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Using the data sets of Mars Atmosphere and Volatile EvolutioN and OMNI for the period 2014 October 10-2020 February 14 and the heliocentric distance of 1-1.66 au, we investigate the statistical properties of solar wind upstream of Mars for the first time. The key parameters, including interplanetary magnetic field (IMF), proton density (N), bulk velocity (|V|), and dynamic pressure (P dyn), are surveyed with regard to variations of solar activity level and heliocentric distance. We find that the parameters |IMF|, N, and P dyn monotonously decrease with heliocentric distance. Both |IMF| and P dyn are generally stronger at a higher solar activity level (F 10.7 70 sfu), while such activity has little relevance to N. In contrast, |V| basically keeps a median of about 370 km s −1 and is insensitive to the solar activity level and heliocentric distance. We also find that the IMF upstream of Mars at the higher solar activity level has a much smaller spiral angle in the inward sector; thus, IMF seems "straighter" than that in the outward sector, although that is not so for the inward sector of the upstream of Earth. Our statistical survey can be used as a reference for upstream solar wind of Mars at 1.4 ∼ 1.7 au, and could benefit the studies on solar wind as well as the Martian space environment.
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Statistical Properties of Solar Wind Upstream of Mars: MAVEN Observations
Di Liu
1,2
, Zhaojin Rong
1,2,3
, Jiawei Gao
1,2
, Jiansen He
4
, Lucy Klinger
5
, Malcolm Wray Dunlop
6,7
, Limei Yan
1,2,3
,
Kai Fan
1,2,3
, and Yong Wei
1,2,3
1
Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, Peopleʼs Republic of China
rongzhaojin@mail.iggcas.ac.cn
2
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, Peopleʼs Republic of China
3
Mohe Observatory of Geophysics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, Peopleʼs Republic of China
4
School of Earth and Space Sciences, Peking University, Beijing, Peopleʼs Republic of China
5
Beijing International Center for Mathematical Research, Peking University, Beijing, Peopleʼs Republic of China
6
Space Science Institute, School of Astronautics, Beihang University, Beijing, Peoples Republic of China
7
RAL, Chilton, Oxfordshire, UK
Received 2021 January 11; revised 2021 March 7; accepted 2021 March 8; published 2021 April 23
Abstract
Using the data sets of Mars Atmosphere and Volatile EvolutioN and OMNI for the period 2014 October 102020
February 14 and the heliocentric distance of 11.66 au, we investigate the statistical properties of solar wind
upstream of Mars for the rst time. The key parameters, including interplanetary magnetic eld (IMF), proton
density (N), bulk velocity (|V|), and dynamic pressure (P
dyn
), are surveyed with regard to variations of solar
activity level and heliocentric distance. We nd that the parameters |IMF|,N, and P
dyn
monotonously decrease
with heliocentric distance. Both |IMF|and P
dyn
are generally stronger at a higher solar activity level
(F
10.7
70 sfu), while such activity has little relevance to N. In contrast, |V|basically keeps a median of about
370 km s
1
and is insensitive to the solar activity level and heliocentric distance. We also nd that the IMF
upstream of Mars at the higher solar activity level has a much smaller spiral angle in the inward sector; thus, IMF
seems straighterthan that in the outward sector, although that is not so for the inward sector of the upstream of
Earth. Our statistical survey can be used as a reference for upstream solar wind of Mars at 1.4 1.7 au, and could
benet the studies on solar wind as well as the Martian space environment.
Unied Astronomy Thesaurus concepts: Solar wind (1534);Mars (1007);Solar activity (1475);Interplanetary
magnetic elds (824);Planetary science (1255)
1. Introduction
It is well known that solar wind is a continuous ow of charged
particles that emits radially outward from the Sun with an average
speed of 400500 km s
1
at 1 au (1au1.5 ×10
8
km).Solar
wind is composed mainly of electrons, protons, and alpha particles
(3%4%), and carries the frozen-ininterplanetary magnetic
eld (IMF).
Earlier spacecraft missions measured the solar wind widely,
such as Helio 1 and 2 (at 0.31.0 au; Rosenbauer et al. 1977);
Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 (in the outer
heliosphere; Burlaga et al. 1980,1984; Collard et al. 1982);
Ulysses (with the larger elliptical orbit covering almost all the
heliolatitudes; Goldstein et al. 1995); and the Advanced
Composition Explorer (ACE; at 1 au, monitoring the upstream
solar wind of Earth; Chiu et al. 1998). The combined observations
of these missions have largely improved our knowledge of solar
wind (e.g., Behannon 1978; Slavin et al. 1984;Gruesbeck2017;
Hanneson et al. 2020)and conrm the Parker theory regarding the
general pattern of solar wind and IMF (Parker 1958,1963).
