Comparing extrapolations of the coronal magnetic field structure at 2.5 solar radii with multi-viewpoint coronagraphic observations
C. Sasso, R. F. Pinto, V. Andretta, R. A. Howard, A. Vourlidas, A. Bemporad, S. Dolei, D. Spadaro, R. Susino, E. Antonucci, L. Abbo, V. Da Deppo, S. Fineschi, F… 展开作者
C. Sasso, R. F. Pinto, V. Andretta, R. A. Howard, A. Vourlidas, A. Bemporad, S. Dolei, D. Spadaro, R. Susino, E. Antonucci, L. Abbo, V. Da Deppo, S. Fineschi, F. Frassetto, F. Landini, G. Naletto, G. Nicolini, P. Nicolosi, M. Pancrazzi, M. Romoli, D. Telloni, R. Ventura
The magnetic field shapes the structure of the solar corona but we still know little about the interrelationships between the coronal magnetic field configurations and the resulting quasi-stationary structures observed in coronagraphic images (as streamers, plumes, coronal holes). One way to obtain information on the large-scale structure of the coronal magnetic field is to extrapolate it from photospheric data and compare the results with coronagraphic images. Our aim is to verify if this comparison can be a fast method to check systematically the reliability of the many methods available to reconstruct the coronal magnetic field. Coronal fields are usually extrapolated from photospheric measurements typically in a region close to the central meridian on the solar disk and then compared with coronagraphic images at the limbs, acquired at least 7 days before or after to account for solar rotation, implicitly assuming that no significant changes occurred in the corona during that period. In this work, we combine images from three coronagraphs (SOHO/LASCO-C2 and the two STEREO/SECCHI-COR1) observing the Sun from different viewing angles to build Carrington maps covering the entire corona to reduce the effect of temporal evolution to ~ 5 days. We then compare the position of the observed streamers in these Carrington maps with that of the neutral lines obtained from four different magnetic field extrapolations, to evaluate the performances of the latter in the solar corona. Our results show that the location of coronal streamers can provide important indications to discriminate between different magnetic field extrapolations.
The existence of shocks driven by Coronal Mass Ejections (CMEs) has always been assumed based on the superalfvenic speeds for some of these events and on indirect evidence such as radio bursts and distant streamer deflections. However, the direct signature of the plasma enhancement at the shock front has escaped detection until recently. Since 2003, work on LASCO observations has shown that CME-driven shocks can be detected by white light coronagraph observations from a few solar radii to at least 20 Rsun. Shock properties, such as the density compression ratio and their direction can be extra…
Identifying coronal sources of L1 solar wind disturbances using the Fisk heliospheric magnetic field and potential field extrapolations during three solar minima
We present a new improved version of our force-free electrodynamics (FFE) numerical code in spherical coordinates that extrapolates the magnetic field in the inner solar corona from a photospheric vector magnetogram. The code satisfies the photospheric boundary condition and the condition divB=0 to machine accuracy. The performance of our method is evaluated with standard convergence parameters, and is found to be comparable to that of other nonlinear force-free extrapolations.
In the solar corona, magnetic flux rope is believed to be a fundamental structure accounts for magnetic free energy storage and solar eruptions. Up to the present, the extrapolation of magnetic field from boundary data is the primary way to obtain fully three-dimensional magnetic information of the corona. As a result, the ability of reliable recovering coronal magnetic flux rope is important for coronal field extrapolation. In this paper, our coronal field extrapolation code (CESE-MHD-NLFFF, Jiang & Feng 2012) is examined with an analytical magnetic flux rope model proposed by Titov & Demouli…
It is widely believed that loops observed in the solar atmosphere trace out magnetic field lines. However, the degree to which magnetic field extrapolations yield field lines that actually do follow loops has yet to be studied systematically. In this paper we apply three different extrapolation techniques - a simple potential model, a NLFF model based on photospheric vector data, and a NLFF model based on forward fitting magnetic sources with vertical currents - to 15 active regions that span a wide range of magnetic conditions. We use a distance metric to assess how well each of these models …
The coronal magnetic field cannot be directly observed, but in principle it can be reconstructed from the comparatively well observed photospheric magnetic field. A popular approach uses a nonlinear force-free model. Non-magnetic forces at the photosphere are significant meaning the photospheric data are inconsistent with the force-free model, and this causes problems with the modeling (De Rosa et al., Astrophys. J. 696, 1780, 2009). In this paper we present a numerical implementation of the Grad-Rubin method for reconstructing the coronal magnetic field using a magnetostatic model. This model…
