1,491 research outputs found
Discriminant analysis of solar bright points and faculae II. Contrast and morphology analysis
Taken at a high spatial resolution of 0.1 arcsec, Bright Points (BPs) are
found to coexist with faculae in images and the latter are often resolved as
adjacent striations. Understanding the properties of these different features
is fundamental to carrying out proxy magnetometry. To shed light on the
relationship between BPs and faculae, we studied them separately after the
application of a classification method, developed and described in a previous
paper) on active region images at various heliocentric angles. In this Paper,
we explore different aspects of the photometric properties of BPs and faculae,
namely their G-band contrast profiles, their peak contrast in G-band and
continuum, as well as morphological parameters. We find that: (1) the width of
the contrast profiles of the classified BPs and faculae are consistent with
studies of disk center BPs at and limb faculae, which indirectly confirms the
validity of our classification, (2) the profiles of limb faculae are limbward
skewed on average, while near disk center they exhibit both centerward and
limbward skewnesses due to the distribution of orientations of the faculae, (3)
the relation between the peak contrasts of BPs and faculae and their apparent
area discloses a trend reminiscent of magnetogram studies. The skewness of
facular profiles provides a novel constraint for 3D MHD models of faculae. As
suggested by the asymmetry and orientation of their contrast profiles, faculae
near disk center could be induced by inclined fields, while apparent BPs near
the limb seem to be in fact small faculae misidentified. The apparent area of
BPs and faculae could be possibly exploited for proxy magnetometry
Magnetohydrostatic equilibrium in starspots: dependences on color (T_{eff}) and surface gravity (g)
Temperature contrasts and magnetic field strengths of sunspot umbrae broadly
follow the thermal-magnetic relationship obtained from magnetohydrostatic
equilibrium. Using a compilation of recent observations, especially in
molecular bands, of temperature contrasts of starspots in cool stars, and a
grid of Kurucz stellar model atmospheres constructed to cover layers of
sub-surface convection zone, we examine how the above relationship scales with
effective temperature T_{eff}, surface gravity g and the associated changes in
opacity of stellar photospheric gas. We calculate expected field strengths in
starpots and find that a given relative reduction in temperatures (or the same
darkness contrasts) yield increasing field strengths against decreasing T_{eff}
due to a combination of pressure and opacity variations against T_{eff}.Comment: 4 pages, 3 figures, to appear in the Proceedings of IAUS 273:
"Physics of Sun and Star Spots", eds. D.P. Choudhary and K. Strassmeier 2010,
Cambridge University Pres
Diagnostics of Coronal Magnetic Fields Through the Hanle Effect in UV and IR Lines
The plasma thermodynamics in the solar upper atmosphere, particularly in the
corona, are dominated by the magnetic field, which controls the flow and
dissipation of energy. The relative lack of knowledge of the coronal vector
magnetic field is a major handicap for progress in coronal physics. This makes
the development of measurement methods of coronal magnetic fields a high
priority in solar physics. The Hanle effect in the UV and IR spectral lines is
a largely unexplored diagnostic. We use magnetohydrodynamic (MHD) simulations
to study the magnitude of the signal to be expected for typical coronal
magnetic fields for selected spectral lines in the UV and IR wavelength ranges,
namely the H I Ly- and the He I 10830 {\AA} lines. We show that the
selected lines are useful for reliable diagnosis of coronal magnetic fields.
The results show that the combination of polarization measurements of spectral
lines with different sensitivities to the Hanle effect may be most appropriate
for deducing coronal magnetic properties from future observations.Comment: 15 pages, 5 figures, Frontiers in Astronomy and Space Sciences, 201
Modified p-modes in penumbral filaments?
Aims: The primary objective of this study is to search for and identify wave
modes within a sunspot penumbra.
Methods: Infrared spectropolarimetric time series data are inverted using a
model comprising two atmospheric components in each spatial pixel. Fourier
phase difference analysis is performed on the line-of-sight velocities
retrieved from both components to determine time delays between the velocity
signals. In addition, the vertical separation between the signals in the two
components is calculated from the Stokes velocity response functions.
