explanation blue bibcodes open ADS page with paths to full text
Author name code: christopoulou
ADS astronomy entries on 2022-09-14
author:"Christopoulou, E.B."
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Title: Planets In Your Hand (2017-2019)
Authors: Palafouta, Sofia; Gazeas, Kosmas; Christopoulou, Elena;
Karampotsiou, Efsevia; Athanasopoulos, Dimitrios; Konstantinou, Anna;
Tzouganatos, Lefteris; Kefala, Kyriaki; Papadami, Argiro; Trivyza,
Marilia; Skliris, Vasilis; Chliaras, Vangelis
2019EPSC...13.1816P Altcode:
The Planets In Your Hand project was granted by Europlanet in the
frame of Public Engagement Funding Scheme in 2017. It consists of a
portable interactive exhibition of planetary surface models, embedded
in square frames. The project offers the visitors a chance to see,
learn and understand the diversity of the planetary surfaces in our
Solar System, including the different conditions such as temperature,
wind and atmosphere. Since the beginning of the project, the planetary
surfaces have been presented to a wide range of audience, including
visually impaired people, preliminary school and high school students
as well as university researchers.
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Title: 120 Minima timings of eclipsing binaries
Authors: Palafouta, S.; Gazeas, K.; Christopoulou, E.; Bakogianni,
V.; Dervou, M.; Loukaidou, G.
2017IBVS.6218....1P Altcode:
We present 120 times of minima of 8 eclipsing binaries.
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Title: Modeling of Detached Binary Systems in LMC from OGLE III
Authors: Theodosiou, A.; Christopoulou, E.; Papageorgiou, A.
2013hell.confQ..46T Altcode:
Using BVI data from OGLE II and OGLE III survey, we analyze light curves
for detached binary systems with nearly circular orbits in the Large
Magellanic Cloud (LMC). We make use of orbital periods, classification,
empirical relations as well as the distance to the LMC to present
light curve solutions with the software PHOEBE. The assumptions made
for the procedure of light curve solution are thoroughly discussed
and compared with previously determined global stellar parameters of
other detached binaries in LMC from spectroscopic and photometric data.
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Title: The first comprehensive photometric study of the overcontact
binary TY Boo
Authors: Christopoulou, E.
2012hell.conf...25C Altcode:
New precision BVRI light curves of the overcontact binary TY Boo have
been obtained using the 35.5 cm Schmidt-Cassegrain telescope at the
University of Patras, Observatory equipped with an SBIG ST-10XME CCD
made during 2010 and 2011. The complete light curves are analyzed with
the Wilson-Devinney code (PHOEBE) and new geometric and photometric
elements are derived. A period investigation based on all available data
spanning 85 years, shows that there exists a cyclic variation overlaying
a secular period decrease. The presence of a third body is investigated
but also spotted models as resulting from magnetic activity.
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Title: CCD photometric study of the misclassified binary system
V380 Cas
Authors: Papageorgiou, A.; Christopoulou, E.
2012hell.confQ..29P Altcode:
A photometric study of the eclipsing binary V380 Cas is presented for
the first time with the 35.5cm Schmidt-Cassegrain telescope at the
University of Patras, Observatory from June 2009 to July 2010. The
BVRI light curves were observed and 7 new minima were calculated. No
secondary minimum has been detected, indicating that its original
period was wrong. The complete light curves are analyzed with the
Wilson-Devinney code (PHOEBE) and the first photometric solution is
derived. The system is probably a detached one with equal components
and double period, contrary to its previous classification as Algol
type. The first O-C analysis of the system based on all timings of
minima is presented and the first precision ephemeris is estimated. Also
the probability of third light is examined from both O-C and photometric
solution.
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Title: Promises and Pitfalls of Solar Hα Zeeman Spectropolarimetry
Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H.
