Bright aurora displays were seen in the night skies of both hemispheres in the early morning of Tuesday 17 March 2015, with reports of displays as far south as Oregon, Kansas and Virginia in the US and England, Germany and Poland in Europe, and as far north as New South Wales and Western Australia. The displays followed a coronal mass ejection (stream of charged particles, mainly electrons and protons but with some ionized atoms of heavier elements such as helium or oxygen) which was ejected from the Sun on 15 March 2015. The displays were expected, but turned out to be much more dramatic than predicted, rated as a level 4 geomagnetic storm, rather than a predicted level 1 storm.
Aurora Borealis (Northern Lights) seen from Jyväskylä in Central Finland on 17 March 2015. Juha Kinnunen/Space Weather.
Aurora displays come in a variety of colours, caused by electrons from the coronal mass ejection striking different atoms in the Earth's atmosphere. This is because the energy of the atoms increases each time it is struck by an electron, but atoms can only absorb so much energy before they must release some, and each atom always releases energy as light (photons) at a specific wavelengths. In the Earth's atmosphere this is effected by altitude, thus Oxygen releases either green or red light and Nitrogen releases either blue or violet light. Typically auroras shimmer as different reactions occur, photographs do not really do them justice.
Gasses release light at specific wavelengths in response to other stimuli besides coronal mass ejections. Thus the blue colour of the daytime sky is the colour of Nitrogen in the lower atmosphere reacting to the (steady) energy input from sunlight, whereas the red colour of sunrises and sunsets is the colour of oxygen higher in the atmosphere reacting to the same; we see this at dawn and dusk because the sun is no longer in line of sight with the lower atmosphere. Neon lights are red because Neon gas releases red light in response to electrical charge, and Sodium lights orange for the same reason. Molecules made up of more than one sort of atom, such as Carbon Dioxide (CO₂), Water (H₂O) or Methane (CH₄) release light in the infra-red part of the spectrum, which can lead to warming of the atmosphere (the Greenhouse Effect), hence the current concerns about the release of such gasses into the atmosphere by industrial processes, and the effect this might have on our climate.
Aurora Borealis (Northern Lights) seen from the Kola Peninsula in Russia on 17 March 2015. Трифонова Любовь/Space Weather.
Coronal mass ejections are a product of solar flares from sunspots. Sunspots are magnetic storms on the face of the Sun. These inhibit convection currents in the Sun's photosphere, causing localized cooling; the surface of the Sun can drop from its usual 5778 K to as low as 3000 K in a Sunspot, causing them to darken compared to the rest of the Sun (though they are in fact still pretty bright). Since Sunspots are magnetic they have magnetic poles, with positive and negative charges. These can be connected by coronal loops, streams of magnetic flux carrying plasma above the surface of the Sun. This can lead to a short circuit in which a large amount of magnetic energy is released suddenly, producing a brightening we perceive as a Solar Flare. This in turn can lead to the release of a coronal mass ejection, a stream of charged particles which takes an average of two days to reach the Earth.
Solar Heliospheric Observatory movie of the 15 March 2015 coronal mass ejection. Space Weather.
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