PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2015GL065280
Special Section:
First Results from the MAVEN
Mission to Mars
Key Points:
• First in situ measurements of the
electron temperature at Mars
• Electron density and temperature
profiles at Mars
Correspondence to:
R. E. Ergun,
[email protected]
Citation:
Ergun, R. E., M. W. Morooka, L. A. Andersson,
C. M. Fowler, G. T. Delory, D. J. Andrews,
A. I. Eriksson, T. McEnulty, and B. M.
Jakosky (2015), Dayside electron temperature and density profiles at Mars: First
results from the MAVEN Langmuir probe
and waves instrument, Geophys. Res. Lett.,
42, doi:10.1002/2015GL065280.
Dayside electron temperature and density profiles
at Mars: First results from the MAVEN Langmuir
probe and waves instrument
R. E. Ergun1,2, M. W. Morooka2, L. A. Andersson2, C. M. Fowler1,2, G. T. Delory3, D. J. Andrews4,
A. I. Eriksson4, T. McEnulty2, and B. M. Jakosky2
1
Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, Boulder, Colorado, USA, 2Laboratory
of Atmospheric and Space Sciences, University of Colorado Boulder, Boulder, Colorado, USA, 3Space Sciences Laboratory,
University of California, Berkeley, California, USA, 4Swedish Institute of Space Physics, Uppsala, Sweden
Abstract We present Mars’ electron temperature (Te) and density (ne) altitude profiles derived from the
MAVEN (Mars Atmosphere and Volatile EvolutioN) mission deep dip orbits in April 2015, as measured
by the Langmuir probe instrument. These orbits had periapsides below 130 km in altitude at low solar
zenith angles. The periapsides were above the peak in ne during this period. Using a Chapman function fit,
we find that scale height and projected altitude of the ne peak are consistent with models and previous
measurements. The peak electron density is slightly higher than earlier works. For the first time, we
present in situ measurements of Te altitude profiles in Mars’ dayside in the altitude range from ~130 km to
500 km and provide a functional fit. Importantly, Te rises rapidly with altitude from ~180 km to ~300 km.
These results and functional fit are important for modeling Mars’ ionosphere and understanding
atmospheric escape.
1. Introduction
Received 8 JUL 2015
Accepted 1 AUG 2015
The primary science goal of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission is to understand
the loss of Mars’ atmosphere and water, in particular the loss of O. There is substantial evidence of loss to
space in the ratios of isotopic abundances [Pepin, 1994]. Mars’ gravitational acceleration is such that Jeans
escape can explain the loss of H, but O is bound by ~2 eV (depending on altitude) and requires an
additional energy source for escape. The primary escape mechanism of O is believed to be from dissociative
recombination of O2+ [e.g., McElroy, 1972; Rohrbaugh et al., 1979; Barth, 1985; Nagy and Cravens, 1988; Zhang
et al., 1993; Fox and Hac, 2009]:
Oþ
2 þe →OþO
This process can impart sufficient kinetic energy into the O products for escape, up to 6.99 eV, depending on
excitation state. The reaction rate is proportional to the product of the O2+ density with ne and depends on
Te0.7 [Alge et al., 1983, and references therein]. Thus, Te and ne can strongly influence atmospheric escape
through O2+ dissociative recombination.
The exobase is typically defined as the altitude above which the vertical column density is equal to the
inverse of the collisional cross section, which is tantamount to the boundary where collisions no longer
dominate photochemistry. This region is particularly important for the O escape process via dissociative
recombination of O2+ [e.g., Fox and Hac, 2009]. If the dissociative recombination reaction occurs above
the exobase, at least one of the O atoms is likely to escape, since it is unlikely to suffer a collision.
However, ne and the density of O2+ rapidly decrease with increasing altitude, so the reaction rate is low
at the exobase and rapidly falls with altitude. Below the exobase, the reaction rate dramatically increases
with increased electron and O2+ densities, but the escape efficiency falls significantly due to collisions,
which are dominated by the neutral atmosphere. As a result, the region within a several scale heights
(roughly 50 km) of the exobase (roughly 180 km for O) is critical to O escape. The ne and Te profile in this
region is essential for determining O escape via dissociative recombination of O2+.
©2015. American Geophysical Union.
All Rights Reserved.
