Scientists have obtained in situ measurements of Earth's magnetosphere, demonstrating a phenomenon that's long been thought to happen but not yet directly been shown: energy is transferred from hydrogen ions to plasma waves, and then from the waves to helium ions.
As plasma interactions are common throughout the Universe, the results from this study will provide a better understanding of a number of space plasma phenomena, such as those involved in Earth's radiation belt, geomagnetic storms, aurorae, and atmospheric loss from planets.
Plasmas are ionised gases containing negative electrons, positive ions and electromagnetic fields. These fields can oscillate over time, carrying energy as plasma waves.
In principle, the waves could act as intermediaries that transfer energy between two different types of ion, but that process has not been directly observed until now.
As part of the Magnetospheric Multiscale mission, four spacecraft fly in close formation through Earth's magnetosphere, the region of plasma dominated by our planet's magnetic field. Naritoshi Kitamura et al. report that on September 1, 2015, the spacecraft encountered an unusual region of the magnetospheric where there was a transfer of energy from hydrogen ions to plasma waves, as expected.
The authors discovered that there was further transfer of energy from the plasma to neighboring helium ions, explaining how the helium ions were accelerated to high energies. These data provide important insights into wave-particle interactions that occur ubiquitously in space.
"Direct measurements of two-way wave-particle energy transfer in a collisionless space plasma" - N. Kitamura et al. - Science September 7, 2018: Vol. 361, Issue 6406, pp. 1000-1003 - DOI: 10.1126/science.aap8730
Particle acceleration by plasma waves and spontaneous wave generation are fundamental energy and momentum exchange processes in collisionless plasmas. Such wave-particle interactions occur ubiquitously in space.
We present ultrafast measurements in Earth’s magnetosphere by the Magnetospheric Multiscale spacecraft that enabled quantitative evaluation of energy transfer in interactions associated with electromagnetic ion cyclotron waves.
The observed ion distributions are not symmetric around the magnetic field direction but are in phase with the plasma wave fields. The wave-ion phase relations demonstrate that a cyclotron resonance transferred energy from hot protons to waves, which in turn nonresonantly accelerated cold He+ to energies up to ~2 kilo–electron volts. These observations provide direct quantitative evidence for collisionless energy transfer in plasmas between distinct particle populations via wave-particle interactions.
Featured image credit: NASA