Nightside Of Mars Research Articles

Asymmetrical Looping Magnetic Fields and Marsward Flows on the Nightside of Mars

AbstractAs the interplanetary magnetic field (IMF) carried by the solar wind encounters the martian atmosphere, it tends to pile up and drape around the planet, forming looping magnetic fields and inducing marsward ion flows on the nightside. Previous statistical observations revealed asymmetrical distribution features within this morphology; however, the underlying physical mechanism remains unclear. In this study, utilizing a three‐dimensional multi‐fluid magnetohydrodynamic simulation model, we successfully reproduce the asymmetrical distributions of the looping magnetic fields and corresponding marsward flows on the martian nightside. Analyzing the magnetic forces resulting from the bending of the IMF over the polar area, we find that the asymmetry is guided by the orientation of the solar wind motional electric field (ESW). A higher solar wind velocity leads to enhanced magnetic forces, resulting in more tightly wrapped magnetic fields with an increased efficiency in accelerating flows as they approach closer to Mars.

Diffusion limited escape of hydrogen from Mars

Hydrogen escapes from Mars primarily by the Jeans mechanism but the rate is variable and the controlling factors complicated. One of the complications is that the temperature at the Martian exobase varies from ∼100K in the early morning hours to ∼300K in the afternoon. At the cold temperatures on the nightside of Mars, H escape rate is limited by Jeans escape, but on the warm dayside H escape is limited by the diffusion rate through the thermosphere. Nevertheless, the hot and cold regions are coupled by efficient ballistic transport through the exosphere. Because of this, H diffuses upward at the diffusion-limited rate even on the nightside and, once H reaches the exosphere, it is transported rapidly by ballistic flow to the warm dayside, where it escapes. As a result, escape is not at all limited by the cold regions of the exobase. The globally integrated escape flux is equal to the globally integrated diffusive limit. Because of this it is important to precisely calculate the diffusion-limited flux and we present a new formulation that is more accurate than the classical formula.

Magnetic Fluctuations Associated With Small-Scale Magnetic Holes in the Martian Magnetosheath

Small-scale magnetic holes are common magnetic structures with a spatial scale always smaller than one thermal proton gyroradius (ρi) in the turbulent planetary magnetosheath. However, the contribution of small-scale MHs to dissipating energy and transporting particles in turbulent plasmas is still unclear. In this work, we investigate magnetic fluctuations around small-scale MHs in the Martian magnetosheath and compute their spectral indices in the kinetic frequency range. Our statistical results show that the spectral index value gradually decreases from the bow shock to the induced magnetosphere boundary at the nightside of Mars. Small-scale MHs have an obvious influence on the spectral indices of the magnetic fluctuations by their high-frequency components. For most events, these MHs lead to a variation in the spectral index in the kinetic range, and such a variation can be very significant in more than 20% of events. Our results indicate that because of the existence of MHs, using the spectral index alone as a diagnostic sometimes is not enough convincing during the analysis of turbulence in planetary environments.

Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

AbstractThis paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons.

In Situ Heating of the Nightside Martian Upper Atmosphere and Ionosphere: The Role of Solar Wind Electron Precipitation

Abstract In the absence of solar radiation, precipitating electrons from the solar wind (SW) are generally thought to be the dominant source of energy deposition in the nightside Martian upper atmosphere, creating a patchy ionosphere and possibly also affecting the nightside thermal budget of various neutral and ionized species. Previous model calculations have not taken into account in situ heating via SW electron impact. In the present study, we utilize extensive measurements made by several instruments on board the Mars Atmosphere and Volatile Evolution spacecraft, in order to perform data-driven computations of the nightside neutral, ion, and electron heating rates. Considering the large range of energetic electron intensity observed on the nightside of Mars, we divide the entire data set into two subsamples, either with or without energetic electron depletion, a notable feature of the nightside Martian ionosphere. Our calculations indicate that in situ nightside neutral heating is dominated by exothermic chemistry and Maxwell interaction with thermal ions for regions with depletion, and by direct SW impact for regions without. Collisional quenching of excited state species produced from a variety of channels, such as electron impact excitation, dissociation, and ionization, as well as dissociative recombination, makes a substantial contribution to neutral heating, except during depletion. For comparison, nightside ion heating is mainly driven by energetic ion production under all circumstances, which occurs mainly via ion-neutral reaction O+ + CO2 and CO2 + predissociation.

