Solar Pillars: Celestial Mirages and Light Phenomena

Solar Pillars: A Play of Ice Crystals and Sun Rays

Solar pillars are vertical columns of light that appear to rise above or below the Sun. They result from the reflection of light on tiny ice crystals suspended in the atmosphere, often observed during sunrise or sunset when the air is very cold.

Physical Mechanism

The phenomenon is based on the specular reflection of light on plate-shaped or needle-shaped ice crystals. Each crystal acts as a microscopic mirror oriented horizontally. The apparent height of the pillar is given by the projection of the reflected rays at an angle \(\theta\) with the horizontal, the brightness being proportional to the number of aligned crystals.

Variations and Celestial Mirages

Lunar Pillars and Artificial Sources

Light pillars are not limited to solar observations. Similar phenomena can appear around the Moon or artificial light sources. These effects, often grouped under the term "celestial mirages," result from local variations in density and optical index in the atmosphere.

Refraction and Thermal Gradient

The formation of a light mirage is based on the refraction of light in a thermal gradient. When warmer air layers are above colder air, light rays undergo progressive curvature, resulting in distorted, amplified, or duplicated images of the source. The apparent height and shape of these images depend on the viewing angle and atmospheric stratification.

Interaction with Halos and Light Arcs

Under extreme conditions, solar pillars can combine with halos or circumzenithal arcs, creating complex light patterns. These interactions result from the superposition of reflection and refraction phenomena on different types of ice crystals: horizontal plates for pillars, columns for vertical or diagonal arcs.

Influence of Ice Crystals

The variations in light intensity observed in these mirages are also influenced by the concentration and orientation of ice crystals. A high number of perfectly oriented crystals can produce very pronounced light columns, while dispersed crystals create diffuse pillars and partial halos.

Scientific Application

Celestial mirages are a natural tool for studying local atmospheric conditions. By analyzing the height and brightness of pillars and associated arcs, it is possible to estimate the average size of ice crystals and the thermal gradient in the lower layers of the atmosphere.

Comparative Table of Light Pillars