What are aurora australia and aurora borealis? How are these triggered?

What the two auroras are

Both are luminous ovals around the geomagnetic poles, typically in a ring offset from the geographic pole. Green is common. It comes from atomic oxygen. Reds and rare blues and purples come from oxygen and nitrogen at different heights, often tens to a few hundreds of kilometres up. The australis is the southern counterpart over Antarctica and the Southern Ocean. Most people never see it. The borealis is famous over Fennoscandia, Canada, and Alaska. During strong storms the oval expands toward mid-latitudes. That is when people in unusual places report lights.

They are not rainbows, which are refraction in rain. They are not lightning. They can flicker and form arcs, curtains, and coronas. They are accompanied by magnetic disturbance on the ground.

• Borealis: This is the northern oval. It is better observed because more inhabited land sits under it.
• Australis: This is the southern oval. It is the same process with fewer witnesses.
• Colour and height: Colour depends on the chemistry of oxygen and nitrogen and on the energy of incoming particles.

How they are triggered

The Sun emits a solar wind of plasma. Coronal mass ejections and fast streams from coronal holes add extra energy and a southward interplanetary magnetic field that couples efficiently to Earth’s magnetosphere. Reconnection on the dayside opens the magnetic shield. Energy is stored in the magnetotail and then released along field lines into the polar ionosphere. Electrons and protons precipitate, collide with upper-air atoms, and excite them. Light is released as the atoms relax. The auroral oval is the footprint of that circuit, closed through field-aligned currents.

A geomagnetic storm, measured operationally by a high Kp index, is the trigger for vivid, expanded displays. The 11-year sunspot cycle changes how often such storms occur. Earth’s dipole tilt and the season affect how well coupling works.

• Solar source: Wind, flares, and CMEs supply the trigger on the Sun.
• Magnetospheric gate: Only certain field orientations dump energy into the poles.
• Atmospheric screen: Collisions in the ionosphere turn particle energy into photons.

Without a magnetic field, the air would be stripped over geological time. With a magnetic field, particle energy appears as polar lights instead of a stripped atmosphere. That is the chain from the photosphere to the polar sky.

Why two names, one mechanism

Borealis and australis are geographic labels. They are not two different kinds of light. Earth’s dipole has two polar funnels. The same solar storm can light both ovals, sometimes with a time lag as the magnetosphere responds. The south is mostly ocean, so the australis is photographed less often. It is not weaker. During extreme events the ovals swell. Radio, GPS, and power grids feel the same storm that observers see as coloured light. The GS point is that the trigger is solar and magnetic, and the place is the polar ionosphere in both hemispheres.