# Different Faces of the Martian Magnetosphere

Eduard Dubinin, Markus Fraenz, Ronan Modolo, Martin Pätzold, Silvia

Tellmann, Gina Dibraccio

► To cite this version:

Eduard Dubinin, Markus Fraenz, Ronan Modolo, Martin Pätzold, Silvia Tellmann, et al. Different Faces of the Martian Magnetosphere. Journal of Geophysical Research Space Physics, 2025, 130 (4), pp.e2025JA033717. 10.1029/2025JA033717. hal-05024053

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RESEARCH ARTICLE

10.1029/2025JA033717

**Key Points:**

* The Martian ionosphere contains components of the induced and intrinsic magnetospheres

* Using different coordinate systems and averaging on the different data set we can separate these components

* A dipole-like obstacle for solar wind is revealed

**Correspondence to:**

E. Dubinin,
dubinin@mps.mpg.de

**Citation:**

Dubinin, E., Fraenz, M., Modolo, R., Pätzold, M., Tellmann, S., & DiBraccio, G. (2025). Different faces of the martian magnetosphere. *Journal of Geophysical Research: Space Physics*, 130, e2025JA033717. https://doi.org/10.1029/2025JA033717

Received 23 JAN 2025

Accepted 18 MAR 2025

©2025. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

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Different Faces of the Martian Magnetosphere

E. Dubinin¹ ID, M. Fraenz¹ ID, R. Modolo² ID, M. Pätzold³ ID, S. Tellmann³, and G. DiBraccio⁴ ID

¹Max-Planck-Institute for Solar System Research, Göttingen, Germany, ²LATMOS, Université de Versailles Saint Quentin, CNRS, Sorbonne Université, CNES, Guyancourt, France, ³Rheinisches Institut Fuer Umweltforschung, Abteilung Planetforschung, Cologne, Germany, ⁴NASA Goddard Space Flight Center, Greenbelt, MD, USA

**Abstract** The Martian magnetosphere contains components of the induced and intrinsic magnetospheres. Since we can not get a global snapshot of the magnetosphere we use a statistical picture based on a large number of measurements. Depending on the choice of coordinate system and the data selection we can observe different faces of the Martian magnetosphere. If we map data in the Martian Solar Electric coordinate system, then we can separate the induced features of the magnetosphere which appears similar to the magnetosphere of Venus. If we use the Martian Solar Orbital coordinate system and select together the orbits with positive and negative *B*<sub>y</sub>-component of the IMF, then the effects related to draped magnetic field and the high order harmonics of the crustal magnetic field are significantly weakened because of averaging over many orbits and a dipole-like magnetosphere of Mars becomes visible. If we select separately the orbits with positive and negative *B*<sub>y</sub>-component we observe a twist of the magnetotail in the direction determined by the sign of *B*<sub>y</sub> that is typical for a hybrid magnetosphere with the induced and intrinsic components. The intrinsic and induced components are also well separated when we select the orbits with northward IMF. Then we observe the features that are somewhat similar to those at the Earth magnetosphere. When we use the MSO coordinates and separate by the phase of Mars rotation, the tail topology occurs more complex. This indicates that the effects of the local crustal magnetic field turn out as being also important.

**Plain Language Summary** The Martian magnetosphere contains features typical for induced and intrinsic magnetospheres. To describe these features we use a picture based on a statistical approach with a large number of measurements. It was shown that utilizing different coordinate systems and selecting orbits with different cross-flow components of the interplanetary magnetic field in the solar wind we can separate the induced and intrinsic components inherent to the Martian magnetosphere. We can isolate the induced component and observe the features that are in many ways similar to those we see at Venus. Isolating the intrinsic component we have found a dipole-like obstacle for solar wind.

