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Our Sun is not exactly a serene ball of hot plasma. In fact, it spits out colossal rashes on a fairly frequent basis; such coronal mass ejections, when directed towards the Earth, are the cause of geomagnetic storms.
From near-Earth space, we can measure them pretty well with satellites and other spacecraft. But in 1998, something incredibly fortuitous happened. Not only was a spacecraft in near-Earth space able to measure a coronal mass ejection (CME), but another spacecraft beyond Mars aligned in the right way to receive the blast as well. solar.
This means that both spacecraft were able to measure the same CME at different points on its journey from the Sun, providing a rare opportunity to understand how these powerful flares evolve.
Coronal mass ejections may not be as noticeable as solar flares (which they sometimes accompany), but they are much more powerful. They occur when twisted magnetic field lines on the Sun reconnect, converting and releasing huge amounts of energy in the process.
This occurs in the form of a CME, in which large amounts of ionized plasma and electromagnetic radiation, stored in a helical magnetic field, are launched into space by the solar wind. When passing beyond Earth, CMEs can interact with the magnetosphere and ionosphere, creating observable effects such as satellite communication problems and auroras.
But what happens to CMEs when they are off Earth, in interplanetary space, has been much more difficult to study. We have a lot, a lot less instruments there, on the one hand. The chances of two spacecraft at widely separated distances from the Sun detecting the same CME are incredibly low.
Fortunately, this is what happened in 1998 with two spacecraft designed to study the solar wind. NASA’s Wind spacecraft, at Lagrangian point L1 about 1 astronomical unit (the distance between Earth and the Sun), first observed a CME on March 4, 1998.
Eighteen days later, that same CME arrived at Ulysses, a spacecraft which at the time was 5.4 astronomical units away, roughly equivalent to Jupiter’s average orbital distance.
Astronomers have now looked at data from these two encounters to characterize, for the first time, how a CME changes as it moves deeper into the solar system. In particular, they studied the magnetohydrodynamic evolution of the onboard magnetic cloud.
Wind data (left) and Ulysses data (right). (Telloni et al., ApJL, 2020)
They found that, in the 4.4 astronomical units between the two spacecraft, the helical structure of the magnetic cloud eroded considerably. The team believe this was likely due to an interaction with a second trailing magnetic cloud that traveled faster than the first, catching up with and compressing it by the time it reached Odysseus.
This could explain why the helical structure of the magnetic cloud in the CME became more twisted by the time it hit 5.4 astronomical units – rather than less, as one would expect. The magnetic interaction between the two clouds could degrade the outer layer, leaving behind a more twisted core.
“What emerges clearly from this analysis is that at 5.4 astronomical units, the second magnetic cloud interacts strongly with the first,” the researchers wrote in their article.
“As a result, the magnetic structure of the preceding magnetic cloud is greatly distorted. In fact, its large-scale rotation extends well beyond the back of the following magnetic cloud and de facto represents a form of rotation of the magnetic field of background.”
It would be fascinating to see more studies on this topic – and, as lucky as this observation was, we might just get them. The researchers note that we are in the early stages of what could be considered a “golden age” of solar physics.
With NASA’s Parker Solar Probe, ESA’s BepiColombo, and ESA’s JAXA and Solar Orbiter orbiting the Sun at varying distances, it might only be a matter of time before the stars – or the spacecraft. spatial, in this case – line up.
The research was published in Letters from the astrophysical journal.
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