Perceiving Particles Receiving Push From Alfvén Wave

Alfvén waves, named after the man who suggested them in 1942, are waves that exist in conductive fluids. Normally they are talked about when it comes to plasmas but they were originally shown to exist in mercury as this too is a conductive liquid that a magnetic field can permeate. The magnetic field acts as the restoring force for the oscillations that make the Alfvén waves exist in the first place. For an ideal Alfvén wave the particles receive no energy as the wave propagates and this is close enough to being true for a significant amount of the time. However, as the oscillations increase in speed, or the wave decreases in size, it can begin to excite the ions through the production of its own electric and magnetic fields. This naturally results in a loss of energy from the wave and is actually one of the most important mechanisms is solar physics for the transfer of energy from electromagnetic waves to the kinetic energy of particles.

Although they are strongly believed to exist and have been shown through tangential evidence, these kinetic scale Alfvén waves have very few details known about them. This is because by their nature they are small and lose energy rapidly existing for a very short time. It is believed that it could be Alfvén waves in the kinetic scale that transfer heat into particles in the upper atmosphere or provide the warming effect in turbulent astrophysical plasmas. Luckily, two years ago, NASA launched their magnetospheric multiscale mission which we hoped would give us insight into the Alfvén waves. This involves four identical observatories positioned in a tetrahedron (the points of a triangle based pyramid) which can provide three dimensional particle distributions many times more quickly then previous space missions.By taking observations from these four stations the details about the interactions of a monochromatic (only one frequency) Alfvén wave was analysed. The wave was directly observed, in a closed transfer, giving energy from its electromagnetic store to the charged particles. Understanding these dynamics will hopefully provide more insight into plasma physics back here on Earth in order to understand how kinetic waves can develop in laboratory conditions.

Paper links: Wave-particle energy exchange directly observed in a kinetic Alfvén-branch wave


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