Modeling Io's and Europa's Plasma Interaction with the Jovian Magnetosphere: Influence of Global Atmospheric Asymmetries and Plumes

We apply a three-dimensional (3D) magnetohydrodynamic (MHD) model to study the influence of inhomogeneities in Europa’s and Io’s atmospheres, as, for example, water vapor plumes and volcanic plumes, on the plasma interaction with the Jovian magnetosphere. The ideal MHD equations have been extended in order to account for the effects of the moons’ atmospheres and plumes on the plasma interaction. We have included collisions between ions and neutrals, plasma production and loss due to electron impact ionization and dissociative recombination. Moreover, electromagnetic induction in a subsurface water ocean was also considered by the model in modeling of Europa’s plasma interaction. In addition to the MHD model we apply an analytic model based on the model of Saur et al. (2007) to understand the role of steep gradients and discontinuities in Europa’s interaction. We find that Europa’s global atmosphere weakens the effect of the hemisphere coupling and generates steep gradients in the magnetic field. Volcanic eruptions on Io and water vapor plumes on Europa locally enhance the neutral density of the atmosphere and thus modify the plasma interaction. We show that an inhomogeneity near the north or south pole affects the plasma interaction in a way that a pronounced north-south asymmetry is generated. We find that an Alfvén winglet develops within the main Alfvén wing on that side where the inhomogeneity is located. Since Europa’s atmosphere is much thinner (by a factor of 100 compared to Io’s atmosphere) we show that dense atmospheric inhomogeneities affect the Alfvénic far-field much stronger compared to Io. At Europa the plasma velocity experiences a decrease up to 95% of the upstream velocity in the Alfvén winglet and a decrease up to 60% of the upstream velocity in the ambient Alfvén wing. Whereas at Io the plasma flow is decelerated by up to 93% in the Alfvén winglet and by more than 80% in the ambient Alfvén wing. Simultaneously, the Alfvén waves perturb also the magnetic field in the Alfvénic far-field so that the magnetic field perturbations are stronger in the Alfvén winglet than in the ambient Alfvén wing. The global form of the Alfvén wings is unchanged because the Alfvén velocity in the far-field is uninfluenced by the distribution of the neutral density in the atmosphere. Additionally to the effect of volcanic plumes on Io’s plasma interaction, we analyze the role of volcanic plumes on the supply rate of the Io plasma torus. We estimate that the contribution to the mass loading by the volcanic plumes is nearly negligible compared to the total mass loading rate of the global atmosphere and that the ejected neutrals, associated with the plume, contribute by less than 7 % to the total atmospheric sputtering rate. Furthermore, we apply our MHD model to analyze the effects of an asymmetric atmosphere on the plasma interaction. Therefore, we use different atmosphere models with longitudinal and latitudinal dependencies. We compare our model results with Io’s plasma environment measured with the instruments of the Galileo spacecraft during two Io passes: I24 and I27. We demonstrate that parts of the magnetic field perturbations, linked to the induction signals of a subsurface magma ocean (Khurana et al., 2011) can alternatively be explained by considering a global asymmetric atmosphere without considering induced fields from a subsurface magma ocean. Our analytic model results show that the resultant discontinuities for a plume that contains 50% of the mass content of Europa’s atmosphere would only contribute to about 5% for the magnetic field amplitudes generated by the global atmosphere. Furthermore we compare our model results with the measured magnetic field data from three flybys of the Galileo spacecraft at Europa which included Alfvén wing crossings: E17, E25A, and E26, to investigate if signals of plumes are visible in the magnetic field measurements. Our analysis suggests that the magnetic field perturbations measured along the E26 trajectory could be consistent with a plume on the southern hemisphere

Constraints on Europa's water group torus from HST/COS observations

In-situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa's atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in-situ. Here we present observations by the Cosmic Origins Spectrograph of the Hubble Space Telescope (HST/COS) from 2020 and 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter. No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modelled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected

Variability of Io's poynting flux : A parameter study using MHD simulations

Io's plasma interaction creates an electromagnetic coupling between Io and Jupiter through Alfvén waves triggering the generation of auroral footprints in Jupiter's southern and northern hemispheres. The brightness of Io's footprints undergoes periodic variations that are primarily modulated by Io's local plasma interaction through the Poynting flux radiated away from the moon. The periodic pattern with two maxima near 110&lt;SUP&gt;∘&lt;/SUP&gt; and 290&lt;SUP&gt;∘&lt;/SUP&gt; Jovian longitude where Io crosses the dense plasma sheet is generally understood. However, some characteristics, like the 2-4 times stronger brightening of the southern footprint near Jovian longitude 110&lt;SUP&gt;∘&lt;/SUP&gt; or the lack of response to Io's eclipse passage, are not fully understood. We systematically study variations in Io's plasma interaction and the Poynting flux using a 3D magnetohydrodynamic model, performing a series of simulations with different upstream plasma conditions and models of Io's atmosphere. Our results indicate that the strong Jovian magnetic field near 110&lt;SUP&gt;∘&lt;/SUP&gt; plays a more important role than previously estimated for the strong brightening there. We find that the Poynting flux is not fully saturated for a wide range of possible atmospheric densities (6 ×10&lt;SUP&gt;18&lt;/SUP&gt; - 6 ×10&lt;SUP&gt;21&lt;/SUP&gt; m&lt;SUP&gt;-2&lt;/SUP&gt;) and that density changes in the atmosphere by a factor of &amp;gt; 3, as possibly happening during Io's eclipse passage, lead to a change of the Poynting flux by &amp;gt; 20%. Assuming that these expected changes in Poynting flux also apply to the footprints, the non-detection of a dimming in the footprint during the eclipse by Juno-UVS suggests that Io's global atmospheric density decreases by a factor of &amp;lt; 2.5. We show that for smaller atmospheric scale heights (i.e. a more confined atmosphere), changes in the atmospheric density have less effect on the Poynting flux. The missing response of the footprint to the eclipse hence might also be consistent with a density decrease by a factor of &amp;gt; 3, if the effective atmospheric scale height is small (&amp;lt; 120 km). Finally, we provide new analytical approximations that can be used for analyzing the effect of the local interaction responsible for the footprint variability in future studies.QC 20200909</p

Plasmoids in the Jovian Magnetotail: Statistical Survey of Ion Acceleration Using Juno Observations

Transient magnetic reconnection plays an important role in energetic particle acceleration in planetary magnetospheres. Jupiter's magnetosphere provides a unique natural laboratory to study processes of energy transport and transformation. Strong electric fields in spatially confined structures such as plasmoids can be responsible for ion acceleration to high energies. In this study we focus on the effectiveness of ion energization and acceleration in plasmoids. Therefore, we present a statistical study of plasmoid structures in the predawn magnetotail, which were identified in the magnetometer data of the Juno spacecraft from 2016 to 2018. We additionally use the energetic particle observations from the Jupiter Energetic Particle Detector Instrument which discriminates between different ion species. We are particularly interested in the analysis of the acceleration and energization of oxygen, sulfur, helium, and hydrogen ions. We investigate how the event properties, such as the radial distance and the local time of the observed plasmoids in the magnetotail, affect the ion intensities close to the current sheet center. Furthermore, we analyze if ion acceleration is influenced by magnetic field turbulence inside the plasmoids. We find significant heavy ion acceleration in plasmoids close to the current sheet center which is in line with the previous statistical results based on Galileo observations conducted by Kronberg et al. (2019, ). The observed effectiveness of the acceleration is dependent on the position of Juno in the magnetotail during the plasmoid event observation. Our results show no correlation between magnetic field turbulence and nonadiabatic acceleration for heavy ions during plasmoids

Table3_Turbulent dipolarization regions in the Earth’s magnetotail: ion fluxes and magnetic field changes.pdf

