Calculating and Testing Nonlinear Force-Free Fields

Improvements to an existing method for calculating nonlinear force-free magnetic fields (Wheatland, Solar Phys. 238, 29, 2006) are described. In particular a solution of the 3-D Poisson equation using 2-D Fourier transforms is presented. The improved nonlinear force-free method is demonstrated in application to linear force-free test cases with localized nonzero values of the normal component of the field in the boundary. These fields provide suitable test cases for nonlinear force-free calculations because the boundary conditions involve localized nonzero values of the normal components of the field and of the current density, and because (being linear force-free fields) they have more direct numerical solutions. Despite their simplicity, fields of this kind have not been recognized as test cases for nonlinear methods before. The examples illustrate the treatment of the boundary conditions on current in the nonlinear force-free method, and in particular the limitations imposed by field lines that connect outside of the boundary region.     

Ground penetrating radar imaging of cap rock, caliche and carbonate strata

Field experiments show ground penetrating radar (GPR) can be used to image shallow carbonate stratigraphy effectively in a variety of settings. In south Florida, the position and structure of cap rock cover on limestone can be an important control on surface water flow and vegetation, but larger scale outcrops (tens of meters) of cap rock are sparse. GPR mapping through south Florida prairie, cypress swamp and hardwood hammock resolves variations in thickness and structure of cap rock to 3 m and holds the potential to test theories for cap rock–vegetation relationships. In other settings, carbonate strata are mapped to test models for the formation of local structural anomalies. A test of GPR imaging capabilities on an arid caliche (calcrete) horizon in southeastern Nevada shows depth penetration to 2 m with resolution of the base of caliche. GPR profiling also succeeds in resolving more deeply buried (5 m) limestone discontinuity surfaces that record subaerial exposure in south Florida.     

构造变形过程中盖层封闭性研究

研究了埋藏-成岩-抬升变形过程中盖层封闭性的演化特点,开展了构造变形过程中盖层破裂实验.认为泥岩在高演化阶段封闭性变差并不具有普遍性,在深埋条件下和构造稳定区,高演化泥岩仍可具有良好的封闭性;构造抬升是盖层封闭性发生破坏的主要因素;力学实验表明,膏盐岩盖层在极小的埋深(1 000 m左右)下就有很强的塑性,其极限强度很...     

An Improved Approach to Non-Force-Free Coronal Magnetic Field Extrapolation

We develop an approach to deriving the three-dimensional non-force-free coronal magnetic field from vector magnetograms. Based on the principle of minimum dissipation rate, a general non-force-free magnetic field is expressed as the superposition of one potential field and two constant-α (linear) force-free fields. Each is extrapolated from its bottom boundary data, providing the normal component only. The constant-α parameters are distinct and determined by minimizing the deviations between the numerically computed and measured transverse magnetic field at the bottom boundary. The boundary conditions required are at least two layers of vector magnetograms, one at the photospheric level and the other at the chromospheric level, presumably. We apply our approach to a few analytic test cases, especially to two nonlinear force-free cases examined by Schrijver et al. (Solar Phys. 235, 161, 2006). We find that for one case with small α parameters, the quantitative measures of the quality of our result are better than the median values of those from a set of nonlinear force-free methods. The reconstructed magnetic-field configuration is valid up to a vertical height of the transverse scale. For the other cases, the results remain valid to a lower vertical height owing to the limitations of the linear force-free-field solver. Because our method is based on the fast-Fourier-transform algorithm, it is much faster and easy to implement. We discuss the potential usefulness of our method and its limitations.     

Analysis of Reconstruction Methods for Nonlinear Force-Free Fields

We focus on quantitative evaluation of several methods for the reconstruction of force-free magnetic fields in the solar corona. We have studied two topics. The first is to carry out a comparison test of two approaches to implement the optimization method: (1) using a boundary layer with a weighting function and (2) involving an optimization algorithm for unknown boundary field vector values. The second approach is shown to provide a better approximation to a desired true solution that is finite in an unlimited semispace. The approximation obtained by the second approach is reasonably close to the true solution up to the boundaries of the reconstruction region. Among the applications of the optimization method to real data, we demonstrate its ability to obtain accurate enough energy estimates and find that the pre- to post-flare energy difference is sufficient in powering the flare.     

Limits for the Firehose Instability in Space Plasmas

Electromagnetic instabilities in high-β plasmas, where β is the ratio of the kinetic plasma energy to the magnetic energy, have a broad range of astrophysical applications. The presence of temperature anisotropies T _∥ /T _⊥ >1 (where ∥ and ⊥ denote directions relative to the background magnetic field) in solar flares and the solar wind is sustained by the observations and robust acceleration mechanisms that heat plasma particles in the parallel direction. The surplus of parallel kinetic energy can excite either the Weibel-like instability (WI) of the ordinary mode perpendicular to the magnetic field or the firehose instability (FHI) of the circularly polarized waves at parallel propagation. The interplay of these two instabilities is examined. The growth rates and the thresholds provided by the kinetic Vlasov – Maxwell theory are compared. The WI is the fastest growing one with a growth rate that is several orders of magnitude larger than that of the FHI. These instabilities are however inhibited by the ambient magnetic field by introducing a temperature anisotropy threshold. The WI admits a larger anisotropy threshold, so that, under this threshold, the FHI remains the principal mechanism of relaxation. The criteria provided here by describing the interplay of the WI and FHI are relevant for the existence of these two instabilities in any space plasma system characterized by an excess of parallel kinetic energy.     

Quasi-Periodic Releases of Streamer Blobs and Velocity Variability of the Slow Solar Wind near the Sun

We search for persistent and quasi-periodic release events of streamer blobs during 2007 with the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory and assess the velocity of the slow solar wind along the plasma sheet above the corresponding streamer by measuring the dynamic parameters of blobs. We find ten quasi-periodic release events of streamer blobs lasting for three to four days. In each day of these events, we observe three – five blobs. The results are in line with previous studies using data observed near the last solar minimum. Using the measured blob velocity as a proxy for that of the mean flow, we suggest that the velocity of the background slow solar wind near the Sun can vary significantly within a few hours. This provides an observational manifestation of the large velocity variability of the slow solar wind near the Sun.     

3D Coronal Structures and Magnetic Field During the Total Solar Eclipse of 29 March 2006

The good quality of the observing sequence of about 60 photographs of the white-light corona taken during the total solar eclipse observations on 29 March 2006, in Al Sallum, Egypt, enable us to use a new method of image processing for enhancement of the fine structure of coronal phenomena. We present selected magnetic-field lines derived for different parameters of the extrapolation model. The coincidence of the observed coronal white-light fine structures and the computed field-line positions provides a 3D causal relationship between coronal structures and the coronal magnetic field.     

2010.0年代蒙古地区岩石磁异常

采用2011~2012年期间在蒙古国境内地磁测量得到的119点地磁数据,利用以球冠谐和和曲面样条方法得到的蒙古地区地磁场数值为基础数据,获得了2010.0年代蒙古地区岩石磁异常的空间分布,对蒙古地区岩石磁异常的空间分布进行了研究。结果表明,蒙古地区的岩石磁异常表现出东西差异,异常幅度在西部较大东部较小。蒙古西部南边界处表现为类弧形结构和贝加尔—乌兰巴托条带结构为蒙古地区较为显著的磁异常结构。