alissandrakis

explanation blue bibcodes open ADS page with paths to full text

Author name code: alissandrakis
ADS astronomy entries on 2022-09-14
author:"Alissandrakis, Costas E."

---------------------------------------------------------
Title: The quiet Sun at mm Wavelengths as Seen by ALMA
Authors: Alissandrakis, Costas; Bastian, Timothy; Brajša, Roman
2022arXiv220902569A Altcode:
Solar observations at sub-mm, mm and cm wavelengths offer a
straightforward diagnostic of physical conditions in the solar
atmosphere because they yield measurement of brightness temperature
which, for optically thick features, equals intrinsic temperature -
much unlike solar diagnostics in other spectral ranges. The Atacama
Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a
new, hitherto underexplored, spectral window for studying the enigmatic
solar chromosphere. In this review we discuss initial ALMA studies of
the quiet chromosphere that used both single-dish and compact-array
interferometric observing modes. We present results on the temperature
structure of the chromosphere, comparison with classic empirical models
of the chromosphere, and observations of the chromospheric network and
spicules. Furthermore, we discuss what may be expected in the future,
since the ALMA capabilities continuously expand and improve towards
higher angular resolution, wavelength coverage, and polarization
measurement for magnetometry.

---------------------------------------------------------
Title: The solar atmosphere and kappa distributions
Authors: Alissandrakis, Costas
2022cosp...44.1220A Altcode:
The sun and its environment is an immense plasma physics laboratory,
where the magnetic field plays a crucial role. The magnetic field
is responsible for a large variety of energy release phenomena, in
all observable temporal an spatial scales, from tiny bright points
to huge flares and coronal mass ejections. In the most energetic
phenomena, electrons are accelerated to energies of hundreds of keV,
giving observational signatures in hard X-rays and in the microwave
range, whereas electron beams traveling at 1/3 of the speed of light
in the corona and the solar wind produce plasma emission in metric
and longer wavelengths. In latter case a bump-in-tail distribution is
invoked, while in the first case a power-law distribution on top of
a Maxwellian is usually employed. However, there have been efforts
to describe suprathermal electrons in terms of kappa-distributions,
both in energetic phenomena an in the quiet Sun. In this review I
will give an overview of the solar atmosphere and of phenomena where
kappa-distributions may play a role.

---------------------------------------------------------
Title: Where is the base of the Transition Region? Evidence from
TRACE, SDO, IRIS and ALMA observations
Authors: Alissandrakis, C. E.
2022arXiv220703159A Altcode:
Classic solar models put the Chromosphere-Corona Transition Region
(CCTR} at $\sim2$Mm above the $\tau_{5000} = 1$ level, whereas rMHD
models place it in a wider range of heights. Observational verification
is scarce. We review and discuss recent results from various instruments
and spectral domains. In SDO and TRACE images spicules appear in
emission in the 1600, 1700 and 304 A bands and in absorption in the
EUV bands; the latter is due to photo-ionization of H and He I. At the
shortest available AIA wavelength and taking into account that the
photospheric limb is $\sim0.34$Mm above the $\tau_{5000}=1$ level,
we found that CCTR emission starts at $\sim3.7$Mm; extrapolating
to $\lambda=0$, where there is no chromospheric absorption, we
deduced a height of $3.0\pm 0.5$Mm, above the value of 2.14Mm of
the Avrett &amp; Loeser model. Another indicator of the extent of
the chromosphere is the height of the network structures. Height
differences produce a limbward shift of features with respect to
their counterparts in magnetograms. Using this approach, we measured
heights of $0.14\pm0.04$Mm (at 1700 A), $0.31\pm0.09$Mm (at 1600 A)
and $3.31\pm 0.18$Mm (at 304 A) for the center of the solar disk. A
previously reported possible solar cycle variation is not confirmed. A
third indicator is the position of the limb in the UV, where IRIS
observations of the Mg II triplet lines show that they extend up to
$\sim2.1$Mm above the 2832 A limb, while AIA/SDO images give a limb
height of $1.4 \pm 0.2$Mm (1600 A) and $5.7\pm 0.2$Mm (304 A). Finally,
ALMA mm-$\lambda$ full-disk images provide useful diagnostics, though
not very accurate; values of $2.4\pm0.7$Mm at 1.26mm and $4.2\pm2.5$Mm
at 3mm were obtained. Putting everything together, we conclude that the
average chromosphere extends higher than homogeneous models predict,
but within the range of rMHD models.

---------------------------------------------------------
Title: First detection of metric emission from a solar surge
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bouratzis,
C.; Hillaris, A.
2022A&A...662A..14A Altcode: 2022arXiv220301043A
We report the first detection of metric radio emission from a surge,
observed with the Nançay Radioheliograph (NRH), STEREO, and other
instruments. The emission was observed during the late phase of the
M9 complex event SOL2010-02-012T11:25:00, described in a previous
publication. It was associated with a secondary energy release,
also observed in STEREO 304 Å images, and there was no detectable
soft X-ray emission. The triangulation of the STEREO images allowed
for the identification of the surge with NRH sources near the central
meridian. The radio emission of the surge occurred in two phases and
consisted of two sources, one located near the base of the surge,
apparently at or near the site of energy release, and another in the
upper part of the surge; these were best visible in the frequency
range of 445.0 to about 300 MHz, whereas a spectral component of
a different nature was observed at lower frequencies. Sub-second
time variations were detected in both sources during both phases,
with a 0.2-0.3 s delay of the upper source with respect to the lower,
suggesting superluminal velocities. This effect can be explained if
the emission of the upper source was due to scattering of radiation
from the source at the base of the surge. In addition, the radio
emission showed signs of pulsations and spikes. We discuss possible
emission mechanisms for the slow time variability component of
the lower radio source. Gyrosynchrotron emission reproduced the
characteristics of the observed total intensity spectrum at the
start of the second phase of the event fairly well, but failed to
reproduce the high degree of the observed circular polarization or the
spectra at other instances. On the other hand, type IV-like plasma
emission from the fundamental could explain the high polarization
and the fine structure in the dynamic spectrum; moreover, it gives
projected radio source positions on the plane of the sky, as seen from
STEREO-A, near the base of the surge. Taking all the properties into
consideration, we suggest that type IV-like plasma emission with
a low-intensity gyrosynchrotron component is the most plausible
mechanism. Movie associated to Fig. A.2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243169/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: A first look at the submillimeter Sun with ALMA
Authors: Alissandrakis, C. E.; Bastian, T. S.; Nindos, A.
2022A&A...661L...4A Altcode: 2022arXiv220501008A
We present the first full-disk solar images obtained with the Atacama
Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm;
347 GHz). In spite of the low spatial resolution (21″), several
interesting results were obtained. During our observation, the sun
was practically devoid of active regions. Quiet Sun structures on
the disk are similar to those in Atmospheric Imaging Assembly images
at 1600 Å and 304 Å, after the latter are smoothed to the ALMA
resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm)
images; they are also similar to negative Hα images of equivalent
resolution. Polar coronal holes, which are clearly seen in the 304
Å band and small Hα filaments, are not detectable at 0.86 mm. We
computed the center-to-limb variation of the brightness temperature,
Tb, in Band 7, as well as in Bands 6 and 3, which were
obtained during the same campaign, and we combined them to a unique
curve of Tb(log μ100), where μ100
is the cosine of the heliocentric angle reduced to 100 GHz. Assuming
that the absolute calibration of the Band 3 commissioning observations
is accurate, we deduced a brightness temperature at the center of the
disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated
from the recommended values for Bands 3 and 6. More importantly, the
Tb(log μ100) curve flattens at large values
of μ100, and so does the corresponding Te(log
τ100) at large τ100. This is probably an
indication that we are approaching the temperature minimum.

---------------------------------------------------------
Title: High-resolution observations with ARTEMIS/JLS and the
NRH. IV. Imaging spectroscopy of spike-like structures near the
front of type-II bursts
Authors: Armatas, S.; Bouratzis, C.; Hillaris, A.; Alissandrakis,
C. E.; Preka-Papadema, P.; Kontogeorgos, A.; Tsitsipis, P.; Moussas, X.
2022A&A...659A.198A Altcode: 2022arXiv220107832A
Context. Narrowband bursts (spikes) are very small duration and
bandwidth bursts which appear on dynamic spectra from microwave to
decametric frequencies. They are believed to be manifestations of
small-scale energy release through magnetic reconnection. Aims:
We study the position of the spike-like structures relative to the front
of type-II bursts and their role in the burst emission. Methods:
We used high-sensitivity, low-noise dynamic spectra obtained with the
acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph,
in conjunction with high-time-resolution images from the Nançay
Radioheliograph (NRH) in order to study spike-like bursts near the front
of a type-II radio burst recorded at the west limb during the November
3, 2003 extreme solar event. The spike-like emission in the dynamic
spectrum was enhanced by means of high-pass-time filtering. 
Results: We identified a number of spikes in the NRH images. Due to
the lower temporal resolution of the NRH, multiple spikes detected in
the dynamic spectrum appeared as single structures in the images. These
spikes had an average size of ≈200″ and their observed brightness
temperature was 1.4 to 5.6 × 109 K, providing a significant
contribution to the emission of the type-II burst front. At variance
with a previous study on the type-IV associated spikes, we found no
systematic displacement between the spike emission and the emission
between spikes. At 327.0 MHz, the type II emission was located about
0.3 R⊙ above the pre-existing continuum emission, which,
in turn, was located 0.1 R⊙ above the western limb. Conclusions: This study, combined with our previous results,
indicates that the spike-like chains aligned along the type II burst
MHD shock front are not a perturbation of the type II emission, as
in the case of type IV spikes, but a manifestation of the type II
emission itself. The preponderance of these chains, together with the
lack of isolated structures or irregular clusters, points towards some
form of small-scale magnetic reconnection, organized along the type-II
propagating front. This work is dedicated to the memory of Costas
Caroubalos (1928-2021), founder of the ARTEMIS radiospectrograh.

---------------------------------------------------------
Title: Multiwavelength observations of a metric type-II event
Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Hillaris,
A.
2021A&A...654A.112A Altcode: 2021arXiv210802855A
We have studied a complex metric radio event that originated in a
compact flare, observed with the ARTEMIS-JLS radiospectrograph on
February 12, 2010. The event was associated with a surge observed at 195
and 304 Å and with a coronal mass ejection observed by instruments on
board STEREO A and B near the eastern and western limbs respectively. On
the disk the event was observed at ten frequencies by the Nançay
Radioheliograph (NRH), in Hα by the Catania observatory, in soft
X-rays by GOES SXI and Hinode XRT, and in hard X-rays by RHESSI. We
combined these data, together with MDI longitudinal magnetograms,
to get as complete a picture of the event as possible. Our emphasis
is on two type-II bursts that occurred near respective maxima in
the GOES light curves. The first, associated with the main peak
of the event, showed an impressive fundamental-harmonic structure,
while the emission of the second consisted of three well-separated
bands with superposed pulsations. Using positional information for the
type-IIs from the NRH and triangulation from STEREO A and B, we found
that the type-IIs were associated neither with the surge nor with the
disruption of a nearby streamer, but rather with an extreme ultraviolet
(EUV) wave probably initiated by the surge. The fundamental-harmonic
structure of the first type-II showed a band split corresponding to
a magnetic field strength of 18 G, a frequency ratio of 1.95 and a
delay of 0.23−0.65 s of the fundamental with respect to the harmonic;
moreover it became stationary shortly after its start and then drifted
again. The pulsations superposed on the second type-II were broadband
and had started before the burst. In addition, we detected another
pulsating source, also before the second type-II, polarized in the
opposite sense; the pulsations in the two sources were out of phase
and hence hardly detectable in the dynamic spectrum. The pulsations
had a measurable reverse frequency drift of about 2 s−1.

---------------------------------------------------------
Title: ALMA observations of the variability of the quiet Sun at
millimeter wavelengths
Authors: Nindos, A.; Patsourakos, S.; Alissandrakis, C. E.; Bastian,
T. S.
2021A&A...652A..92N Altcode: 2021arXiv210604220N
 Aims: We address the variability of the quiet solar chromosphere
at 1.26 mm and 3 mm with a focus on the study of spatially resolved
oscillations and transient brightenings, which are small, weak
events of energy release. Both phenomena may have a bearing on the
heating of the chromosphere. Methods: We used Atacama Large
Millimeter/submillimeter Array (ALMA) observations of the quiet Sun
at 1.26 mm and 3 mm. The spatial and temporal resolution of the data
were 1 − 2″ and 1 s, respectively. The concatenation of light
curves from different scans yielded a frequency resolution in spectral
power of 0.5−0.6 mHz. At 1.26 mm, in addition to power spectra of
the original data, we degraded the images to the spatial resolution
of the 3 mm images and used fields of view that were equal in area
for both data sets. The detection of transient brightenings was made
after the effect of oscillations was removed. Results: At both
frequencies, we detected p-mode oscillations in the range 3.6−4.4
mHz. The corrections for spatial resolution and field of view at 1.26 mm
decreased the root mean square (rms) of the oscillations by a factor of
1.6 and 1.1, respectively. In the corrected data sets, the oscillations
at 1.26 mm and 3 mm showed brightness temperature fluctuations of
∼1.7 − 1.8% with respect to the average quiet Sun, corresponding to
137 and 107 K, respectively. We detected 77 transient brightenings at
1.26 mm and 115 at 3 mm. Although their majority occurred in the cell
interior, the occurrence rate per unit area of the 1.26 mm events was
higher than that of the 3 mm events; this conclusion does not change
if we take into account differences in spatial resolution and noise
levels. The energy associated with the transient brightenings ranged
from 1.8 × 1023 to 1.1 × 1026 erg and from 7.2
× 1023 to 1.7 × 1026 erg for the 1.26 mm and
3 mm events, respectively. The corresponding power-law indices of the
energy distribution were 1.64 and 1.73. We also found that ALMA bright
network structures corresponded to dark mottles or spicules that can be
seen in broadband Hα images from the GONG network. Conclusions:
The fluctuations associated with the p-mode oscillations represent
a fraction of 0.55−0.68 of the full power spectrum. Their energy
density at 1.26 mm is 3 × 10−2 erg cm−3. The
computed low-end energy of the 1.26 mm transient brightenings is among
the smallest ever reported, irrespective of the wavelength of the
observation. Although the occurrence rate per unit area of the 1.26
mm transient brightenings was higher than that of the 3 mm events,
their power per unit area is smaller likely due to the detection of
many weak 1.26 mm events.

---------------------------------------------------------
Title: Radio Measurements of the Magnetic field in the Solar
Chromosphere and the Corona
Authors: Alissandrakis, Costas E.; Gary, Dale E.
2021FrASS...7...77A Altcode:
The structure of the upper solar atmosphere, on all observable scales,
is intimately governed by the magnetic field. The same holds for a
variety of solar phenomena that constitute solar activity, from tiny
transient brightening to huge Coronal Mass Ejections. Due to inherent
difficulties in measuring magnetic field effects on atoms (Zeeman and
Hanle effects) in the corona, radio methods sensitive to electrons
are of primary importance in obtaining quantitative information about
its magnetic field. In this review we explore these methods and point
out their advantages and limitations. After a brief presentation of
the magneto-ionic theory of wave propagation in cold, collisionless
plasmas, we discuss how the magnetic field affects the radio emission
produced by incoherent emission mechanisms (free-free, gyroresonance
and gyrosynchrotron processes) and give examples of measurements of
magnetic filed parameters in the quiet sun, active regions and radio
CMEs. We proceed by discussing how the inversion of the sense of
circular polarization can be used to measure the field above active
regions. Subsequently we pass to coherent emission mechanisms and
present results of measurements from fiber bursts, zebra patterns
and type II burst emission. We close this review with a discussion
of the variation of the magnetic field, deduced by radio measurement,
from the low corona up to 10 solar radii and with some thoughts about
future work.

---------------------------------------------------------
Title: Observations of Solar Spicules at Millimeter and Ultraviolet
Wavelengths
Authors: Bastian, T.; De Pontieu, B.; Shimojo, M.; Iwai, K.;
Alissandrakis, C.; Nindos, A.; Vial, J. C.; White, S. M.
2020AGUFMSH004..08B Altcode:
Solar spicules are a ubiquitous chromospheric phenomenon in which
multitudes of dynamic jets with temperatures of order 104
K extend thousands of kilometers into the solar atmosphere. Recent
progress has been made refining the observational characteristics
of spicules using the Hinode Solar Optical Telescope (SOT) and the
Interface Region Imaging Spectrograph (IRIS) observations at optical
and ultraviolet wavelengths, respectively. Two types of spicule
have been identified. Type I spicules, prevalent in solar active
regions, have upward speeds of order 25 km/s and lifetimes of 3-7
min. They may be the limb counterpart to shock-wave-driven fibrils
commonly seen against the solar disk in active regions. In contrast,
type II spicules, more common in quiet regions and coronal holes,
display upward speeds of 50-150 km/s, lifetimes of 30-110 s, and
appear to be partially heated to temperatures of 105 K and
higher. These observations have provoked intense interest in spicules
and have led to proposals that type II spicules play a central role
as a source of hot plasma in the corona. Nevertheless, their role in
mass and energy transport between the lower and upper layers of the
solar atmosphere remains an outstanding problem. Here, we report
imaging observations of solar spicules at millimeter wavelengths using
the Atacama Large Millimeter-submillimeter Array (ALMA) with arcsecond
angular resolution. Continuum millimeter wavelength radiation forms
under conditions of local thermodynamic equilibrium, thereby providing a
complementary tool to UV lines, which form under non-LTE conditions. The
observations were made on 2018 December 24-25 at λ=1.25 mm and λ=3
mm. The ALMA observations pose special challenges, particularly at
1.25 mm, where the limited field of view of the instrument motivated
us to use a novel mosaic imaging technique: multiple pointings were
assembled to form a single map with an angular resolution of 1" x 0.7"
on a cadence of roughly 2 min. In contrast, we were able to image at 3
mm continuously, with a map cadence of 2 s and an angular resolution of
2.3" x 1.3". We compare and contrast the morphology and dynamics
of mm-λ observations of spicules with those obtained by IRIS at UV
wavelengths and place constraints on spicule temperatures and masses
using the joint millimeter-wavelength observations.

---------------------------------------------------------
Title: Structure of the solar atmosphere: a radio perspective
Authors: Alissandrakis, Costas E.
2020FrASS...7...74A Altcode:
Solar radio emission has been providing information about the Sun for
over half a century. In order to fully exploit this information, one
needs to have a broader view of the solar atmosphere, which cannot be
provided by radio observations alone. The purpose of this review is to
present this background information, which is necessary to understand
the physical processes that determine the solar radio emission
and to link the radio domain with the rest of the electromagnetic
spectrum. Both classic and modern results are presented in a
concise manner. After a brief discussion of the solar interior, the
basic physics of the solar atmosphere and some elements of radiative
transfer are presented. Subsequently the atmospheric structure as
a function of height is examined and one -dimensional models of the
photosphere, the chromosphere, the transition region and the corona are
presented and discussed. An introduction to basic magnetohydrodynamics
precedes the discussion of the rich fine structure of the solar
atmosphere as a 3D object. Active regions are briefly discussed in a
separate section, and this is followed by a section on the problem of
heating of the chromosphere and the corona. I finish with some thoughts
on what to expect from the new instruments currently under development.

---------------------------------------------------------
Title: Modeling the quiet Sun cell and network emission with ALMA
Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T. S.; Patsourakos,
S.
2020A&A...640A..57A Altcode: 2020arXiv200609886A
Observations of the Sun at millimeter wavelengths with the Atacama
Large Millimeter/submillimeter Array (ALMA) offer a unique opportunity
to investigate the temperature structure of the solar chromosphere. In
this article we expand our previous work on modeling the chromospheric
temperature of the quiet Sun, by including measurements of the
brightness temperature in the network and cell interiors, from
high-resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We
also examine the absolute calibration of ALMA full-disk images. We
suggest that the brightness temperature at the center of the solar disk
in Band 6 is ∼440 K above the value recommended by White et al. (2017,
Sol. Phys., 292, 88). In addition, we give improved results for the
electron temperature variation of the average quiet Sun with optical
depth and the derived spectrum at the center of the disk. We found
that the electron temperature in the network is considerably lower
than predicted by model F of Fontenla et al. (1993, ApJ, 406, 319)
and that of the cell interior considerably higher than predicted by
model A. Depending on the network/cell segregation scheme, the electron
temperature difference between network and cell at τ = 1 (100 GHz)
ranges from ∼660 K to ∼1550 K, compared to ∼3280 K predicted
by the models; similarly, the electron temperature, Te
ratio ranges from ∼1.10 to 1.24, compared to ∼1.55 of the model
prediction. We also found that the network/cell Te(τ)
curves diverge as τ decreases, indicating an increase of contrast
with height and possibly a steeper temperature rise in the network
than in the cell interior.

---------------------------------------------------------
Title: Transient brightenings in the quiet Sun detected by ALMA at
3 mm
Authors: Nindos, A.; Alissandrakis, C. E.; Patsourakos, S.; Bastian,
T. S.
2020A&A...638A..62N Altcode: 2020arXiv200407591N
 Aims: We investigate transient brightenings, that is, weak,
small-scale episodes of energy release, in the quiet solar chromosphere;
these episodes can provide insights into the heating mechanism of the
outer layers of the solar atmosphere. Methods: Using Atacama
Large Millimeter/submillimeter Array (ALMA) observations, we performed
the first systematic survey for quiet Sun transient brightenings at 3
mm. Our dataset included images of six 87″ × 87″ fields of view
of the quiet Sun obtained with angular resolution of a few arcsec at
a cadence of 2 s. The transient brightenings were detected as weak
enhancements above the average intensity after we removed the effect
of the p-mode oscillations. A similar analysis, over the same fields
of view, was performed for simultaneous 304 and 1600 Å data obtained
with the Atmospheric Imaging Assembly. Results: We detected 184
3 mm transient brightening events with brightness temperatures from
70 K to more than 500 K above backgrounds of ∼7200 - 7450 K. All
events showed light curves with a gradual rise and fall, strongly
suggesting a thermal origin. Their mean duration and maximum area were
51.1 s and 12.3 Mm2, respectively, with a weak preference
of appearing at network boundaries rather than in cell interiors. Both
parameters exhibited power-law behavior with indices of 2.35 and 2.71,
respectively. Only a small fraction of ALMA events had either 304
or 1600 Å counterparts but the properties of these events were not
significantly different from those of the general population except
that they lacked their low-end energy values. The total thermal
energies of the ALMA transient brightenings were between 1.5 ×
1024 and 9.9 × 1025 erg and their frequency
distribution versus energy was a power law with an index of 1.67 ±
0.05. We found that the power per unit area provided by the ALMA events
could account for only 1% of the chromospheric radiative losses (10%
of the coronal ones). Conclusions: We were able to detect, for
the first time, a significant number of weak 3 mm quiet Sun transient
brightenings. However, their energy budget falls short of meeting the
requirements for the heating of the upper layers of the solar atmosphere
and this conclusion does not change even if we use the least restrictive
criteria possible for the detection of transient brightenings.

---------------------------------------------------------
Title: Modeling of the Brightness of the Chromospheric Network Based
on ALMA High Resolution Observations of the Quiet Sun
Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T.; Patsourakos, S.
2020AAS...23610607A Altcode:
ALMA observations of the Sun at mm-λ offer a unique opportunity
to investigate the temperature/density structure of the solar
chromosphere. In a previous work (Alissandrakis et al 2017, A&amp;A
605, A78) we measured the center-to-limb variation of the brightness
temperature, Tb, using low resolution ALMA full-disk
observations in Band 3 (3mm) and Band 6 (1.26 mm), together with data
at 0.85 mm from Bastian et al. 1993 (ApJ, 415, 364). Combining all
data and inverting the solution of the transfer equation we found
that the electron temperature, Te, in the range of 0.34
&lt; τ100 &lt; 12, where τ100 is the optical
depth at 100 GHz, was ~5% (~300 K) below the one predicted by model C
(average quiet sun) of FAL93 (Fontenla, Avrett, &amp; Loeser, 1993,
ApJ, 406, 319). Here we expand that work by including measurements
of the brightness temperature in the network and cell interiors,
from high resolution ALMA images in Bands 3 and 6. We found that
the observed Tb in the network is considerably lower
than predicted by the FAL93 model F and that of the cell interior
considerably higher than predicted by the FAL93 model A. The observed
network/cell difference of brightness temperature at the center of the
disk, at 100 GHz is about 920 K, compared to ~3250 K predicted by the
FAL93 models; similarly, the Tb, ratio is ~1.14, against
~1.51 of the model prediction. After inversion of the observed data,
the electron temperature of cell interior at τ100=1 is
~390 K below the average (~600 K above model A) and of the network
~400 K above the average (~1800 K below model A). The implications of
these results will be discussed. We will also discuss the question of
the normalization of brightness temperature observed by ALMA.

