Abstract With the aid of the recent Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made onboard the Mars Atmosphere and Volatile Evolution spacecraft, we construct empirical models for the vertical density distributions of 12 species in the dayside Martian ionosphere over the altitude range of 150–450 km, from abundant species such as , , and O + to trace species such as and OCOH + . Two different sets of formulism are proposed to parameterize the ion density distribution characterized either by a distinctive layer structure or by a near exponential decay above 150 km. Both the solar control and magnetic control of ion distribution are taken into account in our modelings. In general, our empirical models are in good agreement with the NGIMS measurements. The full set of parameters constrained by data‐model comparison is tabulated for reference. This allows the easy computation of any individual ion density profile for a unique combination of solar zenith angle, solar extreme ultraviolet flux, and ambient magnetic elevation, which in turn facilitates comparisons with photochemical model results. The empirical models reported in this study are an extension of previous empirical models constructed for the electron distribution in the dayside Martian ionosphere using both radar sounding and radio occultation data.
Abstract The dynamic pressure of solar wind, which is determined by both solar wind density and velocity, is a crucial factor influencing the Martian plasma environment. In this study, we employ a multifluid magnetohydrodynamic (MHD) model to investigate the distinct effects of variations in solar wind velocity and density on boundary layers and the ion escape process. The simulation results indicate that, when the solar wind dynamic pressure is held constant, an increase in solar wind density leads to a significant expansion of the bow shock (BS) and a slight contraction of the magnetic pile‐up boundary. Under conditions of elevated solar wind density, the electric fields that typically inhibit solar wind penetration weaken, allowing a greater number of solar wind protons to traverse the BS. This results in enhanced energy inputs, leading to increased thermal and magnetic pressures. Consequently, the tailward ion escape flux rises substantially due to the increased planetary ion density associated with the higher solar wind proton density. Furthermore, under these conditions, the magnetic field lines exhibit greater piling‐up, with the interplanetary magnetic field penetrating to lower altitudes within the ionosphere, thereby creating additional tailward transport channels for planetary ions. Additionally, as solar wind density increases, the current sheet shifts toward the dawn side, resulting in a more pronounced asymmetry structure.
Abstract Global dust storms (GDS) are an important dynamical phenomenon of the Martian lower atmosphere but are known to have important impacts on the Martian middle/upper atmosphere and ionosphere. Despite extensive studies over the past several decades, how the composition of the Martian ionosphere is modified during the GDS has only been studied from a theoretical point of view. Here we present for the first time the observations of the compositional variation of the Martian ionosphere during the GDS in 2018, using the ion density measurements made by the Neutral Gas and Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile Evolution. At a representative altitude of 170 km, the variations of ionospheric species during the GDS show either enhancement ( , Ar + , , H 2 O + , , ArH + ) or depletion ( , O + , /CO + , OH + ). Despite the apparent diversity, the observations are mostly understandable within the established framework of ionospheric chemistry on Mars, which further demonstrates that the variation of ion species during the GDS is a good diagnostic of the variation of relevant neutral species in the thermosphere. In particular, the observed ionospheric variation strongly supports a scenario that H 2 O is substantially enhanced in the Martian thermosphere during the GDS. However, the variations of and are inconsistent with predictions from ionospheric chemistry and require further investigation.
Abstract Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed in regions with near horizontal magnetic fields and under low solar wind (SW) dynamic pressures. We also find that below 160 km, the occurrence increases with increasing CO 2 density, a trend mostly visible in weakly magnetized regions. These observations have important implications on the formation of electron depletions: (1) Near strong magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; and (2) in weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered, but at sufficiently low altitudes, they are either “absorbed” due to inelastic collisions with ambient neutrals or shielded again in response to a change in magnetic connectivity from open to closed. Our analysis further reveals that both the ionospheric plasma content and thermal electron temperature are reduced in regions with depletions compared to regions without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere.
Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO2++ in this case, in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO2++ distribution declining steadily from 5.6 cm−3 at 160 km to below 1 cm−3 above 200 km. The dominant CO2++ production mechanisms are double photoionization of CO2 below 190 km and single photoionization of CO2+ at higher altitudes; CO2++ destruction is dominated by natural dissociation, but reactions with atmospheric CO2 and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger solar control of the CO2++ density than of the CO2+ density .
