ALMA as the ideal probe of the solar chromosphere
Astrophysics and Space Science manuscript No.
(will be inserted by the editor)
ALMA as the ideal probe of the solar chromosphere
Maria A. Loukitcheva · Sami K. Solanki · Stephen White
Received: date / Accepted: date
Abstract The very nature of the solar chromosphere,
its structuring and dynamics, remains far from being
properly understood, in spite of intensive research. Here
we point out the potential of chromospheric observa-
tions at millimeter wavelengths to resolve this long-
standing problem. Computations carried out with a so-
phisticated dynamic model of the solar chromosphere
due to Carlsson and Stein demonstrate that millimeter
emission is extremely sensitive to dynamic processes in
the chromosphere and the appropriate wavelengths to
look for dynamic signatures are in the range 0.8-5.0
mm. The model also suggests that high resolution ob-
servations at mm wavelengths, as will be provided by
ALMA, will have the unique property of reacting to
both the hot and the cool gas, and thus will have the
potential of distinguishing between rival models of the
solar atmosphere. Thus, initial results obtained from
the observations of the quiet Sun at 3.5 mm with the
BIMA array (resolution of 12′′) reveal significant oscil-
lations with amplitudes of 50-150 K and frequencies of
1.5-8 mHz with a tendency toward short-period oscil-
lations in internetwork and longer periods in network
regions. However higher spatial resolution, such as that
provided by ALMA, is required for a clean separation
between the features within the solar atmosphere and
M.A. Loukitcheva · S.K. Solanki
Max-Planck-Institut für Sonnensystemforschung, D-37191
Katlenburg-Lindau, Germany
E-mail: lukicheva@mps.mpg.de
M. A. Loukitcheva
Astronomical Institute, St. Petersburg University, 198504 St.
Petersburg, Russia
S. White
Astronomy Department, University of Maryland, College Park,
MD 20742, USA
for an adequate comparison with the output of the com-
prehensive dynamic simulations.
Keywords the Sun · solar chromosphere · millimeter
observations
1 Introduction
The chromosphere remains the least understood layer of
the solar atmosphere, with the very basics of its struc-
ture being hotly debated: is it better described by the
classical picture of a steady temperature rise as a func-
tion of height, with superposed weak oscillations (e.g.
semi empirical models of Vernazza et al. [8], Fontenla
et al. [5]), or does the temperature keep dropping out-
wards, with very hot shocks producing strong localized
heating (radiation hydrodynamic simulations of Carls-
son & Stein [3], [4], and Wedemeyer et al. [9])? The
latter concept is consistent with the IR observations of
carbon monoxide, which require cool gas to be present
at chromospheric heights (see, e.g. Ayres [1]).
Thus, existing models cannot provide a complete de-
scription of the solar chromosphere. Consequently now-
adays two alternative pictures of the chromosphere co-
exist and the role played by chromospheric dynamics in
the structuring of this atmospheric layer is a subject of
intense scientific debate.
One reason for conflicting models is that they are
based either on atomic chromospheric lines and con-
tinua in the UV or on molecular lines in the IR, since
UV observations are practically blind to cool gas in a
dynamic chromosphere, while the IR observations sam-
ple only the cool part of the chromosphere. Improved
and more sensitive diagnostics of the chromospheric
structure and dynamics, that sample both the hot and
http://arxiv.org/abs/0704.0023v1
MDI TRACE 1600
CaII K BIMA 3.5 mm
Fig. 1 Portrait of the solar chromosphere at the center of the Sun’s disk at 4 different wavelengths on May 18, 2004. From top left to
bottom right: MDI longitudinal photospheric magnetogram, UV 1600 A image from TRACE, CaII K line center image from BBSO
and BIMA image at 3.5 mm.
the cool gas and should distinguish between the ri-
val models, are provided by observations at millime-
ter wavelengths with an acceptable spatial resolution
as was proposed by Loukitcheva et al. [6]. In this con-
tribution we review the unique chromospheric obser-
vations at 3.5 mm with the Berkeley-Illinois-Maryland
Array and the analysis of the intensity variations ex-
pected from the model of Carlsson & Stein for mm
wavelengths. We postulate the requirements for mm ob-
servations with the future instruments, with emphasis
on spatial and temporal resolution. Finally we discuss
the prospects for chromospheric studies with ALMA.
