Quiet Sun Internetwork Research Articles

Strategy for the Inversion of Hinode Spectropolarimetric Measurements in the Quiet +n

Abstract In this paper we propose an inversion strategy for the analysis of spectropolarimetric measurements taken by Hinode in the quiet Sun. The Spectro-Polarimeter of the Solar Optical Telescope aboard Hinode records the Stokes spectra of the Fe I line pair at 630.2 nm with unprecendented angular resolution, high spectral resolution, and high sensitivity. We discuss the need to consider a local stray-light contamination to account for the effects of telescope diffraction. The strategy is applied to observations of a wide quiet Sun area at disk center. Using these data we examine the influence of noise and initial guess models in the inversion results. Our analysis yields the distributions of magnetic field strengths and stray-light factors. They show that quiet Sun internetwork regions consist mainly of hG fields with stray-light contamination of about 0.8.

Emergence of Small-Scale Magnetic Loops in the Quiet-Sun Internetwork

We study the emergence of magnetic flux at very small spatial scales (less than 2'') in the quiet-Sun internetwork. To this aim, a time series of spectropolarimetric maps was taken at disk center using the instrument SP/SOT on board Hinode. The LTE inversion of the full Stokes vector measured in the Fe I 6301 and 6302 A lines allows us to retrieve the magnetic flux and topology in the region of study. In the example presented here, the magnetic flux emerges within a granular structure. The horizontal magnetic field appears prior to any significant amount of vertical field. As time goes on, the traces of the horizontal field disappear, while the vertical dipoles drift—carried by the plasma motions—toward the surrounding intergranular lanes. These events take place within typical granulation timescales.

Quiet-Sun Magnetic Fields from Space-borne Observations: Simulating Hinode's Case

We examine whether it is possible to derive the field strength distribution of quiet-Sun internetwork regions from very high spatial resolution polarimetric observations in the visible. In particular, we consider the case of the spectropolarimeter attached to the Solar Optical Telescope aboard Hinode. Radiative magnetoconvection simulations are used to synthesize the four Stokes profiles of the Fe I 630.2 nm lines. Once the profiles are degraded to a spatial resolution of 0.32'' and added noise, we infer the atmospheric parameters by means of Milne-Eddington inversions. The comparison of the derived values with the real ones indicates that the visible lines yield correct internetwork field strengths and magnetic fluxes, with uncertainties smaller than ~150 G, when a stray-light contamination factor is included in the inversion. Contrary to the results of ground-based observations at 1'', weak fields are retrieved wherever the field is weak in the simulation.

DOT tomography of the solar atmosphere

High-quality simultaneous image sequences from the Dutch Open Telescope (DOT) in the G band and the Ca  H line are used to quantify the occurrence of reversed granulation as a constituent of the subsonic brightness pattern observed as a background to acoustic oscillations in the quiet-Sun internetwork atmosphere. In the middle photosphere re- versed granulation constitutes a much larger part of this background than at the larger heights sampled by ultraviolet radiation. The anticorrelation with the underlying granulation reaches about 50% at a temporal delay of 2-3 min, and increases with spa- tial image smoothing to mesogranular resolution. We discuss the nature of reversed granulation in terms of convection reversal, gravity waves, acoustic waves, and intergranular magnetism, suggest that the internetwork background pattern is primarily a mixture of the first two ingredients, and speculate that it is also an inverse canopy mapper.

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.

A Study of Chromospheric Oscillations Using theSOHOandTRACESpacecraft

We analyze line and continuum time-series data of the solar atmosphere, with between 10 and 60 s cadence, using the MDI and SUMER instruments on the SOHO spacecraft and the UV bandpasses on the T RACE satellite. The co-aligned data sets sample spectral features formed from photosphere to the middle transition region, spanning —ve decades in pressure, under quiet-Sun and plage conditions. We discuss power, phase diUerence, and coherence spectra, and examine data in the time domain. The observed photospheric and chromospheric oscillations are strongly coupled for frequencies between 2 and 8 mHz. Phase coherences decrease with increasing height, with only occasional periods and locations of observable coherence up to heights where transition region emission lines are formed. The middle chromosphere (in the SUMER continua) oscillates in several megameter (Mm) diameter coherent patches with power predominantly in the 5¨7 mHz range. The T RACE data, formed in the upper photosphere, show smaller patterns superimposed on these large-scale oscillations, resulting (at least in part) from granulation. At the observed spatial scales, all the observed properties point to p-modes, especially the ii pseudomodes ˇˇ just above the acoustic cutoU frequency, as the dominant mode of the chromospheric dynamics. Smaller scale ii acoustic event ˇˇ drivers, associated with granular dynamics, appear to be less important. The predominant internetwork chromospheric oscillations arise from regions much larger horizontally than vertically. If propagating largely vertically, this can naturally explain why the one-dimensional simulations of Carlsson & Stein might be more successful than expected. The chromospheric response to the p-mode driving is, however, intermittent in space and time. Some of the intermittency appears to result from the interaction of the upward-propagating waves with magnetic —elds. Evidence for this includes suppressed intensities and oscillations near quiet-Sun network elements (which we dub ii magnetic shadows ˇˇ), absence of oscillations in internetwork regions neighboring plage magnetic —elds, and a change in character of the quiet-Sun internetwork oscillations between the 119 and 104 nm continua formed at 1 and 1.2 Mm. The latter might be caused by canopy —elds that form between these heights under typical quiet-Sun conditions. A SUMER-only data set reported by et al. has a Wikst‘l factor of 3 more oscillatory power in the 104 nm continuum than the data analyzed here, with stronger coherences extending into the solar transition region. Together, these data support the general picture that the chromosphere oscillates primarily in response to forcing by the p-modes, they are therefore large-scale (several Mm across) waves, and they are often strongly in—uenced by magnetic eUects (internetwork —elds, or the overlying canopy), before the oscillations even reach the transition region.

Dynamics of the Solar Chromosphere. II. CaiiH2Vand K2VGrains versus Internetwork Fields

We use the Advanced Stokes Polarimeter at the NSO/Sacramento Peak Vacuum Tower Telescope to search for spatio-temporal correlations between enhanced magnetic fields in the quiet solar internetwork photosphere and the occurrence of Ca II H2V grains in the overlying chromosphere. We address the question of whether the shocks that produce the latter are caused by magnetism-related processes, or whether they are of purely hydrodynamic nature. The observations presented here are the first in which sensitive Stokes polarimetry is combined synchronously with high-resolution Ca II H spectrometry. We pay particular attention to the nature and significance of weak polarization signals from the internetwork domain, obtaining a robust estimate of our magnetographic noise level at an apparent flux density of only 3 Mx cm-2. For the quiet Sun internetwork area analyzed here, we find no direct correlation between the presence of magnetic features with apparent flux density above this limit and the occurrence of H2V brightenings. This result contradicts the one-to-one correspondence claimed by Sivaraman & Livingston. We also find no correspondence between H2V grains and the horizontal-field internetwork features discovered by Lites et al.