Martian Ionosphere Research Articles

Influence of upstream solar wind on magnetic field distribution in the Martian nightside ionosphere

Influence of upstream solar wind on magnetic field distribution in the Martian nightside ionosphere

Effects of Solar Wind Density and Velocity Variations on the Martian Ionosphere and Plasma transport—A MHD Model Study

AbstractSolar wind dynamic pressure, consisting solar wind density nsw and velocity Vsw, is an important external driver that controls Martian plasma environment. In this study, a 3D magnetohydrodynamic model is applied to investigate the separate influences of solar wind density and velocity on the Martian ionosphere. The spatial distributions of ions in the dayside and near nightside ionosphere under different nsw and Vsw are analyzed, as well as the ion transport process. We find that for the same dynamic pressure condition, the ionosphere extends to higher altitudes under higher solar wind density, indicating that a solar wind velocity enhancement event is more efficient at compressing the Martian ionosphere. A higher Vsw will result in a stronger induced magnetic field, shielding the Martian ionosphere, preventing the penetration of solar wind particles. For the same dynamic pressure, increasing nsw (decreasing Vsw) leads to a higher horizontal ion velocity, facilitating day‐to‐night plasma transport. As a result, the ionosphere extends farther into the nightside. Also, the ion outflow flux is larger for high nsw, which may lead to a higher escape rate. Moreover, the strong crustal fields in the southern hemisphere also cause significant effect to the ionosphere, hindering horizontal ion transport. An additional outflow channel is also provided by the crustal field on the southern dayside, causing different responses of flow pattern between local and global scale while the solar wind condition is varied.

Analysis of Two Selected Solar Events in 2011 and 2015 With Mars Express Radio Occultation Data

AbstractThe temporal behavior of the Martian ionosphere is highly variable due to various dynamic processes including space weather events. Here, we study the effect of solar flares and coronal mass ejections (CMEs) on the Martian ionosphere for two selected solar events in 2011 and 2015, using the publicly available Mars EXpress (MEX) radio occultation (RO) data (MaRS). We developed a data processing software that converts the calibrated Radio Occultation (RO) Doppler data to scientifically valuable atmospheric profiles. Using this software and previously unexplored MaRS observations, the variations in ionospheric parameters (electron density profiles and total electron content (TEC)) are calculated in order to evaluate the ionospheric changes due to solar flares and CMEs. The RO measurements mostly available 1–4 days apart from the peak events, showed no evident change in the TEC nor in the shape of electron density profiles except for a possible gradual decrease in altitude of M2 (main layer) peak density following the arrival of CMEs. To better quantify the effect of solar events on electron density profiles, RO observations near the time of arrival of solar flares and CMEs are crucial. This can be achieved by frequent RO measurements by various Mars orbiters including spacecraft‐to‐spacecraft measurements assisted by multi‐instrument monitoring of the ionosphere.

The Day the Solar Wind Disappeared at Mars

AbstractIn December 2022, an extremely low‐density solar wind stream encountered first the Earth and then Mars, shortly after the radial alignment of the two planets (i.e., Mars opposition). As the low‐density stream passed Mars, the properties of the Mars‐solar wind interaction changed dramatically in response to the low solar wind Alfvénic Mach number and dynamic pressure. The Martian magnetosphere and its boundaries expanded by thousands of kilometers, extending outside of the nominal average bow shock location. The low upstream Mach number resulted in a low ratio of plasma to magnetic field pressure in the magnetosheath, allowing the formation of a very high but stable ion temperature anisotropy, together with a very low level of electromagnetic instabilities. Meanwhile, the decrease in solar wind dynamic pressure caused the Martian upper ionosphere at the terminator to transition from a magnetized state to an unmagnetized state. This event provides an opportunity to study a unique end‐member state of the Mars‐solar wind interaction.

Radio Absorption in the Nightside Ionosphere of Mars During Solar Energetic Particle Events

AbstractCharacterization, understanding, and prediction of the Martian radio environment are of increasing importance to the forthcoming human exploration of Mars. Here we investigate 3–5 MHz radio absorption in the nightside ionosphere of Mars caused by enhanced ionization at <100 km altitudes during solar energetic particle (SEP) events. We conduct a quantitative analysis of radio absorption and SEP flux data that have been accumulated by two spacecraft currently orbiting Mars, thereby demonstrating that radio absorption is clearly correlated with SEP fluxes. A comparison of the observations with radio absorption properties predicted by a numerical model indicates that the relative temporal changes, radio frequency dependence, and SEP energy dependence of the observed radio absorption are in agreement with the model prediction. Meanwhile, the model systematically overestimates the radio absorption in the ionosphere by a factor of 3.7. We explore several sources of uncertainty, including the electron‐neutral collision frequency, absolute sensitivity of the SEP instrument, and limited transport of SEPs to the atmosphere, but the ultimate cause of the systematic discrepancy between the measured and modeled radio absorption is yet to be identified. Further efforts should be put into the development of a comprehensive and observationally validated model of radio absorption in the Martian ionosphere to assist the future crew and spacecraft activities on the surface of Mars.

