Our sun is not exactly a serene ball of scorching hot plasma. In fact, on a somewhat frequent basis it blows out colossal eruptions; such coronal mass ejections, when aimed at the Earth, are the cause of geomagnetic storms.
From space near Earth, we can measure them reasonably well with satellites and other spacecraft. But in 1998, something incredibly coincidental happened. Not only was a spacecraft in Earth’s near space able to measure a coronal mass ejection (CME), another spacecraft beyond Mars was lined up just the right way to receive the sunbeam as well.
This meant that the two spacecraft could measure the same CME at different points in their journey from the sun, providing a rare opportunity to understand how these powerful eruptions evolve.
Coronal mass ejections may not be as visible as solar flares (which sometimes accompany them), but they are much more powerful. They occur when twisted magnetic field lines on the sun reconnect, converting and releasing enormous amounts of energy.
This happens in the form of a CME, in which enormous amounts of ionised plasma and electromagnetic radiation, bundled in a spiral magnetic field, are launched into space with the solar wind. As they flow past Earth, CMEs can interact with the magnetosphere and ionosphere, creating observable effects such as problems with satellite communication and aurorae.
But what happens to CMEs when they are outside of Earth, in interplanetary space, is much more difficult to study. For example, we have many, much fewer instruments. The odds of two spacecraft at very distant distances from the sun detecting the same CME is incredibly low.
Fortunately, that happened in 1998 with two spacecraft designed to study solar wind. NASA’s Wind spacecraft, at the L1 Lagrangian point at about 1 astronomical unit (the distance between the Earth and the sun), first observed a CME on March 4, 1998.
Eighteen days later, that same CME arrived at Ulysses, a spacecraft that at the time was at a distance of 5.4 astronomical units, roughly equal to Jupiter’s average orbit.
Now astronomers have examined the data from both 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 spiral structure of the magnetic cloud had significantly eroded. The team thinks this was likely due to an interaction with a second, underlying magnetic cloud that traveled faster than the first, overtaking it and compressing it by the time it reached Ulysses.
This could explain why the spiral structure of the magnetic cloud in the CME became more twisted by the time it reached 5.4 astronomical units – rather than less, as might be expected. The magnetic interaction between the two clouds could erode the outer layer, leaving a more twisted core.
“What is clear from this analysis is that at 5.4 astronomical units, the second magnetic cloud strongly interacts with the first,” the researchers wrote in their paper.
As a result, the magnetic structure of the preceding magnetic cloud is highly distorted. In fact, its large-scale rotation extends far beyond the rear of the next magnetic cloud and de facto represents a form of rotation of the magnetic field on the planet. background.”
It would be fascinating to see more studies on this topic – and, happy as the 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 and JAXA’s BepiColombo and ESA’s Solar Orbiter all orbiting the sun at different distances, it may only be a matter of time for the stars – or in this case, the spacecraft – to align .
The research is published in The Astrophysical Journal Letters.