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Author name code: georgakilas
ADS astronomy entries on 2022-09-14
author:"Georgakilas, A.A." 

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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: 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: 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

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Title: Polar surges and macrospicules: simultaneous Hα and He BT
    II 304 Angstroms observations
Authors: Georgakilas, A. A.; Koutchmy, S.; Alissandrakis, C. E.
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.

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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.

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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.
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.

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Title: About polar ejection events and surges
Authors: Loucif, M. L.; Koutchmy, S.; Stellmacher, G.; Georgakilas,
   A.; Bocchialini, K.; Delaboudinière, J. -P.
1998ESASP.421..299L    Altcode: 1998sjcp.conf..299L
  No abstract at ADS

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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

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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

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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.

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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.

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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.

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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.

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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

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Title: Evolution of an Active Region and Associated Hα Arch
    Structures
Authors: Georgakilas, A. A.; Zachariadis, Th. G.; Alissandrakis, C. E.
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<SUP>−1</SUP>. 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<SUP>−1</SUP>.

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Title: Time evolution of arch filaments
Authors: Tsiropoula, G.; Georgakilas, A. A.; Alissandrakis, C. E.;
   Mein, P.
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.
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.
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<SUP>−1</SUP>. 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.
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.
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<SUP>−1</SUP>. 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.
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.
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.
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.
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.