Author name code: georgoulis ADS astronomy entries on 2022-09-14 author:"Georgoulis, Manolis K." ------------------------------------------------------------------------ Title: Towards coupling full-disk and active region-based flare prediction for operational space weather forecasting Authors: Pandey, Chetraj; Ji, Anli; Angryk, Rafal A.; Georgoulis, Manolis K.; Aydin, Berkay Bibcode: 2022FrASS...9.7301P Altcode: Solar flare prediction is a central problem in space weather forecasting and has captivated the attention of a wide spectrum of researchers due to recent advances in both remote sensing as well as machine learning and deep learning approaches. The experimental findings based on both machine and deep learning models reveal significant performance improvements for task specific datasets. Along with building models, the practice of deploying such models to production environments under operational settings is a more complex and often time-consuming process which is often not addressed directly in research settings. We present a set of new heuristic approaches to train and deploy an operational solar flare prediction system for ≥M1.0-class flares with two prediction modes: full-disk and active region-based. In full-disk mode, predictions are performed on full-disk line-of-sight magnetograms using deep learning models whereas in active region-based models, predictions are issued for each active region individually using multivariate time series data instances. The outputs from individual active region forecasts and full-disk predictors are combined to a final full-disk prediction result with a meta-model. We utilized an equal weighted average ensemble of two base learners' flare probabilities as our baseline meta learner and improved the capabilities of our two base learners by training a logistic regression model. The major findings of this study are: 1) We successfully coupled two heterogeneous flare prediction models trained with different datasets and model architecture to predict a full-disk flare probability for next 24 h, 2) Our proposed ensembling model, i.e., logistic regression, improves on the predictive performance of two base learners and the baseline meta learner measured in terms of two widely used metrics True Skill Statistic (TSS) and Heidke Skill Score (HSS), and 3) Our result analysis suggests that the logistic regression-based ensemble (Meta-FP) improves on the full-disk model (base learner) by ∼9% in terms TSS and ∼10% in terms of HSS. Similarly, it improves on the AR-based model (base learner) by ∼17% and ∼20% in terms of TSS and HSS respectively. Finally, when compared to the baseline meta model, it improves on TSS by ∼10% and HSS by ∼15%. Title: A physics-based prototype tool to forecast the probability of SEP occurrence using modelled shock wave parameters Authors: Kouloumvakos, Athanasios; Dalmasse, Kévin; Georgoulis, Manolis K.; Rouillard, Alexis; Papaioannou, Athanasios; Anastasiadis, Anastasios; Paouris, Evangelos; Vasalos, George; Indurain, Mikel Bibcode: 2022cosp...44.1180K Altcode: Over the last decade, great efforts have been taken place focusing on the development and the improvement of forecasting models of space weather (SW) hazards. These models are essential in support of space operations and for human space exploration. The most important drivers of intense SW are the coronal mass ejections (CMEs) and their associated shock waves. These can accelerate particles to relativistic energies which are one of the main safety concerns for humans in space and space-based technology. We present a physics-based multi-module scheme that is developed to forecast the probability of SEP occurrence. The developed prototype consists of three main modules, namely, (1) a shock propagation model, (2) a magnetic field connectivity module that provides the connectivity solutions for different input magnetograms and various observers from the low corona to interplanetary (IP) space, (3) a coronal plasma module that utilizes different density and solar wind speed models (empirical or MHD). The two main inputs to the tool are the identified active regions (ARs) at the visible solar disk and the expected maximum CME speed, both provided by the Advanced Solar Particle Events Casting System (ASPECS) forecasting system of the National Observatory of Athens (NOA). The magnetic connectivity is provided by the Institut de Recherche en Astrophysique et Planétologie's magnetic connectivity tool (IRAP-MCT). The model calculates the wave kinematics, and the connection times and shock parameters at the field lines connected to the observers. These calculations are performed for each identified AR. The evolution of the shock parameters can be used to construct a binary prediction model of SEPs. We will present results from the prototype and we will show how a binary classification of solar events to SEP/non-SEP events is possible. We will discuss limitations and further improvements of the model. Integration of the tool as a part of the IRAP-MCT is currently underway. The development of this prototype has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 870405 (EUHFORIA 2.0). Title: The Importance of Method Redundancy in Studying Pre-Eruption Evolution in Solar Active Regions Authors: Georgoulis, Manolis K.; Pariat, Etienne; Liu, Yang; Thalmann, Julia K. Bibcode: 2022cosp...44.1358G Altcode: In a recent synergistic work stemming from a prior International Space Science Institute (ISSI) Working Group, the evolution of magnetic helicity in an intensely eruptive solar active region was studied using several different helicity calculation methods. This was the first time all these methods were tested on real solar data, without the possibility of a ground truth. Focusing on the pre-eruption evolution prior to an eruptive X-class flare (SOL2006-12-13T02:14X3.4) in NOAA active region (AR) 10930, we reveal a more complex picture than what any single method might convey. Through imperfect but overall converging calculations from different methods, we find artifacts that could mislead conclusions. More importantly, we find evidence of competing physical tendencies in the active region whose omission could lead to counterintuitive, hence misleading, again, conclusions. While for the Sun we have the capability to use different data and methods for related purposes, this is not the case for other eruptive stars, which is a fact calling for robust modeling approaches, relying on scarce and indirect observations of stellar magnetic fields and CME properties. Confluence of any data available and modeling could offer the redundancy needed to critically assess partial findings and reconcile them into a physically consistent picture of stellar eruptions, quite possibly with qualitative / quantitative similarities and differences from the eruptions of our own Sun. Title: 4π Heliospheric Observing System - 4π-HeliOS: Exploring the Heliosphere from the Solar Interior to the Solar Wind Authors: Raouafi, Nour E.; Gibson, Sarah; Ho, George; Laming, J. Martin; Georgoulis, Manolis K.; Szabo, Adam; Vourlidas, Angelos; Mason, Glenn M.; Hoeksema, J. Todd; Velli, Marco; Berger, Thomas; Hassler, Donald M.; Kinnison, James; Viall, Nicholeen; Case, Anthony; Newmark, Jeffrey; Lepri, Susan; Krishna Jagarlamudi, Vamsee; Raouafi, Nour; Bourouaine, Sofiane; Vievering, Juliana T.; Englander, Jacob A.; Shannon, Jackson L.; Perez, Rafael M.; Chattopadhyay, Debarati; Mason, James P.; Leary, Meagan L.; Santo, Andy; Casti, Marta; Upton, Lisa A. Bibcode: 2022cosp...44.1530R Altcode: The 4$\pi$ Heliospheric Observing System (4$\pi$-HeliOS) is an innovative mission concept study for the next Solar and Space Physics Decadal Survey to fill long-standing knowledge gaps in Heliophysics. A constellation of spacecraft will provide both remote sensing and in situ observations of the Sun and heliosphere from a full 4$\pi$-steradian field of view. The concept implements a holistic observational philosophy that extends from the Sun's interior, to the photosphere, through the corona, and into the solar wind simultaneously with multiple spacecraft at multiple vantage points optimized for continual global coverage over much of a solar cycle. The mission constellation includes two spacecraft in the ecliptic and two flying as high as $\sim$70$^\circ$ solar latitude. 4$\pi$-HeliOS will provide new insights into the fundamental processes that shape the whole heliosphere. The overarching goals of the 4$\pi$-HeliOS concept are to understand the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the origin of the solar cycle, the causes of solar activity, and the structure and dynamics of the corona as it creates the heliosphere. The mission design study is underway at the Johns Hopkins Applied Physics Laboratory Concurrent Engineering Laboratory (ACE Lab), a premier mission design center, fostering rapid and collaborative mission design evolutions. Title: The SAWS-ASPECS Solar Energetic Particle (SEP) Advanced Warning System Authors: Anastasiadis, Anastasios; Aran, Angels; Georgoulis, Manolis K.; Jiggens, Piers; Balasis, Georgios; Vainio, Rami; Dierckxsens, Mark; Giannakis, Omiros; Sandberg, Ingmar; Papaioannou, Athanasios; Paassilta, Miikka; Paouris, Evangelos; Vasalos, George; Iqbal, Zafar; Aminalragia-Giamini, Sigiava Bibcode: 2022cosp...44.1184A Altcode: Solar particle events (SPEs) are radiation storms induced by eruptive processes on the Sun, namely solar flares and, more prominently, Coronal Mass Ejections (CMEs). These SPEs represent a concern for spacecraft launch and early orbit phase, mission and aircraft operators given the effects on electronics and the threat for human physiology. The SAWS-ASPECS system is a web based tool (http://phobos-srv.space.noa.gr/), that collates and combines outputs from different modules providing forecasts of solar phenomena, solar proton event occurrence and solar proton flux and duration characteristics. The system incorporates two basic operational modes: the forecasting (pre-event) and the nowcasting (post-event) mode. Concerning the pre-event forecasting mode, the starting point is the flare prediction. The outputs of the flare prediction are utilized in the provision of a conditional likelihood of SPE occurrence and an estimation of the expected characteristics (e.g. peak flux and duration) of a forthcoming SPE, with errors for different energies and as a function of different forecasting horizons. Concerning the post-event mode, the predictions for the probability of SPE occurrence and expected peak flux continuously evolve through updates based on near-real time inputs (e.g. solar flare and coronal mass ejections data/characteristics) received by the system. In addition, for the first time the complete time profile of the SPE at respective energies is provided in near real-time, utilizing both simulations and observations. This presentation will give an overview of the SAWS_ASPECS system. \underline{Acknowledgements:} The SAWS-ASPECS system was developed, receiving funding through the ESA activity Solar Energetic Particle (SEP) Advanced Warning System (SAWS) ESA Contract No. 4000120480/NL/LF/hh. Title: Assessment of near sun axial CME magnetic field. Authors: Koya, Shifana; Patsourakos, Spiros; Georgoulis, Manolis K.; Nindos, Alexander Bibcode: 2022cosp...44.1405K Altcode: The magnetic origin and the role of magnetic helicity in solar eruptions are known for several years. Here we present a survey of near-Sun axial coronal mass ejection (CME) magnetic fields that are obtained by applying a semi-analytical method that calculates the magnetic helicity of the source active region relying primarily on photospheric vector magnetograms. The geometrical parameters of CMEs observed by STEREO/SECCHI and SOHO/LASCO are obtained by fitting the GCS magnetic flux rope model. We use the estimated near-Sun CME magnetic fields to infer ICME magnetic fields and to validate them with existing in-situ magnetometer observation at L1. We conclude that the proposed method, including the proposed inferences from the survey, is useful for CME magnetic field forecasting purposes, solar-stellar connection and projecting towards potential properties of stellar CMEs. Title: Exploratory Analysis of Magnetic Polarity Inversion Line Metadata and Eruptive Characteristics of Solar Active Regions Authors: Aydin, Berkay; Georgoulis, Manolis K.; Martens, Petrus; Angryk, Rafal A.; Ji, Anli; Khasayeva, Nigar Bibcode: 2022cosp...44.3223A Altcode: Magnetic polarity inversion lines (PILs) detected in solar active regions and features engineered from them have been recognized as one of the essential features for predicting key characteristics of explosive and eruptive instabilities, such as solar flares and coronal mass ejections. We have built a systematic and comprehensive dataset of polarity inversion lines from line-of-sight magnetograms in HMI Active Region Patches (HARPs) data series. Our dataset covers the series ranging from May 2010 to December 2021. The dataset includes PIL-related binary masks of rasters (i.e., thinned PIL, the region of polarity inversion (RoPI), and the convex hull of PIL) as well as time series metadata extracted from these masks. We will introduce this multi-modal solar dataset and present some key results of our first exploratory analysis. We will further highlight relationships between the time evolution characteristics of magnetic PILs and provide an empirical analysis and predictive heuristics to demonstrate the usefulness of multi-modal PIL data in forecasting solar flares, both confined and eruptive. In particular, we will show the similarities and differences between pre-flare series of eruptive and confined flares and explore the relationships between PIL characteristics and the cumulative flare index. While this line of work is just starting, we emphasize the use of machine-learning-ready datasets for both physical and operational purposes, from understanding the key ingredients of the pre-instability phase in active regions to assigning fully validated forecast probabilities for major solar events that largely shape heliospheric space weather. Title: Automatic detection of small-scale EUV brightenings observed by the Solar Orbiter/EUI Authors: Alipour, N.; Safari, H.; Verbeeck, C.; Berghmans, D.; Auchère, F.; Chitta, L. P.; Antolin, P.; Barczynski, K.; Buchlin, É.; Aznar Cuadrado, R.; Dolla, L.; Georgoulis, M. K.; Gissot, S.; Harra, L.; Katsiyannis, A. C.; Long, D. M.; Mandal, S.; Parenti, S.; Podladchikova, O.; Petrova, E.; Soubrié, É.; Schühle, U.; Schwanitz, C.; Teriaca, L.; West, M. J.; Zhukov, A. N. Bibcode: 2022A&A...663A.128A Altcode: 2022arXiv220404027A Context. Accurate detections of frequent small-scale extreme ultraviolet (EUV) brightenings are essential to the investigation of the physical processes heating the corona.
Aims: We detected small-scale brightenings, termed campfires, using their morphological and intensity structures as observed in coronal EUV imaging observations for statistical analysis.
Methods: We applied a method based on Zernike moments and a support vector machine (SVM) classifier to automatically identify and track campfires observed by Solar Orbiter/Extreme Ultraviolet Imager (EUI) and Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA).
Results: This method detected 8678 campfires (with length scales between 400 km and 4000 km) from a sequence of 50 High Resolution EUV telescope (HRIEUV) 174 Å images. From 21 near co-temporal AIA images covering the same field of view as EUI, we found 1131 campfires, 58% of which were also detected in HRIEUV images. In contrast, about 16% of campfires recognized in HRIEUV were detected by AIA. We obtain a campfire birthrate of 2 × 10−16 m−2 s−1. About 40% of campfires show a duration longer than 5 s, having been observed in at least two HRIEUV images. We find that 27% of campfires were found in coronal bright points and the remaining 73% have occurred out of coronal bright points. We detected 23 EUI campfires with a duration greater than 245 s. We found that about 80% of campfires are formed at supergranular boundaries, and the features with the highest total intensities are generated at network junctions and intense H I Lyman-α emission regions observed by EUI/HRILya. The probability distribution functions for the total intensity, peak intensity, and projected area of campfires follow a power law behavior with absolute indices between 2 and 3. This self-similar behavior is a possible signature of self-organization, or even self-organized criticality, in the campfire formation process.

