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Mem. S.A.It. Vol. 81, 806
c
SAIt 2010 Memorie della
Coupling photosphere and chromosphere
through plasma waves
M. Stangalini1, F. Berrilli1, D. Del Moro1, A. Egidi1, S. Giordano2,
P. F. Moretti3, and B. Viticchi`
e1
1Universit`
a degli studi di Roma ”Tor Vergata”, Via della Ricerca Scientifica 1, I-00133
Rome, Italy, e-mail: marco.stangalini@roma2.infn.it
2Altran Italia SpA
3CNR P.le A. Moro 7 - I-00185 Rome, Italy
Abstract. The new capabilities of fast bidimensional spectropolarimetric scanning, al-
lowed by recent instrumental development, provide a new insight into the study of chro-
mospheric active regions. We present results from the analysis of datasets acquired with
Interferometric BIdimensional Spectrometer operating at the Dunn Solar Telescope in spec-
trometric and spectropolarimetric mode. The high spatial and temporal resolution allows us
to study oscillations and MHD wave propagation between photosphere and chromosphere.
In particular we focused on the coupling between photospheric magnetic field and wave
transmission. Among other findings, we observe a shift of the cross-correlation spectrum,
above those photospheric regions where the magnetic field vector is strongly inclined with
respect to the line of sight. Such a result could offer a new perspective for the understanding
of plasma wave reprocessing.
Key words. Sun: magnetic fields – Sun: oscillations
1. Introduction
Magnetic field inclination has been identi-
fied as one of the causes of cutofffrequency
shift for acoustic waves (Jefferies et al. 2006;
McIntosh & Jefferies 2006; Cally & Schunker
2006). More specifically, it has been shown
that the cutofffrequency νcutoff=γg/4πc=
5.2 mHz can be lowered below this value
due to magnetic field inclination (Bel & Leroy
1977) in regions where the plasma parameter
β << 1.
This can allow the transmission of waves
with frequencies below 5.2 mHz.
Send offprint requests to: M. Stangalini
Acoustic waves are also found to interact
with the canopy around magnetic structures
(Moretti et al. 2007).
An alternative scenario for such a lowering
in the cutofffrequency for acoustic waves can
be found in radiative losses that can occur in
the photosphere (Roberts 1983).
In this work we present a study of the prop-
erties of acoustic waves propagation and we
compare these to magnetic field geometry in-
ferred from full Stokes signals of a magnetic
structure acquired with the Interferometric
Bidimensional Spectrometer (IBIS) installed at
the Dunn Solar Telescope in the Fe 617.3 nm
and Ca 854.2 nm lines. We find that 3 mHz
Stangalini et al.: Coupling photosphere and chromosphere through plasma waves 807
Fig. 1. Upper left panel: the average continuum image at ∼617.3 nm with an isophote defining the sunspot
contour overplotted. Upper right panel: the integrated circular polarized component of the incoming radia-
tion associated to the Fe I 617.3 nm line. Lower left panel: the integrated linear polarized component of the
incoming radiation associated to the Fe I 617.3 nm line. Lower right panel: Selected regions for the analysis
of wave transmission. Region A indicates the region where the Stokes Vis greater than 3σthreshold and B
is the region where the U+Qsignal is greater than a 3σthreshold. On all images the same isophote of the
average continuum image is overplotted.
waves propagate upward mainly in the region
surrounding the magnetic structure forming a
ring shaped region outside the umbra, while
5 mHz waves propagate at the borders of the
umbra. We also find that the cross-correlation
spectrum is fairly affected by the field geom-
etry, showing a shift toward low frequencies
(3 mHz) in those regions where the magnetic
field is strongly inclined with respect to the
Line-of-Sight (LoS) and the Uand QStokes
signals concentrate.
2. Observations and data analysis
The observation run was performed 2008
October 15 in full-Stokes mode with IBIS.
IBIS is based on a dual Fabry-P´
erot interfer-
ometric system. It combines high spectral res-
olution with short exposure times and a large
field of view, as well as the ability to measure
the polarization (Cavallini 2006).
The region tracked was the AR11005
which, seen in SOHO and Hinode images, ap-
pears as a small bipolar region in the northern
hemisphere at high latitude ('30◦N), belong-
ing to the new magnetic cycle. We observed
the only structure evident in continuum light,
at [25.2◦N, 10.0◦W]. Such a structure is a
sunspot, probably in the decay phase, which
exhibits a light-bridge and several umbral-dots
(upper left panel in Fig. 1). Spectopolarimetric
observations (upper right and lower left pan-
els in Fig. 1) reveal that the magnetic field
leans towards the photosphere in the upper part
808 Stangalini et al.: Coupling photosphere and chromosphere through plasma waves
of the structure (towards disk center), however
a penumbral structure is not visible in broad-
band images.
