Juno Observations of Io's Alfvén Wing from 23 Io Radii

On 13 June, day 165 of 2024, Juno passed through Io's main Alfvén wing at a distance of some 23 Io radii (R_I) below the moon during perijove (PJ) 62.  Evidence for this passage was clearly seen in the Juno plasma wave, magnetometer, and ion plasma data. The plasma wave signature was an intensification of quasi-electrostatic waves below about 1 kHz with a weaker magnetic component, all lasting for about 90 seconds.  A strong modification of the magnetic field was observed primarily in the co-rotation direction but with a significant component in the direction away from Jupiter. Ions in the range below about 1 keV/q were slowed within the Alfvén wing. The Juno mission has afforded multiple opportunities to examine the Io-Jupiter interaction near the planet and two close flybys through the Alfvén wing during perijoves 57 and 58.  Hence, PJ62 provided observations of the Io-magnetosphere interaction at an intermediate distance.  The broadband electromagnetic emission below 1 kHz was observed during PJs 57 and 58, however, the magnetic component is markedly reduced from those. An estimate of the power in the interaction obtained by scaling the Poynting flux and integrating over the cross section of the flux tube is ~500x10⁹ W.  And modeling of the current suggests filamentation of the Alfvén waves as observed in other Io Alfvén wings.