Continuous monitoring of nightside upper thermospheric mass densities in the martian southern hemisphere over 4 martian years using electron reflectometry

Details are presented of an improved technique to use atmospheric absorption of magnetically reflecting solar wind electrons to constrain neutral mass densities in the nightside martian upper thermosphere. The helical motion of electrons on converging magnetic field lines, through an extended neutral atmosphere, is modeled to enable prediction of loss cone pitch angle distributions measured by the Magnetometer/Electron Reflectometer (MAG/ER) experiment on Mars Global Surveyor at 400 km altitude. Over the small fraction of Mars' southern hemisphere (∼2.5%) where the permanent crustal magnetic fields are both open to the solar wind and sufficiently strong as to dominate the variable induced martian magnetotail field, spherical harmonic expansions of the crustal fields are used to prescribe the magnetic field along the electron's path, allowing least-squares fitting of measured loss cones, in order to solve for parameters describing the vertical neutral atmospheric mass density profile from 160 to 230 km. Results are presented of mass densities in the southern hemisphere at 2 a.m. LST at the mean altitude of greatest sensitivity, 180 km, continuously over four martian years. Seasonal variability in densities is largely explained by orbital and latitudinal changes in dayside insolation that impacts the nightside through the resulting thermospheric circulation. However, the physical processes behind repeatable rapid, late autumnal cooling at mid-latitudes and near-aphelion warming at equatorial latitudes is not fully clear. Southern winter polar warming is generally weak or nonexistent over several Mars years, in basic agreement with MGS and MRO accelerometer observations. The puzzling response of mid-latitude densities from 160° to 200° E to the 2001 global dust storm suggests unanticipated localized nightside upper thermospheric lateral and vertical circulation patterns may accompany such storms. The downturn of the 11-year cycle of solar EUV flux is likely responsible for lower aphelion densities in 2004 and 2006 (Mars years 27 and 28).     

Dayside induced magnetic field in the ionosphere of Mars

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express spacecraft has occasionally displayed surprising features. One such feature is the occurrence of a series of broadband, low-frequency echoes at equally spaced delay times after the sounder transmitter pulse. The interval between the echoes has been shown to be at the cyclotron period of electrons orbiting in the local magnetic field. The electrons are believed to be accelerated by the large voltages applied to the antenna by the sounder transmitter. Measurements of the period of these “electron cyclotron echoes” provide a simple technique for determining the magnitude of the magnetic field near the spacecraft. These measurements are particularly useful because Mars Express carries no magnetometer, so this is the only method available for measuring the magnetic field magnitude. Using this technique, results are presented showing the large scale structure of the draped field inside the magnetic pile-up boundary. The magnitude of the draped field is shown to vary from about 40 nT at a solar zenith angle of about 25°, to about 25 nT at a solar zenith angle of 90°. The results compare favorably with similar results from the Mars Global Surveyor spacecraft. A fitting technique is developed to derive the vector direction and magnitude of the draped magnetic field in cases where the spacecraft passes through regions with significant variation in the crustal field. The magnetic field directions are consistent with current knowledge of the draping geometry of the magnetic field around Mars.     

Early martian dynamo generation due to giant impacts

During late-stage planet formation, giant impacts produce localized mantle melt regions within which impactor iron droplets settle to the bottom near a permeability horizon. After accumulation, iron heated by the impact migrates downward to the core through colder, mostly solid mantle. The degree of thermal equilibration and partitioning of viscous heating between impactor iron and silicates depends on the mechanism of iron transport to the core. Simple estimates suggest that, following a giant impact, the temperature difference between iron delivered to the core and the mantle outside the impact heated region can be ∼10³ K. Hot impactor iron mergers with the core where it may be efficiently mixed or remain stratified due to thermal buoyancy. In either case, collisional energy carried to the core by impactor iron helps establish conditions favorable for early core cooling and dynamo generation. In this study, we consider the end-member scenario in which impactor iron forms a layer at the top of the core. Energy transfer from the impactor iron layer to the mantle is sufficient to power a dynamo for up to ∼30 Myr even in the limit of a very viscous mantle and heat flux limited by conduction. Using two-dimensional finite element calculations of mantle convection, we show that large-scale mantle flow driven by the buoyancy of the impact thermal anomaly focuses plumes in the impact region and increases both dynamo strength and duration. Melting within the mantle thermal boundary layer likely leads to formation of a single superplume in the location of the impact anomaly driven upwelling. We suggest that formation of magnetized southern highland crust may be related to spreading and differentiation of an impact melt region during the impact-induced dynamo episode.     

