The icy surface of Jupiter’s moon, Europa, is thought to lie on top of a global ocean1–4. Signatures in some Hubble Space Telescope images have been associated with putative water plumes rising above Europa’s surface5,6, providing support for the ocean theory. However, all telescopic detections reported were made at the limit of sensitivity of the data5–7 , thereby calling for a search for plume signatures in in-situ measurements. Here, we report in-situ evidence of a plume on Europa from the magnetic field and plasma wave observations acquired on Galileo’s closest encounter with the moon. During this flyby, which dropped below 400 km altitude, the magnetometer8 recorded an approximately 1,000-kilometre-scale field rotation and a decrease of over 200 nT in field magnitude, and the Plasma Wave Spectrometer9 registered intense localized wave emissions indicative of a brief but substantial increase in plasma density. We show that the location, duration and variations of the magnetic field and plasma wave measurements are consistent with the interaction of Jupiter’s corotating plasma with Europa if a plume with characteristics inferred from Hubble images were erupting from the region of Europa’s thermal anomalies. These results provide strong independent evidence of the presence of plumes at Europa.

1. Letters
https://doi.org/10.1038/s41550-018-0450-z
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
1
Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA. 2
Department of Earth, Planetary and Space
Sciences, UCLA, Los Angeles, CA, USA. 3
Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA. *e-mail: xzjia@umich.edu
The icy surface of Jupiter’s moon, Europa, is thought to lie
on top of a global ocean1–4
. Signatures in some Hubble Space
Telescope images have been associated with putative water
plumes rising above Europa’s surface5,6
, providing support for
the ocean theory. However, all telescopic detections reported
were made at the limit of sensitivity of the data5–7
, thereby call-
ing for a search for plume signatures in in-situ measurements.
Here, we report in-situ evidence of a plume on Europa from
the magnetic field and plasma wave observations acquired on
Galileo’s closest encounter with the moon. During this flyby,
which dropped below 400 km altitude, the magnetometer8
recorded an approximately 1,000-kilometre-scale field rota-
tion and a decrease of over 200 nT in field magnitude, and
the Plasma Wave Spectrometer9
registered intense localized
wave emissions indicative of a brief but substantial increase
in plasma density. We show that the location, duration and
variations of the magnetic field and plasma wave measure-
ments are consistent with the interaction of Jupiter’s corotat-
ing plasma with Europa if a plume with characteristics inferred
from Hubble images were erupting from the region of Europa’s
thermal anomalies. These results provide strong independent
evidence of the presence of plumes at Europa.
Plumes thus far identified from Hubble images are similar in
spatial scale, appearing to rise ~200 km above the disk of Europa’s
solid body5–7,10
. The ones near the equator are located on Europa’s
trailing hemisphere south of the equator in a region of compara-
tively high surface temperature11
. Additional indirect evidence of
a ‘unique feature’, possibly associated with a plume on the trailing
edge of Europa, has been reported12
. Magnetometer (MAG) data8
were acquired on eight targeted passes by Europa during Galileo’s
eight years in orbit around Jupiter, but only two passes (E12 and
E26) came closer to the surface than 400 km—a height at which
the reported plumes5–7
might impose a plasma and field signature.
Both passes crossed the trailing hemisphere of Europa (E12 near
the equator and E26 at high southern latitude) and recorded short
time-duration, large-amplitude perturbations accompanied by
a sharp decrease of the field magnitude near closest approach. A
previous study of the potential effect of plumes on Europa’s plasma
interaction suggested that the magnetic perturbations during the
E26 flyby could be associated with atmospheric inhomogeneity
resulting from a plume13
. Here, we show that a localized signature in
the MAG data acquired on Galileo’s closest encounter with Europa
(E12 flyby) is fully consistent with the perturbations expected if the
spacecraft crossed a plume rising above the nearby surface.
Figure 1 shows the MAG data from E12, which remained
below 400 km altitude between 12:00:59 and 12:05:37 ut on 16
December 1997. Both the magnetic field magnitude and the plasma
density were exceptionally high upstream of Europa14,15
, and the
field fluctuated on time scales of minutes. Closest approach (alti-
tude: 206 km) was at 12:03:20 ut. Between ~12:00 and 12:03 ut,
significant changes in all 3 components of the magnetic field were
observed for ~3 min. About 1 min before closest approach, the mag-
netic field changed by hundreds of nT in 16 s. It is this short time-
scale fluctuation appearing in the context of slower background
fluctuations that we regard as marking the passage through a plume
of the characteristics extracted from Hubble images. Given Galileo’s
6 km s−1
speed relative to Europa, 3 min corresponds to a transverse
spatial scale of ~1,000 km, comparable to the characteristic scale of
imaged plumes at the spacecraft altitude of ~400 km.
The plasma wave spectrum obtained by Galileo’s Plasma Wave
Spectrometer (PWS)9
reveals an isolated change concurrent with
the short time-scale magnetic field perturbations. The sudden,
short-duration jump in the frequency of intense emissions can be
interpreted as consistent with a highly localized source of plasma,
thereby supporting the hypothesis that the magnetic perturba-
tions arise from passage through a localized plume. In the electric
field spectra (Fig. 2a), a narrow-banded enhancement of power
evident through the entire pass is interpreted as the upper hybrid
resonance (UHR) emissions. In conjunction with the known
electron cyclotron frequency, this reveals that the electron plasma
density remained above or near 600 cm−3
on the inbound leg of
this pass and fell abruptly on the outbound leg (Fig. 2b). This
upstream density is of order three times greater than observed
during all other Europa flybys14
, immediately highlighting this
flyby as remarkable. A pink arrow in Fig. 2 indicates the time
(~12:02 ut) of the rapid field rotation identified in the MAG data.
