explanation blue bibcodes open ADS page with paths to full text
Author name code: dara
ADS astronomy entries on 2022-09-14
author:"Dara, H.C."
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Title: Heating Distribution Along Coronal Loops in two Active Regions
Using a Simple Electrodynamic Calculation
Authors: Gontikakis, C.; Georgoulis, M.; Contopoulos, I.; Dara, H. C.
2010ASPC..424...25G Altcode:
The heating along hundreds of coronal loops of non flaring active
regions is computed using a simple electrodynamic model. Photospheric
motions generate electric fields inducing, electric potential
differences at the footpoints of loops. These potential differences
generate electric currents that lead to Ohmic heating. We computed
the magnetic field extrapolation from the magnetograms of two
active regions, namely NOAA AR 9366 (SOHO/MDI) and NOAA AR 10963,
(HINODE/SOT). Closed magnetic field lines model the coronal loops. For
each loop we computed the heating function and obtained the hydrostatic
distribution of temperature and pressure. We found that the coronal
heating is stronger near the footpoints of the loops and asymmetric
along them. We obtained scaling laws that correlate the mean volumetric
heating with the loop length, and the heating flux, through the loop
footpoints with the magnetic field strength at the footpoints. Our
results are in qualitative agreement with observations (see Gontikakis
et al. 2008 for more details).
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Title: Determination of the True Shape of Coronal Loops
Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C.
2008SoPh..252...73A Altcode: 2008SoPh..tmp..143A
Using line of sight velocity measurements from the SUMER and CDS
instruments aboard SOHO, in conjunction with a simple geometrical
model, we reconstructed the true, 3D shape and the velocity of plasma
flow along coronal loops. The projection of the loop on the sky and
the position of the footpoints define a family of curves. Assuming
that the loop is located on a plane, the line of sight velocity
can be used to select the most plausible solution. For two loops,
observed in the Ne VIII 770 Å and O V 630 Å spectral lines,
we find asymmetric, subsonic uni-directional flows, with velocity
maxima of ≈ 80 km s<SUP>−1</SUP> near the footpoints. The loops
are highly inclined with respect to the vertical, by 55<SUP>°</SUP>
and 70<SUP>°</SUP>, respectively; thus the true height of the loop
tops from the photospheric level is ≈ 20<SUP>”</SUP>, comparable
to the isothermal scale height.
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Title: Distribution of coronal heating in a solar active region
Authors: Gontikakis, C.; Contopoulos, I.; Dara, H. C.
2008A&A...489..441G Altcode:
Aims: We investigate the distribution of heating of coronal loops
in a non-flaring solar active region, using a simple electrodynamic
model: the random displacements of the loop footpoints, caused by
photospheric plasma motions, generate electric potential differences
between the footpoints and, as a result, electric currents flow along
the loops, producing Ohmic heating. <BR />Methods: We implement our
model on the closed magnetic field lines in the potential magnetic
field extrapolation of an MDI active region magnetogram. For each
one of the magnetic field lines, we compute the heating function and
obtain the hydrostatic distribution of temperature and pressure. We
find that coronal heating is stronger close to the footpoints of the
loops and asymmetric along them. We obtain scaling laws that relate
both the mean volumetric heating to the loop length, and the heating
flux through the loop footpoints to the magnetic field strength at the
footpoints. Our results agree with observational data. <BR />Results:
According to our model, we attribute the observed small coronal-loop
width expansion to both the preferential heating of coronal loops of
small cross-section variation, and the cross-section confinement due to
the random electric currents flowing along the loops. <BR />Conclusions:
We conclude that our model can be used as a simple working tool in
the study of solar active regions.
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Title: Heating Distribution along Coronal Loops of an Active Region
using a Simple Electrodynamic Calculation
Authors: Gontikakis, C.; Contopoulos, I.; Dara, H. C.
2008ESPM...12.3.46G Altcode:
The heating along hundreds of coronal loops of a non flaring active
region is computed using a simple electrodynamic model. The random
plasma displacements caused by the photospheric motions generate
electric fields and, as a consequence, electric potential differences at
the footpoints of loops. These potential differences generate electric
currents which are the cause of Ohmic heating. <P />We computed the
potential magnetic field extrapolation using the MDI magnetogram of
the NOAA 9366 active region observed on March 6, 2001 and used the
closed magnetic field lines to model the coronal loops. For each loop
we compute the heating function and obtain the hydrostatic distribution
of temperature and pressure. <P />We find that the coronal heating is
stronger near the footpoints of the loops and asymmetric along them. We
obtain scaling laws that relate the mean volumetric heating with the
loop length, and the heating flux through the loop footpoints with the
magnetic field strength at the footpoints. We simulated the emission of
the hydrostatic loops in the 171 and 284 Angstroms spectral bands and
compared the resulting images with EIT observations. Our results are
in qualitative agreement with observations. We conclude that our model
can be used as a simple working tool for the study of active regions.
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Title: Evolution of a Coronal Loop System
Authors: Tsiropoula, G.; Tziotziou, K.; Wiegelmann, T.; Zachariadis,
Th.; Gontikakis, C.; Dara, H.
