Detection of magnetospheric ion drift patterns at Mars

Domain: earth_science
Published: 2023-10-27
Editorial outcome: major_revision

Abstract
Mars lacks a global magnetic field, and instead possesses small-scale crustal magnetic fields, making its magnetic environment fundamentally different from intrinsic magnetospheres like those of Earth or Saturn. Here we report the discovery of magnetospheric ion drift patterns, typical of intrinsic magnetospheres, at Mars using measurements from Mars Atmosphere and Volatile EvolutioN mission. Specifically, we observe wedge-like dispersion structures of hydrogen ions exhibiting butterfly-shaped distributions (pitch angle peaks at 22.5°−45° and 135°−157.5°) within the Martian crustal fields, a feature previously observed only in planetary-scale intrinsic magnetospheres. These dispersed structures are the results of drift motions that fundamentally resemble those observed in intrinsic magnetospheres. Our findings indicate that the Martian magnetosphere embodies an intermediate case where both the unmagnetized and magnetized ion behaviors could be observed because of the wider range of strengths and spatial scales of the crustal magnetic fields around Mars.

Introduction
Unlike Earth or Saturn, which possess strong planetary-scale intrinsic magnetospheres, Mars offers a unique magnetic environment in the solar system due to the absence of a planetary-scale intrinsic magnetic field1,2 and the presence of localized crustal magnetic fields3,4. It is widely accepted that the Martian crustal fields have a significant impact on the solar wind interaction with Mars and potentially plays a key role in the planet’s atmospheric evolution5,6. Numerous studies found that the crustal fields act as magnetic umbrella, shielding the ionosphere from erosion by the solar wind7–13. However, contrasting views have been put forth by other researchers, reporting that the crustal fields could locally enhance the escape rate of ions by facilitating energy transfer between the solar wind and the Martian ionosphere5,6,14. To comprehend the role of crustal fields in ion escape, we must address the question, how do the crustal fields control the ion behavior? Answering this question could advance our understanding of the solar wind interaction with Mars and the Martian atmospheric evolution15–17.

The localized crustal fields are often called mini-magnetospheres because they exhibit morphological similarities to scaled-down intrinsic magnetospheres18–21. Nonetheless, due to the differences in strength and spatial scales between the small-scale crustal fields and planetary-scale intrinsic magnetospheres, it remains uncertain whether the crustal fields assume a comparable role to intrinsic magnetospheres in governing plasma (ions and electrons) dynamics. Charged particles within planetary-scale intrinsic magnetospheres tend to become magnetized since they offer a large electromagnetic environment compared with the particle’s gyroradius. In this case, the presence of electric fields and inhomogeneity of magnetic field induces particle’s drift motion, consequently aiding the formation of drift dispersion structures of magnetized particles22–28. Drawing from Earth’s magnetosphere as an illustrative example, the magnetic gradient and curvature drift velocity depend on the particles’ energy, and give rise to westward motion. Electric drift, however, is independent of energy, resulting in motions to any directions depending on magnetic local times and L (L-shell) values. Consequently, distinct spatial distributions of particles at different energies are generated. For example, previous studies have demonstrated that the low-energy portion of injected ions can drift more eastward than the high-energy portion does on the dawnside because of the presence of the convection electric field. The discrepancy is significant at particular L22–27. As a result, when a spacecraft crosses radially into or out of the inner magnetosphere, it records wedge-like energy dispersion structures in energy versus time spectrograms of ions, characterized by an increase followed by a decrease in ion’s energy along the spacecraft trajectory. Hence, these drift dispersion structures commonly manifest as wedge-like energy dispersion patterns in particle’s energy versus time spectrograms, which essentially represent distinct spatial distributions of particles at different energies.

Moreover, it is crucial to recognize that the wedge-like dispersion structures of particles require a stable electromagnetic environment where the particle’s motions are adiabatic22,26, otherwise, the chaotic motions of particles would lead to their rapid dissipation. The wedge-like dispersion structures of plasma, are commonly observed within the planetary-scale intrinsic magnetospheres with strong magnetic field strength, such as those of Earth22–27 and Saturn28. Hence, the wedge-like dispersion structures of plasma serve as indicators that the spatial scale of the electromagnetic environment is significantly larger than the plasma’s gyroradius.

Recent observations of drift dispersion structures of magnetized electrons within the Martian crustal fields have indicated that the crustal fields exhibit behavior akin to an intrinsic magnetosphere on the scale of the gyroradius of electrons20. Consequently, it is expected that the electron dynamics in the crustal fields are similar to those found within intrinsic magnetospheres. In contrast, ions having larger gyroradius are commonly thought to be unmagnetized within the localized Martian crustal fields29,30, which have weak magnetic field strength and large spatial-temporal inhomogeneity (in contrast to Earth or Saturn’s intrinsic dipole fields). Thus, the prospect of observing the magnetized ion behaviors typical for intrinsic magnetospheres (e.g., wedge-like dispersion structures of ions) within the Martian crustal fields is less likely.

Here we present the observations of wedge-like dispersion structures of ions within the Martian crustal fields, based on the high-resolution magnetic fields and plasma measurements obtained by Mars Atmosphere and Volatile EvolutioN mission (MAVEN)31. This finding suggests that the magnetized ion behavior commonly observed in intrinsic magnetospheres also occurs at Mars, enhancing our understanding of how the crustal fields control the ion dynamics.

Results
Event overview
During 22.50-23.00 UTC, April 15, 2018, MAVEN traversed the nightside terminator region of Mars. The average location of MAVEN was (−0.62, 0.69, −0.74) RM (radius of Mars, RM = 3396 km) in Mars Solar Orbital (MSO) coordinates. Supplementary Fig. 1 provides a visual representation of the MAVEN spacecraft’s path. MAVEN was located above the region with the strongest crustal field, characterized by a longitude of approximately 180°, latitude of approximately 60° S, and altitude of around 650 km.

An interesting observational feature is that MAVEN detected a series of energy-time dispersed ion signatures (refer to Fig. 1a) with a period of approximately 20-30 seconds during the time interval of 22.52-22.58. MAVEN observed four consecutive rising-tone dispersed structures (energy increase with time), starting at 22.52.32, 22.53, 22.53.25, and 22.53.50 UT (identified by the black dashed curves), with later structures at clearly lower energy than the preceding ones. Later, four falling-tone dispersed structures characterized by energy monotonically decrease with time starting at 22.55.24, 22.55.34, 22.55.48, and 22.56.04 UT were detected. The presence of both rising and falling tones indicates that MAVEN captured spatial dispersion structures of ions rather than a purely temporal effect, which would only produce falling tones14,30. These observed features closely resemble the wedge-like dispersion structures commonly observed in Earth’s magnetosphere22–27 and in Saturn’s magnetosphere28.

