Internet Work Research Articles

Solar internetwork magnetic fields: Statistical comparison between observations and MHD simulations

Context. Although the magnetic fields in the quiet Sun account for the majority of the magnetic energy in the solar photosphere, inferring their exact spatial distribution, origin, and evolution poses an important challenge because the signals lie at the limit of today’s instrumental precision. This severely hinders and biases our interpretations, which are mostly made through nonlinear model-fitting approaches. Aims. Our goal is to directly compare simulated and observed polarization signals in the Fe I 6301 Å and 6302 Å spectral lines in the very quiet Sun, the so-called solar internetwork (IN). This way, we aim to constrain the mechanism responsible for the generation of the quiet Sun magnetism while avoiding the biases that plague other diagnostic methods. Methods. We used three different three-dimensional radiative magneto-hydrodynamic simulations representing different scenarios of magnetic field generation in the internetwork: small-scale dynamo, decay of active regions, and horizontal flux emergence. We synthesized Stokes profiles at different viewing angles and degraded them according to the instrumental specifications of the spectro-polarimeter (SP) on board the Hinode satellite. Finally, we statistically compared the simulated spectra to the Hinode/SOT/SP observations at the appropriate viewing angles. Results. Of the three simulations, the small-scale dynamo best reproduced the statistical properties of the observed polarization signals. This is especially prominent for the disk center viewing geometry, where the agreement is excellent. Moving toward more inclined lines of sight, the agreement worsens slightly. Conclusions The agreement between the small-scale dynamo simulation and observations at the disk center suggests that small-scale dynamo action plays an important role in the generation of quiet Sun magnetism. However, the magnetic field around 50 km above the continuum layer in this simulation does not reproduce observations as well as at the very base of the photosphere.

Global Impact of Emerging Internetwork Fields on the Low Solar Atmosphere

Small-scale, newly emerging internetwork (IN) magnetic fields are considered a viable source of energy and mass for the solar chromosphere and possibly the corona. Multiple studies show that single events of flux emergence can indeed locally heat the low solar atmosphere through interactions of the upward propagating magnetic loops and the preexisting ambient field lines. However, the global impact of the newly emerging IN fields on the solar atmosphere is still unknown. In this paper, we study the spatiotemporal evolution of IN bipolar flux features and analyze their impact on the energetics and dynamics of the quiet-Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph, Hinode, and the Solar Dynamics Observatory to identify emerging IN magnetic fields and follow their evolution. Our observational results suggest that only the largest IN bipoles are capable of heating locally the low solar atmosphere, while the global contribution of these bipoles appears to be marginal. However, the total number of bipoles detected and their impact estimated in this work is limited by the sensitivity level, spatial resolution, and duration of our observations. To detect smaller and weaker IN fields that would maintain the basal flux, and examine their contribution to the chromospheric heating, we will need higher resolution, higher sensitivity, and longer time series obtained with current and next-generation ground- and space-based telescopes.

Investigating the magnetic field of the quiet Sun internetwork

Abstract We analyze the magnetism of the quiet Sun internetwork (IN) using high-spatial-resolution data obtained by the spectropolarimeter (SP) of the Solar Optical Telescope aboard the Hinode satellite near the disk center of the Sun. The SP data were inverted using the Stokes Inversion based on Response functions (SIR) inversion code with a single-component atmosphere with depth dependent in the solar photosphere, assuming gradients in physical parameters along the line of sight (LOS). To avoid the effect of noise, only pixels with Stokes U and/or Q signals above 4.5 times the noise level are considered. The inversion results show that the magnetic field of the IN has mainly hG field strength and the inclination distribution is quasi-isotropic at the solar surface. The field strength decreases with height and becomes predominantly horizontal at the upper layers. At the mid photosphere, the distributions of field strength and inclination are consistent with those derived by Milne–Eddington inversion. The mean transverse and longitudinal flux densities are 66 Mx cm−2 and 13 Mx cm−2 at log τ = −1.0; we also study the ratio between the transverse and longitudinal components in the IN region as a function of depth in the photosphere, finding that the ratio is almost 2.7 in the deep layer, increasing to 5.1 in the upper layer. The mean field strength is greater than 100 G in the upper photosphere, which is consistent with the results based on the Hanle effect. We present the LOS velocity probability distribution function for IN at different optical depths, where its distribution is mainly associated with upflow velocities of VLOS = 1.2 and 0.6 km s−1 in the deeper and upper layers, respectively. In addition, there exists a reliable inversion analysis, which is obvious from the comparison between the observed and calculated area asymmetries in both Fe i lines of Hinode SP data.

