The corona is the outermost shell of the Sun; 10 million times less dense than the surface, extending 8 million kilometres into Outer Space and with a temperature of 1 million ºC. However, each of these numbers has a story to tell, since the corona is a very complex, and little understood, part of the Sun’s atmosphere .
One of the reasons for this lack of understanding lies in the first figure: 10 million. The corona isn’t very dense, and is therefore dimmer than the rest of the Sun. This leads to it being washed out by the Sun’s fiery heart and observers on Earth not being able to see it .
However, during a total solar eclipse, the corona becomes visible. This is where the Moon is positioned between the Earth and the Sun, leading to the Moon blocking out the majority of the Sun, whilst leaving the corona intact and visible to observers back on Earth. Because of this, many scientists flock to wherever a total solar eclipse is taking place in order to snap up their measurements in the short time they have (a few seconds to 7.5 minutes), before waiting for the next one, which is on average just every 18 months !
Some scientists are a bit impatient though, so they use a coronagraph to see it, without the need for a total solar eclipse. It uses an occulting disc (an opaque circle) which sits within the telescope, covering the Sun to a similar extent to what the Moon does during a total solar eclipse, but for a much longer time .
The corona during the total solar eclipse in 2019 at ESO's La Silla Observatory in Chile.
Credit: ESA/CESAR/Observatorio Astrofisico di Torino, CC BY-SN 3.0 IGO
Windy… in Space!
The next figure, 8 million kilometres, was the subject of a discovery in mid 2014 that the solar corona was a lot larger than previously thought. Using data gathered from studying ripples in the Sun’s magnetic field known as Alfvén waves (like sound waves, but oscillating around every 4 hours instead of hundreds of times per second, and are 10 times Earth’s diameter instead of up to 17 metres -audibly ) caused by interactions of large solar events within the corona.
These large events in question come in many forms; most notably though, solar flares and their larger cousins: coronal mass ejections (CMEs). Solar flares are large explosions of energy, whilst CMEs are bubbles of hot gas (plasma) ejected by the Sun: both can cause worldwide blackouts and both are caused by the Sun’s peculiar and dynamic magnetic field .
Due to the Sun being an entirely fluid object instead of a rocky body like Earth, it has a complex magnetic field. Individual parts of the field (known as flux ropes ) twist and build up energy, before snapping back to become flat again like an elastic band (magnetic reconnection ), whilst ejecting plasma in the process, in the form of solar flares and CMEs. However, most of these ejections of magnetic energy aren’t directed at Earth (since their direction is random), but the odd time that it does happen, it creates disruption from as small as a few satellites being damaged, to as large as a worldwide power outage (although this is extremely unlikely) .
Coronagraph of Coronal Mass Ejection on 23 July 2012, which narrowly missed Earth
The final figure: 1 million ºC This is the corona’s temperature. However, given it’s the outer extremity of the Sun, it’s nearly 200 times hotter than the Sun’s ‘surface’ (the photosphere), at a measly 5,500ºC !
Seeing as the photosphere is closer to the centre of the Sun than the corona, it would follow that it would be hotter. However, that’s evidently not the case… but why? There are a few theories to explain this.
First off is the wave theory. It explains it by saying that the Alfvén waves described earlier are launched at a certain frequency from within the Sun to the corona. This then starts exciting charged particles and thereby heats the corona.
And then there’s the mystery of nanoflares: tiny solar flares that are the result of the same process of magnetic reconnection. However, the energy produced then heats up the surrounding environment by accelerating nearby particles.
Notably, these two theories do involve similar processes of changes in the Sun’s dynamic magnetic field, and therefore they may not in fact be different processes. Instead, some believe the Alfvén waves are caused by nanoflares, heating the plasma around the event .
Excitingly though, there has been a recent development on the nanoflare front to add to the evidence for this theory, under the nickname ‘campfires’. Solar flares may be a million to a billion times their size (at a microscopic 400-4000km in width), and they may only last from 10 to 200 seconds, but when put together there is the possibility that they are responsible for the heating of the corona.
ESA and NASA’s Solar Orbiter found around 20,000 campfires in just 70 minutes of observing, meaning this could even be a viable constant source of heat (something people have been searching for for decades). And what’s more, a team at the Max Planck Institute for Solar System Research in Germany, along with a PhD student from Peking University in China, were able to model these campfires and a few of their predictions closely matched observations taken by Solar Orbiter. This meant they could utilise the magnetic field lines used in the model to see how the phenomenon worked, and how viable it would be as the corona’s heat source .
Credit: Solar Orbiter/EUI Team/ESA & NASA; CSL, IAS, MPS, PMOD/WRC, ROB, UCL/MSSL
However, it isn’t ‘case closed’ for the corona’s mysterious heating, since there are still many more observations to be made until we’re certain what the full story is.
by George Abraham, ADAS member
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