Martian M2 peak behavior in the dayside near-terminator ionosphere during interplanetary coronal mass ejections

The interplanetary coronal mass ejections (ICMEs) can pose significant impacts on the Martian ionosphere, resulting in plasma depletion, variability, and escape to space. However, the connections between the ICMEs and the associated responses of the dayside near-terminator Martian ionospheric primary peak (M2) are not well understood. The present study primarily investigates the behavior of the ionospheric peak density (Nm) and height (hm) during the passage of ICMEs using observations from the Radio Occultation Science Experiment (ROSE) aboard MAVEN spacecraft. We have selected 8 such ICMEs (during 2017–2022) at Mars from the existing catalogs and studied the ROSE electron density profiles during quiet and disturbed time (ICMEs) for identical solar zenith angle range. We observed the elevation of the M2 peak (hm ∼ 4–16 km) during disturbed time (ICMEs) with a decrease in Nm (0.41–2.8 × 1010 m−3) in comparison to the quiet time. The present study, for the first time, addressed the influence of ICMEs on the M2 peak parameters (Nm and hm). We have proposed that the development of large vertical pressure gradient and electron temperature enhancement are plausible causes for ionospheric variability. Therefore, the present study provides new insights to understand peak plasma behavior in the dayside near-terminator ionosphere during ICMEs.

Observations of a Mini‐Magnetosphere Above the Martian Crustal Magnetic Fields

AbstractMars is typically regarded as a non‐magnetic planet. Currents in the Martian ionosphere generate a Venus‐like induced magnetosphere which deflects the solar wind flows and piles up the interplanetary magnetic fields. However, crustal magnetic fields in the southern hemisphere influence local plasma properties. Using observations from the MAVEN mission, we characterize the distinguishing plasma characteristics of a mini‐magnetosphere that forms on the Martian dayside. We establish three criteria to differentiate this mini‐magnetosphere from the induced magnetosphere. Notably, the mini‐magnetosphere exhibits higher plasma beta (values near 1), with a balance between planetary ions, crustal magnetic fields, and the solar wind at the magnetopause. Observations show that the crustal magnetosphere reaches an altitude of 1,300 km, larger than one‐third of the Martian radius, indicating a dichotomy between the induced northern and the crustal southern magnetospheres. These findings offer novel insights into the distinctive properties of hybrid magnetospheres in the near‐Mars space.

Local Electron Density Depletions in the Martian Upper Ionosphere Obtained From MARSIS: Comparison With ASPERA‐3, and MAVEN, and the Connection With Crustal Magnetic Fields

AbstractSteep electron density depletions in the Martian ionosphere are important, especially because they could be regions of significant plasma escape. Local electron density profiles of the Martian ionosphere obtained from the radar sounder onboard the Mars Express spacecraft often show large fluctuations. In some cases, these fluctuations are observed as deep depletions in the electron density. Investigation over the past 12 years of Mars Advanced Radar for Subsurface and Ionospheric Sounding data have revealed over 200 cases of density depletions where the lowest density is less than one fifth of the density in the surrounding regions. These density dips occur both on the dayside and the nightside, with an abundance between solar zenith angles of 50–70° and 100–120°. They occur at all altitudes in the ionosphere. Many of the depletions occur in the southern hemisphere where the crustal magnetic field is stronger and there are more regions where the crustal magnetic field is mostly vertical. In more than half of the cases, the depletion in the local electron density corresponds to a decrease in the electron flux measured with the Analyzer of Space Plasmas and Energetic Atoms electron spectrometer. Several different mechanisms could be responsible for the formation of the depletions, including regions of localized electric fields or magnetic fields creating areas of increased field pressure, escape of the plasma along magnetic field lines depleting density locally, and the density decrease around the photoelectron boundary followed by an increase in the plasma density due to plasma clouds.

