Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | L4 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/200913875 | |
Published online | 26 February 2010 |
LETTER TO THE EDITOR
The SUMER Ly-
line profile in quiescent prominences
W. Curdt1 - H. Tian1,2 - L. Teriaca1 - U. Schühle1
1 - Max-Planck-Institut für Sonnensystemforschung,
Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
2 - School of Earth and Space Sciences, Peking University, PR China
Received 15 December 2009 / Accepted 5 February 2010
Abstract
Aims. As the result of a novel observing technique, we publish for the first time SoHO-SUMER observations of the true spectral line profile of hydrogen Lyman-
in quiescent prominences. Because SoHO
is not in Earth orbit, our high-quality data set is free of geocoronal
absorption. We studied the line profile to complement earlier
observations of the higher Lyman lines and to substantiate recent model
predictions.
Methods. We applied the reduced-aperture observing mode to two
prominence targets and did a statistical analysis of the line profiles
in both data sets. In particular, we investigated the shape of the
profile, the radiance distribution, and the line shape-to-radiance
interrelation. We also compared Ly- data to co-temporal
1206 Si III data.
Results. We find that the average profile of Ly- has
a blue-peak dominance and is reversed more if the line-of-sight is
perpendicular to the field lines. The contrast of Ly-
prominence
emission rasters is very low, and the radiance distribution differs
from the log-normal distribution of the disk. Features in the Si III line are not always co-spatial with Ly-
emission.
Conclusions. Our empirical results support recent multi-thread
models, which predict that asymmetries and depths of the self-reversal
depend on the orientation of the prominence axis relative to the
line-of-sight.
Key words: Sun: UV radiation - Sun: filaments, prominences - line: formation - line: profiles - opacity
1 Introduction
Prominences protruding out of the perfect sphere of the visible solar disk are even
visible with the naked eye, when the bright disk is occulted. These enigmatic
features, which apparently withstand gravity, have attracted scientists for centuries, but despite substantial
progress and effort during the past decades in understanding the
physics of prominences, important aspects are still not understood.
We refer to review articles and reference material that show this effort and are relevant
to our work: e.g., Wilhelm et al. (2007); Patsourakos & Vial (2002); Tandberg-Hanssen (1995); Parenti et al. (2005). Our own work
focuses on prominence observations of the hydrogen Ly- line profile,
which reveal information on the physical conditions for the line formation.
The Ly- line profile of all disk features is self-reversed (Curdt et al. 2001).
Among other parameters the reversal is related to the amount of neutral
hydrogen in its ground state, which by itself is a complex function of the
temperature and density structure of the emitting plasma. In addition,
flows of the emitting or the absorbing plasma and magnetic field may modulate
the sizes of the red or the blue peak and the symmetry of the profile (Curdt et al. 2008; Tian et al. 2009a).
Early observations of the Ly- line profile in prominences were completed with
the LPSP instrument on OSO 8 (Vial 1982) and the UVSP instrument on
SMM (Fontenla et al. 1988). These photoelectric measurements had to be corrected
for the geocoronal absorption. They have already shown signatures of asymmetry
and a wide parameter range for the depth of the reversal of the profile,
features that at that time could not be reproduced by radiative transfer calculations.
Later on, modeling made it clear that the overall emergent profile
strongly depends on the physical conditions in the prominence (e.g., Gouttebroze et al. 1993).
In particular, the imprint of the incident profile and the role of the
prominence corona transition region (PCTR) were now employed to reproduce
observations (e.g., Vial et al. 2007).
Recently, 2D-multithread models have been established, which are based on the theoretical work of
Heinzel & Anzer (2001) and predict - depending on the orientation of the prominence axis
relative to the line of sight (LOS) - opposite asymmetries for the Ly- and Ly-
lines (Gunár et al. 2007,2008)
and deeply or less deeply self-reversed profiles (Schmieder et al. 2007).
