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# The electric wind of Venus: A global and persistent “polar wind”-like ambipolar electric field sufficient for the direct escape of heavy ionospheric ions: Venus Has Potential

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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 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.
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The electric wind of Venus: A global and persistent
polar wind-like ambipolar electric eld sufcient
for the direct escape of heavy ionospheric ions
Glyn A. Collinson
1,2,3
, Rudy A. Frahm
4
, Alex Glocer
1
, Andrew J. Coates
2
, Joseph M. Grebowsky
1
,
Stas Barabash
5
, Shawn D. Domagal-Goldman
1
, Andrei Fedorov
6,7
, Yoshifumi Futaana
5
, Lin K. Gilbert
2
,
George Khazanov
1
, Tom A. Nordheim
2,8
, David Mitchell
9
, Thomas E. Moore
1
, William K. Peterson
10
,
John D. Winningham
4
, and Tielong L. Zhang
11
1
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA,
2
Mullard Space Science Laboratory, University College
London, Surrey, UK,
3
Institute for Astrophysics and Computational Sciences, Catholic University of America, Washington,
District of Columbia, USA,
4
Southwest Research Institute, San Antonio, Texas, USA,
5
Institutet för rymdfysik, Swedish
Institute of Space Physics, Kiruna, Sweden,
6
LInstitut de Recherche en Astrophysique et Planétologie, CNRS, Toulouse,
France,
7
University Paul Sabatier, Toulouse, France,
8
Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
California, USA,
9
Space Sciences Laboratory, University of California, Berkeley, California, USA,
10
Laboratory for Atmospheric
and Space Physics, Boulder, Colorado, USA,
11
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 ambipolarelectric eld that helps ions overcome gravity. We report the discovery
and rst quantitative extraterrestrial measurements of such a eld at the planet Venus. Unexpectedly, despite
comparable gravity, we show the eld to be ve times stronger than in Earths 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 nd that it is possible for planets to lose heavy ions to
space entirely through electric forces in their ionospheres and such an electric windmust 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 specically 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 ve 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
+
and
O
+
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 eld [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
+
,D
+
) ion escape [Hartle and Grebowsky, 1990, 1993,
1995; Barabash et al., 2007b] is that of a long hypothesized ambipolarelectric eld (also referred to as a
polarizationelectric eld 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
PUBLICATION
S
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL068327
Key Points:
We report the discovery and rst
quantitative extraterrestrial measure-
ments of an ionospheric ambipolar
electric eld 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
their ionospheres
Supporting Information:
Supporting Information S1
Correspondence to:
G. A. Collinson,
glyn.a.collinson@nasa.gov
Citation:
Collinson, G. A., et al. (2016), The electric
wind of Venus: A global and persistent
polar wind-like ambipolar electric eld
sufcient for the direct escape of heavy
ionospheric ions, Geophys. Res. Lett.,43,
doi:10.1002/2016GL068327.
Received 18 FEB 2016
Accepted 4 APR 2016
©2016. American Geophysical Union.
the pull of gravity guided along the draped magnetic eld. However, the Coulomb force restricts their motion
away from the ions. As the electrons pull away, an ambipolar electric eld 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 eld must be to restrain them. Superthermal(170 eV) photoelectrons,
generated by photoionization of the atmosphere, play an especially potent role in generating this eld
[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 eld 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 difcult to measure and thus has remained theoretical. This potential drop is critical to the
formation of Earthspolar wind,which ows outward along open magnetic elds above our polar caps
[Hanks and Holzer, 1968]. However, given that that the scientictermpolar windalso encompasses other
acceleration mechanisms, and that at an unmagnetized planet it would not be conned to the poles, we adopt
the nomenclature of electric windas a shorthand to refer to this specic mechanism: the ambipolar electric
eld, electric potential drop, and outow of decelerated electrons and accelerated ions.
