The red auroral arc capture from the Tashi Choeling Gompa nunnery in Hanle, Ladakh. Credit: Wangchuk Namgyal, Stanzin Norlha, & Stanzin Norboo

The strongest solar storm in two decades coloured skies last weekend across the United States, Canada, Europe and China in bright hues of violet, blue and green. This was an uncommon shift in latitudinal display of the Aurora Borealis or northern lights.

But it was an even bigger surprise for sky gazers in the Indian Himalaya, who caught the spectacle at lower latitudes beyond conventional 'auroral zones'. The sky in Ladakh's Hanle, situated on the westernmost Himalayan stretch at a latitude of roughly 32°N, glowed a stunning crimson on the night of May 10, prompting solar scientists to analyse the event’s extreme nature.

Cameras continuously capturing the sky at Hanle and Merak astronomical observatories in Ladakh recorded a stable red auroral arc display from past midnight till dawn. This was the third such capture over the region within a year. A similar aurora was visible in Ladakh during the night of April 22-23, 2023 after the strong solar storm of April 21.

Auroras occur near the poles and are dynamic light patterns of pink, purple, blue and green. "However, the Ladakh aurora was a stable red and endured for hours," said Dorje Angchuk, astrophotographer and engineer-in-charge at Hanle. Besides the red arc, blue and violet bands caused by particles in the lower atmosphere, were also captured.

His colleague Stanzin Norla, an engineer at the Hanle Dark Sky Reserve, captured the red glow during routine telescope observations on the northwest horizon."It lasted till dawn and we were lucky to capture it on our all-sky camera," he said.

These were 2024's strongest solar flares so far, triggered from the active sunspot AR 13664. A series of at least seven coronal mass ejections (CMEs) began entering Earth's outer atmosphere on May 10, resulting in auroral spectacles across the world and the red display in Ladakh.

A significant increase in solar flare activity is responsible for low-latitude auroral displays. Head of the IISER Kolkata-based Center of Excellence in Space Sciences India (CESSI) Dibyendu Nandi said such auroral red arcs are likely caused by heating of the atmosphere by electric currents circulating between the earth and outer space.

CESSI solar physicist Yoshita Baruah said a rapid-fire of CMEs within a short period may have created a sustained impact. The centre has been recording expected heightened activity as the sun peaks its 11-year solar maximum cycle, when it displays the most sunspots.

The eruptions of plasma and magnetic fields from the sun’s corona caused a fierce geomagnetic storm. The last extremes were the ‘Halloween storms’ of October 2003, according to the United States’ National Oceanic and Atmospheric Administration.

Other establishments of the Indian Institute of Astrophysics (IIA) in Kodaikanal and Gauribidanur kept a close watch on signatures of the incoming geomagnetic storm. They traced the heightened activity to four coronal mass ejections (CMEs). “The CMEs’ magnetic fields and other properties remained strong even after entering Earth’s atmosphere,” says solar astronomer Wageesh Mishra at IIA in Bengaluru.

His initial analysis suggests that the CMEs collided with one another as the sun shot them off in a series. They then merged creating a huge jump in magnetic field as they reached the Earth.

Though rare, red aurorae have been sighted during other extreme solar storms in the past from low latitude observatories1,2.