Author name code: christopoulou ADS astronomy entries on 2022-09-14 author:"Christopoulou, E.B." ------------------------------------------------------------------------ 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 Bibcode: 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. Title: 120 Minima timings of eclipsing binaries Authors: Palafouta, S.; Gazeas, K.; Christopoulou, E.; Bakogianni, V.; Dervou, M.; Loukaidou, G. Bibcode: 2017IBVS.6218....1P Altcode: We present 120 times of minima of 8 eclipsing binaries. Title: Modeling of Detached Binary Systems in LMC from OGLE III Authors: Theodosiou, A.; Christopoulou, E.; Papageorgiou, A. Bibcode: 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. Title: The first comprehensive photometric study of the overcontact binary TY Boo Authors: Christopoulou, E. Bibcode: 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. Title: CCD photometric study of the misclassified binary system V380 Cas Authors: Papageorgiou, A.; Christopoulou, E. Bibcode: 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. Title: Promises and Pitfalls of Solar Hα Zeeman Spectropolarimetry Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H. Bibcode: 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. Title: Wavelet Analysis of Umbral Oscillations Authors: Christopoulou, E. B.; Skodras, A.; Georgakilas, A. A.; Koutchmy, S. Bibcode: 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.

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. Title: Chromospheric Evershed flow Authors: Georgakilas, A. A.; Christopoulou, E. B.; Skodras, A.; Koutchmy, S. Bibcode: 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.

Based on observations performed on the NSO/SPO Richard B. Dunn Solar Telescope (DST). Title: Temporal Behavior of the Evershed Effect Authors: Georgakilas, A. A.; Christopoulou, E. B. Bibcode: 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-1. 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.

Based on observations performed on the NSO/SPO Richard B. Dunn Solar Telescope (DST). Title: Simultaneous Chromospheric and Photospheric Spectropolarimetry of a Sunspot Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H. Bibcode: 2003ASPC..286..227B Altcode: 2003ctmf.conf..227B No abstract at ADS Title: Ultraviolet Observations of Periodic Annular Intensity Fluctuations Propagating around Sunspots Authors: Georgakilas, A. A.; Muglach, K.; Christopoulou, E. B. Bibcode: 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-1, 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. Title: Multi-wavelength study of Network Bright Points near the limb* Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S. Bibcode: 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 b1−0.4 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. Title: Oscillations and running waves observed in sunspots. III. Multilayer study Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S. Bibcode: 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 180o, 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-1. 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) Title: Polar surges and macrospicules. II. Dynamics of an eruptive event from off-limb observations Authors: Georgakilas, A. A.; Koutchmy, S.; Christopoulou, E. B. Bibcode: 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. 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. Bibcode: 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. Title: Oscillations and running waves observed in sunspots: Analysis of an extended sample of sunspots Authors: Georgakilas, A. A.; Christopoulou, E. B.; Zirin, H. Bibcode: 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-1. 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. Title: Oscillations and running waves observed in sunspots. II. Photospheric waves Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S. Bibcode: 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-1 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). Title: Oscillations and running waves observed in sunspots Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S. Bibcode: 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-1; 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. Title: Multiwavelength Observations of Ellerman Bombs Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S. Bibcode: 1999ESASP.448..279G Altcode: 1999mfsp.conf..279G; 1999ESPM....9..279G No abstract at ADS Title: Fine Structure of the Solar Chromosphere: Dynamics of Spicules and Fine Dark Mottles Authors: Georgakilas, A. A.; Christopoulou, E. B.; Koutchmy, S. Bibcode: 1999ESASP.448..285G Altcode: 1999mfsp.conf..285G; 1999ESPM....9..285G No abstract at ADS Title: New Results about Running Penumbral Waves Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S. Bibcode: 1999ESASP.448..245C Altcode: 1999mfsp.conf..245C; 1999ESPM....9..245C No abstract at ADS Title: Chromospheric Mass Motions Associated with an Emerging Flux Region Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, S. Bibcode: 1999ESASP.448..593C Altcode: 1999ESPM....9..593C; 1999mfsp.conf..593C No abstract at ADS Title: Dynamics and Nature of Macrospicules Authors: Georgakilas, A. A.; Koutchmy, S.; Christopoulou, E. B. Bibcode: 1999ESASP.448..291G Altcode: 1999mfsp.conf..291G; 1999ESPM....9..291G No abstract at ADS Title: Running Penumbral Waves in Sunspots Authors: Christopoulou, E. B.; Georgakilas, A. A.; Koutchmy, Serge Bibcode: 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 s1; and the average period is about 190 sec. 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. Bibcode: 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. 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. Bibcode: 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.