Although vital to our understanding of the evolution of our atmosphere, this eld is extremely challenging to
measure given its small magnitude. The few attempts made to measure it in Earths 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 elds, 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 innity [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 (specically 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 elds 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 eld from observations of escaping H
+
and D
+
, 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 eld 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 likeprocess may be occurring [Tsang et al., 2015; Coates et al., 2011, 2015b]. Similar out-
ows of photoelectrons and associated outowing 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-
ow. In addition, the photoelectron and escaping ion uxes 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 Agencys (ESA) Venus
Express at altitudes of up to 2.3 Venus radii (r
v
)[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 eld 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 planets 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 sideof the planet, (b) xversus y, the top downview 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 eld),
so that we examine only eld-
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 sufcient 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 ying on a
midnight-midday pass with periap-
sis in the ionosphere over the north
geographic pole. Shortly after the
Venus Express ew 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 ux [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 ux, 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 eld of view, was looking
planetward (see Figure 3d), and had
excellent pitch angle coverage to
observe photoelectrons outowing
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 Liouvilles
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 nd 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 nd that these ionospheric photoelectrons have been retarded by a signicant planetary electric
potential of Φ
Venus
= 10.6 V.
Although the electrons have lost energy, such an electric potential would impart +10.6 eV to all planetary
ions. This is sufcient 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 outowing ionospheric O
+
and H
+
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: specically, the conuence 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 identied 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 nal
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
Venus
= +9.9 V ± 1.1 V.
Figure 5 shows a map of the location where these observations occurred. We nd that this force drives escape
from all electric latitudes, whereas at magnetized planets, it is conned 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]), nding 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, owing 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 ve times greater than the upper limits we have in Earths 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 innity) [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 signicantly different polariza-
tion electric elds. 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 eld needs to be to restrain then and satisfy quasineutrality. For a
rst-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.
[1997], 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 eld. For full details of how this approximation
was made, see Section S3, and for the source code of the Theis et al. [1980] model (converted into IDL from
the original Fortran), see Section S4.
The rst 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
a
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
a
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 sufcient all
by itself for any ion lighter than
18 amu (which includes O
+
and all
water group ions) to overcome grav-
ity and be directly accelerated to
escape velocities. Thus we nd that
it is possible for planets to lose heavy
ions to space entirely through elec-
trical forces in their ionospheres
and that ambipolar elds can play
an even more dominant role in pla-
netary atmospheric escape than pre-
viously considered. We also nd 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 eld 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 elds 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
Earths (or another Earth-like planets) 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 eld, leading
to higher rates of loss. Signicant 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 [Chasseère,
1997]. Such a strong escape mechanism could also impact the redox evolution of a planetary surface
[Caitling et al., 2001]. Given that we believe Venusstronger polarization eld 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
Acknowledgments
The ASPERA-4 team are grateful to
NASA for allowing the ELS ight spare
(constructed under contract NASW-
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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
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... Current theory and simulations predict that it could be as weak as ≈ 0.4 V (calculated from our Polar Wind Outflow model, Glocer et al. 2007Glocer et al. , 2009Glocer et al. , 2012Glocer et al. , 2017 across the exobase transition region (< 780 km). The first successful direct measurement of an ionospheric ambipolar potential drop was at the planet Venus (Collinson et al. 2016). Surprisingly, Venus' potential drop was found to be +10 V. ...
... Once these spectra have been corrected for the effects of e − scattering and spacecraft potential, the electric potential drop below the spacecraft may be determined from the resulting shift in energy of these known spectral features. Figure 4A shows an example of this technique being used to successfully measure the (10 V) ambipolar potential drop in the ionosphere of Venus (Collinson et al. 2016). These data were collected by the Venus Express ASPERA-4 Electron Spectrometer (ELS) ) at an altitude of approximately 600 km above Venus. ...
... V. Figure 4B shows measurements of He-II photoelectron peaks at Earth (Doering et al. 1973;Su et al. 1998) by NASA's Atmospheric Explorer E spacecraft. At Earth, the predicted (Collinson et al. 2016); Panel B.) Endurance will measure Earth's photoelectrons at higher resolution than Atmospheric Explorer E (Doering et al. 1976;Su et al. 1998) total potential drop is only 0.4 V (at 900 km altitude). Thus, to measure Earth's ambipolar potential drop ( Earth ), Endurance needs to resolve these photopeaks with a minimum resolution of at least 0.4 eV at 22 eV, or E/E = 1.8%. ...
