Showing papers on "Solar transition region published in 2012"
TL;DR: The Atmospheric Imaging Assembly (AIA) as discussed by the authors provides multiple simultaneous high-resolution full-disk images of the corona and transition region up to 0.5 R ⊙ above the solar limb with 1.5-arcsec spatial resolution and 12-second temporal resolution.
Abstract: The Atmospheric Imaging Assembly (AIA) provides multiple simultaneous high-resolution full-disk images of the corona and transition region up to 0.5 R ⊙ above the solar limb with 1.5-arcsec spatial resolution and 12-second temporal resolution. The AIA consists of four telescopes that employ normal-incidence, multilayer-coated optics to provide narrow-band imaging of seven extreme ultraviolet (EUV) band passes centered on specific lines: Fe xviii (94 A), Fe viii, xxi (131 A), Fe ix (171 A), Fe xii, xxiv (193 A), Fe xiv (211 A), He ii (304 A), and Fe xvi (335 A). One telescope observes C iv (near 1600 A) and the nearby continuum (1700 A) and has a filter that observes in the visible to enable coalignment with images from other telescopes. The temperature diagnostics of the EUV emissions cover the range from 6×104 K to 2×107 K. The AIA was launched as a part of NASA’s Solar Dynamics Observatory (SDO) mission on 11 February 2010. AIA will advance our understanding of the mechanisms of solar variability and of how the Sun’s energy is stored and released into the heliosphere and geospace.
4,321 citations
TL;DR: In this article, it was shown that the phase of the reflected fast wave can be interpreted as a travel speed perturbation, with implications for the practical seismic probing of active regions.
Abstract: Alfven waves may be difficult to excite at the photosphere due to low ionization fraction and suffer near-total reflection at the transition region (TR). Yet they are ubiquitous in the corona and heliosphere. To overcome these difficulties, we show that they may instead be generated high in the chromosphere by conversion from reflecting fast magnetohydrodynamic waves, and that Alfvenic transition region reflection is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored. This phase can potentially be misinterpreted as a travel speed perturbation, with implications for the practical seismic probing of active regions.
54 citations
TL;DR: In this article, it was shown that the phase of the reflected fast wave can be interpreted as a travel speed perturbation with implications for the practical seismic probing of active regions.
Abstract: Alfven waves may be difficult to excite at the photosphere due to low-ionization fraction and suffer near-total reflection at the transition region (TR). Yet they are ubiquitous in the corona and heliosphere. To overcome these difficulties, we show that they may instead be generated high in the chromosphere by conversion from reflecting fast magnetohydrodynamic waves, and that Alfvenic TR reflection is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored. This phase can potentially be misinterpreted as a travel speed perturbation with implications for the practical seismic probing of active regions.
49 citations
TL;DR: In this article, the authors show that the joint action of partial frequency redistribution and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings.
Abstract: Recent theoretical investigations have pointed out that the cores of the Ly? lines of H I and He II should show measurable scattering polarization signals when observing the solar disk, and that the magnetic sensitivity, through the Hanle effect, of such linear polarization signals is suitable for exploring the magnetism of the solar transition region. Such investigations were carried out in the limit of complete frequency redistribution (CRD) and neglecting quantum interference between the two upper J-levels of each line. Here we relax both approximations and show that the joint action of partial frequency redistribution and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings. Such wing polarization signals turn out to be very sensitive to the temperature structure of the atmospheric model, so that they can be exploited for constraining the thermal properties of the solar chromosphere. Finally, we show that the approximation of CRD without J-state interference is however suitable for estimating the amplitude of the linear polarization signals in the core of the lines, where the Hanle effect operates.
47 citations
TL;DR: In this paper, the authors show that partial frequency redistribution (PRD) and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings.
