The minimum-energy configuration for the magnetic field above the solar photosphere is curl-free (hence, by Ampere's law, also current-free), so can be represented as the gradient of a scalar potential. Since magnetic fields are divergence free, this scalar potential obeys Laplace's equation, given an appropriate boundary condition (BC). With measurements of the full magnetic vector at the photosphere, it is possible to employ either Neumann or Dirichlet BCs there. Historically, the Neumann BC was used with available line-of-sight magnetic field measurements, which approximate the radial field needed for the Neumann BC. Since each BC fully determines the 3D vector magnetic field, either choice will, in general, be inconsistent with some aspect of the observed field on the boundary, due to the presence of both currents and noise in the observed field. We present a method to combine solutions from both Dirichlet and Neumann BCs to determine a hybrid, "least-squares" potential field, which minimizes the integrated square of the residual between the potential and actual fields. This has advantages in both not overfitting the radial field used for the Neumann BC, and maximizing consistency with the observations. We demonstrate our methods with SDO/HMI vector magnetic field observations of AR 11158, and find that residual discrepancies between the observed and potential fields are significant, and are consistent with nonzero horizontal photospheric currents. We also analyze potential fields for two other active regions observed with two different vector magnetographs, and find that hybrid potential fields have significantly less energy than the Neumann fields in every case --- by more than 10^(32) erg in some cases. This has major implications for estimates of free magnetic energy in coronal field models, e.g., non-linear force-free field extrapolations.
The most violent space weather events (eruptive solar flares and coronal mass ejections) are driven by the release of free magnetic energy stored in the solar corona. Energy can build up on timescales of hours to days, and then may be suddenly released in the form of a magnetic eruption, which then propagates through interplanetary space, possibly impacting the Earth's space environment. Can we use the observed evolution of the magnetic and velocity fields in the solar photosphere to model the evolution of the overlying solar coronal field, including the storage and release of magnetic energy …
Filaments may be mistaken for coronal holes when observed in extreme ultraviolet (EUV) images; however, a closer and more careful look reveals that their photometric properties are different. The combination of EUV images with photospheric magnetograms shows some characteristic differences between filaments and coronal holes. We have performed analyses with 7 different SDO/AIA wavelengths (94, 131, 171, 211, 193, 304, 335~Å) and SDO/HMI magnetograms obtained in September 2011 and March 2012 to study coronal holes and filaments from the photometric, magnetic, and also geometric point of view, s…
Validating Coronal Magnetic Field Models Using Gaussian Separation
Sydney Institute for Astronomy, School of Physics, The University of Sydney（悉尼大学）；University of Wisconsin-Green Bay（威斯康星大学格林贝分校）；W.W. Hansen Experimental Physics Laboratory, Stanford University（斯坦福大学）；The University of Newcastle（纽卡斯尔大学）
The Coronal Global Evolutionary Model (CGEM) provides data-driven simulations of the magnetic field in the solar corona to better understand the build-up of magnetic energy that leads to eruptive events. The CGEM project has developed six capabilities. CGEM modules (1) prepare time series of full-disk vector magnetic field observations to (2) derive the changing electric field in the solar photosphere over active-region scales. This local electric field is (3) incorporated into a surface flux transport model that reconstructs a global electric field that evolves magnetic flux in a consistent w…
The coronal magnetic field evolution of AR 11437 is simulated by applying the magnetofrictional relaxation technique of Mackay et al. (2011). A sequence of photospheric line-of-sight magnetograms produced by SDO/HMI are used to drive the simulation and continuously evolve the coronal magnetic field of the active region through a series of non-linear force-free equilibria. The simulation is started during the first stages of the active region emergence so that its full evolution from emergence to decay can be simulated. A comparison of the simulation results with SDO/AIA observations show that …