Additionally, the variations of solar wind and IMF also show a
correlation with the solar cycle (Neugebauer 1975;Gazis1996;
Richardson & Kasper 2008).
The solar wind plays an important role in driving the
dynamics of the planetary magnetosphere and the evolution of
planetary climate (Lammer et al. 2008; Arridge 2020). Due to
the wide measurements of earlier spacecraft missions, e.g., the
ACE mission (Chiu et al. 1998), we are able to measure and
monitor the upstream solar wind of Earth constantly. The big
data set of solar wind, like the OMNI data set (King &
Papitashvili 2005),
8
facilitates studies of the Earthʼs magneto-
sphere dynamics and terrestrial space weather.
In contrast to Earth, we know little about the solar wind
around other planets, although our knowledge of solar wind in
the heliosphere has improved signicantly in recent decades.
Measurements of solar wind, including IMF, require well-
calibrated plasma instruments and magnetometers. Fortunately,
the recent mission of the Mars Atmosphere and Volatile
EvolutioN (MAVEN)carried both high-performing plasma
instruments and a magnetometer to explore the Martian space
environment (Jakosky et al. 2015). This provided a good
opportunity to study solar wind upstream of Mars within
R
MS
=1.38 1.66 au (R
MS
is the heliocentric distance of
Mars). MAVEN has successfully operated for about six years
since its orbit insertion around Mars on 2014 September 21.
MAVEN has an elliptical orbit with an apoapsis of 6200 km
and a periapsis of 150 km (Jakosky et al. 2015), and can
regularly sample upstream solar wind every 4 or 5 months. The
data set accumulated by MAVEN makes us able to survey the
upstream solar wind more comprehensively.
Investigating solar wind upstream of Mars is crucial to probe
the processes by which solar wind interacts with Mars. However,
to our knowledge, there is no comprehensive study that targets
the solar wind upstream of Mars exclusively, though some
The Astrophysical Journal, 911:113 (10pp), 2021 April 20 https://doi.org/10.3847/1538-4357/abed50
© 2021. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s)and the title
of the work, journal citation and DOI.
8
See https://omniweb.gsfc.nasa.gov.
1
preliminary results of solar wind have been reported in Halekas
et al. (2017)and Lee et al. (2017). Therefore, in this study, we are
motivated to focus on characterizing the properties of solar wind
upstream of Mars based on the data set collected by MAVEN
during 2014 October to 2020 February.
This study is organized as follows. We describe the
instruments and data sets used in Section 2. In Section 3,we
show the statistical histograms of solar wind upstream of Mars
and make a comparison with solar wind upstream of Earth.
Variations of solar wind with heliocentric distance and with
solar activity level are studied in Section 4. Finally, we discuss
and summarize the results in Sections 5and 6, respectively.
2. Instruments and Data Set
2.1. Instruments
The MAVEN data set we use is derived from measurements
of the Solar Wind Ion Analyzer (SWIA; Halekas et al. 2015)
and the Magnetometer (MAG; Connerney et al. 2015a).
SWIA is a toroidal electrostatic analyzer that measures ion
uxes over a broad energy range from 25 eV to 25 keV, and a
maximum angular range of 360°×90°, with a time resolution
of 4 s (Halekas et al. 2015). SWIA measures the 3D ion
velocity distribution of solar wind and provides the plasma
moments including solar wind density and bulk velocity. The
plasma moments of SWIA were computed by assuming that all
detected ions are protons. The assumption is invalid within the
Martian magnetosphere but reasonable for solar wind. For solar
wind, 94%97% of ions are protons; the derived density and
velocity moments are thus reliable in most cases, whereas the
temperature derived cannot be safely reliable since even a small
alpha population (3%6%)could introduce an articially large
temperature moment (Halekas et al. 2017). Thus, we only focus
on moments of velocity and number density measured
by SWIA.
MAG can measure magnetic eld vectors at 32 Hz frequency
(Connerney et al. 2015a,2015b). In this study, we use the
magnetic eld data with 1 Hz for IMF measurement in Mars-
centered Solar Orbital (MSO)coordinates, where +Xpoints
sunward, +Zis perpendicular to Marsʼs orbital plane and
positive toward the ecliptic north, and +Ycompletes the right-
handed system.
2.2. Data Set
To extract observations of pristine solar wind by MAVEN,
we selected the interval when MAVEN was far enough from
the Martian bow shock. We rst identied bow shock crossings
manually by looking at the jump variation of magnetic eld
strength and the energy spectrum of SWIA per orbit
(Gruesbeck et al. 2018).
