E. Wiehr1 - M. Bianda2
1 - Universitäts-Sternwarte,
Geismarlandstraße 11, 37083 Göttingen, Germany
2 -
Istituto Ricerche Solari, Via Patocchi, 6644 Locarno-Orselina,
Switzerland
Received 7 April 2003 / Accepted 6 May 2003
Abstract
We measure the resonance polarization in solar prominences in
H,
H
and HeD3. A two-dimensional set-up with narrow-band
filter, polarization analyzer and CCD camera is used to take prominence images
in polarized light at high spatial resolution. Placed on a coudé telescope's
hour axis, the observations near the equinoxia are free from purely instrumental
polarization. Above the 0.1% noise limit, the Balmer lines do not show a
polarization in contrast to the HeD3 line. Here, we determine the complete
polarization profile after exchange of filter and CCD with the spectrograph,
keeping the polarization analyzer fixed. In most prominences the Stokes-U and
-Q profiles are not similar to Stokes-I: occasionally the blue and the red
components of the emission are equal or even show a reverse ratio. This
fits calculations for magnetic field strengths of the order of 50 Gauß
being markedly stronger than commonly assumed.
Key words: Sun: atmosphere - Sun: prominences - instrumentation: polarimeters - techniques: polarimetry
Solar prominences are predominantly illuminated from below. This asymmetric
incidence of light yields a partial polarization of that part of the emission
which is formed by scattering processes. The already small amount of resonance
polarization is further diminished by the Hanle effect if the emitting plasma
is embedded in a magnetic field
(Sahal-Bréchot et al. 1977). The resulting
small amount of polarized light is superposed by the usually much larger
polarization of the instrument. Extended measurements of linear polarization
in prominences were made by Leroy et al. (1984) through filters integrating
the whole emission line profiles. They found values up to several percent in
H,
H
,
and He D3.
Landi degl'Innocenti (1982) shows that the distribution of polarization through the He D3 fine-structure components offers a possibility to determine more than the two magnetic field parameters deduced from integrated data of the whole emission line by Leroy et al. (1984). This, however, requires full spectral resolution of polarization profile of He D3. Athay et al. (1983) find that the spectral distribution of the intensity differs from that of the linear polarization through the He triplet. They argue that this effect can hardly be due to optical thickness effects and might be due to different sensibility of the He fine-structure components to the Hanle effect. Since the determination of the prominence magnetic fields essentially depends on such measurements, it is worth-while to verify the prominence polarization with different methods.
A telescope with a "German type'' coudé mounting (as, e.g., the Gregory telescopes at Locarno and at Tenerife; Wiehr 1987), is most suitable for polarimetry. Its instrumental polarization originates almost entirely from the two folding flat mirrors, the relative orientation of which varying only with the solar declination. Their combined influence is thus small and largely constant over a day (cf. Wiehr 1974); it even vanishes for zero solar declination at the equinoxia, where the two deflections are precisely orthogonal. We observed close to several equinoxia with the Gregory Coudé Telescope at Tenerife until its dismantling in spring 2002, and in autumn 2002 with its "twin'' at Locarno.
The polarized light was measured with a
plate followed by a
calcite (Savart plate), a narrow-band filter, and a CCD camera placed
directly on the telescope's hour axis (Wiehr & Bianda 2002). The filters
of a few Å widths were accurately centered on H
,
H
,
and
HeD3, respectively, by electronically controlled heating and suitable tilt,
controlled with the spectrograph. The finite field-of-view required for the
Savart plate, was defined by a
aperture in front
of the polarization optics which was imaged by telecentric optics on the CCD.
Lenses, filter, and CCD could readily be removed for an alternate use of the
spectrograph, keeping the polarization analyzer unchanged. This allowed
intermediate spectroscopic measurements of the whole polarization line profile
for such prominences which show a signal in the 2-D polarimetric setup.
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Figure 1:
Prominence observed on Sep. 26, 2002, at the east limb,
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The linear polarization was measured with the "beam-exchange technique''
(Semel et al. 1993) where two exposures are taken sucessively
with 0
and 45
orientation of the half wave plate in front of the
Savart plate. The circular
polarization was measured replacing the
by a
plate.
Using the algorithm described by Bianda et al. (1998),
uncertainties of the CCD's gain table do not affect the measurements. The
"beam-exchange technique'' requires largely equal location and sharpness
of the solar features in the two exposures. The first condition is assured
by the high accuracy of the primary image guider (Küveler et al. 1998)
which compensates drifts in the telescope pointing with high accuracy.
Remaining differences in the features' precise locations on
the CCD, as well as different image sharpness, were minimized by frame
selection among several exposures, yielding a highly similar pair of
sub-images. The fixed orientation of the Savart plate affects that
the Q+ direction is parallel to the CCD rows and not to the solar limb.
Although the instrumental polarization is expected to be very small near
zero solar declination (see above), we measured it at disk center with
reasonable diminution of the light level by neutral filters. The obtained
value of typically
shows that near the equinox a coudé
type telescope with "German mounting'' is, indeed, largely free from linear
polarization, and our final data is unaffected by the telescope. The small
disk center value is nevertheless subtracted from the prominence observations,
assuming that it does not vary over the disk (the maximum declination
difference being only 0.25
).
