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

One of the most striking properties of Kinetic Alfven Waves (KAW) is that, unlike the also Alfvenic 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 Alfvenic 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.

Automated summary

Kinetic Alfven Waves (KAW) are right-hand polarized in the plasma frame, and the dispersion properties in multi-species plasmas can be affected by the presence of O+ ions. Results show that O+ ions allow the existence of KAW in a wider wave-number range and smaller wave-normal angles, but isotropic O+ ions tend to reduce or inhibit the growth rates of unstable KAW triggered by anisotropic protons. These findings suggest that magnetospheric ions may play a crucial role in energy transfer during intense geomagnetic storms.