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\begin{document}

\title{Energetic Electron Imaging of Ganymede's Magnetic Field by the Juno
Spacecraft's Advanced Stellar Compass}



\author[1]{Matija Herceg}%
\author[2]{John L Jorgensen}%
\author[1]{Jose M.G. Merayo}%
\author[1]{Troelz Denver}%
\author[1]{Peter S Jorgensen}%
\author[3]{John E. P. Connerney}%
\author[2]{Mathias Benn}%
\author[3]{Stavros Kotsiaros}%
\affil[1]{Technical University of Denmark}%
\affil[2]{DTU Space, National Space Institute, Technical University of Denmark}%
\affil[3]{NASA Goddard Space Flight Center}%


\vspace{-1em}



  
  \date{May 24, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
The micro Advanced Stellar Compass (\selectlanguage{greek}µ\selectlanguage{english}ASC), an attitude reference for the
Juno Magnetic Field investigation, also continuously monitors high
energy particle fluxes in Jupiter's magnetosphere. The \selectlanguage{greek}µ\selectlanguage{english}ASC camera head
unit (CHU) shielding is sufficient to stop electrons with energy
\textless{}15MeV. By recording the number of particles that penetrate
\selectlanguage{greek}µ\selectlanguage{english}ASC CHU shielding and deposit energy in the CCD sensor, the \selectlanguage{greek}µ\selectlanguage{english}ASC
functions as an energetic particle sensor with a detection threshold
well above that of the Juno Energetic Particle Detector Instrument
(JEDI) flown for that purpose. Radiation data gathered by the \selectlanguage{greek}µ\selectlanguage{english}ASC is
used to monitor the radiation environment of Jupiter and mapping of the
trapped high energy particles. Comparison of the particle population
around Jupiter with individual perijove particle observations reveals
disturbances when Juno is traversing Ganymede's M-shell. We present
highly energetic electrons interaction with Ganymede's magnetic field,
magnitude and extend of the particle depletion associated with the
Ganymede interaction.%
\end{abstract}%



\sloppy


M. Herceg\textsuperscript{1}, J. L. J\selectlanguage{ngerman}ørgensen\textsuperscript{1}, J. M.
G. Merayo\textsuperscript{1}, T. Denver\textsuperscript{1}, P. S.
Jørgensen\textsuperscript{1}, M. Benn\textsuperscript{1}, S.
Kotsiaros\textsuperscript{1}, and J. E. P.
Connerney\textsuperscript{2,3}

\textsuperscript{1}Technical University of Denmark (DTU), Lyngby,
Denmark.

\textsuperscript{2}Space Research Corporation, Annapolis, MD, United
States

\textsuperscript{3}NASA Goddard Space Flight Center, Greenbelt, MD,
United States

Corresponding author: Matija Herceg (mher@space.dtu.dk)

Key Points:

\begin{itemize}
\tightlist
\item
  The \selectlanguage{greek}µ\selectlanguage{english}ASC observations confirms Ganymede's magnetic lensing and
  indicate its impact on the particle population and the wake region.
\item
  Ganymede exhibits relative decrease (up to 52\%) in high energy
  particle flux which is central and symmetrical to the Ganymede
  position.
\item
  The magnetic shadow of Ganymede has a width of \textasciitilde{}26,000
  km, or \textasciitilde{}10 Ganymede radii.
\end{itemize}

Abstract

The micro Advanced Stellar Compass (\selectlanguage{greek}µ\selectlanguage{english}ASC), an attitude reference for the
Juno Magnetic Field investigation, also continuously monitors high
energy particle fluxes in Jupiter's magnetosphere. The \selectlanguage{greek}µ\selectlanguage{english}ASC camera head
unit (CHU) shielding is sufficient to stop electrons with energy
\textless{}15MeV. By recording the number of particles that penetrate
\selectlanguage{greek}µ\selectlanguage{english}ASC CHU shielding and deposit energy in the CCD sensor, the \selectlanguage{greek}µ\selectlanguage{english}ASC
functions as an energetic particle sensor with a detection threshold
well above that of the Juno Energetic Particle Detector Instrument
(JEDI) flown for that purpose. Radiation data gathered by the \selectlanguage{greek}µ\selectlanguage{english}ASC is
used to monitor the radiation environment of Jupiter and mapping of the
trapped high energy particles. Comparison of the particle population
around Jupiter with individual perijove particle observations reveals
disturbances when Juno is traversing Ganymede's M-shell. We present
highly energetic electrons interaction with Ganymede's magnetic field,
magnitude and extend of the particle depletion associated with the
Ganymede interaction.