Abstract The Martian magnetic pile‐up boundary (MPB) delineates the interface between the magnetosheath and the induced magnetosphere, but its global ion‐scale characteristics remaining unclear. Utilizing a three‐dimensional Hall magnetohydrodynamic (MHD) model, this study aims to reveal the features of the MPB layer, including magnetic field, current density, electric fields, and energy transfer between the fields and solar wind as well as planetary ions. Simulation results indicate that magnetic fields tend to pile‐up, drape, bend, and slip at the MPB, leading to the emergence of associated currents () from the nightside electric pole and its flow toward the dayside electric pole along the MPB. Furthermore, energy transfer analysis demonstrates that the solar wind transfers its energy to planetary ions through the motional electric field while simultaneously acquiring some energy from the Hall and ambipolar electric fields at the MPB, resulting in an asymmetrical flow of solar wind and planetary ions.
Abstract Photoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X‐ray ionization and are also frequently observed on the nightside as a result of field‐aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross‐terminator variations, in that the He II peak becomes less pronounced and the 10 − 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of “atmospheric absorption” via photoelectron collisions with ambient neutrals during day‐to‐night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X‐ray radiation, all using the 10–50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross‐terminator spectral reshaping is proposed as the outcome of varying day‐to‐night magnetic connectivity which naturally implies varying field‐aligned atmospheric column for “absorption.” Crude estimates suggest that an “absorbing” atmospheric column of ≈1.5 × 10 15 cm −2 is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross‐terminator magnetic field lines, as complementary to the day‐to‐night magnetic connectivity inferred from the occurrence of nightside photoelectrons.
Abstract In situ produced photoelectrons and precipitating solar wind electrons are two distinct hot electron populations in the dayside Martian upper atmosphere. While each population has been known for decades, its relative contribution to the measured hot electron flux has not been adequately characterized up to now. In this study, we implement a two‐stream kinetic model to compute the hot electron flux for a open magnetic field configuration. By comparing model results to realistic data acquired by the Mars Atmosphere and Volatile Evolution mission, we show that the electron fluxes predicted by the pure photoelectron model are enormously underestimated, especially in the direction toward Mars, but the measurements can be adequately reproduced once solar wind electron precipitation is taken into account. Such a precipitation plays a crucial role above 180 km, not only for hot electrons at all energies that move toward Mars but also for electrons above 1,000 eV that move away from Mars due to the backscattering of precipitating electrons. In contrast, the hot electron population is predicted to be mostly composed of photoelectrons below 180 km. The significance of precipitating solar wind electrons is also evaluated, revealing an appreciable impact on the structure of the dayside Martian upper atmosphere at high altitudes, where electron impact ionization is enhanced by a factor of 7 and cold electron heating enhanced by a factor of 4.
With the aid of a multi-instrument data set gathered by the Mars Atmosphere and Volatile Evolution (MAVEN) during ten selected periods, we make detailed calculations of the CO2+ Ultraviolet Doublet (UVD) emission brightness profiles which are then compared to the Imaging Ultraviolet Spectrometer limb observations. Our calculations confirm that the photoionization of atmospheric CO2 is the predominant process driving CO2+ UVD emission at high altitudes, whereas the photoelectron impact ionization of CO2 becomes more important at low altitudes. The data–model comparisons show good agreement near and above the emission peak at around 120 km with an intensity of 27–45 kR. A special case is found for period 3 coincident with a regional dust storm during which the peak altitude rose by 20 km. Of particular interest is the significant discrepancy below the peak, which is likely associated with the uncertainties in either atmospheric density or incident solar irradiance. A detailed investigation suggests that the latter uncertainty is more likely responsible for such a discrepancy, in that the solar irradiance shortward of a wavelength threshold below 30 nm should be adjusted to achieve reasonable data–model agreement over the entire altitude range. This result highlights the necessity to improve the accuracy of any solar irradiance model used for planetary aeronomical studies.
Abstract As incident solar wind encounters the martian upper atmosphere, it undergoes deflection particularly in the magnetosheath. However, the plasma flow exhibits asymmetrical distribution features within this transition region, which is investigated by employing a three‐dimensional Hall magnetohydrodynamic (MHD) model from an energy transfer perspective in this study. Simulation results reveal that solar wind protons transfer momentum to ionospheric heavy ions through motional electric field in the hemisphere where the motional electric field points outward from the planet. In the opposite hemisphere, solar wind flow tends to be effectively accelerated by ambipolar and Hall electric fields. The distinct dynamics of solar wind protons in both hemispheres result in the asymmetrical deflection. Furthermore, the extent of asymmetry grows as the cross‐flow component of the upstream interplanetary magnetic field increases, but diminishes as the density of the solar wind proton increases, contingent upon the energy effectively acquired from ambipolar and Hall electric fields.
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