Getting to the bottom of the Sun's plasma eruptions
When the Sun sends a great mass of solar material hurtling through space, the repercussions can be felt here on Earth in the form of a geomagnetic storm. One EU-funded scientist set out to understand why these eruptions happen, and to create a methodology for predicting the timeframe between the explosion and its impacts 150 million kilometres away on our planet.
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From time to time, the Sun ejects plasma and magnetic fields in a massive burst. The phenomenon is known as a coronal mass ejection (CME), and can occur up to five times every day.
We earthlings are usually oblivious to these cosmic goings-on. But for those rare occasions when a CME makes its presence felt on Earth – in a worst case scenario by temporarily shutting down electricity supplies – it makes sense to understand the origins of the beast, and when it will arrive.
Spiros Patsourakos took on this challenge when he received one of the EU’s Marie Curie reintegration grants to allow him to continue research in his field on returning to his native Greece after more than 10 years abroad. Over four years, the grant enabled him and his colleagues – through the SEP project – to shed new light on the genesis of CMEs, to solve an age-old discussion on some ‘disturbances’ observed on the Sun’s surface, and to take great strides towards predicting when CMEs will arrive on Earth.
CMEs are an “interesting physics problem”, says Patsourakos. “And we don’t yet know the details.” What we do know is that CMEs are major drivers of space weather.
Not every CME will reach Earth. And not every CME reaching Earth will have a visible impact. A CME would need to be particularly powerful and transporting a south-facing magnetic field if it is to disrupt the Earth’s magnetic fields, disrupt currents and switch off electricity. But it can happen, and it did in the Canadian province of Quebec in 1989. A geomagnetic storm tripped circuit breakers on the power grid, leaving the region without power for nine hours.
“These events are not frequent, but you only need one with the appropriate conditions to cause major disruption,” says Patsourakos.
Knowing the ropes
Before a CME is expelled, sets of magnetic field lines – known as flux rope – begin to twist and turn around the future CME launch pad. A “hot topic” within solar physics is whether these flux ropes are a pre-requisite to an eruption or not.
While Patsourakos’ research hasn’t quite provided an answer to this debate once and for all, it did provide radical new insights into the build-up to a CME. By observing images of the Sun at various temperatures through a powerful telescope providing very regular and high-resolution images, Patsourakos was able to witness the formation of magnetic flux rope and then – seven hours later – the eruption.
Traditional studies involve going back and looking at images from one hour before a CME took place, while Patsourakos’ approach showed that the build-up starts much earlier. “If you only focus on smaller temporal intervals, you may interpret the data differently,” says Patsourakos. He believes that his revelation could change the way scientists approach observations.
As for whether flux ropes are necessary for a CME to take place – Patsourakos and his colleagues are now working on statistical studies in a bid to see how common the flux ropes are, and eventually to answer this all-important question.
Another question dividing space scientists is whether or not the disturbances travelling across the solar surface during the initial stages of a CME are caused by CME-driven waves, or whether they are simply a shadow of the fledgling CME.
“The debate has been settled,” says Patsourakos – and the answer will please both camps. “During the same phenomenon, there are some elements that can be explained as being waves, others as non-wave,” he says.
Estimating arrival time
Predicting when a CME will arrive on Earth still requires a little more work, says Patsourakos. Speed can vary from a few hundred kilometres per second to more than 2000 kilometres per second. By looking in depth at the factors affecting the speed at which a CME travels, and in particular perturbed solar wind, he has already improved on previous methodologies. More extensive testing is now needed – and planned.
Since the SEP project ended in 2014, Patsourakos and his colleagues have continued to work on the many open questions from his lab in the physics department at the University of Ioannina. The next steps are statistical studies of CMEs’ pre-eruptive configuration and propagation.