ISSN  2096-3955

CN  10-1502/P

2019 Vol.3(1)

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Excitation of extremely low-frequency chorus emissions: The role of background plasma density
XiongDong Yu, ZhiGang Yuan, ShiYong Huang, Fei Yao, Zheng Qiao, John R. Wygant, Herbert O. Funsten
2019, 3(1): 1-7. doi: 10.26464/epp2019001
Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts. However, the mechanism (s) generating these low-frequency chorus emissions have not been well understood. . In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 fce (typical ordinary chorus) to 0.02 fce (extremely low-frequency chorus). Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density Ne was found to be several times larger than has been associated with observations of ordinary chorus waves. For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates (calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 fce when the background plasma density Ne increases. Especially when Ne reaches 90 cm–3 or more, the lowest unstable frequency can extend to 0.02 fce or even less, which is consistent with satellite observations. Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely low-frequency chorus waves by controlling the wave growth rates.
Magnetosphere response to the IMF turning from north to south
JianYong Lu, HanXiao Zhang, Ming Wang, ChunLi Gu, HaiYan Guan
2019, 3(1): 8-16. doi: 10.26464/epp2019002
In this paper, the Space Weather Modeling Framework (SWMF) is used to simulate the real-time response of the magnetosphere to a solar wind event on June 5, 1998, in which the interplanetary magnetic field shifted its direction from north to south. Since most current models do not take into account convective effects of the inner magnetosphere, we first study the importance of Rice Convection Model (RCM) in the global model. We then focus on the following four aspects of the magnetosphere’s response: the magnetosphere’s density distribution, the structure of its magnetic field lines, the area of the polar cap boundary, and the corresponding ionospheric current change. We find that (1) when the IMF changes from north to south in this event, high magnetosheath density is observed to flow downstream along the magnetopause with the solar wind; low-latitude reconnection at dayside occurs under the southward IMF, while the magnetic field lines in the tail lobe caudal, caused by the nightside high latitude reconnection, extend into the interplanetary space. Open magnetic field lines exist simultaneously at both high and low latitudes at the magnetopause; (2) the area of the polar cap is obviously increased if the IMF turns from the north to the south; this observation is highly consistent with empirical observations; (3) the ionospheric field align current in the northern hemisphere is stronger than in the southern hemisphere and also increases as the IMF changes from north to south. SWMF with the Rice Convection effect provides reliable modeling of the magnetospheric and ionospheric response to this solar wind variation.
Electron acceleration in interaction of magnetic islands in large temporal-spatial turbulent magnetic reconnection
BoJing Zhu, Hui Yan, David A Yuen, YaoLin Shi
2019, 3(1): 17-25. doi: 10.26464/epp2019003
A new combined Fermi, betatron, and turbulent electron acceleration mechanism is proposed in interaction of magnetic islands during turbulent magnetic reconnection evolution in explosive astrophysical phenomena at large temporal-spatial scale (LTSTMR), the ratio of observed current sheets thickness to electron characteristic length, electron Larmor radius for low-β and electron inertial length for high-β, is on the order of 1010–1011; the ratio of observed evolution time to electron gyroperiod is on the order of 107–109). The original combined acceleration model is known to be one of greatest importance in the interaction of magnetic islands; it assumes that the continuous kinetic-dynamic temporal-spatial scale evolution occurs as two separate independent processes. In this paper, we reconsider the combined acceleration mechanism by introducing a kinetic-dynamic-hydro full-coupled model instead of the original micro-kinetic or macro-dynamic model. We investigate different acceleration mechanisms in the vicinity of neutral points in magnetic islands evolution, from the stage of shrink and breakup into smaller islands (kinetic scale), to the stage of coalescence and growth into larger islands (dynamic scale), to the stages of constant and quasi-constant (contracting-expanding) islands (hydro scale). As a result, we give for the first time the acceleration efficiencies of different types of acceleration mechanisms in magnetic islands’ interactions in solar atmosphere LTSTMR activities (pico-, 10–2–105 m; nano-, 105–106 m; micro-, 106–107 m; macro-, 107–108 m; large-, 108–109 m).
