2.12 microns during the total solar eclipse of 1991 July 11 are employed to separate the contribution of the electron-scattered component and the remaining, nonpolarized component, the latter dominating in the outer corona. After corrections are applied to account for a two-component sky or instrument background, the brightness of the ecliptic and polar corona are fitted by r<SUP>-1.9</SUP> and r<SUP>-2.3</SUP>, respectively, over the radial distance range 3-8 solar radii from Sun center. The ecliptic outer-coronal brightness is compared with a Mie-scattering model of interplanetary dust particles based upon three particle-size distributions deduced from, respectively, lunar microcrater counts (Lamy &amp; Perrin 1986), interplanetary dust flux measurements (Gruen et al. 1985), and for an arbitrary population of large particles (radii greater than 3 x 10<SUP>-5</SUP> cm). Particle physical characteristics and spatial distributions are those assumed in past studies of the zodiacal cloud. For reasonable assumed space number densities of particles, the models agree with the magnitude of observed ecliptic coronal brightness. But in all cases, the models predict a steeper brightness falloff with radial distance that that observed, with those models for which the power-law exponent for the space distribution is v = 1.3 being the most discrepant with the observed radial gradient.