---------------------------------------------------------
Title: Observations of solar chromospheric oscillations at 3 mm
with ALMA
Authors: Patsourakos, S.; Alissandrakis, C. E.; Nindos, A.; Bastian,
T. S.
2020A&A...634A..86P Altcode: 2019arXiv191203480P
 Aims: We aim to study spatially resolved chromospheric
oscillations of the quiet Sun (QS) in the mm-domain at a resolution
of a few arcsec, typically 2.4″ × 4.5″. Methods: We used
Atacama Large millimeter and submillimeter Array (ALMA) time series
of interferometric observations of the QS obtained at 3 mm with a 2-s
cadence and a spatial resolution of a few arcsec. The observations were
performed on March 16, 2017 and seven 80″ × 80″ fields of view
(FoV) going from disk center to limb were covered, each one observed for
10 min, therefore limiting the frequency resolution of the power spectra
to 1.7 mHz. For each FoV, masks for cell and network were derived,
and the averaged power spectral densities (PSDs) for the entire FoV,
cell, and network were computed. The resulting power spectra were
fit with an analytical function in order to derive the frequency
and the root-mean-square (rms) power associated with the peaks. The
same analysis, over the same FoVs and for the same intervals, was
performed for simultaneous Atmospheric Imaging Assembly (AIA) image
sequences in 1600 Å. Results: Spatially resolved chromospheric
oscillations at 3 mm, with frequencies of 4.2 ± 1.7 mHz are observed
in the QS, in both cell and network. The coherence length-scale of
the oscillations is commensurate with the spatial resolution of our
ALMA observations. Brightness-temperature fluctuations in individual
pixels could reach up to a few hundred K, while the spatially averaged
PSDs yield rms in the range ≈55-75 K, i.e., up to ≈1% of the
averaged brightness temperatures and exhibit a moderate increase
towards the limb. For AIA 1600 Å, the oscillation frequency is 3.7
± 1.7 mHz. The relative rms is up to 6% of the background intensity,
with a weak increase towards the disk center (cell, average). ALMA
3 mm time-series lag AIA 1600 Å by ≈100 s, which corresponds to
a formation-height difference of ≈1200 km, representing a novel
determination of this important parameter. Conclusions: The
ALMA oscillations that we detected exhibit higher amplitudes than those
derived from previous lower (≈10″) resolution observations at 3.5 mm
by the Berkeley-Illinois-Maryland Array. Chromospheric oscillations are,
therefore, not fully resolved at the length-scale of the chromospheric
network, and possibly not even at the spatial resolution of our ALMA
observations. Any study of transient brightenings in the mm-domain
should take into account the oscillations.

---------------------------------------------------------
Title: Measurement of the Height of the Chromospheric Network Emission
from Solar Dynamics Observatory Images
Authors: Alissandrakis, C. E.
2019SoPh..294..161A Altcode: 2019arXiv191100758A
We measured the height of the chromospheric network in the 1700,
1600, and 304 Å wavelength bands of the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO) from the shift
of features on the disk with respect to corresponding features in
SDO/Helioseismic and Magnetic Imager (HMI) images of the absolute
value of the longitudinal magnetic field. We found that near the limb
the 304 Å network emission forms 3.60 ±0.24 Mm above the 1600 Å
emission, which, in turn, forms 0.48 ±0.10 Mm above the HMI (6173 Å)
level. At the center of the disk the corresponding height differences
are 2.90 ±0.02 Mm and 0.39 ±0.06 Mm respectively. We also found that
the 1600 Å network emission forms 0.25 ±0.02 Mm above the 1700 Å
emission near the limb and 0.20 ±0.02 Mm at the disk center. Finally,
we examined possible variations with the solar cycle. Our results can
help to check and refine atmospheric models.

---------------------------------------------------------
Title: Erratum: Correction to: Structure of the Transition Region
and the Low Corona from TRACE and SDO Observations Near the Limb
Authors: Alissandrakis, C. E.; Valentino, A.
2019SoPh..294..146A Altcode:
In computing the height with respect to the tau = 1 level, the height
difference between the white light limb and that level was subtracted
rather than added to the values in Table 4. This affects Sections 3.3
and 3.4, Figures 14 and 15, as well as Table 5. The correct versions
are given below.

---------------------------------------------------------
Title: Structure of the Transition Region and the Low Corona from
TRACE and SDO Observations Near the Limb
Authors: Alissandrakis, C. E.; Valentino, A.
2019SoPh..294...96A Altcode: 2019arXiv190609497A
We examined the structure near the solar limb in TRACE images of the
continuum and in the 1600 and 171 Å bands as well as in SDO images in
the continuum (from HMI) and all AIA bands. The images in different
wavelength bands were carefully coaligned by using the position of
Mercury for TRACE and Venus for SDO during their transit in front of
the solar disk in 1999 and 2012, respectively. Chromospheric absorbing
structures in the TRACE 171-Å band are best visible 7”
above the white-light limb, very close to the inner limb, defined as
the inflection point of the rising part of the center-to-limb intensity
variation. They are correlated with, but are not identical to, spicules
in emission, seen in the 1600-Å band. Similar results were obtained
from AIA and SOT images. Tall spicules in 304 Å are not associated
with any absorption in the higher temperature bands. Performing
azimuthal averaging of the intensity over 15∘ sectors
near the N, S, E and W limbs, we measured the height of the limb and
of the peak intensity in all AIA bands. We found that the inner limb
height in the transition region AIA bands increases with wavelength,
consistent with a bound-free origin of the absorption from neutral
H and He. From that we computed the column density and the density
of neutral hydrogen as a function of height. We estimated a height
of (2300 ±500 )km for the base of the transition region. Finally,
we measured the scale height of the AIA emission of the corona and
associated it with the temperature; we deduced a value of (1.24 ±0.25
)×106K for the polar corona

---------------------------------------------------------
Title: High-resolution observations with ARTEMIS-JLS and the
NRH. III. Spectroscopy and imaging of fiber bursts
Authors: Alissandrakis, C. E.; Bouratzis, C.; Hillaris, A.
2019A&A...627A.133A Altcode: 2019arXiv190603434A
 Aims: We study the characteristics of intermediate drift
bursts (fibers) embedded in a large type-IV event. Methods:
We used high-sensitivity, low-noise dynamic spectra obtained with the
acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph,
in conjunction with high time-resolution images from the Nançay
radioheliograph (NRH) and extreme ultraviolet (EUV) images from the
Transition Region and Coronal Explorer (TRACE) to study fiber bursts
during the large solar event of July 14, 2000. We computed both 2D and
1D images and applied high pass time filtering to the images and the
dynamic spectrum in order to enhance the fiber-associated emission. For
the study of the background continuum emission we used images averaged
over several seconds. Results: Practically all fibers visible in
the SAO dynamic spectra are identifiable in the NRH images. Fibers were
first detected after the primary energy release in a moving type-IV
event, probably associated with the rapid eastward expansion of the
flare and the post-flare loop arcade. We found that fibers appeared as
a modulation of the continuum intensity with a root mean square value
of the order of 10%. Both the fibers and the continuum were strongly
circularly polarized in the ordinary mode sense, indicating plasma
emission at the fundamental. We detected a number of discrete fiber
emission sources along two parallel stripes of ∼300 Mm in length,
apparently segments of large-scale loops encompassing both the EUV loops
and the CME-associated flux rope. We found cases of multiple fiber
emissions appearing at slightly different positions and times; their
consecutive appearance can give the impression of apparent motion with
supra-luminal velocities. Images of individual fibers were very similar
at 432.0 and 327.0 MHz. From the position shift of the sources and the
time delays at low and high frequencies, we estimated the exciter speed
and the frequency scale length along the loops for a well-observed group
of fibers; we obtained consistent values from imaging and spectral
data, supporting the whistler origin of the fiber emission. Finally
we found that fibers in emission and fibers in absorption are very
similar, confirming that they are manifestations of the same wave
train. Movies associated to Figs. 2, 7, and 10 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935627/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: High resolution observations with Artemis-JLS. II. Type IV
associated intermediate drift bursts
Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.;
Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.;
Kontogeorgos, A.
2019A&A...625A..58B Altcode: 2019arXiv190402262B
 Aims: We examined the characteristics of isolated intermediate
drift bursts and their morphologies on dynamic spectra, in particular
the positioning of emission and absorption ridges. Furthermore we
studied the repetition rate of fiber groups. These were compared with a
model in order to determine the conditions under which the intermediate
drift bursts appear and exhibit the above characteristics. 
Methods: We analyzed sixteen metric type IV events with embedded
intermediate drift bursts, observed with the Artemis-JLS radio
spectrograph from July 1999 to July 2005 plus an event on 1st
August 2010. The events were recorded with the SAO high resolution
(10 ms cadence) receiver in the 270-450 MHz range with a frequency
resolution of 1.4 MHz. We developed cross- and autocorrelation
techniques to measure the duration, spectral width, and frequency
drift of fiber bursts in 47 intermediate drift bursts (IMD) groups
embedded within the continuum of the sixteen events mentioned above. We
also developed a semi-automatic algorithm to track fibers on dynamic
spectra. Results: The mean duration of individual fiber bursts
at fixed frequency was δt ≈ 300 ms, while the instantaneous relative
bandwidth was fw/f ≈ 0.90% and the total frequency extent
was Δftot ≈ 35 MHz. The recorded intermediate drift bursts
had frequency drift, positive or negative, with average values of df/fdt
equal to -0.027 and 0.024 s-1 respectively. Quite often
the fibers appeared in groups; the burst repetition rate within groups
was, on average, ∼0.98 s. We distinguish six morphological groups of
fibers, based on the relative position of the emission and absorption
ridges. These included fibers with emission or absorption ridges
only, fibers with the absorption ridge at lower or higher frequency
than the emission, or with two absorption ridges above and below the
emission. There were also some fibers for which two emission ridges
were separated by an absorption ridge. Some additional complex groups
within our data set were not easy to classify. A number of borderline
cases of fibers with very high drift rate (∼0.30 s-1)
or very narrow total bandwidth (∼8 MHz) were recorded; among them
there was a group of rope-like fibers characterized by fast repetition
rate and relatively narrow total frequency extent. We found that
the whistler hypothesis leads to reasonable magnetic field values
(∼4.6 G), while the Alfven-wave hypothesis requires much higher
field. From the variation of the drift rate with time we estimated
the ratio of the whistler to the cyclotron frequency, x, to be in
the range of 0.3-0.6, varying by ∼0.05-0.1 in individual fibers;
the same analysis gives an average value of the frequency scale along
the loop of ∼220 Mm. Finally, we present empirical relations between
fiber burst parameters and discuss their possible origin.

---------------------------------------------------------
Title: Detection of spike-like structures near the front of type-II
bursts
Authors: Armatas, S.; Bouratzis, C.; Hillaris, A.; Alissandrakis,
C. E.; Preka-Papadema, P.; Moussas, X.; Mitsakou, E.; Tsitsipis, P.;
Kontogeorgos, A.
2019A&A...624A..76A Altcode: 2019arXiv190210617A
 Aims: We examine high time resolution dynamic spectra for fine
structures in type II solar radio bursts Methods: We used data
obtained with the acousto-optic spectrograph receiver of the Artemis-JLS
(ARTEMIS-IV) solar radio spectrograph in the 450-270 MHz range at 10 ms
cadence and identified more than 600 short, narrowband features. Their
characteristics, such as instantaneous relative bandwidth and total
duration were measured and compared with those of spikes embedded
in type IV emissions. Results: Type II associated spikes
occur mostly in chains inside or close to the slowly drifting type
II emission. These spikes coexist with herringbone and pulsating
structures. Their average duration is 96 ms and their average relative
bandwidth 1.7%. These properties are not different from those of type
IV embedded spikes. It is therefore possible that they are signatures
of small-scale reconnection along the type II shock front.

---------------------------------------------------------
Title: Modeling of the Sunspot-Associated Microwave Emission Using
a New Method of DEM Inversion
Authors: Alissandrakis, C. E.; Bogod, V. M.; Kaltman, T. I.;
Patsourakos, S.; Peterova, N. G.
2019SoPh..294...23A Altcode: 2018arXiv181205751A
We develope a method to compute the temperature and density structure
along the line of sight by inversion of the differential emission
measure (DEM), under the assumptions of stratification and hydrostatic
equilibrium. We apply this method to the DEM obtained from the
Atmospheric Imaging Assembly (AIA) observations and use the results,
together with potential extrapolations of the photospheric magnetic
field, to compute the microwave emission of three sunspots, which we
compare with observations from the Academy of Sciences Radio Telescope
- 600 (RATAN-600) radio telescope and the Nobeyama Radioheliograph
(NoRH). Our DEM-based models reproduce very well the observations of
the moderate-size spot on October 2011 and within 25% the data of a
similar sized spot on March 2016, but predict too low values for the
big spot of 14 April 2016. The latter is better fitted by a constant
conductive flux atmospheric model which, however, cannot reproduce the
peak brightness temperature of 4.7 ×106K and the shape of
the source at the NoRH frequency. We propose that these deviations are
due to the low intensity non-thermal emission associated to a moving
pore and to an opposite polarity light bridge. We also find that the
double structure of the big spot at high RATAN-600 frequencies can be
interpreted in terms of the variation of the angle between the magnetic
field and the line of sight along the sunspot.

---------------------------------------------------------
Title: First high-resolution look at the quiet Sun with ALMA at 3mm
Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos,
S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu,
T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V.
2018A&A...619L...6N Altcode: 2018arXiv181005223N
We present an overview of high-resolution quiet Sun observations,
from disk center to the limb, obtained with the Atacama Large
millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun
regions were observed at a resolution of up to 2.5″ by 4.5″. We
produced both average and snapshot images by self-calibrating the ALMA
visibilities and combining the interferometric images with full-disk
solar images. The images show well the chromospheric network, which,
based on the unique segregation method we used, is brighter than the
average over the fields of view of the observed regions by ∼305
K while the intranetwork is less bright by ∼280 K, with a slight
decrease of the network/intranetwork contrast toward the limb. At 3
mm the network is very similar to the 1600 Å images, with somewhat
larger size. We detect, for the first time, spicular structures,
rising up to 15″ above the limb with a width down to the image
resolution and brightness temperature of ∼1800 K above the local
background. No trace of spicules, either in emission or absorption,
is found on the disk. Our results highlight the potential of ALMA for
the study of the quiet chromosphere.

---------------------------------------------------------
Title: IRIS Observations of Spicules and Structures Near the
Solar Limb
Authors: Alissandrakis, C. E.; Vial, J. -C.; Koukras, A.; Buchlin,
E.; Chane-Yook, M.
2018SoPh..293...20A Altcode: 2018arXiv180102082A
We have analyzed Interface Region Imaging Spectrograph (IRIS)
spectral and slit-jaw observations of a quiet region near the South
Pole. In this article we present an overview of the observations, the
corrections, and the absolute calibration of the intensity. We focus
on the average profiles of strong (Mg II h and k, C II and Si IV),
as well as of weak spectral lines in the near ultraviolet (NUV) and
the far ultraviolet (FUV), including the Mg II triplet, thus probing
the solar atmosphere from the low chromosphere to the transition
region. We give the radial variation of bulk spectral parameters as
well as line ratios and turbulent velocities. We present measurements
of the formation height in lines and in the NUV continuum from which
we find a linear relationship between the position of the limb and
the intensity scale height. We also find that low forming lines,
such as the Mg II triplet, show no temporal variations above the limb
associated with spicules, suggesting that such lines are formed in a
homogeneous atmospheric layer and, possibly, that spicules are formed
above the height of 2″. We discuss the spatio-temporal
structure of the atmosphere near the limb from images of intensity as
a function of position and time. In these images, we identify p-mode
oscillations in the cores of lines formed at low heights above the
photosphere, slow-moving bright features in O I and fast-moving bright
features in C II. Finally, we compare the Mg II k and h line profiles,
together with intensity values of the Balmer lines from the literature,
with computations from the PROM57Mg non-LTE model, developed at the
Institut d' Astrophysique Spatiale, and estimated values of the physical
parameters. We obtain electron temperatures in the range of ∼8000 K
at small heights to ∼20 000 K at large heights, electron densities
from 1.1 ×1011 to 4 ×1010cm−3
and a turbulent velocity of ∼24 kms−1.

---------------------------------------------------------
Title: Center-to-limb observations of the Sun with ALMA . Implications
for solar atmospheric models
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian,
T. S.
2017A&A...605A..78A Altcode:
 Aims: We seek to derive information on the temperature structure
of the solar chromosphere and compare these results with existing
models. Methods: We measured the center-to-limb variation of
the brightness temperature, Tb, from ALMA full-disk images
at two frequencies and inverted the solution of the transfer equation
to obtain the electron temperature, Te as a function of
optical depth, τ. Results: The ALMA images are very similar
to AIA images at 1600 Å. The brightness temperature at the center
of the disk is 6180 and 7250 K at 239 and 100 GHz, respectively,
with dispersions of 100 and 170 K. Plage regions stand out clearly in
the 239/100 GHz intensity ratio, while faculae and filament lanes do
not. The solar disk radius, reduced to 1 AU, is 961.1 ± 2.5” and 964.1
± 4.5” at 239 and 100 GHz, respectively. A slight but statistically
significant limb brightening is observed at both frequencies. 
Conclusions: The inversion of the center-to-limb curves shows that
Te varies linearly with the logarithm of optical depth for
0.34 &lt;τ100 GHz&lt; 12, with a slope dTe/
dlnτ = -608 K. Our results are 5% lower than predicted by the average
quiet Sun model C of Fontenla et al. (1993, ApJ. 406, 319), but do
not confirm previous reports that the mm-λ solar spectrum is better
fitted with models of the cell interior.

---------------------------------------------------------
Title: Evidence for two-loop interaction from IRIS and SDO
observations of penumbral brightenings
Authors: Alissandrakis, C. E.; Koukras, A.; Patsourakos, S.; Nindos, A.
2017A&A...603A..95A Altcode: 2017arXiv170407344A
 Aims: We investigate small scale energy release events which
can provide clues on the heating mechanism of the solar corona. 
Methods: We analyzed spectral and imaging data from the Interface
Region Imaging Spectrograph (IRIS), images from the Atmospheric
Imaging Assembly (AIA) aboard the Solar Dynamics Observatoty (SDO),
and magnetograms from the Helioseismic and Magnetic Imager (HMI)
aboard SDO. Results: We report observations of small flaring
loops in the penumbra of a large sunspot on July 19, 2013. Our main
event consisted of a loop spanning 15”, from the umbral-penumbral
boundary to an opposite polarity region outside the penumbra. It lasted
approximately 10 min with a two minute impulsive peak and was observed
in all AIA/SDO channels, while the IRIS slit was located near its
penumbral footpoint. Mass motions with an apparent velocity of 100 km
s-1 were detected beyond the brightening, starting in the
rise phase of the impulsive peak; these were apparently associated
with a higher-lying loop. We interpret these motions in terms of
two-loop interaction. IRIS spectra in both the C II and Si iv lines
showed very extended wings, up to about 400 km s-1, first
in the blue (upflows) and subsequently in the red wing. In addition
to the strong lines, emission was detected in the weak lines of Cl
I, O I and C I, as well as in the Mg II triplet lines. Absorption
features in the profiles of the C II doublet, the Si iv doublet and
the Mg II h and k lines indicate the existence of material with a
lower source function between the brightening and the observer. We
attribute this absorption to the higher loop and this adds further
credibility to the two-loop interaction hypothesis. Tilts were
detected in the absorption spectra, as well as in the spectra of Cl I,
O I, and C I lines, possibly indicating rotational motions from the
untwisting of magnetic flux tubes. Conclusions: We conclude
that the absorption features in the C II, Si iv and Mg II profiles
originate in a higher-lying, descending loop; as this approached
the already activated lower-lying loop, their interaction gave rise
to the impulsive peak, the very broad line profiles and the mass
motions. Movies associated to Figs. A.1-A.3 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201730643/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Center-to-limb observations of the Sun with ALMA
Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian,
T. S.
2017arXiv170509008A Altcode:
We measured the center-to-limb variation of the brightness temperature,
$T_b$, from ALMA full-disk images at two frequencies and inverted the
solution of the transfer equation to obtain the electron temperature,
$T_e$ as a function of optical depth, $\tau$. The ALMA images are very
similar to AIA images at 1600Å. The brightness temperature at the
center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively,
with dispersions of 100 and 170 K. Plage regions stand out clearly
in the 239/100 GHz intensity ratio, while faculae and filament lanes
do not. The solar disk radius, reduced to 1 AU, is $961.1\pm2.5$
arcsec and $964.1\pm4.5$ arcsec at 239 and 100 GHz respectively. A
slight but statistically significant limb brightening is observed at
both frequencies. The inversion of the center-to-limb curves shows
that $T_e$ varies linearly with the logarithm of optical depth for
$0.34&lt;\tau_{100\,GHz}&lt;12$, with a slope $d\ln T_e/d\tau=-608$
K. Our results are 5% lower than predicted by the average quiet sun
model C of Fontenla et al. (1993), but do not confirm previous reports
that the mm-$\lambda$ solar spectrum is better fitted with models of
the cell interior.

---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.

---------------------------------------------------------
Title: High resolution observations with Artemis-IV and the
NRH. I. Type IV associated narrow-band bursts
Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.;
Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.;
Kontogeorgos, A.
2016A&A...586A..29B Altcode: 2015arXiv151202481B
Context. Narrow-band bursts appear on dynamic spectra from microwave
to decametric frequencies as fine structures with very small duration
and bandwidth. They are believed to be manifestations of small
scale energy release through magnetic reconnection. Aims: We
analyzed 27 metric type IV events with embedded narrow-band bursts,
which were observed by the ARTEMIS-IV radio spectrograph from 30 June
1999 to 1 August 2010. We examined the morphological characteristics of
isolated narrow-band structures (mostly spikes) and groups or chains of
structures. Methods: The events were recorded with the SAO high
resolution (10 ms cadence) receiver of ARTEMIS-IV in the 270-450 MHz
range. We measured the duration, spectral width, and frequency drift of
~12 000 individual narrow-band bursts, groups, and chains. Spike sources
were imaged with the Nançay radioheliograph (NRH) for the event of 21
April 2003. Results: The mean duration of individual bursts at
fixed frequency was ~100 ms, while the instantaneous relative bandwidth
was ~2%. Some bursts had measurable frequency drift, either positive
or negative. Quite often spikes appeared in chains, which were closely
spaced in time (column chains) or in frequency (row chains). Column
chains had frequency drifts similar to type-IIId bursts, while most
of the row chains exhibited negative frequently drifts with a rate
close to that of fiber bursts. From the analysis of NRH data, we found
that spikes were superimposed on a larger, slowly varying, background
component. They were polarized in the same sense as the background
source, with a slightly higher degree of polarization of ~65%, and
their size was about 60% of their size in total intensity. 
Conclusions: The duration and bandwidth distributions did not show any
clear separation in groups. Some chains tended to assume the form of
zebra, lace stripes, fiber bursts, or bursts of the type-III family,
suggesting that such bursts might be resolved in spikes when viewed
with high resolution. The NRH data indicate that the spikes are not
fluctuations of the background, but represent additional emission such
as what would be expected from small-scale reconnection.

---------------------------------------------------------
Title: ALMA Observations of the Sun in Cycle 4 and Beyond
Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.;
Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.;
Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson,
M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.;
Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov,
V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões,
P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig,
A.; White, S.; Yagoubov, P.; Zaqarashvili, T.
2016arXiv160100587W Altcode:
This document was created by the Solar Simulations for the Atacama
Large Millimeter Observatory Network (SSALMON) in preparation of
the first regular observations of the Sun with the Atacama Large
Millimeter/submillimeter Array (ALMA), which are anticipated to start
in ALMA Cycle 4 in October 2016. The science cases presented here
demonstrate that a large number of scientifically highly interesting
observations could be made already with the still limited solar
observing modes foreseen for Cycle 4 and that ALMA has the potential
to make important contributions to answering long-standing scientific
questions in solar physics. With the proposal deadline for ALMA Cycle
4 in April 2016 and the Commissioning and Science Verification campaign
in December 2015 in sight, several of the SSALMON Expert Teams composed
strategic documents in which they outlined potential solar observations
that could be feasible given the anticipated technical capabilities
in Cycle 4. These documents have been combined and supplemented
with an analysis, resulting in recommendations for solar observing
with ALMA in Cycle 4. In addition, the detailed science cases also
demonstrate the scientific priorities of the solar physics community
and which capabilities are wanted for the next observing cycles. The
work on this White Paper effort was coordinated in close cooperation
with the two international solar ALMA development studies led by
T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be
further updated until the beginning of Cycle 4 in October 2016. In
particular, we plan to adjust the technical capabilities of the solar
observing modes once finally decided and to further demonstrate the
feasibility and scientific potential of the included science cases by
means of numerical simulations of the solar atmosphere and corresponding
simulated ALMA observations.

