Detection of wave coupling cascade in space plasmas
Final Report Abstract
Large-amplitude waves in space plasma such as Alfv´n waves are theoretically expected to collapse into various linear and nonlinear modes through wave-wave couplings (called the para- metric instabilities) due to the nonlinearities originating in the complex plasma and magnetic field motions. Understanding the wave-wave couplings in three-dimensional space plasma and comparing with the observations of waves and turbulent fields in space plasma are the goals of the project. Among others, the key questions are put as to the importance of three dimen- sional space, in particular the differences from the low- dimensional treatments (in particular, one-dimensional simulations) which the earlier numerical studies could not perform due to the limited computational capacity. To the first goal, the wave-wave couplings of the large-amplitude Alfvén waves are studied for the first time systematically using a numerical hybrid plasma code of AIKEF. The numerical simulations were conducted under different conditions of spatial dimensions and plasma param- eter beta. Wave coupling patterns, comparison with the analytical expectations, and efficiency in the ion heating by the waves were studied in detail. The highlight of the discoveries out of the project is the multi-channel wave couplings, that is, the wave-wave couplings occur simul- taneously for various coupling pairs. The multiple wave couplings are observed not only in the parallel, oblique, and perpendicular directions to the mean magnetic field, but also symmetri- cally in the azimuthal directions around the mean field. The multi-channel wave couplings occur in various types of parametric instabilities, e.g., decay instability (which has a wave coupling sense of parallel propagation to the mean magnetic field) and filament formation (coupling sense of perpendicular propagation to the mean field). In other words, the wave-wave couplings make full use of the dimensional degree of freedom and generate all possible daughter waves (e.g., sound wave excitation and filament formation), and run simultaneously to evolve into turbu- lence. The multi-channel wave couplings in three-dimensional space plasma are found to be an efficient way to heat the ions compared to that in lower dimensions (1-D or 2-D). The parametric instabilities can qualitatively explain the heating of solar wind ions. To the second goal, the MMS (Magnetospheric Multiscale) spacecraft data quality or accuracy was not sufficient to perform the wave-coupling studies. The spacecraft particle detector was designed for the magnetospheric plasmas, and not for the solar wind or shock-upstream plasmas. As a contingency plan, the Cluster spacecraft data were analyzed in search of finding evidence for the wave-wave couplings in the solar wind. The energy spectra show no discrete peaks but a continuum in the wavevector-frequency domain, and the spectral shape is characterized by the Doppler shift (the spatial structures are swept by the mean flow and pass by the spacecraft) and by the sideband waves that appear as the frequency broadening around the Doppler shift. Different interpretations are possible to explain the origin of the sideband waves. A scenario of finite-time wave evolution (or wave packet formation) is one of the candidates to explain the sideband formation. The sideband formation can also be explained by the wave-wave couplings, and there is a variety of linear-mode waves (such as whistler mode, ion Bernstein mode, kinetic Alfvén mode, kinetic slow mode) for the wave-wave couplings that can generate sideband waves. To sum up, plasmas can evolve into turbulence even without the presence of fluid-type nonlin- earities (e.g., eddies, wakes, and coherent structures). Large-amplitude waves in space plasma have the capability to develop into turbulence with the parametric instabilities, exciting a set of linear and nonlinear modes in a multiple way over all the possible coupling types available in the system. The observational test so far fails to detect a clear signal of wave-wave couplings predicted by the parametric instabilities. Most likely the wave field evolves quickly into a more turbulent state, and the observations need to be made at a sufficiently close distance to the source region of the pump wave.
Publications
- Ion-Scale Sideband Waves and Filament Formation: Alfvénic Impact on Heliospheric Plasma Turbulence, Front. Phys. 5, article ID 8., 2017
Yasuhito Narita, Uwe Motschmann
(See online at https://doi.org/10.3389/fphy.2017.00008) - Lifetime estimate for plasma turbulence, Nonlin. Processes Geophys., 24, 673–679, 2017
Yasuhito Narita and Zoltán Vörös
(See online at https://doi.org/10.5194/npg-24-673-2017) - On heating of solar wind protons by the parametric decay of large-amplitude Alfvén waves, Ann. Geophys., 36, 1647–1655, 2018
Horia Comişel, Yasuhiro Nariyuki, Yasuhito Narita, Uwe Motschmann
(See online at https://doi.org/10.5194/angeo-36-1647-2018) - Space-time structure and wavevector anisotropy in space plasma turbulence, Living Rev. Solar Phys., 15, 2, 2018
Yasuhito Narita
(See online at https://doi.org/10.1007/s41116-017-0010-0) - Multi-channel coupling of decay instability in three-dimensional low-beta plasma, Ann. Geophys., 37, 835–842, 2019
Horia Comişel, Yasuhito Narita, Uwe Motschmann
(See online at https://doi.org/10.5194/angeo-37-835-2019) - On perpendicular coupling of Alfvén waves in 3-D plasma, Earth, Planets Space, 72, article ID 32, 2020
Horia Comişel, Yasuhito Narita, Uwe Motschmann
(See online at https://doi.org/10.1186/s40623-020-01156-8)
