# Effect of a magnetospheric compression on Jovian radio emissions: in situ
case study using Juno data

###### Abstract

During its polar orbits around Jupiter, Juno often crosses the boundaries of
the Jovian magnetosphere (namely the magnetopause and bow shock). From the
boundary locations, the upstream solar wind dynamic pressure can be inferred,
which in turn illustrates the state of compression or relaxation of the
system. The aim of this study is to examine Jovian radio emissions during
magnetospheric compressions, in order to determine the relationship between
the solar wind and Jovian radio emissions. In this paper, we give a complete
list of bow shock and magnetopause crossings (from June 2016 to August 2022),
and the associated solar wind dynamic pressure and standoff distances inferred
from 2002JGRA..107.1309J. We then select two sets of magnetopause crossings
with moderate to strong compression of the magnetosphere for two case studies
of the response of the Jovian radio emissions. We confirm that magnetospheric
compressions lead to the activation of new radio sources. Newly-activated
broadband kilometric emissions are observed almost simultaneously with
compression of the magnetosphere, with sources covering a large range of
longitudes. Decametric emission sources are seen to be activated more than one
rotation later only at specific longitudes and dusk local times. Finally, the
activation of narrowband kilometric radiation is not observed until the
magnetosphere is in its expansion phase.

JGR: Space Physics

School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for
Advanced Studies, Dublin 15, Ireland Observatoire Radioastronomique de Nançay,
Observatoire de Paris, Université PSL, CNRS, University Orléans, Nancay,
France Department of Physics and Astronomy, University of Iowa, Iowa City,
Iowa, USA School of Physics, Trinity College Dublin, Dublin, Ireland LESIA,
Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université,
UPMC University Paris 06, University Paris Diderot, Sorbonne Paris Cité,
Meudon, France Southwest Research Institute, San Antonio, Texas, USA
Department of Physics and Astronomy, University of Texas at San Antonio, San
Antonio, Texas, USA Department of Computer Science, Aalto University, Aalto,
Finland Department of Physics and Astronomy, University of Southampton,
Highfield Campus, Southampton, SO17 1BJ, UK Space Research Corporation,
Annapolis, MD IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
Jet Propulsion Laboratory, Pasadena, California, USA

Corentin Kenelm<EMAIL_ADDRESS>
This paper provides a list of the Jovian magnetosphere boundary crossings by
the Juno spacecraft from June 2016 to August 2022.

Jovian magnetospheric compressions lead to increased bKOM radio emissions
(immediately) and DAM on the dusk sector (more than one rotation later).

nKOM radio emission appears later during relaxation phase of the compression.

## Plain Language Summary

##  1 Introduction

Planetary studies often face the challenge of interpreting in situ spacecraft
observations without the benefit of an upstream monitor revealing the
prevailing conditions in the interplanetary medium. This is particularly true
of the outer planets. Radio emissions provide a direct probe of the site of
particle acceleration and have potential to be used as a proxy for
magnetospheric dynamics (see e.g., 2022FrASS…9.0279C for Saturn;
2022JGRA..12730209F for Earth). At Jupiter, the radio spectrum is composed of
at least six components, from low-frequency emissions, such as quasi-periodic
(QP) bursts or trapped continuum radiation (from a few kHz to 10s of kHz), up
to decametric (DAM) emissions ranging from a few MHz to 40 MHz [Gurnett Scarf
(1983), Zarka (1998), CK. Louis . (2021a)].

In this study, we focus on three types of radio emissions observable with
Juno: narrowband kilometric (nKOM), broadband kilometric (bKOM) and auroral
DAM emissions (i.e., not induced by Galilean moons). The nKOM is attributed to
a mode conversion mechanism producing emissions inside Io’s torus at or near
the local electron plasma frequency [Barbosa (1982), Gurnett Scarf (1983),
Jones (1988), Ronnmark (1992)]. The last two components (bKOM and DAM) are
auroral emissions, produced by the cyclotron maser instability (CMI), near the
local electron cyclotron frequency. The sources of these emissions are located
on magnetic field lines of magnetic apex (M-Shell) between $10$ and $60$
(unitless distance of the magnetic field line at the magnetic equator
normalized to Jovian radius 71492 km). These emissions are very anisotropic
and beamed along the edges of a hollow cone with an opening of $\sim
75^{\circ}\pm 5^{\circ}$ to $\sim 90^{\circ}$ with respect to the local
magnetic field lines [Ladreiter . (1994), Zarka (1998), Treumann (2006),
Louarn . (2017), Louarn . (2018), Imai . (2019), CK. Louis, Prangé . (2019)].

The relation of the different components of Jupiter’s radio emissions to both
internal and external drivers is complex, as shown by several previous
studies. These studies show a relationship between some of the components and
external (solar wind) or internal (rotation, magnetic reconfiguration)
drivers. Recently, 2021JGRA..12629780Z have re-analyzed data from Cassini’s
flyby of Jupiter, and found that hectometric (HOM) and DAM emissions are
dominantly rotation-modulated (i.e. emitted from lighthouse-like sources fixed
in Jovian longitude), whereas bKOM is modulated more strongly by the solar
wind than by the rotation (i.e. emitted from sources more active within a
given Local Time sector). This last study extends earlier results by
1983Natur.306..767Z,1987A&A…182..159G, 2008GeoRL..3517103I,
2011JGRA..11612233I. 1998GeoRL..25.2905L, using Galileo radio observations,
have shown a sudden onset, and increased intensity (up to $2\times 10^{-7}$
V.m-1.Hz-1/2 at $5$ MHz) of bKOM and DAM radio emissions, as well as the
activation of new nKOM radio emissions, during periods of magnetospheric
disturbance. They postulated large-scale energetic events as reconfigurations
of the magnetosphere and plasmasheet somewhat analogous to terrestrial
substorms. The results obtained by 2010A&A…519A..84E, using Ulysses spacecraft
data during the distant Jupiter encounter and Nançay Decameter Array (NDA)
data, show that non-Io DAM radio emissions occur during intervals of enhanced
solar wind dynamic pressure, but without any direct correlation between the
emission duration or power versus the solar wind pressure or the
interplanetary shock Mach number. Using 50 days of observations from Cassini
and Galileo, 2002Natur.415..985G showed that HOM emissions were triggered by
the arrival of interplanetary shocks at Jupiter. 2012P&SS…70..114H,
2014PSS…99..136H have also shown that an increase of the solar wind pressure
affects the non-Io-DAM radio emissions, using ground-based radio measurements
[Hess . (2012)] and Cassini and Galileo radio and magnetic measurements [Hess
. (2014)]. These two studies have compared the type of shocks with the region
of source activation. There are two type of shocks [Kilpua . (2015)]: fast
forward shocks (FFS) and fast reverse shocks (FRS). These shocks are driven by
solar coronal mass ejections (CME) or corotating interaction regions (CIR).
The sudden explosion of a CME, at a higher velocity than the ambient solar
wind, usually drives a FFS. As this fast CME expands into the solar system and
overtakes the slower background solar wind, a compressed interaction region is
usually formed, which is delimited by FFS on one side and FRS on the other
side [Smith Wolfe (1976), Tsurutani . (2006)]. A FFS is characterized by a
sharp or discontinuous increase of the solar wind velocity, density,
temperature and magnetic field amplitude. A FRS is characterized by an
increase of the solar wind velocity, but a decrease of the solar wind
temperature, density and magnetic field amplitude. Both 2012P&SS…70..114H,
2014PSS…99..136H studies have shown that FFS trigger mostly dusk emissions,
whereas FRS trigger both dawn and dusk emissions, with a time delay depending
on the strength/direction of the interplanetary magnetic field (IMF). All the
shock-triggered radio sources were found to sub-corotate (i.e. rotating slower
than the rotation period of Jupiter) with a rate ranging from $50$% to $80$%
depending on the intensity of the IMF. These rates could respectively
correspond to the extended and compressed states of the Jovian magnetosphere.