We examined the data of SWIA and MAG, spanning from
2014 October 10 to 2020 February 14, and identied 7684
orbits that had signicant signatures of crossing bow shock for
both inbound and outbound crossings. We further specied the
solar wind interval between neighboring crossings of bow
shock should be larger than 1.5 hr, and nally selected 5780
intervals. For each selected interval, we resampled the MAG
data and the SWIA data of plasma moments, using a cadence of
45 s in a 30 minute window at the center of each interval. In
this way, we could minimize the disturbance brought by bow
shock and foreshock to some extent. The resampled data points
in these 30 minute windows are grouped as the data set for this
study.
The average locations of MAVEN within these 30 minute
windows are labeled as dots in Figure 1(a). As expected, most
of the locations are far away from the nominal bow shock
location (Trotignon et al. 2006); thus, the disturbance induced
by bow shock or foreshock could be negligible.
We show the time series of R
MS
of our data set in Figure 1(b).
Obviously, for the whole data set, R
MS
is periodically varied
within the range 1.381.66 au. Meanwhile, to study the correlation
with solar activity, the adjusted daily F
10.7
solar ux (solar radio
ux at a wavelength of 10.7 cm), which was observed by the solar
radio telescope on Earth, was used as a proxy of solar activity
(Tapping 2013). The corresponding F
10.7
ux of our data set is
shown in Figure 1(c), which demonstrates that the solar activity
was in a declining phase of the 24th solar cycle from 2014
to 2020.
Figure 1. Panels from top to bottom show (a)the average locations MAVEN
sampled in the upstream solar wind, (b)the time series of R
MS
, and (c)the time
series of daily F
10.7
solar ux, respectively. The black curve in (a)marks the
average shape of bow shock according to the empirical model of Trotignon
et al. (2006), and the color of the dots corresponds to the R
MS
colors. The
horizontal red line in (c)represents F
10.7
=70 sfu (sfu is the unit of F
10.7
ux,
and 1 sfu =10
22
Wm
2
Hz
1
), above which the solar activity is higher, and
vice versa.
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The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
To describe the orientation of IMF, we dened its cone angle
and clock angle in the XY and XZ planes of MSO, respectively. As
shown in Figure 2, the cone angle θ
cone
is dened as the angle
between the projected IMF and +Xin the XY plane, and
rotationally increased from +Xtoward +Y. The Parker spiral angle
for inward IMF is estimated as 360°θ
cone
when θ
cone
270°,
while the Parker spiral angle for outward IMF is estimated as
180°θ
cone
when 90°θ
cone
180°. Similarly, the clock angle
θ
clock
is dened as the angle between the projected IMF and +Zin
the YZ plane, which is rotationally increased from +Ztoward +Y.
θ
clock
=90°(270°)means that IMF points toward a +Y(Y)
direction.
In order to compare contemporaneous solar wind observa-
tions at Earth and at Mars, the OMNI database that provides the
shifted solar wind observations to the nose of Earth bow shock
from several spacecraft was also used (King & Papitashvili
2005). Since the time resolution of our MAVEN solar wind
data set is 45 s, we used an OMNI data set with a resolution of
1 minute covering the same period. The coordinate system used
for this OMNI data set is geocentric solar ecliptic (GSE), where
+Xpoints sunward, +Zis perpendicular to Earthʼs orbital
plane and positive toward ecliptic north, and +Ycompletes the
right-handed system. Note that GSE is basically the same as
MSO, but the origin is at the Earthʼs center, not Mars.
3. Distributions
In order to study how the statistical properties of solar wind
varies with R
MS
and the level of solar activity, the whole MAVEN
data set is partitioned into four subsets: (1)near the perihelion with
lower solar activity (LSA; R
MS
=1.381.52 au and F
10.7
<70 sfu);
(2)near the perihelion with higher solar activity (HSA;
R
MS
=1.381.52 au and F
10.7
70 sfu);(3)near the aphelion
with LSA (R
MS
=1.521.66 au and F
10.7
<70 sfu);(4)near the
aphelion with HSA (R
MS
=1.521.66 au and F
10.7
70 sfu).The
reason for choosing F
10.7
=70 sfu as the reference is that the
median of F
10.7
for our data set is about 72.3 sfu (the histogram of
F
10.7
is not shown here). The two data set groups partitioned by
F
10.7
=70 sfu have comparable amounts of data points, which
could lower statistical bias to some extent.