Abstract: Recent theoretical investigations have pointed out that the cores of the Ly-alpha lines of H I and He II should show measurable scattering polarization signals when observing the solar disk, and that the magnetic sensitivity, through the Hanle effect, of such linear polarization signals is suitable for exploring the magnetism of the solar transition region. Such investigations were carried out in the limit of complete frequency redistribution (CRD) and neglecting quantum interference between the two upper J-levels of each line. Here we relax both approximations and show that the joint action of partial frequency redistribution (PRD) and J-state interference produces much more complex fractional linear polarization (Q/I) profiles, with large amplitudes in their wings. Such wing polarization signals turn out to be very sensitive to the temperature structure of the atmospheric model, so that they can be exploited for constraining the thermal properties of the solar chromosphere. Finally, we show that the approximation of CRD without J-state interference is however suitable for estimating the amplitude of the linear polarization signals in the core of the lines, where the Hanle effect operates.
40 citations
TL;DR: In this article, it was shown that ultraviolet backradiation from the solar transition region and upper chromosphere strongly affects the degree of ionization of minority stages at the top of the photosphere.
Abstract: A recent paper states that ultraviolet backradiation from the solar transition region and upper chromosphere strongly affects the degree of ionization of minority stages at the top of the photosphere, i.e., in the temperature minimum of the one-dimensional static model atmospheres presented in that paper. We show that this claim is incompatible with observations and we demonstrate that the pertinent ionization balances are instead dominated by outward photospheric radiation, as in older static models. We then analyze the formation of Hα in the above model and show that it has significant backradiation across the opacity gap by which Hα differs from other strong scatttering lines.
39 citations
TL;DR: In this article, the authors measured and modeled the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Lyα line, to understand the heating of the solar corona.
Abstract: In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Lyα line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here, we report on the Lyα scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation magnetohydrodynamic simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Lyα. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent Q/I and U/I profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Lyα and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal, and dynamic structure of the upper chromosphere and transition region of the Sun.
32 citations
TL;DR: In this article, it was shown that the scattering polarization of the Ly-alpha line of He II at 304 Angstrom-Sign is sensitive to the Hanle effect for magnetic strengths B {approx}> 100 G.
Abstract: The Ly{alpha} line of He II at 304 Angstrom-Sign is one of the spectral lines of choice for EUV channels of narrowband imagers on board space telescopes, which provide spectacular intensity images of the outer solar atmosphere. Since the magnetic field information is encoded in the polarization of the spectral line radiation, it is important to investigate whether the He II line radiation from the solar disk can be polarized, along with its magnetic sensitivity. Here we report some theoretical predictions concerning the linear polarization signals produced by scattering processes in this strong emission line of the solar transition region, taking into account radiative transfer and the Hanle effect caused by the presence of organized and random magnetic fields. We find that the fractional polarization amplitudes are significant ({approx}1%), even when considering the wavelength-integrated signals. Interestingly, the scattering polarization of the Ly{alpha} line of He II starts to be sensitive to the Hanle effect for magnetic strengths B {approx}> 100 G (i.e., for magnetic strengths of the order of and larger than the Hanle saturation field of the hydrogen Ly{alpha} line at 1216 Angstrom-Sign ). We therefore propose simultaneous observations of the scattering polarization in both Ly{alpha} lines to facilitatemore » magnetic field measurements in the upper solar chromosphere. Even the development of a narrowband imaging polarimeter for the He II 304 Angstrom-Sign line alone would be already of great diagnostic value for probing the solar transition region.« less
32 citations
TL;DR: In this article, it was shown that ultraviolet backradiation from the solar transition region and upper chromosphere strongly affects the degree of ionization of minority stages at the top of the photosphere, i.e., in the temperature minimum of the one-dimensional static model atmospheres.
Abstract: A recent paper states that ultraviolet backradiation from the solar transition region and upper chromosphere strongly affects the degree of ionization of minority stages at the top of the photosphere, i.e., in the temperature minimum of the one-dimensional static model atmospheres presented in that paper. We show that this claim is incompatible with bservations and we demonstrate that the pertinent ionization balances are instead dominated by outward photospheric radiation, as in older static models. We then analyze the formation of Halpha in the above model and show that it has significant backradiation across the opacity gap by which Halpha differs from other strong scatttering lines.
