36 research outputs found
About the directional properties of Solar Spicules from Hough Transform analysis
Spicules are intermittently rising above the surface of the Sun eruptions;
EUV jets are now also reported in immediately above layers. The variation of
spicule orientation with respect to the solar latitude, presumably reflecting
the confinement and the focusing of ejecta by the surrounding global coronal
magnetic field, is an important parameter to understand their dynamical
properties. A wealth of high resolution images of limb spicules are made
available in H CaII emission from the SOT Hinode mission. Furthermore, the
Hough transform is applied to the resulting images for making a statistical
analysis of spicule orientations in different regions around the solar limb,
from the pole to the equator. Results show a large difference of spicule
apparent tilt angles in: (i) the solar pole regions, (ii) the equatorial
regions, (iii) the active regions and (iv) the coronal hole regions. Spicules
are visible in a radial direction in the polar regions with a tilt angle (less
than 200). The tilt angle is even reduced to 10 degrees inside the coronal hole
with open magnetic field lines and at the lower latitude the tilt angle reaches
values in excess of 50 degree. Usually, which is in close resemblance to the
rosettes made of dark mottles and fibrils in projection on the solar disk. The
inference of these results for explaining the so-called chromospheric
prolateness observed at solar minimum of activity in cool chromospheric lines
is considered.Comment: 13 pages, 6 figure
Contribution to the modeling of solar spicules
Solar limb and disc spicule quasi- periodic motions have been reported for a
long time, strongly suggesting that they are oscillating. In order to clear up
the origin and possibly explain some solar limb and disc spicule quasi-periodic
recurrences produced by overlapping effects, we present a simulation model
assuming quasi- random positions of spicules. We also allow a set number of
spicules with different physical properties (such as: height, lifetime and tilt
angle as shown by an individual spicule) occurring randomly.
Results of simulations made with three different spatial resolutions of the
corresponding frames and also for different number density of spicules, are
analyzed. The wavelet time/frequency method is used to obtain the exact period
of spicule visibility. Results are compared with observations of the
chromosphere from i/ the Transition Region and Coronal Explorer (TRACE)
filtergrams taken at 1600 angstrom, ii/ the Solar Optical Telescope (SOT) of
Hinode taken in the Ca II H-line and iii/ the Sac-Peak Dunn's VTT taken in
H{\alpha} line. Our results suggest the need to be cautious when interpreting
apparent oscillations seen in spicule image sequences when overlapping is
present, i.e.; when the spatial resolution is not enough to resolve individual
components of spicules.Comment: 20 pages, 7 figures, 1 tabl
Alfvenic waves in polar spicules
Context. For investigating spicules from the photosphere to coronal heights,
the new Hinode/SOT long series of high resolution observations from Space taken
in CaII H line emission offers an improved way to look at their remarkable
dynamical behavior using images free of seeing effects. They should be put in
the context of the huge amount of already accumulated material from
ground-based instruments, including high- resolution spectra of off-limb
spicules. Results. The surge-like behavior of solar polar region spicules
supports the untwisting multi-component interpretation of spicules exhibiting
helical dynamics. Several tall spicules are found with (i) upward and downward
flows similar at lower and middle-levels, the rate of upward motion being
slightly higher at high levels; (ii) the left and right-hand velocities are
also increasing with height; (iii) a large number of multi-component spicules
show shearing motion of both left-handed and right-handed senses occurring
simultaneously, which might be understood as twisting (or untwisting) threads.
The number of turns depends on the overall diameter of the structure made of
components and changes from at least one turn for the smallest structure to at
most two or three turns for surge-like broad structures; the curvature along
the spicule corresponds to a low turn number similar to a transverse kink mode
oscillation along the threads.Comment: 8 pages, 10 figures, Accepted in Astronomy and Astrophysic
Solar gravitational energy and luminosity variations
Due to non-homogeneous mass distribution and non-uniform velocity rate inside
the Sun, the solar outer shape is distorted in latitude. In this paper, we
analyze the consequences of a temporal change in this figure on the luminosity.
To do so, we use the Total Solar Irradiance (TSI) as an indicator of
luminosity. Considering that most of the authors have explained the largest
part of the TSI modulation with magnetic network (spots and faculae) but not
the whole, we could set constraints on radius and effective temperature
variations (dR, dT). However computations show that the amplitude of solar
irradiance modulation is very sensitive to photospheric temperature variations.
