Our sun is not exactly a serene ball of burning hot plasma. In fact, it releases colossal eruptions on a somewhat frequent basis; such coronal mass ejections, when directed to Earth, are the cause of geomagnetic storms.
From space close to Earth, we can measure them quite well with satellites and other spacecraft. But in 1998 something incredibly fortuitous happened. Not only was a spacecraft close to Earth able to measure a coronal mass ejection (CME), another spacecraft near Mars was properly aligned to receive the solar explosion.
This meant that the two 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 eruptions evolve.
Coronary mass ejections may not be as visible as solar flares (which sometimes accompany them), but they are much stronger. They occur when the twisted lines of the Sun’s magnetic field reconnect, transforming and releasing huge amounts of energy in the process.
This happens in the form of a CME, in which large amounts of ionized plasma and electromagnetic radiation, grouped in a helical magnetic field, are launched into space on the solar wind. When flowing past the Earth, CMEs can interact with the magnetosphere and ionosphere, creating observable effects such as satellite communications problems and auroras.
But what happens to CMEs as they pass by Earth, in interplanetary space, has been much more difficult to study. We have far, many fewer tools there, for one thing. The chances of two spacecraft at great distances separated from the Sun that detect the same CME are incredibly small.
Fortunately, this was the case in 1998 with two spacecraft designed to study the solar wind. NASA’s Wind spacecraft, at the Lagrangian point L1 at about 1 astronomical unit (the distance between Earth and the Sun), first observed a CME on March 4, 1998.
Eighteen days later, the same CME reached Ulysses, a spacecraft that at that time was at a distance of 5.4 astronomical units, more or less equivalent to Jupiter’s average orbital distance.
Astronomers have now examined data from both meetings to characterize, for the first time, how a CME changes as it travels deeper into the Solar System. In particular, they studied the magnetohydrodynamic evolution of the embedded 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 significantly. The team believes this was probably due to an interaction with a second magnetic cloud, which traveled faster than the first, reaching it and compressing it until it reached Odysseus.
This could explain why the helical structure of the magnetic cloud in the CME became more twisted when it reached 5.4 astronomical units – rather than less, as would be expected. The magnetic interaction between the two clouds could degrade the outer layer, leaving behind a more twisted core.
“What is clear from this analysis is that at 5.4 astronomical units, the second magnetic cloud interacts strongly with the first,” the researchers wrote in their paper.
“As a result, the magnetic structure of the previous magnetic cloud is strongly deformed. In fact, its large-scale rotation extends far beyond the back of the next magnetic cloud and is de facto a form of rotation of the background magnetic field.”
It would be fascinating to see more studies on this topic – and no matter how lucky the observation was, we could get them. 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 and BepiColombo from JAXA and ESA’s Solar Orbiter orbiting the Sun at varying distances, it could only be a matter of time before the stars – or spacecraft, in this case – line up.
The research was published in The Astrophysical Journal Letters.