Reflection Driven MHD Turbulence in the Solar Atmosphere and Wind

Alfvénic turbulence is usually invoked and used in many solar wind models (Isenberg and Hollweg, 1982, J. Geophys. Res. 87:5023; Tu et al. 1984, J. Geophys. Res. 89:9695; Hu et  al. 2000, J. Geophys. Res. 105:5093; Li 2003, Astron. Astrphys. 406:345; Isenberg 2004, J. Geophys. Res. 109:3101) as a process responsible for the transfer of energy, released at large scale in the photosphere, towards small scale in the corona, where it is dissipated. Usually an initial spectrum is prescribed since the closest constraint to the spectrum is given by Helios measurements at 0.3 AU. With this work we intend to study the efficiency of the reflection as a driver for the nonlinear interactions of Alfvén waves, the development of a turbulent spectrum and its evolution in the highly stratified solar atmosphere inside coronal holes. Our main finding is that the perpendicular spectral slope changes substantially at the transition region because of the steep density gradient. As a result a strong turbulent heating occurs, just above the transition region, as requested by current solar wind models.     

Multi-Scale Gaussian Normalization for Solar Image Processing

Extreme ultra-violet images of the corona contain information over a wide range of spatial scales, and different structures such as active regions, quiet Sun, and filament channels contain information at very different brightness regimes. Processing of these images is important to reveal information, often hidden within the data, without introducing artefacts or bias. It is also important that any process be computationally efficient, particularly given the fine spatial and temporal resolution of Atmospheric Imaging Assembly on the Solar Dynamics Observatory (AIA/SDO), and consideration of future higher resolution observations. A very efficient process is described here, which is based on localised normalising of the data at many different spatial scales. The method reveals information at the finest scales whilst maintaining enough of the larger-scale information to provide context. It also intrinsically flattens noisy regions and can reveal structure in off-limb regions out to the edge of the field of view. We also applied the method successfully to a white-light coronagraph observation.     

Automatic Solar Filament Detection Using Image Processing Techniques

We present an automatic solar filament detection algorithm based on image enhancement, segmentation, pattern recognition, and mathematical morphology methods. This algorithm cannot only detect filaments, but can also identify spines, footpoints, and filament disappearances. It consists of five steps: (1) The stabilized inverse diffusion equation (SIDE) is used to enhance and sharpen filament contours. (2) A new method for automatic threshold selection is proposed to extract filaments from local background. (3) The support vector machine (SVM) is used to differentiate between sunspots and filaments. (4) Once a filament is identified, morphological thinning, pruning, and adaptive edge linking methods are used to determine the filament properties. (5) Finally, we propose a filament matching method to detect filament disappearances. We have successfully applied the algorithm to Hα full-disk images obtained at Big Bear Solar Observatory (BBSO). It has the potential to become the foundation of an automatic solar filament detection system, which will enhance our capabilities of forecasting and predicting geo-effective events and space weather.     

High- And Low-Latitude Solar Activities: Who Leads Whom In Time Phase?

Using the data from observations of polar faculae by the National Astronomical Observatory of Japan from July 1951 to December 1998, we investigate whether there is a time lag between high-latitude solar activity and low-latitude solar activity. The cross-correlation analysis of the smoothed monthly numbers of the polar faculae with the smoothed monthly sunspot numbers shows that, high-latitude solar activity should lead low-latitude solar activity in time phase. The periodic characteristics of both of them also indicate that high-latitude activity evidently leads low-latitude activity.     