The above cited studies relied on sparse datasets (flybys or remote
measurements) but the once-in-a-generation Juno dataset gives the opportunity
for longer-term monitoring of the Jovian system and its radio response. In
particular, the apojoves early in the mission, which took Juno out to radial
distances of $\sim 110$ RJ on the dawn side, place the spacecraft near the
nominal magnetopause and bow shock locations, and afford the opportunity to
sample snippets of in situ solar wind, as well as to determine the positions
of the magnetospheric boundaries at various points in time. All the while, the
Juno radio instrument is constantly monitoring the Jovian radio spectrum. In
this study we utilise this unique dataset to explore the connection between
the solar wind and Jupiter’s radio emissions by presenting the first case
study of its kind.

Section 2 describes the datasets and processing methodology. Section 3
presents case studies of the Jovian radio emission response to two moderate to
strong magnetospheric compressions inferred from multiple magnetopause
crossings while Juno is on the outbound leg of its trajectory. Finally in
Section 4, we summarise and discuss the results of this study and present the
perspectives.

##  2 Methodology

Since July 2016, Juno has been in orbit around Jupiter, making a polar orbit
every 53 days during its prime mission. Since the Ganymede flyby in June 2021,
the orbits have been shortened to 43 days, before being reduced to 38 days in
September 2022 with the Europa flyby. During its first 44 orbits, with an
apojove of up to $\sim 110$ RJ, Juno crossed the boundaries of the
magnetosphere several times [Hospodarsky . (2017), Ranquist . (2019),
Montgomery . (2022), Collier . (2020)], as shown in Figure 1 projected into
the equatorial plane. Figure 1a displays the magnetopause crossings while
Figure 1b displays the bow shock crossings. In both of these panels are drawn
the $10^{th}$ and $90^{th}$ quantile position of the magnetopause and bow
shock, respectively, based on the 2002JGRA..107.1309J model. Note that this
model was built on crossings from Ulysses, Voyager and Galileo, and thus may
not be representative of all local times (especially the previously poorly
explored dusk flank) or high-latitudes. The coordinate system used in this
figure is the Juno-de-Spun-Sun (JSS), as this is the coordinate system used in
the 2002JGRA..107.1309J model. In this system, X points towards the Sun, Z is
aligned with the Jovian spin axis, and Y closes the right-handed system
(positive towards dusk). A 3D projection plot (in the Jupiter-Sun-Orbit (JSO)
coordinate system) of the Jovian magnetosphere boundary crossings is shown in
Figure S1 in Supporting Information (SI). In the JSO system, X is aligned with
the Jupiter-Sun vector, Y indicates the Sun’s motion in Jupiter frame, and Z
closes the system.

Figure 1: Projection of the Juno trajectory into the equatorial plane, with
the (a) magnetopause and (b) bow-shock crossings overplotted. The magnetopause
crossings studied in this article are highlighted in red in panel (a). The
coordinate system used here is the Jupiter-de-Spun-Sun (JSS). In this system,
X points towards the Sun, Z is aligned with the Jovian spin axis, and Y closes
the right-handed system (positive towards dusk). In panel (a) the dashed line
represents the $10^{\mathrm{th}}$ quantile position of the magnetopause
($0.03$ nPa), the dotted line its $90^{\mathrm{th}}$ quantile position
($0.518$ nPa). In panel (b) these same lines represent the $10^{\mathrm{th}}$
($0.063$ nPa) and $90^{\mathrm{th}}$ ($0.579$ nPa) quantile positions of the
bow shock [<]values from¿[]2002JGRA..107.1309J. Panel (c) displays the solar
wind dynamic pressure $P_{\mathrm{dyn}}$ values inferred from
2002JGRA..107.1309J, for each crossing (“+”: magnetopause; “o”: bow shock), as
a function of time and Local Time (1200: direction of the Sun; 0000:
opposition to the Sun). The colour code corresponds to the orbit number. The
cases studied in this article are highlighted in red.

In this study, the boundary crossings displayed Figure 1 were determined using
the radio measurements of the low frequency receiver of the Juno/Waves
instrument [Kurth . (2017)], and the magnetic field measurements of the
Juno/MAG instrument using the Fluxgate Magnetometer measurements [Connerney .
(2017)], following the work done by 2017GeoRL..44.4506H. Three examples are
shown in Figure 2, with Juno/Waves data [<]using¿[estimated flux density data
set]2021JGRA..12629435L, 2021Juno_Waves_Calibrated_data_collection displayed
in the top panels, and Juno/MAG data (in spherical JSO coordinates system) in
the bottom panels. The “out” crossings (black dashed lines) correspond to a
boundary moving towards Jupiter, e.g., Figure 2a,d, Juno crosses the bow shock
going from the magnetosheath to the solar wind. The “in” crossings (grey
shaded lines) define a boundary moving away from Jupiter, e.g., Juno crosses
the bow shock, leaving the solar wind to enter the magnetosheath.

The bow shock is a discontinuity formed when the supersonic solar wind is
slowed to subsonic by interaction with the Jovian magnetic obstacle. A bow
shock crossing is detected from the change in magnetic field amplitude and in
the level of field fluctuations in the Juno/MAG data between the solar wind
and the magnetosheath (Figure 2d). In the Juno/Waves measurements (Figure 2a)
one can observe (i) an intense and broadband signal at the crossing and (ii)
Langmuir waves when Juno is inside the solar wind, visible here at $\sim 10$
kHz, which are produced by solar electrons reflected back into the solar wind
from the shock boundary [Scarf . (1971), Filbert Kellogg (1979)].

