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We investigate how the Martian dayside ionospheric structure is modified by crustal magnetic field (CMF) strength and upstream solar wind pressure by analyzing electron density data from the Langmuir Probe and Waves instrument onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. We find that the electron density above the exobase is anticorrelated with the ratio of solar winds normal dynamic pressure ( P SW ⊥ ) to CMF magnetic pressure ( P CMF ). We also analyze the electron density behavior across different magnetic topologies as a function of P SW ⊥ / P CMF . The extremely low electron density in the draped topology relates to ionopause-like structures. The lower electron density in the closed and open topology under higher P SW ⊥ / P CMF may be attributed to a downward force, potentially the J × B force in the case of closed topology. This study highlights the complex interplay between solar wind and CMF in influencing the Martian dayside upper ionosphere.

Although the Moon does not have a global intrinsic magnetic field, lunar crustal magnetic anomalies (LMAs) are nonuniformly distributed over the lunar surface. The interaction between the solar wind and LMAs leads to the formation of mini-magnetospheres. Since the spatial scales of LMAs are very small, below several tens of kilometers, solar wind ions are demagnetized while electrons are still magnetized, forming Hall electric fields typically at low altitudes ( < ∼ 30 km). Since direct observations of these interaction regions are challenging from typical nominal altitudes of lunar orbiters ( > ∼ 100 km), the solar wind-LMA interaction has not been fully understood. In this study, we analyze low-altitude data obtained by Kaguya over various LMAs to comprehensively characterize the plasma environment and electromagnetic fields in the solar wind-LMA interaction region. We observe strong solar wind ion reflection and whistler mode waves at 1–10 Hz under high solar wind dynamic pressure and strong interplanetary magnetic field conditions, respectively. These trends are particularly clear over spatially extended LMAs. Over both spatially isolated and extended LMAs, strong Broadband Electrostatic Noise at 1–10 kHz tends to be observed when the spacecraft is magnetically connected to the lunar surface. In addition, our results suggest that anti-moonward electrostatic fields at low altitudes contribute to the acceleration, deceleration, and reflection of incident solar wind particles, and the resulting modification of particle velocity distribution functions can strongly influence the nature of the solar wind-LMA interaction including plasma wave excitation. Based on Kaguya data, we also develop a predictable indicator of the central interaction region where solar wind ions and electrons are decoupled. We propose that this indicator can be utilized to define regions of interest for future low-altitude or lander missions to LMA. [ABSTRACT FROM AUTHOR]

The Martian bow shock has been extensively studied through magnetic field and plasma instrument observations from various Mars space missions. However, prior investigations primarily involve statistical analyses based on single spacecraft crossings, leaving the asymmetry of the Martian bow shock unstudied through simultaneous two‐spacecraft observations. In this study, utilizing simultaneous observations from Tianwen‐1 and MAVEN, we examine the instantaneous asymmetry of the Martian bow shock. We present the asymmetry of the Martian bow shock in the Mars‐Solar‐Electric and Mars‐Solar‐Orbital reference frames, possibly influenced by the solar wind motional electric field and Martian crustal magnetic field, respectively. Moreover, we suggest that the bow shock exhibits increased asymmetry under stronger solar wind motional electric field conditions. This study highlights how a two‐point observation approach offers valuable insights into the dynamic behavior of the Martian induced magnetosphere. Plain Language Summary: Scientists have been studying the Martian bow shock, which slows down the solar wind from supersonic to subsonic, using instruments on different Mars missions. But most studies only use data from one spacecraft, so we don't know much about how the bow shock varies in different places at the same time. In this study, we looked at data from two spacecraft, Tianwen‐1 and MAVEN, at the same time to see if we could find any differences. We found that the bow shock is different in the northern and southern parts of Mars in different reference frames, and this might be because of the solar wind electric field and the Martian crustal field. We also noticed that the bow shock is more different when the solar wind electric field is stronger. Such two‐spacecraft observations helps us understand more about how the magnetic environment around Mars works. Key Points: We investigated the asymmetry of Martian bow shock utilizing two‐spacecraft observations at distinct locationsThe dayside Martian crustal field may induce the north‐south asymmetry of the bow shockThe solar wind electric field may cause bow shock asymmetry in the Mars‐Solar‐Electric frame, with stronger fields increasing the effect [ABSTRACT FROM AUTHOR]

