gontikakis

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Author name code: gontikakis
ADS astronomy entries on 2022-09-14
author:"Gontikakis, Costis"

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Title: Study of the transition region inside a coronal hole using
IRIS and SDO observations
Authors: Koletti, Myrto; Gontikakis, Costis; Tsinganos, Kanaris
2022cosp...44.1343K Altcode:
We study a mid-latitude coronal hole, observed on October 9, 2013 with
the Interface Region Imaging Spectrograph (IRIS) in coordination with
the Atmospheric Imaging Assembly (AIA) imager and the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). We
analyze the \siiv\ 1393.755\AA\ and \siiv\ 1402.770\AA\ spectral lines,
formed in the transition region at logT=4.9, and recorded with IRIS. We
performed Gaussian fits on the individual profiles and co-aligned the
derived maps with a co-temporal HMI magnetogram. The spectral parameters
from the Coronal Hole (CH) area and the surrounding Quiet Sun (QS) are
separated in different distributions. The CH shows different parameter
values than the QS at locations for $|B_{LOS}|$ values higher than 60
Gauss, in agreement with previous publications. Moreover, the integrated
$I\scriptscriptstyle_{1393.755}\displaystyle/I\scriptscriptstyle_{1402.770}\displaystyle$
presents values lower than 2 in more that 50% of the profiles indicating
non negligible opacity. Furthermore, we observe lower ratios in the CH
relative to QS regions, when comparing profiles with large $|B_{LOS}|$
indicating a higher opacity inside the CH. Lastly, magnetic field
extrapolation indicates the locations with open field lines, defining
the coronal hole, while the analysis of the Doppler shift reveal
that $\sim$ 60$%$ of all spectral profiles are red-shifted with a LOS
velocity from 0 to 10 km/s, correspondingly. We also observe that the
Doppler velocity for both spectral lines increases with increasing
$|B_{LOS}|$ values within the CH, relative to QS. Spectral data
from other IRIS lines will be analyzed in order to acquire a better
understanding of the transition region inside the coronal hole.

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Title: The lower solar atmosphere inside and outside coronal holes
and the base of the Solar Wind
Authors: Gontikakis, Costis; Patsourakos, Spiros; Tsinganos, Kanaris;
Koletti, Myrto
2022cosp...44.1336G Altcode:
In this review, we will present the crucial observations that gave rise
to current concepts on the formation of the solar wind, low in the solar
atmosphere. We will discuss the differences between the fast solar wind
originating from coronal holes and the slow solar wind emanating around
solar streamers and closed solar magnetic structures. Observations
from remote spectrographs that may constitute critical tests for the
different solar wind acceleration models will be emphasized. Phenomena
such as plumes and jets will also be examined. Finally, we will
introduce the most recent SolO and PSP results on the origins and
early stages of the solar wind.

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Title: Differential Emission Measure Evolution as a Precursor of
Solar Flares
Authors: Gontikakis, C.; Kontogiannis, I.; Georgoulis, M. K.; Guennou,
C.; Syntelis, P.; Park, S. H.; Buchlin, E.
2020arXiv201106433G Altcode:
We analyse the temporal evolution of the Differential Emission Measure
(DEM) of solar active regions and explore its usage in solar flare
prediction. The DEM maps are provided by the Gaussian Atmospheric
Imaging Assembly (GAIA-DEM) archive, calculated assuming a Gaussian
dependence of the DEM on the logarithmic temperature. We analyse
time-series of sixteen solar active regions and a statistically
significant sample of 9454 point-in-time observations corresponding to
hundreds of regions observed during solar cycle 24. The time-series
analysis shows that the temporal derivatives of the Emission Measure
dEM/dt and the maximum DEM temperature dTmax/dt frequently exhibit
high positive values a few hours before M- and X-class flares,
indicating that flaring regions become brighter and hotter as the flare
onset approaches. From the point-in-time observations we compute the
conditional probabilities of flare occurrences using the distributions
of positive values of the dEM/dt, and dTmax/dt and compare them with
corresponding flaring probabilities of the total unsigned magnetic flux,
a conventionally used, standard flare predictor. For C-class flares,
conditional probabilities have lower or similar values with the ones
derived for the unsigned magnetic flux, for 24 and 12 hours forecast
windows. For M- and X-class flares, these probabilities are higher
than those of the unsigned flux for higher parameter values. Shorter
forecast windows improve the conditional probabilities of dEM/dt,
and dTmax/dt in comparison to those of the unsigned magnetic flux. We
conclude that flare forerunner events such as preflare heating or small
flare activity prior to major flares reflect on the temporal evolution
of EM and Tmax. Of these two, the temporal derivative of the EM could
conceivably be used as a credible precursor, or short-term predictor,
of an imminent flare.

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Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
2020A&A...642A...3Z Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.

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Title: Emergence of small-scale magnetic flux in the quiet Sun
Authors: Kontogiannis, I.; Tsiropoula, G.; Tziotziou, K.; Gontikakis,
C.; Kuckein, C.; Verma, M.; Denker, C.
2020A&A...633A..67K Altcode: 2019arXiv191202496K
Context. We study the evolution of a small-scale emerging flux region
(EFR) in the quiet Sun, from its emergence in the photosphere to
its appearance in the corona and its decay. Aims: We track
processes and phenomena that take place across all atmospheric layers;
we explore their interrelations and compare our findings with those from
recent numerical modelling studies. Methods: We used imaging
as well as spectral and spectropolarimetric observations from a suite
of space-borne and ground-based instruments. Results: The EFR
appears in the quiet Sun next to the chromospheric network and shows all
morphological characteristics predicted by numerical simulations. The
total magnetic flux of the region exhibits distinct evolutionary phases,
namely an initial subtle increase, a fast increase with a Co-temporal
fast expansion of the region area, a more gradual increase, and a slow
decay. During the initial stages, fine-scale G-band and Ca II H bright
points coalesce, forming clusters of positive- and negative-polarity
in a largely bipolar configuration. During the fast expansion, flux
tubes make their way to the chromosphere, pushing aside the ambient
magnetic field and producing pressure-driven absorption fronts that
are visible as blueshifted chromospheric features. The connectivity
of the quiet-Sun network gradually changes and part of the existing
network forms new connections with the newly emerged bipole. A few
minutes after the bipole has reached its maximum magnetic flux, the
bipole brightens in soft X-rays forming a coronal bright point. The
coronal emission exhibits episodic brightenings on top of a long
smooth increase. These coronal brightenings are also associated
with surge-like chromospheric features visible in Hα, which can
be attributed to reconnection with adjacent small-scale magnetic
fields and the ambient quiet-Sun magnetic field. Conclusions:
The emergence of magnetic flux even at the smallest scales can be the
driver of a series of energetic phenomena visible at various atmospheric
heights and temperature regimes. Multi-wavelength observations reveal
a wealth of mechanisms which produce diverse observable effects during
the different evolutionary stages of these small-scale structures.

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Title: Effects of resonant scattering of the Si IV doublet near 140
nm in a solar active region
Authors: Gontikakis, C.; Vial, J. -C.
2018A&A...619A..64G Altcode:
 Aims: In a previous study we analysed the C IV 1548.189 Å and
1550.775 Å lines observed with the Solar Ultraviolet Measurements
of Emitted Radiation (SUMER), showing cases where the 1548.189 Å
spectral profile was noticeably different from the 1550.775 Å one,
profiles that we dubbed differentially shaped profiles. We explained
this differential behaviour by an important radiative contribution,
affecting multiple plasma motions happening at the instrument
sub-resolution scale. In the present study we examine more general
cases where radiative effects may contribute to the emission from
the transition region of an active region. Here we analyse the
lines Si IV 1393.757 Å and 1402.772 Å observed with the Interface
Region Imaging Spectrograph (IRIS). Methods: We study active
region NOAA 12529, observed with IRIS on 18 April 2016. Using sorting
techniques we selected individual profiles for which the intensity line
ratio 1393.757 Å/1402.772 Å is significantly higher or lower than
2 and we also tracked differentially shaped profiles. We analyse the
physical conditions that create these profiles and in some cases we
estimate electron densities. Results: We found more than 4000
individual profiles with line ratios higher than 2, about 500 profiles
for which the line ratio is in the range 1.3-1.6, and 15 differentially
shaped profiles. Line ratios higher than 2, are found along loops, and
mostly at the y = 250 to 300″ part of the plage. There, we estimated
the incident radiation and derived electron densities that can vary
from 109 to a few times 1011 cm-3,
depending on the plasma temperature. For the low line ratios, the
sources are concentrated at the periphery of the active region plage,
mostly along fibrils and present optical depths, τ, between 1.5 and
3. in most cases. The electron densities calculated from these Si IV
profiles are comparable with electron densities derived using the
O IV 1399.766 Å-1401.163 Å ratios. Conclusions: We found
that about 2.4% of the individual profiles for which we can perform
a Gaussian fit present a line ratio higher than 2. In profiles with
a high line ratio, the resonant scattering appears to be due to the
combination of an average incident radiation field with a relatively
low local electron density and not due to the vicinity of an ephemeral
strong light source. As far as low intensity ratios are concerned,
non-negligible optical depths are found at the edge of the plage,
near the footpoints of fibrils that are oriented towards quiet
Sun areas, where the electron density can be as high as (7 - 9) ×
1011 cm-3 if we assume a plasma in ionization
equilibrium. The movie associated to Fig. 3 is only available at <A
href="https://www.aanda.org/10.1051/0004-6361/201732563/olm">https://www.aanda.org</A>

