The electric wind of Venus: A global and persistent
“polar wind”-like ambipolar electric ﬁeld sufﬁcient
for the direct escape of heavy ionospheric ions
Glyn A. Collinson
, Rudy A. Frahm
, Alex Glocer
, Andrew J. Coates
, Joseph M. Grebowsky
, Shawn D. Domagal-Goldman
, Andrei Fedorov
, Yoshifumi Futaana
, Lin K. Gilbert
, Tom A. Nordheim
, David Mitchell
, Thomas E. Moore
, William K. Peterson
John D. Winningham
, and Tielong L. Zhang
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA,
Mullard Space Science Laboratory, University College
London, Surrey, UK,
Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington,
District of Columbia, USA,
Southwest Research Institute, San Antonio, Texas, USA,
Institutet för rymdfysik, Swedish
Institute of Space Physics, Kiruna, Sweden,
L’Institut de Recherche en Astrophysique et Planétologie, CNRS, Toulouse,
University Paul Sabatier, Toulouse, France,
Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
Space Sciences Laboratory, University of California, Berkeley, California, USA,
Laboratory for Atmospheric
and Space Physics, Boulder, Colorado, USA,
Space Research Institute, Austrian Academy of Sciences, Graz, Austria
Abstract Understanding what processes govern atmospheric escape and the loss of planetary water is of
paramount importance for understanding how life in the universe can exist. One mechanism thought to be
important at all planets is an “ambipolar”electric ﬁeld that helps ions overcome gravity. We report the discovery
and ﬁrst quantitative extraterrestrial measurements of such a ﬁeld at the planet Venus. Unexpectedly, despite
comparable gravity, we show the ﬁeld to be ﬁve times stronger than in Earth’s similar ionosphere. Contrary to our
understanding, Venus would still lose heavy ions (including oxygen and all water-group species) to space, even if
there were no stripping by the solar wind. We therefore ﬁnd that it is possible for planets to lose heavy ions to
space entirely through electric forces in their ionospheres and such an “electric wind”must be considered when
studying the evolution and potential habitability of any planet in any star system.
1. Venus and the Polarization Electric Field
Discovering what processes govern the evolution of atmospheres, and speciﬁcally the loss of planetary water
and oxygen, is key to determining what makes planets habitable and is a driving science objective behind
recent missions including the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, the
European Space Agency (ESA) Mars Express, and the ESA Venus Express. Of all other planets, Venus is in many
respects the most Earth-like. Its atmosphere, however, is incredibly dry, with four to ﬁve orders of magnitude
less water than Earth [De Bergh et al., 1991]. The high deuterium-to-hydrogen ratio [McElroy and Hunten, 1969;
Donahue et al., 1982; De Bergh et al., 1991] is indicative that this was not always the case, and that Venus once
had a substantial quantity of water [Donahue and Hodges, 1992; Hartle et al., 1996; Donahue, 1999], possibly
even forming Earth-like oceans [Svedhem et al., 2007]. Although it is thought that Venus lost much of its water
early in its history [Kulikov et al., 2006], one of the major early discoveries of the ESA Venus Express [Svedhem
et al., 2009] mission was that the primary ion species escaping down the comet-like plasma tail were H
ions in a water-like stoichiometric ratio of 2:1. Thus, regardless of the original water inventory, atmospheric
escape mechanisms at Venus today appear to be far more effective at driving water and oxygen loss than at
nearby Earth, with a comparable size and gravity. Without an intrinsic magnetic dipole ﬁeld [Zhang et al.,
2008; Bridge et al., 1967], the prevailing wisdom has been that atmospheric loss is dominated by mechanisms
resulting from stripping by the solar wind [Dubinin et al., 2011]. However, one planetary-driven mechanism
thought to play a supportive (but important) role in both atmospheric evolution [Barabash et al., 2007b;
Dubinin et al., 2011], and the enrichment of light (H
) ion escape [Hartle and Grebowsky, 1990, 1993,
1995; Barabash et al., 2007b] is that of a long hypothesized “ambipolar”electric ﬁeld (also referred to as a
“polarization”electric ﬁeld at Venus [Barabash et al., 2007b]).