\textbf{Plain Language Summary}

The \selectlanguage{greek}µ\selectlanguage{english}ASC camera onboard Juno, that serves primary as an attitude
reference spacecraft, also continuously monitors high energy particle
fluxes in Jupiter's magnetosphere. These observations, captured by the
\selectlanguage{greek}µ\selectlanguage{english}ASC, reveal particle flux disturbances caused by the Jupiter's moon
Ganymede, their extent and magnitude.

1 Introduction

NASA's Juno spacecraft entered polar orbit about Jupiter on July 4th,
2016 (Bolton et al., 2017) and has now completed the 35 science orbits
of its primary mission, systematically mapping the 3D magnetosphere of
Jupiter (\emph{Bagenal et al., 2017} ). Located on the tip of the one of
Juno's three solar arrays, the Magnetic Field Experiment, MAG
(\emph{Connerney et al., 2017} ) carries an attitude sensor; the fully
autonomous micro Advanced Stellar Compass (\selectlanguage{greek}µ\selectlanguage{english}ASC) designed and built at
the Technical University of Denmark. In addition to its primary attitude
determination function, the \selectlanguage{greek}µ\selectlanguage{english}ASC is a star camera with a broad range of
observational capabilities made possible by its versatile design. These
include optical imaging of solar system bodies, autonomous detection and
tracking of objects (\emph{Benn et al., 2017; Jorgensen et al., 2020} )
and detection of the high energy particles (e.g., \emph{Connerney et
al., 2020} ). Electrons with energy \textgreater{}15MeV, protons with
energy \textgreater{}80MeV and heavier elements with energy
\textless{}\textasciitilde{}GeV penetrate the heavily shielded optical
head of the Camera Head Unit (CHU) and are detected by analysis of the
CCD imagery; individual pixel counts are recorded in every telemetry
packet sent to the spacecraft Command and Data Handling (C\&DH)
subsystem for eventual transmission to Earth. This data provides a
continuous record of the energetic particle environment traversed by
Juno. The observed particle flux distribution provides an excellent
in-situ measurement of the global energetic particle environment and its
interaction with the moons of Jupiter.

Juno regularly traverses M-shells of Galilean moons during its orbital
motion around Jupiter and often observes variations in the energetic
particle population associated with interactions between Jupiter's
magnetosphere and the moons. These particle signatures may take many
forms, displaying different characteristics for different moons, and are
often heavily influenced by the angular separation (``phase angle'')
between Juno and the moon during transit. The motion of these charged
particles within the magnetic field of Jupiter is periodic and depends
on particle energy, pitch angle (\selectlanguage{greek}\emph{α} \selectlanguage{english}) and strength of the magnetic
field (\emph{B} ). Trapped charged particles execute motion along the
magnetic field in three superimposed motions: gyro, bounce, and drift
motion, as illustrated in \emph{Figure 1} . This illustration traces the
motion of a 20, 60 and 100 MeV electron injected with a pitch angle of
10 degrees in a magnetic field described by the JRM09 internal magnetic
field (\emph{Connerney et al., 2018} ) combined with that of the
magnetodisc (Connerney et al., 2020). We simulate the motion of
electrons with these energies to span the spectrum of energetic
particles that the ASC instrument is sensitive to.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1} : Motion of a 20 (green), 60 (blue) and 100 MeV (red)
electron injected with an equatorial pitch angle of 10\selectlanguage{ngerman}° at 15
R\textsubscript{J} (Ganymede's orbital distance), bouncing between
mirror points and slowly drifting westward around Jupiter with a drift
period of 29, 87, and 144s respectively, and half bounce drift of
\textasciitilde{}2° along the magnetic equator.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2} : Computed energetic particle trajectories passing by
Ganymede illustrating a particle shadow cast by the moon. Simulation
uses electrons with energies of 20, 60 and 100MeV undergoing motion in a
magnetic field computed using a Ganymede dipole and the Jovian magnetic
field (JRM09 model).

