330 research outputs found
Diagnostics for spectropolarimetry and magnetography
An assessment on the capabilities of modern spectropolarimeters and
magnetographs is in order since most of our astrophysical results rely upon the
accuracy of the instrumentation and on the sensitivity of the observables to
variations of the sought physical parameters. A contribution to such an
assessment will be presented in this talk where emphasis will be made on the
use of the so-called response functions to gauge the probing capabilities of
spectral lines and on an analytical approach to estimate the uncertainties in
the results in terms of instrumental effects. The Imaging Magnetograph
eXperiment (IMaX) and the Polarimetric and Helioseismic Imager (PHI) will be
used as study cases.Comment: To be published in "Physics of Sun and Star Spots", Proceedings of
IAU Symp. 273, D.P. Choudhary & A.C. Cadavid (eds.), Cambridge, UK: Cambridge
University Pres
Magnetic loops in the quiet Sun
We investigate the fine structure of magnetic fields in the atmosphere of the
quiet Sun. We use photospheric magnetic field measurements from {\sc
Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the
photospheric magnetic field into higher layers of the solar atmosphere with the
help of potential and force-free extrapolation techniques. We find that most
magnetic loops which reach into the chromosphere or higher have one foot point
in relatively strong magnetic field regions in the photosphere. of the
magnetic energy in the mid chromosphere (at a height of 1 Mm) is in field
lines, whose stronger foot point has a strength of more than 300 G, i.e. above
the equipartition field strength with convection. The loops reaching into the
chromosphere and corona are also found to be asymmetric in the sense that the
weaker foot point has a strength G and is located in the
internetwork. Such loops are expected to be strongly dynamic and have short
lifetimes, as dictated by the properties of the internetwork fields.Comment: accepted for ApJL Sunrise special issue, 8 Pages, 4 Figure
Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE
Localized strongly Doppler-shifted Stokes V signals were detected by
IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that
are observed as linear polarization features. We aim to set constraints on the
physical nature and causes of these highly Doppler-shifted signals. In
particular, the temporal relation between the appearance of transverse fields
and the strong Doppler shifts is analyzed in some detail. We calculated the
time difference between the appearance of the strong flows and the linear
polarization. We also obtained the distances from the center of various
features to the nearest neutral lines and whether they overlap or not. These
distances were compared with those obtained from randomly distributed points on
observed magnetograms. Various cases of strong flows are described in some
detail. The linear polarization signals precede the appearance of the strong
flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are
closer (0.19") to magnetic neutral lines than randomly distributed points
(0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a
neutral line that is located between the emerging field and pre-existing
fields. That the remaining 20% do not show a close-by pre-existing field could
be explained by a lack of sensitivity or an unfavorable geometry of the
pre-existing field, for instance, a canopy-like structure. Transverse fields
occurred before the observation of the strong Doppler shifts. The process is
most naturally explained as the emergence of a granular-scale loop that first
gives rise to the linear polarization signals, interacts with pre-existing
fields (generating new neutral line configurations), and produces the observed
strong flows. This explanation is indicative of frequent small-scale
reconnection events in the quiet Sun.Comment: 11 pages, 8 figure
Influence of phase-diversity image reconstruction techniques on circular polarization asymmetries
Full Stokes filter-polarimeters are key instruments for investigating the
rapid evolution of magnetic structures on the solar surface. To this end, the
image quality is routinely improved using a-posteriori image reconstruction
methods. We analyze the robustness of circular polarization asymmetries to
phase-diversity image reconstruction techniques. We use snapshots of
magneto-hydrodynamical simulations carried out with different initial
conditions to synthesize spectra of the magnetically sensitive Fe I line at
5250.2 A. We degrade the synthetic profiles spatially and spectrally to
simulate observations with the IMaX full Stokes filter-polarimeter. We also
simulate the focused/defocused pairs of images used by the phase-diversity
algorithm for reconstruction and the polarimetric modulation scheme. We assume
that standard optimization methods are able to infer the projection of the
wavefront on the Zernike polynomials with 10% precision. We also consider the
less favorable case of 25% precision. We obtain reconstructed monochromatic
modulated images that are later demodulated and compared with the original
maps. Although asymmetries are often difficult to define in the quiet Sun due
to the complexity of the Stokes V profiles, we show how asymmetries are
degraded with spatial and spectral smearing. The results indicate that,
although image reconstruction techniques reduce the spatial smearing, they can
modify the asymmetries of the profiles, mainly caused by the appearance of
spatially-correlated noise.Comment: 10 pages, accepted for publication in A&
Properties of sunspots in cycle 23: I. Dependence of brightness on sunspot size and cycle phase
In this paper we investigate the dependence of umbral core brightness, as
well as the mean umbral and penumbral brightness on the phase of the solar
cycle and on the size of the sunspot. Albregtsen & Maltby (1978) reported an
increase in umbral core brightness from the early to the late phase of solar
cycle from the analysis of 13 sunspots which cover solar cycles 20 and 21. Here
we revisit this topic by analysing continuum images of more than 160 sunspots
observed by the MDI instrument on board the SOHO spacecraft for the period
between 1998 March to 2004 March, i.e. a sizable part of solar cycle 23. The
advantage of this data set is its homogeneity, with no seeing fluctuations. A
careful stray light correction, which is validated using the Mercury transit of
7th May, 2003, is carried out before the umbral and penumbral intensities are
determined. The influence of the Zeeman splitting of the nearby NiI spectral
line on the measured 'continuum' intensity is also taken into account. We did
not observe any significant variation in umbral core, mean umbral and mean
penumbral intensities with solar cycle, which is in contrast to earlier
findings for the umbral core intensity. We do find a strong and clear
dependence of the umbral brightness on sunspot size, however. The penumbral
brightness also displays a weak dependence. The brightness-radius relationship
has numerous implications, some of which, such as those for the energy
transport in umbrae, are pointed out.Comment: 16 pages, 21 postscript figures, accepted for publication in A&
Where the granular flows bend
Based on IMaX/Sunrise data, we report on a previously undetected phenomenon
in solar granulation. We show that in a very narrow region separating granules
and intergranular lanes the spectral line width of the Fe I 5250.2 A line
becomes extremely small. We offer an explanation of this observation with the
help of magneto-convection simulations. These regions with extremely small line
widths correspond to the places where the granular flows bend from mainly
upflow in granules to downflow in intergranular lanes. We show that the
resolution and image stability achieved by IMaX/Sunrise are important
requisites to detect this interesting phenomenon.Comment: Accepted for the Sunrise Special Issue of ApJ
Supersonic Evershed flow outside Sunspots
We report on the discovery of mostly horizontal field channels just outside
sunspot penumbrae (in the so-called `moat' region) that are seen to sustain
supersonic flows (line-of-sight component of 6 km s{-1}). The spectral
signature of these supersonic flows corresponds to circular polarization
profiles with an additional, satellite, third lobe of the same sign as the
parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is
consistent with an outward directed flow that we interpret as the continuation
of the magnetized Evershed flow outside sunspots at supersonic speeds. In
Stokes Q and U, a clear signature of a transverse field connecting the two flow
streams is observed. Such an easily detectable spectral signature should allow
for a clear identification of these horizontal field channels in other
spectropolarimetric sunspot data. For the spot analyzed in this paper, a total
of 5 channels with this spectral signature have been unambiguously found
- …