2007SoPh..240...37T Altcode:
The temporal variation of a loop system that appears to be changing
rapidly is examined. The analyzed data were obtained on 15 May 1999,
with the Transition Region and Coronal Explorer (TRACE) during an
observing campaign and consist of observations in the Fe IX/Fe X 171
Å and Fe XII 195 Å passbands taken at a cadence of ∼10 min. The
special interest in this loop system is that it looks like one expanding
loop; however, careful examination reveals that the loop consists of
several strands and that new loop strands become visible successively
at higher altitudes and lower loop strands fade out during the one hour
of our observations. These strands have different widths, densities,
and temperatures and are most probably consisting of, at least, a few
unresolved thinner threads. Several geometric and physical parameters
are derived for two of the strands and an effort is made to determine
their 3D structure based on the extrapolation of the magnetic field
lines. Electron density estimates allow us to derive radiative and
conductive cooling times and to conclude that these loop strands are
cooling by radiation.
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Title: S OHO Observations Of A Coronal Loop Compared With A 2D MHD
Loop Model
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2007ESASP.641E..65G Altcode:
We present SOHO/CDS observations of a coronal loop where Doppler
shifts and proper motions indicate a unidirectional mass flow. The
measurements of velocity, electron density and temperature (about 380
000 K) along its length are compared with the results of a 2D MHD
loop model. The derived energy balance, showing a stronger heating
at the loop footprints, as well as the knowledge gained from the MHD
model are presented. The implications of Solar Orbiter observations
for studying coronal loops are discussed.
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Title: S imulation Of An Active Region With A Simple Electrodynamic
Model
Authors: Contopoulos, I.; Gontikakis, C.; Dara, H. C.
2007ESASP.641E..59C Altcode:
We propose a simple model that allows us to investigate the development
of the slender magnetic loops seen in TRACE observations of active
regions. We argue that random displacements of the loop footpoints
caused by the observed rapid intermittent photospheric plasma flow
generate electric potential differences between the foot- points. As
a result, random electric currents flow along the loops. Our model
allows us to estimate the distribu- tion of intermittent Ohmic (DC)
heating in the corona. We propose that the thickness and duration of
the bright loops is related to the photospheric granulation motion
coherence length and turnover time respectively.
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Title: M odeling The 3-D Geometry of Coronal Loops
Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C.
2007ESASP.641E..52A Altcode:
Loops are the basic structural elements of the solar atmosphere in
low beta regions, particularly in the corona. Important progress
has been made in recent years thanks to the SOHO and TRACE space
missions, which provided opportunities for better studies of the loop
morphology and their physical properties. In this work we use line of
sight velocity measurements from the SUMER and CDS instruments aboard
SOHO, in conjunction with a simple geometrical model, to reconstruct
the velocity of plasma flow along loops. The flow appears to be
uni-directional, with velocity maxima near the footpoints.
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Title: Preliminary results from the March 29, 2006 total eclipse
observations in Egypt
Authors: Koutchmy, S.; Daniel, J. -Y.; Mouette., J.; Vilinga, J.;
Noëns, J. -C.; Damé, L.; Faurobert, M.; Dara, H.; Hady, A.; Semeida,
M.; Sabry, M.; Domenech, A.; Munier, J. -M.; Jimenez, R.; Legault,
Th.; Viladrich, Ch.; Kuzin, S.; Pertsov, A.; O. A. Team
2006sf2a.conf..547K Altcode:
A coordinated effort has been carried in the framework of
the French-Egyptian scientific cooperation to permit joined
simultaneous eclipse observations of the solar corona during the
total solar eclipse of March 29, 2006. Spaceborne EIT and Lasco (SoHO)
observations were also planned at the same time and were successfully
collected. Scientists from other countries collaborated on different
experiments. The synthetic image showing the magnetic coronal structure
of this quasi-minimum corona seen in W-L is given. Some preliminary
results are presented; a White Light (W.L.) movie has been also taken
during the totality.
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Title: Multiwavelength Analysis of an Active Region
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.;
Alissandrakis, C. E.; Nindos, A.; Vial, J. -C.; Tsiropoula, G.
2006SoPh..233...57G Altcode:
We study active region NOAA 8541, observed with instruments on board
SOHO, as well as with TRACE. The data set mainly covers the transition
region and the low corona. In selected loops studied with SUMER on SOHO,
the VIII 770 Å line is systematically redshifted. In order to estimate
the plasma velocity, we combine the Doppler shifts with proper motions
(TRACE) along these loops. In the case of an ejection, apparently
caused by the emergence of a parasitic polarity, proper motions and
Doppler shifts give consistent results for the velocity. A cooler loop,
observed in the same active region with CDS, shows a unidirectional
motion reminiscent of a siphon flow. The derived electron temperature
and density along a large steady loop confirm that it cannot be
described by hydrostatic models.
