Ionospheric Plasma Transported Into the Martian Magnetosheath

Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (<20 eV) and dense (∼>1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature.


Introduction
The lack of a global dipole magnetic field at Mars leads to nearly direct interaction of solar wind protons with the dense population of the upper ionosphere.As interplanetary magnetic field (IMF) flux tubes approach the planet, they collect ionized heavy planetary ions and become mass-loaded, resulting in deceleration of the mass-loaded part of magnetic flux tube due to conservation of momentum.The difference in speed between parts of the same tube, caused by mass-loading and ionospheric pressure, starts to bend (Yeroshenko et al., 1990) and drape it around the planet.At Mars, the draped IMF can penetrate deep in the ionosphere, because solar wind dynamic pressure can exceed the thermal pressure of the Martian ionosphere (see, e.g., Sanchez-Cano et al., 2020).Solar wind mass-loading at Mars is mainly driven by the hot neutral oxygen corona which extends far beyond the bow shock into the solar wind region and is produced mostly through dissociative recombination of O 2 + ions (e.g., Lee et al., 2015).Being ionized by solar extreme ultraviolet radiation, these particles are subsequently picked-up by solar wind and are referred to as "exospheric oxygen ions".Charged particles accelerated from the ionosphere by solar wind induced electric field -V SW × B, where V SW is the solar wind velocity, are often referred to as "ion plume" (e.g., Dong et al., 2015Dong et al., , 2017;;Dubinin et al., 2021;Vaisberg et al., 2018) and are addressed as "ionospheric pick-up" in the current paper.Given the considerable gyroradii of oxygen ions within the solar wind, spanning multiple Martian radii (R M ), the velocities of picked-up ionospheric ions are predominantly oriented nearly perpendicular to the Mars-Sun axis, aligning with the induced electric field within the vicinity of the planet.For large planetary systems, where the gyroradii are small compared to the system size, the exospheric oxygen ions are observed as a ring distribution with a bulk speed of the solar wind that then drives instabilities and identifiable wave signatures (e.g., Wilson, 2016, and references therein).
• Ions of different species gain similar energies in the Martian magnetosheath by Hall electric fields associated with magnetic curvature • A high concentration of ionospheric ions correlates with a near void of shocked solar wind protons and a magnetic field reversal The current paper reports observations of localized high density (∼>1 cm 3 ) O + and O 2 + ion populations accelerated (i.e., with increased bulk velocity) tailward to energies of ∼1 keV in the nightside Martian magnetosheath.In this paper, the term "nightside" refers to locations beyond the terminator plane of the planet, and does not necessarily indicate positions in the shadow of Mars.This type of ion distribution was first observed in the Martian tail from Phobos-2 measurements and reported in Rosenbauer et al., 1989, Dubinin et al., 1993, and referred to as "plasma sheet" (Barabash et al., 2007;Dubinin et al., 2017;Dubinin & Fraenz, 2015;Fedorov et al., 2006Fedorov et al., , 2008;;Halekas et al., 2006;Kallio et al., 1995).
Unlike plasma sheet ions located in the Martian magnetotail, the fluxes of heavy ions presented in this paper are located in the nightside magnetosheath of Mars at altitudes up to ∼4,500 km.Despite being transported so far from ionosphere, where typical ion temperatures are on the order of several eV or even less (Hanley et al., 2021), the observed ions remain almost unheated with temperatures below ∼20 eV.These observations are co-located with the reversal of magnetic field sunward component sign, which implies that magnetic field curvature plays an important role in the reported phenomenon.Recently, similar populations have been reported in the vicinity of magnetosheath terminator region by Dubinin et al., 2023, and were interpreted as sheath protons flowing around a slower population of oxygen ions extracted from ionosphere via a pick-up process.
The current paper addresses a different region is space, located on the nightside of the planet, allowing to study evolutional aspects of these populations.It contains analysis of 1 month of magnetic field reversal region crossings by Mars Atmosphere and Volatile Evolution spacecraft (MAVEN) spacecraft in the nightside magnetosheath and discussion of key forces driving the ion acceleration.
The paper's organization unfolds as follows.Description of instruments and data products used in the study is given in Section 2. Section 3 contains a detailed description of one of the observational events.Statistical observations of the reported phenomenon are given in Section 4, where subsection 4.1 contains description of event selection algorithm and subsection 4.2 furnishes statistical insights concerning the analysis of 28 selected cases.Section 5 is dedicated to a comprehensive discussion of the observed data, culminating in a concluding section to wrap up the paper.

