Large scale MHD properties of interplanetary magnetic clouds

Magnetic Clouds (MCs) are the interplanetary manifestation of Coronal Mass Ejections. These huge astrophysical objects travel from the Sun toward the external heliosphere and can reach the Earth environment. Depending on their magnetic field orientation, they can trigger intense geomagnetic storms. The details of the magnetic configuration of clouds and the typical values of their magnetohydrodynamic magnitudes are not yet well known. One of the most important magnetohydrodynamic quantities in MCs is the magnetic helicity. The helicity quantifies several aspects of a given magnetic structure, such as the twist, kink, number of knots between magnetic field lines, linking between magnetic flux tubes, etc. The helicity is approximately conserved in the solar atmosphere and the heliosphere, and it is very useful to link solar phenomena with their interplanetary counterpart. Since a magnetic cloud carries an important amount of helicity when it is ejected from the solar corona, estimations of the helicity content in clouds can help us to understand its evolution and its coronal origin. In situ observations of magnetic clouds at one astronomical unit are in agreement with a local helical magnetic structure. However, since spacecrafts only register data along a unique direction, several aspects of the global configuration of clouds cannot be observed. In this paper, we review the general properties of magnetic clouds and different models for their magnetic structure at one astronomical unit. We describe the corresponding techniques to analyze in situ measurements. We also quantify their magnetic helicity and compare it with the release of helicity in their solar source for some of the analyzed cases.     

On the nature of coronal loops above the quiet sun network

The structure and dynamics of a box in a stellar corona can be modeled employing a 3D MHD model for different levels of magnetic activity. Depending on the magnetic flux through the surface the nature of the resulting coronal structures can be quite different. We investigate a model of an active region for two sunspots surrounded by magnetic field patches comparable in magnetic flux to the sunspots. The model results in emission from the model corona being concentrated in loop structures. In Gudiksen and Nordlund (2005) the loops seen in EUV and X-ray emission outline the magnetic field, following the general paradigm. However, in our model, where the magnetic field is far from a force-free state, the loops seen in X-ray emission do not follow the magnetic field lines. This result is of interest especially for loops as found in areas where the magnetic field emerging from active regions interacts with the surrounding network.     

Dynamical magnetic reconnection in Parker''s coronal heating model

The excitation (flares, ejections, heating, …) of the corona can be understood in terms of the dynamics of the confectively driven magnetized plasma. In particular, anomalous ohmic heating may be a consequence of the formation and rapid dissipation of small-scale magnetic fields in the corona. We have performed numerical simulations of the loop heating model proposed by Parker (1972, 1994), and have studied its dynamics and global power balance in order to assess its viability as a coronal heating candidate, with promising results. Our results suggest the following view of the small-scale dynamics of coronal loops. First of all, photospheric granular motions quasi-statically twist the magnetic field of the corona in a random-walk fashion. In topologically closed structures, the perpendicular magnetic energy increases, causing magnetic shear to build up at the quasi-separatrices of the resulting close-packed magnetic flux tubes. At some point, the boundary driving causes this stressed configuration to cross the threshold of an ideal time-scale MHD instability (possibly magnetic coalescence or resistive tearing) or a point of nonequilibrium and the field lines pinch toward a small-scale sheared configuration. It then becomes energetically favorable for dynamic reconnection to occur, producing narrow current sheets and an Ohmic heating rate sufficient to balance the input Poynting flux.     

Coronal index as a solar activity index applied to space weather

All possible changes of the solar activity can be expressed by the coronal index of solar activity that represents the averaged daily power of the green corona emitted from the Sun’s visible hemisphere. The representative character of this index allows us to study long-term, intermediate and short-term variations of the Sun as a star. This index can be expressed well as a function of other solar indices. As green line reflects the distribution of the photospheric magnetic fields in the solar corona, the dependence of this index on the solar magnetic field is confirmed by means of statistical analysis of these two parameters. Daily values of the coronal index, as well as of the magnetic field data obtained from the Wilcox Solar Observatory, has been analysed by Fast Fourier analysis and Wavelet Transform analysis for the time period 1966–1998 covering more than three solar cycles. Periodicities of 11.4, 3.2, 2.3, 1.7, 1, 0.29, 0.07 and 0.04 years have been found in both parameters that means once again that the coronal index is probably related to the underlying photospheric magnetic fields and can be used as a global index of solar activity useful for Space Weather studies.     