At this time, just before closest approach, the spectrum changes
abruptly; there is a brief burst of electron cyclotron harmon-
ics, including one band above the upper hybrid band previously
identified14
. The authors of the original study14
were reluctant to
associate this feature specifically with Europa since such waves
are also a characteristic of Jupiter’s magnetic equator. However,
the brief excursion of the upper hybrid band to about 400 kHz
might be interpreted as a local spike in the plasma density. Given
the recent evidence for plumes, the changes would be consistent
with entry into a different plasma regime and, if the emissions
near 400 kHz are UHR waves, the electron density exceeded
2,000 cm−3
(Fig. 2b).
To test the hypothesis that the signatures seen on E12 are
imposed by a plume, we modelled the effect of a plume on plasma
and field properties near Europa using three-dimensional (3D)
multi-fluid magnetohydrodynamic (MHD) simulations16,17
. The
simulation model tracks O+
(representative of magnetospheric
plasma), O2
+
(representative of ions originating from Europa) and
electron fluids separately, and includes ionization, charge-exchange
and recombination processes occurring in Europa’s atmosphere18
.
Evidence of a plume on Europa from Galileo
magnetic and plasma wave signatures
Xianzhe Jia 1
*, Margaret G. Kivelson1,2
, Krishan K. Khurana2
and William S. Kurth3
Nature Astronomy | www.nature.com/natureastronomy

2. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Letters Nature Astronomy
We placed a plume in the region of Europa’s thermal anomaly11
using the structural and density parameters consistent with plume
properties inferred from the previous telescopic observations5–7
.
Our simulation assumes that upstream conditions are steady, with a
background plasma density of ~600 cm−3
. The strong upper hybrid
frequency emissions in Fig. 2a indicate that the background den-
sity remained quite steady at this value until 12:06 ut, after which
it began to decrease, dropping to ~100 cm−3
by the end of the pass.
It is not clear whether the change of background density was a
temporal or spatial feature, such as exit from a possible region of
anomalously cold, high-density plasma referred to as a cold dense
blob19
. In either case, the density decrease should not have affected
the analysis of the plume because it occurred only after the pertur-
bations we were focusing on had diminished. The region of abrupt
large-amplitude fluctuations that we link to a plume lasted <​3 min,
ending by ~12:03 ut, after which high-frequency fluctuations of
the magnetic field are consistent with nominal background noise.
For the purpose of this investigation, changes in the background
conditions that were encountered about 0.7 Europa’s radius (RE)
beyond the signature of the plume and some 3 min after the space-
craft exited the region perturbed by its presence should not affect
the results. As shown below, the simulation reproduced the rapidly
changing magnetic field and plasma density signatures identified in
the Galileo E12 data.
0
100
200
300
400
Bx(nT)
–200
–100
0
100
200
By(nT)
–800
–700
–600
–500
–400
Bz(nT)
11:45 11:50 11:55 12:00 12:05 12:10 12:15 12:20 12:25
400
500
600
700
800
Time
∣B∣(nT)
X (RE)
Y (RE)
Z (RE)
R (RE)
–3.7
2.4
–0.2
4.5
–3.0
1.5
–0.2
3.3
–2.2
0.7
–0.2
2.3
–1.4
–0.2
–0.2
1.4
–0.5
–1.1
–0.2
1.2
0.4
–1.9
–0.2
1.9
1.2
–2.7
–0.2
3.0
2.1
–3.5
–0.2
4.1
3.0
–4.3
–0.2
5.2
Galileo MAG data
MHD model (with plume)
MHD model (no plume)
CA
CA
CA
CA
Fig. 1 | Galileo MAG data for the E12 flyby. Black lines show the MAG data. Green and red lines, respectively, show traces extracted from the MHD
simulations without and with a plume included. Results are presented in EphiO coordinates, with x parallel to Europa’s orbital velocity, y directed towards
Jupiter and z completing the right-handed system. The interval of anomalous changes (~12:00 to 12:03 ut) is bounded by vertical dashed lines, and closest
approach is marked by ‘CA’ and a grey arrow. In the range used for these measurements, the MAG sensor digitization step8
was 0.25 nT. Between 11:55 and
12:05 ut, one or another of the 3 sensors saturated at the spacecraft rotation period. This required special processing, which increased the uncertainty to a
level of a few nT. X, Y, Z and R given at the bottom of the figure indicate the spacecraft location and its radial distance from Europa’s centre in units of RE in
EphiO coordinates.
Nature Astronomy | www.nature.com/natureastronomy

3. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
LettersNature Astronomy
Hubble observations6,7
imply that plumes may erupt over a
range of Europa latitudes and longitudes within the region of
elevated nighttime temperatures11
. Thus, we regard the location of
the base of the plume as weakly constrained and take the location
of the base as parameters. Images from Saturn’s moon Enceladus20
show that plumes have complex structures. Their axes diverge from
radial and there can be localized structures within a single plume.
