The Energetic Oxygen Ion Beams in the Martian Magnetotail Current Sheets: Hints From the Comparisons Between Two Types of Current Sheets

Using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we explore the plasma properties of Martian magnetotail current sheets (CS), to further understand the solar wind interaction with Mars and ion escape. There are some CS exhibit energetic oxygen ions that show narrow beam structures in the energy spectrum, which primarily occurs in the hemisphere where the solar wind electric field (Esw) is directed away from the planet. On average, these CS have a higher escaping flux than that of the CS without energetic oxygen ion beams, suggesting different roles in ion escape. The CS with energetic oxygen ion beams exhibits different proton and electron properties to the CS without energetic oxygen ion beams, indicating their different origins. Our analysis suggests that the CS with energetic oxygen ion beams may result from the interaction between the penetrated solar wind and localized oxygen ion plumes.

Note that, apart from the classical induced magnetotail current sheets that develop due to the stretched and antiparallel draping fields in two lobes (Halekas et al., 2006;Jarvinen et al., 2018), there exist other mechanisms that contribute to the formation of CS.For example, the solar wind discontinuities can be transported across the bow shock, further extending into the magnetotail, potentially emerging as CS (Halekas & Brain, 2010).The CS may also form at the interface between crustal fields and induced magnetic fields, when the direction of the crustal magnetic field lines nearly oppose those of the induced fields (Harada et al., 2018).Recent research by Dubinin et al. (2023) reported that a mini-induced magnetosphere could be formed when the solar wind interacts with localized heavy ions.This structure occasionally appears as a CS, which primarily occur in the plume region and adjacent to the induced magnetosphere boundary.Considering that the solar wind plasma could intrude into the magnetosphere (Dubinin et al., 2006), such CS might also arise within the Martian magnetosphere upon the interaction between intruded solar wind and planetary ions.
In essence, the CS may have various origins and are more complicated than we expected.Our comprehensive understanding of their characteristics, origins, evolution, and formation processes remains limited due to the constrained capabilities of earlier space instruments, such as the low time-resolution and narrow field-of-view (FOV) of plasma instruments.With more comprehensive and high-resolution measurements from MAVEN (Jakosky et al., 2015), we are now able to reevaluate the properties of the CS with greater precision.Based on the pattern of energy distribution of oxygen ions in the energy spectrum, we categorize the CS into two distinct groups: those accompanied by energetic oxygen ion beams, and those without.Then we highlight the unique properties of each CS category in order to reveal their origin and impact on ion escape.

Data Sets and Coordinates
Here we utilize magnetic field data from the Magnetometer (Connerney et al., 2015), ion data from the Suprathermal and Thermal Ion Composition (STATIC) instrument (McFadden et al., 2015) and the Solar Wind Ion Analyzer (SWIA) (Halekas et al., 2013), along with electron data from the SolarWind Electron Analyzer (SWEA) onboard MAVEN (Mitchell et al., 2016).This study employs two Cartesian coordinate systems.The first is the Mars Solar Orbital (MSO) coordinates, where X → MSO points from Mars to the Sun, Y → MSO points opposite to the component of the orbital velocity perpendicular to X → MSO , and Z → MSO completes the right-handed system (e.g., Li et al., 2023;Zhang, Rong, et al., 2023).The second is the Mars Solar Electric coordinates (MSE), where X → MSE points antiparallel to the upstream solar wind flow, Y → MSE points along the cross-flow magnetic field component of the upstream IMF, and Z → MSE points along the direction of the convection electric field in the solar wind.Based on the bow shock crossings (Liu et al., 2021), and following Zhang, Rong, et al. (2022), we construct the MSE coordinates for orbits with steady IMF conditions, characterized by that the angle between the inbound and outbound IMF is less than 30°.Furthermore, we utilize the plasma moments of H + , O + , and O 2 + , derived from STATIC measurements, following methodologies outlined in Fränz et al. (2006), Dubinin et al. (2017, 2021), and Li et al. (2023).For each ion species, its 3D velocity distribution is constructed by integrating various STATIC telemetry products (c0, cf., d1, ce, d0, cd, cc, and ca) into a common matrix with a resolution of 32 energy bins, 4 mass bins, 64 spatial bins, and a 4-s time resolution.Subsequently, it is corrected for the spacecraft potential and its velocity based on Liouville's theorem (Lavraud & Larson, 2016).Finally, we apply the integral method to calculate the moments, including the density, velocity (Zhang, Futaana, et al., 2022).