Choice of suitable regional and residual gravity maps, the case of the South-West Cameroon zone
Fidèle Koumetio, Donatien Njomo, Constant Tatchum Noutchogwe, Eric Ndoh Ndikum, Sévérin Nguiya, Alain-Pierre Kamga Tokam
2019, 3(1): 26-32. doi: 10.26464/epp2019004
The quantitative interpretation of gravity anomalies due to shallow structures needs separation between long wavelength anomalies (regional anomalies) and short wavelength anomalies (residual anomalies). The regional-residual field separation can be carried out using the polynomial method. In this case, the so-called regional field of order n is treated as a polynomial of degree n. The present study shows that the degree n must vary between a smallest value nmin and a maximum value nmax. This article presents a method to process gravity data that allows determination of nmin and nmax for a given study area. We apply the method to gravity data of the South-West Cameroon zone. In this chosen study area, we find that regional anomaly maps of orders ranging from 1 to 9 and residual anomaly maps of orders ranging from 1 to 8 can be used for suitable interpretation. The analyses show that one may need residual anomaly maps of several orders to perform satisfactory quantitative interpretation of the different intrusive bodies found in a given area.
Lineament characteristics using gravity data in the Garoua Zone, North Cameroon: Natural risks implications
Kokea Ariane Darolle Fofie, Fidèle Koumetio, Jean Victor Kenfack, David Yemele
2019, 3(1): 33-44. doi: 10.26464/epp2019009
The Garoua Zone in North Cameroon, the subject of this study, is known to have undergone tectonic movements during the Cretaceous, but the zone’s structural data remain poorly known. This study exploits the Bouguer anomaly to improve knowledge of Garoua tectonics structures. In order to characterize these structures, two methods are used: Euler’s deconvolution method and the method of the horizontal gradient of the vertical derivative. Superposition of the Euler’s solutions map for index N=1 with the map from the horizontal gradient of the vertical derivative method allows determination of gravimetric lineaments, interpreted as faults or as linear contacts, from which we deduce a structural map of the study area. Based on this map, we identify sixteen lineaments, of which we count eight as linear contacts and eight as faults. Among the faults, we denote one of depth between 4 and 8 km, five faults of depth ranging between 8 and 13 km, and two faults of depths between 13 and 36 km. Analysis of these faults shows that the seven deepest faults might present a natural risk in our study area. For purposes of civil protection, such deep faults should be monitored and taken into consideration in the implementation of large public works. The structural map, established herein from data on the in-depth extension of each fault, thus increases scientific knowledge in the area that can be used to site public works in ways that reduce risk.
Crustal strain rates of southeastern Tibetan Plateau derived from GPS measurements and implications to lithospheric deformation of the Shan-Thai terrane
KeLiang Zhang, ShiMing Liang, WeiJun Gan
2019, 3(1): 45-52. doi: 10.26464/epp2019005
The link between the crustal deformation and mantle kinematics in the Tibetan Plateau has been well known thanks to dense GPS measurements and the relatively detailed anisotropy structure of the lithospheric mantle. However, whether the crust deforms coherently with the upper mantle in the Shan-Thai terrane (also known as the Shan-Thai block) remains unclear. In this study, we investigate the deformation patterns through strain rate tensors in the southeastern Tibetan Plateau derived from the latest GPS measurements and find that in the Shan-Thai terrane the upper crust may be coupled with the lower crust and the upper mantle. The GPS-derived strain rate tensors are in agreement with the slipping patterns and rates of major strike-slip faults in the region. The most prominent shear zone, whose shear strain rates are larger than 100×10–9 a–1, is about 1000-km-long in the west, trending northward along Sagaing fault to the Eastern Himalayan Syntaxis in the north, with maximum rate of compressive strain up to –240×10–9 a–1. A secondary shear zone along the Anninghe-Xiaojiang Fault in the east shows segmented shear zones near several conjunctions. While the strain rate along RRF is relatively low due to the low slip rate and low seismicity there, in Lijiang and Tengchong several local shear zones are present under an extensional dominated stress regime that is related to normal faulting earthquakes and volcanism, respectively. Furthermore, by comparing GPS-derived strain rate tensors with earthquake focal mechanisms, we find that 75.8% (100 out of 132) of the earthquake T-axes are consistent with the GPS-derived strain rates. Moreover, we find that the Fast Velocity Direction (FVDs) at three depths beneath the Shan-Thai terrane are consistent with extensional strain rate with gradually increasing angular differences, which are likely resulting from the basal shear forces induced by asthenospheric flow associated with the oblique subduction of the India plate beneath the Shan-Thai terrane. Therefore, in this region the upper crust deformation may be coherent with that of the lower crust and the lithospheric mantle.