In order to understand discrepancies between our best fit and recent
observations of Livingston et al. (2005), showing no effective surface
temperature variation during the solar cycle, we investigated small effective
temperature variation in irradiance modeling. We emphasized a phase-shift
(correlated or anticorrelated radius and irradiance variations) in the (dR,
dT)-parameter plane. We further obtained an upper limit on the amplitude of
cyclic solar radius variations, deduced from the gravitational energy
variations. Our estimate is consistent with both observations of the
helioseismic radius through the analysis of f-mode frequencies and observations
of the basal photospheric temperature at Kitt Peak. Finally, we suggest a
mechanism to explain faint changes in the solar shape due to variation of
magnetic pressure which modifies the granules size. This mechanism is supported
by our estimate of the asphericity-luminosity parameter, which implies an
effectiveness of convective heat transfer only in very outer layers of the Sun.Comment: 17 pages, 2 figure, 1 table, published in New Astronom
Increasing the Fine Structure Visibility of the Hinode SOT Ca II H Filtergrams
We present the improved so-called Madmax (OMC) operator selecting maxima of
convexities computed in multiple directions around each pixel rewritten in
MatLab and shown to be very efficient for pattern recognition.
The aim of the algorithm is to trace the bright hair-like features (for ex.
chromospheric thin jets or spicules) of solar ultimate observations polluted by
a noise of different origins. This popular spatial operator uses the second
derivative in the optimally selected direction for which its absolute value has
a maximum value. Accordingly, it uses the positivity of the resulting intensity
signal affected by a superposed noise. The results are illustrated using a test
artificially generated image and real SOT (Hinode) images are also used, to
make your own choice of the sensitive parameters to use in improving the
visibility of images.Comment: 12 pages, 3 figurs, submitted in Solar Physic
An investigation of the line of sight towards QSO PKS 0237-233
We present a detailed analysis of absorption systems along the line of sight
towards QSO PKS 0237-233 using a high resolution spectrum of signal-to-noise
ratio (SNR) ~ 60-80 obtained with the Ultraviolet and Visual Echelle
Spectrograph mounted on the Very Large Telescope. This line of sight is known
to show a remarkable overdensity of CIV systems that has been interpreted as
revealing the presence of a supercluster of galaxies. A detailed analysis of
each of these absorption systems is presented. In particular, for the z_abs =
1.6359 (with two components of logN(HI) = 18.45, 19.05) and z_abs = 1.6720
(logN(H I) = 19.78) sub-Damped Ly-alpha systems (sub-DLAs), we measure accurate
abundances (resp. [O/H] = -1.63(0.07) and [Zn/H] = - 0.57(0.05) relative to
solar). While the depletion of refractory elements onto dust grains in both
sub-DLAs is not noteworthy, photoionization models show that ionization effects
are important in a part of the absorbing gas of the sub-DLA at z_abs = 1.6359
(HI is 95 percent ionized) and in part of the gas of the sub-DLA at z_abs =
1.6359. The CIV clustering properties along the line of sight is studied in
order to investigate the nature of the observed overdensity. We conclude that
despite the unusually high number of CIV systems detected along the line of
sight, there is no compelling evidence for the presence of a single unusual
overdensity and that the situation is consistent with chance coincidence.Comment: Accepted for publication in MNRAS. 23 pages, 16 figures, 12 table
Are non-magnetic mechanisms such as temporal solar diameter variations conceivable for an irradiance variability?
Irradiance variability has been monitored from space for more than two
decades. Even if data are coming from different sources, it is well established
that a temporal variability exists which can be set to as approximately 0.1%,
in phase with the solar cycle. Today, one of the best explanation for such an
irradiance variability is provided by the evolution of the solar surface
magnetic fields. But if some 90 to 95% can be reproduced, what would be the
origin of the 10 to 5% left? Non magnetic effects are conceivable. In this
paper we will consider temporal variations of the diameter of the Sun as a
possible contributor for the remaining part. Such an approach imposes strong
constraints on the solar radius variability. We will show that over a solar
cycle, variations of no more than 20 mas of amplitude can be considered. Such a
variability (far from what is reported by observers conducting measurements by
means of ground-based solar astrolabes) may explain a little part of the
irradiance changes not explained by magnetic features. Further requirements are
needed that may help to reach a conclusion. Dedicated space missions are
necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around
2008); it is also proposed to reactivate SDS flights for such a purpose.Comment: 8 pages, 2 eps figures, published in Solar Physic
On the long-term variability of the green corona accordingly to four existing databases
We examine four long-term databases of monthly means of the whole disc green coronal brightness which cover the period of the last five and a half solar cycles to verify them, and compare their usefulness, as indices characterize the solar activity. We use the data from (1) the Norikura Observatory, (2) the Kislovodsk Observatory, (3) intensities detected by several coronographs and converted to the common photometric scale of Pic du Midi, (4) coronal indices obtained from detections of several observatories and converted to the photometric scale of Lomnicky Stit. We compare them by means of correlative analysis with the time series of sunspot numbers. The data (2) demonstrate the best agreement with sunspot number as the reference (r = 0.88)