On the propagation of transversally small-scale standing Alfven waves in a three-dimensionally inhomogeneous magnetosphere

The Jovian Infrared Auroral Mapper (JIRAM) on board Juno is a spectro-imager which is observing the atmosphere of Jupiter and its auroral emission using its two imagers in the L (3.3-3.6&#956;m) and M bands (4.5- 5.0&#956;m) and a spectrometer (2-5 &#956;m spectral range). The highly elliptic orbit of Juno and the unprecedented resolution of the JIRAM imager allowed to retrieve wealth of details about the morphology of moon-related aurorae. This phenomenon is due to the jovian magnetic field sweeping past the Galiean moons, which generate Alfven waves travelling towards the ionosphere and set up field aligned currents. When the associated electrons reach the ionosphere, they interact with the hydrogen and make it to glow. In particular, the tails of the footprints showed a spot-like substructure consistently, which were investigated using the L-band of the imager from perijove 4 to perijove 30. This feature was observed close to the footprints, where the the typical distance between spots lies between 250km and 500km. This distance decreases to 150km in a group of three observations in the northern emisphere when each moon is close to 250 &#9702; west longitude. No correlation with orbital parameters such as the longitude of the moons was found so far, which suggests that such morphology is almost purely due to ionospheric processes. Moreover, during PJ 13 a long sequence of images of the Io footprint was shot and it revealed that the secondary spots appears to corotate with Jupiter. This behaviour is observed also during orbits 14 and 26. During these sequences JIRAM clearly observed the Io footprint leaving behind a trail of &#8221;footsteps&#8221; as bright spots. The characteristics of these spots are incompatible with multiple reflection of Alfven waves between the two emispheres. Instead, we are currently investigating ionospheric processes like the feedback instability (FI) as a potential candidate to explain the generation of the observed small scale structure. This process relies on local enhacement of conductivity in the ionosphere, which is affected by electron precipitation. Order of magnitude estimates from the FI are compatible with the inter-spot distance and the stillness of the spots.

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A dynamical model of plasma turbulence in the solar wind

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2014.0145 ◽

2015 ◽

Vol 373 (2041) ◽

pp. 20140145 ◽

Cited By ~ 55

Author(s):

G. G. Howes

Keyword(s):

Solar Wind ◽

Solar Wind Plasma ◽

Plasma Turbulence ◽

Alfven Waves ◽

Dynamical Model ◽

Alfvén Waves ◽

Small Scale ◽

Current Sheets ◽

Turbulent Fluctuations ◽

Physical Mechanisms

A dynamical approach, rather than the usual statistical approach, is taken to explore the physical mechanisms underlying the nonlinear transfer of energy, the damping of the turbulent fluctuations, and the development of coherent structures in kinetic plasma turbulence. It is argued that the linear and nonlinear dynamics of Alfvén waves are responsible, at a very fundamental level, for some of the key qualitative features of plasma turbulence that distinguish it from hydrodynamic turbulence, including the anisotropic cascade of energy and the development of current sheets at small scales. The first dynamical model of kinetic turbulence in the weakly collisional solar wind plasma that combines self-consistently the physics of Alfvén waves with the development of small-scale current sheets is presented and its physical implications are discussed. This model leads to a simplified perspective on the nature of turbulence in a weakly collisional plasma: the nonlinear interactions responsible for the turbulent cascade of energy and the formation of current sheets are essentially fluid in nature, while the collisionless damping of the turbulent fluctuations and the energy injection by kinetic instabilities are essentially kinetic in nature.

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