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The Juno satellite is the first satellite in polar orbit around Jupiter, leading to unprecedented coverage of Jupiter’s magnetosphere. During the Juno extended mission, the satellite samples the polar regions of Jupiter at altitudes of less than 0.5 Jovian radii. Observations from Juno have indicated very low densities, as low as 10−3 cm−3, on polar cap field lines at Jupiter (Sulaiman et al., 2023). This region is strongly magnetized, with surface magnetic fields up to 20 G, or 2 mT (Connerney et al., 2022), leading to the unusual situation that the electron plasma frequency is less than the ion gyrofrequency. For example, in a 1 G (100 μT) field, the proton gyrofrequency is about 1.5 kHz, corresponding to the electron plasma frequency for a density of 0.03 cm−3. In a more typical plasma where the ion gyrofrequency is lower than the plasma frequency, the Alfvén mode transitions to an electromagnetic ion cyclotron wave, sometimes called the Alfvén-ion cyclotron mode at large wave number. However, in this extremely low-density plasma, the Alfven wave becomes a plasma oscillation at the plasma frequency. 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.
Connerney, J. E. P., et al. (2022). Journal of Geophysical Research: Planets, 127, e2021JE007055. https://doi.org/10.1029/2021JE007055
Sulaiman, A. H., et al., (2023), in Planetary, Solar and Heliospheric Radio Emissions IX, editors: Fischer, G., Jackman, C. M., Louis, C. K., Sulaiman, A. H., Zucca, P., doi: https://doi.org/10.25546/103098