Supplementary material (S1-S3) is available at https://www.aanda.org Title: All-Clear in Solar Energetic Particles (SEP) Event Forecasting: Feasibility and Challenges Authors: Georgoulis, Manolis K. Bibcode: 2022cosp...44.3220G Altcode: Equally important to the prediction of a solar weather phenomenon affecting space weather in the heliosphere is the prediction of no such phenomenon occurring from the Sun within a certain forecast window. This has been conventionally coined an "All-Clear" forecast. Predicting All Clear is imperative for sensitive operations in space involving vulnerable humans and equipment in orbit or in future deep space expeditions on the surface of the Moon or even Mars. All Clear is the antipode of an event forecasting, but I will argue that simply taking the complementary of the full-Sun event probability is crudely oversimplified, particularly for rare events, completely ignoring False Negatives. Conversely, precluding an All Clear if a hint of a possible event exists in a solar source is infeasible, completing ignoring False Positives, particularly in solar maximum conditions. I will attempt to specify All Clear for Solar Energetic Particle (SEP) event forecasting and will aim to highlight the issues, ambiguity and ambivalence surrounding it. Different event definitions and potential refined estimates on the basis of an outlier analysis will also be discussed. Particular emphasis will be further given to False Negative (Misses) due to solar sources that have not yet rotated to the earthward solar hemisphere. This is a discussion and debate in progress but will shape the state-of-the-art if and when a consensus within the community is reached. Title: Identifying the Terrestrial Exoplanets which Deserve More Scrutiny for Atmosphere Viability: the mASC method Authors: Samara, Evangelia; Patsourakos, Spiros; Georgoulis, Manolis K. Bibcode: 2022cosp...44.1395S Altcode: We introduce a practical and physically intuitive method to assess whether a given exoplanet is a viable candidate for the existence of an atmosphere, thanks to an efficient magnetospheric shielding from intense space weather activity from its host star. Our proposed mASC (magnetic Atmosphere Sustainability Constraint) relies on a best-case scenario for a dynamo-generated planetary magnetic field and subsequent magnetic pressure, and a worst-case scenario for the magnetic pressure of stellar CMEs. The method estimates a dimensionless ratio R whose excursion from unity implies accordingly an "atmosphere likely" (R < 1) or an "atmosphere unlikely" (R > 1) scenario. In this work, we implement our mASC on six "famous" exoplanets whose discovery was greeted with praise and hopes of habitability. These are Kepler-438b, Proxima-Centauri b, and Trappist-1d, -1e, -1f, -1g. We conclude that for none of them the existence of an atmosphere is likely while our findings are robust for five out of six cases. We conclude that the mASC ratio could help set observing priorities and suggest which exoplanets deserve further scrutiny, possibly toward the ultimate search of potential biosignatures, among other objectives. Title: Investigating possible EUV precursors of major solar flares Authors: AndrÉ-Hoffmann, Augustin; Patsourakos, Spiros; Georgoulis, Manolis K.; Nindos, Alexander Bibcode: 2022cosp...44.2481A Altcode: Large-scale solar eruptions that produce major flares and fast coronal mass ejections are often associated with precursor activity that may start several hours before the main event. Such activity may be observed from the photosphere all the way to the transition region and corona but it is not clear whether it plays an essential role in the eruption initiation. To investigate this question we search for precursor activity in Extreme Ultra-violet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) instrument on board Solar Dynamics Observatory (SDO) within a 24-hour window prior to large eruptive events and investigate whether they contribute to the restructuring and overall evolution of the magnetic field that leads to eruptions. We cross-check our findings by performing the same search in relatively quiet active regions. By comparing the results from the eruptive and quiet active regions we attempt to identify possible signatures that could be useful in both the short-term prediction of major flare events and an enhanced physical understanding of the preflare phase. Title: Exploring Heuristics in Full-Disk Aggregation from Individual Active Region Prediction of Solar Flares Authors: Pandey, Chetraj; Georgoulis, Manolis K.; Aydin, Berkay; Angryk, Rafal A.; Ji, Anli Bibcode: 2022cosp...44.3457P Altcode: Ensemble modeling can boost the predictive confidence of base learners (weak learners) by producing a more optimal model suitable for operational forecasting. In cases where the base learners have uniformity in the problem formulation and datasets, it becomes a relatively easier task to generate a new meta-model that enhances prediction capabilities. In this work, we deal with coupling two solar flare prediction models in which our base learners are trained with two different paradigms: (i) a full-disk mode, where prediction models are trained using full-disk line-of-sight magnetograms and deep learning architectures to issue a full-disk flare probability, and (ii) an active region-based mode, where models utilize multivariate time series data and a multivariate Time Series Forest (TSF) classifier to issue a prediction for each valid active region individually. So far in the literature, individual active region forecasts are aggregated by computing the probability of flare from at least one active region assuming conditional independence and then these flare probabilities are used to compute a full-disk flare occurrence probability by means of a meta-model. However, our empirical analysis on active region aggregation shows that the method of aggregation can be considered rather subjective depending on the distribution of individual models' output. We observe that exploring different heuristics while aggregation impacts the predictive performance of the AR-based models and consequently the meta-models. This shows a need for searching an AR-aggregation heuristic that suits the overall probability distribution of our two base learners. In this research, we present different AR-aggregation heuristics in consideration to the flare probability distribution of our base learners and introduce a more effective heuristic for active region aggregation by verifying performance improvement using metrics such as the True Skill Statistic and the Heidke Skill Score. Title: Influence of coronal hole morphology on the solar wind speed at Earth Authors: Samara, Evangelia; Magdalenić, Jasmina; Rodriguez, Luciano; Heinemann, Stephan G.; Georgoulis, Manolis K.; Hofmeister, Stefan J.; Poedts, Stefaan Bibcode: 2022A&A...662A..68S Altcode: 2022arXiv220400368S Context. It has long been known that the high-speed stream (HSS) peak velocity at Earth directly depends on the area of the coronal hole (CH) on the Sun. Different degrees of association between the two parameters have been shown by many authors. In this study, we revisit this association in greater detail for a sample of 45 nonpolar CHs during the minimum phase of solar cycle 24. The aim is to understand how CHs of different properties influence the HSS peak speeds observed at Earth and draw from this to improve solar wind modeling.
Aims: The CHs were extracted based on the Collection of Analysis Tools for Coronal Holes which employs an intensity threshold technique applied to extreme-ultraviolet filtergrams. We first examined all the correlations between the geometric characteristics of the CHs and the HSS peak speed at Earth for the entire sample. The CHs were then categorized in two different groups based on morphological criteria, such as the aspect ratio and the orientation angle. We also defined the geometric complexity of the CHs, a parameter which is often neglected when the formation of the fast solar wind at Earth is studied. The quantification of complexity was done in two ways. First, we considered the ratio of the maximum inscribed rectangle over the convex hull area of the CH. The maximum inscribed rectangle provides an estimate of the area from which the maximum speed of the stream originates. The convex hull area is an estimate of how irregular the CH boundary is. The second way of quantifying the CH complexity was carried out by calculating the CH's fractal dimension which characterizes the raggedness of the CH boundary and internal structure.
Methods: When treating the entire sample, the best correlations were achieved between the HSS peak speed observed in situ, and the CH longitudinal extent. When the data set was split into different subsets, based on the CH aspect ratio and orientation angle, the correlations between the HSS maximum velocity and the CH geometric characteristics significantly improved in comparison to the ones estimated for the whole sample. By further dividing CHs into subsets based on their fractal dimension, we found that the Pearson's correlation coefficient in the HSS peak speed - CH area plot decreases when going from the least complex toward the most complex structures. Similar results were obtained when we considered categories of CHs based on the ratio of the maximum inscribed rectangle over the convex hull area of the CH. To verify the robustness of these results, we applied the bootstrapping technique. The method confirmed our findings for the entire CH sample. It also confirmed the improved correlations, compared to the ones found for the whole sample, between the HSS peak speed and the CH geometric characteristics when we divided the CHs into groups based on their aspect ratio and orientation angle. Bootstrapping results for the CH complexity categorizations are, nonetheless, more ambiguous.
Results: Our results show that the morphological parameters of CHs such as the aspect ratio, orientation angle, and complexity play a major role in determining the HSS peak speed at 1 AU. Therefore, they need to be taken into consideration for empirical models that aim to forecast the fast solar wind at Earth based on the observed CH solar sources. Title: First Application of a Theoretically Derived Coupling Function in Cosmic-Ray Intensity for the Case of the 10 September 2017 Ground-Level Enhancement (GLE 72) Authors: Xaplanteris, L.; Gerontidou, M.; Mavromichalaki, H.; Rodriguez, J. V.; Livada, M.; Georgoulis, M. K.; Sarris, T. E.; Spanos, V.; Dorman, L. Bibcode: 2022SoPh..297...73X Altcode: In this work we implement an analytically derived coupling function between ground-level and primary proton particles for the case of ground-level enhancement events (GLEs). The main motivation for this work is to determine whether this coupling function is suitable for the study of both major cases of cosmic-ray (CR) variation events, namely GLEs and Forbush decreases. This version of the coupling function, which relies on formalism used in quantum field theory (QFT) computations, has already been applied to Forbush decreases yielding satisfactory results. In this study, it is applied to a GLE event that occurred on 10 September 2017. For the analytical derivations, normalized ground-level cosmic-ray data were used from seven neutron-monitor stations with low cutoff rigidities. To assess and evaluate the results for the normalized proton intensity, we benchmark them with the time series for the proton flux, as recorded by the GOES 13 spacecraft during the same time period. The theoretically calculated results for proton energy ≥1 GeV are in general agreement with the recorded data for protons with energy >700 MeV, presenting a least-squares linear best fit with slope 0.75 ±0.17 and a Pearson correlation coefficient equal to 0.62. We conclude that the coupling function presented in this work is the first coupling function that is well applicable to both cases of cosmic-ray intensity events, namely GLEs and Forbush decreases. Title: Integrated Geostationary Solar Energetic Particle Events Catalog: GSEP Authors: Rotti, Sumanth; Aydin, Berkay; Georgoulis, Manolis K.; Martens, Petrus C. Bibcode: 2022arXiv220412021R Altcode: We present a catalog of solar energetic particle (SEP) events covering solar cycles 22, 23 and 24. We correlate and integrate three existing catalogs based on Geostationary Operational Environmental Satellite (GOES) integral proton flux data. We visually verified and labeled each event in the catalog to provide a homogenized data set. We have identified a total of 341 SEP events of which 245 cross the space weather prediction center (SWPC) threshold of a significant proton event. The metadata consists of physical parameters and observables concerning the possible source solar eruptions, namely flares and coronal mass ejections for each event. The sliced time series data of each event, along with intensity profiles of proton fluxes in several energy bands, have been made publicly available. This data set enables researchers in machine learning (ML) and statistical analysis to understand the SEPs and the source eruption characteristics useful for space weather prediction. Title: Revisiting the Solar Research Cyberinfrastructure Needs: A White Paper of Findings and Recommendations Authors: Nita, Gelu; Ahmadzadeh, Azim; Criscuoli, Serena; Davey, Alisdair; Gary, Dale; Georgoulis, Manolis; Hurlburt, Neal; Kitiashvili, Irina; Kempton, Dustin; Kosovichev, Alexander; Martens, Piet; McGranaghan, Ryan; Oria, Vincent; Reardon, Kevin; Sadykov, Viacheslav; Timmons, Ryan; Wang, Haimin; Wang, Jason T. L. Bibcode: 2022arXiv220309544N Altcode: Solar and Heliosphere physics are areas of remarkable data-driven discoveries. Recent advances in high-cadence, high-resolution multiwavelength observations, growing amounts of data from realistic modeling, and operational needs for uninterrupted science-quality data coverage generate the demand for a solar metadata standardization and overall healthy data infrastructure. This white paper is prepared as an effort of the working group "Uniform Semantics and Syntax of Solar Observations and Events" created within the "Towards Integration of Heliophysics Data, Modeling, and Analysis Tools" EarthCube Research Coordination Network (@HDMIEC RCN), with primary objectives to discuss current advances and identify future needs for the solar research cyberinfrastructure. The white paper summarizes presentations and discussions held during the special working group session at the EarthCube Annual Meeting on June 19th, 2020, as well as community contribution gathered during a series of preceding workshops and subsequent RCN working group sessions. The authors provide examples of the current standing of the solar research cyberinfrastructure, and describe the problems related to current data handling approaches. The list of the top-level recommendations agreed by the authors of the current white paper is presented at the beginning of the paper. Title: A Systematic Magnetic Polarity Inversion Line Detection Dataset from SDO/HMI Magnetogram Authors: Ji, Anli; Cai, Xumin; Aydin, Berkay; Georgoulis, Manolis; Angryk, Rafal Bibcode: 2021AGUFMSH55A1818J Altcode: Magnetic polarity inversion line (PIL) detected from solar active regions and features engineered from them have been recognized as one of the essential features for predicting key characteristics of eruptive events such as flares and coronal mass ejections. However, in most cases, PIL masks are generated as a byproduct and they are not comprehensively available for public use. To fill the gap in this field, we introduce a large-scale PIL dataset that can be used for various space weather forecasting tasks. The dataset is created using the geometries from the HMI Active Region Patches (HARPs) and involves 4,090 HARP series ranging from May 2010 to March 2019. This dataset includes PIL-related binary masks as rasters (i.e., thinned PIL, the region of polarity inversion (RoPI), and the convex hull of PIL) and time series metadata extracted from these masks. The PIL detection procedure is based on an edge detection technique along with magnetic field strength and PIL size filter. First, we identify positive and negative polarity regions with a magnetic field strength threshold. Then, we utilize the Canny edge detector and morphological operations to both positive and negative regions to identify coarse PILs. Finally, we generate PILs by applying magnetic field strength and PIL size filter to the coarse PILs as mentioned above. We envision that this comprehensive PIL dataset will benefit space weather analytics research, specifically in understanding the PIL structure and evolution, and complementing the existing datasets used for space weather forecasting. Title: Which Terrestrial Exoplanets Deserve More Scrutiny for Atmosphere Viability? Authors: Samara, Evangelia; Patsourakos, Spiros; Georgoulis, Manolis Bibcode: 2021AGUFM.U44B..05S Altcode: We introduce a practical and physically intuitive method to assess whether a given exoplanet is a viable candidate for the existence of an atmosphere thanks to an efficient magnetospheric shielding from intense space weather activity originating from its host star. Our proposed mASC (magnetic Atmosphere Sustainability Constraint) relies on a best-case scenario for the dynamo-generated planetary magnetic field and subsequent magnetic pressure, and a worst-case scenario for the magnetic pressure of stellar CMEs. It provides a dimensionless ratio R whose excursion from unity implies accordingly an atmosphere likely (R < 1) or an atmosphere unlikely (R > 1) scenario. In this work, we implement our mASC on six famous exoplanets whose discovery was greeted with praise and hopes of habitability. These are Kepler-438b, Proxima-Centauri b, and Trappist-1d, -1e, -1f, -1g. The results show that for none of them the existence of an atmosphere is likely while our findings are robust for five out of six cases. We conclude that the mASC ratio could help set observing priorities and suggest which exoplanets deserve further scrutiny, possibly toward the ultimate search of potential biosignatures, among other objectives. Title: Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. IV. Application to Solar Observations Authors: Thalmann, J. K.; Georgoulis, M. K.; Liu, Y.; Pariat, E.; Valori, G.; Anfinogentov, S.; Chen, F.; Guo, Y.; Moraitis, K.; Yang, S.; Mastrano, Alpha; ISSI Team on Magnetic Helicity Bibcode: 2021ApJ...922...41T Altcode: 2021arXiv210808525T In this ISSI-supported series of studies on magnetic helicity in the Sun, we systematically implement different magnetic helicity calculation methods on high-quality solar magnetogram observations. We apply finite-volume, discrete flux tube (in particular, connectivity-based) and flux-integration methods to data from Hinode's Solar Optical Telescope. The target is NOAA Active Region 10930 during a 1.5-day interval in 2006 December that included a major eruptive flare (SOL2006-12-13T02:14X3.4). Finite-volume and connectivity-based methods yield instantaneous budgets of the coronal magnetic helicity, while the flux-integration methods allow an estimate of the accumulated helicity injected through the photosphere. The objectives of our work are twofold: a cross-validation of methods, as well as an interpretation of the complex events leading to the eruption. To the first objective, we find (i) strong agreement among the finite-volume methods, (ii) a moderate agreement between the connectivity-based and finite-volume methods, (iii) an excellent agreement between the flux-integration methods, and (iv) an overall agreement between finite-volume- and flux-integration-based estimates regarding the predominant sign and magnitude of the helicity. To the second objective, we are confident that the photospheric helicity flux significantly contributed to the coronal helicity budget and that a right-handed structure erupted from a predominantly left-handed corona during the X-class flare. Overall, we find that the use of different methods to estimate the (accumulated) coronal helicity may be necessary in order to draw a complete picture of an active region corona, given the careful handling of identified data (preparation) issues, which otherwise would mislead the event analysis and interpretation. Title: Space Weather research in the Digital Age and across the full data lifecycle: Introduction to the Topical Issue Authors: McGranaghan, Ryan M.; Camporeale, Enrico; Georgoulis, Manolis; Anastasiadis, Anastasios Bibcode: 2021JSWSC..11...50M Altcode: The onset and rapid advance of the Digital Age have brought challenges and opportunities for scientific research characterized by a continuously evolving data landscape reflected in the four V's of big data: volume, variety, veracity, and velocity. The big data landscape supersedes traditional means of storage, processing, management, and exploration, and requires adaptation and innovation across the full data lifecycle (i.e., collection, storage and processing, analytics, and representation). The Topical Issue, "Space Weather research in the Digital Age and across the full data lifecycle", collects research from across the full data lifecycle (collection, management, analysis, and communication; collectively "Data Science") and offers a tractable compendium that illustrates the latest computational and data science trends, tools, and advances for Space Weather research. We introduce the paradigm shift in Space Weather and the articles in the Topical Issue. We create a network view of the research that highlights the contribution to the change of paradigm and reveals the trends that will guide it hereafter. Title: Improved Approach in the Coupling Function Between Primary and Ground Level Cosmic Ray Particles Based on Neutron Monitor Data Authors: Xaplanteris, L.; Livada, M.; Mavromichalaki, H.; Dorman, L.; Georgoulis, M. K.; Sarris, T. E. Bibcode: 2021SoPh..296...91X Altcode: In this work an improved approach of existing approximations on the coupling function between primary and ground-level cosmic-ray particles is presented. The proposed coupling function is analytically derived based on a formalism used in Quantum Field Theory calculations. It is upgraded compared to previous versions with the inclusion of a wider energy spectrum that is extended to lower energies, as well as an altitude correction factor, also derived analytically. The improved approximations are applied to two cases of Forbush decreases detected in March 2012 and September 2017. In the analytical procedure for the derivation of the primary cosmic-ray spectrum during these events, we also consider the energy spectrum exponent γ to be varied with time. For the validation of the findings, we present a direct comparison between the primary spectrum and the amplitude values derived by the proposed method and the obtained time series of the cosmic-ray intensity at the rigidity of 10 GV obtained from the Global Survey Method. The two sets of results are found to be in very good agreement for both events as denoted by the Pearson correlation factors and slope values of their scatter plots. In such way we determine the validity and applicability of our method to Forbush decreases as well as to other cosmic-ray phenomena, thus introducing a new, alternative way of inferring the primary cosmic-ray intensity. Title: Verification