Our sun is not just a quiet member of scorching hot plasma. In fact, it withstands large explosions relatively often; coronal mass ejections are therefore, when directed on Earth, most responsible for geomagnetic storms.
From a location close to Earth, we can measure them well by satellites and other spacecraft. But in 1998 something surprisingly fortunate happened. Not only was a spacecraft in space near Earth capable of measuring a large coronal explosion (CME), another spacecraft out past Mars was lined in the right way to get a solar explosion too.
This meant that the two spacecraft were able to measure the same CME at different stages of their journey from the Sun, offering a rare opportunity to understand how these powerful explosions unfold.
Coronal mass ejections may not be as noticeable as solar flares (to which they sometimes go), but they are much more powerful. They occur when complex magnetic field lines on the Sun reconnect, change and emit a lot of energy in the process.
This occurs in the form of CME, in which large amounts of irradiated plasma and electromagnetic radiation, wrapped in a helical magnetic field, are launched into space by the solar wind. As they flow past the Earth, CMEs can interact with the magnetosphere and ionosphere, creating obvious effects such as satellite and aurorae communication problems.
But it has become much more difficult to study what happens to CMEs when they are out of Earth, in an interplanetary space. We have far fewer instruments out there, for one thing. Both spacecraft at long distances from the Sun find the same CME very low.
Fortunately, that is what happened in 1998 with two spacecraft designed to study solar winds. A NASA spacecraft, at the L1 Lagrangian point at about 1 astronomical unit (distance between Earth and Sun) first saw CME on March 4, 1998.
Eighteen days later, that same CME reached Ulysses, a spacecraft that was, at the time, at a distance of 5.4 celestial units, more or less equal to the average orbital speed of Jupiter.
Now astronomers have analyzed the data from these two encounters to show, for the first time, how CME changes as it travels deeper into the Solar System. In particular, they studied the magnetohydrodynamic evolution of the rooted magnetic cloud.
Wind data (left) and Ulysses data (right). (Telloni et al., ApJL, 2020)
They found that, in the 4.4 celestial units between the two spacecraft, the helical structure of the magnetic cloud declined significantly. The team believes this was likely due to a second interaction, dragging magnetic clouds that traveled faster than the first one, catching up with him and compacting him by the time he reached Ulysses.
This may explain why the helical structure of the magnetic cloud in the CME was more complex before it reached 5.4 celestial units – rather than less, as might be expected. The magnetic interaction between the two clouds could degrade the outer layer, leaving a more complex heart.
“What emerges clearly from this analysis is that the second magnetic cloud interacts sharply at the first at 5.4 astronomical units,” the researchers wrote in their paper .
“As a result, the magnetic structure of the former magnetic cloud is strongly deformed. In fact, its large circumference extends far behind the following magnetic cloud and represents de facto. as a kind of magnetic background reversal. “
It would be interesting to see more studies on the subject – and, as fortunate as possible, we may find them. The researchers note that we are at an early stage of what could be seen as the “golden age” of solar physics.
With NASA’s Parker Solar Probe, ESA’s BepiColombo and JAXA’s BepiColombo and ESA’s Solar Orbiter all orbiting the sun at different distances, it could be just a matter of time before the stars – or in this case the spacecraft – ‘alignment.
The research was published in Letters of the Astrophysical Journal.