Author name code: alissandrakis ADS astronomy entries on 2022-09-14 author:"Alissandrakis, Costas E." ------------------------------------------------------------------------ Title: The quiet Sun at mm Wavelengths as Seen by ALMA Authors: Alissandrakis, Costas; Bastian, Timothy; Brajša, Roman Bibcode: 2022arXiv220902569A Altcode: Solar observations at sub-mm, mm and cm wavelengths offer a straightforward diagnostic of physical conditions in the solar atmosphere because they yield measurement of brightness temperature which, for optically thick features, equals intrinsic temperature - much unlike solar diagnostics in other spectral ranges. The Atacama Large Millimeter and sub-millimeter Array (ALMA) has therefore opened a new, hitherto underexplored, spectral window for studying the enigmatic solar chromosphere. In this review we discuss initial ALMA studies of the quiet chromosphere that used both single-dish and compact-array interferometric observing modes. We present results on the temperature structure of the chromosphere, comparison with classic empirical models of the chromosphere, and observations of the chromospheric network and spicules. Furthermore, we discuss what may be expected in the future, since the ALMA capabilities continuously expand and improve towards higher angular resolution, wavelength coverage, and polarization measurement for magnetometry. Title: The solar atmosphere and kappa distributions Authors: Alissandrakis, Costas Bibcode: 2022cosp...44.1220A Altcode: The sun and its environment is an immense plasma physics laboratory, where the magnetic field plays a crucial role. The magnetic field is responsible for a large variety of energy release phenomena, in all observable temporal an spatial scales, from tiny bright points to huge flares and coronal mass ejections. In the most energetic phenomena, electrons are accelerated to energies of hundreds of keV, giving observational signatures in hard X-rays and in the microwave range, whereas electron beams traveling at 1/3 of the speed of light in the corona and the solar wind produce plasma emission in metric and longer wavelengths. In latter case a bump-in-tail distribution is invoked, while in the first case a power-law distribution on top of a Maxwellian is usually employed. However, there have been efforts to describe suprathermal electrons in terms of kappa-distributions, both in energetic phenomena an in the quiet Sun. In this review I will give an overview of the solar atmosphere and of phenomena where kappa-distributions may play a role. Title: Where is the base of the Transition Region? Evidence from TRACE, SDO, IRIS and ALMA observations Authors: Alissandrakis, C. E. Bibcode: 2022arXiv220703159A Altcode: Classic solar models put the Chromosphere-Corona Transition Region (CCTR} at $\sim2$Mm above the $\tau_{5000} = 1$ level, whereas rMHD models place it in a wider range of heights. Observational verification is scarce. We review and discuss recent results from various instruments and spectral domains. In SDO and TRACE images spicules appear in emission in the 1600, 1700 and 304 A bands and in absorption in the EUV bands; the latter is due to photo-ionization of H and He I. At the shortest available AIA wavelength and taking into account that the photospheric limb is $\sim0.34$Mm above the $\tau_{5000}=1$ level, we found that CCTR emission starts at $\sim3.7$Mm; extrapolating to $\lambda=0$, where there is no chromospheric absorption, we deduced a height of $3.0\pm 0.5$Mm, above the value of 2.14Mm of the Avrett & Loeser model. Another indicator of the extent of the chromosphere is the height of the network structures. Height differences produce a limbward shift of features with respect to their counterparts in magnetograms. Using this approach, we measured heights of $0.14\pm0.04$Mm (at 1700 A), $0.31\pm0.09$Mm (at 1600 A) and $3.31\pm 0.18$Mm (at 304 A) for the center of the solar disk. A previously reported possible solar cycle variation is not confirmed. A third indicator is the position of the limb in the UV, where IRIS observations of the Mg II triplet lines show that they extend up to $\sim2.1$Mm above the 2832 A limb, while AIA/SDO images give a limb height of $1.4 \pm 0.2$Mm (1600 A) and $5.7\pm 0.2$Mm (304 A). Finally, ALMA mm-$\lambda$ full-disk images provide useful diagnostics, though not very accurate; values of $2.4\pm0.7$Mm at 1.26mm and $4.2\pm2.5$Mm at 3mm were obtained. Putting everything together, we conclude that the average chromosphere extends higher than homogeneous models predict, but within the range of rMHD models. Title: First detection of metric emission from a solar surge Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bouratzis, C.; Hillaris, A. Bibcode: 2022A&A...662A..14A Altcode: 2022arXiv220301043A We report the first detection of metric radio emission from a surge, observed with the Nançay Radioheliograph (NRH), STEREO, and other instruments. The emission was observed during the late phase of the M9 complex event SOL2010-02-012T11:25:00, described in a previous publication. It was associated with a secondary energy release, also observed in STEREO 304 Å images, and there was no detectable soft X-ray emission. The triangulation of the STEREO images allowed for the identification of the surge with NRH sources near the central meridian. The radio emission of the surge occurred in two phases and consisted of two sources, one located near the base of the surge, apparently at or near the site of energy release, and another in the upper part of the surge; these were best visible in the frequency range of 445.0 to about 300 MHz, whereas a spectral component of a different nature was observed at lower frequencies. Sub-second time variations were detected in both sources during both phases, with a 0.2-0.3 s delay of the upper source with respect to the lower, suggesting superluminal velocities. This effect can be explained if the emission of the upper source was due to scattering of radiation from the source at the base of the surge. In addition, the radio emission showed signs of pulsations and spikes. We discuss possible emission mechanisms for the slow time variability component of the lower radio source. Gyrosynchrotron emission reproduced the characteristics of the observed total intensity spectrum at the start of the second phase of the event fairly well, but failed to reproduce the high degree of the observed circular polarization or the spectra at other instances. On the other hand, type IV-like plasma emission from the fundamental could explain the high polarization and the fine structure in the dynamic spectrum; moreover, it gives projected radio source positions on the plane of the sky, as seen from STEREO-A, near the base of the surge. Taking all the properties into consideration, we suggest that type IV-like plasma emission with a low-intensity gyrosynchrotron component is the most plausible mechanism.

Movie associated to Fig. A.2 is available at https://www.aanda.org Title: A first look at the submillimeter Sun with ALMA Authors: Alissandrakis, C. E.; Bastian, T. S.; Nindos, A. Bibcode: 2022A&A...661L...4A Altcode: 2022arXiv220501008A We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21″), several interesting results were obtained. During our observation, the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly images at 1600 Å and 304 Å, after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative Hα images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 Å band and small Hα filaments, are not detectable at 0.86 mm. We computed the center-to-limb variation of the brightness temperature, Tb, in Band 7, as well as in Bands 6 and 3, which were obtained during the same campaign, and we combined them to a unique curve of Tb(log μ100), where μ100 is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3 commissioning observations is accurate, we deduced a brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for Bands 3 and 6. More importantly, the Tb(log μ100) curve flattens at large values of μ100, and so does the corresponding Te(log τ100) at large τ100. This is probably an indication that we are approaching the temperature minimum. Title: High-resolution observations with ARTEMIS/JLS and the NRH. IV. Imaging spectroscopy of spike-like structures near the front of type-II bursts Authors: Armatas, S.; Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.; Preka-Papadema, P.; Kontogeorgos, A.; Tsitsipis, P.; Moussas, X. Bibcode: 2022A&A...659A.198A Altcode: 2022arXiv220107832A Context. Narrowband bursts (spikes) are very small duration and bandwidth bursts which appear on dynamic spectra from microwave to decametric frequencies. They are believed to be manifestations of small-scale energy release through magnetic reconnection.
Aims: We study the position of the spike-like structures relative to the front of type-II bursts and their role in the burst emission.
Methods: We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high-time-resolution images from the Nançay Radioheliograph (NRH) in order to study spike-like bursts near the front of a type-II radio burst recorded at the west limb during the November 3, 2003 extreme solar event. The spike-like emission in the dynamic spectrum was enhanced by means of high-pass-time filtering.
Results: We identified a number of spikes in the NRH images. Due to the lower temporal resolution of the NRH, multiple spikes detected in the dynamic spectrum appeared as single structures in the images. These spikes had an average size of ≈200″ and their observed brightness temperature was 1.4 to 5.6 × 109 K, providing a significant contribution to the emission of the type-II burst front. At variance with a previous study on the type-IV associated spikes, we found no systematic displacement between the spike emission and the emission between spikes. At 327.0 MHz, the type II emission was located about 0.3 R above the pre-existing continuum emission, which, in turn, was located 0.1 R above the western limb.
Conclusions: This study, combined with our previous results, indicates that the spike-like chains aligned along the type II burst MHD shock front are not a perturbation of the type II emission, as in the case of type IV spikes, but a manifestation of the type II emission itself. The preponderance of these chains, together with the lack of isolated structures or irregular clusters, points towards some form of small-scale magnetic reconnection, organized along the type-II propagating front.

This work is dedicated to the memory of Costas Caroubalos (1928-2021), founder of the ARTEMIS radiospectrograh. Title: Multiwavelength observations of a metric type-II event Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Hillaris, A. Bibcode: 2021A&A...654A.112A Altcode: 2021arXiv210802855A We have studied a complex metric radio event that originated in a compact flare, observed with the ARTEMIS-JLS radiospectrograph on February 12, 2010. The event was associated with a surge observed at 195 and 304 Å and with a coronal mass ejection observed by instruments on board STEREO A and B near the eastern and western limbs respectively. On the disk the event was observed at ten frequencies by the Nançay Radioheliograph (NRH), in Hα by the Catania observatory, in soft X-rays by GOES SXI and Hinode XRT, and in hard X-rays by RHESSI. We combined these data, together with MDI longitudinal magnetograms, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occurred near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed an impressive fundamental-harmonic structure, while the emission of the second consisted of three well-separated bands with superposed pulsations. Using positional information for the type-IIs from the NRH and triangulation from STEREO A and B, we found that the type-IIs were associated neither with the surge nor with the disruption of a nearby streamer, but rather with an extreme ultraviolet (EUV) wave probably initiated by the surge. The fundamental-harmonic structure of the first type-II showed a band split corresponding to a magnetic field strength of 18 G, a frequency ratio of 1.95 and a delay of 0.23−0.65 s of the fundamental with respect to the harmonic; moreover it became stationary shortly after its start and then drifted again. The pulsations superposed on the second type-II were broadband and had started before the burst. In addition, we detected another pulsating source, also before the second type-II, polarized in the opposite sense; the pulsations in the two sources were out of phase and hence hardly detectable in the dynamic spectrum. The pulsations had a measurable reverse frequency drift of about 2 s−1. Title: ALMA observations of the variability of the quiet Sun at millimeter wavelengths Authors: Nindos, A.; Patsourakos, S.; Alissandrakis, C. E.; Bastian, T. S. Bibcode: 2021A&A...652A..92N Altcode: 2021arXiv210604220N
Aims: We address the variability of the quiet solar chromosphere at 1.26 mm and 3 mm with a focus on the study of spatially resolved oscillations and transient brightenings, which are small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere.
Methods: We used Atacama Large Millimeter/submillimeter Array (ALMA) observations of the quiet Sun at 1.26 mm and 3 mm. The spatial and temporal resolution of the data were 1 − 2″ and 1 s, respectively. The concatenation of light curves from different scans yielded a frequency resolution in spectral power of 0.5−0.6 mHz. At 1.26 mm, in addition to power spectra of the original data, we degraded the images to the spatial resolution of the 3 mm images and used fields of view that were equal in area for both data sets. The detection of transient brightenings was made after the effect of oscillations was removed.
Results: At both frequencies, we detected p-mode oscillations in the range 3.6−4.4 mHz. The corrections for spatial resolution and field of view at 1.26 mm decreased the root mean square (rms) of the oscillations by a factor of 1.6 and 1.1, respectively. In the corrected data sets, the oscillations at 1.26 mm and 3 mm showed brightness temperature fluctuations of ∼1.7 − 1.8% with respect to the average quiet Sun, corresponding to 137 and 107 K, respectively. We detected 77 transient brightenings at 1.26 mm and 115 at 3 mm. Although their majority occurred in the cell interior, the occurrence rate per unit area of the 1.26 mm events was higher than that of the 3 mm events; this conclusion does not change if we take into account differences in spatial resolution and noise levels. The energy associated with the transient brightenings ranged from 1.8 × 1023 to 1.1 × 1026 erg and from 7.2 × 1023 to 1.7 × 1026 erg for the 1.26 mm and 3 mm events, respectively. The corresponding power-law indices of the energy distribution were 1.64 and 1.73. We also found that ALMA bright network structures corresponded to dark mottles or spicules that can be seen in broadband Hα images from the GONG network.
Conclusions: The fluctuations associated with the p-mode oscillations represent a fraction of 0.55−0.68 of the full power spectrum. Their energy density at 1.26 mm is 3 × 10−2 erg cm−3. The computed low-end energy of the 1.26 mm transient brightenings is among the smallest ever reported, irrespective of the wavelength of the observation. Although the occurrence rate per unit area of the 1.26 mm transient brightenings was higher than that of the 3 mm events, their power per unit area is smaller likely due to the detection of many weak 1.26 mm events. Title: Radio Measurements of the Magnetic field in the Solar Chromosphere and the Corona Authors: Alissandrakis, Costas E.; Gary, Dale E. Bibcode: 2021FrASS...7...77A Altcode: The structure of the upper solar atmosphere, on all observable scales, is intimately governed by the magnetic field. The same holds for a variety of solar phenomena that constitute solar activity, from tiny transient brightening to huge Coronal Mass Ejections. Due to inherent difficulties in measuring magnetic field effects on atoms (Zeeman and Hanle effects) in the corona, radio methods sensitive to electrons are of primary importance in obtaining quantitative information about its magnetic field. In this review we explore these methods and point out their advantages and limitations. After a brief presentation of the magneto-ionic theory of wave propagation in cold, collisionless plasmas, we discuss how the magnetic field affects the radio emission produced by incoherent emission mechanisms (free-free, gyroresonance and gyrosynchrotron processes) and give examples of measurements of magnetic filed parameters in the quiet sun, active regions and radio CMEs. We proceed by discussing how the inversion of the sense of circular polarization can be used to measure the field above active regions. Subsequently we pass to coherent emission mechanisms and present results of measurements from fiber bursts, zebra patterns and type II burst emission. We close this review with a discussion of the variation of the magnetic field, deduced by radio measurement, from the low corona up to 10 solar radii and with some thoughts about future work. Title: Observations of Solar Spicules at Millimeter and Ultraviolet Wavelengths Authors: Bastian, T.; De Pontieu, B.; Shimojo, M.; Iwai, K.; Alissandrakis, C.; Nindos, A.; Vial, J. C.; White, S. M. Bibcode: 2020AGUFMSH004..08B Altcode: Solar spicules are a ubiquitous chromospheric phenomenon in which multitudes of dynamic jets with temperatures of order 104 K extend thousands of kilometers into the solar atmosphere. Recent progress has been made refining the observational characteristics of spicules using the Hinode Solar Optical Telescope (SOT) and the Interface Region Imaging Spectrograph (IRIS) observations at optical and ultraviolet wavelengths, respectively. Two types of spicule have been identified. Type I spicules, prevalent in solar active regions, have upward speeds of order 25 km/s and lifetimes of 3-7 min. They may be the limb counterpart to shock-wave-driven fibrils commonly seen against the solar disk in active regions. In contrast, type II spicules, more common in quiet regions and coronal holes, display upward speeds of 50-150 km/s, lifetimes of 30-110 s, and appear to be partially heated to temperatures of 105 K and higher. These observations have provoked intense interest in spicules and have led to proposals that type II spicules play a central role as a source of hot plasma in the corona. Nevertheless, their role in mass and energy transport between the lower and upper layers of the solar atmosphere remains an outstanding problem.

Here, we report imaging observations of solar spicules at millimeter wavelengths using the Atacama Large Millimeter-submillimeter Array (ALMA) with arcsecond angular resolution. Continuum millimeter wavelength radiation forms under conditions of local thermodynamic equilibrium, thereby providing a complementary tool to UV lines, which form under non-LTE conditions. The observations were made on 2018 December 24-25 at λ=1.25 mm and λ=3 mm. The ALMA observations pose special challenges, particularly at 1.25 mm, where the limited field of view of the instrument motivated us to use a novel mosaic imaging technique: multiple pointings were assembled to form a single map with an angular resolution of 1" x 0.7" on a cadence of roughly 2 min. In contrast, we were able to image at 3 mm continuously, with a map cadence of 2 s and an angular resolution of 2.3" x 1.3".

We compare and contrast the morphology and dynamics of mm-λ observations of spicules with those obtained by IRIS at UV wavelengths and place constraints on spicule temperatures and masses using the joint millimeter-wavelength observations. Title: Structure of the solar atmosphere: a radio perspective Authors: Alissandrakis, Costas E. Bibcode: 2020FrASS...7...74A Altcode: Solar radio emission has been providing information about the Sun for over half a century. In order to fully exploit this information, one needs to have a broader view of the solar atmosphere, which cannot be provided by radio observations alone. The purpose of this review is to present this background information, which is necessary to understand the physical processes that determine the solar radio emission and to link the radio domain with the rest of the electromagnetic spectrum.