We describe a new data product from the CGEM (Coronal Global Evolutionary Model) collaboration that estimates the Lorentz force in active regions (ARs) based on HMI vector magnetogram patches. Following Fisher et al. (2012), we compute three components of the integrated Lorentz force over the outer solar atmosphere every 12 minutes throughout an AR's disk passage. These estimates, differenced during solar eruptive events, can provide valuable diagnostics on dynamic processes. We describe the pipeline modules, provide data retrieval examples, and document some systematic uncertainties that user…
The solar coronal magnetic field produces solar activity, including extremely energetic solar flares and coronal mass ejections (CMEs). Knowledge of the structure and evolution of the magnetic field of the solar corona is important for investigating and understanding the origins of space weather. Although the coronal field remains difficult to measure directly, there is considerable interest in accurate modeling of magnetic fields in and around sunspot regions on the Sun using photospheric vector magnetograms as boundary data. In this work, we investigate effects of the size of the domain chos…
We investigate in detail the magnetic cause of minifilament eruptions that drive coronal-hole jets. We study 13 random on-disk coronal hole jet eruptions, using high resolution X-ray images from Hinode/XRT, EUV images from SDO/AIA, and magnetograms from SDO/HMI. For all 13 events, we track the evolution of the jet-base region and find that a minifilament of cool (transition-region-temperature) plasma is present prior to each jet eruption. HMI magnetograms show that the minifilaments reside along a magnetic neutral line between majority-polarity and minority-polarity magnetic flux patches. Thes…
Quantifying coronal magnetic field remains a central problem in solar physics. Nowadays the coronal magnetic field is often modelled using nonlinear force-free field (NLFFF) reconstructions, whose accuracy has not yet been comprehensively assessed. Here we perform a detailed casting of the NLFFF reconstruction tools, such as pi-disambiguation, photospheric field preprocessing, and volume reconstruction methods using a 3D snapshot of the publicly available full-fledged radiative MHD model. Specifically, from the MHD model we know the magnetic field vector in the entire 3D domain, which enables …
It is well known that magnetic fields dominate the dynamics in the solar corona, and new generation of numerical modelling of the evolution of coronal magnetic fields, as featured with boundary conditions driven directly by observation data, are being developed. This paper describes a new approach of data-driven magnetohydrodynamic (MHD) simulation of solar active region (AR) magnetic field evolution, which is for the first time that a data-driven full-MHD model utilizes directly the photospheric velocity field from DAVE4VM. We constructed a well-established MHD equilibrium based on a single v…
Generation and annihilation of three dimensional magnetic nulls in extrapolated solar coronal magnetic field: Data-based Implicit Large Eddy Simulation
Predicting Associations between Solar Flares and Coronal Mass Ejections Using SDO/HMI Magnetograms and a Hybrid Neural Network
Institute for Space Weather Sciences, New Jersey Institute of Technology（新泽西理工学院空间天气科学研究所）；Department of Computer Science, New Jersey Institute of Technology（新泽西理工学院计算机科学系）；Big Bear Solar Observatory, New Jersey Institute of Technology（新泽西理工学院大熊太阳天文台）；Center for Solar-Terrestrial Research, New Jersey Institute of Technology（新泽西理工学院日地研究中心）；Johns Hopkins University Applied Physics Laboratory（约翰霍普金斯大学应用物理实验室）；Research Center for Astronomy and Applied Mathematics, Academy of Athens（雅典学院天文学与应用数学研究中心）；Department of Astrophysical Sciences, Princeton University（普林斯顿大学天体物理科学系）
During the last few decades, the most widely favored models for coronal heating have involved the in situ dissipation of energy, with footpoint shuffling giving rise to multiple current sheets (the "nanoflare" model) or to Alfv{é}n waves that leak into the corona and undergo dissipative interactions (the wave heating scenario). As has been recognized earlier, observations suggest instead that the energy deposition is concentrated at very low heights, with the coronal loops being filled with hot, dense material from below, which accounts for their overdensities and flat temperature profiles. Wh…
Preprocess the Photospheric Vector Magnetograms for NLFFF Extrapolation using a Potential Field Model and an Optimization Method
日冕非线性无力磁场（NLFFF）的数值重建/外推通常以光球矢量磁图作为底边界输入。然而，在光球层观测到的磁场中含有与无力模型基本假设相冲突的力和不利于实际计算的测量噪声。Wiegelmann、Inhester和Sakurai（2006年）提出了对原始磁图进行预处理，以去除力和噪声，为NLFFF建模提供更好的输入。在本文中，我们开发了一种新的磁图预处理代码，它与我们的外推方法CESE-MHD-NLFFF（Jiang，Feng，and Xiang，2012; Jiang and Feng，2012）一致。根据磁场可分裂为有势场和无势场的磁分裂规律，对磁图进行分裂，将两部分分别处理。磁图的有势部分的预处理是基于一个数值势场模型，而无势部分是使用Wiegelmann等人（2006）的类似优化方法进行预处理。将该方法应用于SDO/HMI数据外推，结果表明，该方法能有效地去除外力和噪声，提高外推质量。