Of A Practical Magnetic Helicity Budget Calculation And Its Contribution To Axial Field Estimates Of Solar And Stellar CMEs Authors: Georgoulis, M. K. Bibcode: 2021AAS...23821320G Altcode: Initial work by Georgoulis, Tziotziou & Raouafi (ApJ, 759:1, 2012) on the estimation of the coronal relative magnetic helicity budget in solar active regions has been scrutinized in multiple occasions by comparison to 'ground-truth' calculations of the volume helicity using classical formulas, mainly in synthetic magnetic configurations. The so-called connectivity-based helicity / energy calculation method infers a coronal magnetic connectivity matrix without three-dimensional extrapolation or magnetohydrodynamical modeling and then calculates magnetic energy and relative helicity budgets on a discretized flux tube ensemble as per the inferred connectivity. We briefly report on the comparison of the method with classical 'finite volume' methods applied to real solar active regions, in which the coronal magnetic field has been inferred by nonlinear force-free extrapolations. Then we describe a recent extension of this line on work that relies on the fundamental conservation principle for the magnetic helicity to infer a worst-case axial magnetic field (Bz) for solar and stellar coronal mass ejections (CMEs) at various locations in the helio-/astro-sphere. Besides connecting the flaring with the CME manifestations in the eruptive active regions, this approach can contribute to an improved understanding of stellar forcing on exoplanetary systems and has, in fact, led to the recent derivation of an atmosphere sustainability constraint for terrestrial exoplanets. Title: How to Train Your Flare Prediction Model: Revisiting Robust Sampling of Rare Events Authors: Ahmadzadeh, Azim; Aydin, Berkay; Georgoulis, Manolis K.; Kempton, Dustin J.; Mahajan, Sushant S.; Angryk, Rafal A. Bibcode: 2021ApJS..254...23A Altcode: 2021arXiv210307542A We present a case study of solar flare forecasting by means of metadata feature time series, by treating it as a prominent class-imbalance and temporally coherent problem. Taking full advantage of pre-flare time series in solar active regions is made possible via the Space Weather Analytics for Solar Flares (SWAN-SF) benchmark data set, a partitioned collection of multivariate time series of active region properties comprising 4075 regions and spanning over 9 yr of the Solar Dynamics Observatory period of operations. We showcase the general concept of temporal coherence triggered by the demand of continuity in time series forecasting and show that lack of proper understanding of this effect may spuriously enhance models' performance. We further address another well-known challenge in rare-event prediction, namely, the class-imbalance issue. The SWAN-SF is an appropriate data set for this, with a 60:1 imbalance ratio for GOES M- and X-class flares and an 800:1 imbalance ratio for X-class flares against flare-quiet instances. We revisit the main remedies for these challenges and present several experiments to illustrate the exact impact that each of these remedies may have on performance. Moreover, we acknowledge that some basic data manipulation tasks such as data normalization and cross validation may also impact the performance; we discuss these problems as well. In this framework we also review the primary advantages and disadvantages of using true skill statistic and Heidke skill score, two widely used performance verification metrics for the flare-forecasting task. In conclusion, we show and advocate for the benefits of time series versus point-in-time forecasting, provided that the above challenges are measurably and quantitatively addressed. Title: The flare likelihood and region eruption forecasting (FLARECAST) project: flare forecasting in the big data & machine learning era Authors: Georgoulis, Manolis K.; Bloomfield, D. Shaun; Piana, Michele; Massone, Anna Maria; Soldati, Marco; Gallagher, Peter T.; Pariat, Etienne; Vilmer, Nicole; Buchlin, Eric; Baudin, Frederic; Csillaghy, Andre; Sathiapal, Hanna; Jackson, David R.; Alingery, Pablo; Benvenuto, Federico; Campi, Cristina; Florios, Konstantinos; Gontikakis, Constantinos; Guennou, Chloe; Guerra, Jordan A.; Kontogiannis, Ioannis; Latorre, Vittorio; Murray, Sophie A.; Park, Sung-Hong; von Stachelski, Samuelvon; Torbica, Aleksandar; Vischi, Dario; Worsfold, Mark Bibcode: 2021JSWSC..11...39G Altcode: 2021arXiv210505993G The European Union funded the FLARECAST project, that ran from January 2015 until February 2018. FLARECAST had a research-to-operations (R2O) focus, and accordingly introduced several innovations into the discipline of solar flare forecasting. FLARECAST innovations were: first, the treatment of hundreds of physical properties viewed as promising flare predictors on equal footing, extending multiple previous works; second, the use of fourteen (14) different machine learning techniques, also on equal footing, to optimize the immense Big Data parameter space created by these many predictors; third, the establishment of a robust, three-pronged communication effort oriented toward policy makers, space-weather stakeholders and the wider public. FLARECAST pledged to make all its data, codes and infrastructure openly available worldwide. The combined use of 170+ properties (a total of 209 predictors are now available) in multiple machine-learning algorithms, some of which were designed exclusively for the project, gave rise to changing sets of best-performing predictors for the forecasting of different flaring levels, at least for major flares. At the same time, FLARECAST reaffirmed the importance of rigorous training and testing practices to avoid overly optimistic pre-operational prediction performance. In addition, the project has (a) tested new and revisited physically intuitive flare predictors and (b) provided meaningful clues toward the transition from flares to eruptive flares, namely, events associated with coronal mass ejections (CMEs). These leads, along with the FLARECAST data, algorithms and infrastructure, could help facilitate integrated space-weather forecasting efforts that take steps to avoid effort duplication. In spite of being one of the most intensive and systematic flare forecasting efforts to-date, FLARECAST has not managed to convincingly lift the barrier of stochasticity in solar flare occurrence and forecasting: solar flare prediction thus remains inherently probabilistic. Title: A Readily Implemented Atmosphere Sustainability Constraint for Terrestrial Exoplanets Orbiting Magnetically Active Stars Authors: Samara, Evangelia; Patsourakos, Spiros; Georgoulis, Manolis K. Bibcode: 2021ApJ...909L..12S Altcode: 2021arXiv210207837S With more than 4300 confirmed exoplanets and counting, the next milestone in exoplanet research is to determine which of these newly found worlds could harbor life. Coronal mass ejections (CMEs), spawned by magnetically active, superflare-triggering dwarf stars, pose a direct threat to the habitability of terrestrial exoplanets, as they can deprive them of their atmospheres. Here we develop a readily implementable atmosphere sustainability constraint for terrestrial exoplanets orbiting active dwarfs, relying on the magnetospheric compression caused by CME impacts. Our constraint focuses on an understanding of CMEs propagation in our own Sun-heliosphere system that, applied to a given exoplanet requires as key input the observed bolometric energy of flares emitted by its host star. Application of our constraint to six famous exoplanets, Kepler-438b, Proxima Centauri b, and Trappist-1d, -1e, -1f, and -1g, within or in the immediate proximity of their stellar host's habitable zones showed that only for Kepler-438b might atmospheric sustainability against stellar CMEs be likely. This seems to align with some recent studies that, however, may require far more demanding computational resources and observational inputs. Our physically intuitive constraint can be readily and en masse applied, as is or generalized, to large-scale exoplanet surveys to detect planets that warrant further scrutiny for atmospheres and, perhaps, possible biosignatures at higher priority by current and future instrumentation. Title: Data Benchmarking for Solar Flare, CME and SEP Event Forecasting: Different Prediction and Verification Needs, Unified Authors: Georgoulis, Manolis K.; Martens, Petrus; Aydin, Berkay; Ahmadzadeh, Azim; Kempton, Dustin J.; Angryk, Rafal A. Bibcode: 2021cosp...43E2357G Altcode: In this synergistic, interdisciplinary work we convey two principles that we consider central to improving space weather forecast capabilities of current and future modeling efforts: first, that data, model and performance verification tasks are equally important and should be treated on equal footing. Second, that the solar end of adverse space weather, comprising flares, coronal mass ejections (CMEs) and Solar Energetic Particle (SEP) events should be viewed and treated as a single, albeit multi-faceted, physical problem, rather than as a set of standalone problems corresponding to each facet. We present paradigms for both of these principles: first, we discuss a solar flare benchmark dataset in which the data are fully verified and we have taken steps to verify different forecast models and their performance. We show that unverified models lead to much degraded performance. Second, we outline the main aspects of a methodology to forecast SEP events in terms of temporal profile and peak proton flux starting from forecasting their source eruptions in the Sun. Performance needs dictate the introduction of two modeling tiers, one for eruption forecasting and projected SEP properties and another for updated, forecast SEP properties in case of SEP-eligible eruptions. This practice enables a dual validation of the performance for both tiers, at the same time constraining applicable uncertainties that could otherwise render the overall task untenable. Benefits of this approach in terms of both operations-to-research and research-to-operations are profound and can lead to both an improved physical understanding of vast swaths of the heliosphere, along with future prediction services combining computational efficiency with proven, quantifiable value. Title: Properties Determining Eruption Initiation and Planeto-Effectiveness of Eruptive Transients in Magnetically Active Stars Authors: Georgoulis, Manolis K.; Patsourakos, Spiros; Zhang, Hongqi; Nindos, Alexander; Samara, Evangelia; Sadykov, Viacheslav M. Bibcode: 2021cosp...43E.993G Altcode: We present a combined theoretical and data analysis approach to, first, understand why magnetic eruptions and corresponding ejecta are triggered in strong-field regions of the Sun and magnetically active stars and, second, assess the key physical parameters responsible for the planeto-effectiveness of these eruptions, both on Earth and in other (exo-)planets. This approach converges on one physical parameter besides magnetic energy, at least for stellar coronae of high magnetic Reynolds numbers allowing this parameter to be conserved even under confined energy release: magnetic helicity. Helicity, via the magnetic energy-helicity diagram, should be treated equally with magnetic energy. Due to magnetic helicity accumulation in solar active regions and its inverse cascading, solar - and stellar, correspondingly - eruptions may become inevitable after a certain 'point-of-no-return' is reached. We identify this critical instant as the time when magnetic polarity inversion lines in active-region photospheres accumulate fluxes that generate fields stronger than local equipartition values. Furthermore, using the conserved helicity budgets we abstractly model post-eruption flux ropes and their transit through astrospheres, reaching exoplanets and compressing their magnetospheres via magnetic pressure effects. A rudimentary validation between the near-Sun and L1 axial magnetic field values of these data-constrained flux ropes is encouraging and allows us to further constrain scaling laws appropriate for the astrospheric transit of these ropes. Importantly, we also find that exoplanets orbiting magnetically active dwarf stars at orbital radii that are fractions of an astronomical unit seem to be strong contenders for eruption-driven atmospheric erosion that may gradually even deprive them from their atmospheres. Some famous exoplanet cases are examined under this prism. Future improvements are expected by widely anticipated space- (Parker Solar Probe and Solar Orbiter) and ground-based (Daniel K. Inouye Solar Telescope) observations. Title: ESA's SEP Advanced Warning System: The ASPECS Project Authors: Jiggens, Piers; Aran, Angels; Georgoulis, Manolis K.; Balasis, Georgios; Vainio, Rami; Dierckxsens, Mark; Giannakis, Omiros; Sandberg, Ingmar; Papaioannou, Athanasios; Paassilta, Miikka; Anastasiadis, Anastasios; Paouris, Evangelos; Vasalos, George; Iqbal, Zafar; Aminalragia-Giamini, Sigiava; Tsigkanos, Antonis Bibcode: 2021cosp...43E1041J Altcode: Solar particle events (SPEs) are radiation storms induced by eruptive processes on the Sun, namely solar flares and, more prominently, Coronal Mass Ejections (CMEs). These SPEs represent a concern for the spacecraft, launch, human spaceflight and aircraft operators given the effects on electronics and human physiology. The outcome of the ASPECS (Advanced Solar Particle Event Casting System) project represents the first tool for forecasting Solar Energetic Particles (SEPs) from several days before the commencement of the radiation storm up to nowcasting of the evolution of the storm. This is achieved by a 3-tier system combining the forecasting of flares, the statistical forecast of events based on flare and CME characteristics and physical and analytical modelling for predicting particle flux profiles. The module in the process is the prediction of flares. The flare forecast used as a proxy of SPEs due to the relative maturity of this element of space weather forecasting relative to CME forecasting. The present model for forecasting flares is based on the B-effective parameter of Georgoulis et al. that captures complexity in addition to magnetic field strength of active region. This model has been extended to make predictions for periods from 6 hours to 3 days and fits made to extend forecasts to the highest intensity flares. The central plank of the system forecasts the likelihood of SPEs from input flare and CME characteristics of location and magnitude, and velocity and width respectively by use of a method based on Bayesian statistics. When both sets of inputs are available, the system combines the forecasts and adapts them when neutron monitors trigger a GLE (from the fastest particles arriving first indicating an SPE is commencing) and electron fluxes (whose absence after a given time indicates no event). This module outputs peak fluxes forecasts for events for protons at four energies (>10, >30, >100 and >300 MeV). Before a solar event, this module uses a separate sub-module for data-driven forecasting of SPE likelihood across the different energies based on a historical SPE occurrence as a function of active region location and flare size (taken from the first module). The final module forecasts the flux profile of the SPE for each energy based on a set of physically-reconstructed profiles using the SOLPENCO(-2) tool given input parameters of solar source longitude and the peak flux prediction. This profile is adjusted with observed fluxes as and when the particles are detected in space. If the initially profile is found to not be well fit to the observed data then the system chooses another physicallyreconstructed profile from the database. If the event progresses in a way that no profile can fit the observations then an analytical profile, based on the work of Kahler and Ling, replaces the SOLPENCO(-2) output. This is especially important in the downstream region which is not modelled by SOLEPNCO(-2). The result is the pre-operational version of ESA's SEP Advanced Warning System (SAWS) spanning energies of interest for different end users. The software is developed in a modular way to take advantage of new modelling developments and data as it becomes available. An extensive verification and validation process is underway to arrive at a stable system to be used by a range of operators. This work has been carried out by the Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS) at the National Observatory of Athens and the Space Applications and Research Consultancy (SPARC) in Greece, the University of Turku in Finland, the University of Barcelona in Spain and the Belgian Institute for Space Aeronomy under ESA contract No. 4000120480/17/NL/LF/hh. Title: Coronal Sigmoids and Chromospheric Filaments - A relation? Authors: Venkataramanasastry, Aparna; Georgoulis, Manolis K.; Martens, Petrus Bibcode: 2021cosp...43E1765V Altcode: Sigmoids are forward (S-shaped) or inverse (Z-shaped) features on the Sun that are seen at coronal heights in X-ray or high temperature extreme ultraviolet (EUV) wavelengths. The sharpest and brightest of them are highly eruptive. They usually deform or disappear via coronal mass ejections. This makes it important to understand X-ray sigmoids because of their relevance for space-weather forecasting purposes. Chromospheric filaments are generally observed as absorption features in H$\alpha$ wavelengths. In this work, we observe chromospheric filaments lying under the sigmoids in order to correlate the chiralities of the two features. We expect that if the formation mechanisms of filaments and sigmoids are similar then they should have same chirality. We have conducted a joint survey of the sigmoids and the underlying filaments between 2007 and 2017. We use Hinode soft and hard X-ray data for sigmoid images and GONG H$\alpha$ data for the filaments. We find a total of 84 sigmoids with filaments within the said time period. Among these we have 41 forward and 43 inverse sigmoids. In the 41 forward sigmoids, 8 (20%) filaments are dextral, 21 (50%) are sinistral and 12 (30%) ambiguous. In the inverse sigmoids, 16 filaments (37%) are dextral, 13 (30%) sinistral and 14 (33%) ambiguous. It is evident from this analysis that there is no clear correspondence between filament chirality and the sigmoid handedness. This result warrants further investigation. We therefore perform calculations of magnetic helicity in the photospheric regions that encloses the footprints of sigmoids and filaments to primarily obtain the sign of their helicities. For many of the cases mentioned above, we use SHARP (Space-weather HMI Active Region Patch) vector magnetograms to serve as the photospheric boundary condition to which we apply the linear force-free magnetic helicity and energy formulations of Georgoulis \& LaBonte (2007). We will report on the findings of this study during the 43rd COSPAR General Assembly. References Georgoulis, M. K. \& LaBonte, B. J., ApJ, 2007, 671, 1034 Title: Differential Emission Measure Evolution as a Precursor of Solar Flares Authors: Gontikakis, C.; Kontogiannis, I.; Georgoulis, M. K.; Guennou, C.; Syntelis, P.; Park, S. H.; Buchlin, E. Bibcode: 2020arXiv201106433G Altcode: We analyse the temporal evolution of the Differential Emission Measure (DEM) of solar active regions and explore its usage in solar flare prediction. The DEM maps are provided by the Gaussian Atmospheric Imaging Assembly (GAIA-DEM) archive, calculated assuming a Gaussian dependence of the DEM on the logarithmic temperature. We analyse time-series of sixteen solar active regions and a statistically significant sample of 9454 point-in-time observations corresponding to hundreds of regions observed during solar cycle 24. The time-series analysis shows that the temporal derivatives of the Emission Measure dEM/dt and the maximum DEM temperature dTmax/dt frequently exhibit high positive values a few hours before M- and X-class flares, indicating that flaring regions become brighter and hotter as the flare onset approaches. From the point-in-time observations we compute the conditional probabilities of flare occurrences using the distributions of positive values of the dEM/dt, and dTmax/dt and compare them with corresponding flaring probabilities of the total unsigned magnetic flux, a conventionally used, standard flare predictor. For C-class flares, conditional probabilities have lower or similar values with the ones derived for the unsigned magnetic flux, for 24 and 12 hours forecast windows. For M- and X-class flares, these probabilities are higher than those of the unsigned flux for higher parameter values. Shorter forecast windows improve the conditional probabilities of dEM/dt, and dTmax/dt in comparison to those of the unsigned magnetic flux. We conclude that flare forerunner events such as preflare heating or small flare activity prior to major flares reflect on the temporal evolution of EM and Tmax. Of these two, the temporal derivative of the EM could conceivably be used as a credible precursor, or short-term predictor, of an imminent flare. Title: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: Models and data analysis tools for the Solar Orbiter mission Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.; Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.; Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.; Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi, N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla, T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.; Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.; Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.; Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.; Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot, V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.; Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.; Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp, E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.; Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.; Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti, S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.; Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez, L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca, L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.; Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov, A. N. Bibcode: 2020A&A...642A...2R Altcode: Context. The Solar Orbiter spacecraft will be equipped with a wide range of remote-sensing (RS) and in situ (IS) instruments to record novel and unprecedented measurements of the solar atmosphere and the inner heliosphere. To take full advantage of these new datasets, tools and techniques must be developed to ease multi-instrument and multi-spacecraft studies. In particular the currently inaccessible low solar corona below two solar radii can only be observed remotely. Furthermore techniques must be used to retrieve coronal plasma properties in time and in three dimensional (3D) space. Solar Orbiter will run complex observation campaigns that provide interesting opportunities to maximise the likelihood of linking IS data to their source region near the Sun. Several RS instruments can be directed to specific targets situated on the solar disk just days before data acquisition. To compare IS and RS, data we must improve our understanding of how heliospheric probes magnetically connect to the solar disk.
