Dynamic Chromosphere and Quiet Sun

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, nonequilibrium 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 wavemodes and details about their
excitation and propagation





Acoustic waves (everywhere)
Slow and fast mode waves (MHD)
Kink waves (MHD)
MHD waves excited by granular
buffetting of magnetic features
Evidence of wave conversion
Transverse waves seen in
Sunrise data sampling
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
radiation,
Centeno et
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
shock
waves
on EUV
radiation

Dashed:
temperature at
τ ν =1
 Solid: intensity
& radiation
temperature at
τ ν =1
Carlsson & Stein
1995
Ly continuum
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
Loukitcheva et al. 2004a
Sub-mm and mm “observations” of
Carlsson-Stein model
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