1. Introduction

The nature of the obstacle to solar wind at Mars has been widely discussed since the first space missions to this planet. The position of the bow shock observed during the flyby by Mariner 4 and by the Soviet spacecraft Mars 2, 3, and 5 allowed to estimate the size of obstacle. Analyzing these data Slavin and Holzer (1982) have concluded that a modest intrinsic magnetic dipole moment (1.4 (±0.6) × 10²² G cm³) would be necessary to explain the effective bow shock shape and location. On the other hand, a composition transition (ion cushion) between the solar wind and obstacle observed by Bogdanov and Vaisberg (1975) on Mars-2 and 3 pointed to the atmospheric origin of the obstacle and the existence of the induced magnetosphere at Mars which was first simulated in laboratory experiments (Podgorny & Andrijanov, 1978; Podgorny et al., 1980) and then was comprehensively studied at Venus (see e.g. Luhmann, 1986; Luhmann & Cravens, 1991; Luhmann et al., 1980; Russell & Vaisberg, 1983; Russell et al., 1979). The lack of consensus on the nature of the obstacle has stimulated laboratory studies of the case when both induced and the dipole components are present at the same time. Such a magnetosphere was called *combined* (Dubinin et al., 1980) and later *hybrid*. There was no a final answer about the nature of the solar wind interaction with Mars and after the measurements made by the Phobos-2 spacecraft because the closest approach to Mars was only about 800 km. The magnetic field and plasma data in the tail were consistent with draped configuration of the induced magnetosphere (Dubinin et al., 1996; Yeroshenko et al., 1990). On the other hand, tracing of the solar wind electrons in the tail gave a hint on the existence of the magnetic field lines connected to Mars (Dubinin et al., 1994). M'ohlmann et al. (1991) have also found spectral peaks at harmonics of Mars' rotation period in the magnetic field data that were attributed to a weak intrinsic magnetic field. This
dualism was resolved after the measurements made by Mars Global Surveyor (MGS). MGS has discovered the existence of strong crustal magnetic fields at Mars (Acuña et al., 1999). Such fields are able to modify essentially the whole pattern of the solar wind interaction with Mars. Distinct signatures of reconnection indicated the coexistence of the draped and intrinsic components (Eastwood et al., 2008; Halekas et al., 2006, 2009). Deviations from the ideal draping geometry and its dependence on sign of the $B_y$ component in the solar wind also indicated a complex field geometry (Brain et al., 2006).

Although the Mars Express spacecraft has no onboard magnetometer, the plasma and optical measurements have brought up new important information about the structure of the Martian magnetosphere (Barabash et al., 2007; Bertaux et al., 2005; Dubinin et al., 2006, 2008, 2011; Fedorov et al., 2006; Fraenz et al., 2010; Gurnett et al., 2005; Leblanc et al., 2008; Lundin et al., 2008, 2009; P'atzold et al., 2016).

The Mars Atmosphere and Volatile Evolution (MAVEN) mission (Jakosky et al., 2015) has provided a new extensive set of magnetic field and plasma measurements and demonstrated that the Martian magnetosphere is unique in our Solar system and contains induced as well as intrinsic components. Direct interaction of solar wind with the Martian ionosphere produces an induced magnetosphere with a magnetotail. The magnetotail is formed by the draped interplanetary magnetic field (IMF) lines which have been slowed down when they pass through the ionosphere. Solar wind also interacts with the crustal magnetic field that leads to the features typical for an intrinsic magnetosphere. The strong crustal field produces a local obstacle to solar wind shifting upward the magnetosphere boundary and bow shock by peculiar bulges on the obstacle surface rotating with Mars (Brain et al., 2005; Edberg et al., 2008; Gruesbeck et al., 2018). A drift motion of trapped electrons within local tiny mini-magnetospheres resembles their behavior in the Earth magnetosphere (Harada et al., 2016; Zhang et al., 2023). The crustal field-dominated Martian magnetosphere topology is constantly changing by the planet's rotation. As a result, a complex magnetosphere can be formed with different classes of the magnetic field lines (Weber et al., 2017, 2019; Xu et al., 2019)-draped field lines typical for the induced magnetosphere, closed field lines typical for the intrinsic magnetosphere and open field lines with one end connected to the Mars ionosphere and the other end to the solar wind. Open field lines can be either connected to the surface after reconnection between the draped and crustal field lines or deeply draped (i.e., have a draped topology but penetrate below the electron exobase). The topology of the magnetic field lines around regions with crustal field becomes very intricate, due to reconnection between the IMF and crustal fields (Harada et al., 2017, 2018, 2020). Precipitation of electrons along the open field lines produce a discrete aurora typical for the Earth magnetosphere (Girazian et al., 2022; Lillis et al., 2018; Schneider et al., 2021). It is worth noting that crustal magnetic fields on Mars not only change the field topology but also significantly vary the density distribution in the upper ionosphere (Andrews et al., 2013; Dubinin et al., 2012, 2016) and impact the ion loss (Dubinin, Fraenz et al., 2020; Dubinin, Luhmann et al., 2020; Fan et al., 2019; Fang et al., 2015, 2017; Weber et al., 2021). The MAVEN observations have also disclosed a global response of the magnetic tail on the sign of the IMF $B_y$ component in the solar wind: a twist of the magnetotail lobes and plasma sheet in the clockwise or anticlockwise direction depending on the sign of the $B_y$ component (DiBraccio et al., 2018, 2022; Dubinin et al., 2017). According to DiBraccio et al. (2022) this twisting can reach ~60° away from its expected location based on IMF draping.