2004ApJ...606.1233B Altcode:
Zeeman spectropolarimetry of the solar disk in the Hα line is a
daunting, yet promising diagnostic for measuring solar chromospheric
magnetic fields. As a demonstration of its potential we used the
National Solar Observatory (NSO)/High Altitude Observatory (HAO)
Advanced Stokes Polarimeter to obtain simultaneous measurements
of polarimetric signals from the photosphere in the Fe I 6301.5,
6302.5 Å lines and from the chromosphere in Hα. With these
measurements, we explore the promises and pitfalls of Hα Zeeman
spectropolarimetry. Remarkable features observed in Hα include
a reversal of Zeeman polarity in the emission core across flaring
active regions and highly redshifted and smeared Stokes V profiles in
prominences. We reproduce the Hα Zeeman reversals using radiative
transfer diagnostics, and we compare photospheric and chromospheric
line-of-sight magnetic fields.
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Title: Wavelet Analysis of Umbral Oscillations
Authors: Christopoulou, E. B.; Skodras, A.; Georgakilas, A. A.;
Koutchmy, S.
2003ApJ...591..416C Altcode:
We study the temporal behavior of the intensity and velocity
chromospheric umbral oscillations, applying wavelet analysis techniques
to four sets of observations in the Hα line and one set of simultaneous
observations in the Hα and the nonmagnetic Fe I (5576.099 Å) line. The
wavelet and Fourier power spectra of the intensity and the velocity at
chromospheric levels show both 3 and 5 minute oscillations. Oscillations
in the 5 minute band are prominent in the intensity power spectra; they
are significantly reduced in the velocity power spectra. We observe
multiple peaks of closely spaced cospatial frequencies in the 3 minute
band (5-8 mHz). Typically, there are three oscillating modes present:
(1) a major one near 5.5 mHz, (2) a secondary near 6.3 mHz, and (3)
oscillations with time-varying frequencies around 7.5 mHz that are
present for limited time intervals. In the frame of current theories,
the oscillating mode near 5.5 mHz should be considered as a fingerprint
of the photospheric resonator, while the other two modes can be better
explained by the chromospheric resonator. The wavelet spectra show a
dynamic temporal behavior of the 3 minute oscillations. We observed
(1) frequency drifts, (2) modes that are stable over a long time
and then fade away or split up into two oscillation modes, and (3)
suppression of frequencies for short time intervals. This behavior
can be explained by the coupling between modes closely spaced in
frequency or/and by long-term variations of the driving source of the
resonators. <P />Based on observations performed on the National Solar
Observatory/Sacramento Peak Observatory Richard B. Dunn Solar Telescope
(DST) and on the Big Bear Solar Observatory Harold Zirin Telescope.
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Title: Chromospheric Evershed flow
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Skodras, A.;
Koutchmy, S.
2003A&A...403.1123G Altcode:
We studied the chromospheric Evershed flow from filtergrams obtained
at nine wavelengths along the Hα profile. We computed line-of-sight
velocities based on Becker's cloud model and we determined the
components of the flow velocity vector as a function of distance
from the center of the sunspot, assuming an axial symmetry of both
the spot and the flow. We found that the flow velocity decreases
with decreasing height and that the maximum of the velocity shifts
towards the inner penumbral boundary. The flow related to some fibrils
deviates significantly from the average Evershed flow. The profile of
the magnitude of the flow velocity as a function of distance from the
spot center, indicates that the velocity attains its maximum value in
the downstream part of the flow channels (assumed to have the form of a
loop). This behavior can be understood in terms of a critical flow that
pass from subsonic to supersonic near the apex of the loop, attains
its higher velocity at the downstream part of the loop and finally
relaxes to subsonic through a tube shock. We computed the average flow
vector from segmented line-of-sight velocity maps, excluding bright
or dark fibrils alternatively. We found that the radial component of
the velocity does not show a significant difference, but the magnitude
of the vertical component of the velocity related to dark fibrils is
higher than that related to bright fibrils. <P />Based on observations
performed on the NSO/SPO Richard B. Dunn Solar Telescope (DST).
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Title: Temporal Behavior of the Evershed Effect
Authors: Georgakilas, A. A.; Christopoulou, E. B.
