Centre of Excellence researchers and collaborators uncover how foreshock waves transmit towards Earth

The interaction of the solar wind, a stream of charged particles emitted by the Sun, with Earth’s magnetic field creates a wealth of interesting physical phenomena in the vicinity of our planet. Near-Earth space forms a natural laboratory enabling the study of universal plasma processes such as magnetic reconnection, particle acceleration and plasma instabilities, which are known to occur in distant astrophysical environments and also play an important role in fusion plasmas.

As it approaches our planet, the supersonic solar wind is slowed down and deflected by a shock wave arched ahead of the protective bubble formed by Earth’s magnetic field, the magnetosphere. When they hit the shock, part of the solar wind particles are reflected back towards the Sun, and create plasma waves in a region of space called the foreshock. What makes these waves particularly intriguing is that while the foreshock extends well outside the outer boundary of Earth’s magnetosphere, spacecraft and ground-based observations show that foreshock waves can penetrate Earth’s magnetic shield and propagate along Earth’s magnetic field lines. How this transmission occurs is however unclear, because a major obstacle lie in their way: Earth’s bow shock.

In a study , Lucile Turc, researcher at the University of Helsinki, and collaborators propose a new scenario to explain the wave transmission. Their work makes use of cutting-edge numerical simulations and spacecraft observations to investigate the processes at play when foreshock waves interact with Earth’s bow shock. Numerical simulations were performed with the , developed at the University of Helsinki. The model describes foreshock waves in their global context and allows tracking the waves on their earthward journey. To confirm the numerical results, observations from NASA’s Magnetospheric Multiscale mission were analysed upstream and downstream of Earth’s bow shock.

The results of the study show that the transmitted waves share very similar properties with their counterparts in the foreshock. At first glance, it may seem that the waves traverse the shock unchanged, as predicted in earlier works. However, one key difference between the waves on both sides of the shock, namely their propagation direction, invalidates the direct transmission hypothesis. The numerical simulations made it possible to take a closer look at the physical processes occurring near the shock, revealing that the foreshock waves modulate the shock properties, such that new waves are created on the other side of the shock, resembling closely the foreshock waves.

This work provides the missing link in the transmission of foreshock waves from their source region upstream of the shock into the Earth’s magnetosphere, thus improving our understanding of near-Earth space dynamics. Shocks, such as the bow shock forming ahead of Earth’s magnetosphere, are found everywhere in space, near other planets, supernovae remnants or active galactic nuclei, and are one of the main sources of high energy particles in our universe. Understanding how plasma waves interact with a bow shock, how they modify it and how they are transmitted to the other side of the shock brings us crucial new insight into collisionless shock waves in general.

Reference

L. Turc, O.W. Roberts, D. Verscharen, A.P. Dimmock, P. Kajdič, M. Palmroth, Y. Pfau-Kempf, A. Johlander, M. Dubart, E.K.J. Kilpua, J. Soucek, K. Takahashi, N. Takahashi, M. Battarbee, U. Ganse: Transmission of foreshock waves through Earth’s bow shock, Nature Physics, 2023,

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