3.1. The Distributions of IMF Upstream of Mars
In Figures 3(a)(c), we show histograms of IMF for the whole
MAVEN data set. The distribution shape of IMF strength, |IMF|,
is similar to a chi-square distribution (Larrodera & Cid 2020,and
references therein)and reaches the peak at 1.87 nT. (We tted the
probability density function using a kernel smoothing method
(Hill 1985)and calculated the most probable value (MPV)that
corresponds to the maxima of the probability density function.)
Correspondingly, θ
clock
reaches its MPV at (89°.6, 270°.8),
implying that IMF is mostly orientating toward either a +Yor
Ydirection with a minor B
z
component. Meanwhile, θ
cone
reaches its MPV at (115°.9, 310°.2), which means that the MPV of
the Parker spiral angle is about 64°. 1 for the outward sector and
49°. 8 for the inward sector.
For the data set of perihelion with LSA, as shown in
Figures 3(d)(f),|IMF|reaches the peak at 2.01 nT, while the
MPVs of θ
clock
and θ
cone
are (96°. 3, 271°.9)and (122°.3, 301°.6),
respectively. The distribution of θ
cone
implies that the Parker spiral
angle is about 57°.7 for the outward sector and 58°. 4 for the inward
sector. In contrast, when Mars was subjected to HSA near
perihelion, we nd |IMF|becomes stronger (MPV =2.35 nT).
Meanwhile, the MPV of θ
cone
is 113°. 8 in the outward sector
(spiral angle is 66°.2)and 313°. 5 in the inward sector (spiral angle
is 46°.5), indicating a signicant spiral angle discrepancy between
the two sectors. The clock angle still keeps MPV as (84°.7,
267°.6), though solar activity level is higher.
In Figures 3(g)(i), we can nd similar kinds of solar activity
when Mars is near the aphelion. In this case, |IMF|in HSA is
comparable (MPV =1.80 nT)to |IMF|in LSA (MPV =1.75 nT).
Again, we notice the signicant discrepancy in spiral angle
between the inward and outward sectors in HSA: the MPV of
θ
cone
is 110°. 8 for the outward sector (spiral angle is 69°.2)and
310°. 4 for the inward sector (spiral angle is 49°.6).InLSA,
however, the MPV of θ
cone
is 122°. 6 in the outward sector (spiral
angle is 57°.4),and308°. 0 in the inward sector (spiral angle is
52°.0).TheMPVofθ
clock
in HSA is (91°.6, 273°.8),whichis
basically the same in LSA as (90°. 7, 271°.1).
A comparison between Figures 3(d)(f)and (g)(i)demon-
strates that the MPV of |IMF|, under the same solar activity
level, is stronger near perihelion. However, we do not nd
Figure 2. Diagrams illustrating the denition of clock angle (left)and cone angle (right).
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The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
signicant dependence of the MPV of θ
clock
or θ
cone
(spiral
angle)on R
MS
.
We tabulate the MPV of these parameters in Table 1.
3.2. The Distributions of Solar Wind Upstream of Mars
Using the same format of Figure 3,Figure4shows histograms
of number density N, bulk speed |V|, and dynamic pressure
P
dyn
(P
dyn
=mNV
2
,mis proton mass), respectively, in the left,
middle, and right columns.
For histograms of the whole data set shown in Figures 4(a)
(c),wend that the MPVs for N,|V|, and P
dyn
are 1.40 cm
3
,
368.9 km s
1
, and 0.39 nPa, respectively. The bulk speed is
dominated by the V
x
component and the MPV for the V
z
component is negligible (see Table 1). The MPV of the V
y
Figure 3. The columns from left to right show histograms of |IMF|,θ
clock
, and θ
cone
, respectively. The upper panels (a)(c)show histograms for the whole data set.
The middle panels (d)(f)show histograms of when Mars is near perihelion. The lower panels (g)(i)show histograms of when Mars is near aphelion. In both panels,
red lines represent HSA, while blue lines represent LSA.
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The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
component (22.2 km s
1
)was compared with the average
orbital speed of Mars around the Sun (24.1 km s
1
), which
suggested that SWIA resolves the off-axis ow velocity with an
excellent accuracy and precision of 1kms
1
, as previously
pointed out by Halekas et al. (2017).
Histograms for solar activity near the perihelion (see
Figures 4(d)(f)) demonstrate that the MPVs of N,|V|, and
P
dyn
in LSA are comparable to those in HSA. Similarly, near
the aphelion (see Figures 4(g)(i)) we nd that, except for P
dyn
,
the MPVs of Nand |V|in LSA are also comparable to those in
HSA. Thus, it seems that the solar activity level cannot
signicantly affect the MPVs of Nand |V|.
