# EPP

ISSN  2096-3955

CN  10-1502/P

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SOLID EARTH: SEISMOLOGY
Recently Published , doi: 10.26464/epp2020026
[Abstract](92) [FullText HTML](26) [PDF 4108KB](8)
Abstract:
The Tanlu 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 80 km × 70 km 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 Tanlu 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 Tanlu fault zone, which were probably partially derived from the magmatic activity of Luzong volcanic basin. From the Late Cretaceous to Early Tertiary, the Tanlu 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 Tanlu fault zone in the west. Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation.
PLANETARY SCIENCES
Recently Published , doi: 10.26464/epp2020025
[Abstract](105) [FullText HTML](58) [PDF 335KB](4)
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Solar energy is the primary driving force behind a planet’s climate system, and surface albedo plays a key role in determining the energy budget of the planet. Coupling the Snow, Ice, and Aerosol Radiation (SNICAR) with the Laboratoire de Météorologie Dynamique (LMD) Mars General Circulation Model (MGCM) to create a new coupled model leads to an approximately 4% drop in the net CO2 ice deposition on Mars. Newly simulated surface albedo affects the concentration of gaseous species in the Martian atmosphere (condensation-sublimation cycle). The new set-up also impacts the solar energy available in the atmosphere. These two effects together lead to subsequent and significant changes in other chemical species in the Martian atmosphere. Compared with results of the MGCM model alone, in the new coupled model CO2 (gas) and O3 show a drop of about 1.17% and 8.59% in their respective concentrations, while H2O (vapor) and CO show an increase of about 13.63% and 0.56% in their respective concentrations. Among trace species, OH shows a maximum increase of about 29.44%, while the maximum drop of 11.5% is observed in the O concentration. Photochemically neutral species such as Ar and N2 remain unaffected by the albedo changes.
ATMOSPHERIC PHYSICS
Recently Published , doi: 10.26464/epp2020024
[Abstract](104) [FullText HTML](58) [PDF 1442KB](6)
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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.
PLANETARY SCIENCES
Recently Published , doi: 10.26464/epp2020013
[Abstract](136) [FullText HTML](82) [PDF 960KB](4)
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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 \begin{document}${{T_{\rm e}}\gg {T_{\rm i}}}$\end{document}. We find that the time evolution of ⅡA instability and the formation of electrostatic shock strongly depend on initial cavity density. The initial position of the electrostatic shock is dependent on the ratio between initial cavity density and background solar wind density, i.e., the farther away the initial position, the lower is the ratio. When the initial cavity density is low enough, the density and electric field profile across the wake become much complex. Meanwhile, the back-to-back electrostatic shock is unstable in the case of lower cavity densities; at the late evolution stage, a new shock-like structure can be formed at the central region of the lunar wake.

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Accepted Articles , doi: 10.26464/epp2020034
[Abstract](38) [PDF](0)
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Using the 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 the statistical results of plasmaspheric hiss distribution, we establish a third-order polynomial function of L-shell, magnetic local time (MLT), magnetic latitude (MLAT) and AE* as the fitting model. Quantitative comparisons indicate that the empirical model in terms of fitted functions can reflect favorably the major features of the global distribution of hiss wave intensity including the MLT asymmetry and substorm dependence. Our results therefore provide an empirically useful analytic model to be readily used for future global simulations of radiation belt electron dynamics under the impact of plasmaspheric hiss waves in geospace.