We study the average global distribution of the external magnetic field at Mars, and its variability with the upstream solar wind dynamic pressure and interplanetary magnetic field as well as with the ambient crustal magnetic field strength. Our approach involves excluding the intrinsic planetary field from the total magnetic field by applying a crustal field model previously derived using low altitude measurements. The distribution of the average external field that remains is statistically analyzed using nearly 8 years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations and several global, time‐dependent magnetohydrodynamic simulations. Overall consistent results have been obtained from the data and model, which are complementary to each other and cross validate the findings. It is found that the external field is significantly enhanced from the upstream across the bow shock (BS) and further intensifies closer to the planet in the topside ionosphere. It peaks at ∼170 km altitude near the subsolar point, significantly decreasing with increasing solar zenith angle. There is a strong day‐night asymmetry in the external field, with a typical dayside intensity of ∼15–50 nT and a nightside intensity of ∼5–15 nT. Under high solar wind dynamic pressures and IMFs, the external field may be enhanced by a factor of ∼2 everywhere below the BS, on both the dayside and nightside. In addition, our model results suggest that strong crustal fields, which effectively withstand the penetration of the solar wind, reduce the external field at low altitudes. Plain Language Summary: We use nearly 8 years of satellite observations and several global numerical simulations to analyze the average distribution of the external magnetic field induced in the solar wind‐Mars interaction. Our approach is to separate the intrinsic and external components both in field measurements from the MAVEN spacecraft and in simulation results from a global Mars‐solar wind interaction model. The intrinsic crustal magnetic field is rooted beneath the surface and is organized in the rotating, planet‐fixed reference frame. The external magnetic field is better described in a Sun‐Mars reference frame in light of the complex solar wind interaction with the Mars obstacle (combined magnetosphere and ionosphere). Mixing them together results in the appearance of complex magnetic field distributions and affects the understanding of physical processes. Our exclusion of the intrinsic component using a crustal field model enables us to focus on the distribution of the external magnetic field itself, which is relatively poorly understood. We investigate the variability of the external field distribution due to the changes of the upstream solar wind and magnetic field conditions and the ambient crustal field strength. Our work shows that the average external field distribution follows basic patterns despite complex variabilities. Key Points: We use Mars Atmosphere and Volatile EvolutioN observations and magnetohydrodynamic simulations to investigate the external magnetic field environment after the crustal field is excludedThe external field has a typical dayside (nightside) intensity of ∼15–50 nT (∼5–15 nT) and peaks at ∼170 km altitude at the subsolar pointThe external field intensity can be enhanced by a factor of ∼2 on a global scale below the bow shock during high solar activity [ABSTRACT FROM AUTHOR]

We perform a statistical analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) project on the global distribution of protons in the Martian magnetosheath. Our results show that the proton number density distribution has a south-north asymmetry. This south-north asymmetry is most likely caused by the south-north asymmetric distributions of the crustal magnetic fields at Mars. The strong crustal magnetic fields push the inner boundary of magnetosheath to a higher altitude in the southern hemisphere. Due to the outward movement of the inner boundary of the magnetosheath, a compressed magnetosheath forms, causing subsequent increases in proton number density, thermal pressure, and total pressure. Eventually, a balance is reached between the increased total pressure inside the magnetosheath and the increased magnetic pressure inside the induced magnetosphere. Our statistical study suggests that the Martian crustal magnetic fields can strongly affect the proton number density distribution in the Martian magnetosheath.

