THE FORMATION OF THE Hα LINE IN THE SOLAR CHROMOSPHERE

We use state-of-the-art radiation-MHD simulations and three-dimensional (3D) non-LTE radiative transfer computations to investigate Hα line formation in the solar chromosphere and apply the results of this investigation to develop the potential of Hα as a diagnostic of the chromosphere. We show that one can accurately model Hα line formation assuming statistical equilibrium and complete frequency redistribution provided the computation of the model atmosphere included non-equilibrium ionization of hydrogen and the Lyα and Lyβ line profiles are described by Doppler profiles. We find that 3D radiative transfer is essential in modeling hydrogen lines due to the low photon destruction probability in Hα. The Hα opacity in the upper chromosphere is mainly sensitive to the mass density and only weakly sensitive to the temperature. We find that the Hα line-core intensity is correlated with the average formation height: The larger the average formation height is, the lower the intensity will be. The line-core width is a measure of the gas temperature in the line-forming region. The fibril-like dark structures seen in Hα line-core images computed from our model atmosphere are tracing magnetic field lines. These structures are caused by field-aligned ridges of enhanced chromospheric mass density that raise their average formation height, and therefore make them appear dark against their deeper-formed surroundings. We compare with observations, and find that the simulated line-core widths are very similar to the observed ones, without the need for additional microturbulence.

[1]  A. Tritschler,et al.  Fabry-Perot versus slit spectropolarimetry of pores and active network. Analysis of IBIS and Hinode data , 2010, 1001.0561.

[2]  M. Carlsson,et al.  ON-DISK COUNTERPARTS OF TYPE II SPICULES IN THE Ca ii 854.2 nm AND Hα LINES , 2009, 0909.2115.

[3]  John M. Davis,et al.  First Results From Hinode , 2008 .

[4]  A. Tritschler,et al.  The solar chromosphere at high resolution with IBIS. IV. Dual-line evidence of heating in chromospheric network , 2009, 0906.2083.

[5]  Han Uitenbroek,et al.  Multilevel Radiative Transfer with Partial Frequency Redistribution , 2001 .

[6]  Mats G. Lofdahl,et al.  CRISP Spectropolarimetric Imaging of Penumbral Fine Structure , 2008, 0806.1638.

[7]  R. Skartlien.  A Multigroup Method for Radiation with Scattering in Three-Dimensional Hydrodynamic Simulations , 2000 .

[8]  Mark Shand,et al.  Adaptive optics system for the new Swedish solar telescope , 2003, SPIE Astronomical Telescopes + Instrumentation.

[9]  M. Carlsson,et al.  Dynamic Fibrils Are Driven by Magnetoacoustic Shocks , 2006, astro-ph/0607332.

[10]  Mats G. Löfdahl,et al.  Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity , 2005 .

[11]  R. Rutten,et al.  DOT Tomography of the Solar Atmosphere VII. Chromospheric Response to Acoustic Events , 2008, 0801.0374.

[12]  E. Dorfi,et al.  Simple adaptive grids for 1-d initial value problems , 1987 .

[13]  D. Mihalas,et al.  Erratum Resonance-Line Transfer with Partial Redistribution: a Preliminary Study of Lyman a in the Solar Chromospherer , 1973 .

[14]  Robert F. Stein,et al.  Does a nonmagnetic solar chromosphere exist , 1995 .

[15]  M. Carlsson,et al.  Dynamic Hydrogen Ionization , 2002, astro-ph/0202313.

[16]  K. Reardon,et al.  THE QUIET SOLAR ATMOSPHERE OBSERVED AND SIMULATED IN Na i D1 , 2009, 0912.2206.

[17]  USA,et al.  High-Resolution Observations and Modeling of Dynamic Fibrils , 2007, astro-ph/0701786.

[18]  M. Carlsson,et al.  Non-LTE Radiating Acoustic Shocks and CA II K2V Bright Points , 1992 .