---------------------------------------------------------
Title: A tiny event producing an interplanetary type III burst
Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.;
Kontogeorgos, A.; Tsitsipis, P.
2015A&A...582A..52A Altcode: 2015arXiv150708423A
 Aims: We investigate the conditions under which small-scale
energy release events in the low corona gave rise to strong
interplanetary (IP) type III bursts. Methods: We analyzed
observations of three tiny events, detected by the Nançay Radio
Heliograph (NRH), two of which produced IP type III bursts. We
took advantage of the NRH positioning information and of the high
cadence of AIA/SDO data to identify the associated extreme-UV (EUV)
emissions. We measured positions and time profiles of the metric and
EUV sources. Results: We found that the EUV events that produced
IP type III bursts were located near a coronal hole boundary, while the
one that did not was located in a closed magnetic field region. In all
three cases tiny flaring loops were involved, without any associated
mass eruption. In the best observed case, the radio emission at the
highest frequency (435 MHz) was displaced by ~55” with respect to
the small flaring loop. The metric type III emission shows a complex
structure in space and in time, indicative of multiple electron
beams, despite the low intensity of the events. From the combined
analysis of dynamic spectra and NRH images, we derived the electron
beam velocity as well as the height, ambient plasma temperature, and
density at the level of formation of the 160 MHz emission. From the
analysis of the differential emission measure derived from the AIA
images, we found that the first evidence of energy release was at the
footpoints, and this was followed by the development of flaring loops
and subsequent cooling. Conclusions: Even small energy release
events can accelerate enough electrons to give rise to powerful IP
type III bursts. The proximity of the electron acceleration site to
open magnetic field lines facilitates the escape of the electrons
into the interplanetary space. The offset between the site of
energy release and the metric type III location warrants further
investigation. The movie is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526265/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Properties of solar energetic particle events inferred from
their associated radio emission
Authors: Kouloumvakos, A.; Nindos, A.; Valtonen, E.; Alissandrakis,
C. E.; Malandraki, O.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.;
Hillaris, A.
2015A&A...580A..80K Altcode: 2015arXiv150703776K
 Aims: We study selected properties of solar energetic particle
(SEP) events as inferred from their associated radio emissions. Methods: We used a catalogue of 115 SEP events, which consists
of entries of proton intensity enhancements at one AU, with complete
coverage over solar cycle 23 based on high-energy (~68 MeV) protons from
SOHO/ERNE. We also calculated the proton release time at the Sun using
velocity dispersion analysis (VDA). After an initial rejection of cases
with unrealistic VDA path lengths, we assembled composite radio spectra
for the remaining events using data from ground-based and space-borne
radio spectrographs. We registered the associated radio emissions for
every event, and we divided the events in groups according to their
associated radio emissions. In cases of type III-associated events, we
extended our study to the timings between the type III radio emission,
the proton release, and the electron release as inferred from VDA
based on Wind/3DP 20-646 keV data. Results: The proton release
was found to be most often accompanied by both type III and II radio
bursts, but a good association percentage was also registered in cases
accompanied by type IIIs only. The worst association was found for the
cases only associated with type II. In the type III-associated cases,
we usually found systematic delays of both the proton and electron
release times as inferred by the particles' VDAs, with respect to the
start of the associated type III burst. The comparison of the proton
and electron release times revealed that, in more than half of the
cases, the protons and electrons were simultaneously released within
the statistical uncertainty of our analysis. For the cases with type
II radio association, we found that the distribution of the proton
release heights had a maximum at ~2.5 R⊙. Most (69%) of
the flares associated with our SEP events were located in the western
hemisphere, with a peak within the well-connected region of 50°-60°
western longitude. Conclusions: Both flare- and shock-related
particle release processes are observed in major proton events at
&gt;50 MeV. Typically, the protons are released after the start of the
associated type III bursts and simultaneously or before the release of
energetic electrons. Our study indicates that a clear-cut distinction
between flare-related and CME-related SEP events is difficult to
establish. Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424397/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: RATAN-600 Observations of Small-Scale Structures with High
Spectral Resolution
Authors: Bogod, V. M.; Alissandrakis, C. E.; Kaltman, T. I.;
Tokhchukova, S. K.
2015SoPh..290....7B Altcode: 2014arXiv1403.7658B; 2014SoPh..tmp...65B
We present observations of quiet-Sun small-scale structures (SSS)
in the microwave range with the Radio Astronomical Telescope of the
Academy of Sciences 600 (RATAN-600) spectral-polarization facility
in a wide range of frequencies. The SSS are regularly recorded in
routine observations of the large reflector-type radio telescope and
represent manifestations in the radio range of various structures of
the quiet Sun: supergranulation network, bright points, plage patches,
and so on. A comparison with images from the Solar Dynamics Observatory
(SDO) showed that the microwave emission comes from a region extending
from the chromosphere to the low transition region. We measured the
properties of the SSS as well as the degree of circular polarization
averaged over the beam of the radio telescope, and from this we
estimated the magnetic field at the formation level of the radiation.

---------------------------------------------------------
Title: Fine Structure of Metric Type IV Radio Bursts Observed with
the ARTEMIS-IV Radio-Spectrograph: Association with Flares and
Coronal Mass Ejections
Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.;
Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.;
Kontogeorgos, A.
2015SoPh..290..219B Altcode: 2014arXiv1406.1202B; 2014SoPh..tmp..101B
Fine structures embedded in type IV burst continua may be used as
diagnostics of the magnetic-field restructuring and the corresponding
energy release associated with the low-corona development of
flare or coronal mass ejection (CME) events. A catalog of 36 type
IV bursts observed with the SAO receiver of the ARTEMIS-IV solar
radio-spectrograph in the 450 - 270 MHz range at high cadence (0.01
sec) was compiled; the fine structures were classified into five basic
classes with two or more subclasses each. The time of fine-structure
emission was compared with the injection of energetic electrons as
recorded by hard X-ray and microwave emission, the soft X-ray (SXR)
light curves and the CME onset time. Our results indicate a very tight
temporal association between energy release episodes and pulsations,
spikes, narrow-band bursts of the type III family, and zebra bursts. Of
the remaining categories, the featureless broadband continuum starts
near the time of the first energy release, between the CME onset
and the SXR peak, but extends for several tens of minutes after that,
covering almost the full extent of the flare-CME event. The intermediate
drift bursts, fibers in their majority, mostly follow the first energy
release, but have a wider distribution than other fine structures.

---------------------------------------------------------
Title: Microwave and EUV Observations of an Erupting Filament and
Associated Flare and Coronal Mass Ejections
Authors: Alissandrakis, Costas E.; Kochanov, Alexey A.; Patsourakos,
Spiros; Altyntsev, Alexander T.; Lesovoi, Sergey V.; Lesovoya, Nadya N.
2013PASJ...65S...8A Altcode: 2013arXiv1309.1703A
A filament eruption was observed with the Siberian Solar Radio Telescope
(SSRT) on 2012 June 23, starting at around 06:40 UT, beyond the west
limb. The filament could be followed in SSRT images to heights above 1
R⊙, and coincided with the core of the CME, seen in LASCO
C2 images. We briefly discuss the dynamics of the eruption: the top of
the filament showed a smooth acceleration up to an apparent velocity
of ∼ 1100 km s-1. Images behind the limb from STEREO-A
show a two-ribbon flare and the interaction of the main filament,
located along the primary neutral line, with an arch-like structure,
oriented in the perpendicular direction. The interaction was accompanied
by strong emission and twisting motions. The microwave images show
a low-temperature component, a high-temperature component associated
with the interaction of the two filaments and another high-temperature
component apparently associated with the top of flare loops. We
computed the differential emission measure from the high-temperature
AIA bands and from this the expected microwave brightness temperature;
for emission associated with the top of the flare loops, the computed
brightness was 35% lower than the observed value.

---------------------------------------------------------
Title: Hot coronal loops associated with umbral brightenings
Authors: Alissandrakis, C. E.; Patsourakos, S.
2013A&A...556A..79A Altcode: 2013arXiv1307.3392A
 Aims: We aim to investigate the association of umbral
brightenings with coronal structures. Methods: We analyzed
AIA/SDO high-cadence images in all bands, HMI/SDO data, soft X-ray
images from SXI/GOES-15, and Hα images from the GONG network. Results: We detected umbral brightenings that were visible
in all AIA bands as well as in Hα. Moreover, we identified
hot coronal loops that connected the brightenings with nearby
regions of opposite magnetic polarity. These loops were initially
visible in the 94 Å band, subsequently in the 335 Å band, and
in one case in the 211 Å band. A differential emission measure
analysis revealed plasma with an average temperature of about 6.5
× 106 K. This behavior suggests cooling of impulsively
heated loops. Two movies are available in electronic from at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Study of the Three-Dimensional Shape and Dynamics of Coronal
Loops Observed by Hinode/EIS
Authors: Syntelis, P.; Gontikakis, C.; Georgoulis, M. K.;
Alissandrakis, C. E.; Tsinganos, K.
2012SoPh..280..475S Altcode: 2012SoPh..tmp..119S; 2012arXiv1206.0126S
We study plasma flows along selected coronal loops in NOAA Active
Region 10926, observed on 3 December 2006 with Hinode'sEUVImaging
Spectrograph (EIS). From the shape of the loops traced on intensity
images and the Doppler shifts measured along their length we compute
their three-dimensional (3D) shape and plasma flow velocity using a
simple geometrical model. This calculation was performed for loops
visible in the Fe VIII 185 Å, Fe X 184 Å, Fe XII 195 Å, Fe XIII
202 Å, and Fe XV 284 Å spectral lines. In most cases the flow is
unidirectional from one footpoint to the other but there are also cases
of draining motions from the top of the loops to their footpoints. Our
results indicate that the same loop may show different flow patterns
when observed in different spectral lines, suggesting a dynamically
complex rather than a monolithic structure. We have also carried out
magnetic extrapolations in the linear force-free field approximation
using SOHO/MDI magnetograms, aiming toward a first-order identification
of extrapolated magnetic field lines corresponding to the reconstructed
loops. In all cases, the best-fit extrapolated lines exhibit left-handed
twist (α&lt;0), in agreement with the dominant twist of the region.

---------------------------------------------------------
Title: On the shape of active region coronal loops observed by
Hinode/EIS.
Authors: Syntelis, P.; Gontikakis, C.; Alissandrakis, C.; Georgoulis,
M.; Tsinganos, K.
2012hell.confQ..14S Altcode:
We study plasma flows in NOAA Active Region (AR) 10926, observed on
December 3, 2006 with Hinode's EUV Imaging Spectrograph (EIS). We
measured the line-of-sight velocity along coronal loops in the Fe
VIII 185A, Fe X 184A , Fe XII 195A, Fe XIII 202A, and Fe XV 284A
spectral lines and reconstructed the three dimensional (3D) shape
and velocity of plasma flow using a simple geometrical model. In
most cases the flow is unidirectional from one footpoint to the other,
resembling siphon flow. However there are also cases of draining motions
from the top of the loops to their footpoints. The multi-wavelength
observations of the AR indicate that similar loops may show different
flow patterns if observed in different spectral lines. We have also
carried out magnetic field extrapolations using an SOHO/MDI and an
SOT/Spectropolarimeter (SP) magnetogram, in order to identify magnetic
field lines corresponding to the reconstructed 3D loops.

---------------------------------------------------------
Title: Radio Emission Associated with Solar Energetic Particle Events
Authors: Kouloumvakos, A.; Nindos, A.; Preka-Papadema, P.; Hillaris,
A.; Caroubalos, C.; Moussas, X.; Alissandrakis, C.; Tsitsipis, P.;
Kontogeorgos, A.
2012hell.confR..12K Altcode:
The corona probed at meter and decimeter wavelengths is a crucial region
for the acceleration and propagation of solar energetic particles
(SEPs), and radio diagnostics in this plasma plays a major role in
assessing the origin of SEP events. Using data from the ARTEMIS IV solar
radio spectro-graph, we report the properties of the radio emission
associated with several major SEP events. The association of the radio
emission with the related flares and CMEs is also investigated.

---------------------------------------------------------
Title: Multi-wavelength Observations of a Metric Type-II Event
Authors: Alissandrakis, C.; Nindos, A.; Patsourakos, S.; Hillaris,
Al.; Artemis Group
2012hell.conf....6A Altcode:
We have studied a complex metric radio event,observed with the ARTEMIS
radiospectrogarph on February 12, 2010. The event was associated with
a surge observed at 195 and 304 A and with a coronal mass ejection
observed by STEREO A and B instruments near the East wnd West limbs
respectively. On the disk the event was observed at 10 frequencies by
the Nancay Radioheliograph (NRH), in H-alpha by the Catania observatory
and in soft x-rays by GOES SXI. We combined these data, together with
MDI longitudinal magnetic field, to get as complete a picture of the
event as possible. Our emphasis is on two type-II bursts that occured
near respective maxima in the GOES light curves. The first, associated
with the main peak of the event, showed a clear foundamental-harmonic
structure, while the emission of the second consisted of three
well-separated bands. Using positional infornation for the type-IIs
from the NRH we explore their possible association with the surge,
the coronal front and the CME. We also studied fine structured and
foundamental harmonic structure in the metric dynamic spectrum.

---------------------------------------------------------
Title: Radio Emission of the Quiet Sun and Active Regions (Invited
Review)
Authors: Shibasaki, K.; Alissandrakis, C. E.; Pohjolainen, S.
2012esrs.book....3S Altcode:
Solar radio emission provides valuable information on the structure
and dynamics of the solar atmosphere above the temperature minimum. We
review the background and most recent observational and theoretical
results on the quiet Sun and active region studies, covering the entire
radio range from millimeter to decameter wavelengths. We examine
small- and large-scale structures, at short and long time scales,
as well as synoptic aspects. Open questions and challenges for the
future are also identified.

---------------------------------------------------------
Title: Radio Emission of the Quiet Sun and Active Regions (Invited
Review)
Authors: Shibasaki, K.; Alissandrakis, C. E.; Pohjolainen, S.
2011SoPh..273..309S Altcode: 2011SoPh..tmp..313S; 2011SoPh..tmp..246S; 2011SoPh..tmp..122S;
2011SoPh..tmp..288S; 2011SoPh..tmp..177S
Solar radio emission provides valuable information on the structure
and dynamics of the solar atmosphere above the temperature minimum. We
review the background and most recent observational and theoretical
results on the quiet Sun and active region studies, covering the entire
radio range from millimeter to decameter wavelengths. We examine
small- and large-scale structures, at short and long time scales,
as well as synoptic aspects. Open questions and challenges for the
future are also identified.

---------------------------------------------------------
Title: On the relationship of shock waves to flares and coronal
mass ejections
Authors: Nindos, A.; Alissandrakis, C. E.; Hillaris, A.;
Preka-Papadema, P.
2011A&A...531A..31N Altcode: 2011arXiv1105.1268N
Context. Metric type II bursts are the most direct diagnostic
of shock waves in the solar corona. Aims: There are two
main competing views about the origin of coronal shocks: that they
originate in either blast waves ignited by the pressure pulse of a
flare or piston-driven shocks due to coronal mass ejections (CMEs). We
studied three well-observed type II bursts in an attempt to place
tighter constraints on their origins. Methods: The type II
bursts were observed by the ARTEMIS radio spectrograph and imaged by
the Nançay Radioheliograph (NRH) at least at two frequencies. To take
advantage of projection effects, we selected events that occurred away
from disk center. Results: In all events, both flares and CMEs
were observed. In the first event, the speed of the shock was about
4200 km s-1, while the speed of the CME was about 850 km
s-1. This discrepancy ruled out the CME as the primary shock
driver. The CME may have played a role in the ignition of another shock
that occurred just after the high speed one. A CME driver was excluded
from the second event as well because the CMEs that appeared in the
coronagraph data were not synchronized with the type II burst. In
the third event, the kinematics of the CME which was determined by
combining EUV and white light data was broadly consistent with the
kinematics of the type II burst, and, therefore, the shock was probably
CME-driven. Conclusions: Our study demonstrates the diversity
of conditions that may lead to the generation of coronal shocks.

---------------------------------------------------------
Title: CME on CME Interaction on January 17, 2005
Authors: Bouratzis, C.; Preka-Papadima, P.; Moussas, X.; Hillaris,
A. E.; Caroubalos, C.; Alissandrakis, C. E.; Tsitsipis, P.;
Kontogeorgos, A.
2010ASPC..424...39B Altcode: 2010arXiv1009.3025B
On January 17, 2005 a complex radio event associated with an X3.8 SXR
flare and two fast Halo CMEs (CME1 &amp; CME2
henceforward) in close succession was observed. We present combined
ARTEMIS-IV &amp; WIND WAVES dynamic spectra which provide a complete
view of the radio emission induced by shock waves and electron beams
from the low corona to about 1 A.U. These are supplemented with data,
from the Nançay Radioheliograph (NRH), GOES, EIT and LASCO for the
study of the associated flare and CME activity.

---------------------------------------------------------
Title: On the Origin of Coronal Shocks
Authors: Nindos, Alexander; Alissandrakis, C. E.; Hillaris, A.;
Caroubalos, C.
2009SPD....40.1924N Altcode:
Type II bursts represent the passage of a shock wave through the
tenuous plasma of the solar corona. Their exact origin has not been
unambiguously determined, although they are associated with eruptive
phenomena and are identified either with a flare blast wave or with
a CME forward shock or with a shock driven by the flanks of a CME. We
investigate the location of the shock formation and its association with
flares and/or CMEs by studying the properties of three Type II radio
bursts associated with eruptions that occurred relatively close to the
limb where the influence of projection effects is rather small. The
Type II bursts were identified in 110-687 MHz range observations of
the ARTEMIS-IV Solar Radio spectrograph and observed simultaneously
with the Nancay Radioheliograph (NRH). The NRH makes images of the
Sun at several fixed frequencies between 150 and 432 MHz, and our
events could be mapped at two or three of its frequencies, allowing
us to determine the propagation of the shocks in the low corona. The
information provided by the radio data combined with observations of
the associated flares and CMEs by space-borne instruments has been
used to constrain the shock wave kinematics and discuss whether they
were flare-ignited or CME-driven.

---------------------------------------------------------
Title: New Developments in ARTEMIS IV Solar Radio Spectrograph
Authors: Alissandrakis, C. E.; Kontogeorgos, A.; Tsitsipis, P.;
Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hilaris, A.; Bougeret,
J. -L.; Dumas, G.
2009EM&P..104...93A Altcode:
We present recent developments of the ARTEMIS IV solar radio
spectrograph operating at Thermopylae, central Greece. Observations
are obtained daily in total intensity and in the frequency range from
20 to 650 MHz, using two antennas and two receivers. We are now in
the process of developing a new system that will record consecutively
the intensity of right-hand and left-hand polarized waves using one
of the antennas and the same receivers.

---------------------------------------------------------
Title: Metric radio bursts and fine structures observed on 17
January, 2005
Authors: Bouratzis, C.; Preka-Papadema, P.; Moussas, X.; Alissandrakis,
C.; Hillaris, A.
2009AdSpR..43..605B Altcode: 2010arXiv1009.3577B
A complex radio event was observed on January 17, 2005 with the
radio-spectrograph ARTEMIS-IV, operating at Thermopylae, Greece;
it was associated with an X3.8 SXR flare and two fast Halo CMEs in
close succession. We present dynamic spectra of this event; the high
time resolution (1/100 s) of the data in the 450-270 MHz range, makes
possible the detection and analysis of the fine structure which this
major radio event exhibits. The fine structure was found to match,
almost, the comprehensive Ondrejov Catalogue which it refers to the
spectral range 0.8-2 GHz, yet seems to produce similar fine structure
with the metric range.

---------------------------------------------------------
Title: A Statistical Study of Microwave Flare Morphologies
Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.
2008SoPh..253...79T Altcode: 2008SoPh..tmp..166T
This study has been motivated by the detection of a small number of
optically thin microwave bursts with maximum emission near the loop
top, which is contrary to the prediction of isotropic gyrosynchrotron
models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution
images at 17 and 34 GHz, we study the morphology at the radio peak of
104 flares that occurred relatively close to the limb. Using data from
the Nobeyama Polarimeter we were able to determine whether the 17-
and 34-GHz emissions came from optically thin or thick sources. We
identified single-loop events, taking into account supplementary
information from EUV and soft X-ray (SXR) images. We found optically
thin emission from the top of the loop in 36% of single-loop events. In
agreement with standard models, in this sample 46% and 18% of the events
showed optically thin emission from the footpoints and optically thick
emission from the entire loop, respectively. The derived percentage
of events with gyrosynchrotron emission from isotropic populations
of energetic electrons is possibly an upper limit. This point is
illustrated by the analysis of an optically thin event that shows
footpoint emission during the rise phase and loop-top emission during
the decay phase. A model that takes into account both anisotropies in
the distribution function of nonthermal electrons and time evolution
can reproduce the observed transition from footpoint to loop-top
morphology, if electrons with pitch-angle anisotropy are injected near
one of the footpoints.

---------------------------------------------------------
Title: Determination of the True Shape of Coronal Loops
Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C.
2008SoPh..252...73A Altcode: 2008SoPh..tmp..143A
Using line of sight velocity measurements from the SUMER and CDS
instruments aboard SOHO, in conjunction with a simple geometrical
model, we reconstructed the true, 3D shape and the velocity of plasma
flow along coronal loops. The projection of the loop on the sky and
the position of the footpoints define a family of curves. Assuming
that the loop is located on a plane, the line of sight velocity
can be used to select the most plausible solution. For two loops,
observed in the Ne VIII 770 Å and O V 630 Å spectral lines,
we find asymmetric, subsonic uni-directional flows, with velocity
maxima of ≈ 80 km s−1 near the footpoints. The loops
are highly inclined with respect to the vertical, by 55°
and 70°, respectively; thus the true height of the loop
tops from the photospheric level is ≈ 20”, comparable
to the isothermal scale height.

---------------------------------------------------------
Title: ARTEMIS, a Solar Radiospectrograph
Authors: Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hillaris,
A.; Mitsakou, E.; Pothitakis, G.; Bouratzis, C.; Tsitsipis, P.;
Kontageorgos, A.; Alissandrakis, C. E.; Bourgeret, J. -L.; Dumas, G.
2008RoAJ...18S.215C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: M otions Near The Solar Limb From TRACE Ly-α O bservations
Authors: Zachariadis, G.; Alissandrakis, C. E.
2007ESASP.641E..53Z Altcode:
We presenty preliminary results of a study of flows near the limb. The
observations were obtained with TRACE in a quiest region near the
limb in the Ly-α, (1216 Å) band, and cover two periods of about 19
and 16 min respectively with a cadence of ~ 10 sec. The high temporal
resolution of the images allows one to follow the evolution of small
bright features of the solar disk and the activity of the spicules
beyond the solar limb. The analysis of the images shows that many bright
feature, with lifetime of a few minutes, shift toward the solar limb,
with a velocity of about 14 km/sec. Spicules show motions both in the
radial direction and parallel to the solar limb.

---------------------------------------------------------
Title: M odeling The 3-D Geometry of Coronal Loops
Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C.
2007ESASP.641E..52A Altcode:
Loops are the basic structural elements of the solar atmosphere in
low beta regions, particularly in the corona. Important progress
has been made in recent years thanks to the SOHO and TRACE space
missions, which provided opportunities for better studies of the loop
morphology and their physical properties. In this work we use line of
sight velocity measurements from the SUMER and CDS instruments aboard
SOHO, in conjunction with a simple geometrical model, to reconstruct
the velocity of plasma flow along loops. The flow appears to be
uni-directional, with velocity maxima near the footpoints.

---------------------------------------------------------
Title: Type II and IV radio bursts in the active period
October-November 2003
Authors: Petoussis, V.; Tsitsipis, P.; Kontogeorgos, A.; Moussas,
X.; Preka-Papadema, P.; Hillaris, A.; Caroubalos, C.; Alissandrakis,
C. E.; Bougeret, J. -L.; Dumas, G.
2006AIPC..848..199P Altcode: 2010arXiv1009.4485P
In this report we present the Type II and IV radio bursts observed
and analyzed by the radio spectrograph ARTEMIS IV1, in the
650-20MHz frequency range, during the active period October-November
2003. These bursts exhibit very rich fine structures such fibers,
pulsations and zebra patterns which is associated with certain
characteristics of the associated solar flares and CMEs.

---------------------------------------------------------
Title: A Statistical Study of Microwave Flare Morphologies
Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.; Shibasaki, K.
2006AIPC..848..248T Altcode:
Using Nobeyama Radioheliograph (NoRH) high spatial resolution images
at 17 and 34 GHz, we study the morphology of several flare events that
occured relatively close to the limb. Our study has been motivated by
the recent detection of a small number of optically thin flares whose
maximum emission peaks close to the loop top. These events may show
significant anisotropy of the density and pitch angle distribution
of the nonthermal electrons. However, we do not know how often they
occur. Our sample consists of 104 flare events. Using data from the
Nobeyama Polarimeter we were able to determine whether the 17 and 34
GHz emissions are optically thin or thick. Almost half of our events
appear unresolved in the NoRH images. Among the resolved events,
special attention is paid to those with circular polarization (V)
showing two distinct sources. Such V sources are considered proxies to
the flaring loop footpoints if the total intensity (I) morphology is
consistent with a single flaring loop configuration. A small number of
events, as expected at such high frequencies, are optically thick with
I maximum between the loop footpoints. The number of optically thin
events showing I maxima co-spatial with the V sources is approximately
the same as the number of those optically thin resolved events showing
emission peak between the V sources. Our future plans concerning the
study of our database are also outlined.

---------------------------------------------------------
Title: Ten Years of the Solar Radiospectrograph ARTEMIS-IV
Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.;
Preka-Papadema, P.; Polygiannakis, J.; Moussas, X.; Tsitsipis, P.;
Kontogeorgos, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J. -L.;
Dumas, G.; Nindos, A.
2006AIPC..848..864C Altcode: 2010arXiv1009.4150C
The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV)
is in operation at the Thermopylae Satellite Communication Station
since 1996. The observations extend from the base of the Solar Corona
(650 MHz) to about 2 Solar Radii (20 MHz) with time resolution
1/10-1/100 sec. The instruments recordings, being in the form of
dynamic spectra, measure radio flux as a function of height in the
corona; our observations are combined with spatial data from the
Nancay Radioheliograph whenever the need for 3D positional information
arises. The ARTEMIS-IV contribution in the study of solar radio bursts
is two fold- Firstly, in investigating new spectral characteristics
since its high sampling rate facilitates the study of fine structures in
radio events. On the other hand it is used in studying the association
of solar bursts with interplanetary phenomena because of its extended
frequency range which is, furthermore, complementary to the range of the
WIND/WAVES receivers and the observations may be readily combined. This
reports serves as a brief account of this operation. Joint observations
with STEREO/WAVES and LOFAR low frequency receivers are envisaged in
the future.

---------------------------------------------------------
Title: Type II Shocks Characteristics: Comparison with associated
CMEs and Flares
Authors: Pothitakis, G.; Mitsakou, E.; Preka-Papadema, P.; Moussas,
X.; Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Tsitsipis,
P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G.
2006AIPC..848..238P Altcode: 2010arXiv1009.4294P
A number of metric (100-650 MHz) typeII bursts was recorded by
the ARTEMIS-IV radiospectrograph in the 1998-2000 period; the
sample includes both CME driven shocks and shocks originating from
flare blasts. We study their characteristics in comparison with
characteristics of associated CMEs and flares.