Magnetic field measurements show that Mars possesses strong crustal magnetic anomalies that formed when the planet had an active dynamo. To investigate the origin of this magnetization, we used localized power spectrum analyses to constrain the equivalent depths of thin magnetic layers in the crust. Using a new martian magnetic field model that incorporates data from both the Mars Atmosphere and Volatile EvolutioN and Mars Global Surveyor missions, we found that the equivalent magnetization depths on Mars vary from the surface to 72 km. In the northern hemisphere the magnetization depths are found to be on average 9 km, whereas in the southern hemisphere the depths are on average 32 km. If these depths are interpreted in terms of a thick magnetic layer, magnetization in the northern lowlands could extend from the surface to about 18 km depth, whereas for the southern highlands the magnetic layer could extend from about 20 km depth to the base of the crust. The strongest magnetic anomalies are, in general, associated with deep source depths. Our results are consistent with the hypothesis where strong primordial magnetic materials of the northern hemisphere were excavated from the Borealis impact basin, leaving only strong deep remanent magnetization in the southern hemisphere. Plain Language Summary: Mars does not have a global magnetic field at present, but portions of the martian crust were strongly magnetized in the past when a dynamo operated. The origin of these crustal magnetic anomalies is uncertain, and knowledge of the depth of the magnetic minerals could help to discriminate among competing hypotheses. We investigate how the magnetization depth varies across the surface. North of the dichotomy boundary, the magnetization lies close to the surface, while in the southern highlands, the magnetization is located on average 32 km below the surface. One interpretation for the origin and distribution of crustal magnetization is that the strongest magnetic anomalies formed early in martian history and that a large impact event that created the dichotomy boundary removed pre‐existing deep crustal magnetization in the northern hemisphere. Key Points: The depth of magnetization on Mars is investigated using localized power spectrum analysesMagnetization is on average 9 km deep north of the dichotomy boundary, but is 32 km deep south of the dichotomyThe strongest magnetic anomalies are associated with deep magnetization [ABSTRACT FROM AUTHOR]

The observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the topside (≥200 km) ionosphere of Mars show that this region is very responsive to the variations of the external (solar extreme ultraviolet flux, solar wind, and interplanetary magnetic field [IMF]) and internal (the crustal magnetic field) drivers. With the growth of the solar irradiance the ionosphere broadens while with increase of the solar wind dynamic pressure it shrinks. As a result, the upper ionospheric boundary at solar zenith angles of 60–70° can move from ∼400 to ∼1,200 km. Similar trends are observed at the nightside ionosphere. At Pdyn ≥ 1–2 nPa the nightside ionosphere becomes very fragmented and depleted. On the other hand, the ion density in the nightside ionosphere significantly (up to a factor of 10) increases with the rise of the solar extreme ultraviolet flux. Large‐amplitude motions of the topside ionosphere also occur with variations of the value of the cross‐flow component of the IMF. The upper dayside ionosphere at altitudes of more than 300–400 km is sensitive also to the direction of the cross‐flow component of the IMF or, correspondingly, to the direction of the motional electric field in the solar wind. The ionosphere becomes very asymmetrical with respect to the Vsw×BIMF direction and the asymmetry strongly enhances at the nightside. The topside ionosphere above the areas with strong crustal magnetic field in the dayside southern hemisphere is significantly denser and expands to higher altitudes as compared to the ionosphere above the northern nonmagnetized lowlands. The crustal magnetic field also protects the nightside ionosphere from being filled by plasma transported from the dayside. The draping IMF penetrates deeply into the ionosphere and actively influences its structure. Weak fields and, correspondingly, weak magnetic field forces only slightly affect the ionosphere. With increase of the induced magnetic field strength the transport motions driven by the magnetic field pressure and field tensions seem to be intensified and we observe that the local ion densities at the dayside considerably decrease. A different trend is observed at the nightside. The ion density in the nightside ionosphere above the northern lowlands is higher than in the southern hemisphere indicating that plasma transport from the dayside is the main source of the nightside ionosphere. Nonstop variations in the solar wind, the IMF and the solar irradiance together with planetary rotation of the crustal magnetic field sources lead to a continuous expansion/shrinking and reconfiguration of the topside ionosphere of Mars. Key Points: Upper ionosphere of Mars is very sensitive to the solar wind variationsUpper ionosphere of Mars is very sensitive to the variations of the interplanetary magnetic fieldUpper ionosphere of Mars is very sensitive to the planetary crustal field [ABSTRACT FROM AUTHOR]

The ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies electron temperatures in the dayside for two regions where (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal‐field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet conditions, which is associated with the local extreme ultraviolet energy deposition. The current models underestimate the electron temperature above 250‐km altitude in the crustal‐field region. Electron heat conduction associated with a photoelectron transport in the crustal‐field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron's upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal‐field region should be considered for future numerical simulations. Key Points: At high altitudes, the electron temperature is lower for the crustal‐field region than for the IMF‐dominated draped regionsThe low temperatures for the crustal‐field region are due to a heat transport along the field lines between lower and upper atmosphereKinetic effects such the magnetic mirror and the ambipolar electric field may affect the heat transport along crustal magnetic field lines [ABSTRACT FROM AUTHOR]

