分类： 地球科学 >> 空间物理学 提交时间： 2016-12-26
摘要： Independent clocks provide time tags for the precision orbit determination (POD) equipment and the radar altimeter onboard the HY-2A satellite, and a bias between POD data' time tag and corresponding range observation's time tag from the HY-2A altimeter exists. The time tag bias contributes a bias in the sea surface height observation due to the nonzero time rate of change of the HY-2A altimeter's height. A transponder for in-orbit radar altimeter calibration provides an approach to estimate the time tag bias. The altimeter receives the responding signals from the transponder and generates ranges. Pertinent reference ranges are obtained fromthe POD data and the transponder's coordinate. Using the ranges from the radar altimeter and the reference ranges, the time tag bias between the POD data and the altimeter observations can be estimated. During an in situ HY-2A altimeter calibration campaign using a reconstructive transponder from August 9, 2012, to July 20, 2014, 17 estimations of the altimeter's time tag bias were obtained. The preliminary results are presented in this letter.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-26
摘要： Derivation of equivalent current systems (ECS) from a global magnetospheric magnetohydrodynamics (MHD) model is very useful in studying magnetosphere-ionosphere coupling, ground induction effects, and space weather forecast. In this study we introduce an improved method to derive the ECS from a global MHD model, which takes account of the obliqueness of the magnetic field lines. By comparing the ECS derived from this improved method and the previous method, we find that the main characteristics of the ECS derived from the two methods are generally consistent with each other, but the eastward-westward component of the geomagnetic perturbation calculated from the ECS derived from the improved method is much stronger than that from the previous method. We then compare the geomagnetic perturbation as a function of the interplanetary magnetic field (IMF) clock angle calculated from the ECS derived from both methods with the observations. The comparison indicates that the improved method can improve the performance of the simulation. Furthermore, it is found that the incomplete counterbalance of the geomagnetic effect produced by the ionospheric poloidal current and field-aligned current (FAC) contributes to most of the eastward-westward component of geomagnetic perturbation.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-26
摘要： With the non-local observables such as two point correlation function and holographic entanglement entropy, we probe the phase structure of the Born-Infeld-anti-de Sitter black holes. For the case bQ > 0.5, where b is the Born-Infeld parameter and Q is the charge of the black hole, the phase structure is found to be similar to that of the Van der Waals phase transition, namely the black hole undergoes a first order phase transition and a second order phase transition before it reaches a stable phase. While for the case bQ < 0.5, a new phase branch emerges besides the Van der Waals phase transition. For the first order phase transition, the equal area law is checked, and for the second order phase transition, the critical exponent of the heat capacity is obtained. All these results are found to be the same as that observed in the entropy-temperature plane.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： With the assumption of radial motion and uniform longitudinal distribution of coronal mass ejections (CMEs), we propose a method to eliminate projection effects from the apparent observed CME latitude distribution. This method has been applied to SOHO LASCO data from 1996 January to 2006 December. As a result, we find that the real CME latitude distribution had the following characteristics: (1) High-latitude CMEs (theta > 60 degrees, where theta is the latitude) constituted 3% of all CMEs and mainly occurred during the time when the polar magnetic fields reversed sign. The latitudinal drift of the high-latitude CMEs was correlated with that of the heliospheric current sheet. (2) Four percent of all CMEs occurred in the range 45 degrees <= theta <= 60 degrees. These midlatitude CMEs occurred primarily in 2000, near the middle of 2002, and in 2005, forming a prominent three-peak structure. (3) The highest occurrence probability of low-latitude (theta < 45 degrees) CMEs was at the minimum and during the declining phase of the solar cycle. However, the highest occurrence rate of low-latitude CMEs was at the maximum and during the declining phase of the solar cycle. The latitudinal evolution of low-latitude CMEs did not follow the Sporer sunspot law, which suggests that many CMEs originated outside of active regions.