Polar mesosphere summer echoes (PMSEs) are very strong radar echoes in the polar mesopause in local summer. Here we present the frequency dependence of the volume reflectivity and the effect of energetic particle precipitation on modulated PMSEs by using PMSEs observations carried out by European Incoherent SCATter (EISCAT) heating equipment simultaneously with very high frequency (VHF) radar and ultra high frequency (UHF) radar on 12 July 2007. According to the experimental observations, the PMSEs occurrence rate at VHF was much higher than that at UHF, and the altitude of the PMSEs maximum observed at VHF was higher than that at UHF. Overlapping regions were observed by VHF radar between high energetic particle precipitation and the PMSEs. In addition, high-frequency heating had a very limited impact on PMSEs when the UHF electron density was enhanced because of energetic particle precipitation. In addition, an updated qualitative method was used to study the relationship between volume reflectivity and frequency. The volume reflectivity was found to be inversely proportional to the fourth power of radar frequency. The theoretical and experimental results provide a definitive data foundation for further analysis and investigation of the physical mechanism of PMSEs.
In this paper, we use wind observations by a Doppler wind LiDAR near Delingha (37.4°N, 97.4°E), Qinghai, Northwestern China to study the characteristics of inertial gravity waves in the stratosphere. We focus on 10–12 December 2013, a particularly interesting case study. Most of the time, the inertial gravity waves extracted from the LiDAR measurements were stationary with vertical wavelengths of about 9–11 km and horizontal wavelengths of about 800–1000 km. However, for parts of the observational period in this case study, a hodograph analysis indicates that different inertial gravity wave propagation features were present at lower and upper altitudes. In the middle and upper stratosphere (~30–50 km), the waves propagated downward, especially during a period of stronger winds, and to the northwest–southeast. In the lower stratosphere and upper troposphere (~10–20 km), however, waves with upward propagation and northeast–southwest orientation were dominant. By taking into account reanalysis data and satellite observations, we have confirmed the presence of different wave patterns in the lower and upper stratosphere during this part of the observational period. The combined data sets suggest that the different wave patterns at lower and upper height levels are likely to have been associated with the presence of lower and upper stratospheric jet streams.
Over the past two decades, the development of the ambient noise cross-correlation technology has spawned the exploration of underground structures. In addition, ambient noise-based monitoring has emerged because of the feasibility of reconstructing the continuous Green’s functions. Investigating the physical properties of a subsurface medium by tracking changes in seismic wave velocity that do not depend on the occurrence of earthquakes or the continuity of artificial sources dramatically increases the possibility of researching the evolution of crustal deformation. In this article, we outline some state-of-the-art techniques for noise-based monitoring, including moving-window cross-spectral analysis, the stretching method, dynamic time wrapping, wavelet cross-spectrum analysis, and a combination of these measurement methods, with either a Bayesian least-squares inversion or the Bayesian Markov chain Monte Carlo method. We briefly state the principles underlying the different methods and their pros and cons. By elaborating on some typical noise-based monitoring applications, we show how this technique can be widely applied in different scenarios and adapted to multiples scales. We list classical applications, such as following earthquake-related co- and postseismic velocity changes, forecasting volcanic eruptions, and tracking external environmental forcing-generated transient changes. By monitoring cases having different targets at different scales, we point out the applicability of this technology for disaster prediction and early warning of small-scale reservoirs, landslides, and so forth. Finally, we conclude with some possible developments of noise-based monitoring at present and summarize some prospective research directions. To improve the temporal and spatial resolution of passive-source noise monitoring, we propose integrating different methods and seismic sources. Further interdisciplinary collaboration is indispensable for comprehensively interpreting the observed changes.