2 Results
2.1 Analysis of the BIMA observations at 3.5 mm
The Berkeley-Illinois-Maryland Array (BIMA) operat-
ing at a wavelength of 3.5 mm (frequency of 85 GHz)
has been the only interferometer in the mm range fre-
quently used for solar observations. The BIMA tele-
scopes are now part of the CARMA array which will
also carry out such observations. With the BIMA data
obtained in the years 2003 and 2004 we have constructed
two-dimensional maps of the solar chromosphere with a
resolution of 12′′, which represents the highest spatial
resolution achieved so far at this wavelength for non-
flare solar observations. The BIMA images have led
to new insights in to chromospheric structure and to
the detection of spatially-resolved chromospheric oscil-
lations at mm wavelengths. The details of the restora-
tion procedure and extensive tests of the sensitivity of
the BIMA data to the detection of dynamic signatures
can be found in White et al. [11].
With the currently available resolution the contrast
of the brightness structures is evaluated to be up to
30% of the quiet-sun brightness at 3.5 mm (White et
al. [11]). However, the similarity of brightness struc-
tures, derived from the mm images and seen in other
chromospheric emissions (Fig.1), in spite of the differ-
ence in resolution of the images (1-2′′ resolution of the
UV images), implies that the BIMA resolution is not
enough to resolve the millimeter fine structure and ob-
servations with spatial resolution much higher than 12′′
are required. A detailed analysis of the relations be-
tween the millimeter emission, magnetic field and other
chromospheric diagnostics is in preparation.
In the millimeter brightness we detected intensity
oscillations with typical amplitudes of 50-150 K in the
range of periods from 120 to 700 seconds (frequency
range 1.5-8 mHz). We found a tendency toward short
period oscillations in internetwork and longer periods in
network regions in the quiet Sun, which is in good agree-
ment with the results obtained at other wavelengths. At
3 mm the inner parts of the chromospheric cells exhibit
a behavior typical of the internetwork with the maxi-
mum of the Fourier power in the 3-minute range, how-
ever, most of the oscillations are quasi-periodic, show-
ing up in wave trains of finite duration lasting for typi-
cally 1-3 wave periods (see also Loukitcheva et al. [7]).
2.2 Analysis of the CS model millimeter spectrum
The response of the submillimeter and millimeter ra-
diation to a time-series generated by Carlsson & Stein
(CS) was computed under the assumption of thermal
free-free radiation by Loukitcheva et al. [6]. The results
are depicted in Fig. 2 as the excess intensity as a func-
tion of wavelength and time.
400      720    1040    1360    1680    2000    2320    2640    2960    3280    3600
time(s)
Fig. 2 Evolution of the Carlsson & Stein model millimeter spec-
trum with time. Negative grey scale representing excess intensity
as a function of time and wavelength.
Wave periods of approximately 3 min can be clearly
distinguished in the intensity at all considered wave-
lengths. Though the dominant frequency of the oscilla-
tions changes slightly with wavelength, for all mm wave-
lengths it lies in the range of 3 minutes. The difference
from one period of time to another can be explained
by the presence of merging shocks during certain time
intervals. The differences in the light curves at differ-
ent wavelengths are caused primarily by the difference
in the formation heights of the emitted radiation. In
general the amplitudes of the oscillations compared to
the radiation temperature are large, in this sense mm
wavelength radiation combines the advantages of the
CO lines, which mainly see the cool gas, with those of
atomic lines and UV continua, which mainly sample the
hot gas.
On the whole, the brightness temperatures are ex-
tremely time-dependent at millimeter wavelengths, fol-
lowing changes in the atmospheric parameters. With
increasing wavelength the amplitude of the brightness
oscillations grows significantly, reaches its maximum
value at 2.2 mm (expected to be 15% of the quiet-Sun
brightness temperature), and decreases rapidly towards
longer wavelengths. Thus we can identify the range 0.8-
5.0 mm as the appropriate range of mm wavelengths at
which one can expect the clearest signatures of dynamic
effects. A careful look at the mm brightness spectrum
as a function of time (see Fig. 2) reveals a time delay
between the oscillations at long and short millimeter
wavelengths. Hence, it is possible to study wave modes
traveling in the chromosphere by comparing sub-mm
with mm observations.