Assuming that Europa plumes may exhibit similar properties, we
allow the central axis of our simulated plume to be inclined relative
to the radial direction. The plume is given a conical structure and
the opening angle and scale height are taken as free parameters
(see Methods).
Based on the constraint on the approximate location of the
plume imposed by the MAG and PWS data, a number of differ-
ent combinations of plume parameters were tested. The results
presented here were extracted from the run in which the plume
emerged from a longitude of 245° W and a latitude of 5° S, some-
what east of the plume imaged6
in a region with elevated nighttime
temperatures (90–110° on Europa’s thermal maps11
). The plume is
tilted in the azimuthal direction by 15° and in the latitudinal direc-
tion by 25°. The location of the plume used is shown on maps of
Europa’s surface features in Fig. 3. The central column density of
the modelled plume is 3 ×​ 1020
m−2
, which falls within the range of
column densities (1.5 ×​ 1020
–2.3 ×​ 1021
m−2
) inferred from spectro-
scopic observations5,6
.
The modelled magnetic fields extracted from the MHD simu-
lations with and without a plume were compared with the mea-
sured field in Fig. 1. The measured and modelled plasma densities
11:40 11:50 12:00 12:10 12:20 12:30
Electricspectraldensity(V2
m–2
Hz–1
)
fUH
102
103
104
105
106
Frequency(Hz)
10–16
10–14
10–12
10–10
10–8
Time
11:40 11:50 12:00 12:10 12:20 12:30
Density(cm–3
)
102
103
PWS inferred density
MHD model density
Time
a
b
CA
12:02 UT
12:02 UT
Fig. 2 | Galileo plasma wave data and derived plasma density for the
E12 flyby. a, Electric field power spectra. The upper hybrid frequency (fUH)
that establishes the electron density is marked by black dashed (from
the previous study14
) and solid lines (this study). A pink arrow at 12:02 ut
indicates the central time of the magnetic perturbation in Fig. 1.
b, Comparison of the plasma number density inferred from the PWS spectra
(black dashed line from the previous study14
and black solid line from the
new interpretation of wave emissions in this study) with the MHD model
result (red line). The interval of anomalous changes (~12:00 to 12:03 ut) is
bounded by vertical dashed lines and closest approach is marked by ‘CA’
and a grey arrow. The uncertainty in the inferred density is about 20%
based on the spectral resolution of the PWS instrument9
.
a
Jupiter
Flow
x
Galileo
b
Jupiter
Flow
G
alileo
Fig. 3 | Location of the modelled plume on Europa’s surface. a,b,
Perspectives from a longitude of 270° W (a) and from above the northern
pole (b). The surface features shown on the sphere were extracted from
a global map of Europa compiled by the United States Geological Survey.
The cyan iso-surface of constant neutral density illustrates the shape and
location of the modelled plume, and the magenta traces show the Galileo
trajectory. The green ellipse in a indicates the location of a putative
plume imaged by Hubble6
.
Nature Astronomy | www.nature.com/natureastronomy

4. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Letters Nature Astronomy
are plotted in Fig. 2b. Overall, the simulation corresponds well
with the smoothed data, although the shocklets (very small ampli-
tude shocks15
) present in the upstream portion of the pass are not
reproduced. Some discrepancies in the smoothed data are read-
ily accounted for. On the E12 pass, the spacecraft crossed Jupiter’s
equatorial plane at 11:59 ut, shortly before reaching the region of
anomalous variations. The background By component reverses sign
across the equator, but the simulation was carried out in a fixed
background field with By =​ 0. This explains why in Fig. 1 the mea-
sured By is positive at the start of the interval and negative at the end
of the interval, but the simulated field starts and ends with By =​ 0.
The plasma conditions assumed are close to the upstream condi-
tions measured, but the gradient of background density after closest
approach is not part of the model, which is why the model field
diverges from measurements after 12:08 ut. The high upstream den-
sity leads to pile-up of the field close to Europa, which is well repro-
duced by the model. Shocklets in the upstream flow may account for
the changes in field magnitude between 11:58 and 12:00 ut.
Overlooking these discrepancies between the model and mea-
surements on the large scale, the effect of a plume can be established
by comparing the two simulation results within the region marked
by the vertical lines in Fig. 1. Flow diversion around an obstacle
twists the magnetic field. The rapid changes in By near the centre
of the plume, where the y-component of the field abruptly turns
positive (125 nT) and then negative (−​219 nT), result from flow
diversion around a plume. The simulation reproduces the rotation
of By arising through a localized diversion of the flow both towards
and away from Jupiter as the trajectory crosses the plume, albeit at
a reduced amplitude.
The change of Bx imposed by the plume can be interpreted in
terms of an Alfvén wing structure21
. The slowing of plasma flow
on approach to Europa (Supplementary Fig. 3d) generates Alfvénic
perturbations that bend the field in the direction of the flow, result-
ing in negative Bx perturbations above the moon and positive Bx
perturbations below it. The Bx signature at ~12:02 ut can be under-
stood as characteristic of a mini Alfvén wing formed below a con-
fined plume (Supplementary Fig. 5).
Starting at 12:00 ut, the variations of Bz and |B| arise from a local
flow stagnation and diversion. The field magnitude becomes large
as a result of field pile-up where the high-density ambient plasma14
is substantially slowed as it approaches the plume (Supplementary
Fig. 4f). The strong decrease in field strength then occurs where the
flow is re-accelerated in the wake of the plume. The peak plasma
density at the centre of the plume extracted from the simulation is
~2,000 cm−3
(Fig. 2b), very close to the enhanced electron density
inferred from PWS if the anomalous emissions at 12:02 ut corre-
spond to UHR waves.