Figure 2: Examples of magnetospheric boundary crossings. Top panels (a-c)
display Juno/Waves measurements [<]using¿[estimated flux density data
set]2021JGRA..12629435L, 2021Juno_Waves_Calibrated_data_collection, while
bottom panels (d-f) display Juno/MAG measurements in spherical JSO
coordinates. Outbound crossings (boundary moving towards Jupiter) are
highlighted by the black-dashed lines, while inbound crossings (boundary
moving away from Jupiter) are highlighted by the grey-shaded lines. (left
(a,d)) Bow shock crossings; (middle (b,e)) Magnetopause crossings; (right
(c,f)) Example where the Juno spacecraft partially crossed the magnetopause
without ever actually passing from the magnetosphere to the magnetosheath
(i.e. moved around the border). The numbers above the Waves data indicate the
region where Juno is located: (1): Magnetosphere, (2) Magnetosheath, (3) Solar
Wind, (1.5): “in” the magnetopause boundary.

The position of the magnetopause is determined by the balance between the
solar wind dynamic pressure and the plasma pressure in the outer magnetosphere
[Mauk . (2004)]. A magnetopause crossing is detected by the
appearance/disappearance in the Juno/Waves data (see Figure 2b) of the trapped
continuum radiation, usually observed between $0.5$ kHz and $2$ kHz. This
signal is only seen when the observer is inside Jupiter’s magnetosphere, in
this example before the black-dashed line at $\sim 2017$-$06$-$18$T$09$:$00$,
and after the grey-shaded line at $\sim 2017$-$06$-$19$T$03$:$00$. This
trapped continuum radiation propagates at a frequency lower than the plasma
frequency inside the magnetosheath and therefore can not propagate into the
magnetosheath (hence the name “trapped”). Juno/MAG measurements of the
magnetic field amplitude (Figure 2e) also show a change as Juno crosses the
magnetopause, passing from the magnetosphere into the magnetosheath (see,
e.g., black-dashed line at $\sim 2017$-$06$-$18$T$09$:$00$), with a decrease
in magnetic field total amplitude $|B|$ and a much more disturbed signal than
in the magnetosphere.

In some observations (see Figure 2c, between black-dashed and grey-shaded
lines), low and high cut-off frequencies of the trapped continuum increase.
Before $\sim 2018$-$08$-$04$T$00$:$00$ (black-dashed line) and after $\sim
2018$-$08$-$05$T$07$:$00$ (grey-shaded line), the trapped-continuum radiation
is visible between $\sim 0.3$ kHz and $\sim 4$ kHz. In-between, the trapped-
continuum radiation is no longer visible at low frequency, but is shifted to
higher frequencies (between $\sim 0.6$ and $\sim 8$ kHz) and is very bursty.
The high frequency part never completely disappears, and no drastic change in
magnetic field components (Figure 2f) is observed, although they are more
disturbed than in the magnetosphere, but less than in the magnetosheath. In
the observation shown in Figures 2c,f, Juno is on the outbound part of its
trajectory and is therefore moving away from Jupiter. We interpret these
observations as the movement of the magnetopause towards Juno at first
(increase of low and high cut-off frequencies, see black-dashed line).
Subsequently, the magnetopause stops moving towards Jupiter, and Juno never
completely crosses the magnetopause to end up in the magnetosheath (between
black-dashed and grey-shaded lines). Juno is however close enough to the
magnetopause , or even in the boundary layer [Went . (2011)], to observe an
increase of the low-frequency cutoff of the trapped continuum by the
increasing density when approaching the boundary. Finally, the magnetopause is
moving away from Jupiter (faster than Juno’s velocity), and high and low cut-
off frequencies decrease (Juno is again completely in the magnetosphere).

From the boundary positions, we can infer the solar wind dynamic pressure
$P_{\mathrm{dyn}}$ using the 2002JGRA..107.1309J model, by solving their
second order polynomial equation [<]equation 1 of¿[]2002JGRA..107.1309J. From
this, we can determine if the crossings of the magnetospheric boundaries are
due to compressions of the magnetosphere, by comparing the inferred
$P_{\mathrm{dyn}}$ values to either 2002JGRA..107.1309J quantile values, or
observed solar wind $P_{\mathrm{dyn}}$ distributions upstream of Jupiter
[Jackman Arridge (2011)]. One should note that the $P_{\mathrm{dyn}}$ value
determined using Juno’s position is not absolute, but a lower limit of the
dynamic pressure. Although Juno is outbound, we cannot directly infer how far
the magnetopause boundary is pushed back towards Jupiter.

Figure 1c displays the inferred $P_{\mathrm{dyn}}$ for all crossings (“+”:
magnetopause; “o”: bow shock) as a function of time and Local Time. Note that
there is a trend of increasing $P_{\mathrm{dyn}}$ values with time and
decreasing Local Time. This is due to the procession of orbits, taking Juno
more and more towards the night side of the magnetosphere (midnight Local
Time), and thus deep into the magnetotail. This means that the magnetosphere
has to be more compressed for Juno to cross the magnetospheric boundaries from
this location. The bow shock is even further out again and thus Juno did not
encounter the dawn side bow shock after the first few Juno orbits.

In the absence of an upstream monitor, we can compare these inferred
$P_{\mathrm{dyn}}$ values with those provided by solar wind propagation models
[<]e.g.,¿[]2005JGRA..11011208T. For this, we must take into account any
uncertainty on the propagation model values due to angle from opposition where
predictions are most reliable. From this propagation model, we can also infer
the type of shock (FFS or FRS) that compresses the magnetosphere as discussed
in Section 1.

The full list of magnetopause and bow shock crossings (from 2016-06-24 to
2022-07-26, i.e. up to orbit 41) are available in Table S1 and S2 in
Supplementary Information (SI), along with the position of Juno (in cartesian
JSS –mandatory to use 2002JGRA..107.1309J model– and cartesian and spherical
International Astronomical Union (IAU) System III (SIII) coordinates system),
the inferred solar wind dynamic pressure and the position of the magnetosphere
standoff distances (bow shock and magnetopause) inferred from the
2002JGRA..107.1309J model [CK. Louis . (2022e)]. Figure S2 displays
statistical distributions based on the magnetosphere boundary crossings (Local
Time, Solar Wind dynamic pressure, magnetopause and bow shock positions).

