Solar transition region above sunspots

Curdt, W.; Teriaca, L.; Landi, E.; Marsch, E.

doi:10.1051/0004-6361/200912114

Cited by 37 publications

(42 citation statements)

References 76 publications

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“…The atmosphere of a sunspot umbra is strongly irradiated from the sides by the penumbra in the walls produced by the Wilson depression, and this will affect the observed spectra. An example is the fact that Lyman-α has centrally-peaked profiles in the umbra but centrally-reversed profiles elsewhere (e.g., Tian et al 2009): centrally-peaked profiles are expected if the umbral Lyα emission is dominated by scattered and redistributed emission coming in from the penumbral walls, rather than emission intrinsic to the umbral atmosphere. One-dimensional models cannot handle the complication introduced by scattering from a source with a different atmospheric structure.…”

Section: Discussionmentioning

confidence: 99%

The chromosphere above sunspots at millimeter wavelengths

Loukitcheva

Solanki

White

2014

A&A

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Aims. The aim of this paper is to demonstrate that millimeter wave data can be used to distinguish between various atmospheric models of sunspots, whose temperature structure in the upper photosphere and chromosphere has been the source of some controversy. Methods. We use observations of the temperature contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm observations from Lindsey & Kopp (1995) and 2 cm observations with the Very Large Array. These are compared with the umbral contrast calculated from various atmospheric models of sunspots. Results. Current mm and submm observational data suggest that the brightness observed at these wavelengths is low compared to the most widely used sunspot models. These data impose strong constraints on the temperature and density stratifications of the sunspot umbral atmosphere, in particular on the location and depth of the temperature minimum and the location of the transition region. Conclusions. A successful model that is in agreement with millimeter umbral brightness should have an extended and deep temperature minimum (below 3000 K). Better spatial resolution as well as better wavelength coverage are needed for a more complete determination of the chromospheric temperature stratification above sunspot umbrae.

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Section: Discussionmentioning

confidence: 99%

The chromosphere above sunspots at millimeter wavelengths

Loukitcheva

Solanki

White

2014

A&A

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“…Observations have shown that the self-reversals are almost absent in sunspot regions (Fontenla et al 1988;Curdt et al 2001;Tian et al 2009). In contrast, the lower Lyman line profiles observed in the plage region are obviously reversed, a phenomenon also found in the normal quiet Sun (Tian et al 2009). These results indicate a much smaller opacity above sunspots, as compared to the surrounding plage region.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen Lyman-α and Lyman-β spectral radiance profiles in the quiet Sun

Curdt

Marsch

Schühle

2009

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Aims. We extend earlier work by studying the line profiles of the hydrogen Lyman-α and Lyman-β lines in the quiet Sun. They were obtained quasi-simultaneously in a raster scan with a size of about 150 × 120 near disk center. Methods. The self-reversal depths of the Ly-α and Ly-β profiles. we are quantified by measuring the maximum spectral radiances of the two horns and the minimum spectral radiance of the central reversal. The information on the asymmetries of the Ly-α and Ly-β profiles is obtained through calculating the 1st and 3rd-order moments of the line profiles. By comparing maps of self-reversal depths and the moments with radiance images of the Lyman lines, photospheric magnetograms, and Dopplergrams of two other optically thin lines, we studied the spatial distribution of the Ly-α and Ly-β profiles with different self-reversal depths, and investigated the relationship between profile asymmetries and flows in the solar atmosphere. Results. We find that the emissions of the Lyman lines tend to be more strongly absorbed in the internetwork, as compared to those in the network region. Almost all of the Ly-α profiles are self-reversed, while about 17% of the Ly-β profiles are not reversed. The ratio of Ly-α and Ly-β intensities seems to be independent of the magnetic field strength. Most Ly-α profiles are stronger in the blue horn, whereas most Ly-β profiles are stronger in the red horn. However, the opposite asymmetries of Ly-α and Ly-β are not correlated pixelto-pixel. We also confirm that when larger transition-region downflows are present, the Ly-α and Ly-β profiles are more enhanced in the blue and red horns, respectively. The first-order moment of Ly-β, which reflects the combined effects of the profile asymmetry and motion of the emitting material, strongly correlates with the Doppler shifts of the Si iii and O vi lines, while this correlation is much weaker for Ly-α. Our analysis shows that both Ly-α and Ly-β might be more redshifted if stronger transition-region downflows are present. We also find that the observed average Ly-β profile is redshifted with respect to its rest position.

