A Relationship Between Transition Region Brightenings,Abundances, and Magnetic Topology

We present multi-instrument observations of active region (AR) 8048, made between 3 June and 5 June 1997, as part of the SOHO Joint Observing Program 33. This AR has a sigmoid-like global shape and undergoes transient brightenings in both soft X-rays and transition region (TR) lines. We compute a magneto-hydrostatic model of the AR magnetic field, using as boundary condition the photospheric observations of SOHO/MDI. The computed large-scale magnetic field lines show that the large-scale sigmoid is formed by two sets of coronal loops. Shorter loops, associated with the core of the SXT emission, coincide with the loops observed in the hotter CDS lines. These loops reveal a gradient of temperature, from 2 MK at the top to 1 MK at the ends. The field lines most closely matching these hot loops extend along the quasi-separatrix layers (QSLs) of the computed coronal field. The TR brightenings observed with SOHO/CDS can also be associated with the magnetic field topology, both QSL intersections with the photosphere, and places where separatrices issuing from bald patches (sites where field lines coming from the corona are tangent to the photosphere) intersect the photosphere. There are, furthermore, suggestions that the element abundances measured in the TR may depend on the type of topological structure present. Typically, the TR brightenings associated with QSLs have coronal abundances, while those associated with BP separatrices have abundances closer to photospheric values. We suggest that this difference is due to the location and manner in which magnetic reconnection occurs in two different topological structures. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013302317042     

Three-Dimensional Structure of the Active Region Photosphere as Revealed by High Angular Resolution

Blue continuum images of active regions at ∼ 60° from the center of the solar disk obtained with the new Swedish 1-m Solar Telescope reveal heretofore unreported structure of the magnetized solar atmosphere. Perhaps the most striking aspect of these images is that, at an angular resolution of 0.12″, they show clearly the three-dimensional structure of the photosphere. In particular, the Wilson depression of the dark floors of pores is readily apparent. Conversely, the segmented structure of light bridges running through sunspots and pores reveal that light bridges are raised above the dark surroundings. The geometry of light bridges permits estimates of the height of their central (slightly darker) ridge: typically in the range 200–450 km. These images also clearly show that facular brightenings outside of sunspots and pores occur on the disk-center side of those granules just limbward of intergranular lanes that presumably harbor the associated plage magnetic flux. In many cases the brightening extends 0.5″ or more over those granules. Furthermore, a very thin, darker lane is often found just centerward of the facular brightening. We speculate that this feature is the signature of cool down flows that surround flux tubes in dynamical models. These newly recognized observational aspects of photospheric magnetic fields should provide valuable constraints for MHD models of the magnetized photosphere, and examination of those models as viewed from oblique angles is encouraged.     

Time-Distance Seismology and the Solar Transition Region

Time-Distance ‘travel time’ perturbations (as inferred from wave phase) are calculated relative to the quiet-Sun as a function of wave orientation and field inclination in a uniform inclined magnetic field. Modelling indicates that the chromosphere-corona Transition Region (TR) profoundly alters travel times at inclinations from the vertical θ for which the ramp-reduced acoustic cutoff frequency ω _c cosθ is similar to the wave frequency ω. At smaller inclinations phase shifts are much smaller as the waves are largely reflected before reaching the TR. At larger inclinations, the shifts resume their quiet-Sun values, although with some resonant oscillatory behaviour. Changing the height of the TR in the model atmosphere has some effect, but the thickness and temperature jump do not change the results substantially. There is a strong correspondence between travel-time shifts and the Alfvén flux that emerges at the top of the modelled region as a result of fast/Alfvén mode conversion. We confirm that the TR transmission coefficient for Alfvén waves generated by mode conversion in the chromosphere is far larger (typically 30 % or more) than for Alfvén waves injected from the photosphere.     

Active-Region Magnetic Structure Observed in the Photosphere and Chromosphere

The full magnetic vector has been measured in both the photosphere and chromosphere across sunspots and plage in NOAA Active Region 8299. We investigate the vertical magnetic structure above the umbral, penumbral and plage regions using quantitative statistical comparisons of the photospheric and chromospheric magnetic data. The results include: (1) a general decrease in average magnetic flux density with height; (2) the direct detection of the superpenumbral canopy in the chromosphere; (3) values for dB/dz which are consistent with earlier investigations when derived from a straight difference between the two measurements, but which are somewhat small when derived from the B=0 condition, (4) a monolithic structure in the umbrae which extends well into the upper chromosphere, with a very complex and varied structure in penumbrae and plage, as evidenced by (5) a uniform magnetic scale height in the umbrae with an abrupt jump to widely varying scale heights in penumbral and plage regions. Further, we find (6) evidence that field extrapolations using the photospheric flux as the boundary may not agree with expectations or with observed coronal structures as well as those which use the chromospheric magnetic flux as the extrapolation starting point.     