---------------------------------------------------------
Title: Type III events, flares and CMEs, in the extremely active
period October-November 2003
Authors: Mitsakou, E.; Thanasa, M.; Preka-Papadema, P.; Moussas, X.;
Hillaris, A.; Caroubalos, C.; Alissandrakis, C. E.; Tsitsipis, P.;
Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G.
2006AIPC..848..234M Altcode: 2010arXiv1009.3592M
The type III observations trace the propagation of energetic electron
populations through the Solar Corona which, more often than not,
precede or are associated with energy release on the Sun. A sample
of Type III bursts in the range 20-650 MHz during the period of
extraordinary solar activity (20 October to 4 November 2003) recorded
by the ARTEMIS-IV1 radio spectrograph is analysed; its
parameters are compared with characteristics of associated flares
(Ha and GOES SXR) and CMEs, observed in the same period and reported
in the SGR and the LASCO archives respectively. In this report we
attempt to establish a correlation between energetic particles and
major manifestations of solar activity such as flares and CMEs.

---------------------------------------------------------
Title: Periodicities in Fine Structure of Type IV Radio Solar Bursts
Authors: Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Preka-Papadema,
P.; Hillaris, A.; Petoussis, V.; Caroubalos, C.; Alissandrakis, C. E.;
Bougeret, J. -L.; Dumas, G.
2006AIPC..848..243T Altcode:
A study of periodicities appearing in the type-IV bursts fine structure
(fibers and pulsations in particular) is presented. Our data set
includes recordings by ARTEMIS IV with high time resolution; this
facilitates the detection of lower than 1 sec periodicities. We
introduce a method which employs directional filtering for the
separation of intertwined fibers and pulsations for independent analysis
of different events within complex radio bursts. For further analysis
we use Short Time Fourier and Wavelet transforms which enable the study
of the evolution of periodicities with time over specified periods
of solar radio activity. Since, according to existing theoretical
models, periodicities depend on basic parameters of active regions,
this methodology provides the appropriate diagnostic tools.

---------------------------------------------------------
Title: Detection of slopes of linear and quasi-linear structures in
noisy background, using 2D-FFT
Authors: Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Preka-Papadema,
P.; Hillaris, A.; Petoussis, V.; Caroubalos, C.; Alissandrakis, C. E.;
Bougeret, J. -L.; Dumas, G.
2006AIPC..848..874T Altcode:
We present a fast algorithm for slope detection on grey scale images,
based on 2D-FFT, which may be used for line or edge detection. Our
approach is based on the calculation of “energy” per direction of the
image, thus obtaining the Energy Spectrum on Slope (α). This exhibits
local maxima at the points where α equals the slopes of linear or
quasi-linear segments within the image, yet it is not affected by their
position within it. The process thus outlined was motivated by the study
of astrophysical images (Solar Dynamic Radio Spectra) which necessitated
the introduction of a method for fast extraction of drifting structures,
since they appear as linear or quasi linear segments on these spectra.

---------------------------------------------------------
Title: The improved ARTEMIS IV multichannel solar radio spectrograph
of the University of Athens
Authors: Kontogeorgos, A.; Tsitsipis, P.; Caroubalos, C.; Moussas,
X.; Preka-Papadema, P.; Hilaris, A.; Petoussis, V.; Bouratzis, C.;
Bougeret, J. -L.; Alissandrakis, C. E.; Dumas, G.
2006ExA....21...41K Altcode: 2006ExA...tmp...16K; 2010arXiv1009.3628K
We present the improved solar radio spectrograph of the University
of Athens operating at the Thermopylae Satellite Telecommunication
Station. Observations now cover the frequency range from 20 to
650 MHz. The spectrograph has a 7-meter moving parabola fed by a
log-periodic antenna for 100 650 MHz and a stationary inverted V fat
dipole antenna for the 20 100 MHz range. Two receivers are operating in
parallel, one swept frequency for the whole range (10 spectrums/sec,
630 channels/spectrum) and one acousto-optical receiver for the range
270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data
acquisition system consists of two PCs (equipped with 12 bit, 225
ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU
and 30 SFU in the 20 100 MHz and 100 650 MHz range respectively. The
daily operation is fully automated: receiving universal time from a
GPS, pointing the antenna to the sun, system calibration, starting
and stopping the observations at preset times, data acquisition,
and archiving on DVD. We can also control the whole system through
modem or Internet. The instrument can be used either by itself or in
conjunction with other instruments to study the onset and evolution
of solar radio bursts and associated interplanetary phenomena.

---------------------------------------------------------
Title: Observing the Sun at 20 650 MHz at Thermopylae with Artemis
Authors: Kontogeorgos, A.; Tsitsipis, P.; Moussas, X.; Preka-Papadema,
G.; Hillaris, A.; Caroubalos, C.; Alissandrakis, C.; Bougeret, J. -L.;
Dumas, G.
2006SSRv..122..169K Altcode:
Fine structure of type IV radio solar bursts with a great variety and
complexity often give much information in different ways and enable
estimation of various coronal characteristics. In this work, we expose
our new method for fine structure revealing and separation of two
basic kinds of type IV fine structure, as fibers and pulsations. We
also estimate frequency drift of fibers from dynamic spectra, clean
from continuous background, with a prototype method using 2-D Fourier
transform and we estimate periodicities of fibers as well as pulsations
with continuous wavelet transform. Working with the last method we
found periodicities close to 3 min umbral oscillations and 5 min global
solar oscillations.

---------------------------------------------------------
Title: Solar flares with and without SOHO/LASCO coronal mass ejections
and type II shocks
Authors: Hillaris, A.; Petousis, V.; Mitsakou, E.; Vassiliou, C.;
Moussas, X.; Polygiannakis, J.; Preka-Papadema, P.; Caroubalos, C.;
Alissandrakis, C.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.;
Dumas, G.
2006AdSpR..38.1007H Altcode: 2010arXiv1009.3636H
We analyse of a set of radio rich (accompanied by type IV or II
bursts) solar flares and their association with SOHO/LASCO Coronal
Mass Ejections in the period 1998 2000. The intensity, impulsiveness
and energetics of these events are investigated. We find that,
on the average, flares associated both with type IIs and CMEs are
more impulsive and more energetic than flares associated with type
IIs only (without CME reported), as well as flares accompanied by
type IV continua but not type II shocks. From the last two classes,
flares with type II bursts (without CMEs reported) are the shortest
in duration and the most impulsive.

---------------------------------------------------------
Title: Multiwavelength Analysis of an Active Region
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.;
Alissandrakis, C. E.; Nindos, A.; Vial, J. -C.; Tsiropoula, G.
2006SoPh..233...57G Altcode:
We study active region NOAA 8541, observed with instruments on board
SOHO, as well as with TRACE. The data set mainly covers the transition
region and the low corona. In selected loops studied with SUMER on SOHO,
the VIII 770 Å line is systematically redshifted. In order to estimate
the plasma velocity, we combine the Doppler shifts with proper motions
(TRACE) along these loops. In the case of an ejection, apparently
caused by the emergence of a parasitic polarity, proper motions and
Doppler shifts give consistent results for the velocity. A cooler loop,
observed in the same active region with CDS, shows a unidirectional
motion reminiscent of a siphon flow. The derived electron temperature
and density along a large steady loop confirm that it cannot be
described by hydrostatic models.

---------------------------------------------------------
Title: Trace Observations of Solar Spicules Beyond the Limb in -
and CIV
Authors: Alissandrakis, C. E.; Zachariadis, Th.; Gontikakis, C.
2005ESASP.600E..54A Altcode: 2005ESPM...11...54A; 2005dysu.confE..54A
No abstract at ADS

---------------------------------------------------------
Title: Complex Solar Events Observed with the Artemis-Iv
Radio-Spectrograph in October/november 2003
Authors: Alissandrakis, C. E.; Nindos, A.; Hilaris, A.; Caroubalos,
C.; Artemis Team
2005ESASP.600E.106A Altcode: 2005dysu.confE.106A; 2005ESPM...11..106A
No abstract at ADS

---------------------------------------------------------
Title: RATAN-600 Observations of Unusual Inversion of Polarization
in Sunspot Associated Microwave Sources
Authors: Gizani, Nectaria A. B.; Alissandrakis, C.; Bogod, V.;
Garaimov, V.; Zheleznyakov, V.; Zlotnik, E.
2005HiA....13..137G Altcode:
We present multi-wavelength observations of an active region in the
range of 1.7 to 30 cm obtained with the RATAN-600 radio telescope with
high polarization accuracy (up to 0.5%). The active region was observed
on six consecutive days in May 96. At short wavelengths the circular
polarization scans show a bipolar structure in conformity with the
bipolar magnetic field. However at 14.5 cm the circular polarization
drops practically to zero and at longer wavelengths the sense of the
polarization is reversed in {both components}. This was consistently
observed through all our six days of observation while the region
was moving from the center of the disk to the west limb. This unusual
polarization inversion cannot be interpreted in terms of the well-known
propagation effects under conditions of weak and strong coupling.

---------------------------------------------------------
Title: Solar type II and type IV radio bursts observed during
1998-2000 with the ARTEMIS-IV radiospectrograph
Authors: Caroubalos, C.; Hillaris, A.; Bouratzis, C.; Alissandrakis,
C. E.; Preka-Papadema, P.; Polygiannakis, J.; Tsitsipis, P.;
Kontogeorgos, A.; Moussas, X.; Bougeret, J. -L.; Dumas, G.; Perche, C.
2004A&A...413.1125C Altcode:
A catalogue of the type II and type IV solar radio bursts in the 110-687
MHz range, observed with the radio spectrograph ARTEMIS-IV operated
by the University of Athens at Thermopylae, Greece from 1998-2000 is
presented. These observations are compared with the LASCO archives
of Coronal Mass Ejections and the Solar Geophysical Reports of solar
flares (Hα &amp; SXR) and examined for possible associations. The
main results are: 68% of the catalogue events were associated
with CMEs. 67% of the type II events were associated with CMEs,
in accordance with previous results. This percentage rises to 79%
in the case of composite type II/IV events. 77% of the type IV
continua were associated with CMEs, which is higher that the CME-type
II association probability. The type II associated CMEs had an
average velocity of (835 ± 380) km s-1, while the CMEs
not associated with type IIs had an average velocity of (500 ± 150)
km s-1. All events, but one, were well associated with
H\alpha and/or SXR flares. Most of the CME launch times precede
by 5-60 min (30 min on average) the associated SXR flare peak; an
important fraction (72%) precede the flare onset as well. Appendix
A is only available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: The Improved Multichannel Solar Radiospectrograph ARTEMIS IV
(Abstract)
Authors: Kontogeorgos, P.; Tsitsipis, P.; Caroubalos, C.; Moussas, X.;
Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis,
J.; Bougeret, J. -L.; Dumas, G.; Perche, C.
2004hell.conf..338K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar Flares with and without Coronal Mass Ejections and Type
II shocks
Authors: Mitsakou, E.; Petousis, V.; Vassiliou, C.; Moussas, X.;
Polygiannakis, J.; Hillaris, A.; Preka-Papadema, P.; Caroubalos, C.;
Alissandrakis, C.; Tsitsipis, P.; Artemis Team
2004cosp...35.2293M Altcode: 2004cosp.meet.2293M
An analysis is presented of a set of “radio rich” solar SXR flares,
i.e. accompanied by type IV or II metric radio bursts, associated with
Coronal Mass Ejections and MHD shocks, recorded as type II events, in
the period 1998-2000. The relative size, impulsiveness and energetics
of these events are investigated. We find that, on the average, the
flares with type II bursts and CME's are larger, more impulsive and
energetic from the flares with type II but without CME's. The latter are
more energetic than flares associated with relatively “slow CME's”
accompanied by type IV continua but not type II shocks. Although a
simple classification may not be readily determined the results imply
that associated energetic events originate, more often than not,
from sources with characteristics fairly well correlated.

---------------------------------------------------------
Title: Study of 40 Solar Energetic Events Based on GOES, LASCO and
ARTEMIS IV Data (1998-2000)
Authors: Mitsakou, E.; Vassiliou, C.; Moussas, X.; Polygiannakis,
J.; Hillaris, A.; Preka-Papadema, P.; Caroubalos, C.; Alissandrakis,
C. E.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G.;
Perche, C.
2004hell.confR..77M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fine Structure in Type IV Solar Radio Bursts during three
major Solar Events of October-November 2003 observed by Artemis IV.
Authors: Tsitsipis, P.; Kontogeorgos, A.; Caroubalos, C.; Moussas,
X.; Hillaris, A.; Preka-Papadema, P.; Bougeret, J. -L.; Alissandrakis,
C.; Dumas, G.; Polygiannakis, J.
2004cosp...35.2705T Altcode: 2004cosp.meet.2705T
The fine structure in three solar type IV radio bursts was studied
using the French-Greek ARTEMIS-IV multichannel radio spectrograph. The
bursts were recorded during three major solar events, on the 26 and 28
October.and 3 Nov. 2003; these we associated with intense flares and
CMEs. The observed fine structure includes intermediate drift bursts
(fibers) and pulsations. The complexity and overlapping of the various
spectral characteristics necessitated the utilization of certain, 2D-FFT
based, filtering processes, aiming firstly at structure detection in
the frequency-time plane (ie the dynamic spectrum) and secondly in the
separation of fibers and pulsations and the suppression of the type
IV continuum. This methodology results also to the fibers frequency
drift rate distributions and their evolution in time. These fiber
frequency drift distributions may be interpreted as the exciter velocity
distributions, using a Newkirk coronal density model; the latter may
be used as diagnostics of the magnetic field restructuring and energy
release during a solar energetic event (flare and/or CME). Artemis IV
will provide complementary data to the STEREO/WAVES experiment.

---------------------------------------------------------
Title: Fine Radio Structures During a Major Solar Event Observed
with Artemis IV (Abstract)
Authors: Tsitsipis, P.; Kontogeorgos, A.; Caroubalos, C.; Moussas, X.;
Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis,
J.; Bougeret, J. -L.; Dumas, G.; Perche, C.
2004hell.confQ..64T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Unusual Inversion of Polarization in Sunspot Sources
Authors: Gizani, Nectaria A.; Alissandrakis, C. E.; Bogod, V.;
Garaimov, V.; Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E.
2003IAUJD...3E...2G Altcode:
We present multi-wavelength observations of an active region in the
range of 1.7 to 30 cm obtained with the RATAN-600 radio telescope with
high polarization accuracy (up to 0.5%). The active region was observed
on six consecutive days in May 96. At short wavelengths the circular
polarization scans show a bipolar structure in conformity with the
bipolar magnetic field. However at 14.5 cm the circular polarization
drops practically to zero and at longer wavelengths the sense of the
polarization is reversed in {both components}. This was consistently
observed through all our six days of observation while the region
was moving from the center of the disk to the west limb. This unusual
polarization inversion cannot be interpreted in terms of the well-known
propagation effects under conditions of weak and strong coupling.

---------------------------------------------------------
Title: Radio signatures of magnetic restructuring during the 2000
July 14 major solar event
Authors: Caroubalos, C.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.;
Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis,
J.; Bougeret, J. -L.; Dumas, G.; Kurt, V.; Vassiliou, C.; Perche,
C.; Gazeas, K.; Kolovos, G.
2002ESASP.505..377C Altcode: 2002IAUCo.188..377C; 2002solm.conf..377C
We analyse the fine structure of the type IV radio burst, during
the main phase of the 14th July 2000 major event recorded by the
radiospectrograph ARTEMIS-IV, in the 110-650 MHz (10 Hz sampling
rate) and the 270-450 MHz (100 Hz rate) range. In this report we
focus on variations in frequency drift rate and repetition rate of
fibers bursts. A pronounced asymmetry between the number of bursts
with positive and negative frequency drift is noted at certain time
periods. These variations follow closely the evolution of the magnetic
structure associated with the major solar event and the onset of the
corresponding CME.

---------------------------------------------------------
Title: Oscillations and waves near the limb from TRACE UV observations
Authors: Alissandrakis, C. E.; Zachariadis, Th. G.; Gontikakis, C.
2002ESASP.505..329A Altcode: 2002solm.conf..329A; 2002IAUCo.188..329A
We analyze TRACE observations near the limb in the 1550, 1600 and 1700
Å UV bands. Intensity-time images were computed at various distances
from the limb. Two dimensional Fourier analysis on the ω-k plane
shows very well the well-known p-mode ridges which persist up to the
limb in all three bands. Their power decreases almost proportionally
to the cosine of the heliocentric angle at 1600 and 1700 Å, being
more flat in the C IV 1550 Å band.

---------------------------------------------------------
Title: Study of coronal loops using TRACE and SOHO
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Nindos, A.;
Alissandrakis, C.; Tsiropoula, G.; Vial, J. -C.
2002ESASP.505..417G Altcode: 2002solm.conf..417G; 2002IAUCo.188..417G
We analysed coronal loops observed in many spectral lines from TRACE and
SUMER, at the active region NOAA 8541, on May 15, 1999. For the loops,
which are identified in a number of wavelengths, we try to combine
the good temporal and spatial resolution of TRACE with the spectral
information obtained by the SUMER rasters, in order to determine some
physical parameters such as temperature, flow velocity and electron
density. The morphology of the magnetic field of the loops is also
approximated by a force-free extrapolation of the photospheric magnetic
field, measured with MDI, and is compared to the loop morphology.

---------------------------------------------------------
Title: Spatially resolved microwave oscillations above a sunspot
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Bogod,
V. M.; Gontikakis, C.
2002A&A...386..658N Altcode:
Using high quality VLA observations, we detected for the first time
spatially resolved oscillations in the microwave total intensity
(I) and circular polarization (V) emission of a sunspot-associated
gyroresonance (g-r) source. Oscillations were detected at 8.5 and 5
GHz during several time intervals of our 10-hour-long dataset. The
oscillations are intermittent: they start suddenly and are damped
somehow more gradually. Despite their transient nature when they
are observed they show significant positional, amplitude and phase
stability. The spatial distribution of intensity variations is
patchy and the location of the patches of strong oscillatory power
is not the same at both frequencies. The strongest oscillations are
associated with a small region where the 8.5 GHz emission comes from
the second harmonic of the gyrofrequency while distinct peaks of weaker
oscillatory power appear close to the outer boundaries of the 8.5 and 5
GHz g-r sources, where the emissions come from the third harmonic of the
gyrofrequency. Overall, the 5 GHz oscillations are weaker than the 8.5
GHz oscillations (the rms amplitudes of the I oscillations are 1.3-2.5 x
104 K and 0.2-1.5 x 105 K, respectively). At both
frequencies the oscillations have periods in the three-minute range: the
power spectra show two prominent peaks at 6.25-6.45 mHz and 4.49-5.47
mHz. Our models show that the microwave oscillations are caused by
variations of the location of the third and/or second harmonic surfaces
with respect to the base of the chromosphere-corona transition region
(TR), i.e. either the magnetic field strength or/and the height of the
base of the TR oscillates. The best-fit model to the observed microwave
oscillations can be derived from photospheric magnetic field strength
oscillations with an rms amplitude of 40 G or oscillations of the height
of the base of the TR with an rms amplitude of 25 km. Furthermore,
small variations of the orientation of the magnetic field vector yield
radio oscillations consistent with the observed oscillations. Tables
1-3 are only available in electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Multiwavelength analysis of an active region observed with
SOHO and TRACE
Authors: Dara, H. C.; Gontikakis, C.; Zachariadis, Th.; Tsiropoula,
G.; Alissandrakis, C. E.; Vial, J. -C.
2002ESASP.477...95D Altcode: 2002scsw.conf...95D
We study the solar active region NOAA 8541, observed on May 15, 1999,
from 13:00 to 15:00 UT, with the instruments aboard SOHO (SUMER,
CDS, MDI) and TRACE. The SOHO observations produced a set of raster
scans of the region with a field of view of 159"×120" for SUMER and
of 244"×240" for CDS. TRACE gave a sequence of high time resolution
images for a much larger (510"×510") field of view, in several spectral
passbands, while MDI gave the photospheric full disk longitudinal
magnetic field. The data set mainly covers the transition region and
the low corona. We have used the data to construct intensity, velocity
and magnetic field maps of the region. The comparison of the intensity
images in various wavelengths with the velocity images, as well as
with the magnetic field, gives us information about the dynamical
characteristics of the observed features and their relationship to
the photospheric magnetic field.

---------------------------------------------------------
Title: Relation between intensity and Dopplershifts in the quiet
Sun transition region
Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th.; Alissandrakis,
C. E.; Vial, J. -C.
2002ESASP.477..107G Altcode: 2002scsw.conf..107G
In this work we used CDS and SUMER (SOHO) observations of a quiet
region (41°W 41°N) on the 28th of July 1996. The study of the
histograms of the velocities corresponding to the network and the
internetwork regions, shows that the network plasma is redshifted
with respect to the internetwork one. The shifts in the histograms
are small, but they are systematic for all lines. It should be
mentioned that the internetwork is also statistically redshifted,
with the exception of the He I line. In the low transition region the
morphology in the intensity images is different from the morphology
in the Dopplergrams. It seems that the network intensity structures
correspond to more than one in the Dopplergrams, since the latter
are quite smaller than the intensity structures. This was verified by
calculating the average autocorrelation function for intensity images
and Dopplergrams of spectral lines from SUMER and CDS. On the average,
the FWHM of the autocorrelation function of the intensity images is
two times larger than that of the corresponding Dopplergram ones.

---------------------------------------------------------
Title: Cooling and evacuation of an active region loop complex
observed with TRACE
Authors: Tsiropoula, G.; Gontikakis, C.; Dara, H. C.; Zachariadis,
Th.; Alissandrakis, C.; Vial, J. -C.
2002ESASP.477..179T Altcode: 2002scsw.conf..179T
We analyse the temporal variation of temperature and emission measure
at the top of a coronal loop system observed with the Transition
Region and Coronal Explorer (TRACE). Loops delineate regions of highly
localized heating and are thus typically the focus of coronal heating
theories. The analyzed data consist of observations in the 171 Å and
195 Å passbands taken at a cadence of ~10 min obtained on May 15,
1999 during an observing campaign. The temperature and emission measure
diagnostic is based in the 171 Å / 195 Å filter-ratio technique. The
loop system evacuates after the plasma at the top of the loops has
cooled. Estimates of the timescales of energy losses by radiation
and by conduction clearly suggest that the cooling is mainly due to
radiation losses.

---------------------------------------------------------
Title: ARTEMIS IV Radio Observations of the 14 July 2000 Large
Solar Event
Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Nindos,
A.; Tsitsipis, P.; Moussas, X.; Bougeret, J. -L.; Bouratzis, K.;
Dumas, G.; Kanellakis, G.; Kontogeorgos, A.; Maroulis, D.; Patavalis,
N.; Perche, C.; Polygiannakis, J.; Preka-Papadema, P.
2001SoPh..204..165C Altcode: 2010arXiv1009.3654C
In this report we present a complex metric burst, associated with
the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio
spectrograph at Thermopylae. Additional space-borne and Earth-bound
observational data are used, in order to identify and analyze the
diverse, yet associated, processes during this event. The emission at
metric wavelengths consisted of broad-band continua including a moving
and a stationary type IV, impulsive bursts and pulsating structures. The
principal release of energetic electrons in the corona was 15-20 min
after the start of the flare, in a period when the flare emission spread
rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation
of the CME also puts its origin in the same time interval, however,
the uncertainty of the extrapolation does not allow us to associate
the CME with any particular radio or X-ray signature. Finally, we
present high time and spectral resolution observations of pulsations
and fiber bursts, together with a preliminary statistical analysis.

---------------------------------------------------------
Title: Dopplershifts in the solar transition region
Authors: Gontikakis, C.; Dara, H. C.; Alissandrakis, C. E.;
Zachariadis, Th. G.; Vial, J. -C.
2001A&A...378..257G Altcode:
We study the dynamics of the quiet sun transition region, using
observations obtained with the SOHO CDS/NIS and SUMER spectrographs. We
examine the morphology of the network as a function of temperature and
we compare the intensity features with those of the dopplergrams. The
velocity distributions have a different behaviour for the bright
features which outline the network and the dark ones, located in
the internetwork. A redshift and a smaller standard deviation are
observed for the bright feature distributions relative to the dark
ones. It should be mentioned that the internetwork is also statistically
redshifted, with the exception of the He I line. Velocity distributions
from different lines are compared.

---------------------------------------------------------
Title: Ratan-600 observations of unusual inversion of polarization
Authors: Gizani, N. A. B.; Alissandrakis, C. E.; Bogod, V.; Garaimov,
V.; Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E.
2001A&AT...20..543G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Multi-wavelength analysis of a solar active region loop system
with SOHO, TRACE and ground-based telescopes
Authors: Tsiropoula, G.; Gontikakis, C.; Dara, H. C.; Zachariadis,
Th. G.; Alissandrakis, C.; Vial, J. -C.
2001hell.confE..40T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Statistical analysis of type IV radio bursts fine structure
observed during the 14th July 2000 large solar event.
Authors: Tsitsipis, P.; Kontogeorgos, A.; Hillaris, A.; Polygiannakis,
J.; Moussas, X.; Preka-Papadema, P.; Bouratzis, C.; Caroubalos, C.;
Alissandrakis, C.; Maroulis, D.; Patavalis, N.; Bougeret, J. L.;
Dumas, G.; Perche, C.
2001hell.confE..37T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Statistics on the occurence of CMEs related radio bursts
observed by ARTEMIS IV radio spectrograph during the maximum of the
solar cycle 23.
Authors: Bouratzis, C.; Desilas, C.; Hillaris, A.; Preka-Papadema,
P.; Moussas, X.; Caroubalos, C.; Alissandrakis, C.; Maroulis, D.;
Patavalis, N.; Polygiannakis, J.; Bougeret, J. -L.; Dumas, G.; Perche,
C.; Kontogeorgos, A.; Tsitsipis, P.
2001hell.confE..36B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Joint observations of solar radio bursts and cosmic ray events
recorded by the ARTEMIS IV radio spectrograph and Athens Cosmic Ray
Station (Super 6NM-64) of the University of Athens
Authors: Caroubalos, C.; Hillaris, A.; Bouratzis, C.; Mavromichalaki,
H.; Souvatzoglou, G.; Sarlanis, C.; Tatsis, S.; Maroulis, D.;
Patavalis, N.; Polygiannakis, J.; Moussas, X.; Preka-Papadema,
P.; Alissandrakis, C.; Bougeret, J. -L.; Dumas, G.; Perche, C.;
Kontogeorgos, A.; Tsitsipis, P.
2001hell.confE..35C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Fine Structure of the Quiet Solar Chromosphere: Limb-Crossing
Features
Authors: Zachariadis, Th. G.; Dara, H. C.; Alissandrakis, C. E.;
Koutchmy, S.; Gontikakis, C.
2001SoPh..202...41Z Altcode:
In this article we study chromospheric structures (spicules) crossing
the solar limb in Hα images corrected for limb darkening. This
correction enabled us to view structures both on the disk and beyond the
limb in the same image. The observations were obtained at the Sacramento
Peak Observatory at Hα±0.75 Å. The processed images reveal both
bright and dark (relative to the local background) features crossing
the limb. We also observed bushes (rosettes) crossing the limb, as well
as structures indicating probably arch-shaped mottles beyond the limb.