With the method of Hough mode decomposition (HMD), the tidal sources of the three main tidal components, namely, the migrating components DW1 (diurnal westward propagating wavenumber 1) and SW2 (semidiurnal westward propagating wavenumber 2) and the non-migrating component DE3 (diurnal eastward propagating wavenumber 3), at the tropospheric altitudes (1–12 km) and in the latitude range of ±60°, were obtained from National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data during the interval from 1988 to 2011. We analyzed these sources in detail at 6 km and obtained the main properties of their yearly variations. The DW1 source was found to present a weak seasonal variation in the lower latitudes (about ±10°–15°). That is, the amplitudes of the DW1 sources were larger in the summer months than in the winter months, and DW1 presented semi-annual variation near the equator (±10°) such that the DW1 source was larger at the equinoxes than at the solstices. In addition, the SW2 source was symmetric and was stronger in the southern hemisphere than in the northern hemisphere. The SW2 source presented remarkable annual and semi-annual variation such that the amplitudes were largest during the March equinox months and larger during the June solstice months. In contrast, DE3 appeared mainly around the equatorial latitudes within about ±30°. The DE3 source presented remarkable semi-annual variation that was larger around the solstices than the equinoxes in the southern hemisphere, and it was opposite in the northern hemisphere. By HMD, we found that the tropospheric tides were primarily dominated by some leading propagating Hough modes, specifically, the (1, 1), (2, 3), and (3, 3) modes; the influences of the other Hough modes were negligible. The consequences of an El Niño–Southern Oscillation modulation of tidal amplitudes for the energy and momentum budgets of the troposphere may now be expected to attract attention. In summary, the above yearly variations of the main tidal sources and the Hough coefficients demonstrate that an HMD analysis can be used to investigate the tropospheric tides.
Diurnal variations in the planetary boundary layer height (PBLH) at different latitudes over different surface characteristics are described, based on 45 years (1973−2017) of radiosonde observations. The PBLH is determined from the radiosonde data by the bulk Richardson number (BRN) method and verified by the parcel method and the potential temperature gradient method. In general, the BRN method is able to represent the height of the convective boundary layer (BL) and neutral residual layer cases but has relatively large uncertainty in the stable BL cases. The diurnal cycle of the PBLH over land is quite different from the cycle over ocean, as are their seasonal variations. For stations over land, the PBLH shows an apparent diurnal cycle, with a distinct maximum around 15:00 LT, and seasonal variation, with higher values in summer. Compared with the PBLH over land, over oceans the PBLH diurnal cycles are quite mild, the PBLHs are much lower, and the seasonal changes are less pronounced. The seasonal variations in the median PBLH diurnal cycle are positively correlated with the near-surface temperature and negatively correlated with the near-surface relative humidity. Finally, although at most latitudes the daytime PBLH exhibits, over these 45 years, a statistically significant increasing trend at most hours between 12:00 LT and 18:00 LT over both land and ocean, there is no significant trend over either land or ocean in the nighttime PBLH for almost all the studied latitudes.
Using Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) data in the northern hemisphere at the 10 hPa level, we compared the stratospheric evolution of temperature and geopotential height during two major sudden stratosphere warming events (SSWs) that occurred in the Arctic winter of 2018 and 2019. In the prewarming period, poleward temperature-enhanced regions were mainly located around 120°E with a displaced vortex and around 120°E and 60°W with splitting vortices. The evolution of geopotential height indicated that these temperature-enhanced regions were both on the western side of high-latitude anticyclones. In the postwarming period, the polar vortex turned from splitting to displacement in the 2018 SSW but from displacement to splitting in the 2019 SSW. Both transitions were observed over the Atlantic region, which may have been caused by anticyclones moving through the polar region. Our findings revealed that the evolution of the anticyclone is important during SSWs and is closely related to temperature-enhanced regions in the prewarming periods and to transitions of the polar vortices in postwarming periods.