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Title: Evidence for coronal plasma oscillations over supergranular
cells
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2006IAUS..233..189G Altcode:
Evidence of coronal oscillations over the interior of supergranular
cells was found through SUMER observations. The observations are
rasters of quiet Sun regions and the oscillations were detected,
in the Ne VIII 770 Å Doppler maps, as a characteristic pattern. It
should be noted that the Ne VIII ion has coronal formation temperature
(650 000 K) and that reports of oscillations in the quiet Sun corona
are scarce. Magnetic extrapolation from MDI magnetogram showed that at
the location where the oscillation was detected, the gas and magnetic
pressures get equalized (β=1) higher in the atmosphere, compared
to the surrounding, non oscillating quiet Sun. This could indicate a
non-compressible wave propagating inside the gas dominated medium of
the cell causing the detected oscillation.
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Title: Coronal oscillation above a supergranular cell of the quiet
Sun chromospheric network?
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2005A&A...441.1191G Altcode:
We have detected an oscillation in the low corona, using a raster of
the SUMER EUV spectrograph in the Ne viii, 770 Å line formed at about
700 000 K. The oscillation was found in the Ne viii Doppler map above
the interior of a supergranular cell of the chromospheric network in
the quiet Sun, while it is absent in line radiance. The photospheric
magnetic field, extrapolated to coronal levels, was used to relate
this phenomenon to the magnetic structure. This oscillation phenomenon,
reported here for the first time, seems to occur only above the darkest
cells of the chromospheric network. We interpret our findings as a
collective non-compressible oscillation of the corona above the whole
cell interior. This oscillation may originate at the chromosphere and
the driving wave may undergo a mode conversion at the top chromosphere,
where the magnetic pressure equals the gas pressure. Our interpretation
cannot be definitive and should be verified with more data.
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Title: A solar active region loop compared with a 2D MHD model
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2005A&A...434.1155G Altcode: 2005astro.ph..3694G
We analyzed a coronal loop observed with the Normal Incidence
Spectrometer (NIS), which is part of the Coronal Diagnostic Spectrometer
(CDS) on board the Solar and Heliospheric Observatory (SOHO). The
measured Doppler shifts and proper motions along the selected loop
strongly indicate unidirectional flows. Analysing the Emission Measure
Curves of the observed spectral lines, we estimated that the temperature
along the loop was about 380 000 K. We adapted a solution of the ideal
MHD steady equations to our set of measurements. The derived energy
balance along the loop, as well as the advantages/disadvantages of
this MHD model for understanding the characteristics of solar coronal
loops are discussed.
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Title: Oscillations Over a Supergranular Cell Observed with SUMER
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2004ESASP.575..131G Altcode: 2004soho...15..131G
No abstract at ADS
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Title: Study of a Solar Active Region Loop Using EUV Spectra
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2004hell.conf...25G Altcode:
No abstract at ADS
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Title: 2D MHD Modelling of Heated Coronal Loops Compared to TRACE
Observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2004hell.conf...31P Altcode:
No abstract at ADS
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Title: Comparing 3D observations and magnetic field extrapolation
for helicity budget, coronal physics understanding and space weather
forecasting
Authors: Portier-Fozzani, F.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.
2004cosp...35.2683P Altcode: 2004cosp.meet.2683P
We compared parameters obtained by 3D stereoscopic methods with
magnetic extrapolations in 2 different cases : 1/ We studied the long
scale variation of the morphology of an active region during several
days. The active region was selected from movies we made using EIT
images in a transition region line (171 A) for all the years of the
SOHO mission (1996 - 2003). The morphology of the magnetic loops,
using force-free magnetic field extrapolation, were compared with
the 3D parameters found by stereoscopic analysis. Conclusions for
magnetic helicity budget versus the dynamic variations are derived. 2/
We studied variations encontered after a flare while a sigmoid (made
of several twisted loops) was transformed into an arcade. The change
of topology was attributed to a reconnection of the sigmoid with new
emerging active region loops, and the 3D information obtained with
SOHO/EIT 195A was compared with magnetic field extrapolation. These two
analyses give us information for a better coronal physics understanding,
criteria for stability and thus space weather forecasting.
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Title: 2D MHD modelling of compressible and heated coronal loops
obtained via nonlinear separation of variables and compared to TRACE
and SoHO observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2003A&A...409.1065P Altcode: 2003astro.ph..3373P
An analytical MHD model of coronal loops with compressible flows and
including heating is compared to observational data. The model is
constructed via a systematic nonlinear separation of the variables
method used to calculate several classes of exact MHD equilibria in
Cartesian geometry and uniform gravity. By choosing a particularly
versatile solution class with a large parameter space we are able to
calculate models whose loop length, shape, plasma density, temperature
and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS
and SoHO/SUMER. Synthetic emission profiles are also calculated and
fitted to the observed emission patterns. An analytical discussion is
given of the two-dimenional balance of the Lorentz force and the gas
pressure gradient, gravity and inertial forces acting along and across
the loop. These models are the first to include a fully consistent
description of the magnetic field, 2D geometry, plasma density and
temperature, flow velocity and thermodynamics of loops. The consistently
calculated heating profiles which are largely dominated by radiative
losses and concentrated at the footpoints are influenced by the flow
and are asymmetric, being biased towards the upflow footpoint.
