anticorona

An anticorona is a luminous ring or halo that appears directly opposite the sun, centered on the antisolar point, when you look at your own shadow projected onto a surface like fog, cloud, or fine spray. The effect is brightest immediately around the shadow of your head, which is why it is sometimes called a heiligenschein, German for "holy light." It occurs because light scattered back toward the observer by water droplets or dust particles is enhanced in the backward direction through a combination of diffraction, refraction, and geometric focusing.

The anticorona differs fundamentally from a corona, which is a set of colored rings that appear around the sun or moon itself when seen through cloud or fog. A corona results from diffraction by uniformly sized droplets; an anticorona results from backscatter enhancement and is centered on the antisolar point rather than on the light source. The two phenomena can coexist in the same sky but are optically distinct and occur at different angular positions relative to the observer.

Formation and visibility

The anticorona is most readily observed from an aircraft, where a clear antisolar point is visible through cloud layers below. On the ground, it appears in morning dew, fog banks, or sea spray when the observer stands with the sun directly behind them. The ring is not sharply defined but a soft brightening, typically a few degrees wide, with maximum intensity at roughly 10 to 20 degrees from the antisolar point. Polarization of the scattered light is significant; the anticorona is often strongest when viewed through polarizing filters aligned perpendicular to the scattering plane.

In practical optics and remote sensing, the anticorona is a source of noise and glint in imaging systems operating at or near the antisolar direction. Thermal cameras and radiometers measuring downwelling radiation from clouds must account for backscatter enhancement, as it can elevate apparent brightness temperature in the backward hemisphere. Similarly, lidar and radar systems operating in a backscatter geometry encounter suppressed noise and improved signal contrast due to the preferential return of photons along the incident direction, an effect related to the anticorona's optical mechanism.

The term "anticorona" is less common in modern scientific literature than "glory," which refers specifically to the colored rings seen in backscattered light from cloud droplets. However, "anticorona" remains useful in optical instrumentation contexts to distinguish the general brightening and backscatter enhancement near the antisolar point from the finer colored-ring phenomena characteristic of monodisperse aerosol layers.