We next investigate the response of bKOM and DAM emissions to magnetospheric
compression in a case study. For that, we use the 2021JGRA..12629435L dataset
[CK. Louis . (2021b)] and catalogue of the radio emissions [CK. Louis .
(2021c)]. This catalogue contains the Jovian radio emissions identified in the
Juno/Waves observations, only from 2016-04-09 to 2019-06-24 (e.g. up to the
21st apojove of Juno). The radio components were visually identified according
to their time-frequency morphology and then manually encircled by contours and
labeled, using a dedicated program that records the coordinates of the
contours and the label of each emission patch [C. Louis . (2022a), CK. Louis .
(2022b)]. While nKOM patches can be identified individually (fuzzy patches of
emission elongated in time), the bKOM and DAM components have not been
explicitly catalogued because they are the most frequent emissions in their
respective frequency range. They can be selected and studied by excluding all
other components and restricting to the adequate frequency range. For example,
excluding nKOM in the range $20$-$140$ kHz allows one to select the bKOM
component only. In the [$3.5$-$40.5$] MHz frequency range, only decametric
emissions induced by the Galilean moons Io, Europa and Ganymede have been
labelled (based on 2019A&A…627A..30L simulations of those radio emissions, see
2020ExPRES_simulations_data_collection for more details). Therefore, by
excluding them, only auroral DAM emissions remain in this range. Given that
HOM emissions can extend up to a few MHz, the highest part of the hectometric
emission could be present in this range, but would only represent a minority
of the emissions observed.

For the case studies described in Section 3, we decided to select the
magnetopause crossings that took place between 2016-12-19 and 2016-12-23,
highlighted in red in Figure 1. This choice is based on three factors: (i) in
2016-2017, the Jovian Auroral Distributions Experiment
[<]JADE,¿[]2017SSRv..213..547M was not activated during excursions into the
solar wind, excluding in situ plasma information, and thus a direct
measurement of $P_{\mathrm{dyn}}$. Therefore, we decided to choose among one
of the (more numerous) magnetopause crossing cases; (ii) the case chosen had
to be within the time interval covered by the catalogue of ¡¿[i.e. between
2016-04-09 and 2019-06-24]2021JGRA..12629435L; (iii) in order to avoid any
bias related to an extremely exceptional case, we did not select the case with
the highest $P_{\mathrm{dyn}}$ value (second half of 2018, orbit 15).

The time interval chosen presents two main advantages. (i) There are two sets
of crossings in a row. The $P_{\mathrm{dyn}}$ value determined for the first
crossing (2016-12-19T01:50) is $0.70$ nPa. The dynamic pressure associated
with the second set of crossings (2016-12-21T08:48) is $0.48$ nPa. The
distribution of $P_{\mathrm{dyn}}$ at Jupiter published by ¡¿[see their Figure
4b]2011SoPh..274..481J reveals a peak at $0.05$ nPa and a maximum slightly
above $1$ nPa. The $0.48$ and $0.70$ values therefore lie towards the tail of
this distribution. Moreover, these inferred values are close to the
$90^{\mathrm{th}}$ quantile value ($0.518$ nPa) of the magnetopause position
given by 2002JGRA..107.1309J. Therefore, these two sets of magnetopause
crossings correspond to a strong and a moderate compression. (ii) Based on
Figure 1c (red points) the $P_{\mathrm{dyn}}$ values associated with these
magnetopause crossings are well above the “trend”, and therefore correspond to
the strongest compressions during orbit $4$. Recall that this “trend” is due
to the procession of Juno’s orbit, taking the spacecraft deep into the
magnetotail, implying that the magnetosphere needs to be more compressed for
Juno to cross the magnetospheric boundaries.

##  3 Jovian auroral radio emission response to compressions of the
magnetosphere

###  3.1 Determination of the compression

Figure 3: (a-c) Juno Waves and MAG measurements during a series of
magnetopause crossings. Panels (a) and (b) show Juno Waves frequency-time
spectrograms covering two different frequency ranges (from $3.5$ to $40.5$ MHz
and between $3$ kHz and $140$ kHz, respectively), with the black polygons in
the top panel denoting the radio emissions induced by the interaction between
Jupiter and its moons [<]e.g. Io, Europa or Ganymede, based on
¿[]2021_Juno_Waves_catalog. Panel (c) shows the three components of magnetic
field (in JSO spherical coordinates system, red, yellow and green lines) and
total field strength (blue). The black line displays the 2002JGRA..107.1196K,
2004jpsm.book..593K magnetic field variation fit. Panels (d-f) display time
series of integrated flux density (normalized at 1 Astronomical Unit (AU),
$15$ sec time resolution) for (d) the auroral decametric (DAM, in the
$3.5$-$40.5$ MHz range) not induced by the interaction between Jupiter and its
moons (i.e. all the non-labelled emissions), (e) broadband kilometric (bKOM,
in the $20$-$140$ kHz range) and (f) narrowband kilometric (nKOM, in the
$40$-$140$ kHz range) radio emissions. The black-dashed lines represent the
outbound magnetopause crossings (from the magnetosphere to the magnetosheath)
while the grey-shaded lines represent the inbound magnetopause crossings (from
the magnetosheath to the magnetosphere). The red-dashed line represents the
time when Juno starts to measure magnetic fluctuations and
$|B|>|B_{\mathrm{lobe}}|$ (panel c), and an increase in the low and high cut-
off frequencies of the trapped-continuum radiation (panel b). The numbers
above the Waves data indicate the region where Juno is located: (1):
Magnetosphere, (2) Magnetosheath, (1.5): “in” the magnetopause boundary.

Figure 3 displays Juno measurements during magnetopause crossings for a 7-day
interval from $2016$-$12$-$17$T$00$:$00$ to $2016$-$12$-$24$T$04$:$15$. Black-
dashed lines show when Juno crossed the magnetopause from the magnetosphere to
the magnetosheath (outbound crossings), while grey-shaded lines show inbound
crossings. Figures 3a,b display Juno/Waves measurements for two different
frequency ranges: (a) [3–40.5] MHz and (b) [0.3–140.0] kHz. Figure 3c displays
Juno/MAG measurements: total amplitude $|B|$, and ($r$,$\theta$,$\phi$)
components in JSO spherical coordinates system. The black line displays the
2002JGRA..107.1196K, 2004jpsm.book..593K magnetic field variation fit in the
lobes (beyond $r=30$ Jovian radii, the lobe magnetic field falls off as
$B_{\mathrm{lobe}}\mathrm{(nT)}=(2.94\pm 0.07)\times 10^{3}$ $r^{-1.37\pm
0.01}$). Therefore, for an observer inside the magnetosphere, and if the
magnetosphere is in a steady-state, $|B|$ should follow $|B_{\mathrm{lobe}}|$.
Figures 3d,e,f display integrated time series of the radio signal measured by
Juno/Waves for three different radio components: (d) auroral DAM (i.e.
excluding the satellite-related DAM emissions), (e) bKOM, and (f) nKOM.

