Author name code: georgakilas ADS astronomy entries on 2022-09-14 author:"Georgakilas, A.A." ------------------------------------------------------------------------ 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: 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: 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. Bibcode: 1999SoPh..187..227Z Altcode: No abstract at ADS Title: Polar surges and macrospicules: simultaneous Hα and He BT II 304 Angstroms observations Authors: Georgakilas, A. A.; Koutchmy, S.; Alissandrakis, C. E. Bibcode: 1999A&A...341..610G Altcode: We compare simultaneous sequences of Hα and He ii 304 Angstroms images near the solar limb. We propose to distinguish polar surges and giant spicules (macrospicules), among the He ii structures observed beyond the solar limb. Polar surges have a complex structure when observed in Hα and an eruptive nature, reminiscent of normal surges in small scale. Giant spicules are simple spikes, very narrow compared to surges. They don't go as high as polar surges and apparently have shorter lifetimes. We found that most polar surges and giant spicules observed in He ii are associated with Hα spikes; however there were cases without any corresponding Hα structure. Furthermore we observed Hα spikes that did not have He ii counterparts. We analyze an example of a polar surge that appeared in He ii well before its appearance in Hα and remained visible for a longer time. Most polar surges that we observed showed a similar behavior. 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. Bibcode: 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: The prolate solar chromosphere Authors: Auchere, F.; Boulade, S.; Koutchmy, S.; Smartt, R. N.; Delaboudiniere, J. P.; Georgakilas, A.; Gurman, J. B.; Artzner, G. E. Bibcode: 1998A&A...336L..57A Altcode: We present a comparative analysis of the chromospheric solar limb prolateness, using strictly simultaneous H_alpha ground-based observations and Heriptsize{II} space-based observations. The typical prolateness is found to be Delta D/D=5.5*E(-3) in Heriptsize{II} and 1.2*E(-3) in H_alpha . The first measurements in the 30.4 nm Heriptsize{II} line over a period of two years, as well as coronal data, are discussed to explore further the origin of the prolateness and its possible consequences. Title: About polar ejection events and surges Authors: Loucif, M. L.; Koutchmy, S.; Stellmacher, G.; Georgakilas, A.; Bocchialini, K.; Delaboudinière, J. -P. Bibcode: 1998ESASP.421..299L Altcode: 1998sjcp.conf..299L 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 b1 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, b1 - 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. Bibcode: 1997ESASP.404..327D Altcode: 1997cswn.conf..327D No abstract at ADS Title: Evolution of an Active Region and Associated Hα Arch Structures Authors: Georgakilas, A. A.; Zachariadis, Th. G.; Alissandrakis, C. E. Bibcode: 1993SoPh..146..241G Altcode: We have studied the early stages of development of two adjacent active regions observed at the center and the wings of Hα for six days. From the growth of spots and arch structures we found that periods of slow flux emergence were followed by periods of vigorous flux emergence. We observed arch filaments covering an appreciable range of sizes (from a length of about 27 000 km and a height of 2000-3000 km to a length of 45 000 km and a height of about 15 000 km). Individual arch filaments within the same arcade sometimes have different inclinations of their planes with respect to the vertical. We observed isolated cases of arches crossing each other at an angle of ∼45°. During their early stages arch filament systems are short and they expand at a rate of about 0.8 km s−1. The rate of growth of arch filament systems is faster when the orientation of the flux tubes is nearly parallel to the equator. Our observations suggest that the early part of the evolution of individual arch filaments in a grown system is not visible; however, in a few cases we observed arch filaments appearing as dark features near one footpoint and expanding towards the other, with a mean velocity of about 30 km s−1. Title: Time evolution of arch filaments Authors: Tsiropoula, G.; Georgakilas, A. A.; Alissandrakis, C. E.; Mein, P. Bibcode: 1992A&A...262..587T Altcode: We have studied the spatial structure and the temporal evolution of the velocity in active region arch filaments. The observations were obtained with the Multichannel Double Pass Spectrograph operating in H-alpha at the Pic du Midi Observatory. Line profiles were reconstructed and monochromatic images and Doppler shift velocities were derived over a 2D FOV. Using Beckers' (1964) cloud model we derived physical parameters, in particular the line of sight velocity. The arches showed the 'classical' type of motion, with material moving towards the observer near the apex and away from the observer near the footpoints. Assuming a symmetric loop, we reconstructed the velocity vector along the arch filaments. The results are consistent with the picture where material is draining out of the filament, while the whole structure is ascending. In one case we observed changes in the geometry and the velocity vector; however, other arch filaments did not change appreciably in a time period of about 13.5 min. Title: Mass Motions Under the Influence of the Magnetic Field in Solar Active Regions Authors: Georgakilas, A. A. Bibcode: 1992PhDT.......172G Altcode: We have studied the development of active region arch structures in Hα and mass motions associated with them. Our emphasis was on the spatial structure and the temporal evolution of the velocity. We reconstructed the velocity vector along the arch filaments and computed models for the material flow. A large developing active region (Mc Math No 16315) was observed for six days from September 25 to September 30 1979. The observations were obtained with a Halle Hα filter (0.5 Passband) mounted on the 50 cm "Tourelle" refractor of the Pic-du-Midi Observatory. On the first day of our observations the active region consisted of only two well developed spots, while on the sixth day it had many large spots and its structure was quite complex. We also used observations of an arch filament system (AFS) in NOAA region 4819 made on June 22, 1987 covering a time interval of 15 m 27 sec. These observations were obtained with the Multichannel Double Pass Spectograph (MSDP) mounted on the 50 cm "Tourelle" refractor of the Pic-du-Midi observatory. The MSDP provides 10 simultaneous intensity images of the same region, 0.256 Angstrem apart in the Hα line. We developed a method for the computation of the line-of-sight velocity from photographs at Hα+- 0.5 Angstrem, under the assumption of Becker's cloud model and resonable assumptions about the Doppler width and optical depth of the arches. For the MSDP data we reconstructed line profiles and monochromatic images and we measured Doppler velocities. Furthemore, using Becker's cloud model, we derived physical parameters and in particular the line of sight velocity. from the line of sight component of the velocity we computed the horizontal and the vertical components along the arch filaments. For this purpose we reconstructed the three dimensional shape of the loops, assuming that they were symmetric with respect to the apex. Finally we have studied the motion of an inviscid isothermal plasma under the influence of gravity in an translational symmetric magnetic field. We considered a time varying flow along a steady loop. The solutions of the MHD eauations were compared with the observations of the material flow in the arch filaments. Title: Dynamic Phenomena in the Chromospheric Layer of a Sunspot Authors: Alissandrakis, C. E.; Georgakilas, A. A.; Dialetis, D. Bibcode: 1992SoPh..138...93A Altcode: We have studied running penumbral waves, umbral oscillations, umbral flashes and their interrelations from Hα observations of a large isolated sunspot. Using a subtraction image processing technique we removed the sharp intensity gradient between the umbra and the penumbra and enhanced the low contrast, fine features. We observed running penumbral waves which started in umbral elements with a size of a few arcseconds, covered the umbra and subsequently propagated through the penumbra. The period of the waves was 190 s and the mean propagation velocity was about 15 km s−1. We detected intense brightenings, located between umbral elements from where waves started, which had the characteristics of umbral flashes. There are indications that umbral flashes are related to the propagation of the waves through the umbra and their coupling. The subtraction images also show considerable fine structure in the chromospheric umbra, with size between 0.3″ and 0.8″. Title: Κίνηση πλάσματος κάτω από την επίδραση μαγνητικών πεδίων σε ηλιακά Κέντρα Δράσης Title: Κίνηση πλάσματος κάτω από την επίδραση μαγνητικών πεδίων σε ηλιακά Κέντρα Δράσης Title: Movement of plasma in solar active regions under the influence of the magnetic fields; Authors: Georgakilas, Alexander A. Bibcode: 1992PhDT.......209G Altcode: No abstract at ADS Title: Mass Motions Associated with Hα Active Region Arch Structures Authors: Georgakilas, A. A.; Alissandrakis, C. E.; Zachariadis, Th. G. Bibcode: 1990SoPh..129..277G Altcode: We have studied mass motions associated with active region arch structures from observations of a developing active region near the center of the solar disk. We present a method for the computation of the line-of-sight velocity from photographs at Hα ± 0.5 under the assumption of Beckers' cloud model and reasonable assumptions about the Doppler width and optical depth of the arches. Some arches show motions typical to arch filaments (the material moves towards the observer near the apex of the arch and away from the observer near the footpoints), while in others the velocity field is more complex. Assuming a symmetric loop, we reconstructed the velocity vector along an arch filament. The results are consistent with the picture where material is draining out of the filament while the whole structure is ascending with a velocity near that of the apex, which does not exceed 10 km s−1. The motion is systematically slower than expected from a free-fall model. Title: Note on some problems of optimum mapping Authors: Petropoulos, B.; Georgakilas, A.; Gautier, D.; Coustenis, A.; Bezard, B. Bibcode: 1990AdSpR..10a.109P Altcode: 1990AdSpR..10..109P Physical parameters have been computed based on Voyager observations for the atmosphere of Uranus from 1 to 140 km altitude above the cloud tops. They include temperature, pressure, density, number density, viscosity, sound speed, pressure scale, mean free path, density scale, and mean particle velocity. Thermal profiles deduced from radio occultation measurements at about 6-7 deg latitude are also considered. Title: The atmosphere of Titan. Authors: Petropoulos, B.; Georgakilas, A. Bibcode: 1988imdk.conf..295P Altcode: The authors summarize the basic astronomical parameters which characterize Titan and present a critial review of Voyager's measurements, on the basis of which they calculate parameters of Titan's atmosphere. These parameters can be used in order to study the meteorology of Titan and to define the design requirements for the construction and testing of future artificial satellites, aiming at Titan's exploration. Title: Physical parameters for the Saturn atmosphere computed by using voyager data Authors: Petropoulos, B.; Georgakilas, A. Bibcode: 1987AdSpR...7l..33P Altcode: 1987AdSpR...7...33P We have computed the following physical parameters for the atmosphere of Saturn: 1) Temperature, 2) Pressure, 3) Density, 4) Density Scale, 5) Number Density, 6) Viscosity, 7) Mean Pressure Scale, 8) Mean Particle Velocity, 9) Mean Collisional Frequency, 10) Columnar Mass, and 11)Mean Free Path. Voyager 2 measurements have been used in order to compute the above parameters from 0 to 300 km above the top of the clouds. From 0 to 300 km below the top of the clouds, ground based measurements have been used. Title: A reference model for the atmosphere of Titan. Authors: Petropoulos, B.; Georgakilas, A. A. Bibcode: 1987PAICz..67..291P Altcode: 1987eram....2..291P The authors have computed the following physical parameters for the atmosphere of Titan, using Voyager's measurements: temperature, pressure, density, speed of sound, density scale, number density, mean free path, viscosity, pressure scale, mean particle velocity, mean collisional frequency, columnar mass.