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With orbiting telescopes now monitoring the Sun minute by minute, astronomers are witnessing ever more spectacular solar phenomena on our nearest star
In early March 2012 amateur astronomers watched with excitement as a monster began to appear over the limb of the Sun. The beast in question was an enormous new sunspot, slowly moving into view as our star rotated. Seething with energy and occasional flickers of activity, the sunspot quickly caught the attention of solar scientists. But unbeknown to those studying it here on Earth, its most violent act was yet to come.
Just after midnight on 7 March 2012 the sunspot let loose a violent blast known as a solar flare, sending waves of energetic radiation surging into the Solar System. Along with this intense burst a huge cloud of plasma, a coronal mass ejection, was flung out at over 3.5 million km/h.
Ferocious solar explosions are not the only violent events that cause the Sun to shudder.
In the 1980s and early 1990s solar scientist Alexander Kosovichev began working in the fledgling field of helioseismology — the study of the Sun's numerous surface and subsurface oscillations. Kosovichev wanted to know what effect solar flares have on their surroundings.
With the help of Valentina Zharkova at Glasgow University, Kosovichev modelled how the shockwave from a powerful solar flare could perturb the solar surface beneath it. Earlier research had indicated that solar flares might be able to boost the oscillations of the solar surface, but Kosovichev and Zharkova's models predicted that something far more dramatic could be produced — a sunquake. "We found that sunquake waves should be observed as expanding circular ripples on the solar surface," recalls Kosovichev. To see if the predictions were correct the scientists would need some way to study the Sun's tumultuous surface in great detail. Thankfully one spacecraft was about to revolutionise our view of our star. Its name was the Solar and Heliospheric Observatory (SOHO).
On 2 December 1995, NASA and ESA launched SOHO and sent it to orbit the Sun 1.5 million km from Earth. Its job was to scrutinise the Sun by observing it across a variety of wavelengths. On the ground, researchers like Kosovichev eagerly awaited the bounty of information the new solar observatory would provide.
"The first flare data was obtained in July 1996. I analysed it, and to my great surprise found the ripples," says Kosovichev. "The observational data corresponded very well to the previously developed flare and helioseismic models. The observations revealed a compact impact on the Sun's surface caused by the downward moving shock, and circular wave ripples travelling away from the impact place." They had found a sunquake.
Rage and Grace
SOHO's Michelson Doppler Imager (MDI) allowed the team to see, for the first time, the surface of the Sun rising and falling due to the shock of a solar flare. "The 'ripple' is observed as a displacement of the solar gases on the visible surface of the Sun," explains Kosovichev. "The MDI instrument measured the gas motions via the Doppler effect, by measuring displacements of the nickel [spectral] line in the Sun's radiation spectrum. When the solar material is moving toward us the wavelength of the radiation is shortened, called blueshift; when it is moving away from us the wavelength of the radiation is lengthened, called redshift. Using this technique the MDI instrument measured the up and down motions of the solar gases very accurately, to a precision of about 20m/s."
The size and speed of the sunquake was sobering. At their fastest, the 3km-high ripples raced across the Sun at more than 400,000km/h. Since that first observation of a sunquake by SOHO, solar scientists have been trying to understand exactly how these incredible events happen and what they can tell us about the Sun. Sergei Zharkov from the Mullard Space Science Laboratory in the UK has recently used NASA's Solar Dynamics Observatory (SDO) to study a pair of sunquakes. "SDO provides essentially continuous coverage of the Sun," says Zharkov. "The spacecraft provides a 4,096 pixel by 4,096 pixel image every 45 seconds."
It's this remarkable rate of observation that makes SDO so powerful, as it allows Zharkov and other solar scientists to watch our star's every twitch. There's a slight problem, however. "Sunquakes are relatively rare events," says Zharkov. "Since SDO's launch a couple of years ago we have seen four more or less confirmed sunquakes. It's possible that they happen but we just don't see them."
Finding a sunquake requires scientists to identify a ripple in a roiling sea of plasma. "The solar surface itself is constantly oscillating," explains Zharkov. "Depending on the strength of the source it may not be easy to find the signal behind all the oscillation that already exists, so that's one possibility [for why we are not seeing them]." Kosovichev also notes that location is a factor in whether a sunquake will be produced. "In many flares the magnetic energy is released high above the solar surface in the corona," he says. "Such flares don't cause significant impacts on the surface and don't produce seismic waves on the Sun."