A Survey of Photoelectrons on the Nightside of Mars

AbstractDespite produced exclusively on the dayside, photoelectrons, as an important population of the Martian ionosphere, have also been observed on the nightside. Here we present a statistical survey of nightside photoelectrons using the suprathermal electron measurements, made by the Solar Wind Electron Analyzer onboard the Mars Atmosphere and Volatile Evolution. We find that nearly 30% of the available nightside suprathermal electron spectra show clear photoelectron signatures. Nightside photoelectrons have an occurrence rate that decreases with increasing solar zenith angle and they are characterized by a preferentially field‐aligned pitch angle distribution. Our analysis also suggests that nightside photoelectrons are less likely observed under the high solar wind condition. These observations are favorably interpreted by the scenario of photoelectron transport along cross‐terminator magnetic field lines, as supported by a simplified test particle model. Our study highlights a complex plasma environment near Mars modulated by both internal and external conditions.

The Influence of Interplanetary Magnetic Field Direction on Martian Crustal Magnetic Field Topology

AbstractCrustal magnetic fields influence a range of plasma processes at Mars, guiding the flow of energy from the solar wind into the planet's atmosphere at some locations while shielding the atmosphere at others. In this study we investigate how the topology of crustal fields varies with changes in the direction of the incoming interplanetary magnetic field (IMF). Using plasma measurements from Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Global Surveyor (MGS), we identify magnetic topology throughout the Martian ionosphere and perform a statistical analysis of crustal magnetic field topology during different IMF configurations. We find that the topology of crustal field cusp regions is dependent on IMF direction and that cusps transition between open and closed topology regularly as they rotate through the nightside of Mars. Finally, we determine that cusps often become topologically closed due to reconnection with open magnetic fields in the Martian magnetotail, creating large closed loops that connect the dayside and nightside of Mars.

MARSIS Observations of the Martian Nightside Ionosphere During the September 2017 Solar Event

AbstractWe present topside ionospheric sounding on the nightside of Mars during the September 2017 solar event by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express along with complementary dayside observations from Mars Atmosphere and Volatile EvolutioN (MAVEN). The MARSIS and MAVEN observations during the event suggest that (i) the nightside bottomside ionosphere was significantly enhanced by solar energetic particles, (ii) the nightside peak electron density was increased to unusually high values of ∼1–2 ×104 cm−3 around 120 km altitudes owing to enhanced electron impact ionization, and (iii) the ionospheric magnetic field was globally amplified by the high dynamic pressure during the interplanetary coronal mass ejection passage. These multipoint measurements help elucidate the global response of the Martian upper ionosphere to the solar event.

Imprints of Quasi‐Adiabatic Ion Dynamics on the Current Sheet Structures Observed in the Martian Magnetotail by MAVEN

AbstractNumerous studies of the current sheets (CS) in the Earth's magnetotail showed that quasi‐adiabatic ion dynamics plays an important role in the formation of complicated multilayered current structures. In order to check whether the similar mechanisms operate in the Martian magnetotail, we analyzed 80 CS crossings using MAVEN measurements on the nightside of Mars at radial distances ~1.0–2.8RM. We found that CS structures experience similar dependence on the value of the normal component of the magnetic field at the neutral plane (BN) and on the ratio of the ion drift velocity outside the CS to the thermal velocity (VT/VD) as it was observed for the CSs in the Earth's magnetotail. For the small values of BN, a thin and intense CS embedded in a thicker one is observed. The half‐thickness L of this layer is ~30–100 km ≤ ρH+ (ρH+ is a gyroradius of thermal protons outside the CS). With the increase of BN, the L also increases up to several hundred kilometers (~ρO+, ρO2+), the current density decreases, and the embedding feature disappears. Our statistical analysis showed a good agreement between L values observed by MAVEN and the CS scaling obtained from the quasi‐adiabatic model, if the plasma characteristics in Martian CSs are used as input parameters. Thus, we may conclude that in spite of the differences in magnetic topology, ion composition, and plasma thermal characteristics observed in the Earth's and Martian magnetotails, similar quasi‐adiabatic mechanisms contribute to the formation of the CSs in the magnetotails of both planets.

