Observations of the Solar F-corona from Space

TL;DR

This paper reviews 25 years of space-based observations of the solar F-corona, revealing its radiance profiles, secular variations, and the structure of the zodiacal cloud, providing new insights into dust dynamics near the Sun.

Contribution

It presents the first detection of a secular variation in the F-corona and offers detailed mappings of the inner zodiacal cloud's structure and composition from multiple space missions.

Findings

Power-law radiance profiles with constant exponents

First observational evidence of secular increase in F-corona radiance

Identification of the dust-free zone boundary at 5-19 solar radii

Abstract

The observations of the solar F-corona from space are reviewed emphasizing the 25 years of continuous monitoring achieved by the LASCO-C2 and C3 coronagraphs. Our work includes images obtained by the Clementine cameras, and the STEREO/SECCHI/HI-1A and PSP/WISPR heliospheric imagers. The characteristic radiance profiles along the equatorial and polar directions follow power laws in the 5{\deg}-50{\deg} range of elongation, with constant power exponents of -2.33 and -2.55. Both profiles connect extremely well to the corresponding standard profiles of the zodiacal light. The LASCO equatorial profile exhibits a shoulder implying a 17% decrease of the radiance within 10Rsun that may be explained by the disappearance of organic materials within 0.3 AU. LASCO detected for the first time a secular variation of the F-corona, an increase at a rate of 0.46% per year of the integrated radiance in the LASCO-C3 FoV. This is likely the first observational evidence of the role of collisions in the inner zodiacal cloud. A composite of C2 and C3 images produced the LASCO reference map of the radiance of the F-corona from 2 to 30Rsun and, by combining with ground-based measurements, the LASCO extended map from 1 to 6 Rsun. The plane of symmetry of the inner zodiacal cloud is strongly warped, its inclination increasing towards the planes of the inner planets and ultimately the solar equator. In contrast, its longitude of ascending node is found to be constant and equal to 87.6{\deg}. LASCO did not detect any small-scale structures such as putative rings occasionally reported during solar eclipses. The outer border of the depletion zone where interplanetary dust particles start to be affected by sublimation appears well constrained at 19Rsun. This zone extends down to 5Rsun, thus defining the boundary of the dust-free zone where the most refractory materials, likely moderately absorbing silicates, disappear.