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Title: Probing the Quiet Solar Atmosphere from the Photosphere to
the Corona
Authors: Kontogiannis, Ioannis; Gontikakis, Costis; Tsiropoula,
Georgia; Tziotziou, Kostas
2018SoPh..293...56K Altcode: 2018arXiv180307934K
We investigate the morphology and temporal variability of a quiet-Sun
network region in different solar layers. The emission in several
extreme ultraviolet (EUV) spectral lines through both raster and
slot time-series, recorded by the EUV Imaging Spectrometer (EIS) on
board the Hinode spacecraft is studied along with Hα observations and
high-resolution spectropolarimetric observations of the photospheric
magnetic field. The photospheric magnetic field is extrapolated up to
the corona, showing a multitude of large- and small-scale structures. We
show for the first time that the smallest magnetic structures at both
the network and internetwork contribute significantly to the emission
in EUV lines, with temperatures ranging from 8 ×104K to
6 ×105K. Two components of transition region emission
are present, one associated with small-scale loops that do not reach
coronal temperatures, and another component that acts as an interface
between coronal and chromospheric plasma. Both components are associated
with persistent chromospheric structures. The temporal variability
of the EUV intensity at the network region is also associated with
chromospheric motions, pointing to a connection between transition
region and chromospheric features. Intensity enhancements in the
EUV transition region lines are preferentially produced by Hα
upflows. Examination of two individual chromospheric jets shows that
their evolution is associated with intensity variations in transition
region and coronal temperatures.

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Title: The Next Level in Automated Solar Flare Forecasting: the EU
FLARECAST Project
Authors: Georgoulis, M. K.; Bloomfield, D.; Piana, M.; Massone,
A. M.; Gallagher, P.; Vilmer, N.; Pariat, E.; Buchlin, E.; Baudin,
F.; Csillaghy, A.; Soldati, M.; Sathiapal, H.; Jackson, D.; Alingery,
P.; Argoudelis, V.; Benvenuto, F.; Campi, C.; Florios, K.; Gontikakis,
C.; Guennou, C.; Guerra, J. A.; Kontogiannis, I.; Latorre, V.; Murray,
S.; Park, S. H.; Perasso, A.; Sciacchitano, F.; von Stachelski, S.;
Torbica, A.; Vischi, D.
2017AGUFMSA21C..07G Altcode:
We attempt an informative description of the Flare Likelihood And
Region Eruption Forecasting (FLARECAST) project, European Commission's
first large-scale investment to explore the limits of reliability
and accuracy achieved for the forecasting of major solar flares. We
outline the consortium, top-level objectives and first results of
the project, highlighting the diversity and fusion of expertise
needed to deliver what was promised. The project's final product,
featuring an openly accessible, fully modular and free to download
flare forecasting facility will be delivered in early 2018. The
project's three objectives, namely, science, research-to-operations and
dissemination / communication, are also discussed: in terms of science,
we encapsulate our close-to-final assessment on how close (or far)
are we from a practically exploitable solar flare forecasting. In
terms of R2O, we briefly describe the architecture of the FLARECAST
infrastructure that includes rigorous validation for each forecasting
step. From the three different communication levers of the project we
finally focus on lessons learned from the two-way interaction with the
community of stakeholders and governmental organizations. The FLARECAST
project has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No. 640216.

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Title: Enabling Solar Flare Forecasting at an Unprecedented Level:
the FLARECAST Project
Authors: Georgoulis, Manolis K.; Pariat, Etienne; Massone, Anna
Maria; Vilmer, Nicole; Jackson, David; Buchlin, Eric; Csillaghy,
Andre; Bommier, Veronique; Kontogiannis, Ioannis; Gallagher, Peter;
Gontikakis, Costis; Guennou, Chloé; Murray, Sophie; Bloomfield,
D. Shaun; Alingery, Pablo; Baudin, Frederic; Benvenuto, Federico;
Bruggisser, Florian; Florios, Konstantinos; Guerra, Jordan; Park,
Sung-Hong; Perasso, Annalisa; Piana, Michele; Sathiapal, Hanna;
Soldati, Marco; Von Stachelski, Samuel; Argoudelis, Vangelis;
Caminade, Stephane
2016cosp...41E.657G Altcode:
We attempt a brief but informative description of the Flare
Likelihood And Region Eruption Forecasting (FLARECAST) project,
European Commission's first large-scale investment to explore the
limits of reliability and accuracy for the forecasting of major solar
flares. The consortium, objectives, and first results of the project
- featuring an openly accessible, interactive flare forecasting
facility by the end of 2017 - will be outlined. In addition, we will
refer to the so-called "explorative research" element of project,
aiming to connect solar flares with coronal mass ejections (CMEs)
and possibly pave the way for CME, or eruptive flare, prediction. We
will also emphasize the FLARECAST modus operandi, namely the diversity
of expertise within the consortium that independently aims to science,
infrastructure development and dissemination, both to stakeholders and
to the general public. Concluding, we will underline that the FLARECAST
project responds squarely to the joint COSPAR - ILWS Global Roadmap
to shield society from the adversities of space weather, addressing
its primary goal and, in particular, its Research Recommendations
1, 2 and 4, Teaming Recommendations II and III, and Collaboration
Recommendations A, B, and D. The FLARECAST project has received funding
from the European Union's Horizon 2020 research and innovation programme
under grant agreement No. 640216.

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Title: Non-symmetrical profiles of the C IV 1548A and 1550A lines for
small dynamic events and their interpretation in terms of resonant
scattering in solar active region
Authors: Gontikakis, Costis; Vial, Jean-Claude
2016cosp...41E.702G Altcode:
We present observations of small areas in solar active regions
recorded with the SUMER/SOHO spectrograph where the C IV 1548A and
1550A lines have spectral profiles of different shapes, although they
are recorded simultaneously and at the same location. This asymmetry
may be explained by resonant scattering associated with relative
velocities of the emitting plasmas. We present detailed artificial
spectral profiles that may explain the observations. The profiles have
been computed using several physical parameters such as the plasma
temperature and electron density and/or the incident radiation on
the emitting volumes. We conclude that the study of asymmetries in
the C IV 1548A, 1550A lines, and also in different doublets, can be
a valuable diagnostic tool for deriving the physical conditions in
solar/stellar plasmas, especially where the radiation processes (such as
flare-emission) are enhanced at the vicinity of the observed structures.

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Title: Evidence of scattering effects influenced by plasma flows in
C VI 1548 Å, 1550 Å spectral lines emitted from the Sun
Authors: Gontikakis, C.; Vial, J. -C.
2016A&A...590A..86G Altcode:
 Aims: We search for, and study, individual spectral profiles
where the complex shape of the C iv 1548 Å line is different from
the shape of the simultaneously recorded C iv 1550 Å line. Such an
asymmetry is not expected for line emission resulting from collisional
excitation. Therefore we propose an explanation of these observations
through the differential effect of velocity fields on resonant
scattering. Methods: We analyse spectra recorded with the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) on the Solar and
Heliospheric Observatory (SOHO) over active region, NOAA 8541 as well
as a second data set on the quiet Sun. We perform Gaussian fits on the
individual profiles with two or three Gaussian functions. Moreover,
we parameterize the profile asymmetries by calculating the intensity
ratios I1548/I1550, from the derived Gaussian
functions. We also calculate artificial spectral profiles emitted from
two plasma volumes, which have different line of sight motions and
where the plasma emission is influenced by resonant scattering. 
Results: We locate three small regions in NOAA 8541 which have spectral
asymmetries. There the profiles have two or three spectral components,
with different intensity ratios. Artificial profiles show that two
plasma volumes, having distinct velocities relative to the observer,
may reproduce the observed profiles, under the influence of resonant
scattering. Conclusions: Asymmetric profiles, found in an active
region, can be used as a diagnostic for the importance of resonant
scattering in transition region plasma.