Both classic and modern results are presented in a concise manner. After a brief discussion of the solar interior, the basic physics of the solar atmosphere and some elements of radiative transfer are presented. Subsequently the atmospheric structure as a function of height is examined and one -dimensional models of the photosphere, the chromosphere, the transition region and the corona are presented and discussed. An introduction to basic magnetohydrodynamics precedes the discussion of the rich fine structure of the solar atmosphere as a 3D object. Active regions are briefly discussed in a separate section, and this is followed by a section on the problem of heating of the chromosphere and the corona. I finish with some thoughts on what to expect from the new instruments currently under development. Title: Modeling the quiet Sun cell and network emission with ALMA Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T. S.; Patsourakos, S. Bibcode: 2020A&A...640A..57A Altcode: 2020arXiv200609886A Observations of the Sun at millimeter wavelengths with the Atacama Large Millimeter/submillimeter Array (ALMA) offer a unique opportunity to investigate the temperature structure of the solar chromosphere. In this article we expand our previous work on modeling the chromospheric temperature of the quiet Sun, by including measurements of the brightness temperature in the network and cell interiors, from high-resolution ALMA images at 3 mm (Band 3) and 1.26 mm (Band 6). We also examine the absolute calibration of ALMA full-disk images. We suggest that the brightness temperature at the center of the solar disk in Band 6 is ∼440 K above the value recommended by White et al. (2017, Sol. Phys., 292, 88). In addition, we give improved results for the electron temperature variation of the average quiet Sun with optical depth and the derived spectrum at the center of the disk. We found that the electron temperature in the network is considerably lower than predicted by model F of Fontenla et al. (1993, ApJ, 406, 319) and that of the cell interior considerably higher than predicted by model A. Depending on the network/cell segregation scheme, the electron temperature difference between network and cell at τ = 1 (100 GHz) ranges from ∼660 K to ∼1550 K, compared to ∼3280 K predicted by the models; similarly, the electron temperature, Te ratio ranges from ∼1.10 to 1.24, compared to ∼1.55 of the model prediction. We also found that the network/cell Te(τ) curves diverge as τ decreases, indicating an increase of contrast with height and possibly a steeper temperature rise in the network than in the cell interior. Title: Transient brightenings in the quiet Sun detected by ALMA at 3 mm Authors: Nindos, A.; Alissandrakis, C. E.; Patsourakos, S.; Bastian, T. S. Bibcode: 2020A&A...638A..62N Altcode: 2020arXiv200407591N
Aims: We investigate transient brightenings, that is, weak, small-scale episodes of energy release, in the quiet solar chromosphere; these episodes can provide insights into the heating mechanism of the outer layers of the solar atmosphere.
Methods: Using Atacama Large Millimeter/submillimeter Array (ALMA) observations, we performed the first systematic survey for quiet Sun transient brightenings at 3 mm. Our dataset included images of six 87″ × 87″ fields of view of the quiet Sun obtained with angular resolution of a few arcsec at a cadence of 2 s. The transient brightenings were detected as weak enhancements above the average intensity after we removed the effect of the p-mode oscillations. A similar analysis, over the same fields of view, was performed for simultaneous 304 and 1600 Å data obtained with the Atmospheric Imaging Assembly.
Results: We detected 184 3 mm transient brightening events with brightness temperatures from 70 K to more than 500 K above backgrounds of ∼7200 - 7450 K. All events showed light curves with a gradual rise and fall, strongly suggesting a thermal origin. Their mean duration and maximum area were 51.1 s and 12.3 Mm2, respectively, with a weak preference of appearing at network boundaries rather than in cell interiors. Both parameters exhibited power-law behavior with indices of 2.35 and 2.71, respectively. Only a small fraction of ALMA events had either 304 or 1600 Å counterparts but the properties of these events were not significantly different from those of the general population except that they lacked their low-end energy values. The total thermal energies of the ALMA transient brightenings were between 1.5 × 1024 and 9.9 × 1025 erg and their frequency distribution versus energy was a power law with an index of 1.67 ± 0.05. We found that the power per unit area provided by the ALMA events could account for only 1% of the chromospheric radiative losses (10% of the coronal ones).
Conclusions: We were able to detect, for the first time, a significant number of weak 3 mm quiet Sun transient brightenings. However, their energy budget falls short of meeting the requirements for the heating of the upper layers of the solar atmosphere and this conclusion does not change even if we use the least restrictive criteria possible for the detection of transient brightenings. Title: Modeling of the Brightness of the Chromospheric Network Based on ALMA High Resolution Observations of the Quiet Sun Authors: Alissandrakis, C. E.; Nindos, A.; Bastian, T.; Patsourakos, S. Bibcode: 2020AAS...23610607A Altcode: ALMA observations of the Sun at mm-λ offer a unique opportunity to investigate the temperature/density structure of the solar chromosphere. In a previous work (Alissandrakis et al 2017, A&A 605, A78) we measured the center-to-limb variation of the brightness temperature, Tb, using low resolution ALMA full-disk observations in Band 3 (3mm) and Band 6 (1.26 mm), together with data at 0.85 mm from Bastian et al. 1993 (ApJ, 415, 364). Combining all data and inverting the solution of the transfer equation we found that the electron temperature, Te, in the range of 0.34 < τ100 < 12, where τ100 is the optical depth at 100 GHz, was ~5% (~300 K) below the one predicted by model C (average quiet sun) of FAL93 (Fontenla, Avrett, & Loeser, 1993, ApJ, 406, 319). Here we expand that work by including measurements of the brightness temperature in the network and cell interiors, from high resolution ALMA images in Bands 3 and 6. We found that the observed Tb in the network is considerably lower than predicted by the FAL93 model F and that of the cell interior considerably higher than predicted by the FAL93 model A. The observed network/cell difference of brightness temperature at the center of the disk, at 100 GHz is about 920 K, compared to ~3250 K predicted by the FAL93 models; similarly, the Tb, ratio is ~1.14, against ~1.51 of the model prediction. After inversion of the observed data, the electron temperature of cell interior at τ100=1 is ~390 K below the average (~600 K above model A) and of the network ~400 K above the average (~1800 K below model A). The implications of these results will be discussed. We will also discuss the question of the normalization of brightness temperature observed by ALMA. Title: Observations of solar chromospheric oscillations at 3 mm with ALMA Authors: Patsourakos, S.; Alissandrakis, C. E.; Nindos, A.; Bastian, T. S. Bibcode: 2020A&A...634A..86P Altcode: 2019arXiv191203480P
Aims: We aim to study spatially resolved chromospheric oscillations of the quiet Sun (QS) in the mm-domain at a resolution of a few arcsec, typically 2.4″ × 4.5″.
Methods: We used Atacama Large millimeter and submillimeter Array (ALMA) time series of interferometric observations of the QS obtained at 3 mm with a 2-s cadence and a spatial resolution of a few arcsec. The observations were performed on March 16, 2017 and seven 80″ × 80″ fields of view (FoV) going from disk center to limb were covered, each one observed for 10 min, therefore limiting the frequency resolution of the power spectra to 1.7 mHz. For each FoV, masks for cell and network were derived, and the averaged power spectral densities (PSDs) for the entire FoV, cell, and network were computed. The resulting power spectra were fit with an analytical function in order to derive the frequency and the root-mean-square (rms) power associated with the peaks. The same analysis, over the same FoVs and for the same intervals, was performed for simultaneous Atmospheric Imaging Assembly (AIA) image sequences in 1600 Å.
Results: Spatially resolved chromospheric oscillations at 3 mm, with frequencies of 4.2 ± 1.7 mHz are observed in the QS, in both cell and network. The coherence length-scale of the oscillations is commensurate with the spatial resolution of our ALMA observations. Brightness-temperature fluctuations in individual pixels could reach up to a few hundred K, while the spatially averaged PSDs yield rms in the range ≈55-75 K, i.e., up to ≈1% of the averaged brightness temperatures and exhibit a moderate increase towards the limb. For AIA 1600 Å, the oscillation frequency is 3.7 ± 1.7 mHz. The relative rms is up to 6% of the background intensity, with a weak increase towards the disk center (cell, average). ALMA 3 mm time-series lag AIA 1600 Å by ≈100 s, which corresponds to a formation-height difference of ≈1200 km, representing a novel determination of this important parameter.
Conclusions: The ALMA oscillations that we detected exhibit higher amplitudes than those derived from previous lower (≈10″) resolution observations at 3.5 mm by the Berkeley-Illinois-Maryland Array. Chromospheric oscillations are, therefore, not fully resolved at the length-scale of the chromospheric network, and possibly not even at the spatial resolution of our ALMA observations. Any study of transient brightenings in the mm-domain should take into account the oscillations. Title: Measurement of the Height of the Chromospheric Network Emission from Solar Dynamics Observatory Images Authors: Alissandrakis, C. E. Bibcode: 2019SoPh..294..161A Altcode: 2019arXiv191100758A We measured the height of the chromospheric network in the 1700, 1600, and 304 Å wavelength bands of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) from the shift of features on the disk with respect to corresponding features in SDO/Helioseismic and Magnetic Imager (HMI) images of the absolute value of the longitudinal magnetic field. We found that near the limb the 304 Å network emission forms 3.60 ±0.24 Mm above the 1600 Å emission, which, in turn, forms 0.48 ±0.10 Mm above the HMI (6173 Å) level. At the center of the disk the corresponding height differences are 2.90 ±0.02 Mm and 0.39 ±0.06 Mm respectively. We also found that the 1600 Å network emission forms 0.25 ±0.02 Mm above the 1700 Å emission near the limb and 0.20 ±0.02 Mm at the disk center. Finally, we examined possible variations with the solar cycle. Our results can help to check and refine atmospheric models. Title: Erratum: Correction to: Structure of the Transition Region and the Low Corona from TRACE and SDO Observations Near the Limb Authors: Alissandrakis, C. E.; Valentino, A. Bibcode: 2019SoPh..294..146A Altcode: In computing the height with respect to the tau = 1 level, the height difference between the white light limb and that level was subtracted rather than added to the values in Table 4. This affects Sections 3.3 and 3.4, Figures 14 and 15, as well as Table 5. The correct versions are given below. Title: Structure of the Transition Region and the Low Corona from TRACE and SDO Observations Near the Limb Authors: Alissandrakis, C. E.; Valentino, A. Bibcode: 2019SoPh..294...96A Altcode: 2019arXiv190609497A We examined the structure near the solar limb in TRACE images of the continuum and in the 1600 and 171 Å bands as well as in SDO images in the continuum (from HMI) and all AIA bands. The images in different wavelength bands were carefully coaligned by using the position of Mercury for TRACE and Venus for SDO during their transit in front of the solar disk in 1999 and 2012, respectively. Chromospheric absorbing structures in the TRACE 171-Å band are best visible 7'' above the white-light limb, very close to the inner limb, defined as the inflection point of the rising part of the center-to-limb intensity variation. They are correlated with, but are not identical to, spicules in emission, seen in the 1600-Å band. Similar results were obtained from AIA and SOT images. Tall spicules in 304 Å are not associated with any absorption in the higher temperature bands. Performing azimuthal averaging of the intensity over 15 sectors near the N, S, E and W limbs, we measured the height of the limb and of the peak intensity in all AIA bands. We found that the inner limb height in the transition region AIA bands increases with wavelength, consistent with a bound-free origin of the absorption from neutral H and He. From that we computed the column density and the density of neutral hydrogen as a function of height. We estimated a height of (2300 ±500 )km for the base of the transition region. Finally, we measured the scale height of the AIA emission of the corona and associated it with the temperature; we deduced a value of (1.24 ±0.25 )×106K for the polar corona Title: High-resolution observations with ARTEMIS-JLS and the NRH. III. Spectroscopy and imaging of fiber bursts Authors: Alissandrakis, C. E.; Bouratzis, C.; Hillaris, A. Bibcode: 2019A&A...627A.133A Altcode: 2019arXiv190603434A
Aims: We study the characteristics of intermediate drift bursts (fibers) embedded in a large type-IV event.
Methods: We used high-sensitivity, low-noise dynamic spectra obtained with the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high time-resolution images from the Nançay radioheliograph (NRH) and extreme ultraviolet (EUV) images from the Transition Region and Coronal Explorer (TRACE) to study fiber bursts during the large solar event of July 14, 2000. We computed both 2D and 1D images and applied high pass time filtering to the images and the dynamic spectrum in order to enhance the fiber-associated emission. For the study of the background continuum emission we used images averaged over several seconds.
Results: Practically all fibers visible in the SAO dynamic spectra are identifiable in the NRH images. Fibers were first detected after the primary energy release in a moving type-IV event, probably associated with the rapid eastward expansion of the flare and the post-flare loop arcade. We found that fibers appeared as a modulation of the continuum intensity with a root mean square value of the order of 10%. Both the fibers and the continuum were strongly circularly polarized in the ordinary mode sense, indicating plasma emission at the fundamental. We detected a number of discrete fiber emission sources along two parallel stripes of ∼300 Mm in length, apparently segments of large-scale loops encompassing both the EUV loops and the CME-associated flux rope. We found cases of multiple fiber emissions appearing at slightly different positions and times; their consecutive appearance can give the impression of apparent motion with supra-luminal velocities. Images of individual fibers were very similar at 432.0 and 327.0 MHz. From the position shift of the sources and the time delays at low and high frequencies, we estimated the exciter speed and the frequency scale length along the loops for a well-observed group of fibers; we obtained consistent values from imaging and spectral data, supporting the whistler origin of the fiber emission. Finally we found that fibers in emission and fibers in absorption are very similar, confirming that they are manifestations of the same wave train.

Movies associated to Figs. 2, 7, and 10 are available at https://www.aanda.org Title: High resolution observations with Artemis-JLS. II. Type IV associated intermediate drift bursts Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.; Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2019A&A...625A..58B Altcode: 2019arXiv190402262B
Aims: We examined the characteristics of isolated intermediate drift bursts and their morphologies on dynamic spectra, in particular the positioning of emission and absorption ridges. Furthermore we studied the repetition rate of fiber groups. These were compared with a model in order to determine the conditions under which the intermediate drift bursts appear and exhibit the above characteristics.
Methods: We analyzed sixteen metric type IV events with embedded intermediate drift bursts, observed with the Artemis-JLS radio spectrograph from July 1999 to July 2005 plus an event on 1st August 2010. The events were recorded with the SAO high resolution (10 ms cadence) receiver in the 270-450 MHz range with a frequency resolution of 1.4 MHz. We developed cross- and autocorrelation techniques to measure the duration, spectral width, and frequency drift of fiber bursts in 47 intermediate drift bursts (IMD) groups embedded within the continuum of the sixteen events mentioned above. We also developed a semi-automatic algorithm to track fibers on dynamic spectra.
Results: The mean duration of individual fiber bursts at fixed frequency was δt ≈ 300 ms, while the instantaneous relative bandwidth was fw/f ≈ 0.90% and the total frequency extent was Δftot ≈ 35 MHz. The recorded intermediate drift bursts had frequency drift, positive or negative, with average values of df/fdt equal to -0.027 and 0.024 s-1 respectively. Quite often the fibers appeared in groups; the burst repetition rate within groups was, on average, ∼0.98 s. We distinguish six morphological groups of fibers, based on the relative position of the emission and absorption ridges. These included fibers with emission or absorption ridges only, fibers with the absorption ridge at lower or higher frequency than the emission, or with two absorption ridges above and below the emission. There were also some fibers for which two emission ridges were separated by an absorption ridge. Some additional complex groups within our data set were not easy to classify. A number of borderline cases of fibers with very high drift rate (∼0.30 s-1) or very narrow total bandwidth (∼8 MHz) were recorded; among them there was a group of rope-like fibers characterized by fast repetition rate and relatively narrow total frequency extent. We found that the whistler hypothesis leads to reasonable magnetic field values (∼4.6 G), while the Alfven-wave hypothesis requires much higher field. From the variation of the drift rate with time we estimated the ratio of the whistler to the cyclotron frequency, x, to be in the range of 0.3-0.6, varying by ∼0.05-0.1 in individual fibers; the same analysis gives an average value of the frequency scale along the loop of ∼220 Mm. Finally, we present empirical relations between fiber burst parameters and discuss their possible origin. Title: Detection of spike-like structures near the front of type-II bursts Authors: Armatas, S.; Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.; Preka-Papadema, P.; Moussas, X.; Mitsakou, E.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2019A&A...624A..76A Altcode: 2019arXiv190210617A
Aims: We examine high time resolution dynamic spectra for fine structures in type II solar radio bursts
Methods: We used data obtained with the acousto-optic spectrograph receiver of the Artemis-JLS (ARTEMIS-IV) solar radio spectrograph in the 450-270 MHz range at 10 ms cadence and identified more than 600 short, narrowband features. Their characteristics, such as instantaneous relative bandwidth and total duration were measured and compared with those of spikes embedded in type IV emissions.
Results: Type II associated spikes occur mostly in chains inside or close to the slowly drifting type II emission. These spikes coexist with herringbone and pulsating structures. Their average duration is 96 ms and their average relative bandwidth 1.7%. These properties are not different from those of type IV embedded spikes. It is therefore possible that they are signatures of small-scale reconnection along the type II shock front. Title: Modeling of the Sunspot-Associated Microwave Emission Using a New Method of DEM Inversion Authors: Alissandrakis, C. E.; Bogod, V. M.; Kaltman, T. I.; Patsourakos, S.; Peterova, N. G. Bibcode: 2019SoPh..294...23A Altcode: 2018arXiv181205751A We develope a method to compute the temperature and density structure along the line of sight by inversion of the differential emission measure (DEM), under the assumptions of stratification and hydrostatic equilibrium. We apply this method to the DEM obtained from the Atmospheric Imaging Assembly (AIA) observations and use the results, together with potential extrapolations of the photospheric magnetic field, to compute the microwave emission of three sunspots, which we compare with observations from the Academy of Sciences Radio Telescope - 600 (RATAN-600) radio telescope and the Nobeyama Radioheliograph (NoRH). Our DEM-based models reproduce very well the observations of the moderate-size spot on October 2011 and within 25% the data of a similar sized spot on March 2016, but predict too low values for the big spot of 14 April 2016. The latter is better fitted by a constant conductive flux atmospheric model which, however, cannot reproduce the peak brightness temperature of 4.7 ×106K and the shape of the source at the NoRH frequency. We propose that these deviations are due to the low intensity non-thermal emission associated to a moving pore and to an opposite polarity light bridge. We also find that the double structure of the big spot at high RATAN-600 frequencies can be interpreted in terms of the variation of the angle between the magnetic field and the line of sight along the sunspot. Title: First high-resolution look at the quiet Sun with ALMA at 3mm Authors: Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos, S.; De Pontieu, B.; Warren, H.; Ayres, T.; Hudson, H. S.; Shimizu, T.; Vial, J. -C.; Wedemeyer, S.; Yurchyshyn, V. Bibcode: 2018A&A...619L...6N Altcode: 2018arXiv181005223N We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5″ by 4.5″. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15″ above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere. Title: IRIS Observations of Spicules and Structures Near the Solar Limb Authors: Alissandrakis, C. E.; Vial, J. -C.; Koukras, A.; Buchlin, E.; Chane-Yook, M. Bibcode: 2018SoPh..293...20A Altcode: 2018arXiv180102082A We have analyzed Interface Region Imaging Spectrograph (IRIS) spectral and slit-jaw observations of a quiet region near the South Pole. In this article we present an overview of the observations, the corrections, and the absolute calibration of the intensity. We focus on the average profiles of strong (Mg II h and k, C II and Si IV), as well as of weak spectral lines in the near ultraviolet (NUV) and the far ultraviolet (FUV), including the Mg II triplet, thus probing the solar atmosphere from the low chromosphere to the transition region. We give the radial variation of bulk spectral parameters as well as line ratios and turbulent velocities. We present measurements of the formation height in lines and in the NUV continuum from which we find a linear relationship between the position of the limb and the intensity scale height. We also find that low forming lines, such as the Mg II triplet, show no temporal variations above the limb associated with spicules, suggesting that such lines are formed in a homogeneous atmospheric layer and, possibly, that spicules are formed above the height of 2. We discuss the spatio-temporal structure of the atmosphere near the limb from images of intensity as a function of position and time. In these images, we identify p-mode oscillations in the cores of lines formed at low heights above the photosphere, slow-moving bright features in O I and fast-moving bright features in C II. Finally, we compare the Mg II k and h line profiles, together with intensity values of the Balmer lines from the literature, with computations from the PROM57Mg non-LTE model, developed at the Institut d' Astrophysique Spatiale, and estimated values of the physical parameters. We obtain electron temperatures in the range of ∼8000 K at small heights to ∼20 000 K at large heights, electron densities from 1.1 ×1011 to 4 ×1010cm−3 and a turbulent velocity of ∼24 kms−1. Title: Center-to-limb observations of the Sun with ALMA . Implications for solar atmospheric models Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian, T. S. Bibcode: 2017A&A...605A..78A Altcode:
Aims: We seek to derive information on the temperature structure of the solar chromosphere and compare these results with existing models.
Methods: We measured the center-to-limb variation of the brightness temperature, Tb, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, Te as a function of optical depth, τ.
Results: The ALMA images are very similar to AIA images at 1600 Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz, respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is 961.1 ± 2.5'' and 964.1 ± 4.5'' at 239 and 100 GHz, respectively. A slight but statistically significant limb brightening is observed at both frequencies.
Conclusions: The inversion of the center-to-limb curves shows that Te varies linearly with the logarithm of optical depth for 0.34 <τ100 GHz< 12, with a slope dTe/ dlnτ = -608 K. Our results are 5% lower than predicted by the average quiet Sun model C of Fontenla et al. (1993, ApJ. 406, 319), but do not confirm previous reports that the mm-λ solar spectrum is better fitted with models of the cell interior. Title: Evidence for two-loop interaction from IRIS and SDO observations of penumbral brightenings Authors: Alissandrakis, C. E.; Koukras, A.; Patsourakos, S.; Nindos, A. Bibcode: 2017A&A...603A..95A Altcode: 2017arXiv170407344A
Aims: We investigate small scale energy release events which can provide clues on the heating mechanism of the solar corona.
Methods: We analyzed spectral and imaging data from the Interface Region Imaging Spectrograph (IRIS), images from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatoty (SDO), and magnetograms from the Helioseismic and Magnetic Imager (HMI) aboard SDO.
Results: We report observations of small flaring loops in the penumbra of a large sunspot on July 19, 2013. Our main event consisted of a loop spanning 15'', from the umbral-penumbral boundary to an opposite polarity region outside the penumbra. It lasted approximately 10 min with a two minute impulsive peak and was observed in all AIA/SDO channels, while the IRIS slit was located near its penumbral footpoint. Mass motions with an apparent velocity of 100 km s-1 were detected beyond the brightening, starting in the rise phase of the impulsive peak; these were apparently associated with a higher-lying loop. We interpret these motions in terms of two-loop interaction. IRIS spectra in both the C II and Si iv lines showed very extended wings, up to about 400 km s-1, first in the blue (upflows) and subsequently in the red wing. In addition to the strong lines, emission was detected in the weak lines of Cl I, O I and C I, as well as in the Mg II triplet lines. Absorption features in the profiles of the C II doublet, the Si iv doublet and the Mg II h and k lines indicate the existence of material with a lower source function between the brightening and the observer. We attribute this absorption to the higher loop and this adds further credibility to the two-loop interaction hypothesis. Tilts were detected in the absorption spectra, as well as in the spectra of Cl I, O I, and C I lines, possibly indicating rotational motions from the untwisting of magnetic flux tubes.
Conclusions: We conclude that the absorption features in the C II, Si iv and Mg II profiles originate in a higher-lying, descending loop; as this approached the already activated lower-lying loop, their interaction gave rise to the impulsive peak, the very broad line profiles and the mass motions.

Movies associated to Figs. A.1-A.3 are available at http://www.aanda.org Title: Center-to-limb observations of the Sun with ALMA Authors: Alissandrakis, C. E.; Patsourakos, S.; Nindos, A.; Bastian, T. S. Bibcode: 2017arXiv170509008A Altcode: We measured the center-to-limb variation of the brightness temperature, $T_b$, from ALMA full-disk images at two frequencies and inverted the solution of the transfer equation to obtain the electron temperature, $T_e$ as a function of optical depth, $\tau$. The ALMA images are very similar to AIA images at 1600Å. The brightness temperature at the center of the disk is 6180 and 7250 K at 239 and 100 GHz respectively, with dispersions of 100 and 170 K. Plage regions stand out clearly in the 239/100 GHz intensity ratio, while faculae and filament lanes do not. The solar disk radius, reduced to 1 AU, is $961.1\pm2.5$ arcsec and $964.1\pm4.5$ arcsec at 239 and 100 GHz respectively. A slight but statistically significant limb brightening is observed at both frequencies. The inversion of the center-to-limb curves shows that $T_e$ varies linearly with the logarithm of optical depth for $0.34<\tau_{100\,GHz}<12$, with a slope $d\ln T_e/d\tau=-608$ K. Our results are 5% lower than predicted by the average quiet sun model C of Fontenla et al. (1993), but do not confirm previous reports that the mm-$\lambda$ solar spectrum is better fitted with models of the cell interior. Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: High resolution observations with Artemis-IV and the NRH. I. Type IV associated narrow-band bursts Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.; Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2016A&A...586A..29B Altcode: 2015arXiv151202481B Context. Narrow-band bursts appear on dynamic spectra from microwave to decametric frequencies as fine structures with very small duration and bandwidth. They are believed to be manifestations of small scale energy release through magnetic reconnection.
Aims: We analyzed 27 metric type IV events with embedded narrow-band bursts, which were observed by the ARTEMIS-IV radio spectrograph from 30 June 1999 to 1 August 2010. We examined the morphological characteristics of isolated narrow-band structures (mostly spikes) and groups or chains of structures.
Methods: The events were recorded with the SAO high resolution (10 ms cadence) receiver of ARTEMIS-IV in the 270-450 MHz range. We measured the duration, spectral width, and frequency drift of ~12 000 individual narrow-band bursts, groups, and chains. Spike sources were imaged with the Nançay radioheliograph (NRH) for the event of 21 April 2003.
Results: The mean duration of individual bursts at fixed frequency was ~100 ms, while the instantaneous relative bandwidth was ~2%. Some bursts had measurable frequency drift, either positive or negative. Quite often spikes appeared in chains, which were closely spaced in time (column chains) or in frequency (row chains). Column chains had frequency drifts similar to type-IIId bursts, while most of the row chains exhibited negative frequently drifts with a rate close to that of fiber bursts. From the analysis of NRH data, we found that spikes were superimposed on a larger, slowly varying, background component. They were polarized in the same sense as the background source, with a slightly higher degree of polarization of ~65%, and their size was about 60% of their size in total intensity.
Conclusions: The duration and bandwidth distributions did not show any clear separation in groups. Some chains tended to assume the form of zebra, lace stripes, fiber bursts, or bursts of the type-III family, suggesting that such bursts might be resolved in spikes when viewed with high resolution. The NRH data indicate that the spikes are not fluctuations of the background, but represent additional emission such as what would be expected from small-scale reconnection. Title: ALMA Observations of the Sun in Cycle 4 and Beyond Authors: Wedemeyer, S.; Fleck, B.; Battaglia, M.; Labrosse, N.; Fleishman, G.; Hudson, H.; Antolin, P.; Alissandrakis, C.; Ayres, T.; Ballester, J.; Bastian, T.; Black, J.; Benz, A.; Brajsa, R.; Carlsson, M.; Costa, J.; DePontieu, B.; Doyle, G.; Gimenez de Castro, G.; Gunár, S.; Harper, G.; Jafarzadeh, S.; Loukitcheva, M.; Nakariakov, V.; Oliver, R.; Schmieder, B.; Selhorst, C.; Shimojo, M.; Simões, P.; Soler, R.; Temmer, M.; Tiwari, S.; Van Doorsselaere, T.; Veronig, A.; White, S.; Yagoubov, P.; Zaqarashvili, T. Bibcode: 2016arXiv160100587W Altcode: This document was created by the Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) in preparation of the first regular observations of the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), which are anticipated to start in ALMA Cycle 4 in October 2016. The science cases presented here demonstrate that a large number of scientifically highly interesting observations could be made already with the still limited solar observing modes foreseen for Cycle 4 and that ALMA has the potential to make important contributions to answering long-standing scientific questions in solar physics. With the proposal deadline for ALMA Cycle 4 in April 2016 and the Commissioning and Science Verification campaign in December 2015 in sight, several of the SSALMON Expert Teams composed strategic documents in which they outlined potential solar observations that could be feasible given the anticipated technical capabilities in Cycle 4. These documents have been combined and supplemented with an analysis, resulting in recommendations for solar observing with ALMA in Cycle 4. In addition, the detailed science cases also demonstrate the scientific priorities of the solar physics community and which capabilities are wanted for the next observing cycles. The work on this White Paper effort was coordinated in close cooperation with the two international solar ALMA development studies led by T. Bastian (NRAO, USA) and R. Brajsa, (ESO). This document will be further updated until the beginning of Cycle 4 in October 2016. In particular, we plan to adjust the technical capabilities of the solar observing modes once finally decided and to further demonstrate the feasibility and scientific potential of the included science cases by means of numerical simulations of the solar atmosphere and corresponding simulated ALMA observations. Title: A tiny event producing an interplanetary type III burst Authors: Alissandrakis, C. E.; Nindos, A.; Patsourakos, S.; Kontogeorgos, A.; Tsitsipis, P. Bibcode: 2015A&A...582A..52A Altcode: 2015arXiv150708423A
Aims: We investigate the conditions under which small-scale energy release events in the low corona gave rise to strong interplanetary (IP) type III bursts.
Methods: We analyzed observations of three tiny events, detected by the Nançay Radio Heliograph (NRH), two of which produced IP type III bursts. We took advantage of the NRH positioning information and of the high cadence of AIA/SDO data to identify the associated extreme-UV (EUV) emissions. We measured positions and time profiles of the metric and EUV sources.
Results: We found that the EUV events that produced IP type III bursts were located near a coronal hole boundary, while the one that did not was located in a closed magnetic field region. In all three cases tiny flaring loops were involved, without any associated mass eruption. In the best observed case, the radio emission at the highest frequency (435 MHz) was displaced by ~55'' with respect to the small flaring loop. The metric type III emission shows a complex structure in space and in time, indicative of multiple electron beams, despite the low intensity of the events. From the combined analysis of dynamic spectra and NRH images, we derived the electron beam velocity as well as the height, ambient plasma temperature, and density at the level of formation of the 160 MHz emission. From the analysis of the differential emission measure derived from the AIA images, we found that the first evidence of energy release was at the footpoints, and this was followed by the development of flaring loops and subsequent cooling.
Conclusions: Even small energy release events can accelerate enough electrons to give rise to powerful IP type III bursts. The proximity of the electron acceleration site to open magnetic field lines facilitates the escape of the electrons into the interplanetary space. The offset between the site of energy release and the metric type III location warrants further investigation.