In addition to E12, the only other Galileo pass that includes
an interval at an altitude below 400 km is E26 (closest approach
at 17:59:43 on 3 January 2000). Again, the spacecraft speed was
~6 km s−1
, but given the relatively high altitude of this pass, a plume
signature might have been encountered for only ~1 min. In the low-
altitude region, no field perturbations last for even 1 min, although
there is a diamagnetic decrease lasting for ~5 s. As this signature
does not approximate the characteristics we have previously identi-
fied, we suggest that the magnetic perturbation observed near clos-
est approach on the E26 pass is unlikely to be produced by a plume.
The consistency of the particles and field perturbations dur-
ing Galileo’s E12 flyby with the expected signature of a plume of
the scale and density inferred from analysis of Hubble images
underscores the value of acquiring in-situ data at low altitude on
upcoming missions carrying appropriate instrumentation. The
European Space Agency’s JUICE mission22
to Ganymede plans two
passes with its closest approach 400 km above Europa’s surface. The
National Aeronautics and Space Administration (NASA)’s Europa
Clipper mission23
will make ~40 passes at altitudes <​400 km. In-situ
and remote-sensing measurements from these missions—espe-
cially those in regions close to the thermal anomaly that may be
a preferred location of plumes—will be highly desirable to obtain
detailed characterization of plume activity at Europa.
Methods
Galileo trajectories for all Europa encounters. During its entire mission, the
Galileo spacecraft encountered Europa on 11 different orbits. Supplementary Fig. 1
shows the Galileo trajectories for all Europa encounters in EphiO coordinates,
where x is parallel to Europa’s orbital velocity, y is directed towards Jupiter and
z completes the right-handed system. MAG data were acquired on eight passes15
,
whereas PWS data were acquired on nine passes14
. Recorded MAG data were lost
on E6, E16 and E18, and recorded PWS data were lost on E16 and E18. Among
the 11 flybys, only the E12 and E26 flybys came within 400 km of the surface with
closest approach altitudes of 196 km for E12 and 348 km for E26.
Multi-fluid MHD model and simulation results. Model basics. We modelled the
plasma interaction with Europa and its exosphere using a well-established MHD
code, BATSRUS (Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme)16,24,25
.
The BATSRUS code solves the governing MHD equations in finite-volume form.
It has been widely applied to simulations of planetary magnetospheres and plasma
interactions with planetary moons, including Europa. Previous modelling studies
on Europa using BATSRUS include single-fluid26
, multi-species27
and multi-fluid17
MHD simulations.
In this work, we employed the multi-fluid version of BATSRUS16,17
to
separately track the plasma species originating from different sources at Europa.
In particular, the multi-fluid MHD model includes separate continuity, momentum
and energy equations for two ion fluids: an O2
+
fluid representing the main ion
species produced from Europa’s O2-dominated exosphere18,28,29
and an O+
fluid
consisting of Jupiter’s ambient magnetospheric plasma and O+
ions produced as a
daughter species from Europa’s O2 exosphere. In our model, each of the ion fluids
is simulated with its own density, bulk velocity and temperature. Electrons in the
system are treated as a charge-neutralizing fluid with a separate pressure equation
that describes the evolution and spatial distribution of the electron temperature.
Source terms for the electron pressure in our model include the thermal energy
contributed by the newly implanted electrons through impact ionization and the
excess energy of photoelectrons, as well as the energies transferred through elastic
collisions with the ions and neutrals. Loss terms for the electron pressure account
for the reduction due to ion–electron recombination and the ionization energy
provided by the ionizing electrons.
Mass-loading model. The key to modelling the plasma interaction with Europa
and its exosphere is to include the important mass-loading processes present in
the vicinity of the moon, such as ionization due to photo- and electron impact
ionization, charge-exchange and dissociative recombination. Our multi-fluid MHD
model incorporates these mass-loading processes by adding appropriate source and
loss terms in the set of MHD equations17
.
The mass-loading model in our simulations assumes a prescribed density
distribution for Europa’s exosphere, which is composed predominantly of
molecular oxygen18,29
. The assumed neutral density (Nn) distribution is described
in a form that contains two exponential functions with different surface densities
and scale heights representing the sublimated and sputtered components of the
exosphere30–33
.