2003ApJ...584..509G Altcode:
We study the Evershed flow in the photosphere and the reverse Evershed
flow in the chromosphere from simultaneous observations, giving
emphasis to the temporal evolution of the phenomena. We compute the
components of the velocity vector as a function of distance from the
center of the sunspot, assuming an axial symmetry of both the spot and
the flow. A five-minute oscillatory pattern is obvious in the penumbra
at photospheric level. Our results verify that the velocity of the
Evershed flow has a maximum above the penumbra in the photosphere and
well outside the penumbra in the chromosphere. We find evidence of
temporal variations prominent in the radial component of the average
photospheric velocity with a characteristic timescale of 25 minutes. We
consider a transient siphon flow or a wave superimposed on a steady flow
as possible explanations for the temporal behavior of the photospheric
Evershed flow. The radial component of the chromospheric reverse
Evershed flow shows a repetitive temporal variation with a typical
timescale of 15 minutes. The variation consists of enhanced velocity
amplitudes that propagate to an opposite direction from the flow with a
velocity of about 5-6 km s<SUP>-1</SUP>. This behavior cannot be easily
explained in the frame of a transient flow and strongly suggests that it
is related to the propagation of a wave. We examine the possibility of
its being associated with the propagation of running penumbral waves
in the superpenumbra. The temporal evolution of the line-of-sight
velocity across superpenumbral fibrils presents alterations that can
be associated with a time-dependent flow. However, we also observe
propagating velocity packets that can be associated with a wave. <P
/>Based on observations performed on the NSO/SPO Richard B. Dunn Solar
Telescope (DST).
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Title: Simultaneous Chromospheric and Photospheric Spectropolarimetry
of a Sunspot
Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H.
2003ASPC..286..227B Altcode: 2003ctmf.conf..227B
No abstract at ADS
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Title: Ultraviolet Observations of Periodic Annular Intensity
Fluctuations Propagating around Sunspots
Authors: Georgakilas, A. A.; Muglach, K.; Christopoulou, E. B.
2002ApJ...576..561G Altcode:
We analyze UV observations of an isolated sunspot taken with the
Transition Region and Coronal Explorer. We produced “space/time
slice images,” which reveal periodic, inward-propagating, intensity
fluctuations in the inner photospheric penumbra and periodic,
outward-propagating, annular intensity fluctuations in the outer
penumbra and the area around the sunspot. The average propagation
velocity of the inward- and outward-propagating intensity flows is about
0.5 and 1 km s<SUP>-1</SUP>, respectively, and the distance between
subsequent crests is about 2500 km. Using observations obtained with
the Michelson Doppler Imager, we found propagating magnetic field
enhancements that present a spatial and temporal coherence with the
UV intensity fluctuations. We have not observed any waves similar or
related to the chromospheric 5 minute waves. Based on observations
taken with the Transition Region and Coronal Explorer and the Michelson
Doppler Imager (MDI) on board the Solar and Heliospheric Observatory.
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Title: Multi-wavelength study of Network Bright Points near the limb*
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S.
2001SoPh..202..241G Altcode:
We analyze the morphology of network bright points (NBPs), their
relation to fine dark mottles and their temporal variations,
using observations of a quiet region near the polar solar limb,
obtained during the solar minimum. For our analysis we used an
image-processing method for the selection and identification of
NBPs. Further we constructed 'maximum power frequency' maps computing
the power spectrum for each pixel of a field and selecting the frequency
of maximum spectral power as the most representative oscillating
frequency for this pixel. The morphological analysis of our data
indicates that the enhancement of NBPs is part of a complicated process
that is responsible for the formation of mottles. The analysis of the
lifetimes of NBPs indicates that although the general patterns of NBPs
remain constant for time intervals larger than an hour, the lifetime
of individual bright points is of the order of 16 min. Furthermore,
our results indicate an association of the lifetime of NBPs with that
of mottles. From the analysis of temporal variations we confirm that the
power of NBPs is smaller than the corresponding power for intra-network
points at both the 3-min and the 5-min oscillating modes and in both
the Mg b<SUB>1</SUB>−0.4 Å and the continuum. Our analysis revealed
enhanced power in the 7-min range, which is theoretically considered
a candidate frequency for transverse waves related to NBPs at the base
of the chromosphere (Kalkofen, 1997). However, we consider more likely
that this period is related to the evolution of individual NBPs.