The energy range of the dispersed ions extends up to approximately 200 eV and down to the lower energy limit of the Solar Wind Ion Analyzer (SWIA) instrument32, at 25 eV. Figures 1b, c show the energy spectrum of light ions (m/q < 8) and heavy ions (m/q > 12) based on the Suprathermal and Thermal Ion Composition (STATIC) data33, where it is evident that the energy-dispersed signatures are dependent on the mass of the ions. Specifically, the light ions with energies ranging from 20 eV to 200 eV exhibit clear energy dispersion signatures, whereas the heavy ions have relatively low energies (E < 50 eV) and do not show dispersed signatures. The mass-energy spectrum in Fig. 2 indicates that the dispersed ions are primarily hydrogen ions (H+) with m/q = 1, and possibly a small fraction of H2+ or He2+ with m/q = 2. The dominance of H+ applies to all dispersion structures, indicating that these different dispersions are not due to the difference in the mass, which sometimes happens on the Earth34. While it is challenging to ascertain how much H+ or He2+ contributes to the dispersions, here we assume that the dispersed ions are entirely H+. Figure 2 also indicates that heavy ions of Marian origin (O+ and O2+) do not contribute to the dispersion (the energy is much lower than dispersion structures). Therefore, we suggest that the observed dispersed H+ was likely originated from the solar wind rather than Mars since their energy distribution is distinctly different to that of heavy ions.

The presence of electron voids, characterized by the decrease in suprathermal electron flux (as depicted in Fig. 1d) detected by Solar Wind Electron Analyzer (SWEA)35, combined with the good agreement between the magnetic fields observed by the Magnetometer (MAG)36 and the latest spherical harmonic model of the crustal fields37 (as shown in Fig. 1e), strongly suggests that MAVEN entered a region dominated by crustal fields. However, there are slight differences between the observed magnetic fields and the model fields37, which maybe attributed to the influence of induced fields. Upon scrutinizing it further, it appears that the observed fields can be approximated as a linear superposition of the crustal fields and steady Bx-dominated fields (as depicted in Fig. 1f), represented by Bobs = Bmodel + [14.8, −0.93, −4.15] nT, where Bobs, Bmodel correspond to the observed magnetic fields and crustal fields model, respectively. The presence of double-sided loss cone distributions of electrons and the magnetic topology analysis38 (as shown in Supplementary Fig. 2) further indicates that the field lines primarily consisted of closed crustal field lines with both foot-points located in the nightside ionosphere (MAVEN traversed from about 5 to 3 local time in the early morning sector). Based on these features, we can infer that the observed wedge-like dispersed H+ ions occurred within the mini-magnetosphere formed by the closed field lines of the crustal fields, albeit with additional [14.8, −0.93, −4.15] nT magnetic fields caused by the influence of induced fields. The observations highlight that the ion drift patterns typically occurred in the intrinsic magnetospheres, the wedge-like dispersion structures, do also occur within the small-scale Martian crustal fields.

Asymmetric butterfly pitch angle distributions
Figure 3b–d provide the pitch angle distribution of ions in three energy ranges, 30–50 eV, 50–100 eV, and 100–200 eV. It is evident that the dispersed H+ ions across all energy ranges exhibit butterfly-shaped pitch angle distributions39, characterized by pitch angle peaks at 22.5–45° and 135–157.5°. Upon scrutiny, a notable asymmetry in the butterfly-shaped distribution dictated by the polarity of the radial component of the magnetic fields (Br, refer to Fig. 3e), becomes apparent. Prior to 22.56.09 during which Br is negative (as indicated by the blue vertical dashed line in Fig. 3), the flux of H+ ions with a pitch angle of 22.5–45° (ions moved toward the planet) is higher than that of ions with a pitch angle of 135–157.5° (ions moved away the planet). Conversely, after 22.56.09 during which Br is positive, there is a higher flux of H+ ions with a pitch angle of 135–157.5°. These results indicate that there are more inward-moving H+ ions compared to outward-moving ones. The majority of ions with pitch angles of 22.5–45° and 135–157.5° fall within the field-of-view of SWIA (see Supplementary Fig. 3), thereby reinforcing the reliability of the aforementioned results.

Methods
Instruments
Here we adoption data from the Solar Wind Ion Analyzer (SWIA)32 and the Suprathermal, Thermal Ion Composition (STATIC) instrument33, electron data from Solar Wind Electron Analyzer (SWEA)35, and magnetic field measurements from the Magnetometer (MAG)36, onboard MAVEN. The MAG is a flux gate magnetometer that measures three-dimensional magnetic field vectors with frequencies of 32 Hz and 1 Hz. Here we use the 1 Hz data. SWIA can measure the ions with energy between 25 eV/q-25 keV/q at time resolution of 4 seconds during this event. The time resolution of full three-dimensional distribution of ions provided by SWIA is 8 seconds. Therefore, MAG and SWIA measurements can produce energy-resolved pitch angle distributions per 8 seconds. The angular resolution of SWIA for 20-200 eV ions is 22.5° × 22.5°, thereby we set that the bin size of pitch angle is 22.5°, which is the upper limit of resolution of pitch angle. Here we use the c0 and c6 data of STATIC that provides the ions spectrum with energy between 0-30 keV/q at time resolution of 4 seconds. STATIC consists of a time-of-flight sensor which can measure the mass-per-charge of ions and then determine the specie of ions. The field-of-view of both SWIA and STATIC are 360° × 90°. During this event, SWEA could measure a full three-dimensional distribution of electrons with range from 3 eV/q to 46 KeV/q within 2 s. The field-of-view of SWEA is 360° × 120°, but about 8% field-of-view is blocked by the spacecraft.

MSO coordinates
The coordinates utilized in this study are the Mars Solar Orbital (MSO) coordinates51, where XMSO points from Mars to the Sun, YMSO points opposite to the component of the orbital velocity perpendicular to XMSO, and ZMSO completes the right-handed system.

Magnetic Topology Analysis
The magnetic topology is categorized into seven types38, (1) closed-to-day (C-D); (2) cross-terminator-closed (C-X); (3) closed-trapped (C-T); (4) closed-voids (C-V); (5) open-to-day (O-D); (6) open-to-night (O-N); (7) draped (DP). The closed-to-day type denotes field lines that have both foot-points on the dayside ionosphere, which can be identified by photoelectrons traveling in both parallel and antiparallel directions. The cross-terminator-closed type field line has one of the foot-points that ends in the dayside ionosphere, and the other one connected to the nightside ionosphere. For this type, both away-to-Mars and toward-to-Mars moving electrons are photoelectrons, but the flux of away-to-Mars moving electrons is higher. The closed-trapped type field lines are characterized by the electrons with pitch angle close to 90°. If the trapped electrons are scattered into the loss cone, forming an electron void signature, which is identified by closed-voids type. For the open-to-day type, the field line has one of the foot-points that ends in the dayside ionosphere, which can be identified by the precipitation of solar wind electrons and an outflow of photoelectrons. The draped field lines are identified by solar wind electrons in both field-aligned directions, as well as by the absence of loss cones in the away directions. Therefore, (1), (2), (3), (4) typically represent the closed crustal fields. While (5), (6) represent the open field lines (OP). Supplementary Fig. 2 shows that the field lines are mainly (3) or (4), where both are closed crustal fields.

Normalized Pitch Angle Distribution
The normalized pitch Angle distribution are defined as the pitch angle distribution in units of differential energy flux as normalized by the average differential energy flux for each time step. That is Norm, DEF = DEF / <DEF>. <DEF> represents the average differential energy flux for each time step.

Tracing the magnetic field lines
We trace the magnetic field lines based on the superposed fields consisting of the crustal fields and induced fields with [14.8, −0.93, −4.15] nT. The crustal fields were derived from the latest spherical harmonic model of the crustal magnetic field37.