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Title: Sizes of quiet Sun transition region structures
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2003A&A...408..743G Altcode:
We studied the morphology of the transition region of the quiet Sun,
with data obtained by the Solar Ultraviolet Measurements of Emitted
Radiation spectrometer (SUMER) and the Extreme-Ultraviolet Imaging
Telescope (EIT), in September 1996. We analyzed lines emitted in the
chromosphere, the low transition region and the low corona. We computed
the mean autocorrelation function for the radiance images in order
to estimate the characteristic size of the features present in the
transition region. Moreover different estimate McCly-mont. Moreover,
we calculated autocorrelation functions for the dopplergrams and
line width images deduced from the SUMER data. In addition to the
line core of the C IV line, we investigated a broader tail component,
whose origin is still unclear. We found that the size of the bright
radiance features is always larger than the size of the structures
of the dopplergrams and Doppler widths, at any altitude. The network
features seem to diminish at a temperature around 10<SUP>5</SUP> K,
due to the thermodynamic properties of the transition region. The mean
size of the structures of the tail component radiance is smaller than
the one of the core radiance.
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Title: Two-component structure of the solar transition region
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2002ESASP.506..625G Altcode: 2002svco.conf..625G; 2002ESPM...10..625G
We studied the morphology of the quiet Sun in the Transition Region,
using observations of the SUMER spectrograph in three emission lines (Si
II, 1533 Å, C IV, 1548 Å and Ne VIII, 770 Å), emitted respectively
in the chromosphere, the low transition region and the low corona, as
well as EIT images in the four passbands of the instrument. We computed
the mean autocorrelation function for the intensity images in order
to estimate the characteristic size of the features present in the
Transition Region. Moreover, we deduced autocorrelation functions for
the Dopplergrams and line width images. Specifically, using images of
the C IV emission lines tail component, as this is estimated by a double
Gaussian fit, we studied the morphology of the coronal funnels, as they
appear at 10<SUP>5</SUP>K (formation temperature of the C IV line). We
found that the size of the intensity bright features is always larger
than the size of the structures of the dopplergrams and dopplerwidths,
at all altitudes. The mean size of the structures of the tail component
intensity is smaller than the one of the core intensity component.
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Title: Exact 2D MHD modelling of coronal loops observed with TRACE
and CDS including comprssible flow and heating.
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H.; Tsinganos, K.
2002ESASP.506..713P Altcode: 2002ESPM...10..713P; 2002svco.conf..713P
We present exact models of coronal loops including heating, which can be
compared quantitatively to observational data. The systematic nonlinear
separation of variables method used to calculate the equilibria has
already been developed in spherical geometry and in Cartesian geometry
generalising many known solutions. Using a particularly versatile
solution class in Cartesian geometry with a large parameter space we
are able to calculate models whose loop length, loop shape, plasma
density, temperature and velocity profiles are fitted to loops observed
with TRACE and with SoHO CDS, while consistent heating profiles are
also given.
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Title: Exact 2D MHD modelling of coronal loops observed with TRACE
including compressible flow and heating
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H.; Tsinganos, K.
2002ESASP.505..149P Altcode: 2002IAUCo.188..149P; 2002solm.conf..149P
We present exact models of coronal loops including heating, which can be
compared quantitatively to observational data. The systematic nonlinear
separation of variables method used to calculate the equilibria has
already been developed in spherical geometry and in Cartesian geometry
generalising many known solutions. Using a particularly versatile
solution class in Cartesian geometry with a large parameter space we
are able to calculate models whose loop length, loop shape, plasma
density, temperature and velocity profiles are fitted to loops observed
with TRACE and with SoHO CDS, while consistent heating profiles are
also given.
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Title: Study of coronal loops using TRACE and SOHO
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Nindos, A.;
Alissandrakis, C.; Tsiropoula, G.; Vial, J. -C.
2002ESASP.505..417G Altcode: 2002solm.conf..417G; 2002IAUCo.188..417G
We analysed coronal loops observed in many spectral lines from TRACE and
SUMER, at the active region NOAA 8541, on May 15, 1999. For the loops,
which are identified in a number of wavelengths, we try to combine
the good temporal and spatial resolution of TRACE with the spectral
information obtained by the SUMER rasters, in order to determine some
physical parameters such as temperature, flow velocity and electron
density. The morphology of the magnetic field of the loops is also
approximated by a force-free extrapolation of the photospheric magnetic
field, measured with MDI, and is compared to the loop morphology.
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Title: Multiwavelength analysis of an active region observed with
SOHO and TRACE
Authors: Dara, H. C.; Gontikakis, C.; Zachariadis, Th.; Tsiropoula,
G.; Alissandrakis, C. E.; Vial, J. -C.