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Title: Oscillations and running waves observed in
sunspots. III. Multilayer study
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S.
2001A&A...375..617C Altcode:
We continue our study of waves and oscillations observed in sunspots
using an improved method for enhancing the waves, giving the opportunity
to identify them and determine their properties in far Hα wings. We
found that the running penumbral waves are observable at least up to
the formation height of the Hα +/- 0.5 Å line, but not in the Hα +/-
0.75 Å or the Fe I+/-0.12 Å. We found a time lag between the waves
in the blue and the red wing of the Hα line corresponding to a phase
shift of 180<SUP>o</SUP>, that indicates a pure Doppler shift of the
line. There is a lag in the propagation of the waves seen at Hα center
and at Hα wings. Also there is a lag in the variation of the umbral
oscillations as they are observed from lower to higher atmospheric
layers. The correlation between umbral oscillations at various
atmospheric heights and running penumbral waves strongly indicates
that the latter are excited by photospheric umbral oscillations and
not the chromospheric ones. We found a new category of photospheric
waves that originate at approximately 0.7 of the distance between the
umbra and the penumbra boundary and propagate beyond the outer penumbra
boundary with a velocity of the order of 3-4 km s<SUP>-1</SUP>. Further,
we found 3 min penumbral oscillations apparent in the inner penumbra
at lower chromospheric layers (far Hα wings). Based on observations
performed on the NSO/SPO Richard B. Dunn Solar Telescope (DST)
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Title: Polar surges and macrospicules. II. Dynamics of an eruptive
event from off-limb observations
Authors: Georgakilas, A. A.; Koutchmy, S.; Christopoulou, E. B.
2001A&A...370..273G Altcode:
We continue our study of polar surges and macrospicules at the
period of solar minimum, analyzing high resolution multiwavelength
limb observations that provide a clearer picture of the dynamical
phenomena occurring well above the chromosphere of a polar cap. The
time sequence of an erupting and impulsive polar event is examined
from the low chromosphere to coronal heights, deriving both proper
motions and Doppler velocities. Our observations suggest that there is
a close association of polar surges with explosive events, supporting
the hypothesis that magnetic reconnection triggered by emerging flux
provides the accelerative mechanism for this polar region event.
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Title: Fine Structure of the Magnetic Chromosphere: Near-Limb Imaging,
Data Processing and Analysis of Spicules and Mottles
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S.
2001SoPh..199...61C Altcode:
The origin and the dynamical evolution of spicules and mottles continue
to be a highly interesting research subject. Using high-resolution
Hα observations obtained with the Dunn Solar Telescope of the
Sacramento Peak Observatory and an image processing technique for
the enhancement of near-limb solar images, we study the dynamics of
spicules and mottles as well as their relation. Our image-processing
technique is based on the correction for the limb darkening and the
use of a directionally sensitive operator, the `MadMax'. The temporal
evolution of characteristic cases of spicules, dark and bright mottles,
indicates an association between them and supports the suggestion that
the magnetic field and probably related forces play a fundamental
role in their generation and dynamics. We present characteristic
cases of fine bright mottles, observable in the Hα far wings, that
appear in close juxtaposition to dark mottles. The phenomenon appears
to be common, suggesting that the velocities derived from marginal
resolution spectroscopic observations could be underestimated. Typical
examples of individual mottles crossing the solar limb further
support the association between spicules and mottles. Finally we
show images of arch-shaped mottles above the limb and especially on
the disk, confirming the existence of chromospheric small loops. Our
image-processing method substantially enhances near-limb observations
and permits an insight into the studies of the very fine chromospheric
structures.