Solar atmosphere wave dynamics generated by solar global oscillating eigenmodes

The solar atmosphere exhibits a diverse range of wave phenomena, where one of the earliest discovered was the five-minute global acoustic oscillation, also referred to as the p-mode. The analysis of wave propagation in the solar atmosphere may be used as a diagnostic tool to estimate accurately the physical characteristics of the Sun’s atmospheric layers.In this paper, we investigate the dynamics and upward propagation of waves which are generated by the solar global eigenmodes. We report on a series of hydrodynamic simulations of a realistically stratified model of the solar atmosphere representing its lower region from the photosphere to low corona. With the objective of modelling atmospheric perturbations, propagating from the photosphere into the chromosphere, transition region and low corona, generated by the photospheric global oscillations the simulations use photospheric drivers mimicking the solar p-modes. The drivers are spatially structured harmonics across the computational box parallel to the solar surface. The drivers perturb the atmosphere at 0.5?Mm above the bottom boundary of the model and are placed coincident with the location of the temperature minimum. A combination of the VALIIIC and McWhirter solar atmospheres are used as the background equilibrium model.We report how synthetic photospheric oscillations may manifest in a magnetic field free model of the quiet Sun. To carry out the simulations, we employed the magnetohydrodynamics code, SMAUG (Sheffield MHD Accelerated Using GPUs).Our results show that the amount of energy propagating into the solar atmosphere is consistent with a model of solar global oscillations described by Taroyan and Erdélyi (2008) using the Klein-Gordon equation. The computed results indicate a power law which is compared to observations reported by Ireland et al. (2015) using data from the Solar Dynamics Observatory/Atmospheric Imaging Assembly.     

The creation of fine structure by magnetic fields

The solar plasma is strongly structured by the presence of magnetic field. This structuring is manifest in the photosphere in the form of flux tubes, from the readily visible sunspots to the sub-telescopic intense tubes, so that the atmosphere is divided into strong-field media or field-free media. In the corona, by contrast, the magnetic field permeates the whole of the atmosphere and structuring consists principally of density and temperature inhomogeneities. We discuss some of the causes of magnetic structuring, including kinematic concentration, convective collapse and magnetoconvection for photospheric tubes, spicules in the chromosphere, and thermal instability for coronal loops.     

Magnetic reconnection on the Sun: ESPD Senior Prize Lecture

Magnetic reconnection is a fundamental process for changing the magnetic topology and converting magnetic energy into other forms on the Sun, such as heat, flow energy and fast particle energy. In two dimensions it is fairly well understood, although some aspects still need to be developed. In three dimensions, it behaves very differently and a substantial body of theory and numerical experiment has now been built up, including reconnection at null points, separators and quasi-separators.Some aspects of solar flares can be understood with 2D reconnection models, but other aspects such as the shapes of flare ribbons, the acceleration of particles and the creation of twist in erupting flux ropes need a 3D understanding. A paradigm shift in our understanding of coronal heating by reconnection has been stimulated by dramatic new observations of photospheric flux cancellation from SUNRISE and from SST together with the realisation that it may well be driving nanoflare heating events and possibly campfires.     

2011年8月4日螺旋状日冕物质抛射爆发事件

利用多卫星多波段的综合观测数据,通过追踪光球表面等离子体速度分析计算了耀斑爆发前后磁螺度的变化,发现耀斑爆发前活动区中光球表面存在强的水平剪切运动,活动区磁螺度的注入主要由这种剪切运动所产生;使用CESE-MHD-NLFFF重建了耀斑爆发前后活动区的磁场位形,推测出耀斑过程中存在磁绳结构的抛射.基于这些分析,给出了这一螺旋状抛射结构的形成机制:爆发前暗条西侧足点的持续剪切运动驱动磁通量绳增加扭转,高度扭缠的通量绳与东侧足点附近的开放磁力线重联并与东侧足点断开,进而向外抛出并伴随解螺旋运动.另外,利用1AU处WIND卫星的观测数据在对应的行星际日冕物质抛射中找到典型磁云的观测特征.这表明除了传统上双足点均在太阳表面的磁云模型,这种单足点固定于太阳表面的磁通量绳爆发图景同样可能在行星系际空间形成磁云结构.研究结果对进一步认识磁云结构具有重要意义.      

