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Auroras and space weather
Bud Kuenzli
What are auroras?
Auroras are light displays typically seen in the skies of high latitude
areas centered on the Earth's magnetic north and south poles. The
Aurora Borealis (Northern Lights) are normally visible from the most
northerly regions of Europe, Russia, Canada, and Alaska; the Aurora
Australis (Southern Lights) are usually seen from the southernmost
regions of Australia, New Zealand, and South America.
David Cartier Sr.
NASA/GSFC
Aurora over the Chena Lakes in North Pole, Alaska, in September 2007.
Auroras are caused by electrically-charged particles from the Sun
colliding with the high-altitude part of the atmosphere known as the
thermosphere, roughly 85–500 km above Earth’s surface. These
collisions temporarily ‘excite’ the atoms and molecules of the air
which then emit light as they return to their ‘ground’ state.
The colour of the light emitted is unique to the atom or molecule
involved; oxygen emits green or brownish-red; nitrogen produces
reds or blues. The exact colours will depend on the collision energy
and gas density (altitude), and other colours such as pink and yellow
can be created through mixing.
The Sun throws off a constant stream of diffuse plasma (a gas of free
electrons, protons, and positive ions) from its outer atmosphere, the
corona. Known as the solar wind, it is emitted in all directions but
varies greatly in density, particle energy, and speed depending on
coronal conditions. At times, ‘explosions’ on the surface of the Sun
cause a massive amount of additional plasma to be violently ejected
from the corona; this is known as a coronal mass ejection. All these
fluctuations directly affect the extent and intensity of aurora; periods
of high solar activity often cause aurora to appear at lower latitudes.
Sun-Earth interactions – including aurora – are referred to as space
weather, and as with terrestrial weather there are good reasons for us
to study it. Auroras often show a ‘curtain’ structure composed of
parallel rays, aligned with Earth's magnetic field lines. Massive
electrical currents flow along these arcs, known as auroral electrojets
or Birkeland currents. These can damage or destroy spacecraft in
orbit. On the ground they can induce massive power surges in
electrical grids, burning out transformers and causing blackouts.
Radio and satellite communications can be disrupted, and astronauts
are put at risk by high-energy particle streams. Understanding and
predicting space weather allows us to prepare for solar ‘storms’ and
minimise damage. Solar physicists at Queen’s use satellites and
ground-based observatories to assist in this worldwide effort.
The solar wind typically takes a few days to arrive at Earth. Most
particles are deflected by the planet’s magnetic field but some are
trapped and accelerated towards the poles, causing auroras.
Auroras have been observed on most planets: Venus, Mars, Jupiter,
Saturn, Uranus, and Neptune. They vary based on planetary magnetic
fields and atmospheric composition, but are fundamentally similar in
origin. Jupiter is unique though – it has strong interactions with its
moons, particularly Io, that generate additional aurora and give it the
finest displays in the solar system.
Left: Saturn with aurora, combined from separate shots of ultraviolet and visible light taken with the Hubble Space Telescope.
Right: X-ray auroras on Jupiter observed by the Chandra X-ray Observatory overlaid on a simultaneous optical image from the
Hubble Space Telescope.
Find out more about the work of Queen’s Astrophysics Research Centre at http://star.pst.qub.ac.uk/
X-ray: NASA/CXC/SwRI/R.Gladstone et al.; Optical: NASA/ESA/Hubble Heritage
Where do the particles come from?
What causes auroras?
Left: Earth’s magnetosphere safely deflects most particles, protecting us from the harsh particle radiation of interplanetary space. Right: To supplement ground and satellite observations, scientists fire sounding rockets into live aurora to probe them. This launch
took place on 3rd March 2014 over Venetie, Alaska.
NASA/ESA/J. Clarke
On August 31st 2012 a long filament of plasma that had been hovering in the sun’s corona erupted out into space at over 900 miles per
second (SDO image left). The coronal mass ejection did not travel directly toward Earth, but did connect with the magnetosphere,
causing extensive aurora to appear for several nights from 3rd September. The photo on the right was taken in Whitehorse, Canada.
The Aurora Borealis seen from the International Space Station on September 26, 2011.
NASA/Aaron Kaase
Why study the aurora?
NASA
There are two types of aurora – diffuse and discrete. Diffuse aurora
are a featureless background glow, often invisible to the naked eye.
Discrete aurora are what most people think of as ‘the aurora’ –
sharply defined structures that vary in intensity from barely visible to
bright enough to read by, and in activity from ‘quiet arcs’ to ‘active
aurora’ which evolve and change constantly.
NASA/Christopher Perry
The ghostly glow of the aurora has fascinated humans since ancient times. Some believed they were the spirits of the dead, others that they were fires lit by dwarves. Many cultures linked them to Gods,
either at play or at war! One of the most charming stories is a Finnish folk tale in which an arctic fox is running far in the north and touching the mountains with its fur, so that sparks fly off into the sky as the
northern lights. The Finnish term for the Northern Lights, Revontulet, literally means “foxfire”. These days we know more about the physics involved, but that doesn’t dilute the beauty of the auroras.