Author name code: dara ADS astronomy entries on 2022-09-14 author:"Dara, H.C." ------------------------------------------------------------------------ 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: 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: 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: 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: Preliminary results from the March 29, 2006 total eclipse observations in Egypt Authors: Koutchmy, S.; Daniel, J. -Y.; Mouette., J.; Vilinga, J.; Noëns, J. -C.; Damé, L.; Faurobert, M.; Dara, H.; Hady, A.; Semeida, M.; Sabry, M.; Domenech, A.; Munier, J. -M.; Jimenez, R.; Legault, Th.; Viladrich, Ch.; Kuzin, S.; Pertsov, A.; O. A. Team Bibcode: 2006sf2a.conf..547K Altcode: A coordinated effort has been carried in the framework of the French-Egyptian scientific cooperation to permit joined simultaneous eclipse observations of the solar corona during the total solar eclipse of March 29, 2006. Spaceborne EIT and Lasco (SoHO) observations were also planned at the same time and were successfully collected. Scientists from other countries collaborated on different experiments. The synthetic image showing the magnetic coronal structure of this quasi-minimum corona seen in W-L is given. Some preliminary results are presented; a White Light (W.L.) movie has been also taken during the totality. 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: 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: 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: 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: 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: 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: 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: JOSO national report 2000-2001 - Greece Authors: Dara, H. C. Bibcode: 2002joso.book...72D Altcode: No abstract at ADS 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: Brightness Variations of mg ib Bright Features Authors: Dara, H. C.; Koutchmy, S.; Zachariadis, Th. G.; Alissandrakis, C. E. Bibcode: 2001SoPh..198..313D Altcode: We study the temporal intensity variations of Mgib bright features and investigate the corresponding Hα velocity pattern. The network bright features are well visible in the continuum, in images averaged over the duration of the observations (130 min). We detected `flashing' bright features, which appear and disappear within two to five minutes, while the rest of the bright features undergo small variations of either their shape and/or their intensity. A power spectrum analysis reveals a 10-min oscillation for approximately half of the stable bright features. The 5-min oscillations are detected mainly at the network boundaries, where stable bright features are located, while 3-min oscillations coincide with few bright features, but are also quite intense inside the network cells. The majority of bright features are associated with Hα downflows. The downflow is very intense at the location of `flashing' bright features. Title: DEM Study of Selected Quiet Sun Regions Authors: Gontikakis, C.; Landi, E.; Dara, H. C.; Alissandrakis, C. E.; Vial, J. -C. 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: Erratum: "Fine structure of the solar chromosphere: arch-shaped mottles" [Sol. Phys., Vol. 184, No. 1, p. 77 - 86 (Jan 1999)]. Authors: Zachariadis, T. G.; Georgakilas, A. A.; Koutchmy, S.; Dara, H. C.; Alissandrakis, C. E. Bibcode: 1999SoPh..187..227Z Altcode: No abstract at ADS Title: Mg bright points and the corresponding velocity pattern. Authors: Dara, H. C.; Zachariadis, T.; Alissandrakis, C.; Koutchmy, S. Bibcode: 1999joso.proc..135D Altcode: The authors study the temporal intensity variations of the Mg b1 bright points, in an effort to detect oscillations and investigate the corresponding Hα velocity pattern. Apart from the "flashing bright points", bright points which appear and disappear within two to three minutes, the rest of the bright points undergo small intensity variations. Power spectrum analysis revealed a 10-min oscillation for half of the stable bright points. The 5-min oscillations are detected mainly at sites where there are no bright points, while 3-min oscillations coincide with some of the bright points. The majority of the bright points are associated with Hα downflows. The downflow is very intense at the "flashing bright points". Title: Fine structure of the Solar Chromosphere: Arch-Shaped Mottles Authors: Zachariadis, Th. G.; Georgakilas, A. A.; Koutchmy, S.; Alissandrakis, C. E.; Dara, H. C. Bibcode: 1999SoPh..184...77Z Altcode: We analyze a time series of high resolution observations near the solar limb, obtained in Hα and the Mg b1 line. We identified arch-shaped dark mottles, which are thin, faint Hα structures observable under very good seeing conditions, best seen in Hα +0.75 Å. Their mean length is about 15'', their mean height about 6'' and indicative lifetimes is of the order of 5 min. They show negative (away from the observer) line-of-sight velocities. A possible interpretation is that material flows from the apex towards the feet of the arches. Title: Properties of Hα spicules from disk and limb high-resolution observations Authors: Dara, H. C.; Koutchmy, S.; Suematsu, Y. Bibcode: 1998ESASP.421..255D Altcode: 1998sjcp.conf..255D No abstract at ADS Title: Spicules and Macrospicules: Simultaneous Hα and He II (304 Å) Observations Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis, C. E.; Koutchmy, S.; Delannée, C.; Delaboudinière, J. -P.; Hochedez, J. -F. Bibcode: 