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： We study a holographic model with vector condensate by coupling the anti-de Sitter gravity to an Abelian gauge field and a charged vector field in (3 + 1) dimensional spacetime. In this model there exists a non-minimal coupling of the vector field to the gauge field. We find that there is a critical temperature below which the charged vector condenses via a second order phase transition. The DC conductivity becomes infinite and the AC conductivity develops a gap in the condensed phase. We study the effect of a background magnetic field on the system. It is found that the background magnetic field can induce the condensate of the vector field even in the case without chemical potential/charge density. In the case with non-vanishing charge density, the transition temperature raises with the applied magnetic field, and the condensate of the charged vector operator forms a vortex lattice structure in the spatial directions perpendicular to the magnetic field.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： We continue to study the holographic p-wave superconductor model in the Einstein-Maxwell-complex vector field theory with a non-minimal coupling between the complex vector field and the Maxwell field. In this paper we work in the AdS soliton background which describes a conformal field theory in the confined phase and focus on the probe approximation. We find that an applied magnetic field can lead to the condensate of the vector field and the AdS soliton instability. As a result, a vortex lattice structure forms in the spatial directions perpendicular to the applied magnetic field. As a comparison, we also discuss the vector condensate in the Einstein-SU(2) Yang-Mills theory and find that in the setup of the present paper, the Einstein-Maxwell-complex vector field model is a generalization of the SU(2) model in the sense that the vector field has a general mass and gyromagnetic ratio.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： We study competition between s-wave order and d-wave order through two holographic superconductor models. We find that once the coexisting phase appears, it is always thermodynamically favored, and that the coexistence phase is narrow and one condensate tends to kill the other. The phase diagram is constructed for each model in terms of temperature and the ratio of charges of two orders. We further compare the behaviors of some thermodynamic quantities, and discuss the different aspects and identical ones between two models.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-22
摘要： In a recent paper, arXiv:1309.4877, a holographic p-wave model has been proposed in an Einstein Maxwell-complex vector field theory with a negative cosmological constant. The model exhibits rich phase structure depending on the mass and the charge of the vector field. We investigate the behavior of the entanglement entropy of dual field theory in this model. When the above two model parameters change, we observe the second order, first order and zeroth order phase transitions from the behavior of the entanglement entropy at some intermediate temperatures. These imply that the entanglement entropy can indicate not only the occurrence of the phase transition, but also the order of the phase transition. The entanglement entropy is indeed a good probe to phase transition. Furthermore, the "retrograde condensation" which is a sub-dominated phase is also reflected on the entanglement entropy. (C) 2015 The Authors. Published by Elsevier B.V.
分类： 地球科学 >> 空间物理学 提交时间： 2016-12-01
摘要： With the assumption of radial motion and uniform longitudinal distribution of coronal mass ejections (CMEs), we propose a method to eliminate projection effects from the apparent observed CME latitude distribution. This method has been applied to SOHO LASCO data from 1996 January to 2006 December. As a result, we find that the real CME latitude distribution had the following characteristics: (1) High-latitude CMEs (theta > 60 degrees, where theta is the latitude) constituted 3% of all CMEs and mainly occurred during the time when the polar magnetic fields reversed sign. The latitudinal drift of the high-latitude CMEs was correlated with that of the heliospheric current sheet. (2) Four percent of all CMEs occurred in the range 45 degrees <= theta <= 60 degrees. These midlatitude CMEs occurred primarily in 2000, near the middle of 2002, and in 2005, forming a prominent three-peak structure. (3) The highest occurrence probability of low-latitude (theta < 45 degrees) CMEs was at the minimum and during the declining phase of the solar cycle. However, the highest occurrence rate of low-latitude CMEs was at the maximum and during the declining phase of the solar cycle. The latitudinal evolution of low-latitude CMEs did not follow the Sporer sunspot law, which suggests that many CMEs originated outside of active regions.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-13
