Multi-wavelength Analysis of a Quiet Solar Region
The Physics of Chromospheric Plasmas
ASP Conference Series, Vol. 368, 2007
Petr Heinzel, Ivan Dorotovič and Robert J. Rutten, eds.
Multi-wavelength Analysis of a Quiet Solar Region
G. Tsiropoula1, K. Tziotziou1, J. Giannikakis1, P. Young2, U. Schühle3
and P. Heinzel4
1Institute for Space Applications and Remote Sensing, Athens, Greece
2CCLRC Rutherford Appleton Laboratory, United Kingdom
3MPI für Sonnensystemforschung, Katlenburg-Lindau, Germany
4Astronomical Institute AS, Ondřejov, Czech Republic
Abstract. We present observations of a solar quiet region obtained by the
ground-based Dutch Open Telescope (DOT), and by instruments on the space-
craft SOHO and TRACE. The observations were obtained during a coordinated
observing campaign on October 2005. The aim of this work is to present the rich
diversity of fine-scale structures that are found at the network boundaries and
their appearance in different instruments and different spectral lines that span
the photosphere to the corona. Detailed studies of these structures are crucial
to understanding their dynamics in different solar layers, as well as the role such
structures play in the mass balance and heating of the solar atmosphere.
1. Introduction
In the quiet regions of the solar surface the magnetic field is mainly concen-
trated at the boundaries of the network cells. Over the past decade, apart from
the well-known mottles and spicules, several other structures residing at the
network boundaries such as explosive events, blinkers, network flares, upflow
events have been mentioned in the literature. However, their interpretation,
inter-relationship and their relation to the underlying photospheric magnetic
concentrations remain ambiguous, because the same feature has a different ap-
pearance when observed in different spectral lines and by different instruments.
For most of the events mentioned above magnetic reconnection is suggested as
the driving mechanism. This is not surprising, since it is now well established
from investigations of high resolution magnetograms, that new bipolar elements
emerge continuously inside the cell interiors and are, subsequently, swept at the
network boundaries by the supergranular flow (Wang et al. 2006; Schrijver et al.
1997). Interactions of the magnetic fields have as a result either the enhance-
ment of the flux concentration in the case of same polarities or its cancellation in
the case of opposite polarities. Observations support the idea that flux cancel-
lation most likely invokes magnetic reconnection. In this context, the study and
comprehension of the dynamical behaviour of the different fine-scale structures
is crucial to the understanding of the dynamics of the solar atmosphere.
In this work we present observations of a solar quiet region and some of the
properties of several different structures appearing at the network boundaries
and observed in different wavelengths by the different instruments involved in a
coordinated campaign.
http://arxiv.org/abs/0704.1592v1
172 Tsiropoula et al.
Figure 1. Left : C IV TRACE image. Right : MDI magnetogram. The white
rectangle inside the images marks the DOT’s field-of-view.
2. Observations and Data Reduction
In October 2005 we ran a 12 days observational campaign. The aim of that
campaign was the collection of multi-wavelength observations both from the
ground and space that could be used for the study of the dynamical behaviour
of mottles/spicules and other fine structures, observed in different layers of the
solar atmosphere.
Three ground-based telescopes were involved in that campaign: DOT on La
Palma, THEMIS on Tenerife and SOLIS at Kitt Peak. From space telescopes two
spacecraft were involved: SOHO (with CDS, SUMER, and MDI) and TRACE.
