Semiempirical Models of the Solar Atmosphere. II. The Quiet‐Sun Low Chromosphere at Moderate Resolution

Fontenla, J. M.; Balasubramaniam, Krishnan; Harder, J. W.

doi:10.1086/520319

Cited by 84 publications

(78 citation statements)

References 64 publications

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“…If the CS97 shocks correctly represent the acoustic heating mechanism, the non-magnetic chromosphere would be essentially cold. This is contrary to the common view, derived from semi-empirical static models, of a chromosphere characterized everywhere by a steady temperature rise (Vernazza et al 1981;Fontenla et al 2007;Avrett & Loeser 2008). The failure of the CS97 model to reproduce spatially and temporally resolved quiet Sun UV observations obtained with SUMER in chromospheric lines ) has further sharpened the controversy.…”

Section: Introductioncontrasting

confidence: 64%

The solar chromosphere at high resolution with IBIS

Vecchio

Cauzzi

Reardon

2008

A&A

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Context. The exact nature of the quiet solar chromosphere and especially its temporal variation, are still subjects of intense debate. One of the contentious issues is the possible role of the magnetic field in structuring the quieter solar regions. Aims. We characterize the dynamics of the quiet inter-network chromosphere by studying the occurrence of acoustic shocks and their relation with the concomitant photospheric structure and dynamics, including small scale magnetic structures.Methods. We analyze a comprehensive data set that includes high-resolution chromospheric (Ca ii 854.2 nm) and photospheric (Fe i 709.0 nm) spectra obtained with the IBIS imaging spectrometer in two quiet-Sun regions. This is complemented by highresolution sequences of MDI magnetograms of the same targets. From the chromospheric spectra we identify the spatio-temporal occurrence of the acoustic shocks. We compare it with the photospheric dynamics by means of both Fourier and wavelet analysis and study the influence of magnetic structures on the phenomenon. Results. Mid-chromospheric shocks occur within the general chromospheric dynamics pattern of acoustic waves propagating from the photosphere. In particular, they appear as a response to underlying powerful photospheric motions at periodicities nearing the acoustic cut-off, consistent with 1-D hydrodynamical modeling. However, their spatial distribution within the supergranular cells is highly dependent on the local magnetic topology, both at the network and internetwork scale. We find that large portions of the internetwork regions undergo very few shocks, since they are "shadowed" by the horizontal component of the magnetic field. The latter is betrayed by the presence of chromospheric fibrils, observed in the core of the Ca ii line as slanted structures with distinct dynamical properties. The shadow mechanism also appears to operate on the very small scales of inter-network magnetic elements, and provides for a very pervasive influence of the magnetic field even in the quietest region analyzed. Conclusions. The magnetic field might play a larger role in structuring the quiet solar chromosphere than normally assumed. The presence of fibrils highlights a clear disconnection between the photospheric dynamics and the response of the geometrically overlaying chromosphere. As these results hold for a mid-chromospheric indicator such as the Ca ii 854.2 line, it is expected that diagnostics formed in higher layers, such as UV lines and continua, will be affected to a greater extent by the presence of magnetic fields, even in quiet regions. This is relevant for the chromospheric models that make use of such diagnostics.

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Section: Introductioncontrasting

confidence: 64%

The solar chromosphere at high resolution with IBIS

Vecchio

Cauzzi

Reardon

2008

A&A

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“…Observations and/or models of the quiet Sun have been published by, for example, Vernazza et al (1981); Loukitcheva et al (2004);Fontenla et al (2007); Avrett & Loeser (2008), and De la Luz et al (2014). Evidence is, however, mounting that the heating of the solar chromosphere and corona is dominated by active, magnetically controlled, processes (Carlsson & Stein 1995), and it remains to be seen what quiet stellar chromosphere models could add to the more complete understanding of the physics of the Sun and other stars.…”