ϕ ϕ
ϕ
=
+ ⋅ + ≤
+ >
− −
∘
− − − −
∘







































N
n e n e
n e n e
(1 2 cos ) , if 90
, if 90
(1)
r R
h
r R
h
r R
h
r R
h
n
10 20
10 20
s s
s s
E
1
E
2
E
1
E
2
Here, r is the radial distance in units of RE (RE =​ 1,570 km is Europa’s radius),
n10 =​ 4 ×​ 107
cm−3
and hs1 =​ 100 km are the surface density and scale height
for the relatively confined thermal component of the exosphere34
, whereas
n20 =​ 1 ×​ 106
cm−3
and hs1 =​ 500 km are used to represent the relatively extended
sputtered component of the exosphere32,33
. For the upstream (or trailing)
hemisphere, the sum of the two exponential functions is multiplied by the
coefficient dependent on ϕ, which is the azimuthal angle measured from the
–x axis in EphiO coordinates. The coefficient involving ϕ results in higher neutral
densities on the trailing hemisphere than on the leading hemisphere, consistent
with the idea that sputtering by Jovian magnetospheric particles is a dominant
process in producing Europa’s exosphere35
. The resulting neutral distribution for
the exosphere is shown in Supplementary Fig. 2a. The total column density arising
from the sum of the two components ranges from 4.5 ×​ 1018
m−2
on the leading
hemisphere to 1.3 ×​ 1019
m−2
on the trailing hemisphere. The range of exospheric
column densities used in our simulations is consistent with those obtained from
observations (2.4 ×​ 1018
–1.4 ×​ 1019
m−2
(refs 18,28
)) and those used in previous
simulations of Europa’s plasma–exosphere interaction13,17,36,37
. The exosphere model
described in equation (1) is used as the background neutral exosphere model in
Nature Astronomy | www.nature.com/natureastronomy

5. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
LettersNature Astronomy
simulations with and without a plume included. For simulations with a plume, a
localized plume-related neutral distribution, which is detailed below, is added to
this background neutral model.
Based on the prescribed neutral exosphere, the MHD model calculates the
corresponding ion production rate by multiplying the neutral density by a given
ionization rate. Here, we adopt a constant ionization rate of 1 ×​ 10−6
s−1
for O2
+
production and 1 ×​ 10−7
s−1
for O+
production according to the mean ionization
rates derived previously17
. Although the ionization rates are constants in our
model, the ion production rates are asymmetric, with higher rates on the upstream
than on the downstream side because the neutral exosphere density is asymmetric
between the leading and trailing hemispheres. The total ion production rate in our
simulation without plume amounts to ~14.7 kg s−1
or 2.9 ×​ 1026
ions s−1
.
The charge-exchange interaction between O2 and O2
+
, which does not yield
additional mass input into the system but contributes to the momentum loading
of the flow, is also included in our model through ion-neutral friction. The rate for
the ion-neutral friction is given by38
:
̄ ̄= . × × − . ×−
k N T T2 59 10 (1 0 073 log ) (2)in
17
n 10
2
where Nn is the exospheric neutral density and ̄= + +T T T( )1
2 O O2 2
is the mean
temperature between O2 and O2
+
. The resulting total charge-exchange rate
integrated over the entire interaction region is ~27 kg s−1
or 5 ×​ 1026
ions s−1
.
Plasma generated from Europa’s exosphere may be lost by dissociative
recombination between ions and electrons. The recombination rates for O+
(α +O )
and O2
+
(α +O2
) are prescribed according to the following functions given by38
:
α α= . × = . ×−
.
−
.
+ +











T T
3 7 10
250
, 2 4 10
300
(3)O
18
e
0 7
O
13
e
0 7
2
where Te is the electron temperature calculated from the electron energy equation.
The total recombination rate in our model is ~8 ×​ 10−4
kg s−1
—several orders of
magnitude smaller than the ion production rate. This result is consistent with the
previous finding that recombination appears to be an insignificant loss process for
Europa’s ionosphere17,37
.
Plume model. To model the effect of a plume on the plasma interaction, we
incorporated in our MHD model an analytical form for the plume neutral
density distribution similar to that derived based on the Hubble observations5
.
As described in equation (4), the density distribution (NP) has a conical structure
governed by several parameters, including the central surface density (NP0), scale
height (HP) and opening angle (θP).
= ⋅ ⋅
θ
θ
−
−
−











N N e e
(4)
r R
H
P P0
E
P P
2
Here, r is the radial distance in units of RE and θ is the polar angle measured relative
to the central axis of the plume.
The location of the plume on Europa’s surface is also a free parameter in our
model, but the Galileo MAG and PWS observations of the anomalous variations
in particles and field conditions provide a key constraint on the general region
where the plume should be located. Specifically, the signatures seen around
12:02 ut in the MAG and PWS data that we identify to be associated with a plume
were encountered at a longitude of 235° W and a latitude of 9° S. Images from
Saturn’s moon Enceladus20
show that plumes possess a complicated structure
in that their axes diverge from the radial direction and there can be localized
structures within a single plume. Assuming that Europa plumes may exhibit similar
properties, we allow the central axis of our simulated plume to bend relative to
the radial direction. Guided by this information, we tested a number of different
combinations of the parameters controlling the location and spatial distribution of
the plume. The results shown in this paper were extracted from the run in which
the base of the plume is located at a longitude of 245° W and a latitude of 5° S,
and the plume’s central axis is tilted relative to the radial direction by 15° in the
azimuthal direction towards the east and by 25° in the latitudinal direction towards
the south. Other parameters used are NP0 =​ 2 ×​ 109
cm−3
, HP =​ 150 km and θP =​ 15°.
These parameters yield a central column density of Ncol =​ 3 ×​ 1020
m−2
, which falls
right within the range of plume column densities inferred from spectroscopic
observations: 1.5 ×​ 1020
m−2
(ref. 5
) to 2.3 ×​ 1021
m−2
(ref. 6
). Supplementary Fig. 2b
shows the distribution of the total neutral density, which is the sum of the
background exosphere density and plume density. The net ion production rate
associated with the modelled plume is estimated to be ~0.3 kg s−1
.