Coronal heating by reconnection

The outermost atmosphere of the Sun, called the corona, is some 200 times hotter than the surface of the Sun. The main source of energy for heating the corona is believed to be the magnetic field which dominates the corona. Magnetic reconnection is probably the most important mechanism for releasing magnetic energy and may, therefore, be important for coronal heating or micro-flaring. The best observational examples of reconnection in the corona are thought to be X-ray bright points, which are small-scale brightenings seen randomly throughout the whole corona. Theoretical models can not only explain the key observations relating to bright points, but they can also explain the complex three-dimensional structures often seen in bright points. In these models magnetic neutral points play a significant role as the centres for reconnection both in two and three dimensions.     

Solar X-ray and EUV corona and their relation to the magnetic activity

We have studied the solar magnetic cycle in corona using X-ray data from YOHKOH and Extreme Ultraviolet data from SOHO/EIT. Soft X-ray data last the period from after the maximum cycle 22 to the maximum cycle 23 (1991–2001). The SOHO/EIT Extreme Ultraviolet data are used for the period from 1996 to 2003. These data provide us a unique opportunity to look at the solar corona on the solar disc and to compare with the magnetic activity, directly.Our studies reveal a close relationship between the coronal emissions and the photospheric magnetic field in the axisymmetrical case. The evolution of coronal structures in X-ray and EUV can be considered as a proxy of the coronal magnetic field and demonstrates a development of the solar magnetic cycle in corona. It is shown that the most important feature of the coronal cycle is the forming of giant loops structure visible in X-ray and, partially, in EUV (284A) on the solar disk.     

Emission heights of coronal bright points on Fe XII radiance map

The study of coronal bright points (BPs) is important for understanding coronal heating and the origin of the solar wind. Previous studies indicated that coronal BPs have a highly significant tendency to coincide with magnetic neutral lines in the photosphere. Here we further studied the emission heights of the BPs above the photosphere in the bipolar magnetic loops that are apparently associated with them. As BPs are seen in projection against the disk their true emission heights are unknown. The correlation of the BP locations on the Fe XII radiance map from EIT with the magnetic field features (in particular neutral lines) was investigated in detail. The coronal magnetic field was determined by an extrapolation of the photospheric field (derived from 2-D magnetograms obtained from the Kitt Peak observatory) to different altitudes above the disk. It was found that most BPs sit on or near a photospheric neutral line, but that the emission occurs at a height of about 5 Mm. Some BPs, while being seen in projection, still seem to coincide with neutral lines, although their emission takes place at heights of more than 10 Mm. Such coincidences almost disappear for emissions above 20 Mm. We also projected the upper segments of the 3-D magnetic field lines above different heights, respectively, on to the tangent x–y plane, where x is in the east–west and y in the south–north direction. The shape of each BP was compared with the respective field-line segment nearby. This comparison suggests that most coronal BPs are actually located on the top of their associated magnetic loops. Finally, we calculated for each selected BP region the correlation coefficient between the Fe XII intensity enhancement and the horizontal component of the extrapolated magnetic field vector at the same x–y position in planes of different heights, respectively. We found that for almost all the BP regions we studied the correlation coefficient, with increasing height, increases to a maximal value and then decreases again. The height corresponding to this maximum was defined as the correlation height, which for most bright points was found to range below 20 Mm.     

Flux emergence,flux imbalance,magnetic free energy and solar flares

Emergence of complex magnetic flux in the solar active regions lead to several observational effects such as a change in sunspot area and flux embalance in photospheric magnetograms. The flux emergence also results in twisted magnetic field lines that add to free energy content. The magnetic field configuration of these active regions relax to near potential-field configuration after energy release through solar flares and coronal mass ejections. In this paper, we study the relation of flare productivity of active regions with their evolution of magnetic flux emergence, flux imbalance and free energy content. We use the sunspot area and number for flux emergence study as they contain most of the concentrated magnetic flux in the active region. The magnetic flux imbalance and the free energy are estimated using the HMI/SDO magnetograms and Virial theorem method. We find that the active regions that undergo large changes in sunspot area are most flare productive. The active regions become flary when the free energy content exceeds 50% of the total energy. Although, the flary active regions show magnetic flux imbalance, it is hard to predict flare activity based on this parameter alone.     