Variability of Draped Interplanetary Magnetic Field in the Subsolar Martian Ionosphere

In the absence of an intrinsic magnetic field, the solar wind interacts directly with the Martian atmosphere, and the interplanetary magnetic field (IMF) drapes around the ionosphere. With the aid of multi-instrument measurements from Mars Atmosphere and Volatile Evolution (MAVEN), we investigate the altitudinal profiles of the draped magnetic field in the subsolar Martian ionosphere. Our analysis of 141 profiles found that they have a median profile with a nearly constant field strength of about 30 nT above 160 km. However, these profiles also show large variability, for example, with a standard deviation of 16 nT at 250 km. Timescale analysis found that magnetic field transport with downward ion plasma motion dominates the draping of IMF at high altitudes. At low altitudes, magnetic diffusion takes over due to increasing collisions. At even lower altitudes, approaching the ionospheric peak, the draped field is quickly damped owing to high conductivity. We used a 1D model simulation to confirm the above processes involved and further indicate that undulating magnetic fields with oscillation periods longer than tens of minutes can penetrate deeply into the ionosphere. This causes the large variability of draped field observed down to below 200 km. The diverse profiles of draped field observed by MAVEN are thus likely due to varying IMF conditions.

Effects of stealth CME to ion energisation at ionospheric altitudes of Mars

The declining phase of Solar cycle 24 produced a slow coronal mass ejection (CME), which was categorized as stealth CME based on the absence of associated low coronal signatures. This CME impacted Mars and caused a major space weather event during 27–28 August 2018 (max vsw = 580 km s−1 near 1.5 AU). The impact of the stealth CME on the topside of the Martian ionosphere is investigated using the data from Solar Wind Ion Analyzer (SWIA), Solar Energetic Particle (SEP) and Suprathermal and Thermal Ion Composition (STATIC) instruments onboard Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The variations of ion and electron fluxes within the plasma boundaries during the passage of ICME event were observed using Electron Spectrometer (ELS) and the Ion Mass Analyzer (IMA) sensors onboard Mars Express (MEX) mission. The plasma boundaries of the induced magnetosphere of Mars showed considerable enhancement in the observed fluxes during the event. SWIA observations during the interaction of the CME with Martian plasma environment showed enhancement in the ion flux. STATIC observations showed energisation of planetary heavy ions like O+ and O2+ to energies between 10 eV up to hundreds of eV at altitudes below 300 km. SEP measurements of energetic O+ pick-up ions at energies below 50 keV further support the claim on planetary ion energisation.

Tianwen-1 and MAVEN observations of Martian oxygen ion plumes

There is a significant Martian ion escape channel called the “plume”. Due to the lack of multi-point, high-resolution field and particle detections, the energization mechanism and source region identification of ion plumes by in situ measurements remain unclear. China's first Mars exploration mission, Tianwen-1, provides us an opportunity to study ion escape owing to the high altitude of its apoapsis (∼3 RM, RM is Mars radius). In this study, we take advantage of the joint observations of Tianwen-1 and Mars Atmosphere and Volatile EvolutioN (MAVEN) to study oxygen ion plumes. Our statistical results confirm the convection electric field acceleration as the energization mechanism of plume ions (up to >15 keV) in a more direct way than ever before. Our estimation suggests that most ion plumes originate from middle and low MSE (Mars Solar Electric) latitude of the northern dayside Martian ionosphere. This work improves our understanding of the pick-up process of the Martian escaping ions.

Tianwen-1 and MAVEN Observations of the Response of Mars to an Interplanetary Coronal Mass Ejection

Interplanetary coronal mass ejections (ICMEs) are solar transients that have significant effects on the upper atmosphere and ionosphere of Mars. The simultaneous spacecraft observations from Tianwen-1/MOMAG in solar wind and multiple instruments on board the Mars Atmosphere and Volatile Evolution (MAVEN) in the Martian upper atmosphere are used to study the response of Mars to an ICME. The ICME was observed at Mars by Tianwen-1 and MAVEN at 00:00 UT on 2021 December 10, which was earlier observed by BepiColombo upstream of Mars at 22:32 UT on 2021 December 6. During 2021 December 6–15, MAVEN measured the nightside ionosphere and Tianwen-1 measured the dayside ionosphere while both were inside the Martian bow shock. The rapid drop in densities of ionospheric ions and electrons, which is typically identified as the end of the ionosphere at altitudes between 300–800 km, is known as the ionopause. The altitude of the Martian ionopause location was lowered by the high dynamic pressure of the solar wind during the ICME passage. The depletion of the plasma density in the topside Martian ionosphere on the nightside reveals the presence of substantial ion and electron escape to space through the interaction between the ICME and Mars. The column abundance of plasma dramatically decreased, with 34% e−, 61% , and 73% O+ reduced. This study highlights the significant impact of the space weather associated with the intense magnetic field and high dynamic pressure of the ICME on Mars’s atmosphere, which is particularly important for future human exploration missions to Mars.