The ionosphere of any planet consists of ions and electrons in approximately equal numbers. In the absence
of electrical forces, electrons, being three to four orders of magnitude lighter than ions, would easily escape
COLLINSON ET AL. VENUS HAS POTENTIAL 1
Geophysical Research Letters
•We report the discovery and ﬁrst
quantitative extraterrestrial measure-
ments of an ionospheric ambipolar
electric ﬁeld at the planet Venus
•A persistent, stable, and global phe-
nomenon, accelerating any ion lighter
than 18 amu to escape velocity
•Planets can lose heavy ions to space
entirely through electrical forces in
•Supporting Information S1
G. A. Collinson,
Collinson, G. A., et al. (2016), The electric
wind of Venus: A global and persistent
“polar wind”-like ambipolar electric ﬁeld
sufﬁcient for the direct escape of heavy
ionospheric ions, Geophys. Res. Lett.,43,
Received 18 FEB 2016
Accepted 4 APR 2016
©2016. American Geophysical Union.
All Rights Reserved.
the pull of gravity guided along the draped magnetic ﬁeld. However, the Coulomb force restricts their motion
away from the ions. As the electrons pull away, an ambipolar electric ﬁeld forms to resist their separation, pre-
venting a net charge from forming and satisfying quasineutrality (see Figure 1). The more energetic the elec-
trons, the stronger the electric ﬁeld must be to restrain them. “Superthermal”(1–70 eV) photoelectrons,
generated by photoionization of the atmosphere, play an especially potent role in generating this ﬁeld
[Lemaire, 1972] even though they make up a small fraction of the total electron population [Khazanov et al.,
1997]. The potential drop that results from this electric ﬁeld assists terrestrial atmospheric escape [Moore
et al., 1997] since it reduces the potential barrier required for heavier ions (such as O
) to escape and accelerates
light ions (such as H
) to escape velocity. An identical physical process is also hypothesized to occur in the solar
wind [Lemaire and Scherer, 1973] (and in the stellar winds from all stars [Scudder and Karimabadi, 2013]),
although it is very difﬁcult to measure and thus has remained theoretical. This potential drop is critical to the
formation of Earth’s“polar wind,”which ﬂows outward along open magnetic ﬁelds above our polar caps
[Hanks and Holzer, 1968]. However, given that that the scientiﬁcterm“polar wind”also encompasses other
acceleration mechanisms, and that at an unmagnetized planet it would not be conﬁned to the poles, we adopt
the nomenclature of “electric wind”as a shorthand to refer to this speciﬁc mechanism: the ambipolar electric
ﬁeld, electric potential drop, and outﬂow of decelerated electrons and accelerated ions.
Although vital to our understanding of the evolution of our atmosphere, this ﬁeld is extremely challenging to
measure given its small magnitude. The few attempts made to measure it in Earth’s ionosphere were only
able to estimate an upper bound on the electric potential drop (i.e., the total drop below the observing space-
craft) of ≤2V[Coates et al., 1985; Fung and Hoffman, 1991]. In addition to these weak ionospheric ﬁelds, larger
(≈20 V) [Kitamura et al., 2012; Winningham and Gurgiolo, 1983; Wilson et al., 1997] parallel potential drops are
frequently observed above spacecraft (i.e., between ≈3800 km and inﬁnity [Kitamura et al., 2012]). However,
these higher-altitude potential drops occur entirely above the bulk of our ionosphere and are not to be con-
fused with the ionospheric (≤500 km) potential drops discussed in this study.
Similarly, recent investigations of the ionospheric potential drop at other worlds (speciﬁcally Mars [Collinson
et al., 2015] and Titan [Coates et al., 2015a] have also only been able to put an upper limit of <±2 V due to
instrumentational limitations and the ﬁeld’s diminutive strength. Although an ambipolar electric potential
has never been successfully measured in a planetary ionosphere, there is abundant indirect evidence for
the presence of an electric wind at Venus. Hartle and Grebowsky [1990, 1993, 1995] theorized the presence
of an ambipolar electric ﬁeld from observations of escaping H
, by the NASA Pioneer Venus
Figure 1. The induced magnetosphere of Venus and formation of the electric wind.
Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 2
Orbiter and presumed that this electric ﬁeld dominated light ion escape. Additionally, the observation of hot
superthermal photoelectrons (created in the dayside ionosphere [Coates et al., 2008] and observed in the
magnetotail of Venus [Tsang et al., 2015; Coates et al., 2011, 2015b]) was also presented as indirect evidence
that a “polar wind like”process may be occurring [Tsang et al., 2015; Coates et al., 2011, 2015b]. Similar out-
ﬂows of photoelectrons and associated outﬂowing ions have also been observed in the induced magneto-
tails of Mars [Frahm et al., 2006, 2010; Coates et al., 2011] and Titan [Coates et al., 2007; Coates, 2009;
Coates et al., 2011; Wellbrock et al., 2012; Coates et al., 2015a] and are also possibly indicative of ambipolar out-
ﬂow. In addition, the photoelectron and escaping ion ﬂuxes were used to estimate the relevant electric wind-
related escape rates at Titan [Coates et al., 2012], Mars [Frahm et al., 2010], and Venus [Coates et al., 2015b].
Although an electric potential drop should theoretically occur at any planet or moon with an atmosphere,
without an understanding of the magnitude of the potential, it is not possible to determine the role and rela-
tive importance that the electric wind plays in atmospheric escape.
2. Measuring the Electric Potential of a Planet
The ionosphere of Venus is a rich source of hot photoelectrons [Coates et al., 2008], which were observed
escaping down the plasma tail on practically every orbit of the European Space Agency’s (ESA) Venus
Express at altitudes of up to 2.3 Venus radii (r
)[Coates et al., 2015b]. Photoelectrons are key to both the gen-
eration and measurement of the electric wind. The energy spectra of Cytherean photoelectrons exhibit bright
spectral peaks resulting from the photoionization of atomic oxygen by ultraviolet (He-II 30.4nm) photons
including two peaks at 22.3 eV and 23.7 eV, resolved by our instrument as a single merged peak at 23 eV
[Coates et al., 2008, 2011], in addition to a third peak at 27.2 eV. The energy of photopeaks is dictated by atomic
physics. Therefore, any observed shift in their energy can be used to determine the presence, polarity, and mag-
nitude of an external electric potential drop between the ionosphere where the photoelectrons are generated
and the detector onboard the spacecraft [Coates et al., 1985, 2015a; Collinson et al., 2015].This potential drop has
two components: one associated with the electric wind and the other due to spacecraft charging.
To measure the strength of the electric wind using the ESA Venus Express, we therefore require the following:
(1) The spacecraft must be in the right location: on an open magnetic ﬁeld line connected to both the solar
wind and ionosphere. (2) The spacecraft must carry an electron spectrometer capable of resolving any shift in
known spectral features. (3) The spacecraft must carry a magnetometer so that electrons can be binned by
Figure 2. Map of orbit №357 of the ESA Venus Express in Venus Solar Orbital coordinates where Xpoints toward the Sun, Y
is perpendicular to Xand points in the opposite direction to the planet’s velocity vector, and Zcompletes the right-handed
system pointing up out of the plane of the Cytherean ecliptic. The progress of the spacecraft is marked at 5 min intervals in
Greenwich Mean Time. (a) xversus zview from the “side”of the planet, (b) xversus y, the “top down”view over the north
geographic pole. Approximate locations of the bow shock [Slavin et al., 1980] (solid line) and ionopause [Martinecz et al.,
2008] (dashed line) are included for orientation.
Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 3
“pitch angle”(the angle of observa-
tion relative to the magnetic ﬁeld),
so that we examine only ﬁeld-
aligned electrons coming from the
ionospheric source. (4) We require a
measurement of the electrostatic
potential due to spacecraft charging
in order to determine what portion
of the observed potential drop is
due to the electric wind. (5) The
spacecraft potential must remain
constant for the 60 s integration
required to gain sufﬁcient counting
statistics with our instrument.
