Aurora Borealis and Aurora Australis – UPSC

• An aurora (polar light) is a natural display of coloured light in the night sky. It is produced when energetic charged particles, mostly electrons, from the Earth’s magnetosphere collide with oxygen and nitrogen in the upper atmosphere, at about 100–400 km, in the ionosphere and thermosphere.
• In the north it is the aurora borealis (northern lights); in the south, the aurora australis (southern lights).
  • The northern name comes from Aurora, the Roman goddess of dawn, and Boreas, the Greek north wind, and was popularised by Galileo Galilei (1619) and Pierre Gassendi (1621).
  • James Cook (1773) named the southern display.
• Auroras are common at high latitudes, rarer in middle latitudes, and almost never seen near the equator, except during great geomagnetic storms.

How an Aurora Forms

• Source:
  • The Sun continually emits the solar wind, a stream of protons and electrons (plasma) carrying the Sun’s magnetic field.
  • Bursts of activity add coronal mass ejections (CMEs): billions of tonnes of magnetised plasma, often following solar flares from sunspot groups.
  • Both are driven by magnetic reconnection in the corona, which converts magnetic energy into heat and motion.
• The magnetosphere:
  • Bow shock: a shock wave on the sunward side, where the solar wind is slowed and deflected around the Earth through the magnetosheath
  • Magnetopause: the outer boundary of the Earth’s field, compressed to about 6–10 Earth radii on the day side
  • Magnetotail: the night-side field, stretched by the solar wind to hundreds of Earth radii
  • Van Allen radiation belts: zones of trapped particles inside the magnetosphere
• Energy transfer: when the interplanetary magnetic field points southward, it reconnects with the Earth’s field and loads energy into the magnetotail. Sudden releases (substorms) shoot electrons down the field lines toward the poles.
• Light emission:
  • Electrons strike oxygen and nitrogen atoms and molecules and excite them.
  • As the atoms return to their ground state they emit photons of characteristic colours, like gas in a neon tube.
  • Ionised nitrogen regaining an electron also glows.
• Auroral ovals: the field lines funnel particles into two rings centred on the geomagnetic poles, not the geographic poles.
  • The rings lie at about 65–70° magnetic latitude: over northern Scandinavia, Iceland, southern Greenland, Alaska and northern Canada, and around Antarctica.
  • Both ovals usually light up at the same time, as near mirror images.
  • In strong storms the ovals expand towards the equator.

Why Auroras Have Different Colours and Shapes

• The colour depends on which gas is struck, at what height (air density) and how energetic the electrons are:
  • energetic electrons penetrate deeper and excite green oxygen and blue nitrogen light
  • low-energy electrons stop high up and give red
• Shapes: arcs, bands, rays, curtains that trace the magnetic field lines, and diffuse glows. They may change within seconds or glow steadily for hours.
• Auroras also emit ultraviolet and X-rays, which satellites image from space.

When and Where Auroras Are Seen

• Darkness and clear skies are needed: auroras occur in daylight too, but cannot be seen.
• The best seasons are near the equinoxes in the long nights of autumn and spring; geomagnetic activity also peaks around the equinoxes.
• Solar cycle: auroras are most frequent and reach lowest latitudes near solar maximum of the ~11-year sunspot cycle.
  • Near maximum the Sun produces about three CMEs a day; near minimum, about one in five days.
  • Solar Cycle 25 began in December 2019 and peaked in October 2024 (smoothed sunspot number ~161), stronger than forecast.

Great Geomagnetic Storms and Low-Latitude Auroras

• Carrington Event (1–2 September 1859): the largest storm on record. Auroras were seen near the tropics, and telegraph lines sparked.
• Quebec blackout (March 1989): induced currents collapsed the Hydro-Québec power grid for about nine hours.
• Halloween storms (October–November 2003):
  • included the largest solar flare ever recorded by GOES satellites
  • damaged satellites and disrupted communications
  • aircraft were rerouted away from polar routes
• May 2024 (10–11 May):
  • the first extreme (G5) storm since 2003, from sunspot region AR 3664
  • auroras seen as far south as Mexico and northern India
  • all-sky cameras of the Indian Astronomical Observatory at Hanle and at Merak, Ladakh, recorded the aurora
  • India’s Aditya-L1 solar observatory, at the Sun–Earth L1 point since January 2024, observed the eruptions
• October 2024 (10–11 October): an intense red aurora was visible to the naked eye from Hanle and Merak, the fourth such sighting in Ladakh in the current cycle.
  • Low-latitude auroras are usually red, because only the high-altitude oxygen glow of a greatly expanded oval rises above the northern horizon.

Effects of Auroras and Geomagnetic Storms

• Radio: high-frequency radio blackouts and disturbed long-distance radio and polar flight communication.
• Navigation: GPS errors from ionospheric disturbance.
• Satellites: atmospheric heating increases drag on low-orbit satellites. About 40 Starlink satellites were lost after a minor storm in February 2022.
• Ground infrastructure: geomagnetically induced currents in power grids, pipelines and railway signalling.
• Astronauts and high-altitude polar flights receive higher radiation doses.

Auroras on Other Planets

• Any planet with an atmosphere and a magnetic field can have auroras.
  • Jupiter and Saturn have powerful, permanent auroral ovals; Jupiter’s are fed partly by volcanic material from its moon Io.
  • Uranus and Neptune have auroras tilted with their offset magnetic fields.
  • Mars, with only patchy crustal magnetism, shows diffuse auroras over large areas.

Missions Studying the Aurora

• THEMIS (NASA, 2007): five spacecraft that found what triggers the sudden brightening of auroras during substorms (magnetic reconnection in the magnetotail).
• Arase / ERG (JAXA, 2016): studies how particles in the radiation belts are accelerated and lost during storms.
• Aditya-L1 (ISRO, 2023): India’s first solar mission, watching CMEs and the solar wind before they reach the Earth.