太阳过渡区的结构与特性

太阳过渡区是位于色球与日冕之间的一个高度动态的等离子体区域.通过对太阳紫外光谱的研究发现,过渡区的主要结构是以磁场集中为特征的网络组织.首先回顾太阳过渡区的研究历史,接着从整体上介绍太阳过渡区的结构及观测特征,然后介绍过渡区各种结构和现象的主要模型和物理解释,并结合作者的认识进行必要的评论,最后对未来的研究方向提出看法.     

Observations of Transition Region Plasma

Recent space missions have changed our view of the solar transition region. In particular the ESA/NASA Solar and Heliospheric Observatory (SOHO) and NASA's TRACE satellite have provided a unique opportunity to explore the solar atmosphere in detail. The combination of high spatial, spectral and temporal observations has made it possible to derive three dimensional images of the emission and velocity structures of solar features. Active region loop structures at transition region temperatures appear to be extremely time variable and dynamic, a result with profound implications for our understanding and modeling of the upper solar atmosphere. Large Dopplershifts have also been observed in these structures. A 3-minute transition region oscillation has been observed above sunspots suggesting upward-propagating acoustic waves. Clear evidence of velocity oscillations in the internetwork regions has also been observed in both the chromosphere and the transition region. The longstanding and puzzling problem of the apparent net red shift of emission lines from the transition region has been revisited. The extensive wavelength coverage of the SOHO spectrometers has made it possible to extend the measurements to much higher temperatures compared to previous instruments. The combination of magnetograms, EUV spectral imaging and the high resolution broad-band images from TRACE has also given us new insight concerning the structure of the transition region and its relation with the photospheric magnetic field. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005224709046     

On the Magnetic Correspondence between the Photosphere and the Heliosphere

The solar magnetic field maps every point in the corona to a corresponding place on the solar surface. Identifying the magnetic connection map is difficult at low latitudes near the heliospheric current sheet, but remarkably simple in coronal hole interiors. We present a simple analytic magnetic model (‘pseudocurrent extrapolation’) that reproduces the global structure of the corona, with significant physical advantages over other nearly analytic models such as source-surface potential field extrapolation. We use the model to demonstrate that local horizontal structure is preserved across altitude in the central portions of solar coronal holes, up to at least 30 R_s, in agreement with observations. We argue that the preserved horizontal structure may be used to track the magnetic footpoint associated with the location of a hypothetical spacecraft traveling through the solar corona, to relate in situ measurements of the young solar wind at ∼10–30 R_s to particular source regions at the solar surface. Further, we discuss the relationship between readily observable geometrical distortions and physical parameters of interest such as the field-aligned current density.     

The Nature of Blinkers and the Solar Transition Region

Solar plasma that exists at around 10⁵ K, which has traditionally been referred to as the solar transition region, is probably in a dynamic and fibril state with a small filling factor. Its origin is as yet unknown, but we suggest that it may be produced primarily by one of five different physical mechanisms, namely: the heating of cool spicular material; the containment of plasma in low-lying loops in the network; the thermal linking of cool and hot plasma at the feet of coronal loops; the heating and evaporating of chromospheric plasma in response to a coronal heating event; and the cooling and draining of hot coronal plasma when coronal heating is switched off. We suggest that, in each case, a blinker could be produced by the granular compression of a network junction, causing subtelescopic fibril flux tubes to spend more of their time at transition-region temperatures and so to increase the filling factor temporarily.     

UV Observations with the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer is a space-borne solar telescope featuring high spatial and temporal resolution. TRACE images emission from solar plasmas in three extreme-ultraviolet (EUV) wavelengths and several ultraviolet (UV) wavelengths, covering selected ion temperatures from 6000 K to 1 MK. The TRACE UV channel employs special optics to collect high-resolution solar images of the H i L line at 1216 Å, the C iv resonance doublet at 1548 and 1550 Å, the UV continuum near 1550 Å, and also a white-light image covering the spectrum from 2000–8000 Å.We present an analytical technique for creating photometrically accurate images of the C iv resonance lines from the data products collected by the TRACE UV channel. We use solar spectra from several space-borne instruments to represent a variety of solar conditions ranging from quiet Sun to active regions to derive a method, using a linear combination of filtered UV images, to generate an image of solar C iv 1550 Å emission. Systematic and statistical error estimates are also presented. This work indicates that C iv measurements will be reliable for intensities greater than 1014 photons s–1 cm–2 sr–1. This suggests that C iv 1550 Å images will be feasible with statistical error below 20% in the magnetic network, bright points, active regions, flares and other features bright in C iv. Below this intensity the derived image is dominated by systematic error and read noise from the CCD.     