ERGUN ET AL.
Measurements of ne have been made of the Martian ionosphere over range of solar zenith angles and
altitudes using a variety of techniques. Near the peak in ne, the Martian ionosphere is reasonably well
DAYSIDE ELECTRON TEMPERATURES AT MARS
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Geophysical Research Letters
10.1002/2015GL065280
approximated by a Chapman layer with peak densities in the range ~1.5–2 × 105 cm3 at altitudes of ~125–130 km.
Scale heights are less well constrained with values ranging from ~5 to ~25 km [Morgan et al., 2008; Němec
et al., 2011; Fallows et al., 2015a, 2015b].
The altitude profile of Te at Mars, which is critical to modeling efforts [e.g., Fox and Yeager, 2006; Withers et al.,
2014; Cui et al., 2015; Fallows et al., 2015a, 2015b], is not well established. The only previous in situ measurements
of Te and ion temperatures were from the Viking landers [Hanson et al., 1977; Hanson and Mantas, 1988]. Indirect
estimates of Te, for example, from the vertical ne gradient, vary significantly [e.g., Cui et al., 2015], which makes in
situ measurements of Te valuable. A more general description of Mar’s ionosphere and discussion on the importance of Te and ne altitude profiles are provided by Nagy et al. [2004]; Witasse et al. [2008], and Withers et al. [2012].
In this article, we present the measurements of ne and Te during the MAVEN deep dip campaign [Jakosky
et al., 2015] from 15 April 2015 to 22 April 2015 from the Langmuir probe and waves instrument (LPW).
These data summarize 28 orbits in which MAVEN probed altitudes just below 130 km in Mars’ dayside ionosphere, which is a critical region for modeling Mars’ atmospheric photochemistry. Our data include altitudes
up to 500 km and contains the region critical for O2+ dissociative recombination. Several of the orbits showed
unusual density structure, which is the focus of several future articles.
2. Instrument
The MAVEN LPW instrument is described by Andersson et al. [2015], so our description here is brief. The LPW
has two independent cylindrical Langmuir Probes (LPs), each sensor being 0.0625 cm in diameter and 40 cm
long. The sensors are mounted at the end of 7 m booms resulting in excellent isolation from spacecraft
disturbances of the electron distribution and limited photoelectron fluxes emitted by the spacecraft.
During the deep dip campaign, the Langmuir probes made a 128-step sweep, stepping the voltage from
+5.4 V to 5.4 V with respect to an estimated plasma potential [see Andersson et al., 2015 for details] while
measuring the current with ~1 nA accuracy, resulting in a current-voltage (I/V) characteristic. A sweep is made
over a 1 s period. Each of the two probes is swept every 4 s, resulting in 2 s time resolution when combined.
The electron density, ion density, electron temperature, and spacecraft potential are derived from the I/V
characteristics via a fitting process enhanced from that of, e.g., Allen [1992] or Brace [1998]. The ne also is
derived from wave sounding [Andersson et al., 2015]. The sensors, coated with TiN, have undergone some
contamination from atomic oxygen (AO). Starting January 2015, the LPW sensors display a deviation from
the expected I/V characteristic in regions of high density (ne > 104 cm3) and low temperatures
(Te < 0.15 eV). This deviation is shown in Figure 1 (left). The deviation appears as the probe potential is
positive with respect to the plasma (Vsweep > VSC, where VSC is the spacecraft potential) and persists until
Vsweep > VSC + ~1.3 V. The AO contamination does not appear to influence the electron retardation region
of the I/V curve, so Te and ne are derived to be better than 20% accuracy in most conditions. In addition to
the AO contamination, the RAM ions can distort the sheath around the sensor and cause deviations from
the expected I/V characteristic in high plasma densities if Vsweep > VSC.
To ensure an accurate fit, the electron current (Ie) is separated from the sensor-emitted photoelectron current
(IPheSens), the spacecraft photoelectron current (IPheSC), and the ion current (II):
Ie ¼ Im IPheSens IPheSC II
Here Im is the measured current and II is optimized so that Ie is near zero at negative sweep voltages.
IPheSens and IPheSC are established by examining I/V characteristics in low-density plasmas. IPheSens is typically between 10 and 20 nA (depending on the spacecraft orientation) in if Vsweep < VSC. IPheSC reaches
~30–50 nA if Vsweep >> VSC.