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

Appendices are available in electronic form at http://www.aanda.org Title: RATAN-600 Observations of Small-Scale Structures with High Spectral Resolution Authors: Bogod, V. M.; Alissandrakis, C. E.; Kaltman, T. I.; Tokhchukova, S. K. Bibcode: 2015SoPh..290....7B Altcode: 2014arXiv1403.7658B; 2014SoPh..tmp...65B We present observations of quiet-Sun small-scale structures (SSS) in the microwave range with the Radio Astronomical Telescope of the Academy of Sciences 600 (RATAN-600) spectral-polarization facility in a wide range of frequencies. The SSS are regularly recorded in routine observations of the large reflector-type radio telescope and represent manifestations in the radio range of various structures of the quiet Sun: supergranulation network, bright points, plage patches, and so on. A comparison with images from the Solar Dynamics Observatory (SDO) showed that the microwave emission comes from a region extending from the chromosphere to the low transition region. We measured the properties of the SSS as well as the degree of circular polarization averaged over the beam of the radio telescope, and from this we estimated the magnetic field at the formation level of the radiation. Title: Fine Structure of Metric Type IV Radio Bursts Observed with the ARTEMIS-IV Radio-Spectrograph: Association with Flares and Coronal Mass Ejections Authors: Bouratzis, C.; Hillaris, A.; Alissandrakis, C. E.; Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2015SoPh..290..219B Altcode: 2014arXiv1406.1202B; 2014SoPh..tmp..101B Fine structures embedded in type IV burst continua may be used as diagnostics of the magnetic-field restructuring and the corresponding energy release associated with the low-corona development of flare or coronal mass ejection (CME) events. A catalog of 36 type IV bursts observed with the SAO receiver of the ARTEMIS-IV solar radio-spectrograph in the 450 - 270 MHz range at high cadence (0.01 sec) was compiled; the fine structures were classified into five basic classes with two or more subclasses each. The time of fine-structure emission was compared with the injection of energetic electrons as recorded by hard X-ray and microwave emission, the soft X-ray (SXR) light curves and the CME onset time. Our results indicate a very tight temporal association between energy release episodes and pulsations, spikes, narrow-band bursts of the type III family, and zebra bursts. Of the remaining categories, the featureless broadband continuum starts near the time of the first energy release, between the CME onset and the SXR peak, but extends for several tens of minutes after that, covering almost the full extent of the flare-CME event. The intermediate drift bursts, fibers in their majority, mostly follow the first energy release, but have a wider distribution than other fine structures. Title: Microwave and EUV Observations of an Erupting Filament and Associated Flare and Coronal Mass Ejections Authors: Alissandrakis, Costas E.; Kochanov, Alexey A.; Patsourakos, Spiros; Altyntsev, Alexander T.; Lesovoi, Sergey V.; Lesovoya, Nadya N. Bibcode: 2013PASJ...65S...8A Altcode: 2013arXiv1309.1703A A filament eruption was observed with the Siberian Solar Radio Telescope (SSRT) on 2012 June 23, starting at around 06:40 UT, beyond the west limb. The filament could be followed in SSRT images to heights above 1 R, and coincided with the core of the CME, seen in LASCO C2 images. We briefly discuss the dynamics of the eruption: the top of the filament showed a smooth acceleration up to an apparent velocity of ∼ 1100 km s-1. Images behind the limb from STEREO-A show a two-ribbon flare and the interaction of the main filament, located along the primary neutral line, with an arch-like structure, oriented in the perpendicular direction. The interaction was accompanied by strong emission and twisting motions. The microwave images show a low-temperature component, a high-temperature component associated with the interaction of the two filaments and another high-temperature component apparently associated with the top of flare loops. We computed the differential emission measure from the high-temperature AIA bands and from this the expected microwave brightness temperature; for emission associated with the top of the flare loops, the computed brightness was 35% lower than the observed value. Title: Hot coronal loops associated with umbral brightenings Authors: Alissandrakis, C. E.; Patsourakos, S. Bibcode: 2013A&A...556A..79A Altcode: 2013arXiv1307.3392A
Aims: We aim to investigate the association of umbral brightenings with coronal structures.
Methods: We analyzed AIA/SDO high-cadence images in all bands, HMI/SDO data, soft X-ray images from SXI/GOES-15, and Hα images from the GONG network.
Results: We detected umbral brightenings that were visible in all AIA bands as well as in Hα. Moreover, we identified hot coronal loops that connected the brightenings with nearby regions of opposite magnetic polarity. These loops were initially visible in the 94 Å band, subsequently in the 335 Å band, and in one case in the 211 Å band. A differential emission measure analysis revealed plasma with an average temperature of about 6.5 × 106 K. This behavior suggests cooling of impulsively heated loops.

Two movies are available in electronic from at http://www.aanda.org 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: 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: Radio Emission Associated with Solar Energetic Particle Events Authors: Kouloumvakos, A.; Nindos, A.; Preka-Papadema, P.; Hillaris, A.; Caroubalos, C.; Moussas, X.; Alissandrakis, C.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2012hell.confR..12K Altcode: The corona probed at meter and decimeter wavelengths is a crucial region for the acceleration and propagation of solar energetic particles (SEPs), and radio diagnostics in this plasma plays a major role in assessing the origin of SEP events. Using data from the ARTEMIS IV solar radio spectro-graph, we report the properties of the radio emission associated with several major SEP events. The association of the radio emission with the related flares and CMEs is also investigated. Title: Multi-wavelength Observations of a Metric Type-II Event Authors: Alissandrakis, C.; Nindos, A.; Patsourakos, S.; Hillaris, Al.; Artemis Group Bibcode: 2012hell.conf....6A Altcode: We have studied a complex metric radio event,observed with the ARTEMIS radiospectrogarph on February 12, 2010. The event was associated with a surge observed at 195 and 304 A and with a coronal mass ejection observed by STEREO A and B instruments near the East wnd West limbs respectively. On the disk the event was observed at 10 frequencies by the Nancay Radioheliograph (NRH), in H-alpha by the Catania observatory and in soft x-rays by GOES SXI. We combined these data, together with MDI longitudinal magnetic field, to get as complete a picture of the event as possible. Our emphasis is on two type-II bursts that occured near respective maxima in the GOES light curves. The first, associated with the main peak of the event, showed a clear foundamental-harmonic structure, while the emission of the second consisted of three well-separated bands. Using positional infornation for the type-IIs from the NRH we explore their possible association with the surge, the coronal front and the CME. We also studied fine structured and foundamental harmonic structure in the metric dynamic spectrum. Title: Radio Emission of the Quiet Sun and Active Regions (Invited Review) Authors: Shibasaki, K.; Alissandrakis, C. E.; Pohjolainen, S. Bibcode: 2012esrs.book....3S Altcode: Solar radio emission provides valuable information on the structure and dynamics of the solar atmosphere above the temperature minimum. We review the background and most recent observational and theoretical results on the quiet Sun and active region studies, covering the entire radio range from millimeter to decameter wavelengths. We examine small- and large-scale structures, at short and long time scales, as well as synoptic aspects. Open questions and challenges for the future are also identified. Title: Radio Emission of the Quiet Sun and Active Regions (Invited Review) Authors: Shibasaki, K.; Alissandrakis, C. E.; Pohjolainen, S. Bibcode: 2011SoPh..273..309S Altcode: 2011SoPh..tmp..313S; 2011SoPh..tmp..246S; 2011SoPh..tmp..122S; 2011SoPh..tmp..288S; 2011SoPh..tmp..177S Solar radio emission provides valuable information on the structure and dynamics of the solar atmosphere above the temperature minimum. We review the background and most recent observational and theoretical results on the quiet Sun and active region studies, covering the entire radio range from millimeter to decameter wavelengths. We examine small- and large-scale structures, at short and long time scales, as well as synoptic aspects. Open questions and challenges for the future are also identified. Title: On the relationship of shock waves to flares and coronal mass ejections Authors: Nindos, A.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P. Bibcode: 2011A&A...531A..31N Altcode: 2011arXiv1105.1268N Context. Metric type II bursts are the most direct diagnostic of shock waves in the solar corona.
Aims: There are two main competing views about the origin of coronal shocks: that they originate in either blast waves ignited by the pressure pulse of a flare or piston-driven shocks due to coronal mass ejections (CMEs). We studied three well-observed type II bursts in an attempt to place tighter constraints on their origins.
Methods: The type II bursts were observed by the ARTEMIS radio spectrograph and imaged by the Nançay Radioheliograph (NRH) at least at two frequencies. To take advantage of projection effects, we selected events that occurred away from disk center.
Results: In all events, both flares and CMEs were observed. In the first event, the speed of the shock was about 4200 km s-1, while the speed of the CME was about 850 km s-1. This discrepancy ruled out the CME as the primary shock driver. The CME may have played a role in the ignition of another shock that occurred just after the high speed one. A CME driver was excluded from the second event as well because the CMEs that appeared in the coronagraph data were not synchronized with the type II burst. In the third event, the kinematics of the CME which was determined by combining EUV and white light data was broadly consistent with the kinematics of the type II burst, and, therefore, the shock was probably CME-driven.
Conclusions: Our study demonstrates the diversity of conditions that may lead to the generation of coronal shocks. Title: CME on CME Interaction on January 17, 2005 Authors: Bouratzis, C.; Preka-Papadima, P.; Moussas, X.; Hillaris, A. E.; Caroubalos, C.; Alissandrakis, C. E.; Tsitsipis, P.; Kontogeorgos, A. Bibcode: 2010ASPC..424...39B Altcode: 2010arXiv1009.3025B On January 17, 2005 a complex radio event associated with an X3.8 SXR flare and two fast Halo CMEs (CME1 & CME2 henceforward) in close succession was observed. We present combined ARTEMIS-IV & WIND WAVES dynamic spectra which provide a complete view of the radio emission induced by shock waves and electron beams from the low corona to about 1 A.U. These are supplemented with data, from the Nançay Radioheliograph (NRH), GOES, EIT and LASCO for the study of the associated flare and CME activity. Title: On the Origin of Coronal Shocks Authors: Nindos, Alexander; Alissandrakis, C. E.; Hillaris, A.; Caroubalos, C. Bibcode: 2009SPD....40.1924N Altcode: Type II bursts represent the passage of a shock wave through the tenuous plasma of the solar corona. Their exact origin has not been unambiguously determined, although they are associated with eruptive phenomena and are identified either with a flare blast wave or with a CME forward shock or with a shock driven by the flanks of a CME. We investigate the location of the shock formation and its association with flares and/or CMEs by studying the properties of three Type II radio bursts associated with eruptions that occurred relatively close to the limb where the influence of projection effects is rather small. The Type II bursts were identified in 110-687 MHz range observations of the ARTEMIS-IV Solar Radio spectrograph and observed simultaneously with the Nancay Radioheliograph (NRH). The NRH makes images of the Sun at several fixed frequencies between 150 and 432 MHz, and our events could be mapped at two or three of its frequencies, allowing us to determine the propagation of the shocks in the low corona. The information provided by the radio data combined with observations of the associated flares and CMEs by space-borne instruments has been used to constrain the shock wave kinematics and discuss whether they were flare-ignited or CME-driven. Title: New Developments in ARTEMIS IV Solar Radio Spectrograph Authors: Alissandrakis, C. E.; Kontogeorgos, A.; Tsitsipis, P.; Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hilaris, A.; Bougeret, J. -L.; Dumas, G. Bibcode: 2009EM&P..104...93A Altcode: We present recent developments of the ARTEMIS IV solar radio spectrograph operating at Thermopylae, central Greece. Observations are obtained daily in total intensity and in the frequency range from 20 to 650 MHz, using two antennas and two receivers. We are now in the process of developing a new system that will record consecutively the intensity of right-hand and left-hand polarized waves using one of the antennas and the same receivers. Title: Metric radio bursts and fine structures observed on 17 January, 2005 Authors: Bouratzis, C.; Preka-Papadema, P.; Moussas, X.; Alissandrakis, C.; Hillaris, A. Bibcode: 2009AdSpR..43..605B Altcode: 2010arXiv1009.3577B A complex radio event was observed on January 17, 2005 with the radio-spectrograph ARTEMIS-IV, operating at Thermopylae, Greece; it was associated with an X3.8 SXR flare and two fast Halo CMEs in close succession. We present dynamic spectra of this event; the high time resolution (1/100 s) of the data in the 450-270 MHz range, makes possible the detection and analysis of the fine structure which this major radio event exhibits. The fine structure was found to match, almost, the comprehensive Ondrejov Catalogue which it refers to the spectral range 0.8-2 GHz, yet seems to produce similar fine structure with the metric range. Title: A Statistical Study of Microwave Flare Morphologies Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E. Bibcode: 2008SoPh..253...79T Altcode: 2008SoPh..tmp..166T This study has been motivated by the detection of a small number of optically thin microwave bursts with maximum emission near the loop top, which is contrary to the prediction of isotropic gyrosynchrotron models. Using Nobeyama Radioheliograph (NoRH) high-spatial-resolution images at 17 and 34 GHz, we study the morphology at the radio peak of 104 flares that occurred relatively close to the limb. Using data from the Nobeyama Polarimeter we were able to determine whether the 17- and 34-GHz emissions came from optically thin or thick sources. We identified single-loop events, taking into account supplementary information from EUV and soft X-ray (SXR) images. We found optically thin emission from the top of the loop in 36% of single-loop events. In agreement with standard models, in this sample 46% and 18% of the events showed optically thin emission from the footpoints and optically thick emission from the entire loop, respectively. The derived percentage of events with gyrosynchrotron emission from isotropic populations of energetic electrons is possibly an upper limit. This point is illustrated by the analysis of an optically thin event that shows footpoint emission during the rise phase and loop-top emission during the decay phase. A model that takes into account both anisotropies in the distribution function of nonthermal electrons and time evolution can reproduce the observed transition from footpoint to loop-top morphology, if electrons with pitch-angle anisotropy are injected near one of the footpoints. Title: Determination of the True Shape of Coronal Loops Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C. 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: ARTEMIS, a Solar Radiospectrograph Authors: Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hillaris, A.; Mitsakou, E.; Pothitakis, G.; Bouratzis, C.; Tsitsipis, P.; Kontageorgos, A.; Alissandrakis, C. E.; Bourgeret, J. -L.; Dumas, G. Bibcode: 2008RoAJ...18S.215C Altcode: No abstract at ADS Title: M otions Near The Solar Limb From TRACE Ly-α O bservations Authors: Zachariadis, G.; Alissandrakis, C. E. Bibcode: 2007ESASP.641E..53Z Altcode: We presenty preliminary results of a study of flows near the limb. The observations were obtained with TRACE in a quiest region near the limb in the Ly-α, (1216 Å) band, and cover two periods of about 19 and 16 min respectively with a cadence of ~ 10 sec. The high temporal resolution of the images allows one to follow the evolution of small bright features of the solar disk and the activity of the spicules beyond the solar limb. The analysis of the images shows that many bright feature, with lifetime of a few minutes, shift toward the solar limb, with a velocity of about 14 km/sec. Spicules show motions both in the radial direction and parallel to the solar limb. Title: M odeling The 3-D Geometry of Coronal Loops Authors: Alissandrakis, C. E.; Gontikakis, C.; Dara, H. C. 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: Type II and IV radio bursts in the active period October-November 2003 Authors: Petoussis, V.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Preka-Papadema, P.; Hillaris, A.; Caroubalos, C.; Alissandrakis, C. E.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AIPC..848..199P Altcode: 2010arXiv1009.4485P In this report we present the Type II and IV radio bursts observed and analyzed by the radio spectrograph ARTEMIS IV1, in the 650-20MHz frequency range, during the active period October-November 2003. These bursts exhibit very rich fine structures such fibers, pulsations and zebra patterns which is associated with certain characteristics of the associated solar flares and CMEs. Title: A Statistical Study of Microwave Flare Morphologies Authors: Tzatzakis, V.; Nindos, A.; Alissandrakis, C. E.; Shibasaki, K. Bibcode: 2006AIPC..848..248T Altcode: Using Nobeyama Radioheliograph (NoRH) high spatial resolution images at 17 and 34 GHz, we study the morphology of several flare events that occured relatively close to the limb. Our study has been motivated by the recent detection of a small number of optically thin flares whose maximum emission peaks close to the loop top. These events may show significant anisotropy of the density and pitch angle distribution of the nonthermal electrons. However, we do not know how often they occur. Our sample consists of 104 flare events. Using data from the Nobeyama Polarimeter we were able to determine whether the 17 and 34 GHz emissions are optically thin or thick. Almost half of our events appear unresolved in the NoRH images. Among the resolved events, special attention is paid to those with circular polarization (V) showing two distinct sources. Such V sources are considered proxies to the flaring loop footpoints if the total intensity (I) morphology is consistent with a single flaring loop configuration. A small number of events, as expected at such high frequencies, are optically thick with I maximum between the loop footpoints. The number of optically thin events showing I maxima co-spatial with the V sources is approximately the same as the number of those optically thin resolved events showing emission peak between the V sources. Our future plans concerning the study of our database are also outlined. Title: Ten Years of the Solar Radiospectrograph ARTEMIS-IV Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis, J.; Moussas, X.; Tsitsipis, P.; Kontogeorgos, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J. -L.; Dumas, G.; Nindos, A. Bibcode: 2006AIPC..848..864C Altcode: 2010arXiv1009.4150C The Solar Radiospectrograph of the University of Athens (ARTEMIS-IV) is in operation at the Thermopylae Satellite Communication Station since 1996. The observations extend from the base of the Solar Corona (650 MHz) to about 2 Solar Radii (20 MHz) with time resolution 1/10-1/100 sec. The instruments recordings, being in the form of dynamic spectra, measure radio flux as a function of height in the corona; our observations are combined with spatial data from the Nancay Radioheliograph whenever the need for 3D positional information arises. The ARTEMIS-IV contribution in the study of solar radio bursts is two fold- Firstly, in investigating new spectral characteristics since its high sampling rate facilitates the study of fine structures in radio events. On the other hand it is used in studying the association of solar bursts with interplanetary phenomena because of its extended frequency range which is, furthermore, complementary to the range of the WIND/WAVES receivers and the observations may be readily combined. This reports serves as a brief account of this operation. Joint observations with STEREO/WAVES and LOFAR low frequency receivers are envisaged in the future. Title: Type II Shocks Characteristics: Comparison with associated CMEs and Flares Authors: Pothitakis, G.; Mitsakou, E.; Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AIPC..848..238P Altcode: 2010arXiv1009.4294P A number of metric (100-650 MHz) typeII bursts was recorded by the ARTEMIS-IV radiospectrograph in the 1998-2000 period; the sample includes both CME driven shocks and shocks originating from flare blasts. We study their characteristics in comparison with characteristics of associated CMEs and flares. Title: Type III events, flares and CMEs, in the extremely active period October-November 2003 Authors: Mitsakou, E.; Thanasa, M.; Preka-Papadema, P.; Moussas, X.; Hillaris, A.; Caroubalos, C.; Alissandrakis, C. E.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AIPC..848..234M Altcode: 2010arXiv1009.3592M The type III observations trace the propagation of energetic electron populations through the Solar Corona which, more often than not, precede or are associated with energy release on the Sun. A sample of Type III bursts in the range 20-650 MHz during the period of extraordinary solar activity (20 October to 4 November 2003) recorded by the ARTEMIS-IV1 radio spectrograph is analysed; its parameters are compared with characteristics of associated flares (Ha and GOES SXR) and CMEs, observed in the same period and reported in the SGR and the LASCO archives respectively. In this report we attempt to establish a correlation between energetic particles and major manifestations of solar activity such as flares and CMEs. Title: Periodicities in Fine Structure of Type IV Radio Solar Bursts Authors: Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Preka-Papadema, P.; Hillaris, A.; Petoussis, V.; Caroubalos, C.; Alissandrakis, C. E.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AIPC..848..243T Altcode: A study of periodicities appearing in the type-IV bursts fine structure (fibers and pulsations in particular) is presented. Our data set includes recordings by ARTEMIS IV with high time resolution; this facilitates the detection of lower than 1 sec periodicities. We introduce a method which employs directional filtering for the separation of intertwined fibers and pulsations for independent analysis of different events within complex radio bursts. For further analysis we use Short Time Fourier and Wavelet transforms which enable the study of the evolution of periodicities with time over specified periods of solar radio activity. Since, according to existing theoretical models, periodicities depend on basic parameters of active regions, this methodology provides the appropriate diagnostic tools. Title: Detection of slopes of linear and quasi-linear structures in noisy background, using 2D-FFT Authors: Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Preka-Papadema, P.; Hillaris, A.; Petoussis, V.; Caroubalos, C.; Alissandrakis, C. E.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AIPC..848..874T Altcode: We present a fast algorithm for slope detection on grey scale images, based on 2D-FFT, which may be used for line or edge detection. Our approach is based on the calculation of ``energy'' per direction of the image, thus obtaining the Energy Spectrum on Slope (α). This exhibits local maxima at the points where α equals the slopes of linear or quasi-linear segments within the image, yet it is not affected by their position within it. The process thus outlined was motivated by the study of astrophysical images (Solar Dynamic Radio Spectra) which necessitated the introduction of a method for fast extraction of drifting structures, since they appear as linear or quasi linear segments on these spectra. Title: The improved ARTEMIS IV multichannel solar radio spectrograph of the University of Athens Authors: Kontogeorgos, A.; Tsitsipis, P.; Caroubalos, C.; Moussas, X.; Preka-Papadema, P.; Hilaris, A.; Petoussis, V.; Bouratzis, C.; Bougeret, J. -L.; Alissandrakis, C. E.; Dumas, G. Bibcode: 2006ExA....21...41K Altcode: 2006ExA...tmp...16K; 2010arXiv1009.3628K We present the improved solar radio spectrograph of the University of Athens operating at the Thermopylae Satellite Telecommunication Station. Observations now cover the frequency range from 20 to 650 MHz. The spectrograph has a 7-meter moving parabola fed by a log-periodic antenna for 100 650 MHz and a stationary inverted V fat dipole antenna for the 20 100 MHz range. Two receivers are operating in parallel, one swept frequency for the whole range (10 spectrums/sec, 630 channels/spectrum) and one acousto-optical receiver for the range 270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data acquisition system consists of two PCs (equipped with 12 bit, 225 ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU and 30 SFU in the 20 100 MHz and 100 650 MHz range respectively. The daily operation is fully automated: receiving universal time from a GPS, pointing the antenna to the sun, system calibration, starting and stopping the observations at preset times, data acquisition, and archiving on DVD. We can also control the whole system through modem or Internet. The instrument can be used either by itself or in conjunction with other instruments to study the onset and evolution of solar radio bursts and associated interplanetary phenomena. Title: Observing the Sun at 20 650 MHz at Thermopylae with Artemis Authors: Kontogeorgos, A.; Tsitsipis, P.; Moussas, X.; Preka-Papadema, G.; Hillaris, A.; Caroubalos, C.; Alissandrakis, C.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006SSRv..122..169K Altcode: Fine structure of type IV radio solar bursts with a great variety and complexity often give much information in different ways and enable estimation of various coronal characteristics. In this work, we expose our new method for fine structure revealing and separation of two basic kinds of type IV fine structure, as fibers and pulsations. We also estimate frequency drift of fibers from dynamic spectra, clean from continuous background, with a prototype method using 2-D Fourier transform and we estimate periodicities of fibers as well as pulsations with continuous wavelet transform. Working with the last method we found periodicities close to 3 min umbral oscillations and 5 min global solar oscillations. Title: Solar flares with and without SOHO/LASCO coronal mass ejections and type II shocks Authors: Hillaris, A.; Petousis, V.; Mitsakou, E.; Vassiliou, C.; Moussas, X.; Polygiannakis, J.; Preka-Papadema, P.; Caroubalos, C.; Alissandrakis, C.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G. Bibcode: 2006AdSpR..38.1007H Altcode: 2010arXiv1009.3636H We analyse of a set of radio rich (accompanied by type IV or II bursts) solar flares and their association with SOHO/LASCO Coronal Mass Ejections in the period 1998 2000. The intensity, impulsiveness and energetics of these events are investigated. We find that, on the average, flares associated both with type IIs and CMEs are more impulsive and more energetic than flares associated with type IIs only (without CME reported), as well as flares accompanied by type IV continua but not type II shocks. From the last two classes, flares with type II bursts (without CMEs reported) are the shortest in duration and the most impulsive. Title: Multiwavelength Analysis of an Active Region Authors: Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.; Alissandrakis, C. E.; Nindos, A.; Vial, J. -C.; Tsiropoula, G. 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: 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: Complex Solar Events Observed with the Artemis-Iv Radio-Spectrograph in October/november 2003 Authors: Alissandrakis, C. E.; Nindos, A.; Hilaris, A.; Caroubalos, C.; Artemis Team Bibcode: 2005ESASP.600E.106A Altcode: 2005dysu.confE.106A; 2005ESPM...11..106A No abstract at ADS Title: RATAN-600 Observations of Unusual Inversion of Polarization in Sunspot Associated Microwave Sources Authors: Gizani, Nectaria A. B.; Alissandrakis, C.; Bogod, V.; Garaimov, V.; Zheleznyakov, V.; Zlotnik, E. Bibcode: 2005HiA....13..137G Altcode: We present multi-wavelength observations of an active region in the range of 1.7 to 30 cm obtained with the RATAN-600 radio telescope with high polarization accuracy (up to 0.5%). The active region was observed on six consecutive days in May 96. At short wavelengths the circular polarization scans show a bipolar structure in conformity with the bipolar magnetic field. However at 14.5 cm the circular polarization drops practically to zero and at longer wavelengths the sense of the polarization is reversed in {both components}. This was consistently observed through all our six days of observation while the region was moving from the center of the disk to the west limb. This unusual polarization inversion cannot be interpreted in terms of the well-known propagation effects under conditions of weak and strong coupling. Title: Solar type II and type IV radio bursts observed during 1998-2000 with the ARTEMIS-IV radiospectrograph Authors: Caroubalos, C.; Hillaris, A.; Bouratzis, C.; Alissandrakis, C. E.; Preka-Papadema, P.; Polygiannakis, J.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Bougeret, J. -L.; Dumas, G.; Perche, C. Bibcode: 2004A&A...413.1125C Altcode: A catalogue of the type II and type IV solar radio bursts in the 110-687 MHz range, observed with the radio spectrograph ARTEMIS-IV operated by the University of Athens at Thermopylae, Greece from 1998-2000 is presented. These observations are compared with the LASCO archives of Coronal Mass Ejections and the Solar Geophysical Reports of solar flares (Hα & SXR) and examined for possible associations. The main results are:

