Auroral displays at high latitudes are one of the notable space weather events that result from Sun-Earth interactions. Stable auroral red (SAR) phenomena originated at the sub-auroral latitudes of the equator. Unlike the Polar Aurora, Stable Auroral Red (SAR) arcs are among the lesser-known auroras and were first discovered in 1956. SAR is an exceptional atmospheric phenomenon observed amidst a strong G3-class geomagnetic storm. SAR arcs are caused by extremely high temperatures and kinetic energy in the Earth's atmosphere, whereas aurorae are produced by energetic particles showering down from above. Even though the source of the SAR arc's energy is situated 10,000–20,000 km above the Earth's surface, SAR arc emissions can be seen as high as 450 km. SAR arc emissions are monochromatic at 630.0 nm (red-line) and occur at the optical region. The excitation of oxygen atoms (OI) in the topside of the Fregion ionosphere (300–500 km in altitude) is the cause of these emissions. Global in scope, SAR arcs span over 100° longitudinally. However, they are small in latitude, measuring only 1° to 2° wide, and are primarily created during geomagnetic storms.
Mechanism
Several mechanisms have been proposed earlier for the formations of SAR e.g. increased dissociative recombination, enhanced electric field orthogonal to the local magnetic field etc, but discarded later. It has been suggested that electron heat flows or very low-energy particle fluxes from an energy source region high in the magnetosphere cause SAR arcs in the upper atmosphere. Along geomagnetic field lines, energy deposited in the thermal plasma at high altitudes is transferred into the ionosphere. At SAR arc altitudes of about 400 km, the energy input flux causes the electron temperature at the foot of the field line within the ionosphere to rise to values typically above -3000 K. This magnitude of temperature is necessary to produce observable 6300 Angstrom missions.
Electrons above 1.96 eV in the high-energy segment of the ambient thermal population collide with atmospheric atomic oxygen, causing the oxygen atoms to be excited to a 1DÂ state. This collisional de-excitation process is called quenching. The process of the collision should be slow for releasing the energy of excited oxygen radiatively rather than through collision. This process results in significant emissions produced at altitudes above ~200 km and between 300 to 500 km the rate of the emission maximizes. Fa actors like neutral oxygen density, ionospheric electron density, and electron temperature at given altitude control the intensity and rate of emissions. The red glow that distinguishes SAR arcs is caused by heated electrons connected to high plasma temperatures.
SAR event captured in Ladakh, India
In November 2023 the strange phenomenon of SAR was observed in the skies of Ladakh, a region in the northernmost part of India. A mysterious redness has appeared in the clouds, creating a stunning contrast between snow-capped mountains and the blue sky. The locals have different explanations for this phenomenon, ranging from religious to scientific. Some believe that it is a sign of the gods, while others think that it is caused by dust particles or pollution. The rare atmospheric phenomenon began on November 5 and was ascribed to a Stable Auroral Red (SAR) event, that contrasts the more common green and blue curtains of light typically associated with auroras at higher latitudes with vivid red skies. From 10 p.m. until midnight, the red aurora phenomenon illuminated the northern horizon, which peaked in intensity at 10:40 p.m. This remarkable occurrence was recorded using an All-sky Camera by the Indian Astrophysics Observatory (IAO), run by the Indian Institute of Astrophysics (IIA), Bengaluru. These kinds of uncommon events can be captured on camera since they continuously scan the whole sphere of space.
Merak, another place in Ladakh on the banks of Pangong Tso, was also able to catch this amazing show of stars. Higher mountains in the area largely blocked Merak's view, which was known as the planned location for the National Large Solar Telescope. Hanle and Merak weren't the only places where this SAR event was seen. This unusual show was observed in several places worldwide, making it a worldwide astronomical event. Such an auroral emission at lower latitudes, at 33° North Ladakh, is an uncommon and exciting event, enhancing Hanle's appeal as a centre for scientific research and astro-tourism. Studying more of these auroral events from Hanle, especially during the Sun's active periods, excited the IIA. Hanle remains a popular place for astronomy enthusiasts and scholars due to these uncommon celestial phenomena.