Aims: The aim of the present paper is to briefly review how the current modelling of the Sun and its atmosphere can support Solar Orbiter science. We describe the results of a community-led effort by European Space Agency's Modelling and Data Analysis Working Group (MADAWG) to develop different models, tools, and techniques deemed necessary to test different theories for the physical processes that may occur in the solar plasma. The focus here is on the large scales and little is described with regards to kinetic processes. To exploit future IS and RS data fully, many techniques have been adapted to model the evolving 3D solar magneto-plasma from the solar interior to the solar wind. A particular focus in the paper is placed on techniques that can estimate how Solar Orbiter will connect magnetically through the complex coronal magnetic fields to various photospheric and coronal features in support of spacecraft operations and future scientific studies.
Methods: Recent missions such as STEREO, provided great opportunities for RS, IS, and multi-spacecraft studies. We summarise the achievements and highlight the challenges faced during these investigations, many of which motivated the Solar Orbiter mission. We present the new tools and techniques developed by the MADAWG to support the science operations and the analysis of the data from the many instruments on Solar Orbiter.
Results: This article reviews current modelling and tool developments that ease the comparison of model results with RS and IS data made available by current and upcoming missions. It also describes the modelling strategy to support the science operations and subsequent exploitation of Solar Orbiter data in order to maximise the scientific output of the mission.
Conclusions: The on-going community effort presented in this paper has provided new models and tools necessary to support mission operations as well as the science exploitation of the Solar Orbiter data. The tools and techniques will no doubt evolve significantly as we refine our procedure and methodology during the first year of operations of this highly promising mission. Title: The Solar Orbiter Science Activity Plan. Translating solar and heliospheric physics questions into action Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.; Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.; Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.; Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.; Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.; Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio, L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun, A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso, F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.; Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.; Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.; van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi, L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine, D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot, S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham, G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler, D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier, K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins, J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis, I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.; Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis, G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.; Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.; Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis, K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien, H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.; Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.; Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines, J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.; Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.; Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.; Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.; Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.; Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula, G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio, A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.; Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann, T.; Young, P. R.; Zhukov, A. N. Bibcode: 2020A&A...642A...3Z Altcode: 2020arXiv200910772Z Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed. Title: Solar Flare Prediction Using Magnetic Field Diagnostics above the Photosphere Authors: Korsós, M. B.; Georgoulis, M. K.; Gyenge, N.; Bisoi, S. K.; Yu, S.; Poedts, S.; Nelson, C. J.; Liu, J.; Yan, Y.; Erdélyi, R. Bibcode: 2020ApJ...896..119K Altcode: 2020arXiv200512180K In this article, we present the application of the weighted horizontal gradient of magnetic field (WGM) flare prediction method to three-dimensional (3D) extrapolated magnetic configurations of 13 flaring solar active regions (ARs). The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WGM method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential and nonlinear force-free field extrapolations, are constructed to study a vertical range from the photosphere up to the low corona with a 45 km step size. The WGM method is applied as a function of height to all 13 flaring AR cases that are subject to certain selection criteria. We found that applying the WGM method between 1000 and 1800 km above the solar surface would improve the prediction of the flare onset time by around 2-8 hr. Certain caveats and an outlook for future work along these lines are also discussed. Title: Machine Learning in Heliophysics and Space Weather Forecasting: A White Paper of Findings and Recommendations Authors: Nita, Gelu; Georgoulis, Manolis; Kitiashvili, Irina; Sadykov, Viacheslav; Camporeale, Enrico; Kosovichev, Alexander; Wang, Haimin; Oria, Vincent; Wang, Jason; Angryk, Rafal; Aydin, Berkay; Ahmadzadeh, Azim; Bai, Xiaoli; Bastian, Timothy; Filali Boubrahimi, Soukaina; Chen, Bin; Davey, Alisdair; Fereira, Sheldon; Fleishman, Gregory; Gary, Dale; Gerrard, Andrew; Hellbourg, Gregory; Herbert, Katherine; Ireland, Jack; Illarionov, Egor; Kuroda, Natsuha; Li, Qin; Liu, Chang; Liu, Yuexin; Kim, Hyomin; Kempton, Dustin; Ma, Ruizhe; Martens, Petrus; McGranaghan, Ryan; Semones, Edward; Stefan, John; Stejko, Andrey; Collado-Vega, Yaireska; Wang, Meiqi; Xu, Yan; Yu, Sijie Bibcode: 2020arXiv200612224N Altcode: The authors of this white paper met on 16-17 January 2020 at the New Jersey Institute of Technology, Newark, NJ, for a 2-day workshop that brought together a group of heliophysicists, data providers, expert modelers, and computer/data scientists. Their objective was to discuss critical developments and prospects of the application of machine and/or deep learning techniques for data analysis, modeling and forecasting in Heliophysics, and to shape a strategy for further developments in the field. The workshop combined a set of plenary sessions featuring invited introductory talks interleaved with a set of open discussion sessions. The outcome of the discussion is encapsulated in this white paper that also features a top-level list of recommendations agreed by participants. Title: A Comparison of Flare Forecasting Methods. IV. Evaluating Consecutive-day Forecasting Patterns Authors: Park, Sung-Hong; Leka, K. D.; Kusano, Kanya; Andries, Jesse; Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey, Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter T.; Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo; Lobzin, Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek A. M.; Qahwaji, Rami; Sharpe, Michael; Steenburgh, R. A.; Steward, Graham; Terkildsen, Michael Bibcode: 2020ApJ...890..124P Altcode: 2020arXiv200102808P A crucial challenge to successful flare prediction is forecasting periods that transition between "flare-quiet" and "flare-active." Building on earlier studies in this series in which we describe the methodology, details, and results of flare forecasting comparison efforts, we focus here on patterns of forecast outcomes (success and failure) over multiday periods. A novel analysis is developed to evaluate forecasting success in the context of catching the first event of flare-active periods and, conversely, correctly predicting declining flare activity. We demonstrate these evaluation methods graphically and quantitatively as they provide both quick comparative evaluations and options for detailed analysis. For the testing interval 2016-2017, we determine the relative frequency distribution of two-day dichotomous forecast outcomes for three different event histories (I.e., event/event, no-event/event, and event/no-event) and use it to highlight performance differences between forecasting methods. A trend is identified across all forecasting methods that a high/low forecast probability on day 1 remains high/low on day 2, even though flaring activity is transitioning. For M-class and larger flares, we find that explicitly including persistence or prior flare history in computing forecasts helps to improve overall forecast performance. It is also found that using magnetic/modern data leads to improvement in catching the first-event/first-no-event transitions. Finally, 15% of major (I.e., M-class or above) flare days over the testing interval were effectively missed due to a lack of observations from instruments away from the Earth-Sun line. Title: Multivariate time series dataset for space weather data analytics Authors: Angryk, Rafal A.; Martens, Petrus C.; Aydin, Berkay; Kempton, Dustin; Mahajan, Sushant S.; Basodi, Sunitha; Ahmadzadeh, Azim; Cai, Xumin; Filali Boubrahimi, Soukaina; Hamdi, Shah Muhammad; Schuh, Michael A.; Georgoulis, Manolis K. Bibcode: 2020NatSD...7..227A Altcode: We introduce and make openly accessible a comprehensive, multivariate time series (MVTS) dataset extracted from solar photospheric vector magnetograms in Spaceweather HMI Active Region Patch (SHARP) series. Our dataset also includes a cross-checked NOAA solar flare catalog that immediately facilitates solar flare prediction efforts. We discuss methods used for data collection, cleaning and pre-processing of the solar active region and flare data, and we further describe a novel data integration and sampling methodology. Our dataset covers 4,098 MVTS data collections from active regions occurring between May 2010 and December 2018, includes 51 flare-predictive parameters, and integrates over 10,000 flare reports. Potential directions toward expansion of the time series, either "horizontally" - by adding more prediction-specific parameters, or "vertically" - by generalizing flare into integrated solar eruption prediction, are also explained. The immediate tasks enabled by the disseminated dataset include: optimization of solar flare prediction and detailed investigation for elusive flare predictors or precursors, with both operational (research-to-operations), and basic research (operations-to-research) benefits potentially following in the future. Title: Which Machine- or Deep-Learning Methods are Most Appropriate for Solar Flare Prediction? Possibly, a Misleading Question Authors: He, H.; Georgoulis, M. K. Bibcode: 2019AGUFMSH34A..01H Altcode: Over the past decade, machine-learning methods for solar flare prediction have been established as a necessity in order to browse through the immense parameter space of promising flare-predictive properties in solar active regions. This obvious big data problem requires advanced computer science methods to be tackled, in an endeavor that has demonstrably reached out to communities beyond heliophysics. However, as in any new research avenue, the landscape of techniques and methodologies is constantly changing in an unplanned and unpredictable way, with groundbreaking results yet to be produced and shown conclusively. Recent collaborative works compare very different methods that, however, deliver very similar, far from perfect, results. We aim to provide a preliminary interpretation of this situation by asserting that the specific machine- (and deep-, very recently) learning methods employed for solar flare forecasting may not be the most important element here: their training and testing practices, however, are. Training and testing over active-region property metadata can significantly change the results of any method, in a way that if very different methods share the same testing / testing pattern, they tend to give similar, invariably imperfect, results. To this one must add the relative deficiency of timeseries analysis methods for flare forecasting, along with the unmitigated severe class imbalance problem, stemming from the intrinsic rarity of major flares: machine-learning methods require class-balanced event / no-event data sets (this remains to be seen for deep-learning ones) so their performance is consistently below expectations. We identify some early attempts toward this second level of efforts, in hopes of instigating further discussion and, hopefully, progress. Title: Magnetic Impact of Propagating Interplanetary Coronal Mass Ejections in the Solar and Stellar Habitability Zones Authors: Georgoulis, M. K.; Samara, E.; Patsourakos, S. Bibcode: 2019AGUFMSH43A..05G Altcode: We recount recent results of a statistical method that assigns an axial magnetic field to CME flux ropes, inferred via the fundamental conservation principle of magnetic helicity in solar active region sources. We then extrapolate the near-Sun CME magnetic field to 1 AU, juxtaposing the extrapolation with tens of magnetic-cloud observations. Uncertainties given, we manage to statistically reproduce observations, thereby proposing a simple method that alleviates unnecessary complexity, while featuring applicability on a case-by-case basis. At a second level, we generalize CME magnetic configurations and stellar activity, expanding to flaring M-dwarf and Sun-like stars. We correlate the magnetic energy of stellar, assumed eruptive, flares with their helicity and extrapolate again to stellar habitable zones. From assumed planetary equatorial magnetic fields we predict atmospheric erosion by CME activity for a number of recently discovered exoplanets (Keppler 438b; Proxima b; the TRAPPIST system), thought promising for harboring life. Preliminary results show that knowledge of the planetary equatorial magnetic field can impose a valuable constraint for exoplanet habitability. Meanwhile, (1) terrestrial atmospheric erosion seems unlikely even for unrealistically intense solar eruptions and (2) the likelihood of absence of atmosphere due to CME-induced erosion in many of the studied exoplanets seems high. Title: Challenges with Extreme Class-Imbalance and Temporal Coherence: A Study on Solar Flare Data Authors: Ahmadzadeh, Azim; Hostetter, Maxwell; Aydin, Berkay; Georgoulis, Manolis K.; Kempton, Dustin J.; Mahajan, Sushant S.; Angryk, Rafal A. Bibcode: 2019arXiv191109061A Altcode: In analyses of rare-events, regardless of the domain of application, class-imbalance issue is intrinsic. Although the challenges are known to data experts, their explicit impact on the analytic and the decisions made based on the findings are often overlooked. This is in particular prevalent in interdisciplinary research where the theoretical aspects are sometimes overshadowed by the challenges of the application. To show-case these undesirable impacts, we conduct a series of experiments on a recently created benchmark data, named Space Weather ANalytics for Solar Flares (SWAN-SF). This is a multivariate time series dataset of magnetic parameters of active regions. As a remedy for the imbalance issue, we study the impact of data manipulation (undersampling and oversampling) and model manipulation (using class weights). Furthermore, we bring to focus the auto-correlation of time series that is inherited from the use of sliding window for monitoring flares' history. Temporal coherence, as we call this phenomenon, invalidates the randomness assumption, thus impacting all sampling practices including different cross-validation techniques. We illustrate how failing to notice this concept could give an artificial boost in the forecast performance and result in misleading findings. Throughout this study we utilized Support Vector Machine as a classifier, and True Skill Statistics as a verification metric for comparison of experiments. We conclude our work by specifying the correct practice in each case, and we hope that this study could benefit researchers in other domains where time series of rare events are of interest. Title: Feature Ranking of Active Region Source Properties in Solar Flare Forecasting and the Uncompromised Stochasticity of Flare Occurrence Authors: Campi, Cristina; Benvenuto, Federico; Massone, Anna Maria; Bloomfield, D. Shaun; Georgoulis, Manolis K.; Piana, Michele Bibcode: 2019ApJ...883..150C Altcode: 2019arXiv190612094C Solar flares originate from magnetically active regions (ARs) but not all solar ARs give rise to a flare. Therefore, the challenge of solar flare prediction benefits from an intelligent computational analysis of physics-based properties extracted from AR observables, most commonly line-of-sight or vector magnetograms of the active region photosphere. For the purpose of flare forecasting, this study utilizes an unprecedented 171 flare-predictive AR properties, mainly inferred by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO/HMI) in the course of the European Union Horizon 2020 FLARECAST project. Using two different supervised machine-learning methods that allow feature ranking as a function of predictive capability, we show that (i) an objective training and testing process is paramount for the performance of every supervised machine-learning method; (ii) most properties include overlapping information and are therefore highly redundant for flare prediction; (iii) solar flare prediction is still—and will likely remain—a predominantly probabilistic challenge. Title: Which Photospheric Characteristics Are Most Relevant to Active-Region Coronal Mass Ejections? Authors: Kontogiannis, Ioannis; Georgoulis, Manolis K.; Guerra, Jordan A.; Park, Sung-Hong; Bloomfield, D. Shaun Bibcode: 2019SoPh..294..130K Altcode: 2019arXiv190906088K We investigate the relation between characteristics of coronal mass ejections