Magnetic field intensification: comparison of 3D MHD simulations with Hinode/SP results

Recent spectro-polarimetric observations have provided detailed measurements of magnetic field, velocity and intensity during events of magnetic field intensification in the solar photosphere. We consider the temporal evolution of the relevant physical quantities for three cases of magnetic field intensification in a numerical simulation. We determine the evolution of the intensity, magnetic flux density and zero-crossing velocity derived from the synthetic Stokes parameters by taking into account the spectral and spatial resolution of the spectropolarimeter (SP) on board Hinode. The three events considered show a similar evolution: advection of magnetic flux to a granular vertex, development of a strong downflow, evacuation of the magnetic feature, increase of the field strength and the appearance of the bright point. We find that synthetic and real observations are qualitatively consistent and, for one of the cases considered, agree very well also quantitatively. The effect of finite resolution (spatial smearing) is most pronounced in the case of small features, for which the synthetic Hinode/SP observations miss the bright point formation and also the high-velocity downflows during the formation of the smaller magnetic features.Comment: accepted in A&

Probing quiet Sun magnetism using MURaM simulations and Hinode/SP results: support for a local dynamo

We obtain information about the magnetic flux present in the quiet Sun by comparing radiative MHD simulations with Hinode/SP observations, with particular emphasis on the role of surface dynamo action. Simulation runs with different magnetic Reynolds numbers (Rm) are used together with observations at different heliocentric angles with different levels of noise. The results show that simulations with an imposed mixed-polarity field and Rm below the threshold for dynamo action reproduce the observed vertical flux density, but do not display a sufficiently high horizontal flux density. Surface dynamo simulations at the highest Rm feasible at the moment yield a ratio of the horizontal and vertical flux density consistent with observational results, but the overall amplitudes are too low. Based on the properties of the local dynamo simulations, a tentative scaling of the magnetic field strength by a factor 2 - 3 reproduces the signal observed in the internetwork regions. We find an agreement with observations at different heliocentric angles. The mean field strength in internetwork, implied by our analysis, is roughly 170 G at the optical depth unity. Our study shows that surface dynamo could be responsible for most of the magnetic flux in the quiet Sun outside the network given that the extrapolation to higher Rm is valid.Comment: accepted in A&

STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations

The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magnetic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. To apply these techniques to spectral lines that sample the chromosphere, NLTE effects must be included in the calculations. We developed a new inversion code STiC to study spectral lines that sample the upper chromosphere. The code is based the RH synthetis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.Comment: Accepted for publication in Astronomy & Astrophysic

Transport of magnetic flux from the canopy to the internetwork

Recent observations have revealed that 8% of linear polarization patches in the internetwork quiet Sun are fully embedded in downflows. These are not easily explained with the typical scenarios for the source of internetwork fields which rely on flux emergence from below. We explore using radiative MHD simulations a scenario where magnetic flux is transported from the magnetic canopy overlying the internetwork into the photosphere by means of downward plumes associated with convective overshoot. We find that if a canopy-like magnetic field is present in the simulation, the transport of flux from the canopy is an important process for seeding the photospheric layers of the internetwork with magnetic field. We propose that this mechanism is relevant for the Sun as well, and it could naturally explain the observed internetwork linear polarization patches entirely embedded in downflows.Comment: Accepted to Ap

High-frequency Oscillations in Small Magnetic Elements Observed with Sunrise/SuFI

We characterize waves in small magnetic elements and investigate their propagation in the lower solar atmosphere from observations at high spatial and temporal resolution. We use the wavelet transform to analyze oscillations of both horizontal displacement and intensity in magnetic bright points found in the 300 nm and the Ca II H 396.8 nm passbands of the filter imager on board the Sunrise balloon-borne solar observatory. Phase differences between the oscillations at the two atmospheric layers corresponding to the two passbands reveal upward propagating waves at high frequencies (up to 30 mHz). Weak signatures of standing as well as downward propagating waves are also obtained. Both compressible and incompressible (kink) waves are found in the small-scale magnetic features. The two types of waves have different, though overlapping, period distributions. Two independent estimates give a height difference of approximately 450+-100 km between the two atmospheric layers sampled by the employed spectral bands. This value, together with the determined short travel times of the transverse and longitudinal waves provide us with phase speeds of 29+-2 km/s and 31+-2 km/s, respectively. We speculate that these phase speeds may not reflect the true propagation speeds of the waves. Thus, effects such as the refraction of fast longitudinal waves may contribute to an overestimate of the phase speed.Comment: 14 pages, 7 figure

Inclinations of small quiet-Sun magnetic features based on a new geometric approach

High levels of horizontal magnetic flux have been reported in the quiet-Sun internetwork, often based on Stokes profile inversions. Here we introduce a new method for deducing the inclination of magnetic elements and use it to test magnetic field inclinations from inversions. We determine accurate positions of a set of small, bright magnetic elements in high spatial resolution images sampling different photospheric heights obtained by the Sunrise balloon-borne solar observatory. Together with estimates of the formation heights of the employed spectral bands, these provide us with the inclinations of the magnetic features. We also compute the magnetic inclination angle of the same magnetic features from the inversion of simultaneously recorded Stokes parameters. Our new, geometric method returns nearly vertical fields (average inclination of around 14 deg with a relatively narrow distribution having a standard deviation of 6 deg). In strong contrast to this, the traditionally used inversions give almost horizontal fields (average inclination of 75+-8 deg) for the same small magnetic features, whose linearly polarised Stokes profiles are adversely affected by noise. The almost vertical field of bright magnetic features from our geometric method is clearly incompatible with the nearly horizontal magnetic fields obtained from the inversions. This indicates that the amount of magnetic flux in horizontal fields deduced from inversions is overestimated in the presence of weak Stokes signals, in particular if Stokes Q and U are close to or under the noise level. By combining the proposed method with inversions we are not just improving the inclination, but also the field strength. This technique allows us to analyse features that are not reliably treated by inversions, thus greatly extending our capability to study the complete magnetic field of the quiet Sun.Comment: 12 pages, 9 figures, 1 table; Accepted for publication in Astronomy & Astrophysic