We divide the spatial region within the latitude range of −65° to −55° and the longitude range of 175° to 190° into a grid with a spacing of 1° for both longitude and latitude. The altitude is then set at 500 km, resulting in 150 initial points, which are represented by r0, then the next position could be obtained by r1 = r0 + b * δl, where b is the unit magnetic field vector at r0, and δl is the step length, here we set δl = 1 km. Applying the same procedures, we could know the ith point of the field lines, ri (i = 1, 2, 3...). We stop the tracing when the altitude reaches 200 km since the crustal field model could only well represent the crustal fields at altitudes above 120 km. After obtaining all points of the field lines, we regard the point with the smallest Bt value as the magnetic equator point.

In addition, it should be noted that the gradients of the magnetic fields and the magnetic drift velocity are also calculated based on the superposed fields throughout this study.

Fitting the magnetic equator plane
Assuming that the normal direction of a plane is n = Ax + By + Cz in the cartesian coordinate system {x, y, z}. Then all the data points in this plane should satisfy the equation, A * Px + B * Py + C * Pz + D = 0, where P = Px x + Py y + Pz z represent the positions of data points, D is constant. In other words, the data points on the plane would satisfy that, P * n + D = 0. As the estimate of n, the least-square method requires that it identifies (fP(m) * n) (m = 1, 2, 3... M) has the minimum variance, i.e., σ2 = Σm=1M [|P(m) - <P>| * n]2 / M reaches the minimum, where the average <P> is defined as <P> = Σm=1M P(m) / M. Furthermore, this minimization is subject to the normalization constraint |n|2 = 1. Then we could readily derive the n by the minimum variance analysis (MVA)52, where n is the eigenvector corresponding to the minimum eigenvalue of the variance matrix Mij = <Pi Pj> - <Pi><Pj>, where the subscripts i, j = 1, 2, 3 denote cartesian components along the {x, y, z} system. For our case, n is (−0.7387, 0.5966, 0.3136). Then the azimuthal direction is deduced as a = (b × n) / |b × n|, where b is the unit magnetic fields vector on the center of magnetic equator points, the radial direction (r) completes the right-handed system. Here r is (-0.1469, 0.3117, −0.9388), a is (−0.6578, −0.7395, −0.1426).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents very interesting observations and proposes a compelling explanation for the observations.\nOverall, I think that the manuscript is well written and well organized.\nI do feel, however, that the title is a bit over-the-top. The fact that the observations indicate energy-dependent drifts is not a property of intrinsic magnetospheres (though that is where they have been observed so far); it is a property of magnetic field configurations larger than the gyroradius of the particles. Such behavior has been hypothesized to occur in smaller-than-global magnetic field configurations elsewhere including the Moon and Mars. I think these observations are very interesting, and a first for Mars, but not necessarily surprising.\nAdditionally, I do have some relatively minor comments below.\nFigure 4: I am having some trouble understanding Figure 4b. I guess in general I understand that it illustrates the magnitude of the magnetic field along the magnetic equatorial plane, but I don’t understand the axes – particularly the vertical axis. What is the radial distance measured with respect to, and how does it go negative?\nFigure 5 caption: I don’t think that “polygon” is the best descriptor for the shaded shapes in Figure 5. Perhaps it is more of a crescent shape (polygon implies straight rather than curved edges).\nLine 433: On this line SWIA should be SWEA since it has to do with electron measurements.\nTo what altitude are the magnetic field lines traced? High order spherical harmonic magnetic field models become unphysical at too low an altitude. The manuscript states that the magnetic field model represents the crustal fields well at altitudes above 120 km. Are the field lines traced lower than that altitude?\nAdditionally, although it is not explicitly stated, I assume that the gradient in the magnetic field is calculated using the magnetic field model plus the constant offset rather than just the model (this is mentioned for the field line tracing but not for the other magnetic field calculations).\nSupplementary Figure 5 (and line 220): It is mentioned and shown that no proton cyclotron waves are present locally. However, Supplementary Figure 5c does appear to show some wave power at a slightly lower frequency on the order of 0.1 Hz. Are these waves significant and can they be identified?\nSupplementary Table 1: I am curious as to why the solar wind dynamic pressure is included in this table. I don’t recall dynamic pressure mentioned elsewhere in the manuscript. Additional, it brings up questions as to how the dynamic pressure was calculated given that MAVEN is not in the solar wind during the time of observation. Personally, I would be interested in the possibility of variations in the upstream dynamic pressure during the observations; could dynamic pressure variations drive upstream waves (as mentioned on line 207)? But I wouldn’t think that this information would be available given the spacecraft orbit.", "reviewer_2": "The paper presents interesting MAVEN plasma and magnetic field observations around closest approach, when the spacecraft traverse through a region of crustal magnetic fields. The authors report, for the first-time, observations of wedgelike dispersion signatures of protons, exhibiting butterfly-shaped distributions within the Martian crustal fields, a feature observed in intrinsic magnetospheres. The authors argue that the observed structures can be understood in terms of drifts motions, despite the relatively weak and large inhomogeneity of crustal magnetic fields, compared to planetary global intrinsic magnetic fields. The authors also report the identification of 16 additional wedge-like dispersion events within the crustal magnetic field of Mars, observed by MAVEN.\nIn my opinion, this manuscript presents new, original observations suggesting wedge-like dispersion signatures of protons at Mars. This is worthy of further investigation and opens the possibility for complementary studies involving magnetic reconnection and planetary ion escape, in connection with crustal magnetic fields. The main concern I have with this paper is that the authors do not sufficiently explain the observed signatures in terms of particle drifts within crustal magnetic fields. I think the manuscript would benefit from additional estimations of relevant parameters, based on the properties observed by MAVEN and current crustal magnetic field models available in the literature.\nPlease find below my specific comments.\nLine 98: How are the black dashed curves shown in panels 1a and 1b computed?\nLine 109: Is there any physical explanation for a 20-30 s periodicity? What is the periodicity observed in the other 16 events the authors were able to identify?\nLine 149: What is the interpretation for the additional [14.8, -0.93, -4.15] nT magnetic field?\nLine 208-210: I think the observed periodicity should be explained in more detail.\nLine 230-232: Why would the source be already anisotropic? Is it the case with all 16 additionally identified cases?\nLine 362-364: According to the authors, the observations cannot be explained by magnetic drift, but by the electric drift velocity. Is there any evidence supporting this claim, in addition to the fact that the magnetic field drift would likely be azimuthal?\nLine 377-379: If the estimated electric field is one order of magnitude larger than the corotation electric field, can the authors provide evidence in favor of the presence of convective or impulsive electric fields with the expected order of magnitude?\nLines 366-379: I think the manuscript would benefit from additional estimations. For instance, what is the proton Larmor radii range for the conditions observed in the analyzed event? How does it compare with the scale length over which the local (crustal) magnetic field varies? What is the expected electric field velocity drift that can explain the observations? How does it compare with the local proton velocity?", "reviewer_3": "Comments on “Discovery of Intrinsic Magnetospheric Ion Behaviour at Mars” by Zhang et al.