Solar magnetic fields play a key role in understanding the nature of the coronal phenomena. Global coronal magnetic fields are usually extrapolated from photospheric fields, for which farside data is taken when it was at the frontside, about two weeks earlier. For the first time we have constructed the extrapolations of global magnetic fields using frontside and artificial intelligence (AI)-generated farside magnetic fields at a near-real time basis. We generate the farside magnetograms from three channel farside observations of Solar Terrestrial Relations Observatory (STEREO) Ahead (A) and Be…
Shocks driven by Coronal Mass Ejections (CMEs) are primary agents of space weather. They can accelerate particles to high energies and can compress the magnetosphere thus setting in motion geomagnetic storms. For many years, these shocks were studied only in-situ when they crossed over spacecraft or remotely through their radio emission spectra. Neither of these two methods provides information on the spatial structure of the shock nor on its relationship to its driver, the CME. In the last decade, we have been able to not only image shocks with coronagraphs but also measure their properties r…
Due to the absence of direct measurement, the magnetic field in the solar corona is usually extrapolated from the photosphere in numerical way. At the moment, the nonlinear force-free field (NLFFF) model dominates the physical models for field extrapolation in the low corona. Recently we have developed a new NLFFF model with MHD relaxation to reconstruct the coronal magnetic field. This method is based on CESE--MHD model with the conservation-element/solution-element (CESE) spacetime scheme. In this paper, we report the application of the CESE--MHD--NLFFF code to \SDO/HMI data with magnetogram…
The determination of the speed of Coronal Mass Ejections (CMEs) is usually done by tracking brighter features (such as the CME front and core) in visible light coronagraphic images and by deriving unidimensional profiles of the CME speed as a function of altitude or time. Nevertheless, CMEs are usually characterized by the presence of significant density inhomogeneities propagating outward with different radial and latitudinal projected speeds, resulting in a complex evolution eventually forming the Interplanetary CME. In this work, we demonstrate for the first time how coronagraphic image seq…
Magnetic reconnections (MRs) for various magnetic field line (MFL) topologies are believed to be the initiators of various solar eruptive events like flares and coronal mass ejections (CMEs). Consequently, important is a thorough understanding and quantification of the MFL topology and their evolution which leads to MRs. Contemporary standard is to extrapolate the coronal MFLs using equilibrium models where the Lorentz force on the coronal plasma is zero everywhere, because either there is no current or the current is parallel to the magnetic field. In tandem, a non-force-free-field (NFFF) ext…
We develop a nonlinear force-free field (NLFFF) extrapolation code based on the magnetohydrodynamic (MHD) relaxation method. We extend the classical MHD relaxation method in two important ways. First, we introduce an algorithm initially proposed by cite{2002JCoPh.175..645D} to effectively clean the numerical errors associated with $nabla cdot vec{B}$. Second, the multi-grid type method is implemented in our NLFFF to perform direct analysis of the high-resolution magnetogram data. As a result of these two implementations, we successfully extrapolated the high resolution force-free field introdu…
Two extrapolation models of the solar coronal magnetic field are compared using magnetogram data from the SDO/HMI instrument. The two models, a horizontal current-current sheet-source surface (HCCSSS) model and a potential field-source surface (PFSS) model differ in their treatment of coronal currents. Each model has its own critical variable, respectively the radius of a cusp surface and a source surface, and it is found that adjusting these heights over the period studied allows better fit between the models and the solar open flux at 1 AU as calculated from the Interplanetary Magnetic Field…
Solar filament are commonly thought to be supported in magnetic dips, in particular, of magnetic flux ropes (FRs). In this Letter, from the observed photospheric vector magnetogram, we implement a nonlinear force-free field (NLFFF) extrapolation of a coronal magnetic FR that supports a large-scale intermediate filament between an active region and a weak polarity region. This result is the first in that current NLFFF extrapolations with presence of FRs are limited to relatively small-scale filaments that are close to sunspots and along main polarity inversion line (PIL) with strong transverse …
The SDO/HMI instruments provide photospheric vector magnetograms with a high spatial and temporal resolution. Our intention is to model the coronal magnetic field above active regions with the help of a nonlinear force-free extrapolation code. Our code is based on an optimization principle and has been tested extensively with semi-analytic and numeric equilibria and been applied before to vector magnetograms from Hinode and ground based observations. Recently we implemented a new version which takes measurement errors in photospheric vector magnetograms into account. Photospheric field measure…