Effects of magnetic field direction on [formula omitted] vertical transport in the Martian ionosphere

The directions of Martian magnetic field, which depends on the interaction between the crustal magnetic field and the time-various interplanetary magnetic field (IMF), is in a state of disorder and plays a significant role in ions vertical transport, such as ions upwelling and precipitation. In general, vertical diffusion transport of ionospheric plasma tends to be promoted around vertical magnetic field regions but be suppressed around horizontal magnetic field area. However, the mechanism behind this phenomenon is not clear to date. Based on three-dimensional multi-fluid Hall magneto-hydrodynamic (MHD) equations in conjunction with an equivalent source dipole (ESD) model, we investigated mechanism behind the control of vertical transport by different magnetic field directions in the Martian ionosphere. Numerical results showed that the same direction of interplanetary magnetic field with ESD results in the formation of mini-magnetosphere to protect the ionosphere, which reflected in higher density distributions comparing to the no-ESD case due to joint effects of thermal pressure increasing and plasma vertical transport. At low altitude of ionosphere, O+ ion inward transport (precipitation) tends to be inhibited particularly around horizontal magnetic field, with high contribution from the motional electric force. In addition, at high altitude, O+ outward transport (upwelling) is likely to be facilitated over vertical-field areas in significant part controlled by the ambipolar electric force, whereas it is likely to be inhibited in horizontal-field region, contributed mainly from the Hall electric force. These results will enrich our understanding of mechanisms behind the control of ions vertical transport by magnetic field directions.

Science results from sixteen years of MRO SHARAD operations

In operation for >16 years to date, the Mars Reconnaissance Orbiter (MRO) Shallow Radar (SHARAD) sounder has acquired data at its nominal 300–450 m along-track and 3-km cross-track resolution covering >55% of the Martian surface, with nearly 100% overlap in coverage at that scale in the polar regions and in a number of smaller mid-latitude areas. While SHARAD data have opened a new window into understanding the interior structures and properties of Martian ices, volcanics, and sedimentary deposits up to a few kilometers in depth, they have also led to new revelations about the deeper interior and the behavior of the planet's ionosphere. Here we summarize the data collected by SHARAD over this time period, the methods used in the analysis of that data, and the resulting scientific findings. The polar data are especially rich, revealing complex structures that comprise up to several dozen reflecting interfaces that extend to depths of 3 km, which inform the evolution of Martian climate in the late Amazonian period. SHARAD observations of mid-latitude lobate debris aprons and other glacier-like landforms detect strong basal reflections and low dielectric loss, confirming that they are ice-rich debris-covered glaciers. In other mid-latitude terrains, SHARAD data demonstrate the presence of widespread ground ices, likely at lower concentrations. SHARAD signals also probe non-icy materials, mapping out stacked lava flows, probing low-density materials thought to be ash-fall deposits, and occasionally penetrating sedimentary deposits, all of which reveal the structures and interior properties diagnostic of emplacement processes. SHARAD signals are impacted by their passage through the Martian ionosphere, revealing variations in time and space of the total electron content linked with the remanent magnetic field. Advanced techniques developed over the course of the mission, which include subband and super-resolution processing, coherent and incoherent summing, and three-dimensional (3D) radar imaging, are enabling new discoveries and extending the utility of the data. For 3D imaging, a cross-track spacing at the nominal 3-km resolution is more than sufficient to achieve good results, but finer spacing of 0.5 km or less significantly improves the spatially interpolated radar images. Recent electromagnetic modeling and a flight test show that SHARAD's signal-to-noise ratio can be greatly improved with a large (∼120°) roll of the spacecraft to reduce interference with the spacecraft body. Both MRO and SHARAD are in remarkably fine working order, and the teams look forward to many more years in which to pursue improvements in coverage density, temporal variability in the ionosphere, and data quality that promise exciting new discoveries at Mars.

Variabilities and dependencies of suprathermal electron exobase altitudes in the martian ionosphere: Insights from MAVEN measurements

Variabilities and dependencies of suprathermal electron exobase altitudes in the martian ionosphere: Insights from MAVEN measurements

MAVEN Observations of the Interloop Magnetic Reconnections at Mars

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere.