Results: The inversion yields two atmospheric components, one permeated by a
nearly horizontal magnetic field, the other with a less-inclined magnetic
field. Time delays between the oscillations in the two components in the
frequency range 2.5-4.5 mHz are combined with speeds of atmospheric wave modes
to determine wave travel distances. These are compared to expected path lengths
obtained from response functions of the observed spectral lines in the
different atmospheric components. Fast-mode (i.e., modified p-mode) waves
exhibit the best agreement with the observations when propagating toward the
sunspot at an angle ~50 degrees to the vertical.Comment: 8 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
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Simplified Production of Large Prototypes using Visible Slicing
Rapid Prototyping (RP) is a totally automatic generative manufacturing
technique based on a âdivide-and-conquerâ strategy called âslicingâ. Simple
slicing used on 2.5-axis kinematics of the existing RP machines is responsible for
the staircase error. Although thinner slices will have less error, the slice thickness
has practical limits. Visible Slicing overcomes these limitations. A few visible
slices exactly represent the object. Each visible slice can be realized using a 3- axis kinematics machine from two opposite directions. Visible slicing is
implemented on Segmented Object Manufacturing (SOM) machine under
development. SOM can produce soft large prototypes faster and cheaper with
accuracy comparable to that of CNC machining.Mechanical Engineerin
Nonlinear Force-Free Field Modeling of the Solar Magnetic Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations
In the quiet solar photosphere, the mixed polarity fields form a magnetic
carpet, which continuously evolves due to dynamical interaction between the
convective motions and magnetic field. This interplay is a viable source to
heat the solar atmosphere. In this work, we used the line-of-sight (LOS)
magnetograms obtained from the Helioseismic and Magnetic Imager (HMI) on the
\textit{Solar Dynamics Observatory} (\textit{SDO}), and the Imaging
Magnetograph eXperiment (IMaX) instrument on the \textit{Sunrise} balloon-borne
observatory, as time dependent lower boundary conditions, to study the
evolution of the coronal magnetic field. We use a magneto-frictional relaxation
method, including hyperdiffusion, to produce time series of three-dimensional
(3D) nonlinear force-free fields from a sequence of photospheric LOS
magnetograms. Vertical flows are added up to a height of 0.7 Mm in the modeling
to simulate the non-force-freeness at the photosphere-chromosphere layers.
Among the derived quantities, we study the spatial and temporal variations of
the energy dissipation rate, and energy flux. Our results show that the energy
deposited in the solar atmosphere is concentrated within 2 Mm of the
photosphere and there is not sufficient energy flux at the base of the corona
to cover radiative and conductive losses. Possible reasons and implications are
discussed. Better observational constraints of the magnetic field in the
chromosphere are crucial to understand the role of the magnetic carpet in
coronal heating.Comment: Accepted for publication in The Astrophysical Journal (13 pages, 10
figures
Nature of the energy source powering solar coronal loops driven by nanoflares
Magnetic energy is required to heat the corona, the outer atmosphere of the
Sun, to millions of degrees. We study the nature of the magnetic energy source
that is probably responsible for the brightening of coronal loops driven by
nanoflares in the cores of solar active regions. We consider observations of
two active regions (ARs), 11890 and 12234, in which nanoflares have been
detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV)
images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) for coronal loop diagnostics. These images are combined with
the co-temporal line-of-sight magnetic field maps from the Helioseismic and
Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal
loops and their magnetic roots in the photosphere. The core of these ARs
exhibit loop brightening in multiple EUV channels of AIA, particularly in its
9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex
mixed polarity magnetic field distribution at the base of these loops. We
detect the cancellation of photospheric magnetic flux at these locations at a
rate of about Mx s. The associated compact coronal
brightenings directly above the cancelling magnetic features are indicative of
plasma heating due to chromospheric magnetic reconnection. We suggest that the
complex magnetic topology and the evolution of magnetic field, such as flux
cancellation in the photosphere and the resulting chromospheric reconnection,
can play an important role in energizing active region coronal loops driven by
nanoflares. Our estimate of magnetic energy release during flux cancellation in
the quiet Sun suggests that chromospheric reconnection can also power the quiet
corona.Comment: A&A Letters (in press
First Stereoscopic Coronal Loop Reconstructions from Stereo Secchi Images
We present the first reconstruction of the three-dimensional shape of
magnetic loops in an active region from two different vantage points based on
simultaneously recorded images. The images were taken by the two EUVI
telescopes of the SECCHI instrument onboard the recently launched STEREO
spacecraft when the heliocentric separation of the two space probes was 12
degrees. We demostrate that these data allow to obtain a reliable
three-dimensional reconstruction of sufficiently bright loops. The result is
compared with field lines derived from a coronal magnetic field model
extrapolated from a photospheric magnetogram recorded nearly simultaneously by
SOHO/MDI. We attribute discrepancies between reconstructed loops and
extrapolated field lines to the inadequacy of the linear force-free field model
used for the extrapolation.Comment: 6 pages, 5 figure
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