Instrumentation
The data employed in the present study is sourced from the MAVEN spacecraft (Jakosky et al., 2015), which has been in orbit around Mars since late 2014.During the period under investigation, MAVEN maintained an orbit characterized by a roughly 3.5-hr period and an inclination of ∼75°.Furthermore, its periapsis and apoapsis were situated at ∼200 and ∼4,400 km from the Martian surface, respectively.The study addresses data from Solar Wind Ion Analyzer (SWIA) and Supra-Thermal and Thermal Ion Composition (STATIC) top-hat ion spectrometers, Solar Wind Electron Analyzer (SWEA) electron spectrometer and magnetometer MAG.

An Example of a Single Event
The paper uses the Mars-Solar-Orbit (MSO) coordinate system to describe spacecraft position, in which the X axis is directed to the Sun, the Z axis is codirectional with Martian orbital angular momentum with respect to the Sun, and Y completes the system to the right-handed triple.beginning of the shown interval till 10:55 UT and is characterized by broad proton energy spectra with a peak at ∼250 eV (panels (a) and (b)).
The initial impetus for this study was the observation of an unusually high electron density in the magnetosheath region for a short period of time from ∼10:20 to ∼10:25 UT, as depicted by the solid black line in Figure 1e.While this data was roughly estimated from spacecraft potential by Langmuir probe and Waves (LPW, Andersson et al., 2015) experiment (I-V curve doesn't provide reliable density estimation at these altitudes), our research subsequently leverages more precise ion density measurements acquired by the STATIC instrument.
The B XMSO component, presented in panel (h), remains steady at ∼+10 nT until it abruptly changes its sign at ∼10:22 UT.At the same time, magnetic field magnitude experiences a decrease from ∼10 to ∼3.5 nT and then returns to its previous value.This change of B XMSO sign is accompanied by the presence of O + and O 2 + ions, observed at energies exceeding 1 keV, which are clearly seen between 10:15 and 10:30 UT (panels (a), (c), (d)).
Simultaneously with the observation of oxygen ions, magnetosheath proton number density drops from ∼2 to 3 cm 3 to ∼0.2 cm 3 .At the same time, number densities of O + and O 2 + reach their maximums at ∼1.5 and 5 cm 3 , respectively, while these two populations are almost absent outside of the magnetic field reversal region.Thus, the number density of heavy ions inside the magnetic field reversal region is more than double the number density of the protons outside.The bulk velocity of accelerated O + during magnetic field reversal is ∼150 km/s, O 2 + -100 km/s, while proton bulk velocity drops from nearly 200 km/s in the ambient shocked solar wind to ∼110 km/s.Another notable feature of the observed oxygen ions is that their measured velocity varies with time by ∼30% and reaches its maximum at the moment when the magnetic field magnitude has a minimum, which can be clearly seen in panels (a) and (g).An increase in electron energy flux around 10:20-10:25 UT (panel (e)) is observed, interpreted as necessary for maintaining quasi-neutrality in response to the heightened ion number density coinciding with the magnetic field reversal.

Event Selection
The statistical research is based on analysis of 28 events of magnetic field sunward component sign reversal accompanied by observation of high-density oxygen ion populations.The whole month of December 2020 has been considered for the selection of these events, as during this period of time spacecraft's apoapsis was in the nightside magnetosheath at X MSO close to ∼ -1.8 R M while the orbit did not cross the shadow of the planet and magnetotail region.
As these events are associated with magnetic field reversals, the magnetic field data was used for their primary automatic identification, followed by a manual assessment of the candidate events.First, B XMSO component data has been smoothed by applying 5-min moving average window to reduce the number of B XMSO = 0 crossings associated with magnetic field noise.Subsequently, points with a change of sign in the smoothed data were identified as candidate events.
Second, candidates with an even number of events identified within a 5-min interval were excluded from consideration in order to choose only those instances demonstrating a clear transition from a steady B XMSO > 0 region to a B XMSO < 0 region (or vice versa).If this number was odd, it was treated as a single "noisy" transition, and therefore only the middle event was left for consideration.
Finally, the times of the candidate events have been shifted from the B XMSO reversal points to the moments of time with a maximum relative O + and O 2 + number density over all ion species within a 5-min interval of the identified event in order to adjust the position of heavy ions escape channel.
This procedure resulted in identification of 476 B X sign reversals associated with an increase of a relative oxygen number density composition on the nightside of the planet within the time interval from 1-31 December 2020.
Figure 2 represents a scatter plot diagram of all the 476 primarily selected events in O 2 + number density-bulk velocity coordinates, where color indicates relative proton concentration (n p and n h are proton and sum of O + and O 2 + number densities, respectively).These parameters have been calculated as zeroth-and first-order moments of ion distribution function measured by STATIC.The diagram reveals the presence of 4 clusters, each of which represents its own region of Martian plasma environment or the physical process occurring in it.This clustering is considered to be tentative, and a more rigorous selection procedure is undertaken further.