---------------------------------------------------------
Title: On-the-Disk Development of the Halo Coronal Mass Ejection on
1998 May 2
Authors: Pohjolainen, S.; Maia, D.; Pick, M.; Vilmer, N.; Khan, J. I.;
Otruba, W.; Warmuth, A.; Benz, A.; Alissandrakis, C.; Thompson, B. J.
2001ApJ...556..421P Altcode:
A halo coronal mass ejection (CME) was observed at 15:03 UT on 1998 May
2 by the Solar and Heliospheric Observatory Large-Angle Spectrometric
Coronagraph. The observation of the CME was preceded by a major soft
X-ray flare in NOAA Active Region 8210, characterized by a delta spot
magnetic configuration and some activity in region 8214. A large
transequatorial interconnecting loop (TIL) seen in the soft X-rays
connected AR 8210 to a faint magnetic field region in the periphery
of region 8214. Smaller loop systems were also connecting AR 8210 to
other fainter bipolar magnetic structures, the interconnecting loop
(IL) east of AR 8210 being one of the most visible. We present here
a multiwavelength analysis of the large- and small-scale coronal
structures associated with the development of the flare and of the
CME, with emphasis placed on radio-imaging data. In the early phases
of the flare, the radio emission sources traced the propagation paths
of electrons along the TIL and the IL, which are accelerated in the
vicinity of AR 8210. Furthermore, jetlike flows were observed in soft
X-rays and in Hα in these directions. Significantly, the TIL and
IL loop systems disappeared at least partially after the CME. An EUV
Imaging Telescope (EIT) dimming region of similar size and shape to the
soft X-ray TIL, but noticeably offset from it, was also observed. During
the “flash” phase of the flare, new radio sources appeared, presenting
signatures of destabilization and reconnection at discrete locations of
the connecting loops. We interpret these as possible signatures of the
CME liftoff on the disk. An Hα Moreton wave (blast wave) and an “EIT
wave” were also observed, originating from the flaring AR 8210. The
signatures in radio, after the wave propagated high into the corona,
include type II-like emissions in the spectra. The radio images link
these emissions to fast-moving sources, presumably formed at locations
where the blast wave encounters magnetic structures. The opening of
the CME magnetic field is revealed by the radio observations, which
show large and expanding moving sources overlying the later-seen EIT
dimming region.

---------------------------------------------------------
Title: Radio observations of filaments at metric and decimetric
wavelengths
Authors: Marqué, Ch.; Lantos, P.; Delouis, J. M.; Alissandrakis, C. E.
2001Ap&SS.277..329M Altcode:
A filament eruption at decimetric wavelength is illustrated here,
involving a quiescent filament seen in absorption. A CME occurs in
the vicinity of the event.

---------------------------------------------------------
Title: Joint EUV/Radio Observations of a Solar Filament
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bastian, T.;
Bocchialini, K.; Harrison, R. A.
2001SoPh..199..115C Altcode:
In this paper we compare simultaneous extreme ultraviolet (EUV) line
intensity and microwave observations of a filament on the disk. The
EUV line intensities were observed by the CDS and SUMER instruments on
board SOHO and the radio data by the Very Large Array and the Nobeyama
radioheliograph. The main results of this study are the following: (1)
The Lyman continuum absorption is responsible for the lower intensity
observed above the filament in the EUV lines formed in the transition
region (TR) at short wavelengths. In the TR lines at long wavelengths
the filament is not visible. This indicates that the proper emission of
the TR at the filament top is negligible. (2) The lower intensity of
coronal lines and at radio wave lengths is due to the lack of coronal
emission: the radio data supply the height of the prominence, while EUV
coronal lines supply the missing hot matter emission measure (EM). (3)
Our observations support a prominence model of cool threads embedded
in the hot coronal plasma, with a sheath-like TR around them. From the
missing EM we deduce the TR thickness and from the neutral hydrogen
column density, derived from the Lyman continuum and He i absorption,
we estimate the hydrogen density in the cool threads.

---------------------------------------------------------
Title: The New Multichannel Radiospectrograph ARTEMIS-IV/HECATE,
of the University of Athens
Authors: Caroubalos, C.; Maroulis, D.; Patavalis, N.; Bougeret, J. -L.;
Dumas, G.; Perche, C.; Alissandrakis, C.; Hillaris, A.; Moussas, X.;
Preka-Papadema, P.; Kontogeorgos, A.; Tsitsipis, P.; Kanelakis, G.
2001ExA....11...23C Altcode:
We present the new solar radiospectrograph of the University of Athens
operating at the Thermopylae Station since 1996. Observations cover the
frequency range from 110 to 688 MHz. The radiospectrograph has a 7-meter
parabolic antenna and two receivers operating in parallel. One is a
sweep frequency receiver and the other a multichannel acousto-optical
receiver. The data acquisition system consists of a front-end VME based
subsystem and a Sun Sparc-5 workstation connected through Ethernet. The
two subsystems are operated using the VxWorks real-time package. The
daily operation is fully automated: pointing of the antenna to the
sun, starting and stopping the observations at pre-set times, data
acquisition, data compression by `silence suppression', and archiving
on DAT tapes. The instrument can be used either by itself to study
the onset and evolution of solar radio bursts or in conjunction with
other instruments including the Nançay Decametric Array and the
WIND/WAVES RAD1 and RAD2 low frequency receivers to study associated
interplanetary phenomena.

---------------------------------------------------------
Title: Brightness Variations of mg ib Bright Features
Authors: Dara, H. C.; Koutchmy, S.; Zachariadis, Th. G.; Alissandrakis,
C. E.
2001SoPh..198..313D Altcode:
We study the temporal intensity variations of Mgib bright features and
investigate the corresponding Hα velocity pattern. The network bright
features are well visible in the continuum, in images averaged over the
duration of the observations (130 min). We detected `flashing' bright
features, which appear and disappear within two to five minutes, while
the rest of the bright features undergo small variations of either their
shape and/or their intensity. A power spectrum analysis reveals a 10-min
oscillation for approximately half of the stable bright features. The
5-min oscillations are detected mainly at the network boundaries,
where stable bright features are located, while 3-min oscillations
coincide with few bright features, but are also quite intense inside
the network cells. The majority of bright features are associated
with Hα downflows. The downflow is very intense at the location of
`flashing' bright features.

---------------------------------------------------------
Title: DEM Study of Selected Quiet Sun Regions
Authors: Gontikakis, C.; Landi, E.; Dara, H. C.; Alissandrakis, C. E.;
Vial, J. -C.
2001IAUS..203..390G Altcode:
In the present work EUV spectra of quiet Sun regions, observed with
the Coronal Diagnostic Spectometer (CDS), are analysed in order to
determine the Differential Emission Measure (DEM) of selected areas
of the field of view. In particular, we study the differences between
the DEM curves of the quiet Sun cell center areas, network areas and
cell-network boundaries. The results are discussed in the light of
theoretical models for the solar upper atmospheres.

---------------------------------------------------------
Title: Association of chromospheric sunspot umbral oscillations and
running penumbral waves. I. Morphological study
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Mein, P.
2000A&A...355..375T Altcode:
Observations of a sunspot region located near the center of the solar
disk were obtained on October 3, 1994, with the Multichannel Subtractive
Double Pass Spectrograph (MSDP). This instrument, operating in Hα ,
was installed at the focus of the VTT at Tenerife (Canary Islands)
and provided Hα intensity profiles at every pixel of the field of
view. Reconstruction of the Hα profile allowed the computation of
two dimensional intensity and Doppler velocity images at different
wavelengths within the line. We analyse a time series of 1 hour and
8 min, obtained with a cadence of 36 sec and investigate the relation
between umbral oscillations and running penumbral waves. The Doppler
velocity as a function of time, along radial cuts through the center of
the spot, shows several clear cases where waves that originate inside
the umbra continue to propagate in the penumbra. In one case we were
able to follow the evolution of an oscillating element for 216 sec,
from the inner part of the umbra to the penumbra and we describe the
propagation characteristics. We confirm the close association between
sunspot oscillations and running penumbral waves and suggest that they
are probably due to the same resonator.

---------------------------------------------------------
Title: RATAN-600 Observations of Unusual Inversion of Polarization
in Microwave Sources
Authors: Alissandrakis, C.; Gizani, N.; Bogod, V.; Garaimov, V.;
Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E.
1999ESASP.448.1225A Altcode: 1999mfsp.conf.1225A; 1999ESPM....9.1225A
No abstract at ADS

---------------------------------------------------------
Title: The Expedition of the University of Ioannina and the Academy
of Athens to Shabla, Bulgaria
Authors: Alissandrakis, C. E.; Zachariadis, Th.; Nakas, Ch.;
Ladoyanni, G.
1999ESASP.448.1287A Altcode: 1999mfsp.conf.1287A; 1999ESPM....9.1287A
No abstract at ADS

---------------------------------------------------------
Title: Association between Umbral Oscillations and Running Penumbral
Waves
Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P.
1999ESASP.448..217A Altcode: 1999ESPM....9..217A; 1999mfsp.conf..217A
No abstract at ADS

---------------------------------------------------------
Title: Structure and Dynamics in the Transition Region
Authors: Gontikakis, C. P.; Dara, H. C.; Alissandrakis, C. E.;
Zachariadis, Th. G.; Vial, J. -C.; Bastian, T.; Chiuderi Drago, F.
1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Field Diagnostics in the Low Corona from Microwave
Circular Polarization Inversion
Authors: Alissandrakis, C. E.
1999spro.proc...53A Altcode:
The characteristics of the observed polarization of radio waves
are determined by the emission mechanism and by the propagation
conditions in the corona. In the case of weak coupling between
the two electromagnetic wave modes, the polarization changes
along the ray path, reflecting the local physical conditions; this
results in inversion of the sense of circular polarization when a
transverse field region (TFR) is crossed. On the contrary, if the wave
coupling is strong, the polarization is fixed and its sense does not
change when a TFR is crossed. As a result, Stokes V maps of active
regions do not always correspond to the magnetic polarities shown in
photospheric magnetograms. The differences depend on the wavelength,
the heliographic position of the region, as well as on the density
and the magnetic field of the corona, at about 0.1 Rsolar
above the photosphere. In this short review I present older and recent
observation of polarization inversion and will discuss the diagnostics
they provide on the magnetic field.

---------------------------------------------------------
Title: Analysis of coronal emissions observed at meter wavelengths
Authors: Lantos, P.; Alissandrakis, C. E.
1999A&A...351..373L Altcode:
We present an extensive analysis of large and medium scale radio
sources observed with the Nançay Radioheliograph at 169 MHz, over
more than four solar rotations in 1984 (during the declining phase of
the solar cycle). The large scale emission is dominated by the coronal
plateau, the radio counterpart of the coronal plasma sheet, which had
an oscillatory structure around the equator. Medium scale local sources
include both faint noise storm continua and thermal sources. Noise storm
continua have greater brightness (8 105 to 3.8 106
K) and smaller diameter than thermal sources. We present distributions
of brightness temperatures, sizes as well as longitude and latitude
of the thermal sources and we study their location with respect to
faculae and neutral lines. Based on their rotation rate, we estimate
their altitude at 70 000 km. We find that most thermal sources at 169
MHz are located between faculae and neutral lines and we present a
geometrical model to account for this observation, according to which
these sources are located in the upper leg of large scale loops. A
small number of thermal sources is closer to neutral lines; these
might be loops at the base of isolated coronal streamers.

---------------------------------------------------------
Title: Metric and Decimetric Observations of the Quiet Solar Corona
Authors: Marqué, Ch.; Lantos, P.; Delouis, J. M.; Alissandrakis, C. E.
1999ESASP.446..483M Altcode: 1999soho....8..483M
We describe a multi-wavelength analysis of the solar corona in the
metric and decimetric range for a period in May 1992. The four radio
wavelengths, available with the Nancay Radioheliograph provide
a 3-dimensional view of the corona from about 1.05 to 1.2 solar
radius. Some of the radio features are relative to the large scale
structure of the corona (coronal plateau, coronal holes) and to medium
scale structures (streamers) we can follow during their transit on the
disk. We also observe thermal emission associated to active regions
and we compare locations of radio emissions with structures of the
X-ray loops.

---------------------------------------------------------
Title: Magnetic Field Diagnostics from Microwave Circular Polarization
Inversion
Authors: Alissandrakis, C. E.
1999ASPC..184...23A Altcode:
The characteristics of the observed polarization of radio waves
are determined by the emission mechanism and by the propagation
conditions in the corona. In the case of weak coupling between the two
electromagnetic wave modes, the polarization changes along the ray path,
reflecting the local physical conditions; this results in inversion
of the sense of circular polarization when a transverse field region
(TFR) is crossed. On the contrary, if the wave coupling is strong,
the polarization is fixed and its sense does not change when a TFR
is crossed. In this article I present older and recent observation
of polarization inversion and discuss the diagnostics they provide on
the magnetic field.

---------------------------------------------------------
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

---------------------------------------------------------
Title: Coronal Magnetography of an Active Region From Microwave
Polarization Inversion
Authors: Ryabov, B. I.; Pilyeva, N. A.; Alissandrakis, C. E.;
Shibasaki, K.; Bogod, V. M.; Garaimov, V. I.; Gelfreikh, G. B.
1999SoPh..185..157R Altcode:
The microwave circular polarization of the active region (AR) NOAA 7260
on 21-23 August 1992 is analyzed. Two-dimensional images at 1.76 cm
with spatial resolution of θ=10” from the Nobeyama radioheliograph
and one-dimensional scans at 9 wavelengths in the range of 1.81-3.43
cm and θ=16.3”-31.1” from the radio telescope RATAN-600 were
used. An inversion of the sense of circular polarization through
the wavelength range was recorded on 22 August. It is shown that
both the wavelength and the time dependence of the inversion are
consistent with quasi-transverse (QT) propagation of the radiation
in the solar corona. From this, the strength of the coronal magnetic
field in the active region was found to be H=20-65 G at a height of h=
(5.7-8.7)×109 cm above the photosphere on 22 and 30 August and 125
G at the lower height of (3.7-6.4)×109 cm on 23 August. We present a
new technique, based on the radio mapping (in both Stokes I and V) of
an AR undergoing circular polarization inversion; applying this method
to the Nobeyama data we obtained, for the first time, a magnetogram of
the coronal magnetic field. For AR 7260 we found values in the range of
70-100 G at heights of (4-6)×109 cm on 23 August, adopting a constant
value of N Lα (where N is the electron density and Lα is the scale
of the coronal field divergence) of 2.5×1018 cm−2. We compare our
results with force-free extrapolations of the photospheric magnetic
field from a MSFC magnetogram obtained on 20 August.

---------------------------------------------------------
Title: Dynamics of subrelativistic electron beams in the solar
corona. Type III group analysis
Authors: Hillaris, A.; Alissandrakis, C. E.; Bougeret, J. -L.;
Caroubalos, C.
1999A&A...342..271H Altcode:
A number of type III groups is studied. Wave particle interactions
are neglected and the so called drift approximation is used to model
the electron beam evolution. We extrapolated the observed Flux-Time
profiles towards higher frequencies and estimated the injection time
for the individual beams responsible for the excitation of the type
III's. A simple clustering algorithm, based on the temporal separation
of nearest neighbour injections as a proximity criterion, was used to
identify patterns of burst subgroups within the type III groups. The
question of whether the acceleration and injection of component bursts
exciters is coherently modulated in a single source, or this process
is driven by a statistical flare in a spatially fragmented energy
release site, is addressed.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Gradient of the line-of-sight velocities in the dark
superpenumbral fibrils.
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Mein, P.; Mein, N.
1999joso.proc..116T Altcode:
An isolated sunspot located near the centre of the solar disk was
observed on October 3, 1994. The present high resolution observations
were obtained with the Multichannel Subtractive Double Pass Spectrograph
(MSDP) operating in Hα. Two dimensional intensity and Doppler velocity
maps at several depths in the Hα line were computed. The monochromatic
intensities at two wavelengths on either side of the line are used in
terms of the photographic subtraction method expressing the "Doppler
signal" to give, under some assumptions, the velocities at different
depths in the Hα line. The values of the velocities obtained by
this method are compared to those given by the bisector method while
comparison of the velocities at different depths can give a picture
of the gradient of the velocity in the dark superpenumbral fibrils.

---------------------------------------------------------
Title: Mg bright points and the corresponding velocity pattern.
Authors: Dara, H. C.; Zachariadis, T.; Alissandrakis, C.; Koutchmy, S.
1999joso.proc..135D Altcode:
The authors study the temporal intensity variations of the Mg b1
bright points, in an effort to detect oscillations and investigate
the corresponding Hα velocity pattern. Apart from the "flashing
bright points", bright points which appear and disappear within two to
three minutes, the rest of the bright points undergo small intensity
variations. Power spectrum analysis revealed a 10-min oscillation
for half of the stable bright points. The 5-min oscillations are
detected mainly at sites where there are no bright points, while 3-min
oscillations coincide with some of the bright points. The majority of
the bright points are associated with Hα downflows. The downflow is
very intense at the "flashing bright points".

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Correction [to “A shock associated (SA) radio event and
related phenomena observed from the base of the solar corona to
1 AU”]
Authors: Bougeret, J. -L.; Zarka, P.; Caroubalos, C.; Karlický, M.;
Leblanc, Y.; Maroulis, D.; Hillaris, A.; Moussas, X.; Alissandrakis,
C. E.; Dumas, G.; Perche, C.
1998GeoRL..25.4103B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Microwave, Soft and Hard X-Ray Observations of Solar Flares -
a Self-Consistent Model of the Flare Site
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bentley, R. D.;
Philips, A. T.
1998SoPh..182..459C Altcode:
High-resolution microwave observations of several flares performed
with the Westerbork Synthesis Radio Telescope (WRST) on 3 and 4 July
1993 are compared with Yohkoh observations in the soft and hard X-ray
domain. Only for one flare, among the six analyzed, was the hard
X-ray spectrum between 20 and 200 keV available from the Wide Bragg
Spectrometer, supplying the energy spectrum of non-thermal particles
responsible for this radiation and for the radio emission. A complete
model of this flare is derived which accounts for all available
observations in the X-ray and radio wavelengths.

---------------------------------------------------------
Title: A shock associated (SA) radio event and related phenomena
observed from the base of the solar corona to 1 AU
Authors: Bougeret, J. -L.; Zarka, P.; Caroubalos, C.; Karlický, M.;
Leblanc, Y.; Maroulis, D.; Hillaris, A.; Moussas, X.; Alissandrakis,
C. E.; Dumas, G.; Perche, C.
1998GeoRL..25.2513B Altcode:
We present for the first time an almost complete frequency coverage of a
Shock Associated (SA) radio event and related phenomena observed on May
6, 1996 at 9:27 UT. It is observed from the base of the solar corona
up to almost 1 Astronomical Unit (AU) from the Sun by the following
radio astronomical instruments: the Ondřejov spectrometer operating
between 4.5 GHz and 1 GHz (radiation produced near the chromosphere);
the Thermopyles Artemis-IV spectrograph operating between 600 MHz
and 110 MHz (distance range about 1.1-1.4 R⊙ from sun
center); the Nançay Decameter Array operating between 75 and 25 MHz
(distance range about 1.4-2 R⊙) and the RAD2 and RAD1
radio receivers on the WIND spacecraft covering the range from 14
MHz to about 20 kHz (distance range between 3 R⊙ and
about 1 AU). Observations at the Nançay Decameter Array clearly show
that the SA event starts from a coronal type II radio burst which
traces the progression of a shock wave through the corona above 1.8
R⊙-2 R⊙ from the sun center. This SA event
has no associated radio emission in the decimetric-metric range, thus
there is no evidence for electron injection in the low/middle corona.

---------------------------------------------------------
Title: Umbral and Penumbral Waves in a Chromospheric Sunspot
Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P.
1998ASPC..155...49A Altcode: 1998sasp.conf...49A
No abstract at ADS

---------------------------------------------------------
Title: Second Advances in Solar Physics Euroconference:
Three-Dimensional Structure of Solar Active Regions
Authors: Alissandrakis, Costas E.; Schmieder, Brigitte
1998ASPC..155.....A Altcode: 1998sasp.conf.....A
No abstract at ADS

---------------------------------------------------------
Title: EUV and Microwave Observations of a Filament
Authors: Chiuderi-Drago, F.; Bocchialini, K.; Lamartinie, S.;
Vial, J. -C.; Bastian, T.; Alissandrakis, C. E.; Harrison, R.;
Delaboudiniere, J. -P.; Lemaire, P.
1998ASPC..150...55C Altcode: 1998IAUCo.167...55C; 1998npsp.conf...55C
No abstract at ADS

---------------------------------------------------------
Title: Quiescent filament diagnostic.
Authors: Lamartinie, S.; Bocchialini, K.; Vial, J. -C.; Chiuderi-Drago,
F.; Harrison, R.; Alissandrakis, C. E.
1998joso.proc..139L Altcode:
UV observations of a quiescent filament were performed on July 28,
1996 by SUMER and CDS, two spectrometers onboard SOHO.

---------------------------------------------------------
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

---------------------------------------------------------
Title: Sunspot-associated Sources, a Peculiar Source and a Halo-like
Source as Basic Components of the 3D Structure of a Large Active
Region from RATAN-600 and SSRT Observations
Authors: Kaltman, T. I.; Korzhavin, A. N.; Peterova, N. G.; Lubyshev,
B. I.; Maksimov, V. P.; Alissandrakis, C. E.; Fu, Q.
1998ASPC..155..140K Altcode: 1998sasp.conf..140K
No abstract at ADS

---------------------------------------------------------
Title: Unusual inversion of polarization in sunspot associated
microwave sources: RATAN-600 observations and their interpretation
Authors: Alissandrakis, C.; Gizani, Nectaria A. B.; Bogod, V.;
Garaimov, V.; Zheleznyakov, V.; Zlotnik, E.
1998cee..workE..72A Altcode:
Results of RATAN-600 observations of unusual sunspot-associated
microwave sources on May 12-16, 1996 and their interpretation are
given. The intensity and circular polarization of radio emission were
recorded at 39 frequencies in the wave band 1.74 cm - 30 cm. The
peculiarity of polarizational pattern is that the microwave source
was bipolar one, and at the wavelength about 15 cm the sense of
circular polarization reversed for both sunspots. The most important
and unexpected feature is that the frequency of reversal is the
same for preceding and following spots, and the picture as a whole
was invariable during 5 days of observations. It is easy to see that
the picture can't be explained by standard effect of linear coupling
of electromagnetic modes in transverse magnetic field originating
from bipolar group, because the reversal of polarization occurred
at both parts of bipolar source and the picture was invariable for
several days. It also can't be due to coupling in a neutral current
sheet, since transformation coefficient in zero magnetic field is
independent on frequency. Moreover, it can't be due to linear coupling
in transverse magnetic field in current sheet (in a case when magnetic
field in current sheet has small transverse component which makes
waves to couple not in zero, but transverse magnetic field), because
the angle in which reversal can take place is extremely small, and the
picture must vary with solar rotation. So, the only conceivable way
to explain observed polarization by linear coupling is to consider
magnetic field perpendicular to the plane connecting the spots of
the group, that is parallel to a solar meridian. It is obviously that
only such a field can provide independence of polarizational pattern
on solar rotation and reversal at the same frequency for both parts
of the group. We cannot say where this field originates from: it may
be a general magnetic field of the Sun (the form of magnetic field
created by vertical dipole in the center of the Sun suits observational
pattern) or some large-scale structure in the upper corona. Numerical
estimations are quite appropriate for reversal at 15 cm. This work
is supported by INTAS Grant No.94-4625 and partially by RFBR Grants
No.98-02-16238 and No.96-02-16598.

---------------------------------------------------------
Title: Three-dimensional structure of solar active regions. 2nd
Euroconference ASPE97 (Advances in Solar Physics Euroconference 1997),
Preveza (Greece), 7 - 11 Oct 1997. Poster contributions.
Authors: Schmieder, B.; Alissandrakis, C.
1998joso.proc...78S Altcode:
All of the oral presentations of ASPE97 will be published in a separate
volume of the ASP Conference Series, while this volume contains the
poster presentations.

---------------------------------------------------------
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.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Unusual inversion of polarization in microwave sources
associated with sunspot group: RATAN-600 observations and their
interpretation.
Authors: Alissandrakis, C. E.; Bogod, V. M.; Garaimov, V.; Gelfreikh,
G. B.; Zheleznyakov, V. V.; Zlotnik, E.
1998joso.proc..101A Altcode:
The data of multi-wavelength observations (in the range from 1.7 cm to
32 cm) on the reflector type radiotelescope RATAN-600 were obtained with
high polarization accuracy up to 0.5%. They detect a new polarization
phenomenon in radio emission of active region on the Sun. The signs
of polarization of the leading and following spot associated sources
are changed simultaneously for both sources at decimeter part of
the spectrum. One of the interpretation versions of the phenomenon
is proposed.

