MPAe T1ex127-.45ex51275atigkeitsbericht 2002/2003 - Max-Planck ...

II.1. SONNE UND HELIOSPHÄRE/SUN AND HELIOSPHERE 29
velocity of the Ne 7+ ions, which can then be used as
markers of the proton flow. Therefore one has,
ftot = fbp + fdr = Ne,bpvbpAdp + Ne,drvdrAdr ,
(1)
where f , v and A are the mass flux, measured
(Doppler) outflow velocity and flux tube area, which
could be inferred from the simultaneously measured
magnetic flux. Ne,bp and Ne,dr denote the electron
density in bright points and dark regions. Their ratio
can be estimated from the line intensity ratio, by simply
assuming the emission volume for a given line has
the same bottom area. Empirically one finds that
Ne,bp
Ne,dr
≈
Ibp
Idr
≈ 1.8 . (2)
The mass flux contributed by bright points to the total
outflow within the measured hole area, fbp/ftot,
is thus estimated to be about 12%. This estimate is
consistent with previous values in the literature, and
suggests that the portion of the mass flux contributed
by bright points to the fast solar wind is at most comparable
to the relative area they cover and therefore
negligable. Funnels make the major contributions to
the mass flux of the nascent solar wind.
(L.D. Xia, E. Marsch, W. Curdt)
Network structures and Doppler shifts in solar
equatorial coronal holes
By combining observations of the Sun made by
SUMER and the Michelson Doppler Imager (MDI)
aboard SOHO, the network structures in equatorial
coronal holes have been studied, in particular the relationship
between the ultraviolet emission-line parameters
(line radiance, Doppler shift and line width) and
the magnetic field. The bases of coronal holes as seen
in chromospheric spectral lines with relatively low formation
temperatures generally have similar properties
as normal quiet-Sun regions, i.e., small bright patches
with a size of about 2 arcsec to 10 arcsec are the dominant
features in the network as well as cell interiors.
With increasing formation temperature, these features
become more diffuse and larger. Loop-like structures
are the most prominent features in the transition region.
In coronal holes, they seem to have one footpoint
rooted in the intra-network and to extend into the cell
interiors. Some of them appear as star-shape clusters.
In Dopplergrams of typical transition region lines,
such as O VI (103.2 nm), there are also many fine
structures with apparent blue shifts, although, on average
they appear red shifted. Structures with blue
shifts usually have also broader line widths. They
seem to represent plasma above large concentrations
Fig. 24: Difference between the average Doppler shift
in quiet Sun regions and equatorial coronal holes for
various emission lines versus the line formation (electron)
temperature.
of unipolar magnetic field, without obvious bipolar
photospheric magnetic features nearby. The difference
between the average Doppler shift in quiet Sun regions
and coronal holes around the equator are for various
emission lines shown in Fig. 24.
(L.D. Xia, E. Marsch, K. Wilhelm)
Spatial mapping of scattering polarisation at the
solar limb
To investigate the topology of scattering polarisation
at the solar limb we recorded polarised filtergrams
in the core of the Ca II K line with the Zurich Imaging
Polarimeter ZIMPOL II at the New Swedish Solar
Telescope on La Palma. The Ca II K line is chosen for
the following reasons: (i) It exhibits a strong scattering
polarization signal. (ii) The relatively large line
width better matches the relatively broad filter pass
band (0.1 nm) than is the case for most other scattering
lines in the solar spectrum. (iii) This line has a rich
and intriguing spectral structure with considerable diagnostic
potential. (iv) Its properties have been well
explored in the spectral domain.
A first inspection of the data (see Fig. 25) shows clear
signatures of scattering polarisation not only in a narrow
limb zone but also above the visible solar limb
in spicules, as well as in regions corresponding to
network boundaries. While circular polarisation due
to the Zeeman effect is not detected due to spectral
smearing, signatures of Hanle rotation are prominent
in the limb zone as well as in the observed spicules.
Observations of this type harbour potential for diagnostics
of chromospheric magnetic fields. The preliminary
data are promising and justify further investigation.