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An improved method to derive to equivalent current systems from global MHD simulations

Zhang JiaoJiao; Wang Chi; Tang BinBinSubjects: Geosciences >> Space Physics

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. |

Phase structure of the Born-Infeld-anti-de Sitter black holes probed by non-local observables

Zeng, Xiao-Xiong; Liu, Xian-Ming; Li, Li-FangSubjects: Geosciences >> Space Physics

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. |

A holographic study on vector condensate induced by a magnetic field

Cai, Rong-Gen; He, Song; Li, Li; Li, Li-FangSubjects: Geosciences >> Space Physics

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. |

Vector condensate and AdS soliton instability induced by a magnetic field

Cai, Rong-Gen; Li, Li; Li, Li-Fang; Wu, YouSubjects: Geosciences >> Space Physics

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. |

Competition between s-wave order and d-wave order in holographic superconductors

Li, Li-Fang; Cai, Rong-Gen; Li, Li; Wang, Yong-QiangSubjects: Geosciences >> Space Physics

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. |

Entanglement entropy in a holographic p-wave superconductor model

Li, Li-Fang; Cai, Rong-Gen; Li, Li; Shen, ChaoSubjects: Geosciences >> Space Physics

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. |

[1 Pages/ 6 Totals]