What are aurora australis and aurora borealis? How are these triggered? (Answer in 250 words)

Aurora borealis (northern lights) and aurora australis (southern lights) are natural light displays in the polar upper atmosphere, mainly in the thermosphere/ionosphere at about 100–400 km. They occur when charged particles from the Sun enter Earth’s magnetosphere and collide with atmospheric gases.

The trigger begins with solar activity. The Sun emits a solar wind of electrons and protons, usually in the eV–keV range, and during coronal mass ejections or high-speed streams it can send denser plasma that may be further accelerated toward Earth. When this plasma reaches Earth, it interacts with the magnetosphere. Energy enters mainly through magnetic reconnection, when the southward-pointing field of the solar wind links with Earth's field; energy stored in the magnetotail is released in substorms that accelerate electrons down the field lines. Magnetic field lines channel the particles toward the polar regions, where they precipitate into the atmosphere along the auroral oval, roughly at 65–75° magnetic latitude.

In the thermosphere/ionosphere, these particles excite oxygen and nitrogen molecules. Oxygen emits green light at lower auroral altitudes and red at higher altitudes; nitrogen gives blue and purple hues. The emitted photons create the curtains, arcs and rays seen as aurora. Because the same field lines connect the two polar regions, aurorae usually appear in both hemispheres at once as near-mirror images.

Auroral intensity is linked to geomagnetic storms, measured by the Kp index, and to the 11-year solar cycle. The extreme G5 storm of May 2024 made aurora visible far south, including Hanle, Ladakh. India’s Hanle Dark Sky Sanctuary and the Indian Astronomical Observatory provide ideal sites for observing such events, while ISRO’s Aditya-L1 mission studies the solar drivers behind them. Thus, auroras are visible signatures of Sun–Earth coupling.