A search for turbulent wave heating and acceleration
signatures with SOHO/SUMER observations :
Measurements of the widths of off-limb Iron lines
L. Dolla , P. Lemaire , J. Solomon and J.-C. Vial
Institut d’Astrophysique Spatiale, Unité Mixte CNRS-Université Paris XI, Bât. 121, 91405 Orsay, France
Abstract. The widths of coronal ions lines may contain important information about the energetics of the solar wind and
corona. We present a method to measure these widths, taking into account the problems of instrumental stray light inherent
to SoHO/SUMER. The Iron lines are interesting to set an upper limit on the ”unresolved” velocity, that may be a signature of
turbulent or wave motion in the corona.
INTRODUCTION
THE DATA
For many years, and more particularly since Ulysses
observations of a fast solar wind at high heliolatitudes, it
has appeared that the heating of the solar corona and the
acceleration of the solar wind require additionnal (nonthermal) energy input. One possible energy input could
result from wave-particle interactions (Alfvén waves, ion
cyclotron resonance, turbulence, e.g. [1], [2]). A possible
signature of such turbulent or Alfvénic motion could
be the apparent non-thermal broadening of coronal ions
lines as discussed in [3], [4], for instance.
The SUMER instrument is described in [5]. We just remind here some characteristics of the data. The data
that we get from the detector are made of images of
1024 360 pixels (wavelength versus spatial dimension).
Each pixel dimension corresponds to 43 mÅ and
1 arcsec.
We use data acquired during the MEDOC Campaign
#7 (May 2001), in the corona above a "Quiet Sun" region.
The position of the slit was (860",-600") (in solar disc
coordinates). The altitudes covered are from 13 to 180
arcsec above the solar limb. Two Iron lines are visible in
the same spectral range : Fe X and Fe XI, at 1463.50 Å
and 1467.06 Å, respectively.
A 1” wide slit is used for a better accuracy in measuring the width.
To reduce the noise, several files can be summed, thus
increasing the total exposure time. For the same purpose,
we can average the data over several pixels in the spatial
dimension.
The accuracy of the width given by a gaussian fit can
be estimated by simulating fits of noisy lines (lines with
known width, plus some noise proportional to the square
root of the total number of counts in the lines). In this
simulation, we reproduce the main characteristics of the
studied lines (average width, discrete sampling on the
detector pixels, etc. . . ). With sufficient counts in the line,
the width uncertainty is a fraction of a pixel.
Using the results of the simulation, we can perform
temporal and/or spatial sums to get the counts necessary
for a given accuracy.
The non-thermal broadening of spectral
lines of coronal ions
The width of an optically thin line is a signature of the
particle velocity distribution of the plasma which emitted
the light (unfortunately modified by the integration along
the line of sight). Coronal (and transition region) spectral
lines show a width larger than the thermal width corresponding to the ion formation temperature.To account for
this excess broadening, we can add a non-thermal or ”unresolved” velocity ξ , so that the Gaussian width is given
by :
σ2
λ 2 2kT
2c2 M
ξ 2 σI2
(where σI is the instrumental width)
As the thermal contribution is inversely proportional
to the ion mass, it is smaller for heavy ions, like iron.
Therefore, the width of iron lines can be used to set an
upper limit on this unresolved velocity.
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
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We can, next, compare I λ z stray light to the ob-
DEALING WITH THE INSTRUMENTAL
STRAY LIGHT
served spectrum I λ z (Fig. 1) : it appears that the two
Iron lines are blended by stray light from chromospheric
lines.
For the sake of comparison, we tried to use an on-disc
spectrum as an alternative stray light reference, instead
of the high altitude spectrum. Without a doubt, this latter
better matched the intensities of the known cold lines appearing as stray light in the off limb spectra. An accurate
determination of the stray light spectrum is very important for width measurements, which are more sensitive to
the line profile than intensity measurements.
Two kinds of blending by stray light are possible :
As the SUMER instrument has been designed to observe
the solar disc, there is no occulter. Therefore, when observing off-limb corona, the SUMER slit is ”dazzled” by
the light coming from the solar disc. Thus, in the spectra
acquired in off-limb observations, one must take into account the stray light, especially for radial analysis of the
linewidth [6].
Yet, SUMER remains the unique spectrometer able
to provide information about the corona below 1.5 solar
radii, complementing SoHO/UVCS field of view.
1. by the same resonance line, if this one is very intense on the disc
2. by a cold ion line, which should not exist in the hot
corona.
Choice of a “stray light spectrum”
We first have to establish a stray light spectrum, i.e. the
spectral distribution that contributes to the off-limb spectrum as stray light. We can use a reference spectrum directly acquired on the disc, but this solution is not satisfactory. Indeed, such data were not available at the time
of our observations. Moreover, an on-disc spectrum is local, while the off-limb stray light is made of contributions
from all the points of the solar disc, including structures
such as active regions or sunspots, which are known to
exhibit different kinds of spectra (see [8]).