2002ESASP.477...95D Altcode: 2002scsw.conf...95D
We study the solar active region NOAA 8541, observed on May 15, 1999,
from 13:00 to 15:00 UT, with the instruments aboard SOHO (SUMER,
CDS, MDI) and TRACE. The SOHO observations produced a set of raster
scans of the region with a field of view of 159"×120" for SUMER and
of 244"×240" for CDS. TRACE gave a sequence of high time resolution
images for a much larger (510"×510") field of view, in several spectral
passbands, while MDI gave the photospheric full disk longitudinal
magnetic field. The data set mainly covers the transition region and
the low corona. We have used the data to construct intensity, velocity
and magnetic field maps of the region. The comparison of the intensity
images in various wavelengths with the velocity images, as well as
with the magnetic field, gives us information about the dynamical
characteristics of the observed features and their relationship to
the photospheric magnetic field.
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Title: Relation between intensity and Dopplershifts in the quiet
Sun transition region
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Alissandrakis,
C. E.; Vial, J. -C.
2002ESASP.477..107G Altcode: 2002scsw.conf..107G
In this work we used CDS and SUMER (SOHO) observations of a quiet
region (41°W 41°N) on the 28th of July 1996. The study of the
histograms of the velocities corresponding to the network and the
internetwork regions, shows that the network plasma is redshifted
with respect to the internetwork one. The shifts in the histograms
are small, but they are systematic for all lines. It should be
mentioned that the internetwork is also statistically redshifted,
with the exception of the He I line. In the low transition region the
morphology in the intensity images is different from the morphology
in the Dopplergrams. It seems that the network intensity structures
correspond to more than one in the Dopplergrams, since the latter
are quite smaller than the intensity structures. This was verified by
calculating the average autocorrelation function for intensity images
and Dopplergrams of spectral lines from SUMER and CDS. On the average,
the FWHM of the autocorrelation function of the intensity images is
two times larger than that of the corresponding Dopplergram ones.
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Title: Cooling and evacuation of an active region loop complex
observed with TRACE
Authors: Tsiropoula, G.; Gontikakis, C.; Dara, H. C.; Zachariadis,
Th.; Alissandrakis, C.; Vial, J. -C.
2002ESASP.477..179T Altcode: 2002scsw.conf..179T
We analyse the temporal variation of temperature and emission measure
at the top of a coronal loop system observed with the Transition
Region and Coronal Explorer (TRACE). Loops delineate regions of highly
localized heating and are thus typically the focus of coronal heating
theories. The analyzed data consist of observations in the 171 Å and
195 Å passbands taken at a cadence of ~10 min obtained on May 15,
1999 during an observing campaign. The temperature and emission measure
diagnostic is based in the 171 Å / 195 Å filter-ratio technique. The
loop system evacuates after the plasma at the top of the loops has
cooled. Estimates of the timescales of energy losses by radiation
and by conduction clearly suggest that the cooling is mainly due to
radiation losses.
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Title: JOSO national report 2000-2001 - Greece
Authors: Dara, H. C.
2002joso.book...72D Altcode:
No abstract at ADS
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Title: Dopplershifts in the solar transition region
Authors: Gontikakis, C.; Dara, H. C.; Alissandrakis, C. E.;
Zachariadis, Th. G.; Vial, J. -C.
2001A&A...378..257G Altcode:
We study the dynamics of the quiet sun transition region, using
observations obtained with the SOHO CDS/NIS and SUMER spectrographs. We
examine the morphology of the network as a function of temperature and
we compare the intensity features with those of the dopplergrams. The
velocity distributions have a different behaviour for the bright
features which outline the network and the dark ones, located in
the internetwork. A redshift and a smaller standard deviation are
observed for the bright feature distributions relative to the dark
ones. It should be mentioned that the internetwork is also statistically
redshifted, with the exception of the He I line. Velocity distributions
from different lines are compared.
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Title: Multi-wavelength analysis of a solar active region loop system
with SOHO, TRACE and ground-based telescopes
Authors: Tsiropoula, G.; Gontikakis, C.; Dara, H. C.; Zachariadis,
Th. G.; Alissandrakis, C.; Vial, J. -C.
2001hell.confE..40T Altcode:
No abstract at ADS
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Title: Fine Structure of the Quiet Solar Chromosphere: Limb-Crossing
Features
Authors: Zachariadis, Th. G.; Dara, H. C.; Alissandrakis, C. E.;
Koutchmy, S.; Gontikakis, C.
2001SoPh..202...41Z Altcode:
In this article we study chromospheric structures (spicules) crossing
the solar limb in Hα images corrected for limb darkening. This
correction enabled us to view structures both on the disk and beyond the
limb in the same image. The observations were obtained at the Sacramento
Peak Observatory at Hα±0.75 Å. The processed images reveal both
bright and dark (relative to the local background) features crossing
the limb. We also observed bushes (rosettes) crossing the limb, as well
as structures indicating probably arch-shaped mottles beyond the limb.