Cases Study
Here, we briefly examine a typical CS with energetic oxygen ion beams that took place on 25 September 2016, and compare it with a CS without energetic oxygen ion beams observed on 5 December 2014.

A Case of Current Sheet With Oxygen Ion Beams
The left panels of Figure 1 (Figures 1a1-1i1) present an overview of a typical CS with oxygen ion beams.The variations of magnetic field strength (Figure 1a1) and ion spectrums (Figure 1b1) suggest that MAVEN initially detected the magnetosphere and traversed the MPB at approximately 10:10 marked by the presence of highly fluctuating magnetic fields and heated ions.Moreover, the magnetic field strength predicted by the latest crustal fields model (Gao et al., 2021) remained practically negligible throughout (red dashed line in Figure 1a), implying that MAVEN entered the region filled by induced fields.The CS crossings occurred between 09:54-09:59 (see the black vertical lines in Figures 1a1-1b1), with MAVEN situated at approximately (X MSO = 1.16,YMSO = 0.43, Z MSO = 1.19)R M (radius of Mars, R M = 3,390 km) in Mars Solar Orbital (MSO) coordinates (refer to Figure S1 in Supporting Information S1), which was within the nominal magnetotail region.
Figures 1c1-1i1 offer a detail observation of this event.The CS was pinpointed through the reversal of B x around 09:56:40 (Figure 1c1).The interval during which the B x exhibited a monotonic variation is considered the entire CS crossing, spanning from 09:55:10 to 09:58:10, which is highlighted by the yellow shaded region.
Notably, it can be seen that the CS was associated with the oxygen ion beam, as both O + (Figure 1d1) and O 2 + (Figure 1e1) show a narrow beam shape in the energy spectrum (e.g., Carlsson et al., 2008).Fundamentally, this beam structure suggest that its temperature is relatively low.Concurrently with the beam, there is a presence of cold O 2 + ions, having energies in the range of 10-100 eV/q, within the CS.A distinctive characteristic of this CS is the surrounding presence of hot H + with energies ranging from 10 to 300 eV/q (Figure 1f1), alongside 30-300 eV electrons (Figure 1g1).Note that, in the center of CS, the H + exhibit two distinct populations.The first is the 10-300 eV H + , which resemble those in the surrounding region but exhibit a reduced flux.The second population is characterized by ∼1 keV H + , displaying a narrow beam structure, similar to that of O + and O 2 + .
The number density of oxygen ions exhibits significant variation during this CS crossing (Figure 1h1).In the external region of CS during 09:58:20-09:58:50, the number densities of O + and O 2 + are about 0.7 and 0.5 cm 3 , respectively.They increase to about 3.9 and 26.2 cm 3 in the center of CS, respectively.The density of H + decreased from 2 cm 3 in the outer region to 0.4 cm 3 in the center of CS.Taking into account the above results, we propose that the 10-300 eV H + ions originate from the solar wind.Conversely, the ∼1 keV H + ions, which display a beam-like structure similar to that of oxygen ions, are likely from the ionosphere.
Figure 1i1 illustrates that the tailward velocity of oxygen ions depends on their mass, specifically ) are about 2.5 × 10 8 cm 2 s 1 .
Figure 1j1 illustrates the variations among different pressure components.It's evident that the magnetic fields pressure (P M ) dominates in both two edges of CS, while the total thermal pressure of oxygen ions (O + and O 2 + ) (P OX ) dominates in the center of CS.The total pressure (P Total ) remains relatively constant across the CS, suggesting that the CS is in a state of pressure equilibrium.