Large density fluctuations in the martian ionosphere as observed by the Mars Express radar sounder

The MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express spacecraft provides both local and remote measurements of electron densities and measurements of magnetic fields in the martian ionosphere. The density measurements show a persistent level of large fluctuations, sometimes as much as a factor of three or more at high altitudes. Large magnetic field fluctuations are also observed in the same region. The power spectrums of both the density and magnetic field fluctuations have slopes on a log-log plot that are consistent with the Kolmogorov spectrum for isotropic fluid turbulence. The fractional density fluctuation, Δn_e/n_e, of the turbulence increases with altitude, and reaches saturation, Δn_e/n_e ∼ 1, at an altitude of about 400 km, near the nominal boundary between the ionosphere and the magnetosheath. The fluctuations are usually so large that a well-defined ionopause-like boundary between the ionosphere and the solar wind is seldom observed. Of mechanisms that could be generating this turbulence, we believe that the most likely are (1) solar wind pressure perturbations, (2) an instability in the magnetosheath plasma, such as the mirror-mode instability, or (3) the Kelvin-Helmholtz instability driven by velocity shear between the rapidly flowing magnetosheath and the ionosphere.     

Electron reflectometry in the martian atmosphere

The technique of electron reflectometry, a method for remote estimation of planetary magnetic fields, is expanded from its original use of mapping crustal magnetic fields at the Moon to achieving the same purpose at Mars, where the presence of a substantial atmosphere complicates matters considerably. The motion of solar wind electrons, incident on the martian atmosphere, is considered in detail, taking account of the following effects: the electrons' helical paths around the magnetic field lines to which they are bound, the magnetic mirror force they experience due to converging field lines in the vicinity of crustal magnetic anomalies, their acceleration/deceleration by electrostatic potentials, their interactions with thermal plasma, their drifts due to magnetic field line curvature and perpendicular electric fields and their scattering off, and loss of energy through a number of different processes to, atmospheric neutrals. A theoretical framework is thus developed for modeling electron pitch angle distributions expected when a spacecraft is on a magnetic field line which is connected to both the martian crust and the interplanetary magnetic field. This framework, along with measured pitch angle distributions from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER) experiment, can be used to remotely measure crustal magnetic field magnitudes and atmospheric neutral densities at ∼180 km above the martian datum, as well as estimate average parallel electric fields between 200 and 400 km altitude. Detailed analysis and full results, concerning the crustal magnetic field and upper thermospheric density of Mars, are left to two companion papers.     

Electric fields within the martian magnetosphere and ion extraction: ASPERA-3 observations

Observations made by the ASPERA-3 experiment onboard the Mars Express spacecraft found within the martian magnetosphere beams of planetary ions. In the energy (E/q)-time spectrograms these beams are often displayed as dispersive-like, ascending or descending (whether the spacecraft moves away or approach the planet) structures. A linear dependence between energy gained by the beam ions and the altitude from the planet suggests their acceleration in the electric field. The values of the electric field evaluated from ion energization occur close to the typical values of the interplanetary motional electric field. This suggests an effective penetration of the solar wind electric field deep into the martian magnetosphere or generation of large fields within the magnetosphere. Two different classes of events are found. At the nominal solar wind conditions, a ‘penetration’ occurs near the terminator. At the extreme solar wind conditions, the boundary of the induced magnetosphere moves to a more dense upper atmosphere that leads to a strong scavenging of planetary ions from the dayside regions.     