\nThe manuscript describes observations of protons in Martian crustal magnetic fields that have dispersed energy signatures and oblique (or “butterfly”) pitch angle distributions that are asymmetric and correlated with the radial component of the crustal magnetic field. The manuscript discusses how the protons are injected into the crustal fields (reconnection), what causes their periodic occurrence in the crustal fields (periodic injections, heating, or variations in the source population), how the pitch angle distributions came to be asymmetric (scattering/drift of field-aligned/90-degree pitch angles), and how the energy dispersion signatures formed (higher energy ions drift deeper into the crustal fields).\nOverall the phenomenon reported in the manuscript is novel for ions in the Martian magnetosphere (and any small-scale magnetosphere). I therefore believe that the community should be made aware of this work through publication, perhaps after some relatively minor comments below are considered. The main question in my mind is whether the work should be published in a high impact journal such as Nature Communications. Here, I am less certain, for three related reasons:\n1. The signatures reported here for ions are similar in many ways (energy-time dispersion, bursty and periodic) to a previous report of observations of electrons in Mars crustal fields in 2016 by one of the manuscript’s co-authors, with similar interpretation about mechanisms. These similarities are not mentioned in the present manuscript at all, even though the paper (Harada et al.) is referenced. Though it is very interesting to see the same signatures in ions, what is learned overall from this manuscript about plasma behaviour in Mars crustal fields is therefore not particularly new.\n2. The manuscript makes many references to “intrinsic magnetospheric behaviour” that the ion observations provide evidence for - saying in the opening paragraph that “…we report the discovery of intrinsic ion magnetospheric behavior at Mars…”. I am not fully convinced that this is a discovery, since there have been many previous reports of intrinsic magnetospheric behavior at Mars (e.g. aurora, magnetic reconnection). It may be that the emphasis in this phrase is on “ion” as opposed to \"intrinsic magnetosphere\", but I find this less compelling since the community is already aware that the crustal fields behave as intrinsic magnetospheres in other regards, and there are published studies of ion observations near crustal fields from both Mars Express and MAVEN.\n3. The manuscript mentions that only 17 examples of these observations have been found in 5 years of observations. This suggests that the evidence of intrinsic magnetospheric behaviour is the exception, and not the rule.\nI note that the figures are informative and useful, and the paper is generally well-written.\nMy detailed comments follow:\nL26-28: The wording of this sentence was awkward for me. I think I understand the intent: “The Martian magnetosphere is unusual and different from intrinsic magnetospheres”. But I might have worded it a little differently: “Mars lacks a global dynamo magnetic field, and instead possess small-scale crustal magnetic fields, making its magnetic environment fundamentally different from intrinsic magnetospheres like those of Earth or Saturn.”\nL31: Clarify “butterfly-shaped pitch angle distributions”\nL36-37: This last sentence of the paragraph is vague, and ultimately not revisited in the manuscript in any detail.\nL43: suggest change to “…play a key role in the planet’s atmospheric…”\nL48: suggest change to “…how do the crustal fields control…”\nL49: re: “provide new insights” - this statement would be stronger if the kinds of new insights that are possible are described.\nL52: suggest change to “mini-magnetospheres”\nL53: The Harada et al. paper is referenced here, in a general statement about scaled-down intrinsic magnetospheres. But the electron signatures are so similar to the ion signatures that they should really be called out. Otherwise it feels like they are being deliberately minimized in the present work. (I’m not surmising this is the authors’ intent. I’m just pointing out the impression it may give!)\nL76-77: “can be regarded as key plasma evidence of intrinsic magnetosphere behaviour”. This statement is setting up the argument for the present manuscript that dispersed ion structures are evidence of an intrinsic magnetosphere. It reads to me like a bit of an overstatement - that dispersed ion structures are the -only- evidence of intrinsic magnetosphere behaviour. However, there are many signatures of intrinsic magnetospheres, and several have been previously reported for Mars. And dispersed energy-time ion signatures have been previously reported in the Martian magnetosphere - just not near crustal fields.\nL83: suggest changing “could” to “does”\nL105-106: suggest change to “…traversed the nightside terminator…”\nL133: add “the” before “energy”\nL134: suggest changing “were” to “was”\nL144: please provide a reference here in the text to the crustal field model, since there are many in the community.\nL164: remove “meticulous”\nL195-197: What is the evidence that magnetic reconnection causes the injections of ions? Are there other possibilities? This section has two sentences - one basically gives the question and the next gives the answer with no substantive justification. I suggest adding more discussion here.\nL202-203: “the pitch angle…Br”. I may have misunderstood this phrase, but think it could be reworded to be more clear. The specific pitch angle (e.g. 140 degrees) is not determined by Br. Instead, the asymmetry between parallel and antiparallel pitch angles is determined by Br.\nL210: How could these three options be distinguished from each other?\nL231: anisotropy —> anisotropic\nL269: coordinates —> coordinate\nL280-281: It seems like a lot of effort went into proving this statement that interior crustal field lines have higher field strength at the magnetic equator. This seems self-evident to me, without any demonstration required. Am I missing something? Second comment: The “MFL” and “MEP” acronyms are not standard in the literature, as far as I am aware. I understand why they were used - to avoid repetition in the discussion. But I ended up getting repeatedly lost while reading this paragraph and the several preceding paragraphs.\nL303: suggest change to “According to the above results, the complete scenario of observed…”\nL390-391: Are there peculiarities in the external conditions for the 17 identified events that could explain why they are only rarely observed?\nL415-416: “crustal fields can locally provide a plasma environment similar to the planetary-scale intrinsic magnetospheres”. I agree with this statement, but don’t think this manuscript was the first to demonstrate this. Perhaps give other examples from the literature?"}, "author_response": "Response to the Reviewers\nReviewer #1 (Remarks to the Author):\nThis manuscript presents very interesting observations and proposes a compelling\nexplanation for the observations.\nOverall, I think that the manuscript is well written and well organized.\nI do feel, however, that the title is a bit over-the-top. The fact that the observations\nindicate energy-dependent drifts is not a property of intrinsic magnetospheres (though\nthat is where they have been observed so far); it is a property of magnetic field\nconfigurations larger than the gyroradius of the particles. Such behavior has been\nhypothesized to occur in smaller-than-global magnetic field configurations elsewhere\nincluding the Moon and Mars. I think these observations are very interesting, and a first\nfor Mars, but not necessarily surprising.