The analysed data were obtained on October 14 and consist of time se-
quences of observations of a quiet region found at the solar disk center recorded
by different instruments. Sequences recorded by the DOT were obtained be-
tween 10:15:43 – 10:30:42 UT and consist of 26 speckle reconstructed images
taken simultaneously at a cadence of 35 s with a pixel size of 0.071′′ in 5 wave-
lengths along the Hα line profile (i.e. at −0.7 Å, −0.35 Å, line centre, 0.35 Å
and 0.7 Å), in the G band with a 10 Å filter, in the Ca IIH line taken with a
narrow band filter and in the blue and red continuum. TRACE obtained high
cadence filter images at 1550 Å, 1600 Å and 1700 Å. SUMER obtained raster
scans and sit-and-stare observations from 8:15 to 10:30 UT. CDS obtained sit-
and-stare observations from 6:44 to 10:46 UT and six 154′′ × 240′′ raster scans
(each one having a duration of 30min) from 10:46 to 13:52 UT. Both SOHO
instruments (i.e., CDS and SUMER) observed in several spectral lines spanning
the upper solar atmosphere. Using the standard software the raw measurements
were corrected for flat field, cosmic rays and other instrumental effects. A single
Gaussian with a linear background and Poisson statistics were used for fitting
each spectral line profile. MDI obtained high cadence images at its high resolu-
tion mode.
Multi-wavelength Analysis of a Quiet Region 173
Figure 2. DOT images of a rosette region. Left : Hα -0.7 Å (first row),
Hα+0.7 Å (second row). Right : Ca IIH (first row), G band (second row).
Extensive work went into collecting, scaling and co-aligning the various
data sets to a common coordinate system (see Fig. 1 showing the coalignment
of TRACE, MDI, and DOT images).
3. Analysis and Results
3.1. DOT observations
DOT’s field-of-view (FOV) is 80′′ in the X direction and 63′′ in the Y direction.
For the present study we selected a smaller region which contains a rosette
with several mottles pointing to a common center (see Fig. 2) and is found at
the middle upper part of the DOT’s FOV. In the G-band image (Fig. 2, right,
second row) isolated bright points show up in the regions of strong magnetic
field as can be seen in the MDI image. These seem to be passively advected
with the general granular flow field in the intergranular lanes. While we have
not conducted an exhaustive study of bright point lifetimes we find that bright
points can be visible from some minutes up to almost the entire length of the
time series. In the contemporal Ca IIH images (Fig. 2, right, first row bright
points are less sharp due to strong scattering in this line and possibly due to
increasing flux tube with height. Reversed granulation caused by convection
reversal is obvious in this image.
In the Hα − 0.7 Å (Fig. 2, left, first row) the dark streaks are part of the
elongated Hα mottles seen better at Hα line center. Some mottle endings appear
174 Tsiropoula et al.
Figure 3. Cross correlation function vs time for Left : the intensity at Hα−
0.7 Å, Middle: the intensity at Hα line center. Right : the velocity at Hα −
0.7 Å.
extra dark in the blue wing image through Doppler blueshift. Near the mottle
endings one can see bright points. These are sharper in the G-band but stand
out much clearer in the Hα wing. Thus Hα wing represents a promising proxy
magnetometer to locate and track isolated intermittent magnetic elements. This
is because the Hα wing has a strong photospheric contribution, as it is shown
by Leenaarts et al. (2006) who arrived to this conclusion by using radiative
transfer calculations and convective simulations. In the Hα+0.7 Å (Fig. 2, left,
second row) dark streaks around the rosette’s center are signatures of redshifts.
Blueshifts in the outer endings and redshifts in the inner endings of mottles
provide evidence for the presence of bi-directional flows along these structures.
An important parameter for the study of the dynamics of mottles is their
velocity. For its determination, when filtergrams at two wavelengths of equal
intensity at the blue and the red side of the line are available, a technique
based on the subtraction of images can be used. In this technique, by using the
well known representation of the line intensity profile and assuming a Gaussian
wavelength dependence of the optical thickness, we can define the parameter
ΣI − 2I0λ
, (1)
where ∆I = I(−∆λ) − I(+∆λ), ΣI = I(−∆λ) + I(+∆λ) and I0(∆λ) is the
reference profile emitted by the background. DS is called Doppler signal, has
the same sign as the velocity and can be used for a qualitative description of the
velocity field (for a description of the method see Tsiropoula 2000). When an
optical depth less than one is assumed then quantitative values of the velocity
can be obtained from the relation:
1 +DS
, (2)
since in that case the velocity depends only on DS (obtained from the obser-
vations) and the Doppler width, ∆λD (obtained from the literature). By using
these relations we have constructed 2-D intensity and velocity images for the
whole time series. We found out that it is difficult to follow each one mottle for
more than two or three frames and that the general appearance of the region
Multi-wavelength Analysis of a Quiet Region 175
Figure 4. CDS raster image obtained at the OV 629.7 Å line with an over-
plotted SUMER image at NeVIII 770 Å.