In this paper, we focus on signatures observed by Juno's \selectlanguage{greek}µ\selectlanguage{english}ASC when Juno
is traversing Ganymede's M-shell, defined as the set of magnetic field
lines that cross Jupiter's magnetic equator at Ganymede's radial
distance from Jupiter. Similar to L-shell, calculated using the magnetic
dipole (\emph{Mcllwain, 1961} ), an M-shell is computed using non-dipole
fields.

To understand these signatures, we perform particle motion simulations
for particles moving along field lines connecting the Juno spacecraft
and Ganymede. Electron particle motion around Jupiter is governed by
three adiabatic invariants, bouncing, gyration and westward drift motion
(\selectlanguage{ngerman}\emph{Alfvén 1950, Northrop, 1966., Kruskal, 1958., Chew et. al, 1955}
), as shown in \emph{Figure 1} . North-south particle bouncing motion,
also called the second adiabatic invariant, is expressed as the
longitudinal integral between the two turning points for the particle
trapped in a magnetic mirror. Particles with weaker energies have mirror
points at lower latitudes and they drift faster (the third adiabatic
invariant). If both the first and the second adiabatic invariants are
conserved in the magnetic field, the particle will drift (the third
adiabatic invariant) due to the radial gradient of the field (`gradient
drift') and the curvature of the magnetic field lines (`curvature
drift'). As the particle gyrates about the field, it will enter a weaker
magnetic field when further from the planet causing it to drift east or
west in longitude depending on the sign of its charge. Figure shows how
the electron's equatorial pitch angle increases with eah bounce and the
electron drifts more rapidly westward. A 60 MeV electron injected with
10° pitch angle at 15 R\textsubscript{J} drifts around Jupiter in 87sec.
Since each particle bounce between the mirror points increases the
Larmor radius (gyration radius of circular motion), the drift rate
increases also. This leads to the particle's loss of stability and it
exits the drift shell after \textasciitilde{}260 seconds. In the
simulation, we do not account for the an energy loss mechanism such as
synchrotron radiation and collisional loss.

A charged particle injected near Ganymede will be guided by the magnetic
field of Jupiter (JRM09 model), the magnetodisc (\emph{Connerney et al.,
2020} ) and the internal magnetic field of Ganymede (\emph{Kivelson et
al., 1997; Kivelson et al., 2002} ). Particle motions were computed
using the DTU particle simulation toolbox for the electrons with
energies of 20, 60 and 100 MeV (\emph{Figure2)} . Verification of the
DTU particle simulation toolbox was confirmed by comparison with
previously published simulated particle trajectories (\emph{Öztürk,
2012} ). Nevertheless, uncertainties in particle trajectory calculations
may be caused by accumulation of errors in numerical integration and the
fidelity of the magnetic models used. Since particle motion is simulated
15R\textsubscript{J} away from Jupiter, the magnetic model uncertainties
are minimal but for variability of the magnetodisc. However,
uncertainties in Ganymede's magnetic field (\emph{Kivelson, 2002} ) and
those due to the interaction with Jupiter's field will directly impact
simulated particle motions, and to a lesser extent, the total lensing
effect.

Particle trajectory calculations were performed using JRM09
(\emph{Connerney et al., 2018} ) for Jupiter's field, augmented by a
model magnetodisc (Connerney et al, 2020), and a Ganymede magnetic
dipole model (\emph{Kivelson, 2002} ) with coefficients {[}g10, g11,
h10{]} = {[}-728, 66, -11{]} nT. The field due to JRM09 and magnetodisc
at Ganymede (\textasciitilde{}15 R\textsubscript{J})

is \textasciitilde{}75nT, almost 10\% of Ganymede's equatorial magnetic
field; particle motion further away (\textgreater{}3 R\textsubscript{G})
from Ganymede will be governed primarily by the magnetic field of
Jupiter and the magnetodisc.