Global Forces in Eruptive Solar Flares: The Lorentz Force Acting on the Solar Atmosphere and the Solar Interior

We compute the change in the Lorentz force integrated over the outer solar atmosphere implied by observed changes in vector magnetograms that occur during large, eruptive solar flares. This force perturbation should be balanced by an equal and opposite force perturbation acting on the solar photosphere and solar interior. The resulting expression for the estimated force change in the solar interior generalizes the earlier expression presented by Hudson, Fisher, and Welsch (Astron. Soc. Pac. CS-383, 221, 2008), providing horizontal as well as vertical force components, and provides a more accurate result for the vertical component of the perturbed force. We show that magnetic eruptions should result in the magnetic field at the photosphere becoming more horizontal, and hence should result in a downward (toward the solar interior) force change acting on the photosphere and solar interior, as recently argued from an analysis of magnetogram data by Wang and Liu (Astrophys. J. Lett. 716, L195, 2010). We suggest the existence of an observational relationship between the force change computed from changes in the vector magnetograms, the outward momentum carried by the ejecta from the flare, and the properties of the helioseismic disturbance driven by the downward force change. We use the impulse driven by the Lorentz-force change in the outer solar atmosphere to derive an upper limit to the mass of erupting plasma that can escape from the Sun. Finally, we compare the expected Lorentz-force change at the photosphere with simple estimates from flare-driven gasdynamic disturbances and from an estimate of the perturbed pressure from radiative backwarming of the photosphere in flaring conditions.     

太阳大气中环状结构的形状

根据力平衡条件,求得了与静止分层大气中一条孤立细磁通量管形状有关的一个积分,(1-M_a~2)Bcosθ=const.,这里M_a是阿尔芬马赫数,B是磁场强度,θ是管切向与水平方向的夹角,并简单地讨论了这个积分在等温与静平衡条件下的应用.     

The Validity of Dynamical Models of the Solar Atmosphere

Important results on the structure and dynamics of the nonmagnetic solar chromosphere are based on hydrodynamic models that oversimplify either the geometry of the atmosphere or the interaction of radiation and matter. Although the observed granulation pattern is well reproduced by the three-dimensional (3D) models, oversimplification of radiative relaxation leads to the prediction of temperature fluctuations that are too high (by a factor of 10 to 100) and result in a monotonic decrease with height in the chromosphere of the horizontally and temporally averaged temperature, and hence in the prediction of absorption lines at wavelengths where only emission lines are observed on the Sun. New values of solar abundances of oxygen and other metals are based on 3D hydrodynamic models with temporal and spatial fluctuations that are far greater than those observed. These new abundances destroy the previous agreement of observed modes with acoustic eigenmodes that had been predicted for the old abundances from a solar model for which the sound speed throughout most of the Sun was determined to an accuracy of a few parts in 10⁴. One expects that, when radiative relaxation is properly accounted for, 3D models will reproduce the essential characteristics of the solar atmosphere, among them a positive temperature gradient in the outward direction and hence exclusively emission lines in the extreme ultraviolet at all times and positions in the nonmagnetic chromosphere. A?minimum characteristic length of 0.1?arcsec is identified for the solar atmosphere, below which there is no significant structure in the actual Sun, only in wave models of the Sun. This criticism does not detract from the notable success of hydrodynamic modeling to explain the mechanism by which chromospheric H_2V and K_2V bright points are formed.     

Web-Based Data Processing System for Automated Detection of Oscillations with Applications to the Solar Atmosphere

A web-based, interactive system for the remote processing of imaging data sets (i.e., EUV, X-ray, and microwave) and the automated interactive detection of wave and oscillatory phenomena in the solar atmosphere is presented. The system targets localized, but spatially resolved, phenomena such as kink, sausage, and longitudinal propagating and standing waves. The system implements the methods of Periodmapping for pre-analysis, and Pixelized Wavelet Filtering for detailed analysis of the imaging data cubes. The system is implemented on the dedicated data-processing server , which is situated at the Institute of Solar-Terrestrial Physics, Irkutsk, Russia. Input data in the .sav, .fits, or .txt formats can be submitted via the local and/or global network (the Internet). The output data can be in the png, jpeg, and binary formats, on the user’s request. The output data are periodmaps; narrowband amplitude, power, phase and correlation maps of the wave’s sources at significant harmonics and in the chosen spectral intervals, and mpeg movies of their evolution. The system was tested by the analysis of the EUV and microwave emission from the active region NOAA 10756 on 4 May 2005 observed with TRACE and the Nobeyama Radioheliograph. The similarity of the spatial localization of three-minute propagating waves, near the footpoint of locally open magnetic-field lines determined by the potential-field extrapolation, in both the transition region and the corona was established. In the transition region the growth of the three-minute amplitude was found to be accompanied by the decrease in the line-of-sight angle to the wave-propagation direction.     