Figure 1 (center) shows the separated electron current. Te is derived from an isolated region of the I/V
characteristic where 7.5 nA < Ie < 1000 nA to avoid effects of AO contamination. This region can contain
as few as six points in the highest densities but contains more than 10 points in most cases. Te is also
derived from the slope of dIm/dV (Figure 1, right), which requires no modification of Im since II is nearly
constant through the electron retardation region [see example in Andersson et al., 2015]. In almost all
(>95%) of the derivations with ne > 104 cm3 reported here, the two derivations of Te agree to within
20% on a given sensor. For each sensor, the reported values of Te are a weighted average of two Te values.
ERGUN ET AL.
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Figure 1. A single sweep from LPW sensor 1 with high density and low temperature. (left) The circles represent the measured current from sensor 1, and the line is the fit
to the data; red are negative values. The fit deviates from the data in the region from 0.7 V (VSC) to ~2 V due to sheath barriers and possible AO contamination. (center)
Te is derived from a fit of the measured data with the ion current, the sensor photoelectron current, and the spacecraft photoelectron current removed. (right) Te also is
derived from the slope of dIm/dV. In this example and most sweeps, the two derived values of Te agree to within 5%.
The value of Te derived from dIm/dV receives a lower weighting in lower densities since the derivative
amplifies measurement error. A difference between the two derived values results in a larger uncertainty
in Te. The isolation procedure is not needed if ne < 104 cm3 or if Te > 0.15 eV.
Surface resistance or capacitance on the sensor appears to prevent a measurement of Te below 0.045 eV
(~525°K) on sensor 1 and below 0.60 eV (~700°K) on sensor 2. As a result, the lowest temperature measurements,
those below ~0.1 eV, have larger uncertainties and are systematically biased to yield a higher measurement of Te
than the actual value. Furthermore, Te from sensor 1 and sensor 2 can disagree by ~20%. Surface resistance,
surface capacitance, and other measurement errors can be estimated from hysteresis in I/V characteristics
(the difference between a positive-stepped sweep and a negative-stepped sweep). Regular hysteresis tests
show that the hysteresis in LPW sensor 1 is very small (and lower than that of sensor 2), which allows us to
estimate a lower bound of Te. Sensor 1 is given higher weight if Te < 0.1 eV in the reported average altitude
profile. We present Te data as derived from the slope of Ie and from the slope of dIm/dV (weighted average)
from the LPs with these known uncertainties and include estimates of lower and upper uncertainties. At
low values of Te, the reported value of Te is near the upper bound.
The density is measured on both sensors using ion current and electron current, resulting in four measurements of plasma density from the two sensors. The density from the ion current is derived assuming the
ion ram velocity is from the spacecraft motion. The ion-derived density is systematically lower than the
electron-derived density by ~15% when ne > 104 cm3; the ion-derived density is used primarily to verify
the electron-derived density.
The wave sounding can allow for an accurate determination of ne at times when valid returns can be
identified. However, stimulated Langmuir waves from sounding appear to reside in density cavities created
by the spacecraft suggestive of Langmuir eigenmodes [Ergun et al., 2008; Malaspina and Ergun, 2008;
Andrews et al., 2015]. Thus, sounding returns are best considered a lower bound of ne. The LPs and wave
sounding agree within ~5% when ne is ~103 cm3. The LPs systematically underestimate ne from that of wave
sounding by as much as ~ 25% when ne is ~ 102 cm3 or lower. The LP-derived density is systematically
higher (by a factor of ~1.33, causing a 25% uncertainty) from that wave sounding when
2 × 104 cm3 < ne < 5 × 104 cm3. Wave sounding is not available if ne > 5 × 104 cm3 (frequency limit of
wave receiver) and is highly unreliable/variable when 2 × 103 cm3 < ne < 2 × 104 cm3, presumably because
of eigenmode formation in the spacecraft wake. At this time, we are unable to determine if the wave sounding is underestimating ne (eigenmode formation) or if the LPs are overestimating ne in regions of high density
ERGUN ET AL.
