Our lidar systems operate at different wavelengths. They use a process called resonance fluorescence which requires very specific wavelengths so that the metal atoms absorb the light and electrons raise to a higher energy level (an “excited state”). When the electrons naturally fall back to their ground state, they release a new photon, and we are able to capture some of these in our telescopes. Our Na lidar system operates at 589 nm and our Fe lidar system uses 2 lasers that operate at 372 nm and 374 nm.

The visible range of the electromagnetic system is approximately 380 nm to 700 nm (with colors ranging from violet at 380 nm to red at 700 nm). This means that the Na system laser (operating at 589 nm) falls into the yellow/orange area of the spectrum and is well within the visible spectrum, making it easy to see. However, the Fe lasers operate just outside the visible spectrum, in the ultraviolet (UV) spectrum. This means that we can’t see them most of the time. However, there are situations where they become visible.

One such situation is if we hold a piece of paper in front of the light. The paper scatters the light, and with enough of the light being scattered towards us our eyes are able to pick it up. In addition, many types of paper have optical brighteners which enhance this effect. Another situation is with enough fog. Fog mostly occurs in summer, when it’s too bright outside to see the laser. However, we recently got a rare bit of winter fog. This scattered the Fe light and we were able to see the beams. They appeared in the fog in a blue/violet color. I attempted to take a photo, but the STAR laser was overwhelming (also being scattered by the fog, making it even brighter than normal).

Above: The outgoing lidar lasers in a thin fog. The bright laser is the Na STAR laser and to the right you can see the two Fe lasers very close together, shooting up between the two brightest stars. While they appeared violet/blue in person, the light from the Na laser made it difficult to get an accurate photo.

On the same day (before the fog rolled in), I was able to capture some photos of the auroras as well.

Left: Auroras over Mt. Erebus and the USAP Arrival Heights building.
Right: An Aurora over Scott base and the wind farm.