The simulation shows that electrons with energies of 20-100MeV passing
within \textasciitilde{}5 Ganymede radii (R\textsubscript{G}) may be
redirected up to \textasciitilde{}4 R\textsubscript{G} away from
Ganymede, producing a particle ``shadow'' of up to \textasciitilde{}8
R\textsubscript{G in} width. Juno has completed 35 orbits around
Jupiter, having crossed Ganymede's M-shell more than 110 times. Even
though the majority of these M-shell crossings occurred while Ganymede
was well separated from Juno in longitude, a few crossings occurred with
Juno and Ganymede in or near magnetic conjugacy. These few revealed a
repeatable signature in the energetic particle population that
illustrates the lensing effect that Ganymede's magnetic field has on
particles of this energy.

2 Observations

The Juno spacecraft experiences multiple traversals of Ganymede's
M-shell on approach to Jupiter, a consequence of its polar orbit and the
\textasciitilde{}10-hour rotation of Jupiter's magnetic field. These may
occur at radial distances spanning Ganymede's orbital radius (14.96 RJ),
their number and location largely determined by Juno's System III
longitude on approach (Figure 3, right panel). When Juno crosses an
M-shell corresponding to the orbit of one of Jupiter's moons, sensors on
the spacecraft often detect variations in the charged particle
environment (Sulaiman et al., 2020, Szalay et al., 2020, Paranicas et
al., 2021), radio emissions (Louis et al., 2020), infrared observations
(Mura et al., 2020), and magnetic field (Szalay et al., 2020) associated
with the moon's interaction with the Jovian magnetosphere. The approach
to Juno's 11th perijove presented a particularly auspicious opportunity
to observe Ganymede's interaction with the magnetosphere, with numerous
traversals of Ganymede's M-shell. Three of the traversals occurred while
Juno and Ganymede were separated by a large radial distance (few to
\textasciitilde{}10 RJ) and within a few degrees of System III (1965)
longitude. As shown in top left panel of figure 3, Juno's trajectory is
illustrated with a black dashed line, and magnetic field lines that Juno
crosses during the second Ganymede M-shell traversal are colored in red.
The equatorial crossing point of the field line intersecting the
spacecraft trajectory is marked with a blue line, while the equatorial
crossing point of the field line passing through Ganymede's orbit is
colored in red; both calculated using the JRM09 magnetic field model and
the Connerney et al (2020) magnetodisc model. Where the two cross, Juno
is traversing Ganymede's M-shell.

An overview of Juno's approach to perijove 11 appears in the top right
panel of figure 3, a rendering in the magnetic dipole coordinate system.
Juno traverses Ganymede's M-shell 5 times while inbound. Another way to
illustrate M-shell traversals appears in the second row of figure 3,
which shows the radial distance to the equatorial crossing point of the
Juno spacecraft and the Galilean satellites as a function of time.
Similarly, the third row shows the variation in time of the System III
longitude of Juno and the major moons. Where Juno's and Ganymede's
curves intersect in the second and third panel, Juno is traversing
Ganymede's M-shell and on, or near, the same magnetic field line as
Ganymede. This alignment is observed by \selectlanguage{greek}µ\selectlanguage{english}ASC three times during perijove
11 (crossings 1, 2 and 3 on bottom of the figure 3).

Energetic particle flux observations (figure 3, bottom panel) show many
features: a large, gradual increase in the particle population (between
crossing 1 and 2) as Juno approaches the magnetic equator between the
orbits of Ganymede and Europa; passage through the radiation belt
``horn'' right after crossing 3 at a distance of a few RJ; passage
through perijove and again crossing of the radiation belt horn while
outbound, this time much closer to Jupiter where the radiation is most
intense. Apart from these large and expected radiation signatures, there
a several small disturbances. Disturbances observed around perijove
mapping to high magnetic latitudes are mostly associated with crossing
of the main aurora oval (Kotsiaros et al., 2019, Mauk et al., 2020) and
episodic particle events mapping to very large radial distances (paper
in preparation by DTU). The signatures we are concerned with here are
those observed prior to perijove (marked 1, 2, 3) that correspond to the
predicted traversal of Ganymede's M-shell depicted in the second and the
third panels.