32 citations
TL;DR: In this paper, the authors measured and modeled the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Ly$\alpha$ line, to understand the heating of the solar corona.
Abstract: In order to understand the heating of the solar corona it is crucial to obtain empirical information on the magnetic field in its lower boundary (the transition region). To this end, we need to measure and model the linear polarization produced by scattering processes in strong UV lines, such as the hydrogen Ly$\alpha$ line. The interpretation of the observed Stokes profiles will require taking into account that the outer solar atmosphere is highly structured and dynamic, and that the height of the transition region may well vary from one place in the atmosphere to another. Here we report on the Ly$\alpha$ scattering polarization signals we have calculated in a realistic model of an enhanced network region, resulting from a state-of-the-art radiation MHD simulation. This model is characterized by spatially complex variations of the physical quantities at transition region heights. The results of our investigation lead us to emphasize that scattering processes in the upper solar chromosphere should indeed produce measurable linear polarization in Ly$\alpha$. More importantly, we show that via the Hanle effect the model's magnetic field produces significant changes in the emergent $Q/I$ and $U/I$ profiles. Therefore, we argue that by measuring the polarization signals produced by scattering processes and the Hanle effect in Ly$\alpha$ and contrasting them with those computed in increasingly realistic atmospheric models, we should be able to decipher the magnetic, thermal and dynamic structure of the upper chromosphere and transition region of the Sun.
30 citations
TL;DR: In this paper, a 1D wave simulation for the solar chromosphere by studying tubes of different spreading as a function of height aimed at representing tubes in environments of different magnetic filling factors is presented.
Abstract: In the present work we expand the study of time-dependent ionization previously identified to be of pivotal importance for acoustic waves in solar magnetic flux tube simulations. We focus on longitudinal tube waves (LTW) known to be an important heating agent of solar magnetic regions. Our models also consider new results of wave energy generation as well as an updated determination of the mixing length of convection now identified as 1.8 scale heights in the upper solar convective layers. We present 1D wave simulations for the solar chromosphere by studying tubes of different spreading as a function of height aimed at representing tubes in environments of different magnetic filling factors. Multilevel radiative transfer has been applied to correctly represent the total chromospheric emission function. The effects of time-dependent ionization are significant in all models studied. They are most pronounced behind strong shocks and in low-density regions, i.e. the middle and high chromosphere. Concerning our models of different tube spreading, we attained pronounced differences between the various types of models, which were largely initiated by different degrees of dilution of the wave energy flux as well as the density structure partially shaped by strong shocks, if existing. Models showing a quasi-steady rise of temperature with height are obtained via monochromatic waves akin to previous acoustic simulations. However, longitudinal flux tube waves are identified as insufficient to heat the solar transition region and corona in agreement with previous studies.
14 May 2012
TL;DR: In this paper, a non-LTE multilevel radiative transfer code for modeling the intensity and linear polarization produced by scattering processes in spectral lines and its modification by the Hanle effect is presented.
Abstract: In order to obtain empirical information on the magnetism of the solar transition region we need to measure and interpret the linear polarization produced by scattering processes in FUV and EUV spectral lines. Via the Hanle effect such linear polarization signals are sensitive to the magnetic fields expected for the quiet and active regions of the outer solar atmosphere. For example, the Ly$\alpha$ line of H\,{\sc i} at 1216\,\AA\ is mainly sensitive to magnetic strengths between 10 and 100 G. The interpretation of the observed spectral line polarization requires the development of suitable modeling tools. To this end, we have developed a three-dimensional (3D), non-LTE multilevel radiative transfer code for modeling the intensity and linear polarization produced by scattering processes in spectral lines and its modification by the Hanle effect.