As described in Section 2 (see Figures 2b,e), the magnetopause crossings are
clearly seen in Figure 3b from the disappearing of the trapped continuum
radiation and in Figure 3c from the change in the magnetic field components
and total amplitude (see the black-dashed and grey-shaded lines). Looking in
more detail at Juno/MAG measurements (Figure 3c), one can notice at $\sim
2016$-$12$-$18$T$09$:$00$ (indicated by the red dotted line), i.e., $\sim 18$
h before the crossing of the magnetopause, an increase of the $|B|$ (blue
curve) and $B_{\phi}$ (green curve) components while the $B_{\mathrm{r}}$ (red
curve) and $B_{\theta}$ (yellow curve) components decrease. This is followed
by turbulence observed in all magnetic field components, but without the sharp
decrease in $|B|$ characteristic of magnetic measurements in the
magnetosheath. We also see, approximately at the same time, that the cut-off
frequencies of the trapped continuum are increasing (Figure 3b): the trapped
continuum is observable in the [$\sim 0.4$-$3$] kHz frequency range before the
red-dashed line, and in the [$\sim 0.8$-$5$] kHz frequency range between the
red-dashed and black-dashed lines. This change in the cut-off frequencies is
due to the inward motion of the magnetopause during the compression. Because
of this, the local density along Juno’s path is increasing, and therefore the
low-frequency part of the trapped continuum cannot propagate, resulting in an
increase in the cut-off frequencies of the trapped continuum. All these
characteristics are the signature of the inward motion of the magnetopause
boundary towards the spacecraft (see Figures 2c,f).

Furthermore, comparing the total amplitude of the magnetic field $|B|$ (blue
curve) to 2002JGRA..107.1196K, 2004jpsm.book..593K magnetic field variation
fit $|B_{\mathrm{lobe}}|$, one can see that before $\sim
2016$-$12$-$18$T$09$:$00$ (red dotted line), $|B|$ and $|B_{\mathrm{lobe}}|$
follow the same trend. However, between $\sim 2016$-$12$-$18$T$09$:$00$ and
the crossing of the magnetopause (first black dashed-line), $|B|$ is above
$|B_{\mathrm{lobe}}|$, which is a clear sign that the magnetosphere is being
compressed [<]see e.g.,¿[]2010JGRA..11510240J.

All these elements lead us to interpret this as representative of the
beginning of the impact of a stronger solar wind on the magnetosphere, and
thus the beginning of compression. On the other hand, after Juno crosses the
magnetopause for the second time (back into the magnetosphere, grey-shaded
line) on $\sim 2016$-$12$-$19$T$14$:$12$ and until the next outward crossing
of the magnetopause ($\sim 2016$-$12$-$21$T$08$:$48$), we observe the same
features: a variable low and high cut-off frequencies of the trapped
continuum, small perturbations in the magnetic field components, and
$|B|>|B_{\mathrm{lobe}}|$. We interpret this as the relaxation phase of the
magnetosphere, but not to a fully extended state. From the observations, we
can deduce that Juno remains very close to the magnetopause (same
characteristics as in Figures 2c,f), before the second compression takes place
and the spacecraft is again in the magnetosheath.

By comparing the time spent by Juno inside the magnetosheath during the two
compression events, we can infer whether one of the compressions was stronger
than the other, i.e., lasted longer or the magnetopause was pushed further
inwards. During the first pass from the magnetosphere to the magnetosheath,
Juno stayed in it for $\sim 12$ h $20$ min, whereas during the second pass,
Juno stayed inside the magnetosheath less than $7$ h, before going back into
the magnetosphere very quickly twice for a few minutes. Therefore, we can
deduce that the first compression either lasted longer or the magnetopause was
pushed further inwards. In any case, we can infer that the magnetosphere was
probably more disturbed by the first compression.

The 2005JGRA..11011208T solar wind propagation model is more reliable when
Earth and Jupiter are in conjunction as seen from the Sun (Jupiter-Sun-Earth
angle equal to $0^{\circ}$). During the time range displayed in Figure 3, the
Jupiter-Sun-Earth angle is $-110^{\circ}$ (in average). Therefore, the error
in timing on 2005JGRA..11011208T solar wind propagation model can be as large
as 2 days or more, the time interval between the shocks can also be shifted,
and $P_{\mathrm{dyn}}$ can be misjudged. Therefore, the outputs from the
2005JGRA..11011208T model should be used here only as a guide. For that
reason, they are only displayed in the SI (Figure S3-S4), for information.
According to 2005JGRA..11011208T model, two shocks arrive at Jupiter
successively in a time interval of two and a half days. The model predicts the
arrival of the first compression at the beginning of day 2016-12-16, i.e. two
days before the first compression observed by Juno. By shifting the model
outputs by two days (see Figure S4), we obtain a good match between the
arrival of the two shocks at Jupiter and the compressions observed by Juno.
These two shocks have very different characteristics (see Figure S3): (i) the
first one shows an increase in the solar wind speed and a sharp decrease in
the solar wind density and temperature, while (ii) the second shock shows an
increase in the solar wind speed, density and temperature. Thus, if we take
the outputs of 2005JGRA..11011208T model as reliable, the first shock would be
a Fast Reverse Shock (FRS) while the second would be a Fast Forward Shock
(FFS).

###  3.2 Response of the auroral radio emission to the first compression

Having determined the start time of the compression and the associated dynamic
pressure, let us now study the response of the radio emissions to the first
compression.

####  3.2.1 Broadband kilometric (bKOM) emission

The bKOM emissions (Figure 3e) are the first to show a strong variation.
Before the onset of the compression, we can see some peaks in the integrated
intensity, but restricted to a narrow frequency range (few 10s of kHz, see
Figure 3b). Immediately after (dashed-red line at $\sim
2016$-$12$-$18$T$09$:$00$), we observe emissions almost continuously, with an
increase in the integrated intensity. This increase can be explained by both
the observations of bKOM emissions over a much wider frequency range, i.e.
from 20 kHz to 140 kHz (see Figure 3b), and by the increase intensity of the
emission. Very low frequency extensions of the emission, i.e., emissions
extended down to $20$ kHz, are only visible over $\sim 1$ h $15$ min, thus
only for specific sources. The bKOM emissions seen at almost every longitude
are then observed until $\sim 2016$-$12$-$21$T$02$:$00$, thus over more than
$60$ hours. The observation of emissions on an almost continuous basis tells
us that sources have been activated at almost all longitudes. It should be
noted that no bKOM emissions seem to be observed between 2016-12-18T17:00 and
19:00. A sector of longitude therefore seems to have no associated bKOM
emissions, at least during the first rotation. This could be due to various
reasons, such as emissions that are too weak to be detected, geometric effects
preventing the emission from being beamed towards the observer, or a sector
that is completely non-activated.