Numerical grid. The MHD simulations were conducted on a non-uniform,
high-resolution spherical mesh that ensures high grid resolution of the near-Europa
interaction region and an accurate prescription of boundary conditions at the
moon. The outer boundary of the simulation corresponds to a sphere of radius of
56 RE, and the inner boundary corresponds to Europa’s surface of radius of 1 RE.
Using the adaptive mesh refinement capability of BATSRUS16
, we generated a
spherical mesh with the multi-level refined grid structure shown in Supplementary
Fig. 2c,d. The grid resolution at r =​ 2 RE is ~0.02 RE (or 31 km). Given the small-scale
size of the plume of interest, we refined the grid resolution even further in the
vicinity of the plume, which was essential for resolving the fine structure in the
magnetic and density perturbations associated with the plume. The smallest grid
size around the plume is ~0.004 RE or 6 km.
Boundary conditions. At the upstream outer boundary, we specify the magnetic
field according to the Galileo MAG observations obtained around closest
approach. The vector components (in EphiO coordinates) used in the simulation
were (Bx, By, Bz) =​ (78, 0, −​395) nT. Based on the Galileo PWS and PLS
measurements14,39
, a uniform electron number density of 500 cm−3
, a flow velocity
of 100 km s–1
in the corotation direction and a temperature of 100 eV were specified
for the upstream plasma flowing into the simulation domain. On the downstream
outer boundary, floating boundary conditions were applied to allow the plasma to
freely leave the simulation domain.
At the inner boundary (Europa’s surface), we included the induced dipole field
arising from the inductive response of a subsurface ocean to the time-varying
external magnetic field1,4
. Assuming a 100% induction efficiency40
, an equatorial
dipole with its axis parallel to the –x axis (in EphiO coordinates) and an equatorial
surface strength of 39 nT were prescribed as initial conditions and fixed in our
simulations to represent the induced field. In regions at the surface where the
plasma flow in the computational cell next to the surface had an inward radial
component, floating boundary conditions were applied to the plasma parameters
(that is, zero gradients in density, pressure and velocity), such that Europa’s surface
absorbed incoming flows. In regions where the plasma flow had an outward radial
flow near the surface, fixed densities of 1 cm−3
for O+
and 10 cm−3
for O2
+
and a
temperature of 600 K for both ion fluids were specified to represent the situation
where Europa’s surface contributes only a minimal amount of cold plasma to
the ionosphere.
Comparison of model results between simulations with and without a plume. With
the input parameters described above, we performed two sets of MHD simulations:
one with only the background exosphere and another with the plume (described
in the text above) added to the existing exosphere. In all simulations, the upstream
parameters and inner boundary conditions were kept the same and the MHD
model was iterated until the system had reached a quasi-steady state, from which
the simulation results presented here were extracted.
Supplementary Figs. 3–5 present a series of comparisons of the model results
between the two cases to reveal the perturbations to plasma and magnetic field
conditions caused by the plume. In particular, Supplementary Fig. 3a,b compares
the density distribution for the dominant ionospheric species, O2
+
, in the xy
plane (at z =​ −​0.2 RE) that contained the Galileo trajectory during the E12 flyby.
In the simulation with just the background exosphere, during the interval of
interest (12:00–12:03 ut) the Galileo spacecraft would have encountered a plasma
population with a roughly constant density of ~700 cm−3
. In contrast, in the
simulation with the plume included, the ionization of the plume particles produced
localized density enhancements with a peak density of ~2,000 cm−3
around
12:02 ut at the spacecraft altitude, consistent with the enhanced electron density
inferred from PWS at this time. Without a plume, as the ambient Jovian plasma
flow approaches Europa from upstream, it is slowed by ion pickup and by Europa’s
ionosphere21
; some of the flow then diverts around the moon and is re-accelerated
on the flanks (Supplementary Fig. 3c). The flow speeds during the interval of
interest range from 40 to 60 km s−1
. In the presence of the plume (Supplementary
Fig. 3d), the flow that is originally diverted around Europa is further slowed down
locally due to the pickup processes occurring in the vicinity of the plume. The flow
speeds in the same region are reduced to 25–40 km s−1
.
Supplementary Fig. 4 compares the magnetic perturbations in the xy plane
that contains the Galileo trajectory for the two cases. The left column applies
to the simulation without a plume and illustrates the large-scale structure of
the magnetic perturbations arising from the interaction between the ambient
Jovian plasma and Europa’s exosphere. The slowing down of the ambient flow
near Europa causes the magnetic field bendback above and below the equatorial
plane21
with positive Bx perturbations below Europa’s equator in the plane of
Galileo’s E12 pass (Supplementary Fig. 4a). The slowing down of the flow also
produces compressional waves that result in enhancement of the magnitude of
the dominant component Bz (Supplementary Fig. 4e) and of the field magnitude
upstream of Europa. The ambient field for the E12 pass is approximately aligned
with the z axis near closest approach, and the plasma interaction produces minimal
perturbations in the By component upstream of Europa. Shown in the right column
of Supplementary Fig. 4 are the results from the case with the plume included.