The Solar Internetwork. III. Unipolar versus Bipolar Flux Appearance

Abstract Small-scale internetwork (IN) magnetic fields are considered to be the main building blocks of quiet Sun magnetism. For this reason, it is crucial to understand how they appear on the solar surface. Here, we employ a high-resolution, high-sensitivity, long-duration Hinode/NFI magnetogram sequence to analyze the appearance modes and spatiotemporal evolution of individual IN magnetic elements inside a supergranular cell at the disk center. From identification of flux patches and magnetofrictional simulations, we show that there are two distinct populations of IN flux concentrations: unipolar and bipolar features. Bipolar features tend to be bigger and stronger than unipolar features. They also live longer and carry more flux per feature. Both types of flux concentrations appear uniformly over the solar surface. However, we argue that bipolar features truly represent the emergence of new flux on the solar surface, while unipolar features seem to be formed by the coalescence of background flux. Magnetic bipoles appear at a faster rate than unipolar features (68 as opposed to 55 Mx cm−2 day−1), and provide about 70% of the total instantaneous IN flux detected in the interior of the supergranule.

Constraining the magnetic vector in the quiet solar photosphere and the impact of instrumental degradation

Context. With the advent of next generation high resolution telescopes, our understanding of how the magnetic field is organized in the internetwork (IN) photosphere is likely to advance significantly. Aims. We aim to evaluate the extent to which we can retrieve accurate information about the magnetic vector in the IN photosphere using inversion techniques. Methods. We use a snapshot produced from high resolution three-dimensional magnetohydrodynamic (MHD) simulations and employ the Stokes Inversions based on Response functions (SIR) code to produce synthetic observables in the same near infrared spectral window as observed by the GREGOR Infrared Spectrograph (GRIS), which contains the highly magnetically sensitive photospheric Fe I line pair at 15 648.52 Å and 15 652.87 Å. We then use a parallelized wrapper to SIR to perform nearly 14 million inversions of the synthetic spectra to test how well the ‘true’ MHD atmospheric parameters can be constrained statistically. Finally, we degrade the synthetic Stokes vector spectrally and spatially to GREGOR resolutions and examine how this influences real observations, considering the impact of stray light, spatial resolution and signal-to-noise (S/N) in particular. Results. We find that the depth-averaged parameters can be recovered by the inversions of the undegraded profiles, and by adding simple gradients to magnetic field strength, inclination, and line of sight velocity we show that an improvement in the χ2 value is achieved. We also evaluate the extent to which we can constrain these parameters at various optical depths, with the kinematic and thermodynamic parameters sensitive deeper in the atmosphere than the magnetic parameters. We find the S/N and spatial resolution both play a significant role in determining how the degraded atmosphere appears. At the same time, we find that the magnetic and kinematic parameters are invariant upon inclusion of an unpolarized stray light. We compare our results to recent IN observations obtained by GREGOR. We studied a linear polarization feature which resembles those recently observed by GRIS in terms of appearing as ‘loop-like’ structures and exhibiting very similar magnetic flux density. Thus, we demonstrate that realistic MHD simulations are capable of showing close agreement with real observations, and the symbiosis between them and observations continues to prove essential. We finally discuss the considerations that must be made for DKIST-era observations.

Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields

Abstract The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by the magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper, we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ∼3 minutes in the photosphere and ∼12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg ii h and k lines, we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

Dynamics of internetwork chromospheric fibrils: Basic properties and magnetohydrodynamic kink waves

Using the spectroscopic imaging capabilities of the Swedish Solar Telescope, we aim to provide the first investigation on the nature and dynamics of elongated absorption features (fibrils) observed in H$\alpha$ in the internetwork. We observe and identify a number of internetwork fibrils, which form away from the kilogauss, network magnetic flux, and we provide a synoptic view on their behaviour. The internetwork fibrils are found to support wave-like behaviour, which we interpret as Magnetohydrodynamic (MHD) kink waves. The properties of these waves, that is, amplitude, period, and propagation speed, are measured from time-distance diagrams and we attempt to exploit them via magneto-seismology in order to probe the variation of plasma properties along the wave-guides. We found that the Internetwork (IN) fibrils appear, disappear, and re-appear on timescales of tens of minutes, suggesting that they are subject to repeated heating. No clear photospheric footpoints for the fibrils are found in photospheric magnetograms or H$\alpha$ wing images. However, we suggest that they are magnetised features as the majority of them show evidence of supporting propagating MHD kink waves, with a modal period of $120$~s. Additionally, one IN fibril is seen to support a flow directed along its elongated axis, suggesting a guiding field. The wave motions are found to propagate at speeds significantly greater than estimates for typical chromospheric sound speeds. Through their interpretation as kink waves, the measured speeds provide an estimate for local average Alfv\'en speeds. Furthermore, the amplitudes of the waves are also found to vary as a function of distance along the fibrils, which can be interpreted as evidence of stratification of the plasma in the neighbourhood of the IN fibril.