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Title: The spectroscopic imprint of the pre-eruptive configuration
resulting into two major coronal mass ejections
Authors: Syntelis, P.; Gontikakis, C.; Patsourakos, S.; Tsinganos, K.
2016A&A...588A..16S Altcode: 2016arXiv160203680S
 Aims: We present a spectroscopic analysis of the pre-eruptive
configuration of active region NOAA 11429, prior to two very fast
coronal mass ejections (CMEs) on March 7, 2012 that are associated
with this active region. We study the thermal components and the
dynamics associated with the ejected flux ropes. Methods: Using
differential emission measure (DEM) analysis of Hinode/EIS and SDO/AIA
observations, we identify the emission components of both the flux rope
and the host active region. We then follow the time evolution of the
flux rope emission components by using AIA observations. The plasma
density and the Doppler and non-thermal velocities associated with
the flux ropes are also calculated from the EIS data. Results:
The eastern and western parts of the active region, in which the two
different fast CMEs originated during two X-class flares, were studied
separately. In both regions we identified an emission component in the
temperature range of log T = 6.8-7.1 associated with the presence of
flux ropes. The time evolution of the eastern region showed an increase
in the mean DEM in this temperature range by an order of magnitude, 5
h prior to the first CME. This was associated with a gradual rise and
heating of the flux rope as manifested by blue-shifts and increased
non-thermal velocities in Ca xv 200.97 Å, respectively. An overall
upward motion of the flux ropes was measured (relative blue-shifts of
~ 12 km s-1). The measured electron density was found to
be 4 × 109-2 × 1010 cm-3 (using
the ratio of Ca xv 181.90 Å over Ca xv 200.97 Å). We compare our
findings with other works on the same AR to provide a unified picture
of its evolution.

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Title: The Major Geoeffective Solar Eruptions of 2012 March 7:
Comprehensive Sun-to-Earth Analysis
Authors: Patsourakos, S.; Georgoulis, M. K.; Vourlidas, A.; Nindos,
A.; Sarris, T.; Anagnostopoulos, G.; Anastasiadis, A.; Chintzoglou,
G.; Daglis, I. A.; Gontikakis, C.; Hatzigeorgiu, N.; Iliopoulos, A. C.;
Katsavrias, C.; Kouloumvakos, A.; Moraitis, K.; Nieves-Chinchilla, T.;
Pavlos, G.; Sarafopoulos, D.; Syntelis, P.; Tsironis, C.; Tziotziou,
K.; Vogiatzis, I. I.; Balasis, G.; Georgiou, M.; Karakatsanis, L. P.;
Malandraki, O. E.; Papadimitriou, C.; Odstrčil, D.; Pavlos, E. G.;
Podlachikova, O.; Sandberg, I.; Turner, D. L.; Xenakis, M. N.; Sarris,
E.; Tsinganos, K.; Vlahos, L.
2016ApJ...817...14P Altcode:
During the interval 2012 March 7-11 the geospace experienced a
barrage of intense space weather phenomena including the second
largest geomagnetic storm of solar cycle 24 so far. Significant
ultra-low-frequency wave enhancements and relativistic-electron dropouts
in the radiation belts, as well as strong energetic-electron injection
events in the magnetosphere were observed. These phenomena were
ultimately associated with two ultra-fast (&gt;2000 km s-1)
coronal mass ejections (CMEs), linked to two X-class flares launched
on early 2012 March 7. Given that both powerful events originated from
solar active region NOAA 11429 and their onsets were separated by less
than an hour, the analysis of the two events and the determination
of solar causes and geospace effects are rather challenging. Using
satellite data from a flotilla of solar, heliospheric and magnetospheric
missions a synergistic Sun-to-Earth study of diverse observational
solar, interplanetary and magnetospheric data sets was performed. It was
found that only the second CME was Earth-directed. Using a novel method,
we estimated its near-Sun magnetic field at 13 R⊙ to be
in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun
CME magnetic field are required to match the magnetic fields of the
corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream
solar-wind conditions, as resulting from the shock associated with the
Earth-directed CME, offer a decent description of its kinematics. The
magnetospheric compression caused by the arrival at 1 AU of the shock
associated with the ICME was a key factor for radiation-belt dynamics.

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Title: Emergence of non-twisted magnetic fields in the Sun: Jets
and atmospheric response
Authors: Syntelis, P.; Archontis, V.; Gontikakis, C.; Tsinganos, K.
2015A&A...584A..10S Altcode: 2015arXiv150902850S
 Aims: We study the emergence of a non-twisted flux tube
from the solar interior into the solar atmosphere. We investigate
whether the length of the buoyant part of the flux tube (i.e. λ)
affects the emergence of the field and the dynamics of the evolving
magnetic flux system. Methods: We perform three-dimensional
(3D), time-dependent, resistive, compressible magnetohydrodynamic
(MHD) simulations using the Lare3D code. Results: We find that
there are considerable differences in the dynamics of the emergence
of a magnetic flux tube when λ is varied. In the solar interior,
for larger values of λ, the rising magnetic field emerges faster
and expands more due to its lower magnetic tension. As a result, its
field strength decreases and its emergence above the photosphere occurs
later than in the smaller λ case. However, in both cases, the emerging
field at the photosphere becomes unstable in two places, forming two
magnetic bipoles that interact dynamically during the evolution of the
system. Most of the dynamic phenomena occur at the current layer, which
is formed at the interface between the interacting bipoles. We find the
formation and ejection of plasmoids, the onset of successive jets from
the interface, and the impulsive heating of the plasma in the solar
atmosphere. We discuss the triggering mechanism of the jets and the
atmospheric response to the emergence of magnetic flux in the two cases.

---------------------------------------------------------
Title: Flux emergence of a non twisted magnetic flux tube
Authors: Syntelis, P., Archontis, V.; Gontikakis, C.; Tsinganos, K.
2013hell.confR..10S Altcode:
We study the numerical simulation of a weakly twisted magnetic flux
tube emergence in a non magnetized corona. We find that this flux tube
creates a system of two expanding magnetic lobes, that exist over the
photospheric bipolar magnetic region and are separated by the polarity
inversion line. Those structures expand due to the magnetic pressure,
creating a current sheet that leads to the reconnection of the magnetic
field lines. This continuous interaction ejects chromospheric plasma to
the upper atmosphere and heats plasma up to 2MK, creating a sequence of
cool and hot coronal jets. Due to the reconnection, the initial emerging
field creates hot and cool loops in the active region, and leads to
the formation of a confined twisted flux rope inside the magnetic
envelope. We created synthetic AIA and XRT images of the simulation
to see whether those structures could be observed. We find that only
some of the ejection can be identified in the synthetic images, and
that some cool and dense structures could be misinterpreted as very
warm in high temperature filters.

---------------------------------------------------------
Title: On the possible importance of coherent resonant scattering
in the solar transition region. A study of the C IV 154.8; 155.0
nm doublet.
Authors: Gontikakis, C.; Jean-Claude, V.
2013hell.conf....6G Altcode:
We study the importance of resonant, coherent scattering of photons,
in the C IV 154.8 and 155.0 nm spectral lines formed in the solar
transition region. We show that under the effect of coherent scattering,
the two lines of the doublet can present different spectral shapes as
found in some solar and stellar observations which have not yet been
explained. In order to model the combination of resonant scattering
and opacity effects we simulate the spectral profiles of the C IV
lines using Monte Carlo methods. Finally we present estimations of the
importance of resonant scattering with respect to electron collisions
inherent in the Differential Emission Measure function, a diagnostic
tool much used for transition region plasma.

