The effect of oxygen ions on the stability and polarization of Kinetic Alfvén Waves in the magnetosphere

Abstract

One of the most striking properties of Kinetic Alfvén Waves (KAW) is that, unlike the also Alfvénic Electromagnetic Ion Cyclotron (EMIC) waves, these waves are right-hand polarized in the plasma frame. In particular, this signature property is key for the identification of KAW from in situ measurements of plasma waves. From the theoretical point of view, both the dispersion relation and the polarization of KAW has been mostly studied in proton-electron plasmas. However, most astrophysical and space plasmas are multi-species, and therefore in these systems the dispersion properties of the KAW may not depend only on the macroscopic parameters of proton and electron distributions, but also on the parameters of heavier ions. Here, using Vlasov linear theory we study the dispersion properties of Alfvénic modes in multi-species plasmas composed by electrons, protons, and O + ions, with macroscopic plasma parameters relevant to the inner magnetosphere. In consistency with recent observations, our numerical results show that the presence of O + ions allows the existence of KAW in a wider wave-number range and at smaller wave-normal angles compared to the electron-proton case, but at the same time isotropic O + ions tend to reduce (or even inhibiting) the growth rates of unstable KAW triggered by anisotropic protons. These results suggest that magnetospheric ions may play an important role on the energy transfer from large macroscopic scales to sub-ionic and electronic scales, especially during intense geomagnetic storms in which O + ions can dominate the plasma composition in the inner magnetosphere.

The role of O+ and He+ on the propagation of Kinetic Alfvén Waves in the Earth's inner magnetosphere

“…We then compare the dispersion relation and dispersion properties spectra (polarization, helicity, magnetic compressibility, and parallel electric field to transverse magnetic field ratio) obtained for the multi-species magnetospheric plasma with an electron-proton plasma of the same density and temperature for each considered L and θ. The calculation and comparison was done without the use of any approximation to the elements of the dispersion tensor something possible as we use our own kinetic dispersion solver written in Python (Moya et al 2021). Fig.…”

“…However, most astrophysical and space plasmas are multi-species. Thus, in these systems it is expected that the dispersion properties of EMIC or KAW modes to be dependent not only on the parameters of proton and electron distributions, but also on the plasma composition and the parameters of heavier ions such as O+ ions (Moya et al 2021), O+ and He+ ions (Moya et al 2015), or N+ ions (Bashir & Ilie 2018).…”

“…In this context, during the recent years, studies have shown that ion beams can provide the free energy necessary to generate KAWs in the inner magnetosphere and auroral region (Lakhina 2008;Barik et al , 2021. In addition, Moya et al (2021) computed the exact numerical dispersion relation of KAWs, without the use of approximations, considering different abundances of O+, using plasma parameters relevant to the Earth inner magnetosphere and cometary environments. They found that the presence of O+ ions allows the propagation of weakly damped or unstable KAW modes at smaller wave-normal angles than if the plasma were composed only by protons and electrons.…”

“…DISPERSION TENSOR AND DISPERSION PROPERTIESHere we briefly present basic expressions for the dispersion tensor and dispersion properties of electromagnetic waves propagating through a magnetized, colissionless, and uniform bi-Maxwellian plasma with distribution functions given by Eq.(1).A.1. Dispersion tensorConsidering the background magnetic field as B 0 = B 0 ẑ, and wave-vectors for parallel propagating waves given by k = k ⊥ x + k ẑ (where k = k cos(θ) and k ⊥ = k sin(θ), with θ the wave-normal angle), the dispersion tensor D(ω, k) can be written in the following form(Stix 1992;Viñas et al 2000;Moya et al 2021): xx D xy D xz −D xy D yy D yz D xz −D yz D zz s − 1) + µ s Z(ξ ns ) ξns ] , s nΛ (λ s )Z(ξ ns ) ξns , (A3)…”

The Role of O+ and He+ in the Propagation of Kinetic Alfvén Waves in the Earth’s Inner Magnetosphere

“…However, most astrophysical and space plasmas are multispecies. Thus, in these systems, it is expected that the dispersion properties of the EMIC or KAW modes will be dependent not only on the parameters of the proton and electron distributions, but also on the plasma composition and the parameters of heavier ions, such as O+ ions (Moya et al 2021), O+ and He+ ions (Moya et al 2015), or N+ ions (Bashir & Ilie 2018). The impact of heavier ions may be especially relevant in the inner magnetosphere during geomagnetic storms, as there is a strong dependence between the relative abundance of protons, O+ ions and He+ ions, and geomagnetic activity.…”