A Study of the Distribution of Electric Currents and Current Helicity in the Photosphere at the Growth Stage of a Bipolar Active Region

The process of active region formation was researched by analyzing the densities of electric current and electric current helicity in the photosphere. The observational data were obtained with the vector magnetograph of the Sayan observatory. The appearance (as the sunspot developed) of the part of current helicity which is determined by the vertical components of the magnetic field and electric current density was studied. It is concluded that the loop-like magnetic flux tube which is responsible for the active region emergence contains thinner tubes with the same structure. The electric current system in a sunspot is simplified as the sunspot forms perhaps because the thinner flux tubes are merged together.     

Variation in the Width of Transition Region Network Boundaries

The transition region network seen in solar extreme ultraviolet (EUV) lines is the extension of the chromospheric network. The network appears as an irregular web-like pattern over the solar surface outside active regions. The average width of transition region network boundaries is obtained from the two-dimensional autocorrelation function of SOlar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer (CDS) synoptic images of the Sun in two emission lines, He i 586 Å and O v 630 Å during 1996?–?2012. The width of the network boundaries is found to be roughly correlated with the solar cycle variation with a lag of about ten months. A comparison of the widths in the two emission lines shows that they are larger for the He i line. The SOHO/CDS data also show large asymmetry in boundary widths in the horizontal (x) and vertical (y) image directions, which is shown to be caused by image distortions that are due to instrumental effects. Since the network boundary widths are related to the magnetic flux concentration along the boundaries, the results are expected to have implications on the flux transport on the solar surface, solar cycle, and the mass and energy budget of network loops and jets.     

Magnetic Field Diagnostics and Spatio-Temporal Variability of the Solar Transition Region

Magnetic field diagnostics of the transition region from the chromosphere to the corona faces us with the problem that one has to apply extreme-ultraviolet (EUV) spectro-polarimetry. While for the coronal diagnostics techniques already exist in the form of infrared coronagraphy above the limb and radio observations on the disk, one has to investigate EUV observations for the transition region. However, so far the success of such observations has been limited, but various current projects aim to obtain spectro-polarimetric data in the extreme UV in the near future. Therefore it is timely to study the polarimetric signals we can expect from these observations through realistic forward modeling. We employ a 3D magneto-hydrodynamic (MHD) forward model of the solar corona and synthesize the Stokes I and Stokes V profiles of C?iv (1548 Å). A signal well above 0.001 in Stokes V can be expected even if one integrates for several minutes to reach the required signal-to-noise ratio, and despite the rapidly changing intensity in the model (just as in observations). This variability of the intensity is often used as an argument against transition region magnetic diagnostics, which requires exposure times of minutes. However, the magnetic field is evolving much slower than the intensity, and therefore the degree of (circular) polarization remains rather constant when one integrates in time. Our study shows that it is possible to measure the transition region magnetic field if a polarimetric accuracy on the order of 0.001 can be reached, which we can expect from planned instrumentation.     

Linear Polarization Features in the Quiet-Sun Photosphere: Structure and Dynamics

We present detailed characteristics of linear polarization features (LPFs) in the quiet-Sun photosphere from high-resolution observations obtained with Sunrise/IMaX. We explore differently treated data with various noise levels in linear polarization signals, from which structure and dynamics of the LPFs are studied. Physical properties of the detected LPFs are also obtained from the results of Stokes inversions. The number of LPFs and their sizes and polarization signals are found to be strongly dependent on the noise level and on the spatial resolution. While the linear polarization with a signal-to-noise ratio \(\geq4.5\) covers about 26% of the entire area in the least noisy data in our study (with a noise level of \(1.7\times10^{-4}\) in the unit of Stokes \(I\) continuum), the detected (spatially resolved) LPFs cover about 10% of the area at any given time, with an occurrence rate on the order of \(8\times10^{-3}\mbox{ s}^{-1}\)?arcsec^?2. The LPFs were found to be short lived (in the range of 30?–?300 s), relatively small structures (radii of \(\approx0.1\)?–?1.5 arcsec), highly inclined, posing hG fields, and they move with an average horizontal speed of 1.2 km?s^?1. The LPFs were observed (almost) equally on both upflow and downflow regions, with an intensity contrast always larger than that of the average quiet Sun.     