How to disentangle the induced and intrinsic components of Mars's magnetosphere? Since we can't get a snapshot of the magnetosphere we are forced to use a statistical approach based on a large number of measurements performed in different parts of the Martian magnetosphere. Then, we can choose, for example, different coordinate systems in order to isolate the purely induced and intrinsic components and to find their main features. In this study we utilized the magnetic field data measured by the MAVEN spacecraft (Connerney et al., 2015). We derive the IMF orientation by taking the median value of the magnetic field vector components from 30 min around the time where the maximum proton velocity measured by the solar wind monitor SWIA (Halekas et al., 2015) was observed. We processed all orbits in order to study the global features without any assumptions about stationary nature of the IMF orientation. During the orbits when MAVEN was not in the solar wind we use the magnetic field measurements in the magnetosheath. Note that for these cases errors in clock angle due to the IMF draping may appear (Dong et al., 2019). The field vectors were averaged over the orbits in bins of $0.1 R_M \times 0.1 R_M$. The magnetic field data were complemented by the ion measurements made by the Supra-Thermal And Thermal Ion Composition (STATIC) instrument (McFadden et al., 2015).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Map of the magnetic field vectors near the central XY plane in MSE coordinates. Red and green color is used to distinguish the vectors with a different sign of the Bₓ component. (b) Map of the Bᵧ component in the YZ-MSE plane in the tail (−2 R_M < X < 0 R_M). (c) Map of the magnetic field vectors near the XZ-MSO plane obtained by averaging on all IMF directions. A sampling of the utilized IMF directions (the cross-flow component) is given above the (c) panel. Arrows depict several possible IMF orientations. (d) Map of the Bₓ component in the YZ-MSO plane in the tail. (e) Map of the unit vectors of the magnetic field in the XZ-MSO plane from the Cain' model obtained after the averaging procedure. Color shows the model Bₓ-component. (f) Map of the model Bᵧ component in the tail.</figcaption></figure>

## 2. Observations

To isolate the induced magnetosphere, we can rotate the magnetic field data and the spacecraft orbits from the Mars Solar Orbital (MSO) coordinate system to the Mars Solar Electric (MSE). In the MSO system the X axis is directed from the center of Mars to Sun, the Z axis is perpendicular to the planetary orbital plane and directed to the north, and Y axis completes the right-handed system. In MSE coordinate frame the cross-flow component of the IMF is always along one axis (e.g., Y axis), the X axis antiparallel to the upstream solar wind flow (the solar wind aberration was not taken into account), and the Z axis pointing in the direction of the solar wind motional electric field (−**V**<sub>sw</sub> × **B**<sub>IMF</sub>), where **V**<sub>sw</sub> and **B**<sub>IMF</sub> are the vectors of the velocity and the magnetic field in solar wind, respectively. Due to such a rotation and by averaging over many orbits the contribution of the local crustal magnetic field is strongly attenuated and we clearly observe a draping of the IMF around Mars (Figure 1a) and the formation of an induced magnetosphere with a magnetic tail, two lobes of which are separated by the current sheet along the Z axis (Figure 1b). Here we utilized the magnetometer data from November. 2014 to May 2018.

It is more difficult to separate intrinsic components of the Martian magnetosphere because the strong crustal magnetic field spreads not homogeneously in latitude and longitude. Thus their effects must rotate with Mars. Therefore, people often use the geographic coordinates to investigate, for example, how the crustal magnetic field impacts on the ionosphere and ion escape. To observe the global effect of the planetary magnetic field caused by the solar wind interaction we can use the MSO coordinates averaging over many orbits with different IMF conditions. Then the effect of draped field lines might be almost canceled and we can expect to observe a global intrinsic component of the Martian magnetosphere. The middle column of Figure 1 presents the magnetic field data for this type of mapping. On the top, the sampling of the cross-flow component of the IMF in solar wind
tagged to the selected MAVEN orbits is shown. Averaging over all orbits strongly reduces the resultant cross-flow component of the IMF. As a result, in the magnetosheath region we do not observe any draping features (Figure 1c), in contrast to Figure 1a, where a draping of the magnetic field in the magnetosheath is well seen. However, within the magnetosphere, we observe a regular and well ordered pattern of the magnetic field lines stretching tailward, implying likely their planetary origin. Here, the vectors of the magnetic field with different sign of the Bx component are depicted by red and green color, respectively. These field lines represent only their magnetospheric portions and might be connected to the IMF lines which are not visible here due to the used averaging procedure. Indeed, Xu et al. (2023) have shown that in the MSO coordinates the field lines in the lobes contain an open component, with one end on the Mars surface and another one in the solar wind.