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10.1002/2015GL065280
Figure 2. (left) The coverage of the MAVEN deep dip campaign. The 28 orbital tracks move from the lower left to the upper
right. The inbound (descending) legs of the orbits are closest to 0° solar zenith angle. The red circle represents Mars. (right)
Te and ne from single periapsis pass during a MAVEN deep dip campaign. (top) ne. The blue line represents sensor 1; the
black line represents sensor 2. (bottom) Te from the two sensors.
(> ~104 cm3). In this article, we present uncorrected LP measurements with appropriate uncertainties
(typically 25% + 10%) assigned.
3. Coverage
Figure 2 (left) displays the tracks of 28 orbits of the MAVEN deep dip campaign from 17 April 2015, 05:43:08
UT, until 22 April 2015, 06:45:33 UT. The horizontal axis represents the Y position, and the vertical axis represents the Z position in Mars-centered Solar Orbital (MSO) coordinates. The red circle outlines Mars. The orbital
tracks run from the bottom right to top left, and the color represents the altitude. We confine this study to
altitudes below 500 km. The periapsis line is just north and east of the subsolar point of Mars. The inbound
leg (below 500 km to periapsis) of the orbits has the lower solar zenith angles, whereas the outbound legs
(from periapsis to 500 km) show strong variation in ne and Te (see below), so we use only inbound legs in this
study. The average solar zenith angle is ~12° at periapsis, ~6° at 150 km in altitude, ~15° at 225 km in altitude,
and as high as 40° at 500 km. The MAVEN orbital period (~4 h) results in broad coverage in longitude.
4. Results
Figure 2 (right) displays ne and Te as a function of time for a single periapsis pass as MAVEN descended
from 500 km in altitude to ~130 km and then ascended back 500 km over a ~24 min period. The blue lines
are ne and Te values derived from sensor 1, and the black lines represent values from sensor 2. The two
sensors have excellent agreement in ne but occasionally display differences in Te as large as 20%. The
inbound (descending) leg shows a nearly steady increase in ne and a mostly steady decline in Te. The
outbound (ascending) leg shows interesting structure that is seen on many orbits. At ~1100 s into the pass,
Te sharply rises to nearly 0.4 eV as ne decreases by roughly an order of magnitude. This event is followed at
1150 s by a sharp decrease in Te and increase in ne, then at ~1210 s there is another sharp increase in Te and
decrease in ne. The MAVEN spacecraft (velocity of ~4.1 km/s) travels ~400 km during this period. It is unclear
at the time of this writing if these abrupt changes are due to changes in altitude, due to horizontal motion,
or are temporal variations.
Figure 3 displays the altitude profile of ne and Te over 28 orbits. These data have several interesting features.
The density profiles show large variation at altitudes above ~180 km, sometimes in individual passes and
sometimes from orbit to orbit. On the other hand, the variation is <10% below 150 km in altitude, indicating
that this part of the ionosphere was stable for the 6 day deep dip campaign.
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Figure 3. The altitude profile of ne and Te from the 28 deep dip orbits of MAVEN. (left) The blue dots are the ne measured by
sensor 1. The black dots are ne from sensor 2. There are ~10,000 sweeps. The red line is mean value of the ~28 orbits.
The green line is a fit to the Chapman function for comparison to models and other measurements. (right) The blue dots are
Te from sensor 1. The black dots are Te from sensor 2. The green lines are an upper bound to the mean value of Te and a
lower bound to the mean value of Te combining measurement uncertainties and deviation.
The density profile at the lowest altitudes was fit to a Chapman function for comparison to previous works
[e.g., Fallows et al., 2015a]:
N ¼ Ne1 þ Ne2
e
1
z Zi
Z Zi
1
Nei ¼ Ni exp
exp
2
Hi
Hi
Here z is the altitude, Ne1 is the density of the M1 layer, and Ne2 is the density of the M2 layer. Since the
MAVEN spacecraft does not probe the M1 layer, we use predetermined values for N1 (peak density), Z1
(altitude of the peak density), and H1 (the scale height), which are on Figure 3 [e.g., Fox and Yeager, 2006;
Liao et al., 2006; Fallows et al., 2015a, 2015b]. We include the M1 contribution to improve the M2 fit.
The values of N2, Z2, and H2 are determined by fit to the average measured value of ne (red line in Figure 3, left).