In this work, we consider the dynamics of ion fluxes and magnetic field changes in turbulent regions of magnetotail dipolarizations. The data from the Cluster-II mission (magnetic field measurements from fluxgate magnetometers and energetic charged particle observations from RAPID spectrometers) were used for the analysis. We study individual events and investigate statistically the changes of charged particle fluxes during magnetic field dipolarizations observed during 2001–2015. Received changes in the spectral index indicate that CNO+ ions undergo stronger acceleration during dipolarization than protons and helium ions. Before dipolarization front monotonic growth the ions flux is observed (the maximum of flux is observed at 1–1,5 min after the start of dipolarization) in the range of ∼ 92–374 keV for proton; in the energy range ∼ 138–235 keV for He+ and in the energy range of 414–638 keV for CNO+ ions. Flux increase before arriving dipolarization front may result from the reflection of plasma sheet ions at the dipolarization front and the result of the resonant interactions of ions with low-frequency electromagnetic waves.</p

Table2_Turbulent dipolarization regions in the Earth’s magnetotail: ion fluxes and magnetic field changes.pdf

In this work, we consider the dynamics of ion fluxes and magnetic field changes in turbulent regions of magnetotail dipolarizations. The data from the Cluster-II mission (magnetic field measurements from fluxgate magnetometers and energetic charged particle observations from RAPID spectrometers) were used for the analysis. We study individual events and investigate statistically the changes of charged particle fluxes during magnetic field dipolarizations observed during 2001–2015. Received changes in the spectral index indicate that CNO+ ions undergo stronger acceleration during dipolarization than protons and helium ions. Before dipolarization front monotonic growth the ions flux is observed (the maximum of flux is observed at 1–1,5 min after the start of dipolarization) in the range of ∼ 92–374 keV for proton; in the energy range ∼ 138–235 keV for He+ and in the energy range of 414–638 keV for CNO+ ions. Flux increase before arriving dipolarization front may result from the reflection of plasma sheet ions at the dipolarization front and the result of the resonant interactions of ions with low-frequency electromagnetic waves.</p

Table1_Turbulent dipolarization regions in the Earth’s magnetotail: ion fluxes and magnetic field changes.pdf

In this work, we consider the dynamics of ion fluxes and magnetic field changes in turbulent regions of magnetotail dipolarizations. The data from the Cluster-II mission (magnetic field measurements from fluxgate magnetometers and energetic charged particle observations from RAPID spectrometers) were used for the analysis. We study individual events and investigate statistically the changes of charged particle fluxes during magnetic field dipolarizations observed during 2001–2015. Received changes in the spectral index indicate that CNO+ ions undergo stronger acceleration during dipolarization than protons and helium ions. Before dipolarization front monotonic growth the ions flux is observed (the maximum of flux is observed at 1–1,5 min after the start of dipolarization) in the range of ∼ 92–374 keV for proton; in the energy range ∼ 138–235 keV for He+ and in the energy range of 414–638 keV for CNO+ ions. Flux increase before arriving dipolarization front may result from the reflection of plasma sheet ions at the dipolarization front and the result of the resonant interactions of ions with low-frequency electromagnetic waves.</p

Constraints on Europa’s Water Group Torus from HST/COS Observations

In situ plasma measurements as well as remote mapping of energetic neutral atoms around Jupiter provide indirect evidence that an enhancement of neutral gas is present near the orbit of the moon Europa. Simulations suggest that such a neutral gas torus can be sustained by escape from Europa’s atmosphere and consists primarily of molecular hydrogen, but the neutral gas torus has not yet been measured directly through emissions or in situ. Here we present observations by the Cosmic Origins Spectrograph (COS) of the Hubble Space Telescope (HST) from 2020 to 2021, which scanned the equatorial plane between 8 and 10 planetary radii west of Jupiter. No neutral gas emissions are detected. We derive upper limits on the emissions and compare these to modeled emissions from electron impact and resonant scattering using a Europa torus Monte Carlo model for the neutral gases. The comparison supports the previous findings that the torus is dilute and primarily consists of molecular hydrogen. A detection of sulfur ion emissions radially inward of the Europa orbit is consistent with emissions from the extended Io torus and with sulfur ion fractional abundances as previously detected