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NASA’s Endurance sounding rocket (yard No. 47.001) will launch from Ny Ålesund, Svalbard in May 2022 on a solid fueled Oriole III-A launch vehicle. Its ∼19\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim19$\end{document} minute flight will carry it to an altitude of ∼780km\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sim780~\text{km}$\end{document} above Earth’s sunlit polar cap. Its objective is to make the first measurement of the weak “ambipolar” electric field generated by Earth’s ionosphere. This field is thought to play a critical role in the upwelling and escape of ionospheric ions, and thus potentially in the evolution of Earth’s atmosphere. The results will enable us to determine the importance to ion escape of this previously unmeasured fundamental property of our planet, which will aid in a better understanding of what makes Earth habitable. Endurance will carry six science instruments (with 16 sensors) that will measure the total electrical potential drop below the spacecraft, and the physical parameters required to understand the physics of what generates the ambipolar field. The mission will be supported by simultaneous observations of solar and geomagnetic activity.
... However, they acknowledged that this could be because they simply were no longer measurable above this altitude. Additionally, the orbital Brace et al. (1987), the first conceptual sketch of the wake of Venus from collected PVO observations; (b) A revised picture of the Venusian magnetotail from the Venus Express era, based on Futaana et al. (2017); Collinson et al. (2014); Collinson, Frahm, et al. (2016); Collinson, Sibeck, et al. (2017) (c) Adapted from Brecht and Ledvina (2021, Figure 4): A tail ray structure consistent with Pioneer Venus Orbiter measurements appears in their hybrid simulations. Over-plotted are the boundaries of the tail ray simulated by Brecht and Ledvina (2021) (pink) and approximate average locations of the bow shock (solid white line, as per Slavin et al., 1980) and magnetopause (dashed white line, as per Martinecz et al., 2008); (d) A new, unified concept of the structure of the magnetotail and induced magnetosphere of Venus following Parker Solar Probe VGA 4 flyby. ...
... While Venus Express made substantial advances in our understanding of the space environment around Venus (Futaana et al., 2017, Figure 1b), direct comparisons to Pioneer Venus Orbiter plasma observations were found to be challenging (Collinson et al., 2014). This is because, ironically, without a Langmuir Probe, Venus Express was blind to the cold thermal electrons measured by the Pioneer Venus OETP (Collinson, Frahm, et al., 2016;Collinson et al., 2019). Without a measurement of total plasma density it is very difficult to identify with Venus Express phenomena reported by Pioneer Venus Orbiter. ...
... Without a measurement of total plasma density it is very difficult to identify with Venus Express phenomena reported by Pioneer Venus Orbiter. While there have been numerous studies of ionospheric outflow in the Venusian tail (Dubinin et al., 2012(Dubinin et al., , 2013Dubinin & Fraenz, 2015;Coates et al., 2015;Collinson, Frahm, et al., 2016) to date no PVO-like tail rays have been explicitly identified in the Venus Express dataset. ...
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Plain Language Summary Like a comet, the planet Venus has a tail made of plasma. This tail was first discovered and explored by NASA's 1979–1991 Pioneer Venus Orbiter (PVO) mission. The next mission capable of measuring plasmas at Venus was the 2006–2014 ESA Venus Express (VEX) mission. However, by the time of VEX, the technology of space plasma analyzers had advanced so much that even if PVO and VEX flew through the same phenomenon, each would present data very differently. On February 20, 2021 NASA's Parker Solar Probe made its fourth close flyby of Venus, spending 10 min in the wake on the nightside. For two of these minutes, Parker encountered a plume of cold plasma escaping from Venus with all the properties of a Venusian tail ray. These measurements confirm the predictions of recent simulations which predict that tail rays can extend away from the nightside of Venus to an altitude equivalent to the radius of the planet. Furthermore, some of the modern instrumentation aboard Parker are similar to that flown aboard Venus Express enabling us to identify tail rays in that dataset as well, allowing us to directly compare and combine VEX and PVO datasets.
