1,750 research outputs found
Link between the chromospheric network and magnetic structures of the corona
Recent work suggested that the traditional picture of the corona above the
quiet Sun being rooted in the magnetic concentrations of the chromospheric
network alone is strongly questionable. Building on that previous study we
explore the impact of magnetic configurations in the photosphere and the low
corona on the magnetic connectivity from the network to the corona.
Observational studies of this connectivity are often utilizing magnetic field
extrapolations. However, it is open to which extent such extrapolations really
represent the connectivity found on the Sun, as observations are not able to
resolve all fine scale magnetic structures. The present numerical experiments
aim at contributing to this question. We investigated random
salt-and-pepper-type distributions of kilo-Gauss internetwork flux elements
carrying some to Mx, which are hardly distinguishable by
current observational techniques. These photospheric distributions are then
extrapolated into the corona using different sets of boundary conditions at the
bottom and the top. This allows us to investigate the fraction of network flux
which is connected to the corona, as well as the locations of those coronal
regions which are connected to the network patches. We find that with current
instrumentation one cannot really determine from observations, which regions on
the quiet Sun surface, i.e. in the network and internetwork, are connected to
which parts of the corona through extrapolation techniques. Future
spectro-polarimetric instruments, such as with Solar B or GREGOR, will provide
a higher sensitivity, and studies like the present one could help to estimate
to which extent one can then pinpoint the connection from the chromosphere to
the corona.Comment: 8 pages, 5 figures, acceped for publication in A&
Solar Coronal Structures and Stray Light in TRACE
Using the 2004 Venus transit of the Sun to constrain a semi-empirical
point-spread function for the TRACE EUV solar telescope, we have measured the
effect of stray light in that telescope. We find that 43% of 171A EUV light
that enters TRACE is scattered, either through diffraction off the entrance
filter grid or through other nonspecular effects. We carry this result forward,
via known-PSF deconvolution of TRACE images, to identify its effect on analysis
of TRACE data. Known-PSF deconvolution by this derived PSF greatly reduces the
effect of visible haze in the TRACE 171A images, enhances bright features, and
reveals that the smooth background component of the corona is considerably less
bright (and hence much more rarefied) than commonly supposed. Deconvolution
reveals that some prior conlclusions about the Sun appear to have been based on
stray light in the images. In particular, the diffuse background "quiet corona"
becomes consistent with hydrostatic support of the coronal plasma; feature
contrast is greatly increased, possibly affecting derived parameters such as
the form of the coronal heating function; and essentially all existing
differential emission measure studies of small features appear to be affected
by contamination from nearby features. We speculate on further implications of
stray light for interpretation of EUV images from TRACE and similar
instruments, and advocate deconvolution as a standard tool for image analysis
with future instruments such as SDO/AIA.Comment: Accepted by APJ; v2 reformatted to single-column format for online
readabilit
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
pres
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
Distinction between clouds and ice/snow covered surfaces in the identification of cloud-free observations using SCIAMACHY PMDs
SCIAMACHY on ENVISAT allows measurement of different trace gases including those most abundant in the troposphere (e.g. CO<sub>2</sub>, NO<sub>2</sub>, CH<sub>4</sub>, BrO, SO<sub>2</sub>). However, clouds in the observed scenes can severely hinder the observation of tropospheric gases. Several cloud detection algorithms have been developed for GOME on ERS-2 which can be applied to SCIAMACHY. The GOME cloud algorithms, however, suffer from the inadequacy of not being able to distinguish between clouds and ice/snow covered surfaces because GOME only covers the UV, VIS and part of the NIR wavelength range (240-790 nm). As a result these areas are always flagged as clouded, and therefore often not used. Here a method is presented which uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 µm to make a distinction between clouds and ice/snow covered surfaces. The algorithm is developed using collocated MODIS observations. The algorithm presented here is specifically developed to identify cloud-free SCIAMACHY observations. The SCIAMACHY Polarisation Measurement Devices (PMDs) are used for this purpose because they provide higher spatial resolution compared to the main spectrometer measurements
Group divisible designs with block-size four
AbstractIt is proved that the obvious necessary conditions for the existence of a group divisible design with k = 4 are sufficient, except for the cases corresponding to the non-existing transversal designs T[4, 1; 2] and T[4, 1; 6]
C IV fluxes from the Sun as a star, and the correlation with magnetic flux
A total of 144 C IV wavelength 1548 Solar Maximum Mission (SMM)-UVSP spectroheliograms of solar plages were analyzed, some of which are series of exposures of the same region on the same day. Also analyzed were the C IV wavelength 1551 rasters of plages and C IV wavelength 1548 rasters of the quiet sun. The sample contained data on 17 different plages, observed on 50 different days. The center-to-limb variations of the active regions show that the optical thickness effects in the C IV wavelength 1548 line can be neglected in the conversion from intensity to flux density. As expected for the nearly optically thin situation, the C IV wavelength 1548 line is twice as bright as the C IV 1551 line. The average C IV wavelength 1548 flux density for a quiet region is 2700 ergs/cm/s and, with surprisingly little scatter, 18,000 erg/cm/s for plages. The intensity histograms of rasters obtained at disk center can be separated into characteristic plage and quiet sun contributions with variable relative filling factors. The relationship between the C IV and magnetic flux densities for spatially resolved data is inferred to be almost the same, with only an additional factor of order unity in the constant of proportionality
Thermal Diagnostics with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions
We present a new method for performing differential emission measure (DEM)
inversions on narrow-band EUV images from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO). The method yields positive
definite DEM solutions by solving a linear program. This method has been
validated against a diverse set of thermal models of varying complexity and
realism. These include (1) idealized gaussian DEM distributions, (2) 3D models
of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a
non-linear force-free field, and (3) thermodynamic models from a
fully-compressible, 3D MHD simulation of AR corona formation following magnetic
flux emergence. We then present results from the application of the method to
AIA observations of Active Region 11158, comparing the region's thermal
structure on two successive solar rotations. Additionally, we show how the DEM
inversion method can be adapted to simultaneously invert AIA and XRT data, and
how supplementing AIA data with the latter improves the inversion result. The
speed of the method allows for routine production of DEM maps, thus
facilitating science studies that require tracking of the thermal structure of
the solar corona in time and space.Comment: 21 pages, 18 figures, accepted for publication in Ap
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