Interplanetary Coronal Mass Ejections and Stream Interaction Regions Observed by Tianwen-1 and MAVEN at Mars

The Tianwen-1 spacecraft is China's first Mars exploration mission. The Mars Orbiter Magnetometer (MOMAG) is a scientific instrument on board the Tianwen-1 mission that is designed to study magnetic fields at Mars, including the solar wind to the magnetosheath and the ionosphere. Using the first Tianwen-1/MOMAG data that is publicly available, we present an interplanetary coronal mass ejection (ICME) and stream interaction region (SIR) list based on in situ observations at Mars between 2021 November 16 and 2021 December 31. We compared the magnetic field intensity and vector magnetic field measurements from Tianwen-1/MOMAG and Mars Atmospheric Volatile EvolutioN (MAVEN)/Magnetometer (MAG) during the ICME and SIR interval and found a generally good consistency between them. Due to MAVEN's orbital adjustment since 2019, the Tianwen-1/MOMAG instrument is almost unique in its status as an interplanetary magnetic field monitor currently at Mars. The observations indicate that the MOMAG instrument on Tianwen-1 is performing well and can provide accurate measurements of the vector magnetic field in the near-Mars solar wind space. The multipoint observations combining MOMAG, MINPA, and MEPA on board Tianwen-1 with MAG, SWIA, and STATIC on board MAVEN will help develop systematic studies of the characteristics of ICMEs and SIRs at Mars, and their influences on the Martian atmosphere and ionosphere.

Local Generation of Magnetosonic Waves by Ring Beam Hot Protons in the Martian Ionosphere

AbstractMagnetosonic (MS) waves are dominant plasma waves causing severe Martian ionospheric erosion. They are generally considered to originate upstream of Martian bow shock with frequencies near the upstream proton gyrofrequency. However, whether MS waves can be locally excited lacks theoretical analysis. Here we present an event of MS waves with frequencies above and closely related to the local proton gyrofrequency in the Martian ionosphere. Concurrently, ring beam hot proton distributions are observed due to the penetration of magnetosheath protons. By employing the observed plasma and magnetic field data, the calculated linear growth rates for MS waves agree well with the observed wave power spectra, demonstrating that they can be locally excited by unstable ring beam hot protons at Mars. Our results could be of great help in understanding the excitation of MS waves in a heavy ion‐rich environment around unmagnetized planets.

Fourier Analysis of Magnetic Fields in the Ionosphere of Mars

AbstractThe two main sources of the magnetic field in the Martian ionosphere are the solar wind interaction with the planet, and, mainly in the southern hemisphere, remnant crustal magnetization. The magnetic fields measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Global Surveyor spacecraft displayed a wide range of spatial scales, from the global (i.e., L ≈ 103 km) to mesoscale (L ≈ 102 km) to small‐scale (L < 10 km). Hamil et al. (2022) used MAVEN magnetometer and Langmuir Probe data to study these structures and suggested that they might be advected into the ionosphere from the solar wind and magnetosheath. In the current study, we apply a Fourier analysis to the fields and interpret the resulting power spectral density profiles versus frequency. The power spectral density function found from MAVEN data resembles that of the solar wind magnetic field (or interplanetary magnetic field) (i.e., power law with an index of about −2), but shifted upward in frequency by a factor of about 100. From a comparison of ionospheric power spectra with solar wind power spectra, we deduce that plasma, carrying a magnetic field with it moves from the magnetic pile‐up region downward into the ionosphere with speeds of roughly tens of meters per second. The derived power spectra in the ionosphere, in addition to the basic power law shape, show hints of extra power at a spatial scale of about 10 km, and this might be due to the creation of a magnetic structure within the ionosphere itself.

Alteration of Magnetic Field Strength in the Strong Crustal Field Region at Mars by Currents Induced by the Solar Wind Interaction

AbstractCrustal magnetic fields on Mars were first found by the Mars Global Surveyor and reported in 1998. One interesting aspect of the interaction between crustal fields and the Martian ionosphere is the impact on total ion loss over time, which is a central question addressed by the Mars Atmospheric and Volatile EvolutioN (MAVEN) spacecraft currently orbiting Mars. In this work, we seek to improve understanding of the impact of crustal fields on plasma energetics, and consequent ion loss, by examining the effects of ionospheric current sheets that may form at the boundary between crustal and induced magnetic fields. These current sheets will not just affect the external induced field regions but will also affect the observed magnetic fields in the crustal field regions. Variations in field strength should be evident by comparing crustal field models and measured magnetic fields collected by the MAVEN magnetometer instrument (MAG). One way to confirm such variation is to compare dayside and nightside orbit pairs that occur over the same crustal field regions. The induced magnetic field exists mostly on the dayside and is largely absent on the nightside. We examine these orbit pairs and discuss the perturbation of magnetic fields in the dayside ionosphere. We confirm that deviations on the dayside are larger than on the nightside, and that current sheets in the ionosphere could be responsible for these deviations, particularly in the strong crustal field regions.