The dataset consists of 80 sequences, con-
taining a full Stokes 21 points scan of the
Fe 617.3 nm line and a 21 points scan of the
Ca 854.2 nm line. The wavelength distance in
between the spectral points for the Fe line is
20.0 mÅ. The wavelength distance in between
the spectral points for the Ca line is 60.0 mÅ.
The exposure time for each image was set to
80 ms and each spectral scan took 52 seconds
to complete, thus setting the time resolution.
The pixel scale of these 512 ×512 pixel im-
ages was set at 0.
00167. For each spectral im-
age a broad-band (WL) and a G-Band coun-
terpart, approximately imaging the same FOV,
have been acquired as ancillary images. The
pixel scale of the 1024×1024 pixel WL image
(621.3±5 nm) was set at 0.
000835 and the expo-
sure time was 80 ms (shared shutter with IBIS
spectral images).The pixel scale of the 1024 ×
1024 pixel G-band image (430.5±0.5 nm)
was set at 0.
000514 and the integration time was
10 ms.
The pipeline provided by the IBIS team
takes care of normal calibration processes
(dark frame, flat field, etc.) and also corrects
for blue-shift effects (Reardon & Cavallini
2008) and instrumental polarization. For fur-
ther details on the calibration pipeline see
Viticchi`
e et al. (2009).
The ancillary images have been re-
stored with Multi-Frame Blind Deconvolution
(MFBD) (van Noort et al. 2005), obtaining a
single frame for each scan both for the G-band
and the broad-band images. Using these im-
ages, the spectropolarimetric images have been
registered and destretched to fix the AO uncor-
rected seeing effects and to achieve the highest
spatial resolution.
The estimated mean spatial resolution of
the LoS velocity fields computed from the
spectropolarimetric scans used in this work is
0.
0036.
3. Cross-correlation spectra
Using the LoS velocity fields for both the
Fe 617.3 nm and Ca 854.2 nm lines, we studied
Fig. 2. Cross-correlation spectrum obtained in the
region where the magnetic field is highly linearly
polarized (upper panel) and highly circularly polar-
ized (lower panel). Low-frequency peaks at 0.5 and
1 mHz have been filtered out as they are caused by
seeing conditions modulations as deduced by fur-
ther analysis (not reported here). The sampling time
scale is 52 s and dataset duration is 70 minutes.
the cross-correlation spectrum and the phase
lag, focusing on two regions: one with strong
circular polarization and one with strong linear
polarizations signals.
Despite the complexity of line formation in
a highly structured atmosphere, Doppler shifts
of the line Ca 854.2 nm are a reliable diag-
nostic of the low-chromosphere (Vecchio et al.
2007) We selected two regions (see Fig. 1) set-
ting a 3σthreshold on U+QStokes signals
(region B in the lower right panel of Fig. 1)
and on VStokes (region A in the lower right
panel of Fig. 1).
This selection allows us to study the waves
propagation in two different regimes of mag-
netic field inclination.
Stangalini et al.: Coupling photosphere and chromosphere through plasma waves 809
Fig. 3. Phase map for 5 mHz component (panel A)
and for 3 mHz (panel B).
It is worth noting how the linear polariza-
tion signal is not symmetric with respect to the
magnetic structure center and it is mainly out-
side the visible umbra edges.
From this analysis we obtained the cross-
correlation spectra shown in Fig. 2.
As evident, the spectrum shape is depen-
dent on the region analyzed. Pixels with in-
clined magnetic field (region B in Fig. 1) show
a higher cross-correlation amplitude at lower
frequency (∼3.5 mHz) with respect to the re-
gion where the polarization is mainly circular
(region A in Fig. 1). This umbral region shows
the highest peaks in the cross-correlation spec-
trum at higher frequencies, between 4 mHz and
5.5 mHz.
4. Phase lag
We investigated the phase lag between the pho-
tosphere and chromosphere at small spatial
scales in and around the magnetic structure. In
Fig. 3 the phase maps corresponding to 5 mHz
(panel A) and 3 mHz (panel B) components are
reported.
The maps clearly show that the 3 mHz
waves are propagating upward (phase greater
than zero) mainly around the magnetic struc-
ture. Whereas, 5 mHz waves are propagating
upward at the edges of the umbra and in the
left lobe of the sunspot.
This scenario seems to be compatible with
that described by de Wijn et al. (2009). The
authors reported that 3 mHz waves propa-
gated mostly in regions surrounding magnetic
structures where the field is most likely to be
inclined, thus contributing to the cutofffre-
quency shift.
Acknowledgements. We thank Alexandra Tritschler
for providing the IBIS data reduction pipeline. IBIS
was built with contributions from INAF/Arcetri
Observatory, the University of Florence, the
University of Rome Tor Vergata, and MIUR. NSO
is operated by the Association of Universities for
Research in Astronomy, Inc. (AURA), under co-
operative agreement with the National Science
Foundation.
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