\nReply: We extend our gratitude to the reviewer for their valuable comment. Based on\nreviewer’s suggestion, we have replaced the title by “Discovery of typical intrinsic\nmagnetospheric ion drift patterns at Mars”, so that the study content can be\nspecifically reflected by the title for the better. The drift dispersion structures indeed\nindicate that the scale-sizes of gradient/curvature of the magnetic field configurations\nare larger than the gyroradius of the particles, but also requires stable configuration for\nboth the magnetic field and electric field over the time scale that causes the dispersion.\nSince the scale-size magnetic field is small and complicated for the Mars, its field\nconfigurations may not be stable as the rotation of Mars.\nIt was found that the electron drift dispersion structures could occur within the small-\nscale crustal fields at Mars (Harada et al. 2015). Nevertheless, when considering ions\n(of ~100 eV) with larger gyroradius, drift dispersion structures of ions would not be\nanticipated at Mars, as indicated by Halekas et al. (2016), since the ions usually exhibit\nnon-magnetized states characterized by the non-adiabatic motion of ions (Soobiah et al.\n2019).\nTherefore, our study reports the drift dispersion structures of ions at the first time\ndespite MAVEN’s continuous exploration of Mars for over 9 years. Our study also\nsuggests that the Martian magnetosphere exhibits an intermediate case wherein both\nunmagnetized and magnetized ion behaviors are observable because of the wide range\nof strengths and spatial scales of induced and crustal magnetic fields.\nPlease see the revised Title, Abstract (lines 26-37), Introduction (lines 52-97), and\nConclusion part (lines 450-454) of our paper.\nAdditionally, I do have some relatively minor comments below.\nFigure 4: I am having some trouble understanding Figure 4b. I guess in general I\nunderstand that it illustrates the magnitude of the magnetic field along the magnetic\nequatorial plane, but I don’t understand the axes – particularly the vertical axis. What\nis the radial distance measured with respect to, and how does it go negative?\nReply: Thanks for this comment. In fact, we set the innermost magnetic equator point\nsituated in the upper left corner as the origin point (we have marked it as the black star\nin the revised Figure 4). Consequently, both radial and azimuth distances are computed\nwith respect to this origin point. We have clarified it in the paper. Please see the lines\n278-279.\nFigure 5 caption: I don’t think that “polygon” is the best descriptor for the shaded\nshapes in Figure 5. Perhaps it is more of a crescent shape (polygon implies straight\nrather than curved edges).\nReply: We have replaced it by the crescent-shaped area. Please see the revised Figure\n5.\nLine 433: On this line SWIA should be SWEA since it has to do with electron\nmeasurements.\nReply: Thank you for catching the typo. We have revised it, please see line 469.\nTo what altitude are the magnetic field lines traced? High order spherical harmonic\nmagnetic field models become unphysical at too low an altitude. The manuscript states\nthat the magnetic field model represents the crustal fields well at altitudes above 120\nkm. Are the field lines traced lower than that altitude?\nReply: Here we traced all field lines starting at an altitude of 500 km, and we stopped\ntracing when the altitude reaches 200 km. We have described it in more detail, please\nsee the subsection of “Tracing the magnetic field lines” of the section of “Method”\n(lines 499-507).\nAdditionally, although it is not explicitly stated, I assume that the gradient in the\nmagnetic field is calculated using the magnetic field model plus the constant offset\nrather than just the model (this is mentioned for the field line tracing but not for the\nother magnetic field calculations).\nReply: Exactly. The gradient and magnetic drift velocity are all calculated based on the\nsuperposed fields, which are the magnetic field model plus the constant offset. We have\nemphasized in the subsection of “Tracing the magnetic field lines” of the section of\n“Method” (lines 508-509).\nSupplementary Figure 5 (and line 220): It is mentioned and shown that no proton\ncyclotron waves are present locally. However, Supplementary Figure 5c does appear to\nshow some wave power at a slightly lower frequency on the order of 0.1 Hz. Are these\nwaves significant and can they be identified?\nReply: Thanks for this comment. We think these waves are not significant, since the\nvariation of magnetic fields (32 Hz) (Supplementary Figure 5b) does not clearly show\nany wave signatures.\nSupplementary Table 1: I am curious as to why the solar wind dynamic pressure is\nincluded in this table. I don’t recall dynamic pressure mentioned elsewhere in the\nmanuscript. Additionally, it brings up questions as to how the dynamic pressure was\ncalculated given that MAVEN is not in the solar wind during the time of observation.\nPersonally, I would be interested in the possibility of variations in the upstream dynamic\npressure during the observations; could dynamic pressure variations drive upstream\nwaves (as mentioned on line 207)? But I wouldn’t think that this information would be\navailable given the spacecraft orbit.\nReply: Thanks for this comment. We have deleted the information of solar wind\ndynamic pressure. The upstream conditions are considered to be the average values of\nthe solar wind conditions in the 30 minutes preceding (subsequent to) the inbound\n(outbound) bow shock crossing for each orbit (see Halekas et al. 2017).\nReviewer #2 (Remarks to the Author):\nThe paper presents interesting MAVEN plasma and magnetic field observations around\nclosest approach, when the spacecraft traverse through a region of crustal magnetic\nfields. The authors report, for the first-time, observations of wedgelike dispersion\nsignatures of protons, exhibiting butterfly-shaped distributions within the Martian\ncrustal fields, a feature observed in intrinsic magnetospheres. The authors argue that the\nobserved structures can be understood in terms of drifts motions, despite the relatively\nweak and large inhomogeneity of crustal magnetic fields, compared to planetary global\nintrinsic magnetic fields. The authors also report the identification of 16 additional\nwedge-like dispersion events within the crustal magnetic field of Mars, observed by\nMAVEN.\nIn my opinion, this manuscript presents new, original observations suggesting wedge-\nlike dispersion signatures of protons at Mars. This is worthy of further investigation and\nopens the possibility for complementary studies involving magnetic reconnection and\nplanetary ion escape, in connection with crustal magnetic fields. The main concern I\nhave with this paper is that the authors do not sufficiently explain the observed\nsignatures in terms of particle drifts within crustal magnetic fields. I think the\nmanuscript would benefit from additional estimations of relevant parameters, based on\nthe properties observed by MAVEN and current crustal magnetic field models available\nin the literature.\nWe thank the reviewer for providing these insightful comments. Taking each comment\ninto careful consideration, we have revised the paper, highlighting the changes in red\ntext. Our paper has been improved based on your comments. We hope that our\nmodifications have met the requirements of reviewer.\nPlease find below my specific comments.\nLine 98: How are the black dashed curves shown in panels 1a and 1b computed?\nReply: The black dashed curves shown in panels 1a and 1b were manually fitted to\naccentuate the dispersed signatures, which does not impact the analysis presented in\nour paper. Although they may appear as curves, they are actually straight lines when\nviewed on a linear Y scale. However, for visual representation, they appear as curves\nwhen viewed on a logarithmic Y scale. Please see the line 108.\nLine 109: Is there any physical explanation for a 20-30 s periodicity? What is the\nperiodicity observed in the other 16 events the authors were able to identify?\nReply: As we discussed in the text, the bounce motion could not explain the periodicity\nof our event.\nIt is worth noting that periodical dynamics with periods of several tens of seconds have\nbeen commonly observed on Mars, as highlighted in previous studies by Zhang et al.\n(2021), Halekas et al. (2015, 2020), Grigoriev et al. (2007), and Collinson et al. (2018).