3 Discussion
The CS model predicts that spatially and temporally
resolved observations should clearly exhibit the signa-
tures of the strong shock waves. However, a direct com-
parison of the observational data products (RMS val-
ues, histogram skewness, Fourier and wavelet spectra,
etc.), referring to regions with weak magnetic field like
the quiet Sun internetwork, with the corresponding prod-
ucts expected from the simulations of Carlsson & Stein
exhibits large differences. In particular, the RMS of the
brightness temperature is nearly an order of magnitude
larger in the model (800 K at 3 mm) than in the ob-
servations (100 K). Another difference is the absence of
longer periods in the model power spectrum. But these
discrepancies do not rule out the CS models. On the
one hand the model is one dimensional and hence does
not predict a coherence length of the oscillations, while
on the other hand we are not able to resolve individual
oscillating elements due to the limited spatial resolution
of the observations.
Consequently we estimated the influence of the spa-
tial smearing on the model parameters of chromospheric
dynamics and on the observed oscillatory power. Thus
we confirmed that the very limited spatial resolution
currently available hinders a clean separation between
cells and network and typically both network and in-
ternetwork areas contribute to the recorded BIMA ra-
diation. From the analysis of the observational data it
was found that power in all frequency ranges increases
significantly with improving resolution. Consistency be-
tween the power predicted by the CS model and the
observed power is obtained if the coherence length of
oscillating elements is on the order of 1′′.
Our results are consistent with Wedemeyer et al.
[10], who computed the millimeter wave signature re-
sulting from the 3-D simulations of Wedemeyer et al.
[9]. Although the 3-D simulations suffer from the fact
that the radiative transfer of energy is computed en-
tirely in LTE, which becomes a poor assumption at
chromospheric heights, the authors believe that the chro-
mospheric pattern and its temporal evolution is repre-
sentative of the non-magnetic internetwork regions of
the solar chromosphere. The simulations display a com-
plex 3D structure of the chromospheric layers, which is
highly dynamical on temporal scales of 20-25 s and on
spatial scales comparable to solar granulation, which is
in good agreement with the 1′′ size of oscillating ele-
ments that we deduced. According to Wedemeyer et al.
[9] the chromospheric temperature structure is charac-
terized by a pattern of hot shock waves, which originate
from convective motions, and cool gas lying between the
shocks. The intensity distribution at mm wavelengths
follows the pattern of the shocks in the chromosphere
with a sub arcsecond size of the features associated with
the shocks. All this complex and dynamic 3D structure
can be deduced from observations at mm wavelengths
with a sufficiently high spatial resolution of better than
4 Summary
Simultaneous mm-submm observations at different wave-
lengths can be used for the tomography of the solar
atmosphere, as radiation at the different wavelengths
originates from different layers, with the average for-
mation height increasing with wavelength. Such obser-
vations also provide a strong test of present and future
models. However, observations that might be able to
uncover the nature of the chromosphere should meet
the following requirements:
– multiband observations in mm-submm domain (0.8-
5.0 mm) to address shock waves and chromospheric
oscillation modes
– arcsecond spatial resolution to resolve fine structure
– temporal resolution better than a few seconds to
follow its evolution in time
– FOV size of order of 1′
– accurate absolute calibration of the observations (Bas-
tian [2])
These requirements look very similar to the techni-
cal specification of the continuum observations with the
Atacama Large Millimeter Array (ALMA), which rep-
resents an enormous advance over existing instrumenta-
tion operating at mm-submm wavelengths. ALMA will
produce images of the highest resolution available for
the foreseeable future (although the technical problem
of sampling both large and small spatial scales simulta-
neously, required for high–quality imaging of the chro-
mosphere, will remain a challenge) and will be the most
sensitive instrument operating at submm-mm wavelengths.
To summarize, ALMA will be an extraordinarily pow-
erful instrument for studying the solar chromosphere. It
will finally allow the mapping of the three-dimensional
thermal structure of the solar chromosphere which will
be a real breakthrough in solar studies.
Acknowledgements The use of BIMA for scientific research
carried out at the University of Maryland is supported by NSF
grant AST–0028963. Solar research at the University of Maryland
is supported by NSF grant ATM 99-90809 and NASA grants NAG
5-8192, NAG 5-10175, NAG 5-12860 and NAG 5-11872.
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	Introduction
	Results
	Discussion
	Summary