The global atmospheric static stability (N2) in the middle atmosphere and its relation to gravity waves (GWs) were investigated by using the temperature profiles measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument from 2002 to 2018. At low latitudes, a layer with enhanced N2 occurs at an altitude of ~20 km and exhibits annual oscillations caused by tropopause inversion layers. Above an altitude of ~70 km, enhanced N2 exhibits semiannual oscillations at low latitudes caused by the mesosphere inversion layers and annual oscillations at high latitudes resulting from the downward shift of the summer mesopause. The correlation coefficients between N2 and GW amplitudes can be larger than 0.8 at latitudes poleward of ~40°N/S. This observation provides factual evidence that a large N2 supports large-amplitude GWs and indicates that N2 plays a dominant role in maintaining GWs at least at high latitudes of the middle atmosphere. This evidence also partially explains the previous results regarding the phase changes of annual oscillations of GWs at high latitudes.
The wavenumber spectral components WN4 at the mesosphere and low thermosphere (MLT) altitudes (70–10 km) and in the latitude range between ±45° are obtained from temperature data (T) observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments on board the National Aeronautics and Space Administration (NASA)’s Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics (TIMED) spacecraft during the 11-year solar period from 2002 to 2012. We analyze in detail these spectral components WNk and obtain the main properties of their vertical profiles and global structures. We report that all of the wavenumber spectral components WNk occur mainly around 100 km altitude, and that the most prominent component is the wavenumber spectral component WN4 structure. Comparing these long duration temperature data with results of previous investigations, we have found that the yearly variation of spectral component WN4 is similar to that of the eastward propagating non-migrating diurnal tide with zonal wavenumber 3 (DE3) at the low latitudes, and to that of the semi-diurnal tide with zonal wavenumber 2 (SE2) at the mid-latitudes: the amplitudes of the A4 are larger during boreal summer and autumn at the low-latitudes; at the mid-latitudes the amplitudes have a weak peak in March. In addition, the amplitudes of component WN4 undergo a remarkable short period variation: significant day-to-day variation of the spectral amplitudes A4 occurs primarily in July and September at the low-latitudes. In summary, we conclude that the non-migrating tides DE3 and SE2 are likely to be the origins, at the low-latitudes and the mid-latitudes in the MLT region, respectively, of the observed wavenumber spectral component WN4.
Through respectively adding June tide and December tide at the low boundary of the GCITEM-IGGCAS model (Global Coupled Ionosphere–Thermosphere–Electrodynamics Model, Institute of Geology and Geophysics, Chinese Academy of Sciences), we simulate the influence of atmospheric tide on the annual anomalies of the zonal mean state of the ionospheric electron density, and report that the tidal influence varies with latitude, altitude, and solar activity level. Compared with the density driven by the December tide, the June tide mainly increases lower ionospheric electron densities (below roughly the height of 200 km), and decreases electron densities in the higher ionosphere (above the height of 200 km). In the low-latitude ionosphere, tides affect the equatorial ionization anomaly structure (EIA) in the relative difference of electron density, which suggests that tides affect the equatorial vertical E×B plasma drifts. Although the tide-driven annual anomalies do not vary significantly with the solar flux level in the lower ionosphere, in the higher ionosphere the annual anomalies generally decrease with solar activity.
In this research, the roles of gravity waves and planetary waves in the change to middle atmospheric residual circulation during a sudden stratospheric warming period are differentiated and depicted separately by adopting the downward control principle. Our analysis shows clear anomalous poleward residual circulation patterns from the equator to high latitudes in the lower winter stratosphere. At the same time, upward mean flows are identified at high latitudes of the winter upper stratosphere and mesosphere, which turn equatorward in the mesosphere and reach as far as the tropical region, and consequently the extratropical region in the summer hemisphere. The downward control principle shows that anomalous mesospheric residual circulation patterns, including interhemispheric coupling, are solely caused by the change in gravity wave forcing resulting from the reversal of the winter stratospheric zonal wind. Nevertheless, both planetary waves and gravity waves are important to variations in the winter stratospheric circulation, but with opposite effects.