By comparing Figures 4(d)(f)and (g)(i),wend that,
under the same solar activity level, the MPV of Nis stronger
near the perihelion, while the MPV of |V|seems independent of
R
MS
.
3.3. Comparison with Solar Wind Observations at 1 au
In order to check the results we obtained in Sections 3.1 and
3.2, we compared MAVENʼs data set with a contemporaneous
OMNI data set at 1 au. The OMNI data set covering 2014 October
102020 February 14 was similarly partitioned into two subsets,
that is, LSA (F
10.7
³70 sfu)and HSA (F
10.7
=70 sfu).
Using the same format as Figures 3and 4, the histograms of
IMF and solar wind moments at 1 au are plotted in Figures 5
and 6, respectively. Accordingly, the MPVs of these histo-
grams are tabulated in Table 1.
Obviously, as a whole, IMF at 1 au has a stronger eld
strength (MPV =4.13 nT)relative to Mars. The MPVs for both
θ
clock
and θ
cone
suggest that IMF is mostly orientating toward a
+Yor Ydirection and the Parker spiral angle is about 43°.4
(45°.1)for the outward (inward)sector. The increased spiral
angle and decreased |IMF|from Earth to Mars are consistent
with the general IMF pattern. The response of IMF to solar
activity level at 1 au shows some similarities with the case at
Mars. We nd that |IMF|in HSA (MPV =4.50 nT)is stronger
than that in LSA (MPV =3.65 nT). However, in contrast to the
case of Mars, no matter what the solar activity level is,
the Parker spiral angle at 1 au shows little discrepancy between
the inward and outward sectors (the largest discrepancy is 12°.8
in LSA).
With regard to the plasma moments, the MPV of solar wind
at 1 au has a higher density (3.67 cm
3
), a comparable ow
speed (351.1 km s
1
)with a much smaller V
y
component, and a
higher dynamic pressure (1.51 nPa)relative to the solar wind
upstream of Mars. In contrast to the case of Mars, the MPV of
solar wind in HSA at 1 au shows higher ow speed
(369.4 km s
1
)and lower density (3.13 cm
3
)relative to that
in LSA, while the MPV of dynamic pressure still seems
insensitive to the solar activity level.
4. Variations in Heliocentric Distance
In this section, we examine the radial dependence of solar
wind parameters (|IMF|,|V|,N, and P
dyn
). To study the radial
prole, the range of R
MS
(1.381.66 au)observed by MAVEN
is equally divided into six bins of the same width, and the
median within each bin is calculated. At the same time, the
medians at 1 au are calculated using the OMNI database. In
comparison with MPV, the median value better indicates the
whole level of data points within the bin. In Figures 7(a)(d),
using the calculated medians, we show the radial variations of
|IMF|,|V|,N, and P
dyn
, respectively, as well as responses to
different solar activity levels.
These radial proles covering heliocentric distance
11.66 au demonstrate that:
1. |IMF|in LSA monotonously decreases with heliocentric
distance, whereas |IMF|in HSA decreases with heliocentric
distance but is almost constant beyond 1.45 au. We also
note that |IMF|in HSA is generally stronger than that in
LSA by about 0.5 nT. The enhanced |IMF|in HSA could be
induced by solar wind structures of a large scale, i.e.,
interplanetary coronal mass ejections or corotating interac-
tion regions, which appear more frequently in HSA.
2. Like the prole of |IMF|,Nalso monotonously decreases
with heliocentric distance in LSA, but keeps almost
constant beyond 1.45 au in HSA. In contrast to |IMF|,it
seems that the prole of Nis insensitive to the level of
solar activity within 1.381.66 au.
3. The median of |V|is basically varied within 360
420 km s
1
, which has no signicant correlation with
heliocentric distance or the level of solar activity.
4. The comparison between Figures 7(b)and (c)suggests
that |V|is anticorrelated with the variation of N. This
anticorrelation has been reported in some previous studies
(e.g., Richardson & Kasper 2008).
