Evidence of quiet Sun chromospheric activity related to an emerging small-scale magnetic loop

Aims: We investigate the temporal evolution of magnetic flux emergence in the quiet Sun atmosphere close to disk center. Methods: We combine high-resolution SoHO/MDI magnetograms with TRACE observations taken in the 1216 {\AA} channel in order to analyze the temporal evolution of an emerging small-scale magnetic loop and its traces in the chromosphere. Results: At first place, we find signatures of flux emergence very close to the edge of a supergranular network boundary located at disk center. The new emerging flux appears first in the MDI magnetograms in form of an asymmetric bipolar element, i.e. the patch with negative polarity is roughly two-times weaker than the corresponding patch with opposite polarity. The average values of magnetic flux and magnetic flux densities reach 1.6 x 10^18 Mx, -8.5 x 10^17 Mx, and 55 Mx cm^-2, -30 Mx cm^-2, respectively. The spatial distance between the opposite polarity patches of the emerged feature increases from about 2.5" to 5.0" during the lifetime of the loop which was not longer than 36 min. A more precise lifetime-estimate of the feature was not possible because of a gap in the temporal sequence of the MDI magnetograms. The chromospheric response to the emerged magnetic dipole occurs ~ 9 minutes later with respect to the photospheric magnetograms. It consists of a quasi-periodic sequence of time-localized brightenings visible in the 1216 {\AA} TRACE channel apparent for ~ 14 minutes and being co-spatial with the axis connecting the two patches of opposite magnetic polarity. Conclusions: We identify the observed event as a small-scale magnetic loop emerging at photospheric layers and subsequently rising up to the chromosphere. We discuss the possibility that the fluctuations detected in the chromospheric emission probably reflect magnetic field oscillations which propagate to the chromosphere in form of waves.Comment: 6 pages, 4 figures, 1 table, Astronomy and Astrophysics, in pres

Can spicules be detected at disc centre in broad-band Ca II H filter imaging data ?

We estimate the formation height range contributing to broad-band and narrow-band filter imaging data in Ca II H to investigate whether spicules can be detected in such observations at the centre of the solar disc. We apply spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca line profiles to simulate Ca imaging data. We estimate the relative intensity contributions of off-limb and on-disc structures. We compare the synthetic Ca filter imaging data with intensity maps of Ca spectra at different wavelengths and temperature maps at different optical depths. We determine the intensity response function for the wavelengths covered by the filters of different FWHM. The intensity emitted off the solar limb is about 5% of the intensity at disc centre. For a 0.3 nm-wide Ca II H filter, up to about 1/3 of the off-limb intensity comes from emission in Hepsilon. On the disc, only about 15% of the intensity transmitted through a broad-band filter comes from the line-core region. No traces of elongated fibrillar structures are visible in imaging data at disc centre, opposite to the line-core images of the Ca spectra. The response function for a 0.3 nm-wide filter peaks at about 200 km. Relative contributions from atmospheric layers above 800 km are about 10%. The inversion results suggest that the slightly enhanced emission around the photospheric magnetic network in broad-band Ca imaging data is caused by a thermal canopy at a height of about 600 km. Broad-band Ca II H imaging data do not trace upper chromospheric structures such as spicules in observations at the solar disc because of the too small relative contribution of the line core to the total wavelength-integrated filter intensity.Comment: 10 pages, 11 figures, accepted for publication in A&

Spectroscopy at the solar limb: II. Are spicules heated to coronal temperatures ?

Spicules of the so-called type II were suggested to be relevant for coronal heating because of their ubiquity on the solar surface and their eventual extension into the corona. We investigate whether solar spicules are heated to transition-region or coronal temperatures and reach coronal heights (>6 Mm) using multi-wavelength observations of limb spicules in different chromospheric spectral lines (Ca II H, Hepsilon, Halpha, Ca II IR at 854.2 nm, He I at 1083 nm). We determine the line width of individual spicules and throughout the field of view and estimate the maximal height that different types of off-limb features reach. We derive estimates of the kinetic temperature and the non-thermal velocity from the line width of spectral lines from different chemical elements. We find that most regular spicules reach a maximal height of about 6 Mm above the solar limb. The majority of features found at larger heights are irregularly shaped with a significantly larger lateral extension than spicules. Both individual and average line profiles in all spectral lines show a decrease in their line width with height above the limb with very few exceptions. Both the kinetic temperature and the non-thermal velocity decrease with height above the limb. We find no indications that the spicules in our data reach coronal heights or transition-region or coronal temperatures.Comment: Accepted for publication in Solar Physics, 52 pages, 32 figure