The ratio between vertical and radial amplitudes of Rayleigh waves (hereafter, the Rayleigh wave ZH ratio) is an important parameter used to constrain structures beneath seismic stations. Some previous studies have explored crust and upper mantle structures by joint inversion of the Rayleigh wave ZH ratio and surface wave dispersion. However, all these studies have used a 1-D depth sensitivity kernel, and this kernel may lack precision when the structure varies a great deal laterally. Here, we present a systematic investigation of the two-dimensional (2-D) Rayleigh wave ZH ratio kernel based on the adjoint-wavefield method and perform two synthetic tests using the new kernel. The 2-D ZH ratio kernel is consistent with the traditional 1-D sensitivity kernel but has an asymmetric pattern with a preferred orientation toward the source. The predominant effect caused by heterogeneity can clearly be seen from kernels calculated from models with 2-D heterogeneities, which confirms the necessity of using the new 2-D kernel in some complex regions. Inversion tests using synthetic data show that the 2-D ZH ratio kernel has the potential to resolve small anomalies as well as complex lateral structures.
The fault branching phenomenon, which may heavily influence the patterns of rupture propagation in fault systems, is one of the geometric complexities of fault systems that is widely observed in nature. In this study, we investigate the effect of the branching angle on the rupture inclination and the interaction between branch planes in two-fork branching fault systems by numerical simulation and theoretical analysis based on Mohr’s circle. A friction law dependent on normal stress is used, and special attention is paid to studying how ruptures on the upper and lower branch planes affect the stress and rupture on each other separately. The results show that the two branch planes affect each other in different patterns and that the intensity of the effect changes with the branching angle. The rupture of the lower branch plane has a negative effect on the rupture of the upper branch plane in the case of a small branching angle but has almost no negative effect in the case of a large branching angle. The rupture of the upper branch plane, however, suppresses the rupture of the lower branch plane regardless of whether the branching angle is large or small.
In recent studies of the Martian atmosphere, strong diurnal variation in the dust was discovered in the southern hemisphere during major dust storms, which provides strong evidence that the commonly recognized meridional transport process is driven by thermal tides. This process, when coupled with deep convection, could be an important part of the short-term atmospheric dynamics of water escape. However, the potential of this process to alter the horizontal distribution of moist air has not been systematically investigated. In this work, we conducted pre-research on the horizontal transport of water vapor associated with the migrating diurnal tide (DW1) at 50 Pa in the upper troposphere during major dust storms based on the Mars Climate Database (MCD) 5.3, a state-of-the-art database for Martian atmospheric research that has been validated as simulating the relevant short-period atmospheric dynamics well. We found westward-propagating diurnal patterns in the global water vapor front during nearly all the major dust storms from Martian years (MYs) 24 to 32. Statistical and correlation analyses showed that the diurnal transport of water vapor during global and A-season regional dust storms is dominated by the DW1. The effect of the tidal transport of water vapor varies with the types of dust storms in different seasons. During regional dust storms, the tidal transport induces only limited diurnal motion of the water vapor. However, the horizontal tidal wind tends to increase the abundance of daytime water vapor at mid- to low latitudes during the MY 28 southern summer global dust storm while decreasing it during the MY 25 southern spring global dust storm. The tidal transport process during these two global dust storms can induce opposite effects on water escape.
The global geopotential models do not contain the very high frequencies of the Earth’s external gravity field. This is called omission error. The omission error become more important in mountainous areas (areas with highly variable topography). This work consists in reducing the omission error (refinement) on Bouguer gravity anomalies derived from the global geopotential model EGM2008 using the spectral enhancement method. This method consists in computing the residual terrain effects and then coupling them to the gravimetric signal of the global geopotential model. To compute the residual terrain effects, we used the Residual Terrain Model (RTM) technique. A reference surface (ETOPO1) developed up to degree 2190 (maximum degree of the EGM2008 model) and a detailed elevation model (AW3D30) were needed to perform it. The computation was done with TC program of the GRAVSOFT package. The topography of the study area is taken with a constant density of 2670 kg/m3. For the inner and outer zones, the respective integration radius of 10 km and 200 km have been chosen. We obtained very important RTM values ranging from -53.59 to 34.79 mGal. These values were added to the gravity anomalies grid of the EGM2008 model to improve their accuracy at high frequencies. On a part of the Cameroon Volcanic Line and its surroundings (mountainous area), we made a comparison between the residual Bouguer anomalies before and after refinement. We get differences ranging from -37.40 to 26.40 mGal. We concluded that the impact of omission error on gravimetric signatures is more felt on areas with a high variable topography, especially on the Cameroon Volcanic Line and around the localities of Takamanda, Essu, Dumbo and Ngambe. This shows the great influence that topography has on these gravity anomalies when it is not taken into account. We can conclude that in preparing a global geopotential model, a high resolution DTM has to be used to decrease the omission error. This means the degree of expansion has to increase in order to take the higher frequencies into account. The new refined Bouguer anomalies grid can be used in addition to terrestrial gravity anomalies in the study area, especially in mountainous areas where gravimetric data are very sparse or non-existent.
