The overall structure of the main auroral spots is consistent with the expectations from ray-tracing models (e.g., Hinton et al., 2019). Our model indicates that the spacing of the main and reflected Alfvén wings is consistent with previous work and depends upon the density profile along the flux tube. Furthermore, the structure of the field-aligned currents produced by the model is consistent with the transverse displacement in secondary spots observed by JIRAM (Mura et al., 2018). The results suggest that this may be due to the generation of field-aligned currents at the surface of Io and its ionosphere. In addition, the secondary structure in the currents is consistent with the reflection of Alfvén waves from the boundary of plasma torus, which can occur over time scales of about a minute. However, one puzzle in the results is that when the parallel electric fields due to current limitation are included, these secondary reflections do not produce significant potential drops.
The complicated structure after the passage of the RAW suggests that the waves are subject to phase mixing, particularly while passing through the dense plasma torus. This makes the current structure more complicated, especially after the passage of the reflected Alfvén wing. Recently, Schlegel and Saur (2022) considered the effect of different travel times on the alternating spots and concluded that this was only a minor effect. However, in our model, the difference in the passage time of an Alfvén wave from one ionosphere to another between L=5.90 and 5.95, a difference of about 1 Io radius, is 22.5 seconds in the low-density model and 30.6 seconds in high-density model, much larger than their assumed time of 3.7 seconds. In addition, the RAW spot occurs after the wave has traveled through the torus 1.5 times, so the difference is 50% larger. So it appears plausible that the travel time difference, i.e., phase mixing, can be an important effect down the tail from the main spot. This may be related to the bifurcated auroral tail sometimes seen downstream from main spot (Szalay et al., 2018; Mura et al., 2018).
In summary, this new model for the propagation of Alfvén waves generated by Io has revealed some interesting points and raised some questions about this interaction:
While this model shows some interesting features, there are still many unanswered questions. Although our model includes the electron inertial effect thought to be responsible for the electric field producing a broadband electron distribution, there does not seem to be significant electric fields produced by this effect. This may be due to the large size of Juno with respect to the electron inertial length. However, if the currents are strongly filamented, the importance of the electron inertial effect would be increased. Including the possibility of a turbulent cascade to smaller scales (Hess et al., 2010; Saur et al., 2002) or ionospheric feedback effects (Lysak, 1991; Lysak and Song, 2002; Moirano et al., 2021) would give a more complete understanding of the propagation of Io-generated Alfvén waves in Jupiter’s magnetosphere.