---------------------------------------------------------
Title: Particle acceleration and nanoflare heating in coronal loops
Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.;
Anastasiadis, A.; Georgoulis, M.
2013hell.conf...18G Altcode:
We model nanoflare heating of extrapolated active-region coronal loops
via the acceleration of electrons and protons in Harris-type current
sheets. The kinetic energy of the accelerated particles is estimated
using semi-analytical and test-particle-tracing approaches. Vector
magnetograms and photospheric Doppler velocity maps of NOAA active
region 09114, recorded by the Imaging Vector Magnetograph (IVM),
were used for this analysis in order to compute a current-free field
extrapolation of the active-region corona. The corresponding Poynting
fluxes at the footpoints of 5000 extrapolated coronal loops were then
calculated. Assuming that reconnecting current sheets develop along
these loops, we utilized previous results to estimate the kinetic-energy
gain of the accelerated particles and we related this energy to
nanoflare heating and macroscopic loop characteristics. Kinetic
energies of 0.1 to 8~keV (for electrons) and 0.3 to 470~keV (for
protons) were found to cause heating rates ranging from 10^-6 to 1
erg s^-1 cm^-3. Hydrodynamic simulations show that such heating rates
can sustain plasma in coronal conditions inside the loops and generate
plasma thermal distributions which are consistent with active region
observations. We concluded the analysis by computing the form of Xray
spectra generated by the accelerated electrons using the thick target
approach that were found to be in agreement with observed X-ray spectra,
thus supporting the plausibility of our nanoflare-heating scenario. This
work is supported by EU's Seventh Framework Programme via a Marie Curie
Fellowship and by the Hellenic National Space Weather Research Network
(HNSWRN) via the THALIS Programme.

---------------------------------------------------------
Title: Combining Particle Acceleration and Coronal Heating via
Data-constrained Calculations of Nanoflares in Coronal Loops
Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.;
Anastasiadis, A.; Georgoulis, M. K.
2013ApJ...771..126G Altcode: 2013arXiv1305.5195G
We model nanoflare heating of extrapolated active-region coronal loops
via the acceleration of electrons and protons in Harris-type current
sheets. The kinetic energy of the accelerated particles is estimated
using semi-analytical and test-particle-tracing approaches. Vector
magnetograms and photospheric Doppler velocity maps of NOAA active
region 09114, recorded by the Imaging Vector Magnetograph, were
used for this analysis. A current-free field extrapolation of the
active-region corona was first constructed. The corresponding Poynting
fluxes at the footpoints of 5000 extrapolated coronal loops were then
calculated. Assuming that reconnecting current sheets develop along
these loops, we utilized previous results to estimate the kinetic
energy gain of the accelerated particles. We related this energy
to nanoflare heating and macroscopic loop characteristics. Kinetic
energies of 0.1-8 keV (for electrons) and 0.3-470 keV (for protons)
were found to cause heating rates ranging from 10-6 to 1
erg s-1 cm-3. Hydrodynamic simulations show
that such heating rates can sustain plasma in coronal conditions
inside the loops and generate plasma thermal distributions that are
consistent with active-region observations. We concluded the analysis
by computing the form of X-ray spectra generated by the accelerated
electrons using the thick-target approach. These spectra were found
to be in agreement with observed X-ray spectra, thus supporting the
plausibility of our nanoflare-heating scenario.

---------------------------------------------------------
Title: Spectral diagnostic of a microflare. Evidences of resonant
scattering in C IV 1548 Å, 1550 Å lines
Authors: Gontikakis, C.; Winebarger, A. R.; Patsourakos, S.
2013A&A...550A..16G Altcode:
 Aims: We study a microflare, classified as a GOES-A1 after
background subtraction, which was observed in active region NOAA 8541
on May 15, 1999. Methods: We used TRACE filtergrams to study
the morphology and time evolution. SUMER spectral lines were used to
diagnose the chromospheric plasma (Si ii 1533 Å), transition region
plasma (C iv 1548, 1550 Å), and coronal plasma (Ne viii 770 Å). Results: In the 171 Å and 195 Å filtergrams, we measure apparent
mass motions along two small loops that compose the microflare from
the eastern toward the western footpoints. In SUMER, the microflare is
detected as a small (47 Mm2), bright area at the western
footpoints of the TRACE loops. The spectral profiles recorded over
the bright area are complex. The Si ii 1533 Å line is self-reversed
owing to opacity, and the coronal Ne viii line profile is composed of
two Gaussian components, one of them systematically redshifted. The
C iv 1548 Å and 1550 Å profiles are badly distorted because of the
temporary depression of the detector local gain caused by the very
high count rates reached in the flaring region and we can only confirm
the presence of strong blueshifts of ≃ -200 km s-1. Few,
unaffected C iv profiles show two spectral components. In the northern
part of the bright area, all SUMER spectral lines have at least one
blueshifted spectral component. In the southern region of the bright
area the spectral lines are redshifted. Adjacent to the microflare we
measure, for the first time on the solar disk, an intensity ratio of the
1548 Å line to 1550 Å line with values of three to four, indicating
that resonance scattering prevails in the lines formation. Moreover,
the scattering region is found to be cospatial to a solar pore. 
Conclusions: The blueshifts in the footpoints of the microflare and
the apparent mass motions observed with TRACE can be explained by
a gentle chromospheric evaporation triggered by the microflare. The
redshifted spectral components can be explained as cooling material
that is falling back on the solar surface. The presence of resonant
scattering can be explained by the low electron density expected in the
transition region of a solar pore, combined with the high photon flux
coming from the nearby microflare. We estimate that the lower limit
of the electron density in the pore lies in the range 108
cm-3 to 109 cm-3.

---------------------------------------------------------
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: Nanoflare heating of coronal loops in an active region
triggered by reconnecting current sheets
Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.;
Anastasiadis, A.; Georgoulis, M.
2012hell.conf....7G Altcode:
The purpose of this work is to study the heating of coronal loops,
produced by the acceleration of particles inside reconnecting current
sheets (RCS) which represent nanoflares. We also study the hydrodynamic
response of the loops atmosphere to such a heating event. The RCS are
formed as discontinuities of the loop magnetic field caused by the
photospheric shuffling motions. The coronal loops are represented
by the closed magnetic lines of force calculated by the magnetic
field extrapolation of the active region NOAA 9114 magnetogram. The
photospheric motions at the loops footpoints are measured using local
correlation tracking. The magnetic and electric fields accelerating
particles at the RCS are computed using the loop magnetic fields and
the photospheric motions. We further discuss the question of energy
conservation inside the current sheet, and we present the statistical
distributions of quantities relevant for particles acceleration and
coronal heating for a number of the active region's coronal loops.

---------------------------------------------------------
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: Study of a microflare observed with SUMER and TRACE
Authors: Gontikakis, C.; Winebarger, A. R.
2012hell.confQ..11G Altcode:
We study a GOES-A1 microflare, observed in active region NOAA 8541 on
May 15, 1999 with TRACE images, SUMER spectra and MDI magnetograms. In
TRACE filtergrams of 171A and 195A, the microflare is composed of two
interacting, 20Mm long, loops. SUMER observations include four spectral
lines: the Si II 1533A (a chromospheric line), the C IV 1548A, 1550 A
(transition region lines) and the Ne VIII 770 A (a coronal line). These
spectra record the impulsive stage of the microflare, which appears
as a bright feature at the west footpoint of the TRACE loops. In an
area adjacent to the microflare we observe, for the first time on
the solar disk, a region where the lines intensity ratio 1548A/1550A
equals to 4 which means that resonant scattering dominates the emission
process. Over the microflare, the SUMER spectral lines are blue shifted,
indicating upflows due to explosive evaporation, as well as red shifted,
indicating, cooling downward motions. Moreover, the C IV microflare
spectral profiles, indicate upflows of ~200 km/s even if most of them
are damaged due to the SUMER detector over exposure, while the Si II
1533A profiles are self-reversed due to opacity effects.

---------------------------------------------------------
Title: On the initiation of Coronal Mass Ejections observed by
STEREO/EUVI
Authors: Syntelis, P.; Tsinganos, K.; Vourlidas, A.; Gontikakis, C.
2012hell.confR..14S Altcode:
This study examines different stages of a Coronal Mass Ejection's (CME)
initiation in NOAA Active Region (AR) 10980, observed on January 2,
2008 by STEREO's Extreme UltraViolet Imager (EUVI). We identify a
first phase consisting of an upward motion, which at 1.58R? reaches the
velocity of 70 (4) km/s. Those measurements are extrapolated to later
time frames to examine whether this initial acceleration drives the
CME's propagation later on. We also identify an ascending new flux-rope
beneath the CME. During the CME's rise, there are indications that
some adjacent loops incline to the main CME body. At the later phase
of the initiation, some moving blob-like structures appear along the
CME flanks. Propagation speeds of these blobs are measured. These
blobs could be indications that a siphon flow exists along the CME.