China's Mars probe, named Tianwen-1, including an orbiter and a landing rover, will be launched during the July-August 2020 Mars launch windows. Selected to be among the rover payloads is a Subsurface Penetrating Radar module (RoSPR). The main scientific objective of the RoSPR is to characterize the thickness and sub-layer distribution of the Martian soil. The RoSPR consists of two channels. The low frequency channel of the RoSPR will penetrate the Martian soil to depths of 10 to 100 m with a resolution of a few meters. The higher frequency channel will penetrate to a depth of 3 to 10 m with a resolution of a few centimeters. This paper describes the design of the instrument and some results of field experiments.
The first Mars exploration mission of China (Tianwen-1) is scheduled to be launched in 2020; a charged particle telescope, the Mars Energetic Particle Analyzer (MEPA), is carried as one of the payloads on the orbiter. The MEPA is designed to measure solar energetic particles (SEPs) and galactic cosmic rays (GCRs) in the near-Mars space and in the transfer orbit from Earth to Mars. Before the launch, the MEPA was calibrated in ground experiments with radioactive sources, electronic pulses, and accelerator beams. The calibration parameters, such as energy conversion constants, threshold values for the triggers, and particle identification criteria, were determined and have been stored for onboard use. The validity of the calibration parameters has been verified with radioactive sources and beams. The calibration results indicate that the MEPA can measure charged particles reliably, as designed, and that it can satisfy the requirements of the Tianwen-1 mission.
China's first Mars exploration mission will carry out comprehensive global surveys of the planet from data collected by instruments carried in orbit and roving on the planet itself. Goals of the mission include detailed inspections and surveys of key areas on the surface of Mars. One of the main scientific payloads installed on the orbiter is the moderate resolution camera. Its mission is to image the surface of Mars sufficiently to produce a global remote sensing image map of the planet, and to explore and record changes to the topography of Mars, including major geological structures, and to advance research on topography and geomorphology in general. The moderate resolution camera uses a lightweight and compact integrated design; its primary components are an optical module, a focal plane module, a camera control module, a power and interface module, a camera support module, a thermal control module, and a reference module. Radiometric calibration, color calibration, and geometric calibration have been carried out to ensure that the camera can acquire sufficient accurate data to complete mission goals. This paper introduces the camera's detection mission, its system composition, and its working principle; it also describes the camera's ground calibration tests and their results, and provides a reference for processing the camera's scientific data and for future applications.
The background and scientific objectives of the Mars Climate Station (MCS) for Tianwen-1 are introduced, accompanied by a comparative review of the status of related meteorological observation missions and of advanced sensing technologies. As one of the China Tianwen-1 Mission’s principal scientific payloads, the MCS contains four measurement sensors and one electronic processing unit that are specially designed to measure local temperature, pressure, wind, and sound on the Martian surface. The MCS’s measurement principles, technical schemes, ground calibration techniques, and adaptability evaluation to the Mars surface environment of MCS are introduced in details. The conclusion presents measurement performance specifications of the MCS, based on ground test results, that will provide guidance to future research based on data from the Tianwen-1 and later Mars missions.