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Title: Brightness Variations of mg ib Bright Features
Authors: Dara, H. C.; Koutchmy, S.; Zachariadis, Th. G.; Alissandrakis,
C. E.
2001SoPh..198..313D Altcode:
We study the temporal intensity variations of Mgib bright features and
investigate the corresponding Hα velocity pattern. The network bright
features are well visible in the continuum, in images averaged over the
duration of the observations (130 min). We detected `flashing' bright
features, which appear and disappear within two to five minutes, while
the rest of the bright features undergo small variations of either their
shape and/or their intensity. A power spectrum analysis reveals a 10-min
oscillation for approximately half of the stable bright features. The
5-min oscillations are detected mainly at the network boundaries,
where stable bright features are located, while 3-min oscillations
coincide with few bright features, but are also quite intense inside
the network cells. The majority of bright features are associated
with Hα downflows. The downflow is very intense at the location of
`flashing' bright features.
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Title: DEM Study of Selected Quiet Sun Regions
Authors: Gontikakis, C.; Landi, E.; Dara, H. C.; Alissandrakis, C. E.;
Vial, J. -C.
2001IAUS..203..390G Altcode:
In the present work EUV spectra of quiet Sun regions, observed with
the Coronal Diagnostic Spectometer (CDS), are analysed in order to
determine the Differential Emission Measure (DEM) of selected areas
of the field of view. In particular, we study the differences between
the DEM curves of the quiet Sun cell center areas, network areas and
cell-network boundaries. The results are discussed in the light of
theoretical models for the solar upper atmospheres.
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Title: Dynamics of the transition region
Authors: Gontikakis, C.; Dara, H. C.
2000NewAR..44..599G Altcode:
In this review, we consider the problem of the apparent redshifts of
the UV lines in the transition region and review the basic observations
made over the last decades, especially the observations of the last
few years from satellite observatories. Moreover, we revise the most
popular theoretical explanations for the motions in the transition
region. This review is a contribution to the understanding of the
physical processes in this important layer of the solar atmosphere
and it points out the pending problems.
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Title: Structure and Dynamics in the Transition Region
Authors: Gontikakis, C. P.; Dara, H. C.; Alissandrakis, C. E.;
Zachariadis, Th. G.; Vial, J. -C.; Bastian, T.; Chiuderi Drago, F.
1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G
No abstract at ADS
---------------------------------------------------------
Title: Erratum: "Fine structure of the solar chromosphere: arch-shaped
mottles" [Sol. Phys., Vol. 184, No. 1, p. 77 - 86 (Jan 1999)].
Authors: Zachariadis, T. G.; Georgakilas, A. A.; Koutchmy, S.; Dara,
H. C.; Alissandrakis, C. E.
1999SoPh..187..227Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mg bright points and the corresponding velocity pattern.
Authors: Dara, H. C.; Zachariadis, T.; Alissandrakis, C.; Koutchmy, S.
1999joso.proc..135D Altcode:
The authors study the temporal intensity variations of the Mg b1
bright points, in an effort to detect oscillations and investigate
the corresponding Hα velocity pattern. Apart from the "flashing
bright points", bright points which appear and disappear within two to
three minutes, the rest of the bright points undergo small intensity
variations. Power spectrum analysis revealed a 10-min oscillation
for half of the stable bright points. The 5-min oscillations are
detected mainly at sites where there are no bright points, while 3-min
oscillations coincide with some of the bright points. The majority of
the bright points are associated with Hα downflows. The downflow is
very intense at the "flashing bright points".
---------------------------------------------------------
Title: Fine structure of the Solar Chromosphere: Arch-Shaped Mottles
Authors: Zachariadis, Th. G.; Georgakilas, A. A.; Koutchmy, S.;
Alissandrakis, C. E.; Dara, H. C.
1999SoPh..184...77Z Altcode:
We analyze a time series of high resolution observations near the solar
limb, obtained in Hα and the Mg b1 line. We identified arch-shaped dark
mottles, which are thin, faint Hα structures observable under very
good seeing conditions, best seen in Hα +0.75 Å. Their mean length
is about 15”, their mean height about 6” and indicative lifetimes
is of the order of 5 min. They show negative (away from the observer)
line-of-sight velocities. A possible interpretation is that material
flows from the apex towards the feet of the arches.
---------------------------------------------------------
Title: Properties of Hα spicules from disk and limb high-resolution
observations
Authors: Dara, H. C.; Koutchmy, S.; Suematsu, Y.
1998ESASP.421..255D Altcode: 1998sjcp.conf..255D
No abstract at ADS
---------------------------------------------------------
Title: Spicules and Macrospicules: Simultaneous Hα and He II (304
Å) Observations
Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis,
C. E.; Koutchmy, S.; Delannée, C.; Delaboudinière, J. -P.; Hochedez,
J. -F.
1998ASPC..155..376G Altcode: 1998sasp.conf..376G
No abstract at ADS
---------------------------------------------------------
Title: Polar jets and plasmoids:Results from JOP 57
Authors: Delannee, C.; Koutchmy, S.; Delaboudiniere, J. -P.; Hochedez,
J. -F.; Vial, J. -C.; Dara, H.; Georgakilas, A.
1998ESASP.421..129D Altcode: 1998sjcp.conf..129D
No abstract at ADS
---------------------------------------------------------
Title: Kinematics of Hα spicules from near-limb observations.