Table 1
The MPVs of Solar Wind and IMF Parameters for Different Data Sets
Data Set |IMF|θ
clock
θ
cone
Spiral Angle |V|Vx Vy Vz N P
dyn
(nT)(°)(°)(°)(km s
1
)(km s
1
)(km s
1
)(km s
1
)(cm
3
)(nPa)
Upstream of Mars covering 1.381.66 au
Whole MAVEN data set 1.87 89.6, 270.8 115.9, 310.2 64.1, 49.8 368.9 367.5 22.2 1.1 1.40 0.39
Perihelion and HSA 2.35 84.7, 267.6 113.8, 313.5 66.2, 46.5 344.8 344.8 23.2 3.1 1.50 0.44
Perihelion and LSA 2.01 96.3, 271.9 122.3, 301.6 57.7, 58.4 352.6 348.6 24.8 1.9 1.65 0.43
Aphelion and HSA 1.80 91.6, 273.8 110.8, 310.4 69.2, 49.6 372.6 371.6 21.3 0.0 1.38 0.45
Aphelion and LSA 1.75 90.7, 271.1 122.6, 308.0 57.4, 52.0 369.1 367.6 18.7 2.6 1.32 0.28
Upstream of Earth at 1 au
Whole OMNI data set 4.13 89.5, 267.3 136.6, 314.9 43.4, 45.1 351.1 350.0 2.5 1.5 3.67 1.51
HSA 4.50 86.1, 266.6 135.5, 316.7 44.5, 43.3 369.4 369.8 2.5 2.2 3.13 1.47
LSA 3.65 95.8, 269.6 143.7, 310.9 36.3, 49.1 348.9 348.8 2.1 0.0 3.81 1.51
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5. P
dyn
monotonously decreases with heliocentric distance
regardless of the solar activity level, and P
dyn
is generally
stronger in HSA than in LSA by about 0.1 nPa.
5. Discussion
It is well known that typical solar wind near the Earthʼs orbit
(1au)has N36cm
3
,|V|400 km s
1
,P
dyn
12 nPa,
|IMF|45 nT, and a Parker spiral angle of 45°
(Neugebauer & Snyder 1966; Bothmer & Zhukov 2007;
Hansteen et al. 2010; Larrodera & Cid 2020). Our statistical
analysis of the OMNI data set in Figures 5and 6conrms these
parameters. In comparison, our statistical investigation of solar
wind upstream of Mars (R
MS
=1.381.66 au)suggests that
solar wind has a comparable |V|(370 km s
1
), a lower
N(1.4 cm
3
), a lower P
dyn
(0.39 nPa), a lower |IMF|
Figure 4. Histograms of N(left column),|V|(middle column), and P
dyn
(right column). The format is the same as Figure 3.
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The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
(1.87 nT), and a larger spiral angle of 57°(see Table 1). This
|V|, the radial decreased trend of N,P
dyn
, and |IMF|, and the
increased spiral angle from Earth to Mars are roughly
consistent with the Parker theory (Parker 1958,1963).Itis
worth noting from Table 1and Figure 7(b)that the radial
prole of Nbasically satises the falloff of the inverse square
of the heliocentric distance within 11.66 au, which implies
that the ux of solar wind stays constant within this range.
However, considering the scale of a heliosphere of 100 au
(Burlaga et al. 2005), the narrow range of 11.66 au from Earth
to Mars does not favor a direct comparison with the Parker
theory. Interested readers can refer to many previous solar
winds regarding the radial prole (Richardson et al. 1995;
Köhnlein 1996; Richardson & Wang 1999; Hanneson et al.
2020).
Previous studies suggested that each solar wind parameter can
experience a corresponding solar cycle variation. |V|is usually
higher and Nis lower during the solar maximum, because the
coronal holes with tenuous high-speed ow could excurse to the
elliptical plane during the solar maximum. Meanwhile, P
dyn
usually reaches a minimum near the solar maximum, and arrives
atapeakinthedecliningphase,roughly three years after the solar
maximum (Richardson et al. 1995,2003; Richardson & Wang
1999;Richardson&Kasper2008). Due to the available period of
the MAVEN data set, which covers 2014 to 2020, however, we
are unable to study the variation of solar wind parameters over a
complete solar cycle. The F
10.7
solar ux shown in Figure 1(c)
demonstrates that the highest period of HSA (20152017)
corresponds to the declining phase of the 24th solar cycle, and
P
dyn
in HSA, which we reported as stronger, is consistent with the
previous conclusion that P
dyn
usually peaks in the declining phase.
Therefore, although we found that Nand |V|are insensitive to the
solar activity level as we dened it here, this does not necessarily
contradict previous studies on the modulation of solar cycles.
From Table 1, we notice that solar wind speed upstream of
Mars has a minor V
y
component (22 km s
1
), but a negligible
V
y
component (2.5 km s
1
), at 1 au. Considering the orbital
speed of Earth (30 km s
1
)and Mars (24 km s
1
), this may
suggest that the traverse component of solar wind velocity in
the elliptical plane is comparable to the Earth orbital velocity at
1 au, and that the traverse component becomes negligible at the
orbit of Mars.