3. Evidence for an Electric
Potential: Orbit №357
Figure 2 shows a map of orbit №
357, representing our current best
example satisfying our measure-
ment criteria, and Figure 3 shows
Venus Express co-incident observa-
tions from near periapsis. The space-
craft, orbiting in a highly elliptical
24 h polar orbit, was ﬂying on a
midnight-midday pass with periap-
sis in the ionosphere over the north
geographic pole. Shortly after the
Venus Express ﬂew into daylight
there was a brief window (06:15
Greenwich Mean Time (GMT) to
06:17 GMT) where it was possible to
make a direct measurement of
electric potential drop at Venus.
This region of interest was addition-
ally fortuitous in that magnetic condi-
tions (Figure 3b) were calm, resulting
in stable magnetic connectivity to the
same approximate region of the
ionosphere during our electron
observations. Before 06:14 GMT, the
only particles measured by the
Analyser of Space Plasmas and
Energetic Atoms (ASPERA-4) Electron
Spectrometer (ELS) [Barabash et al.,
2007a; Collinson et al., 2009] (Figure 3c) were a hot population of shocked solar wind electrons. At lower
altitudes (>06:19 GMT) this population disappears, indicating that the spacecraft was no longer in magnetic con-
nection to the solar wind, and instead ELS measured solely ionospheric photoelectrons with the He-II photopeak
visible as a line in the spectrogram (Figure 3c). This photoelectron population is highly typical in terms of spectral
features and ﬂux [Cui et al., 2011]. In the region of interest, both populations are observed, and therefore the Venus
Express was in the right place, with the right magnetic connection to both ionosphere and solar wind, to observe
the electric wind.
Figure 3. Magnetic and particle observations from the ESA Venus Express on
13 April 2007. (a) Spacecraft altitude for context, (b) magnetic observations
from the Venus Express Magnetometer, and (c) ASPERA-ELS electron
spectrogram (time versus energy), with the color scale showing the log of
the differential energy ﬂux, integrated over all 16 anodes; spacecraft
potential measured from the spacecraft charging line, providing a known
and stable electrostatic environment during the region of interest; and the
pitch angle of ELS anodes 14 and 6, showing a fortuitously good and stable
pitch angle coverage during the region of interest.
Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 4
Between 06:14 and 06:19 GMT, we
observe a spacecraft charging line
from which we may determine the
electrostatic potential of the space-
craft [Johnstone et al., 1997]. Any
electron with energy below the
spacecraft potential is accelerated,
resulting in a sharp cutoff in the spec-
trum. Therefore, the energy of this
cutoff directly corresponds to the
spacecraft potential. In the region of
interest, this held stable for 2 min,
permitting two 60 s integrations
using the ASPERA-4 ELS. Anode 14
was selected to measure the electric
wind on the grounds that it had an
unimpeded ﬁeld of view, was looking
planetward (see Figure 3d), and had
excellent pitch angle coverage to
observe photoelectrons outﬂowing
from the ionosphere.
Figure 4 shows the results of these
two ASPERA-4 ELS integrations.
Original uncorrected spectra as mea-
sured at Venus are shown in red.
Final spectra corrected for the +6 V
spacecraft potential (using Liouville’s
theorem; see Section S1 in the sup-
porting information) are shown in
blue. Having corrected the spectra
we may now examine the He-II photo-
peak for any shift and ﬁnd that
although it must have been gener-
ated in the ionosphere at 23 eV, it
arrived at the spacecraft at 12.4 eV.
We thus ﬁnd that these ionospheric photoelectrons have been retarded by a signiﬁcant planetary electric
potential of Φ
= 10.6 V.
Although the electrons have lost energy, such an electric potential would impart +10.6 eV to all planetary
ions. This is sufﬁcient to counter the gravitational binding energy of even an O
ion and directly accelerate
it to escape velocity. Although the ASPERA plasma suite carried an Ion Mass Analyzer (IMA) [Barabash
et al., 2007a], taking into account the spacecraft potential, the lowest energy that IMA could see in the region
of interest was 18 eV and the peak of a 10.6 eV ion distribution would be below its reach. Although their
populations could not be fully resolved (full details of ASPERA-IMA observations can be found in
Section S2), clear evidence for cold outﬂowing ionospheric O
ions was observed on orbit №357.