and parameterizations of the eruptive capability of solar active regions widely used in solar flare-prediction schemes. These parameters, some of which are explored for the first time, are properties related to topological features, namely, magnetic polarity-inversion lines (MPILs) that indicate large amounts of stored non-potential (i.e. free) magnetic energy. We utilize the Space Weather Database of Notifications, Knowledge, Information (DONKI) and the Large Angle and Spectrometric Coronograph (LASCO) databases to find flare-associated coronal mass ejections and their kinematic characteristics, while properties of MPILs are extracted from Helioseismic and Magnetic Imager (HMI) vector magnetic-field observations of active regions to extract the properties of source-region MPILs. The correlation between all properties and the characteristics of CMEs ranges from moderate to very strong. More significant correlations hold particularly for fast CMEs, which are most important in terms of adverse space-weather manifestations. Non-neutralized currents and the length of the main MPIL exhibit significantly stronger correlations than the rest of the properties. This finding supports a causal relationship between coronal mass ejections and non-neutralized electric currents in highly sheared, conspicuous MPILs. In addition, non-neutralized currents and MPIL length carry distinct, independent information as to the eruptive potential of active regions. The combined total amount of non-neutralized electric currents and the length of the main polarity-inversion line, therefore, reflect more efficiently than other parameters the eruptive capacity of solar active regions and the CME kinematic characteristics stemming from these regions. Title: A Comparison of Flare Forecasting Methods. III. Systematic Behaviors of Operational Solar Flare Forecasting Systems Authors: Leka, K. D.; Park, Sung-Hong; Kusano, Kanya; Andries, Jesse; Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey, Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter T.; Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo; Lobzin, Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek A. M.; Qahwaji, Rami; Sharpe, Michael; Steenburgh, Robert A.; Steward, Graham; Terkildsen, Michael Bibcode: 2019ApJ...881..101L Altcode: 2019arXiv190702909L A workshop was recently held at Nagoya University (2017 October 31-November 2), sponsored by the Center for International Collaborative Research, at the Institute for Space-Earth Environmental Research, Nagoya University, Japan, to quantitatively compare the performance of today’s operational solar flare forecasting facilities. Building upon Paper I of this series, in Paper II we described the participating methods for this latest comparison effort, the evaluation methodology, and presented quantitative comparisons. In this paper, we focus on the behavior and performance of the methods when evaluated in the context of broad implementation differences. Acknowledging the short testing interval available and the small number of methods available, we do find that forecast performance: (1) appears to improve by including persistence or prior flare activity, region evolution, and a human “forecaster in the loop” (2) is hurt by restricting data to disk-center observations; (3) may benefit from long-term statistics but mostly when then combined with modern data sources and statistical approaches. These trends are arguably weak and must be viewed with numerous caveats, as discussed both here and in Paper II. Following this present work, in Paper IV (Park et al. 2019) we will present a novel analysis method to evaluate temporal patterns of forecasting errors of both types (i.e., misses and false alarms). Hence, most importantly, with this series of papers, we demonstrate the techniques for facilitating comparisons in the interest of establishing performance-positive methodologies. Title: A Comparison of Flare Forecasting Methods. II. Benchmarks, Metrics, and Performance Results for Operational Solar Flare Forecasting Systems Authors: Leka, K. D.; Park, Sung-Hong; Kusano, Kanya; Andries, Jesse; Barnes, Graham; Bingham, Suzy; Bloomfield, D. Shaun; McCloskey, Aoife E.; Delouille, Veronique; Falconer, David; Gallagher, Peter T.; Georgoulis, Manolis K.; Kubo, Yuki; Lee, Kangjin; Lee, Sangwoo; Lobzin, Vasily; Mun, JunChul; Murray, Sophie A.; Hamad Nageem, Tarek A. M.; Qahwaji, Rami; Sharpe, Michael; Steenburgh, Robert A.; Steward, Graham; Terkildsen, Michael Bibcode: 2019ApJS..243...36L Altcode: 2019arXiv190702905L Solar flares are extremely energetic phenomena in our solar system. Their impulsive and often drastic radiative increases, particularly at short wavelengths, bring immediate impacts that motivate solar physics and space weather research to understand solar flares to the point of being able to forecast them. As data and algorithms improve dramatically, questions must be asked concerning how well the forecasting performs; crucially, we must ask how to rigorously measure performance in order to critically gauge any improvements. Building upon earlier-developed methodology of Paper I (Barnes et al. 2016), international representatives of regional warning centers and research facilities assembled in 2017 at the Institute for Space-Earth Environmental Research, Nagoya University, Japan to, for the first time, directly compare the performance of operational solar flare forecasting methods. Multiple quantitative evaluation metrics are employed, with the focus and discussion on evaluation methodologies given the restrictions of operational forecasting. Numerous methods performed consistently above the “no-skill” level, although which method scored top marks is decisively a function of flare event definition and the metric used; there was no single winner. Following in this paper series, we ask why the performances differ by examining implementation details (Leka et al. 2019), and then we present a novel analysis method to evaluate temporal patterns of forecasting errors in Paper IV (Park et al. 2019). With these works, this team presents a well-defined and robust methodology for evaluating solar flare forecasting methods in both research and operational frameworks and today’s performance benchmarks against which improvements and new methods may be compared. Title: The source and engine of coronal mass ejections Authors: Georgoulis, Manolis K.; Nindos, Alexander; Zhang, Hongqi Bibcode: 2019RSPTA.37780094G Altcode: Coronal mass ejections (CMEs) are large-scale expulsions of coronal plasma and magnetic field propagating through the heliosphere. Because CMEs are observed by white-light coronagraphs which, by design, occult the solar disc, supporting disc observations (e.g. in EUV, soft X-rays, Halpha and radio) must be employed for the study of their source regions and early development phases. We review the key properties of CME sources and highlight a certain causal sequence of effects that may occur whenever a strong (flux-massive and sheared) magnetic polarity inversion line develops in the coronal base of eruptive active regions (ARs). Storing non-potential magnetic energy and helicity in a much more efficient way than ARs lacking strong polarity inversion lines, eruptive regions engage in an irreversible course, making eruptions inevitable and triggered when certain thresholds of free energy and helicity are crossed. This evolution favours the formation of pre-eruption magnetic flux ropes. We describe the steps of this plausible path to sketch a picture of the pre-eruptive phase of CMEs that may apply to most events, particularly the ones populating the high end of the energy/helicity distribution, that also tend to have the strongest space-weather implications.

This article is part of the theme issue 'Solar eruptions and their space weather impact'. Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic Configuration of Coronal Mass Ejections: Diagnostics, Challenges and Future Observables Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.; Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2019shin.confE.194P Altcode: Our thinking about the pre-eruptive magnetic configuration of Coronal Mass Ejections has been effectively dichotomized into two opposing and often fiercely contested views: namely, sheared magnetic arcades and magnetic flux ropes. Finding a solution to this issue will have important implications for our understanding of CME initiation. We first discuss the very value of embarking into the arcade vs. flux rope dilemma and illustrate the corresponding challenges and difficulties to address it. Next, we are compiling several observational diagnostics of pre-eruptive sheared magnetic arcades stemming from theory/modeling, discuss their merits, and highlight potential ambiguities that could arise in their interpretation. We finally conclude with a discussion of possible new observables, in the frame of upcoming or proposed instrumentation, that could help to circumvent the issues we are currently facing. Title: Deriving the Near-Sun Magnetic Field of Coronal Mass Ejections from Magnetic Helicity Conservation Authors: Patsourakos, Spiros; Georgoulis, M. K.; Petroulea, G.; Vourlidas, A.; Nieves-Chinchilla, T. Bibcode: 2019shin.confE.222P Altcode: The near-Sun magnetic field of Coronal Mass Ejections represents a key parameter for assessing their energetics and structuring, and additionally, it is a major element of methods/applications/simulations aiming to predict the magnetic field of Earth-directed CMEs upon impact at geospace. Diagnostics of CME magnetic fields in the corona can be achieved via observations in the radio domain, which however, are currently not available on a regular basis. Therefore, several methods to infer the CME magnetic field in the corona have recently emerged. We developed one such method which is based on the magnetic helicity conservation principle applied to flux rope CMEs. Its input parameters could be readily retrieved from the analysis of HMI magnetograms and SOHO/STEREO WL coronagraph images. We present parametric and case-study applications of this method, and discuss how it be could be used to predict the CME magnetic field magnitude at 1 AU. Title: Machine Learning in Solar Eruption Forecasting: a Scene-Setting Attempt Authors: Georgoulis, Manolis K.; Martens, P. C.; Angryk, R. A.; Aydin, B.; Ahmadzadeh, A. Bibcode: 2019shin.confE..89G Altcode: Over the nearly three decades of space weather forecasting efforts, increased awareness suggests that conventional solar physics may not suffice to fully understand, and ultimately predict, eruptive solar activity, particularly the rarest trio of major flare, coronal mass ejection and solar energetic particle events. Both statistical and computer science techniques and approaches, such as machine and deep learning, seem apt to break ground in both pursuits, namely, in enhancing physical understanding and in enabling forecasts reliable enough to become practical. Like any promising new trend, however, machine and deep learning methods entail major challenges. In this brief account, we discuss a subset of these challenges whose efficient tackling can spur further progress. These major issues became evident in the framework of the European Union Flare Likelihood and Region Eruption Forecasting (FLARECAST) project on solar flare forecasting. The most substantial of them, that can easily mislead results, comparisons and interpretation, include climatology and uncertainties thereof, the construction of the training and test samples in machine learning methods and the class imbalance problem that becomes conspicuous when forecasting increasingly rare events. The above underline the need for benchmarking standards against which new methods are to be tested, verified and validated. We discuss a such benchmark, namely the Space Weather data ANalytics (SWAN) benchmark dataset for flare prediction, established at the Georgia State University Astroinformatics Cluster, that aims to tackle climatology and the construction of training and test samples. In addition, we show indicative, preliminary results of efforts tackling the class imbalance problem on the GSU SWAN flare prediction data. Title: Coronal Mass Ejection Initiation: a Likely Irreversible Evolutionary Process Authors: Georgoulis, Manolis K. Bibcode: 2019shin.confE..92G Altcode: Combining original results and review assessments, we shed new light to the asserted necessity of the Sun to relieve itself from the excess magnetic helicity it produces via coronal mass ejections (CMEs). In particular, we argue that fast, active-region CME initiation is an irreversible consequence of the formation of intense magnetic polarity inversion lines (PILs) in the low solar atmosphere. By “intense” we mean PILs on which the magnetic field strength is well above the equipartition value, even at photospheric altitudes. The formation of intense PILs is exclusively related to the action of non-neutralized (net) electric currents along the PILs that give rise to a Lorentz tension force able to move the plasma and cause the observed velocity shear along PILs. Shear gives rise to quasi-steady, confined magnetic reconnection along the PIL that efficiently converts the substantial mutual magnetic helicity of the sheared magnetic arcade (SMA) into self helicity (twist and writhe), enabling a pre-eruption magnetic flux rope (MFR) that will eventually destabilize if the SMA does not erupt already via breakout. Invariably, either SMAs or MFRs shed a significant fraction of the source active region helicity into the heliosphere. This progression, leading to at least one major eruption, lasts for as long as there is flux emergence to sustain the strong PIL and during a significant part of the active region decay phase, until Lorentz tension weakens and helicity expulsion is significant enough to allow the dissolution of the active region. This mechanism is yet to be validated conclusively, but its validation path is clear. If proven, it will show that the dichotomy between SMA and MFR in the pre-eruption evolution of eruptive active regions is of little practical meaning: with the same end result, namely the expulsion of magnetic helicity, pre-eruption MFRs will form if the coronal magnetic topology is not sufficient to enable a direct SMA eruption. Title: Preface: Solar physics advances from the interior to the heliosphere Authors: Georgoulis, Manolis K.; Kontar, Eduard P. Bibcode: 2019AdSpR..63.1387G Altcode: Solar Physics has been experiencing a golden era of unprecedented observations and voluminous data for nearly three decades. While much of the progress in the 1990s and 2000s was spurred by flagship space missions, the latest decade has also seen the culmination of game-changing ground-based facilities, long sought after by the community. Observing the Sun from space has indisputable benefits; however, space missions have a relatively limited lifespan, mainly because of the unforgiving deep space or orbital conditions and our limited ability to maintain them after launch. This is not the case for ground-based facilities that can, in principle, serve and train entire generations of researchers. Title: Analysis and interpretation of inner-heliospheric SEP events with the ESA Standard Radiation Environment Monitor (SREM) onboard the INTEGRAL and Rosetta Missions Authors: Georgoulis, Manolis K.; Papaioannou, Athanasios; Sandberg, Ingmar; Anastasiadis, Anastasios; Daglis, Ioannis A.; Rodríguez-Gasén, Rosa; Aran, Angels; Sanahuja, Blai; Nieminen, Petteri Bibcode: 2018JSWSC...8A..40G Altcode: Using two heliospheric vantage points, we study 22 solar energetic particle (SEP) events, 14 of which were detected at both locations. SEP proton events were detected during the declining phase of solar cycle 23 (November 2003-December 2006) by means of two nearly identical Standard Radiation Environment Monitor (SREM) units in energies ranging between 12.6 MeV and 166.3 MeV. In this work we combine SREM data with diverse solar and interplanetary measurements, aiming to backtrace solar eruptions from their impact in geospace (i.e., from L1 Lagrangian point to Earth's magnetosphere) to their parent eruptions at the Sun's low atmosphere. Our SREM SEP data support and complement a consistent inner-heliospheric description of solar eruptions (solar flares and coronal mass ejections [CMEs]) and their magnetospheric impact. In addition, they provide useful information on the understanding of the origin, acceleration, and propagation of SEP events at multi-spacecraft settings. All SEP events in our sample originate from major eruptions consisting of major (>M-class) solar flares and fast (>1800 km/s, on average), overwhelmingly (>78%) halo, CMEs. All but one SEP event studied are unambiguously associated with shock-fronted CMEs, suggesting a CME-driven shock acceleration mechanism. Moreover, a significant correlation is found between the SEP event peak and the onset of the storm sudden commencement, that might help improve prediction of magnetospheric disturbances. In general, SEP events correlate better with interplanetary (i.e., in-situ; L1-based) than with solar eruption features. Our findings support (a) the routine use of cost-effective SREM units, or future improvements thereof, for the detection of SEP events and (b) their implementation in multi-spacecraft settings as a means to improve both the physical understanding of SEP events and their forecasting. Title: On the Evolution of Pre-Flare Patterns of a 3-Dimensional Model of AR 11429 Authors: Korsós, M. B.; Poedts, S.; Gyenge, N.; Georgoulis, M. K.; Yu, S.; Bisoi, S. K.; Yan, Y.; Ruderman, M. S.; Erdélyi, R. Bibcode: 2018IAUS..335..294K Altcode: 2018arXiv180100433K We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation encompassing a vertical range from the photosphere through the chromosphere and transition region into the low corona. The basis of our proxy measure of activity prediction is the so-called weighted horizontal gradient of magnetic field (WGM) defined between spots of opposite polarities close to the polarity inversion line of an active region. The temporal variation of the distance of the barycenter of the opposite polarities is also found to possess potentially important diagnostic information about the flare onset time estimation as function