The Quiet Chromosphere - Sami K. Solanki Max Planck Institute for Solar System Research

The Quiet Chromosphere

       Sami K. Solanki

    Max Planck Institute for
    Solar System Research

The active
   chromosphere
Coronal mass ejection

                        Flares

The quiet chromosphere

Chromosphere

Simplified sketch of the QUIET photosphere
             and chromosphere

  Wedemeyer-Böhm et al. 2008; similar to an earlier sketch by Rob Rutten

What is special about the chromosphere?
   Possibly Sun’s most complex layer: waves, gas (thermal
    energy), flows, magnetic field all have similar energy
    densities. Radiation needs non-LTE RT treatment, non-
    equilibrium ionization, ambipolar diffusion, etc.
   Extremely dynamic and
    strongly structured
   Magnetic field and waves
     structure & dynamics
   Energy transfer layer: from
    photosphere to corona

                Spicules at limb
             (in Ca II H at SST)

What is special about the
                     chromosphere?
   Chromosphere far less understood than
    photosphere. Neglected for many years
   This is changing:
       TIP II, IBIS, CRISP, etc. increasingly used
        to probe the chromosphere
       IRIS (launch in April) will mainly study upper
        chromosphere (Mg II lines) & TR
       Solar C (launch ~2020), will concentrate on
        the chromosphere and its interaction with
        photosphere and corona

   ALMA has chance of studying the solar
    chromosphere along with IRIS, NST,
    Gregor; well before Solar C & ATST

Blue continuum

Observed with SST
by Hirzberger and Zakharov

                      Ca II K

Flux Tubes, Canopies, Loops and
            Funnels

     Hot and bright

                    Expanding
                  field

Flux Tubes, Canopies, Loops and
            Funnels

     Hot and bright

                    Expanding
                  field

Evolving structures with height

The convection
dominated image of
the photosphere
(seen in g-band) is
replaced by the
magnetic field and
wave dominated
chromosphere when
observing in strong
spectral lines
(Ca II K and Hα)

                      DOT data (Rutten and Sütterlin)

Chromospheric dynamics (DOT)

Chromospheric dynamics: waves
   At low to medium resolution:
       Power at 3 min in internetwork
       Power at longer periods in
        Network

   At high spatial resolution:
    identification of many wave-
    modes and details about their
    excitation and propagation
       Acoustic waves (everywhere)
       Slow and fast mode waves (MHD)
       Kink waves (MHD)
       MHD waves excited by granular     Transverse waves seen in
        buffetting of magnetic features     Sunrise data sampling
       Evidence of wave conversion           different heights

Energy transported by MHD waves is sufficient to heat corona

Chromospheric dynamics: How
          important are shock waves?
   As acoustic waves propagate
    upwards, they steepen and
    shock
   Early 1-D models: Do take into
    account non-local
              Centeno etradiation,
                          al. 2009
    non-locally controlled atomic
    levels (time dep. excitations)
   Start with piston in CZ,
    consistent with obs. of
    photospheric oscillations
                      He 10830
    Carlsson & Stein 1993, 1995,
    1997, 2001

Effect of           Ly continuum

   shock
  waves
  on EUV
 radiation
 Dashed:
  temperature at
  τ ν =1
 Solid: intensity
  & radiation
  temperature at
  τ ν =1

 Carlsson & Stein
            1995

Chromospheric MHD simulation
    Various MHD codes
   allow chromospheric
structure and dynamics
   to be computed. The
      most realistic such
    code is currently the
Bifrost code of the Oslo
                   group.

         Upper panels:
        intensity & BZ in
            photosphere

      Lower panels:
Temperature at 1.7 Mm
    & B-field direction     J. Leenaarts et al. 2012

Do atomic spectral lines sample cool
       regions of the chromosphere?
 de la Cruz et al. (2012): Take a
  NLTE-RT 3D MHD simulatiion of
  the solar photosphere and
  chromosphere, compute in 3D a
  popular chromospheric line (Ca
  II 8542 Å), invert these synthetic
  profiles to get the 3D structure of
  the chromosphere and then
  compare with the original
 Results:
       The inversion works reasonably
        well in warmer part of
        chromosphere
       For cooler gas the inversion can
        get things wrong

Do atomic spectral lines sample cool
       regions of the chromosphere?
 de la Cruz et al. (2012): Reason
  for fitting computed line profile
  with too high temperature is that
  the forward calculation was done
  in 3D, but the inversion in 1D
 In reality the effect will be much
  stronger:
       Their work assumes perfect
        spatial resolution, i.e. no PSF,
        no scattered light
       Both the inner part of the PSF
        and the presence of scattered
        light hide small cool pockets
        (Ayres effect)

Evidence for cool gas: CO lines at 5 μm

                                                From
                                                atomic lines

                                                From CO
                                                lines

Ayres 1981, 2002 ApJ, Ayres et al. 1986, 1998 ApJ, Solanki et al. 1994 Science
              Uitenbroek et al. 1994 ApJ, Uitenbroek 2000a, b 2000

More evidence for inhomogeneous
    chromosphere with cool component
   Holzreuter et al. (2006); Holzreuter & Stenflo (2007): in order
    to reproduce scattering polarization of Ca II K line, two
    components with rather different temperatures are needed

Sub-mm and mm data as a diagnostic
   Sub-mm and mm radiation in quiet chromosphere:
    H--free-free, with source function in LTE: Planck law
     Rayleigh-Jeans: intensity depends linearly on
    temperature  Radiation sees both hot & cool gas
   Combines advantages of CO (sees cool gas) &
    atomic lines (sees hot gas). Additional weighting
    according to the electron density (not in equilibrium)
   Main disadvantage: poor spatial resolution of
    instruments available so far (BIMA, CARMA, VLA)
   There is a strong need for a higher resolution
    telescope such as ALMA

Sub-mm and mm “observations” of
     Carlsson-Stein model

                                  Loukitcheva et al. 2004a

Brightness
temperature
 for different
wavelengths

                 9mm           3mm       1mm           0.1mm

                         Wedemeyer-Boehm et al. 2007

  Brightness
temperature
 for different
       spatial
  resolutions

                  0.9”         0.6”       0.3”         0.06”

Radiation at mm wavelengths from 3-D
   MHD simulations of rising bipole

          1mm radiation                          3mm radiation
Movies by M. Loukitcheva, from Bifrost computations, provided by M. Carlsson

Ca II H vs. mm wavelengths

            mm wavelength radiation allows probing the
             chromosphere at different heights reaching
         considerably higher than even the strongest lines in
                              the visible

Images by M. Loukitcheva, from Bifrost computations, provided by M. Carlsson

Sunspots
Chromospheric layers of sunspots are rather poorly known.
                                 Various umbral models exist (based
                                 on atomic spectral lines); they differ
                                 rather strongly
                                 At sub-mm and mm wavelengths they
                                 give vastly different signatures

 About penumbrae, even less is known. Only 3 models, with
                even larger differences

                 Loukitcheva et al., in preparation

Conclusions
   The chromosphere is an exciting and relatively poorly
    studied part of the solar atmosphere
   Sub-mm and mm-wavelength data have the potential to
    provide information beyond that given by atomic lines
   ALMA will have the necessary spatial resolution to fulfil this
    potential
   See also talks by
       M. Loukitcheva and S. Wedemeyer-Böhm for more on modelling
       S. White for more on observations

Thank you for your attention