Observed Variability of the Solar Mg II h Spectral Line

The Mg II h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double gaussian model to fit the Mg II h profile for a full-Sun mosaic dataset taken 24-Aug-2014. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles which do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared with predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core which is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg II profiles observed by IRIS.Comment: Accepted for publicatio

Link between the chromospheric network and magnetic structures of the corona

Recent work suggested that the traditional picture of the corona above the quiet Sun being rooted in the magnetic concentrations of the chromospheric network alone is strongly questionable. Building on that previous study we explore the impact of magnetic configurations in the photosphere and the low corona on the magnetic connectivity from the network to the corona. Observational studies of this connectivity are often utilizing magnetic field extrapolations. However, it is open to which extent such extrapolations really represent the connectivity found on the Sun, as observations are not able to resolve all fine scale magnetic structures. The present numerical experiments aim at contributing to this question. We investigated random salt-and-pepper-type distributions of kilo-Gauss internetwork flux elements carrying some $10^{15}$ to $10^{17}$ Mx, which are hardly distinguishable by current observational techniques. These photospheric distributions are then extrapolated into the corona using different sets of boundary conditions at the bottom and the top. This allows us to investigate the fraction of network flux which is connected to the corona, as well as the locations of those coronal regions which are connected to the network patches. We find that with current instrumentation one cannot really determine from observations, which regions on the quiet Sun surface, i.e. in the network and internetwork, are connected to which parts of the corona through extrapolation techniques. Future spectro-polarimetric instruments, such as with Solar B or GREGOR, will provide a higher sensitivity, and studies like the present one could help to estimate to which extent one can then pinpoint the connection from the chromosphere to the corona.Comment: 8 pages, 5 figures, acceped for publication in A&

Small-scale solar magnetic fields

As we resolve ever smaller structures in the solar atmosphere, it has become clear that magnetism is an important component of those small structures. Small-scale magnetism holds the key to many poorly understood facets of solar magnetism on all scales, such as the existence of a local dynamo, chromospheric heating, and flux emergence, to name a few. Here, we review our knowledge of small-scale photospheric fields, with particular emphasis on quiet-sun field, and discuss the implications of several results obtained recently using new instruments, as well as future prospects in this field of research.Comment: 43 pages, 18 figure

Photometric Trends in the Visible Solar Continuum and Their Sensitivity to the Center-to-Limb Profile

Solar irradiance variations over solar rotational time-scales are largely determined by the passage of magnetic structures across the visible solar disk. Variations on solar cycle time scales are thought to be similarly due to changes in surface magnetism with activity. Understanding the contribution of magnetic structures to total solar irradiance and solar spectral irradiance requires assessing their contributions as a function of disk position. Since only relative photometry is possible from the ground, the contrasts of image pixels are measured with respect to a center-to-limb intensity profile. Using nine years of full-disk red and blue continuum images from the Precision Solar Photometric Telescope at the Mauna Loa Solar Observatory (PSPT/MLSO), we examine the sensitivity of continuum contrast measurements to the center-to-limb profile definition. Profiles which differ only by the amount of magnetic activity allowed in the pixels used to determine them yield oppositely signed solar cycle length continuum contrast trends; either agreeing with the result of Preminger et al. (2011) showing negative correlation with solar cycle or disagreeing and showing positive correlation with solar cycle. Changes in the center-to-limb profile shape over the solar cycle are responsible for the contradictory contrast results, and we demonstrate that the lowest contrast structures, internetwork and network, are most sensitive to these. Thus the strengths of the full-disk, internetwork, and network photometric trends depend critically on the magnetic flux density used in the quiet-sun definition. We conclude that the contributions of low contrast magnetic structures to variations in the solar continuum output, particularly to long-term variations, are difficult, if not impossible, to determine without the use of radiometric imaging.Comment: Accepted to ApJ. 11 pages, 5 figure

On the validity of the 630 nm Fe I nm lines for the magnetometry of the internetwork quiet Sun

The purpose of this work is to analyze the reliability of the magnetic field strengths inferred from the 630 nm pair of Fe I lines at internetwork quiet Sun regions. Some numerical experiments have been performed that demonstrate the inability of these lines to recover the magnetic field strength in such low flux solar regions. It is shown how different model atmospheres, with magnetic field strengths ranging from few hundred Gauss to kiloGauss, give rise to Stokes profiles that can not be distinguished. The reasons for this degeneracy are discussed.Comment: Accepted for publication in A&

On the fine structure of the quiet solar \Ca II K atmosphere

We investigate the morphological, dynamical, and evolutionary properties of the internetwork and network fine structure of the quiet sun at disk centre. The analysis is based on a $\sim$6 h time sequence of narrow-band filtergrams centred on the inner-wing \Ca II K$_{\rm 2v}$ reversal at 393.3 nm. The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. The average evolutionary time scale of the internetwork in our observations is 52 sec. Internetwork grains show a tendency to appear on a mesh-like pattern with a mean cell size of $\sim$4-5 arcsec. Based on this size and the spatial organisation of the mesh we speculate that this pattern is related to the existence of photospheric downdrafts as predicted by convection simulations. The image segmentation shows that typical sizes of both network and internetwork grains are in the order of 1.6 arcs.Comment: 8 pages, 9 figure

Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun

The Ca II infrared triplet is one of the few magnetically sensitive chromospheric lines available for ground-based observations. We present spectropolarimetric observations of the 8498 A and 8542 A lines in a quiet Sun region near a decaying active region and compare the results with a simulation of the lines in a high plasma-beta regime. Cluster analysis of Stokes V profile pairs shows that the two lines, despite arguably being formed fairly close, often do not have similar shapes. In the network, the local magnetic topology is more important in determining the shapes of the Stokes V profiles than the phase of the wave, contrary to what our simulations show. We also find that Stokes V asymmetries are very common in the network, and the histograms of the observed amplitude and area asymmetries differ significantly from the simulation. Both the network and internetwork show oscillatory behavior in the Ca II lines. It is stronger in the network, where shocking waves, similar to those in the high-beta simulation, are seen and large self-reversals in the intensity profiles are common.Comment: 23 pages, 17 figures, accepted to ApJ some figures are low-res, for high-res email [email protected]