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Title: Oscillations and running waves observed in sunspots: Analysis
of an extended sample of sunspots
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Zirin, H.
2000AAS...197.5109G Altcode: 2000BAAS...32Q1489G
Sunspots show oscillatory behavior, identifiable as intensity and
velocity variations in photospheric and chromospheric lines. In
the chromospheric layers 3 min standing oscillations are dominant
in the umbra (umbral oscillations), while 5 min running waves are
dominant in the penumbra and superpenumbra. Penumbral waves (RPW)
were first observed in Hα by Zirin and Stein (1972) and independently
by Giovanelli (1972). Since their discovery a number of authors have
tried to determine the physical properties of umbral oscillations
and running penumbral waves (like the frequency and the propagation
velocity) and to identify their nature, as well as the relation between
them. In order to clarify discrepancies, due to the analysis of limited
number of sunspots, we present comprehensive results from the study of
8 sunspots observed from the Big Bear Solar Observatory (BBSO) during
1999 and 2000. Our results show that RPW are propagating with a mean
velocity of the order of 15 km s<SUP>-1</SUP>. The fourier analysis
of 'time slice images' (created taking cross sections of every image
of a time series) indicates that there is not a clear relationship
between umbral oscillations and running penumbral waves. This work
was supported by NSF grant ATM-9726147.
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Title: Oscillations and running waves observed in
sunspots. II. Photospheric waves
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S.
2000A&A...363..306G Altcode:
To continue our study of waves related to sunspots in the photosphere,
we analyzed CCD, high resolution sunspot observations obtained
in the Fe i 557.6 nm line. We produced “time slice images" which
reveal inward slow propagating waves in the photospheric penumbra and
outward propagating waves in the area around the sunspot. The phase
velocity of the waves is near 0.5 km s<SUP>-1</SUP> in both cases and
their horizontal wavelength about 2500 km. The waves could be related
either to solar p-modes or to the subphotospheric layer large-scale
convection. Based on observations performed on the NSO/SPO Dunn's
Solar Telescope (DST).
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Title: Oscillations and running waves observed in sunspots
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S.
2000A&A...354..305C Altcode:
In order to study umbral oscillations, running penumbral waves and
the relationship between them, we analyzed CCD, high resolution,
sunspot observations obtained at center and the wings of the Hα line
and the Fe I 5576 Ä line. The UBF filter was used in order to produce
high cadence sequences of filtergrams. Images were processed to remove
the sharp intensity gradient between the umbra and the penumbra. They
show the waves to start out around umbral oscillating elements and to
propagate outwards forming concentric cycles around the elements. The
waves appear to propagate beyond the outer edge of the photospheric
penumbra, in the superpenumbra, where they dilute. Comparing images
in 9 wavelengths along the Hα profile we found out that the waves
are definitely better observed near the Hα center and near the blue
wing -0.35 Ä. This indicates a possible vertical upward mass motion
in the oscillating penumbral structure and that the oscillation is not
symmetric about zero. We found different oscillating modes. Standing
umbral oscillations are dominant in the umbra and inner penumbra;
their frequency is around 6.5 mHz. Similar oscillations are observed
in the penumbra - superpenumbra boundary but with considerably lower
frequency (2 mHz). Oscillations are absent or have reduced magnitude
in the central part of the penumbra. Penumbral waves are running waves
propagating with a constant phase velocity around 13 km s<SUP>-1</SUP>;
their frequency is remaining constant over the whole penumbra, in the
band of 3 mHz. We produced “time slice images" which show, that there
is not a clear relationship between umbral oscillations and running
penumbral waves.
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Title: Multiwavelength Observations of Ellerman Bombs
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S.
1999ESASP.448..279G Altcode: 1999mfsp.conf..279G; 1999ESPM....9..279G
No abstract at ADS
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Title: Fine Structure of the Solar Chromosphere: Dynamics of Spicules
and Fine Dark Mottles
Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S.
1999ESASP.448..285G Altcode: 1999mfsp.conf..285G; 1999ESPM....9..285G
No abstract at ADS
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Title: New Results about Running Penumbral Waves
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S.