We nd that IMF upstream of Mars is straighterin the
inward sector (spiral angle 46°.5 near perihelion, and 49°.6
near aphelion)than that in the outward sector (spiral angle
66°.2 near perihelion, and 69°. 2 near aphelion)in HSA.
Meanwhile, both sectors in LSA show comparable spiral angles
(57°.457°. 7 for the outward sector and 52°.0 58°. 4 for the
inward sector). One may argue that smaller spiral angles in the
inward sector would imply a higher |V
x
|there. However, by
Figure 5. Histograms of |IMF|(left column),θ
clock
(middle column), and θ
cone
(right column)at 1 au. The format is the same as Figure 3.
7
The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
comparing the medians of |V
x
|for the two sectors in HSA, we
did not observe a higher |V
x
|in the inward sector (not shown
here). From Table 1, we do not nd a signicant discrepancy of
spiral angles between the two sectors at 1 au. Thus, the true
reason why IMF upstream of Mars in HSA is straighterin
inward sector remains an unknown and open question.
It is noteworthy that we chose F
10.7
=70 sfu to distinguish
the solar activity level. Although a minor change of criterion
would slightly affect the quantitative results, the qualitative
results of our conclusion remain robust.
6. Summary
In this study, we use the data set of MAVEN, spanning from
2014 October 10 to 2020 February 14, to characterize the
properties of solar wind upstream of Mars for the rst time. Our
results demonstrate that typical solar wind density is about
1.4 cm
3
, velocity is almost radially outward with a speed
about 370 km s
1
, and dynamic pressure is about 0.4 nPa. The
IMF frozen-into the solar wind basically lies on the ecliptic
plane with a eld strength about 1.9 nT. The spiral angle of
IMF is about 50°in the inward sector and 64°in the outward
sector. Thus, IMF in the inward sector seems straighterthan
that in the outward sector.
Combining our results with the data set of solar wind
upstream of Earth, the variations of solar wind with the solar
activity level and heliocentric distance within 1 1.66 au
demonstrate that:
1. The strength of IMF decreases with heliocentric distance,
and is generally stronger in HSA.
2. IMF upstream of Earth has a comparable spiral angle
(45°)for both inward and outward sectors, regardless of
the solar activity level. On the other hand, IMF upstream
of Mars evidently shows a smaller angle or straighter
eld lines in inward sectors in HSA.
3. The density of solar wind decreases with heliocentric
distance, roughly satisfying a falloff of inverse square of
heliocentric distance, but does not show any signicant
correlation with a solar activity level within 1.381.66 au.
4. The median of solar wind speed basically stays at
370 km s
1
, and is insensitive to the solar activity level
and heliocentric distance.
5. The solar wind speed is anticorrelated with the variation
of solar wind density.
6. The median of dynamic pressure monotonously decreases
with heliocentric distance, and is generally stronger
in HSA.
Figure 6. Histograms of N(left column),|V|(middle column), and P
dyn
(right column)at 1 au. The format is the same as Figure 4.
8
The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
The data set of MAVEN is publicly available in
NASAʼs Planetary Data System (https://pds-ppi.igpp.ucla.
edu/search/?t=Mars&sc=MAVEN&facet=SPACECRAFT_
NAME&depth=1). OMNI data were obtained from the
GSFC/SPDF OMNIWeb interface (https://omniweb.gsfc.
nasa.gov),andthedataoftheF
10.7
solar ux were provided
by the National Research Council and Natural Resources
Canada (https://www.spaceweather.gc.ca/solarux/sx-3-en.
php). This work was supported by the National Natural
Science Foundation of China (grant Nos. 41922031, 41774188,
and 42074207), the Strategic Priority Research Program of
Chinese Academy of Sciences (grant No. XDA17010201),and
the Key Research Program of the Institute of Geology &
Geophysics, CAS (grant No. IGGCAS-201904).
ORCID iDs
Di Liu https://orcid.org/0000-0001-7636-7245
Zhaojin Rong https://orcid.org/0000-0003-4609-4519
Jiawei Gao https://orcid.org/0000-0003-4432-1132
Jiansen He https://orcid.org/0000-0001-8179-417X
Limei Yan https://orcid.org/0000-0002-1402-923X
Kai Fan https://orcid.org/0000-0003-2572-1587
Yong Wei https://orcid.org/0000-0001-7183-0229
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The Astrophysical Journal, 911:113 (10pp), 2021 April 20 Liu et al.