---------------------------------------------------------
Title: Heating Distribution Along Coronal Loops in two Active Regions
Using a Simple Electrodynamic Calculation
Authors: Gontikakis, C.; Georgoulis, M.; Contopoulos, I.; Dara, H. C.
2010ASPC..424...25G Altcode:
The heating along hundreds of coronal loops of non flaring active
regions is computed using a simple electrodynamic model. Photospheric
motions generate electric fields inducing, electric potential
differences at the footpoints of loops. These potential differences
generate electric currents that lead to Ohmic heating. We computed
the magnetic field extrapolation from the magnetograms of two
active regions, namely NOAA AR 9366 (SOHO/MDI) and NOAA AR 10963,
(HINODE/SOT). Closed magnetic field lines model the coronal loops. For
each loop we computed the heating function and obtained the hydrostatic
distribution of temperature and pressure. We found that the coronal
heating is stronger near the footpoints of the loops and asymmetric
along them. We obtained scaling laws that correlate the mean volumetric
heating with the loop length, and the heating flux, through the loop
footpoints with the magnetic field strength at the footpoints. Our
results are in qualitative agreement with observations (see Gontikakis
et al. 2008 for more details).

---------------------------------------------------------
Title: Study of a Solar Active Region Jet
Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K.
2010ASPC..424...19G Altcode:
We present the observations of an active region jet originating from the
east side of NOAA 8531 on May 15 1999. The observations include a series
of TRACE 171 Å filtergrams, and simultaneous observations from SUMER
in Ne VIII, 770 Å, C IV 1548 Å, as well as MDI magnetograms. The
observations were compared with the results of a 3D MHD numerical
simulation of magnetic flux emergence and its subsequent reconnection
with preexisting magnetic flux. The numerical simulation reproduces
the observed 100 km/s outflow at the right temperature range
(0.6-1×106 Kelvin). Moreover, the observations seem to
suggest that the jet plasma falls back on the solar surface along an
active region loop, in agreement with our model.

---------------------------------------------------------
Title: Recurrent solar jets in active regions
Authors: Archontis, V.; Tsinganos, K.; Gontikakis, C.
2010A&A...512L...2A Altcode: 2010arXiv1003.2349A
 Aims: We study the emergence of a toroidal flux tube into the
solar atmosphere and its interaction with a pre-existing field of
an active region. We investigate the emission of jets as a result of
repeated reconnection events between colliding magnetic fields. Methods: We perform 3D simulations by solving the time-dependent,
resistive MHD equations in a highly stratified atmosphere. 
Results: A small active region field is constructed by the emergence
of a toroidal magnetic flux tube. A current structure is build up
and reconnection sets in when new emerging flux comes into contact
with the ambient field of the active region. The topology of the
magnetic field around the current structure is drastically modified
during reconnection. The modification results in a formation of new
magnetic systems that eventually collide and reconnect. We find that
reconnection jets are taking place in successive recurrent phases in
directions perpendicular to each other, while in each phase they release
magnetic energy and hot plasma into the solar atmosphere. After a series
of recurrent appearance of jets, the system approaches an equilibrium
where the efficiency of the reconnection is substantially reduced. We
deduce that the emergence of new magnetic flux introduces a perturbation
to the active region field, which in turn causes reconnection between
neighboring magnetic fields and the release of the trapped energy in the
form of jet-like emissions. This is the first time that self-consistent
recurrency of jets in active regions is shown in a three-dimensional
experiment of magnetic flux emergence.

---------------------------------------------------------
Title: Observations and 3D MHD simulations of a solar active
region jet
Authors: Gontikakis, C.; Archontis, V.; Tsinganos, K.
2009A&A...506L..45G Altcode:
Aims: We study an active region jet originating from NOAA 8531 on
May 15 1999. We perform 3D MHD numerical simulations of magnetic flux
emergence and its subsequent reconnection with preexisting magnetic
flux. Then, we compare the physical properties of the observed jet with
the reconnecting outflow produced in the numerical model. Methods:
We report observations of this jet using a series of TRACE 171 Å
filtergrams, simultaneous observations from SUMER in Ne viii 770Å and
C iv 1548 Å as well as MDI magnetograms. In the numerical simulation,
the full compressible and resistive MHD equations are solved, including
viscous and Ohmic heating. Results: A high-velocity upflow
(≃100 km s-1) is observed after the emergence of new
magnetic flux at the edge of the active region. The jet is recorded
over a range of temperatures between 105 K and 1.5 ×
106 K. In our numerical experiments, we find that the jet
is the result of magnetic reconnection between newly emerging flux and
the preexisting magnetic field of the active region. Conclusions:
The hot and high-velocity bidirectional flows occur as a result of the
interaction between oppositely directed magnetic fields. Observations
and numerical results are strongly suggestive of effective reconnection
process being responsible for producing jets when emerging flux appears
in solar active regions.

---------------------------------------------------------
Title: Charged particles' acceleration through Reconnecting Current
Sheets in Solar Flares
Authors: Gontikakis, C.; Efthymiopoulos, C.; Anastasiadis, A.
2009ASSP....8..449G Altcode: 2009chas.book..449G
No abstract at ADS

---------------------------------------------------------
Title: Particle Interactions with Single or Multiple 3D Solar
Reconnecting Current Sheets
Authors: Anastasiadis, A.; Gontikakis, C.; Efthymiopoulos, C.
2008SoPh..253..199A Altcode: 2008arXiv0806.4854A; 2008SoPh..tmp..135A
The acceleration of charged particles (electrons and protons) in flaring
solar active regions is analyzed using numerical experiments. The
acceleration is modeled as a stochastic process taking place by the
interaction of the particles with local magnetic reconnection sites via
multiple steps. Two types of local reconnecting topologies are studied:
the Harris-type and the X-point. A formula for the maximum kinetic
energy gain in a Harris-type current sheet, found in a previous work of
ours, fits well the numerical data for a single step of the process. A
generalization is then given approximating the kinetic energy gain
through an X-point. In the case of the multiple step process, in both
topologies the kinetic energy distribution of the particles is found to
acquire a practically invariant form after a small number of steps. This
tendency is interpreted theoretically. Other characteristics of the
acceleration process are given, such as the mean acceleration time
and the pitch angle distributions of the particles.

---------------------------------------------------------
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: Distribution of coronal heating in a solar active region
Authors: Gontikakis, C.; Contopoulos, I.; Dara, H. C.
2008A&A...489..441G Altcode:
Aims: We investigate the distribution of heating of coronal loops
in a non-flaring solar active region, using a simple electrodynamic
model: the random displacements of the loop footpoints, caused by
photospheric plasma motions, generate electric potential differences
between the footpoints and, as a result, electric currents flow along
the loops, producing Ohmic heating. Methods: We implement our
model on the closed magnetic field lines in the potential magnetic
field extrapolation of an MDI active region magnetogram. For each
one of the magnetic field lines, we compute the heating function and
obtain the hydrostatic distribution of temperature and pressure. We
find that coronal heating is stronger close to the footpoints of the
loops and asymmetric along them. We obtain scaling laws that relate
both the mean volumetric heating to the loop length, and the heating
flux through the loop footpoints to the magnetic field strength at the
footpoints. Our results agree with observational data. Results:
According to our model, we attribute the observed small coronal-loop
width expansion to both the preferential heating of coronal loops of
small cross-section variation, and the cross-section confinement due to
the random electric currents flowing along the loops. Conclusions:
We conclude that our model can be used as a simple working tool in
the study of solar active regions.

---------------------------------------------------------
Title: Heating Distribution along Coronal Loops of an Active Region
using a Simple Electrodynamic Calculation
Authors: Gontikakis, C.; Contopoulos, I.; Dara, H. C.
2008ESPM...12.3.46G Altcode:
The heating along hundreds of coronal loops of a non flaring active
region is computed using a simple electrodynamic model. The random
plasma displacements caused by the photospheric motions generate
electric fields and, as a consequence, electric potential differences at
the footpoints of loops. These potential differences generate electric
currents which are the cause of Ohmic heating. We computed the
potential magnetic field extrapolation using the MDI magnetogram of
the NOAA 9366 active region observed on March 6, 2001 and used the
closed magnetic field lines to model the coronal loops. For each loop
we compute the heating function and obtain the hydrostatic distribution
of temperature and pressure. We find that the coronal heating is
stronger near the footpoints of the loops and asymmetric along them. We
obtain scaling laws that relate the mean volumetric heating with the
loop length, and the heating flux through the loop footpoints with the
magnetic field strength at the footpoints. We simulated the emission of
the hydrostatic loops in the 171 and 284 Angstroms spectral bands and
compared the resulting images with EIT observations. Our results are
in qualitative agreement with observations. We conclude that our model
can be used as a simple working tool for the study of active regions.