Impact demagnetization of the martian crust

The lack of magnetic anomalies within the giant martian impact basins, Hellas, Argyre, and Isidis suggests that the impacts demagnetized the crust. Our analysis of the magnetic anomaly intensity shows that the interior parts of the basins are completely demagnetized, while the outer parts and surroundings are partially demagnetized. We investigate the shock pressure and impact heating resulting from the impacts. The crust has been completely demagnetized within ∼0.8 basin radius by a combination of thermal and shock effects, and the surroundings have been partially demagnetized by shock to a distance of at least 1.4 radii. We also investigate magnetic signatures of intermediate-size craters. From the pressures generated by both the large and intermediate-sized impacts, we conclude that the remanent magnetization is carried at least in part by high coercivity rocks. Since the crust beneath the basins does not appear to have been remagnetized as it cooled following the impacts, we conclude that the martian core dynamo was inactive or very weak for at least 100 Myr following the Hellas impact.     

An improved crustal magnetic field map of Mars from electron reflectometry: Highland volcano magmatic history and the end of the martian dynamo

We apply improved kinetic modeling of electron transport in the martian thermosphere to fit pitch angle distributions measured by the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER), together with appropriate filtering, binning, averaging and error correction techniques, to create the most reliable ER global map to date of crustal magnetic field magnitude at 185 km altitude, with twice the spatial resolution and considerably higher sensitivity to crustal fields than global maps of magnetic field components produced with MAG data alone. This map compares favorably to sparsely sampled dayside MAG data taken at similar altitudes, insofar as a direct comparison is meaningful. Using this map, we present two case studies. The first compares the magnetic signatures of two highland volcanoes, concluding that the comparatively greater thermal demagnetization at Syrtis Major compared with Tyrrhena Patera is likely due to a higher ratio of intruded to extruded magmas. The second uses the map along with topographic data to compare the magnetic signatures and crater retention ages of the demagnetized Hellas impact basin and magnetized Ladon impact basin. From this comparison, we determine that the martian global dynamo magnetic field went from substantial to very weak or nonexistent in the absolute model age time interval 4.15±0.05 to 4.07±0.05 Ga ago.     

Localized ionization patches in the nighttime ionosphere of Mars and their electrodynamic consequences

Using an electron transport model, we calculate the electron density of the electron impact-produced nighttime ionosphere of Mars and its spatial structure. As input we use Mars Global Surveyor electron measurements, including an interval when accelerated electrons were observed. Our calculations show that regions of enhanced ionization are localized and occur near magnetic cusps. Horizontal gradients in the calculated ionospheric electron density on the night side of Mars can exceed 10⁴ cm⁻³ over a distance of a few tens of km; the largest gradients produced by the model are over 600 cm⁻³ km⁻¹. Such large gradients in the plasma density have several important consequences. These large pressure gradients will lead to localized plasma transport perpendicular to the ambient magnetic field which will generate horizontal currents and electric fields. We calculate the magnitude of these currents to be up to 10 nA/m². Additionally, transport of ionospheric plasma by neutral winds, which vary in strength and direction as a function of local time and season, can generate large (up to 1000 nA/m²) and spatially structured horizontal currents where the ions are collisionally coupled to the neutral atmosphere while electrons are not. These currents may contribute to localized Joule heating. In addition, closure of the horizontal currents and electric fields may require the presence of vertical, field-aligned currents and fields which may play a role in high altitude acceleration processes.     

Observations of magnetic anomaly signatures in Mars Express ASPERA-3 ELS data

Mars Express (MEX) Analyser of Space Plasmas and Energetic Atoms (ASPERA-3) data is providing insights into atmospheric loss on Mars via the solar wind interaction. This process is influenced by both the interplanetary magnetic field (IMF) in the solar wind and by the magnetic ‘anomaly’ regions of the martian crust. We analyse observations from the ASPERA-3 Electron Spectrometer near to such crustal anomalies. We find that the electrons near remanent magnetic fields either increase in flux to form intensified signatures or significantly reduce in flux to form plasma voids. We suggest that cusps intervening neighbouring magnetic anomalies may provide a location for enhanced escape of planetary plasma. Initial statistical analysis shows that intensified signatures are mainly a dayside phenomenon whereas voids are a feature of the night hemisphere.     

