Ion cyclotron instability at Io: Hybrid simulation results compared to in situ observations

Abstract

We present analysis of global three‐dimensional hybrid simulations of Io's interaction with Jovian magnetospheric plasma. We apply a single‐species model with simplified neutral‐plasma chemistry and downscale Io in order to resolve the ion kinetic scales. We consider charge exchange, electron impact ionization, and photoionization by using variable rates of these processes to investigate their impact. Our results are in a good qualitative agreement with the in situ magnetic field measurements for five Galileo flybys around Io. The hybrid model describes ion kinetics self‐consistently. This allows us to assess the distribution of temperature anisotropies around Io and thereby determine the possible triggering mechanism for waves observed near Io. We compare simulated dynamic spectra of magnetic fluctuations with in situ observations made by Galileo. Our results are consistent with both the spatial distribution and local amplitude of magnetic fluctuations found in the observations. Cyclotron waves, triggered probably by the growth of ion cyclotron instability, are observed mainly downstream of Io and on the flanks in regions farther from Io where the ion pickup rate is relatively low. Growth of the ion cyclotron instability is governed mainly by the charge exchange rate.

Dynamic Plasma Interaction at Io: Multispecies Hybrid Simulations

“…Results from the multispecies hybrid simulations exhibit significant improvement over those obtained from the single‐species simulations used before by Šebek et al (). The main improvements in the multispecies simulation model were inclusion of multiple plasma and neutral species in the environment and modified treatment of electron impact ionization.…”

“…The unitary plasma density n e is represented by 100 macroparticles of singly ionized ion species or 50 macroparticles of the doubly ionized sulfur. In our single‐species hybrid simulations (Šebek et al, ), we found that a value of resistivity in the Ohm's law resulted in artificial acceleration of ions in the Alfvén wings. For this reason we use a lower value of in this model.…”

“…On the other hand, an equivalent simulation without induced dipole field exhibits lower peaks of magnetic field B z and temperature T i on the anti‐Jovian side of the wake in contrast to observations. The induced dipole field was not included in our previous single‐species simulation model (Šebek et al, ), and results of this model also exhibit incorrect asymmetry of the magnetic field and temperature peaks observed during the I0 flyby. As discussed by Šebek et al () the reversed asymmetry of magnetic field and temperature peaks may result from the assumption of spherically symmetric atmosphere and that longitudinal asymmetry in the atmosphere model with higher neutral density on anti‐Jovian side could produce better results.…”

“…The induced dipole field was not included in our previous single‐species simulation model (Šebek et al, ), and results of this model also exhibit incorrect asymmetry of the magnetic field and temperature peaks observed during the I0 flyby. As discussed by Šebek et al () the reversed asymmetry of magnetic field and temperature peaks may result from the assumption of spherically symmetric atmosphere and that longitudinal asymmetry in the atmosphere model with higher neutral density on anti‐Jovian side could produce better results. In the context of the multispecies model results presented here, it follows that the incorrect asymmetry of magnetic field B z and temperature T i observed in the single‐species simulations may partly result from the absence of an induced dipole field in the single‐species simulation model.…”

“…The interaction of Io with its plasma torus has been studied intensively by means of numerical simulations based on fluid and hybrid models (e.g., Combi et al, ; Dols et al, ; Kabin et al, ; Linker et al, ; Lipatov & Combi, ; Saur et al, ; Wolf‐Gladrow et al, ). Recently, Šebek et al () investigated the plasma interaction at Io using a single‐species hybrid model. This model was designed to investigate the importance of the overall ionization rate around Io on the interaction.…”