The magnetic field plays an essential role in the initiation and evolution of different solar phenomena in the corona. The structure and evolution of the 3D coronal magnetic field are still not very well known. A way to get the 3D structure of the coronal magnetic field is by performing magnetic field extrapolations from the photosphere to the corona. In previous work, it was shown that by prescribing the 3D reconstructed loops' geometry, the magnetic field extrapolation finds a solution with a better agreement between the modeled field and the reconstructed loops. Also, it improves the qualit…
The Plasma-prescribed Active Region Static Extrapolation (PARSE) Dataset: A Machine-Learning-Ready Collection of Magnetohydrostatic Coronal Active Regions
Accurate forecasting of the solar wind has grown in importance as society becomes increasingly dependent on technology that is susceptible to space weather events. This work describes an inner boundary condition for ambient solar wind models based on tomography maps of the coronal plasma density gained from coronagraph observations, providing a novel alternative to magnetic extrapolations. The tomographical density maps provide a direct constraint of the coronal structure at heliocentric distances of 4 to 8Rs, thus avoiding the need to model the complex non-radial lower corona. An empirical in…
The solar wind consists of continuous streams of charged particles that escape into the heliosphere from the Sun, and is split into fast and slow components, with the fast wind emerging from the interiors of coronal holes. Near the ecliptic plane, the fast wind from low-latitude coronal holes is interspersed with a highly structured slow solar wind, the source regions and drivers of which are poorly understood. Here we report extreme-ultraviolet observations that reveal a spatially complex web of magnetized plasma structures that persistently interact and reconnect in the middle corona. Corona…
We developed a {\sl coronal non-linear force-free field (COR-NLFFF)} forward-fitting code that fits an approximate {\sl non-linear force-free field (NLFFF)} solution to the observed geometry of automatically traced coronal loops. In contrast to photospheric NLFFF codes, which calculate a magnetic field solution from the constraints of the transverse photospheric field, this new code uses coronal constraints instead, and this way provides important information on systematic errors of each magnetic field calculation method, as well as on the non-forcefreeness in the lower chromosphere. In this s…
We use rotation stereoscopy to estimate the height of a steady-state solar feature relative to the photosphere, based on its apparent motion in the image plane recorded over several days of observation. The stereoscopy algorithm is adapted to work with either one- or two-dimensional data (i.e. from images or from observations that record the projected position of the source along an arbitrary axis). The accuracy of the algorithm is tested on simulated data, and then the algorithm is used to estimate the coronal radio source heights associated with the active region NOAA 10956, based on multifr…
Refinement of global coronal and interplanetary magnetic field extrapolations constrained by remote-sensing and in-situ observations at the solar minimum
针对太阳磁场外推中的 Open Flux Problem，提出三个评估参数结合遥感与就地观测约束日冕和行星际磁场，确定了太阳极小期 PFSS+PFCS 和 CSSS 模型的最优自由参数，发现使用 HMI 综观图可缓解该问题。
We have performed numerical magnetohydrodynamic (MHD) simulations of two closed active regions (AR). The input magnetic field values were the coronal magnetic field computed as extrapolation coronal from observations of the photospheric magnetic field. The studied active regions, NOAA AR12565 and AR12567, were registered as different bipolar region. Our investigation, the 3D coronal extrapolations, as well as the numerical MHD experiments, revealed that actually they evolved together as a quadrupolar active region. The second region emerged later under the loops system of AR12565 and separated…
Magnetic reconnection is one of the primary mechanisms for triggering solar eruptive events, but direct observation of its rapid process has been of challenge. In this Letter we present, using a nonlinear force-free field (NLFFF) extrapolation technique, a visualization of field line connectivity changes resulting from tether-cutting reconnection over about 30 minutes during the 2011 February 13 M6.6 flare in NOAA AR 11158. Evidence for the tether-cutting reconnection was first collected through multiwavelength observations and then by the analysis of the field lines traced from positions of f…
We study the role of the coronal magnetic field configuration of an active region in determining the propagation direction of a coronal mass ejection (CME). The CME occurred in the active region 11944 (S09W01) near the disk center on 2014 January 7 and was associated with an X1.2 flare. A new CME reconstruction procedure based on a polarimetric technique is adopted, which shows that the CME changed its propagation direction by around 28$^\circ$ in latitude within 2.5 R$_\odot$ and 43$^\circ$ in longitude within 6.5 R$_\odot$ with respect to the CME source region. This significant non-radial mo…
What Determines the Brightness of the Magnetically Open Solar Corona?: Insights from Three-dimensional Radiative Magnetohydrodynamic Simulations and Observations