Estimation of the Magnetic Flux Emergence Rate in the Quiet Sun from Sunrise Data

Small-scale internetwork (IN) features are thought to be the major source of fresh magnetic flux in the quiet Sun. During its first science flight in 2009, the balloon-borne observatory Sunrise captured images of the magnetic fields in the quiet Sun at a high spatial resolution. Using these data we measure the rate at which the IN features bring magnetic flux to the solar surface. In a previous paper it was found that the lowest magnetic flux in small-scale features detected using the Sunrise observations is 9 x 10^14 Mx. This is nearly an order of magnitude smaller than the smallest fluxes of features detected in observations from the Hinode satellite. In this paper, we compute the flux emergence rate (FER) by accounting for such small fluxes, which was not possible before Sunrise. By tracking the features with fluxes in the range 10^15-10^18 Mx, we measure an FER of 1100 Mx cm^-2 day^-1. The smaller features with fluxes less than or equal to 10^16 Mx are found to be the dominant contributors to the solar magnetic flux. The FER found here is an order of magnitude higher than the rate from Hinode, obtained with a similar feature tracking technique. A wider comparison with the literature shows, however, that the exact technique of determining the rate of the appearance of new flux can lead to results that differ by up to two orders of magnitude, even when applied to similar data. The causes of this discrepancy are discussed and first qualitative explanations proposed.

Wave propagation in a solar quiet region and the influence of the magnetic canopy

Aims. We seek indications or evidence of transmission/conversion of magnetoacoustic waves at the magnetic canopy, as a result of its impact on the properties of the wave field of the photosphere and chromosphere. Methods. We use cross-wavelet analysis to measure phase differences between intensity and Doppler signal oscillations in the Halpha, CaII H, and G-band.We use the height of the magnetic canopy to create appropriate masks to separate internetwork (IN) and magnetic canopy regions. We study wave propagation and differences between these two regions. Results. The magnetic canopy affects wave propagation by lowering the phase differences of progressive waves and allowing the propagation of waves with frequencies lower than the acoustic cut-off. We also find indications in the Doppler signals of Halpha of a response to the acoustic waves at the IN, observed in the CaII H line. This response is affected by the presence of the magnetic canopy. Conclusions. Phase difference analysis indicates the existence of a complicated wave field in the quiet Sun, which is composed of a mixture of progressive and standing waves. There are clear imprints of mode conversion and transmission due to the interaction between the p-modes and small-scale magnetic fields of the network and internetwork.

DIFFUSION OF MAGNETIC ELEMENTS IN A SUPERGRANULAR CELL

Small scale magnetic fields (magnetic elements) are ubiquitous in the solar photosphere. Their interaction can provide energy to the upper atmospheric layers, and contribute to heat the solar corona. In this work, the dynamic properties of magnetic elements in the quiet Sun are investigated. The high number of magnetic elements detected in a supegranular cell allowed us to compute their displacement spectrum $\langle(\Delta r)^2\rangle\propto\tau^\gamma$ (being $\gamma>0$, and $\tau$ the time since the first detection), separating the contribution of the network (NW) and the internetwork (IN) regions. In particular, we found $\gamma=1.27\pm0.05$ and $\gamma=1.08\pm0.11$ in NW (at smaller and larger scales, respectively), and $\gamma=1.44\pm0.08$ in IN. These results are discussed in light of the literature on the topic, as well as the implications for the build up of the magnetic network.