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“…Electron density measurements above sunspots umbrae using transition region spectral lines have shown electron densities 10 times smaller than the electron density of the surrounding active region (Tian et al 2009). Therefore the reason for such low electron densities is that the scattering region is above a small sunspot.…”

Section: Discussionmentioning

confidence: 99%

“…This low magnetic field strength of the solar pore is caused by a low filling factor as the 1.97 spatial resolution of the magnetogram is larger than the size of the solar pore. Studies of the transition region above sunspots have shown that the electron density on the order of 10 10 cm −3 which is 10 times smaller than in the surrounding plage (Tian et al 2009). We expect that the observed solar pore has electron densities of the same order, or even higher, than the ones measured in Tian et al (2009).…”

Section: Relation Of the Scattering Region To The Solar Porementioning

confidence: 97%

“…Studies of the transition region above sunspots have shown that the electron density on the order of 10 10 cm −3 which is 10 times smaller than in the surrounding plage (Tian et al 2009). We expect that the observed solar pore has electron densities of the same order, or even higher, than the ones measured in Tian et al (2009). However, we measure electron densities that are one or two orders of magnitude lower (10 8 to 10 9 cm −3 ).…”

Section: Relation Of the Scattering Region To The Solar Porementioning

confidence: 97%

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Spectral diagnostic of a microflare. Evidences of resonant scattering in C IV 1548 Å, 1550 Å lines

Gontikakis

Winebarger

Patsourakos

2013

A&A

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Aims. We study a microflare, classified as a GOES-A1 after background subtraction, which was observed in active region NOAA 8541 on May 15, 1999. Methods. We used TRACE filtergrams to study the morphology and time evolution. SUMER spectral lines were used to diagnose the chromospheric plasma (Si ii 1533 Å), transition region plasma (C iv 1548, 1550 Å), and coronal plasma (Ne viii 770 Å).Results. In the 171 Å and 195 Å filtergrams, we measure apparent mass motions along two small loops that compose the microflare from the eastern toward the western footpoints. In SUMER, the microflare is detected as a small (47 Mm 2 ), bright area at the western footpoints of the TRACE loops. The spectral profiles recorded over the bright area are complex. The Si ii 1533 Å line is self-reversed owing to opacity, and the coronal Ne viii line profile is composed of two Gaussian components, one of them systematically redshifted. The C iv 1548 Å and 1550 Å profiles are badly distorted because of the temporary depression of the detector local gain caused by the very high count rates reached in the flaring region and we can only confirm the presence of strong blueshifts of -200 km s −1 .Few, unaffected C iv profiles show two spectral components. In the northern part of the bright area, all SUMER spectral lines have at least one blueshifted spectral component. In the southern region of the bright area the spectral lines are redshifted. Adjacent to the microflare we measure, for the first time on the solar disk, an intensity ratio of the 1548 Å line to 1550 Å line with values of three to four, indicating that resonance scattering prevails in the lines formation. Moreover, the scattering region is found to be cospatial to a solar pore. Conclusions. The blueshifts in the footpoints of the microflare and the apparent mass motions observed with TRACE can be explained by a gentle chromospheric evaporation triggered by the microflare. The redshifted spectral components can be explained as cooling material that is falling back on the solar surface. The presence of resonant scattering can be explained by the low electron density expected in the transition region of a solar pore, combined with the high photon flux coming from the nearby microflare. We estimate that the lower limit of the electron density in the pore lies in the range 10 8 cm −3 to 10 9 cm −3 .

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Solar transition region above sunspots

Cited by 37 publications

References 76 publications

The chromosphere above sunspots at millimeter wavelengths

The chromosphere above sunspots at millimeter wavelengths

Hydrogen Lyman-α and Lyman-β spectral radiance profiles in the quiet Sun

Spectral diagnostic of a microflare. Evidences of resonant scattering in C IV 1548 Å, 1550 Å lines

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