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 …
This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions.Our results show no correlation between the fractal dimensions of the observed 2d photospheric structures and the extrapolated unstable volumes in the corona, when nonlinear force-free extrapolation is used. This result is independent of efforts to (1) bring the photospheric magnetic fields closer to a nonlinear force-free equilibrium and (2) omit the lower part of the modeled magnetic field volume that is almost completely filled…
Magnetic field extrapolation is an important tool to study the three-dimensional (3D) solar coronal magnetic field which is difficult to directly measure. Various analytic models and numerical codes exist but their results often drastically differ. Thus a critical comparison of the modeled magnetic field lines with the observed coronal loops is strongly required to establish the credibility of the model. Here we compare two different non-potential extrapolation codes, a non-linear force-free field code (CESE-MHD-NLFFF) and a non-force-free field (NFFF) code in modeling a solar active region (A…
Similar to the Sun, other stars shed mass and magnetic flux via ubiquitous quasi-steady wind and episodic stellar coronal mass ejections (CMEs). We investigate the mass loss rate via solar wind and CMEs as a function of solar magnetic variability represented in terms of sunspot number and solar X-ray background luminosity. We estimate the contribution of CMEs to the total solar wind mass flux in the ecliptic and beyond, and its variation over different phases of the solar activity cycles. The study exploits the number of sunspots observed, coronagraphic observations of CMEs near the Sun by SOH…
Because of its distinctive compositional properties and variability, low-speed ($\lesssim 450$ km s$^{-1}$) solar wind is widely believed to originate from coronal streamers, unlike high-speed wind, which comes from coronal holes. An alternative scenario is that the bulk of the slow wind (excluding that in the immediate vicinity of the heliospheric current sheet) originates from rapidly diverging flux tubes rooted inside small coronal holes or just within the boundaries of large holes. This viewpoint is based largely on photospheric field extrapolations, which are subject to considerable uncer…
Extrapolation codes for modelling the magnetic field in the corona in cartesian geometry do not take the curvature of the Sun's surface into account and can only be applied to relatively small areas, \textit{e.g.}, a single active region. We apply a method for nonlinear force-free coronal magnetic field modelling of photospheric vector magnetograms in spherical geometry which allows us to study the connectivity between multi-active regions. We use vector magnetograph data from the Synoptic Optical Long-term Investigations of the Sun survey (SOLIS)/Vector Spectromagnetograph(VSM) to model the c…
Getting insights into the 3D structure of the solar coronal magnetic field have been done in the past by two completely different approaches: (1.) Nonlinear force-free field (NLFFF) extrapolations, which use photospheric vector magnetograms as boundary condition. (2.) Stereoscopy of coronal magnetic loops observed in EUV coronal images from different vantage points. Both approaches have their strength and weaknesses. Extrapolation methods are sensitive to noise and inconsistencies in the boundary data and the accuracy of stereoscopy is affected by the ability of identifying the same structure …
Potential field extrapolations are widely used as minimum-energy models for the Sun's coronal magnetic field. As the reference to which other magnetic fields are compared, they have -- by any reasonable definition -- no global (signed) magnetic helicity. Here we investigate the internal topological structure that is not captured by the global helicity integral, by splitting it into individual field line helicities. These are computed using potential field extrapolations from magnetogram observations over Solar Cycle 24, as well as for a simple illustrative model of a single bipolar region in a…
Magnetic field in the solar lower atmosphere can be measured by the use of the Zeeman and Hanle effects. In contrast, the coronal magnetic field well above the solar surface, which directly controls various eruptive phenomena, can not be precisely measureed with the traditional techniques. Several attempts are being made to probe the coronal magnetic field, such as force-free extrapolation based on the photospheric magnetograms, gyroresonance radio emissions, and coronal seismology based on MHD waves in the corona. Compared to the waves trapped in the localized coronal loops, EIT waves are the…
Magnetic null points can be located numerically in a potential field extrapolation or their average density can be estimated from the Fourier spectrum of a magnetogram. We use both methods to compute the null point density from a quiet Sun magnetogram made with Hinode's NFI and from magnetograms from SOHO's MDI in both its high-resolution and low-resolution modes. All estimates of the super-chromospheric column density (z>1.5 Mm) agree with one another and with the previous measurements: 0.003 null points per square Mm of solar surface.