A Case of Current Sheet Without Oxygen Ion Beam
Figures 1a2-1i2 display an overview of a typical CS without oxygen ion beams.MAVEN initially detected the magnetosheath and traversed the MPB at approximately 22:50, characterized by the vanishing of the highly fluctuating magnetic field and heated ions.The CS crossings took place between 23:16 and 23:24, as indicated by the black vertical lines in Figures 1a2-1b2.During this period, MAVEN was positioned at approximately ( 1.43, 0.08, 0.2) R M (refer to Figure S1 in Supporting Information S1).Similar to the Case 1, the strength of crustal fields remained negligible during this case (refer to Figure 1a2).1e2).Cold H + and electrons, with energies less than 10 eV, are also evident in Figures 1f2 and 1g2.The increase of the energy of H + is due to the enhanced spacecraft potential, as indicated by the black line in Figure 1f2.The analogous energy distribution between H + and oxygen ions indicates that these H + ions are of ionospheric origin, different from Case 1.
Additionally, this CS displays a markedly higher density of oxygen ions.ionosphere (Lundin et al., 2009).While the oxygen ion density is greater in this case compared to Case 1, the net tailward flux of oxygen ions is about 1.8 × 10 8 cm 2 s 1 , slightly lower than the Case 1 since the tailward speed of O + and O 2 + are limited to approximately 18.6 km/s and 14 km/s, respectively (Figure 1i2).The , indicates that O + and O 2 + were also accelerated to the similar energy.In this case, the velocity distribution of oxygen ions is quite spread out and not concentrated in the tail direction, which is different from Case 1 (see Figure S2 in Supporting Information S1).
The variations of pressure terms observed in this case exhibit similarities to those in Case 1, where P M is predominant at both edges of the CS while P OX prevails in the center of CS (Figure 1j2).Moreover, this CS also maintains a state of pressure equilibrium, as evidenced by the relative constancy of P Total across the CS.
For both cases, the typical ion flow directions in the magnetotail-sunward and tailward-are within the field-ofview of the STATIC instrument (refer to the Figure S3 in Supporting Information S1).

Selecting Criteria and Classification
In order to comprehensively examine the properties of CS, we conducted a statistical study of the CS based on the MAVEN data set spanning from October 2014 to December 2020.We established the following criteria to identify and categorize the CS.
1.The spatial region should be within the nominal magnetotail, defined by, X MSO < 0 and 2. To guarantee the accuracy of the moment data, we only consider data when both the Sun and tail directions are within STATIC's FOV, and its energy range spans from below 10 eV to above 1,000 eV. 3. The CS are initially identified by a significant and rapid reversal in B x .Specifically, we define the magnetic field characteristic of the CS as a change in the polarity of B x , accompanied by a variation in its magnitude exceeding 5 nT within a 60-s period (Zhang, Rong, et al., 2023).Additionally, we visually check each CS, to eliminate the pseudo-crossings in the external magnetosheath, which exhibits a continuous spectrum of solar wind ions with energies typically above 800 eV and a highly fluctuating magnetic field.4. Since the CS identification is only reliable in non-crustal fields region (Halekas & Brain, 2010;Harada et al., 2020) Based on the above criteria, we identified 1918 CS in total.These were subsequently divided into two distinct categories.The first type, termed "With Beam" CS, is distinguished by the presence of energetic oxygen ion beams.Initially, we sum up the differential energy flux (DEF) of the O + and O 2 + , and determine the energy corresponding to its maximum DEF, denoted as E peak .Given our concentration on the energetic beam, a "With Beam" CS must meet the condition E peak > 200 eV.Assuming that the E peak corresponds to the i peak energy step, we aggregate the DEF between adjacent steps, calculated as DEF peak = DEF i peak 1 + DEF i peak + DEF i peak +1 , where DEF i represents the different energy flux at ith energy step.Then we calculate the ratio, DEF peak /DEF all , where DEF all represents the total differential energy flux across all energy steps.A high ratio signifies a narrow oxygen ion beam pattern.Hence, we classify the CS with the ratio exceeding 0.7 as the "With Beam" CS.For example, for the Case 1 described in Section 3, the E peak and DEF peak /DEF all are about 1.12 keV and 0.97, satisfying all criteria of "With Beam" CS.Using these criteria, we identify 587 "With Beam" CS in total.The other 1331 CS were classified as the "Without Beam" type.