####  3.2.2 Decametric (DAM) emission

After compression, an increase in the integrated intensity of the DAM radio
emissions is also observed. However, unlike the bKOM emissions, this is not
observed simultaneously with the onset of the magnetic disturbances, nor is it
continuous over time. DAM emissions visible before the compression [<]non-
labelled vertex early arc up to 15 MHz, see Figure 3a, statistically reported
by¿[]2017GeoRL..44.4584I are still visible during the compression with the
same rotation period, however their intensity has increased compared to before
the compression. Therefore, the appearance of these emissions is probably
modulated by rotation and independent of any compression. However, compression
seems to have an impact on their intensity. New emissions, more intense and
extending up to 25-30 MHz, appear at $\sim 2016$-$12$-$19$T$12$:$00$, i.e.
$\sim 28$ hours after the compression, and last for $\sim 30$ hours. Their
rotation period is longer than the previously visible DAM emissions, visible
with the double peak in the integrated time series Figure 3d, which means that
the sources are sub-corotating (see below).

Figure 4: (a) Juno Waves and (b) Nançay Decameter Array (NDA) routine receiver
observations. Decametric radio emissions are clearly visible (a) between 11:00
and 12:30 (Spacecraft Event Time) and (b) between 09:00 and 09:30 (UT time).
The light travel time between Juno and Earth is $\sim$ $47$ minutes. The data
gap after 11:10 is due to the fact that Jupiter is no longer visible in the
sky from the NDA observatory. (c) Observers’ configuration. (d) Cartoon of the
geometry and nomenclature of the auroral radio emissions and corresponding
arc-shape in the (time, frequency) plane. If the source is located to the West
of Jupiter for the observer (sources “B” or “D”), the emission will have a
vertex early arc shape. If on the contrary the source is located to the East
of Jupiter for the observer (sources “A” or “C”), the emission will have the
shape of a vertex late arc. The arcs observed in panels (a) and (b) originate
from the same source. NDA sees the emission cone exiting its field of view
(vertex late arc) while Juno sees the emission cone entering its field of view
(hence vertex early arc).

Since the CMI emissions are not isotropically emitted, but only emitted at the
edge of a hollow cone, with an angle of $\sim 75^{\circ}$ to $\sim 90^{\circ}$
with respect to the local magnetic field line (see Section 1), geometry
effects are important, and emission is mostly seen by an observer when the
sources are at a longitude $\sim 75^{\circ}$ to $90^{\circ}$ greater or lower
than the longitude of the observer. It can thus be complicated to disentangle
between “no emission” and “non-visible emission”, because the observer is not
in the beam of the emission. For this, it can be interesting to have multi-
point observations, e.g., including ground-based radio telescopes such as the
Nançay Decameter Array (NDA). Figures 4 displays observations taken by Juno
(4a) and NDA (4b) on 2016-12-19. The observation geometry is shown in Figure
4c, with Juno located at a mean local time of $5.2$ hours, and NDA at a mean
local time of $12.64$ hours, at the moment of the observations of the radio
emissions. Finally, Figure 4d shows the shape of the radio emission as a
function of the position of the sources relative to the observer.

Multiple “B” arcs are observed by Juno up to almost $30$ MHz, between 11:00
and 12:30 [<]Figure 4a, see also¿[who statistically reported these
arcs]2017GeoRL..44.4584I. The type of the arcs and the position of Juno
indicates that the emissions come from the midnight-to-dusk side as seen from
Juno (see Figures 4c,d). On the other hand, between 09:00 and 09:30, “A”
emissions are observed by the NDA (Figure 4b) up to almost 30 MHz. The type of
the emissions seen by the NDA, and its position relative to Jupiter, indicates
the emissions come from the dusk side as seen from Earth (see Figures 4c,d).
By studying the time delay (e.g., at $24.5$ MHz) between the first emission
seen on 2016-12-19T09:08 by the NDA (Earth Time, i.e. $\sim
2016$-$12$-$19$T$08$:$23$ Juno Time, taking into account the light travel
time) and the first emission seen on 2016-12-19T12:27 (Juno Time) by Juno, we
obtain a $\delta t=4.1$ h. According to the local time positions of the two
observers, this is consistent with an emission originating from the same
source, seen from both side of the beaming cone, and rotating with a sub-
corotation rate of $70\pm 5$ %, meaning that the source is rotating at $70$ %
of Jupiter’s rotation angular frequency [<]taking into account that the
emission at $24.5$ MHz is beamed along a hollow cone with aperture angle of
$75^{\circ}\pm 5^{\circ}$,¿[]2017GeoRL..44.9225L.

The beaming angle allowed by the CMI is in the range
$75^{\circ}$–$90^{\circ}$, and Juno does not see a “B” radio emission before
the NDA. Therefore the onset region must be located in a region greater than
Juno’s local time plus $75^{\circ}$–$90^{\circ}$, and lower than NDA’s local
time minus $75^{\circ}$–$90^{\circ}$, therefore in the local time range
[1110–1740] $\pm$ 0100 hours.

The lack of emission observed by Juno is therefore partly due to geometry
effects, but probably also to a delay in the activation of the sources and in
a specific region (dusk). Indeed, the NDA sees an emission before Juno, but no
emission is seen by Juno at the previous rotation, indicating that a time
delay exists between the compression of the magnetosphere and the activation
of newly activated DAM sources. This exact time delay is difficult to
determine here, and would require a more statistical study or more observers,
but it seems that at least two Jovian rotations are needed before new DAM
sources are activated.

####  3.2.3 Narrowband kilometric (nKOM) emission

Finally, the delay for new nKOM emissions to be visible is far longer than for
bKOM and DAM emissions. The first new emission appears at $\sim
2016$-$12$-$20$T$00$:$00$, i.e. $39$ hours after the first visible bKOM
emission. The interval between the peaks in the integrated intensity is not
regular, and varies between $\sim 9$ h $14$ min, $\sim 10$ h $22$ min and
$\sim 9$ h $44$ min. A closer look to the intensity peaks at different
frequencies (see Figure S5b) shows that the signal at lower frequencies (e.g.,
from $70.862$ kHz to $112.43$ kHz) is triggered before the signal at higher
frequencies (e.g., at 126.16 kHz and 141.54 kHz), and then disappears first.
The interval between the peaks seems to be different depending on the
frequency, which implies different source locations (see Section 3.3, and
Section 4, for more details).

###  3.3 Response of the auroral radio emission to the second compression

As mentioned at the beginning of this section, the dynamic pressure of the
solar wind during the second compression event is potentially weaker than
during the first event. This is suggested by both (i) the position of the
magnetopause, further away from Jupiter (see the second dotted black line at
$2016$-$12$-$21$T$08$:$48$), and (ii) the time spent in the magnetosphere
which is shorter than during the first event.