On the effect of the martian crustal magnetic field on atmospheric erosion

Without the shielding of a strong intrinsic magnetic field, the martian atmosphere directly interacts with the impacting solar wind. The neutral constituents of the atmospheric corona can be ionized, and then picked up and accelerated by the magnetic field and convection electric field in the solar wind. A significant fraction of pickup ions escape Mars’ gravitational pull and are lost to space. This non-thermal escape process of heavy species is an important mechanism responsible for atmospheric erosion. While there is a perception that the martian magnetic anomalies are significant for the ionospheric density distribution and the bow shock standoff location, little is known about the quantitative influence of the martian crustal magnetic field on the global distribution of escaping pickup ions. In this paper, we apply a newly developed Monte Carlo ion transport model to resolve the crustal field effect on the pickup oxygen ion distribution around Mars. The background magnetic and electric fields, in which test particles are followed, are calculated using an independent three-dimensional multispecies MHD model. The effects of the crustal magnetic field on particle escape are quantified by varying the crustal field orientation in the model setup and comparing the corresponding test particle simulation results. The comparison is made by turning on or off the crustal field or changing the local time of the strongest field from the dayside to the dawnside. It is found that without the protection of the crustal magnetic field, the total amount of atmospheric escape through the tail region would be enhanced by more than a factor of two. It is shown that the crustal magnetic field not only regionally deflects the solar wind around the martian atmosphere, but also has an important global effect on atmospheric erosion and thus on long-term atmospheric evolution.     

A statistical study of flux ropes in the Martian magnetosphere

Using minimum variance analysis of the circular mapping data from the Mars Global Surveyor (MGS) spacecraft during four selected weeks of observation, we identify 360 magnetic field structures in the Martian topside ionosphere with characteristic signatures of flux ropes. Physical parameters including size, peak field strength, helicity, orientation, and external conditions at the time of each observation are compiled for the events in each population. We observe that Martian flux ropes typically have a peak field amplitude of ∼15 nT and a diameter of ∼80–100 km assuming they are stationary. Flux ropes tend to be aligned approximately parallel to the planetary surface, and perpendicular to the direction from which the solar wind flows. They are more frequently observed during times of low solar wind pressure, but do not show a clear preference for a particular Interplanetary Magnetic Field (IMF) draping direction. Flux rope characteristics of peak field amplitude, diameter, and helicity vary with solar zenith angle. Amplitudes tend to be higher during periods of high solar wind pressure. The events are sorted into three populations based on the location at which they were observed, possibly corresponding to distinct formation mechanisms. Flux ropes observed in eclipse tend to have smaller peak amplitudes and are larger than those observed in sunlight, and are less likely to be oriented parallel to the planetary surface. Proximity to crustal fields does not appear to influence the characteristics of flux ropes observed at the 400 km spacecraft altitude. The frequent observation of flux rope structures near Mars in a variety of locations suggests that the low-altitude plasma environment is quite dynamic, with magnetic shear playing a prominent role in determining magnetic field structure near the planet.     

East-west trending magnetic anomalies in the Southern Hemisphere of Mars: Modeling analysis and interpretation

Maps of the vector components of the martian crustal magnetic field over the strongly magnetized Terra Cimmeria/Sirenum region are constructed using Mars Global Surveyor magnetometer data. Although pronounced east-west trending anomalies are present on the radial and north field component maps at the mapping altitude (∼360-380 km), these trends are much less prominent at the lower aerobraking altitude (∼90-150 km). Comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of ∼500 km and maximum aspect ratios of ∼2. Iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data. On this basis, it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions. Using this additional constraint, iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude. The model magnetization distribution, which includes 41 discrete sources, again indicates no highly elongated sources. An examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions (e.g., subsurface dike swarms), cooling in the presence of water, are the most likely sources of the magnetic anomalies.     