Statistics
In this section, we present the statistical results for both types of CS to highlight their average characteristics.

Spatial Distributions
Figures 2a and 2b illustrate the spatial distributions of 587 "With Beam" CS and 1,331 "Without Beam" CS on the X MSO R MSO plane.The "With Beam" CS largely cluster near the wake boundary, progressively penetrating deeper into the Martian wake as they move tailward (Figure 2a).In contrast, the "Without Beam" CS are distributed more randomly (Figure 2b). Figure S4 in Supporting Information S1 demonstrates that neither type of current sheet exhibits a preference for occurring in the southern hemisphere, where the crustal fields are predominantly located.This indicates that crustal fields have a minimal influence in controlling these two types of CS.Figures 2c and 2d show the spatial distributions on the Y MSE Z MSE plane.Only 82 "With Beam" CS and 334 "Without Beam" CS met the criteria for to the construction of MSE coordinates (see Section 2).Most "With Beam" CS (approximately 89% or 73 events) were observed in the +E hemisphere (Figure 2c), consistent with the previously identified ion beams observed by Mars Express (e.g., Carlsson et al., 2008).In contrast, the "Without Beam" CS predominantly occur in the E hemisphere, representing around 74% of total events (248 events) (Figure 2d).

Dependence on the Magnitude of Pdy and Esw
Figures 2e and 2f display the relationship between the normalized occurrence rate of both "With Beam" and "Without Beam" type CS with respect to the magnitude of the solar wind dynamic pressure (Pdy) and solar wind convective electric fields (Esw), respectively.There are 134 instances of "With Beam" CS and 825 instances of "Without Beam" CS that correspond with upstream solar wind conditions.The normalized occurrence rate is defined as N events /N total , where N events represents the number of cases in each bin, N total is the total events number of each type, which equals to 134 and 825 for the "With Beam" and "Without Beam" CS.However, no distinct disparity between the two types of CS is observed.Note that some events occurred during solar wind transits, such as interplanetary coronal mass ejections.However, given their relatively low occurrence, we believe they do not significantly impact the results.

Average Plasma Properties
Figure 3 compares the average plasma properties as traversing the "With Beam" type and "Without Beam" type CS.We identify the time interval for each CS crossing based on the methodology outlined in Section 4. The peak value of | B x | is identified as B 0 .Consequently, B x /B 0 ∼ ±1 signify the outer regions of the CS, whereas B x /B 0 ∼ 0 denotes its central region.
Figures 3a1-3a2 demonstrate that for both types, the average densities of O + (N O + ) and O 2 + (N O + 2 ) increase when transitioning from the two-side edges | B x /B 0 |≈1) to the center of CS (B x /B 0 ≈ 0).For the "With Beam" CS type, N O + and N O + 2 shift from roughly 0.5 and 1 cm 3 to about 1.7 and 5.3 cm 3 from edges to the center, respectively.In contrast, the "Without Beam" CS exhibits more oxygen ions, with its average densities being 5-8 times greater than the "With Beam" CS.Specifically, N O + and N O + 2 in the "Without Beam" CS range from about 4, 8 cm 3 to approximately 10, 24 cm 3 from edges to the center, respectively.
The profile of density of H + (N H + ) exhibits differently to that of oxygen ions.In the "With Beam" type CS, N H + decreases from approximately ∼0.95 to 0.55 cm 3 from edge to the center of CS.Conversely, in the "Without Beam" type CS, it increases from roughly 3.4 to 6.8 cm 3 from edge to the center of CS.Figures 3b1-3b2 display the density ratio of H + , calculated by 2 ).The density ratio of H + drops from around 0.4 to approximately 0.1 from edge to the center of "With Beam" CS, whereas it maintains at about 0.2 without significant change across the "Without Beam" CS.Therefore, H + in the "With Beam" type likely originates from the solar wind, while it appears to be of ionospheric origin in the "Without Beam" type.
Figures 3c1-3c2 contrast the net tailward flux of oxygen ions as traversing the "With Beam" and "Without Beam" type CS.It's evident that the average flux of oxygen ions rises from both edges of CS toward the center of the CS in both types.However, even though the "Without Beam" type CS exhibit denser oxygen ions, the average flux within the "With Beam" type CS, ranging from 1.32 × 10 7 to 5.09 × 10 7 cm 2 s 1 , is approximately 2-4 times greater than in the "Without Beam" CS, which ranges from 0.65 × 10 7 to 1.55 × 10 7 cm 2 s 1 .This can be attributed to that the oxygen ions have a higher tailward speed in the "With Beam" CS.
There are marked differences in the electron distribution between the "With Beam" and "Without Beam" CS types (see Figures 3d1-d2).The "With Beam" CS seems to feature two distinct electron populations: one with energies spanning 30-300 eV, and another comprised of low-energy electrons (below 10 eV).In contrast, the "Without Beam" CS predominantly showcases a single electron population, with energies under 100 eV in the edges and accelerating to around 100 eV at the center.The different electron properties between the two CS types also suggest their different origins.