As one of the seven scientific payloads on board the Tianwen-1 orbiter, the Mars Orbiter Magnetometer (MOMAG) will measure the magnetic fields of and surrounding Mars to study its space environment and the interaction with the solar wind. The instrument consists of two identical triaxial fluxgate magnetometer sensors, mounted on a 3.19 meter-long boom with a seperation of about 90 cm. The dual-magnetometers configuration will help eliminate the magnetic field interference generated by the spacecraft platform and payloads. The sensors are controlled by an electric box mounted inside the orbiter. Each magnetometer measures the ambient vector magnetic field over a wide dynamic range (to 10,000 nT per axis) with a resolution of 1.19 pT. Both magnetometers sample the ambient magnetic field at an intrinsic frequency of 128 Hz, but will operate in a model with alternating frequency between 1 and 32 Hz to meet telemetry allocations.
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 .
Electron pitch angle distributions similar to bidirectional electron conics (BECs) have been reported at Mars in previous studies based on analyses of Mars Global Surveyor measurements. BEC distribution, also termed “butterfly” distribution, presents a local minimum flux at 90° and a maximum flux before reaching the local loss cone. Previous studies have focused on 115 eV electrons that were produced mainly via solar wind electron impact ionization. Here using Solar Wind Electron Analyzer measurements made onboard the Mars Atmosphere and Volatile Evolution spacecraft, we identify 513 BEC events for 19–55 eV photoelectrons that were generated via photoionization only. Therefore, we are investigating electrons observed in regions well away from their source regions, to be distinguished from 115 eV electrons observed and produced in the same regions. We investigate the spatial distribution of the 19–55 eV BECs, revealing that they are more likely observed on the nightside as well as near strong crustal magnetic anomalies. We propose that the 19–55 eV photoelectron BECs are formed due to day-to-night transport and the magnetic mirror effect of photoelectrons moving along cross-terminator closed magnetic field lines.
Profiles of the Martian dayside ionosphere can be used to derive the neutral atmospheric densities at 130 km, which can also be obtained from the Mars Climate Database (MCD) and spacecraft aerobraking observations. In this research, we explain the method used to calculate neutral densities at 130 km via ionosphere observations and three long-period 130-km neutral density data sets at northern high latitudes (latitudes > 60°) acquired through ionospheric data measured by the Mars Global Surveyor (MGS) Radio Occultation Experiment. The calculated 130-km neutral density data, along with 130-km density data from the aerobraking observations of the MGS and Mars Odyssey (ODY) in the northern high latitudes, were compared with MCD outputs at the same latitude, longitude, altitude, solar latitude, and local time. The 130-km density data derived from both the ionospheric profiles and aerobraking observations were found to show seasonal variations similar to those in the MCD data. With a negative shift of about 2 × 1010 cm−3, the corrected 130-km neutral densities derived from MCD v4.3 were consistent with those obtained from the two different observations. This result means that (1) the method used to derive the 130-km neutral densities with ionospheric profiles was effective, (2) the MCD v4.3 data sets generally overestimated the 130-km neutral densities at high latitudes, and (3) the neutral density observations from the MGS Radio Science Experiment could be used to calibrate a new atmospheric model of Mars.
Unlike Earth, Mars lacks a global dipolar magnetic field but is dominated by patches of a remnant crustal magnetic field. In 2021, the Chinese Mars Rover will land on the surface of Mars and measure the surface magnetic field along a moving path within the possible landing region of 20°W–50°W, 20°N–30°N. One scientific target of the Rover is to monitor the variation in surface remnant magnetic fields and reveal the source of the ionospheric current. An accurate local crustal field model is thus considered necessary as a field reference. Here we establish a local crust field model for the candidate landing site based on the joint magnetic field data set from Mars Global Explorer (MGS) and Mars Atmosphere and Volatile Evolution (MAVEN) data combined. The model is composed of 1,296 dipoles, which are set on three layers but at different buried depths. The application of the dipole model to the joint data set allowed us to calculate the optimal parameters of their dipoles. The calculated results demonstrate that our model has less fitting error than two other state-of-the art global crustal field models, which would indicate a more reasonable assessment of the surface crustal field from our model.