The inspection of the radio emission time series shows that one DAM emission
is observed at $\sim 2016$-$12$-$21$T$21$:$30$, also observed one rotation
later with greater intensity. This emission is most likely the reactivation of
previously observed sources (as observed during the first compression event).
Indeed DAM emission with decreasing intensity is observed $\sim 20$ hours
before ($\sim 2016$-$12$-$21$T$01$:$30$) with the same shape. Since the NDA is
observing only one third of the time we have no contemporaneous observations
for this event.

New bKOM emission sources are activated at $\sim 2016$-$12$-$21$T$08$:$00$.
However, in contrast to the first event, fewer bKOM sources seem to have been
activated, since the bKOM emission is not visible at all times, and the
sources are activated for a shorter period of time (only visible for $\sim 30$
hours vs. $\sim 60$ hours).

Finally, regarding the nKOM emission, new nKOM emissions are activated,
starting at $\sim 2016$-$12$-$22$T$15$:$00$, and lasting for $\sim 40$ hours
(same duration as for the first compression), with integrated intensity higher
than for the first event. This time, the delay between the activation of the
bKOM and the nKOM emissions is only $\sim 31$ hours. Again, it can be seen
that the period between the peaks in the integrated intensity is not regular.
It varies between $\sim 10$ h $30$ min, $\sim 9$ h $50$ min and $\sim 10$ h
$54$ min. A closer look to the intensity peaks at different frequencies (see
Figure S5c) shows that the signal is first triggered at the lowest frequencies
before being triggered at the highest frequencies. Then the signal disappears,
or fades, in the same order. The interval between two peaks is different
depending on the frequency. Focusing on distribution peaks at each frequency,
it can be seen that periodicity increases with decreasing frequency. When the
new nKOM emissions are activated, all peaks are almost centered at the same
time ($\sim 2016$-$12$-$22$T$15$:$45$); one rotation later, the peaks are
distributed in order of decreasing frequency, with the 141.54 kHz signal seen
first and the 89.172 kHz signal peak seen last. This could be explained by the
fact that the lower frequency nKOM is generated at lower density, hence,
larger radial distances from Jupiter: the deviation from rigid co-rotation
would be greater farther from the planet, and the periodicity should be
longer.

##  4 Summary, Discussion and Perspectives

Table 1: Table summarising the results of the study of the response time of
radio emissions to compression, as seen by Juno. For each compression, the
dynamic pressure of the solar wind [<]determined from the model of
¿[]2002JGRA..107.1309J, the type of shock [<]determined from the model of
¿[]2005JGRA..11011208T, the response time of each component of the radio
emission (main band of the bKOM, low frequency extension (LFE) of the bKOM,
DAM and nKOM) and the activation time (as seen by Juno) are given.

Compression | Pdyn | Type of shock | Auroral radio emission | Activation time | Duration
---|---|---|---|---|---
2002JGRA..107.1309J |  2005JGRA..11011208T
1st compression | 0.70 | FRS | bKOM | Main band |  $\leq 10$s min |  $\sim 60$ hours
LFE |  $\sim 34$ hours |  $1$ h $15$ min
DAM |  $\sim 28$ hours |  $\sim 30$ hours
|  | nKOM |  $\sim 39$ hours |  $\sim$40 hours
2nd compression | 0.48 | FFS | bKOM | Main band |  $\leq 10$ min |  $\sim 30$ hours
LFE |  $\leq 10$ min |  $\sim 15$ hours
DAM |  $\sim 12$ h $45$ min |  $10$ hours
|  | nKOM |  $\sim 31$ hours |  $\sim 40$ hours

In this paper, we have presented in Section 2 a set of magnetospheric boundary
crossings (See Figure 1). More detailed information on each crossing, such as
their exact time, their positions in different coordinate systems, and several
added values ($P_{\mathrm{dyn}}$, magnetopause and bow shock standoff
distances) are given in Supporting Information (Tables S1, S2), as well as
statistical distributions for these added values (Figure S2). The files
corresponding to Tables S1, S2 are accessible through
2022_boundary_crossings_lists.

In Section 3, we presented case studies of the response of Jovian radio
emission to strong to moderate magnetospheric compressions, inferred by
magnetopause crossings. Using the 2002JGRA..107.1309J model, we calculated the
dynamic pressure (lower limit) of the solar wind (see Table 1), and its main
characteristics and type of shocks associated with these events using the
2005JGRA..11011208T. We determined that the first magnetopause crossing is
potentially due to (i) either a stronger and shorter compression, (ii) or
higher solar wind dynamic pressure, based on the time spent by Juno in the
magnetosheath.

We chose to study the magnetopause crossings occurring between
2016-12-17T00:00 and 2016-12-24T04:15 (fourth orbit of Juno). These
magnetopause crossings are among the innermost cases (see Figure 1a and S1a),
corresponding to strong compressions ($P_{\mathrm{dyn}}\subset$ [$0.5$-$0.7$]
according to the 2002JGRA..107.1309J model). These compressions occur when
Juno is still on the dawn side of the magnetosphere, i.e. in a region where
the model of 2002JGRA..107.1309J is valid, in contrast to the dusk side where
it is less constrained. Moreover, during this 7-day interval, we observe
several magnetopause crossings, which can be grouped into 2 phases of
magnetospheric compression. These two cases also seem to correspond to two
different types of shock: FFS and FRS, according to the propagation model of
2005JGRA..11011208T, with different responses observed in the radio components
(see Table 1).

Concerning the radio emission response to the compressions, we have determined
that the bKOM sources are the first to be triggered, at almost every
longitude, almost immediately after the observation of the first magnetic
disturbances and density perturbations. The bKOM emission is then observed
over $60$ hours for the first compression and for $30$ hours for the second
one. Low Frequency Extensions, i.e. emissions going down to $20$ kHz, are
observed in both cases for a shorter duration.

In both cases, the DAM emissions are the second ones to be observed, at least
one rotation after the start of the compression, and only in the noon-dusk
sector, i.e. inside the local time range [1110–1740]. This sector includes
that determined by 2012P&SS…70..114H, 2014PSS…99..136H, but is necessarily
less precise given that we are only studying two cases here. A statistical
study with Juno will provide further constraints, given the evolution of
Juno’s local time position during its mission. Our results seem to show that
both FRS and FFS activate new, or re-activate, DAM emissions on the dusk side
only. This is partially in agreement with 2012P&SS…70..114H, 2014PSS…99..136H,
who showed that FFS mainly trigger DAM emission on the dusk side, while FRS
trigger emissions on the dusk and dawn sides. However, since we are measuring
radio emission only above $3.5$ MHz in this study (due to Waves sensitivity)
we are missing part of the DAM and most of the HOM emissions, that can go down
to $0.3$ MHz, while 2012P&SS…70..114H, 2014PSS…99..136H used Cassini radio
measurements, down to $0.1$ MHz. The DAM emission lasts for $30$ hours in the
first case, and $10$ hours in the second case. In both cases, sources rotate
in subcorotation, with a rate of $70\pm 5$ % of rigid corotation. This value
is comparable with the values obtained by 2012P&SS…70..114H, 2014PSS…99..136H.