Figure 1d shows the Bx component in the YZ cross-section at -2 R<sub>M</sub> < X < 0. We observe two magnetic field lobes in the northern and southern hemispheres separated by the current sheet in Y direction. Such a field topology in the tail is typical for a dipole-like magnetosphere, while at the dayside we can only speculate about a degenerated dipole. A signature of a dipole-like configuration at the dayside is tentatively seen in the southern hemisphere (see Figure 1c with the pink colored vectors).

To examine how the global planetary magnetic field might look like in the MSO coordinates we plot the crustal magnetic field taken from the model (e.g., from the Cain’ model), taking into account the geographical configuration and the crustal contribution along the spacecraft tracks, and average over the same set of the spacecraft orbits. Figure 1e depicts the unit vectors of the model magnetic field projected onto the XZ plane. Color gives the value of the Bx component. Indeed, we can see the dipole magnetic field. It occurs that in first approximation higher-order harmonics caused by the strong local crustal field are almost canceled by averaging over many Mars rotations and the dipole field becomes well visible. The distribution of the Bx-component in the YZ plane (Figure 1f) is similar to the one in Figure 1d although the values are smaller. An increase of the Bx component might be caused by a stretching of the field lines in the tail, similar as it happens in the Earth magnetosphere.

To observe at the same time components of the induced and a dipole-like magnetosphere one can use the MSO coordinates, but select, for example, only orbits with B<sub>ysw</sub> > 0 or B<sub>ysw</sub> < 0 in solar wind. Such an approach was used by DiBraccio et al. (2018). These authors have observed a twist of the magnetic lobes in the tail which occurs in the opposite directions for B<sub>ysw</sub> > 0 and B<sub>ysw</sub> < 0. Reconnection of the draped IMF with dipole-like magnetic field lines can produce a magnetic torque which turns the magnetic tail either clockwise or anticlockwise depending on sign of the B<sub>ysw</sub> component in solar wind. Such a twist of the magnetic lobes is a typical feature of the hybrid magnetosphere with elements of both induced and intrinsic magnetospheres (Axford, 1991; Dubinin et al., 1980).

Figure 2 compares the magnetic field maps in the XY and XZ planes for orbits with B<sub>ysw</sub> > 0 (left column) and B<sub>ysw</sub> < 0 (right column), respectively. In the XY plane, we observe a reversal of the sign of the B<sub>x</sub> component in the tail lobes for cases with B<sub>ysw</sub> > 0 and B<sub>ysw</sub> < 0. In the XZ plane, the picture has not changed. Thus, in the polar regions of the hybrid magnetosphere of Mars, the field lines probably have a planetary origin, while at lower latitudes, the draped field lines dominate. Although the contributions from southward and northward components of the IMF which are also present in the averaging procedure can cancel each other out when combining all the IMF with B<sub>ysw</sub> > 0 (duskward) or B<sub>ysw</sub> < 0 (dawnward) (see a top panel in Figures 3a and 3b), a bias in the northward/southward component may change the twist of the lobes. Figures 3c and 3d show that by selecting the orbits with duskward and southward, and dawnward and southward components the twisting increases. By selecting the orbits with duskward and northward, and dawnward and northward components the twisting almost disappears (Figures 3e and 3f). Bowers et al. (2023) have suggested that conditions for reconnection between the IMF and the crustal magnetic field might be more favorable for the southward IMF and can be responsible for the observed twisting. However, if we collect only orbits with clock angle 80°–100° (almost duskward orientation) (Figure 3g) or −80°–−100° (almost dawnward orientation) (Figure 3h) we still find a distinct twisting that supports the assumption of a dipole-like component. Dubinin et al. (1980) have found that twisting of the tail lobes in the laboratory model is also controlled by the ratio of values of the cross-flow component and the dipole magnetic field at the equator. With decrease of the ratio, contribution of the dipole field increases and twisting raises. At Mars, we observe a similar trend. For selection of orbits with B<sub>ysw</sub> > 0, clock-angle 80°–100° and B<sub>ysw</sub> < 1.5 nT (Figure 3i) the current sheet separating two lobes is almost along the Y axis, that is, the dipole-like
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Maps of the magnetic field vectors for positive and negative $B_y$ components in the solar wind. Color gives the value of the $B_x$ component. The upper row shows the sampling of the used IMF directions. The left (right) column corresponds to cases with $B_{y,sw} > 0$ ($B_{y,sw} < 0$), respectively. Panels (a, c) show the maps in the XY-MSO plane ($-0.25 R_M < Z < 0.25 R_M$). Panels (b, d) show the maps in the XZ-MSO plane ($-0.25 R_M < Y < 0.25 R_M$).</figcaption></figure>

component in the tail probably dominates. For orbits with $B_{y,sw}<0$, clock-angle $-80°$--$100°$ and $-1.5$ nT $<B_{y,sw} < 0$ (Figure 3j) the twisting only slightly enlarges.

