Measuring the Flattening of the Outer F-corona Using STEREO-A/HI-1 Images

Abstract

Abstract The white-light Fraunhofer-corona (F-corona) arises from light scattered by the circumsolar dust. Using weekly minimum background models of ST-A/HI-1 observations, we characterized the flattening of the F-corona between 5° and 24° elongation by measuring the radii of constant-intensity contours along, and at a 25° angle to, the photometric axis. The ratio of these quantities (the pseudo-flattening index f ˜ = R eq / R ( 25 ° ) − 1 ) is analogous to the definition of the flattening index ( f = R eq / R pol − 1 ). Measurements of the pseudo-flattening in the north and south hemispheres reveal a periodic asymmetry in the appearance of the F-corona (attributable to changes in polar brightness due to the elevation of the spacecraft from the dust symmetry surface), as well as a north/south asymmetry possibly introduced by the warped dust symmetry surface. The north/south averaged pseudo-flattening was used to infer the flattening for each weekly model. We found that the inferred flattening index (1) varies periodically with spacecraft position, reaching a maximum in the range 262° ≲ λ ≲ 290° due to a brightening along the photometric axis when the spacecraft passes through the surface of maximum dust density, and a minimum at λ ≈ 200°, and (2) slightly varies as a function of time (at least partially due to the displacement of the circumsolar dust with respect to the Sun). Comparison of the flattening index with previous works suggests a cubic dependence of the flattening index with log elongation for ∣ ϵ ∣ ≲ 24 ° .