A new study indicates that solar tornadoes may have been misnamed because scientists have so far only been able to observe them using 2D images.
Composite image of an erupting solar prominence observed by NASA’s Solar Dynamics Observatory on August 31, 2012. Image credit: NASA / SDO / GSFC.
Solar tornadoes — gigantic structures several times the size of the Earth — have been observed on the Sun for around a hundred years.
They are so called because of their striking shape and apparent resemblance to tornadoes on Earth, but that is where the comparison ends.
Whereas terrestrial tornadoes are formed from intense winds and are very mobile, solar tornadoes are instead magnetized gas. They seem to be rooted somewhere further down the solar surface, and so stay fixed in place.
The term was re-popularized a few years ago when solar astronomers looked at movies obtained by the Atmospheric Imaging Assembly (AIA) instrument on NASA’s Solar Dynamics Observatory. These show hot plasma in extreme UV light apparently rotating to form a giant structure taking the shape of a tornado.
Now, using the Doppler effect to add a third dimension to their data, a team of European astronomers has been able to measure the speed of the moving plasma, as well as its direction, temperature and density.
Using several years’ worth of observations, they were able to build up a more complete picture of the magnetic field structure that supports the plasma, in structures known as prominences.
“We found that despite how prominences and tornadoes appear in images, the magnetic field is not vertical, and the plasma mostly moves horizontally along magnetic field lines,” said team leader Dr. Nicolas Labrosse, from the University of Glasgow.
“However we see tornado-like shapes in the images because of projection effects, where the line of sight information is compressed onto the plane of the sky.”
“The overall effect is similar to the trail of an aeroplane in our skies: the aeroplane travels horizontally at a fixed height, but we see that the trail starts above our heads and ends up on the horizon. This doesn’t mean that it has crashed,” added team member Dr. Arturo López Ariste, from the University of Toulouse.
“Solar tornadoes sound scary but in fact they normally have no noticeable consequences for us,” said team member Dr. Brigitte Schmieder, from Paris Observatory.
“However, when a tornado prominence erupts, it can cause what’s known as space weather, potentially damaging power, satellite and communication networks on Earth.”
The scientists presented their results April 6 at the European Week of Astronomy and Space Science (EWASS) in Liverpool, UK.
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Nicolas Labrosse et al. High-resolution diagnostics of solar prominences and prominence-like tornadoes. EWASS 2018, abstract # 274