The magnetic perturbations remain similar to those found in the case without
a plume, except in the immediate vicinity of the plume where the perturbed
Jovian plasma interacts with plume particles to generate strong local magnetic
and plasma perturbations. The base of the plume introduced in our model lies
above the Galileo trajectory, and consequently the Alfvénic perturbations caused
by the locally modified flow cause additional bendback of the field lines, seen as
locally enhanced positive Bx perturbations in Supplementary Fig. 4b. The localized
positive and negative By perturbations (Supplementary Fig. 4d) near the plume
arise from the diversion of flow around the plume. Upstream of the plume, the
slowing down of the flow leads to pile-up of the magnetic field producing negative
Nature Astronomy | www.nature.com/natureastronomy

6. © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Letters Nature Astronomy
perturbations of Bz (Supplementary Fig. 4f), analogous to those that develop where
the ambient Jovian flow interacts with Europa’s global exosphere upstream of the
moon. The effect appears on a much smaller spatial scale because of the small
spatial size of the plume. Downstream of the plume, the diverted flow around the
plume centre is re-accelerated, producing a decrease in the field magnitude.
The variations of the simulated magnetic field components and strength
associated with the plume discussed above are entirely consistent with the Galileo
MAG observations.
Supplementary Fig. 5 compares the 3D configuration of the interaction region
for cases with and without the plume. An iso-surface with a flow speed of 25 km s−1
is
chosen to illustrate, as an approximation, the morphology of the interaction region.
Colour contours of Bx are plotted on the iso-surface to show the bendback of the
field, which leads to negative Bx perturbations above and positive Bx perturbations
below the central plane where the peak mass-loading occurs. Supplementary Fig. 5a,b
reveals the 3D structure of the plume-generated perturbations. Supplementary Fig.
5b shows that a ridge-like structure forms surrounding the plume, with negative Bx
perturbations above and positive Bx perturbations below the central axis of the plume.
These perturbations are characteristic of those expected from an Alfvén wing-like
structure21
. The figure also shows the trajectory geometry during the E12 flyby relative
to the plume-generated mini Alfvén wing.
Data availability. The data that support the plots within this paper and
other findings of this study are available from the corresponding author upon
reasonable request.
Received: 4 December 2017; Accepted: 19 March 2018;
Published: xx xx xxxx
References
1. Khurana, K. K. et al. Induced magnetic fields as evidence for subsurface
oceans in Europa and Callisto. Nature 395, 777–780 (1998).
2. Carr, M. H. et al. Evidence for a subsurface ocean on Europa. Nature 391,
363–365 (1998).
3. Pappalardo, R. T. et al. Does Europa have a subsurface ocean? Evaluation of
the geological evidence. J. Geophys. Res. 104, 24105–24055 (1999).
4. Kivelson, M. G. et al. Galileo magnetometer measurements strengthen the
case for a subsurface ocean at Europa. Science 289, 1340–1343 (2000).
5. Roth, L. et al. Transient water vapor at Europa’s south pole. Science 343,
171–174 (2014).
6. Sparks, W. B. et al. Probing for evidence of plumes on Europa with HST/
STIS. Astrophys. J. 829, 121 (2016).
7. Sparks, W. B. et al. Active cryovolcanism on Europa? Astrophys. J. Lett. 839,
L18 (2017).
8. Kivelson, M. G., Khurana, K. K., Means, J. D., Russell, C. T. & Snare, R. C.
The Galileo magnetic field investigation. Space Sci. Rev. 60, 357–383 (1992).
9. Gurnett, D. A. et al. The Galileo plasma wave investigation. Space Sci. Rev. 60,
341–355 (1992).
10. Roth, L. et al. Orbital apocenter is not a sufficient condition for HST/STIS
detection of Europa’s water vapor aurora. Proc. Natl Acad. Sci. USA 111,
E5123–E5132 (2014).
11. Spencer, J. R., Tamppari, L. K., Martin, T. Z. & Travis, L. D. Temperatures on
Europa from Galileo photopolarimeter-radiometer: nighttime thermal
anomalies. Science 284, 1514–1516 (1999).
12. McGrath, M. & Sparks, W. B. Galileo ionosphere profile coincident with
repeat plume detection location at Europa. Res. Notes AAS 1, 14 (2017).
13. Blöcker, A., Saur, J. & Roth, L. Europa’s plasma interaction with an
inhomogeneous atmosphere: development of Alfvén winglets within the
Alfvén wings. J. Geophys. Res. 121, 9794–9828 (2016).
14. Kurth, W. S. et al. The plasma wave environment of Europa. Planet. Space Sci.
49, 345–363 (2001).
15. Kivelson, M. G., Khurana, K. K. & Volwerk, M. in Europa (eds Pappalardo, R.
T., McKinnon, W. B. & Khurana, K. K.) 545–570 (Univ. Arizona Press,
Tucson, AZ, 2009).
16. Toth, G. et al. Adaptive numerical algorithms in space weather modeling.
J. Comp. Phys. 231, 870–903 (2012).
17. Rubin, M. et al. Self-consistent multi-fluid MHD simulations of Europa’s
exospheric interaction with Jupiter’s magnetosphere. J. Geophys. Res. 120,
3503–3524 (2015).
18. Hall, D. T., Strobel, D. F. & Feldman, P. D., McGrath, M. A. & Weaver, H. A.
Detection of an oxygen atmosphere on Jupiter’s moon Europa. Nature 373,
677–679 (1995).