ANALYSIS OF QUIET-SUN INTERNETWORK MAGNETIC FIELDS BASED ON LINEAR POLARIZATION SIGNALS

We present results from the analysis of Fe i 630 nm measurements of the quiet Sun taken with the spectropolarimeter of the Hinode satellite. Two data sets with noise levels of 1.2 × 10−3 and 3 × 10−4 are employed. We determine the distribution of field strengths and inclinations by inverting the two observations with a Milne–Eddington model atmosphere. The inversions show a predominance of weak, highly inclined fields. By means of several tests we conclude that these properties cannot be attributed to photon noise effects. To obtain the most accurate results, we focus on the 27.4% of the pixels in the second data set that have linear polarization amplitudes larger than 4.5 times the noise level. The vector magnetic field derived for these pixels is very precise because both circular and linear polarization signals are used simultaneously. The inferred field strength, inclination, and filling factor distributions agree with previous results, supporting the idea that internetwork (IN) fields are weak and very inclined, at least in about one quarter of the area occupied by the IN. These properties differ from those of network fields. The average magnetic flux density and the mean field strength derived from the 27.4% of the field of view with clear linear polarization signals are 16.3 Mx cm−2 and 220 G, respectively. The ratio between the average horizontal and vertical components of the field is approximately 3.1. The IN fields do not follow an isotropic distribution of orientations.

Asymmetries of the StokesVprofiles observed by HINODE SOT/SP in the quiet Sun

We present the first classification of SOT/SP circular polarization measurements with the aim of highlighting exhaustively the whole variety of Stokes V shapes emerging from the quiet Sun. k-means is used to classify HINODE SOT/SP Stokes V profiles observed in the quiet Sun network and internetwork (IN). We analyze a 302 x 162 square arcsec field-of-view (FOV) which can be considered a complete sample of quiet Sun measurements performed at at the disk center with 0.32 arcsec angular resolution and 0.001 polarimetric sensitivity. Such a classification allows us to divide the whole dataset in classes, with each class represented by a cluster profile, i.e., the average of the profiles in the class. The set of 35 cluster profiles derived from the analysis completely characterizes SOT/SP quiet Sun measurements. The separation between network and IN profile shapes is evident - classes in the network are not present in the IN, and vice versa. Asymmetric profiles are approximatively 93 % of the total number of profiles. Among these, approximatively 34 % of the profiles are strongly asymmetric profiles, and they can be divided in three families: blue-lobe, red-lobe, and Q-like profiles. The blue-lobe profiles tend to be associated with upflows (granules), whereas the red-lobe and Q-like ones appear in downflows (intergranular lanes). Such profiles need to be interpreted considering model atmospheres different from a uniformly magnetized Milne-Eddington (ME) atmosphere, i.e., characterized by gradients and/or discontinuities in the magnetic field and velocity along the line-of-sight (LOS).

Interpretation ofHINODESOT/SP asymmetric Stokes profiles observed in the quiet Sun network and internetwork

We present the first interpretation of the Stokes profile asymmetries measured in the FeI 630 nm lines by SOT/SP, in both quiet Sun internetwork (IN) and network regions. The inversion is carried out under the hypothesis of MISMA, where the unresolved structure is assumed to be optically thin. We analyze a 29.52"x31.70" subfield carefully selected to be representative of the properties of a 302"x162" quiet Sun field-of-view at disk center. The inversion code is able to reproduce the observed asymmetries in a very satisfactory way. The inversion code interprets 25% of inverted profiles as emerging from pixels in which both positive and negative polarities coexist. kG field strengths are found at the base of the photosphere in both network and IN; in the case of the latter, both kG fields and hG fields are admixed. When considering the magnetic properties at the mid photosphere most kG fields are gone, and the statistics is dominated by hG fields. We constrain the magnetic field of only 4.5% of the analyzed photosphere (and this percentage reduces to 1.3% when referred to all pixels, including those with low polarization not analyzed). The rest of the plasma is consistent with the presence of weak fields not contributing to the detected polarization signals. The average flux densities derived in the full subfield and in IN regions are higher than the ones derived from the same dataset by Milne-Eddington inversion. The existence of large asymmetries in SOT/SP polarization profiles is uncovered. These are not negligible in quiet Sun data. The MISMA inversion code reproduces them in a satisfactory way, and provides a statistical description of the magnetized IN and network which partly differs and complements the results obtained so far. From this it follows the importance of having a complete interpretation of the line profile shapes.

