Europa's plasma interaction with an inhomogeneous atmosphere: Development of Alfvén winglets within the Alfvén wings

Abstract

We apply a three‐dimensional magnetohydrodynamic (MHD) model to study the influence of inhomogeneities in Europa's atmosphere, as, for example, water vapor plumes, on Europa's plasma interaction with the Jovian magnetosphere. In our model we have included electromagnetic induction in a subsurface water ocean, collisions between ions and neutrals, plasma production and loss due to electron impact ionization, and dissociative recombination. We present a systematic study of the plasma interaction when a local inhomogeneity in the neutral density is present within a global sputtering generated atmosphere. 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 Europa's main Alfvén wing on that side where the inhomogeneity is located. 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 the interaction. 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. Our analysis suggests that the magnetic field might be influenced by atmospheric inhomogeneities during the E26 flyby. The findings of this work will aid in the search for plumes at Europa in future plasma and field observations.

Magnetic Signatures of a Plume at Europa During the Galileo E26 Flyby

“…Both of these flybys occurred in the orbital trailing hemisphere of Europa: The E12 flyby occurred at equatorial latitudes, whereas the E26 flyby occurred at more southern latitudes (see Figure 1). As suggested by Blöcker et al (2016), transverse currents within the plumes should leave a clear imprint in Galileo magnetometer data from such close flybys. As suggested by Blöcker et al (2016), transverse currents within the plumes should leave a clear imprint in Galileo magnetometer data from such close flybys.…”

“…We use similar magnetospheric upstream parameters as Rubin et al (2015) and Blöcker et al (2016) to facilitate a comparison to their results for the E26 flyby. Since the bulk velocity of the (nearly) corotating plasma relative to Europa is not constrained through observations from E26, we use a value of u 0 = 100 km/s in agreement with Bagenal et al (2015).…”

“…We consider two cases: A = 0, similar to the symmetric, barometric profile of Blöcker et al (2016) and A = 10. The symbol denotes the angle between the position ⃗ r and the negative x axis (which points into the trailing/ramside hemisphere).…”

“…A localized inhomogeneity in the atmosphere generates a tube-like region of locally enhanced current density and flow deceleration within the main Alfvén wing, referred to as an Alfvén winglet by Blöcker et al (2016). A localized inhomogeneity in the atmosphere generates a tube-like region of locally enhanced current density and flow deceleration within the main Alfvén wing, referred to as an Alfvén winglet by Blöcker et al (2016).…”

DETECTION OF A HYDROGEN CORONA IN HST Lyα IMAGES OF EUROPA IN TRANSIT OF JUPITER

“…For this estimation, we assume that electron-impact excitation produces Lyα emission that partially balances the absorption. The auroral emission yield does not linearly increase with increasing plume density, but is limited by the fraction of hot electrons that reaches the dense regions of the plume (Roth et al 2011;Blöcker et al 2016). We, therefore, assume a fixed Lyα aurora brightness of 600R as detected by Roth et al (2014b).…”