... The other solar wind parameters are set as follows: the solar wind velocities V Y and V Z are chosen to be 0, the upstream solar wind ion temperature T i = 5 × 10 4 K, and the electron temperature T e = 3 × 10 5 K (for the single fluid approach, the total temperature is set to be 3.5 × 10 5 K). With these parameter sets, which are the observed normal solar wind conditions upstream of Mars (e.g., Halekas et al. 2017;Liu et al. 2021;Andreone et al. 2022), we simulate different conditions of IMF, solar wind dynamic pressure, and the location of the intense crustal field to study their effect on the dayside magnetic reconnection between the solar wind and the Martian crustal field. ...
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The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft has been continuously observing the variability of solar soft X-rays and EUV irradiance, monitoring the upstream solar wind and interplanetary magnetic field conditions and measuring the fluxes of solar energetic ions and electrons since its arrival to Mars. In this paper, we provide a comprehensive overview of the space weather events observed during the first ∼1.9 years of the science mission, which includes the description of the solar and heliospheric sources of the space weather activity. To illustrate the variety of upstream conditions observed, we characterize a subset of the event periods by describing the Sun-to-Mars details using observations from the MAVEN solar Extreme Ultraviolet Monitor, solar energetic particle (SEP) instrument, Solar Wind Ion Analyzer, and Magnetometer together with solar observations using near-Earth assets and numerical solar wind simulation results from the Wang-Sheeley-Arge-Enlil model for some global context of the event periods. The subset of events includes an extensive period of intense SEP electron particle fluxes triggered by a series of solar flares and coronal mass ejection (CME) activity in December 2014, the impact by a succession of interplanetary CMEs and their associated SEPs in March 2015, and the passage of a strong corotating interaction region (CIR) and arrival of the CIR shock-accelerated energetic particles in June 2015. However, in the context of the weaker heliospheric conditions observed throughout solar cycle 24, these events were moderate in comparison to the stronger storms observed previously at Mars.
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We report on the inflight performance of the Solar Wind Ion Analyzer (SWIA) and observations of the Mars-solar wind interaction made during the Mars Atmosphere and Volatile EvolutioN (MAVEN) prime mission and a portion of its extended mission, covering 0.85 Martian years. We describe the data products returned by SWIA and discuss the proper handling of measurements made with different mechanical attenuator states and telemetry modes, and the effects of penetrating and scattered backgrounds, limited phase space coverage, and multi-ion populations on SWIA observations. SWIA directly measures solar wind protons and alpha particles upstream from Mars. SWIA also provides proxy measurements of solar wind and neutral densities based on products of charge exchange between the solar wind and the hydrogen corona. Together, upstream and proxy observations provide a complete record of the solar wind experienced by Mars, enabling organization of the structure, dynamics, and ion escape from the magnetosphere. We observe an interaction that varies with season and solar wind conditions. Solar wind dynamic pressure, Mach number, and extreme ultraviolet flux all affect the bow shock location. We confirm the occurrence of order-of-magnitude seasonal variations of the hydrogen corona. We find that solar wind Alfvén waves, which provide an additional energy input to Mars, vary over the mission. At most times, only weak mass loading occurs upstream from the bow shock. However, during periods with near-radial interplanetary magnetic fields, structures consistent with Short Large Amplitude Magnetic Structures and their wakes form upstream, dramatically reconfiguring the Martian bow shock and magnetosphere.
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The Solar Wind Ion Analyzer (SWIA) on the MAVEN mission will measure the solar wind ion flows around Mars, both in the upstream solar wind and in the magneto-sheath and tail regions inside the bow shock. The solar wind flux provides one of the key energy inputs that can drive atmospheric escape from the Martian system, as well as in part controlling the structure of the magnetosphere through which non-thermal ion escape must take place. SWIA measurements contribute to the top level MAVEN goals of characterizing the upper atmosphere and the processes that operate there, and parameterizing the escape of atmospheric gases to extrapolate the total loss to space throughout Mars' history. To accomplish these goals, SWIA utilizes a toroidal energy analyzer with electrostatic deflectors to provide a broad 360∘×90∘ field of view on a 3-axis spacecraft, with a mechanical attenuator to enable a very high dynamic range. SWIA provides high cadence measurements of ion velocity distributions with high energy resolution (14.5 %) and angular resolution (3.75∘×4.5∘ in the sunward direction, 22.5∘×22.5∘ elsewhere), and a broad energy range of 5 eV to 25 keV. Onboard computation of bulk moments and energy spectra enable measurements of the basic properties of the solar wind at 0.25 Hz.
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The 10.7 cm solar radio flux, or F10.7 is, along with sunspot number, one of the most widely used indices of solar activity. This paper describes the equipment and procedures used to make the measurements and to calibrate them, and discusses some of the "most-asked" questions about the data.