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Planetary Sciences
2020, 4(2): 89 -94   doi: 10.26464/epp2020014
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To infer the internal equilibrium structure of a gaseous planet, especially the equation of state (EOS) and size of its inner core, requires accurate determination of lower-order zonal gravitational coefficients. Modeling of the gravitational signature associated with deep zonal circulation depends critically upon reliable subtraction of the dynamical components from totally derived gravitational coefficients. In the era of the Juno mission and the Grand Finale phase of the Cassini mission, it is timely and necessary to revisit and examine the so-called ‘Thermal Wind Equation (TWE)’, which has been extensively utilized to diagnose the dynamical parts of the gravitational fields measured by the two spacecrafts. TWE treats as negligible a few terms in the full equation of balance. However, the self-gravitational anomaly of the distorted fluid, unlike oblateness effects of solid-body rotation, is not a priori minor and thus should not be neglected in the name of approximation. Another equation, the ‘Thermal Gravitational Wind Equation (TGWE)’, includes this important additional term; we compare it with the TWE and show that physically the TGWE models a fundamentally different balance from the TWE and delivers numerical results considerably different from models based on the TWE. We conclude that the TWE balance cannot be relied upon to produce realistic convection models. Only after the TGWE balance is obtained can the relative importance of terms be assessed. The calculations we report here are based on two types of zonal circulations that are produced by realistically possible convections inside planets, instead of being constructed or assumed.
PLANETARY SCIENCES
2020, 4(2): 95 -104   doi: 10.26464/epp2020015
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Decametric (DAM) radio emissions are one of the main windows through which one can reveal and understand the Jovian magnetospheric dynamics and its interaction with the moons. DAMs are generated by energetic electrons through cyclotron-maser instability. For Io (the most active moon) related DAMs, the energetic electrons are sourced from Io volcanic activities, and quickly trapped by neighboring Jovian magnetic field. To properly interpret the physical processes behind DAMs, it is important to precisely locate the source field lines from which DAMs are emitted. Following the work by Hess et al. (2008, 2010), we develop a method to locate the source region as well as the associated field lines for any given DAM emission recorded in a radio dynamic spectrum by, e.g., Wind/WAVES or STEREO/WAVES. The field lines are calculated by the state-of-art analytical model, called JRM09 (Connerney et al., 2018). By using this method, we may also derive the emission cone angle and the energy of associated electrons. If multiple radio instruments at different perspectives observe the same DAM event, the evolution of its source region and associated field lines is able to be revealed. We apply the method to an Io-DAM event, and find that the method is valid and reliable. Some physical processes behind the DAM event are also discussed.
PLANETARY SCIENCES
2020, 4(2): 105 -110   doi: 10.26464/epp2020016
Abstract:
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.
PLANETARY SCIENCES
2020, 4(2): 111 -119   doi: 10.26464/epp2020017
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Measurements of Jupiter's gravity field by Juno have been acquired with unprecedented precision, but uncertainties in the planet’s hydrogen–helium equation of state (EOS) and the hydrogen–helium phase separation have meant that differences remain in the interior model predictions. We deduce an empirical EOS from Juno gravity field observations in terms of the hydrostatic equation and then investigate the structure and composition of Jupiter by comparison of the empirical EOS with Jupiter's adiabats obtained from the physical EOS. The deduced helium mass fraction suggests depletion of helium in the outermost atmosphere and helium concentration in the inner molecular hydrogen region, which is a signature of helium rain in Jupiter's interior. The deduced envelope metallicity (the heavy-element mass fraction) is as high in the innermost envelope as 11–13 times the solar value. Such a high metallicity provides sharp support to the dilute core model with the heavy elements dissolved in hydrogen and expanded outward. No matter how the core mass is varied, the empirical EOS derived from the two-layer interior model generally suggests higher densities in the innermost envelope than does the best-fit Jupiter's adiabat; this result is, again, a signature of dilute cores in Jupiter's interior. Moreover, no matter the core mass, the empirical EOS is found to exhibit an inflexion point in the deep interior, around 10 Mbar, which can be explained as the combined effect of helium concentration in the upper part and dilute cores in the lower part.
