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 .
The profiles of the Martian dayside ionosphere can be used to derive the 130km neutral atmospheric densities, which can also be obtained from the Mars Climate Database (MCD) and the spacecraft aerobraking observations. In this research, the method to calculate 130km neutral densities through ionosphere observations is explained, and three long-period 130km neutral densities data sets at northern high-latitudes (Lat >60°) are acquired through the ionospheric data measured by MGS Radio Occultation Experiment. The calculated 130km neutral density data, along with the 130km density data from the aerobraking observations of MGS and ODY in the northern high-latitudes, are compared with the MCD outputs with the same latitude, longitude, altitude, solar latitude and local time. It is found that both the 130km density data derived from ionospheric profiles and aerobraking observations show similar seasonal variations as the MCD data. With a negative shift of ~2x1010 cm -3, the corrected 130km neutral densities derived from the MCD v4.3 are consistent with those obtained from two different observations. This result means that 1) the method to derive 130km neutral densities through ionospheric profiles is effective, 2) the MCD v4.3 data sets generally overestimate the 130km neutral density at high-latitudes, and 3) the neutral density observations from MGS Radio Science Experiment can be used to calibrate the new atmospheric model of Mars.
Photoelectrons are an important population of the dayside Martian ionosphere, which are produced by solar Extreme Ultraviolet and X-ray ionization of atmospheric neutrals. A typical photoelectron energy spectrum is characterized by a distinctive peak near 27 eV, related to the strong solar HeII emission line at 30.4 nm, and an additional peak near 500 eV, related to O Auger ionization. In this study, the solar driven scenario for photoelectrons is verified with the aid of the extensive measurements made by the Solar Wind Electron Analyzer on board the recent Mars Atmosphere and Volatile Evolution spacecraft. Our analysis reveals that the photoelectron intensities at the centers of both peaks increase steadily with increasing solar ionizing flux below 90 nm and that the observed solar cycle variation is substantially more prominent near the O Auger peak than near the HeII peak. The latter observation is clearly driven by a larger variability in solar irradiance at shorter wavelengths. When the solar ionizing flux increases from 1 mW m-2 to 2.5 mW m-2, the photoelectron intensity increases by a factor of 3.2 at the HeII peak and by a much larger factor of 10.5 at the O Auger peak, both within the optically thin regions of the Martian atmosphere.
The prototype for investigations of formation mechanisms and related geological effects of large impact basins on planetary bodies has been the Orientale basin on the Moon. Its widespread secondaries, light plains, and near-rim melt flows have been well mapped in previous studies. Flow features are also widely associated with secondaries on planetary bodies, but their physical properties are not well constrained. The nature of flow features associated with large impact basins are critically important to understand the emplacement process of basin ejecta, which is one of the most fundamental processes in shaping the shallow crusts of planetary bodies. Here we use multisource remote sensing data to constrain the physical properties of flow features formed by the secondaries of the Orientale basin. The results suggest that such flows are dominated by centimeter-scale fine debris fines; larger boulders are not abundant. The shattering of target materials during the excavation of the Orientale basin, landing impact of ejecta that formed the secondaries, and grain comminution within the flows have substantially reduced particle sizes, forming the fine flows. The discovery of global-wide fine debris flows formed by large impact basins has profound implications to the interpretation of both previously-returned samples and remote sensing data.
One-dimensional hybrid simulations are carried out to study the plasma refilling process in the lunar wake. Previous theoretical and simulation studies have shown that ion-ion acoustic (ⅡA) instability can be initiated and electrostatic shock can be formed under the condition
On 21 June 2020, an annular solar eclipse will traverse the low latitudes from Africa to Southeast Asia. The highest latitude of the maximum eclipse obscuration is approximately 30°. This low-latitude solar eclipse provides a unique and unprecedented opportunity to explore the impact of the eclipse on the low-latitude ionosphere–thermosphere (I–T) system, especially in the equatorial ionization anomaly region. In this study, we describe a quantitative prediction of the impact of this upcoming solar eclipse on the I–T system by using Thermosphere–Ionosphere–Electrodynamics General Circulation Model simulations. A prominent total electron content (TEC) enhancement of around 2 TEC units occurs in the equatorial ionization anomaly region even when this region is still in the shadow of the eclipse. This TEC enhancement lasts for nearly 4.5 hours, long after the solar eclipse has ended. Further model control simulations indicate that the TEC increase is mainly caused by the eclipse-induced transequatorial plasma transport associated with northward neutral wind perturbations, which result from eclipse-induced pressure gradient changes. The results illustrate that the effect of the solar eclipse on the I–T system is not transient and linear but should be considered a dynamically and energetically coupled system.