1999ESASP.448..245C Altcode: 1999mfsp.conf..245C; 1999ESPM....9..245C
No abstract at ADS
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Title: Chromospheric Mass Motions Associated with an Emerging
Flux Region
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S.
1999ESASP.448..593C Altcode: 1999ESPM....9..593C; 1999mfsp.conf..593C
No abstract at ADS
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Title: Dynamics and Nature of Macrospicules
Authors: Georgakilas, A. A.; Koutchmy, S.; Christopoulou, E. B.
1999ESASP.448..291G Altcode: 1999mfsp.conf..291G; 1999ESPM....9..291G
No abstract at ADS
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Title: Running Penumbral Waves in Sunspots
Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, Serge
1999ASPC..184..103C Altcode:
In order to study running penumbral waves, we analyzed high resolution
sunspot observations obtained at the center and the wings of the
Hα line. The sharp intensity gradient between the umbra and the
penumbra has been removed by using an image processing technique. The
processed images show the waves to start out from the umbral oscillating
elements and to propagate outwards forming concentric circles around the
elements. The propagation velocity is between 6 to 18 km s<SUP>1</SUP>;
and the average period is about 190 sec.
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Title: Two dimensional distribution of physical parameters in dark
mottles assuming constant and non-constant source function
Authors: Tsiropoula, G.; Madi, G.; Christopoulou, E.; Alissandrakis,
G.; Schmieder, B.; Preka-Papadema, P.
1997jena.confE..60T Altcode:
Observations of a rosette region consisting of several dark mottles
located almost at the solar disk centre (N5, W5) were made with the
Multichannel Double Pass (MSDP) spectrograph mounted on the 50 cm
“Tourelle” refractor of the Pic du Midi Observatory, on June 17,
1986. The MSDP having 11 channels provides at every point of the field
of view the profile of the H-alpha line, which is reconstructed from
11 values. The observed contrast profiles of the dark features as a
function of the wavelength are used in terms of Beckers' cloud model
(e.g. assuming that the source function is constant throughout the
feature) in order to derive 4 parameters: the line-of-sight velocity,
the optical depth at line centre, the Doppler width and the source
function. The computation of the four parameters was carried out by an
iterative least square procedure for non linear functions. Allowing
the source function to have a parabolic variation with the optical
depth and using the same iteration procedure the source function at
the centre of the feature and the source function variation factor
are determined. Comparison of the values of the physical parameters
derived under these two different assumptions concerning the source
function is performed. These values are also compared with the values
obtained from multilevel non-LTE models.
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Title: Comparison of line-of-sight velocities of chromospheric
structures derived by three different methods
Authors: Tsiropoula, G.; Christopoulou, E.; Madi, C.; Dialetis, D.;
Mein, P.; Mein, N.
1997jena.confE..61T Altcode:
We have used three different methods to derive line-of-sight velocities
in dark fibrils forming the superpenumbra of an isolated regular sunspot
located near the centre of the solar disk (NOOA/AR 7783). The present
observations were obtained on October 3, 1994. They were performed
with the Multichannel Subtractive Double Pass (MSDP) spectrograph which
operates in H-alpha and it is installed at the focus of the Vacuum Tower
Telescope (VTT) at Tenerife (Canary Islands). This instrument records
a two dimensional field of view on the solar surface with good spatial
and temporal resolution. The observations were made simultaneously in 9
wavelengts, 0.3AA apart in the H-alpha profile. At every pixel of the
2D field of view the line profile can be restored from the measured
values of the intensity in the 9 channels and a third degree spline
interpolation. These profiles were used for the computation of the
line-of-sight velocity by three different methods: (a) the Doppler
shift method, (b) the photographic subtraction method expressing the
“Doppler signal” and (c) the classical Beckers' cloud model. 2D
maps of the velocity are computed, comparison of the values derived
by the 3 different methods is performed and the conditions governing
the validity of the 3 methods are stated.