Dynamics of small-scale magnetic fields on the Sun: observations and numerical simulations

Context. Small-scale magnetic fields play an important role in the structure and the dynamics of the solar atmosphere. Aims. This study aims at revealing the evolution of magnetic fields, together with granular convection in the quiet Sun by means of observations with high spatial, spectral, and temporal resolution and of numerical MHD simulations. Methods. Time sequences from quiet Sun disc centre were obtained with the upgraded “Gottingen” Fabry-Perot spectropolarimeter at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the $\ion{Fe}{i}$ 6173 A line. The data were reconstructed with speckle methods. For comparison with the observations, numerical simulations of granular magnetoconvection were carried out with the MURaM code. The intensities and Stokes vectors emerging from the simulation box were degraded in wavelength, spatial co-ordinates, and noise to the quality of the observations. Results. The noise in the observed magnetograms from the centre-of-gravity method is $\sigma_B\approx2$ G, yielding a polarimetric sensitivity of $3 \times 10^{15}$ Mx, at a cadence of 23 s with 0$\farcs$33 spatial resolution in a field of view of ${\sim}33\arcsec\times 25\arcsec$. Many of the observed V profiles in network and internetwork (IN) areas exhibit strong asymmetries that indicate strong magnetoconvection. The temporal evolutions of IN structures and of a bright point (BP), as seen in broadband and line-minimum images, in Dopplergrams, and in magnetograms, are presented. The magnetic field structure in the numerical MHD simulations is even more complex than seen in the observations. Correspondingly, the emergent Stokes profiles are often very abnormal. The degradation yields a reduction of the intrinsic field strength to the “observed” one by a factor 4–5. The spectral resolution of the spectrometer is adequate, yet the limitation in spatial resolution and by noise filtering swamps the details seen in the non-degraded simulations. A BP was not found in the simulations, presumably because BP's are not so common and the simulated box is quiet small: only 6 Mm wide in both horizontal directions. Conclusions. The combination of high-resolution observations with numerical simulation is a highly valuable means for studying small-scale magnetic fields on the Sun. Two-dimensional, low-noise data with spectral resolution as good as achieved here and with spatial resolution of 0$\farcs$1 and better are needed to better understand this important part of the solar magnetism.

A Web-Based Remote Interactive Laboratory for Internetworking Education

A Web-based remote interactive laboratory (RIL) developed to deliver Internetworking laboratory experience to geographically remote graduate students is presented in this paper. The onsite Internetworking program employs hands-on laboratories in a group setting that correlates with the constructivist and collaborative pedagogical approach. This paper discusses the pedagogical and technical considerations that influence the design and implementation of the remote laboratory environment given the constraints of the special hardware and learning outcomes of the program. For wide-ranging usability, the remote Internetworking (INWK) laboratory uses de facto networking standards and commercial and broad-band Internet connectivity to ensure real-time secure interaction with equipment. A four-tier role architecture consisting of faculty, local facilitators, remote facilitators, and students has been determined appropriate to maintain academic integrity and ensure good quality of interaction with the remote laboratory. A survey employing a five-point scale has been devised to measure the usability of the remote access INWK laboratory.

Inter-Network magnetic fields observed during the minimum of the solar cycle

We analyze a time series of high angular resolution magnetograms of quiet Sun Inter-Network (IN) magnetic fields. These magnetograms have a spatial resolution better than 05, a noise of some 20 G, and they have been obtained at the disk center during the minimum of the solar cycle. The IN regions show a typical unsigned flux density of the order of 15 G. Signals occur, preferentially, in the intergranular lanes, and the strongest signals trace a network with a scale similar to the mesogranulation. All these features are consistent with the IN magnetograms by [CITE], obtained during the maximum of the solar cycle. Consequently, the unsigned magnetic flux of the structures that give rise to the IN polarization signals does not seem to undergo large variations during the solar cycle.

Inter-network magnetic fields observed with sub-arcsec resolution

We analyze a time sequence of Inter-Network (IN) magnetograms observed at the solar disk center. Speckle reconstruction techniques provide a good spatial resolution (0.5 cutoff frequency) yet maintaining a fair sensitivity (some 20G). Patches with signal above noise cover 60% of the observed area, most of which corresponds to intergranular lanes. The large surface covered by signal renders a mean unsigned magnetic flux density between 17G and 21G (1G \equiv 1Mx cm$^{-2}$). The difference depends on the spectral line used to generate the magnetograms (Fe I 6302 or Fe I 6301). Such systematic difference can be understood if the magnetic structures producing the polarization have intrinsic field strengths exceeding 1 kG, and consequently, occupying only a very small fraction of the surface (some 2%). We observe both, magnetic signals changing in time scales smaller than 1 min, and a persistent pattern lasting longer than the duration of the sequence (17 min). The pattern resembles a network with a spatial scale between 5 and 10 arcsec, which we identify as the mesogranulation. The strong dependence of the polarization signals on spatial resolution and sensitivity suggests that much quiet Sun magnetic flux still remains undetected.