As a companion paper to
Using wave measurements from the EMFISIS instrument onboard Van Allen Probes, we investigate statistically the spatial distributions of the intensity of plasmaspheric hiss waves. To reproduce these empirical results, we establish a fitting model that is a third-order polynomial function of L-shell, magnetic local time (MLT), magnetic latitude (MLAT), and AE*. Quantitative comparisons indicate that the model’s fitting functions can reflect favorably the major empirical features of the global distribution of hiss wave intensity, including substorm dependence and the MLT asymmetry. Our results therefore provide a useful analytic model that can be readily employed in future simulations of global radiation belt electron dynamics under the impact of plasmaspheric hiss waves in geospace.
Nine years (2001–2009) of data from the Cluster spacecraft are analyzed in this study of the Earth’s mid- and high-altitude (2–9RE) cusp. Properties of the cusp region, and its location and size in the Solar Magnetic coordinate system, are studied statistically. The survey shows that (1) the relationships between X and Z are nearly linear for the poleward, equatorward boundaries and the center of the cusp; (2) the relationship between cusp width in the X direction and Z can be expressed by a quadratic function; (3) the cusp region is almost dawn-dusk symmetric for the cusp width in the X direction. Based on topology information, a new normalized statistical methodology is developed to organize the measurements of cusp crossings to obtain distributions of magnetic field and plasma parameters in the XZ plane. The statistical results show that (1) Bx is mostly negative and Bz is always negative; (2) proton velocity is found to be positive for Vx and Vz at low altitudes, while Vx and Vz are negative on the equator side and negative Vx and positive Vz on the pole side at high altitudes; (3) proton density is higher on the equator side than on the pole side. Results reported here will be useful in suggesting directions for future cusp research.
This study presents an analysis of the quasi-16-day wave (Q16DW) at three stations in the middle latitudes by using a meteor radar chain in conjunction with Aura Microwave Limb Sounder temperature data and MERRA2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) reanalysis data from 2008 to 2017. The radar chain consists of three meteor radar stations located at Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), and Wuhan (WH, 30.5°N, 114.6°E). The Q16DW wave exhibits similar seasonal variation in the neutral wind and temperature, and the Q16DW amplitude is generally strong during winter and weak around summer. The Q16DW at BJ was found to have secondary enhancement around September in the zonal wind, which is rarely reported at similar latitudes. The latitudinal variations of the Q16DW in the neutral wind and temperature are quite different. The Q16DW at BJ is the most prominent in both neutral wind components among the three stations and the Q16DW amplitudes at MH and WH are comparable, whereas the wave amplitude in temperature decreases with decreasing latitude. The quasi-geostrophic refractive index squared at the three stations in the period from 2008 to 2017 was revealed. The results indicate that the Q16DW in the mesosphere and lower thermosphere (MLT) at MH has a limited contribution from the lower atmosphere. Around March and October, the Q16DW in the troposphere at BJ can propagate upward into the MLT region, whereas at WH, the contribution to the Q16DW in the MLT region is largely from the mesosphere.
By using atmospheric wind data in the mesopause and lower thermosphere (MLT) region, features of seasonal variations in the quasi-6-day wave (6DW) at different latitudes are analyzed, and modulation of the 6DW by the diurnal tide and solar 27-day period is discussed. The data used in the analysis are extracted from a wind dataset collected by a meteor radar chain from December 2008 to November 2017. The meteor radar chain includes four stations, in Mohe, Beijing, Wuhan, and Sanya. Features of seasonal variations in the 6DW indicate that in summer the 6DW is usually strongest during July and August, followed by stronger variations in January and April. At certain altitudes over Wuhan and Sanya, the 6DW is slightly different in different years and altitudes. In our analysis of seasonal variations in the 6DW, we find that it is generally affected by annual oscillations and semiannual oscillations. The annual oscillations of the 6DW in the mid-low latitudes are modulated by the quasibiennial oscillation in the diurnal tide, resulting in seasonal features that are different from those at other latitudes. In addition, the 6DW amplitude at mid-high latitudes has a significant 27-day solar rotation variation, which was prominent in 2016.