10.1029/2023GL107953
The low density and slow speed cluster (lower left in Figure 2, 71 identified cases) is attributed to O 2 + ions typically seen in magnetosheath regardless of B XMSO change of sign event.These are most probably exospheric particles produced by photochemical reactions (Krasnopolsky, 1993) which are in early phase of being picked-up by the solar wind electric field.
The high density and slow speed cluster (lower right in Figure 2, 134 identified cases) represents cases with particle distributions typically observed within the ionosphere.
The low density and high-speed cluster (upper left in Figure 2, 69 identified cases) stands for the ionospheric oxygen ions accelerated via solar wind motional electric field -V SW × B, representing an ionospheric pick-up population.Previous studies (Dong et al., 2015(Dong et al., , 2017;;Dubinin et al., 2021;Vaisberg et al., 2018) suggest that these ions are continuously observed in the "+E hemisphere" of Mars (where the angle θ E between solar wind electric field vector and spacecraft radius-vector in Y-Z MSO plane is less than 90°) as a narrow band in energytime spectra, the energy of which is dependent on the altitude, and the pattern of these ions in spectrograms are not significantly affected by the change of B XMSO sign.
Finally, the high density and high-speed cluster (upper right in Figure 2, 202 identified cases) is attributed to the ions that the current research is focused on.A distinctive oxygen ions density spike is observed at B XMSO reversal within this group, a characteristic not observed in the other three groups.This cluster contains the largest number of dots among the 4 groups and makes up ∼42% of all the primarily identified events of B XMSO sign change.
Out of these 202 events associated with short observations of high density and bulk velocity oxygen ions, 28 cases were manually selected for further analysis satisfying the following criteria: (a) only one change of B XMSO sign is observed within a nightside part of a single orbit, (b) the event is collocated with observations of steady magnetosheath region (i.e., clear shocked solar wind protons and associated electron distributions were observed before and after it).The first criterion was considered in order to exclude cases with unstable magnetic field conditions during observations because that could indicate changes in the solar wind were occurring on timescales shorter than the ions transit time from the planet to the spacecraft.The second criterion allows to study how the observed heavy ion fluxes affect surrounding plasma.While a detailed description of one of these 28 observational cases was provided in the previous section, the full list of events with their properties is given in the supporting information to the paper (see Table S1 in Supporting Information S1).The subset of instances selected for in-depth analysis is denoted by filled circles in Figure 2.

Statistical Features of Tailward Accelerated Fluxes
While the example in Figure 1 is a single representative of the ion population described in this paper, the plasma properties recorded for 28 B XMSO reversal events selected in Section 4.1 are summarized in Figure 3.Although not required by the selection procedure, it was observed that θ E for all the instances didn't exceed 50°, suggesting that these populations were originally picked-up from ionosphere by solar wind electric field.As seen in panel (a), bulk velocities of accelerated O 2 + ions are directed predominantly tailward and their values vary from ∼40 to + bulk velocities inside observed 28 magnetic field reversal regions in cylindrical coordinates, black line is induced magnetosphere boundary from Trotignon et al., 2006, (b) angle between O 2 + velocity during B X sign reversal and magnetic field rotation plane, (c) angle between magnetic field prior and after B XMSO reversal crossing, (d) ratio of all ions number density during B X sign reversal to the ambient sheath plasma versus X MSO , (e) ratio of heavy ions (O + , O 2