---------------------------------------------------------
Title: Charged Particles' Acceleration through Multiple Reconnecting
Regions
Authors: Gontikakis, C.; Efthymiopoulos, A. C. Anastasiadis
2008ESPM...12.3.45G Altcode:
We study the acceleration of charged particles (electrons and
protons) in steady Reconnecting Current Sheets and X-points in the
Solar Corona. We compute the orbits of test particles in simplified
magnetic and electric field topologies where a longitudinal magnetic
field component is included. We study the particles' kinetic energy
gain as a function of the field parameters. The kinetic energy gain of
particles accelerated through a number of subsequent current sheets
converges to a saturation at high energies after a small number of
particles-current sheets encounters. This numerical result can be
explained by an analytical theory.

---------------------------------------------------------
Title: Particle distributions and X-ray spectra in single or multiple
solar current sheets
Authors: Gontikakis, C.; Anastasiadis, A.; Efthymiopoulos, C.
2007MNRAS.378.1019G Altcode:
The acceleration of charged particles in a site of magnetic reconnection
is analysed by detailed numerical simulations. Single or multiple
encounters of the particles with Harris-type reconnecting current
sheets (RCSs) are modelled as an overall stochastic process taking
place within an active region. RCS physical parameters are selected
in a parameter space relevant to solar flares. Initially, the charged
particles form a thermal (Maxwellian) distribution corresponding to
coronal temperature ~=2 × 106 K. Our main goal is to
investigate how the acceleration process changes the shape of the
particles' kinetic energy distribution. The evolution of the kinetic
energy distribution, calculated numerically after one encounter of the
particles with a single RCS, is found to be in good agreement with our
previously published analytical formulae. In the case of consecutive
encounters, we find that the kinetic distribution tends to converge
to a practically invariant form after a relatively small number of
encounters. We construct a discrete stochastic process that reproduces
the numerical distributions and we provide a theoretical interpretation
of the asymptotic convergence of the energy distribution. We finally
compute the theoretical X-ray spectra that would be emitted by the
simulated particles in a thick target model of radiation.

---------------------------------------------------------
Title: Evolution of a Coronal Loop System
Authors: Tsiropoula, G.; Tziotziou, K.; Wiegelmann, T.; Zachariadis,
Th.; Gontikakis, C.; Dara, H.
2007SoPh..240...37T Altcode:
The temporal variation of a loop system that appears to be changing
rapidly is examined. The analyzed data were obtained on 15 May 1999,
with the Transition Region and Coronal Explorer (TRACE) during an
observing campaign and consist of observations in the Fe IX/Fe X 171
Å and Fe XII 195 Å passbands taken at a cadence of ∼10 min. The
special interest in this loop system is that it looks like one expanding
loop; however, careful examination reveals that the loop consists of
several strands and that new loop strands become visible successively
at higher altitudes and lower loop strands fade out during the one hour
of our observations. These strands have different widths, densities,
and temperatures and are most probably consisting of, at least, a few
unresolved thinner threads. Several geometric and physical parameters
are derived for two of the strands and an effort is made to determine
their 3D structure based on the extrapolation of the magnetic field
lines. Electron density estimates allow us to derive radiative and
conductive cooling times and to conclude that these loop strands are
cooling by radiation.

---------------------------------------------------------
Title: Particle Acceleration In Single Or Multiple Solar Current
Sheets: The Final Spectra
Authors: Gontikakis, C.; Anastasiadis, A.; Efthymiopoulos, C.
2007ESASP.641E..64G Altcode:
Numerical simulations are presented of the acceleration of charged
particle in a single multiple Harris-type Reconnecting Current Sheet
(RCS). We start with thermal (Maxwellian) initial particle distributions
at ≅ 2 x 106 K. The main goal is to understand how acceleration is a
reconnection site affects the final form of the energy and pitch angle
distributions, as well as the X-Ray spectra produced on the assumption
of a thick target model. These quantities are expored in a parametric
space including a) the particle species, b) the relative strengths
of the magnetic and electric field components, and c) the number if
successive scattering evens in a reconnection site including multiple
RCSs with random orientation and field parameters. The numberical
results are compared to analytical estimates obtained from our previous
studies (i.e. [4], [5]).

---------------------------------------------------------
Title: S OHO Observations Of A Coronal Loop Compared With A 2D MHD
Loop Model
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2007ESASP.641E..65G Altcode:
We present SOHO/CDS observations of a coronal loop where Doppler
shifts and proper motions indicate a unidirectional mass flow. The
measurements of velocity, electron density and temperature (about 380
000 K) along its length are compared with the results of a 2D MHD
loop model. The derived energy balance, showing a stronger heating
at the loop footprints, as well as the knowledge gained from the MHD
model are presented. The implications of Solar Orbiter observations
for studying coronal loops are discussed.

---------------------------------------------------------
Title: S imulation Of An Active Region With A Simple Electrodynamic
Model
Authors: Contopoulos, I.; Gontikakis, C.; Dara, H. C.
2007ESASP.641E..59C Altcode:
We propose a simple model that allows us to investigate the development
of the slender magnetic loops seen in TRACE observations of active
regions. We argue that random displacements of the loop footpoints
caused by the observed rapid intermittent photospheric plasma flow
generate electric potential differences between the foot- points. As
a result, random electric currents flow along the loops. Our model
allows us to estimate the distribu- tion of intermittent Ohmic (DC)
heating in the corona. We propose that the thickness and duration of
the bright loops is related to the photospheric granulation motion
coherence length and turnover time respectively.

---------------------------------------------------------
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: Electron Orbits in Solar Reconnecting Current Sheets
Authors: Gontikakis, C.; Efthymiopoulos, C.; Anastasiadis, A.
2006AIPC..848...88G Altcode:
We investigate the orbits of charged particles (electrons) that
interact with a 3D-solar type reconnecting current sheet (RCS). The
magnetic field used is a linearized Harris type model. Particles are
accelerated by a super-Dreicer electric field. Taking advantage of
the translational symmetries of the fields, the orbits computation is
simplified to integration of the equations of motion in a Hamiltonian of
two degrees of freedom. The application of canonical transformations,
constructed with Lie series, transforms the Hamiltonian to a normal
form. The conditions for which electrons are trapped in the RCS are
derived analytically. For escaping particles, the amount of final
kinetic energy gain or loss is described with analytical expressions
that are in good agreement with the numerical results.

---------------------------------------------------------
Title: Regular and chaotic dynamics in 3D reconnecting current sheets
Authors: Gontikakis, C.; Efthymiopoulos, C.; Anastasiadis, A.
2006MNRAS.368..293G Altcode:
We consider the possibility of particles being injected at the interior
of a reconnecting current sheet (RCS), and study their orbits by
dynamical systems methods. As an example we consider orbits in a 3D
Harris type RCS. We find that, despite the presence of a strong electric
field, a `mirror' trapping effect persists, to a certain extent,
for orbits with appropriate initial conditions within the sheet. The
mirror effect is stronger for electrons than for protons. In summary,
three types of orbits are distinguished: (i) chaotic orbits leading
to escape by stochastic acceleration, (ii) regular orbits leading to
escape along the field lines of the reconnecting magnetic component,
and (iii) mirror-type regular orbits that are trapped in the sheet,
making mirror oscillations. Dynamically, the latter orbits lie on a set
of invariant KAM tori that occupy a considerable amount of the phase
space of the motion of the particles. We also observe the phenomenon of
`stickiness', namely chaotic orbits that remain trapped in the sheet
for a considerable time. A trapping domain, related to the boundary of
mirror motions in velocity space, is calculated analytically. Analytical
formulae are derived for the kinetic energy gain in regular or chaotic
escaping orbits. The analytical results are compared with numerical
simulations.

---------------------------------------------------------
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: Evidence for coronal plasma oscillations over supergranular
cells
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2006IAUS..233..189G Altcode:
Evidence of coronal oscillations over the interior of supergranular
cells was found through SUMER observations. The observations are
rasters of quiet Sun regions and the oscillations were detected,
in the Ne VIII 770 Å Doppler maps, as a characteristic pattern. It
should be noted that the Ne VIII ion has coronal formation temperature
(650 000 K) and that reports of oscillations in the quiet Sun corona
are scarce. Magnetic extrapolation from MDI magnetogram showed that at
the location where the oscillation was detected, the gas and magnetic
pressures get equalized (β=1) higher in the atmosphere, compared
to the surrounding, non oscillating quiet Sun. This could indicate a
non-compressible wave propagating inside the gas dominated medium of
the cell causing the detected oscillation.