01 Jan 2012
TL;DR: In this paper, it was shown that fast magnetohydrodynamic waves may instead be generated high in the chromosphere by reflecting fast magnetodynamic waves, and that the reflection at the transition region (TR) is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored.
Abstract: Alfv´ en waves may be difficult to excite at the photosphere due to low-ionization fraction and suffer near-total reflection at the transition region (TR). Yet they are ubiquitous in the corona and heliosphere. To overcome these difficulties, we show that they may instead be generated high in the chromosphere by conversion from reflecting fast magnetohydrodynamic waves, and that Alfv´ enic TR reflection is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored. This phase can potentially be misinterpreted as a travel speed perturbation with implications for the practical seismic probing of active regions.
01 May 2012
01 Jan 2012
TL;DR: In this paper, the authors considered the self-similar MHD problem of the oblique interference of a solar rotational discontinuity A and a stationary contact discontinuity C. The interaction between A and C was studied for typical conditions in the solar corona.
Abstract: We consider the self-similar MHD problem of the oblique interference of a solar rotational (Alfven) discontinuity A and a stationary contact discontinuity C. The interaction between A and C is studied for typical conditions in the solar corona. Since solar Alfven waves observed in the solar plasma are numerous, prerequisites exist for the formation of a solar rotational discontinuity that propagates from the chromosphere through the transition region to the corona. Dissipative slow MHD shock waves with insignificant variation of the magnetic field also appear due to the refraction of the solar non-dissipative rotational discontinuities against a contact discontinuity inside the transition region. It is supposed that a real source of plasma heating may exist in the high solar corona due to the well-known mechanism of Landau damping of the dissipative slow MHD shock waves. Frequently observed explosive events may also be triggered in the solar chromospheric plasma. Thereby, we suggest a new model of the coronal plasma heating.
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TL;DR: In this article, a non-LTE multilevel radiative transfer code for modeling the intensity and linear polarization produced by scattering processes in spectral lines and its modification by the Hanle effect is presented.
Abstract: In order to obtain empirical information on the magnetism of the solar transition region we need to measure and interpret the linear polarization produced by scattering processes in FUV and EUV spectral lines. Via the Hanle effect such linear polarization signals are sensitive to the magnetic fields expected for the quiet and active regions of the outer solar atmosphere. For example, the Ly$\alpha$ line of H\,{\sc i} at 1216\,\AA\ is mainly sensitive to magnetic strengths between 10 and 100 G. The interpretation of the observed spectral line polarization requires the development of suitable modeling tools. To this end, we have developed a three-dimensional (3D), non-LTE multilevel radiative transfer code for modeling the intensity and linear polarization produced by scattering processes in spectral lines and its modification by the Hanle effect.
TL;DR: In this article, a 1D wave simulation for the solar chromosphere by studying tubes of different spreading as function of height aimed at representing tubes in environments of different magnetic filling factors is presented.
Abstract: In the present work we expand the study of time-dependent ionization previously identified to be of pivotal importance for acoustic waves in solar magnetic flux tube simulations. We focus on longitudinal tube waves (LTW) known to be an important heating agent of solar magnetic regions. Our models also consider new results of wave energy generation as well as an updated determination of the mixing length of convection now identified as 1.8 scale heights in the upper solar convective layers. We present 1-D wave simulations for the solar chromosphere by studying tubes of different spreading as function of height aimed at representing tubes in environments of different magnetic filling factors. Multi-level radiative transfer has been applied to correctly represent the total chromospheric emission function. The effects of time-dependent ionization are significant in all models studied. They are most pronounced behind strong shocks and in low density regions, i.e., the middle and high chromosphere. Concerning our models of different tube spreading, we attained pronounced differences between the various types of models, which were largely initiated by different degrees of dilution of the wave energy flux as well as the density structure partially shaped by strong shocks, if existing. Models showing a quasi-steady rise of temperature with height are obtained via monochromatic waves akin to previous acoustic simulations. However, longitudinal flux tube waves are identified as insufficient to heat the solar transition region and corona in agreement with previous studies.