Concerning the activated nKOM emissions, we observe a strong difference
compared to the bKOM and DAM emissions, with a long delay between compression
and activation of the nKOM sources ($\sim 30$ to $40$ hours). nKOM emission is
then observed for $\sim 40$ hours in both compression events. The periodicity
of the nKOM peaks is frequency-dependent and increases with decreasing
frequency. This would be related to the mechanism, producing emissions at the
plasma frequency which is proportional to the local plasma density. Therefore,
low-frequency emissions are produced farther from Jupiter than higher-
frequency emissions. The activation of new nKOM sources seems related to the
relaxation/reconfiguration phase of the magnetosphere. As these emissions are
produced by different mechanisms, it is not surprising that the activation of
these emissions is also different. However, it is possible that the energetic
events observed by 1998GeoRL..25.2905L, 2016JGRA..121.9651L could be caused or
amplified by and expansion of the magnetosphere, which would amplify the
centrifugal ejection of matter. It will therefore be mandatory to study in
detail the nKOM during plasmasheet distortion, which will require a list of
magnetic disturbances measured during plasma sheet crossings, simultaneously
to compression events. But this is beyond the scope of this current article,
and will be the subject of an upcoming study.

To get a better estimate of the conditions in the solar wind, such as the
solar wind dynamic pressure and velocity, the 2019JGRA..124.7799T analytical
method could be used, based on Juno/JADE measurements inside the magnetosheath
(Juno/JADE data were not available for the event studied in Section 3). This
will be compared to estimation of the dynamic pressure obtained from
2002JGRA..107.1309J magnetosphere boundaries model and 2005JGRA..11011208T
propagation tool model. We could also use different solar wind propagation
tools, such as “HuXT” model [<]Heliospheric Upwind Extrapolation with time
dependence¿[]2020SoPh..295…43O, “WSA-ENLIL solar wind simulation”, “HelioCast”
[Réville . (2023)] or the “CDPP/Propagation Tool” extended to Jupiter
[Rouillard . (2017)].

To go further on the generalization of the response of Jovian radio emissions,
the activation of new sources or the amplification of existing radio
emissions, and their intensity to magnetospheric compression and solar wind
characteristics (dynamic pressure, velocity, temperature, magnetic field
orientation), a statistical study will be necessary. The same method will be
used and will be applied to all the compression events determined from the
list of magnetopause crossings provided in the SI tables (see also Figures 1
and SI1). This will involve using boundary crossings to infer compressions,
examining the response of associated radio emissions, and grouping case
studies by properties such as solar wind dynamic pressure, or shock type.

There are several benefits to a future statistical study. The first is to
explore the differences between dawn and dusk side responses, and the
different properties of the boundaries of the magnetopshere [<]e.g., Kelvin-
Helmholtz instability,¿[]2021GeoRL..4894002M, or the differences in the
observation of radio sources (beaming constraints). The second aspect is the
opportunity to explore different classes of behaviour in terms of
magnetosperic compression state. Due to the precession of the apojoves, we
observe the compression of the magnetosphere from different positions in the
magnetosphere. As shown in Figure 1c, the nature of the boundary motion is
highly variable, and the number of boundary crossings varies greatly from one
orbit to another. Some orbits have clean boundary crossings, while other
orbits have multiple crossings in a short time. This makes it possible to
study the radio response during the compression and relaxation phases, but
also during the stationary state - see Figures 2c,f for an example. Thirdly,
the long period of time between Juno’s insertion into Jovian orbit (July 2016)
and the latest orbits of the extended mission (perijoves $\geq 50$) covers two
different phases of two different solar cycle and different Jovian seasons,
which could allow us to explore the response of radio emissions to compression
as a function of the solar cycles and Jovian seasons.

At the time of writing, Juno is still crossing the boundaries on the high
southern latitude dusk side, and thus a full statistical exploration of the
broad parameter space should await the completion of these apojove passes.
Moreover, the comprehensive labelled radio emissions catalogue [CK. Louis .
(2021c)] is currently being updated to cover the whole mission.

###### Acknowledgements.

C. K. Louis’, C. M. Jackman’s, A. R. Fogg’s and S. C. McEntee’s work at the
Dublin Institute for Advanced Studies was funded by the Science Foundation
Ireland Grant 18/FRL/6199. The research at the University of Iowa is supported
by NASA through Contract 699041X with Southwest Research Institute. D. M.
Weigt’s work at the Dublin Institute for Advanced Studies was funded by
European Union’s Horizon 2020 research and innovation programme under Grant
agreement No. 952439 and project number AO 2-1927/22/NL/GLC/ov as part of the
ESA OSIP Nanosats for Spaceweather Campaign D. M. Weigt’s work at Aalto
University was funded from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme (project
“SYCOS”, grant agreement no 101101005). The research at the University of Iowa
is supported by NASA through Contract 699041X with the Southwest Research
Institute. WSK acknowledges the use of the Space Physics Data Repository at
the University of Iowa supported by the Roy J. Carver Charitable Trust.

## Data Availability Statements

The Juno/Waves dataset displayed in this paper, produced by
2021JGRA..12629435L, is accessible at https://doi.org/10.25935/6jg4-mk86 [CK.
Louis . (2021b)], and the catalogue can be download at
https://doi.org/10.25935/nhb2-wy29 [CK. Louis . (2021c)] . The Juno/MAG
magnetic field data are accessible through the NASA/PDS website [Connerney
(2017)]. Figure 1 was produced using the Jupiter magnetosphere boundaries
crossings given in the SI Tables S1 and S2 [CK. Louis . (2022e)]. Juno/Waves
and Juno/MAG data were displayed using the Autoplot tool [Faden . (2010)]. The
Nançay Decameter Array dataset displayed in Figure 4 is accessible at
https://doi.org/10.25935/PBPE-BF82 [Lamy . (2021)]. The routine that allows to
determine the dynamic pressure from the 2002JGRA..107.1309J model are
accessible at
https://github.com/DIASPlanetary/jupiter_magnetosphere_boundaries. Juno
ephemeris and MAG data (in JSO coordinates system) were retrieved from
http://amda.cdpp.eu/ [Génot . (2021)]. Juno ephemeris used to inferred the
dynamic pressure (in JSS coordinate) were retrieved from
https://wgc.jpl.nasa.gov:8443/webgeocalc/#StateVector.

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