68% of the catalogue events were associated with CMEs.

67% of the type II events were associated with CMEs, in accordance with previous results. This percentage rises to 79% in the case of composite type II/IV events.

77% of the type IV continua were associated with CMEs, which is higher that the CME-type II association probability.

The type II associated CMEs had an average velocity of (835 ± 380) km s-1, while the CMEs not associated with type IIs had an average velocity of (500 ± 150) km s-1.

All events, but one, were well associated with H\alpha and/or SXR flares.

Most of the CME launch times precede by 5-60 min (30 min on average) the associated SXR flare peak; an important fraction (72%) precede the flare onset as well.

Appendix A is only available in electronic form at http://www.edpsciences.org Title: The Improved Multichannel Solar Radiospectrograph ARTEMIS IV (Abstract) Authors: Kontogeorgos, P.; Tsitsipis, P.; Caroubalos, C.; Moussas, X.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis, J.; Bougeret, J. -L.; Dumas, G.; Perche, C. Bibcode: 2004hell.conf..338K Altcode: No abstract at ADS Title: Solar Flares with and without Coronal Mass Ejections and Type II shocks Authors: Mitsakou, E.; Petousis, V.; Vassiliou, C.; Moussas, X.; Polygiannakis, J.; Hillaris, A.; Preka-Papadema, P.; Caroubalos, C.; Alissandrakis, C.; Tsitsipis, P.; Artemis Team Bibcode: 2004cosp...35.2293M Altcode: 2004cosp.meet.2293M An analysis is presented of a set of ``radio rich'' solar SXR flares, i.e. accompanied by type IV or II metric radio bursts, associated with Coronal Mass Ejections and MHD shocks, recorded as type II events, in the period 1998-2000. The relative size, impulsiveness and energetics of these events are investigated. We find that, on the average, the flares with type II bursts and CME's are larger, more impulsive and energetic from the flares with type II but without CME's. The latter are more energetic than flares associated with relatively ``slow CME's'' accompanied by type IV continua but not type II shocks. Although a simple classification may not be readily determined the results imply that associated energetic events originate, more often than not, from sources with characteristics fairly well correlated. Title: Study of 40 Solar Energetic Events Based on GOES, LASCO and ARTEMIS IV Data (1998-2000) Authors: Mitsakou, E.; Vassiliou, C.; Moussas, X.; Polygiannakis, J.; Hillaris, A.; Preka-Papadema, P.; Caroubalos, C.; Alissandrakis, C. E.; Tsitsipis, P.; Kontogeorgos, A.; Bougeret, J. -L.; Dumas, G.; Perche, C. Bibcode: 2004hell.confR..77M Altcode: No abstract at ADS Title: Fine Structure in Type IV Solar Radio Bursts during three major Solar Events of October-November 2003 observed by Artemis IV. Authors: Tsitsipis, P.; Kontogeorgos, A.; Caroubalos, C.; Moussas, X.; Hillaris, A.; Preka-Papadema, P.; Bougeret, J. -L.; Alissandrakis, C.; Dumas, G.; Polygiannakis, J. Bibcode: 2004cosp...35.2705T Altcode: 2004cosp.meet.2705T The fine structure in three solar type IV radio bursts was studied using the French-Greek ARTEMIS-IV multichannel radio spectrograph. The bursts were recorded during three major solar events, on the 26 and 28 October.and 3 Nov. 2003; these we associated with intense flares and CMEs. The observed fine structure includes intermediate drift bursts (fibers) and pulsations. The complexity and overlapping of the various spectral characteristics necessitated the utilization of certain, 2D-FFT based, filtering processes, aiming firstly at structure detection in the frequency-time plane (ie the dynamic spectrum) and secondly in the separation of fibers and pulsations and the suppression of the type IV continuum. This methodology results also to the fibers frequency drift rate distributions and their evolution in time. These fiber frequency drift distributions may be interpreted as the exciter velocity distributions, using a Newkirk coronal density model; the latter may be used as diagnostics of the magnetic field restructuring and energy release during a solar energetic event (flare and/or CME). Artemis IV will provide complementary data to the STEREO/WAVES experiment. Title: Fine Radio Structures During a Major Solar Event Observed with Artemis IV (Abstract) Authors: Tsitsipis, P.; Kontogeorgos, A.; Caroubalos, C.; Moussas, X.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis, J.; Bougeret, J. -L.; Dumas, G.; Perche, C. Bibcode: 2004hell.confQ..64T Altcode: No abstract at ADS Title: Unusual Inversion of Polarization in Sunspot Sources Authors: Gizani, Nectaria A.; Alissandrakis, C. E.; Bogod, V.; Garaimov, V.; Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E. Bibcode: 2003IAUJD...3E...2G Altcode: We present multi-wavelength observations of an active region in the range of 1.7 to 30 cm obtained with the RATAN-600 radio telescope with high polarization accuracy (up to 0.5%). The active region was observed on six consecutive days in May 96. At short wavelengths the circular polarization scans show a bipolar structure in conformity with the bipolar magnetic field. However at 14.5 cm the circular polarization drops practically to zero and at longer wavelengths the sense of the polarization is reversed in {both components}. This was consistently observed through all our six days of observation while the region was moving from the center of the disk to the west limb. This unusual polarization inversion cannot be interpreted in terms of the well-known propagation effects under conditions of weak and strong coupling. Title: Radio signatures of magnetic restructuring during the 2000 July 14 major solar event Authors: Caroubalos, C.; Tsitsipis, P.; Kontogeorgos, A.; Moussas, X.; Alissandrakis, C. E.; Hillaris, A.; Preka-Papadema, P.; Polygiannakis, J.; Bougeret, J. -L.; Dumas, G.; Kurt, V.; Vassiliou, C.; Perche, C.; Gazeas, K.; Kolovos, G. Bibcode: 2002ESASP.505..377C Altcode: 2002IAUCo.188..377C; 2002solm.conf..377C We analyse the fine structure of the type IV radio burst, during the main phase of the 14th July 2000 major event recorded by the radiospectrograph ARTEMIS-IV, in the 110-650 MHz (10 Hz sampling rate) and the 270-450 MHz (100 Hz rate) range. In this report we focus on variations in frequency drift rate and repetition rate of fibers bursts. A pronounced asymmetry between the number of bursts with positive and negative frequency drift is noted at certain time periods. These variations follow closely the evolution of the magnetic structure associated with the major solar event and the onset of the corresponding CME. Title: Oscillations and waves near the limb from TRACE UV observations Authors: Alissandrakis, C. E.; Zachariadis, Th. G.; Gontikakis, C. 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: ARTEMIS IV Radio Observations of the 14 July 2000 Large Solar Event Authors: Caroubalos, C.; Alissandrakis, C. E.; Hillaris, A.; Nindos, A.; Tsitsipis, P.; Moussas, X.; Bougeret, J. -L.; Bouratzis, K.; Dumas, G.; Kanellakis, G.; Kontogeorgos, A.; Maroulis, D.; Patavalis, N.; Perche, C.; Polygiannakis, J.; Preka-Papadema, P. Bibcode: 2001SoPh..204..165C Altcode: 2010arXiv1009.3654C In this report we present a complex metric burst, associated with the 14 July 2000 major solar event, recorded by the ARTEMIS-IV radio spectrograph at Thermopylae. Additional space-borne and Earth-bound observational data are used, in order to identify and analyze the diverse, yet associated, processes during this event. The emission at metric wavelengths consisted of broad-band continua including a moving and a stationary type IV, impulsive bursts and pulsating structures. The principal release of energetic electrons in the corona was 15-20 min after the start of the flare, in a period when the flare emission spread rapidly eastwards and a hard X-ray peak occurred. Backward extrapolation of the CME also puts its origin in the same time interval, however, the uncertainty of the extrapolation does not allow us to associate the CME with any particular radio or X-ray signature. Finally, we present high time and spectral resolution observations of pulsations and fiber bursts, together with a preliminary statistical analysis. Title: Dopplershifts in the solar transition region Authors: Gontikakis, C.; Dara, H. C.; Alissandrakis, C. E.; Zachariadis, Th. G.; Vial, J. -C. 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: Ratan-600 observations of unusual inversion of polarization Authors: Gizani, N. A. B.; Alissandrakis, C. E.; Bogod, V.; Garaimov, V.; Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E. Bibcode: 2001A&AT...20..543G Altcode: No abstract at ADS Title: Multi-wavelength analysis of a solar active region loop system with SOHO, TRACE and ground-based telescopes Authors: Tsiropoula, G.; Gontikakis, C.; Dara, H. C.; Zachariadis, Th. G.; Alissandrakis, C.; Vial, J. -C. Bibcode: 2001hell.confE..40T Altcode: No abstract at ADS Title: Statistical analysis of type IV radio bursts fine structure observed during the 14th July 2000 large solar event. Authors: Tsitsipis, P.; Kontogeorgos, A.; Hillaris, A.; Polygiannakis, J.; Moussas, X.; Preka-Papadema, P.; Bouratzis, C.; Caroubalos, C.; Alissandrakis, C.; Maroulis, D.; Patavalis, N.; Bougeret, J. L.; Dumas, G.; Perche, C. Bibcode: 2001hell.confE..37T Altcode: No abstract at ADS Title: Statistics on the occurence of CMEs related radio bursts observed by ARTEMIS IV radio spectrograph during the maximum of the solar cycle 23. Authors: Bouratzis, C.; Desilas, C.; Hillaris, A.; Preka-Papadema, P.; Moussas, X.; Caroubalos, C.; Alissandrakis, C.; Maroulis, D.; Patavalis, N.; Polygiannakis, J.; Bougeret, J. -L.; Dumas, G.; Perche, C.; Kontogeorgos, A.; Tsitsipis, P. Bibcode: 2001hell.confE..36B Altcode: No abstract at ADS Title: Joint observations of solar radio bursts and cosmic ray events recorded by the ARTEMIS IV radio spectrograph and Athens Cosmic Ray Station (Super 6NM-64) of the University of Athens Authors: Caroubalos, C.; Hillaris, A.; Bouratzis, C.; Mavromichalaki, H.; Souvatzoglou, G.; Sarlanis, C.; Tatsis, S.; Maroulis, D.; Patavalis, N.; Polygiannakis, J.; Moussas, X.; Preka-Papadema, P.; Alissandrakis, C.; Bougeret, J. -L.; Dumas, G.; Perche, C.; Kontogeorgos, A.; Tsitsipis, P. Bibcode: 2001hell.confE..35C Altcode: No abstract at ADS Title: Fine Structure of the Quiet Solar Chromosphere: Limb-Crossing Features Authors: Zachariadis, Th. G.; Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S.; Gontikakis, C. 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: On-the-Disk Development of the Halo Coronal Mass Ejection on 1998 May 2 Authors: Pohjolainen, S.; Maia, D.; Pick, M.; Vilmer, N.; Khan, J. I.; Otruba, W.; Warmuth, A.; Benz, A.; Alissandrakis, C.; Thompson, B. J. Bibcode: 2001ApJ...556..421P Altcode: A halo coronal mass ejection (CME) was observed at 15:03 UT on 1998 May 2 by the Solar and Heliospheric Observatory Large-Angle Spectrometric Coronagraph. The observation of the CME was preceded by a major soft X-ray flare in NOAA Active Region 8210, characterized by a delta spot magnetic configuration and some activity in region 8214. A large transequatorial interconnecting loop (TIL) seen in the soft X-rays connected AR 8210 to a faint magnetic field region in the periphery of region 8214. Smaller loop systems were also connecting AR 8210 to other fainter bipolar magnetic structures, the interconnecting loop (IL) east of AR 8210 being one of the most visible. We present here a multiwavelength analysis of the large- and small-scale coronal structures associated with the development of the flare and of the CME, with emphasis placed on radio-imaging data. In the early phases of the flare, the radio emission sources traced the propagation paths of electrons along the TIL and the IL, which are accelerated in the vicinity of AR 8210. Furthermore, jetlike flows were observed in soft X-rays and in Hα in these directions. Significantly, the TIL and IL loop systems disappeared at least partially after the CME. An EUV Imaging Telescope (EIT) dimming region of similar size and shape to the soft X-ray TIL, but noticeably offset from it, was also observed. During the ``flash'' phase of the flare, new radio sources appeared, presenting signatures of destabilization and reconnection at discrete locations of the connecting loops. We interpret these as possible signatures of the CME liftoff on the disk. An Hα Moreton wave (blast wave) and an ``EIT wave'' were also observed, originating from the flaring AR 8210. The signatures in radio, after the wave propagated high into the corona, include type II-like emissions in the spectra. The radio images link these emissions to fast-moving sources, presumably formed at locations where the blast wave encounters magnetic structures. The opening of the CME magnetic field is revealed by the radio observations, which show large and expanding moving sources overlying the later-seen EIT dimming region. Title: Radio observations of filaments at metric and decimetric wavelengths Authors: Marqué, Ch.; Lantos, P.; Delouis, J. M.; Alissandrakis, C. E. Bibcode: 2001Ap&SS.277..329M Altcode: A filament eruption at decimetric wavelength is illustrated here, involving a quiescent filament seen in absorption. A CME occurs in the vicinity of the event. Title: Joint EUV/Radio Observations of a Solar Filament Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bastian, T.; Bocchialini, K.; Harrison, R. A. Bibcode: 2001SoPh..199..115C Altcode: In this paper we compare simultaneous extreme ultraviolet (EUV) line intensity and microwave observations of a filament on the disk. The EUV line intensities were observed by the CDS and SUMER instruments on board SOHO and the radio data by the Very Large Array and the Nobeyama radioheliograph. The main results of this study are the following: (1) The Lyman continuum absorption is responsible for the lower intensity observed above the filament in the EUV lines formed in the transition region (TR) at short wavelengths. In the TR lines at long wavelengths the filament is not visible. This indicates that the proper emission of the TR at the filament top is negligible. (2) The lower intensity of coronal lines and at radio wave lengths is due to the lack of coronal emission: the radio data supply the height of the prominence, while EUV coronal lines supply the missing hot matter emission measure (EM). (3) Our observations support a prominence model of cool threads embedded in the hot coronal plasma, with a sheath-like TR around them. From the missing EM we deduce the TR thickness and from the neutral hydrogen column density, derived from the Lyman continuum and He i absorption, we estimate the hydrogen density in the cool threads. Title: The New Multichannel Radiospectrograph ARTEMIS-IV/HECATE, of the University of Athens Authors: Caroubalos, C.; Maroulis, D.; Patavalis, N.; Bougeret, J. -L.; Dumas, G.; Perche, C.; Alissandrakis, C.; Hillaris, A.; Moussas, X.; Preka-Papadema, P.; Kontogeorgos, A.; Tsitsipis, P.; Kanelakis, G. Bibcode: 2001ExA....11...23C Altcode: We present the new solar radiospectrograph of the University of Athens operating at the Thermopylae Station since 1996. Observations cover the frequency range from 110 to 688 MHz. The radiospectrograph has a 7-meter parabolic antenna and two receivers operating in parallel. One is a sweep frequency receiver and the other a multichannel acousto-optical receiver. The data acquisition system consists of a front-end VME based subsystem and a Sun Sparc-5 workstation connected through Ethernet. The two subsystems are operated using the VxWorks real-time package. The daily operation is fully automated: pointing of the antenna to the sun, starting and stopping the observations at pre-set times, data acquisition, data compression by `silence suppression', and archiving on DAT tapes. The instrument can be used either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including the Nançay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequency receivers to study associated interplanetary phenomena. Title: Brightness Variations of mg ib Bright Features Authors: Dara, H. C.; Koutchmy, S.; Zachariadis, Th. G.; Alissandrakis, C. E. 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: Association of chromospheric sunspot umbral oscillations and running penumbral waves. I. Morphological study Authors: Tsiropoula, G.; Alissandrakis, C. E.; Mein, P. Bibcode: 2000A&A...355..375T Altcode: Observations of a sunspot region located near the center of the solar disk were obtained on October 3, 1994, with the Multichannel Subtractive Double Pass Spectrograph (MSDP). This instrument, operating in Hα , was installed at the focus of the VTT at Tenerife (Canary Islands) and provided Hα intensity profiles at every pixel of the field of view. Reconstruction of the Hα profile allowed the computation of two dimensional intensity and Doppler velocity images at different wavelengths within the line. We analyse a time series of 1 hour and 8 min, obtained with a cadence of 36 sec and investigate the relation between umbral oscillations and running penumbral waves. The Doppler velocity as a function of time, along radial cuts through the center of the spot, shows several clear cases where waves that originate inside the umbra continue to propagate in the penumbra. In one case we were able to follow the evolution of an oscillating element for 216 sec, from the inner part of the umbra to the penumbra and we describe the propagation characteristics. We confirm the close association between sunspot oscillations and running penumbral waves and suggest that they are probably due to the same resonator. Title: RATAN-600 Observations of Unusual Inversion of Polarization in Microwave Sources Authors: Alissandrakis, C.; Gizani, N.; Bogod, V.; Garaimov, V.; Gelfreikh, G.; Zheleznyakov, V.; Zlotnik, E. Bibcode: 1999ESASP.448.1225A Altcode: 1999mfsp.conf.1225A; 1999ESPM....9.1225A No abstract at ADS Title: The Expedition of the University of Ioannina and the Academy of Athens to Shabla, Bulgaria Authors: Alissandrakis, C. E.; Zachariadis, Th.; Nakas, Ch.; Ladoyanni, G. Bibcode: 1999ESASP.448.1287A Altcode: 1999mfsp.conf.1287A; 1999ESPM....9.1287A No abstract at ADS Title: Association between Umbral Oscillations and Running Penumbral Waves Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P. Bibcode: 1999ESASP.448..217A Altcode: 1999ESPM....9..217A; 1999mfsp.conf..217A No abstract at ADS Title: Structure and Dynamics in the Transition Region Authors: Gontikakis, C. P.; Dara, H. C.; Alissandrakis, C. E.; Zachariadis, Th. G.; Vial, J. -C.; Bastian, T.; Chiuderi Drago, F. Bibcode: 1999ESASP.448..297G Altcode: 1999mfsp.conf..297G; 1999ESPM....9..297G No abstract at ADS Title: Magnetic Field Diagnostics in the Low Corona from Microwave Circular Polarization Inversion Authors: Alissandrakis, C. E. Bibcode: 1999spro.proc...53A Altcode: The characteristics of the observed polarization of radio waves are determined by the emission mechanism and by the propagation conditions in the corona. In the case of weak coupling between the two electromagnetic wave modes, the polarization changes along the ray path, reflecting the local physical conditions; this results in inversion of the sense of circular polarization when a transverse field region (TFR) is crossed. On the contrary, if the wave coupling is strong, the polarization is fixed and its sense does not change when a TFR is crossed. As a result, Stokes V maps of active regions do not always correspond to the magnetic polarities shown in photospheric magnetograms. The differences depend on the wavelength, the heliographic position of the region, as well as on the density and the magnetic field of the corona, at about 0.1 Rsolar above the photosphere. In this short review I present older and recent observation of polarization inversion and will discuss the diagnostics they provide on the magnetic field. Title: Analysis of coronal emissions observed at meter wavelengths Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1999A&A...351..373L Altcode: We present an extensive analysis of large and medium scale radio sources observed with the Nançay Radioheliograph at 169 MHz, over more than four solar rotations in 1984 (during the declining phase of the solar cycle). The large scale emission is dominated by the coronal plateau, the radio counterpart of the coronal plasma sheet, which had an oscillatory structure around the equator. Medium scale local sources include both faint noise storm continua and thermal sources. Noise storm continua have greater brightness (8 105 to 3.8 106 K) and smaller diameter than thermal sources. We present distributions of brightness temperatures, sizes as well as longitude and latitude of the thermal sources and we study their location with respect to faculae and neutral lines. Based on their rotation rate, we estimate their altitude at 70 000 km. We find that most thermal sources at 169 MHz are located between faculae and neutral lines and we present a geometrical model to account for this observation, according to which these sources are located in the upper leg of large scale loops. A small number of thermal sources is closer to neutral lines; these might be loops at the base of isolated coronal streamers. Title: Metric and Decimetric Observations of the Quiet Solar Corona Authors: Marqué, Ch.; Lantos, P.; Delouis, J. M.; Alissandrakis, C. E. Bibcode: 1999ESASP.446..483M Altcode: 1999soho....8..483M We describe a multi-wavelength analysis of the solar corona in the metric and decimetric range for a period in May 1992. The four radio wavelengths, available with the Nancay Radioheliograph provide a 3-dimensional view of the corona from about 1.05 to 1.2 solar radius. Some of the radio features are relative to the large scale structure of the corona (coronal plateau, coronal holes) and to medium scale structures (streamers) we can follow during their transit on the disk. We also observe thermal emission associated to active regions and we compare locations of radio emissions with structures of the X-ray loops. Title: Magnetic Field Diagnostics from Microwave Circular Polarization Inversion Authors: Alissandrakis, C. E. Bibcode: 1999ASPC..184...23A Altcode: The characteristics of the observed polarization of radio waves are determined by the emission mechanism and by the propagation conditions in the corona. In the case of weak coupling between the two electromagnetic wave modes, the polarization changes along the ray path, reflecting the local physical conditions; this results in inversion of the sense of circular polarization when a transverse field region (TFR) is crossed. On the contrary, if the wave coupling is strong, the polarization is fixed and its sense does not change when a TFR is crossed. In this article I present older and recent observation of polarization inversion and discuss the diagnostics they provide on the magnetic field. Title: Erratum: "Fine structure of the solar chromosphere: arch-shaped mottles" [Sol. Phys., Vol. 184, No. 1, p. 77 - 86 (Jan 1999)]. Authors: Zachariadis, T. G.; Georgakilas, A. A.; Koutchmy, S.; Dara, H. C.; Alissandrakis, C. E. Bibcode: 1999SoPh..187..227Z Altcode: No abstract at ADS Title: Coronal Magnetography of an Active Region From Microwave Polarization Inversion Authors: Ryabov, B. I.; Pilyeva, N. A.; Alissandrakis, C. E.; Shibasaki, K.; Bogod, V. M.; Garaimov, V. I.; Gelfreikh, G. B. Bibcode: 1999SoPh..185..157R Altcode: The microwave circular polarization of the active region (AR) NOAA 7260 on 21-23 August 1992 is analyzed. Two-dimensional images at 1.76 cm with spatial resolution of θ=10'' from the Nobeyama radioheliograph and one-dimensional scans at 9 wavelengths in the range of 1.81-3.43 cm and θ=16.3''-31.1'' from the radio telescope RATAN-600 were used. An inversion of the sense of circular polarization through the wavelength range was recorded on 22 August. It is shown that both the wavelength and the time dependence of the inversion are consistent with quasi-transverse (QT) propagation of the radiation in the solar corona. From this, the strength of the coronal magnetic field in the active region was found to be H=20-65 G at a height of h= (5.7-8.7)×109 cm above the photosphere on 22 and 30 August and 125 G at the lower height of (3.7-6.4)×109 cm on 23 August. We present a new technique, based on the radio mapping (in both Stokes I and V) of an AR undergoing circular polarization inversion; applying this method to the Nobeyama data we obtained, for the first time, a magnetogram of the coronal magnetic field. For AR 7260 we found values in the range of 70-100 G at heights of (4-6)×109 cm on 23 August, adopting a constant value of N Lα (where N is the electron density and Lα is the scale of the coronal field divergence) of 2.5×1018 cm−2. We compare our results with force-free extrapolations of the photospheric magnetic field from a MSFC magnetogram obtained on 20 August. Title: Dynamics of subrelativistic electron beams in the solar corona. Type III group analysis Authors: Hillaris, A.; Alissandrakis, C. E.; Bougeret, J. -L.; Caroubalos, C. Bibcode: 1999A&A...342..271H Altcode: A number of type III groups is studied. Wave particle interactions are neglected and the so called drift approximation is used to model the electron beam evolution. We extrapolated the observed Flux-Time profiles towards higher frequencies and estimated the injection time for the individual beams responsible for the excitation of the type III's. A simple clustering algorithm, based on the temporal separation of nearest neighbour injections as a proximity criterion, was used to identify patterns of burst subgroups within the type III groups. The question of whether the acceleration and injection of component bursts exciters is coherently modulated in a single source, or this process is driven by a statistical flare in a spatially fragmented energy release site, is addressed. Title: Polar surges and macrospicules: simultaneous Hα and He BT II 304 Angstroms observations Authors: Georgakilas, A. A.; Koutchmy, S.; Alissandrakis, C. E. Bibcode: 1999A&A...341..610G Altcode: We compare simultaneous sequences of Hα and He ii 304 Angstroms images near the solar limb. We propose to distinguish polar surges and giant spicules (macrospicules), among the He ii structures observed beyond the solar limb. Polar surges have a complex structure when observed in Hα and an eruptive nature, reminiscent of normal surges in small scale. Giant spicules are simple spikes, very narrow compared to surges. They don't go as high as polar surges and apparently have shorter lifetimes. We found that most polar surges and giant spicules observed in He ii are associated with Hα spikes; however there were cases without any corresponding Hα structure. Furthermore we observed Hα spikes that did not have He ii counterparts. We analyze an example of a polar surge that appeared in He ii well before its appearance in Hα and remained visible for a longer time. Most polar surges that we observed showed a similar behavior. Title: Gradient of the line-of-sight velocities in the dark superpenumbral fibrils. Authors: Tsiropoula, G.; Alissandrakis, C. E.; Mein, P.; Mein, N. Bibcode: 1999joso.proc..116T Altcode: An isolated sunspot located near the centre of the solar disk was observed on October 3, 1994. The present high resolution observations were obtained with the Multichannel Subtractive Double Pass Spectrograph (MSDP) operating in Hα. Two dimensional intensity and Doppler velocity maps at several depths in the Hα line were computed. The monochromatic intensities at two wavelengths on either side of the line are used in terms of the photographic subtraction method expressing the "Doppler signal" to give, under some assumptions, the velocities at different depths in the Hα line. The values of the velocities obtained by this method are compared to those given by the bisector method while comparison of the velocities at different depths can give a picture of the gradient of the velocity in the dark superpenumbral fibrils. Title: Mg bright points and the corresponding velocity pattern. Authors: Dara, H. C.; Zachariadis, T.; Alissandrakis, C.; Koutchmy, S. 