Joining the dots of an energetic particle event
The solar flare event of 21 March 2022 observed by the ESA/NASA Solar Orbiter spacecraft and highlights the complex magnetic environment in which the spacecraft operates. The flare itself took place just behind the visible face of the Sun as seen by Solar Orbiter but was still glimpsed by Solar Orbiter’s Extreme Ultraviolet Imager (EUI) and the X-ray Spectrometer/Telescope (STIX) instrument, as the energy released in the flare rose above the solar limb. In the graphic, the source X-ray flare seen by STIX is marked as a small red dot, the expanding shock wave (green/yellow) was seen by EUI.
The flare triggered an eruption of particles into space, known as a coronal mass ejection, but this did not travel towards Solar Orbiter. Instead, it moved off in a different direction through the Solar System. Nevertheless, a cloud of extremely energetic particles produced in the flare and probably by the shock wave driven by the CME’s expansion did begin to fill space near the Sun, guided by the Sun’s magnetic field. And some of these were travelling towards Solar Orbiter.
The spacecraft’s Radio and Plasma Waves (RPW) experiment saw them coming, picking up the strong characteristic sweep of radio frequencies produced by the accelerated particles – mostly electrons – spiraling outwards along the Sun’s magnetic field lines. RPW then detected oscillations known as Langmuir waves when the particles swept over the spacecraft.
Simultaneously, the Energetic Particle Detector (EPD) also registered a cloud of energetic particles sweeping across the spacecraft. The diagonal detection curve shows that the most energetic of the particles arrived first, followed by those of lower energies that arrived up to several hours later. This indicates that the particles were produced far away from the spacecraft, which allowed the faster particles to pull ahead of the slower ones. The compositions of the particles are also noted.
By combining the results of Solar Orbiter’s remote sensing instruments with those that record the conditions around the spacecraft, researchers can follow the chain of events from near the surface of the Sun to the spacecraft and beyond. This ‘linkage’ science is one of Solar Orbiter’s primary scientific objectives.