2D Feature Recognition And 3d Reconstruction In Solar Euv Images

EUV images show the solar corona in a typical temperature range of T >rsim 1 MK, which encompasses the most common coronal structures: loops, filaments, and other magnetic structures in active regions, the quiet Sun, and coronal holes. Quantitative analysis increasingly demands automated 2D feature recognition and 3D reconstruction, in order to localize, track, and monitor the evolution of such coronal structures. We discuss numerical tools that “fingerprint” curvi-linear 1D features (e.g., loops and filaments). We discuss existing finger-printing algorithms, such as the brightness-gradient method, the oriented-connectivity method, stereoscopic methods, time-differencing, and space–time feature recognition. We discuss improved 2D feature recognition and 3D reconstruction techniques that make use of additional a priori constraints, using guidance from magnetic field extrapolations, curvature radii constraints, and acceleration and velocity constraints in time-dependent image sequences. Applications of these algorithms aid the analysis of SOHO/EIT, TRACE, and STEREO/SECCHI data, such as disentangling, 3D reconstruction, and hydrodynamic modeling of coronal loops, postflare loops, filaments, prominences, and 3D reconstruction of the coronal magnetic field in general.     

太阳大气磁精细结构的射电探讨

太阳大气磁场的研究对于太阳大气物理及太阳活动研究是十分重要的。目前探测光球以外的日够以球，过渡区磁场的几乎唯一办法，是在紧密联系其他频说段取得的信息基础上使用射电观测。根据在微波，米波段有关辐射机制和传播过程，介绍了推导磁场讯息的基本射电方法。     

Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity

An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change, near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up the restorations. We present a number of examples of applications of the restoration method to observations obtained with the Swedish 1-m Solar Telescope on La Palma.     

Observation of Turbulence in Solar Surface Convection: I. Line Parameter Correlations

By using slit observations of solar photospheric lines shifted by 0.4 arcsec, a 2D field on the Sun was scanned to obtain a 16-minute time series of 2D line-parameter variations. The aim was to investigate in detail the occurrence of turbulence that can be measured by line-width variations extracted from the line profiles. The continuum-intensity variation served as a proxy for granular (bright) and intergranular (dark) areas. The results show that turbulence is not limited to the intergranular space but is also produced by horizontal motions that may become supersonic, leading to turbulence. These motions lead to brightenings, as predicted by theoretical models. Thus, enhanced line-width variations are found to occur in both bright and dark areas. A Sobel filter served to detect the areas where strong gradients in the line parameters occur. By applying this filter to the different line-parameter variations over the 2D field observed, we can determine whether there exists a similarity of these strong-gradient patterns with other parameters that characterize granular motions such as intensity variations or velocity fluctuations.     

Light Deflection Near the Sun's Limb: Refraction by the Solar Atmosphere

Light refraction by the Sun's atmosphere is calculated.As detected from the Earth, the refraction can deflect a light ray emitted from the Sun's limb by 13 or a starlight ray grazing the solar limb by 26, an effect 15 times larger than the gravitational deflection.     

The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO

Solar flares presumably have an impact on the deepest layers of the solar atmosphere and yet the observational evidence for such an impact is scarce. Using ten years of measurements of the Na D₁ and Na D₂ Fraunhofer lines, measured by GOLF onboard SOHO, we show that this photospheric line is indeed affected by flares. The effect of individual flares is hidden by solar oscillations, but a statistical analysis based on conditional averaging reveals a clear signature. Although GOLF can only probe one single wavelength at a time, we show that both wings of the Na line can nevertheless be compared. The varying line asymmetry can be interpreted as an upward plasma motion from the lower solar atmosphere during the peak of the flare, followed by a downward motion.     

