PI: Matteini
Alfvén waves play a crucial role in the transport of energy in various magnetised laboratory, space and astrophysical plasmas. They are both linear and non-linear MHD solutions and can travel long distances along the background magnetic field, transporting and depositing energy far from the source. In the presence of compressible fluctuations however, Alfvén waves can couple with acoustic modes in the system and undergo so-called parametric instabilities. The parametric decay instability (PDI) is the coupling of a mother wave with a daughter, reflected, Alfvén wave, through the interaction with a shorter-wavelength sound wave. When the wavelengths of the waves approach the typical ion-characteristic scales, PDI can strongly couple with ion dynamics, leading to preferential ion heating and field-aligned acceleration.
In this project, we have investigated the PDI of kinetic waves (ion-cyclotron and whistler) in a low-beta plasma, in the presence of multi-ion populations using the hybrid-PIC code CAMELIA. This is relevant for e.g. the low-beta Solar Corona, where dispersive Alfvén waves are thought to be responsible for the preferential heating and acceleration of heavier ions.
At the same time, the low-beta regime is also particularly relevant for waves in the Earth’s Ionosphere. In a dedicated study (Recchiuti, Franci, Matteini et al, 2026, J. of Plasma Physics, in press.) we have investigated the preferential acceleration and heating of Ionospheric ions (Oxygen and Hydrogen) by right- and left-handed Alfvén waves propagating along the magnetic field. We found that PDI characterises the evolution of the waves on a short timescale, leading to the rapid generation of compressive wave fronts that accelerate the ions, causing significant deformations of their velocity distribution function (see figure).
Figure: low-beta PDI evolution in a kinetic regime. (top) Spectra as a function of time in a simulation with a mother wave k0=2 (blue dotted line) leading to the generation of a lower-k daughter wave and higher-k compressible fluctuations (black).(bottom) The compressible acoustic fronts preferentially accelerate ions along the magnetic field, leading to significant deformations of the initial Maxwellian VDF (grey contours). As in the low-beta regime the sound speed is much lower than the Alfvén speed, the acoustic perturbation excited is a quasi-steady wave and then interact with resonant ions in both directions.
In the paper the extremely low-beta (strong magnetic field) regime of the Ionosphere is investigated for the first time, exploring the parameter range where the PDI is at work and ion VDFs are more significantly affected. We also found that when the amplitude of the mother waves exceeds a certain threshold, the instability develops with an unexpected strong and disruptive behaviour, leading to substantial ion parallel heating. The threshold is identified as the ratio between the fluctuating magnetic pressure and the ion plasma thermal pressure.
The paper is available at: https://arxiv.org/pdf/2512.01757