---------------------------------------------------------
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.

---------------------------------------------------------
Title: Modeling of a Microwave Burst Emission
Authors: Preka-Papadema, P.; Alissandrakis, C. E.; Dennis, B. R.;
Kundu, M. R.
1997SoPh..172..233P Altcode: 1997ESPM....8..233P
We applied model computations on a microwave burst observed with
the Westerbork Synthesis Radio Telescope (WSRT) at 6 cm. We used
additional data from Hα, soft and hard X-rays in order to reproduce
the flaring loop and to compute the microwave total intensity and
circular polarization. We examined both cases of thin and thick
target. The computations show a large emission source in an optically
thick loop. We compare our results with the observation.

---------------------------------------------------------
Title: Artemis Mark-IV, the New Greek-French Digital Radio
Spectrograph at Thermopyles, Greece
Authors: Maroulis, D.; Dumas, G.; Caroubalos, C.; Bougeret, J. L.;
Moussas, X.; Alissandrakis, C.; Patavalis, N.
1997SoPh..172..353M Altcode: 1997ESPM....8..353M
We present the new digital solar radio spectrograph located at
the Thermopyles station, Greece, operated by the University of
Athens. Observations cover the range from 110 to 600 MHz, using a
7-m parabolic antenna. The reception system uses two techniques in
parallel: sweep frequency and multi-channel, the latter being based
on the Acousto-Optical technique. The data acquisition system is based
on two subsystems, a Sun Sparc-5 workstation and a front end based on
a VME Motorola system. The two subsystems are connected through the
Ethernet and are operated using the VxWorks real-time package. The
daily operation is completely automated: pointing of the antenna to
the sun, starting and stopping the observations at pre-set times,
acquiring data, compressing data by silence suppression in real time,
and archiving the data on a routine manner on DAT tapes. Apart from
its usual function, this instrument will be used in conjunction with
other instruments, including the Nançay decameter array and the low
frequency radio receivers on the Wind spacecraft.

---------------------------------------------------------
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 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: Evolution in Space and Time of Superpenumbral Chromospheric
Fibrils
Authors: Tsiropoula, G.; Dialetis, D.; Alissandrakis, C. E.; Mein,
P.; Mein, N.
1997SoPh..172..139T Altcode: 1997ESPM....8..139T
We have studied the spatial structure and temporal evolution of the
intensity and Doppler velocity of dark fibrils forming the superpenumbra
of an isolated regular sunspot. The observations were obtained with the
Multichannel Subtractive Double Pass (MSDP) spectrograph which operates
in Hα and is installed at the focus of the Vacuum Tower Telescope (VTT)
at Tenerife (Canary Islands). The fibril pattern shows a remarkable
stability during the period of our observations (64 min). Moreover,
almost all individual fibrils are identifiable in all frames, but they
undergo continual changes in contrast, shape and size. Investigating the
temporal evolution of intensity and velocity of individual fibrils,
fluctuations were found which have a quasi periodic behavior. As
mechanisms for these changes we may suggest (a) change of the Doppler
shift due to a wave, (b) periodic changes of the density of the Hα
absorbing material, (c) disappearance and reappearance of fibrils,
in more or less the same magnetic flux tube, at regular intervals.

---------------------------------------------------------
Title: Radio Observations of the Quiet Sun and Their Implications
on Coronal Heating
Authors: Alissandrakis, Costas E.; Einaudi, Giorgio
1997LNP...483...53A Altcode: 1997cprs.conf...53A
We discuss the actual and potential contribution of radio techniques
in the study of the structure and the dynamics of the corona as well as
in the problem of coronal heating. Radio observations provide powerful
diagnostics of the physical conditions in the transition region and the
corona. Recent observational and theoretical results are presented and
their implications are discussed. The prospects of further observations,
in conjunction with other wavelength ranges are given.

---------------------------------------------------------
Title: Testing the Extrapolated Coronal Magnetic Fields by Using
Propagation Effects at Microwaves
Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Borgioli, F.
1997ESASP.404..275C Altcode: 1997cswn.conf..275C
No abstract at ADS

---------------------------------------------------------
Title: Radio Observations of Sunspots
Authors: Alissandrakis, C. E.
1997ASPC..118..150A Altcode: 1997fasp.conf..150A
High spatial resolution observations of sunspots have been made
with aperture synthesis instruments for over two decades. Sunspots
appear as bright features at short cm wavelengths, due to thermal
gyroresonance (g-r) emission from layers where the local gyrofrequency
is a sub-harmonic of the observing frequency, providing a powerful
diagnostic of the sunspot magnetic field above the photosphere.

---------------------------------------------------------
Title: Medium and large scale structures of the low corona from
decimeter and meter wavelength observations.
Authors: Lantos, P.; Coulais, A.; Alissandrakis, C. E.
1997ASIC..494..121L Altcode: 1997topr.conf..121L
Since 1991, the Nançay Radioheliograph provides daily observations of
the Sun at wavelengths from 69 cm (435 MHz) to 1.83 m (164 MHz). Older
observations were available mostly at 169 or 164 MHz. Two-dimensional
maps could be computed with an aperture synthesis method when the
brightness distribution remains constant during the day (i.e. in absence
of strong noise storms). At meter wavelengths, detected large scale
structures are coronal holes, the quiet sun and a new component called
"coronal plateau" which is a signature at low altitude of the coronal
streamer belt and of the heliospheric current sheet. Medium scale
structures observed at meter and decimeter wavelengths are explained
as emissions in loop systems. They may be either faint noise storm
continua or thermal emissions.

---------------------------------------------------------
Title: Microwave and UV observations of filaments with SOHO and
the VLA
Authors: Alissandrakis, C. E.; Drago, F.; Bastian, T.; Bocchialini,
K.; Delaboudiniere, J. -P.; Lemaire, P.; Vial, J. -C.; Harrison,
R. A.; Thompson, B.
1997ASPC..118..289A Altcode: 1997fasp.conf..289A
Observations performed in coordination between SOHO instruments and
ground-based observatories offer the unique possibility to derive
information simultaneously in several wavelengths formed at different
altitudes and/or temperatures in the solar atmosphere. The SUMER and
CDS spectrometers, the imaging telescope EIT aboard SOHO, and the VLA
provide complementary information in the UV and the radio ranges. We
illustrate such a coordination with observations of filaments in the
transition region, performed in July 1996. The observations in the UV
between 10(4) and 10(6) K provide the differential emission measure
as a function of temperature; this can be used to compute the expected
brightness temperature in the microwave range and check models of the
filament-corona transition region.

---------------------------------------------------------
Title: Coronal Magnetic Fields from Microwave Polarization
Observations
Authors: Alissandrakis, C. E.; Borgioli, F.; Chiuderi Drago, F.;
Hagyard, M.; Shibasaki, K.
1996SoPh..167..167A Altcode:
The solar active region (AR) 7530 was observed at 6 cm on July 3
and 4, 1993 with the Westerbork Synthesis Radio Telescope, using a
multi-channel receiver with very narrow bandwidth. We compare the
radio data with Yohkoh SXT observations and with the magnetic field
extrapolated from the Marshall vector magnetograms in the force-free
and current-free approximations. The comparison with soft X-rays shows
that, although a general agreement exists between the shape of the
radio intensity map and the X-ray loops, the brightness temperature,
Tb, obtained using the parameters derived from the SXT is
much lower than that observed. The comparison with the extrapolated
photospheric fields shows instead that they account very well for
the observed Tb above the main sunspots, if gyroresonance
emission is assumed. In the observation of July 4 an inversion and
strong suppression of the circular polarization was clearly present
above different portions of the AR, which indicates that particular
relationships exist between the electron density and the magnetic
field in the region where the corresponding lines of sight cross the
field quasi-perpendicularly. The extrapolated magnetic field at a
much higher level (∼ 1010 cm), satisfies the constraints
required by the wave propagation theory all over the AR. However,
a rather low electron density is derived.

---------------------------------------------------------
Title: Periodic and Non-Periodic Phenomena in a Sunspot Region
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Dialetis, D.; Mein, P.
1996SoPh..167...79T Altcode:
We have studied running penumbral waves, the homogeneous Evershed
effect, and the spatial relation between intensity and Doppler velocity
penumbral features of a chromospheric sunspot. The observations were
obtained with the multichannel subtractive double-pass spectrograph
(MSDP) operating in Hα at the Vacuum Tower Telescope (VTT) installed
at Tenerife (Canary Islands). We derived intensity and Doppler velocity
maps at Hα ± 0.3 Å over a two-dimensional field of view. We have
computed the components of the velocity vector (radial, azimuthal,
vertical) as a function of distance from the center of the spot under
the assumption of axial symmetry. The results show the well-known,
from previous observations, general large-scale characteristics of the
chromospheric Evershed flow. Our measurements show that the axes along
the discrete structures, where the Evershed flow is confined, are not
spatially related to the axes along Hα ± 0.3 Å intensity features,
and we suggest that either the flow is confined in flow channels or that
it takes place along sheared magnetic field lines. We also detected,
for the first time in velocity images, running penumbral waves, which
started in the outer 0.3 of the umbral radius and propagated through
the penumbra with propagation velocities 13-24 km s−1. The
propagation velocity, as well as the velocity amplitude, is greater
for the waves closer to the center of the spot and diminishes as one
moves outward.

---------------------------------------------------------
Title: A Model for Active Region Emission at Centimeter Wavelengths
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Kundu,
M. R.; Dere, K. P.; Korzhavin, A. N.; Bogod, V. M.
1996SoPh..166...55N Altcode:
We present multi-frequency observations and model computations of the
microwave emission of a solar active region. The radio observations
were obtained with the RATAN-600 at several wavelengths between 0.8
and 31.6 cm and with the VLA at 6 and 20 cm. The active region was
also observed in the EUV O Iv lines by the HRTS instrument aboard
the Space Shuttle Spacelab-2 mission. These lines are formed in the
chromosphere-corona transition region and their intensity ratio is
sensitive to pressure. Photospheric magnetograms provided both the
longitudinal and the transverse component of the magnetic field. The
microwave observations were checked against model computations
taking into account both the free-free and the gyro-resonance emission
mechanisms and using the pressure data from the O IV lines. The magnetic
field was computed through constant-α force-free extrapolations of
the longitudinal photospheric field. We computed both the flux from
2 to 20 cm and the spatial structure of the microwave emission at 6
and 20 cm. The comparison of the computed and observed flux spectra
allowed us to estimate the magnetic field strength at the base of the
transition region and in the low corona, as well as the values of the
conductive flux and the height of the base of the transition region. The
model maps at 6 cm and 20 cm showed that α was not constant above the
active region; the same conclusion was reached on the basis of the
photospheric observations. The use of pressure measurements allowed
us to identify microwave structures which were determined by pressure
enhancements. At 6 cm the computations confirmed the fact that the
magnetic field is the principal factor that determines the structure
of sunspot-associated sources and showed that the effect of pressure
variations was small. Pressure variations were more important at 20 cm,
where the peak of the emission was associated with the sunspot and a
diffuse component was associated with the plage which had an average
pressure higher by a factor of 1.54 than the sunspot.

---------------------------------------------------------
Title: Synoptic Study of Coronal Structures Observed at Meter
Wavelengths during the Declining Phase of the Solar Cycle
Authors: Alissandrakis, C. E.; Lantos, P.
1996SoPh..165...61A Altcode:
We have produced synoptic maps from daily images of the solar corona
on the disk obtained with the Nançay Radioheliograph at 169 MHz during
June-August 1984. We discuss briefly the structures seen on the synoptic
maps and we compare the location of radio sources with photospheric
neutral lines and Ha filaments derived from synoptic charts from Meudon,
Boulder, and Stanford. We measured the distance of radio sources from
these optical features and compared their distribution with computed
random distributions. A criterion of confidence is proposed which,
when applied to our data, shows that the metric radio sources of
thermal origin are best associated with the large-scale neutral lines
shown on the low resolution Stanford magnetograms. This association
indicates that the emission comes from arcades of moderately dense
loops spanning the neutral line. However, the radio sources are not
usually located directly above or very close to neutral lines, but
their distribution peaks at distances between 4 and 6°. Our results
show no statistically significant association with Meudon filaments.

---------------------------------------------------------
Title: Coronal Sources at Meter and Optical Wavelengths during the
Declining Phase of the Solar Cycle
Authors: Lantos, P.; Alissandrakis, C. E.
1996SoPh..165...83L Altcode:
We present observations of the corona at 169 MHz with the Nançay
Radioheliograph during the summer of 1984. We compare synoptic maps
of the metric radio emission on the solar disk with synoptic charts of
the K-corona as well as of the green and the red lines. Local sources
of radio emission are not located near regions of enhanced green or
red line emission which, in turn, are in general above chromospheric
faculae. Thus the radio emissions located in the surroundings of
faculae are apparently related to different loop systems, with lower
density. The comparison of the radio data with the K-corona showed one
radio source associated with enhanced emission both at 1.3 and at 1.7
R⊙, apparently a streamer. Other radio sources did not
show any clear associations, but were nevertheless located within
the coronal plasma sheet, delineated by the large-scale K-corona
emission. Moreover the large-scale structure of the corona at 169
MHz was quite similar to the coronal plasma sheet observed at 1.3
R⊙ above the limb. The extent of the radio emission in
latitude is very similar to that of the K-corona, while the coronal
line emission is more concentrated near the solar equator.

---------------------------------------------------------
Title: Two-Dimensional Mapping of the Sun with the RATAN-600
Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Borovik,
V. N.; Korzhavin, A. N.; Bogod, V. M.
1996SoPh..165...41N Altcode:
We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm
computed from one-dimensional observations with the RATAN-600, using
Earth rotation aperture synthesis techniques. Before the calculation
of maps, the position of each scan was corrected with respect to the
center of the solar disk and the scans were calibrated. The circular
polarization scans were corrected for polarization cross-talk between
the I and V channels. Subsequently, the quiet-Sun background emission
was subtracted. After all corrections, a dirty map was computed by
combining the scans at different position angles. The last step of
the processing was an attempt to free the dirty map of the sidelobes,
using the standard CLEAN procedure. The resolution of the clean maps
at 2.7 cm was 0.5' by 6'. Both active regions which were present on
the solar disk were mapped. We studied the flux spectra of different
types of sources: one was associated with a sunspot, the second was
located over the neutral line of an active region, and the other was
associated with the plage. The emission mechanism of the former was
attributed to the gyroresonance process, while the short wavelength
emission of the others was attributed to the free-free process. For
the sunspot-associated source we estimated the magnetic field strengths
at the base of the transition region and in the low corona.

---------------------------------------------------------
Title: Large scale structure of the solar corona observed at meter
and decimeter wavelengths and in X-rays
Authors: Lantos, P.; Alissandrakis, C. E.; YOHKOH Team
1996AdSpR..17d.261L Altcode: 1996AdSpR..17..261L
Maps of the quiet solar corona, obtained at meter and decimeter
wavelengths with the Nancay (France) Radioheliograph, are compared
to Yohkoh soft X-ray images in order to study large scale density
structures of the corona. Of particular interest is a global
restructuring of the corona, observed on 19 May 1992 in X-rays, in
conjunction with a coronal mass ejection. The radio observations show
the CME moving continuum, while the Yohkoh images show the slower
motion of the X-ray loops. After this event the appearance of a new
noise storm, with a duration of several days, was observed.

---------------------------------------------------------
Title: Running Penumbral Waves in a chromospheric Sunspot
Authors: Tsiropoula, G.; Dialetis, D.; Alissandrakis, C. E.; Mein, P.
1996hell.conf...37T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Coronal Magnetic Fields from Polarization Observations at
Microwaves
Authors: Chiuderi Drago, F.; Borgioli, F.; Alissandrakis, C. E.;
Hagyard, M.; Shibasaki, K.
1996mpsa.conf..443C Altcode: 1996IAUCo.153..443C
No abstract at ADS

---------------------------------------------------------
Title: The fine scale Structure of the velocity Field in the
chromospheric Penumbra of a solar Sunspot
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Dialetis, D.; Mein, P.
1996hell.conf...32T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evolution of the coronal plasma sheet from cycle maximum to
cycle minimum observed at meter wavelengths
Authors: Lantos, P.; Alissandrakis, C. E.
1995AdSpR..16i.185L Altcode: 1995AdSpR..16..185L
The Nancay Radioheliograph provides maps of the quiet solar corona
around 160 MHz. In addition to coronal holes, localized emission
sources and quiet sun, an unresolved component can be detected on the
disk. The so-called “coronal plateau” corresponds to the K-corona
dense regions observed on the limb and is thus located at the base of
the interplanetary plasma sheet. Its evolution is studied from 1980
to 1986, during the declining phase of the cycle 21.

---------------------------------------------------------
Title: Large Scale Structure of the Solar Corona in the Declining
Phase of the Solar Cycle
Authors: Lantos, P.; Alissandrakis, C. E.
1995SSRv...72...45L Altcode:
Maps of the corona, obtained at meter wavelengths with the Nançay
Radioheliograph (France), are used to study, on the disk, the radio
counterpart of the coronal plasma sheet observed in K-corona on the
limb. We study here the evolution of the coronal plasma sheet from
the maximum of the activity cycle in 1980 to the minimum in 1986 and
identify some of its large scale structures.

---------------------------------------------------------
Title: Dynamical fine structures of the chromosphere
Authors: Tsiropoula, G.; Schmieder, B.; Alissandrakis, C. E.
1994SSRv...70...65T Altcode:
We have studied the spatial structure and temporal evolution of a
chromospheric region with chains of mottles at the junction of three
supergranules. The observations were obtained with the Multichannel
Subtractive Double Pass spectrograph operating in Hα at the Pic du
Midi Observatory. From a statistical point of view the fine structures
were stable in intensity over the observation period (15 min), while the
line of sight velocity showed remarkable changes within a few minutes. A
detailed analysis shows that the predominant pattern of bulk motion in
the dark mottles is that of downflow at their footpoints and upflow at
their tops, which is followed by downflows along the whole structure and
that this process repeats itself several times during their lifetime.

---------------------------------------------------------
Title: Time evolution of fine structures in the solar chromosphere.
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B.
1994A&A...290..285T Altcode:
We have studied the temporal evolution of two quiet chromospheric
regions, one with a typical rosette and another with chains of
mottles at the junction of three supergranules. The observations
were obtained during 15 minutes with the Multichannel Subtractive
Double Pass spectrograph (MSDP) operating in Hα at the Pic du Midi
Observatory. We derived intensity maps and Doppler shift velocities at
different wavelengths along the Hα profile over a two dimensional
field of view. The observed contrast profiles were matched with
theoretical contrast profiles using Beckers' cloud model for a more
accurate determination of the line of sight velocity. A statistical
analysis with cross correlation functions showed that the fine
structures were stable in intensity over the observation period (15
min), but the line of sight velocity showed important changes within
a few minutes. A detailed analysis of the velocities along the axes
of dark mottles showed that the predominant pattern of bulk motion is
that of downflow at their footpoints and alternating phases of upflow
and downflow at their tops. This motion is consistent with Pikel'ner's
model for spicules, which attributes this pattern to the reconnection of
opposite magnetic filed lines. This picture is also consistent with the
velocity reversals with time observed in spicules and may be associated
to the systematic downflows observed in the transition region. Doppler
shift velocities in dark mottles are too low compared to those derived
with the cloud model; the latter are comparable to those reported for
spicules, strengthening the view that these structures are identical.

---------------------------------------------------------
Title: Detection of Linear Polarization in the Microwave Emission
of Solar Active Regions
Authors: Alissandrakis, C. E.; Chiuderi-Drago, F.
1994ApJ...428L..73A Altcode:
We report the detection for the first time of linear polarization in the
microwave emission above a Solar Active Region. The observations were
made with the Westerbork Synthesis Radio Telescope, taking advantage of
the very small bandwidth of a multichannel spectral line receiver. The
intensity of the Stokes parameter U, measured at several points close
to the line of zero circular polarization, shows a clear sinusoidal
trend as a function of lambda2, in accordance to what is
expected from Faraday rotation. From the measured period, reasonable
values of integral NBLdr were derived. Such observations
could present in the future a unique way to measure coronal magnetic
fields above Active Regions.

---------------------------------------------------------
Title: Meter wavelength observations of the quiet sun and the slowly
varying component during declining phase of the solar cycle
Authors: Lantos, P.; Alissandrakis, C. E.
1994AdSpR..14d..97L Altcode: 1994AdSpR..14...97L
Two dimensional maps of the Sun at 164 MHz, obtained with the Nançay
Radioheliograph from April to August 1984 are presented. The quiet
sun emission and the slowly varying component far from sunspots are
studied. The association of local emissions with the chromospheric
calcium plages is pointed out and interpreted. The altitude of the radio
sources is estimated. The comparison of the location of the sources
with K-corona observations confirms previous results obtained for the
minimum of the cycle. The brightness temperature of the coronal plateau,
which is the radio counterpart of the coronal plasma sheet, is given.

---------------------------------------------------------
Title: Radio observations of the quiet solar corona
Authors: Alissandrakis, C. E.
1994AdSpR..14d..81A Altcode: 1994AdSpR..14...81A
Observations of radio emission from the quiet solar atmosphere and
the associated emission mechanisms are discussed. Long wavelength
observations, with a spatial resolution of ~1 arc min are ideal
for the study of the large scale structure of the corona, including
coronal holes, arches, streamers and the base of the heliospheric
current sheet. These structures are observed both beyond the limb
and on the solar disk, with little line of sight integration. We also
discuss diagnostics of electron temperature and density, as well as
the interpretation problems arising from the inhomogeneity of the
corona and refraction effects.

---------------------------------------------------------
Title: The magnetic field of the solar corona.
Authors: Alissandrakis, C. E.
1994LNP...432..109A Altcode: 1994LNPM...11..109A
The structure of the solar corona, on all observable scales,
is intimately controlled by the magnetic field. Although direct
measurements of the magnetic field are from difficult to impossible,
its presence is evident in all spectral ranges where the corona is
observable. This review discusses the measurement of coronal magnetic
fields and the indirect information provided by radio, white light
and soft X-ray observations.

---------------------------------------------------------
Title: Time Evolution of Chromospheric Fine Structures
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B.
1994emsp.conf..111T Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Magnetic Field of a Bipolar Sunspot Region in the
Photosphere and the Chromosphere
Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E.
1994emsp.conf...61D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evidence for Ordinary Mode Emission from Microwave Bursts
Authors: Alissandrakis, C. E.; Nindos, A.; Kundu, M. R.
1993SoPh..147..343A Altcode:
We analyze high-resolution, one-dimensional observations of simple
microwave bursts, obtained at 4.9 GHz with the Westerbork Synthesis
Radio Telescope in 1980, together with Hα photographs of the associated
flares from the Observatories of Athens and Meudon. In most cases the
polarization structure can be interpreted in terms of extraordinary mode
emission, taking into account the polarity of the underlying magnetic
field and propagation effects, which may lead to inversion of the sense
of polarization in the limbward part of the flaring loop. We found
evidence for ordinary mode emission in two classes of events. In one
class theo-mode comes from regions overlying strong magnetic field,
which we interpret in terms of thermal gyroresonance absorption of
the extraordinary mode at the third harmonic of the gyrofrequency. In
the other class the entire burst emits in theo-mode, which may be
attributed to high gyrosynchrotron optical depth.

---------------------------------------------------------
Title: Photospheric and chromospheric magnetic field structure of
a bipolar sunspot region
Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E.
1993A&A...277..648D Altcode:
We present high resolution magnetograms of a large split spot and an
opposite polarity pore in the 6102.7 Å Ca I photospheric line and in
Hα and we discuss their fine structure. We found important differences
between the photospheric and chromospheric magnetic field. Although
the large spot was apparently unipolar at the photospheric level, we
detected an opposite polarity region at the chromospheric level. We
also found that there was hardly any trace of the pore magnetic field
in the Hα magnetogram. These results cannot be interpreted in terms
of constant-α magnetic fields. Finally we confirm the occurrence
of filamentary magnetic structures in Hα at the location of the
superpenumbra of the large spot.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Asymmetry of FeI lines in solar
spectrum (Stathopoulou+ 1993)
Authors: Stathopoulou, M.; Alissandrakis, C. E.
1993yCat..32740555S Altcode:
75 FeI lines were studied from wide slit FTS spectra obtained at Kitt
Peak at the center of the disk, near the limb (μ=0.2) (1 data file).

---------------------------------------------------------
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−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: A study of the asymmetry of Fe I lines in the solar spectrum
Authors: Stathopoulou, M.; Alissandrakis, C. E.
1993A&A...274..555S Altcode:
We have studied 75 Fe I lines from wide slit FTS spectra obtained
at Kitt Peak at the center of the disk, near the limb (μ=O.2) and
in total flux. We present measurements of the intensity at the line
center, the equivalent width and the asymmetry. We have done a detailed
analysis of bisectors and studied the dependence of the asymmetry on
the optical depth. Plots of the asymmetry versus the effective depth
of formation are similar for all lines, with a small dependence on the
excitation potential. The combined curve from all lines at the center
of the disk shows that the asymmetry decreases with optical depth
with a minimum at log τ = -O.33, which corresponds to the turning
point of the C-shaped bisectors; no such minimum is observed at μ =
O.2, where the measurements go up to log τ ∼ -0.8. We infer that
the asymmetry is a measure of the average line of sight velocity of
the material and we discuss the results in terms of characteristic
heights derived by other observers.