... Many studies have examined the role superthermal electrons play in setting up ambipolar electric fields at Earth, Mars, and Venus and the impact on ion loss to space (e.g. Khazanov et al., 1997;Collinson et al., 2015Collinson et al., , 2016Glocer et al., 2017;Xu et al., 2018a;Akbari et al., 2019). ...
Thesis
The study of the transport of superthermal electrons in planetary space environments is important because they are able to efficiently heat and ionize the upper atmosphere, a contributing factor in atmospheric escape and ionospheric dynamics. The hybrid magnetosphere of Mars, with characteristics of both induced and intrinsic magnetospheres, offers a unique and complicated space environment to study space physics and electron transport. The magnetic topology of Mars is a mix of interplanetary magnetic fields, localized crustal fields connected to the planet, and reconnected crustal fields that allow access of solar wind particles to the lower atmosphere. This system is highly dynamic, both spatially and temporally, as the crustal fields rotate with the planet, in and out of interaction with the solar wind. Electron pitch angle distributions, along with energy spectra, allow us to infer the magnetic topology, which is critical for the interpretation of spacecraft measurements. Previous studies have suggested that our understanding of electron transport on the crustal magnetic fields of Mars is incomplete and that our assumptions of what pitch angle distributions we expect on closed fields are incorrect. First, using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, I have shown that the pitch angle distributions of high energy (100-500 eV) photoelectrons are not explainable by collisions and adiabatic invariants alone, and that the plasma dynamics on the crustal fields are more complex than originally thought. I hypothesize that whistler-mode waves are preferentially energizing trapped electrons to these high energies. Second, I used MAVEN data to study the low frequency (0.03-16 Hz) magnetic wave activity in the Mars' magnetosphere and ionosphere. I found that the magnetic wave power was highest near the boundaries of the planetary-sourced plasma, i.e. the magnetosheath and lower ionosphere, and that the solar wind regulates the injection of waves from the magnetosheath into the hybrid magnetosphere. I also showed that magnetic wave power is strongest over the closed crustal field regions. However, whistler-mode waves typically have higher frequencies than those measurable with MAVEN. Therefore, third, I used quasi-linear theory to show that the space environment of Mars is conducive for whistler-mode waves to interact with photoelectrons, and that the timescales of interaction are faster than other relevant processes (i.e., collisions). Fourth, I built a new model which solves the bounce-averaged quasi-linear diffusion equation for the steady-state velocity distribution of superthermal electrons on a Mars' crustal field in order to quantify the effect of whistler-mode waves. The initial results agree quite well with the statistical pitch angle distributions observed by MAVEN, reconciling both previous data-model discrepancies. In this dissertation, I have shown that the observed pitch angle distributions of photoelectrons on closed crustal fields at Mars indicate ubiquitous wave-particle interactions. I have also demonstrated that whistler-mode waves can be responsible for the change in the photoelectron velocity space distribution away from what collisions alone would produce. The crustal fields of Mars, smaller scale analogs to the Earth's magnetic field, offer a new system to study wave-particle interactions. Using data, theory, and numerical modeling, I am working toward a more complete picture of electron transport at Mars.
... The spacecraft potential is assumed to be insignificant in comparison with the lowest measured ions. Typically, the spacecraft potential varies between ∼±5 eV, as found by both modeling (Garrett, 1981) and measurements (Collinson et al., 2016). Therefore, the potential is assumed to not greatly affect the ion measurements (e.g. ...