The polarization signature of photospheric magnetic fields in 3D MHD simulations and observations at disk center

Before using 3D MHD simulations of the solar photosphere in the determination of elemental abundances, one has to ensure that the correct amount of magnetic flux is present in the simulations. The presence of magnetic flux modifies the thermal structure of the solar photosphere, which affects abundance determinations and the solar spectral irradiance. We compare the polarization signals in disk-center observations of the solar photosphere in quiet-Sun regions with those in Stokes spectra computed on the basis of 3D MHD simulations having average magnetic flux densities of about 20, 56, 112 and 224 G. This approach allows us to find the simulation run that best matches the observations. The observations were taken with the Hinode SP, TIP, POLIS and the GFPI, respectively. We determine characteristic quantities of full Stokes profiles in a few photospheric spectral lines in the visible (630 nm) and near-infrared (1083 and 1565 nm). We find that the appearance of abnormal granulation in intensity maps of degraded simulations can be traced back to an initially regular granulation pattern with numerous bright points in the intergranular lanes before the spatial degradation. The linear polarization signals in the simulations are almost exclusively related to canopies of strong magnetic flux concentrations and not to transient events of magnetic flux emergence. We find that the average vertical magnetic flux density in the simulation should be less than 50 G to reproduce the observed polarization signals in the quiet Sun internetwork. A value of about 35 G gives the best match across the SP, TIP, POLIS and GFPI observations.Comment: 12 pages, 11 figures; accepted for publication in Ap

Twist, Writhe & Helicity in the inner penumbra of a sunspot

The aim of this work is the determination of the twist, writhe, and self magnetic helicity of penumbral filaments located in an inner Sunspot penumbra. To this extent, we inverted data taken with the spectropolarimeter (SP) aboard Hinode with the SIR (Stokes Inversion based on Response function) code. For the construction of a 3D geometrical model we applied a genetic algorithm minimizing the divergence of the magnetic field vector and the net magnetohydrodynamic force, consequently a force-free solution would be reached if possible. We estimated two proxies to the magnetic helicity frequently used in literature: the force-free parameter and the current helicity term. We show that both proxies are only qualitative indicators of the local twist as the magnetic field in the area under study significantly departures from a force-free configuration. The local twist shows significant values only at the borders of bright penumbral filaments with opposite signs on each side. These locations are precisely correlated to large electric currents. The average twist (and writhe) of penumbral structures is very small. The spines (dark filaments in the background) show a nearly zero writhe. The writhe per unit length of the intraspines diminishes with increasing length of the tube axes. Thus, the axes of tubes related to intraspines are less wrung when the tubes are more horizontal. As the writhe of the spines is very small, we can conclude that the writhe reaches only significant values when the tube includes the border of a intraspine.Comment: 7 pages, 4 figures; Astrophysical Journal, in pres

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Numerical 3D radiative (M)HD simulations of solar convection are used to understand the physical properties of the solar photosphere. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer. We investigate the realism of 3D convection simulations carried out with the Stagger code. We compared the characteristic properties of several spectral lines in solar disc centre observations with spectra synthesized from the simulations. We degraded the synthetic spectra to the spatial resolution of the observations using the continuum intensity distribution. We estimated the necessary spectral degradation by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly, we used the SIR code to invert the spectra. Most of the line parameters from the observational data are matched well by the degraded simulation spectra. The inversions predict a macroturbulent velocity below 10 m/s for the simulation at full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution of 0.3". The temperature fluctuations in the inversion of the degraded simulation do not exceed those from the observational data (of the order of 100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change of (average) line properties at a spatial scale between 0.13" and 0.3". Up to a spatial resolution of 0.3", we find no indications of the presence of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even better spatial resolution.Comment: 21 pages, 15 figures + 2 pages Appendix, accepted for publication in A&A; v2 version: corrected for an error in the calculation of stray-light estimates, for details see the Corrigendum to A&A, 2013, 557, 109 (DOI: 10.1051/0004-6361/201321596). Corrected text and numbers are in bold font. Apart from the stray-light estimates, nothing in the rest of the paper was affected by the erro