The induced and intrinsic components are well separated by mapping only orbits with $B_{z,sw} > 0$. A draping of the IMF lines is clearly observed in the magnetosheath adjacent to the magnetosphere (blue vectors in Figure 4a). However, within the magnetosphere, the field configuration occurs different, with a reversal of the sign of the $B_x$ component in both hemispheres. Here the field vectors with $B_x > 0$ and $B_x < 0$ are plotted by red and green, respectively. Correspondingly, in the tail, in the YZ plane we observe draping features in the outer regions and the intrinsic component in the central inner part (Figure 4b). Comparing the map in Figure 4a with maps depicted in
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The first and the third rows give samples of the IMF directions (the cross-flow component) utilized for the selection of the MAVEN orbits used for the averaging procedure and plotting of the $B_x$ component in the tail ($-2 R_M < X < 0 R_M$).</figcaption></figure>

Figures 1c, 2b and 2d we see the same field configuration in the tail for very different IMF orientation. Such a coherent picture indicates the important role of the dipole-like component in the solar wind-Mars interaction. The origin of the field lines within the magnetosphere, at the dayside, at low and middle latitudes (Figure 4a) remains uncertain. They are either draped IMF lines penetrated deep to the ionosphere or dipole-like field lines. A probable answer can be obtained from a similar map for $B_{zsw} < 0$. Figure 4c, plotted for such a case, shows that although it is difficult to separate the induced and intrinsic components, the magnetic field in this area follows the draping direction. Thus we are likely dealing with a degenerated dipole where the dayside is mostly filled by the IMF lines.

We were able to isolate the intrinsic component in Figure 4 because in the XY plane the draped field lines for $B_{ysw} > 0$ and $B_{ysw} < 0$ cancel each other out. But in the YZ plane we observe a field shaping which is not consistent with a dipole-like configuration (Figure 5). Such a picture, which looks like a rotation of the near Mars tail along the *X* axis, appears because the arising $B_z$ component for cases with $B_{ysw} > 0$ and $B_{ysw} < 0$ is not canceled but, instead, is enhanced. This feature is well seen in Figure 6. Panels (a), (b) and panels (d) (e) of Figure 6 show maps of the magnetic field vectors in XZ planes for $Y = -1.2$ to $-0.8 R_M$ and $Y = 0.8-1.2 R_M$ for cases with $B_{ysw} > 0$ and $B_{ysw} < 0$, respectively. As a result, in the YZ plane we observe a picture depicted in Figures 6c and 6f, that is effect of three factors: (a), a shift of the magnetic barrier to the $E^+$ ($Z_{MSE} > 0$) hemisphere in which the solar wind motional electric field ($-\vec{V}_{sw} \times \vec{B}_{IMF}$) is pointing along the $Z_{MSE}$ axis and the field increases above the “northern pole” in the $E^+$ hemisphere, (b) a draping of the field lines along the Z axis, (c) a wrapping of the field lines in the $E^-$ ($Z_{MSE} > 0$) hemisphere. All these features are also typical for an induced magnetosphere (Dubinin et al., 2021). A combined effect of the maps shown in Figures 6c and 6f may result in a false picture of the rotation of the magnetic field lines in the tail. This is seen, for example, on the map in MSO coordinates for all IMF directions (Figure 6g), and also on the maps shown in Figure 5. It is interesting to note that the field configuration
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Panels (a, c) show maps of the magnetic field vectors in the XZ-MSO plane ($-0.25 R_M < Y < 0.25 R_M$) for the MAVEN orbits with $B_{zsw} > 0$ ($B_{zsw} < 0$), respectively. The IMF directions are given on the top row. Panels (b, d) depict the maps of the $B_x$ component in the tail. The bottom row shows cartoons of the field configuration for these cases.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Maps of the magnetic field vectors in the YZ-MSO plane for $B_{zsw} > 0$ and $B_{zsw} < 0$.</figcaption></figure>

in Figures 6a–6c implies the existence of a tailward component of the electric current separating two lobes in the tail, that is, the charged particles, which carry the current, drift not along the Z axis but along an axis inclined to the XY plane (see, e.g. Ramstad et al., 2020). The current might contain several substructures caused by the multi-ion plasma composition and therefore be a rather broad (Zelenyi et al., 2020). Correspondingly, on the magnetosphere boundary, in the tail, the current must have a sunward component. Since at the Mars orbit, the IMF is mostly within the ecliptic plane (DiBraccio et al., 2018; Liu et al., 2021), it is difficult to find out a sufficient number of the MAVEN orbits with a pure northward IMF component and thus to study effects associated with a dipole-like magnetic field for $B_{zsw} > 0$.