摘要： Superhalo electrons appear to be continuously present in the interplanetary medium, even during very quiet times, with a power-law spectrum at energies above similar to 2 keV. Here we numerically investigate the generation of superhalo electrons by magnetic reconnection in the solar wind source region, using magnetohydrodynamics and test particle simulations for both single X-line reconnection and multiple X-line reconnection. We find that the direct current electric field, produced in the magnetic reconnection region, can accelerate electrons from an initial thermal energy of T similar to 10(5) K up to hundreds of keV. After acceleration, some of the accelerated electrons, together with the nascent solar wind flow driven by the reconnection, propagate upwards along the newly-opened magnetic field lines into interplanetary space, while the rest move downwards into the lower atmosphere. Similar to the observed superhalo electrons at 1 AU, the flux of upward-traveling accelerated electrons versus energy displays a power-law distribution at similar to 2 - 100 keV, f (E) similar to E-delta, with a delta of similar to 1.5 - 2.4. For single (multiple) X-line reconnection, the spectrum becomes harder (softer) as the anomalous resistivity parameter alpha (uniform resistivity eta) increases. These modeling results suggest that the acceleration in the solar wind source region may contribute to superhalo electrons.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-13
摘要： We investigate equilibrium height of a flux rope, and its internal equilibrium in a realistic plasma environment by carrying out numerical simulations of the evolution of systems including a current-carrying flux rope. We find that the equilibrium height of a flux rope is approximately described by a power-law function of the relative strength of the background field. Our simulations indicate that the flux rope can escape more easily from a weaker background field. This further confirms that a catastrophe in the magnetic configuration of interest can be triggered by a decrease in strength of the background field. Our results show that it takes some time to reach internal equilibrium depending on the initial state of the flux rope. The plasma flow inside the flux rope due to the adjustment for the internal equilibrium of the flux rope remains small and does not last very long when the initial state of the flux rope commences from the stable branch of the theoretical equilibrium curve. This work also confirms the influence of the initial radius of the flux rope in its evolution;the results indicate that a flux rope with a larger initial radius erupts more easily. In addition, by using a realistic plasma environment and a much higher resolution in our simulations, we notice some different characteristics compared to previous studies in Forbes.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： MHD discontinuities are ubiquitous in the solar wind and are often found at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential discontinuities (TDs) and rotational discontinuities (RDs) are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been observationally investigated. Here, we present numerical results from a three-dimensional MHD simulation with pressure anisotropy and we define new methods for identifying and distinguishing TDs and RDs. Three statistical results obtained for the relative occurrence rates and heating effects are highlighted: (1) RDs tend to take up the majority of the discontinuities along with time;(2) the thermal states embedding TDs tend to be associated with extreme plasma parameters or instabilities while RDs do not;(3) TDs have a higher average T as well as perpendicular temperature T.. The simulation shows that TDs and RDs evolve and contribute to solar wind heating differently. These results will improve our understanding of the mechanisms that generate discontinuities and cause plasma heating.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Using a 2.5-dimensional MHD simulation, we investigate the role played by the inner coronal null point in the formation and evolution of solar quiescent prominences. The flux rope is characterized by its magnetic fluxes, the toroidal magnetic flux Phi(p) and the poloidal flux Phi(phi.) It is found that for a given Phi(p), the catastrophe does not occur in the flux rope system until Phi(phi) increases to a critical point. Moreover, the magnetic flux of the null point is the maximum value of the magnetic flux in the quadrupole background magnetic field, and represented by psi(N). The results show that the bigger psi(N) usually corresponds to the smaller catastrophic point, the lower magnetic energy of the flux rope system, and the lesser magnetic energy inside the flux rope. Our results confirm that catastrophic disruption of the prominence occurs more easily when there is a bigger psi(N). However, psi(N) has little influence on the maximum speed of the coronal mass ejections (CMEs) with an erupted prominence. Thus we argue that a topological configuration with the inner coronal null point is a necessary structure for the formation and evolution of solar quiescent prominences. In conclusion, it is easier for the prominences to form and to erupt as a core part of the CMEs in the magnetic structure with a greater psi(N.)
