The Jovian Ionospheric Alfvén Resonator and Auroral Particle Acceleration

R. L. Lysak; Y. Song; S. Elliott; W. Kurth; A. H. Sulaiman; D. Gershman

doi:10.1029/2021JA029886

The ionospheric Alfvén resonator (IAR) is a structure formed by the rapid decrease in the plasma density above a planetary ionosphere. This results in a corresponding increase in the Alfvén speed that can provide partial reflection of Alfvén waves. At Earth, the IAR on auroral field lines is associated with the broadband acceleration of auroral particles, sometimes termed the Alfvénic aurora. This arises since phase mixing in the IAR reduces the perpendicular wavelength of the Alfvén waves, which enhances the parallel electric field due to electron inertia. This parallel electric field fluctuates at frequencies of 0.1–20.0 Hz, comparable to the electron transit time through the acceleration region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter suggests that a similar process can occur in the Jovian IAR. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions, indicating that the IAR resonant frequencies are in the same range as those at Earth. This model describes the evolution of the electric and magnetic fields in the low‐altitude region close to Jupiter that is sampled during Juno's perijove passes. In particular, the model relates measurement of magnetic fields below the ion cyclotron frequency from the MAG and Waves instruments on Juno and electric fields from Waves to the associated parallel electric fields that can accelerate auroral particles. Plain Language Summary Just like at Earth, the polar regions of the planet Jupiter are circled by a luminous aurora (northern and southern lights) that can be seen from telescopes like the Hubble Space Telescope near Earth. The aurora on both planets is produced by electrons impacting the upper atmosphere, causing the atoms and molecules in this region to emit light. At Earth, these electrons are mainly produced by large voltages that cause all the electrons to be accelerated to nearly the same energy. However, recent observations from the Juno satellite at Jupiter shows that these electrons are mainly accelerated over a broad range of energies. This suggests that the voltages accelerating these electrons are fluctuating rapidly in time. Such fluctuations can be caused by the strong increase in the effective wave speed due to a rapid decrease in the number of electrons as the altitude is increased. We have developed a computer model to help understand these interactions. Key Points Broadband acceleration of auroral particles at Jupiter, can be achieved by Alfvén waves propagating in the ionospheric Alfvén resonator Numerical results indicate that electrons could be accelerated to the 10–100 keV range for observed levels of Alfvén wave activity There is also an Alfvén resonator in the high‐Alfvén speed velocity region between the ionosphere and the dense plasma sheet

The Jovian Ionospheric Alfvén Resonator and Auroral Particle Acceleration

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