We perform a statistical analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) project on the global distribution of protons in the Martian magnetosheath. Our results show that the proton number densities distribution has a south-north asymmetry. This south-north asymmetry is most likely caused by the south-north asymmetric distributions of the crustal magnetic fields at Mars. The strong crustal magnetic fields push the inner boundary of magnetosheath to a higher altitude in the southern hemisphere. Due to the outward movement of the inner boundary of the magnetosheath, a compressed magnetosheath forms, causing subsequent increases in proton number densities, thermal pressure, and total pressure. Eventually, a balance is reached between the increased total pressure inside the magnetosheath and the increased magnetic pressure inside the induced magnetosphere. Our statistical study suggests that the Martian crustal magnetic fields can strongly affect the distributions of proton number densities in the Martian magnetosheath.
O++ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O++ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O++ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O+ at high altitudes, respectively, in response to the varying degree of O ionization. O++ destruction is shown to occur mainly via charge exchange with CO2 at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O++ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O++ escape on Mars, characterized by an escape rate of 6×1022 s–1.
The Chinese Chang'e-3 mission landed close to the eastern rim of the ~450 m diameter Ziwei crater. Regional stratigraphy of the landing site and impact excavation model suggest that the bulk continuous ejecta deposits of the Ziwei crater are composed by Erathothenian-aged mare basalts. Along the traverse of the Yutu rover, the western segment features a gentle topographic uplift (~0.5 m high over ~4 m), which is spatially connected with the structurally-uplifted crater rim. Assuming that this broad topographic uplift has physical properties discontinuous with materials below, we use data returned by the high-frequency lunar penetrating radar onboard the Yutu rover to estimate the possible range of relative permittivity for this topographic uplift. Only when the relative permittivity is ~9 is the observed radar reflection consistent with the observed topography, suggesting that the topographic uplift is composed of basaltic blocks that were excavated by the Ziwei crater. This result is consistent both with the impact excavation model that predicts deeper basaltic materials being deposited closer to the crater rim, and with observation of numerous half-buried boulders on the surface of this hill. We note that this study is the first to use topography and radargram data to estimate the relative permittivity of lunar surface uplifts, an approach that has had many successful applications on Mars. Similar approaches can apply other ground penetrating radar data for the Moon, such as will be available from the ongoing Chang'e-4 mission.
The ion-to-electron temperature ratio is a good indicator of the processes involved in the plasma sheet. Observations have suggested that patchy reconnection and the resulting earthward bursty bulk flows (BBFs) transport may be involved in causing the lower temperature ratios at smaller radial distances during southward IMF periods. In this paper, we estimate theoretically how a patchy magnetic reconnection electric field can accelerate ions and electrons differently. If both ions and electrons are non-adiabatically accelerated only once within each reconnection, the temperature ratio would be preserved. However, when reconnection occurs closer to the Earth where magnetic field lines are shorter, particles mirrored back from the ionosphere can cross the reconnection region more than once within one reconnection; and electrons, moving faster than ions, can have more crossings than do ions, leading to electrons being accelerated more than ions. Thus as particles are transported from tail to the near-Earth by BBFs through multiple reconnection, electrons should be accelerated by the reconnection electric field more times than are ions, which can explain the lower temperature ratios observed closer to the Earth.
Previous studies have shown that the ionospheric responses to a solar flare are significantly dependent on the solar zenith angle (SZA): the ionospheric responses are negatively related to the SZAs. The largest enhancement in electron density always occurs around the subsolar point. However, from 2001 to 2014, the global distribution of total electron content (TEC) responses showed no obvious relationship between the increases in TEC and the SZA during some solar flares. During these solar flares, the greatest enhancements in TEC did not appear around the subsolar point, but rather far away from the subsolar point. The distribution of TEC enhancements showed larger TEC enhancements along the same latitude. The distribution of anomalous ionospheric responses to the solar flares was not structured the same as traveling ionospheric disturbances. This anomaly distribution was also unrelated to the distribution of background neutral density. It could not be explained by changes in the photochemical process induced by the solar flares. Thus, the transport process could be one of the main reasons for the anomaly distribution of ionospheric responses to the solar flares. This anomaly distribution also suggests that not only the photochemical process but also the transport process could significantly affect the variation in ionospheric electron density during some solar flares.
Water vapor in the stratosphere makes a significant contribution to global climate change by altering the radiative energy budget of the Earth’s climate system. Although many previous studies have shown that the El Niño–Southern Oscillation (ENSO) has significant effects on the water vapor content of the stratosphere in terms of the annual or seasonal mean, a comprehensive analysis of the seasonal evolution of these effects is still required. Using reanalysis data and satellite observations, we carried out a composite analysis of the seasonal evolution of stratospheric water vapor during El Niño/La Niña peaks in winter and decays in spring. The ENSO has a distinct hysteresis effect on water vapor in the tropical lower stratosphere. The El Niño/La Niña events moisten/dry out the tropical lower stratosphere in both winter and spring, whereas this wetting/dehydration effect is more significant in spring. This pattern is due to a warmer temperature in the upper troposphere and lower stratosphere during the El Niño spring phase, which causes more water vapor to enter the stratosphere, and vice versa for La Niña. This delayed warming/cooling in the lower stratosphere during the El Niño/La Niña decay in spring leads to the seasonal evolution of ENSO effects on water vapor in the lower stratosphere.