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Plain Language Summary Today, there is barely any water on Venus, but presumably a large amount existed in its earlier history. Therefore, the water must have been lost over the course of the Venusian history. An important process for removal of water is escape to space, induced by the interaction between the Venusian atmosphere and the solar wind (a fast stream of particles ejected from the Sun). In this study, we investigated the connection between the energy available in the upstream solar wind and the energy leaving Venus in the form of oxygen ion escape. By characterizing this relation, we investigate how the Venusian atmosphere reacts to changes in the upstream solar wind and how well it protects itself from atmospheric loss caused by the solar wind. We find that the energy transfer decreases as the available upstream energy increases, a trend that is very similar to that found at Mars. However, the Venusian atmosphere seems to absorb less energy from the solar wind than Mars. This indicates that the Venusian induced magnetosphere efficiently screens the atmosphere from the solar wind. This is important for the understanding of the effect of the solar wind on the Venusian atmospheric evolution.
... Wordsworth & Pierrehumbert, 2014), but the present Venus atmosphere does not show this tracer of ocean loss and potential false positive for an oxygen biosignature. Hydration and oxidation of surface rocks (e.g., Matsui & Abe, 1986) and top-of-the-atmosphere loss processes (Chassefière, 1997;Collinson et al., 2016) may have removed any O 2 that was produced by early ocean loss, although it is uncertain how the present atmospheric loss processes would operate for a different (younger) Venus atmosphere, potentially in the presence of a stronger magnetic field (Curry et al., 2015;Luhmann et al., 2008;Persson et al., 2020). Thus, Venus is an ideal laboratory to test hypotheses for abiotic oxygen production and loss processes. ...
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Over the past several decades, thousands of planets have been discovered outside our Solar System. These planets exhibit enormous diversity, and their large numbers provide a statistical opportunity to place our Solar System within the broader context of planetary structure, atmospheres, architectures, formation, and evolution. Meanwhile, the field of exoplanetary science is rapidly forging onward toward a goal of atmospheric characterization, inferring surface conditions and interiors, and assessing the potential for habitability. However, the interpretation of exoplanet data requires the development and validation of exoplanet models that depend on in situ data that, in the foreseeable future, are only obtainable from our Solar System. Thus, planetary and exoplanetary science would both greatly benefit from a symbiotic relationship with a two-way flow of information. Here, we describe the critical lessons and outstanding questions from planetary science, the study of which are essential for addressing fundamental aspects for a variety of exoplanetary topics. We outline these lessons and questions for the major categories of Solar System bodies, including the terrestrial planets, the giant planets, moons, and minor bodies. We provide a discussion of how many of these planetary science issues may be translated into exoplanet observables that will yield critical insight into current and future exoplanet discoveries.
... Others involve solar wind interactions with the planetary atmosphere and/or ionosphere, such as the removal of pick up ions generated by photoionization or charge exchange, or the formation of detached blobs of ionospheric plasma generated by either the Kelvin-Helmholtz instability at the ionopause or magnetic reconnection with ionospheric or remnant crustal magnetic fields. Even in the absence of solar wind stripping, ambipolar electric fields may cause a planet to lose heavy ions (Collinson et al. , 2016. ...
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This study provides the first characterization of magnetic topology (i.e., the magnetic connectivity to the collisional ionosphere) at Venus, which might give new insights into the Venusian space environment on topics such as the penetration of the interplanetary magnetic field (IMF) into the ionosphere, planetary ion outflow and inflow, and auroral emission. Magnetic topology is inferred from the electron and magnetic field measurements from Venus Express. We demonstrate through a few case studies that various types of magnetic topologies exist at Venus, including typical draped IMF, open magnetic fields connected to the nightside atmosphere or the dayside ionosphere, and unexpected cross‐terminator closed field lines. We also provide a detailed characterization of an ionospheric hole event, where we find an open topology and a field‐aligned potential of ∼[−10,−20] V with respect to the collisional ionosphere, which has important implications for its formation mechanism.
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Our understanding of ancient Venus and its evolution to the present day could be substantially advanced through future Space Physics investigations from orbit. We outline three high-priority strawman investigations, each possible for relative thrift with existing (or near-future) technology. 1.) To understand the physical processes that facilitate Venusian atmospheric escape to space, so that we may extrapolate backward through time; 2.) To explore ancient Venus through the measurement of the escape rates of key species such as Deuterium, Noble elements, and Nitrogen; 3.) To understand and quantify how energy and momentum are transferred from the solar wind, through the ionosphere, and into the atmosphere, so that we may reveal its impact on the dynamics of the atmosphere.