of height similar to its counterpart introduced initially in an application at the photosphere only in Korsós et al. (2015). We apply the photospheric pre-flare behavioural patterns of sunspot groups to the evolution of their associated 3D-constructed AR 11429 as function of height. We found that at a certain height in the lower solar atmosphere the onset time may be estimated much earlier than at the photosphere or at any other heights. Therefore, we present a tool and recipe that may potentially identify the optimum height for flare prognostic in the solar atmosphere allowing to improve our flare prediction capability and capacity. Title: Photospheric Shear Flows in Solar Active Regions and Their Relation to Flare Occurrence Authors: Park, Sung-Hong; Guerra, Jordan A.; Gallagher, Peter T.; Georgoulis, Manolis K.; Bloomfield, D. Shaun Bibcode: 2018SoPh..293..114P Altcode: 2018arXiv180707714P Solar active regions (ARs) that produce major flares typically exhibit strong plasma shear flows around photospheric magnetic polarity inversion lines (MPILs). It is therefore important to quantitatively measure such photospheric shear flows in ARs for a better understanding of their relation to flare occurrence. Photospheric flow fields were determined by applying the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) method to a large data set of 2548 coaligned pairs of AR vector magnetograms with 12-min separation over the period 2012 - 2016. From each AR flow-field map, three shear-flow parameters were derived corresponding to the mean («S »), maximum (Smax) and integral (Ssum) shear-flow speeds along strong-gradient, strong-field MPIL segments. We calculated flaring rates within 24 h as a function of each shear-flow parameter and we investigated the relation between the parameters and the waiting time (τ ) until the next major flare (class M1.0 or above) after the parameter observation. In general, it is found that the larger Ssum an AR has, the more likely it is for the AR to produce flares within 24 h. It is also found that among ARs which produce major flares, if one has a larger value of Ssum then τ generally gets shorter. These results suggest that large ARs with widespread and/or strong shear flows along MPILs tend to not only be more flare productive, but also produce major flares within 24 h or less. Title: Eruptive Flare Initiation and the CME Magnetic Field Authors: Georgoulis, Manolis K.; Patsourakos, Spiros; Kontogiannis, Ioannis Bibcode: 2018cosp...42E1180G Altcode: We recount very recent results on the correlation between photospheric characteristics of eruptive solar active regions and coronal mass ejection (CME) occurrence / characteristics. In particular, we argue that one of the most relevant parameters for CME occurrence is the non-neutralized electric currents appearing exclusively along intense, shear-ridden magnetic polarity-inversion lines (PILs) in the photosphere of eruptive active regions. These currents are simply lacking in the absence of strong PILs and shear. While the physics underlying non-neutralized currents is rich and shows far-reaching ramifications, we will focus on the injection of magnetic helicity due to non-neutralized currents in the pre-eruption phase, that will then be bodily transported via the CME. For a conductive plasma of high magnetic Reynolds number, such as that of the solar corona, we show how the fundamental helicity conservation principle can lead to estimates of, first, the CME's axial magnetic field strength and, second, the anticipated magnetic field strength of the interplanetary CME (ICME) on the verge of geospace. We discuss how this analysis can be viewed as a meaningful initial or boundary condition for more elaborate inner-heliospheric propagation models that further consider the orientation of the ICME magnetic field, thus leading to an improved understanding and prediction of ICME geoeffectiveness. Part of this work has been supported by the EU Horizon-2020 FLARECAST project (grant agreement no. 640216). Title: Towards common validation and verification procedures in the ongoing SSA Space Weather Service Network Authors: Borries, Claudia; Glover, Alexi; Georgoulis, Manolis K.; Perry, Chris; Dierckxsens, Mark; Tsagouri, Ioanna Bibcode: 2018cosp...42E.400B Altcode: In the framework of the Space Situational Awareness Programme (SSA) of the European Space Agency (ESA), the Space WEather (SWE) segment aims to help end-users in a wide range of SWE affected sectors to mitigate the effects on their systems, reducing costs and improving reliability. In the SWE segment, five Expert Service Centres (ESCs), the SSA Space Weather Coordination Centre (SSCC) and the SSA Space Weather Data Centre (SWE-DC) form the SSA SWE network. This network is providing SWE services to users dependent on space weather conditions. The SWE services are composed of an extensive number of products provided by numerous contributing expert groups. Each product is delivered to the SSA SWE network with a full package of documentation and user guidance. This includes results of validation and verification procedures helping the end-user to identify the appropriate products for each application. The currently diverse procedures for product validation and verification hamper the comparison and evaluation of different products. Therefore, the SWE network aims to harmonize these procedures. Here, an overview of the ESCs product validation assessment and planning for the current activity will be given, including examples of best practice and approaches for common validation and verification procedures. Title: A method to assess planetary habitability based on the effects of CME magnetic fields on planetary magnetospheres Authors: Samara, Evangelia; Georgoulis, Manolis K. Bibcode: 2018shin.confE.216S Altcode: In this work, we propose a methodology to assess planetary surface habitability for any newly discovered exoplanet lying in the habitable zone of a low mass, superflare, main-sequence star. While habitability is usually a function of multiple parameters (i.e. the size of the planet, the composition of its atmosphere, the existence of liquid water, orbital or rotational dynamics), the presented method primarily focuses on stellar activity and its effects on potentially existing planetary magnetospheres stemming from dynamo-generated magnetic fields. The area our method applies to, is split in two regimes depending on whether the exoplanet of interest is tidally locked or tidally unlocked to its mother star. In the former case, a ratio between a conditional (Beq) and a best-case equatorial planetary magnetic field (Bstev), is calculated. A smaller-than-unity value of that ratio favors the formation of a stable atmosphere adequate to host surface, atmosphere-dependent habitability. In the latter case (tidally unlocked regime), a ratio between a conditional (Beq) and the terrestrial magnetic field (Bearth) is estimated, which indirectly implies how easy or hard it is for an exoplanet to sustain an atmosphere. For a ratio's value lower than unity, there is hope for surface habitability since its magnetic field is assumed equal to, or larger than, the terrestrial one. On the other hand, a value higher than one casts doubts on surface habitability, because even a strong, terrestrial magnetic field is not sufficient to secure an atmosphere for the exoplanet. Both ratios concerning the two different spatial regimes of our method's application are based on the observed mother star's activity and are calculated on the critical magnetopause distance of two planetary radii, below which atmospheric erosion phenomena are assumed to start taking place. Title: Forecast Verification in the Framework of the EU FLARECAST Project Authors: Georgoulis, Manolis K.; Massone, Anna Maria; Jackson, David; Benvenuto, Federico; Piana, Michele; Bloomfield, Shaun; Florios, Konstantinos; Campi, Cristina; Worsfold, Mark Bibcode: 2018cosp...42E1181G Altcode: We describe the main practices and results of the comprehensive forecast verification effort undertaken in the framework of the EU FLARECAST project for solar flare prediction. In discussing FLARECAST, we compare between previous verification efforts of solar flare forecasting products or services and the potential advances brought by the FLARECAST project, that served as a uniform platform for testing numerous and diverse flare forecasting methods. Results shown include major skill score values and extensive ranking of flare-predictive parameters. We further advocate for a robust error assessment of the scores and parameters involved in the verification process, that would provide the basis of a much-needed, continuous assessment of solar flare forecasting capabilities. In concluding, we list the discussion areas and recommendations addressed by the COSPAR - ILWS Roadmap, indicating the locations where the FLARECAST project has been contributing and providing feedback, potentially enabling further advances in the verification of forecasting activities of solar and heliospheric weather. This work has received support by the EU Horizon-2020 FLARECAST project (grant agreement no. 640216). Title: Testing and Improving a Set of Morphological Predictors of Flaring Activity Authors: Kontogiannis, Ioannis; Georgoulis, Manolis K.; Park, Sung-Hong; Guerra, Jordan A. Bibcode: 2018SoPh..293...96K Altcode: 2018arXiv180706371K Efficient prediction of solar flares relies on parameters that quantify the eruptive capability of solar active regions. Several such quantitative predictors have been proposed in the literature, inferred mostly from photospheric magnetograms and/or white-light observations. Two of them are the Ising energy and the sum of the total horizontal magnetic field gradient. The former has been developed from line-of-sight magnetograms, while the latter uses sunspot detections and characteristics, based on continuum images. Aiming to include these parameters in an automated prediction scheme, we test their applicability on regular photospheric magnetic field observations provided by the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). We test their efficiency as predictors of flaring activity on a representative sample of active regions and investigate possible modifications of these quantities. The Ising energy appears to be an efficient predictor, and the efficiency is even improved if it is modified to describe interacting magnetic partitions or sunspot umbrae. The sum of the horizontal magnetic field gradient appears to be slightly more promising than the three variations of the Ising energy we implement in this article. The new predictors are also compared with two very promising predictors: the effective connected magnetic field strength and the total unsigned non-neutralized current. Our analysis shows that the efficiency of morphological predictors depends on projection effects in a nontrivial way. All four new predictors are found useful for inclusion in an automated flare forecasting facility, such as the Flare Likelihood and Region Eruption Forecasting (FLARECAST), but their utility, among others, will ultimately be determined by the validation effort underway in the framework of the FLARECAST project. Title: The Ambivalent Role of Field-Aligned Electric Currents in the Solar Atmosphere Authors: Georgoulis, Manolis K. Bibcode: 2018GMS...235..371G Altcode: No abstract at ADS Title: Forecasting Solar Flares Using Magnetogram-based Predictors and Machine Learning Authors: Florios, Kostas; Kontogiannis, Ioannis; Park, Sung-Hong; Guerra, Jordan A.; Benvenuto, Federico; Bloomfield, D. Shaun; Georgoulis, Manolis K. Bibcode: 2018SoPh..293...28F Altcode: 2018arXiv180105744F We propose a forecasting approach for solar flares based on data from Solar Cycle 24, taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) mission. In particular, we use the Space-weather HMI Active Region Patches (SHARP) product that facilitates cut-out magnetograms of solar active regions (AR) in the Sun in near-realtime (NRT), taken over a five-year interval (2012 - 2016). Our approach utilizes a set of thirteen predictors, which are not included in the SHARP metadata, extracted from line-of-sight and vector photospheric magnetograms. We exploit several machine learning (ML) and conventional statistics techniques to predict flares of peak magnitude >M1 and >C1 within a 24 h forecast window. The ML methods used are multi-layer perceptrons (MLP), support vector machines (SVM), and random forests (RF). We conclude that random forests could be the prediction technique of choice for our sample, with the second-best method being multi-layer perceptrons, subject to an entropy objective function. A Monte Carlo simulation showed that the best-performing method gives accuracy ACC =0.93 (0.00 ), true skill statistic TSS =0.74 (0.02 ), and Heidke skill score HSS =0.49 (0.01 ) for >M1 flare prediction with probability threshold 15% and ACC =0.84 (0.00 ), TSS =0.60 (0.01 ), and HSS =0.59 (0.01 ) for >C1 flare prediction with probability threshold 35%. Title: Active Region Photospheric Magnetic Properties Derived from Line-of-Sight and Radial Fields Authors: Guerra, J. A.; Park, S. -H.; Gallagher, P. T.; Kontogiannis, I.; Georgoulis, M. K.; Bloomfield, D. S. Bibcode: 2018SoPh..293....9G Altcode: 2017arXiv171206902G The effect of using two representations of the normal-to-surface magnetic field to calculate photospheric measures that are related to the active region (AR) potential for flaring is presented. Several AR properties were computed using line-of-sight (Blos) and spherical-radial (Br) magnetograms from the Space-weather HMI Active Region Patch (SHARP) products of the Solar Dynamics Observatory, characterizing the presence and features of magnetic polarity inversion lines, fractality, and magnetic connectivity of the AR photospheric field. The data analyzed correspond to ≈4 ,000 AR observations, achieved by randomly selecting 25% of days between September 2012 and May 2016 for analysis at 6-hr cadence. Results from this statistical study include: i) the Br component results in a slight upwards shift of property values in a manner consistent with a field-strength underestimation by the Blos component; ii) using the Br component results in significantly lower inter-property correlation in one-third of the cases, implying more independent information as regards the state of the AR photospheric magnetic field; iii) flaring rates for each property vary between the field components in a manner consistent with the differences in property-value ranges resulting from the components; iv) flaring rates generally increase for higher values of properties, except the Fourier spectral power index that has flare rates peaking around a value of 5 /3 . These findings indicate that there may be advantages in using Br rather than Blos in calculating flare-related AR magnetic properties, especially for regions located far from central meridian. Title: The Next Level in Automated Solar Flare Forecasting: the EU FLARECAST Project Authors: Georgoulis, M. K.; Bloomfield, D.; Piana, M.; Massone, A. M.; Gallagher, P.; Vilmer, N.; Pariat, E.; Buchlin, E.; Baudin, F.; Csillaghy, A.; Soldati, M.; Sathiapal, H.; Jackson, D.; Alingery, P.; Argoudelis, V.; Benvenuto, F.; Campi, C.; Florios, K.; Gontikakis, C.; Guennou, C.; Guerra, J. A.; Kontogiannis, I.; Latorre, V.; Murray, S.; Park, S. H.; Perasso, A.; Sciacchitano, F.; von Stachelski, S.; Torbica, A.; Vischi, D. Bibcode: 2017AGUFMSA21C..07G Altcode: We attempt an informative description of the Flare Likelihood And Region Eruption Forecasting (FLARECAST) project, European Commission's first large-scale investment to explore the limits of reliability and accuracy achieved for the forecasting of major solar flares. We outline the consortium, top-level objectives and first results of the project, highlighting the diversity and fusion of expertise needed to deliver what was promised. The project's final product, featuring an openly accessible, fully modular and free to download flare forecasting facility will be delivered in early 2018. The project's three objectives, namely, science, research-to-operations and dissemination / communication, are also discussed: in terms of science, we encapsulate our close-to-final assessment on how close (or far) are we from a practically exploitable solar flare forecasting. In terms of R2O, we briefly describe the architecture of the FLARECAST infrastructure that includes rigorous validation for each forecasting step. From the three different communication levers of the project we finally focus on lessons learned from the two-way interaction with the community of stakeholders and governmental organizations. The FLARECAST project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 640216. Title: A New Spin to Exoplanet Habitability Criteria Authors: Georgoulis, M. K.; Patsourakos, S. Bibcode: 2017AGUFM.P53E2676G Altcode: We describe a physically- and statistically-based method to infer the near-Sun magnetic field of coronal mass ejections (CMEs) and then extrapolate it to the inner heliosphere and beyond. Besides a ballpark agreement with in-situ observations of interplanetary CMEs (ICMEs) at L1, we use our estimates to show that Earth does not seem to be at risk of an extinction-level atmospheric erosion or stripping by the magnetic pressure of extreme solar eruptions, even way above a Carrington-type event. This does not seem to be the case with exoplanets, however, at least those orbiting in the classically defined habitability zones of magnetically active dwarf stars at orbital radii of a small fraction of 1 AU. We show that the combination of stellar ICMEs and the tidally locking zone of mother stars, that quite likely does not allow these exoplanets to attain Earth-like magnetic fields to shield themselves, probably render the existence of a proper atmosphere in them untenable. We propose, therefore, a critical revision of habitability criteria in these cases that would limit the number of target exoplanets considered as potential biosphere hosts. Title: Non-neutralized Electric Currents in Solar Active Regions and Flare Productivity Authors: Kontogiannis, Ioannis; Georgoulis, Manolis K.; Park, Sung-Hong; Guerra, Jordan A. Bibcode: 2017SoPh..292..159K Altcode: 2017arXiv170807087K We explore the association