Section: Discussionmentioning

confidence: 99%

ALMA observations ofαCentauri

Liseau

Vlemmings

Bayo

et al. 2014

A&A

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Context. The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying stellar chromospheres one aims at identifying the relevant physical processes. Defining the permittable extent of the parameter space can also serve as a template for the Sun-as-a-star. This feedback will probably also help identify stars that potentially host planetary systems that are reminiscent of our own. Aims. Earlier observations with Herschel and APEX have revealed the temperature minimum of α Cen, but these were unable to spatially resolve the binary into individual components. With the data reported in this Letter, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870 µm. In the present context, we intend to extend the spectral mapping (SED) to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident. Methods. The Atacama Large Millimeter/submillimeter Array (ALMA) is particularly suited to point sources, such as unresolved stars. ALMA provides the means to achieve our objectives with both its high sensitivity of the collecting area for the detection of weak signals and the high spatial resolving power of its adaptable interferometer for imaging close multiple stars. Results. This is the first detection of main-sequence stars at a wavelength of 3 mm. Furthermore, the individual components of the binary α Cen AB are clearly detected and spatially well resolved at all ALMA wavelengths. The high signal-to-noise ratios of these data permit accurate determination of their relative flux ratios, i.e., S B ν /S A ν = 0.54 ± 0.04 at 440 µm, =0.46 ± 0.01 at 870 µm, and =0.47 ± 0.006 at 3.1 mm, respectively. Conclusions. The previously obtained flux ratio of 0.44 ± 0.18, which was based on measurements in the optical and at 70 µm, is consistent with the present ALMA results, albeit with a large error bar. The observed 3.1 mm emission greatly exceeds what is predicted from the stellar photospheres, and undoubtedly arises predominantly as free-free emission in the ionized chromospheric plasmas of both stars. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising.

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“…At the present time, there is not a uniform correction of the limb weakening. According to the simulated result of Jin et al (2013), who used the thin flux tube model (Fontenla et al 2006(Fontenla et al , 2007, the variation of the observed line-of-sight field conforms to the cosine function far away from the solar disk center, so the observed line-of-sight component is corrected as B B ( ) cos( ) cal obs a a = . The angle α is defined by…”

Section: Observations and Data Analysismentioning

confidence: 99%

Solar Cycle Variation of the Inter-Network Magnetic Field

Jin

Wang

2015

ApJ

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The solar inter-network magnetic field is the weakest component of solar magnetism, but it contributes most of the solar surface magnetic flux. The study of its origin has been constrained by the inadequate tempospatial resolution and sensitivity of polarization observations. With dramatic advances in spatial resolution and detecting sensitivity, the solar spectropolarimetry provided by the Solar Optical Telescope on board Hinode in an interval from the solar minimum to maximum of cycle 24 opens an unprecedented opportunity to study the cyclic behavior of the solar inter-network magnetic field. More than 1000 Hinode magnetograms observed from 2007 January to 2014 August are selected in the study. It has been found that there is a very slight correlation between sunspot number and magnetic field at the inter-network flux spectrum. From solar minimum to maximum of cycle 24, the flux density of the solar inter-network field is invariant, at 10 ± 1 G. The observations suggest that the inter-network magnetic field does not arise from flux diffusion or flux recycling of solar active regions, thereby indicating the existence of a local small-scale dynamo. Combining the full-disk magnetograms observed by the Solar and Heliospheric Observatory/Michelson Doppler Imager and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in the same period, we find that the area ratio of the inter-network region to the full disk of the Sun apparently decreases from solar minimum to maximum but always exceeds 60%, even in the phase of solar maximum.

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Semiempirical Models of the Solar Atmosphere. II. The Quiet‐Sun Low Chromosphere at Moderate Resolution

Cited by 84 publications

References 64 publications

The solar chromosphere at high resolution with IBIS

The solar chromosphere at high resolution with IBIS

ALMA observations ofαCentauri

Solar Cycle Variation of the Inter-Network Magnetic Field

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