Formation of a penumbra in a decaying sunspot

Context : Penumbrae are an important characteristic of sunspots, whose formation is intricately related to the nature of sub-photospheric magnetic fields. Aims : We study the formation of a penumbra in a decaying sunspot and compare its properties with those seen during the development of a proto-spot. Methods : High-resolution spectropolarimetric observations of active region NOAA 11283 were obtained from the spectro-polarimeter on board Hinode. These were complemented with full-disk filtergrams of continuum intensity, line-of-sight magnetograms, and dopplergrams from the Helioseismic and Magnetic Imager at high cadence. Results : The formation of a penumbra in the decaying sunspot occurs after the coalescence of the sunspot with a magnetic fragment/pore, which initially formed in the quiet Sun close to an emerging flux region. At first, a smaller set of penumbral filaments develop near the location of the merger with very bright penumbral grains with intensities of 1.2 I_QS, upflows of 4 km/s, and a lifetime of 10 hr. During the decay of these filaments, a larger segment of a penumbra forms at the location of the coalescence. These new filaments are characterized by nearly supersonic downflows of 6.5 km/s that change to a regular Evershed flow nearly 3 hr later. Conclusions : The coalescence of the pore with the decaying sunspot provided sufficient magnetic flux for the penumbra to form in the sunspot. The emerging flux region could have played a decisive role in this process because the formation occurred at the location of the merger and not on the opposite side of the sunspot.Comment: Accepted for publication in A&A Letters, 6 pages, 4 figure

Three dimensional structure of penumbral filaments from Hinode observations

We analyse spectropolarimetric observations of the penumbra of the NOAA AR 10953 at high spatial resolution (0.3"). The full Stokes profiles of the Fe I lines at 630.1 nm and 630.2 nm have been obtained with the Solar Optical Telescope (SOT) on board the Hinode satellite. The data have been inverted by means of the SIR code, deriving the stratifications of temperature, line of sight velocity, and the components of the magnetic field vector in optical depth. In order to evaluate the gas pressure and to obtain an adequate geometrical height scale, the motion equation has been integrated for each pixel taking into account the terms of the Lorentz force. To establish the boundary condition, a genetic algorithm has been applied. The final resulting magnetic field has a divergence compatible with 0 inside its uncertainties. First analyses of the correlation of the Wilson depression with velocity, temperature, magnetic field strength, and field inclination strongly support the uncombed penumbral model proposed by Solanki & Montavon (1993).Comment: 6 pages, 3 figures, conference proceeding: SEA meeting 2008, Santander, Spai

Astrophysics Evidence of quiet-Sun chromospheric activity related to an emerging small-scale magnetic loop

ABSTRACT Aims. We investigate the temporal evolution of magnetic flux emergence in the quiet-Sun atmosphere close to disk center. Methods. We combined high-resolution SoHO/MDI magnetograms with TRACE observations taken in the 1216 Å channel to analyze the temporal evolution of an emerging small-scale magnetic loop and its traces in the chromosphere. Results. We find signatures of flux emergence very close to the edge of a supergranular network boundary located at disk center. The new emerging flux appeared first in the MDI magnetograms in form of an asymmetric bipolar element, i.e., the patch with negative polarity is roughly twice as weak as the corresponding patch with opposite polarity. The average values of magnetic flux and magnetic flux densities reached 1.6 × 10 18 Mx, −8.5 × 10 17 Mx, and 55 Mx cm −2 , -30 Mx cm −2 , respectively. The spatial distance between the opposite polarity patches of the emerged feature increased from about 2. 5 to 5. 0 during the lifetime of the loop, which was 36 min. A more precise lifetime-estimate of the feature was not possible because of a gap in the temporal sequence of the MDI magnetograms. The chromospheric response to the emerged magnetic dipole occurred ∼9 min later than in the photospheric magnetograms. It consisted of a quasi-periodic sequence of time-localized brightenings visible in the 1216 Å TRACE channel for ∼14 min that were co-spatial with the axis connecting the two patches of opposite magnetic polarity. Conclusions. We identify the observed event as a small-scale magnetic loop emerging at photospheric layers that subsequently rose to the chromosphere. We discuss the possibility that the fluctuations detected in the chromospheric emission probably reflect magneticfield oscillations which propagate to the chromosphere in the form of waves