Characterization of Low‐Altitude Nightside Martian Magnetic Topology Using Electron Pitch Angle Distributions

AbstractMagnetic field lines at Mars act as direct pathways for both energy inflow and ion escape. Local variations in magnetic field topology can therefore directly impact the interaction between the solar wind and the Martian ionosphere. One method of analyzing magnetic topology is through the use of electron pitch angle distributions (PADs). Previous PAD investigations have characterized magnetic topology in the Martian system using data from the Mars Global Surveyor spacecraft, but these studies were orbitally constrained to ∼400 km altitude and 2 a.m./2 p.m. local time. With the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we are now able to extend this analysis to a larger range of altitudes and local times. Here we use electron PADs measured using the Solar Wind Electrostatic Analyzer and Magnetometer instruments on MAVEN to analyze the magnetic topology of the nightside Martian environment. We use several characteristic PAD shapes to determine where Martian magnetic field lines are open or closed to the solar wind and present frequency maps of how these PAD shapes vary both geographically and with altitude. Finally, we present an initial analysis of the variation of the PAD shapes with local time, finding that trapped electron distributions become increasingly frequent as crustal fields rotate from dusk to dawn across the nightside of Mars.

Mars nightside electrons over strong crustal fields

AbstractWe investigated 7 years worth of data from the electron reflectometer and magnetometer aboard Mars Global Surveyor to quantify the deposition of photoelectron and solar wind electron populations on the nightside of Mars, over the strong crustal field region located in the southern hemisphere. Just under 600,000 observations, each including energy and pitch angle distributions, were examined. For solar zenith angles (SZA) less than 110°, photoelectrons have the highest occurrence rate; beyond that, plasma voids occur most often. In addition, for SZA >110°, energy deposition of electrons mainly occurs on vertical field lines with median pitch angle averaged energy flux values on the order of 107–108 eV cm−2 s−1. The fraction of downward flux that is deposited at a given location was typically low (16% or smaller), implying that the majority of precipitated electrons are magnetically reflected or scattered back out. The average energy of the deposited electrons is found to be 20–30 eV, comparable to typical energies of photoelectrons and unaccelerated solar wind electrons. Median electron flux values, from near‐vertical magnetic field lines past solar zenith angle of 110°, calculated in this study produced a total electron content of 4.2 × 1014 m−2 and a corresponding peak density of 4.2 × 103 cm−3.

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 10 4 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/m 2. 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/m 2) 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.

Tailward flow of energetic neutral atoms observed at Mars

The ASPERA‐3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.

Suprathermal electron fluxes on the nightside of Mars: ASPERA-3 observations

Recently aurora-type UV emissions were discovered on the nightside of Mars [Bertaux, J.-L., Leblanc, F., Witasse, O., et al., 2005. Discovery of an aurora on Mars. Nature 439, doi:10.1038/nature03603]. It was suggested that these emissions are produced by suprathermal electrons with energies of tens of eV, rather than by the electrons with spectra peaked above 100 eV [Leblanc, F., Witasse, O., Winningham J., et al., 2006. Origin of the martian aurora observed by spectroscopy for investigation of characteristics of the atmosphere of Mars (SPICAM) onboard Mars Express. J. Geophys. Res. 111, A09313, doi:10.1029/2006JA011763]. In this paper we present observations of fluxes of suprathermal electrons ( E e ≈ 30 –100 eV) on the Martian nightside by the ASPERA-3 experiment onboard the Mars Express spacecraft. Narrow spikes of suprathermal electrons are often observed in energy-time spectrograms of electron fluxes at altitudes between 250 and 600 km. These spikes are spatially organized and form narrow strips in regions with strong upward or downward crustal magnetic field. The values of electron fluxes in such events generally could explain the observed auroral UV emissions although a question of their origin (transport from the dayside or local precipitation) remains open.

Model calculations of electron precipitation induced ionization patches on the nightside of Mars

Using an electron transport model, we investigate the effect of electron precipitation on the electron density and total electron content in the nightside ionosphere of Mars. As input we use Mars Global Surveyor observations of a typical tail electron spectrum and an auroral‐like electron spectrum. The accelerated electron spectrum increases the maximum number density and total electron content by a factor of 3 over that produced by the typical tail spectrum. Our calculations show a secondary electron density peak due to precipitation of several keV electrons not seen in previous modeling efforts. Regions of enhanced ionization are expected to be localized in space, corresponding to magnetic cusps formed by the interaction of crustal sources with the interplanetary magnetic field. Radio and radar measurements from both Mars Global Surveyor and Mars Express agree with this expectation. The horizontally inhomogeneous regions of ionization can affect signals used for subsurface sounding from orbit.