Authors: Zachariadis, T. G.; Georgakilas, A. A.; Dara, H. C.;
Alissandrakis, C. E.; Koutchmy, S.
1998joso.proc..126Z Altcode:
The authors have studied the behavior of dark mottles near the solar
limb. After the corrections for flat field and limb darkening the
authors can identify spicules crossing the solar limb. Most of the
mottles initially show an ascending velocity which gradually turns
to descending. Moreover, the authors observe a group of dark mottles
which have the shape of arches. One of the footpoints of the arch is
in a chromospheric network cell, while the other in a nearby rosette.
---------------------------------------------------------
Title: Flashing Mgb1 bright points.
Authors: Dara, H. C.; Georgakilas, A.; Zachariadis, T.; Alissandrakis,
C.; Koutchmy, S.
1998joso.proc..124D Altcode:
High resolution observations, obtained at the Sacramento Peak
Observatory near the solar limb, are analysed for the study of
the evolution of Mgb1 (5183.22 Å) bright points, as well as their
association to bright features in the continuum (5256.40 Å) and Hα
wings (±0.75 Å). The authors have detected stable bright points from
the beginning to the end of the time series which hardly move and
whose shape undergoes small changes, bright points whose brightness
increases and decreases periodically, and bright points which suddenly
appear and disappear within one or two minutes.
---------------------------------------------------------
Title: Magnetic and velocity field in association with Ellerman bombs.
Authors: Dara, H. C.; Alissandrakis, C. E.; Zachariadis, T. G.;
Georgakilas, A. A.
1997A&A...322..653D Altcode:
We present two dimensional maps of the longitudinal magnetic field,
obtained in the CaI 6103Å line, as well as line of sight velocity maps
in Hα for an active region. The observations were obtained with the
Tower telescope of the Sacramento Peak Observatory and the Universal
Birefrigent Filter, with circular polarization measurements. The
comparison of the photographs in the wing of Hα, where the Ellerman
bombs (EBs) are best observed, with the corresponding magnetic maps
shows that very few EBs are associated with small scale magnetic
elements, while the great majority, especially the brighter ones,
appear at the boundaries of magnetic features. The comparison of
EBs with the velocity maps in the wings of Hα shows that most of
them coincide with blue wing intensity excess; near the brightest EBs
ascending motions are observed. We also found a very good coincidence
of EBs with photospheric facular granules.
---------------------------------------------------------
Title: Fine Structure of the solar atmosphere from near-limb
observations in three wavelengths
Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis,
C. E.; Koutchmy, S.
1997SoPh..172..133G Altcode: 1997ESPM....8..133G
We analyze a time series of high-resolution observations near the
limb, in the continuum, in the Mg b<SUB>1</SUB> line (-0.4 Å off
line center), and in the wings of Hα (+0.75 Å). The observations
were obtained with a CCD camera at the Vacuum Tower telescope of
the Sacramento Peak Observatory. We study the association of facular
points, as they appear at different heights of the solar atmosphere
(continuum, b<SUB>1</SUB> - 0.4 Å, Hα + 0.75 Å) with the feet of
fine dark mottles.
---------------------------------------------------------
Title: Polar Jets and Plasmoids: Preliminary Results from JOP 57
Authors: Delannée, C.; Koutchmy, S.; Delaboudiniè, J. -P.; Hochedez,
J. -F.; Vial, J. -C.; Dara, H.; Georgakilas, A.
1997ESASP.404..327D Altcode: 1997cswn.conf..327D
No abstract at ADS
---------------------------------------------------------
Title: The Magnetic Field of a Bipolar Sunspot Region in the
Photosphere and the Chromosphere
Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E.
1994emsp.conf...61D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photospheric and chromospheric magnetic field structure of
a bipolar sunspot region
Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E.
1993A&A...277..648D Altcode:
We present high resolution magnetograms of a large split spot and an
opposite polarity pore in the 6102.7 Å Ca I photospheric line and in
Hα and we discuss their fine structure. We found important differences
between the photospheric and chromospheric magnetic field. Although
the large spot was apparently unipolar at the photospheric level, we
detected an opposite polarity region at the chromospheric level. We
also found that there was hardly any trace of the pore magnetic field
in the Hα magnetogram. These results cannot be interpreted in terms
of constant-α magnetic fields. Finally we confirm the occurrence
of filamentary magnetic structures in Hα at the location of the
superpenumbra of the large spot.
---------------------------------------------------------
Title: Study of small scale magnetic flux and the corresponding
velocity pattern
Authors: Alissandrakis, C. E.; Dara, H. C.; Koutchmy, S.
1991A&A...249..533A Altcode:
Two-dimensional maps of the longitudinal magnetic field and the
line of sight velocity of two small-scale magnetic regions in
the quiet sun, using a one-hour time sequence of filtergrams in
the magnetically sensitive lambda 6103 A CaI line (with circular
polarization measurements) obtained with the Sacramento Peak Vacuum
Tower Telescope and the universal filter, are calculated. After the
elimination of the effect of the 5-minute photospheric oscillations,
downflows smaller than 300 m/s are found in both magnetic regions. The
magnetic field intensity and the velocity are weakly correlated:
high values of the magnetic field tend to occur in regions of small
negative velocities. The magnetic field intensity increased during
the observations, while some magnetic peaks moved with respect to
others. The difference of the magnetic field intensity measured in
the blue and the red wing indicates a line asymmetry; this asymmetry
was found to be independent of the line of sight velocity.