The observed distribution of plasma in the tail is also strongly affected when we accumulate the orbits in a broad range of IMF clock-angles. Figure 7 shows maps of the total value of the mass velocity of oxygen ions ($O^+$ and $O_2^+$). Ions with high velocities form a plume in the $E^+$ hemisphere. For $B_{y,sw} > 0$ ($B_{y,sw} < 0$) the plume is in the northern (southern) hemisphere, respectively (Figures 7a and 7b). For $B_{z,sw} > 0$ ($B_{z,sw} < 0$) the plume must be in the $-Y$ (+Y) hemisphere, respectively (Figures 7c and 7d). The presence of the $B_y$ component and correspondingly, of the northward/southward components of the motional electric field, for cases with the northward and southward IMF broadens the plume region for $B_{z,sw} > 0$ and $B_{z,sw} < 0$ (Figures 7c and 7d). On the other hand, the $E_y$ component of the motional electric field for the eastward and westward IMF but with contribution of the northward/southward IMF components also widens the plume region (Figures 7a and 7b).

The interesting case is realized when the IMF direction is close to the X axis. Then, the induced effects associated with the motional electric field $-\vec{V}_{sw} \times \vec{B}_{IMF}$ are strongly reduced and the global intrinsic component might be well
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Panels (a, b) and panels (d, e) show maps of the magnetic field vectors in XZ planes for $Y = -1.2-0.8 R_M$ and $Y = 0.8-1.2 R_M$ for cases with $B_{ysw} > 0$ and $B_{ysw} < 0$, respectively. Panels (c, f) depict the corresponding maps in the YZ-MSO plane. Panel (g) shows the map in the YZ-MSO plane when all IMF directions were used in the averaging procedure.</figcaption></figure>

recognized. Figure 8 shows the data for cases with the IMF cone angle of 0°–20° (upper row) and 160°–180° (lower row), respectively. Here, the IMF cone angle is defined as the angle between the IMF direction and the *X* axis in the MSO coordinates. For a better statistics we have utilized the magnetic field measurements carried out from Dec 2014 to May 2023. Panels (a) and (d) depict maps of the $B_x$ component in the XZ-MSO plane
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Maps of the total value of the velocity of oxygen (O⁺ and O₂⁺) ions with energy E > 30 eV in the YZ-MSO plane for different selection of the IMF in the solar wind.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Maps of the magnetic field for almost radial IMF. Red and green vectors depict the unit magnetic field vectors with $B_x > 0$ and $B_x < 0$, respectively.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 9.** The upper row shows maps of the $B_x$ component in the tail (−2 $R_M$ < X < 0 $R_M$) for different IMF conditions and different phases of the Mars rotation. Panels (a, b) present the maps for cases when the strongest crustal field was located near the midnight (±30°). Panels (c, d) correspond to the cases when the strongest crustal field was on the dusk side (±20°). The $B_y$ component of the IMF was either positive (panels (a, c)) or negative (panels (b, d)). A more accurate information about the crustal magnetic field can be obtained from panels (a1–d1) and panels (a2–d2) which depict the total value $B_t$ of the measured magnetic field and the value $B_t$ from the Cain’ model (Cain et al., 2003), respectively.</figcaption></figure>

($-0.5R_M < Y < 0.5R_M$). Panels (b) and (e) show maps with unit vectors of the magnetic field. Red and green are vectors with $B_x > 0$ and $B_x < 0$, respectively. Panels (c) and (f) are maps of the $B_x$ component in YZ-MSO at $-1R_M < X < 0$. We again observe a rather coherent picture with the sunward (antisunward) $B_x$ component in the northern (southern) hemisphere, respectively. Therefore, the data are also consistent with the existence of a small dipole-like obstacle to solar wind.