We present the effects of the local magnetic field configurations on ions precipitating into the upper atmosphere of Mars using Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Precipitating pickup planetary heavy ions (O+, O2+, and CO2+) are of particular interest in the Martian plasma environment because they potentially enhance the sputtering loss of ambient neutral particles. In addition, solar wind protons (and H+ pickup ions) penetrate into the dayside atmosphere due to the direct interaction with the Martian obstacle. We present a statistical study showing that precipitating ion fluxes are typically enhanced by a factor of 2–3 under radial field configurations. We also show that the crustal fields have a shielding effect; the precipitating fluxes are significantly reduced by ∼50% under the strong crustal fields (≳ 100 nT), where the local magnetic field is oriented with a more horizontal component to the surface. These trends are seen consistently regardless of ion species, as well as the observed locations including dayside/nightside, subsolar longitudes, and ±E hemispheres in the Mars‐centered solar electric (MSE) coordinates. In particular, the local magnetic field configurations control precipitating ions with energies lower than a few keV, while precipitating high‐energy ion fluxes are likely independent of the local magnetic field configurations. Precipitating ion fluxes are known to vary by at least an order of magnitude depending on the upstream solar wind. Therefore, the local magnetic field configurations turn out to be the secondary factor in modulating precipitating ion fluxes at Mars. Key Points: We investigated the effects of the local crustal field configurations on precipitating ion fluxes into MarsPrecipitating ion fluxes increase by a factor of 2–3 with the radial crustal fields while decrease by half with horizontal crustal fieldsThe local crustal fields consistently control precipitating ion fluxes regardless of their species and the observed location [ABSTRACT FROM AUTHOR]

Abstract: The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magnetic field lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvénic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic field data acquired from orbit shows that the Moon possesses many magnetic anomalies. Though most of these are not associated with known geologic structures, some are found within large impact basins within the interior peak ring. The primary magnetic carrier in lunar rocks is metallic iron, but indigenous lunar rocks are metal poor and cannot account easily for the observed field strengths. The projectiles that formed the largest impact basins must have contained a significant quantity of metallic iron, and a portion of this iron would have been retained on the Moon's surface within the impact melt sheet. Here we use orbital magnetic field data to invert for the magnetization within large impact basins using the assumption that the crust is unidirectionally magnetized. We develop a technique based on laboratory thermoremanent magnetization acquisition to quantify the relationship between the strength of the magnetic field at the time the rock cooled and the abundance of metal in the rock. If we assume that the magnetized portion of the impact melt sheet is 1 km thick, we find average abundances of metallic iron ranging from 0.11% to 0.45 wt %, with an uncertainty of a factor of about 3. This abundance is consistent with the metallic iron abundances in sampled lunar impact melts and the abundance of projectile contamination in terrestrial impact melts. These results help constrain the composition of the projectile, the impact process, and the time evolution of the lunar dynamo. [ABSTRACT FROM AUTHOR]

We present multi-instrument observations of the effects of the crustal magnetic field on the Martian ionosphere at different altitudes and solar zenith angles by Mars Express. Total electron content (TEC) at solar zenith angles 55 ° ≥ SZA ≥ 105 ° over the ionosphere with crustal sources increases with the strength of the magnetic field. A similar trend is observed in a dependence of the local electron density in the upper ionosphere on the crustal magnetic field. On the nightside, at SZA ≥ 110 ° , the opposite trend of TEC increase with decrease in the magnetic field value is observed. A dependence on the magnetic field inclination also varies between the day and night sides. TEC decreases for vertical field inclination at 90 ° ≥ SZA ≥ 70 ° and increases at SZA ≥ 110 ° . This effect becomes stronger for larger magnetic field values. A different dependence of the local electron densities in the upper ionosphere at small and high SZA is observed too. An ionospheric exhaust for vertical field inclination in the regions with strong crustal sources is probably caused by escape to space along open field lines which arise due to reconnection that is confirmed by the case studies. The existence of such localized ionospheric depressions is also observed by the in-situ plasma observations. In contrast, on the nightside downward plasma transport and electron precipitation along the field lines produce patches of enhanced ionization. [ABSTRACT FROM AUTHOR]