This is the dedicated context and the rationale of our work.
2 Observations
Because of its wavelength range from 660 Å to 1600 Å, its high spectral
resolution, and its vantage point outside of the irritating geocorona,
which absorbs Ly- emission, the SUMER instrument on SoHO (Wilhelm et al. 1995)
is ideally suited to providing information about the line profile.
Its enormous brightness, however, exceeds the capabilities
of the SUMER detectors, and Ly-
can only be observed in small sections of 50 px
on both sides of the detectors beneath a 1:10 attenuating grid.
Unfortunately, the attenuation also exerts a modulation onto
the line profile, which makes it difficult to interpret this data. Attempts
to observe Ly-
in quiet Sun locations on the unattenuated bare section
of the photocathode had difficultiy calibrating the local gain depression.
First results from prominence data acquired in April and May 2005 have
been reported by Gunár et al. (2006), Vial et al. (2007), and Gunár et al. (2008).
![]() |
Figure 1: The prominences observed above the southeast limb on June 15 ( left) and above the southwest limb on June 9 (right). The images were taken in the EIT 304 channel (Delaboudinière et al. 1995), and the area of the SUMER rasters is indicated by rectangles. |
Open with DEXTER |
In July 2008, the SUMER team found a new, unconventional method of observing the extremely
bright Ly- line of neutral hydrogen with partially closed telescope aperture to
reduce the incoming photon flux. The obtained genuine Ly-
profiles in the quiet
Sun and coronal hole regions were analyzed by Curdt et al. (2008), Tian et al. (2009a), and
Tian et al. (2009b). Here we present the first unprecedented Ly-
observations of
two quiescent prominences seen in June 2009 and discuss the results from a
detailed analysis of the line profiles.
![]() |
Figure 2:
Raster scan in Ly- |
Open with DEXTER |
![]() |
Figure 3:
Idem for the observation on June 9. The raster was completed just outside the southwest limb.
We show the average profiles of Ly- |
Open with DEXTER |
The new method reducing the incoming photon flux to a moderate level,
appropriate for Ly-,
was described in earlier work
(Curdt et al. 2008; Tian et al. 2009a,b).
A standard procedure for partially closing the door led to a reproducible
reduction to a 20% level.
In June 2009, this method was applied for the first time to prominence observations.
On June 9 and 15, 2009, we completed raster scans of approximately
at positions near the solar limb with mid-size prominences.
Two spectral windows were transmitted, 100 pixels (px) around Ly-
recorded on the bare
photocathode of the detector and 50 px around
1206 Si III recorded
on the KBr-coated section of the photocathode. All observations were completed with
the 0.3
120
slit. With an exposure time of 14.5 s, both lines
were observed with enough counts for a good line profile analysis.
For both data sets three exposures at each position were completed before the
raster was continued with a very small increment of 0.375
.
A first inspection indicated no temporal variations of the
object during the observing time of 45 min. In our statistical analysis
we keep the temporal information and assume that the subresolution
increment of 0.375
(=3/8
)
is equivalent to three
hypothetical increments of 1/8
.
Exact knowledge of the limb position and distance for each pixel
is very important for prominence observations. Therefore we used additional
information for an independent assessment of the pointing uncertainty,
provided by the hardware encoders in the instrument's housekeeping channel.
Thus, we confirmed that the azimuth movement was as expected and that the
actual east-west pointing was very close to its nominal value.
Similarly, we confirmed that in elevation the absolute positions for both
rasters differ by the nominal value of 5
.
Since the position of the
limb can be determined in the June 9 data set, we can estimate that the overall
pointing uncertainty is about 2
to 3
.
The prominences as seen
in the EIT 304 channel are shown in Fig. 1,
including the area covered by the SUMER rasters.
Both data sets were processed with standard procedures of the SUMER-soft library. We used the dedicated flatfield exposure of April 19, 2009 to complete the flatfield correction.