4. A Persistent, Global Feature
In order to see if this potential drop was typical, we searched for more instances of the electron spectral con-
ditions required for measurement: speciﬁcally, the conﬂuence of solar wind, photoelectrons, and stable
spacecraft charging line. Venus Express spends only 5 to 10 min out of each 24 h orbit in a region where it
might even be theoretically possible to observe the electric wind. This necessitated a search through two
earth-years of data, from which we identiﬁed 14 regions of interest, from six orbits, full details of which are
shown in Table 1. Although the electric wind could only be measured occasionally, we found that (a) any time
Figure 4. Corrected and uncorrected electron spectra from the region of
interest from ELS anode 14, showing key spectral features and the ﬁnal
12.4 eV spectral shift resulting from a 10.6 V electric potential drop below
the ESA Venus Express. (a) First ELS integration. (b) Second integration.
Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 5
it was possible to measure an electric potential drop, one was observed, and (b) the magnitude of this drop
was very consistent, with a mean value of Φ
= +9.9 V ± 1.1 V.
Figure 5 shows a map of the location where these observations occurred. We ﬁnd that this force drives escape
from all electric latitudes, whereas at magnetized planets, it is conﬁned to the magnetic poles. For each orbit we
sampled data from the solar wind as near to the region of interest possible (the standard technique when no
second spacecraft is available [Collinson et al., 2014]), ﬁnding no evidence for any bias in upstream conditions.
Given this, andthe consistency in its strength, our collected observations indicate that the electric wind is a per-
sistent, stable, and global phenomenon, ﬂowing tailward from behind the entire terminator.
5. The Importance of the Electric Wind
Contrary to all expectations [Hartle and Grebowsky, 1993], the electric potential drop in the ionosphere of Venus
is at least ﬁve times greater than the upper limits we have in Earth’s topside ionosphere [Coates et al., 1985; Fung
and Hoffman, 1991]. Although parallel electric potentials of comparable magnitude have been observed at
Earth [Kitamura et al., 2012; Winningham and Gurgiolo, 1983; Wilson et al., 1997], all occur at much higher alti-
tudes (3800 km →inﬁnity) [Kitamura et al., 2012], far above the bulk of the terrestrial ionosphere where they
cannot directly act uponionospheric particles. We therefore discover that it is possible for two terrestrial planets
with similar sizes, surface gravities, and ionospheres [Brace et al., 1983] to have signiﬁcantly different polariza-
tion electric ﬁelds. Based on models developed for Earth [Khazanov et al., 1997], we speculate that one possible
contributing factor in explaining its unexpected strength may be a higher proportion of photoelectrons at
Venus due to its closer distance to the Sun and higher photoionization rates. The greater the admixture of
photoelectrons, the greater the electric ﬁeld needs to be to restrain then and satisfy quasineutrality. For a
ﬁrst-order approximation of this ratio at Venus, we used a model derived from the NASA Pioneer Venus
Orbiter Langmuir probe measurements [Theis et al., 1980] to estimate thermal electron densities. When com-
pared to superthermal densities measured by ASPERA-ELS, it corresponds to a ratio between 0.1 and 1%.
This would be a very high fraction for Earth [Khazanov and Liemohn, 1996], and according to Khazanov et al.
, this would translate to a potential drop of approximately 7 V. This is reasonably consistent with our
measurements and supports our hypothesis that the higher admixture of superthermal electrons at Venus is
a contributor to the enhancement of the polarization electric ﬁeld. For full details of how this approximation
was made, see Section S3, and for the source code of the Theis et al.  model (converted into IDL from
the original Fortran), see Section S4.