\nThese studies have suggested that such periodicity is attributed to upstream waves.\nBased on this existing knowledge, we hypothesized that the periodicity observed in our\nevent could also be attributed to these waves. It is widely accepted that these waves are\nprimarily excited by planetary pickup ions or ions reflected by the bow shock (Dubinin\n& Fraenz, 2016; Halekas et al. 2020). Furthermore, these waves will exhibit a period\napproximately equal to the proton cyclotron period, which is on the order of 10 seconds\nwithin the upstream region.\nWhile there are an additional 16 events demonstrating wedge-like dispersion structures,\nit's important to note that only a subset of these events (No. 4, 5, 9, 15 in Supplementary\nTable 1) display multiple dispersed structures. Remarkably, the periodicity observed in\nthese events is also found to be on the order of 10 seconds, in alignment with our\nproposed mechanism. We have described it in more detail, please see the lines 223-228.\nLine 149: What is the interpretation for the additional [14.8, -0.93, -4.15] nT magnetic\nfield?\nReply: Based on the topology analysis and electron distributions, we believed that the\nmagnetic field lines are mostly generated by the crustal fields. However, the crustal\nfields could interact with the induced fields, leading to the compression and distortion\nof the crustal fields. Therefore, we think the additional fields arise from the influence\nof the induced fields. Please see the lines 166-167.\nLine 208-210: I think the observed periodicity should be explained in more detail.\nReply: Thank you, we have described it in more detail. Please see the lines 223-228.\nLine 230-232: Why would the source be already anisotropic? Is it the case with all 16\nadditionally identified cases?\nReply: Given that the loss cone effect couldn’t account for the difference between ions’\npitch angles of 22.5°-45° and 135°-157.5°, we proposed that the source distribution\nitself might have been already anisotropic, leading to the observed anisotropy.\nIn fact, we found that each event exhibits a different pitch angle distribution. We will\nfurther investigate it in the future.\nLine 362-364: According to the authors, the observations cannot be explained by\nmagnetic drift, but by the electric drift velocity. Is there any evidence supporting this\nclaim, in addition to the fact that the magnetic field drift would likely be azimuthal?\nReply: Thanks for this good comment. There are two indirect evidences pointing to the\npresence of electric fields. The first is the radial transport of ions, a phenomenon beyond\nthe explanatory capacity of magnetic drift alone. The second is that the magnetic drift\nspeed of 50-200 eV ions (7-30 km/s) significantly surpasses the spacecraft’s velocity\n(~3.8 km/s). Thus, the spacecraft would exclusively detect the falling-tone (time-of-\nflight) dispersed structures since the magnetic drift speed is proportion to the energy,\nprovided that we disregard the electric field drift.\nConsequently, we think that the electric field drift (is energy independent and could\ncontribute the radial transport) is involved for our specific case. Moreover, in addition\nto the case studies, if magnetic drift motion were indeed sufficient to elucidate the\nformation of drift dispersion structures, one would anticipate a more prevalent\noccurrence of such structures, rather than the identification of only 17 distinct events,\nsince the configuration of crustal fields is relatively stable while the electric fields are\nhighly variable.\nNevertheless, the measurement of electric fields remains limited for MAVEN,\ndepriving us of further substantiating evidence. Further observations and simulations\nmight help us to shed light on it. We found that the words we wrote in the manuscript\nwas too assertive, so we have also made modifications. Please see the lines 386-394.\nLine 377-379: If the estimated electric field is one order of magnitude larger than the\ncorotation electric field, can the authors provide evidence in favor of the presence of\nconvective or impulsive electric fields with the expected order of magnitude?\nReply: We indirectly deduced the presence of convective or impulsive electric fields by\ncomparing the magnitude of estimated electric field and the corotation electric field.\nBut same to above, there are no confirmed direct evidence about the electric field. But\nthe latest simulation about the global electric fields around Mars suggest that the\nconvective electric field at ~500 km altitude ranges from 0.1-1 mV/m (Wang et al.\n2023), align with our expectations. Please see the lines 402-416.\nLines 366-379: I think the manuscript would benefit from additional estimations. For\ninstance, what is the proton Larmor radii range for the conditions observed in the\nanalyzed event? How does it compare with the scale length over which the local (crustal)\nmagnetic field varies? What is the expected electric field velocity drift that can explain\nthe observations? How does it compare with the local proton velocity?\nReply: We have added the distribution of proton gyroradius, the curvature radius of\nfield lines in the revised Supplementary Figure 6. We find that the gyroradius of 200\neV (the upper limit energy of dispersed ions) H+ with pitch angle of 90° ranges from\n~30 km to ~100km (Supplementary Figure 6a), while the curvature radius of the field\nlines varies from ~300 km to ~1000 km (Supplementary Figure 6b). Therefore, the\ngyroradius of protons are relatively smaller than the curvature radius.\nThe expected electric field velocity drift in the azimuthal direction should nearly\ncounterbalance the magnetic drift velocity (3-30 km/s), while it should be smaller than\nthe speed of MAVEN in the radial direction (~1.1 km/s) to lead to the observations of\nrising tones as MAVEN entered into the interior region of crustal fields.\nThe proton velocity is larger than the drift velocity since their field-aligned velocity\nranges from 30-100 km/s. However, the field-aligned velocity will not affect the\nformation of the drift dispersion structures. Please see the lines 418-424.\nReviewer #3 (Remarks to the Author):\nComments on “Discovery of Intrinsic Magnetospheric Ion Behaviors at Mars” by\nZhang et al.\nThe manuscript describes observations of protons in Martian crustal magnetic fields\nthat have dispersed energy signatures and oblique (or “butterfly”) pitch angle\ndistributions that are asymmetric and correlated with the radial component of the crustal\nmagnetic field. The manuscript discusses how the protons are injected into the crustal\nfields (reconnection), what causes their periodic occurrence in the crustal fields\n(periodic injections, heating, or variations in the source population), how the pitch angle\ndistributions came to be asymmetric (scattering/drift of field-aligned/90-degree pitch\nangles), and how the energy dispersion signatures formed (higher energy ions drift\ndeeper into the crustal fields).\nOverall the phenomenon reported in the manuscript is novel for ions in the Martian\nmagnetosphere (and any small-scale magnetosphere). I therefore believe that the\ncommunity should be made aware of this work through publication, perhaps after some\nrelatively minor comments below are considered. The main question in my mind is\nwhether the work should be published in a high impact journal such as Nature\nCommunications. Here, I am less certain, for three related reasons:\nReply: We are grateful for the valuable suggestions and comments provided by the\nreviewer. We have considered your comments, and revised the paper on a point-by-\npoint basis marked in red text. The paper has been enhanced significantly as a result of\nyour insights. We trust that our revisions have successfully addressed the reviewer's\nrequirements.\n1. The signatures reported here for ions are similar in many ways (energy-time\ndispersion, bursty and periodic) to a previous report of observations of electrons in Mars\ncrustal fields in 2016 by one of the manuscript’s co-authors, with similar interpretation\nabout mechanisms. These similarities are not mentioned in the present manuscript at\nall, even though the paper (Harada et al.) is referenced. Though it is very interesting to\nsee the same signatures in ions, what is learned overall from this manuscript about\nplasma behaviour in Mars crustal fields is therefore not particularly new.