+
) to proton number density versus X MSO , (f) ratio between velocities of different ions inside B XMSO reversal region to the ambient sheath velocity.∼140 km/s.At the same time, panel (b) shows a decrease of angle between O 2 + bulk velocity and the magnetic field rotation plane while point of observation moves further in the -X MSO direction.The magnetic field rotation plane normal n MF was calculated as normalized cross product of magnetic field vector in steady magnetosheath prior and after the reversal of B XMSO component (exact time intervals are provided in Table S1 in Supporting Information S1), and its 180°ambiguity is resolved in a way so that the normal is directed away from X MSO axis.Designation θ V O +

2
:n MF stands for the angle between n MF and O 2 + bulk velocity.As shown in panel (c), the angles between magnetic field vectors prior and after B XMSO reversal crossing θ pre:post are increasing in the analyzed data set from ∼80°to ∼160°, indicating the straightening of magnetic field lines.
Another noticeable feature of the accelerated ion observation events is that the overall plasma number density at the reversal n inside is greater than plasma number density in the surrounding sheath region n outside in almost all cases, which is seen in panel (d) of Figure 3. Furthermore, the ratio n inside /n outside is increasing in the tailward direction.Also, the relative content of heavy ions n h (derived as sum of O + and O 2 + number densities) shows an increasing trend in the same direction (panel (e)).These features of the observed ion fluxes are to be discussed in the next section.
The velocities of heavy ion populations observed inside the B XMSO reversal region (designated as V inside in panel (f)) are lower than the proton bulk velocity V p outside seen in the ambient sheath plasma, and heavier populations are slower than the lighter ones.The bulk velocities of all ions in the reversal region tend to increase at distances X MSO < -1R M , and sometimes proton velocities even exceed that of the ambient sheath region by almost 1.5 times.
In order to estimate escaping O 2 + temperature, O 2 + energy spectra were fitted with Maxwellian curves at the reversal, which included 10 data points for one of the analyzed 28 cases (resulting in ∼19.5 eV temperature evaluation), and from 2 to 5 points for the other cases.The O 2 + mean temperature averaged over all of the observed cases is ∼21.4 eV with ∼18 eV standard deviation.This should be considered as an upper temperature estimation due to relatively high STATIC dE/E ∼15% and high energies of the observed populations.

Discussion of Acceleration Mechanism
The draped magnetic field configuration is expected to prevail in the region of Martian plasma environment considered in the presented study (e.g., Ma et al., 2004).As a magnetic field flux tube passes the planet, it starts to straighten due to the magnetic tension term of j × B force, which is seen in Figure 3c, releasing the stored energy in plasma acceleration.This energy transfer is demonstrated as O 2 + velocities increasingly align with the magnetic field rotation plane (Figure 3b).The straightening process is also evidenced by the reduced magnetic field magnitude observed in the middle of B XMSO reversal (Figure 1h): as the center of magnetic field flux tube accelerates to catch up with its ends, the distance between neighboring tubes increases, which is illustrated in Figure 4. Evidence of particle acceleration is seen in Figure 3f, where the heavy ion velocities exhibit an increment with rising distance from the planet.
As the central part (i.e., with the highest curvature) of a single magnetic flux tube has lower speed than its ends located in the solar wind during its convection around the planet, magnetized solar wind protons with greater speed can escape along the draped magnetic field lines while they are still on the dayside of Mars, leading to local proton depletion in the region of magnetic field reversal.This is consistent with observations showing that the relative number density of heavy ions in its central part keeps increasing at the observed distances from the planet, as illustrated in Figure 3e.Proton bulk velocities exceeding sheath speed observed at X MSO ∼ 1.2-1.6R M can be interpreted as energetic part of surrounding plasma population, the gyroradii of which are greater than distance from adjacent lobes to the location of magnetic field reversal region.
Ion number density inside the magnetic field reversal region is several times higher than in the ambient sheath environment, as shown in Figure 3d.It creates positive electric field potential (shown with blue dashed arrows in Figure 4) which attracts additional electrons into the region to sustain plasma quasi-neutrality.Having small gyroradii and therefore being bound to the magnetic field lines (e.g., Dubinin et al., 1993;Nilsson et al., 2018), electrons pervade into the B XMSO reversal region from the surrounding sheath, gaining energies up to ∼100 eV (Figure 1e), and are subsequently decelerated on their way out.