The fit included a weighting function proportional to average value of the measured ne, so measurements
above ~180 km have little influence. The fits indicate that MAVEN periapsides (~130 km) were just above Z2,
the altitude of the M2 density peak, at 124 km (+2.5 km/5 km); the uncertainty is dominated by the altitude
binning of the data (5 km). Since MAVEN did not penetrate below the M2 peak, there is a larger uncertainty
to the lower bound. Z2 at 124 km is consistent with Z2 in most Mars models [Hantsch and Bauer, 1990; Zhang
et al., 1990; Fox and Weber, 2012], consistent with that determined by the Mars Advanced Radar for
Subsurface and Ionosphere Sounding (MARSIS) instrument [Gurnett et al., 2005; Nielsen et al., 2006; Morgan
et al., 2008; Němec et al., 2011], but slightly below of that derived by the Mars Global Surveyor (MGS) radio
occultation [Fallows et al., 2015a]. Given differences in solar zenith angle coverage and possible variation in
Z2, we find the results from MAVEN, MARSIS, and MGS to be generally consistent.
The peak density of the M2 layer (N2) is projected by fit to be 2.6 × 105 cm3 with a lower limit of 1.95 × 105 cm3
and an upper limit of 2.86 × 105 cm3. The lower limit reflects the uncertainty in the comparison of density from wave sounding and LP-derived densities. These values are higher than most model-predicted
values but very near of that derived from the Mars Global Surveyor (MGS) radio occultation
(1.97 × 105 cm3) [Fallows et al., 2015a, 2015b]. The peak ne is somewhat higher than that determined
ERGUN ET AL.
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10.1002/2015GL065280
by MARSIS [Nielsen et al., 2006; Morgan
et al., 2008; Němec et al., 2011].
Considering that the MAVEN data are
limited to a 6 day period and that seasonal variation, magnetic fields, and
solar output have not been accounted
for, we see good agreement with
previous observations and modeling.
The scale height, H2, at 12.6 km, is relatively accurately measured at ±1 km. It
is generally consistent with that determined from measurements made by
MARSIS [Němec et al., 2011] but somewhat higher than that reported by
MGS radio occultations and many models [e.g., Fox and Weber, 2012; Fallows
et al., 2015a, 2015b].
Figure 4. The measured altitude profile of Te at low solar zenith angles.
The circles are the mean value of Te from Figure 3. The black lines are
the uncertainty range of the mean value of Te. The blue line is a fit. A
hyperbolic tangent fits the data well.
Figure 3 (right) reports the first in situ
measurements of the Te profile near the
subsolar ionosphere at Mars. Above
~300 km, Te is highly variable both within
a single orbit and from orbit to orbit. The
mean value of Te (300 km < z < 500 km)
is 0.270 eV (3130°K) with an uncertainty
range of 0.216 eV to 0.301 eV, considering the standard deviation and possible
systemic errors from the measuring
technique. Te dramatically declines in
the region between 300 km and 200 km
in altitude.
The mean value of Te at 200 km is 0.084 eV (0.017 eV, +0.010 eV). In this region, sensor 2 is near its lower
limit, so the uncertainty, particularly of the lower limit of Te, increases. This region is critical to dissociative
recombination of O2+ and subsequent escape of O. The dramatic change in Te with altitude increases the
importance of the region just below the exobase for O escape since O2+ dissociative recombination reaction
rate is proportional to Te0.7. For example, at ~225 km, the mean value of Te is 0.135 eV (0.027 eV, +0.014 eV)
reducing the O2+ dissociative recombination rate, whereas at 175 km, the mean value of Te reduces to
0.062 eV (0.015 eV, +0.006 eV), greatly increasing the O2+ dissociative recombination rate.
In the region below ~200 km, we rely on more heavily on sensor 1, which has reported temperatures as low as
0.045 eV. The mean value of Te continues to decrease with a decrease in altitude. At the lowest altitudes
measured by MAVEN, sensor 1 indicates a mean value of Te of 0.052 eV. From our best estimates of surface
irregularities, surface resistance, and capacitance of sensor 1, we estimate a lower bound of 0.026 eV and
an upper bound of 0.060 eV. At these low temperatures, the LP measuring technique can overestimate Te,
whereas an underestimation can only come from current measurement error, which is very low in such
high densities.