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Peter, Hardi
Tu, Chuanyi
Wang, Linghua
Zhang, Shaohua
Feng, Xueshang
摘要： The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to similar to 10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfven speed of similar to 1000 km s(-1). In the k-omega. diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15Mm from the wave source region, we estimate the energy flux of FMWs to be E similar to 7.0 x 10(6) erg cm(-2) s(-1), which is similar to 50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： We numerically study the detailed evolutionary features of the wave-like disturbance and its propagation in the eruption. This work is a follow-up to Wang et al., using significantly upgraded new simulations. We focus on the contribution of the velocity vortices and the fast shock reflection and refraction in the solar corona to the formation of the EUV waves. Following the loss of equilibrium in the coronal magnetic structure, the flux rope exhibits rapid motions and invokes the fast-mode shock at the front of the rope, which then produces a type II radio burst. The expansion of the fast shock, which is associated with outward motion, takes place in various directions, and the downward expansion shows the reflection and the refraction as a result of the non-uniform background plasma. The reflected component of the fast shock propagates upward and the refracted component propagates downward. As the refracted component reaches the boundary surface, a weak echo is excited. The Moreton wave is invoked as the fast shock touches the bottom boundary, so the Moreton wave lags the type II burst. A secondary echo occurs in the area where reflection of the fast shock encounters the slow-mode shock, and the nearby magnetic field lines are further distorted because of the interaction between the secondary echo and the velocity vortices. Our results indicate that the EUV wave may arise from various processes that are revealed in the new simulations.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Dynamic pressure pulses (DPPs) in the solar wind are a significant phenomenon closely related to the solar-terrestrial connection and physical processes of solar wind dynamics. In order to automatically identify DPPs from solar wind measurements, we develop a procedure with a three-step detection algorithm that is able to rapidly select DPPs from the plasma data stream and simultaneously define the transition region where large dynamic pressure variations occur and demarcate the upstream and downstream region by selecting the relatively quiet status before and after the abrupt change in dynamic pressure. To demonstrate the usefulness, efficiency, and accuracy of this procedure, we have applied it to the Wind observations from 1996 to 2008 by successfully obtaining the DPPs. The procedure can also be applied to other solar wind spacecraft observation data sets with different time resolutions.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Solar wind dynamic pressure pulse (DPP) structures, across which the dynamic pressure changes abruptly over timescales from a few seconds to several minutes, are often observed in the near-Earth space environment. The space weather effects of DPPs on the magnetosphere-ionosphere coupling system have been widely investigated in the last two decades. In this study, we perform a statistical survey on the properties of DPPs near 1 AU based on nearly 20 years of observations from the WIND spacecraft. It is found that only a tiny fraction of DPPs (around 4.2%) can be regarded as interplanetary shocks. For most DPPs, the total pressure (the sum of the thermal pressure and magnetic pressure) remains in equilibrium, but there also exists a small fraction of DPPs that are not pressure-balanced. The overwhelming majority of DPPs are associated with solar wind disturbances, including coronal mass ejection-related flows, corotating interaction regions, as well as complex ejecta. The annual variations of the averaged occurrence rate of DPPs are roughly in phase with the solar activity during solar cycle 23, and during the rising phase of solar cycle 24.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： Based on a hybrid galactic cosmic-ray transport model, which incorporated MHD global heliospheric data into Parker's cosmic-ray transport equation, we studied the behavior of the transport of galactic cosmic rays and the corresponding gradients in their flux near the heliopause (HP). We found that, (1) by increasing the ratio of the parallel diffusion coefficient to the perpendicular diffusion coefficient in the interstellar magnetic field of the outer heliosheath, the simulated radial flux near the HP increases as well. As the ratio multiplying factor reached 1010, the radial flux experienced a sudden jump near the HP, similar to what Voyager 1 observed in 2012. (2) The effect of changing the diffusion coefficients' ratio on the radial flux variation depends on the energy of the cosmic rays, the lower the energy, the more pronounced the effect is. (3) The magnitude of the diffusion coefficients also affect the radial flux near the HP, the modulation beyond the HP varies by adjusting the magnitude multiplying factor.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： A three-dimensional (3D) imager with a single-pixel detector and complementary intensity modulation of a digital micromirror device (DMD) array, which does not rely on scene raster scanning as in light detection and ranging (LIDAR) or on a two-dimensional array of sensors as used in time-of-flight (TOF) cameras, can not only capture full-color, high-quality images of real-life objects, but also recover the depth information and 3D reflectivity of the scene, reducing the required measurement dimension as well as the complexity, and cutting the cost of the detector array down to a single unit. The imager achieves spatial resolution using compressed sensing to exploit the sparsity of the signal. The disparity maps of the scene are reconstructed using sum of absolute or squared differences to reveal the depth information. This nonscanning, low-complexity 3D reflectivity imaging prototype may be of considerable value to various computer vision applications. (C) 2015 Optical Society of America
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： In this paper, we propose a method based on signal processing techniques to improve the performance of aperture antennas, such as a parabolic reflector, used to estimate the Direction of Arrival (DOA) of a signal. Three signal processing algorithms are investigated, including the correlation method, which is used to make an initial estimate of the incidence angle within a certain range. Then inverse matrix method and singular value decomposition method are subsequently utilized to refine the estimate within this range.
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