New Plasma Regime in Jupiter's Auroral Zones

Observations from the Juno satellite have indicated very low electron densities, as low as 10^{-3}  cm^{-3}, at high-latitudes in Jupiter's magnetosphere. This region is strongly magnetized, with surface magnetic fields at the one-bar level up to 20 G, or 2 mT, leading to the unusual situation that the electron plasma frequency is less than the ion gyrofrequency. In this extremely low-density plasma, the Alfven wave becomes a plasma oscillation at the electron plasma frequency at shorter perpendicular wavelengths. Analysis of this mode with a kinetic low-frequency dispersion solver indicates that at large wave number, this mode has the characteristics of the Langmuir wave. Thus, this mode can be called an Alfvén-Langmuir mode. Below the plasma frequency, the high-wave number behavior of this mode exhibits a resonance cone, with frequency determined by the angle of the wave vector with the background magnetic field. These waves can be excited by the upward electron beams observed by Juno.