seems to change quite rapidly with time. For a quantitative estimate of the
temporal changes we computed the value of the cross correlation (CC) function
over the 2-D FOV both for the intensity and velocity. Figure 3 shows the inten-
sity CC curve at Hα− 0.7 Å (left) and Hα line center (middle) and the velocity
CC curve at Hα± 0.7 Å (right). The decay of the CC curve is a measure of the
lifetime of the structures. The e-folding time for the left curve is found equal to
2min, the middle curve equal to 5min and the right curve is of the order of the
cadence.
3.2. CDS and SUMER observations
In Fig. 4 we show the CDS raster image obtained in the OV 629.7 Å line with the
SUMER image at NeVIII 770 Å overplotted. Although there is a time difference
of 3 hours between the two images the network is constant enough to allowed the
coalignement of the two images. In CDS intensity maps several brightenings are
observed which are called blinkers (Harrison et al. 1999). These events are best
observed in transition region lines and show an intensity increase of 60 - 80%.
Most of them have a repetitive character and reappear at the same position
several times.
In Fig. 5 (left, up) we show an integrated (over the spectral line with the
background included) intensity image in the Ne VIII 770 Å line produced by sit-
and-stare observations of a network region. The image is produced by binning
over 6 spectra in order to improve the signal-to-noise ratio. The Doppler shift
map was derived by applying a single Gaussian fitting (Fig. 5, left, bottom). The
Doppler shift map and the spectral line profiles were visually inspected for any
non-Gaussian profiles with enhancements in both the blue and red wings that
are the main characteristics of the presence of bi-directional jets. Large numbers
of such profiles were found at the network boundaries (Fig. 5, right).
176 Tsiropoula et al.
Figure 5. SUMER sit-and-stare observations in the NeVIII line. Left : in-
tensities (up), Doppler velocities (bottom) (network boundaries are bright in
the intensity image). Right : non-Gaussian profiles in the positions marked by
“x” inside the images in the left.
4. Conclusions
In this work we present observations of a quiet solar region obtained by different
instruments in different spectral lines. Network boundaries are found to be the
locus of several structures which have different appearances when observed by
different instruments e.g., blinkers (when observed with CDS), mottles (when
observed with DOT), jets (when observed with SUMER). Their interrelationship
is to be further explored.
Regarding flows no-clear pattern is found in blinkers, while bi-directional
flows are found in jets. In dark mottles downward velocities are found at their
footpoints and upwards velocities at their upper parts and very fast changes in
their appearance.
The network shows a remarkable constancy when observed in low resolution
images. However when seen in high resolution images several fine structures are
observed which change so fast that it is very hard to follow.
Acknowledgments. K. Tziotziou acknowledges support by Marie Curie European
Reintegration Grant MERG-CT-2004-021626. This work has been partly supported by
a Greek-Czech programme of cooperation.
References
Harrison, A., Lang, J., Brooks, D.H., & Innes, D.E. 1999, A&A, 351, 1115
Leenarts, J., Rutten, R.J., Sütterlin, P., Carlsson, M., & Uitenbroek H. 2006, A&A,
449, 1209
Schrijver, C.J., Title, A.M., Van Ballegooijen, A.A., Hagenaar, H.J., & Shine, R.A.
1997, ApJ, 487, 424
Tsiropoula, G. 2000, New Astronomy, 5, 1
Wang, H., Tang, F., Zirin, H., & Wang, J. 1996, Solar Phys. , 165, 223