The Secrets Within
The driving force behind each sunquake is still thought to be the initial solar flare — a product of a twisted, pent-up region of the Sun's magnetic field. "The magnetic field appears to be stable but within it there is quite a lot of energy," says Zharkov. "At some point it becomes unstable and releases all that energy. We see enormous heating and particles being accelerated, with much of the energy from the flare going into interplanetary space."
Yet some of the flare's almost unfathomable ferocity is directed toward the Sun itself. One of the sunquakes that Zharkov and his colleagues observed with SDO recently blasted across the solar surface with the same energy as the detonation of 478 billion tons of TNT.
Remarkably, the quakes themselves could work like a huge sonar device, allowing helioseismologists to study the interior of the Sun. "The shape of the ripples contains information about how the waves are created," Zharkov explains. "Several groups are now trying to analyze the ripples to decode the information they hold about the subsurface structures that they pass through."
Even with these latest observations, the exact mechanism by which sunquakes are produced is still unclear, says Zharkov, and several theories are being considered. "One involves some of the particles from the flare creating a pressure pulse, which generates a sunquake. The second theory, called 'backwarming', also proposes that a pressure pulse causes the quake, but this time the pulse is caused by the heat from a solar flare radiating towards the photosphere. Another scenario suggests that the recoil of the magnetic field during a flare creates a force with enough energy to generate a sunquake." Zharkov and his colleagues in the solar science community are now looking at the data produced by the SDO to establish which theory fits the bill.
While the mystery surrounding sunquakes rolls on, other images from SDO are throwing up yet more surprises. In September last year researchers from Aberystwyth University stumbled across an incredible event occurring within the Sun's fiery atmosphere.
"My colleague Xing Li had been browsing quick-look, low-resolution data from SDO and happened to see this strange event," recalls Huw Morgan from the Aberystwyth team. "He had the foresight to download the full resolution data, which confirmed the event's uniqueness and beauty."
The high-resolution images from SDO revealed a startling maelstrom of activity, the appearance of which would have been familiar to many of us — Li had spotted a swirling solar tornado. But even the boldest Earthly storm chasers would baulk at the enormous scale of this solar leviathan. The spinning vortex of plasma stretched more than 150,000km above the Sun's surface — that's more than 10 times Earth's diameter — swirling at a staggering 300,000km/h.
Unusual and Unexplained
According to Morgan there have been reports of tornadoes on the Sun as far back as the early 1900s, but the one studied by the Aberystwyth researchers stands out for several reasons. "It was very large, and the rotation was coherent for many hours," Morgan reveals. "Short-lived, smaller tornadoes often become apparent before a prominence eruption. This one remained in place for several hours. The strange dynamics within the structure were also quite unique."
As yet the team aren't sure how often solar tornadoes like the one spotted by Li form, but they're making progress in trying to understand what might be causing them. The tornado probably begins with a prominence — a huge wisp of plasma reaching up from the Sun's surface.
"The prominence that turns into a tornado is quiet for a few days prior to the event," says Morgan. "We believe that the Sun then pumps out helical magnetic fields like a Slinky into the prominence, activating the whole structure. Magnetic disturbances occur at the bases of the helices, which then pump plasma into the magnetic fields. As the plasma follows the helical shape, it appears as a tornado when we view the structure along the axis of the helix."
On the surface, understanding the detailed physics of a ferocious solar tornado might not seem that important to us, 150 million km away here on Earth. Yet the team's research into the helical magnetic fields in the Sun's atmosphere may well help to explain other violent events that can affect us.
"Perhaps this is one way that coronal mass ejections are formed," Morgan says. "Understanding such phenomena is a very important part of our work since they can have an impact on Earth and our society."
As the Sun becomes more and more active, scientists will no doubt relish the opportunities that solar maximum can provide. With the likelihood of more solar flares, eruptions and activity on the Sun's surface, who knows what new events they'll observe. What's clear is that with an impressive collection of solar observatories watching intently from space, we'll have a front-row view of the turbulent phenomena that define our violent star.
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