Characterization of the White-light Brightness of the F-corona between 5° and 24° Elongation

Abstract

Abstract The white-light F-corona arises from light scattered by circumsolar dust. Using weekly models of the eastern side of the F-corona between 5° and 24° elongation, we analyzed the elongation and time dependence of the brightness of its photometric axis. The models were constructed from STEREO-A SECCHI/HI-1 images taken between 2007 December and 2014 March. We found that the brightness profiles can be approximated by power laws, with the coefficients of the models depending upon the observer’s ecliptic longitude. Their variation is not symmetric with respect to the orbital nodes of the dust plane, nor is the behavior similar in the two halves of the spacecraft orbit delimited by the line of nodes. The exponents range between −2.31 and −2.35, the former occurring when the observer is at the nodes. The asymmetry observed in the behavior of the proportionality constant is indicative of the projected center of the dust cloud being offset from the Sun’s center by ∼0.4 R ⊙. The coefficients exhibit a secular variation correlated with the location of the barycenter of the solar system. We also used the HI-1 frames obtained during STEREO-A calibration rolls to model the 360° F-corona. We found that (1) its flattening index ( f = R eq / R pol − 1 ) decreases from ∼0.66 to ∼0.46 with decreasing elongation and (2) the isophotes’ shape can be approximated by a series of superellipses, with the superellipse index n increasing (nonlinearly) with brightness ( ∼ 1.54 < n < ∼ 1.65 ). Cubic extrapolation of the results below 5° elongation points to a circular F-corona below 1° elongation.