On May 12, 2008, an Mw7.9 earthquake occurred in Wenchuan County, Sichuan Province, China. Movement of Yingxiu–Beichuan Fault in the Longmenshan Fault Zone was considered to be the main cause of the earthquake. Earthquakes are closely related to fault activities. Therefore, studying the strain distribution and evolution process around active fault zones is important to the understanding of seismic activities. In this study, we conduct laboratory experiments with uniaxial compression applied to marble sheets with intentionally fabricated cracks. The speckle patterns of the rock samples under different loading conditions are recorded in real time by a digital camera. To calculate the deformation fields of the deliberately cracked marble sheets during different stages of the loading processes, the recorded images are processed by the digital image correlation method. The distribution and variation of the displacement and strain are further analyzed in order to understand the strain localization of and observed damage in the experimental fracture zones. Finally, we compare these laboratory results with the GPS-observed coseismic displacements during the 2008 Wenchuan earthquake, to assess the consistency between our laboratory observations and the field observations of the earthquake, but also to suggest how laboratory results can improve thinking about how earthquake patterns do and do not reflect fault patterns.
Late at night on 17 June 2019, a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County, Sichuan, China, becoming the largest earthquake recorded within the southern Sichuan Basin. A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock, and we thus refer to these earthquakes as the Changning M6 earthquake sequence (or swarm). The mainshock was located very close to a salt mine, into which for ~3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km. It was also near (within ~15 km) the epicenter of the 18 December 2018 M5.7 Xingwen earthquake, which is thought to have been induced by shale gas hydraulic fracturing (HF), prompting questions about the possible involvement of industrial activities in the M6 sequence. Following previous studies, this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block. Except for a period of serious water loss after the start of cross-well injection in 2005–2006, the frequency of earthquakes shows a slightly increasing tendency. Overall, there is a good correlation between the event rate in the Shuanghe area and the loss of injected water. More than 400 M ≥ 3 earthquakes, including 40 M ≥ 4 and 5 M ≥ 5 events, had been observed by the end of August 2019. Meanwhile, in the Shangluo area, seismicity has increased during drilling and HF operations (mostly in vertical wells) since about 2009, and dramatically since the end of 2014, coincident with the start of systematic HF in the area. The event rate shows a progressively increasing background with some fluctuations, paralleling the increase in HF operations. More than 700 M ≥ 3 earthquakes, including 10 M ≥ 4 and 3 M ≥ 5 in spatially and temporally clustered seismic events, are correlated closely with active fracturing platforms. Well-resolved centroid moment tensor results for M ≥ 4 earthquakes were shown to occur at very shallow depths around shale formations with active HF, in agreement with some of the clusters, which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located. After the Xingwen M5.7 earthquake, seismic activity in the salt well area increased significantly. The Xingwen earthquake may have created a unidirectional rupture to the NNW, with an end point close to the NW-trending fault of the Shuanghe earthquake. Thus, a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake, possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.
Low-rise buildings are susceptible to high-frequency ground motion. The high-frequency ground motions at regional distances are mainly controlled by crustal Lg waves whose amplitudes are typically much larger than those of body waves. In this study, we develop a Lg-wave Q model for the Sichuan and Yunnan region in the frequency band of 0.3–2.0 Hz using regional seismic records of 1166 earthquakes recorded at 152 stations. Comparison between the observed pattern of ground motion from real earthquake and model prediction demonstrates the robustness and effectiveness of our Lg-Q model. Then, assuming that the Lg-wave Q structure is the main factor affecting the propagation of the high-frequency ground motions, we calculate the spatial distributions of high-frequency ground motions from scenario earthquakes at different locations in the region using the average Lg-wave attenuation model over the frequency band of 0.3–2.0 Hz. We also use the Lg-Q model to estimate the distribution of cumulative energy of high-frequency ground motions based on the historical seismicity of the Sichuan and Yunnan region. Results show that the Lg-Q model can be used effectively in estimating the spatial distribution of high-frequency seismic energies and thus can contribute to the assessment of seismic hazard to low-rise buildings.
The thermal structure of the lower mantle plays a key role in understanding the dynamic processes of the Earth's evolution and mantle convection. Because intrinsic attenuation in the lower mantle is highly sensitive to temperature, determining of the attenuation of the lower mantle could help us determine its thermal state. We attempted to constrain the attenuation of the lower mantle by measuring the amplitude ratios of p to ScP on the vertical component and s to ScS on the tangential component at short epicentral distances for seismic wave data from deep earthquakes in Northeast China. We calculated the theoretical amplitude ratios of p to ScP and s to ScS by using ray theory and the axial-symmetric spectral element method AxiSEM, as well as by considering the effects of radiation patterns, geometrical spreading, and ScP reflection coefficients. By comparing the observed amplitude ratios with the synthetic results, we constrained the quality factors as Qα ≈ 3,000 and Qβ ≈ 1,300 in the lower mantle beneath Northeast China, which are much larger than those in the preliminary reference Earth model (PREM) model of Qα ~800 and Qβ ~312. We propose that the lower mantle beneath Northeast China is relatively colder than the average mantle, resulting in weaker intrinsic attenuation and higher velocity. We estimated the temperature of the lower mantle beneath Northeast China as approximately 300–700 K colder than the global average value.
The Bayesian inversion method is a stochastic approach based on the Bayesian theory. With the development of sampling algorithms and computer technologies, the Bayesian inversion method has been widely used in geophysical inversion problems. In this study, we conduct inversion experiments using crosshole seismic travel-time data to examine the characteristics and performance of the stochastic Bayesian inversion based on the Markov chain Monte Carlo sampling scheme and the traditional deterministic inversion with Tikhonov regularization. Velocity structures with two different spatial variations are considered, one with a chessboard pattern and the other with an interface mimicking the Mohorovičić discontinuity (Moho). Inversions are carried out with different scenarios of model discretization and source–receiver configurations. Results show that the Bayesian method yields more robust single-model estimations than the deterministic method, with smaller model errors. In addition, the Bayesian method provides the posterior probabilistic distribution function of the model space, which can help us evaluate the quality of the inversion result.