Our Sun is not exactly a serene ball of scorching hot plasma. In fact, it produces colossal eruptions somewhat frequently; these coronal mass expulsions, when directed at the 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. A spacecraft in space close to Earth was not only able to measure a coronal mass expulsion (CME), but another spacecraft that passed behind Mars was aligned in the correct way to receive the solar flare.
This meant that the two ships were able to measure the same CME at different points in their journey from the Sun, offering a rare opportunity to understand how these powerful eruptions evolve.
Coronal mass ejections may not be as visible as sunlight (which sometimes accompanies them), but they are much more powerful. They occur when the Sun’s twisted magnetic field lines reconnect, converting and releasing huge amounts of energy in the process.
This takes the form of a CME, in which large amounts of ionized plasma and electromagnetic radiation, grouped in a helical magnetic field, are thrown into space by the solar wind. When transiting the 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 pass through Earth, in interplanetary space, has been much harder to study. On the one hand, we have many, many fewer instruments. The probabilities that two spacecraft at distances widely separated from the Sun will detect the same CME are incredibly low.
Luckily, this happened 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 the Earth and the Sun), first observed a CME on March 4, 1998.
Eight days later, that same CME reached Ulysses, a probe that, at the time, was at a distance of 5.4 astronomical units, more or less equivalent to the average orbital distance of Jupiter.
Astronomers have now examined data from these two encounters 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 that traveled faster than the first, reaching and compressing it when it reached Ulysses.
This could explain why the helical structure of the magnetic cloud in the CME twisted more when it reached 5.4 astronomical units, rather than less, as might 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 article.
“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 de facto represents a form of field rotation. magnetic background “.
It would be fascinating to see more studies on this topic, and luckily for me to have the observation, we may just get them. Researchers point out 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 JAXA’s BepiColombo, and ESA’s solar orbiter orbiting the Sun at varying distances, it could only be a matter of time before the stars – or spacecraft. spatial, in this case– aligned.
The research has been published in The Astrophysical Journal Letters.