Juno traverses M-shells with different radial distances on approach to
Jupiter. During perijove 11, at around 01:55 UTC and radial distance of
13 RJ, Juno's footprint meets the one of Ganymede (figure 3). Then, Juno
and Ganymede have the same equatorial crossing distance (purple and blue
line on the second row plot of figure 3). At nearly the same time a
significant (\textasciitilde{}25\%) decrease in energetic particle flux
is observed by the \selectlanguage{greek}µ\selectlanguage{english}ASC (bottom plot in figure 3). Similarly, at 06:47
UTC, Juno traverses Ganymede's M-shell even closer in phase (difference
between the crossings S3 longitude) and observes a significant depletion
(\textasciitilde{}50\%) in particle flux. The third traversal occurs at
09:05 UTC, but by then the Juno and Ganymede field lines are separated
by \textasciitilde{}13 degrees S3 longitude resulting in a much weaker
energetic particle depletion.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3} : Observed particle flux disturbance due to Ganymede.
Left top panel illustrates the geometry during event \#2, during which
Juno's equatorial crossing point crosses Ganymede's; magnetic field
lines (red) intersecting the Juno trajectory (dashed black) and magnetic
equator (blue). The top right panel shows Juno's trajectory in magnetic
equatorial coordinates color coded with the intensity of radiation
observed by the \selectlanguage{greek}µ\selectlanguage{english}ASC. Bottom 3 panels show satellite and spacecraft
equatorial crossing distance, S3 longitude, and \selectlanguage{greek}µ\selectlanguage{english}ASC observations as a
function of time (blue and red, averaged). Ganymede interaction events
1, 2, and 3 indicated.

\selectlanguage{greek}µ\selectlanguage{english}ASC observations displayed in the bottom plot of figure 3 show clear
signatures of Ganymede's interaction with the charged particle
environment. However, the signatures of the highly energetic particles
that \selectlanguage{greek}µ\selectlanguage{english}ASC is sensitive to (\textgreater{}20 MeV) are only observed when
Ganymede and Juno have small phase separation.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4} : Three panels show \selectlanguage{greek}µ\selectlanguage{english}ASC observations as a function of
time (1, 2, 3, from top to bottom). The green line shows the difference
in the radial direction (Rj) between the equatorial crossing point of
field lines through Ganymede and Juno, and the yellow line shows angular
difference. Red shaded area spans the Ganymede signature and the red
filled circle indicates where the model predicts the crossing of Juno
and Ganymede M-shells.

4 Results

Juno observes a depletion of energetic particles, presumably electrons,
when it traverses magnetic field line that connected (or close) to
Ganymede. A depletion of \textasciitilde{}15-50\% in particle count is
observed and the magnitude of the depletion depends on the phase angle,
with a reduction of 50\% experienced at small (-2.5 deg) phase angle,
and 15\% at larger (-13 deg) phase angle. Traversals of Ganymede's
M-shell that occur with greater phase separation (
\textgreater{}\textasciitilde{}10\selectlanguage{ngerman}º SIII longitude) evidence no particle
depletion. These signatures could alternatively be explained by the
presence of a cloud of neutrals around Ganymede, closely confined to
Ganymede's position, depleting the energetic electrons within the \selectlanguage{greek}µ\selectlanguage{english}ASC
sensitivity band (15-80MeV).

The particle depletion is similar in spatial scale for all 3 events,
varying between 8.9 and 11.1 Ganymede radii (R\textsubscript{GA}). The
time spent in the particle depletion region is dictated by the angle
between Juno's and Ganymede's magnetic equator crossing footprints; if
small the depletion persists longer.

The width of the depletion region can be compared to the simulations to
illustrate the Ganymede lensing effect (figure 2). The simulation shows
a Ganymede magnetic ``shadow'' of \textasciitilde{}8
R\textsubscript{GA}, compared to the average observed width of
\textasciitilde{}10 R\textsubscript{GA}. The small difference between
model and observation could be due to other contributors to the measured
particle flux, or temporal variation in the magnetic field, not captured
by the models.