3 Prominences in Ly-
and in Si III
The rasters for both days are shown in Figs. 2 and 3.
The x-axis also contains time information (cf., prev. section),
both axes are on a different scale, and the x-dimension is stretched.
The contours delineate the top 15% and the top 40% of the
pixels in the Ly- brightness histogram. These contours have been transferred
to the Si III raster.
In Fig. 2 (observation on June 15), we distinguish six different segments of the raster, separated by the blue contours or red boxes:
- 1.
- disk,
- 2.
- limb and near disk,
- 3.
- sub-prominence void,
- 4.
- inner prominence boundary,
- 5.
- prominence core,
- 6.
- outer prominence boundary.
In Fig. 3 (observation on June 9) we distinguish the
- 1.
- inner prominence core,
- 2.
- prominence interconnection,
- 3.
- sub-prominence void,
- 4.
- prominence interconnection,
- 5.
- outer prominence core,
- 6.
- outer prominence boundary.
The prominence is also seen in Si III. Again, the Ly- radiance contours
have been transferred. These contours show that there are considerable differences in
the Si III spectroheliogram, structures are not co-spatial, and the
prominence appears more granulated and not as diffuse as in Ly
.
The
formation temperature of Si III is 70 000 K, much higher than typical
prominence temperatures of 6000 K to 8000 K. Si III is a typical
transition region line. Since its wavelength is well
above the Lyman-limit at 912 Å, opacity effects by hydrogen can be ruled out.
The prominence is basically transparent (Anzer et al. 2007). These authors also show
that the C I recombination continuum below 1239 Å is negligible,
and consequently the PCTR of each unresolved thread would contribute to the
Si III emission so one would expect an appearance similar to the
cold body. The differences in appearance may indicate the coexistence
of hot and cold plasma with different opacities. Recent observations by Hinode-SOT
(Berger et al. 2008) assume buoyant bubbles of hotter plasma in quiescent prominences,
although on smaller scales. Such a scenario would also be compatible with
our observation. Without Hinode-SOT co-observations, however, our results remain
inconclusive.
We sorted the pixels of all disk locations and of all prominence locations by the total line radiance and defined six equally spaced radiance bins. The profiles for these bins are displayed in Figs. 4 and 5. There are striking differences of prominence profiles compared to disk profiles. In the prominence, the contrast is much lower, reduced by a factor of 4 to 5. The blue-peak dominance is observed in all radiance bins from the brightest areas of the prominence core.
![]() |
Figure 4:
We have sorted the pixels within the top 40% contours (solid in Fig. 2) by their
radiance and show the profiles of Ly- |
Open with DEXTER |
![]() |
Figure 5: Idem for the inner ( top) and outer ( bottom) region in Fig. 3. |
Open with DEXTER |
The central reversals of the Ly- profiles in both prominences differ, the
profiles obtained on June 15 were more reversed than those from June 9. This
may be related to the different orientations of the prominence axes as derived
from EIT 304 (cf., Figs. 2 and 3) and Kanzelhöhe H-
images. On June 15,
the threads were rather perpendicular to the line-of-sight, while
more edge-on (LOS parallel to the field lines) on June 9. This
explanation would be consistent with the model calculations and predictions
of Heinzel et al. (2005). Observational evidence for such a scenario based on spectra
of the higher Lyman lines, Ly2 to Ly7, has been reported by Schmieder et al. (2007).
4 Radiance histograms
We already noted the low contrast of the prominences in Ly-.
In Fig. 6
we show radiance histograms of the prominence core and of the on-disk
locations in Fig. 2. For comparison we add the log-normal
radiance distribution of Ly-
in the quiet Sun as presented in earlier work (Curdt et al. 2008).
Because it has a different bin size, the disk histogram was scaled for
better comparison.