Recent observations show that the buildup and triggering of minifilament eruptions that drive coronal jets result from magnetic flux cancelation at the neutral line between merging majority- and minority-polarity magnetic flux patches. We investigate the magnetic setting of ten on-disk small-scale UV/EUV jets (jetlets), smaller than coronal X-ray jets but larger than chromospheric spicules) in a coronal hole by using IRIS UV images and SDOAIA EUV images and line-of-sight magnetograms from SDO/HMI. We observe recurring jetlets at the edges of magnetic network flux lanes in the coronal hole. Fro…
Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Fr…
{We report on the first of a series of studies on coronal bright points investigating the physical mechanism that generates these phenomena.} {The aim of this paper is to understand the magnetic-field structure that hosts the bright points.} {We use longitudinal magnetograms taken by the Solar Optical Telescope with the Narrowband Filter Imager. For a single case, magnetograms from the Helioseismic and Magnetic Imager were added to the analysis. The longitudinal magnetic field component is used to derive the potential magnetic fields of the large regions around the bright points. A magneto-sta…
A recent study demonstrated that freedom of convection and strength of magnetic field in the photospheric feet of active-region (AR) coronal loops, together, can engender or quench heating in them. Other studies stress that magnetic flux cancellation at the loop-feet potentially drives heating in loops. We follow 24-hour movies of a bipolar AR, using EUV images from SDO/AIA and line-of-sight (LOS) magnetograms from SDO/HMI, to examine magnetic polarities at the feet of 23 of the brightest coronal loops. We derived FeXVIII emission (hot-94) images (using the Warren et al. method) to select the …
We study the link between an expanding coronal shock and the energetic particles measured near Earth during the Ground Level Enhancement (GLE) of 17 May 2012. We developed a new technique based on multipoint imaging to triangulate the 3-D expansion of the shock forming in the corona. It uses images from three vantage points by mapping the outermost extent of the coronal region perturbed by the pressure front. We derive for the first time the 3-D velocity vector and the distribution of Mach numbers, $M_{FM}$, of the entire front as a function of time. Our approach uses magnetic field reconstruc…
3D picture of the coronal magnetic field remains an outstanding problem in solar physics, particularly, in active regions. Nonlinear force-free field reconstructions that employ routinely available full-disk photospheric vector magnetograms represent state-of-the-art coronal magnetic field modeling. Such reconstructions, however, suffer from an inconsistency between a force-free coronal magnetic field and non-force-free photospheric boundary condition, from which the coronal reconstruction is performed. In this study we focus on integrating the additional chromospheric and / or coronal magneti…
We analyse observations of a saddle-like structure in the corona above the western limb of the Sun on 2017 July 18. The structure was clearly outlined by coronal loops with typical coronal temperature no more than 1 MK. The dynamics of loops showed convergence toward the centre of the saddle in the vertical direction and divergence in the horizontal direction. The event is a clear example of smooth coronal magnetic field reconnection. No heating manifestations in the reconnection region or magnetically connected areas were observed. Potential magnetic field calculations, which use as the bound…
Deriving the Coronal Hole Electron Temperature: Electron Density Dependent Ionization/Recombination Considerations
将理论推导出的谱线比与观测数据进行比较，可以估算出等离子体的物理参数，如电子密度或温度。计算谱线比的通常做法是假设电子/质子激发，然后辐射衰变。此外，使用所谓的日冕近似是正常的，即只考虑电离和从基态的复合。更准确的处理方法是包括到亚稳态水平和从亚稳态水平的电离/复合。在这里，我们将其应用于相邻电离阶段的两条线; Mg IX 368 A和Mg X 625 A，这已被证明是一个非常有用的温度诊断。在典型的冕洞条件下的密度，假设零密度极限导出的电子温度与电子密度相关的电离/复合之间的差异很小。然而，对于电子密度大几个数量级的耀斑，情况并非如此。太阳活动最大时冕洞的温度约为1.04 MK，而太阳活动最小时的温度略低于0.82 MK。
The heliospheric magnetic field is of pivotal importance in solar and space physics. The field is rooted in the Sun's photosphere, where it has been observed for many years. Global maps of the solar magnetic field based on full disk magnetograms are commonly used as boundary conditions for coronal and solar wind models. Two primary observational constraints on the models are (1) the open field regions in the model should approximately correspond to coronal holes observed in emission, and (2) the magnitude of the open magnetic flux in the model should match that inferred from in situ spacecraft…