---------------------------------------------------------
Title: Small scale magnetic field mapping with high temporal
resolution.
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1991sopo.work..257D Altcode:
Two dimensional maps of the longitudinal magnetic field can be readily
calculated for small-scale magnetic regions. Here the authors use
an one-hour time sequence of high spatial resolution (0.75 arc sec)
filtergrams, obtained at the Sacramento Peak Vacuum Tower Telescope with
the universal filter (UBF) in the wings of the magnetically sensitive
λ 6103 Ca I line. The time difference between two magnetic maps is
32 sec. The measurements of the magnetic field in the blue wing are
about 40% higher than in the red wing.
---------------------------------------------------------
Title: Erratum: Velocity Pattern of Small Scale Magnetic Fields
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990SoPh..128..431D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Velocity pattern of small scale magnetic fields
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990SoPh..126..403D Altcode:
High resolution observations of horizontal proper motions, as well
as vertical Doppler velocities measured over two selected regions
of small scale magnetic elements show a coherent behaviour. In a
region with two opposite polarities, approching with a velocity of
0.4 km s<SUP>-1</SUP>, the material in between moves downwards with
a velocity of 0.10 to 0.45 km s<SUP>-1</SUP>; while in a region with
two peaks of the same polarity, moving apart with a velocity of 0.3 km
s<SUP>-1</SUP>, the material in between moves predominantly upwards,
with a velocity of up to 0.3 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Small Scale Motions Over Concentrated Magnetic Field Regions
of the Quiet Sun
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990IAUS..138..153D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Small-scale motions over concentrated magnetic regions of
the quiet Sun
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1987SoPh..109...19D Altcode:
We have used a 5.5 min time-sequence of spectra in the Fe I lines
λ5576 (magnetically insensitive), λ6301.5 and λ6302.5 (magnetically
sensitive) to study the association of concentrated magnetic regions
and velocity in the quiet Sun. After the elimination of photospheric
oscillations we found downflows of 100-300 m s <SUP>−1</SUP>,
displaced by about 2″ from the peaks of the magnetic field; this
velocity is comparable to downflow velocity associated with the
granulation and of the same order or smaller than the oscillation
amplitude. Quasi-periodic time variations of the vertical component
of the magnetic field up to ± 40% were also found with a period near
250 s, close to the values found for the velocity field. Finally we
report a possible association of intensity maxima at the line center
with peaks of the oscillation amplitude.
---------------------------------------------------------
Title: Ηλιακές χρωμοσφαιρικές
αναρρίψεις
---------------------------------------------------------
Title: Ηλιακές
χρωμοσφαιρικές αναρρίψεις
---------------------------------------------------------
Title: Solar
chromospheric surges;
Authors: Dara, Helen
1982PhDT........75D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On Time Variations of the Chromospheric Network
Authors: Dara, H. C.; Macris, C. J.
1980BAICz..31..364D Altcode:
A study of the variation of the chromospheric network during the 19th
solar cycle is presented. A change is found of the relative density of
the small emission centres of the K(232) solar chromosphere (flocculi)
with a maximum near the time of the solar activity maximum and a
minimum about one year after the solar activity minimum.
---------------------------------------------------------
Title: Study of the motion of three solar prominences.
Authors: Dara, H. C.; Macris, C. J.
1978AkAtP..52..414D Altcode:
The motion of three characteristic solar prominences observed on
April 14, 1967, August 19, 1967, and November 24, 1967 was studied
by measuring their heights and their velocity changes. The maximum
ascending velocities of 150 km/sec, 30 km/sec, and 170 km/sec are
in agreement with measurements made by other observers. None of the
prominences reached an escape velocity. The plots of velocity versus
time for all points of the prominences are smooth, and no abrupt
accelerations were noted. This is in agreement with Pan Puh's (1939)
conjecture that there are no abrupt changes of velocity in prominence
plasma, which shows that Pettit's laws for eruptive prominences are
not valid.
---------------------------------------------------------
Title: Comparison of the dimensions of the flocculi at periods of
maximum and minimum solar activity.
Authors: Dara, H. C.; Macris, C. J.; Zachariadis, T. G.
1975AkAtP..50..391D Altcode:
Measurements of the dimensions of solar flocculi were taken and
compared for solar activity maxima and minima, and the relative
frequency distributions are cited. The measurements show a 7% increase
in floccular dimensions from solar activity minimum to solar activity
maximum, with large flocculi predominant during maxima and smaller
flocculi more numerous during minima. Statistical tests show the
7% increase in floccular size to be significant. A few short of
600 flocculi were studied for the maxima and for the minima, with
measurements taken in the W-E direction at 55 deg heliographic latitude.
