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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.
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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.