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Figure 2:
Prominence observed on Sep. 20, 2002, at the west limb,
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Figure 3:
Prominence observed on March 24, 2002, at the east limb,
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Figure 4:
Spectral scan of the He D3 polarization in a prominence at
the east limb,
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During the various observing campaigns at four equinoxia, we observed
dozens of prominences occurring above the limb at various solar latitudes
and with large variety of brightness (i.e. optical thickness; cf.
Stellmacher & Wiehr 1995). A measurable linear polarization was not
found in the H or the H
lines. Examples of He D3 linear
polarization images are shown in Figs. 1, 2, and 3. The spatial variation
of the linear polarization differs from that of the intensity features.
This is most pronounced for the prominence in Fig. 3, where the two
branches behave oppositely in U/I as in intensity.
For a more detailed investigation of such a behavior, the spectral
distribution of the linear polarization through the whole HeD3 profile
was measured with the spectrograph mode of our polarimeter. Five of the six
HeD3 components superpose to an (unresolved) blue emission, whereas the
sixth faint component at 343.3 mÅ red-wards remains well separated
(cf. Landi degl'Innocenti 1982). We find that the intensity ratio of the
two observable components is mostly near 6:1, being close to the ranges
found by Athay et al. (1983) and by Lopez-Ariste & Casini (2002). This
significant deviation from the 8:1 ratio, expected from the transition
probabilities, is a strong hint for non-negligible optical thickness.
Similarly, Stellmacher et al. (2003) calculate for the
He 10830 triplet that the 8:1 ratio of its combined two red components
and its separate faint blue component declines to 6:1 already for
.
The HeD3 lines will not behave much differently.
Among those prominences which yield polarization above 10-3, the
majority shows a ratio of the (combined) blue component(s) and the faint
red component which is smaller than the ratio of the Stokes-I. This
result agrees with findings by Paletou et al. (2001) and by
Lopez-Ariste
& Casini (2002). For HeD3 profiles which show
%,
we find a significantly smaller difference between the two HeD3
emissions: in Fig. 4 the Q/I and the U/I maxima of the (single) red
component exceed those of the (stronger) blue component(s). In these
rare cases, we also measure a Stokes-V signal with typically V/I <0.5%
(cf. Fig. 4).
Disregarding the fact that the calculations of fractional linear
polarization by Landi degl'Innocenti (1982) and by Lopez Ariste & Casini
(2002) are not rigorously valid for non-negligible optical thickness,
a nearly equal polarization in the the two components indicates
field strengths up to 50 Gauß. Such strong fields were already measured
via Zeeman effect by Kim et al. (1984) and later by Paletou et al. (2001).
Also Lopez-Ariste & Casini (2002) deduce from the re-considered data by
Athay et al. (1983) similar field strengths which significantly exceed the
values found by Leroy et al. (1984, and references therein). Such
higher field strengths might agree with higher gas-pressures deduced from the
observed ratio of the Ca+8452 and H
emissions by Stellmacher &
Wiehr (2000; and references therein).
The absence of linear polarization in the H
and H
emissions
seems to disagree with Leroy et al. (1984) who found values of several percent,
which should easily have been detected at our 2-D polarimetric accuracy of
10-3. Leroy (1981) finds that the linear polarization decreases with
brightness of the Balmer emissions. This decrease, however, only occurs if
the optical thickness in the H
line exceeds unity optical thickness,
which corresponds to an integrated line emission above
erg/(cm2 s ster) (cf. Stellmacher & Wiehr 1994; Fig. 4). We may
reasonably exclude a preferred selection of brightest prominences for
our observations; the large quantity of prominences observed near four
equinoxia with our 2-D polarimeter, certainly contains much fainter ones
- at least such with less bright locations. It it thus astonishing that
none of the 2-D polarization images indicates any significant signal of
the Balmer lines in excess of 10-3. (A similar finding is reported
by the Zürich group.)
As far as the H
line is concerned, its decrease of linear polarization
with brightness can hardly be explained by optical thickness since that emission
is 5-11 times fainter than H
(Stellmacher & Wiehr 1994; Fig. 3).
Bommier et al. (1986) discuss the influence of electron density; according to
their Fig. 3, our upper limit of the 10-3 level would indicate an electron
density of almost 1011 cm-3 which seems to be rather high - even
with regard to the higher gas-pressure favored by Stellmacher & Wiehr (2000)
and with the higher prominence magnetic fields indicated by the polarimetry.
It remains unclear why the majority of prominences does not yield linear
polarization in the Balmer lines above the 10-3 level. Stronger magnetic
fields might decrease the resonance polarization, but would increase the
circular polarization; this should be the subject of future observations.
Acknowledgements
We are indebted to Drs. G. Stellmacher and E. Landi degl'Innocenti for fruitful discussions and suggestions. The Gregory Coudé telescope at Locarno is operated by the "Istituto Ricerche Solari'' at the Swiss observatory of Orselina near Locarno. The Gregory Coudé telescope on Tenerife is operated by the Universitäts-Sternwarte, Göttingen (USG), at the Spanish "Observatorio del Teide'' of the Instituto de Astrofísica de Canarias.