Geophysical Research Letters
10.1029/2023GL107953 Unlike protons and electrons, oxygen ions of the observed energies have Larmor radii ∼2,000 km, therefore being unmagnetized at the scales of observed distances to the planet, so that electrons and ions become decoupled as a result of magnetic tension term of j × B force acting solely on the electrons.This force provides additional electron acceleration in the tailward direction, which is evidenced by the enhancement in electron energy spectra close to 90°pitch angle in the middle of magnetic field reversal region (see Figure S1 in Supporting Information S1).The emerging charge separation produces Hall electric field (thick blue arrows in Figure 4), which is a primary source of ion acceleration.
Escaping heavy ion temperature estimations provided in Section 4.2 show that their kinetic energy in the Mars rest reference frame (∼1 keV) exceeds their thermal energy (<20 eV) by a factor of at least 50.This is also consistent with the electric field being the main driver of particle acceleration, as it acts equally on each ion and does not lead to population thermalization.However, as O + and O 2 + population have different speeds, these ions are still prone to heating as a result of dissipative processes, probably via wave-particle interactions, occurring between them (e.g., Akbari et al., 2022), as evidenced by the increased oxygen temperature relative to the ionospheric plasma where these ions originate.Further evidence of this dissipative process is the fact that there are slightly different mean energies of heavy ion populations (Figures 1c and 1d)).

Conclusion
Observations of 476 sunward magnetic field component changes of sign on the nightside of Mars during December 2020 revealed a significant (42%) fraction of events accompanied with high density (>1 cm 3 ) fluxes of O + and O 2 + ions accelerated to ∼1 keV, while their temperatures remain <20 eV.These ions are likely to be extracted from ionosphere by solar wind electric field and subsequently accelerated by Hall electric field as a result of unmagnetized ions and magnetized electrons charge separation by magnetic tension term of j × B force applied to the electrons.
As the region with magnetic field sunward component change of sign persists constantly on the nightside of Mars, and the energetic high density oxygen ions are observed in nearly half of magnetic field reversal instances, the reported observations of accelerated heavy ions represent a common atmospheric escape channel.

Figure 1
Figure1shows an example of localized accelerated oxygen ion observations in the nightside magnetosheath during a change of B XMSO component sign occurred at 7 December 2020.The presented time interval covers the nightside part of MAVEN orbit, during which the solar-zenith angle (SZA) maximum value of ∼130°is reached at ∼10:37 UT, when the spacecraft altitude was ∼4,230 km.The magnetosheath region is observed from the

Figure 1 .
Figure 1.An example of accelerated oxygen ions observations in the nightside magnetosheath during B XMSO sign reversal.(a) SWIA energy-time spectrogram of all ions, (b)-(d) STATIC energy-time spectrograms of H + , O + and O 2 + , (e) SWEA electron energy-time spectrogram with overlayed electron density derived from LPW, (f)-(g) ion number densities and bulk velocities, respectively, derived as moments of STATIC distribution functions (velocities for densities less than 0.1 cm 3 are not shown), (h) Magnetic field magnitude, its X MSO component and spacecraft SZA.The bottom panel indicates sign of Z MSO coordinate and height above the surface of Mars in km.

Figure 2 .
Figure 2. Number density-velocity scatter plot for O 2+ ions during B XMSO sign reversal events in December 2020.Upper right dots stand for the events representing phenomena under investigation.Filled dots are instances selected for further analysis.

Figure 3 .
Figure 3. (a) O 2+ bulk velocities inside observed 28 magnetic field reversal regions in cylindrical coordinates, black line is induced magnetosphere boundary fromTrotignon et al., 2006, (b)  angle between O 2 + velocity during B X sign reversal and magnetic field rotation plane, (c) angle between magnetic field prior and after B XMSO reversal crossing, (d) ratio of all ions number density during B X sign reversal to the ambient sheath plasma versus X MSO , (e) ratio of heavy ions (O + , O 2

Figure 4 .
Figure 4. Sketch of tailward acceleration process of planetary ions by bent IMF lines with respect to Mars location.Sizes of B XMSO reversal region and curvature of magnetic field lines are not to scale.The electrons (thin red lines) pass through the region getting accelerated and subsequently decelerated on the way out along magnetic field lines.O + and O 2 + ions locations are schematically presented for 4 separate magnetic flux tubes.