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: Correction [to “A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU”] Authors: Bougeret, J. -L.; Zarka, P.; Caroubalos, C.; Karlický, M.; Leblanc, Y.; Maroulis, D.; Hillaris, A.; Moussas, X.; Alissandrakis, C. E.; Dumas, G.; Perche, C. Bibcode: 1998GeoRL..25.4103B Altcode: No abstract at ADS Title: Microwave, Soft and Hard X-Ray Observations of Solar Flares - a Self-Consistent Model of the Flare Site Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Bentley, R. D.; Philips, A. T. Bibcode: 1998SoPh..182..459C Altcode: High-resolution microwave observations of several flares performed with the Westerbork Synthesis Radio Telescope (WRST) on 3 and 4 July 1993 are compared with Yohkoh observations in the soft and hard X-ray domain. Only for one flare, among the six analyzed, was the hard X-ray spectrum between 20 and 200 keV available from the Wide Bragg Spectrometer, supplying the energy spectrum of non-thermal particles responsible for this radiation and for the radio emission. A complete model of this flare is derived which accounts for all available observations in the X-ray and radio wavelengths. Title: A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU Authors: Bougeret, J. -L.; Zarka, P.; Caroubalos, C.; Karlický, M.; Leblanc, Y.; Maroulis, D.; Hillaris, A.; Moussas, X.; Alissandrakis, C. E.; Dumas, G.; Perche, C. Bibcode: 1998GeoRL..25.2513B Altcode: We present for the first time an almost complete frequency coverage of a Shock Associated (SA) radio event and related phenomena observed on May 6, 1996 at 9:27 UT. It is observed from the base of the solar corona up to almost 1 Astronomical Unit (AU) from the Sun by the following radio astronomical instruments: the Ondřejov spectrometer operating between 4.5 GHz and 1 GHz (radiation produced near the chromosphere); the Thermopyles Artemis-IV spectrograph operating between 600 MHz and 110 MHz (distance range about 1.1-1.4 R from sun center); the Nançay Decameter Array operating between 75 and 25 MHz (distance range about 1.4-2 R) and the RAD2 and RAD1 radio receivers on the WIND spacecraft covering the range from 14 MHz to about 20 kHz (distance range between 3 R and about 1 AU). Observations at the Nançay Decameter Array clearly show that the SA event starts from a coronal type II radio burst which traces the progression of a shock wave through the corona above 1.8 R-2 R from the sun center. This SA event has no associated radio emission in the decimetric-metric range, thus there is no evidence for electron injection in the low/middle corona. Title: Umbral and Penumbral Waves in a Chromospheric Sunspot Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P. Bibcode: 1998ASPC..155...49A Altcode: 1998sasp.conf...49A No abstract at ADS Title: Second Advances in Solar Physics Euroconference: Three-Dimensional Structure of Solar Active Regions Authors: Alissandrakis, Costas E.; Schmieder, Brigitte Bibcode: 1998ASPC..155.....A Altcode: 1998sasp.conf.....A No abstract at ADS Title: EUV and Microwave Observations of a Filament Authors: Chiuderi-Drago, F.; Bocchialini, K.; Lamartinie, S.; Vial, J. -C.; Bastian, T.; Alissandrakis, C. E.; Harrison, R.; Delaboudiniere, J. -P.; Lemaire, P. Bibcode: 1998ASPC..150...55C Altcode: 1998IAUCo.167...55C; 1998npsp.conf...55C No abstract at ADS Title: Quiescent filament diagnostic. Authors: Lamartinie, S.; Bocchialini, K.; Vial, J. -C.; Chiuderi-Drago, F.; Harrison, R.; Alissandrakis, C. E. Bibcode: 1998joso.proc..139L Altcode: UV observations of a quiescent filament were performed on July 28, 1996 by SUMER and CDS, two spectrometers onboard SOHO. Title: Spicules and Macrospicules: Simultaneous Hα and He II (304 Å) Observations Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis, C. E.; Koutchmy, S.; Delannée, C.; Delaboudinière, J. -P.; Hochedez, J. -F. Bibcode: 1998ASPC..155..376G Altcode: 1998sasp.conf..376G No abstract at ADS Title: Sunspot-associated Sources, a Peculiar Source and a Halo-like Source as Basic Components of the 3D Structure of a Large Active Region from RATAN-600 and SSRT Observations Authors: Kaltman, T. I.; Korzhavin, A. N.; Peterova, N. G.; Lubyshev, B. I.; Maksimov, V. P.; Alissandrakis, C. E.; Fu, Q. Bibcode: 1998ASPC..155..140K Altcode: 1998sasp.conf..140K No abstract at ADS Title: Unusual inversion of polarization in sunspot associated microwave sources: RATAN-600 observations and their interpretation Authors: Alissandrakis, C.; Gizani, Nectaria A. B.; Bogod, V.; Garaimov, V.; Zheleznyakov, V.; Zlotnik, E. Bibcode: 1998cee..workE..72A Altcode: Results of RATAN-600 observations of unusual sunspot-associated microwave sources on May 12-16, 1996 and their interpretation are given. The intensity and circular polarization of radio emission were recorded at 39 frequencies in the wave band 1.74 cm - 30 cm. The peculiarity of polarizational pattern is that the microwave source was bipolar one, and at the wavelength about 15 cm the sense of circular polarization reversed for both sunspots. The most important and unexpected feature is that the frequency of reversal is the same for preceding and following spots, and the picture as a whole was invariable during 5 days of observations. It is easy to see that the picture can't be explained by standard effect of linear coupling of electromagnetic modes in transverse magnetic field originating from bipolar group, because the reversal of polarization occurred at both parts of bipolar source and the picture was invariable for several days. It also can't be due to coupling in a neutral current sheet, since transformation coefficient in zero magnetic field is independent on frequency. Moreover, it can't be due to linear coupling in transverse magnetic field in current sheet (in a case when magnetic field in current sheet has small transverse component which makes waves to couple not in zero, but transverse magnetic field), because the angle in which reversal can take place is extremely small, and the picture must vary with solar rotation. So, the only conceivable way to explain observed polarization by linear coupling is to consider magnetic field perpendicular to the plane connecting the spots of the group, that is parallel to a solar meridian. It is obviously that only such a field can provide independence of polarizational pattern on solar rotation and reversal at the same frequency for both parts of the group. We cannot say where this field originates from: it may be a general magnetic field of the Sun (the form of magnetic field created by vertical dipole in the center of the Sun suits observational pattern) or some large-scale structure in the upper corona. Numerical estimations are quite appropriate for reversal at 15 cm. This work is supported by INTAS Grant No.94-4625 and partially by RFBR Grants No.98-02-16238 and No.96-02-16598. Title: Three-dimensional structure of solar active regions. 2nd Euroconference ASPE97 (Advances in Solar Physics Euroconference 1997), Preveza (Greece), 7 - 11 Oct 1997. Poster contributions. Authors: Schmieder, B.; Alissandrakis, C. Bibcode: 1998joso.proc...78S Altcode: All of the oral presentations of ASPE97 will be published in a separate volume of the ASP Conference Series, while this volume contains the poster presentations. Title: Kinematics of Hα spicules from near-limb observations. Authors: Zachariadis, T. G.; Georgakilas, A. A.; Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. 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: Unusual inversion of polarization in microwave sources associated with sunspot group: RATAN-600 observations and their interpretation. Authors: Alissandrakis, C. E.; Bogod, V. M.; Garaimov, V.; Gelfreikh, G. B.; Zheleznyakov, V. V.; Zlotnik, E. Bibcode: 1998joso.proc..101A Altcode: The data of multi-wavelength observations (in the range from 1.7 cm to 32 cm) on the reflector type radiotelescope RATAN-600 were obtained with high polarization accuracy up to 0.5%. They detect a new polarization phenomenon in radio emission of active region on the Sun. The signs of polarization of the leading and following spot associated sources are changed simultaneously for both sources at decimeter part of the spectrum. One of the interpretation versions of the phenomenon is proposed. Title: Magnetic and velocity field in association with Ellerman bombs. Authors: Dara, H. C.; Alissandrakis, C. E.; Zachariadis, T. G.; Georgakilas, A. A. 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: Modeling of a Microwave Burst Emission Authors: Preka-Papadema, P.; Alissandrakis, C. E.; Dennis, B. R.; Kundu, M. R. Bibcode: 1997SoPh..172..233P Altcode: 1997ESPM....8..233P We applied model computations on a microwave burst observed with the Westerbork Synthesis Radio Telescope (WSRT) at 6 cm. We used additional data from Hα, soft and hard X-rays in order to reproduce the flaring loop and to compute the microwave total intensity and circular polarization. We examined both cases of thin and thick target. The computations show a large emission source in an optically thick loop. We compare our results with the observation. Title: Artemis Mark-IV, the New Greek-French Digital Radio Spectrograph at Thermopyles, Greece Authors: Maroulis, D.; Dumas, G.; Caroubalos, C.; Bougeret, J. L.; Moussas, X.; Alissandrakis, C.; Patavalis, N. Bibcode: 1997SoPh..172..353M Altcode: 1997ESPM....8..353M We present the new digital solar radio spectrograph located at the Thermopyles station, Greece, operated by the University of Athens. Observations cover the range from 110 to 600 MHz, using a 7-m parabolic antenna. The reception system uses two techniques in parallel: sweep frequency and multi-channel, the latter being based on the Acousto-Optical technique. The data acquisition system is based on two subsystems, a Sun Sparc-5 workstation and a front end based on a VME Motorola system. The two subsystems are connected through the Ethernet and are operated using the VxWorks real-time package. The daily operation is completely automated: pointing of the antenna to the sun, starting and stopping the observations at pre-set times, acquiring data, compressing data by silence suppression in real time, and archiving the data on a routine manner on DAT tapes. Apart from its usual function, this instrument will be used in conjunction with other instruments, including the Nançay decameter array and the low frequency radio receivers on the Wind spacecraft. Title: Fine Structure of the solar atmosphere from near-limb observations in three wavelengths Authors: Georgakilas, A. A.; Dara, H.; Zachariadis, Th.; Alissandrakis, C. E.; Koutchmy, S. 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: Evolution in Space and Time of Superpenumbral Chromospheric Fibrils Authors: Tsiropoula, G.; Dialetis, D.; Alissandrakis, C. E.; Mein, P.; Mein, N. Bibcode: 1997SoPh..172..139T Altcode: 1997ESPM....8..139T We have studied the spatial structure and temporal evolution of the intensity and Doppler velocity of dark fibrils forming the superpenumbra of an isolated regular sunspot. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph which operates in Hα and is installed at the focus of the Vacuum Tower Telescope (VTT) at Tenerife (Canary Islands). The fibril pattern shows a remarkable stability during the period of our observations (64 min). Moreover, almost all individual fibrils are identifiable in all frames, but they undergo continual changes in contrast, shape and size. Investigating the temporal evolution of intensity and velocity of individual fibrils, fluctuations were found which have a quasi periodic behavior. As mechanisms for these changes we may suggest (a) change of the Doppler shift due to a wave, (b) periodic changes of the density of the Hα absorbing material, (c) disappearance and reappearance of fibrils, in more or less the same magnetic flux tube, at regular intervals. Title: Radio Observations of the Quiet Sun and Their Implications on Coronal Heating Authors: Alissandrakis, Costas E.; Einaudi, Giorgio Bibcode: 1997LNP...483...53A Altcode: 1997cprs.conf...53A We discuss the actual and potential contribution of radio techniques in the study of the structure and the dynamics of the corona as well as in the problem of coronal heating. Radio observations provide powerful diagnostics of the physical conditions in the transition region and the corona. Recent observational and theoretical results are presented and their implications are discussed. The prospects of further observations, in conjunction with other wavelength ranges are given. Title: Testing the Extrapolated Coronal Magnetic Fields by Using Propagation Effects at Microwaves Authors: Chiuderi Drago, F.; Alissandrakis, C. E.; Borgioli, F. Bibcode: 1997ESASP.404..275C Altcode: 1997cswn.conf..275C No abstract at ADS Title: Radio Observations of Sunspots Authors: Alissandrakis, C. E. Bibcode: 1997ASPC..118..150A Altcode: 1997fasp.conf..150A High spatial resolution observations of sunspots have been made with aperture synthesis instruments for over two decades. Sunspots appear as bright features at short cm wavelengths, due to thermal gyroresonance (g-r) emission from layers where the local gyrofrequency is a sub-harmonic of the observing frequency, providing a powerful diagnostic of the sunspot magnetic field above the photosphere. Title: Medium and large scale structures of the low corona from decimeter and meter wavelength observations. Authors: Lantos, P.; Coulais, A.; Alissandrakis, C. E. Bibcode: 1997ASIC..494..121L Altcode: 1997topr.conf..121L Since 1991, the Nançay Radioheliograph provides daily observations of the Sun at wavelengths from 69 cm (435 MHz) to 1.83 m (164 MHz). Older observations were available mostly at 169 or 164 MHz. Two-dimensional maps could be computed with an aperture synthesis method when the brightness distribution remains constant during the day (i.e. in absence of strong noise storms). At meter wavelengths, detected large scale structures are coronal holes, the quiet sun and a new component called "coronal plateau" which is a signature at low altitude of the coronal streamer belt and of the heliospheric current sheet. Medium scale structures observed at meter and decimeter wavelengths are explained as emissions in loop systems. They may be either faint noise storm continua or thermal emissions. Title: Microwave and UV observations of filaments with SOHO and the VLA Authors: Alissandrakis, C. E.; Drago, F.; Bastian, T.; Bocchialini, K.; Delaboudiniere, J. -P.; Lemaire, P.; Vial, J. -C.; Harrison, R. A.; Thompson, B. Bibcode: 1997ASPC..118..289A Altcode: 1997fasp.conf..289A Observations performed in coordination between SOHO instruments and ground-based observatories offer the unique possibility to derive information simultaneously in several wavelengths formed at different altitudes and/or temperatures in the solar atmosphere. The SUMER and CDS spectrometers, the imaging telescope EIT aboard SOHO, and the VLA provide complementary information in the UV and the radio ranges. We illustrate such a coordination with observations of filaments in the transition region, performed in July 1996. The observations in the UV between 10(4) and 10(6) K provide the differential emission measure as a function of temperature; this can be used to compute the expected brightness temperature in the microwave range and check models of the filament-corona transition region. Title: Coronal Magnetic Fields from Microwave Polarization Observations Authors: Alissandrakis, C. E.; Borgioli, F.; Chiuderi Drago, F.; Hagyard, M.; Shibasaki, K. Bibcode: 1996SoPh..167..167A Altcode: The solar active region (AR) 7530 was observed at 6 cm on July 3 and 4, 1993 with the Westerbork Synthesis Radio Telescope, using a multi-channel receiver with very narrow bandwidth. We compare the radio data with Yohkoh SXT observations and with the magnetic field extrapolated from the Marshall vector magnetograms in the force-free and current-free approximations. The comparison with soft X-rays shows that, although a general agreement exists between the shape of the radio intensity map and the X-ray loops, the brightness temperature, Tb, obtained using the parameters derived from the SXT is much lower than that observed. The comparison with the extrapolated photospheric fields shows instead that they account very well for the observed Tb above the main sunspots, if gyroresonance emission is assumed. In the observation of July 4 an inversion and strong suppression of the circular polarization was clearly present above different portions of the AR, which indicates that particular relationships exist between the electron density and the magnetic field in the region where the corresponding lines of sight cross the field quasi-perpendicularly. The extrapolated magnetic field at a much higher level (∼ 1010 cm), satisfies the constraints required by the wave propagation theory all over the AR. However, a rather low electron density is derived. Title: Periodic and Non-Periodic Phenomena in a Sunspot Region Authors: Tsiropoula, G.; Alissandrakis, C. E.; Dialetis, D.; Mein, P. Bibcode: 1996SoPh..167...79T Altcode: We have studied running penumbral waves, the homogeneous Evershed effect, and the spatial relation between intensity and Doppler velocity penumbral features of a chromospheric sunspot. The observations were obtained with the multichannel subtractive double-pass spectrograph (MSDP) operating in Hα at the Vacuum Tower Telescope (VTT) installed at Tenerife (Canary Islands). We derived intensity and Doppler velocity maps at Hα ± 0.3 Å over a two-dimensional field of view. We have computed the components of the velocity vector (radial, azimuthal, vertical) as a function of distance from the center of the spot under the assumption of axial symmetry. The results show the well-known, from previous observations, general large-scale characteristics of the chromospheric Evershed flow. Our measurements show that the axes along the discrete structures, where the Evershed flow is confined, are not spatially related to the axes along Hα ± 0.3 Å intensity features, and we suggest that either the flow is confined in flow channels or that it takes place along sheared magnetic field lines. We also detected, for the first time in velocity images, running penumbral waves, which started in the outer 0.3 of the umbral radius and propagated through the penumbra with propagation velocities 13-24 km s−1. The propagation velocity, as well as the velocity amplitude, is greater for the waves closer to the center of the spot and diminishes as one moves outward. Title: A Model for Active Region Emission at Centimeter Wavelengths Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Kundu, M. R.; Dere, K. P.; Korzhavin, A. N.; Bogod, V. M. Bibcode: 1996SoPh..166...55N Altcode: We present multi-frequency observations and model computations of the microwave emission of a solar active region. The radio observations were obtained with the RATAN-600 at several wavelengths between 0.8 and 31.6 cm and with the VLA at 6 and 20 cm. The active region was also observed in the EUV O Iv lines by the HRTS instrument aboard the Space Shuttle Spacelab-2 mission. These lines are formed in the chromosphere-corona transition region and their intensity ratio is sensitive to pressure. Photospheric magnetograms provided both the longitudinal and the transverse component of the magnetic field. The microwave observations were checked against model computations taking into account both the free-free and the gyro-resonance emission mechanisms and using the pressure data from the O IV lines. The magnetic field was computed through constant-α force-free extrapolations of the longitudinal photospheric field. We computed both the flux from 2 to 20 cm and the spatial structure of the microwave emission at 6 and 20 cm. The comparison of the computed and observed flux spectra allowed us to estimate the magnetic field strength at the base of the transition region and in the low corona, as well as the values of the conductive flux and the height of the base of the transition region. The model maps at 6 cm and 20 cm showed that α was not constant above the active region; the same conclusion was reached on the basis of the photospheric observations. The use of pressure measurements allowed us to identify microwave structures which were determined by pressure enhancements. At 6 cm the computations confirmed the fact that the magnetic field is the principal factor that determines the structure of sunspot-associated sources and showed that the effect of pressure variations was small. Pressure variations were more important at 20 cm, where the peak of the emission was associated with the sunspot and a diffuse component was associated with the plage which had an average pressure higher by a factor of 1.54 than the sunspot. Title: Synoptic Study of Coronal Structures Observed at Meter Wavelengths during the Declining Phase of the Solar Cycle Authors: Alissandrakis, C. E.; Lantos, P. Bibcode: 1996SoPh..165...61A Altcode: We have produced synoptic maps from daily images of the solar corona on the disk obtained with the Nançay Radioheliograph at 169 MHz during June-August 1984. We discuss briefly the structures seen on the synoptic maps and we compare the location of radio sources with photospheric neutral lines and Ha filaments derived from synoptic charts from Meudon, Boulder, and Stanford. We measured the distance of radio sources from these optical features and compared their distribution with computed random distributions. A criterion of confidence is proposed which, when applied to our data, shows that the metric radio sources of thermal origin are best associated with the large-scale neutral lines shown on the low resolution Stanford magnetograms. This association indicates that the emission comes from arcades of moderately dense loops spanning the neutral line. However, the radio sources are not usually located directly above or very close to neutral lines, but their distribution peaks at distances between 4 and 6°. Our results show no statistically significant association with Meudon filaments. Title: Coronal Sources at Meter and Optical Wavelengths during the Declining Phase of the Solar Cycle Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1996SoPh..165...83L Altcode: We present observations of the corona at 169 MHz with the Nançay Radioheliograph during the summer of 1984. We compare synoptic maps of the metric radio emission on the solar disk with synoptic charts of the K-corona as well as of the green and the red lines. Local sources of radio emission are not located near regions of enhanced green or red line emission which, in turn, are in general above chromospheric faculae. Thus the radio emissions located in the surroundings of faculae are apparently related to different loop systems, with lower density. The comparison of the radio data with the K-corona showed one radio source associated with enhanced emission both at 1.3 and at 1.7 R, apparently a streamer. Other radio sources did not show any clear associations, but were nevertheless located within the coronal plasma sheet, delineated by the large-scale K-corona emission. Moreover the large-scale structure of the corona at 169 MHz was quite similar to the coronal plasma sheet observed at 1.3 R above the limb. The extent of the radio emission in latitude is very similar to that of the K-corona, while the coronal line emission is more concentrated near the solar equator. Title: Two-Dimensional Mapping of the Sun with the RATAN-600 Authors: Nindos, A.; Alissandrakis, C. E.; Gelfreikh, G. B.; Borovik, V. N.; Korzhavin, A. N.; Bogod, V. M. Bibcode: 1996SoPh..165...41N Altcode: We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm computed from one-dimensional observations with the RATAN-600, using Earth rotation aperture synthesis techniques. Before the calculation of maps, the position of each scan was corrected with respect to the center of the solar disk and the scans were calibrated. The circular polarization scans were corrected for polarization cross-talk between the I and V channels. Subsequently, the quiet-Sun background emission was subtracted. After all corrections, a dirty map was computed by combining the scans at different position angles. The last step of the processing was an attempt to free the dirty map of the sidelobes, using the standard CLEAN procedure. The resolution of the clean maps at 2.7 cm was 0.5' by 6'. Both active regions which were present on the solar disk were mapped. We studied the flux spectra of different types of sources: one was associated with a sunspot, the second was located over the neutral line of an active region, and the other was associated with the plage. The emission mechanism of the former was attributed to the gyroresonance process, while the short wavelength emission of the others was attributed to the free-free process. For the sunspot-associated source we estimated the magnetic field strengths at the base of the transition region and in the low corona. Title: Large scale structure of the solar corona observed at meter and decimeter wavelengths and in X-rays Authors: Lantos, P.; Alissandrakis, C. E.; YOHKOH Team Bibcode: 1996AdSpR..17d.261L Altcode: 1996AdSpR..17..261L Maps of the quiet solar corona, obtained at meter and decimeter wavelengths with the Nancay (France) Radioheliograph, are compared to Yohkoh soft X-ray images in order to study large scale density structures of the corona. Of particular interest is a global restructuring of the corona, observed on 19 May 1992 in X-rays, in conjunction with a coronal mass ejection. The radio observations show the CME moving continuum, while the Yohkoh images show the slower motion of the X-ray loops. After this event the appearance of a new noise storm, with a duration of several days, was observed. Title: Running Penumbral Waves in a chromospheric Sunspot Authors: Tsiropoula, G.; Dialetis, D.; Alissandrakis, C. E.; Mein, P. Bibcode: 1996hell.conf...37T Altcode: No abstract at ADS Title: Coronal Magnetic Fields from Polarization Observations at Microwaves Authors: Chiuderi Drago, F.; Borgioli, F.; Alissandrakis, C. E.; Hagyard, M.; Shibasaki, K. Bibcode: 1996mpsa.conf..443C Altcode: 1996IAUCo.153..443C No abstract at ADS Title: The fine scale Structure of the velocity Field in the chromospheric Penumbra of a solar Sunspot Authors: Tsiropoula, G.; Alissandrakis, C. E.; Dialetis, D.; Mein, P. Bibcode: 1996hell.conf...32T Altcode: No abstract at ADS Title: Evolution of the coronal plasma sheet from cycle maximum to cycle minimum observed at meter wavelengths Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1995AdSpR..16i.185L Altcode: 1995AdSpR..16..185L The Nancay Radioheliograph provides maps of the quiet solar corona around 160 MHz. In addition to coronal holes, localized emission sources and quiet sun, an unresolved component can be detected on the disk. The so-called ``coronal plateau'' corresponds to the K-corona dense regions observed on the limb and is thus located at the base of the interplanetary plasma sheet. Its evolution is studied from 1980 to 1986, during the declining phase of the cycle 21. Title: Large Scale Structure of the Solar Corona in the Declining Phase of the Solar Cycle Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1995SSRv...72...45L Altcode: Maps of the corona, obtained at meter wavelengths with the Nançay Radioheliograph (France), are used to study, on the disk, the radio counterpart of the coronal plasma sheet observed in K-corona on the limb. We study here the evolution of the coronal plasma sheet from the maximum of the activity cycle in 1980 to the minimum in 1986 and identify some of its large scale structures. Title: Large Scale Structure of the Solar Corona in the Declining Phase of the Solar Cycle Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1995hlh..conf...45L Altcode: No abstract at ADS Title: Dynamical fine structures of the chromosphere Authors: Tsiropoula, G.; Schmieder, B.; Alissandrakis, C. E. Bibcode: 1994SSRv...70...65T Altcode: We have studied the spatial structure and temporal evolution of a chromospheric region with chains of mottles at the junction of three supergranules. The observations were obtained with the Multichannel Subtractive Double Pass spectrograph operating in Hα at the Pic du Midi Observatory. From a statistical point of view the fine structures were stable in intensity over the observation period (15 min), while the line of sight velocity showed remarkable changes within a few minutes. A detailed analysis shows that the predominant pattern of bulk motion in the dark mottles is that of downflow at their footpoints and upflow at their tops, which is followed by downflows along the whole structure and that this process repeats itself several times during their lifetime. Title: Time evolution of fine structures in the solar chromosphere. Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B. Bibcode: 1994A&A...290..285T Altcode: We have studied the temporal evolution of two quiet chromospheric regions, one with a typical rosette and another with chains of mottles at the junction of three supergranules. The observations were obtained during 15 minutes with the Multichannel Subtractive Double Pass spectrograph (MSDP) operating in Hα at the Pic du Midi Observatory. We derived intensity maps and Doppler shift velocities at different wavelengths along the Hα profile over a two dimensional field of view. The observed contrast profiles were matched with theoretical contrast profiles using Beckers' cloud model for a more accurate determination of the line of sight velocity. A statistical analysis with cross correlation functions showed that the fine structures were stable in intensity over the observation period (15 min), but the line of sight velocity showed important changes within a few minutes. A detailed analysis of the velocities along the axes of dark mottles showed that the predominant pattern of bulk motion is that of downflow at their footpoints and alternating phases of upflow and downflow at their tops. This motion is consistent with Pikel'ner's model for spicules, which attributes this pattern to the reconnection of opposite magnetic filed lines. This picture is also consistent with the velocity reversals with time observed in spicules and may be associated to the systematic downflows observed in the transition region. Doppler shift velocities in dark mottles are too low compared to those derived with the cloud model; the latter are comparable to those reported for spicules, strengthening the view that these structures are identical. Title: Detection of Linear Polarization in the Microwave Emission of Solar Active Regions Authors: Alissandrakis, C. E.; Chiuderi-Drago, F. Bibcode: 1994ApJ...428L..73A Altcode: We report the detection for the first time of linear polarization in the microwave emission above a Solar Active Region. The observations were made with the Westerbork Synthesis Radio Telescope, taking advantage of the very small bandwidth of a multichannel spectral line receiver. The intensity of the Stokes parameter U, measured at several points close to the line of zero circular polarization, shows a clear sinusoidal trend as a function of lambda2, in accordance to what is expected from Faraday rotation. From the measured period, reasonable values of integral NBLdr were derived. Such observations could present in the future a unique way to measure coronal magnetic fields above Active Regions. Title: Meter wavelength observations of the quiet sun and the slowly varying component during declining phase of the solar cycle Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1994AdSpR..14d..97L Altcode: 1994AdSpR..14...97L Two dimensional maps of the Sun at 164 MHz, obtained with the Nançay Radioheliograph from April to August 1984 are presented. The quiet sun emission and the slowly varying component far from sunspots are studied. The association of local emissions with the chromospheric calcium plages is pointed out and interpreted. The altitude of the radio sources is estimated. The comparison of the location of the sources with K-corona observations confirms previous results obtained for the minimum of the cycle. The brightness temperature of the coronal plateau, which is the radio counterpart of the coronal plasma sheet, is given. Title: Radio observations of the quiet solar corona Authors: Alissandrakis, C. E. Bibcode: 1994AdSpR..14d..81A Altcode: 1994AdSpR..14...81A Observations of radio emission from the quiet solar atmosphere and the associated emission mechanisms are discussed. Long wavelength observations, with a spatial resolution of ~1 arc min are ideal for the study of the large scale structure of the corona, including coronal holes, arches, streamers and the base of the heliospheric current sheet. These structures are observed both beyond the limb and on the solar disk, with little line of sight integration. We also discuss diagnostics of electron temperature and density, as well as the interpretation problems arising from the inhomogeneity of the corona and refraction effects. Title: The magnetic field of the solar corona. Authors: Alissandrakis, C. E. Bibcode: 1994LNP...432..109A Altcode: 1994LNPM...11..109A The structure of the solar corona, on all observable scales, is intimately controlled by the magnetic field. Although direct measurements of the magnetic field are from difficult to impossible, its presence is evident in all spectral ranges where the corona is observable. This review discusses the measurement of coronal magnetic fields and the indirect information provided by radio, white light and soft X-ray observations. Title: Time Evolution of Chromospheric Fine Structures Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B. Bibcode: 1994emsp.conf..111T Altcode: No abstract at ADS Title: The Magnetic Field of a Bipolar Sunspot Region in the Photosphere and the Chromosphere Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E. Bibcode: 1994emsp.conf...61D Altcode: No abstract at ADS Title: Evidence for Ordinary Mode Emission from Microwave Bursts Authors: Alissandrakis, C. E.; Nindos, A.; Kundu, M. R. Bibcode: 1993SoPh..147..343A Altcode: We analyze high-resolution, one-dimensional observations of simple microwave bursts, obtained at 4.9 GHz with the Westerbork Synthesis Radio Telescope in 1980, together with Hα photographs of the associated flares from the Observatories of Athens and Meudon. In most cases the polarization structure can be interpreted in terms of extraordinary mode emission, taking into account the polarity of the underlying magnetic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward part of the flaring loop. We found evidence for ordinary mode emission in two classes of events. In one class theo-mode comes from regions overlying strong magnetic field, which we interpret in terms of thermal gyroresonance absorption of the extraordinary mode at the third harmonic of the gyrofrequency. In the other class the entire burst emits in theo-mode, which may be attributed to high gyrosynchrotron optical depth. Title: Photospheric and chromospheric magnetic field structure of a bipolar sunspot region Authors: Dara, H. C.; Koutchmy, S.; Alissandrakis, C. E. 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: VizieR Online Data Catalog: Asymmetry of FeI lines in solar spectrum (Stathopoulou+ 1993) Authors: Stathopoulou, M.; Alissandrakis, C. E. Bibcode: 1993yCat..32740555S Altcode: 75 FeI lines were studied from wide slit FTS spectra obtained at Kitt Peak at the center of the disk, near the limb (μ=0.2) (1 data file). Title: Evolution of an Active Region and Associated Hα Arch Structures Authors: Georgakilas, A. A.; Zachariadis, Th. G.; Alissandrakis, C. E. Bibcode: 1993SoPh..146..241G Altcode: We have studied the early stages of development of two adjacent active regions observed at the center and the wings of Hα for six days. From the growth of spots and arch structures we found that periods of slow flux emergence were followed by periods of vigorous flux emergence. We observed arch filaments covering an appreciable range of sizes (from a length of about 27 000 km and a height of 2000-3000 km to a length of 45 000 km and a height of about 15 000 km). Individual arch filaments within the same arcade sometimes have different inclinations of their planes with respect to the vertical. We observed isolated cases of arches crossing each other at an angle of ∼45°. During their early stages arch filament systems are short and they expand at a rate of about 0.8 km s−1. The rate of growth of arch filament systems is faster when the orientation of the flux tubes is nearly parallel to the equator. Our observations suggest that the early part of the evolution of individual arch filaments in a grown system is not visible; however, in a few cases we observed arch filaments appearing as dark features near one footpoint and expanding towards the other, with a mean velocity of about 30 km s−1. Title: A study of the asymmetry of Fe I lines in the solar spectrum Authors: Stathopoulou, M.; Alissandrakis, C. E. Bibcode: 1993A&A...274..555S Altcode: We have studied 75 Fe I lines from wide slit FTS spectra obtained at Kitt Peak at the center of the disk, near the limb (μ=O.2) and in total flux. We present measurements of the intensity at the line center, the equivalent width and the asymmetry. We have done a detailed analysis of bisectors and studied the dependence of the asymmetry on the optical depth. Plots of the asymmetry versus the effective depth of formation are similar for all lines, with a small dependence on the excitation potential. The combined curve from all lines at the center of the disk shows that the asymmetry decreases with optical depth with a minimum at log τ = -O.33, which corresponds to the turning point of the C-shaped bisectors; no such minimum is observed at μ = O.2, where the measurements go up to log τ ∼ -0.8. We infer that the asymmetry is a measure of the average line of sight velocity of the material and we discuss the results in terms of characteristic heights derived by other observers. Title: A new determination of the mean lifetime of bright and dark chromospheric mottles Authors: Bratsolis, E.; Dialetis, D.; Alissandrakis, C. E. Bibcode: 1993A&A...274..940B Altcode: We determined the lifetime of dark and bright mottles of the solar chromosphere using a more accurate method proposed by Alissandrakis et al. (1987). Our study is based on an excellent sequence of photographs taken at the Pic-du-Midi Observatory. We found that the mean lifetime of both bright and dark mottles is 13-14 min. We discuss this result and we compare it with previous investigations. Title: Correction: Two-dimensional solar mapping at 5.2 CM with the Siberian Solar Radio Telescope Authors: Alissandrakis, C. E.; Lubyshev, B. I.; Smol'Kov, G. Ia.; Krissinel', B. B.; Treskov, T. A.; Miller, V. G.; Kardapolova, N. N. Bibcode: 1993SoPh..145..418A Altcode: No abstract at ADS Title: The fine structure of a chromospheric rosette Authors: Tsiropoula, G.; Alissandrakis, C. E.; Schmieder, B. Bibcode: 1993A&A...271..574T Altcode: We have studied the spatial behaviour of the physical properties of dark mottles forming a well-defined rosette. The observations were obtained with the Multichannel Subtractive Double Pass (MSDP) spectrograph, operating in Hα at the Pic du Midi Observatory. From these observations, intensity fluctuations and Doppler shift velocities at different wavelengths were derived over a two dimensional field of view. The observed contrast profiles were matched with theoretical contrast profiles using Beckers' cloud model and 4 parameters were derived for the dark mottles: the source function, the line-of-sight velocity, the Doppler width and the optical depth. From these parameters a range for the temperature and electron density can be derived using Vernazza's et al. (1981) model D. We detected strong downflows at the roots of the mottles, while their upper part is ascending with velocities sometimes greater than 10 km s-1. The values of the cloud parameters are consistent with values given by other authors for this kind of features. Consistency was also found between cloud parameters and the values given for spicules, leading to the conclusion for the identity of these structures. Title: Spectral observations of active region sources with RATAN-600 and WSRT Authors: Alissandrakis, C. E.; Gel'Frejkh, G. B.; Borovik, V. N.; Korzhavin, A. N.; Bogod, V. M.; Nindos, A.; Kundu, M. R. Bibcode: 1993A&A...270..509A Altcode: We present spectral observations of neutral line and sunspot associated sources obtained with the RATAN-600 radio telescope and the WSRT in the wavelength range of 2 to 6 cm. Sources associated with large sunspots have flat spectra, while neutral line sources have very steep spectra. In the case of a large spot we estimated the magnetic field to be at least 2700 G at the base of the transition region and 1800 G in the low corona. We consider possible interpretations of the radio emission above the neutral lines. Gyroresonance emission at the fourth harmonic is inadequate, whereas emission from a small population of nonthermal electrons (total number 10 exp 30 to 10 exp 31) with a delta = 3 power law distribution seems to be sufficient. Title: Two-Dimensional Solar Mapping at 5.2-CM with the Siberian Solar Radio Telescope Authors: Alissandrakis, C. E.; Lubyshev, B. I.; Smol'Kov, G. Ia.; Krissinel', B. B.; Treskov, T. A.; Miller, V. G.; Kardapolova, N. N. Bibcode: 1992SoPh..142..341A Altcode: We present two-dimensional solar maps at 5.2 cm computed from one-dimensinal observations with the Siberian Solar Radio Telescope (SSRT), using Earth rotation aperture synthesis techniques. The resolution attained with the E-W branch of the instrument is 15 by 45″ for a solar declination of about 23°. Maps during the period of June 8 to 13, 1988 clearly show the quiet-Sun background, sunspot and plage associated emission as well as compact sources above the neutral line in some active regions. We found that the latter disappear as the gradient of the longitudinal magnetic field decreases. We also detected emission associated with active regions behind the limb, apparently from unresolved loops, extending up to ∼40″. The prospects of the SSRT, as a dedicated solar instrument, are discussed. Title: Observation of a coronal streamer at meter and decimeter wavelengths. Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1992ESASP.348..141L Altcode: 1992cscl.work..141L Maps obtained at meter and decimeter wavelengths with the Nançay Radioheliograph on October 24 and 25, 1987 are presented. One of the meter wavelength emission sources is located above a chromospheric magnetic field neutral line and a small Hα prominence. The source has decimeter wavelength counterparts on both sides of the neutral line. The decimeter sources are close to regions of enhanced large scale photospheric magnetic field, while the meter wavelength source is located in between. This suggests emissions at different altitudes in a loop system straddling the neutral line. The region corresponds to a Fe XIV coronal streamer seen during the eastern limb passage. The distance between the footpoints is of the order of 370000 km and the angle between the loops and the neutral line is about 50°, projected on a synoptic chart. Title: Time evolution of arch filaments Authors: Tsiropoula, G.; Georgakilas, A. A.; Alissandrakis, C. E.; Mein, P. Bibcode: 1992A&A...262..587T Altcode: We have studied the spatial structure and the temporal evolution of the velocity in active region arch filaments. The observations were obtained with the Multichannel Double Pass Spectrograph operating in H-alpha at the Pic du Midi Observatory. Line profiles were reconstructed and monochromatic images and Doppler shift velocities were derived over a 2D FOV. Using Beckers' (1964) cloud model we derived physical parameters, in particular the line of sight velocity. The arches showed the 'classical' type of motion, with material moving towards the observer near the apex and away from the observer near the footpoints. Assuming a symmetric loop, we reconstructed the velocity vector along the arch filaments. The results are consistent with the picture where material is draining out of the filament, while the whole structure is ascending. In one case we observed changes in the geometry and the velocity vector; however, other arch filaments did not change appreciably in a time period of about 13.5 min. Title: Two-dimensional model maps of flaring loops at cm-wavelengths Authors: Preka-Papadema, P.; Alissandrakis, C. E. Bibcode: 1992A&A...257..307P Altcode: We present a complete set of model computations of the microwave emission from a flaring loop. Two-dimensional maps in total intensity and circular polarization are given, as a function of wavelength, heliocentric distance and the orientation of the loop with respect to the direction of the limb. In agreement with our previous one-dimensional computations, the emission in the optically thin case comes from the one or both feet of the loop, with the primary maximum usually at the diskward side; in the optically thick case the emission comes from the entire loop, with one or two maxima near the top. We discuss in detail the effects of the orientation of the loop, as well as the polarization structure produced by self-absorption and gyroresonance absorption. The latter produce a patchy structure in the V map, with regions polarized both in the extraordinary sense (in the upper part of the loop) and in the ordinary sense (in the lower part of the loop). Finally we compare our computations with high spatial resolution observations of simple bursts. Title: Dynamic Phenomena in the Chromospheric Layer of a Sunspot Authors: Alissandrakis, C. E.; Georgakilas, A. A.; Dialetis, D. Bibcode: 1992SoPh..138...93A Altcode: We have studied running penumbral waves, umbral oscillations, umbral flashes and their interrelations from Hα observations of a large isolated sunspot. Using a subtraction image processing technique we removed the sharp intensity gradient between the umbra and the penumbra and enhanced the low contrast, fine features. We observed running penumbral waves which started in umbral elements with a size of a few arcseconds, covered the umbra and subsequently propagated through the penumbra. The period of the waves was 190 s and the mean propagation velocity was about 15 km s−1. We detected intense brightenings, located between umbral elements from where waves started, which had the characteristics of umbral flashes. There are indications that umbral flashes are related to the propagation of the waves through the umbra and their coupling. The subtraction images also show considerable fine structure in the chromospheric umbra, with size between 0.3″ and 0.8″. Title: Quiet-Sun Emission and Local Sources at Meter and Decimeter Wavelengths and Their Relationship with the Coronal Neutral Sheet Authors: Lantos, P.; Alissandrakis, C. E.; Rigaud, D. Bibcode: 1992SoPh..137..225L Altcode: We analysed multifrequency 2-dimensional maps of the solar corona obtained with the Nançay radioheliograph during two solar rotations in 1986. We discuss the emission of the quiet Sun, coronal holes and local sources and its association with chromospheric and coronal features as well as with large-scale magnetic fields. The brightness temperature of the quiet Sun was 5 to 5.5 × 105 K at 164 MHz and 4.5 to 5 × 105 K at 408 MHz. A coronal hole, also detected in the 10830 Å He I line, had a brightness temperature of 4.5 × 105 at 164 and 2.5 × 105 at 408 MHz. We give statistics of source brightness temperatures (on the average 8% above the background at 164 MHz and 14% at 408 MHz), as well as distributions in longitude and latitude. Although we found no significant center-to-limb effect in the brightness temperature, the sources were not visible far from the central meridian (apparently a refraction effect). The brightest sources at 164 MHz were near, but not directly above active regions and had characteristics of faint type I continua. At 408 MHz some sources were observed directly above active regions and one was unambiguously a type I continuum. The majority of the fainter sources showed no association with chromospheric features seen on Hα synoptic charts, including filaments. Most of them were detected at one frequency only. Sources identified at three frequencies (164, 327, and 408 MHz) were located in regions of enhanced large-scale magnetic field, some of them at the same location as decayed active regions visible one rotation before on synoptic Hα charts. Multifrequency sources are associated with maxima of the green line corona. The comparison with K-corona synoptic charts shows a striking association of the radio sources with dense coronal regions, associated with the coronal neutral sheet. Furthermore, we detected an enhanced brightness region which surrounds the local sources and is stable over at least one solar rotation. We call this feature a coronal plateau and we identify it with the radio counterpart of the coronal neutral sheet. Title: Detection of the coronal plasma sheet on the solar disk Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1992sws..coll..213L Altcode: The large scale coronal neutral sheet, which is considered to be the source of low speed solar wind, is the low altitude counterpart of the interplanetary heliosheet. It is routinely detected with white light corona observations at the limb. Maps of the sun at meter wavelengths, obtained with the Nancay Radioheliograph, lead to the first detection of this density structure on the disk, in the form of a 'plateau' in brightness temperature. The radio observations also show the denser parts of the coronal neutral sheet as discrete emission sources. Some of them may be identified as coronal streamers. Title: VLA observations of solar active regions at 6 and 20 CM Authors: Alissandrakis, C. E.; Kundu, M. R.; Shevgaonkar, K. R. Bibcode: 1991A&A...251..276A Altcode: High-resolution observations are presented of two active regions at 6 and 20 cm over a period of 5 days, together with H-alpha and photospheric magnetic fields. The large-scale emission at 20 cm is associated with the H-alpha plage. In one region the strongest source was over the neutral line, near the tip of an active-region filament, which indicates that the emission probably originated in small-scale coronal loops. In the second region the peak of the emission was near a well-developed sunspot. Neither region showed evidence of large-scale loops joining their preceding and following parts. Several other sources were observed at 20 cm; a source associated with an H-alpha plage region crossed by a filament and one associated with a small bipolar region are briefly discussed. The 6-cm emission from a well-developed spot showed clearly the characteristics expected from gyroresonance model computations. Title: Study of small scale magnetic flux and the corresponding velocity pattern Authors: Alissandrakis, C. E.; Dara, H. C.; Koutchmy, S. 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: Multifrequency radio observations of the solar corona during cycle minimum Authors: Rigaud, D.; Lantos, P.; Alissandrakis, C. E. Bibcode: 1991AdSpR..11a..65R Altcode: 1991AdSpR..11...65R Observations obtained at meter and decimeter wavelengths during a period of very low solar activity (from May to July 1986) are studied to analyse the relationship between the corona and the underlying levels. The location of radio sources observed on the disk is compared with active regions and with Hα filaments. The radio emission is found to trace on the disk the large scale structure of the corona as observed with K-coronameter: the so-called coronal sheet (belt of equatorial streamers) is observed and individual streamers could be located, for the first time, on the disk. Title: Mass Motions Associated with Hα Active Region Arch Structures Authors: Georgakilas, A. A.; Alissandrakis, C. E.; Zachariadis, Th. G. Bibcode: 1990SoPh..129..277G Altcode: We have studied mass motions associated with active region arch structures from observations of a developing active region near the center of the solar disk. We present a method for the computation of the line-of-sight velocity from photographs at Hα ± 0.5 under the assumption of Beckers' cloud model and reasonable assumptions about the Doppler width and optical depth of the arches. Some arches show motions typical to arch filaments (the material moves towards the observer near the apex of the arch and away from the observer near the footpoints), while in others the velocity field is more complex. Assuming a symmetric loop, we reconstructed the velocity vector along an arch filament. The results are consistent with the picture where material is draining out of the filament while the whole structure is ascending with a velocity near that of the apex, which does not exceed 10 km s−1. The motion is systematically slower than expected from a free-fall model. Title: Flow patterns in a sunspot region observed in the photosphere, chromosphere and transition region Authors: Dere, K. P.; Schmieder, B.; Alissandrakis, C. E. Bibcode: 1990A&A...233..207D Altcode: The flow patterns around a sunspot at photospheric, chromospheric, and transition zone temperatures is examined. The spiral fibril pattern observed in H-alpha is consistent with a constant alpha force-free field extrapolation of the photospheric magnetic field. Evidence is found for the first time that C IV fibrils near spots follow the field lines parallel to the chromospheric H-alpha fibrils. The observed large-scale pattern of redshifts and blueshifts in the vicinity of the sunspot is stable and conforms to the typical Evershed flow patterns of radial outflow at the photospheric levels and radial inflows at the chromospheric and transition region levels. The three-dimensional velocity vectors associated with this flow are derived assuming an axially symmetric Evershed flow. Title: Erratum: Velocity Pattern of Small Scale Magnetic Fields Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1990SoPh..128..431D Altcode: No abstract at ADS Title: Physical parameters of solar H-alpha absorption features derived with the cloud model Authors: Alissandrakis, C. E.; Tsiropoula, G.; Mein, P. Bibcode: 1990A&A...230..200A Altcode: Observations of a fibril region and of an arch filament region obtained by the Multichannel Subtractive Double Pass Spectrograph (MSDP) operating in H-alpha at the Pic du Midi Observatory are presented. Intensity fluctuations and Doppler shift velocities were derived over two dimensional fields of view. The observed contrast profiles have been matched with theoretical contrast profiles based on the cloud model which has four parameters: the line-of-sight velocity, the source function, the optical depth, and the Doppler width. Contour maps and histograms of the 4 parameters were made for these regions, and a comparison of the velocities given by 3 different methods (cloud model, Doppler shift, and the '3-optical depths' method of Mein and Mein, 1988) is made. Title: Velocity pattern of small scale magnetic fields Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. 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: VLA Observations of Solar Active Regions at 6 and 20 cm During the Spacelab 2 Mission Authors: Alissandrakis, C. E.; Kundu, M. R.; Shevgaonkar, R. K. Bibcode: 1990BAAS...22..795A Altcode: No abstract at ADS Title: Computation of electron beam parameters for solar type III and J bursts Authors: Hillaris, A.; Alissandrakis, C. E.; Caroubalos, C.; Bougeret, J. -L. Bibcode: 1990A&A...229..216H Altcode: The dynamic evolution of mildly relativistic electrons (10-100 keV) propagating outward in the solar corona following open or closed magnetic lines is studied using the drift approximation. Wave-particle interactions are neglected, since the beam-plasma system is considered as almost decoupled in the range of parameters used in this model. The results of this simulation are used for the computation of observable quantities and the analysis of dynamic spectra of isolated type III or J bursts, obtained with the Digital Multichannel Radio Spectrograph of the Space Research Laboratory of Paris, Meudon. The initial velocity dispersion of the exciter and the coronal scale height are thus determined. The implications of the results on the emission processes are discussed. Title: Small Scale Motions Over Concentrated Magnetic Field Regions of the Quiet Sun Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. Bibcode: 1990IAUS..138..153D Altcode: No abstract at ADS Title: Relationship between a spot and a filament observed during spacelab 2 mission Authors: Schmieder, B.; Dere, K. P.; Raadu, M. A.; Démoulin, P.; Alissandrakis, C. E. Bibcode: 1990AdSpR..10i.195S Altcode: 1990AdSpR..10..195S During the Spacelab 2 Mission, an active region including a sunspot, plages, fibrils and a filament which disappears during the observation period, was observed in Hα and in the C IV lines. The analysis of the observations leads to several conclusions. Shear in the active region filament is observed before its disappearance. Hα and C IV upflows in the filament are well correlated spatially, but the transition zone (C IV) velocities are an order of magnitude larger than the Hα chromospheric ones. Over the sunspot, the reverse-Evershed effect is observed in Hα and in C IV Dopplershift images while strong downflows are also detected in the C IV profiles.