We report one of several homologous non-radial eruptions from NOAA active region (AR) 11158 that are strongly modulated by the local magnetic field as observed with the Solar Dynamic Observatory (SDO). A small bipole emerged in the sunspot complex and subsequently created a quadrupolar flux system. Non-linear force-free field (NLFFF) extrapolation from vector magnetograms reveals its energetic nature: the fast-shearing bipole accumulated ~2e31 erg free energy (10% of AR total) over just one day despite its relatively small magnetic flux (5% of AR total). During the eruption, the ejected plasma…
Coronal Magnetography using Spectropolarimetry with SKA Telescopes
National Centre for Radio Astrophysics, Tata Institute of Fundamental Research（塔塔基础研究院国家射电天体物理中心）；University Corporation for Atmospheric Research（大气研究大学联合会）；Johns Hopkins University Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；Physical Research Laboratory（物理研究实验室）；KU Leuven（荷语鲁汶大学）
In this review, I focus on the structure and evolution of the coronal magnetic fields modelled from observations. The development of instruments measuring the photospheric and chromospheric magnetic fields with a high spatial and time resolutions allows us to improve the modeling of the coronal fields based on extrapolation and evolution techniques. In particular, I detail the advance modelling of quiet-Sun areas, active regions and full-disc evolution. I discuss the structure of coronal magnetic features such as filaments, sigmoids and coronal loops as well as their time evolution and instabi…
Coronal holes are the darkest and least active regions of the Sun, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. This paper reviews measurements of the plasma properties in coronal holes and how these measurements are used to reveal details about the physical processes that heat the solar corona and accelerate the solar wind. It is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wave-l…
We present a numerical investigation of the coronal evolution of a coronal mass ejection (CME) on 2005 August 22 using a 3-D thermodynamics magnetohydrodynamic model, the SWMF. The source region of the eruption was anemone active region (AR) 10798, which emerged inside a coronal hole. We validate our modeled corona by producing synthetic extreme ultraviolet (EUV) images, which we compare to EIT images. We initiate the CME with an out-of-equilibrium flux rope with an orientation and chirality chosen in agreement with observations of a H-alpha filament. During the eruption, one footpoint of the …
Type II radio bursts are thought to be produced by shock waves in the solar atmosphere. However, what magnetic conditions are needed for the generation of type II radio bursts is still a puzzling issue. Here, we quantify the magnetic structure of a coronal shock associated with a type II radio burst. Based on the multi-perspective extreme-ultraviolet observations, we reconstruct the three-dimensional (3D) shock surface. By using a magnetic field extrapolation model, we then derive the orientation of the magnetic field relative to the normal of the shock front ($θ_{\rm Bn}$) and Alfvén Mach num…
We report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than li…
Presently, many models of the coronal magnetic field rely on photospheric vector magnetograms but these data have been shown to be problematic as the sole boundary information for nonlinear force-free field (NLFFF) extrapolations. Magnetic fields in the corona manifest themselves in high-energy images (X-rays and EUV) in the shapes of coronal loops, providing an additional constraint that at present is not used due to the mathematical complications of incorporating such input into numerical models. Projection effects and the limited number of usable loops further complicate the use of coronal …
Context: Knowledge about the coronal magnetic field is important to the understanding the structure of the solar corona. We compute the field in the higher layers of the solar atmosphere from the measured photospheric field under the assumption that the corona is force-free. Aims: Here we develop a method for nonlinear force-free coronal magnetic field medelling and preprocessing of photospheric vector magnetograms in spherical geometry using the optimization procedure. Methods: We describe a newly developed code for the extrapolation of nonlinear force-free coronal magnetic fields in spherica…
Our study attempts to understand the collision characteristics of two coronal mass ejections (CMEs) launched successively from the Sun on 2013 October 25. The estimated kinematics, from three-dimensional (3D) reconstruction techniques applied to observations of CMEs by SECCHI/Coronagraphic (COR) and Heliospheric Imagers (HIs), reveal their collision around 37 $R_\sun$ from the Sun. In the analysis, we take into account the propagation and expansion speeds, impact direction, angular size as well as the masses of the CMEs. These parameters are derived from imaging observations, but may suffer fr…
We investigate the performance of six methods for predicting the CME time of arrival (ToA) and velocity at Earth using a sample of nine Earth-impacting CMEs between May 2010 and June 2011. The CMEs were tracked continuously from the Sun to near Earth in multi-viewpoint imaging data from STEREO SECCHI and SOHO LASCO. We use the Graduate Cylindrical Shell (GCS) model to estimate the three-dimensional direction and height of the CMEs in every image out to $\sim$200 R$_\odot$. We fit the derived three-dimensional (deprojected) height and time data with six different methods to extrapolate the CME …