Viscous flow properties in the transport of solar wind momentum to the Venus upper ionosphere

A comparative study of the viscous transport of solar wind momentum to the upper layers of the Venus ionosphere with that occurring within the trans-terminator flow leads to estimates of the ratio of the viscosity coefficients that are applicable to both cases. Support for viscous forces between the solar wind and the ionospheric plasma in the trans-terminator flow derives from the momentum flux balance between the momentum flux in the latter flow and the deficiency of solar wind momentum along the flanks of the ionosheath. By comparing the relative width of the viscous boundary layer in the Venus ionosheath and the width of the trans-terminator flow we find that the transport of momentum within the upper ionosphere proceeds at a rate similar to that at which momentum is delivered to the upper ionosphere from the solar wind. Comparable values are obtained for the viscosity coefficient of the solar wind that streams over the ionosphere and that implied from momentum transport within the ionospheric trans-terminator flow. It is further suggested that despite the different nature of the processes that give place to the viscous transport of the solar wind momentum to the upper ionosphere (wave-particle interactions) and those responsible for its distribution within the ionosphere (through coulombian collisions) there is a similar response in the behavior of both plasmas to momentum transport. Calculations show that with comparable values of the viscosity coefficient in the ionosheath and in the upper ionospheric plasma the mean free path suitable to wave-particle interactions in the ionosheath is of the same order of magnitude as the mean free path of the planetary O⁺ ions that interact through coulombian collisions in the upper ionosphere. The effects of this similarity are considered in the discussion.     

Slow degradation of ATP in simulated martian environments suggests long residence times for the biosignature molecule on spacecraft surfaces on Mars

Prelaunch planetary protection protocols on spacecraft are designed to reduce the numbers and diversity of viable bioloads on surfaces in order to mitigate the forward contamination of planetary surfaces. In addition, there is a growing appreciation that prelaunch spacecraft cleaning protocols will be required to reduce the levels of biogenic signature molecules on spacecraft to levels that will not compromise life-detection experiments on landers. The biogenic molecule, adenosine triphosphate (ATP) was tested for long-term stability under simulated Mars surface conditions of high UV flux, low temperature, low pressure, Mars atmosphere, and clear-sky dust loading conditions. Data on UV-induced ATP degradation rates were then extrapolated to a diversity of global conditions using a radiative transfer model for UV on Mars. The UV-induced degradation of ATP tested at 4.1 W m⁻² UVC (200-280 nm), −10 °C, 7.1 mb, 95% CO₂ gas composition, and an atmospheric opacity of τ=0.1 yielded a half-life for ATP of 1342 kJ m⁻²; or extrapolated to approximately 22 sols on equatorial Mars with an atmospheric opacity of τ=0.5. Temperature was found to moderately affect ATP degradation rates under martian conditions; tests at −80 or 20 °C yielded ATP half-lives of 2594 or 1183 kJ m⁻², respectively. The ATP degradation rates reported here are over 10 orders of magnitude slower than the UV-induced biocidal rates reported in the literature on the inactivation of strongly UV-resistant bacterial spores from Bacillus pumilus SAFR-032 [Schuerger, A.C., Richards, J.T., Newcombe, D.A., Venkateswaran, K.J., 2006. Icarus 181, 52-62]. Extrapolating results to global Mars conditions, residence times for a 99% reduction of ATP on spacecraft surfaces ranged from 158 sols on Sun-exposed surfaces to approximately 32,000 sols for the undersides of landers similar to Viking. However, spacecraft materials greatly affected the survival times of ATP under martian conditions. Stainless steel was found to enhance the UV degradation of ATP by over 2 orders of magnitude compared to ATP-doped iridited aluminum, graphite, and astroquartz coupons. Extrapolating these results to global conditions, ATP on stainless steel might be expected to persist between 2 and 320 sols for upper and lower surfaces of landers. Liquid chromatography-mass spectrometry data supported the conclusion that UV irradiation acted to remove the γ-phosphate group from ATP, and no evidence was observed for the UV-degradation of d-ribose or adenine moieties. Long residence times for ATP on spacecraft materials under martian conditions suggest that prelaunch cleaning protocols may need to be strengthened to mitigate against possible ATP contamination of life-detection experiments on Mars landers.     