Reconstruction of Horizontal Plasma Motions at the Photosphere from Intensitygrams: A Comparison Between DeepVel,LCT, FLCT,and CST

Direct measurements of plasma motions in the photosphere are limited to the line-of-sight component of the velocity. Several algorithms have therefore been developed to reconstruct the transverse components from observed continuum images or magnetograms. We compare the space and time averages of horizontal velocity fields in the photosphere inferred from pairs of consecutive intensitygrams by the LCT, FLCT, and CST methods and the DeepVel neural network in order to identify the method that is best suited for generating synthetic observations to be used for data assimilation. The Stein and Nordlund (Astrophys. J. Lett.753, L13, 2012) magnetoconvection simulation is used to generate synthetic SDO/HMI intensitygrams and reference flows to train DeepVel. Inferred velocity fields show that DeepVel performs best at subgranular and granular scales and is second only to FLCT at mesogranular and supergranular scales.     

Horizontal Flows in the Photosphere and Subphotosphere of Two Active Regions

We compare horizontal flow fields in the photosphere and in the subphotosphere (a layer 0.5 Mm below the photosphere) in two solar active regions: AR?11084 and AR?11158. AR?11084 is a mature, simple active region without significant flaring activity, and AR?11158 is a multipolar, complex active region with magnetic flux emerging during the period studied. Flows in the photosphere are derived by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) on HMI-observed vector magnetic fields, and the subphotospheric flows are inferred by time–distance helioseismology using HMI-observed Dopplergrams. Similar flow patterns are found for both layers for AR?11084: inward flows in the sunspot umbra and outward flows surrounding the sunspot. The boundary between the inward and outward flows, which is slightly different in the photosphere and the subphotosphere, is within the sunspot penumbra. The area having inward flows in the subphotosphere is larger than that in the photosphere. For AR?11158, flows in these two layers show great similarities in some areas and significant differences in other areas. Both layers exhibit consistent outward flows in the areas surrounding sunspots. On the other hand, most well-documented flux-emergence-related flow features seen in the photosphere do not have counterparts in the subphotosphere. This implies that the horizontal flows caused by flux emergence do not extend deeply into the subsurface.     

Explosive Events: Swirling Transition Region Jets

In this paper, we extend our earlier work to provide additional evidence for an alternative scenario to explain the nature of events called ??explosive events??. The bidirected, fast Doppler motion of explosive events observed spectroscopically in the transition region emission is classically interpreted as a pair of bidirectional jets moving upward and downward from a reconnection site. We discuss the problems of this model. In our previous work, we focused basically on the discrepancy of fast Doppler motion without detectable motion in the image plane. We now suggest an alternative scenario for the explosive events, based on our observations of spectral line tilts and bifurcated structure in some events. Both features are indicative of rotational motion in narrow structures. We explain the bifurcation as the result of rotation of hollow cylindrical structures and demonstrate that this kind of sheath model can also be applied to explain the nature of the puzzling ??explosive events??. We find that the spectral tilt, the lack of apparent motion, the bifurcation, and a rapidly growing number of direct observations support an alternative scenario of linear, spicular-sized jets with a strong spinning motion.     

关于光球横向磁场和电流的测定(英文)

本文给出了一个在观测中辨别光球横向磁场指向的一个判据。并证明了光球横向电流与横向磁场的指向无关,因而完全可以由向量磁象仪测定。     

Probing the Quiet Solar Atmosphere from the Photosphere to the Corona

We investigate the morphology and temporal variability of a quiet-Sun network region in different solar layers. The emission in several extreme ultraviolet (EUV) spectral lines through both raster and slot time-series, recorded by the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft is studied along with \(\mbox{H}\upalpha\) observations and high-resolution spectropolarimetric observations of the photospheric magnetic field. The photospheric magnetic field is extrapolated up to the corona, showing a multitude of large- and small-scale structures. We show for the first time that the smallest magnetic structures at both the network and internetwork contribute significantly to the emission in EUV lines, with temperatures ranging from \(8\times 10^{4}~\mbox{K}\) to \(6\times 10^{5}~\mbox{K}\). Two components of transition region emission are present, one associated with small-scale loops that do not reach coronal temperatures, and another component that acts as an interface between coronal and chromospheric plasma. Both components are associated with persistent chromospheric structures. The temporal variability of the EUV intensity at the network region is also associated with chromospheric motions, pointing to a connection between transition region and chromospheric features. Intensity enhancements in the EUV transition region lines are preferentially produced by \(\mbox{H}\upalpha\) upflows. Examination of two individual chromospheric jets shows that their evolution is associated with intensity variations in transition region and coronal temperatures.