Small-scale loops heated to transition region temperatures and their chromospheric signatures in the simulated solar atmosphere

Recent observations revealed loop-like structures at very small scales visible in observables that sample transition region (TR) and coronal temperatures. Their formation remains unclear. We study an example of a bipolar system in realistic magnetohydrodynamic simulations and forward synthesis of spectral lines to investigate how these features occur. Computations are done using the MURaM code to generate model atmospheres. The synthetic H$\alpha$ and Si IV spectra are calculated at two angles ($\mu = 1$, $\mu = 0.66$) using the Multi3D code. Magnetic field lines are traced in the model and the evolution of the underlying field topology is examined. The synthetic H$\alpha$ dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic H$\alpha$ line profiles show observed asymmetries and doppler shifts in the line core. They, however, also show strong emission peaks in the line wings, even at the slated view. The synthetic Si IV emission features partly coincide with structures visible in H$\alpha$ dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg$(T /K)= [5.0, 5.5]$ temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region. We find that our results largely reproduce the observed features and their characteristics. A bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.Comment: Online material: https://dubshen.astro.su.se/~mosk1498/mov/fig16_4p.mp

Photospheric response to an ellerman bomb-like event—an analogy of Sunrise/IMaX observations and MHD simulations

S. Danilovic et. al.©2017 The American Astronomical Society. All rights reserved.Ellerman Bombs are signatures of magnetic reconnection, which is an important physical process in the solar atmosphere. How and where they occur is a subject of debate. In this paper, we analyze Sunrise/IMaX data, along with 3D MHD simulations that aim to reproduce the exact scenario proposed for the formation of these features. Although the observed event seems to be more dynamic and violent than the simulated one, simulations clearly confirm the basic scenario for the production of EBs. The simulations also reveal the full complexity of the underlying process. The simulated observations show that the Fe i 525.02 nm line gives no information on the height where reconnection takes place. It can only give clues about the heating in the aftermath of the reconnection. However, the information on the magnetic field vector and velocity at this spatial resolution is extremely valuable because it shows what numerical models miss and how they can be improved.The German contribution to Sunrise and its reflight was funded by the Max Planck Foundation, the Strategic Innovations Fund of the President of the Max Planck Society (MPG), DLR, and private donations by supporting members of the Max Planck Society, which are all gratefully acknowledged. This work has benefited from the discussions at the meeting "Solar UV bursts—a new insight to magnetic reconnection" at the International Space Science Institute (ISSI) in Bern. The Spanish contribution was funded by the Ministerio de Economia y Competitividad under Projects ESP2013-47349-C6 and ESP2014-56169-C6, partially using European FEDER funds. The HAO contribution was partly funded through NASA grant number NNX13AE95G. The National Solar Observatory (NSO) is operated by the Association of Universities for Research in Astronomy (AURA) Inc. under a cooperative agreement with the National Science Foundation. This work was also partly supported by the BK21 plus program through the National Research Foundation (NRF), funded by the Ministry of Education of Korea.Peer reviewe

First high-resolution images of the Sun in the 2796 \AA{} Mg II k line

We present the first high-resolution solar images in the Mg II k 2796 \AA{} line. The images, taken through a 4.8 \AA{} broad interference filter, were obtained during the second science flight of SUNRISE in June 2013 by the SuFI instrument. The Mg II k images display structures that look qualitatively very similar to images taken in the core of Ca II H. The Mg II images exhibit reversed granulation (or shock waves) in the internetwork regions of the quiet Sun, at intensity contrasts that are similar to those found in Ca II H. Very prominent in Mg II are bright points, both in the quiet Sun and in plage regions, particularly near disk center. These are much brighter than at other wavelengths sampled at similar resolution. Furthermore, Mg II k images also show fibril structures associated with plage regions. Again, the fibrils are similar to those seen in Ca II H images, but tend to be more pronounced, particularly in weak plage.Comment: Accepted for publication in The Astrophysical Journal Letter

Solar image denoising with convolutional neural networks

The topology and dynamics of the solar chromosphere are greatly affected by the presence of magnetic fields. The magnetic field can be inferred by analyzing polarimetric observations of spectral lines. Polarimetric signals induced by chromospheric magnetic fields are, however, particularly weak, and in most cases very close to the detection limit of current instrumentation. Because of this, there are only few observational studies that have successfully reconstructed the three components of the magnetic field vector in the chromosphere. Traditionally, the signal-to-noise ratio of observations has been improved by performing time-averages or spatial averages, but in both cases, some information is lost. More advanced techniques, like principal-component-analysis, have also been employed to take advantage of the sparsity of the observations in the spectral direction. In the present study, we propose to use the spatial coherence of the observations to reduce the noise using deep-learning techniques. We design a neural network that is capable of recovering weak signals under a complex noise corruption (including instrumental artifacts and non-linear post-processing). The training of the network is carried out without a priori knowledge of the clean signals, or an explicit statistical characterization of the noise or other corruption. We only use the same observations as our generative model. The performance of this method is demonstrated on both, synthetic experiments and real data. We show examples of the improvement in typical signals obtained in current telescopes such as the Swedish 1-meter Solar Telescope. The presented method can recover weak signals equally well no matter on what spectral line or spectral sampling is used. It is especially suitable for cases when the wavelength sampling is scarce.Comment: 13 pages; accepted for publication in A&