Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa

The goal of our study is to present a systematic modeling framework for the identification of water vapor plumes in plasma and magnetic field data from spacecraft flybys of Europa. In particular, we determine the degree to which different plume configurations can be obscured by the interaction of Jupiter's magnetospheric plasma with Europa's induced dipole field and its global atmosphere. We apply the hybrid model AIKEF (kinetic ions, fluid electrons) to investigate the effect of inhomogeneities in Europa's atmosphere (plumes) on the plasma interaction with the Jovian magnetosphere. To systematically assess the magnitude and structure of the perturbations associated with the plume-plasma interaction at Europa, we vary the plume location across Europa's surface while considering different symmetric and asymmetric density profiles of the moon's global atmosphere. To isolate the impact of a plume on Europa's magnetospheric environment, we also conduct model runs without any global atmosphere. To quantify the magnetic perturbations caused by plumes, we analyze the field components along hypothetical spacecraft trajectories through each plume. Conclusions of our study are (1) localized regions of stagnant flow are most indicative of the presence of a plume. (2) The visibility of plumes in the magnetic field strongly depends on the density profile (whether it is symmetric or asymmetric) of the global atmosphere. (3) The presence of an induced dipole complicates the identification of magnetic signatures associated with a plume and dominates Europa's magnetic environment in its intermediate vicinity. (4) Complex fine structures are visible in the tail of escaping plume ions.magnetospheric plasma interacts with the (time-varying) dipole field induced in the moon's subsurface ocean (Kivelson et al., 1999;Zimmer et al., 2000), which is driven by the 9.6 • tilt between Jupiter's magnetic and rotational axes. This induced dipole field is compressed at Europa's ramside and stretched at its wakeside, locally contributing to transverse currents and therefore to the Alfvén wings (Liuzzo et al., 2016). The coupling of the dipole-magnetosphere interaction and the ionospheric mass loading reduces the cross section of the Alfvén wings and generates a slight displacement of the wings with respect to the moon (Neubauer, 1999;Volwerk et al., 2007).However, the view of Europa's neutral gas environment was changed drastically when, in December 2012, Hubble Space Telescope (HST) observations of the moon's UV aurora revealed a localized surplus of UV emission intensity near its south pole, associated with an increase in oxygen and hydrogen column densities. Roth et al. (2014) showed that two water vapor plumes emanating near 180 • W 75 • S and 55 • S, each with a scale height of about 200 km, quantitatively match the HST observations. Through image postprocessing, Sparks et al. (2016) found hints of additional transient plumes, located near the south pole at 271 • W 63 • S and in the equatorial region at 275.7 • W 16.4 • S. However, subsequ...

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“…higher, due to the mass difference between the species), thus being visible in magnetic field data (Desai et al, 2017;Volwerk et al, 2001).…”

Section: Case 2: Plasma Interaction With a Plume And A Symmetric Globmentioning

confidence: 92%

Plasma Interaction Signatures of Plumes at Europa

2020

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“…However, the significance of wave‐particle interactions to different ion populations at Europa’s orbit is not well constrained through the limited available observations. In particular, there is no consensus on the growth times and saturation amplitudes of ICWs (Desai et al., 2017; Volwerk et al., 2001). Furthermore, ICWs emerge mainly from regions of strong temperature anisotropy (Šebek et al., 2016), the locations of which have not yet been constrained for Europa through observations or modeling.…”

Section: Model Descriptionmentioning

confidence: 99%

Influence of Europa’s Time‐Varying Electromagnetic Environment on Magnetospheric Ion Precipitation and Surface Weathering

et al. 2021

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The magnetic field of Jupiter (radius R J = 71,492 km) generates a magnetosphere that extends ∼50 R J at the sun-facing side and hundreds of R J at the sun-averted side (Joy et al., 2002). The magnetic dipole axis of Jupiter is inclined by 9.6° against its rotational axis, causing the magnetic field to "wobble" when viewed from a fixed point at the rotational equator. This wobble creates periodic variations in the magnetospheric field near Jupiter's Galilean satellites. Europa, the smallest of the Galilean satellites (radius R E = 1,560.8 km), orbits its parent planet at a distance of 9.38 R J , well within the magnetosphere. Europa possesses a dilute exosphere (e.g.,

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“…1. Intense plasma waves have been observed at Rhea (Santolík et al, 2011), and right-hand polarized Alfvéncyclotron waves, which would gyroresonantly interact with O − , could be produced by negatively charged pickup ions (Desai, Cowee, et al, 2017). These however feed of the free energy from the pickup ions, and this effect could not be significant over such a short time period.…”

Section: Velocity Space Analysismentioning

confidence: 99%

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

Desai

Taylor

Regoli

et al. 2018

Geophysical Research Letters

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Saturn's largest icy moon, Rhea, hosts a tenuous surface‐sputtered exosphere composed primarily of molecular oxygen and carbon dioxide. In this Letter, we examine Cassini Plasma Spectrometer velocity space distributions near Rhea and confirm that Cassini detected nongyrotropic fluxes of outflowing CO2+ during both the R1 and R1.5 encounters. Accounting for this nongyrotropy, we show that these possess comparable along‐track densities of ∼2 × 10−3 cm−3. Negatively charged pickup ions, also detected during R1, are surprisingly shown as consistent with mass 26 ± 3 u which we suggest are carbon‐based compounds, such as CN−, C2H−, C2−, or HCO−, sputtered from carbonaceous material on the moon's surface. The negative ions are calculated to possess along‐track densities of ∼5 × 10−4 cm−3 and are suggested to derive from exogenic compounds, a finding consistent with the existence of Rhea's dynamic CO2 exosphere and surprisingly low O2 sputtering yields. These pickup ions provide important context for understanding the exospheric and surface ice composition of Rhea and of other icy moons which exhibit similar characteristics.

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Hybrid Simulations of Positively and Negatively Charged Pickup Ions and Cyclotron Wave Generation at Europa

Cited by 13 publications

References 58 publications

Plasma Interaction Signatures of Plumes at Europa

Plasma Interaction Signatures of Plumes at Europa

Influence of Europa’s Time‐Varying Electromagnetic Environment on Magnetospheric Ion Precipitation and Surface Weathering

Cassini CAPS Identification of Pickup Ion Compositions at Rhea

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