Magnetograph data for the whole region are used to calculate the linear force-free magnetic field. A single value of the force-free parameter α is found to give a good fit to both the sheared filament structure and the sunspot spiral structures, indicating a causal relationship.

We propose a model based on the inductive coupling between current systems associated with the filament and the sunspot. Relaxation phenomena due to resistive effects in the filament could lead to irreversible conversion of magnetic energy into kinetic energy and heating. Title: Formation of a Filament around a Magnetic Region Authors: Schmieder, B.; Démoulin, P.; Ferreira, J.; Alissandrakis, C. E. Bibcode: 1990LNP...363..232S Altcode: 1990doqp.coll..232S; 1990IAUCo.117..232S The evolution of the active region AR4682 observed in 1985 during six rotations was dominated by three different phenomena: . the large scale pattern activity: relationship between two active regions, formation of a quiescent filament during the decay phase of the active region, . the presence of two pivot points along the filament surrounding the sunspot-with the long term one is associated the existence of the filament, with the short term one the activity with partial disappearance, . the magnetic shear during one rotation. The magnetic field lines have been extrapolated from photospheric data using Alissandrakis code (1981). The magnetic configuration with the existence of a dip favors the formation of a filament. We note that the shearing of the sunspot region and of the filament are both well described by force-free magnetic fields with the same constant α. This suggests that they are both a consequent of the same shear process. Title: Coronal meter wavelength observations during solar minimum: location of dense streamers. Authors: Rigaud, D.; Lantos, P.; Alissandrakis, C. E. Bibcode: 1990PDHO....7..270R Altcode: No abstract at ADS Title: Observation of the heliosheet at meter wavelengths Authors: Lantos, P.; Rigaut, D.; Alissandrakis, C. E. Bibcode: 1989AdSpR...9d..37L Altcode: 1989AdSpR...9...37L The Nancay Radioheliograph provides detailed radio maps at 169 MHz of the solar corona. The meter wavelength maps are used in particular to map the heliosheet on the limb in a way similar to the HAO K-corona synoptic maps. In absence of X-ray observations, the radio data provides presently the unique possibility to compare directly the heliosheet structure with the solar chromospheric features because the radio emissions are also observed on the disk and may be followed during their transit on the sun. Title: Particle acceleration. Authors: Vlahos, L.; Machado, M. E.; Ramaty, R.; Murphy, R. J.; Alissandrakis, C.; Bai, T.; Batchelor, D.; Benz, A. O.; Chupp, E.; Ellison, D.; Evenson, P.; Forrest, D. J.; Holman, G.; Kane, S. R.; Kaufmann, P.; Kundu, M. R.; Lin, R. P.; MacKinnon, A.; Nakajima, H.; Pesses, M.; Pick, M.; Ryan, J.; Schwartz, R. A.; Smith, D. F.; Trottet, G.; Tsuneta, S.; van Hoven, G. Bibcode: 1989epos.conf..127V Altcode: Contents: 1. Introduction. 2. Phenomena associated with mildly-relativistic electrons. 3. Phenomena associated with ions and relativistic electrons in solar flares. 4. Theoretical studies of particle acceleration. 5. Achievements - outstanding questions. Title: Formation of a Filament around a Magnetic Region Authors: Schmieder, B.; Demoulin, P.; Ferreira, J.; Alissandrakis, C. E. Bibcode: 1989HvaOB..13....1S Altcode: No abstract at ADS Title: The Evershed flow in the solar photosphere, chromosphere and chromosphere-corona transition region Authors: Alissandrakis, C. E.; Dialetis, D.; Mein, P.; Schmieder, B.; Simon, G. Bibcode: 1988A&A...201..339A Altcode: The authors have studied the height dependence of the characteristics of the Evershed flow in the photosphere, chromosphere and chromosphere-corona transition region. The Multichannel Subtractive Double Pass Spectrograph was used to obtain line of sight velocity maps in Hα, the Meudon magnetograph for mapping the photospheric velocity and magnetic field and the Ultraviolet Spectrometer and Polarimeter on the Solar Maximum Mission spacecraft for line-of-sight velocities in C IV, in sunspot regions. The observations are practically simultaneous in the three atmospheric levels and cover three consecutive days (Sep 28, 29 and 30, 1980). The emphasis was on the large scale, quasi-stationary characteristics of the flow. Title: Dynamics of sub-relativistic electron beams in magnetic traps - A model for solar N-bursts Authors: Hillaris, A.; Alissandrakis, C. E.; Vlahos, L. Bibcode: 1988A&A...195..301H Altcode: The dynamic evolution of mildly relativistic electrons (10 - 100 keV) injected into a model magnetic trap is studied numerically, using the drift approximation. Wave-particle interactions are neglected, since the beam plasma system is shown to be non-linearly decoupled in the range of parameters used in this study. The results from the simulation are used to interpret certain observational characteristics of the N-bursts observed by the Nançay radio spectrograph. N-bursts are believed to be the first direct radio evidence for mirror effects in solar magnetic loops. Title: High resolution microwave and X-ray observations of solar flares Authors: Alissandrakis, C. E.; Schadee, A.; Kundu, M. R. Bibcode: 1988A&A...195..290A Altcode: This paper presents an analysis of high spatial resolution observations of 18 weak flares, carried out during the Solar Maximum Year in the microwave range, H-alpha, and soft X-rays, together with observations of the associated hard X-ray emission and the photospheric magnetic field. This combination of observations made it possible to obtain a picture of the flare geometry and the relative position of the various emissions, to study the change of the geometry during the flare, to estimate the electron temperature and emission measure of the thermal plasma, and to compute the contribution of the free-free process in the microwave emission. Results are interpreted as evidence for the presence of energetic electrons even in the post-burst phase of some flares. There was also found a clear case of homologous radio and X-ray bursts, one case of foot-point expansion with a relative velocity of 20 km/s at 6.16 cm, as well as some evidence of triggering of flares by disturbances moving with a velocity of 5000-13,000 km/s. Title: Spatial and spectral structure of a solar flaring loop at centimeter wavelengths Authors: Preka-Papadema, P.; Alissandrakis, C. E. Bibcode: 1988A&A...191..365P Altcode: The authors have studied the behaviour of the gyrosynchrotron emission coefficient and source function as a function of the strength and direction of the magnetic field. The results are applied in the qualitative description of the spatial and spectral structure of solar microwave bursts. In addition the authors present a complete set of model computations of microwave emission from a loop filled with energetic electrons. Their model predicts footpoint emission at short wavelengths and emission from the entire loop at long wavelengths. The computed spectra of the emission show considerable variation along the flaring loop. Title: Quiet Sun and Slowly Varying Component at Meter and Decameter Wavelengths Authors: Lantos, P.; Alissandrakis, C. E.; Gergely, T.; Kundu, M. R. Bibcode: 1987SoPh..112..325L Altcode: Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nançay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations. Title: Observations of Ellerman bombs in H α Authors: Zachariadis, Th. G.; Alissandrakis, C. E.; Banos, G. Bibcode: 1987SoPh..108..227Z Altcode: A developing active region near the center of the solar disk was observed for 80 min at the center and the wings of Hα. Ellerman bombs lying both below an Arch Filament System and near sunspots were studied at Hα - 1.0 Å and Hα - 0.75 Å. We determined their average contrast, lifetime, size and we studied their flux as a function of time. We found evidence that the size of Ellerman bombs increases with height. The time curves of flux provide evidence for both impulsive and gradual energy release. Under the AFS the Ellerman bombs form a cellular pattern with a characteristic size of 3.1 × 103 km. Fifty percent of the bombs appear and disappear in pairs, possibly associated with bipolar emerging magnetic flux tubes. Title: Microwave Emission above Steady and Moving Sunspots Authors: Chiuderi Drago, F.; Alissandrakis, C.; Hagyard, M. Bibcode: 1987SoPh..112...89C Altcode: Two-dimensional maps of radio brightness temperature and polarization, computed assuming thermal emission with free-free and gyroresonance absorption, are compared with observations of active region 2502, performed at Westerbork at λ = 6.16 cm during a period of 3 days in June 1980. The computation is done assuming a homogeneous model in the whole field of view (5' × 5') and a force-free extrapolation of the photospheric magnetic field observed at MSFC with a resolution of 2″.34. The mean results are the following: A very good agreement is found above the large leading sunspot of the group, assuming a potential extrapolation of the magnetic field and a constant conductive flux in the transition region ranging from 2 × 106 to 107 erg cm−2s−1. Title: Determination of the mean lifetime of solar features from photographic observations Authors: Alissandrakis, C. E.; Dialetis, D.; Tsiropoula, G. Bibcode: 1987A&A...174..275A Altcode: Two approaches, one based on a lifetime distribution and the second based on survival functions, are combined in order to determine the mean lifetimes of solar features. The relations between the lifetime distributions and the survival and probability density functions are derived. This method is applied to the determination of the mean lifetimes of solar granules. The accuracy of the method is evaluated by comparing the calculated lifetimes with data obtained using different methods. It is determined that the proposed method provides accurate lifetime data, and a mean lifetime value of 16 min with a standard deviation of about 5 min was obtained for the solar granules. Title: Small-scale motions over concentrated magnetic regions of the quiet Sun Authors: Dara, H. C.; Alissandrakis, C. E.; Koutchmy, S. 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: The Evershed flow in the solar photosphere, chromosphere and chromosphere-corona transition region. Authors: Alissandrakis, C. E.; Dialetis, D.; Mein, P.; Schmieder, B.; Simon, G. Bibcode: 1987PAICz..66..147A Altcode: The authors have studied the height dependent characteristics of the Evershed flow in the photosphere, chromosphere and chromosphere-corona transition region. Title: Microwave emission from steady and moving sunspots. Authors: Alissandrakis, C. E.; Chiuderi Drago, F.; Hagyard, M. J. Bibcode: 1987PAICz..66..267A Altcode: 1987eram....1..267A The authors have used force-free extrapolations of photospheric magnetic field observations from Marshall Space Flight Center to compute the total intensity and circular polarisation of sunspot associated emission from active region 2502 in the period June 13 to 15, 1980. Title: Synoptic study of the corona at meter wavelength. Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1986NASCP2442..257L Altcode: 1986copp.nasa..257L The Mark III Nancay Radioheliograph is used to observe the Sun at 169 MHz with a time resolution of 25 East-West and North-South images per second. When the brightness distribution of the Sun is stable during the eight hours of daily observation, a two dimensional map can be produced using the technique of earth rotation synthesis. The best images are obtained during the period April to August, when the declination of the Sun is high to give a good coverage in the uv plane and a reasonable North-South resolution. The spatial resolution is 1.5' East-West and in summer, 3.5' North-South. The maps are calibrated using Cygnus A as a reference. Examples of the maps are given and discussed. Title: Emission of Lyman-α radiation by solar coronal loops. II. Filtergram analysis. Authors: Tsiropoula, G.; Alissandrakis, C.; Bonnet, R. M.; Gouttebroze, P. Bibcode: 1986A&A...167..351T Altcode: The authors study active region loops, observed on Lα high resolution filtergrams obtained with a rocket borne instrument. Some formulae or diagrams are given for the computations of Lα and Hα intensities emitted by homogeneous loop models. The dimensions and emitted intensities of the observed loops being known, the authors determine the variations of temperature, pressure and density inside these objects under the assumption of constant temperature and hydrostatic equilibrium. The influence of temperature gradient and mass flow on the results is discussed, and an alternative model with radial temperature increase is envisaged. The case of an isolated loop with a larger diameter is also discussed. Title: Gyrosynchrotron Emission of Solar Flares Authors: Alissandrakis, C. E. Bibcode: 1986SoPh..104..207A Altcode: The current status of our knowledge on the theory of radio emission from mildly relativistic electrons and its application in the interpretation of solar radio bursts are reviewed. The recent high spatial resolution microwave observations have given important information about the geometry of the emitting region and have helped in the computation of better inhomogeneous models that reproduce qualitatively several observational characteristics of the emission. The limitations of the observations and the theory (particularly the effect of mode coupling on the observed polarisation) are pointed out and the potential of the gyrosynchrotron process as a diagnostic of the physical conditions is discussed. This will help us to obtain quantitative information about the changes of the magnetic field and the acceleration of particles in solar flares. Title: Coronal structures observed at meter wavelengths. Authors: Lantos, P.; Alissandrakis, C. E. Bibcode: 1986HiA.....7..761L Altcode: Two dimensional maps obtained with the Nançay Radioheliograph at 169 MHz from 1980 are used to describe and identify the features of the quiet sun at meter wavelengths. Title: The Evershed flow as a steady-state homogeneous phenomenon Authors: Dialetis, D.; Mein, P.; Alissandrakis, C. E. Bibcode: 1985A&A...147...93D Altcode: The present observations of photospheric and chromospheric line-of-sight velocity, as well as the photospheric magnetic field, facilitate the comprehensive study of the Evershed (1909) flow, with emphasis on large scale, quasi-stationary characteristics of the flow and its association with the topology of the magnetic field. A multichannel subtractive double pass spectrograph was used to obtain line-of-sight velocity maps in H-alpha, together with the Meudon magnetograph. Computations are made for the components of the velocity vector as a function of distance from the center of the spot, under the assumption of axial symmetry. The radial component is the dominant one in both chromosphere and magnetosphere. The present analysis of the magnetograms, using current-free and force-free models, indicates that the velocity maximum occurs in regions where the magnetic field is almost horizontal in both layers; the results are judged to appear consistent with subsonic flow in magnetic flux tubes. Title: Coronal structures observed at metric wavelengths with the Nançay radioheliograph Authors: Alissandrakis, C. E.; Lantos, P.; Nicolaidis, E. Bibcode: 1985SoPh...97..267A Altcode: We present and discuss two-dimensional maps of the Sun at 169 MHz, obtained with the Nançay radioheliograph used as an Earth rotation aperture synthesis instrument. The maps have been computed on the basis of about 6 hr of one-dimensional observations by the east-west and the north-south arrays of the radioheliograph and have a resolution of 1.5' by 4.2' for a solar declination near 23°. In addition to a broad background component, the maps show several features both brighter and darker than the background. Some of the bright features are sources of noise storm continua, as evidenced by their positions relative to active regions and by the occurrence of type I bursts. Weaker emission regions are apparently associated with neutral lines of the photospheric magnetic field. We found no sources associated with extended quiescent filaments. Some of the depressions on the maps correspond to coronal holes both in the equatorial region and near the poles, while the more shallow ones may be arch regions with low electron temperature and/or emission measure. The distribution of brightness temperature at a height of 0.15 solar radii above the photospheric limb shows a gross similarity with coronal green line observations. The present results indicate that the notion of the slowly varying component at metric wavelengths may have to be reexamined, since sources of different nature may have been grouped in this component in the past. Title: Observations of Ellerman Bombs in H&alpha Authors: Zachariadis, Th.; Alissandrakis, C. E.; Banos, G. Bibcode: 1985LNP...233..304Z Altcode: 1985hrsp.proc..304Z A developing active region near the center of the solar disk was observed for 80 min at the center and the wings of H-alpha. Ellerman bombs lying below an arch filament system (AFS) and near sunspots were studied at H -1 A and H -0.75 A. It was found that 50 percent of the bombs appear and disappear in pairs, the axis of each pair forming a small angle with the magnetic field lines of force as evidenced by the AFS. In addition, the members of each pair show proper motions with an average relative velocity of 600 m/sec. This behavior suggests their association with bipolar emerging magnetic flux tubes. The average contrast (1.29), lifetime (19 minutes), size (0.96 in. x 0.58 in.) and flux of some 70 bombs was also determined. Evidence was found for an increase in size with height and a fluctuation in flux with time. Title: Solar Coronal Holes Observed with Nancay Meter Radioheliograph Authors: Lantos, P.; Alissandrakis, C. Bibcode: 1985ESASP.229..367L Altcode: 1985erbp.symp..367L Observations of the sun at meter wavelengths, with the Nançay radioheliograph, provide maps of the structures of the middle corona. In particular, coronal holes with low temperature and density are well observed both in equatorial regions and near the poles as deep depressions in brightness temperature. This provides a new set of data to study solar wind and geomagnetic activity origin. Title: Dynamic Phenomena in the Chromospheric Umbra and Penumbra of a Sunspot Authors: Alissandrakis, C. E.; Dialetis, D.; Macris, C. J. Bibcode: 1985LNP...233..294A Altcode: 1985hrsp.proc..294A A prominent, short lived (70 sec) umbral flash in H-alpha is observed with the 50 cm 'Tourelle' refractor at Pic du Midi. The main component of the flash was small (1 arcsec by 0.5 arcsec) with a maximum intensity 25 percent above the background. It occurred in a region which showed irregular intensity variations. Umbral