Author name code: gontikakis ADS astronomy entries on 2022-09-14 author:"Gontikakis, Costis" ------------------------------------------------------------------------ Title: Study of the transition region inside a coronal hole using IRIS and SDO observations Authors: Koletti, Myrto; Gontikakis, Costis; Tsinganos, Kanaris Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Effects of resonant scattering of the Si IV doublet near 140 nm in a solar active region Authors: Gontikakis, C.; Vial, J. -C. Bibcode: 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 https://www.aanda.org Title: Probing the Quiet Solar Atmosphere from the Photosphere to the Corona Authors: Kontogiannis, Ioannis; Gontikakis, Costis; Tsiropoula, Georgia; Tziotziou, Kostas Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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 (>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. Title: Emergence of non-twisted magnetic fields in the Sun: Jets and atmospheric response Authors: Syntelis, P.; Archontis, V.; Gontikakis, C.; Tsinganos, K. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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<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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2001IAUS..203..390G Altcode: In the present work EUV spectra of quiet Sun regions, observed with the Coronal Diagnostic Spectometer (CDS), are analysed in order to determine the Differential Emission Measure (DEM) of selected areas of the field of view. In particular, we study the differences between the DEM curves of the quiet Sun cell center areas, network areas and cell-network boundaries. The results are discussed in the light of theoretical models for the solar upper atmospheres. Title: Dynamics of the transition region Authors: Gontikakis, C.; Dara, H. C. Bibcode: 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. 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. Bibcode: 1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G No abstract at ADS Title: Emission of hydrogen lines by moving solar prominences. Authors: Gontikakis, C.; Vial, J. -C.; Gouttebroze, P. Bibcode: 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. Title: Spectral diagnostics for eruptive prominences Authors: Gontikakis, C.; Vial, J. -C.; Gouttebroze, P. Bibcode: 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. Title: Diagnostic and observations of quiescent and Authors: Vial, J. -C.; Gontikakis, C.; Bocchialini, K.; Gouttebroze, P. Bibcode: 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. 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. Bibcode: 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. Title: Prominence Eruption and Coronal Mass Ejections: Diagnostics and Future Observations from SOHO Authors: Gontikakis, C.; Vial, J. -C. Bibcode: 1996ApL&C..34..119G Altcode: No abstract at ADS Title: Dynamics of solar magnetic arches in photosphere and chromosphere Authors: Mein, P.; Mein, N.; Démoulin, P.; Gontikakis, C.; Engvold, O.; Molowby, R. Bibcode: 1994smf..conf..366M Altcode: No abstract at ADS