Discussion and Conclusion
Here we present a statistical study of current sheets with and without the energetic oxygen ion beams in the Martian magnetotail.The key findings can be summarized as follows.
1.Both types of CS exhibit pronounced E asymmetry.The "With Beam" ("Without Beam") CS tends to occur in the +E ( E) hemisphere.Furthermore, no clear correlation can be observed between either type of CS and the magnitude of Pdy and Esw. 2. For both types of CS, a significant increase in the density of oxygen ions is observed from the edges toward the center of the CS.This leads to that the magnetic fields pressure being dominant at the edges of the CS, while the thermal pressure of oxygen ions becomes more pronounced at its center.On average, the density of oxygen ions in the "Without Beam" CS is approximately 5-8 times greater than that in the "With Beam" CS.However, the average net tailward flux is higher in the "With Beam" CS, attributed to the faster speeds of the oxygen ions within it.3. The characteristics of H + and electrons exhibit notable differences between these two types of CS.The energy distribution and density variation of H + indicate that, in the case of the "With Beam" CS, the H + is likely sourced from the solar wind.In contrast, the H + present in the "Without Beam" CS appears to originate from the ionosphere.
We find that the escaping flux of oxygen ions in the CS is related to its type, as evidenced by the higher net tailward flux in the "With Beam" CS.This indicates that despite the majority of escaping oxygen ions in the magnetotail being primarily low energy ions (with energy less than 200 eV) (Curry et al., 2022;Dubinin et al., 2017;Ramstad et al., 2015Ramstad et al., , 2016Ramstad et al., , 2017)), high-energy ions (E > 200 eV) might play a dominant role in ion escape within the CS.This highlights the complexity of ion escape, emphasizing its considerable variability in different regions.
The presence of the low-energy ionospheric ions, combined with a higher occurrence rate, indicates that most of the "Without Beam" CS can be characterized as classical induced CS, resulting from the stretched draped field lines (see Figure 4).Conversely, the "With Beam" CS exhibit plasma characteristics akin to those found in the CS occurred in the oxygen ions plume as described by Dubinin et al. (2023) and Jarvinen et al. (2018), in that the density of oxygen ions increase from two edges to the center of CS, while the density of solar wind (H + ) decrease from two edges to the center of CS.This hints at potentially similar origins or formation processes between the "With Beam" CS and the CS occurred in the oxygen ions plume.Given that the structure of oxygen ion beams in the "With Beam" CS resembles the oxygen plume near the ion composition layers as shown in Halekas et al. (2019), we propose that these beams might be localized oxygen ion plume structures (refer to Figure 4).Similar to CS formation in the oxygen plume, the penetrated solar wind (magenta dots) interacts with localized oxygen ion plume (gray shaded region), leading to the formation of a CS characterized by fewer solar wind protons but an increased presence of oxygen ions.Once the CS is formed, the nearby cold heavy ions would move toward the CS center due to the lower magnetic field pressure, which is in agreement with the appearance of cold heavy ions shown in Figure 1c.In this scenario, we anticipate a greater frequency of CS formation outside the planet's wake region due to the higher incidence of solar wind plasma penetration, consistent with our findings (see Figure 2a).Moreover, considering that the solar wind prefers to move from the +E hemisphere toward the E hemisphere in the magnetotail (Dubinin et al., 2018;Zhang, Rong, et al., 2022), and the localized plume is expected mainly occur in the +E hemisphere, the "With Beam" CS would predominantly form in the +E hemisphere, which aligns with our result.