So far, in all cases discussed above we have averaged over Mars rotations. The strongest crustal magnetic field is located in the southern hemisphere near the 180° E geographic meridian. Figure 9 shows several examples of maps of the $B_x$ component in the YZ plane at $-2 R_M < X < 0 R_M$ for cases when the strongest crustal field was located near the midnight (±30°) (panels (a) and (b)), and on the dusk side (±20°) (panels (c) and (d)). The $B_y$ component of the IMF was either positive ((panels (a) and (c)) or negative (panels (b) and (d)). Panels (a1–d1) and panels (a2–d2) depict the total value $B_t$ of the measured magnetic field and the value $B_t$ from the Cain’ model (Cain et al., 2003) to render regions with strong crustal field. We observe that a strong local magnetic field does not break the global field configuration. However, reconnection of the draped field lines in these local areas and a subsequent motion of the newly formed open field lines can produce a more complicated field topology. Indeed, although a twisting effect is tentatively seen in all these cases, the magnetotail structure occurs more rugged with zigzag current sheets. A separation between the intrinsic and induced components only by using the magnetic field measurements seems questionable.
### 3. Discussion

The Martian magnetosphere contains features typical for induced and intrinsic magnetospheres. To describe these features we use a picture based on a statistical approach with a large number of measurements. It was shown that utilizing different coordinate systems and selecting orbits with different cross-flow components of the IMF in the solar wind we can separate the induced and intrinsic components inherent to the Martian magnetosphere. In the MSE coordinates we isolate the induced component and observe the features that are in many ways similar to those we see at Venus and were widely discussed for Venus and Mars (Dubinin et al., 2021; Dubinin, Fraenz et al., 2020; Dubinin, Luhmann et al., 2020; Futaana et al., 2017; Halekas et al., 2021; Luhmann et al., 1980; Russell & Vaisberg, 1983). The magnetic field tensions of the draped field lines push the ionospheric plasma to a tailward motion providing a continuous loss of planetary ions. The picture, symmetric in the XY plane, containing the IMF, occurs very asymmetric in the XZ plane containing the motional electric field. Among new features which are not discussed in this paper are the ion plume formed by oxygen ions extracted from the ionosphere by the motional electric field (Dong et al., 2015; Dubinin et al., 2011, 2021) and the ion trail in the opposite hemisphere filled by a slowly moving and dense ionospheric plasma (Dubinin et al., 2019). Another interesting characteristic of the induced magnetosphere is a distinct turn around the X direction of the magnetic field lines in both lobes leading to a peculiar draping of the field lines in the opposite direction to the motional electric field (Dubinin et al., 2019, 2021; Ramstad et al., 2020). Such a draping and the associated magnetic field forces push the ionosphere plasma in the transverse direction opening a passage for the formation of an ion trail. The disadvantage of this approach is that we can not observethe contribution of the crustal magnetic field and effects of reconnection of the draped and intrinsic field lines. The dawn-dusk asymmetries due to the Parker Spiral IMF as well as dawn/dusk ionosphere differences (see e.g. Liemohn et al., 2017) are also lost in the MSE coordinates.

Luhmann et al. (2015) have used a global MHD model of the solar wind/Mars interaction trying to reveal the influence of the lower-order moments of the crustal field (dipole component). The authors concluded that such an effect might be compromised by the contribution of the induced component. Fang et al. (2023) have separated the induced and the crustal magnetic fields by subtraction of the crustal field components using the crustal field model. Then, the authors have focused on study of the induced component and its variations with the upstream solar wind dynamic pressure and IMF intensity as well as the ambient crustal field strength.

Using the MSO coordinates and making averaging over many orbits with all IMF directions we strongly restrain the draped component and observe a global and coherent response of the Martian obstacle to the impact of the solar wind. Since the high-order harmonics describing the crustal field at Mars in such averaging are also suppressed, a response of the dipole-like component may be observed. In this case the low-order harmonics of the magnetic field and mainly the dipole component determine the field topology of the obstacle. This conclusion is also confirmed by electron tracing. According to Xu et al. (2023), the lobes in this case are filled by open field lines which are connected at one end to the Mars surface. A rough estimate of the magnetic moment based on the remaining model field value near the equator after the averaging procedure ($\le2$ nT) yields $\le2 \times 10^{18}$ A m$^2$ ($\le2 \times 10^{21}$ G cm$^3$). Such a dipole magnetic field is not sufficient for solar wind stoppage but sufficient for build up of the magnetotail. Xu et al. (2023) have suggested that the dipole-like response is caused not by a global and weak dipole, by several smaller dipole-like loops at low and middle latitudes with moderate values of the crustal field which span over almost all geographic longitudes. However, in such a scenario one can not explain the coherent behavior of the magnetic field in the tail, with the sunward (antisunward) component in the northern (southern) hemisphere, respectively.