5. Discussion
The measured Te altitude profiles from the MAVEN LPW instrument are consistent with that observed by the
Viking 1 lander [Hanson and Mantas, 1988] given the large variability above 200 km. The Viking 1 lander did
not measure Te at altitudes below 200 km. Modeling efforts [e.g., Matta et al., 2014] expect Te to be close to
the neutral temperature (~0.02 eV; ~200°K) below ~125 km in altitude but rise quickly with altitude above
~125 km. An additional heat source is required to reach the Viking lander temperatures (~0.3 eV; ~3000°K).
ERGUN ET AL.
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The MAVEN LPW results indicate Te rises relative slowly from ~130 km to ~180 km in altitude, with a dramatic
rise in temperature from ~180 km to 300 km in altitude. The high values of Te (0.27 eV; 3000°K) above 300 km
in altitude require a substantial energy source.
Since an analytic equation for Te could be most useful in modeling efforts, we attempted to fit the measured
profile to an analytic form. The Te profile as measured during this period, fits surprisingly well to a simple
hyperbolic tangent in altitude:
TH þ TL TH TL
z Zo
Te ¼
þ
tanh
2
2
Ho
While there is no theoretical justification for this functional form, the parameters are useful to describe the Te
profile. TL (0.044 eV; 510°K) represents the asymptotic value of Te at the lowest altitudes, TH (0.271 eV; 3140°K)
is the high-altitude asymptotic value of Te, Zo (241 km) is the altitude of the most rapid change in Te, and Ho
(60 km) is the scale height of the rapid change. Figure 4 displays the mean values of Te as a function of altitude
(identical to those in Figure 3) and the fit.
The measured Te profile is suggestive of plasma heating in the topside ionosphere [e.g., Ergun et al., 2006;
Andersson et al., 2010]. Plasma waves and fluctuations from the solar wind boundaries appear to be absorbed
as they propagate toward the lower ionosphere, heating the topside of the ionosphere but having relatively
little effect below ~150 km. The value of TH is such that the ambipolar electric field, needed to retain the
electron population, could result in a potential on the order of 1 V (3 to 4 times TH). This potential alone
supplies nearly one half of the escape energy needed for O+ and a forth of that needed for O2+. A reinvestigation
of the Mars ionosphere in the light of new data should prove interesting.
6. Conclusions
We report in situ measurements of ne and Te from the LPW during the MAVEN deep dip campaign from 15
April 2015 to 22 April 2015. At the lowest altitudes, MAVEN was within ~15° solar zenith angle during the campaign. The ne altitude profiles show a slightly higher than expected M2 peak ne. Given the measurement
uncertainties, possible seasonal variation, magnetic fields, and solar output are not accounted for; we find
that MAVEN primarily confirms the peak ne and the location of the peak ne in models and in previous observations. The MAVEN LPW reports a Chapman scale height of 12.6 km ± 1 km during this 6 day period. The
MAVEN LPW instrument presented the first in situ measurements of Te at low solar zenith angles. We find that
Te rises with altitude from ~0.052 eV (0.026 eV, +0.008 eV) at 130 km in altitude to 0.084 eV (0.017 eV,
+0.010 eV) at 200 km. From ~180 km in altitude to ~300 km in altitude, Te rapidly rises to 0.239 eV
(0.042 eV, +0.027 eV) at 300 km. The Te profile fits well to a hyperbolic tangent, which may be useful to modeling. Above 300 km, Te is highly variable both within a single pass (~24 min) and from orbit to orbit (~4 h). It is
not know at the time of this writing if the Te variation within an orbit is due to horizontal motion of the
MAVEN spacecraft, if it reflects Te variation in altitude or if it is temporal variation. Future work includes careful
examination and modeling of the LPW sensor response to reduce the uncertainty in these observations.
Acknowledgments
The MAVEN project is supported by
NASA through the Mars Exploration
Program. Work at IRF was supported by
grants from the Swedish National Space
Board (DNR 162/14) and Vetenskapsadet
(DNR 621-2014-5526). Data are from
MAVEN LPW and are to be submitted to
the NASA Planetary Data System by
August 2015.
The Editor thanks two anonymous
reviewers for their assistance in
evaluating this paper.
ERGUN ET AL.
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