For these reasons, we preferred to use data obtained
the same day at a higher altitude z 0 ( 350 arcsec above
limb), which can be considered as due to stray light only.
These data provide a reference spectrum I λ ref .
Presently, we are in the second case : both Iron lines
are blended by C I lines (possibly with some contribution
from Ni II). It is noticeable that the stray light intensity
decreases less rapidly than the intensity of the emission
lines (Fig. 1 : comparison at two different altitudes).
MEASURING THE WIDTH
To get the width of the effective coronal emission line,
we fit the observed spectrum with two gaussians, one
of them fitting the stray light contribution. Nevertheless,
this method is very sensitive to the wavelength of the
blending line. Thus we had to adjust the wavelength
scales between the data and the reference spectrum for
every radial position, to take into account some remnants
of the geometric distorsion of the detector (this distorsion
is not completely corrected by applying the SUMER data
reduction software).
Performing such gaussian fits also produces some errors, as the blending lines do not look like pure gaussians,
mainly because they are blended themselves. These errors become critical when the blending is strong, and will
be taken into account in a future work. Here we present
the results yielded by using gaussian fits.
Estimation of the stray light intensity
A method to estimate the stray light intensity, using
cold lines, is described in [7]. It is based on the assumption that all counts observed in off-limb cold lines are
due to stray light (very low emission of cold ions in the
hot corona).
Here we extend this assumption to the continuum : we
suppose that there is no emission of continuum at the
altitude above limb where we are observing : then, all the
continuum observed off-limb is due to stray light.
By using I λ ref , we have at any altitude z z 0 , and
for any wavelength λ :
I λ zstray light
”Radial” variation
I z(off-limb)
continuum I λ ref
I (ref)
continuum
Figures 2 and 3 show the radial evolution of the widths
of Fe X and Fe XI lines, with a 12600 s total exposure
time. We perform spatial averaging over several pixels
in the slit height, in such a way as to provide a quite
constant size of the errors bars (using at least 10 pixels).
We have plotted both linewidths before and after the
correction from the stray light contribution (removing
During all these intensity measurements, we take into
account the average noise from the detector itself.
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FIGURE 1. Examples of spectra at two altitudes (16 and 159 arcsec above the solar limb) : the observed spectrum (in grey) is the
sum of stray light contribution (in black) and effective off-limb emission.
353
it is the case for the present observations). This may
explain some of the sharp variations displayed by the
Fe XI line.
CONCLUSION
Width measurements of relatively hot Iron lines in the
low corona have been discussed in some details. Particularly, we have stressed the importance of the stray light
spectrum which can increase dramatically the error on
width measurements, if not taken into account. In this
context, analysis of recent observations of various coronal ions lines is in progress.
FIGURE 2. Radial variation of the gaussian width of Fe X :
the large correction necessary to account for stray light make
this line dubious to use for width measurements.
ACKNOWLEDGEMENTS
SoHO is a mission of international cooperation between ESA and NASA. The SUMER project is financially supported by DLR, CNES, NASA and ESA
PRODEX Programme (Swiss contribution). The observations were performed during the MEDOC Campaign
#7 (May 2001).
REFERENCES
1. Cranmer, S. R., Field, G. B., and Kohl, J. L., Ap. J, 518,
937–947 (1999).
2. Marsch, E., and Tu, C.-Y., J. Geophys. Res., 106, 227–238
(2001).
3. Seely, J. F., Feldman, U., Schuehle, U., Wilhelm, K., Curdt,
W., and Lemaire, P., Ap. J. Let., 484, L87–+ (1997).
4. Tu, C.-Y., Marsch, E., Wilhelm, K., and Curdt, W., Ap. J,
503, 475+ (1998).
5. Wilhelm et al., K., Solar Phys., 162, 189–231 (1995).
6. Doschek, G. A., Feldman, U., Laming, J. M., Schühle, U.,
and Wilhelm, K., Ap. J, 546, 559–568 (2001).
7. Feldman, U., Doschek, G. A., Schühle, U., and Wilhelm,
K., Ap. J, 518, 500–507 (1999).
8. Curdt, W., Brekke, P., Feldman, U., Wilhelm, K., Dwivedi,
B. N., Schühle, U., and Lemaire, P., Astron. Astrophys.,
375, 591–613 (2001).
FIGURE 3. Radial variation of the gaussian width of Fe XI
(error bars are only drawn for the values before correction from
the stray light).
the instrumental width contribution). The error bars are
given for the non-corrected results. It is obvious that the
stray light correction is not important for the Fe XI line
(the corrected curve is within the error bars of the noncorrected one), at least for the considered altitude range.
On the contrary, there is no doubt about the critical effect
of the stray light on the width of the Fe X line.
Radial or latitudinal variation ?
One should notice that when moving along the
SUMER slit, both the radial and latitudinal positions
change (13 to 180 arcsec above limb, and -27 o to -41o
in latitude). This way, the field of view may intersect
different structures (a check with EIT image shows that
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