of non-neutralized currents with solar flare occurrence in a sizable sample of observations, aiming to show the potential of such currents in solar flare prediction. We used the high-quality vector magnetograms that are regularly produced by the Helioseismic Magnetic Imager, and more specifically, the Space weather HMI Active Region Patches (SHARP). Through a newly established method that incorporates detailed error analysis, we calculated the non-neutralized currents contained in active regions (AR). Two predictors were produced, namely the total and the maximum unsigned non-neutralized current. Both were tested in AR time-series and a representative sample of point-in-time observations during the interval 2012 - 2016. The average values of non-neutralized currents in flaring active regions are higher by more than an order of magnitude than in non-flaring regions and correlate very well with the corresponding flare index. The temporal evolution of these parameters appears to be connected to physical processes, such as flux emergence and/or magnetic polarity inversion line formation, that are associated with increased solar flare activity. Using Bayesian inference of flaring probabilities, we show that the total unsigned non-neutralized current significantly outperforms the total unsigned magnetic flux and other well-established current-related predictors. It therefore shows good prospects for inclusion in an operational flare-forecasting service. We plan to use the new predictor in the framework of the FLARECAST project along with other highly performing predictors. Title: Predicting Flares and Solar Energetic Particle Events: The FORSPEF Tool Authors: Anastasiadis, A.; Papaioannou, A.; Sandberg, I.; Georgoulis, M.; Tziotziou, K.; Kouloumvakos, A.; Jiggens, P. Bibcode: 2017SoPh..292..134A Altcode: A novel integrated prediction system for solar flares (SFs) and solar energetic particle (SEP) events is presented here. The tool called forecasting solar particle events and flares (FORSPEF) provides forecasts of solar eruptive events, such as SFs with a projection to occurrence and velocity of coronal mass ejections (CMEs), and the likelihood of occurrence of an SEP event. In addition, the tool provides nowcasting of SEP events based on actual SF and CME near real-time data, as well as the SEP characteristics (e.g. peak flux, fluence, rise time, and duration) per parent solar event. The prediction of SFs relies on the effective connected magnetic field strength (Beff) metric, which is based on an assessment of potentially flaring active-region (AR) magnetic configurations, and it uses a sophisticated statistical analysis of a large number of AR magnetograms. For the prediction of SEP events, new statistical methods have been developed for the likelihood of the SEP occurrence and the expected SEP characteristics. The prediction window in the forecasting scheme is 24 hours with a refresh rate of 3 hours, while the respective prediction time for the nowcasting scheme depends on the availability of the near real-time data and ranges between 15 - 20 minutes for solar flares and 6 hours for CMEs. We present the modules of the FORSPEF system, their interconnection, and the operational setup. Finally, we demonstrate the validation of the modules of the FORSPEF tool using categorical scores constructed on archived data, and we also discuss independent case studies. Title: A Helicity-Based Method to Infer the CME Magnetic Field Magnitude in Sun and Geospace: Generalization and Extension to Sun-Like and M-Dwarf Stars and Implications for Exoplanet Habitability Authors: Patsourakos, S.; Georgoulis, M. K. Bibcode: 2017SoPh..292...89P Altcode: 2017arXiv170703579P Patsourakos et al. (Astrophys. J.817, 14, 2016) and Patsourakos and Georgoulis (Astron. Astrophys.595, A121, 2016) introduced a method to infer the axial magnetic field in flux-rope coronal mass ejections (CMEs) in the solar corona and farther away in the interplanetary medium. The method, based on the conservation principle of magnetic helicity, uses the relative magnetic helicity of the solar source region as input estimates, along with the radius and length of the corresponding CME flux rope. The method was initially applied to cylindrical force-free flux ropes, with encouraging results. We hereby extend our framework along two distinct lines. First, we generalize our formalism to several possible flux-rope configurations (linear and nonlinear force-free, non-force-free, spheromak, and torus) to investigate the dependence of the resulting CME axial magnetic field on input parameters and the employed flux-rope configuration. Second, we generalize our framework to both Sun-like and active M-dwarf stars hosting superflares. In a qualitative sense, we find that Earth may not experience severe atmosphere-eroding magnetospheric compression even for eruptive solar superflares with energies ≈104 times higher than those of the largest Geostationary Operational Environmental Satellite (GOES) X-class flares currently observed. In addition, the two recently discovered exoplanets with the highest Earth-similarity index, Kepler 438b and Proxima b, seem to lie in the prohibitive zone of atmospheric erosion due to interplanetary CMEs (ICMEs), except when they possess planetary magnetic fields that are much higher than that of Earth. Title: Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. III. Twist Number Method Authors: Guo, Y.; Pariat, E.; Valori, G.; Anfinogentov, S.; Chen, F.; Georgoulis, M. K.; Liu, Y.; Moraitis, K.; Thalmann, J. K.; Yang, S. Bibcode: 2017ApJ...840...40G Altcode: 2017arXiv170402096G We study the writhe, twist, and magnetic helicity of different magnetic flux ropes, based on models of the solar coronal magnetic field structure. These include an analytical force-free Titov-Démoulin equilibrium solution, non-force-free magnetohydrodynamic simulations, and nonlinear force-free magnetic field models. The geometrical boundary of the magnetic flux rope is determined by the quasi-separatrix layer and the bottom surface, and the axis curve of the flux rope is determined by its overall orientation. The twist is computed by the Berger-Prior formula, which is suitable for arbitrary geometry and both force-free and non-force-free models. The magnetic helicity is estimated by the twist multiplied by the square of the axial magnetic flux. We compare the obtained values with those derived by a finite volume helicity estimation method. We find that the magnetic helicity obtained with the twist method agrees with the helicity carried by the purely current-carrying part of the field within uncertainties for most test cases. It is also found that the current-carrying part of the model field is relatively significant at the very location of the magnetic flux rope. This qualitatively explains the agreement between the magnetic helicity computed by the twist method and the helicity contributed purely by the current-carrying magnetic field. Title: Magnetic helicity estimations in models and observations of the solar magnetic field Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey; Chen, Feng; Georgoulis, Manolis; Guo, Yang; Liu, Yang; Moraitis, Kostas; Thalmann, Julia K.; Yang, Shangbin Bibcode: 2017EGUGA..19.3692V Altcode: Magnetic helicity, as one of the few conserved quantities in magneto-hydrodynamics, is often invoked as the principle driving the generation and structuring of magnetic fields in a variety of environments, from dynamo models in stars and planets, to post-disruption reconfigurations of tokamak's plasmas. Most particularly magnetic helicity has raised the interest of solar physicists, since helicity is suspected to represent a key quantity for the understanding of solar flares and the generation of coronal mass ejections. In recent years, several methods of estimation of magnetic helicity have been proposed and already applied to observations and numerical simulations. However, no systematic comparison of accuracy, mutual consistency, and reliability of such methods has ever been performed. We present the results of the first benchmark of several finite-volume methods in estimating magnetic helicity in 3D test models. In addition to finite volume methods, two additional methods are also included that estimate magnetic helicity based either on the field line's twist, or on the field's values on one boundary and an inferred minimal volume connectivity. The employed model tests range from solutions of the force-free equations to 3D magneto-hydrodynamical numerical simulations. Almost all methods are found to produce the same value of magnetic helicity within few percent in all tests. However, methods show differences in the sensitivity to numerical resolution and to errors in the solenoidal property of input fields. Our benchmark of finite volume methods allows to determine the reliability and precision of estimations of magnetic helicity in practical cases. As a next step, finite volume methods are used to test estimation methods that are based on the flux of helicity through one boundary, in particular for applications to observation-based models of coronal magnetic fields. The ultimate goal is to assess if and how can helicity be meaningfully used as a diagnostic of the evolution of magnetic fields in the solar atmosphere. Title: Solar Magnetic Data Analysis for the FLARECAST Project Authors: Guerra, J. A.; Park, S. H.; Kontogiannis, I.; Bloomfield, D.; Gallagher, P.; Georgoulis, M. K. Bibcode: 2016AGUFMSH11C2234G Altcode: The Flare Likelihood And Region Eruption foreCASTing (FLARECAST) project is an EU H2020-funded consortium project aiming to develop an advanced solar flare forecasting system by implementing state-of-the-art solar data analysis and flare prediction algorithms. The Solar Physics Group at Trinity College Dublin is in charge of the analysis of observational data to extract solar active region properties that serve as input for the prediction algorithms. The calculated active region properties correspond to a non-exhaustive list of parameters that have demonstrated a strong flare association, such as Schrijver's R-value, the Fourier power spectrum exponent, the effective connected magnetic field (Beff), the horizontal field decay index, and the weighted length of strong-gradient polarity inversion lines (WLSG). Parameters were calculated from Spaceweather HMI Active Region Patch (SHARP) magnetograms, a data product of the Helioseismic and Magnetic Imager (HMI) magnetograph on the Solar Dynamics Observatory (SDO). SHARPs provide photospheric vector-magnetic field (B) images in near-realtime. For this study, results from a statistical study performed on a robust subsample of the entire SHARP dataset will be presented. In the framework of the FLARECAST predictor component, this study focuses, for the first time, on differences between parameter values found when the radial magnetic field component, Br, is used instead of the line-of-sight component, Blos. The effect of active region longitudinal position is discussed, as well as the flare association of the properties. Title: Solar flares, coronal mass ejections and solar energetic particle event characteristics Authors: Papaioannou, Athanasios; Sandberg, Ingmar; Anastasiadis, Anastasios; Kouloumvakos, Athanasios; Georgoulis, Manolis K.; Tziotziou, Kostas; Tsiropoula, Georgia; Jiggens, Piers; Hilgers, Alain Bibcode: 2016JSWSC...6A..42P Altcode: A new catalogue of 314 solar energetic particle (SEP) events extending over a large time span from 1984 to 2013 has been compiled. The properties as well as the associations of these SEP events with their parent solar sources have been thoroughly examined. The properties of the events include the proton peak integral flux and the fluence for energies above 10, 30, 60 and 100 MeV. The associated solar events were parametrized by solar flare (SF) and coronal mass ejection (CME) characteristics, as well as related radio emissions. In particular, for SFs: the soft X-ray (SXR) peak flux, the SXR fluence, the heliographic location, the rise time and the duration were exploited; for CMEs the plane-of-sky velocity as well as the angular width were utilized. For radio emissions, type III, II and IV radio bursts were identified. Furthermore, we utilized element abundances of Fe and O. We found evidence that most of the SEP events in our catalogue do not conform to a simple two-class paradigm, with the 73% of them exhibiting both type III and type II radio bursts, and that a continuum of event properties is present. Although, the so-called hybrid or mixed events are found to be present in our catalogue, it was not possible to attribute each SEP event to a mixed/hybrid sub-category. Moreover, it appears that the start of the type III burst most often precedes the maximum of the SF and thus falls within the impulsive phase of the associated SF. At the same time, type III bursts take place within ≈5.22 min, on average, in advance from the time of maximum of the derivative of the SXR flux (Neupert effect). We further performed a statistical analysis and a mapping of the logarithm of the proton peak flux at E > 10 MeV, on different pairs of the parent solar source characteristics. This revealed correlations in 3-D space and demonstrated that the gradual SEP events that stem from the central part of the visible solar disk constitute a significant radiation risk. The velocity of the associated CMEs, as well as the SXR peak flux and fluence, are all fairly significantly correlated to both the proton peak flux and the fluence of the SEP events in our catalogue. The strongest correlation to SEP characteristics is manifested by the CME velocity. Title: An observationally-driven kinetic approach to coronal heating Authors: Moraitis, K.; Toutountzi, A.; Isliker, H.; Georgoulis, M.; Vlahos, L.; Chintzoglou, G. Bibcode: 2016A&A...596A..56M Altcode: 2016arXiv160307129M; 2016arXiv160307129T
Aims: Coronal heating through the explosive release of magnetic energy remains an open problem in solar physics. Recent hydrodynamical models attempt an investigation by placing swarms of "nanoflares" at random sites and times in modeled one-dimensional coronal loops. We investigate the problem in three dimensions, using extrapolated coronal magnetic fields of observed solar active regions.
Methods: We applied a nonlinear force-free field extrapolation above an observed photospheric magnetogram of NOAA active region (AR) 11 158. We then determined the locations, energy contents, and volumes of "unstable" areas, namely areas prone to releasing magnetic energy due to locally accumulated electric current density. Statistical distributions of these volumes and their fractal dimension are inferred, investigating also their dependence on spatial resolution. Further adopting a simple resistivity model, we inferred the properties of the fractally distributed electric fields in these volumes. Next, we monitored the evolution of 105 particles (electrons and ions) obeying an initial Maxwellian distribution with a temperature of 10 eV, by following their trajectories and energization when subjected to the resulting electric fields. For computational convenience, the length element of the magnetic-field extrapolation is 1 arcsec, or 725 km, much coarser than the particles' collisional mean free path in the low corona (0.1-1 km).
Results: The presence of collisions traps the bulk of the plasma around the unstable volumes, or current sheets (UCS), with only a tail of the distribution gaining substantial energy. Assuming that the distance between UCS is similar to the collisional mean free path we find that the low active-region corona is heated to 100-200 eV, corresponding to temperatures exceeding 2 MK, within tens of seconds for electrons and thousands of seconds for ions.
Conclusions: Fractally distributed, nanoflare-triggening fragmented UCS in the active-region corona can heat electrons and ions with minor enhancements of the local resistivity. This statistical result is independent from the nature of the extrapolation and the spatial resolution of the modeled active-region corona. This finding should be coupled with a complete plasma treatment to determine whether a quasi-steady temperature similar to that of the ambient corona can be maintained, either via a kinetic or via a hybrid, kinetic and fluid, plasma treatment. The finding can also be extended to the quiet solar corona, provided that the currently undetected nanoflares are frequent enough to account for the lower (compared to active regions) energy losses in this case. Title: Near-Sun and 1 AU magnetic field of coronal mass ejections: a parametric study Authors: Patsourakos, S.; Georgoulis, M. K. Bibcode: 2016A&A...595A.121P Altcode: 2016arXiv160900134P
Aims: The magnetic field of coronal mass ejections (CMEs) determines their structure, evolution, and energetics, as well as their geoeffectiveness. However, we currently lack routine diagnostics of the near-Sun CME magnetic field, which is crucial for determining the subsequent evolution of CMEs.
Methods: We recently presented a method to infer the near-Sun magnetic field magnitude of CMEs and then extrapolate it to 1 AU. This method uses relatively easy to deduce observational estimates of the magnetic helicity in CME-source regions along with geometrical CME fits enabled by coronagraph observations. We hereby perform a parametric study of this method aiming to assess its robustness. We use statistics of active region (AR) helicities and CME geometrical parameters to determine a matrix of plausible near-Sun CME magnetic field magnitudes. In addition, we extrapolate this matrix to 1 AU and determine the anticipated range of CME magnetic fields at 1 AU representing the radial falloff of the magnetic field in the CME out to interplanetary (IP) space by a power law with index αB.
Results: The resulting distribution of the near-Sun (at 10 R) CME magnetic fields varies in the range [0.004, 0.02] G, comparable to, or higher than, a few existing observational inferences of the magnetic field in the quiescent corona at the same distance. We also find that a theoretically and observationally motivated range exists around αB = -1.6 ± 0.2, thereby leading to a ballpark agreement between our estimates and observationally inferred field magnitudes of magnetic clouds (MCs) at L1.