---------------------------------------------------------
Title: A new determination of the mean lifetime of bright and dark
chromospheric mottles
Authors: Bratsolis, E.; Dialetis, D.; Alissandrakis, C. E.
1993A&A...274..940B Altcode:
We determined the lifetime of dark and bright mottles of the solar
chromosphere using a more accurate method proposed by Alissandrakis et
al. (1987). Our study is based on an excellent sequence of photographs
taken at the Pic-du-Midi Observatory. We found that the mean lifetime
of both bright and dark mottles is 13-14 min. We discuss this result
and we compare it with previous investigations.

---------------------------------------------------------
Title: Correction: Two-dimensional solar mapping at 5.2 CM with the
Siberian Solar Radio Telescope
Authors: Alissandrakis, C. E.; Lubyshev, B. I.; Smol'Kov, G. Ia.;
Krissinel', B. B.; Treskov, T. A.; Miller, V. G.; Kardapolova, N. N.
1993SoPh..145..418A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The fine structure of a chromospheric rosette
Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B.
1993A&A...271..574T Altcode:
We have studied the spatial behaviour of the physical properties
of dark mottles forming a well-defined rosette. The observations
were obtained with the Multichannel Subtractive Double Pass (MSDP)
spectrograph, operating in Hα at the Pic du Midi Observatory. From
these observations, intensity fluctuations and Doppler shift velocities
at different wavelengths were derived over a two dimensional field of
view. The observed contrast profiles were matched with theoretical
contrast profiles using Beckers' cloud model and 4 parameters were
derived for the dark mottles: the source function, the line-of-sight
velocity, the Doppler width and the optical depth. From these parameters
a range for the temperature and electron density can be derived using
Vernazza's et al. (1981) model D. We detected strong downflows at
the roots of the mottles, while their upper part is ascending with
velocities sometimes greater than 10 km s-1. The values of
the cloud parameters are consistent with values given by other authors
for this kind of features. Consistency was also found between cloud
parameters and the values given for spicules, leading to the conclusion
for the identity of these structures.

---------------------------------------------------------
Title: Spectral observations of active region sources with RATAN-600
and WSRT
Authors: Alissandrakis, C. E.; Gel'Frejkh, G. B.; Borovik, V. N.;
Korzhavin, A. N.; Bogod, V. M.; Nindos, A.; Kundu, M. R.
1993A&A...270..509A Altcode:
We present spectral observations of neutral line and sunspot associated
sources obtained with the RATAN-600 radio telescope and the WSRT
in the wavelength range of 2 to 6 cm. Sources associated with large
sunspots have flat spectra, while neutral line sources have very steep
spectra. In the case of a large spot we estimated the magnetic field
to be at least 2700 G at the base of the transition region and 1800 G
in the low corona. We consider possible interpretations of the radio
emission above the neutral lines. Gyroresonance emission at the fourth
harmonic is inadequate, whereas emission from a small population of
nonthermal electrons (total number 10 exp 30 to 10 exp 31) with a
delta = 3 power law distribution seems to be sufficient.

---------------------------------------------------------
Title: Two-Dimensional Solar Mapping at 5.2-CM with the Siberian
Solar Radio Telescope
Authors: Alissandrakis, C. E.; Lubyshev, B. I.; Smol'Kov, G. Ia.;
Krissinel', B. B.; Treskov, T. A.; Miller, V. G.; Kardapolova, N. N.
1992SoPh..142..341A Altcode:
We present two-dimensional solar maps at 5.2 cm computed from
one-dimensinal observations with the Siberian Solar Radio Telescope
(SSRT), using Earth rotation aperture synthesis techniques. The
resolution attained with the E-W branch of the instrument is 15 by
45″ for a solar declination of about 23°. Maps during the period of
June 8 to 13, 1988 clearly show the quiet-Sun background, sunspot and
plage associated emission as well as compact sources above the neutral
line in some active regions. We found that the latter disappear as the
gradient of the longitudinal magnetic field decreases. We also detected
emission associated with active regions behind the limb, apparently
from unresolved loops, extending up to ∼40″. The prospects of the
SSRT, as a dedicated solar instrument, are discussed.

---------------------------------------------------------
Title: Observation of a coronal streamer at meter and decimeter
wavelengths.
Authors: Lantos, P.; Alissandrakis, C. E.
1992ESASP.348..141L Altcode: 1992cscl.work..141L
Maps obtained at meter and decimeter wavelengths with the Nançay
Radioheliograph on October 24 and 25, 1987 are presented. One of the
meter wavelength emission sources is located above a chromospheric
magnetic field neutral line and a small Hα prominence. The source
has decimeter wavelength counterparts on both sides of the neutral
line. The decimeter sources are close to regions of enhanced large
scale photospheric magnetic field, while the meter wavelength source
is located in between. This suggests emissions at different altitudes
in a loop system straddling the neutral line. The region corresponds
to a Fe XIV coronal streamer seen during the eastern limb passage. The
distance between the footpoints is of the order of 370000 km and the
angle between the loops and the neutral line is about 50°, projected
on a synoptic chart.

---------------------------------------------------------
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: Two-dimensional model maps of flaring loops at cm-wavelengths
Authors: Preka-Papadema, P.; Alissandrakis, C. E.
1992A&A...257..307P Altcode:
We present a complete set of model computations of the microwave
emission from a flaring loop. Two-dimensional maps in total intensity
and circular polarization are given, as a function of wavelength,
heliocentric distance and the orientation of the loop with respect
to the direction of the limb. In agreement with our previous
one-dimensional computations, the emission in the optically thin
case comes from the one or both feet of the loop, with the primary
maximum usually at the diskward side; in the optically thick case the
emission comes from the entire loop, with one or two maxima near the
top. We discuss in detail the effects of the orientation of the loop,
as well as the polarization structure produced by self-absorption and
gyroresonance absorption. The latter produce a patchy structure in the
V map, with regions polarized both in the extraordinary sense (in the
upper part of the loop) and in the ordinary sense (in the lower part
of the loop). Finally we compare our computations with high spatial
resolution observations of simple bursts.

---------------------------------------------------------
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−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: Quiet-Sun Emission and Local Sources at Meter and Decimeter
Wavelengths and Their Relationship with the Coronal Neutral Sheet
Authors: Lantos, P.; Alissandrakis, C. E.; Rigaud, D.
1992SoPh..137..225L Altcode:
We analysed multifrequency 2-dimensional maps of the solar corona
obtained with the Nançay radioheliograph during two solar rotations
in 1986. We discuss the emission of the quiet Sun, coronal holes
and local sources and its association with chromospheric and coronal
features as well as with large-scale magnetic fields. The brightness
temperature of the quiet Sun was 5 to 5.5 × 105 K at
164 MHz and 4.5 to 5 × 105 K at 408 MHz. A coronal hole,
also detected in the 10830 Å He I line, had a brightness temperature
of 4.5 × 105 at 164 and 2.5 × 105 at 408
MHz. We give statistics of source brightness temperatures (on the
average 8% above the background at 164 MHz and 14% at 408 MHz), as
well as distributions in longitude and latitude. Although we found no
significant center-to-limb effect in the brightness temperature, the
sources were not visible far from the central meridian (apparently a
refraction effect). The brightest sources at 164 MHz were near, but not
directly above active regions and had characteristics of faint type I
continua. At 408 MHz some sources were observed directly above active
regions and one was unambiguously a type I continuum. The majority
of the fainter sources showed no association with chromospheric
features seen on Hα synoptic charts, including filaments. Most
of them were detected at one frequency only. Sources identified at
three frequencies (164, 327, and 408 MHz) were located in regions of
enhanced large-scale magnetic field, some of them at the same location
as decayed active regions visible one rotation before on synoptic
Hα charts. Multifrequency sources are associated with maxima of the
green line corona. The comparison with K-corona synoptic charts shows a
striking association of the radio sources with dense coronal regions,
associated with the coronal neutral sheet. Furthermore, we detected
an enhanced brightness region which surrounds the local sources and
is stable over at least one solar rotation. We call this feature a
coronal plateau and we identify it with the radio counterpart of the
coronal neutral sheet.

---------------------------------------------------------
Title: Detection of the coronal plasma sheet on the solar disk
Authors: Lantos, P.; Alissandrakis, C. E.
1992sws..coll..213L Altcode:
The large scale coronal neutral sheet, which is considered to be the
source of low speed solar wind, is the low altitude counterpart of the
interplanetary heliosheet. It is routinely detected with white light
corona observations at the limb. Maps of the sun at meter wavelengths,
obtained with the Nancay Radioheliograph, lead to the first detection
of this density structure on the disk, in the form of a 'plateau' in
brightness temperature. The radio observations also show the denser
parts of the coronal neutral sheet as discrete emission sources. Some
of them may be identified as coronal streamers.

---------------------------------------------------------
Title: VLA observations of solar active regions at 6 and 20 CM
Authors: Alissandrakis, C. E.; Kundu, M. R.; Shevgaonkar, K. R.
1991A&A...251..276A Altcode:
High-resolution observations are presented of two active regions
at 6 and 20 cm over a period of 5 days, together with H-alpha and
photospheric magnetic fields. The large-scale emission at 20 cm is
associated with the H-alpha plage. In one region the strongest source
was over the neutral line, near the tip of an active-region filament,
which indicates that the emission probably originated in small-scale
coronal loops. In the second region the peak of the emission was near
a well-developed sunspot. Neither region showed evidence of large-scale
loops joining their preceding and following parts. Several other sources
were observed at 20 cm; a source associated with an H-alpha plage
region crossed by a filament and one associated with a small bipolar
region are briefly discussed. The 6-cm emission from a well-developed
spot showed clearly the characteristics expected from gyroresonance
model computations.

---------------------------------------------------------
Title: Study of small scale magnetic flux and the corresponding
velocity pattern
Authors: Alissandrakis, C. E.; Dara, H. C.; Koutchmy, S.
1991A&A...249..533A Altcode:
Two-dimensional maps of the longitudinal magnetic field and the
line of sight velocity of two small-scale magnetic regions in
the quiet sun, using a one-hour time sequence of filtergrams in
the magnetically sensitive lambda 6103 A CaI line (with circular
polarization measurements) obtained with the Sacramento Peak Vacuum
Tower Telescope and the universal filter, are calculated. After the
elimination of the effect of the 5-minute photospheric oscillations,
downflows smaller than 300 m/s are found in both magnetic regions. The
magnetic field intensity and the velocity are weakly correlated:
high values of the magnetic field tend to occur in regions of small
negative velocities. The magnetic field intensity increased during
the observations, while some magnetic peaks moved with respect to
others. The difference of the magnetic field intensity measured in
the blue and the red wing indicates a line asymmetry; this asymmetry
was found to be independent of the line of sight velocity.

---------------------------------------------------------
Title: Small scale magnetic field mapping with high temporal
resolution.
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1991sopo.work..257D Altcode:
Two dimensional maps of the longitudinal magnetic field can be readily
calculated for small-scale magnetic regions. Here the authors use
an one-hour time sequence of high spatial resolution (0.75 arc sec)
filtergrams, obtained at the Sacramento Peak Vacuum Tower Telescope with
the universal filter (UBF) in the wings of the magnetically sensitive
λ 6103 Ca I line. The time difference between two magnetic maps is
32 sec. The measurements of the magnetic field in the blue wing are
about 40% higher than in the red wing.

---------------------------------------------------------
Title: Multifrequency radio observations of the solar corona during
cycle minimum
Authors: Rigaud, D.; Lantos, P.; Alissandrakis, C. E.
1991AdSpR..11a..65R Altcode: 1991AdSpR..11...65R
Observations obtained at meter and decimeter wavelengths during a
period of very low solar activity (from May to July 1986) are studied
to analyse the relationship between the corona and the underlying
levels. The location of radio sources observed on the disk is compared
with active regions and with Hα filaments. The radio
emission is found to trace on the disk the large scale structure of
the corona as observed with K-coronameter: the so-called coronal sheet
(belt of equatorial streamers) is observed and individual streamers
could be located, for the first time, on the disk.

---------------------------------------------------------
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−1. The
motion is systematically slower than expected from a free-fall model.

---------------------------------------------------------
Title: Flow patterns in a sunspot region observed in the photosphere,
chromosphere and transition region
Authors: Dere, K. P.; Schmieder, B.; Alissandrakis, C. E.
1990A&A...233..207D Altcode:
The flow patterns around a sunspot at photospheric, chromospheric, and
transition zone temperatures is examined. The spiral fibril pattern
observed in H-alpha is consistent with a constant alpha force-free
field extrapolation of the photospheric magnetic field. Evidence
is found for the first time that C IV fibrils near spots follow the
field lines parallel to the chromospheric H-alpha fibrils. The observed
large-scale pattern of redshifts and blueshifts in the vicinity of the
sunspot is stable and conforms to the typical Evershed flow patterns
of radial outflow at the photospheric levels and radial inflows at
the chromospheric and transition region levels. The three-dimensional
velocity vectors associated with this flow are derived assuming an
axially symmetric Evershed flow.

---------------------------------------------------------
Title: Erratum: Velocity Pattern of Small Scale Magnetic Fields
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990SoPh..128..431D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Physical parameters of solar H-alpha absorption features
derived with the cloud model
Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P.
1990A&A...230..200A Altcode:
Observations of a fibril region and of an arch filament region
obtained by the Multichannel Subtractive Double Pass Spectrograph
(MSDP) operating in H-alpha at the Pic du Midi Observatory are
presented. Intensity fluctuations and Doppler shift velocities were
derived over two dimensional fields of view. The observed contrast
profiles have been matched with theoretical contrast profiles based on
the cloud model which has four parameters: the line-of-sight velocity,
the source function, the optical depth, and the Doppler width. Contour
maps and histograms of the 4 parameters were made for these regions,
and a comparison of the velocities given by 3 different methods (cloud
model, Doppler shift, and the '3-optical depths' method of Mein and
Mein, 1988) is made.

---------------------------------------------------------
Title: Velocity pattern of small scale magnetic fields
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990SoPh..126..403D Altcode:
High resolution observations of horizontal proper motions, as well
as vertical Doppler velocities measured over two selected regions
of small scale magnetic elements show a coherent behaviour. In a
region with two opposite polarities, approching with a velocity of
0.4 km s-1, the material in between moves downwards with
a velocity of 0.10 to 0.45 km s-1; while in a region with
two peaks of the same polarity, moving apart with a velocity of 0.3 km
s-1, the material in between moves predominantly upwards,
with a velocity of up to 0.3 km s-1.

---------------------------------------------------------
Title: VLA Observations of Solar Active Regions at 6 and 20 cm During
the Spacelab 2 Mission
Authors: Alissandrakis, C. E.; Kundu, M. R.; Shevgaonkar, R. K.
1990BAAS...22..795A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Computation of electron beam parameters for solar type III
and J bursts
Authors: Hillaris, A.; Alissandrakis, C. E.; Caroubalos, C.; Bougeret,
J. -L.
1990A&A...229..216H Altcode:
The dynamic evolution of mildly relativistic electrons (10-100 keV)
propagating outward in the solar corona following open or closed
magnetic lines is studied using the drift approximation. Wave-particle
interactions are neglected, since the beam-plasma system is considered
as almost decoupled in the range of parameters used in this model. The
results of this simulation are used for the computation of observable
quantities and the analysis of dynamic spectra of isolated type III or
J bursts, obtained with the Digital Multichannel Radio Spectrograph of
the Space Research Laboratory of Paris, Meudon. The initial velocity
dispersion of the exciter and the coronal scale height are thus
determined. The implications of the results on the emission processes
are discussed.

---------------------------------------------------------
Title: Small Scale Motions Over Concentrated Magnetic Field Regions
of the Quiet Sun
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1990IAUS..138..153D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Relationship between a spot and a filament observed during
spacelab 2 mission
Authors: Schmieder, B.; Dere, K. P.; Raadu, M. A.; Démoulin, P.;
Alissandrakis, C. E.
1990AdSpR..10i.195S Altcode: 1990AdSpR..10..195S
During the Spacelab 2 Mission, an active region including a sunspot,
plages, fibrils and a filament which disappears during the observation
period, was observed in Hα and in the C IV lines. The analysis of the
observations leads to several conclusions. Shear in the active region
filament is observed before its disappearance. Hα and C IV upflows
in the filament are well correlated spatially, but the transition
zone (C IV) velocities are an order of magnitude larger than the Hα
chromospheric ones. Over the sunspot, the reverse-Evershed effect is
observed in Hα and in C IV Dopplershift images while strong downflows
are also detected in the C IV profiles. Magnetograph data for
the whole region are used to calculate the linear force-free magnetic
field. A single value of the force-free parameter α is found to give a
good fit to both the sheared filament structure and the sunspot spiral
structures, indicating a causal relationship. We propose a model
based on the inductive coupling between current systems associated with
the filament and the sunspot. Relaxation phenomena due to resistive
effects in the filament could lead to irreversible conversion of
magnetic energy into kinetic energy and heating.

---------------------------------------------------------
Title: Formation of a Filament around a Magnetic Region
Authors: Schmieder, B.; Démoulin, P.; Ferreira, J.; Alissandrakis,
C. E.
1990LNP...363..232S Altcode: 1990doqp.coll..232S; 1990IAUCo.117..232S
The evolution of the active region AR4682 observed in 1985 during six
rotations was dominated by three different phenomena: . the large
scale pattern activity: relationship between two active regions,
formation of a quiescent filament during the decay phase of the
active region, . the presence of two pivot points along the filament
surrounding the sunspot-with the long term one is associated the
existence of the filament, with the short term one the activity with
partial disappearance, . the magnetic shear during one rotation. The
magnetic field lines have been extrapolated from photospheric data
using Alissandrakis code (1981). The magnetic configuration with the
existence of a dip favors the formation of a filament. We note that
the shearing of the sunspot region and of the filament are both well
described by force-free magnetic fields with the same constant α. This
suggests that they are both a consequent of the same shear process.

---------------------------------------------------------
Title: Coronal meter wavelength observations during solar minimum:
location of dense streamers.
Authors: Rigaud, D.; Lantos, P.; Alissandrakis, C. E.
1990PDHO....7..270R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Observation of the heliosheet at meter wavelengths
Authors: Lantos, P.; Rigaut, D.; Alissandrakis, C. E.
1989AdSpR...9d..37L Altcode: 1989AdSpR...9...37L
The Nancay Radioheliograph provides detailed radio maps at 169 MHz
of the solar corona. The meter wavelength maps are used in particular
to map the heliosheet on the limb in a way similar to the HAO K-corona
synoptic maps. In absence of X-ray observations, the radio data provides
presently the unique possibility to compare directly the heliosheet
structure with the solar chromospheric features because the radio
emissions are also observed on the disk and may be followed during
their transit on the sun.

---------------------------------------------------------
Title: Particle acceleration.
Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.;
Alissandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.;
Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.;
Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima,
H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.;
Trottet, G.; Tsuneta, S.; van Hoven, G.
1989epos.conf..127V Altcode:
Contents: 1. Introduction. 2. Phenomena associated with
mildly-relativistic electrons. 3. Phenomena associated with ions and
relativistic electrons in solar flares. 4. Theoretical studies of
particle acceleration. 5. Achievements - outstanding questions.

---------------------------------------------------------
Title: Formation of a Filament around a Magnetic Region
Authors: Schmieder, B.; Demoulin, P.; Ferreira, J.; Alissandrakis,
C. E.
1989HvaOB..13....1S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Evershed flow in the solar photosphere, chromosphere and
chromosphere-corona transition region
Authors: Alissandrakis, C. E.; Dialetis, D.; Mein, P.; Schmieder,
B.; Simon, G.
1988A&A...201..339A Altcode:
The authors have studied the height dependence of the characteristics
of the Evershed flow in the photosphere, chromosphere and
chromosphere-corona transition region. The Multichannel Subtractive
Double Pass Spectrograph was used to obtain line of sight velocity maps
in Hα, the Meudon magnetograph for mapping the photospheric velocity
and magnetic field and the Ultraviolet Spectrometer and Polarimeter on
the Solar Maximum Mission spacecraft for line-of-sight velocities in C
IV, in sunspot regions. The observations are practically simultaneous in
the three atmospheric levels and cover three consecutive days (Sep 28,
29 and 30, 1980). The emphasis was on the large scale, quasi-stationary
characteristics of the flow.

---------------------------------------------------------
Title: Dynamics of sub-relativistic electron beams in magnetic traps -
A model for solar N-bursts
Authors: Hillaris, A.; Alissandrakis, C. E.; Vlahos, L.
1988A&A...195..301H Altcode:
The dynamic evolution of mildly relativistic electrons (10 - 100 keV)
injected into a model magnetic trap is studied numerically, using
the drift approximation. Wave-particle interactions are neglected,
since the beam plasma system is shown to be non-linearly decoupled
in the range of parameters used in this study. The results from the
simulation are used to interpret certain observational characteristics
of the N-bursts observed by the Nançay radio spectrograph. N-bursts
are believed to be the first direct radio evidence for mirror effects
in solar magnetic loops.

---------------------------------------------------------
Title: High resolution microwave and X-ray observations of solar
flares
Authors: Alissandrakis, C. E.; Schadee, A.; Kundu, M. R.
1988A&A...195..290A Altcode:
This paper presents an analysis of high spatial resolution observations
of 18 weak flares, carried out during the Solar Maximum Year in the
microwave range, H-alpha, and soft X-rays, together with observations
of the associated hard X-ray emission and the photospheric magnetic
field. This combination of observations made it possible to obtain a
picture of the flare geometry and the relative position of the various
emissions, to study the change of the geometry during the flare, to
estimate the electron temperature and emission measure of the thermal
plasma, and to compute the contribution of the free-free process in
the microwave emission. Results are interpreted as evidence for the
presence of energetic electrons even in the post-burst phase of some
flares. There was also found a clear case of homologous radio and X-ray
bursts, one case of foot-point expansion with a relative velocity of
20 km/s at 6.16 cm, as well as some evidence of triggering of flares
by disturbances moving with a velocity of 5000-13,000 km/s.

---------------------------------------------------------
Title: Spatial and spectral structure of a solar flaring loop at
centimeter wavelengths
Authors: Preka-Papadema, P.; Alissandrakis, C. E.
1988A&A...191..365P Altcode:
The authors have studied the behaviour of the gyrosynchrotron emission
coefficient and source function as a function of the strength and
direction of the magnetic field. The results are applied in the
qualitative description of the spatial and spectral structure of
solar microwave bursts. In addition the authors present a complete set
of model computations of microwave emission from a loop filled with
energetic electrons. Their model predicts footpoint emission at short
wavelengths and emission from the entire loop at long wavelengths. The
computed spectra of the emission show considerable variation along
the flaring loop.

---------------------------------------------------------
Title: Quiet Sun and Slowly Varying Component at Meter and Decameter
Wavelengths
Authors: Lantos, P.; Alissandrakis, C. E.; Gergely, T.; Kundu, M. R.
1987SoPh..112..325L Altcode:
Comparison of maps of the Sun obtained over the period June 29 to July
8, 1982 at 169 MHz with the Nançay Radioheliograph and at 73.8, 50,
and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly
varying component at meter and decameter wavelengths is not always
thermal emission. During the period under study weak noise storm
continua were the most frequent sources of slowly varying component
at 169 and 73.8 MHz. Most filaments show no radio counterpart on the
disk. A streamer has been detected on the disk from 169 to 30.9 MHz with
an optimum observability at 50 MHz. The brightest source of the slowly
varying component from 73.8 to 30.9 MHz for most of the period was
located above an extended coronal hole in a region where a depression
was observed at 169 MHz. In favorable cases, electron densities can
be derived from the positions of noise storms and radio streamers;
these are in agreement with previous K-corona eclipse observations.

---------------------------------------------------------
Title: Observations of Ellerman bombs in H α
Authors: Zachariadis, Th. G.; Alissandrakis, C. E.; Banos, G.
1987SoPh..108..227Z Altcode:
A developing active region near the center of the solar disk was
observed for 80 min at the center and the wings of Hα. Ellerman
bombs lying both below an Arch Filament System and near sunspots were
studied at Hα - 1.0 Å and Hα - 0.75 Å. We determined their average
contrast, lifetime, size and we studied their flux as a function of
time. We found evidence that the size of Ellerman bombs increases with
height. The time curves of flux provide evidence for both impulsive
and gradual energy release. Under the AFS the Ellerman bombs form a
cellular pattern with a characteristic size of 3.1 × 103
km. Fifty percent of the bombs appear and disappear in pairs, possibly
associated with bipolar emerging magnetic flux tubes.

---------------------------------------------------------
Title: Microwave Emission above Steady and Moving Sunspots
Authors: Chiuderi Drago, F.; Alissandrakis, C.; Hagyard, M.
1987SoPh..112...89C Altcode:
Two-dimensional maps of radio brightness temperature and polarization,
computed assuming thermal emission with free-free and gyroresonance
absorption, are compared with observations of active region 2502,
performed at Westerbork at λ = 6.16 cm during a period of 3 days in
June 1980. The computation is done assuming a homogeneous model in
the whole field of view (5' × 5') and a force-free extrapolation of
the photospheric magnetic field observed at MSFC with a resolution of
2″.34. The mean results are the following: A very good agreement is
found above the large leading sunspot of the group, assuming a potential
extrapolation of the magnetic field and a constant conductive flux in
the transition region ranging from 2 × 106 to 107
erg cm−2s−1.

---------------------------------------------------------
Title: Determination of the mean lifetime of solar features from
photographic observations
Authors: Alissandrakis, C. E.; Dialetis, D.; Tsiropoula, G.
1987A&A...174..275A Altcode:
Two approaches, one based on a lifetime distribution and the second
based on survival functions, are combined in order to determine the
mean lifetimes of solar features. The relations between the lifetime
distributions and the survival and probability density functions
are derived. This method is applied to the determination of the mean
lifetimes of solar granules. The accuracy of the method is evaluated by
comparing the calculated lifetimes with data obtained using different
methods. It is determined that the proposed method provides accurate
lifetime data, and a mean lifetime value of 16 min with a standard
deviation of about 5 min was obtained for the solar granules.