Contemporary crustal tectonic movement in the southern Sichuan-Yunnan block based on dense GPS observation data
HongLin Jin, Yuan Gao, XiaoNing Su, GuangYu Fu
2019, 3(1): 53-61. doi: 10.26464/epp2019006
We analyzed 360 permanent and campaign GPS data from 1999 to 2017 in the southern Sichuan-Yunan block, and obtained crustal horizontal deformation in this region. Then, we derived the strain rate using a multi-scale spherical wavelet method. Results reveal a complex pattern of tectonic movement in the southern Sichuan-Yunnan block. Compared to the stable Eurasian plate, the maximum rate of the horizontal deformation in the southern Sichuan-Yunnan block is approximately 22 mm/a. The Xiaojiang fault shows a significantly lower deformation—a left-lateral strike-slip movement of 9.5 mm/a. The Honghe fault clearly shows a complex segmental deformation from the north to south. The northern Honghe fault shows 4.3 mm/a right strike-slip with 6.7 mm/a extension; the southern Honghe fault shows 1.9 mm/a right strike-slip with 1.9 mm/a extension; the junction zone in the Honghe and Lijiang–Xiaojinhe faults shows an obvious clockwise-rotation deformation. The strain calculation results reveal that the maximum shear-strain rate in this region reaches 70 nstrain/a, concentrated around the Xiaojiang fault and at the junction of the Honghe and Lijiang–Xiaojinhe faults. We note that most of the earthquakes with magnitudes of 4 and above that occurred in this region were within the high shear strain-rate zones and the strain rate gradient boundary zone, which indicates that the magnitude of strain accumulation is closely related to the seismic activities. Comparison of the fast shear-wave polarization direction of the upper-crust with the upper-mantle anisotropy and the direction of the surface principal compressive strain rate obtained from the inversion of the GPS data reveals that the direction of the surface principal compressive strain is basically consistent with the fast shear-wave polarization direction of the upper crust anisotropy, but different from the polarization direction of the upper mantle. Our results support the hypothesis that the principal elements of the deformation mechanism in the southern Sichuan-Yunnan block are decoupling between the upper and lower crust and ductile flow in the lower crust.
Seismic evidence for the existence of an entrained mantle flow coupling the northward advancing Indian plate under Tibet
Yu Zou, XiaoBo Tian, YouQiang Yu, Fa-Bin Pan, LingLing Wang, XiaoBo He
2019, 3(1): 62-68. doi: 10.26464/epp2019007
The Tibetan Plateau, known as " the roof of the world” and " the third pole of the earth”, is a product of the collision between India and Asia during the last ~50 Ma. The regional tectonics–in particular, growth and expansion of the plateau–has been attributed primarily to deformation within the lithosphere. The role and pattern of the underlying asthenospheric flow, however, remain mostly unaddressed. In light of recent seismic tomographic images and published seismic anisotropic descriptions of the upper mantle, here we propose that an entrained mantle flow is likely to exist under Tibet, induced by the northward advancing Indian plate. The direction of mantle flow is characterized by a gradual rotation from northward in the south to eastward in the north as a result of deflection by the deep root of the Tarim block. The presence of an underlying mantle flow is not only able to account for the west-east oriented fast-axis of seismic anisotropy in northern Tibet, but can also adequately explain the sporadic null splitting in southern Tibet. Specifically, the null splitting results, at least in part, from upwellings of asthenospheric flow through tears of the underthrusting Indian plate that have been revealed by various seismic observations. The mantle flow may in turn promote the block extrusion under Tibet that has been observed in GPS measurements; hot asthenospheric upwellings may also lead to widespread post-collisional magmatism in southern Tibet.
Crustal thicknesses and Poisson's ratios beneath the Chuxiong-Simao Basin in the Southeast Margin of the Tibetan Plateau
ZiQi Zhang, Yuan Gao
2019, 3(1): 69-84. doi: 10.26464/epp2019008
In the Southeast Margin of the Tibetan Plateau, low-velocity sedimentary layers that would significantly affect the accuracy of the H-κ stacking of receiver functions are widely distributed. In this study, we use teleseismic waveform data of 475 events from 97 temporary broadband seismometers deployed by ChinArray Phase I to obtain crustal thicknesses and Poisson’s ratios within the Chuxiong-Simao Basin and adjacent area, employing an improved method in which the receiver functions are processed through a resonance-removal filter, and the H-κ stacking is time-corrected. Results show that the crustal thickness ranges from 30 to 55 km in the study area, reaching its thickest value in the northwest and thinning toward southwest, southeast and northeast. The apparent variation of crustal thickness around the Red River Fault supports the view of southeastern escape of the Tibetan Plateau. Relatively thin crustal thickness in the zone between Chuxiong City and the Red River Fault indicates possible uplift of mantle in this area. The positive correlation between crustal thickness and Poisson’s ratio is likely to be related to lower crust thickening. Comparison of results obtained from different methods shows that the improved method used in our study can effectively remove the reverberation effect of sedimentary layers.