Thermal equilibration in collisionless magnetospheric plasmas via entropy-mode turbulence

ABSTRACT A planetary magnetosphere is a peculiar plasma environment where a high-temperature plasma is confined in a strongly inhomogeneous dipolar magnetic field generated by a planet. Turbulence driven by the magnetic curvature and density gradient (the entropy mode) is known to cause the inward pinch whereby particles are transported against the density gradient to achieve high confinement. However, a comprehensive understanding of how the global magnetospheric plasma confinement is determined is still missing. Here, we show that the entropy-mode turbulence equilibrates temperatures between species without collisions in a magnetospheric plasma. The classical stability analysis in terms of energetic consideration reveals the interchangeable roles of electrons and ions for destabilization due to resonance with drift waves depending on their temperature. One of the species has negative energy and grows its energy fed from the background density gradient. Turbulence driven by the microscopic (kinetic) instability tends to rearrange the internal energy between species, pre-dominantly via linear physics, leading to an equal temperature state. Our new finding adds an ingredient to energy transport processes and contributes to a consistent explanation of the global self-organization of magnetospheric plasmas.