Although the small number of prominence pixels only allows a noisy distribution,
the differences are, as expected, very obvious. The histogram is by definition clipped on the
dim side because of the area selection criterion. The main difference is certainly
found in the high-radiance part, because the prominence histogram completely lacks
brighter pixels, which makes it a very narrow distribution. The uniform emergent emission translates,
according to the Barbier-Eddington relation, to a uniform source function at
an optical depth,
,
of unity and is indicative of homogeneous populations
of the 1s and 2p levels, and thus rather homogeneous thermodynamic conditions.
In Fig. 7 we show the radiance distribution of the prominence in
Fig. 3 in Ly- and in Si III emission. This data set has more
prominence pixels and also allows fainter pixels to be included here.
We defined an empirically determined discrimination level to separate prominence emission
from coronal background and defined the lower-15% radiance category as coronal background, which does not belong
to the prominence. The Ly-
histogram has a sharp upper limit and, in
contrast to the disk histogram, a low-radiance wing. The Si III
histogram of this prominence differs significantly, as one could expect, from
both the quiet Sun state and from its Ly-
counter part.
We conclude that the radiance distributions of both prominences are, as a consequence of dissimilar physical conditions, remarkably different from the log-normal distribution of the average quiet Sun (Curdt et al. 2008; Fontenla et al. 1988).
![]() |
Figure 6: Radiance distribution of the limb location (1+2) in Fig. 2 ( left) and of the prominence core ( right). The low contrast of the prominence location translates into a narrow distribution, which differs significantly from the log-normal distribution, found by Curdt et al. (2008) in the quiet Sun at disk center (dotted curve). |
Open with DEXTER |
![]() |
Figure 7: Idem for the prominence in Fig. 3. The coronal background of pixels in the lower-15% radiance category has been excluded. |
Open with DEXTER |
5 Summary and conclusion
We have presented the first SUMER observations of prominences in the light of
the hydrogen Ly- line at 1216 Å with reduced incoming photon flux to avoid
the saturation effects of the SUMER detection system. We completed a statistical
analysis and report salient empirical results derived thereof.
As such, we found clear evidence of models, which predict an effect
of the orientation of the magnetic field relative to the line of sight on
the asymmetry of the Ly-
profile.
The Lyman lines are more reversed if the line of sight is across the prominence
axis as compared to the case where it is aligned along its axis.
Given the great variability in the appearance of prominences
and the wide range of physical parameters, the observation of two
prominences is hardly enough to cover all the issues. We felt, however, that
our results constitute a piece of information that is important enough to be presented
here. More joint observations of prominences and modeling of their Ly-
line profile are
highly desirable.
The SUMER project is financially supported by DLR, CNES, NASA, and the ESA PRODEX Programme (Swiss contribution). SUMER is part of SoHO of ESA and NASA. H.T. is supported by the International Max Planck Research School for his stay at MPS. This non-routine observation was performed with the help of D. Germerott. This paper greatly benefited from the very constructive comments of the referee.
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All Figures
![]() |
Figure 1: The prominences observed above the southeast limb on June 15 ( left) and above the southwest limb on June 9 (right). The images were taken in the EIT 304 channel (Delaboudinière et al. 1995), and the area of the SUMER rasters is indicated by rectangles. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Raster scan in Ly- |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Idem for the observation on June 9. The raster was completed just outside the southwest limb.
We show the average profiles of Ly- |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
We have sorted the pixels within the top 40% contours (solid in Fig. 2) by their
radiance and show the profiles of Ly- |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Idem for the inner ( top) and outer ( bottom) region in Fig. 3. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Radiance distribution of the limb location (1+2) in Fig. 2 ( left) and of the prominence core ( right). The low contrast of the prominence location translates into a narrow distribution, which differs significantly from the log-normal distribution, found by Curdt et al. (2008) in the quiet Sun at disk center (dotted curve). |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Idem for the prominence in Fig. 3. The coronal background of pixels in the lower-15% radiance category has been excluded. |
Open with DEXTER | |
In the text |
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