[19]  S. Wedemeyer,et al.  Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere , 2003, astro-ph/0311273.

[20]  H. Hudson,et al.  GENERATION OF ELECTRIC CURRENTS IN THE CHROMOSPHERE VIA NEUTRAL–ION DRAG , 2010, 1011.5834.

[21]  H. Socas-Navarro,et al.  On the Diagnostic Potential of Hα for Chromospheric Magnetism , 2004 .

[22]  H. Uitenbroek.  The Inverse-C Shape of Solar Chromospheric Line Bisectors , 2006 .

[23]  H. Socas-Navarro,et al.  Are solar chromospheric fibrils tracing the magnetic field , 2011, 1101.3551.

[24]  K. Reardon,et al.  EVIDENCE FOR TWO SEPARATE BUT INTERLACED COMPONENTS OF THE CHROMOSPHERIC MAGNETIC FIELD , 2011 .

[25]  M. Carlsson,et al.  Formation of Solar Calcium H and K Bright Grains , 1997 .

[26]  Juan M. Fontenla,et al.  Energy balance in the solar transition region. III - Helium emission in hydrostatic, constant-abundance models with diffusion , 1993 .

[27]  V. Hansteen,et al.  ON THE ORIGIN OF THE TYPE ii SPICULES: DYNAMIC THREE-DIMENSIONAL MHD SIMULATIONS , 2010, 1011.4703.

[28]  L. R. V. D. Voort,et al.  THREE-DIMENSIONAL NON-LTE RADIATIVE TRANSFER COMPUTATION OF THE CA 8542 INFRARED LINE FROM A RADIATION-MHD SIMULATION , 2009, 0903.0791.

[29]  R. Davies,et al.  Astronomical Society of the Pacific Conference Series , 2010 .

[30]  Tapio K. Korhonen,et al.  The 1-meter Swedish solar telescope , 2003, SPIE Astronomical Telescopes + Instrumentation.

[31]  Eugene H. Avrett,et al.  Structure of the solar chromosphere. III. Models of the EUV brightness components of the quiet sun , 1981 .

[32]  B. Pontieu,et al.  Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind , 2007, Science.

[33]  B. Pontieu,et al.  Spicule-like structures observed in 3D realistic MHD simulations , 2009, 0906.4446.

[34]  B. Pontieu,et al.  SPICULE-LIKE STRUCTURES OBSERVED IN THREE-DIMENSIONAL REALISTIC MAGNETOHYDRODYNAMIC SIMULATIONS , 2009 .

[35]  M. Carlsson,et al.  DOT tomography of the solar atmosphere - VI. Magnetic elements as bright points in the blue wing of H$\mathsf{\alpha}$ , 2006 .

[36]  M. Carlsson,et al.  Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere , 2007, 0709.3751.

[37]  M. Carlsson,et al.  TWISTED FLUX TUBE EMERGENCE FROM THE CONVECTION ZONE TO THE CORONA. II. LATER STATES , 2009 .

[38]  M. Carlsson,et al.  Twisted Flux Tube Emergence From the Convection Zone to the Corona , 2007, 0712.3854.

[39]  Yan Xu,et al.  NONPOTENTIALITY OF CHROMOSPHERIC FIBRILS IN NOAA ACTIVE REGIONS 11092 AND 9661 , 2011 .

[40]  R. Rutten.  Solar Spectroscopy and (Pseudo-)Diagnostics of the Solar Chromosphere , 2009, 0905.2623.

[41]  John W. Leibacher,et al.  Solar MHD theory and observations : a high spatial resolution perspective : in honor of Robert F. Stein : proceedings of a meeting held at the National Solar Obsevatory, Sacramento Peak, Sunspot, New Mexico, USA, 18-22 July 2005 , 2006 .

[42]  Stephen L. Keil,et al.  Innovative Telescopes and Instrumentation for Solar Astrophysics , 2003 .