Figure 1 .
Figure 1.Overview of two typical crossings of CS.A "With Beam" CS case that occurred on 25 September 2016 is displayed in the left column.The right column shows a "Without Beam" CS which occurs on 5 December 2014 (a1), (a2) The magnetic field strength observed by MAVEN (black solid line) and crustal magnetic field strength (red dashed line) estimated by the crustal fields model (Gao et al., 2021) (b1), (b2) the ion's spectrum measured by SWIA.The black vertical dashed lines denote the interval of crossing of the CS (c1), (c2): the time series of the magnetic fields.The black horizontal dashed line represents the zero value (d1), (d2), (e1), (e2), (f1), (f2) show the energy spectrums of O + , O 2 + , H + measured by STATIC, respectively.The black lines in the bottom (d2)-(f2) represent the value of negative spacecraft potential (g1), (g2) show the energy spectrums of electrons (h1), (h2): the density of ions (i1), (i2) the tailward speed of oxygen ions (V x ) (j1), (j2) show the variations of different pressure terms: total pressure (P Total ), total thermal pressure of oxygen ions (O + and O 2 + ) (P OX ), thermal pressure of H + (P H ) and magnetic fields pressure (P M ).The black shaded region denotes the crossing interval of CS.
The CS was identified by reversal of B x components at about 23:20 as depicted in Figure1c2.The time interval of entire CS crossing was considered as 23:17-23:22, when the B x basically exhibited a monotonic variation.Contrary to Case 1, it was predominantly filled with cold O + and O 2 + with energies below 30 eV (Figures 1d2- where (X MSO , Y MSO , Z MSO ) represent the position of MAVEN.R MPB represents the time-averaged radial position of the magnetic pile-up boundary (MPB) from the model of Trotignon et al. (2006).

Figure 2 .
Figure 2. (a), (b) shows the spatial distribution of "With Beam" type CS (red dots), "Without Beam" type CS (blue dots) in the X MSO R MSO plane, where R = ̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ ̅ Y 2 MSO + Z 2 MSO √ .The black curves in (a) and (b) denote the nominal magnetic pile-up boundary (MPB) from Trotignon et al. (2006).(c), (d) show them in Y MSE Z MSE plane.The black circles in panels (a)-(d) represent the planet.The black dashed lines in panel (c), (d) represent the Z MSE = 0 plane.(e), (f) shows the histogram of normalized occurrence rate for two types varied with Pdy, Esw, respectively.The red (blue) color represents the "With Beam" ("Without Beam") CS.

Figure 4 .
Figure 4. Sketch of the formation of two types of current sheet in the Martian magnetotail.The red curves with arrows represent the magnetic field lines of "With Beam" CS, which are formed by the interaction between the penetrated solar wind (marked as magenta dots with the label "H") and localized oxygen ion plume (gray shaded region).The "Without Beam" CS (blue curves with arrows) are filled with low-energy oxygen ions (yellow shaded region).The directions of the solar wind flow (Vsw) and its electric field (Esw) are indicated by the black arrows in the bottom left corner.
implying equivalent energy acceleration for both O + and O 2 + .The O + , O 2 + could reach around 106.83 km/s and 80.2 km/s in the center of CS, respectively.Furthermore, the velocity distribution of both O + and O 2 + shows that the oxygen ion beams are mainly concentrated on the tail direction (refer to Figure S2 of Supporting Information).For this case, the net tailward flux of oxygen ions (O + and O 2 +