The ﬁrst measurement of an ionospheric ambipolar potential drop is noteworthy, but the unexpected discov-
ery that its magnitude can be so large has profound implications for our understanding of atmospheric loss
processes for all planets. Whilst other loss mechanisms are present [Dubinin et al., 2011], the newly discovered
Table 1. Collected Venus Express Measurements of the Total Electric Potential Drop
Date of Observation Start of Region of Interest (GMT) Altitude (km) Photopeak Energy (eV) Potential Drop (V)
2007-03-29 06:27:00 550 km 11.5 eV 11.5 V
2007-04-13 06:15:00 629 km 12.4 eV 10.6 V
06:16:00 532 km 12.4 eV 10.6 V
2007-12-05 02:40:00 353 km 12.4 eV 10.6 V
2008-02-04 04:45:00 349 km 13.5 eV 9.5 V
04:46:00 420 km 12.4 eV 10.6 V
04:47:00 514 km 13.5 eV 9.5 V
04:48:00 628 km 13.5 eV 9.5 V
04:49:00 763 km 14.6 eV 8.4 V
2008-03-31 04:05:30 450 km 15.8 eV 7.2 V
2008-04-08 03:56:00 335 km 12.4 eV 10.6 V
03:57:00 382 km 13.5 eV 9.5 V
03:58:00 453 km 12.4 eV 10.6 V
03:59:00 545 km 13.5 eV 9.5 V
Mean: 493 km 13.1 eV 9.9 ± 1.1 V
Times denote the beginning of a 60 s ASPERA-4 ELS integration; altitude denotes the minimum altitude of the space-
craft during the measurement. The energy is that of the He-II generated photoelectron peak, corrected for spacecraft
potential, and the potential is that of the total integrated potential drop below the spacecraft.
Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 6
electric potential is quite sufﬁcient all
by itself for any ion lighter than
18 amu (which includes O
water group ions) to overcome grav-
ity and be directly accelerated to
escape velocities. Thus we ﬁnd that
it is possible for planets to lose heavy
ions to space entirely through elec-
trical forces in their ionospheres
and that ambipolar ﬁelds can play
an even more dominant role in pla-
netary atmospheric escape than pre-
viously considered. We also ﬁnd that,
contrary to our current understand-
ing, Venus would still lose heavy ions
such as O
to space regardless of any
atmospheric stripping by the solar
wind. Additionally, however, the
electric wind will further enhance
these other escape processes (such
as pickup and acceleration by the
motional electric ﬁeld of the solar
wind) by transporting ions from the
bulk of the ionosphere (150 km
[Brace et al., 1983]) to the ionopause
(300+ km [Martinecz et al., 2008]) and
above, where these solar wind dri-
ven mechanisms can take effect.
Thus, given their importance,
ionospheric ambipolar ﬁelds must
therefore always be included in any
study of the atmospheric evolution
of Venus, Earth, Mars, Titan, or
indeed any planet in any star system.
The discovery of a powerful electric wind at Venus, an Earth-like terrestrial planet, also has important conse-
quences for the study of exoplanets by missions such as Kepler. If, for example, the electric potential drop in
Earth’s (or another Earth-like planet’s) ionosphere was a Venus-like +12 V, then a similar direct loss of heavy
ions would likely occur, regardless of the presence or absence of a planetary dynamo magnetic ﬁeld, leading
to higher rates of loss. Signiﬁcant changes to planetary escape rates could impact the ability of a planet to
retain an atmosphere [Zahnle and Catling, 2013; Cohen and Glocer, 2012] and maintain liquid water oceans
and increase the likelihood that a planet loses its oceans during the moist greenhouse phase [Chasseﬁère,
1997]. Such a strong escape mechanism could also impact the redox evolution of a planetary surface
[Caitling et al., 2001]. Given that we believe Venus’stronger polarization ﬁeld may arise from its closer proxi-
mity to the Sun, and that most known exoplanets have been found relatively close to their stars (since these
are easier to detect), the possibility of a strong electric wind must be considered when assessing planetary
evolution or the potential for habitability on exoplanets.
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Geophysical Research Letters 10.1002/2016GL068327
COLLINSON ET AL. VENUS HAS POTENTIAL 7
The ASPERA-4 team are grateful to
NASA for allowing the ELS ﬂight spare
(constructed under contract NASW-
00003) to be ﬂown on the Venus Express
as part of the ASPERA-4 suite. This work
was supported by NASA Solar System
Workings Program grant NNX15A176G.
We thank Rhiannon Shelagh Lewis for
her assistance in initiating this investi-
gation. Venus Express data are available
from the ESA Planetary Science Archive.
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