Solar magnetic fields and the dynamo theory

Unlike Earth’s dipolar magnetic fields, solar magnetic fields consist of wide ranges of length-scales and strengths, and interestingly, they evolve in a cyclic fashion with a 22-year periodicity. A magnetohydrodynamic dynamo operating in the Sun is most likely responsible for producing the solar magnetic activity cycle. While the first solar dynamo models were built half a century ago, recent views differ significantly from those models. According to widely accepted present concepts, the large-scale solar dynamo is of flux-transport type, which involves three basic processes: (i) generation of toroidal fields by shearing the pre-existing poloidal fields by differential rotation (the Ω-effect); (ii) re-generation of poloidal fields by lifting and twisting the toroidal fluxtubes (the α-effect); (iii) flux transport by meridional circulation. This class of dynamos has been successful in explaining many large-scale solar cycle features, including a particularly difficult one – the correct phase relationship between the equatorward-migrating sunspot belt and the poleward drifting large-scale, diffuse fields. The dynamo cycle period in such models is primarily governed by the meridional flow speed near the bottom of the convection zone. After briefly reviewing the historical background, we will present the successes of flux-transport dynamos, including their predictive capability. For example, we will demonstrate how the meridional circulation plays a key role in governing the Sun’s memory about its own magnetic field, and how a flux-transport dynamo-based predictive tool can explain the cause of the very slow polar reversal in the so-called “peculiar” cycle 23 compared to those in cycles 20, 21 and 22. We will close by presenting explanations for certain long-term variability using these models, such as, what may have maintained the observed cyclic variation in slow solar wind flow during Maunder minima, in the presence of near zero solar activity.     

Combining MHD and kinetic modelling of solar flares

Solar flares are explosive events in the solar corona, representing fast conversion of magnetic energy into thermal and kinetic energy, and hence radiation, due to magnetic reconnection. Modelling is essential for understanding and predicting these events. However, self-consistent modelling is extremely difficult due to the vast spatial and temporal scale separation between processes involving thermal plasma (normally considered using magnetohydrodynamic (MHD) approach) and non-thermal plasma (requiring a kinetic approach). In this mini-review we consider different approaches aimed at bridging the gap between fluid and kinetic modelling of solar flares. Two types of approaches are discussed: combined MHD/test-particle (MHDTP) models, which can be used for modelling the flaring corona with relatively small numbers of energetic particles, and hybrid fluid-kinetic methods, which can be used for modelling stronger events with higher numbers of energetic particles. Two specific examples are discussed in more detail: MHDTP models of magnetic reconnection and particle acceleration in kink-unstable twisted coronal loops, and a novel reduced-kinetic model of particle transport in converging magnetic fields.     

Inferring plasma flow velocities from photospheric vector magnetic field observations for the investigation of flare onsets

The amount of emergence and submergence of magnetized plasma and the horizontal motion of the footpoints of flux tubes might be crucial for the dynamics of the solar atmosphere. Although the rate of flux emergence and submergence can be observationally determined near the polarity inversion line (Chae et al., 2004), the same is not true for regions away from the PIL. Also, the horizontal motions cannot be directly measured in the solar photosphere. In this sense, the evolution of the photospheric magnetic field provides valuable information which can be used to estimate photospheric plasma flows since magnetic field and plasma are closely associated (frozen-in-condition). We used three methods to estimate the photospheric plasma motion from magnetic field observations. The methods were applied to photospheric vector magnetic field data of active region NOAA 9077, observed by the Huairou Solar Observing Station (HSOS) of the National Astronomical Observatories of China before and after the ‘Bastille Day’ flare on July 13th and 14th, 2000.     

What are the physical mechanisms of eruptions and CMEs?

CMEs are due to physical phenomena that drive both, eruptions and flares in active regions. Eruptions/CMEs must be driven from initially force-free current-carrying magnetic field. Twisted flux ropes, sigmoids, current lanes and pattern in photospheric current maps show a clear evidence of currents parallel to the magnetic field. Eruptions occur starting from equilibria which have reached some instability threshold. Revisiting several data sets of CME observations we identified different mechanisms leading to this unstable state from a force free field. Boundary motions related to magnetic flux emergence and shearing favor the increase of coronal currents leading to the large flares of November 2003. On the other hand, we demonstrated by numerical simulations that magnetic flux emergence is not a sufficient condition for eruptions. Filament eruptions are interpreted either by a torus instability for an event occurring during the minimum of solar activity either by the diffusion of the magnetic flux reducing the tension of the restraining arcade. We concluded that CME models (tether cutting, break out, loss of equilibrium models) are based on these basic mechanisms for the onset of CMEs.     