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The accumulation of detailed ion flux measurements from long-lived spacecraft orbiting the solar system’s terrestrial planets have enabled recent studies to estimate the rate of solar wind driven atmospheric ion escape from Venus, Earth, and Mars, as well as the influence of solar wind and solar extreme ultraviolet (EUV) ionizing radiation on the atmospheric ion escape rates. Here, we introduce the basic forces and processes of ion escape, review the recent studies of ion escape rates, and provide a general framework for understanding ion escape as a process that can be limited by potential bottlenecks, such as ion supply, solar wind energy transfer, and transport efficiency, effectively determining the state of the ion escape process at each planet. We find that ion escape from Venus and Earth is energy-limited, though exhibit different dependencies on solar wind and EUV, revealing the influence of Earth’s intrinsic magnetic field. In contrast, ion escape from Mars is in a supply-limited state, mainly due to its low gravity, and has likely contributed relatively little to the total loss of the early Martian atmosphere, in comparison to neutral escape processes. Contrary to the current paradigm, the comparisons between the solar system planets show that an intrinsic magnetic dipole field is not required to prevent stellar wind-driven escape of planetary atmospheres, and the presence of one may instead increase the rate of ion escape. Anticipating the atmospheres of the exoplanets that will begin to be characterized over the coming decade, and the need to explain their evolution, we argue that a modern, nuanced, and evidence-based view of the magnetic field’s role in atmospheric escape is required.
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An important mechanism in the generation of polar wind outflow is the ambipolar electric potential which assists ions in overcoming gravity and is a key mechanism for Terrestrial ionospheric escape. At Mars, open field lines are not confined to the poles, and outflow of ionospheric electrons is observed far into the tail. It has thus been hypothesized that a similar electric potential may be present at Mars, contributing to global ionospheric loss. However, no direct measurements of this potential have been made. In this pilot study, we examine photoelectron spectra measured by the Solar Wind Electron Analyzer instrument on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Mars Scout to put an initial upper bound on the total potential drop in the ionosphere of Mars of Φ♂≼⊥2V , with the possibility of a further ≼4.5 V potential drop above this in the magnetotail. If the total potential drop was close to the upper limit, then strong outflows of major ionospheric species (H+, O+, and O+2) would be expected. However, if most of the potential drop is confined below the spacecraft, as expected by current theory, then such a potential would not be sufficient on its own to accelerate O+2 to escape velocities, but would be sufficient for lighter ions. However, any potential would contribute to atmospheric loss through the enhancement of Jeans escape.
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Neutral particles dominate regions of the Saturn magnetosphere and locations near several of Saturn's moons. Sunlight ionizes neutrals, producing photoelectrons with characteristic energy spectra. The Cassini plasma spectrometer electron spectrometer has detected photoelectrons throughout these regions, where photoelectrons may be used as tracers of magnetic field morphology. They also enhance plasma escape by setting up an ambipolar electric field, since the relatively energetic electrons move easily along the magnetic field. A similar mechanism is seen in the Earth's polar wind and at Mars and Venus. Here we present a new analysis of Titan photoelectron data, comparing spectra measured in the sunlit ionosphere at ~1.4 Titan radii (RT) and at up to 6.8 RT away. This results in an upper limit on the potential of 2.95 V along magnetic field lines associated with Titan at up to 6.8 RT, which is comparable to some similar estimates for photoelectrons seen in Earth's magnetosphere.