Advanced Automated Solar Filament Detection And Characterization Code: Description, Performance, And Results

We present a code for automated detection, classification, and tracking of solar filaments in full-disk Hα images that can contribute to Living With a Star science investigations and space weather forecasting. The program can reliably identify filaments; determine their chirality and other relevant parameters like filament area, length, and average orientation with respect to the equator. It is also capable of tracking the day-by-day evolution of filaments while they travel across the visible disk. The code was tested by analyzing daily Hα images taken at the Big Bear Solar Observatory from mid-2000 until beginning of 2005. It identified and established the chirality of thousands of filaments without human intervention. We compared the results with a list of filament proprieties manually compiled by Pevtsov, Balasubramaniam and Rogers (2003) over the same period of time. The computer list matches Pevtsov's list with a 72% accuracy. The code results confirm the hemispheric chirality rule stating that dextral filaments predominate in the north and sinistral ones predominate in the south. The main difference between the two lists is that the code finds significantly more filaments without an identifiable chirality. This may be due to a tendency of human operators to be biased, thereby assigning a chirality in less clear cases, while the code is totally unbiased. We also have found evidence that filaments obeying the chirality rule tend to be larger and last longer than the ones that do not follow the hemispherical rule. Filaments adhering to the hemispheric rule also tend to be more tilted toward the equator between latitudes 10^° and 30^°, than the ones that do not.     

1D Solar Image Synthesis: A Multifrequency Analysis

The radio emission from the solar corona is related to the configuration of the inner atmosphere. By studying the Sun at multiple frequencies, different layers of plasma in solar atmosphere are probed. We use the Mauritius Radio Telescope. The quiet Sun period, difference maps using synthesized 1D maps reveal a certain regular feature, the origin of which is not thoroughly understood and which is attributed to the solar differential rotation. For the active Sun period, the coronal emission is linked to the magnetic field configuration originating from the inner atmosphere. As expected, a strong correlation exists between the MRT 151 MHz and Nancay 164 MHz radio emission. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Early Diagnostics of the Geoeffectiveness of Solar Eruptions from Photospheric Magnetic Flux Observations: The Transition from SOHO to SDO

In our previous articles (Chertok et al. in Solar Phys. 282, 175, 2013; Chertok et al. in Solar Phys. 290, 627, 2015), we presented a preliminary tool for the early diagnostics of the geoeffectiveness of solar eruptions based on the estimate of the total unsigned line-of-sight photospheric magnetic flux in accompanying extreme ultraviolet (EUV) arcades and dimmings. This tool was based on the analysis of eruptions observed during 1996?–?2005 with the Extreme-ultraviolet Imaging Telescope (EIT) and the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO). Empirical relationships were obtained to estimate the probable importance of upcoming space weather disturbances caused by an eruption, which just occurred, without data on the associated coronal mass ejections. In particular, it was possible to estimate the intensity of a non-recurrent geomagnetic storm (GMS) and Forbush decrease (FD), as well as their onset and peak times. After 2010?–?2011, data on solar eruptions are obtained with the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We use relatively short intervals of overlapping EIT–AIA and MDI–HMI detailed observations, and additionally, a number of large eruptions over the next five years with the 12-hour cadence EIT images to adapt the SOHO diagnostic tool to SDO data. We show that the adopted brightness thresholds select practically the same areas of arcades and dimmings from the EIT 195 Å and AIA 193 Å image, with a cross-calibration factor of 3.6?–?5.8 (5.0?–?8.2) for the AIA exposure time of 2.0 s (2.9 s). We also find that for the same photospheric areas, the MDI line-of-sight magnetic flux systematically exceeds the HMI flux by a factor of 1.4. Based on these results, the empirical diagnostic relationships obtained from SOHO data are adjusted to SDO instruments. Examples of a post-diagnostics based on SDO data are presented. As before, the tool is applicable to non-recurrent GMSs and FDs caused by nearly central eruptions from active regions, provided that the southern component of the interplanetary magnetic field near the Earth is predominantly negative, which is not predicted by this tool.