\nReply: In fact, it is less expected that the drift dispersion of trapped ions on small-scale\ncrustal fields would occur because of their large gyroradius, which is different to the\ncase of electrons (Harada et al. 2015). Previous studies have only found the energy\ndispersed ions caused by the temporal effects (time-of-flight) and the ions were usually\nunmagnetized at Mars (Halekas et al. 2016; Soobiah et al. 2019; Zhang et al. 2021).\nHowever, as opposed to the previous studies, our paper reported the observation of ion\ndrift patterns typical for intrinsic magnetospheres within the Martian crustal fields at\nthe first time despite MAVEN’s continuous exploration of Mars for over 9 years. Our\nfindings suggest that magnetized ion behavior commonly observed in intrinsic\nmagnetospheres, does also occur at the crustal field regions of Mars. Furthermore, our\npaper implies that the Martian magnetosphere embodies an intermediate case where\nboth the unmagnetized and magnetized ion behaviors could be observed because of the\nwide range of strengths and spatial scales of induced and crustal magnetic fields. Thus,\nwe think our paper presents a novel plasma signature at Mars, and might open the\npossibility for complementary studies involving magnetic reconnection and planetary\nion escape, in connection with crustal magnetic fields. Please see the revised Title,\nAbstract (lines 26-37), Introduction (lines 52-97), and Conclusion part (lines 450-454)\nof our paper.\n2. The manuscript makes many references to “intrinsic magnetospheric behaviour” that\nthe ion observations provide evidence for - saying in the opening paragraph that “…we\nreport the discovery of intrinsic ion magnetospheric behavior at Mars…”. I am not fully\nconvinced that this is a discovery, since there have been many previous reports of\nintrinsic magnetospheric behavior at Mars (e.g. aurora, magnetic reconnection). It may\nbe that the emphasis in this phrase is on “ion” as opposed to \"intrinsic magnetosphere\",\nbut I find this less compelling since the community is already aware that the crustal\nfields behave as intrinsic magnetospheres in other regards, and there are published\nstudies of ion observations near crustal fields from both Mars Express and MAVEN.\nReply: We agree with the reviewer. In the revised manuscript, we have removed the\nterms of “intrinsic magnetospheric behavior”, but emphasized “ion behavior” and “Ion\ndrift patterns” rather than “intrinsic magnetosphere”. We have revised the Abstract,\nIntroduction and Discussion part of this paper.\nFor the importance of our paper, we think this paper updates our understanding of how\nthe crustal fields controls the ion dynamics. Our work highlights that both electrons and\nions can become magnetized, giving rise to drift dispersion structures on Mars.\nFurthermore, we suggest that the Martian magnetosphere exhibits an intermediate case\nwherein both unmagnetized and magnetized ion behaviors are observable because of\nthe wide range of strengths and spatial scales of induced and crustal magnetic fields.\nAs a result, we can further learn a rich variety of particle dynamics at Mars.\n3. The manuscript mentions that only 17 examples of these observations have been\nfound in 5 years of observations. This suggests that the evidence of intrinsic\nmagnetospheric behavior is the exception, and not the rule.\nReply: We agree with the reviewer. That is why we think the development of dispersion\nstructures of ions at Mars might hinge upon the setup of both magnetic fields and\nelectric fields. It would be more common if the formation of dispersion structures of\nions only depends on the magnetic field configuration. Please see the lines 429-431.\nI note that the figures are informative and useful, and the paper is generally well-written.\nReply: Thanks for this positive comment.\nMy detailed comments follow:\nL26-28: The wording of this sentence was awkward for me. I think I understand the\nintent: “The Martian magnetosphere is unusual and different from intrinsic\nmagnetospheres”. But I might have worded it a little differently: “Mars lacks a global\ndynamo magnetic field, and instead possess small-scale crustal magnetic fields, making\nits magnetic environment fundamentally different from intrinsic magnetospheres like\nthose of Earth or Saturn.”\nReply: We have revised it. Please see the lines 26-28.\nL31: Clarify “butterfly-shaped pitch angle distributions”\nReply: We have clarified it. Please see the lines 31-32.\nL36-37: This last sentence of the paragraph is vague, and ultimately not revisited in the\nmanuscript in any detail.\nReply: Thanks for this comment. We have deleted it.\nL43: suggest change to “…play a key role in the planet’s atmospheric…”\nReply: Revised, please see line 43.\nL48: suggest change to “…how do the crustal fields control…”\nReply: Revised, please see line 48.\nL49: re: “provide new insights” - this statement would be stronger if the kinds of new\ninsights that are possible are described.\nReply: Deleted, please see line 49.\nL52: suggest change to “mini-magnetospheres”\nReply: Revised, please see line 52.\nL53: The Harada et al. paper is referenced here, in a general statement about scaled-\ndown intrinsic magnetospheres. But the electron signatures are so similar to the ion\nsignatures that they should really be called out. Otherwise it feels like they are being\ndeliberately minimized in the present work. (I’m not surmising this is the authors’ intent.\nI’m just pointing out the impression it may give!)\nReply: We appreciate this comment very much. We have revised this manuscript and\nadded the findings of Harada et al. (2015), please see the lines 83-85. We also have\nemphasized the importance and novelty of our paper. Please see the revised Introduction\npart.\nL76-77: “can be regarded as key plasma evidence of intrinsic magnetosphere\nbehaviour”. This statement is setting up the argument for the present manuscript that\ndispersed ion structures are evidence of an intrinsic magnetosphere. It reads to me like\na bit of an overstatement - that dispersed ion structures are the -only- evidence of\nintrinsic magnetosphere behavior. However, there are many signatures of intrinsic\nmagnetospheres, and several have been previously reported for Mars. And dispersed\nenergy-time ion signatures have been previously reported in the Martian magnetosphere\n- just not near crustal fields.\nReply: We agree with the reviewer. The drift dispersion structures of plasma indicate\nthat the spatial scale of the electromagnetic environment is significantly larger than the\nplasma’s gyroradius. We have revised it throughout this paper. Please see the lines 79-\n91.\nNote that we so far have only found the energy dispersion of unmagnetized ions caused\nby temporal effect at Mars (Halekas et al. 2015; Zhang et al. 2021). The drift dispersion\nstructures of ions have exclusively been documented within planetary-scale intrinsic\nmagnetospheres with strong magnetic field strength, such as those of Earth and Saturn.\nFurthermore, their emergence within small-scale crustal fields is considerably\nunexpected since the ions are generally unmagnetized.\nL83: suggest changing “could” to “does”\nReply: Changed, please see the line 96.\nL105-106: suggest change to “…traversed the nightside terminator…”\nReply: Changed, please see the lines 99-100.\nL133: add “the” before “energy”\nReply: Added, please see the line 144.\nL134: suggest changing “were” to “was”\nReply: Changed, please see the line 145.\nL144: please provide a reference here in the text to the crustal field model, since there\nare many in the community.\nReply: Added.\nL164: remove “meticulous”\nReply: Removed.\nL195-197: What is the evidence that magnetic reconnection causes the injections of\nions? Are there other possibilities? This section has two sentences - one basically gives\nthe question and the next gives the answer with no substantive justification. I suggest\nadding more discussion here.