1998ASPC..155..376G Altcode: 1998sasp.conf..376G No abstract at ADS Title: Polar jets and plasmoids:Results from JOP 57 Authors: Delannee, C.; Koutchmy, S.; Delaboudiniere, J. -P.; Hochedez, J. -F.; Vial, J. -C.; Dara, H.; Georgakilas, A. Bibcode: 1998ESASP.421..129D Altcode: 1998sjcp.conf..129D No abstract at ADS Title: Kinematics of Hα spicules from near-limb observations. Authors: Zachariadis, T. G.; Georgakilas, A. A.; Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1998joso.proc..126Z Altcode: The authors have studied the behavior of dark mottles near the solar limb. After the corrections for flat field and limb darkening the authors can identify spicules crossing the solar limb. Most of the mottles initially show an ascending velocity which gradually turns to descending. Moreover, the authors observe a group of dark mottles which have the shape of arches. One of the footpoints of the arch is in a chromospheric network cell, while the other in a nearby rosette. Title: Flashing Mgb1 bright points. Authors: Dara, H. C.; Georgakilas, A.; Zachariadis, T.; Alissandrakis, C.; Koutchmy, S. Bibcode: 1998joso.proc..124D Altcode: High resolution observations, obtained at the Sacramento Peak Observatory near the solar limb, are analysed for the study of the evolution of Mgb1 (5183.22 Å) bright points, as well as their association to bright features in the continuum (5256.40 Å) and Hα wings (±0.75 Å). The authors have detected stable bright points from the beginning to the end of the time series which hardly move and whose shape undergoes small changes, bright points whose brightness increases and decreases periodically, and bright points which suddenly appear and disappear within one or two minutes. Title: Magnetic and velocity field in association with Ellerman bombs. Authors: Dara, H. C.; Alissandrakis, C. E.; Zachariadis, T. G.; Georgakilas, A. A. Bibcode: 1997A&A...322..653D Altcode: We present two dimensional maps of the longitudinal magnetic field, obtained in the CaI 6103Å line, as well as line of sight velocity maps in Hα for an active region. The observations were obtained with the Tower telescope of the Sacramento Peak Observatory and the Universal Birefrigent Filter, with circular polarization measurements. The comparison of the photographs in the wing of Hα, where the Ellerman bombs (EBs) are best observed, with the corresponding magnetic maps shows that very few EBs are associated with small scale magnetic elements, while the great majority, especially the brighter ones, appear at the boundaries of magnetic features. The comparison of EBs with the velocity maps in the wings of Hα shows that most of them coincide with blue wing intensity excess; near the brightest EBs ascending motions are observed. We also found a very good coincidence of EBs with photospheric facular granules. Title: Fine Structure of the solar atmosphere from near-limb observations in three wavelengths Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1997SoPh..172..133G Altcode: 1997ESPM....8..133G We analyze a time series of high-resolution observations near the limb, in the continuum, in the Mg b1 line (-0.4 Å off line center), and in the wings of Hα (+0.75 Å). The observations were obtained with a CCD camera at the Vacuum Tower telescope of the Sacramento Peak Observatory. We study the association of facular points, as they appear at different heights of the solar atmosphere (continuum, b1 - 0.4 Å, Hα + 0.75 Å) with the feet of fine dark mottles. Title: Polar Jets and Plasmoids: Preliminary Results from JOP 57 Authors: Delannée, C.; Koutchmy, S.; Delaboudiniè, J. -P.; Hochedez, J. -F.; Vial, J. -C.; Dara, H.; Georgakilas, A. Bibcode: 1997ESASP.404..327D Altcode: 1997cswn.conf..327D No abstract at ADS Title: The Magnetic Field of a Bipolar Sunspot Region in the Photosphere and the Chromosphere Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E. Bibcode: 1994emsp.conf...61D Altcode: No abstract at ADS Title: Photospheric and chromospheric magnetic field structure of a bipolar sunspot region Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E. Bibcode: 1993A&A...277..648D Altcode: We present high resolution magnetograms of a large split spot and an opposite polarity pore in the 6102.7 Å Ca I photospheric line and in Hα and we discuss their fine structure. We found important differences between the photospheric and chromospheric magnetic field. Although the large spot was apparently unipolar at the photospheric level, we detected an opposite polarity region at the chromospheric level. We also found that there was hardly any trace of the pore magnetic field in the Hα magnetogram. These results cannot be interpreted in terms of constant-α magnetic fields. Finally we confirm the occurrence of filamentary magnetic structures in Hα at the location of the superpenumbra of the large spot. Title: Study of small scale magnetic flux and the corresponding velocity pattern Authors: Alissandrakis, C. E.; Dara, H. C.; Koutchmy, S. Bibcode: 1991A&A...249..533A Altcode: Two-dimensional maps of the longitudinal magnetic field and the line of sight velocity of two small-scale magnetic regions in the quiet sun, using a one-hour time sequence of filtergrams in the magnetically sensitive lambda 6103 A CaI line (with circular polarization measurements) obtained with the Sacramento Peak Vacuum Tower Telescope and the universal filter, are calculated. After the elimination of the effect of the 5-minute photospheric oscillations, downflows smaller than 300 m/s are found in both magnetic regions. The magnetic field intensity and the velocity are weakly correlated: high values of the magnetic field tend to occur in regions of small negative velocities. The magnetic field intensity increased during the observations, while some magnetic peaks moved with respect to others. The difference of the magnetic field intensity measured in the blue and the red wing indicates a line asymmetry; this asymmetry was found to be independent of the line of sight velocity. Title: Small scale magnetic field mapping with high temporal resolution. Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1991sopo.work..257D Altcode: Two dimensional maps of the longitudinal magnetic field can be readily calculated for small-scale magnetic regions. Here the authors use an one-hour time sequence of high spatial resolution (0.75 arc sec) filtergrams, obtained at the Sacramento Peak Vacuum Tower Telescope with the universal filter (UBF) in the wings of the magnetically sensitive λ 6103 Ca I line. The time difference between two magnetic maps is 32 sec. The measurements of the magnetic field in the blue wing are about 40% higher than in the red wing. Title: Erratum: Velocity Pattern of Small Scale Magnetic Fields Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1990SoPh..128..431D Altcode: No abstract at ADS Title: Velocity pattern of small scale magnetic fields Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1990SoPh..126..403D Altcode: High resolution observations of horizontal proper motions, as well as vertical Doppler velocities measured over two selected regions of small scale magnetic elements show a coherent behaviour. In a region with two opposite polarities, approching with a velocity of 0.4 km s-1, the material in between moves downwards with a velocity of 0.10 to 0.45 km s-1; while in a region with two peaks of the same polarity, moving apart with a velocity of 0.3 km s-1, the material in between moves predominantly upwards, with a velocity of up to 0.3 km s-1. Title: Small Scale Motions Over Concentrated Magnetic Field Regions of the Quiet Sun Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1990IAUS..138..153D Altcode: No abstract at ADS Title: Small-scale motions over concentrated magnetic regions of the quiet Sun Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1987SoPh..109...19D Altcode: We have used a 5.5 min time-sequence of spectra in the Fe I lines λ5576 (magnetically insensitive), λ6301.5 and λ6302.5 (magnetically sensitive) to study the association of concentrated magnetic regions and velocity in the quiet Sun. After the elimination of photospheric oscillations we found downflows of 100-300 m s −1, displaced by about 2″ from the peaks of the magnetic field; this velocity is comparable to downflow velocity associated with the granulation and of the same order or smaller than the oscillation amplitude. Quasi-periodic time variations of the vertical component of the magnetic field up to ± 40% were also found with a period near 250 s, close to the values found for the velocity field. Finally we report a possible association of intensity maxima at the line center with peaks of the oscillation amplitude. Title: Ηλιακές χρωμοσφαιρικές αναρρίψεις Title: Ηλιακές χρωμοσφαιρικές αναρρίψεις Title: Solar chromospheric surges; Authors: Dara, Helen Bibcode: 1982PhDT........75D Altcode: No abstract at ADS Title: On Time Variations of the Chromospheric Network Authors: Dara, H. C.; Macris, C. J. Bibcode: 1980BAICz..31..364D Altcode: A study of the variation of the chromospheric network during the 19th solar cycle is presented. A change is found of the relative density of the small emission centres of the K(232) solar chromosphere (flocculi) with a maximum near the time of the solar activity maximum and a minimum about one year after the solar activity minimum. Title: Study of the motion of three solar prominences. Authors: Dara, H. C.; Macris, C. J. Bibcode: 1978AkAtP..52..414D Altcode: The motion of three characteristic solar prominences observed on April 14, 1967, August 19, 1967, and November 24, 1967 was studied by measuring their heights and their velocity changes. The maximum ascending velocities of 150 km/sec, 30 km/sec, and 170 km/sec are in agreement with measurements made by other observers. None of the prominences reached an escape velocity. The plots of velocity versus time for all points of the prominences are smooth, and no abrupt accelerations were noted. This is in agreement with Pan Puh's (1939) conjecture that there are no abrupt changes of velocity in prominence plasma, which shows that Pettit's laws for eruptive prominences are not valid. Title: Comparison of the dimensions of the flocculi at periods of maximum and minimum solar activity. Authors: Dara, H. C.; Macris, C. J.; Zachariadis, T. G. Bibcode: 1975AkAtP..50..391D Altcode: Measurements of the dimensions of solar flocculi were taken and compared for solar activity maxima and minima, and the relative frequency distributions are cited. The measurements show a 7% increase in floccular dimensions from solar activity minimum to solar activity maximum, with large flocculi predominant during maxima and smaller flocculi more numerous during minima. Statistical tests show the 7% increase in floccular size to be significant. A few short of 600 flocculi were studied for the maxima and for the minima, with measurements taken in the W-E direction at 55 deg heliographic latitude.