19. Bagenal, F., Dougherty, L. P., Bodish, K. M., Richardson, J. D. & Belcher, J. M.
Survey of Voyager plasma science ions at Jupiter: 1. Analysis method.
J. Geophys. Res. 122, 8241–8256 (2017).
20. Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science
311, 1393–1401 (2006).
21. Neubauer, F. M. The sub-Alfvénic interaction of the Galilean satellites with
the Jovian magnetosphere. J. Geophys. Res. 103, 19843–19866 (1998).
22. Grasset, O. et al. JUpiter ICy moons Explorer (JUICE): an ESA mission to
orbit Ganymede and to characterise the Jupiter system. Planet. Space Sci. 78,
1–21 (2013).
23. Pappalardo, R. et al. Science objectives and capabilities of the NASA Europa
mission. In Proc. 47th Lunar and Planetary Science Conf. 3058 (Lunar and
Planetary Institute, 2016).
24. Powell, K. G., Roe, P. L., Linde, T. J., Gombosi, T. I. & De Zeeuw, D. L. A
solution-adaptive upwind scheme for ideal magnetohydrodynamics.
J. Comp. Phys.154, 284–309 (1999).
25. Gombosi, T. I. et al. Semirelativistic magnetohydrodynamics and physics-
based convergence acceleration. J. Comput. Phys. 177, 176–205 (2002).
26. Kabin, K. et al. On Europa’s magnetospheric interaction: a MHD simulation
of the E4 flyby. J. Geophys. Res. 104, 19983–19992 (1999).
27. Liu, Y. et al. Two-species, 3D MHD simulation of Europa’s interaction with
Jupiter’s magnetosphere. Geophys. Res. Lett. 27, 1791–1794 (2000).
28. Hall, D. T., Feldman, P. D., McGrath, M. A. & Strobel, D. F. The far-ultraviolet
oxygen airglow of Europa and Ganymede. Astrophys. J. 499, 475–481 (1998).
29. McGrath, M. A., Hansen, C. J. & Hendrix, A. R. in Europa (eds Pappalardo,
R. T., McKinnon, W. B. & Khurana, K. K.) 485–505 (Univ. Arizona Press,
Tucson, AZ, 2009).
30. Shematovich, V. I., Johnson, R. E., Cooper, J. F. & Wong, M. C. Surface-
bounded atmosphere of Europa. Icarus 173, 480–498 (2005).
31. Smyth, W. H. & Marconi, M. L. Europa’s atmosphere, gas tori, and
magnetospheric implications. Icarus 181, 510–526 (2006).
32. Cassidy, T. A., Johnson, R. E., McGrath, M. A., Wong, M. C. & Cooper, J. F.
The spatial morphology of Europa’s near-surface O2 atmosphere. Icarus 191,
755–764 (2007).
33. Plainaki, C. et al. The role of sputtering and radiolysis in the generation of
Europa exosphere. Icarus 218, 956–966 (2012).
34. Saur, J., Strobel, D. F. & Neubauer, F. M. Interaction of the Jovian
magnetosphere with Europa: constraints on the neutral atmosphere.
J. Geophys. Res. 103, 19947–19962 (1998).
35. Johnson, R. E. et al. in Europa (eds Pappalardo, R. T., McKinnon, W. B. &
Khurana, K. K.) 507–528 (Univ. Arizona Press, Tucson, AZ, 2009).
36. Schilling, N., Neubauer, F. M. & Saur, J. Influence of the internally induced
magnetic field on the plasma interaction of Europa. J. Geophys. Res. 113,
A03203 (2008).
37. Dols, V., Bagenal, F., Cassidy, T., Crary, F. J. & Delamere, P. A. Europa’s
atmospheric neutral escape: importance of symmetrical O2 charge exchange.
Icarus 264, 387–397 (2016).
38. Schunk, R. & Nagy, A. Ionospheres: Physics, Plasma Physics, and Chemistry
(Cambridge Univ. Press, Cambridge, UK, 2009).
39. Bagenal, F. et al. Plasma conditions at Europa’s orbit. Icarus 261, 1–13 (2015).
40. Zimmer, C., Khurana, K. K. & Kivelson, M. G. Subsurface oceans on Europa
and Callisto: constraints from Galileo magnetometer observations. Icarus 147,
329–347 (2000).
Acknowledgements
We thank M. McGrath for an illuminating presentation at a Europa Clipper
Project Science Group meeting on observations of Europa’s plumes, which
led us to re-examine the Galileo MAG data on which this paper is largely based.
The work at the University of Michigan was supported by NASA through grants
#NNX12AM74G and #NNX15AH28G, contract #1532308 through the Jet Propulsion
Laboratory and contract #143448 through the Applied Physics Laboratory at
Johns Hopkins University. The research at the University of Iowa is supported by
NASA through contract UTA16-001080 through the University of Texas at
Austin. Additional funding for work at UCLA was provided by NASA grants
#NNX13AL05G:000002 and #NNX14AO24G.
Author contributions
X.J., M.G.K. and K.K.K. contributed to the analysis of the Galileo magnetic field data.
W.S.K. analysed the Galileo plasma wave data. X.J. performed the simulations and led the
interpretation of the model results. All authors discussed the results and contributed to
writing the manuscript.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/
s41550-018-0450-z.
Reprints and permissions information is available at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to X.J.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Nature Astronomy | www.nature.com/natureastronomy