---------------------------------------------------------
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: Particle dynamics in 3-D reconnecting current sheets in the
solar atmosphere
Authors: Efthymiopoulos, C.; Gontikakis, C.; Anastasiadis, A.
2005A&A...443..663E Altcode:
The orbits of charged particles (electrons and protons), in
a Harris-type 3D field topology of a reconnecting current sheet
(RCS), are analyzed by dynamical systems methods. The focus is on
values of the magnetic and electric fields relevant to RCSs in the
solar atmosphere. First, a perturbative form of the equations of
motion is used to determine the stability perpendicularly to the
plane of reconnection, which is crucial in the efficiency of the RCS
as an accelerator. The problem is shown to correspond to a case of
"parametric resonance". The orbits are then studied with the complete
form of the equations of motion. These can be reduced to a two degrees
of freedom Hamiltonian nonlinear system by exploiting the existence
of an additional integral of motion besides the energy. The orbits
are studied analytically by normal form theory. Regular and chaotic
orbits are identified by the use of appropriate Poincaré surfaces of
section. The kinetic energy gain for escaping particles is calculated
as a function of the initial conditions of injection of an orbit in
the sheet. Formulae relating the kinetic energy gain to the physical
parameters of the sheet and the initial conditions of the orbits are
given both for electrons and protons.

---------------------------------------------------------
Title: Coronal oscillation above a supergranular cell of the quiet
Sun chromospheric network?
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2005A&A...441.1191G Altcode:
We have detected an oscillation in the low corona, using a raster of
the SUMER EUV spectrograph in the Ne viii, 770 Å line formed at about
700 000 K. The oscillation was found in the Ne viii Doppler map above
the interior of a supergranular cell of the chromospheric network in
the quiet Sun, while it is absent in line radiance. The photospheric
magnetic field, extrapolated to coronal levels, was used to relate
this phenomenon to the magnetic structure. This oscillation phenomenon,
reported here for the first time, seems to occur only above the darkest
cells of the chromospheric network. We interpret our findings as a
collective non-compressible oscillation of the corona above the whole
cell interior. This oscillation may originate at the chromosphere and
the driving wave may undergo a mode conversion at the top chromosphere,
where the magnetic pressure equals the gas pressure. Our interpretation
cannot be definitive and should be verified with more data.

---------------------------------------------------------
Title: A solar active region loop compared with a 2D MHD model
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2005A&A...434.1155G Altcode: 2005astro.ph..3694G
We analyzed a coronal loop observed with the Normal Incidence
Spectrometer (NIS), which is part of the Coronal Diagnostic Spectrometer
(CDS) on board the Solar and Heliospheric Observatory (SOHO). The
measured Doppler shifts and proper motions along the selected loop
strongly indicate unidirectional flows. Analysing the Emission Measure
Curves of the observed spectral lines, we estimated that the temperature
along the loop was about 380 000 K. We adapted a solution of the ideal
MHD steady equations to our set of measurements. The derived energy
balance along the loop, as well as the advantages/disadvantages of
this MHD model for understanding the characteristics of solar coronal
loops are discussed.

---------------------------------------------------------
Title: Oscillations Over a Supergranular Cell Observed with SUMER
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2004ESASP.575..131G Altcode: 2004soho...15..131G
No abstract at ADS

---------------------------------------------------------
Title: Electron acceleration and radiation in evolving complex
active regions
Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L.
2004A&A...422..323A Altcode:
We present a model for the acceleration and radiation of solar
energetic particles (electrons) in evolving complex active regions. The
spatio - temporal evolution of active regions is calculated using a
cellular automaton model, based on self-organized criticality. The
acceleration of electrons is due to the presence of randomly placed,
localized electric fields produced by the energy release process,
simulated by the cellular automaton model. We calculate the resulting
kinetic energy distributions of the particles and their emitted X-ray
radiation spectra using the thick target approximation, and we perform a
parametric study with respect to number of electric fields present and
thermal temperature of the injected distribution. Finally, comparing
our results with the existing observations, we find that they are in
a good agreement with the observed X-ray spectra in the energy range
100-1000 keV.

---------------------------------------------------------
Title: Energetic Particle Acceleration and Radiation in Evolving
Complex Active Regions
Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L.
2004hell.conf...71A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Study of a Solar Active Region Loop Using EUV Spectra
Authors: Gontikakis, C.; Petrie, G. J. D.; Dara, H. C.; Tsinganos, K.
2004hell.conf...25G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 2D MHD Modelling of Heated Coronal Loops Compared to TRACE
Observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2004hell.conf...31P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Comparing 3D observations and magnetic field extrapolation
for helicity budget, coronal physics understanding and space weather
forecasting
Authors: Portier-Fozzani, F.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.
2004cosp...35.2683P Altcode: 2004cosp.meet.2683P
We compared parameters obtained by 3D stereoscopic methods with
magnetic extrapolations in 2 different cases : 1/ We studied the long
scale variation of the morphology of an active region during several
days. The active region was selected from movies we made using EIT
images in a transition region line (171 A) for all the years of the
SOHO mission (1996 - 2003). The morphology of the magnetic loops,
using force-free magnetic field extrapolation, were compared with
the 3D parameters found by stereoscopic analysis. Conclusions for
magnetic helicity budget versus the dynamic variations are derived. 2/
We studied variations encontered after a flare while a sigmoid (made
of several twisted loops) was transformed into an arcade. The change
of topology was attributed to a reconnection of the sigmoid with new
emerging active region loops, and the 3D information obtained with
SOHO/EIT 195A was compared with magnetic field extrapolation. These two
analyses give us information for a better coronal physics understanding,
criteria for stability and thus space weather forecasting.

---------------------------------------------------------
Title: 2D MHD modelling of compressible and heated coronal loops
obtained via nonlinear separation of variables and compared to TRACE
and SoHO observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2003A&A...409.1065P Altcode: 2003astro.ph..3373P
An analytical MHD model of coronal loops with compressible flows and
including heating is compared to observational data. The model is
constructed via a systematic nonlinear separation of the variables
method used to calculate several classes of exact MHD equilibria in
Cartesian geometry and uniform gravity. By choosing a particularly
versatile solution class with a large parameter space we are able to
calculate models whose loop length, shape, plasma density, temperature
and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS
and SoHO/SUMER. Synthetic emission profiles are also calculated and
fitted to the observed emission patterns. An analytical discussion is
given of the two-dimenional balance of the Lorentz force and the gas
pressure gradient, gravity and inertial forces acting along and across
the loop. These models are the first to include a fully consistent
description of the magnetic field, 2D geometry, plasma density and
temperature, flow velocity and thermodynamics of loops. The consistently
calculated heating profiles which are largely dominated by radiative
losses and concentrated at the footpoints are influenced by the flow
and are asymmetric, being biased towards the upflow footpoint.

---------------------------------------------------------
Title: Sizes of quiet Sun transition region structures
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2003A&A...408..743G Altcode:
We studied the morphology of the transition region of the quiet Sun,
with data obtained by the Solar Ultraviolet Measurements of Emitted
Radiation spectrometer (SUMER) and the Extreme-Ultraviolet Imaging
Telescope (EIT), in September 1996. We analyzed lines emitted in the
chromosphere, the low transition region and the low corona. We computed
the mean autocorrelation function for the radiance images in order
to estimate the characteristic size of the features present in the
transition region. Moreover different estimate McCly-mont. Moreover,
we calculated autocorrelation functions for the dopplergrams and
line width images deduced from the SUMER data. In addition to the
line core of the C IV line, we investigated a broader tail component,
whose origin is still unclear. We found that the size of the bright
radiance features is always larger than the size of the structures
of the dopplergrams and Doppler widths, at any altitude. The network
features seem to diminish at a temperature around 105 K,
due to the thermodynamic properties of the transition region. The mean
size of the structures of the tail component radiance is smaller than
the one of the core radiance.

---------------------------------------------------------
Title: Particle acceleration and radiation in an evolving active
region based on a Cellular Automaton (CA) model
Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L.
2002ESASP.506..265A Altcode: 2002svco.conf..265A; 2002ESPM...10..265A
We present a model for the acceleration and radiation of solar energetic
particles (electrons) in an evolving active region. The spatio-temporal
evolution of the active region is calculated using a Cellular Automaton
(CA) model for the energy release process. The acceleration of particles
is due to the presence of randomly placed, localized electric fields. We
calculate the resulting kinetic energy distributions of the particles
and the emitted radiation by performing a parametric study with respect
to the trapping time of the injected distribution.