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The presence of photoelectrons in ionospheres, including that of unmagnetised Venus, can be inferred from their characteristic spectral peaks in the electron energy spectrum. The electrons within the peaks are created by the photoionisation of neutrals in the upper atmosphere by the solar HeII 30.4 nm line. Here, we present some case studies of photoelectron spectra observed by the ASPERA-4 instrument aboard Venus Express with corresponding ion data. In the first case study, we observe photoelectron peaks in the sunlit ionosphere, indicating relatively local production. In the second case study, we observe broadened peaks in the sunlit ionosphere near the terminator, which indicate scattering processes between a more remote production region and the observation point. In the third case study, we present the first observation of ionospheric photoelectrons in the induced magnetotail of Venus, which we suggest is due to the spacecraft being located at that time on a magnetic field line connected to the dayside ionosphere at lower altitudes. Simultaneously, low energy ions are observed moving away from Venus. In common with observations at Mars and at Titan, these imply a possible role for the relatively energetic electrons in producing an ambipolar electric field which enhances ion escape.
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The dayside of the Venus ionosphere at the top of the planet's thick atmosphere is sustained by photoionization. The consequent photoelectrons may be identified by specific peaks in the energy spectrum at 20–30 eV which are mainly due to atomic oxygen photoionization. The ASPERA-4 electron spectrometer has an energy resolution designed to identify the photoelectron production features. Photoelectrons are seen not only in their production region, the sunlit ionosphere, but also at more distant locations on the nightside of the Venus environment. Here, we present a summary of the work to date on observations of photoelectrons at Venus, and their comparison with similar processes at Titan and Mars. We expand further by presenting new examples of the distant photoelectrons measured at Venus in the dark tail and further away from Venus than seen before. The photoelectron and simultaneous ion data are then used to determine the ion escape rate from Venus for one of these intervals. We compare the observed escape rates with other rates measured at Venus, and at other planets, moons and comets. We find that the escape rates are grouped by object type when plotted against body radius.
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Ionospheric holes are a Cytherian night-side phenomena discovered by the NASA Pioneer Venus Orbiter, featuring localized plasma depletions driven by prominent and unexplained enhancements in the draped Interplanetary Magnetic Field (IMF). Observed only during solar maximum, the phenomenon remains unexplained, despite their frequent observation during the first three years of the mission and more than thirty years having elapsed since their first description in the literature. We present new observations by the ESA Venus Express showing that ionospheric holes can extend much further into the tail than previously anticipated (1.2 to 2.4 planetary radii), may be observed throughout the solar cycle, and over a wide range of solar wind conditions. We find that ionospheric holes are a manifestation of a deeper underlying phenomenon: Tubes of enhanced draped Interplanetary Magnetic Field that emerge in pairs from below the ionosphere and stretching far down the tail. We speculate on two possible explanations for the magnetic fields underlying the phenomena: magnetic pile-up due to stagnation of ionospheric flow, and internal draping around a metallic core.
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An electron analyzer to measure the three-dimensional velocity distribution of electrons on the multispacecraft Cluster mission is described, with emphasis on the steps taken to meet the special requirements imposed by the novel character of the mission. The technical description includes electron optics, readout systems, onboard data processing equipment, calibration, and flight operations.
Article
Characteristics of photoelectron flows and presence of a field-aligned potential drop on the open magnetic field lines in the polar cap are systematically investigated using the data obtained by the FAST satellite during geomagnetically quiet periods in July 2002. We found high occurrence frequencies of the potential drop larger than ˜10 V, reaching ˜90% (small field-aligned current (FAC) case) and ˜83% (all data). A typical magnitude of the potential drop above ˜3800 km altitude is ˜20 V. This value is significantly larger than the potential drop below ˜3800 km altitude (probably ˜1-3 V), although the typical potential drop is smaller by a factor of ˜2-3 in comparison to the modeling results that suggested presence of a field-aligned potential jump at several earth radii. The net escaping electron number flux negatively correlates with the upward electron number flux and with the magnitude of the potential drop. This relation is contrary to expectation from photoelectron-driven polar wind models that an increase in the photoelectrons drives the larger polar wind flux, since the net escaping electron number flux balances the flux of polar wind ions under zero net FAC conditions. An increase in the upward backscatter of reflected electrons with an increasing potential drop may explain the negative correlations. A potential drop at high altitudes would provide a polar wind system regulated by a negative feedback, and the most appropriate balance for polar wind ions would be achieved near the median of the reflection potential.