Observations of the Solar F-Corona from Space

AbstractWe present a review of the observations of the solar F-corona from space with a special emphasis of the 25 years of continuous monitoring achieved by the LASCO-C2 and C3 coronagraphs. Our work includes images obtained by the navigation cameras of the Clementine spacecraft, the SECCHI/HI-1A heliospheric imager onboard STEREO-A, and the Wide Field Imager for Solar Probe onboard the Parker Solar Probe. The connection to the zodiacal light is considered based on ground- and space-based observations, prominently from the past Helios, IRAS, COBE, and IRAKI missions. The characteristic radiance profiles along the two symmetry axis of the “elliptically” shaped F-corona (aka equatorial and polar directions) follow power laws in the $5^{\circ }$ 5 ∘ –$50^{\circ }$ 50 ∘ 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 $\approx17\%$ ≈ 17 % decrease of the radiance within $\approx10~\text{R}_{\odot }$ ≈ 10 R ⊙ 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 field of view. This is likely the first observational evidence of the role of collisions in the inner zodiacal cloud. The temporal evolution of the integrated radiance in the LASCO-C2 field of view is more complex suggesting possible additional processes. Whereas it is well established that the F-corona is slightly redder than the Sun, the spectral variation of its color index is not yet well established. A composite of C2 and C3 images produced the LASCO reference map of the radiance of the F-corona from 2 to $30~\text{R}_{\odot }$ 30 R ⊙ and, by combining with ground-based measurements, the LASCO extended map from 1 to $6~\text{R}_{\odot }$ 6 R ⊙ . An upper limit of $0.03~\text{R}_{\odot}$ 0.03 R ⊙ is obtained for the offset between the center of the Sun and that of the F-corona with a most likely value of zero. The flattening index of the F-corona starts from zero at an elongation of $0.5^{\circ }\pm0.01^{\circ }$ 0.5 ∘ ± 0.01 ∘ ($1.9~\text{R}_{ \odot }$ 1.9 R ⊙ ) and increases linearly with the logarithm of the elongation to connect to that of the zodiacal light with however a small hump related to the shoulder in the equatorial profile. The shape of the isophotes is best described by super-ellipses with an exponent linked to the flattening index. An ellipsoid model of the spatial density of interplanetary dust is solely capable of reproducing this shape, thus rejecting other classical models such as fan, and cosine. 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 $87.6^{ \circ }$ 87.6 ∘ . 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 $\approx19~\text{R}_{\odot }$ ≈ 19 R ⊙ . This zone extends down to $\approx5~\text{R}_{\odot}$ ≈ 5 R ⊙ , thus defining the boundary of the dust-free zone where the most refractory materials – likely moderately absorbing silicates – disappear.