Earth’s magnetosphere and its bow shock, formed by the interaction of the supersonic solar wind with the terrestrial magnetic field, provide a rich natural laboratory that enables in situ research into universal plasma processes. A foreshock with powerful wave activity forms upstream of the bow shock under the right interplanetary magnetic field conditions. These waves are also observed in the magnetosphere and down to the Earth’s surface, but how they are transmitted by the bow shock remains unknown.
A new study involving UCL has uncovered how magnetic waves are transmitted beyond a standing shock wave, also known as the bow shock. Scientists looked at magnetic waves preceding the Earth’s shock (bow shock), known as foreshock waves. These are made by particles bouncing off the shock and traveling back to the sun.
Using a computer model called Vlasiator, scientists simulated the physical processes involved in sending these waves. They found waves on the opposite side of the shock with nearly identical properties to those in the front shock. They then confirmed the presence of these waves using observational data from NASA’s Magnetospheric Multiscale (MMS) mission.
Co-author Dr. Daniel Verscharen (UCL Mullard Space Science Laboratory), who has world-leading expertise in plasma wave analysis and whose code was used to interpret the MMS data, said: “Shock waves in plasma are much more difficult to understand than when they occur in the air. There is a lot of space between the particles and collisions between them are rare.
“However, it is a universal process that takes place throughout the universe. We can’t send a spacecraft to a supernova – so we’re lucky to be able to study plasma shock waves in our cosmic neighborhood.”
Lead author Dr. Lucile Turc from the University of Helsinki said: “At first we thought that the initial theory proposed in the 1970s was correct: the waves could bridge the shock unchanged. But there was an inconsistency in the wave properties that this theory couldn’t reconcile, so we explored it further.
“Eventually it became clear that things were much more complicated than it seemed. The waves we saw behind the shock were not the same as those in the foreshock, but new waves created at the shock by the periodic impact of foreshock waves.
“The numerical model also indicated that these waves could only be detected in a narrow area behind the shock and could easily be hidden by the turbulence in this area. This probably explains why they had not been observed before.”
- Turc, L., Roberts, OW, Verscharen, D., et al. Transmission of foreshock waves by the Earth’s bow shock. Wet. Physical. (2022). DOI: 10.1038/s41567-022-01837-z