The influence of sunspot canopies on magnetic inclination measurements in solar plages

Sunspots are known to have large, low-lying magnetic canopies, i.e. horizontal magnetic fields overlying a field-free medium, that cover substantial fractions of active region plage. In this paper we consider the influence of such canopies on the inclination of plage magnetic fields. We find that for observations in spectral lines like 5250.2Å the neglect of a sunspot canopy when determining magnetic inclination angles of plage fields can introduce errors exceeding 5–10°. This is particularly true if the observations do not have high spatial resolution. Thus this effect may explain some of the measurements of substantially inclined fields in solar plages. Furthermore we find that the Fe I 15648 Å line is far superior in giving correct flux-tube inclinations in the presence of a sunspot magnetic canopy. Finally, the inversion of full Stokes profiles is shown to produce more reliable results than results obtained by considering only ratios of individual Stokes profile parameters.     

A 3D hybrid simulation study of the electromagnetic field distributions in the lunar wake

A three-dimensional hybrid code is used to study the electromagnetic disturbances in the solar wind that arise due to the absorption effect of the Moon. Due to the nearly insulating nature of the Moon, interplanetary magnetic fields (IMFs) can move through the interior without hindrance. However, the near-vacuum created in the wake region due to the lunar absorption effect will lead to enhancement of the strength of the magnetic field by a factor of about 1.4 in the middle of the lunar wake and lead to depletions at two sides. The situations arising from different orientations of the interplanetary magnetic fields relative to the radial direction are compared. Asymmetries of the inward diffusions both along and perpendicular to the field lines are also observed. The electric field formed from the plasma convection could reach a magnitude of 0.2–0.8 mV/m at the border of the wake. The role of the electric field on the inward accelerations is important to the geometry of the lunar wake.     

Observation of saturnian stream particles in the interplanetary space

In January 2004 the dust instrument on the Cassini spacecraft detected the first high-velocity grain expelled from Saturn - a so-called stream particle. Prior to Cassini’s arrival at Saturn in July 2004 the instrument registered 801 faint impacts, whose impact signals showed the characteristic features of a high-velocity impact by a tiny grain. The impact rates as well as the directionality of the stream particles clearly correlate with the sector structure of the interplanetary magnetic field (IMF). The Cosmic Dust Analyser (CDA) registered stream particles dominantly during periods when the IMF direction was tangential to the solar wind flow and in the prograde direction. This finding provides clear evidence for a continuous outflow of tiny dust grains with similar properties from the saturnian system. Within the compressed part of co-rotating interaction regions (CIRs) of the IMF, characterized by enhanced magnetic field strength and compressed solar wind plasma, CDA observed impact bursts of faster stream particles. We find that the bursts result from the stream particles being sped up inside the compressed CIR regions. Our analysis of the stream-particle dynamics inside rarefaction regions of the IMF implies that saturnian stream particles have sizes between 2 and 9 nm and exit the saturnian systems closely aligned with the planet’s ring plane with speeds in excess of 70 km s⁻¹.     

Impact of the plasma fluctuations in the Martian ionosphere on the performance of the synthetic aperture ground-penetrating radar

The effect of the ionospheric scintillations on the performance of the dekameter wave band synthetic aperture radar (SAR) is considered. The numerical model for computer simulations, based on the multiple phase screen technique and accounting for diffraction, is formulated. The result of numerical simulations of the radar pulses is presented and analyzed. The role of the various features of the ionospheric disturbances and their influence on the radar pulse characteristic parameters is figured out and discussed. Both effects of anisotropy of the correlation function of plasma irregularities and their temporal variations are studied numerically.