SPACE PHYSICS: IONOSPHERIC PHYSICS
2020, 4(2): 120 -130   doi: 10.26464/epp2020018
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As a dispersive wave mode produced by lightning strokes, tweek atmospherics provide important hints of lower ionospheric (i.e., D-region) electron density. Based on data accumulation from the WHU ELF/VLF receiver system, we develop an automatic detection module in terms of the maximum-entropy-spectral-estimation (MESE) method to identify unambiguous instances of low latitude tweeks. We justify the feasibility of our procedure through a detailed analysis of the data observed at the Suizhou Station (31.57°N, 113.32°E) on 17 February 2016. A total of 3961 tweeks were registered by visual inspection; the automatic detection method captured 4342 tweeks, of which 3361 were correct ones, producing a correctness percentage of 77.4% (= 3361/4342) and a false alarm rate of 22.6% (= 981/4342). A Short-Time Fourier Transformation (STFT) was also applied to trace the power spectral profiles of identified tweeks and to evaluate the tweek propagation distance. It is found that the fitting accuracy of the frequency–time curve and the relative difference of propagation distance between the two methods through the slope and through the intercept can be used to further improve the accuracy of automatic tweek identification. We suggest that our automatic tweek detection and analysis method therefore supplies a valuable means to investigate features of low latitude tweek atmospherics and associated ionospheric parameters comprehensively.
SPACE PHYSICS: MAGNETOSPHERIC PHYSICS
2020, 4(2): 131 -137   doi: 10.26464/epp2020019
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In this paper, we present evolutions of the phase space density (PSD) spectra of ring current (RC) ions based on observations made by Van Allen Probe B during a geomagnetic storm on 23–24 August 2016. By analyzing PSD spectra ratios from the initial phase to the main phase of the storm, we find that during the main phase, RC ions with low magnetic moment μ values can penetrate deeper into the magnetosphere than can those with high μ values, and that the μ range of PSD enhancement meets the relationship: S(O+) >S(He+) >S(H+). Based on simultaneously observed ULF waves, theoretical calculation suggests that the radial transport of RC ions into the deep inner magnetosphere is caused by drift-bounce resonance interactions, and the efficiency of these resonance interactions satisfies the relationship: η(O+) > η(He+) > η(H+), leading to the differences in μ range of PSD enhancement for different RC ions. In the recovery phase, the observed decay rates for different RC ions meet the relationship: R(O+) > R(He+) > R(H+), in accordance with previous theoretical calculations, i.e., the charge exchange lifetime of O+ is shorter than those of H+ and He+.
SPACE PLASMA PHYSICS
2020, 4(2): 138 -150   doi: 10.26464/epp2020020
Abstract:
In this paper we show that two significant phenomena of magnetospheric chorus emission can be explained by the participation of beam-like electron structures, created by Landau-resonant interaction with growing oblique whistler waves. The first concerns the widely observed spectral gap near half the electron cyclotron frequency Ωe; the second is related to the observation of very obliquely propagating lower-band waves that cannot be directly generated by temperature anisotropy. Concerning the gap, kinetic dispersion theory reveals that interference of the beam-related cyclotron mode ω~Ωe-kVb with the conventional whistler mode leads to mode splitting and the appearance of a ‘forbidden’ area in the ω-k space. Thereby the beam velocity Vb appears as an essential parameter. It is directly related to the phase velocity of the most unstable whistler wave mode, which is close to VAe/2 for sufficiently hot electrons (VAe is the electron Alfven velocity). To clarify the second point, we show that Landau-resonant beams with Vb < VAe/2, which arise in cold plasmas from unstable upper-band waves, are able to generate lower-band whistler mode waves at very oblique propagation (θ ≥ 60°). Our studies demonstrate the important role of Landau-resonant electrons in nonlinear whistler wave generation in the magnetosphere.
SOLID EARTH: GEODYNAMICS
2020, 4(2): 151 -162   doi: 10.26464/epp2020021
Abstract:
Seismic hazard analysis is gaining increased attention in the present era because of the catastrophic effects of earthquakes. Scientists always have as a goal to develop new techniques that will help forecast earthquakes before their reoccurrence. In this research, we have performed a shear failure experiment on rock samples with prefabricated cracks to simulate the process of plate movement that forms strike-slip faults. We studied the evolution law of the deformation field to simulate the shear failure experiment, and these results gave us a comprehensive understanding of the elaborate strain distribution law and its formation process with which to identify actual fault zones. We performed uniaxial compression tests on marble slabs with prefabricated double shear cracks to study the distribution and evolution of the deformation field during shear failure. Analysis of the strain field at different loading stages showed that with an increase in the load, the shear strain field initially changed to a disordered-style distribution. Further, the strain field was partially concentrated and finally completely concentrated near the crack and then distributed in the shape of a strip along the crack. We also computed coefficients of variation (CVs) for the physical quantities u, v, and exy, which varied with the load. The CV curves were found to correspond to the different loading stages. We found that at the uniform deformation stage, the CV value was small and changed slowly, whereas at the later nonuniform deformation stage, the CV value increased sharply and changed abruptly. Therefore, the precursor to a rock sample breakdown can be predicted by observing the variation characteristics of CV statistics. The correlation we found between our experimental and theoretical results revealed that our crack evolution and sample deformation results showed good coupling with seismic distribution characteristics near the San Andreas Fault.