---------------------------------------------------------
Title: Small-scale motions over concentrated magnetic regions of
the quiet Sun
Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.
1987SoPh..109...19D Altcode:
We have used a 5.5 min time-sequence of spectra in the Fe I lines
λ5576 (magnetically insensitive), λ6301.5 and λ6302.5 (magnetically
sensitive) to study the association of concentrated magnetic regions
and velocity in the quiet Sun. After the elimination of photospheric
oscillations we found downflows of 100-300 m s −1,
displaced by about 2″ from the peaks of the magnetic field; this
velocity is comparable to downflow velocity associated with the
granulation and of the same order or smaller than the oscillation
amplitude. Quasi-periodic time variations of the vertical component
of the magnetic field up to ± 40% were also found with a period near
250 s, close to the values found for the velocity field. Finally we
report a possible association of intensity maxima at the line center
with peaks of the oscillation amplitude.

---------------------------------------------------------
Title: Microwave emission from steady and moving sunspots.
Authors: Alissandrakis, C. E.; Chiuderi Drago, F.; Hagyard, M. J.
1987PAICz..66..267A Altcode: 1987eram....1..267A
The authors have used force-free extrapolations of photospheric magnetic
field observations from Marshall Space Flight Center to compute the
total intensity and circular polarisation of sunspot associated emission
from active region 2502 in the period June 13 to 15, 1980.

---------------------------------------------------------
Title: Synoptic study of the corona at meter wavelength.
Authors: Lantos, P.; Alissandrakis, C. E.
1986NASCP2442..257L Altcode: 1986copp.nasa..257L
The Mark III Nancay Radioheliograph is used to observe the Sun at 169
MHz with a time resolution of 25 East-West and North-South images
per second. When the brightness distribution of the Sun is stable
during the eight hours of daily observation, a two dimensional map
can be produced using the technique of earth rotation synthesis. The
best images are obtained during the period April to August, when the
declination of the Sun is high to give a good coverage in the uv plane
and a reasonable North-South resolution. The spatial resolution is 1.5'
East-West and in summer, 3.5' North-South. The maps are calibrated using
Cygnus A as a reference. Examples of the maps are given and discussed.

---------------------------------------------------------
Title: Emission of Lyman-α radiation by solar coronal
loops. II. Filtergram analysis.
Authors: Tsiropoula, G.; Alissandrakis, C.; Bonnet, R. M.; Gouttebroze,
P.
1986A&A...167..351T Altcode:
The authors study active region loops, observed on Lα high resolution
filtergrams obtained with a rocket borne instrument. Some formulae or
diagrams are given for the computations of Lα and Hα intensities
emitted by homogeneous loop models. The dimensions and emitted
intensities of the observed loops being known, the authors determine
the variations of temperature, pressure and density inside these
objects under the assumption of constant temperature and hydrostatic
equilibrium. The influence of temperature gradient and mass flow
on the results is discussed, and an alternative model with radial
temperature increase is envisaged. The case of an isolated loop with
a larger diameter is also discussed.

---------------------------------------------------------
Title: Gyrosynchrotron Emission of Solar Flares
Authors: Alissandrakis, C. E.
1986SoPh..104..207A Altcode:
The current status of our knowledge on the theory of radio emission from
mildly relativistic electrons and its application in the interpretation
of solar radio bursts are reviewed. The recent high spatial resolution
microwave observations have given important information about the
geometry of the emitting region and have helped in the computation
of better inhomogeneous models that reproduce qualitatively several
observational characteristics of the emission. The limitations of the
observations and the theory (particularly the effect of mode coupling
on the observed polarisation) are pointed out and the potential of the
gyrosynchrotron process as a diagnostic of the physical conditions is
discussed. This will help us to obtain quantitative information about
the changes of the magnetic field and the acceleration of particles
in solar flares.

---------------------------------------------------------
Title: Coronal structures observed at meter wavelengths.
Authors: Lantos, P.; Alissandrakis, C. E.
1986HiA.....7..761L Altcode:
Two dimensional maps obtained with the Nançay Radioheliograph at 169
MHz from 1980 are used to describe and identify the features of the
quiet sun at meter wavelengths.

---------------------------------------------------------
Title: The Evershed flow as a steady-state homogeneous phenomenon
Authors: Dialetis, D.; Mein, P.; Alissandrakis, C. E.
1985A&A...147...93D Altcode:
The present observations of photospheric and chromospheric line-of-sight
velocity, as well as the photospheric magnetic field, facilitate the
comprehensive study of the Evershed (1909) flow, with emphasis on large
scale, quasi-stationary characteristics of the flow and its association
with the topology of the magnetic field. A multichannel subtractive
double pass spectrograph was used to obtain line-of-sight velocity
maps in H-alpha, together with the Meudon magnetograph. Computations
are made for the components of the velocity vector as a function of
distance from the center of the spot, under the assumption of axial
symmetry. The radial component is the dominant one in both chromosphere
and magnetosphere. The present analysis of the magnetograms, using
current-free and force-free models, indicates that the velocity maximum
occurs in regions where the magnetic field is almost horizontal in both
layers; the results are judged to appear consistent with subsonic flow
in magnetic flux tubes.

---------------------------------------------------------
Title: Coronal structures observed at metric wavelengths with the
Nançay radioheliograph
Authors: Alissandrakis, C. E.; Lantos, P.; Nicolaidis, E.
1985SoPh...97..267A Altcode:
We present and discuss two-dimensional maps of the Sun at 169 MHz,
obtained with the Nançay radioheliograph used as an Earth rotation
aperture synthesis instrument. The maps have been computed on the basis
of about 6 hr of one-dimensional observations by the east-west and
the north-south arrays of the radioheliograph and have a resolution of
1.5' by 4.2' for a solar declination near 23°. In addition to a broad
background component, the maps show several features both brighter and
darker than the background. Some of the bright features are sources
of noise storm continua, as evidenced by their positions relative to
active regions and by the occurrence of type I bursts. Weaker emission
regions are apparently associated with neutral lines of the photospheric
magnetic field. We found no sources associated with extended quiescent
filaments. Some of the depressions on the maps correspond to coronal
holes both in the equatorial region and near the poles, while the
more shallow ones may be arch regions with low electron temperature
and/or emission measure. The distribution of brightness temperature at
a height of 0.15 solar radii above the photospheric limb shows a gross
similarity with coronal green line observations. The present results
indicate that the notion of the slowly varying component at metric
wavelengths may have to be reexamined, since sources of different
nature may have been grouped in this component in the past.

---------------------------------------------------------
Title: Observations of Ellerman Bombs in H&amp;alpha
Authors: Zachariadis, Th.; Alissandrakis, C. E.; Banos, G.
1985LNP...233..304Z Altcode: 1985hrsp.proc..304Z
A developing active region near the center of the solar disk was
observed for 80 min at the center and the wings of H-alpha. Ellerman
bombs lying below an arch filament system (AFS) and near sunspots were
studied at H -1 A and H -0.75 A. It was found that 50 percent of the
bombs appear and disappear in pairs, the axis of each pair forming a
small angle with the magnetic field lines of force as evidenced by the
AFS. In addition, the members of each pair show proper motions with an
average relative velocity of 600 m/sec. This behavior suggests their
association with bipolar emerging magnetic flux tubes. The average
contrast (1.29), lifetime (19 minutes), size (0.96 in. x 0.58 in.) and
flux of some 70 bombs was also determined. Evidence was found for an
increase in size with height and a fluctuation in flux with time.

---------------------------------------------------------
Title: Solar Coronal Holes Observed with Nancay Meter Radioheliograph
Authors: Lantos, P.; Alissandrakis, C.
1985ESASP.229..367L Altcode: 1985erbp.symp..367L
Observations of the sun at meter wavelengths, with the Nançay
radioheliograph, provide maps of the structures of the middle
corona. In particular, coronal holes with low temperature and density
are well observed both in equatorial regions and near the poles as
deep depressions in brightness temperature. This provides a new set
of data to study solar wind and geomagnetic activity origin.

---------------------------------------------------------
Title: Dynamic Phenomena in the Chromospheric Umbra and Penumbra of
a Sunspot
Authors: Alissandrakis, C. E.; Dialetis, D.; Macris, C. J.
1985LNP...233..294A Altcode: 1985hrsp.proc..294A
A prominent, short lived (70 sec) umbral flash in H-alpha is observed
with the 50 cm 'Tourelle' refractor at Pic du Midi. The main component
of the flash was small (1 arcsec by 0.5 arcsec) with a maximum intensity
25 percent above the background. It occurred in a region which showed
irregular intensity variations. Umbral oscillations were observed
in an adjacent region and running penumbral waves around part of the
superpenumbra.

---------------------------------------------------------
Title: A siphon flow model for Evershed motion.
Authors: Dialetis, D.; Alissandrakis, C. E.
1984ESASP.220..245D Altcode: 1984ESPM....4..245D
The authors have studied the motion of an inviscid isothermal
plasma under the influence of gravity along the lines of force of an
axisymmetric magnetic field. The general behaviour of the solutions
is in good agreement with the observations.

---------------------------------------------------------
Title: Coronal structures observed at metric wavelengths with the
Nançayradioheliograph.
Authors: Alissandrakis, C. E.; Lantos, P.; Nicolaidis, E.
1984ESASP.220..263A Altcode: 1984ESPM....4..263A
The authors present two-dimensional maps of the Sun at 169 MHz,
obtained with the Nançay radioheliograph, with resolution of 1.5arcmin
by 4.2arcmin.

---------------------------------------------------------
Title: Microwave emission and polarization of a flaring loop
Authors: Alissandrakis, C. E.; Preka-Papadema, P.
1984A&A...139..507A Altcode:
The authors present computations of the microwave emission from a
loop filled with accelerated electrons. At short wavelengths the loop
is optically thin and the emission comes predominantly from the feet
of the loop, while at long wavelengths the emission comes from the
entire loop with a maximum near the top. If the loop is not located
at the center of the solar disk the observed polarization is greatly
affected by propagation effects in the corona which may change the
sense of the circular polarization (V). For the adopted model and for
a loop located 30° away from the center of the disk the sense of V
corresponds to the direction of the magnetic field in the region of
emission only for wavelengths much shorter than 1.4 cm; for wavelengths
much longer than 7.5 cm the sense of V corresponds to the polarity of
the field at the diskward foot of the loop.

---------------------------------------------------------
Title: Center-to-limb variation of a sunspot-associated microwave
source
Authors: Alissandrakis, C. E.; Kundu, M. R.
1984A&A...139..271A Altcode:
Observations of the simple bipolar active region McMath 16862,
obtained with the Westerbork Synthesis Radio Telescope at 6.16 cm
over six consecutive days, indicate two bright sources associated
with the region's two main spots, as well as some weaker emission
that may be associated with loop structures. Application of a novel
analytical method has allowed the mapping of the vertical as well as
the horizontal component of the sunspot magnetic field at specific
locations in the low corona. While the vertical component decreases
away from the source's center, the horizontal component has both a
radial and an azimuthal part. These results are interpreted in terms
of a force-free magnetic field model, as well as in terms of a dipole
whose axis is inclined to the vertical.

---------------------------------------------------------
Title: Structure and Polarization of Active Region Microwave Emission
Authors: Kundu, M. R.; Alissandrakis, C. E.
1984SoPh...94..249K Altcode:
We present observations of active region radio emission at 6.16 cm
wavelength, obtained with an angular resolution of 3″ by 10″
arc using the Westerbork Synthesis Radio Telescope (WSRT) during the
action interval May 20-27, 1980 of the Solar Maximum Year (SMY). We
present maps in both total intensity (I) and circular polarization (V)
of three regions (Hale numbers 16850, 16863, and 16864) and provide a
detailed comparison of these maps with on- and off-band Hα pictures
and with magnetograms. The strongest sources were associated with
neutral lines and soft X-ray arcades. We present evidence that these
neutral lines were characterized by having their two opposite polarities
close to each other, implying a high magnetic field gradient, and by
their association with arch filament systems. The sunspot associated
radio sources had a relatively simple structure in region 16850;
however for the large spots of regions 16864 and 16863 the emission
had a patchy appearance with a tendency of the peaks to lie over the
penumbra. In the V maps we observed for the first time two `islands',
polarized in the sense of the ordinary mode, which were located inside
the sunspot associated sources and were associated with intrusions
of opposite polarity field into the penumbra. These structures can be
accounted for if the electron temperature along the line of sight is
not a monotonically increasing function of height, but has a maximum
near the second harmonic level. Finally we give a detailed analysis of
observations of the inversion of the sense of circular polarization in
region 16863. We find that the large scale structure of the magnetic
field can be approximated by a dipole with its axis inclined by 11°
with respect to the photosphere and with a dipole moment of about
2 × 1031 cgs units; the depolarization line is located
at a height of 0.16-0.19 R⊙ above the dipole, where the estimated
intensity of the magnetic field is 10-20 G.

---------------------------------------------------------
Title: Interpretation of microwave active region structures using
SMM soft X-ray observations
Authors: Strong, K. T.; Alissandrakis, C. E.; Kundu, M. R.
1984ApJ...277..865S Altcode:
The purpose of this paper is to investigate the roles played
by the geometrical effect of gyroresonance absorption and by the
low-temperature (&lt;106K) flux tubes above sunspot regions
with microwave ring structures, by combining WSRT 6 cm observations
with the X-ray observations made with the X-ray Polychromator on the
SMM satellite. Some features associated with Hale regions 16863 and
16864 are analyzed from observations on 1980 May 25 and 26 combined
with model computations of the radio emission. The nature of a cool and
compact coronal feature seen in soft X-rays, which corresponded to some
interesting microwave and magnetic features, is also examined. Finally,
the origin of the X-ray and radio emission associated with neutral
lines of the magnetic field is considered.

---------------------------------------------------------
Title: Observations of ring structure in a sunspot associated source
at 6 centimeter wavelength
Authors: Alissandrakis, C. E.; Kundu, M. R.
1982ApJ...253L..49A Altcode:
The detection of a new kind of sunspot-associated source in which the
emission comes predominantly from a ring structure with size between
that of the umbra and the penumbra is reported. The absence of emission
from the center of the spot is interpreted in terms of the orientation
of the magnetic field and the presence of low temperature material
above the umbra.

---------------------------------------------------------
Title: Measurements of the granule-intergranular lane contrast at
5200 Å and 6300 Å
Authors: Alissandrakis, C. E.; Macris, C. J.; Zachariadis, T. G.
1982SoPh...76..129A Altcode:
We present measurements of the granule-intergranular lane intensity
ratio at 5200 Å and 6300 Å, at the center of the disk. The
observations were obtained at Pic du Midi and Sacramento Peak
observatories between 1967 and 1978. The contrast at 5200 Å was
corrected for the effect of instrumental profile using a two-dimensional
model. At 6300 Å and in one photograph our measurements gave an
average contrast of 1.40, while the values at 5200 Å show a variation
with time, with the highest corrected values in the range of 1.30 to
1.37. The possible origins of the time variation are discussed.

---------------------------------------------------------
Title: On the computation of constant alpha force-free magnetic field
Authors: Alissandrakis, C. E.
1981A&A...100..197A Altcode:
Consideration is given to the boundary value problem of the
determination of the strength and configuration of the magnetic field in
the half space above the solar photosphere on the basis of photospheric
observations for the case of a force-free magnetic field with constant
alpha, a scalar relating the cross product of the currents and the field
vectors to the field vector. Solutions are derived in terms of Fourier
transforms and extended to regions far from the center of the disk. It
is found that the large-scale components of the solutions involve a
quasi-periodic variation of the field and current with height, carry
infinite energy, and can be completely specified only with observations
of a transverse field component at the boundary. The solutions are thus
unphysical, however solutions are required for the small-scale solution
to satisfy the boundary condition. It is pointed out that although
numerical computations based on the fast Fourier transform are about
100 times faster than those based on the Green's function formulation,
the use of a rectangular grid may result in important aliasing effects
unless the observed field is placed in an array somewhat larger than
the observed region.

---------------------------------------------------------
Title: A model for sunspot associated emission at 6 CM wavelength
Authors: Alissandrakis, C. E.; Kundu, M. R.; Lantos, P.
1980A&A....82...30A Altcode:
Two-dimensional maps of total intensity and circular polarization of
a sunspot region at 6 cm have been calculated using a simple model
for the chromosphere-corona transition region and observations of
the longitudinal component of the photospheric magnetic field. The
calculations are in good agreement with the high resolution observations
of the same sunspot region at 6 cm, obtained with the Westerbork
Synthesis Radio Telescope. It is shown that the 6 cm radiation is
predominantly due to gyroresonance absorption process at the second
and third harmonics of the gyrofrequency (H = 900-600 G). Estimates of
the conductive flux and the electron density in the transition region
above the sunspot are also given.

---------------------------------------------------------
Title: Active region magnetic fields and cm-lambda emission
Authors: Alissandrakis, C. E.
1980IAUS...86..101A Altcode:
The influence of the magnetic field on the structure of
sunspot-associated sources at cm wavelengths has been investigated
through model calculations. The case of a source observed with the
Westerbork SRT and the structure of a single sunspot source are
considered.

---------------------------------------------------------
Title: 6 centimeter observations of solar bursts with 6" resolution.
Authors: Alissandrakis, C. E.; Kundu, M. R.
1978ApJ...222..342A Altcode:
During May 1974 data were collected on nine 6-cm solar bursts
originating in two active regions. One-dimensional fan-beam scans
were obtained every 30 seconds in order to study burst structure
and evolution. Maximum burst flux was in the 0.34-12.0 sfu range. At
maximum intensity the estimated brightness temperatures were in the
10 to the 6th to 2 x 10 to the 7th K range. Angular sizes ranged from
7-23 arcsec. In most cases an expansion of the burst core after maximum
intensity was observed along with a drift in the position of the burst
core. Four bursts were associated important H-alpha flares, and in all
cases burst maximum occurred before flare maximum. The bursts were
circularly polarized in the extraordinary mode sense during maximum
intensity. Since the polarization was the same over the extent of the
burst source, it is suggested that if the burst is associated with
loop sources, the emission is associated with one leg of the loop.

---------------------------------------------------------
Title: 6 centimeter observations of solar active regions with 6"
resolution.
Authors: Kundu, M. R.; Alissandrakis, C. E.; Bregman, J. D.; Hin, A. C.
1977ApJ...213..278K Altcode:
We have used the Westerbork Synthesis Radio Telescope for observations
of solar active regions at 6 cm during 1974 May 8-10. The particular
problems of using a sidereal instrument for solar observations are
discussed. Maps of total intensity and circular polarization were
obtained for four active regions with sunspots and a plage region;
the linear polarization was below the noise limit. The brightest
components of the 6 cm emission are associated with sunspots, while
weaker components are associated with the plage magnetic fields, both
longitudinal and transverse. An inversion of the sense of circular
polarization was observed in the preceding part of a region with
bipolar magnetic-field structure, about two days after the region
passed through the central meridian. Nine bursts were also observed,
and their one-dimensional positions with respect to the associated
active regions are given.

---------------------------------------------------------
Title: .
Authors: Alissandrakis, C. E.
1977PhDT.........4A Altcode:
The Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands
was used for Solar observations at 6 cm on May 8-10, 1974. The WSRT
was used for two-dimensional mapping of Solar active regions and for
one-dimensional mapping of transient burst sources. Synthesis maps of
four active regions were produced. Compact sources associated both with
sunspots and with plage regions were detected. The compact plage sources
were associated with areas of enhanced longitudinal magnetic field
or with regions near the neutral line of the photospheric field. The
magnetic field in the transition zone or low corona above the sunspots
was calculated as 600-900 Gauss. This value is approximately two times
larger than the predictions of current-free calculation of the magnetic
field based on the Meudon magnetogram.

---------------------------------------------------------
Title: A comparison of positions and sizes of sources of centimeter
and X-ray bursts.
Authors: Kundu, M. R.; Alissandrakis, C. E.; Kahler, S. W.
1976SoPh...50..429K Altcode:
We have made a parallel study of three cm-λ radio bursts, observed
on 9 August, 1973 with the NRAO 1 three-element interferometer at 3.7
and 11.1 cm and the associated X-ray flares observed with the S-054
telescope aboard Skylab. Within the errors of our measurements (±
5″) the radio and X-ray events are cospatial. We find good agreement
between the size of the X-ray kernel and that of the core of the cm
burst, while there is evidence that much of the impulsive radio flux
was produced in a larger area.

---------------------------------------------------------
Title: Observations of Solar Bursts at 6 cm Wavelength with Angular
Resolution of 6" arc.
Authors: Alissandrakis, C. E.; Kundu, M. R.
1976BAAS....8Q.555A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A search for periodic variations of solar radio emission at
3.7- and 11.1-cm wavelengths.
Authors: Kundu, M. R.; Alissandrakis, C. E.
1975MNRAS.173...65K Altcode:
Periodic time variations in the emission of quiet solar regions were
sought with the NRAO three-element interferometer at 3.7 and 11.1
cm. The visibility function of a source as obtained with a tracking
interferometer is an implicit function of time, because of projected
baseline changes, and therefore only observations taken when the
baseline changes vary slowly with time were used. The visibility
function shows significant time variations, but a power-spectrum
analysis of the data revealed no unique periodicity.

---------------------------------------------------------
Title: Observations at 6 CM of the solar active region
Authors: Kundu, M. R.; Alissandrakis, C. E.
1975Natur.257..465K Altcode:
Preliminary results are reported for the synthesis of a solar active
region from observations with a synthesis radio telescope at a
wavelength of 6 cm. Several individual sources are observed in the
total-intensity and circular-polarization maps. These are shown to
correspond to two large sunspots, two groups of small sunspots, and a
plage. A magnetogram indicates that this active region had a bipolar
structure with positive polarity concentrated on the main sunspots
and negative polarity scattered over the plage and some of the small
sunspots. It is noted that several radio bursts were recorded during
the observation period and that one corresponded to a H-alpha flare
near one of the sunspot groups.

---------------------------------------------------------
Title: High Resolution Polarization Observations of a Solar Active
Region at 6 cm.
Authors: Alissandrakis, C. E.; Kundu, M. R.
1975BAAS....7..473A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 6 cm Observations of A Solar Active Region with the Westerbork
Synthesis Radio Telescope.
Authors: Kundu, M. R.; Alissandrakis, C. E.; van Someren Greve, H. W.
1975BAAS....7..235K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High Resolution Observations of Solar Bursts at 3.7 and 11.1
cm Wavelengths
Authors: Alissandrakis, C. E.; Kundu, M. R.
1975SoPh...41..119A Altcode:
Four bursts were observed on August 9, 1973 with the NRAO 1 3-element
interferometer at 3.7 and 11.1 cm. By using a simple source model we
have calculated the temperature, flux, size and position of the small
scale components of the bursts as a function of time. We obtained
peak temperatures around 107 K for the components with
size of about 10″ at 11.1 cm and 3″ at 3.7cm. The peak flux
of these components lies between 4 and 40 % of the total peak flux
of the bursts. Two of the bursts were found to be right circularly
polarized. There is evidence that the location of these two bursts does
not coincide with the location of the brightest point of the associated
active regions. The existence of burst structures with temperatures
of the order of 107K indicates that at least part of the
radiation in these bursts is generated by a non-thermal mechanism.

---------------------------------------------------------
Title: Absence of Periodic Variation in Solar Radiation at 3.7 cm
and 11.1 cm Wavelengths.
Authors: Alissandrakis, C.; Kundu, M. R.; Becker, R. H.
1974BAAS....6..348A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Long Base Line Interferometry of the Sun at 3.7 and 11.1
cm Wavelengths.
Authors: Kundu, M. R.; Alissandrakis, C.; Becker, R. H.
1974BAAS....6..219K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Properties of the Red SPOT of Jupiter in 1971
Authors: Banos, C. J.; Dialetis, D. G.; Alissandrakis, C. E.
1972A&A....19..381B Altcode:
Isodensity tracings of Jupiter and its Red Spot during the year 1971 as
well as photometric profiles are given. The variation of the relative
intensity of the Red Spot as the Spot moves from the one edge of the
planetary disc to the other for 1971 is studied. Key words: planetary
atmospheres - Jupiter - Red Spot - photometry.

---------------------------------------------------------
Title: Isophotometry of the Chromospheric Bright and Dark Mottles
on the Solar Disk
Authors: Alissandrakis, C. E.; Macris, C. J.
1972ApL....10...59A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Isodensitometry of Jupiter's Red SPOT and Jupiter
Authors: Banos, C. J.; Alissandrakis, C. E.
1971A&A....15..424B Altcode:
Isodensity tracings for Jupiter in 1968 and for the Red Spot in
1968-1969-1970 are given. The photometric proffies of both derived
from the isophotes and for = 5500 A are given in the East-West and
NorthSouth directions. Key words: planetary atmospheres - Jupiter -
red spot isophotes.

---------------------------------------------------------
Title: A Study of the Fine Structure of the Solar Chromosphere at
the Limb
Authors: Alissandrakis, C. E.; Macris, C. J.
1971SoPh...20...47A Altcode:
We have measured the dimensions, distances from the inner limb and
the lifetime of bright mottles at the limb. Spicule lifetimes have
been measured too. The problem of the `dark band', lying just above
the inner limb, as well as the relations between bright mottles and
spicules are discussed.

---------------------------------------------------------
Title: On the relationships between bright mottles and spicules of
the solar chromosphere.
Authors: Alissandrakis, C. E.; Macris, C. J.
1971AkAtP..46..107A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Lifetime of the Dark and Bright Mottles of the Solar
Chromosphere
Authors: Macris, C. J.; Alissandrakis, C. E.
1970SoPh...11...59M Altcode:
No abstract at ADS