Juno's \selectlanguage{greek}µ\selectlanguage{english}ASC observations of Ganymede particle depletion show that
magnetic shadowing effect is present up to \textasciitilde{}13\selectlanguage{ngerman}º
separation in phase. However, at this distance the signature is very
weak and at the edge of \selectlanguage{greek}µ\selectlanguage{english}ASC detection sensitivity, as in the plot for
perijove 1 (figure 5). The stronger particle depletion observed when
Juno is trailing Ganymede can be explained by the wake flow and
magnetospheric plasma not filling the Ganymede particle trail, compared
to the plasma density in Ganymede's ram direction. Signatures might also
originate from the wake region, if accounting for the bend back of the
field due to radial currents and half bounce drift (as shown on figure
1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5} : Footprint of the Galilean moons in the magnetic
equator and the associated depletion of the high energy particle
population. Dots (blue filled circles) mark the S3 longitude of the
equatorial crossing point of the field line through Juno as Juno
transverses the Ganymede M-shell. Shaded area shows how far away in S3
longitude the particle shadow is observed in the radiation data,
illustrated (blue) to the right. Negative [?]\selectlanguage{greek}λ \selectlanguage{english}indicates when Juno
footprint is trailing Ganymede.

Thus far during Juno's prime mission, there have been more than 120
traversals of Ganymede's M-shell. Of these only a handful with small
phase separation show clear signatures (figure 5). Our analysis,
supported by the simulations of energetic electron motion in the
environment of Ganymede, shows that the magnetic shadow of Ganymede has
a width of \textasciitilde{}26,000 km, or \textasciitilde{}10 Ganymede
radii. This novel use of the \selectlanguage{greek}µ\selectlanguage{english}ASC star tracker as an energetic particle
detector, and observations obtained in the Ganymede environment, allows
us to constrain the energy of the population penetrating the \selectlanguage{greek}µ\selectlanguage{english}ASC. The
particle trajectory calculations suggest that the \selectlanguage{greek}µ\selectlanguage{english}ASC mainly observes
\textasciitilde{}80 MeV electrons at the Ganymede M-shell distances.

5 Conclusions

During 35 successful science orbits of its primary mission, Juno's
Advanced Stellar Compass (\selectlanguage{greek}µ\selectlanguage{english}ASC) continuously monitored the population of
high energy particles (\textgreater{}15MeV electron and \textless{}80MeV
protons) in Jupiter's magnetosphere. Comparison of the particle
population model around Jupiter with individual perijove particle
observations reveals disturbances observed when Juno is traversing
M-shell of Ganymede. Particle fluxes observed while traversing Ganymede
M-shell are heavily influenced by the phase angle between Juno and
Ganymede.

The \selectlanguage{greek}µ\selectlanguage{english}ASC observations confirm the presence of the Ganymede magnetic
lensing and its extent along the wake region. Ganymede causes a relative
decrease (up to 52\%) in high energy particle flux which is central and
symmetrical to Ganymede's position, supported by simulations of
energetic electron motion in Ganymede's environment. These simulations
show that the magnetic shadow of Ganymede has a width of
\textasciitilde{}26,000 km, or \textasciitilde{}10 Ganymede radii. Close
agreement between Ganymede magnetic lensing simulation and \selectlanguage{greek}µ\selectlanguage{english}ASC
observations confirms that the JRM09 and current sheet model fits very
well with observations and can be reliably used for magnetic field
tracing and M-shell definition within the inner (\textless{}30
R\textsubscript{J}) inner magnetosphere. The \selectlanguage{greek}µ\selectlanguage{english}ASC observations are a
useful complement to Juno's dedicated radiation monitoring instruments,
extending detection capability to particle energies \textgreater{} 20
MeV.

Acknowledgments

All authors acknowledge support from the Juno project.~Data supporting
the conclusions are available in the permanent archival data repository,
Zenodo (Herceg et al, 2021), and will soon be archived in the NASA
Planetary Data System at https://pds.nasa.gov,

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