Plain Language Summary
Jupiter’s moon Io generated electrical currents when it passes through
Jupiter’s magnetic field. These currents take the form of fluctuations
in the magnetic field lines, much like the waves on a stringed musical
instrument. Due to the motion of Io, these waves follow behind Io and
bounce back and forth between Jupiter and the dense ionized gas emitted
by Io. This process creates auroral emissions that can be observed, for
example, with the Hubble Space Telescope.
1 Introduction
Alfvén waves have long been associated with the coupling of the moon Io
with the ionosphere of Jupiter since the discovery by Bigg (1964) that
the Jovian decametric radio emissions were modulated by the phase of Io
in its orbit. Goldreich and Lynden-Bell (1969) identified Io as the
generator of field-aligned current due to its motion relative to the
co-rotating plasma at Jupiter. Goertz (1980) and Neubauer (1980) noted
that this is due to the launching of Alfvén “wings” from the moving
satellite. These Alfvén waves were identified by the Voyager 1 flyby of
Jupiter (Acuña et al., 1981; Belcher et al., 1981). The reflection of
these Alfvén waves from Jupiter’s ionosphere was invoked by Gurnett and
Goertz (1981) to explain the multiplicity of decametric radio emissions.
The role of the plasma torus produced by the volcanic activity on Io at
modifying the propagation of Alfvén waves was recognized by Bagenal
(1983), who considered the reflecting Alfvén wave model in the context
of plasma measurements from the Voyager mission. She noted that the
travel time for Alfvén waves to propagate from Io to Jupiter depended on
Io’s position with respect to the plasma torus, which peaks at the
centrifugal equator (i.e., the loci of points on each field line that is
farthest from the rotation axis of Jupiter) at a angle of 7° from Io’s
orbital plane. The auroral emissions from Io were observed from the
Infrared Telescope Facility (Connerney et al., 1993) and the Hubble
Space Telescope (Bonfond et al., 2008, 2009; Clarke et al., 1996, 2002).
The pre-Juno understanding of the Io-plasma interaction was summarized
by Saur et al. (2004) and Bagenal and Dols (2020).
With the arrival of the Juno satellite at Jupiter in July, 2016,
detailed measurements of particles and waves in Jupiter’s magnetosphere
became possible. In particular, the Juno orbit has made a number of
high-latitude crossings of field lines magnetically connected to Io,
indicating the presence of accelerated electrons and ions (Szalay et
al., 2018, 2020) and Alfvén waves with a power law spectrum extending up
to the proton gyrofrequency (Gershman et al., 2019; Sulaiman et al.,
2020). Observations of the Io footprint tail have been made by the
Jovian Infrared Auroral Mapper (JIRAM) instrument on Juno (Moirano et
al., 2021; Mura et al., 2017, 2018), which indicate that the footprint
tail is made up from a series of spots that at times appear to alternate
positions with respect to the footprint of Io’s orbit. The purpose of
the present work is to show the first results from a numerical model of
the Io interaction with Jupiter’s magnetosphere and ionosphere to help
understand this interaction.
The propagation of these Alfvén waves was considered qualitatively by
Crary and Bagenal (1997) as well as Bonfond et al. (2008). More
recently, Hinton et al. (2019) used a diffusive equilibrium model of the
plasma density to construct ray paths for the reflecting Alfvén waves.
On the other hand, Crary and Bagenal (1997) and Jacobsen et al. (2007)
assumed a sharp gradient and straight magnetic field lines. A new model
for the interaction of Io was introduced by Schlegel and Saur (2022) who
used a Hall-MHD model to describe the interaction. They included the
Hall conductivity in Io’s ionosphere in an attempt to understand the
alternating auroral spots observed by Mura et al. (2018) and Moirano et
al. (2021). However, this model apparently assumed perfect conductivity
in the Jovian ionosphere and straight magnetic field lines.
In order to improve upon these earlier efforts, we have developed a
model for the propagation of Alfvén waves from Io that is cast in a more
realistic dipole geometry. We note that the orbit of Io is roughly in
the equatorial plane of Jupiter, while the magnetic dipole moment is
displaced by 10.25°, consistent with the dipole term in the spherical
harmonic analysis of JRM33 (Connerney et al., 2022). (It should be noted
that this tilt is 9.52° for the VIP4 model (Connerney et al., 1998) and
10.31° for the JRM09 model (Connerney et al., 2018); however, the
results presented here are not too sensitive to the magnetic field model
used.) The Io plasma torus is centered on the centrifugal equator, which
is two-thirds of the distance from the rotational equatorial plane to
the magnetic equatorial plane (e.g., Bagenal, 1983). Io is modeled as a
cloud of Pedersen conductivity that is moving with respect to the
co-rotating plasma of Jupiter’s magnetosphere. Jupiter’s ionospheres are
modeled by a height-integrated Pedersen conductance. In this work, we
will focus on the effects of the plasma distribution along the Io flux
tube as well as the effect of different conductances on the
field-aligned current signatures from Io. The next section will describe
the model, followed by a discussion of the effect of the density
profile. The following section will consider variations in the
conductance of Jupiter’s ionosphere. Then the effect of parallel
electric fields due to kinetic effects will be considered. We will
conclude with a discussion of the implications of these results for the
structure of the auroral footprint tail of Io.