The Jovian ionospheric Alfvén resonator and auroral particle acceleration

Abstract:

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 Alfvenic 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 0.2-1.0 Hz, comparable to the electron transit time through the region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter (e.g., Mauk et al., 2017; Allegrini et al., 2020) suggests that a similar process can occur in the Jovian IAR. Early estimates (Su et al., 2006) indicate that the IAR frequency is about 20 Hz at Jupiter, which could lead to similar particle acceleration. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions. 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. In addition, measurements of the plasma frequency and lower hybrid resonance from the Waves instrument will be used to determine the density in the model, giving a better estimate for the fundamental IAR frequencies.

Allegrini, F., et al. (2020). Energy flux and characteristic energy of electrons over Jupiter's main auroral emission. Journal of Geophysical Research: Space Physics, 125, e2019JA027693. https://doi.org/10.1029/2019JA027693

Mauk, B. H., et al. (2017), Discrete and broadband electron acceleration in Jupiter’s powerful aurora, Nature, 549, 66, doi: 10.1038/nature23648.

Su, Y., et al. (2006), Io-Jupiter interaction: Alfvén wave propagation and ionospheric Alfvén resonator, J. Geophys. Res., 111, A06211, doi:10.1029/2005JA011252.