Emergence of granular-sized magnetic bubbles through the solar atmosphere. II. Non-LTE chromospheric diagnostics and inversions

Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photo- sphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca II 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain depth-dependent information of the temperature and line-of-sight stratification. Our analysis explains the peculiar Ca II 8542 profiles observed in the flux-emerging region. In addition, we derive detailed temperature and velocity maps describing the ascent of magnetic bubbles from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere.Comment: Submitted to ApJ, 10 Figure

Lateral downflows in sunspot penumbral filaments and their temporal evolution

© 2015. The American Astronomical Society. All rights reserved.. We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited scans of the Fe i 617.3nm line taken with the CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of granular convection in the quiet Sun. There is a good relation between brightness and velocity in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of quiet Sun convection. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells forming filaments where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles that are suggestive of opposite magnetic polarities in the pixel.Financial support by the Spanish Ministerio de Economia y Competitividad through grants AYA2009-14105-C06-06, AYA2012-39636-C06-05, and ESP2013-47349-C6-1 R, including a percentage from European FEDER funds, is gratefully acknowledged.Peer Reviewe

EMERGENCE of GRANULAR-SIZED MAGNETIC BUBBLES THROUGH the SOLAR ATMOSPHERE. II. NON-LTE CHROMOSPHERIC DIAGNOSTICS and INVERSIONS

© 2015. The American Astronomical Society. All rights reserved. Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca ii 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca ii 8542 profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere.he authors thank J. Leenaarts and L. Rouppe van der Voort for illuminating discussions. J. de la Cruz Rodriguez acknowledges financial support from the CHROMOBS project funded by the Knut and Alice Wallenberg Foundation. L. Bellot Rubio is funded by grants AYA2012-39636-C06-05 and ESP2013-47349-C6-1-R of the Spanish Ministerio de Economia y Competitividad, including a percentage from European FEDER funds.Peer Reviewe

The Solar Internetwork. II. Magnetic Flux Appearance and Disappearance Rates

Small-scale internetwork magnetic fields are important ingredients of the quiet Sun. In this paper we analyze how they appear and disappear on the solar surface. Using high resolution Hinode magnetograms, we follow the evolution of individual magnetic elements in the interior of two supergranular cells at the disk center. From up to 38 hr of continuous measurements, we show that magnetic flux appears in internetwork regions at a rate of $120\pm3$ Mx cm$^{-2}$ day$^{-1}$ ($3.7 \pm 0.4 \times 10^{24}$ Mx day$^{-1}$ over the entire solar surface). Flux disappears from the internetwork at a rate of $125 \pm 6$ Mx cm$^{-2}$ day$^{-1}$ ($3.9\pm 0.5 \times 10^{24}$ Mx day$^{-1}$) through fading of magnetic elements, cancellation between opposite-polarity features, and interactions with network patches, which converts internetwork elements into network features. Most of the flux is lost through fading and interactions with the network, at nearly the same rate of about 50 Mx cm$^{-2}$ day$^{-1}$. Our results demonstrate that the sources and sinks of internetwork magnetic flux are well balanced. Using the instantaneous flux appearance and disappearance rates, we successfully reproduce the time evolution of the total unsigned flux in the two supergranular cells.Comment: 8 pages, 6 figures. Accepted in ApJ. An animation of the right panel of Figure 1 is available at http://spg.iaa.es/pub/downloads/gosic/figure1_right_panel.ta

Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots

High resolution spectropolarimetric observations of 3 sunspots taken with Hinode demonstrate the existence of supersonic downflows at or close to the umbra-penumbra boundary which have not been reported before. These downflows are confined to large patches, usually encompassing bright penumbral filaments, and have lifetimes of more than 14 hr. The presence of strong downflows in the center-side penumbra near the umbra rules out an association with the Evershed flow. Chromospheric filtergrams acquired close to the time of the spectropolarimetric measurements show large, strong, and long-lived brightenings in the neighborhood of the downflows. The photospheric intensity also exhibit persistent brightenings comparable to the quiet Sun. Interestingly, the orientation of the penumbral filaments at the site of the downflows is similar to that resulting from the reconnection process described by Ryutova et al. The existence of such downflows in the inner penumbra represents a challenge for numerical models of sunspots because they have to explain them in terms of physical processes likely affecting the chromosphere.Comment: Accepted for publication in Ap

Properties of Umbral Dots from Stray Light Corrected Hinode Filtergrams

High resolution blue continuum filtergrams from Hinode are employed to study the umbral fine structure of a regular unipolar sunspot. The removal of scattered light from the images increases the rms contrast by a factor of 1.45 on average. Improvement in image contrast renders identification of short filamentary structures resembling penumbrae that are well separated from the umbra-penumbra boundary and comprise bright filaments/grains flanking dark filaments. Such fine structures were recently detected from ground based telescopes and have now been observed with Hinode. A multi-level tracking algorithm was used to identify umbral dots in both the uncorrected and corrected images and to track them in time. The distribution of the values describing the photometric and geometric properties of umbral dots are more easily affected by the presence of stray light while it is less severe in the case of kinematic properties. Statistically, umbral dots exhibit a peak intensity, effective diameter, lifetime, horizontal speed and a trajectory length of 0.29 I_QS, 272 km, 8.4 min, 0.45 km/s and 221 km respectively. The 2 hr 20 min time sequence depicts several locations where umbral dots tend to appear and disappear repeatedly with various time intervals. The correction for scattered light in the Hinode filtergrams facilitates photometry of umbral fine structure which can be related to results obtained from larger telescopes and numerical simulations.Comment: Accepted for publication in ApJ : 10 pages, 10 figures, 3 table