How Jupiter's Moons - Io & Europa- Modulate Jupiter's Magnetosphere

 

                     Artist's sketch of the extended Jovian magnetospheric system.

In this post I look at a newly reported link (EOS Space Science, September, p. 42) between Jupiter's two innermost moons:  Io and Europa, - and their modulation of Jupiter's planetary magnetosphere.The new revelations are based on a recent review paper by Bagenal and Dols that collated and analyzed decades of research on the intricate Jupiter-Io-Europa system.  Whereas previous reviews focused on Io's or Europa's space environment alone, the current paper addresses both in tandem.

 The researchers succeed in first painting a detailed picture of what is known about the respective moon's atmospheres. For example, Io's is thought to consist mostly of sulfur dioxide (SO2) while Europa's is mostly oxygen.  Each moon's atmosphere then interacts in specific ways with the  magnetospheric plasma causing clouds of neutral atoms to escape from the atmospheres.  Io plays a particularly key role. At any given moment, 50 to 100 of its many volcanoes are actively erupting - emitting sulfur dioxide into the moon's atmosphere.  Each second roughly a ton of this gas becomes plasma  that feeds into a doughnut-shaped belt (see graphic) of dense plasma that encircles the planet. 

Many of the non-MHD phenomena arise because the neutral atoms and/or molecules that surround the 2 moons can be ionized through photoionization, electron impact, charge exchange and related processes . As a result of photoionization and electron impact ionization, ions and electrons are added to the ambient plasma.

As soon as they are added to the flow, forces act that accelerate them to the speed of the surrounding plasma. The flow then slows because total momentum must be conserved. The momentum that goes into new ions is extracted from the background plasma, reducing u. The new ions acquire gyrospeeds equal to the flow speed but their  V _k  does not change.

The plasma temperature (or second moment of the distribution) is also affected, with the perpendicular temperature increasing if in the background plasma  v  _thermal < u  _plasma and decreasing if in background plasma v  _thermal > u  _plasma.  Here v  _thermal  is the thermal energy of the background plasma and  u  _plasma  is the flow speed. Charge exchange differs from other forms of pickup. It refers to a process in which an incident ion moving with the flow exchanges its charge with a neutral at rest in the moon’s frame. The products of the interaction are a neutral with the momentum of the impacting ion and an ion at rest (because collisional momentum is conserved). Charge exchange does not change mass density because for each plasma ion lost, a new ion is added. However, the ion initially at rest must join the flowing plasma, requiring that momentum be extracted from the flow as in the case of pickup.

If many charge exchange interactions occur, the plasma in the final state will have slowed and large numbers of fast moving neutrals will have been created. The neutral atoms speeding off on straight paths at the local plasma rotation velocity spread to great distances.   For example, near Io in regions where the flow has not yet slowed because of the interaction, the neutral velocity is 57 km/s (relative to an ion moving with Io) tangential to the azimuthal direction at the point of origin, implying that in one Jovian rotation period, the neutral travels 29Rj (Rj ¼ radius of Jupiter)

It is well to point out anisotropic distributions may be unstable to generation of plasma waves if resonant interactions dominate the damping effect of the background plasma. Resonance with a wave of angular frequency w  and wave vector k occurs for particles whose velocity satisfies the condition:

w  +   k V  _||   =   + Ω _ci

With:  w  <   Ω _ci

Thus in a bi-Maxwellian plasma, waves can grow at frequency f _waves with:

f _waves    < f  _ci

 Additional non-MHD processes relevant to the interactions near the moons arise because the gyroradii of energetic ions can become comparable with the scale size of the interaction regions, characterized by the radii of the moons.

 All of this provides only a preliminary picture of the modulation dynamic. For example, we'd like to know the exact compositions of the moons' atmospheres to nail down the plasma aspects.  Also, how do the plasma -atmosphere interactions vary at different locations around each moon?  Then, what impact do changes in Io's volcanic activity have on the system?  I.e. Do we have v  _thermal > u  _plasma  or  v  _thermal <  u  _plasma ?   Also, is the bi-Maxwellian plasma feature preserved?

Fortunately, the ongoing 'Juno'  mission combined with enhanced computational plasma modeling should help to provide new insights into the Io-Europa magnetospheric system.  Most importantly, the authors emphasize that a flyby mission to Io is needed to address many of the questions raised in their research - and others raised in this post.