---------------------------------------------------------
Title: Two-component structure of the solar transition region
Authors: Gontikakis, C.; Peter, H.; Dara, H. C.
2002ESASP.506..625G Altcode: 2002svco.conf..625G; 2002ESPM...10..625G
We studied the morphology of the quiet Sun in the Transition Region,
using observations of the SUMER spectrograph in three emission lines (Si
II, 1533 Å, C IV, 1548 Å and Ne VIII, 770 Å), emitted respectively
in the chromosphere, the low transition region and the low corona, as
well as EIT images in the four passbands of the instrument. We computed
the mean autocorrelation function for the intensity images in order
to estimate the characteristic size of the features present in the
Transition Region. Moreover, we deduced autocorrelation functions for
the Dopplergrams and line width images. Specifically, using images of
the C IV emission lines tail component, as this is estimated by a double
Gaussian fit, we studied the morphology of the coronal funnels, as they
appear at 105K (formation temperature of the C IV line). We
found that the size of the intensity bright features is always larger
than the size of the structures of the dopplergrams and dopplerwidths,
at all altitudes. The mean size of the structures of the tail component
intensity is smaller than the one of the core intensity component.

---------------------------------------------------------
Title: Exact 2D MHD modelling of coronal loops observed with TRACE
and CDS including comprssible flow and heating.
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H.; Tsinganos, K.
2002ESASP.506..713P Altcode: 2002ESPM...10..713P; 2002svco.conf..713P
We present exact models of coronal loops including heating, which can be
compared quantitatively to observational data. The systematic nonlinear
separation of variables method used to calculate the equilibria has
already been developed in spherical geometry and in Cartesian geometry
generalising many known solutions. Using a particularly versatile
solution class in Cartesian geometry with a large parameter space we
are able to calculate models whose loop length, loop shape, plasma
density, temperature and velocity profiles are fitted to loops observed
with TRACE and with SoHO CDS, while consistent heating profiles are
also given.

---------------------------------------------------------
Title: Dynamics of limb features from TRACE EUV observations
Authors: Zachariadis, Th. G.; Gontikakis, C.
2002ESASP.505..619Z Altcode: 2002IAUCo.188..619Z; 2002solm.conf..619Z
We present a preliminary study of spicule dynamics from TRACE UV
observations in the 1216 (Ly-α), 1550 (C IV), 1600 and 1700 Å bands
near the solar north pole. Spicules are visible beyond the in all
bands, being stronger in the Ly-α and C IV bands, where some are seen
crossing the limb. After reaching maximum height of up to 16000 km
(40000 in exceptional cases), some of them fall back, others diffuse
and a small number eject their material into the corona.

---------------------------------------------------------
Title: Exact 2D MHD modelling of coronal loops observed with TRACE
including compressible flow and heating
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H.; Tsinganos, K.
2002ESASP.505..149P Altcode: 2002IAUCo.188..149P; 2002solm.conf..149P
We present exact models of coronal loops including heating, which can be
compared quantitatively to observational data. The systematic nonlinear
separation of variables method used to calculate the equilibria has
already been developed in spherical geometry and in Cartesian geometry
generalising many known solutions. Using a particularly versatile
solution class in Cartesian geometry with a large parameter space we
are able to calculate models whose loop length, loop shape, plasma
density, temperature and velocity profiles are fitted to loops observed
with TRACE and with SoHO CDS, while consistent heating profiles are
also given.

---------------------------------------------------------
Title: Acceleration and radiation model of solar energetic particles
in an evolving active region
Authors: Anastasiadis, A.; Gontikakis, C.; Vilmer, N.; Vlahos, L.
2002ESASP.505..337A Altcode: 2002solm.conf..337A; 2002IAUCo.188..337A
We present a model for the acceleration and radiation of solar energetic
particles (electrons) in an evolving active region. The spatio-temporal
evolution of the active region is calculated using a Cellular Automaton
(CA) model for the energy release process. The acceleration of particles
is due to the presence of randomly placed, localized electric fields. We
calculate the resulting kinetic energy distributions of the particles
by performing a parametric study with respect to the trapping time
of the injected distribution. Our results show a power law or a power
law with an exponential tail behavior for the resulting kinetic energy
distribution, depending on the maximum trapping time of the injected
particles in the acceleration volume. Finally we calculate the emitted
radiation spectrum from the resulting energy distributions.

---------------------------------------------------------
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: 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: 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: 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: 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.

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Title: Dynamics of the transition region
Authors: Gontikakis, C.; Dara, H. C.
2000NewAR..44..599G Altcode:
In this review, we consider the problem of the apparent redshifts of
the UV lines in the transition region and review the basic observations
made over the last decades, especially the observations of the last
few years from satellite observatories. Moreover, we revise the most
popular theoretical explanations for the motions in the transition
region. This review is a contribution to the understanding of the
physical processes in this important layer of the solar atmosphere
and it points out the pending problems.

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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

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Title: Emission of hydrogen lines by moving solar prominences.
Authors: Gontikakis, C.; Vial, J. -C.; Gouttebroze, P.
1997A&A...325..803G Altcode:
We study the radiative transfer processes occurring in a plane-parallel
slab standing vertically above the chromosphere and moving upward
as a solid body. This structure simulates a prominence in the phase
of eruption or a quiescent prominence where the plasma presents bulk
velocities. We use partial redistribution for the description of the
resonance scattering in the hydrogen Lα and Lβ lines. We compute
the Lα, Lβ and Hα emergent line profiles of hydrogen for different
velocities. We derive the variation of the emitted intensities as a
function of the velocity for various temperatures and thicknesses. We
discuss our results in view of the diagnostic of erupting prominences
in Lyman lines that SOHO observations can provide.

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Title: Spectral diagnostics for eruptive prominences
Authors: Gontikakis, C.; Vial, J. -C.; Gouttebroze, P.
1997SoPh..172..189G Altcode: 1997ESPM....8..189G
The diagnostic of eruptive prominences needs the development of
new tools. Here we propose the Lyman and Balmer lines of hydrogen,
which are important in the radiative budget. In the NLTE radiative
transfer calculations, we include the effect of the outward motion
of the structure associated with the eruption of the prominence. The
treatment of the resonance scattering of Lα and Lβ with partial
redistribution gives higher intensities, and a higher ionization than
the complete redistribution, but the two approaches converge to the same
solution as the velocity increases. As a first step in the diagnostic,
we present new results concerning the variation of the integrated
intensities of hydrogen lines with respect to the radial velocity.

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Title: Diagnostic and observations of quiescent and
Authors: Vial, J. -C.; Gontikakis, C.; Bocchialini, K.; Gouttebroze, P.
1997IAUJD..19E..55V Altcode:
We present a new diagnostic tool for investigating the plasma conditions
in a moving structure in the solar corona, such as an eruptive filament
or prominence. It relies on NLTE radiative transfer calculations and
the signatures of outward directed velocities on Lyman and Balmer lines
profiles. Preliminary observations with SUMER on SOHO are discussed.

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Title: Dynamics of solar magnetic arches in the photosphere and
the chromosphere.
Authors: Mein, P.; Demoulin, P.; Mein, N.; Engvold, O.; Molowny-Horas,
R.; Heinzel, P.; Gontikakis, C.
1996A&A...305..343M Altcode:
Mass motions in chromospheric arch filaments have been observed
with imaging spectroscopy (MSDP) at the VTT telescope of the Teide
Observatory. Coordinated observations of time sequences of continuum
images were carried out at the SVST telescope of the Observatorio
del Roque de los Muchachos, which provided transverse velocities of
photospheric tracers, by "local correlation tracking." Hα profiles
along 3 arch filaments are analysed in terms of "differential
cloud MODel", to discuss the Doppler velocities of chromospheric
material. Models of ascending arches with downflows in both legs
are investigated. The gas pressure is neglected, but the free-fall
equations are integrated versus time (non-stationary case). We first
assume circular lines of force with constant radius. We can account
for chromospheric velocities, but we cannot fit the slow motion of
footpoints derived from photospheric tracers. A better agreement is
obtained by assuming lines of force with fixed footpoints and variable
radius. Typical values are: half distance between footpoints of 10 to
15Mm, upward velocity at the top of loops of 4km/s after an integration
time of 800s. The obtained values are consistent with the sizes and
the lifetime of arch filaments.

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Title: Prominence Eruption and Coronal Mass Ejections: Diagnostics
and Future Observations from SOHO
Authors: Gontikakis, C.; Vial, J. -C.
1996ApL&C..34..119G Altcode:
No abstract at ADS

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Title: Dynamics of solar magnetic arches in photosphere and
chromosphere
Authors: Mein, P.; Mein, N.; Démoulin, P.; Gontikakis, C.; Engvold,
O.; Molowby, R.
1994smf..conf..366M Altcode:
No abstract at ADS