Although the approach with averaging over all IMF directions is useful to highlight the intrinsic component, the induced component is mostly lost. A more reasonable way is to take orbits with $B_{ysw} > 0$ or $B_{ysw} < 0$. Then, we might expect that a combination of a dipole-like component and a draped component would lead to a classical hybrid model described in Axford (1991); Dubinin et al. (1980), (2023) and characterized by a twisting of the tail lobes. Such a twist of the magnetic lobes is a typical feature of the hybrid magnetosphere with elements of both induced and intrinsic magnetospheres. The hybrid magnetosphere has been studied earlier in the laboratory (Dubinin et al., 1980). A tiny magnetic dipole was placed into the induced magnetosphere. By reversing polarity of the dipole the authors could separate the draped and dipole field lines. The draped field lines occupied low and middle latitudes of the magnetosphere while the dipole field lines were in the polar regions. In fact, due to reconnection of the magnetic fields, another important class of field lines appears–open field lines. A similar separation might be done for Mars, but not by a reversal of a dipole, but by a reversal of the cross-flow component
of the IMF. Such a twist was indeed observed at Mars (DiBraccio et al., 2018, 2022). The hybrid simulations of the solar wind interaction with the ionospheric obstacle and a small magnetic dipole (Kallio et al., 2008) confirm a twisting of the tail and are in favor of this scenario. Bowers et al. (2023) have suggested that conditions for reconnection between the IMF and the crustal magnetic field might be more favorable for the southward IMF, which is also present in our averaging procedure, and can be responsible for the observed twisting. The authors used a model of antiparallel magnetic reconnection and calculated the probability of reconnection. We have shown that although a bias caused by the northward or the southward IMF components can increase or decrease the twist angle (the bias is more sensitive for $B_{ysw} > 0$), a twisting persists even for a narrow range of the IMF clock-angles, emphasizing the robust origin of twisting. We have also shown that the angle of twist increases with a decrease of the $B_{ysw}$ component, that is in a general agreement with the results of the laboratory experiments (Dubinin et al., 1980). For $0 < B_{ysw} \le 1.5$ nT, the twist almost reaches ~90° implying the dominance of the dipole-like component in the tail structure. On the other hand, for $0 > B_{ysw} \ge -1.5$ nT, variations in twisting are much smaller. Asymmetry in coupling between the induced and intrinsic components for the different polarity of the $B_{ysw}$ was also found in DiBraccio et al. (2022) that is difficult to explain only in terms of the classical hybrid model. All these features probably show an important contribution of reconnection between the draped field lines and the field lines of the local crustal sources that break the symmetry of twisting for $B_{ysw} > 0$ and $B_{ysw} < 0$.

Separation between the induced and intrinsic components is especially evident if we select the orbits with $B_{zsw} > 0$. The structure of the inner part of the tail is again very coherent and consists of two lobes with the sunward $B_x$ component in the northern hemisphere and the antisunward component in the southern hemisphere while in the outer part of the tail the draped field lines have the reversed polarity. At the dayside, we observe a *bald* area filled by the penetrated inward IMF. Such a topology is consistent with a degenerated dipole-like intrinsic magnetosphere. It is interesting to note that Xu et al. (2023) have found that there are more open field lines for $B_{zsw} < 0$, that might be explained by more favorable conditions for reconnection between the draped field lines and the dipole field similar as it happens at the Earth. However, the inference of the additional draping inherent to the induced magnetosphere and antiparallel to the direction of the motional electric field in the solar wind compromises an employment of such data selection for a further study of features of the dipole-like magnetosphere. Signatures of a dipole-like magnetosphere are also revealed when we separate the orbits with almost radial IMF. In this case the induced component related with the motional electric field $-\vec{V}_{sw} \times \vec{B}_{IMF}$ is weakened.

A more complicated picture is observed when we select orbits with a fixed phase of the ~24 hr Mars rotation. The current sheet separating the tail lobes has a zigzag shape implying a stronger mixing of the intrinsic and induced components. We believe that such a structure might be more typical for an actual snapshot face of the Martian magnetosphere. The recent hybrid simulations (Dubinin et al., 2024) show some features of the combined magnetosphere for different phases of the Mars rotation and their changes with solar wind and the IMF variations. Simultaneous magnetic field measurements by several spacecraft (MAVEN, Tianwen and ESCAPE) can help us in differentiating the intrinsic and induced components and a retrieval of the dipole magnetic field at Mars.

## Data Availability Statement

The MAVEN mission is supported by NASA through the Mars Exploration Program. The research described in this manuscript utilizes publicly available data from the MAVEN mission, including data from the MAG (Connerney, 2024). The MAVEN data used in this study are available through the Planetary Data System http://pds-ppi.igpp.ucla.edu/mission/MAVEN (NASA Planetary Data System (PDS), MAVEN Mission [Dataset], 2024).

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## Acknowledgments

Authors ED, MP, ST wish to acknowledge support from DFG for supporting this work by Grant PA 525/25-1. Open Access funding enabled and organized by Projekt DEAL.
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