A turbulence index independent framework for deriving solar wind speed and coronal electron density from radio spectral broadening
Indian Institute of Technology Indore（印度印多尔理工学院）；Indian Space Research Organization, Vikram Sarabhai Space Centre（印度空间研究组织，维克拉玛·萨拉巴伊航天中心）；University of Kerala（喀拉拉大学）；The University of Tokyo（东京大学）；Kyoto Sangyo University（京都产业大学）
Observations of rapidly-rotating cool stars often show coronal slingshot prominences that remove mass and angular momentum when they are ejected. The derived masses of these prominences show a scatter of some two orders of magnitude. In order to investigate if this scatter could be intrinsic, we use a full magnetic cycle of solar magnetograms to model the coronal structure and prominence distribution in a young Sun, where we scale the field strength in the magnetograms with angular velocity according to $ B \propto Ω^{-1.32} $. We reproduce both the observed prominence masses and their scatter…
Solar flares are explosions in the solar atmosphere that release intense bursts of short-wavelength radiation and are capable of producing severe space-weather consequences. Flares release free energy built up in coronal fields, which are rooted in active regions (ARs) on the photosphere, via magnetic reconnection. The exact processes that lead to reconnection are not fully known and therefore reliable forecasting of flares is challenging. Recently, photospheric magnetic-field data has been extensively analysed using machine learning (ML) and these studies suggest that flare-forecasting accura…
Solar coronal rain is classified generally into two categories: flare-driven and quiescent coronal rain. The latter is observed to form along both closed and open magnetic field structures. Recently, we proposed that some of the quiescent coronal rain events, detected in the transition region and chromospheric diagnostics, along loop-like paths could be explained by the formation mechanism for quiescent coronal rain facilitated by interchange magnetic reconnection between open and closed field lines. In this study, we revisited 38 coronal rain reports from the literature. From these earlier wo…
Coronal Mass Ejections (CMEs) are one of the primary drivers of extreme space weather. They are large eruptions of mass and magnetic field from the solar corona and can travel the distance between Sun and Earth in half a day to a few days. Predictions of CMEs at 1 Astronomical Unit (AU), in terms of both its arrival time and magnetic field configuration, are very important for predicting space weather. Magnetohydrodynamic (MHD) modeling of CMEs, using flux-rope-based models is a promising tool for achieving this goal. In this study, we present one such model for CME simulations, based on spher…
Rapid Changes of Photospheric Magnetic Field after Tether-Cutting Reconnection and Magnetic Implosion
光球磁场的快速、不可逆变化被认为是太阳耀斑过程的一个重要因素。这封信报告了2011年2月13日NOAA AR 11158 M6.6耀斑期间磁场的快速变化，我们从日震和磁成像仪的矢量磁图中发现了12分钟的节奏。在非线性无力场（NLFFF）假设下，利用太阳耀斑极大值前后~-5.5，-1.5，1.5和4小时的高分辨率磁象图重建三维日冕磁场。紫外和硬X射线图像也被用来阐明磁场的演变和能量释放。快速变化主要由HMI在位于磁S形中心的紧凑区域中检测到，其中平均水平场强表现出28%的显著增加。该区域位于色球层中的初始强UV和硬X射线源之间，根据NLFFF模型，它们与S形的中央脚是同空间的。NLFFF模型进一步表明，强日冕电流集中在该地区的正上方，更有趣的是，日冕电流系统在S型爆发后经历了明显的向下崩溃。这些结果进行了讨论，有利于系绳切割重联产生的耀斑和随后的内爆的日冕领域所造成的能量释放。
Our recent observational study shows that the majority of coronal bright points (CBPs) in the quiet Sun are sources of one or more eruptions during their lifetime. Here, we investigate the non-potential time dependent structure of the magnetic field of the CBP regions with special emphasis on the time evolving magnetic structure at the spatial locations where the eruptions are initiated. The magnetic structure is evolved in time using a Non-Linear Force Free Field (NLFFF) relaxation approach, based on a time series of Helioseismic and Magnetic Imager (HMI) longitudinal magnetograms. This resul…
We have reported previously on a new method we are developing for using image-based information to improve global coronal magnetic field models. In that work we presented early tests of the method which proved its capability to improve global models based on flawed synoptic magnetograms, given excellent constraints on the field in the model volume. In this follow-up paper we present the results of similar tests given field constraints of a nature that could realistically be obtained from quality white-light coronagraph images of the lower corona. We pay particular attention to difficulties ass…
Multi-channel Observations of Plasma Outflows and the Associated Small-Scale Magnetic Field Cacellations on the Edges of an Active Region
利用SDO/AIA仪器，在活动区边界上沿两个不同的日冕开口环观测到连续和间歇的等离子体流出。%，速度为30 $\sim$ 200 km s $^{-1}$。通过对HMI/SDO时间序列磁图的研究发现，小尺度磁重联可能在冕环等离子体外流的产生中起着重要作用。结果表明，等离子体外流的起源与混合极性的小尺度磁场的位置相吻合，说明等离子体沿冕环的外流可能是小尺度闭合冕环与大尺度开放活动区冕环之间的磁重联的结果。