oscillations were observed in an adjacent region and running penumbral waves around part of the superpenumbra. Title: A siphon flow model for Evershed motion. Authors: Dialetis, D.; Alissandrakis, C. E. Bibcode: 1984ESASP.220..245D Altcode: 1984ESPM....4..245D The authors have studied the motion of an inviscid isothermal plasma under the influence of gravity along the lines of force of an axisymmetric magnetic field. The general behaviour of the solutions is in good agreement with the observations. Title: Coronal structures observed at metric wavelengths with the Nançayradioheliograph. Authors: Alissandrakis, C. E.; Lantos, P.; Nicolaidis, E. Bibcode: 1984ESASP.220..263A Altcode: 1984ESPM....4..263A The authors present two-dimensional maps of the Sun at 169 MHz, obtained with the Nançay radioheliograph, with resolution of 1.5arcmin by 4.2arcmin. Title: Microwave emission and polarization of a flaring loop Authors: Alissandrakis, C. E.; Preka-Papadema, P. Bibcode: 1984A&A...139..507A Altcode: The authors present computations of the microwave emission from a loop filled with accelerated electrons. At short wavelengths the loop is optically thin and the emission comes predominantly from the feet of the loop, while at long wavelengths the emission comes from the entire loop with a maximum near the top. If the loop is not located at the center of the solar disk the observed polarization is greatly affected by propagation effects in the corona which may change the sense of the circular polarization (V). For the adopted model and for a loop located 30° away from the center of the disk the sense of V corresponds to the direction of the magnetic field in the region of emission only for wavelengths much shorter than 1.4 cm; for wavelengths much longer than 7.5 cm the sense of V corresponds to the polarity of the field at the diskward foot of the loop. Title: Center-to-limb variation of a sunspot-associated microwave source Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1984A&A...139..271A Altcode: Observations of the simple bipolar active region McMath 16862, obtained with the Westerbork Synthesis Radio Telescope at 6.16 cm over six consecutive days, indicate two bright sources associated with the region's two main spots, as well as some weaker emission that may be associated with loop structures. Application of a novel analytical method has allowed the mapping of the vertical as well as the horizontal component of the sunspot magnetic field at specific locations in the low corona. While the vertical component decreases away from the source's center, the horizontal component has both a radial and an azimuthal part. These results are interpreted in terms of a force-free magnetic field model, as well as in terms of a dipole whose axis is inclined to the vertical. Title: Structure and Polarization of Active Region Microwave Emission Authors: Kundu, M. R.; Alissandrakis, C. E. Bibcode: 1984SoPh...94..249K Altcode: We present observations of active region radio emission at 6.16 cm wavelength, obtained with an angular resolution of 3″ by 10″ arc using the Westerbork Synthesis Radio Telescope (WSRT) during the action interval May 20-27, 1980 of the Solar Maximum Year (SMY). We present maps in both total intensity (I) and circular polarization (V) of three regions (Hale numbers 16850, 16863, and 16864) and provide a detailed comparison of these maps with on- and off-band Hα pictures and with magnetograms. The strongest sources were associated with neutral lines and soft X-ray arcades. We present evidence that these neutral lines were characterized by having their two opposite polarities close to each other, implying a high magnetic field gradient, and by their association with arch filament systems. The sunspot associated radio sources had a relatively simple structure in region 16850; however for the large spots of regions 16864 and 16863 the emission had a patchy appearance with a tendency of the peaks to lie over the penumbra. In the V maps we observed for the first time two `islands', polarized in the sense of the ordinary mode, which were located inside the sunspot associated sources and were associated with intrusions of opposite polarity field into the penumbra. These structures can be accounted for if the electron temperature along the line of sight is not a monotonically increasing function of height, but has a maximum near the second harmonic level. Finally we give a detailed analysis of observations of the inversion of the sense of circular polarization in region 16863. We find that the large scale structure of the magnetic field can be approximated by a dipole with its axis inclined by 11° with respect to the photosphere and with a dipole moment of about 2 × 1031 cgs units; the depolarization line is located at a height of 0.16-0.19 R⊙ above the dipole, where the estimated intensity of the magnetic field is 10-20 G. Title: Interpretation of microwave active region structures using SMM soft X-ray observations Authors: Strong, K. T.; Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1984ApJ...277..865S Altcode: The purpose of this paper is to investigate the roles played by the geometrical effect of gyroresonance absorption and by the low-temperature (<106K) flux tubes above sunspot regions with microwave ring structures, by combining WSRT 6 cm observations with the X-ray observations made with the X-ray Polychromator on the SMM satellite. Some features associated with Hale regions 16863 and 16864 are analyzed from observations on 1980 May 25 and 26 combined with model computations of the radio emission. The nature of a cool and compact coronal feature seen in soft X-rays, which corresponded to some interesting microwave and magnetic features, is also examined. Finally, the origin of the X-ray and radio emission associated with neutral lines of the magnetic field is considered. Title: Observations of ring structure in a sunspot associated source at 6 centimeter wavelength Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1982ApJ...253L..49A Altcode: The detection of a new kind of sunspot-associated source in which the emission comes predominantly from a ring structure with size between that of the umbra and the penumbra is reported. The absence of emission from the center of the spot is interpreted in terms of the orientation of the magnetic field and the presence of low temperature material above the umbra. Title: Measurements of the granule-intergranular lane contrast at 5200 Å and 6300 Å Authors: Alissandrakis, C. E.; Macris, C. J.; Zachariadis, T. G. Bibcode: 1982SoPh...76..129A Altcode: We present measurements of the granule-intergranular lane intensity ratio at 5200 Å and 6300 Å, at the center of the disk. The observations were obtained at Pic du Midi and Sacramento Peak observatories between 1967 and 1978. The contrast at 5200 Å was corrected for the effect of instrumental profile using a two-dimensional model. At 6300 Å and in one photograph our measurements gave an average contrast of 1.40, while the values at 5200 Å show a variation with time, with the highest corrected values in the range of 1.30 to 1.37. The possible origins of the time variation are discussed. Title: On the computation of constant alpha force-free magnetic field Authors: Alissandrakis, C. E. Bibcode: 1981A&A...100..197A Altcode: Consideration is given to the boundary value problem of the determination of the strength and configuration of the magnetic field in the half space above the solar photosphere on the basis of photospheric observations for the case of a force-free magnetic field with constant alpha, a scalar relating the cross product of the currents and the field vectors to the field vector. Solutions are derived in terms of Fourier transforms and extended to regions far from the center of the disk. It is found that the large-scale components of the solutions involve a quasi-periodic variation of the field and current with height, carry infinite energy, and can be completely specified only with observations of a transverse field component at the boundary. The solutions are thus unphysical, however solutions are required for the small-scale solution to satisfy the boundary condition. It is pointed out that although numerical computations based on the fast Fourier transform are about 100 times faster than those based on the Green's function formulation, the use of a rectangular grid may result in important aliasing effects unless the observed field is placed in an array somewhat larger than the observed region. Title: A model for sunspot associated emission at 6 CM wavelength Authors: Alissandrakis, C. E.; Kundu, M. R.; Lantos, P. Bibcode: 1980A&A....82...30A Altcode: Two-dimensional maps of total intensity and circular polarization of a sunspot region at 6 cm have been calculated using a simple model for the chromosphere-corona transition region and observations of the longitudinal component of the photospheric magnetic field. The calculations are in good agreement with the high resolution observations of the same sunspot region at 6 cm, obtained with the Westerbork Synthesis Radio Telescope. It is shown that the 6 cm radiation is predominantly due to gyroresonance absorption process at the second and third harmonics of the gyrofrequency (H = 900-600 G). Estimates of the conductive flux and the electron density in the transition region above the sunspot are also given. Title: Active region magnetic fields and cm-lambda emission Authors: Alissandrakis, C. E. Bibcode: 1980IAUS...86..101A Altcode: The influence of the magnetic field on the structure of sunspot-associated sources at cm wavelengths has been investigated through model calculations. The case of a source observed with the Westerbork SRT and the structure of a single sunspot source are considered. Title: 6 centimeter observations of solar bursts with 6" resolution. Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1978ApJ...222..342A Altcode: During May 1974 data were collected on nine 6-cm solar bursts originating in two active regions. One-dimensional fan-beam scans were obtained every 30 seconds in order to study burst structure and evolution. Maximum burst flux was in the 0.34-12.0 sfu range. At maximum intensity the estimated brightness temperatures were in the 10 to the 6th to 2 x 10 to the 7th K range. Angular sizes ranged from 7-23 arcsec. In most cases an expansion of the burst core after maximum intensity was observed along with a drift in the position of the burst core. Four bursts were associated important H-alpha flares, and in all cases burst maximum occurred before flare maximum. The bursts were circularly polarized in the extraordinary mode sense during maximum intensity. Since the polarization was the same over the extent of the burst source, it is suggested that if the burst is associated with loop sources, the emission is associated with one leg of the loop. Title: 6 centimeter observations of solar active regions with 6" resolution. Authors: Kundu, M. R.; Alissandrakis, C. E.; Bregman, J. D.; Hin, A. C. Bibcode: 1977ApJ...213..278K Altcode: We have used the Westerbork Synthesis Radio Telescope for observations of solar active regions at 6 cm during 1974 May 8-10. The particular problems of using a sidereal instrument for solar observations are discussed. Maps of total intensity and circular polarization were obtained for four active regions with sunspots and a plage region; the linear polarization was below the noise limit. The brightest components of the 6 cm emission are associated with sunspots, while weaker components are associated with the plage magnetic fields, both longitudinal and transverse. An inversion of the sense of circular polarization was observed in the preceding part of a region with bipolar magnetic-field structure, about two days after the region passed through the central meridian. Nine bursts were also observed, and their one-dimensional positions with respect to the associated active regions are given. Title: . Authors: Alissandrakis, C. E. Bibcode: 1977PhDT.........4A Altcode: The Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands was used for Solar observations at 6 cm on May 8-10, 1974. The WSRT was used for two-dimensional mapping of Solar active regions and for one-dimensional mapping of transient burst sources. Synthesis maps of four active regions were produced. Compact sources associated both with sunspots and with plage regions were detected. The compact plage sources were associated with areas of enhanced longitudinal magnetic field or with regions near the neutral line of the photospheric field. The magnetic field in the transition zone or low corona above the sunspots was calculated as 600-900 Gauss. This value is approximately two times larger than the predictions of current-free calculation of the magnetic field based on the Meudon magnetogram. Title: A comparison of positions and sizes of sources of centimeter and X-ray bursts. Authors: Kundu, M. R.; Alissandrakis, C. E.; Kahler, S. W. Bibcode: 1976SoPh...50..429K Altcode: We have made a parallel study of three cm-λ radio bursts, observed on 9 August, 1973 with the NRAO 1 three-element interferometer at 3.7 and 11.1 cm and the associated X-ray flares observed with the S-054 telescope aboard Skylab. Within the errors of our measurements (± 5″) the radio and X-ray events are cospatial. We find good agreement between the size of the X-ray kernel and that of the core of the cm burst, while there is evidence that much of the impulsive radio flux was produced in a larger area. Title: Observations of Solar Bursts at 6 cm Wavelength with Angular Resolution of 6" arc. Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1976BAAS....8Q.555A Altcode: No abstract at ADS Title: A search for periodic variations of solar radio emission at 3.7- and 11.1-cm wavelengths. Authors: Kundu, M. R.; Alissandrakis, C. E. Bibcode: 1975MNRAS.173...65K Altcode: Periodic time variations in the emission of quiet solar regions were sought with the NRAO three-element interferometer at 3.7 and 11.1 cm. The visibility function of a source as obtained with a tracking interferometer is an implicit function of time, because of projected baseline changes, and therefore only observations taken when the baseline changes vary slowly with time were used. The visibility function shows significant time variations, but a power-spectrum analysis of the data revealed no unique periodicity. Title: Observations at 6 CM of the solar active region Authors: Kundu, M. R.; Alissandrakis, C. E. Bibcode: 1975Natur.257..465K Altcode: Preliminary results are reported for the synthesis of a solar active region from observations with a synthesis radio telescope at a wavelength of 6 cm. Several individual sources are observed in the total-intensity and circular-polarization maps. These are shown to correspond to two large sunspots, two groups of small sunspots, and a plage. A magnetogram indicates that this active region had a bipolar structure with positive polarity concentrated on the main sunspots and negative polarity scattered over the plage and some of the small sunspots. It is noted that several radio bursts were recorded during the observation period and that one corresponded to a H-alpha flare near one of the sunspot groups. Title: High Resolution Polarization Observations of a Solar Active Region at 6 cm. Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1975BAAS....7..473A Altcode: No abstract at ADS Title: 6 cm Observations of A Solar Active Region with the Westerbork Synthesis Radio Telescope. Authors: Kundu, M. R.; Alissandrakis, C. E.; van Someren Greve, H. W. Bibcode: 1975BAAS....7..235K Altcode: No abstract at ADS Title: High Resolution Observations of Solar Bursts at 3.7 and 11.1 cm Wavelengths Authors: Alissandrakis, C. E.; Kundu, M. R. Bibcode: 1975SoPh...41..119A Altcode: Four bursts were observed on August 9, 1973 with the NRAO 1 3-element interferometer at 3.7 and 11.1 cm. By using a simple source model we have calculated the temperature, flux, size and position of the small scale components of the bursts as a function of time. We obtained peak temperatures around 107 K for the components with size of about 10″ at 11.1 cm and 3″ at 3.7cm. The peak flux of these components lies between 4 and 40 % of the total peak flux of the bursts. Two of the bursts were found to be right circularly polarized. There is evidence that the location of these two bursts does not coincide with the location of the brightest point of the associated active regions. The existence of burst structures with temperatures of the order of 107K indicates that at least part of the radiation in these bursts is generated by a non-thermal mechanism. Title: Absence of Periodic Variation in Solar Radiation at 3.7 cm and 11.1 cm Wavelengths. Authors: Alissandrakis, C.; Kundu, M. R.; Becker, R. H. Bibcode: 1974BAAS....6..348A Altcode: No abstract at ADS Title: Long Base Line Interferometry of the Sun at 3.7 and 11.1 cm Wavelengths. Authors: Kundu, M. R.; Alissandrakis, C.; Becker, R. H. Bibcode: 1974BAAS....6..219K Altcode: No abstract at ADS Title: Properties of the Red SPOT of Jupiter in 1971 Authors: Banos, C. J.; Dialetis, D. G.; Alissandrakis, C. E. Bibcode: 1972A&A....19..381B Altcode: Isodensity tracings of Jupiter and its Red Spot during the year 1971 as well as photometric profiles are given. The variation of the relative intensity of the Red Spot as the Spot moves from the one edge of the planetary disc to the other for 1971 is studied. Key words: planetary atmospheres - Jupiter - Red Spot - photometry. Title: Isophotometry of the Chromospheric Bright and Dark Mottles on the Solar Disk Authors: Alissandrakis, C. E.; Macris, C. J. Bibcode: 1972ApL....10...59A Altcode: No abstract at ADS Title: Isodensitometry of Jupiter's Red SPOT and Jupiter Authors: Banos, C. J.; Alissandrakis, C. E. Bibcode: 1971A&A....15..424B Altcode: Isodensity tracings for Jupiter in 1968 and for the Red Spot in 1968-1969-1970 are given. The photometric proffies of both derived from the isophotes and for = 5500 A are given in the East-West and NorthSouth directions. Key words: planetary atmospheres - Jupiter - red spot isophotes. Title: A Study of the Fine Structure of the Solar Chromosphere at the Limb Authors: Alissandrakis, C. E.; Macris, C. J. Bibcode: 1971SoPh...20...47A Altcode: We have measured the dimensions, distances from the inner limb and the lifetime of bright mottles at the limb. Spicule lifetimes have been measured too. The problem of the `dark band', lying just above the inner limb, as well as the relations between bright mottles and spicules are discussed. Title: On the relationships between bright mottles and spicules of the solar chromosphere. Authors: Alissandrakis, C. E.; Macris, C. J. Bibcode: 1971AkAtP..46..107A Altcode: No abstract at ADS Title: Lifetime of the Dark and Bright Mottles of the Solar Chromosphere Authors: Macris, C. J.; Alissandrakis, C. E. Bibcode: 1970SoPh...11...59M Altcode: No abstract at ADS