SOLID EARTH: SEISMOLOGY
2020, 4(2): 163 -177   doi: 10.26464/epp2020022
Abstract:
In order to understand the crustal structure and tectonic background of the Changning–Gongxiang area, southeastern Sichuan Province, where a series of moderate-to-strong earthquakes occurred in recent years, we utilized the seismic phase data both from a local dense array and from the regional seismic networks; we used the tomoDD program to invert for the high-resolution three-dimensional velocity structure within the depth range of 0–10 km and for accurate hypocentral locations in this area. We analyzed the seismogenic structures for the events of Xingwen M5.7 in 2018 and Gongxian M5.3 and Changning M6.0 in 2019. The results show that: (1) widespread lateral inhomogeneity exists in the velocity structure of the study area, and the location of the velocity anomaly is largely consistent with known structures. In the range of distinguishable depth, the inhomogeneity decreases with increasing depth, and the velocity structure anomalies in some areas are continuous in depth; (2) earthquakes occurred in clusters, showing the characteristics of zonal folding trends in the NW-SE and NE-SW directions; the focal depth in the area is generally shallow in both the sedimentary cap and the crystalline basement. The seismogenic structures of small earthquake clusters are different in size and occurrence in different sections, and the clusters occurred mostly in regions with high P- or S-wave velocities; (3) synthesis of a variety of data suggests that the seismogenic structures of the Xingwen M5.7 and Changning M6.0 earthquakes are associated with slip faults that trend NW-SE in, respectively, the south wing and the axis of the Changning–Shuanghe anticline, while that of the Gongxian M5.3 earthquake is associated with thrust faults that trend N-S in the Jianwu syncline region. The dynamic sources of the three earthquakes are all from the SE pushing of the Qinghai–Tibet block on the Sichuan basin; (4) the risk of future strong earthquakes in this area must be reevaluated in light of the facts (a) that in recent years, moderate-to-strong earthquake swarms have occurred frequently in southeast Sichuan; (b) that the complex structural area exhibits the easy-to-trigger characteristic, and (c) that the small-scale faults in this area are characterized by the phenomenon of stress “lock and release”.
2017, 1(1): 2-12   doi: 10.26464/epp2017002
[Abstract](3557) [FullText HTML](1018) [PDF](205)
2017, 1(1): 13-25   doi: 10.26464/epp2017003
[Abstract](3172) [FullText HTML](1204) [PDF](177)
2017, 1(1): 26-34   doi: 10.26464/epp2017004
[Abstract](4491) [FullText HTML](936) [PDF](150)
2018, 2(1): 52-66   doi: 10.26464/epp2018005
[Abstract](2637) [FullText HTML](833) [PDF](142)
2018, 2(4): 257-275   doi: 10.26464/epp2018025
[Abstract](2151) [FullText HTML](318) [PDF](115)
2018, 2(1): 1-14   doi: 10.26464/epp2018001
[Abstract](3514) [FullText HTML](823) [PDF](105)
2018, 2(1): 67-73   doi: 10.26464/epp2018006
[Abstract](2148) [FullText HTML](984) [PDF](101)
2018, (2): 89-111   doi: 10.26464/epp2018011
2017, 1(1): 58-62   doi: 10.26464/epp2017008
[Abstract](2548) [FullText HTML](1017) [PDF](95)
2019, 3(4): 314-329   doi: 10.26464/epp2019033

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