What is the role of magnetic null points in large flares?

We have performed the analysis of the magnetic topology of active region NOAA 10486 before two large flares occurring on October 26 and 28, 2003. The 3D extrapolation of the photospheric magnetic field shows the existence of magnetic null points when using two different methods. We use TRACE 1600 Å and 195 Å brightenings as tracers of the energy release due to magnetic reconnections. We conclude on the three following points:

1. The small events observed before the flares are related to low lying null points. They are long lasting and associated with low energy release. They are not triggering the large flares.

2. On October 26, a high altitude null point is found. We look for bright patches that could correspond to the signatures of coronal reconnection at the null point in TRACE 1600 Å images. However, such bright patches are not observed before the main flare, they are only observed after it.

3. On October 28, four ribbons are observed in TRACE images before the X17 flare. We interpret them as due to a magnetic breakout reconnection in a quadrupolar configuration. There is no magnetic null point related to these four ribbons, and this reconnection rather occurs at quasi-separatrix layers (QSLs).

We conclude that the existence of a null point in the corona is neither a sufficient nor a necessary condition to give rise to large flares.     

Turbulence,complexity, and solar flares

The issue of predicting solar flares is one of the most fundamental in physics, addressing issues of plasma physics, high-energy physics, and modelling of complex systems. It also poses societal consequences, with our ever-increasing need for accurate space weather forecasts. Solar flares arise naturally as a competition between an input (flux emergence and rearrangement) in the photosphere and an output (electrical current build up and resistive dissipation) in the corona. Although initially localised, this redistribution affects neighbouring regions and an avalanche occurs resulting in large scale eruptions of plasma, particles, and magnetic field. As flares are powered from the stressed field rooted in the photosphere, a study of the photospheric magnetic complexity can be used to both predict activity and understand the physics of the magnetic field. The magnetic energy spectrum and multifractal spectrum are highlighted as two possible approaches to this.     

Bright point study

Transition region and coronal observations of bright points by instruments aboard the Solar Maximum Mission were accompanied by high resolution photospheric magnetograph observations on September 11, 1980.In the photosphere a total of 31 bipolar ephemeral regions were observed from birth in 9.3 hours of combined magnetograph observations from three observatories.The Ultraviolet Spectrometer-Polarimeter observed 2 of the 3 ephemeral regions present in its field of view in the C IV 1548Å line. The unobserved ephemeral region was the shortest-lived (2.5 hr) and lowest in magnetic flux density (13G) of the three.In the O VIII 18.969Å line, the Flat Crystal Spectrometer detected only low level signals that are not statistically significant to be positively identified with any of the 16 ephemeral regions observed in the photosphere.The SMM data shows that at any given time there lacked a one-to-one correspondence between observable bright points and photospheric ephemeral regions. More ephemeral regions were observed than their counterparts in the transition region and the corona.     

A model of foreground emission in UV using GALEX deep observations

Foreground emission, mainly airglow and zodiacal light, is a significant contributor in an ultraviolet observation especially from low earth orbit. Its careful estimation and removal are tedious yet unavoidable processes in the study of diffuse UV radiation and by extension interstellar dust studies. Our analysis of deep GALEX observations show that airglow is not only a function of Sun angle but also a strong function of Solar activity at the time of observation. We present here an empirical model of airglow emission, derived from GALEX deep observations, as a function of 10.7 cm Solar flux and Sun angle. We obtained the model by training machine learning models on the data using a variant of the regression algorithm that is both resilient toward outlier data and sensitive to the complexities of the provided data. Our model predictions across various observations show no loss in generalization as well as good agreement with the observed values. We find that the total airglow in an observation is the sum of a baseline part (AG_c) that depends on the Solar flux and Sun angle, and a variable part (AG_v) that depends on the Sun angle and the time of observation with respect to local midnight. We also find that the total airglow can vary between 85 – 390 photon units in FUV and 80 – 465 photon units in NUV.