\nReply: We have discussed two possible mechanisms that result in the ions injection:\nmagnetic reconnection and the non-adiabatic effect. Demonstrating both scenarios is\nchallenging due to limited observations. Further studies might address it. We have\nexpanded our discussion on these aspects. Please see the lines 204-214.\nL202-203: “the pitch angle…Br”. I may have misunderstood this phrase, but think it\ncould be reworded to be more clear. The specific pitch angle (e.g. 140 degrees) is not\ndetermined by Br. Instead, the asymmetry between parallel and antiparallel pitch angles\nis determined by Br.\nReply: Deleted.\nL210: How could these three options be distinguished from each other?\nReply: If the injection position is stationary, the periodic injections and variations in the\nsource population should be identical; otherwise, they would be different. Nonetheless,\ndetermining the stationarity of the injection position proves to be challenging.\nL231: anisotropy —> anisotropic\nReply: Revised, please see the line 249.\nL269: coordinates —> coordinate\nReply: Revised, please see the line 290.\nL280-281: It seems like a lot of effort went into proving this statement that interior\ncrustal field lines have higher field strength at the magnetic equator. This seems self-\nevident to me, without any demonstration required. Am I missing something? Second\ncomment: The “MFL” and “MEP” acronyms are not standard in the literature, as far as\nI am aware. I understand why they were used - to avoid repetition in the discussion. But\nI ended up getting repeatedly lost while reading this paragraph and the several\npreceding paragraphs.\nReply: The crustal magnetic fields exhibit greater complexity and irregularity compared\nto the dipole fields. Proving that interior crustal field lines have higher field strength at\nthe magnetic equator before the further analysis is better, despite the fact that the\nmorphology of localized crustal fields appears dipole-like in our specific case.\nThe “MFL” and “MEP” acronyms are removed throughout this paper.\nL303: suggest change to “According to the above results, the complete scenario of\nobserved…”\nReply: Changed, please see the line 328.\nL390-391: Are there peculiarities in the external conditions for the 17 identified events\nthat could explain why they are only rarely observed?\nReply: We have not identified any distinct peculiarities in the external conditions for\nthe 17 events. We think the development of dispersion structures of ions at Mars might\nhinge upon the setup of both magnetic fields and electric fields. Please see the lines\n429-431. Nevertheless, it is important to note that the absence of concurrent\nmeasurements for both solar wind and crustal fields could impose limitations on our\nfurther research.\nL415-416: “crustal fields can locally provide a plasma environment similar to the\nplanetary-scale intrinsic magnetospheres”. I agree with this statement, but don’t think\nthis manuscript was the first to demonstrate this. Perhaps give other examples from the\nliterature?\nReply: Thanks for this comment. We agree with the reviewer. Our paper actually reports\nthe wedge-like dispersion structures of ions at Mars at the first time, which are\npreviously only reported in intrinsic magnetospheres. We have revised the paper, please\nsee the lines 434-435, 450-454.\nReference:\nCollinson, G. (2018). Solar Wind Induced Waves in the Skies of Mars: Ionospheric\nCompression, Energization, and Escape Resulting From the Impact of Ultralow\nFrequency Magnetosonic Waves Generated Upstream of the Martian Bow Shock.\nJournal of Geophysical Research: Space Physics, 123(5), 4129-4149.\nhttps://doi.org/10.1029/2018JA025414\nDubinin, E., Fraenz, M., (2016). Waves at Venus and Mars. In: Keiling, A., V.\nNakariakov, D.H.L. (Eds.), Low-frequency Waves in Space Plasmas, vol. 103.\nAGU, pp. 343–364.\nGrigoriev, A., Futaana, Y., Barabash, S., & Fedorov, A. (2007). Observations of the\nMartian Subsolar ENA Jet Oscillations. Space Science Reviews, 126(1-4), 299-\n313. https://doi.org/10.1007/s11214-006-9121-y\nHalekas, J. S., et al. (2015), Time-dispersed ion signatures observed in the Martian\nmagnetosphere by MAVEN, Geophys. Res. Lett., 42, 8910–8916,\ndoi:10.1002/2015GL064781.\nHalekas, J. S., et al. (2017), Structure, dynamics, and seasonal variability of the Mars\nsolar wind interaction: MAVEN Solar Wind Ion Analyzer in-flight performance\nand science results, J. Geophys. Res. Space Physics, 122, 547–578,\ndoi:10.1002/2016JA023167.\nHalekas, J. S., Ruhunusiri, S., Vaisberg, O. L., Harada, Y., Espley, J. R., Mitchell, D.\nL., Mazelle, C., Romanelli, N., DiBraccio, G. A., & Brain, D. A. (2020).\nProperties of Plasma Waves Observed Upstream From Mars. Journal of\nGeophysical Research: Space Physics, 125(9).\nhttps://doi.org/10.1029/2020ja028221\nHarada, Y., et al. (2016), MAVEN observations of energy-time dispersed electron\nsignatures in Martian crustal magnetic fields, Geophysical Research Letters, 43,\n939–944, doi:10.1002/2015GL067040.\nSoobiah, Y. I. J., Espley, J. R., Connerney, J. E. P., et al.. (2019). MAVEN Case\nStudies of Plasma Dynamics in Low-Altitude Crustal Magnetic Field at Mars 1:\nDayside Ion Spikes Associated With Radial Crustal Magnetic Fields. Journal of\nGeophysical Research: Space Physics, 124(2), 1239-1261.\nhttps://doi.org/10.1029/2018ja025569\nWang, X.-D., Fatemi, S., Nilsson, H., Futaana, Y., Holmström, M., & Barabash, S.\n(2023). Solar wind interaction with Mars: electric field morphology and source\nterms. Monthly Notices of the Royal Astronomical Society, 521(3), 3597-3607.\nhttps://doi.org/10.1093/mnras/stad247\nZhang, C., Rong, Z., Nilsson, H., Klinger, L., Xu, S., Futaana, Y., Wei, Y., Zhong, J.,\nFränz, M., Li, K., Zhang, H., Fan, K., Wang, L., Holmström, M., Ge, Y., & Cui,\nJ. (2021). MAVEN Observations of Periodic Low-altitude Plasma Clouds at\nMars. The Astrophysical Journal Letters, 922(2). https://doi.org/10.3847/2041-\n8213/ac3a7d"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am very sorry for my late response. The response to the reviewers and the revised manuscript have addressed most of my major concerns.\nThe only question that popped up to me is that of the speed of the MAVEN spacecraft. On line 392, the speed is quoted at “~3.8 km/s” while on line 410 it is “~1.3 km/s”. Is this second speed just the radial component? It is not clear to me.\nAside from that, I personally think the manuscript presents interesting and convincing analyses.", "reviewer_2": "I thank the authors for addressing my comments. I suggest publication in Nature Communications.", "reviewer_3": "I thank the authors for their careful responses to the reviewer comments, and their corresponding revisions to the manuscript. I am happy with their choice to recast the importance of the results in terms of observing ion drift patterns, and the Mars crustal magnetic fields as an intermediate case that can host both magnetized and unmagnetized ions."}, "author_response": "We are thankful for the positive comments provided by the reviewers.\nReviewer #1 (Remarks to the Author):\nI am very sorry for my late response. The response to the reviewers and the\nrevised manuscript have addressed most of my major concerns.\nThe only question that popped up to me is that of the speed of the MAVEN\nspacecraft. On line 392, the speed is quoted at “~3.8 km/s” while on line 410\nit is “~1.3 km/s”. Is this second speed just the radial component? It is not clear\nto me.\nAside from that, I personally think the manuscript presents interesting and\nconvincing analyses.\nReply: Thank you for this comment. It is right, the 3.8 km/s is the total speed, while the\n1.3 km/s is the radial speed. I have added it in the text, please see the line 412.\nReviewer #2 (Remarks to the Author):\nI thank the authors for addressing my comments. I suggest publication in\nNature Communications.\nReviewer #3 (Remarks to the Author):\nI thank the authors for their careful responses to the reviewer comments, and\ntheir corresponding revisions to the manuscript. I am happy with their choice\nto recast the importance of the results in terms of observing ion drift patterns,\nand the Mars crustal magnetic fields as an intermediate case that can host both\nmagnetized and unmagnetized ions."}]

Decision letter