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Jovian magnetospheric plasma
is coupled to the ionosphere through Alfvén waves.
Alfvén waves enable the transport of angular momentum
and energy between the planet and magnetospheric plasma,
a process that ultimately generates Jupiter's bright
auroral emissions. However, past the Alfvén radius, the
location where the radial velocity is greater than the
Alfvén velocity, magnetospheric plasma is effectively
decoupled from the planet, with fluctuations in the
local magnetosphere no longer communicated back to the
planet. Determining Jupiter's Alfvén surface is critical
for interpreting drivers of auroral emissions, in situ
data, and applications of numerical models. Previous
studies that calculated the location of the Alfvén
radius assumed an azimuthally symmetric magnetosphere
and local-time independent magnetic field. Here, we
employ a statistical description of the magnetic field
that includes local time effects. We find a minimum
Alfvén radius of 30 RJ (Jupiter radii) at 6 LT, with
plasma decoupled from the planet in the post-dusk
through dawn sector. Furthermore, no Alfvén radius
exists within 60 RJ between 8 and 20 LT. At distances
greater than 50 RJ, the Alfvén travel time is such that
magnetospheric plasma moves substantially in the
magnetosphere before angular momentum can be efficiently
transferred from the atmosphere. Therefore, the angular
momentum supplied may no longer be sufficient for the
local conditions. Our results highlight the importance
of local time considerations in MI coupling studies and
offer new interpretations for local time dependent
auroral features, such as the polar collar.
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