Chemical organizations in atmospheric photochemistries—A new method to analyze chemical reaction networks

The theory of chemical organizations is proposed as a new method to study atmospheric reaction networks. Here, the method is applied to an atmospheric photochemistry model of Mars. Only considering its stoichiometry, the reaction network is decomposed into self-sustaining sub-networks called organizations. These are arranged into an overlapping hierarchy representing the organizational structure of the reaction network. We analyze both the day- and nightside of Mars, discovering a simple, combinatorial hierarchy of organizations for the nightside and a rich and complex hierarchy for the dayside. One organization representing the recycling pathway for CO 2 could be identified. Molecular species of similar molecular structure are found to often appear together in organizations. For the dayside, only few organizations contain many molecular species, indicating that only few steady states featuring a high diversity in molecular composition exist. Using a simulated concentration profile, organizations can be grouped into four categories with distinct intensity profiles.

Energetic Hydrogen and Oxygen Atoms Observed on the Nightside of Mars

We present measurements of energetic hydrogen and oxygen atoms (ENAs) on the nightside of Mars detected by the neutral particle detector (NPD) of ASPERA-3 on Mars Express. We focus on the observations for which the field-of-view of NPD was directed at the nightside of Mars or at the region around the limb, thus monitoring the flow of ENAs towards the nightside of the planet. We derive energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 105 cm−2 sr−1 s−1, but no oxygen ENA signals above the detection threshold of 104 cm−2 sr−1 s−1 are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements (Galli et al., this issue) point to a hydrogen exobase density of 1010 m−3 and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 1012 m−3 and a temperature of 200 K predicted by (1996). Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENAs.

Locations of Atmospheric Photoelectron Energy Peaks Within the Mars Environment

By identifying peaks in the photoelectron spectrum produced by photoionization of CO2 in the Martian atmosphere, we have conducted a pilot study to determine the locations of these photoelectrons in the space around Mars. The significant result of this study is that these photoelectrons populate a region around Mars bounded externally by the magnetic pileup boundary, and internally by the lowest altitude of our measurements (∼250 km) on the dayside and by a cylinder of approximately the planetary radius on the nightside. It is particularly noteworthy that the photoelectrons on the nightside are observed from the terminator plane tailward to a distance of ∼3 R M, the Mars Express apoapsis. The presence of the atmospherically generated photoelectrons on the nightside of Mars may be explained by direct magnetic field line connection between the nightside observation locations and the Martian dayside ionosphere. Thus the characteristic photoelectron peaks may be used as tracers of magnetic field lines for the study of the magnetic field configuration and particle transport in the Martian environment.

Plasma Acceleration Above Martian Magnetic Anomalies

Auroras are caused by accelerated charged particles precipitating along magnetic field lines into a planetary atmosphere, the auroral brightness being roughly proportional to the precipitating particle energy flux. The Analyzer of Space Plasma and Energetic Atoms experiment on the Mars Express spacecraft has made a detailed study of acceleration processes on the nightside of Mars. We observed accelerated electrons and ions in the deep nightside high-altitude region of Mars that map geographically to interface/cleft regions associated with martian crustal magnetization regions. By integrating electron and ion acceleration energy down to the upper atmosphere, we saw energy fluxes in the range of 1 to 50 milliwatts per square meter per second. These conditions are similar to those producing bright discrete auroras above Earth. Discrete auroras at Mars are therefore expected to be associated with plasma acceleration in diverging magnetic flux tubes above crustal magnetization regions, the auroras being distributed geographically in a complex pattern by the many multipole magnetic field lines extending into space.

Response of Bacillus subtilis spores to dehydration and UV irradiation at extremely low temperatures.

Spores of Bacillus subtilis have been exposed to the conditions of extreme dehydration (argon/silica gel; simulated space vacuum) for up to 12 weeks at 298 K and 80 K in the dark. The inactivation has been correlated with the production of DNA-double strand-breaks. The temperature-dependence of the rate constants for inactivation or production of DNA-double strand-breaks is surprisingly low. Controls kept in the frozen state at 250 K for the same period of time showed no sign of deterioration. In another series of experiments the spores have been UV irradiated (253.7 nm) at 298 K, 200 K and 80 K after exposure to dehydrating conditions for 3 days. Fluence-effect relationships for inactivation, production of DNA-double strand-breaks and DNA-protein cross-links are presented. The corresponding F37-values for inactivation and production of DNA lesions are significantly increased only at 80 K (factor of 4 to 5). The data indicate that the low temperatures that prevail in the outer parts of the Solar System or at the nightside of Mars or the Moon are not sufficiently low to crucially inhibit inactivation by dehydration. Our data place further constraints on the panspermia hypothesis.