Co-seismic gas leakage usually occurs on the edge of seismic faults in petroliferous basins, and it may have an impact on the local environment, such as the greenhouse effect, which can cause thermal infrared brightness anomalies. Using wavelet transform and power spectrum estimation methods, we processed brightness temperature data from the Chinese geostationary meteorological satellite FY-C/E. We report similarities between the co-seismic thermal infrared brightness (CTIB) anomalies before, during and after earthquakes that occurred at the edges of the Sichuan, Tarim, Qaidam, and Junggar basins surrounding the North and East of the Qinghai–Tibet Plateau in western China. Additionally, in each petroliferous basin, the area of a single CTIB anomaly accounted for 50% to 100% of the basin area, and the spatial distribution similarities in the CTIB anomalies existed before, during and after these earthquakes. To better interpret the similarities, we developed a basin warming effect model based on geological structures and topography. The model suggests that in a petroliferous basin with a subsurface gas reservoir, gas leakage could strengthen with the increasing stress before, during, and even after an earthquake. The accumulation of these gases, such as the greenhouse gases CH4 and CO2, results in the CTIB anomalies. In addition, we conclude that the CTIB anomalies are strengthened by the high mountains (altitude ~5000 m) around the basins and the basins’ independent climatic conditions. This work provides a new perspective from which to understand the CTIB anomalies in petroliferous basins surrounding the North and East of the Qinghai–Tibet Plateau.
Thick sediments from foreland basins usually provide valuable information for understanding the relationships between mountain building, rock denudation, and sediment deposition. In this paper, we report environmental magnetic measurements performed on the Miocene sediments in the Subei Basin, northeastern Tibetan Plateau. Our results show two different patterns. First, the bulk susceptibility and SIRM, ARM, and HIRM mainly reflect the absolute-concentration of magnetic minerals; all have increased remarkably since 13.7 Ma, related to provenance change rather than climate change. Second, the ratios of IRM100mT/SIRM, IRM100mT/IRM30mT, and IRM100mT/IRM60mT, together with the redness and S ratio, reflect the relative-concentration of hematite, being climate-dependent. Their vertical changes correlate in general with the long-term Miocene climatic records of marine oxygen isotope variations, marked by the existence of higher ratios between 17 and 14 Ma. This may imply that global climate change, rather than uplift of the Tibetan Plateau, played a dominant role in the long-term climatic evolution of the Subei area from the early to middle Miocene.
The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China. Investigations of the structures of the fault zone and its surrounding areas have attracted much attention. In this study, we used dense-array ambient noise tomography to construct a three-dimensional shear wave velocity model of shallow crust in an area about 80km × 70km in Lujiang, Anhui Province, eastern China. For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region, and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method; we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves. Based on the direct surface wave tomography method, we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km. The results revealed important geological structural features of the study area. In the north region, the sedimentary center of the Hefei Basin — the southwestern part of the Chaohu Lake — shows a significant low-velocity anomaly to a depth of at least 5 km. The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks, respectively, and both show obvious high-speed anomalies; the sedimentary area within the Tan-Lu fault zone (about 10 km wide) shows low-velocity anomalies. However, the volcanic rock intrusion area in the fault zone is shown as high velocity. Our shallow crustal imaging results reflect the characteristics of different structures in the study area, especially the high-speed intrusive rocks in the Tan-Lu fault zone, which were probably partially derived from the magmatic activity of Luzong volcanic basin. From the Late Cretaceous to Early Tertiary, the Tan-Lu fault zone was in a period of extensional activity; the special stress environment and the fractured fault zone morphology provided conditions for magma in the Luzong volcanic basin to intrude into the Tan-Lu fault zone in the west. Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation.