The formation of sunspots and starspots is not yet fully understood and is therefore one of the major open problems in solar and stellar physics. Magnetic flux concentrations can be produced by the negative effective magnetic pressure instability (NEMPI). This instability is strongly suppressed by rotation. However, the presence of an outer coronal envelope was previously found to strengthen the flux concentrations and make them more prominent. It also allows for the formation of bipolar regions (BRs). We want to know whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. We use direct numerical simulations and mean-field simulations. We adopt a simple two-layer model of turbulence that mimics the jump between the convective turbulent and coronal layers below and above the surface of a star, respectively. The computational domain is Cartesian and located at a certain latitude of a rotating sphere. We investigate the effects of rotation on NEMPI by changing the Coriolis number, the latitude, and the box resolution. Rotation has a strong impact on the process of BR formation. Even rather slow rotation is found to suppress their formation. However, increasing the imposed magnetic field strength also makes the structures stronger and alleviates the rotational suppression somewhat. The presence of a coronal layer itself does not significantly alleviate the effects of rotational suppression.
Small, impulsive jets commonly occur throughout the solar corona, but are especially visible in coronal holes. Evidence is mounting that jets are part of a continuum of eruptions that extends to much larger coronal mass ejections and eruptive flares. Because coronal-hole jets originate in relatively simple magnetic structures, they offer an ideal testbed for theories of energy buildup and release in the full range of solar eruptions. We analyzed an equatorial coronal-hole jet observed by SDO/AIA on 09 January 2014, in which the magnetic-field structure was consistent with the embedded-bipole t…
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…
Whilst intense solar flares are almost always accompanied by a coronal mass ejection (CME), reports on stellar CMEs are rare, despite the frequent detection of stellar 'super flares'. The torus instability of magnetic flux ropes is believed to be one of the main driving mechanisms of solar CMEs. Suppression of the torus instability, due to a confining background coronal magnetic field that decreases sufficiently slowly with height, may contribute to the lack of stellar CME detection. Here we use the solar magnetic field as a template to estimate the vertical extent of this 'torus-stable zone' …
Loss of equilibrium of magnetic flux ropes is a leading candidate for the origin of solar coronal mass ejections (CMEs). The aim of this paper is to explore to what extent this mechanism can account for the initiation of CMEs in the global context. A simplified MHD model for the global coronal magnetic field evolution in response to flux emergence and shearing by large-scale surface motions is described and motivated. Using automated algorithms for detecting flux ropes and ejections in the global magnetic model, the effects of key simulation parameters on the formation of flux ropes and the nu…
We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501 on 20 November 2003. Both flares displayed four homologous H-alpha ribbons and were both accompanied by coronal mass ejections (CMEs). The central flare ribbons were located at the site of an emerging bipole in the center of the active region. The negative polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ~ 110 deg within 32 hours. We model the coronal magnetic field and compute its topology, using as boundary condition …
We study an EUV confined ejection observed on 09 May 2012 in active region (AR) NOAA 11476. For the analysis we use observations in multiple wavelengths (EUV, X-rays, H$α$, and magnetograms) from a variety of ground-based and space instruments. The magnetic configuration showed the presence of two rotating bipoles, with decreasing magnetic flux, within the following polarity of the AR. This evolution was present along some tens of hours before the studied event and continued even later. A minifilament with a length of $\approx 30 \arcsec$ lay along the photospheric inversion line of the larges…
Three extreme-ultraviolet (EUV) jets recurred in about one hour on 2010 September 17 in the following magnetic polarity of active region 11106. The EUV jets were observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board SDO measured the vector magnetic field, from which we derive the magnetic flux evolution, the photospheric velocity field, and the vertical electric current evolution. The magnetic configuration before the jets is derived by the nonlinear force-free field (NLFFF) extrapolation. We derive…
In order to understand the flare trigger mechanism, we conducted three-dimensional magnetohydrodynamic simulations using a coronal magnetic field model derived from data observed by the Hinode satellite. Several types of magnetic bipoles were imposed into the photospheric boundary of the Non-linear Force-Free Field (NLFFF) model of Active Region NOAA 10930 on 2006 December 13 to investigate what kind of magnetic disturbance may trigger the flare. As a result, we confirm that certain small bipole fields, which emerge into the highly sheared global magnetic field of an active region, can effecti…
We explore the effects of an outer stably stratified coronal envelope on rotating turbulent convection, differential rotation, and large-scale dynamo action in spherical wedge models of the Sun. We solve the compressible magnetohydrodynamic equations in a two-layer model with unstable stratification below the surface, representing the convection zone, and a stably stratified coronal envelope above. The interface represents a free surface. We compare our model to models that have no coronal envelope. The presence of a coronal envelope is found to modify the Reynolds stress and the $Λ$ effect re…
We present results of convective turbulent dynamo simulations including a coronal layer in a spherical wedge. We find an equatorward migration of the radial and azimuthal fields similar to the behavior of sunspots during the solar cycle. The migration of the field coexist with a spoke-like differential rotation and anti-solar (clockwise) meridional circulation. Even though the migration extends over the whole convection zone, the mechanism causing this is not yet fully understood.
We report the spontaneous formation of bipolar magnetic structures in direct numerical simulations of stratified forced turbulence with an outer coronal envelope. The turbulence is forced with transverse random waves only in the lower (turbulent) part of the domain. Our initial magnetic field is either uniform in the entire domain or confined to the turbulent layer. After about 1--2 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion tim…
We analyse the multiwavelength observations of an M2.9/1N flare that occurred in the active region (AR) NOAA 11112 in the vicinity of a huge filament system on 16 October 2010. SDO/HMI magnetograms reveal the emergence of a bipole (within the existing AR) 50 hours prior to the flare event. During the emergence, both the positive and negative sunspots in the bipole show translational as well as rotational motion. The positive polarity sunspot shows the significant motion/rotation in the south-westward/clockwise direction and continuously pushing/sliding the surrounding opposite polarity field r…
Using detrended fluctuation analysis (DFA) and rescaled range (R/S) analysis, we investigate the scaling properties of EUV intensity fluctuations of low-latitude coronal holes (CHs) and neighboring quiet-Sun (QS) regions in signals obtained with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) instrument. Contemporaneous line-of-sight SDO/Helioseismic and Magnetic Imager (HMI) magnetic fields provide a context for the physical environment. We find that the intensity fluctuations in the time series of EUV images present at each spatial point a scaling symmetry over the rang…
Investigating the Relationship Between Physical Properties and Spatial Irregularities at Coronal Hole Boundaries
Max Planck Institute for Solar System Research（马克斯·普朗克太阳系研究所）；University College London, Mullard Space Science Laboratory（伦敦大学学院穆拉德空间科学实验室）；Dublin City University（都柏林城市大学）；Dublin Institute for Advanced Studies（都柏林高等研究院）；Northumbria University（诺森比亚大学）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Predictive Science Inc.（预测科学公司）；University of Helsinki（赫尔辛基大学）；University of Graz（格拉茨大学）；Catholic University of America（美国天主教大学）；NASA Goddard Space Flight Center（NASA戈达德太空飞行中心）
In this paper, we study in detail the evolution of a mini-sigmiod originating in a cross-equatorial coronal hole, where the magnetic field is mostly open and seriously distinct from the closed background field above active-region sigmoids. The source region first appeared as a bipole, which subsequently experienced a rapid emergence followed by a long-term decay. Correspondingly, the coronal structure initially appeared as arc-like loops, then gradually sheared and transformed into continuously sigmoidal loops, mainly owing to flux cancellation near the polarity inversion line. The temperature…
We report on the results of four convective dynamo simulations with an outer coronal layer. The magnetic field is self-consistently generated by the convective motions beneath the surface. Above the convection zone, we include a polytropic layer that extends to 1.6 solar radii. The temperature increases in this region to $\approx8$ times the value at the surface, corresponding to $\approx1.2$ times the value at the bottom of the spherical shell. We associate this region with the solar corona. We find solar-like differential rotation with radial contours of constant rotation rate, together with…
Coronal bright points (CBP) are ubiquitous small brightenings in the solar corona associated with small magnetic bipoles. We derive the solar differential rotation profile by tracing the motions of CBPs detected by the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). We also investigate problems related to detection of coronal bright points resulting from instrument and detection algorithm limitations. To determine the positions and identification of coronal bright points we used a segmentation algorithm. A linear fit of their central meridian distance…
We examine the initiation of 10 coronal jet eruptions in quiet regions on the central disk, thereby avoiding near-limb spicule-forest obscuration of the slow-rise onset of the minifilament eruption. From the SDO/AIA 171A 12-second-cadence movie of each eruption, we (1) find and compare the start times of the minifilament's slow rise, the jet-base bright point, the jet-base interior brightening, and the jet spire, and (2) measure the minifilament's speed at the start and end of its slow rise. From (a) these data, (b) prior observations showing that each eruption was triggered by magnetic flux c…
We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the pre-existing field of active region NOAA 12529. IRIS data are complemented by full-disk observations of the Solar Dynamics Observatory satellite, relevant to the photosphere and the corona. The photospheric configuration of the EFR is also analyzed by measurements taken with the spectropolarimeter aboard the Hinode satellite, when the EFR was fully developed. Recurrent intense brighteni…
It is now established that the hot Jupiters have extensive gaseous (ionospheric) envelopes, which expanding far beyond the Roche lobe. The envelopes are weakly bound to the planet and affected by strong influence of stellar wind fluctuations. Also, the hot Jupiters are located close to the parent star and therefore the magnetic field of stellar wind is an important factor, determining the structure their magnetosphere. At the same time, for a typical hot Jupiter, the velocity of stellar wind plasma, flowing around the atmosphere, is close to the Alfvén velocity. This should result in stellar w…
We describe and analyze observations by the Solar Dynamics Observatory of the emergence of a small, bipolar active region within an area of unipolar magnetic flux that was surrounded by a circular, quiescent filament. Within only eight hours of the start of the emergence, a partial splitting of the filament and two consecutive coronal mass ejections took place. We argue that all three dynamic events occurred as a result of particular magnetic-reconnection episodes between the emerging bipole and the pre-existing coronal magnetic field. In order to substantiate our interpretation, we consider t…
The coronal mass ejection productivity of solar active region 13664/8
Sun Yat-sen University（中山大学）；CAS center for Excellence in Comparative Planetology（中国科学院比较行星学卓越中心）；University of Science and Technology of China（中国科学技术大学）；Institute of Deep Space Sciences, Deep Space Exploration Laboratory（深空科学研究所，深空探测实验室）
$δ$-sunspots, with highly complex magnetic structures, are very productive in energetic eruptive events, such as X-class flares and homologous eruptions. We here study the formation of such complex magnetic structures by numerical simulations of magnetic flux emergence from the convection zone into the corona in an active-region-scale domain. In our simulation, two pairs of bipolar sunspots form on the surface, originating from two buoyant segments of a single subsurface twisted flux rope, following the approach of Toriumi et al. (2014). Expansion and rotation of the emerging fields in the two…
Context: Relaxation theory offers a straightforward method for estimating the energy that is released when a magnetic field becomes unstable, as a result of continual convective driving. Aims: We present new results obtained from nonlinear magnetohydrodynamic (MHD) simulations of idealised coronal loops. The purpose of this work is to determine whether or not the simulation results agree with Taylor relaxation, which will require a modified version of relaxation theory applicable to unbounded field configurations. Methods: A three-dimensional (3D) MHD Lagrangian-remap code is used to simulate …
Using magnetic field maps acquired by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory we measured rotation rates of 864 active and 322 ephemeral regions observed between 2010 and 2016. We found smaller magnetic tracers to show a tendency to rotate faster as compared to larger ones. Thus, ephemeral regions exhibit on average the fastest rotation rate. We further divided active regions into three classes. Class A comprised magnetic bipoles obeying Hale's polarity law, Joy's law, and exhibiting more coherent leading polarity in comparison with the following one. The s…
Magnetic flux ropes (MFRs) are believed to be at the heart of solar coronal mass ejections (CMEs). A well-known example is the prominence cavity in the low corona that sometimes makes up a three-part white-light (WL) CME upon its eruption. Such a system, which is usually observed in quiet-Sun regions, has long been suggested to be the manifestation of an MFR with relatively cool filament material collecting near its bottom. However, observational evidence of eruptive, filament-hosting MFR systems has been elusive for those originating in active regions. By utilizing multi-passband extreme-ultr…
We have carried out an analysis of the HST STIS archival spectra of the magnetic white dwarf in the Hyades eclipsing-spectroscopic, post-common envelope binary V471 Tauri, time resolved on the orbit and on the X-ray rotational phase of the magnetic white dwarf. An HST STIS spectrum obtained during primary eclipse reveals a host of transition region/chromospheric emission features including N V (1238, 1242), Si IV (1393, 1402), C IV (1548, 1550) and He II (1640). The spectroscopic characteristics and emission line fluxes of the transition region/chromosphere of the very active, rapidly rotating…
We present the first model that couples the formation of the corona of a solar active region to a model of the emergence of a sunspot pair. This allows us to study when, where, and why active region loops form, and how they evolve. We use a 3D radiation MHD simulation of the emergence of an active region through the upper convection zone and the photosphere as a lower boundary for a 3D MHD coronal model. The latter accounts for the braiding of the magnetic fieldlines, which induces currents in the corona heating up the plasma. We synthesize the coronal emission for a direct comparison to obser…
Persistent plasma upflows were observed with Hinode's EUV Imaging Spectrometer (EIS) at the edges of active region (AR) 10978 as it crossed the solar disk. We analyze the evolution of the photospheric magnetic and velocity fields of the AR, model its coronal magnetic field, and compute the location of magnetic null-points and quasi-sepratrix layers (QSLs) searching for the origin of EIS upflows. Magnetic reconnection at the computed null points cannot explain all of the observed EIS upflow regions. However, EIS upflows and QSLs are found to evolve in parallel, both temporarily and spatially. S…
The heating of coronal loops is investigated to understand the observational consequences in terms of the thermodynamics and radiative losses from the Sun as well as the magnetized coronae of stars with an outer convective envelope. The dynamics of the Parker coronal heating model are studied for different ratios of the photospheric forcing velocity timescale $t_p$ to the Alfvén crossing time along a loop $t_A$. It is shown that for $t_p/t_A \gtrsim$ 10--24 the heating rate and maximum temperature are largest and approximately independent of $t_p/t_A$, leading to a strong emission in X-rays an…
We present a magnetic analysis of the formation and eruption of an active region (AR) sigmoid in AR 11283 from 2011 September 4 to 6. To follow the quasi-static evolution of the coronal magnetic field, we reconstruct a time sequence of static fields using a recently developed nonlinear force-free field model constrained by the SDO/HMI vector magnetograms. A detailed analysis of the fields compared with the SDO/AIA observations suggests the following scenario for the evolution of the region. Initially, a new bipole emerges into the negative polarity of a pre-existing bipolar AR, forming a null …
Magnetic activity in cool stars: manifestations and relevance to exoplanets
Max-Planck-Institut für Sonnensystemforschung（马克斯·普朗克太阳系统研究所）；Centro de Rádio Astronomia e Astrofísica Mackenzie, Universidade Presbiteriana Mackenzie（麦肯齐射电天文学与天体物理学中心，麦肯齐长老会大学）；Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM（巴黎萨克雷大学，巴黎西岱大学，法国原子能和替代能源委员会，法国国家科学研究中心，天文信息与管理实验室）；Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory（匈牙利研究网络地球与天文学研究中心，孔科利天文台）；Instituto de Astronomía y Física del Espacio (IAFE, UBA-CONICET)（空间天体物理学与天体物理研究所）；The Catholic University of America（美国天主教大学）；The Hakubi Center for Advanced Research, Kyoto University（京都大学博雅教育研究中心）；Department of Physics, Kyoto University（京都大学物理学系）；University of Helsinki（赫尔辛基大学）
