Astrophysicists find evidence that Alfvén waves lead to heat generation in the magnetosphere

28 Nov 2024, 11:00 by Bob Yirka

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An artist's impression shows the four spacecraft of the Magnetospheric Multiscale Mission, which fly in a tetrahedral formation and gather information about the microphysics of reconnecting magnetic-field lines and the associated processes. Credit: NASA/GSFC

A small team of astrophysicists at the University of California, Los Angeles, working with colleagues from the University of Texas at Dallas and the University of Colorado, Boulder, has found evidence that Alfvén waves in space plasmas speed up ion beams, resulting in the creation of small-scale acoustic waves that in turn generate heat in the magnetosphere.

In their study, published in the journal Physical Review Letters, the group used data from the four-spacecraft Magnetospheric Multiscale (MMS) mission that took place in 2015 to prove a theory about heat generation in the magnetosphere.

For several years, astronomers have been studying the impact of the solar wind striking the magnetopause, which defines the outer edges of the magnetosphere. Prior research has shown that as the solar wind arrives, Alfvén waves are generated and the resulting energy heats up the plasma in the magnetosphere. However, the plasma there is too thin to result in a cascade.

To explain what happens, researchers have theorized that Alfvén waves speed up the ion beams, resulting in the creation of acoustic waves, which in turn generate heat. In this new effort, the researchers found evidence of such a chain of events, backing up the theories.

The researchers analyzed data from MMS. The unique venture involved four spacecraft that flew together in a special configuration through the magnetosphere above regions of the Earth experiencing dusk. This allowed the craft to observe large-scale topical transformations and the movement of an Alfvén wave. The configuration also allowed the spacecraft to monitor the motion of the ions in the surrounding plasma.

Such data, the research team discovered, could be used to prove a theory suggesting that heat generated by ion beams was the means by which Alfvén waves were converted to heat.

Propagation characteristics of Alfvén waves and IAWs in the spatiotemporal domain in the simulation. Credit: arXiv: DOI: 10.48550/arxiv.2406.14782

The instruments aboard the spacecraft showed that magnetic pressure variation in the Alfvén waves was synched with ion density fluctuations and the surrounding electric field. They also showed the speed of the ion beams matched those of the Alfvén wave.

Confident that the data had proven theories surrounding heat generation in the magnetosphere, the researchers created simulations of the action as it unfolded. The simulations matched both the theory and the observations they had made.

More information: Xin An et al, Cross-Scale Energy Transfer from Fluid-Scale Alfvén Waves to Kinetic-Scale Ion Acoustic Waves in the Earth's Magnetopause Boundary Layer, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.225201. On arXiv: DOI: 10.48550/arxiv.2406.14782

Journal information: Physical Review Letters , arXiv