Conclusions: In a statistical sense, our method provides results that are consistent with observations. Title: Magnetic Helicity Estimations in Models and Observations of the Solar Magnetic Field. Part I: Finite Volume Methods Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey; Chen, Feng; Georgoulis, Manolis K.; Guo, Yang; Liu, Yang; Moraitis, Kostas; Thalmann, Julia K.; Yang, Shangbin Bibcode: 2016SSRv..201..147V Altcode: 2016SSRv..tmp...68V; 2016arXiv161002193V Magnetic helicity is a conserved quantity of ideal magneto-hydrodynamics characterized by an inverse turbulent cascade. Accordingly, it is often invoked as one of the basic physical quantities driving the generation and structuring of magnetic fields in a variety of astrophysical and laboratory plasmas. We provide here the first systematic comparison of six existing methods for the estimation of the helicity of magnetic fields known in a finite volume. All such methods are reviewed, benchmarked, and compared with each other, and specifically tested for accuracy and sensitivity to errors. To that purpose, we consider four groups of numerical tests, ranging from solutions of the three-dimensional, force-free equilibrium, to magneto-hydrodynamical numerical simulations. Almost all methods are found to produce the same value of magnetic helicity within few percent in all tests. In the more solar-relevant and realistic of the tests employed here, the simulation of an eruptive flux rope, the spread in the computed values obtained by all but one method is only 3 %, indicating the reliability and mutual consistency of such methods in appropriate parameter ranges. However, methods show differences in the sensitivity to numerical resolution and to errors in the solenoidal property of the input fields. In addition to finite volume methods, we also briefly discuss a method that estimates helicity from the field lines' twist, and one that exploits the field's value at one boundary and a coronal minimal connectivity instead of a pre-defined three-dimensional magnetic-field solution. Title: A Comparison of Flare Forecasting Methods. I. Results from the “All-Clear” Workshop Authors: Barnes, G.; Leka, K. D.; Schrijver, C. J.; Colak, T.; Qahwaji, R.; Ashamari, O. W.; Yuan, Y.; Zhang, J.; McAteer, R. T. J.; Bloomfield, D. S.; Higgins, P. A.; Gallagher, P. T.; Falconer, D. A.; Georgoulis, M. K.; Wheatland, M. S.; Balch, C.; Dunn, T.; Wagner, E. L. Bibcode: 2016ApJ...829...89B Altcode: 2016arXiv160806319B Solar flares produce radiation that can have an almost immediate effect on the near-Earth environment, making it crucial to forecast flares in order to mitigate their negative effects. The number of published approaches to flare forecasting using photospheric magnetic field observations has proliferated, with varying claims about how well each works. Because of the different analysis techniques and data sets used, it is essentially impossible to compare the results from the literature. This problem is exacerbated by the low event rates of large solar flares. The challenges of forecasting rare events have long been recognized in the meteorology community, but have yet to be fully acknowledged by the space weather community. During the interagency workshop on “all clear” forecasts held in Boulder, CO in 2009, the performance of a number of existing algorithms was compared on common data sets, specifically line-of-sight magnetic field and continuum intensity images from the Michelson Doppler Imager, with consistent definitions of what constitutes an event. We demonstrate the importance of making such systematic comparisons, and of using standard verification statistics to determine what constitutes a good prediction scheme. When a comparison was made in this fashion, no one method clearly outperformed all others, which may in part be due to the strong correlations among the parameters used by different methods to characterize an active region. For M-class flares and above, the set of methods tends toward a weakly positive skill score (as measured with several distinct metrics), with no participating method proving substantially better than climatological forecasts. Title: Enabling Solar Flare Forecasting at an Unprecedented Level: the FLARECAST Project Authors: Georgoulis, Manolis K.; Pariat, Etienne; Massone, Anna Maria; Vilmer, Nicole; Jackson, David; Buchlin, Eric; Csillaghy, Andre; Bommier, Veronique; Kontogiannis, Ioannis; Gallagher, Peter; Gontikakis, Costis; Guennou, Chloé; Murray, Sophie; Bloomfield, D. Shaun; Alingery, Pablo; Baudin, Frederic; Benvenuto, Federico; Bruggisser, Florian; Florios, Konstantinos; Guerra, Jordan; Park, Sung-Hong; Perasso, Annalisa; Piana, Michele; Sathiapal, Hanna; Soldati, Marco; Von Stachelski, Samuel; Argoudelis, Vangelis; Caminade, Stephane Bibcode: 2016cosp...41E.657G Altcode: We attempt a brief but informative description of the Flare Likelihood And Region Eruption Forecasting (FLARECAST) project, European Commission's first large-scale investment to explore the limits of reliability and accuracy for the forecasting of major solar flares. The consortium, objectives, and first results of the project - featuring an openly accessible, interactive flare forecasting facility by the end of 2017 - will be outlined. In addition, we will refer to the so-called "explorative research" element of project, aiming to connect solar flares with coronal mass ejections (CMEs) and possibly pave the way for CME, or eruptive flare, prediction. We will also emphasize the FLARECAST modus operandi, namely the diversity of expertise within the consortium that independently aims to science, infrastructure development and dissemination, both to stakeholders and to the general public. Concluding, we will underline that the FLARECAST project responds squarely to the joint COSPAR - ILWS Global Roadmap to shield society from the adversities of space weather, addressing its primary goal and, in particular, its Research Recommendations 1, 2 and 4, Teaming Recommendations II and III, and Collaboration Recommendations A, B, and D. The FLARECAST project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 640216. Title: A Robust Method to Predict the Near-Sun and Interplanetary Magnetic Field Strength of Coronal Mass Ejections: Parametric and Case Studies Authors: Patsourakos, Spiros; Georgoulis, Manolis K. Bibcode: 2016cosp...41E1531P Altcode: Predicting the near-Sun, and particularly the Interplanetary (IP), magnetic field structure of Coronal Mass Ejections (CMEs) and interplanetary counterparts (ICMEs) is a topic of intense research activity. This is because Earth-directed CMEs with strong southward magnetic fields are responsible for the most powerful geomagnetic storms. We have recently developed a simple two-tier method to predict the magnetic field strength of CMEs in the outer corona and in the IP medium, using as input the magnetic-helicity budget of the source solar active region and stereoscopic coronagraphic observations. Near-Sun CME magnetic fields are obtained by utilizing the principle of magnetic helicity conservation of flux-rope CMEs for coronagraphic observations. Interplanetary propagation of the inferred values is achieved by employing power-law prescriptions of the radial evolution of the CME-ICME magnetic fields. We hereby present a parametric study of our method, based on the observed statistics of input parameters, to infer the anticipated range of values for the near-Sun and interplanetary CME-ICME magnetic fields. This analysis is complemented by application of our method to several well-observed major CME-ICME events. Title: Solar Flare Prediction Science-to-Operations: the ESA/SSA SWE A-EFFort Service Authors: Georgoulis, Manolis K.; Tziotziou, Konstantinos; Themelis, Konstantinos; Magiati, Margarita; Angelopoulou, Georgia Bibcode: 2016cosp...41E.656G Altcode: We attempt a synoptical overview of the scientific origins of the Athens Effective Solar Flare Forecasting (A-EFFort) utility and the actions taken toward transitioning it into a pre-operational service of ESA's Space Situational Awareness (SSA) Programme. The preferred method for solar flare prediction, as well as key efforts to make it function in a fully automated environment by coupling calculations with near-realtime data-downloading protocols (from the Solar Dynamics Observatory [SDO] mission), pattern recognition (solar active-region identification) and optimization (magnetic connectivity by simulated annealing) will be highlighted. In addition, the entire validation process of the service will be described, with its results presented. We will conclude by stressing the need for across-the-board efforts and synergistic work in order to bring science of potentially limited/restricted interest into realizing a much broader impact and serving the best public interests. The above presentation was partially supported by the ESA/SSA SWE A-EFFort project, ESA Contract No. 4000111994/14/D/MRP. Special thanks go to the ESA Project Officers R. Keil, A. Glover, and J.-P. Luntama (ESOC), M. Bobra and C. Balmer of the SDO/HMI team at Stanford University, and M. Zoulias at the RCAAM of the Academy of Athens for valuable technical help. Title: Predicting the near-Sun and Interplanetary Magnetic Field of CMEs using photospheric magnetograms and coronagraph images Authors: Patsourakos, Spiros; Georgoulis, Manolis Bibcode: 2016EGUGA..18.4784P Altcode: Earth-directed Coronal Mass Ejections (CMEs) containing a strong southward magnetic-field component upon arrival at 1 AU statistically account for the most powerful geomagnetic storms. Unfortunately, though, we currently lack routine diagnostics of the magnetic field of CMEs and its evolution in the inner heliosphere and the interplanetary (IP) medium. We hereby present a simple, yet powerful and easy-to-implement, method to deduce the near-Sun and IP magnetic field entrained in CMEs, by using photospheric magnetograms of the solar source regions and multi-viewpoint coronagraph images of the corresponding CMEs. The method relies on the principle of magnetic-helicity conservation in low plasma-beta, flux-rope CMEs and a power-law prescription of the radial evolution of the CME magnetic field in the IP medium. We outline a parametric study based on the observed statistics of input parameters to calculate a matrix of magnetic-field solutions for 10000 synthetic CMEs. The robustness and possible limitations / ramifications of the method are deduced by a comparison with the distributions of the predicted CME-ICME magnetic fields at 0.3 and 1 AU using actual Messenger and ACE published observations. Title: 25 Years of Self-organized Criticality: Numerical Detection Methods Authors: McAteer, R. T. James; Aschwanden, Markus J.; Dimitropoulou, Michaila; Georgoulis, Manolis K.; Pruessner, Gunnar; Morales, Laura; Ireland, Jack; Abramenko, Valentyna Bibcode: 2016SSRv..198..217M Altcode: 2015SSRv..tmp...31M; 2015arXiv150608142M The detection and characterization of self-organized criticality (SOC), in both real and simulated data, has undergone many significant revisions over the past 25 years. The explosive advances in the many numerical methods available for detecting, discriminating, and ultimately testing, SOC have played a critical role in developing our understanding of how systems experience and exhibit SOC. In this article, methods of detecting SOC are reviewed; from correlations to complexity to critical quantities. A description of the basic autocorrelation method leads into a detailed analysis of application-oriented methods developed in the last 25 years. In the second half of this manuscript space-based, time-based and spatial-temporal methods are reviewed and the prevalence of power laws in nature is described, with an emphasis on event detection and characterization. The search for numerical methods to clearly and unambiguously detect SOC in data often leads us outside the comfort zone of our own disciplines—the answers to these questions are often obtained by studying the advances made in other fields of study. In addition, numerical detection methods often provide the optimum link between simulations and experiments in scientific research. We seek to explore this boundary where the rubber meets the road, to review this expanding field of research of numerical detection of SOC systems over the past 25 years, and to iterate forwards so as to provide some foresight and guidance into developing breakthroughs in this subject over the next quarter of a century. Title: The Major Geoeffective Solar Eruptions of 2012 March 7: Comprehensive Sun-to-Earth Analysis Authors: Patsourakos, S.; Georgoulis, M. K.; Vourlidas, A.; Nindos, A.; Sarris, T.; Anagnostopoulos, G.; Anastasiadis, A.; Chintzoglou, G.; Daglis, I. A.; Gontikakis, C.; Hatzigeorgiu, N.; Iliopoulos, A. C.; Katsavrias, C.; Kouloumvakos, A.; Moraitis, K.; Nieves-Chinchilla, T.; Pavlos, G.; Sarafopoulos, D.; Syntelis, P.; Tsironis, C.; Tziotziou, K.; Vogiatzis, I. I.; Balasis, G.; Georgiou, M.; Karakatsanis, L. P.; Malandraki, O. E.; Papadimitriou, C.; Odstrčil, D.; Pavlos, E. G.; Podlachikova, O.; Sandberg, I.; Turner, D. L.; Xenakis, M. N.; Sarris, E.; Tsinganos, K.; Vlahos, L. Bibcode: 2016ApJ...817...14P Altcode: During the interval 2012 March 7-11 the geospace experienced a barrage of intense space weather phenomena including the second largest geomagnetic storm of solar cycle 24 so far. Significant ultra-low-frequency wave enhancements and relativistic-electron dropouts in the radiation belts, as well as strong energetic-electron injection events in the magnetosphere were observed. These phenomena were ultimately associated with two ultra-fast (>2000 km s-1) coronal mass ejections (CMEs), linked to two X-class flares launched on early 2012 March 7. Given that both powerful events originated from solar active region NOAA 11429 and their onsets were separated by less than an hour, the analysis of the two events and the determination of solar causes and geospace effects are rather challenging. Using satellite data from a flotilla of solar, heliospheric and magnetospheric missions a synergistic Sun-to-Earth study of diverse observational solar, interplanetary and magnetospheric data sets was performed. It was found that only the second CME was Earth-directed. Using a novel method, we estimated its near-Sun magnetic field at 13 R to be in the range [0.01, 0.16] G. Steep radial fall-offs of the near-Sun CME magnetic field are required to match the magnetic fields of the corresponding interplanetary CME (ICME) at 1 AU. Perturbed upstream solar-wind conditions, as resulting from the shock associated with the Earth-directed CME, offer a decent description of its kinematics. The magnetospheric compression caused by the arrival at 1 AU of the shock associated with the ICME was a key factor for radiation-belt dynamics. Title: 25 Years of Self-Organized Criticality: Solar and Astrophysics Authors: Aschwanden, Markus J.; Crosby, Norma B.; Dimitropoulou, Michaila; Georgoulis, Manolis K.; Hergarten, Stefan; McAteer, James; Milovanov, Alexander V.; Mineshige, Shin; Morales, Laura; Nishizuka, Naoto; Pruessner, Gunnar; Sanchez, Raul; Sharma, A. Surja; Strugarek, Antoine; Uritsky, Vadim Bibcode: 2016SSRv..198...47A Altcode: 2014arXiv1403.6528A; 2014SSRv..tmp...29A Shortly after the seminal paper "Self-Organized Criticality: An explanation of 1/ f noise" by Bak et al. (1987), the idea has been applied to solar physics, in "Avalanches and the Distribution of Solar Flares" by Lu and Hamilton (1991). In the following years, an inspiring cross-fertilization from complexity theory to solar and astrophysics took place, where the SOC concept was initially applied to solar flares, stellar flares, and magnetospheric substorms, and later extended to the radiation belt, the heliosphere, lunar craters, the asteroid belt, the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars, black-hole objects, cosmic rays, and boson clouds. The application of SOC concepts has been performed by numerical cellular automaton simulations, by analytical calculations of statistical (powerlaw-like) distributions based on physical scaling laws, and by observational tests of theoretically predicted size distributions and waiting time distributions. Attempts have been undertaken to import physical models into the numerical SOC toy models, such as the discretization of magneto-hydrodynamics (MHD) processes. The novel applications stimulated also vigorous debates about the discrimination between SOC models, SOC-like, and non-SOC processes, such as phase transitions, turbulence, random-walk diffusion, percolation, branching processes, network theory, chaos theory, fractality, multi-scale, and other complexity phenomena. We review SOC studies from the last 25 years and highlight new trends, open questions, and future challenges, as discussed during two recent ISSI workshops on this theme. Title: Preface: Advances in solar physics Authors: Georgoulis, Manolis K.; Nakariakov, Valery M. Bibcode: 2015AdSpR..56.2677G Altcode: The idea for this special issue of Advances in Space Research (ASR) was formulated during the 14th European Solar Physics Meeting (ESPM-14) that took place in Dublin, Ireland in September 2014. Since ASR does not publish conference proceedings, it was decided to extend a general call to the international solar-physics community for manuscripts pertinent to the following thematic areas:

New and upcoming heliospheric observational and data assimilation facilities. Title: Analysing the Effects of Apodizing Windows on Local Correlation Tracking Using Nirvana Simulations of Convection Authors: Louis, Rohan E.; Ravindra, B.; Georgoulis, Manolis K.; Küker, Manfred Bibcode: 2015SoPh..290.1135L Altcode: 2015arXiv150202530L; 2015SoPh..tmp...25L We employ different shapes of apodizing windows in the local correlation tracking (LCT) routine to retrieve horizontal velocities using numerical simulations of convection. LCT was applied on a time sequence of temperature maps generated by the Nirvana code with four different apodizing windows, Gaussian, Lorentzian, trapezoidal, and triangular, with varying widths. In terms of correlations (between the LCT-retrieved and simulated flow field), the triangular and the trapezoidal perform the best and worst, respectively. By segregating the intrinsic velocities in the simulations on the basis of their magnitudes, we find that for all windows a significantly higher correlation is obtained for the intermediate and high-velocity bins and only modest or weak values in the low-velocity bins. The differences between the LCT-retrieved and simulated flow fields were determined spatially. They show large residuals at or close to the boundary of granules. The extent to which the horizontal flow vectors retrieved by LCT are similar to the simulated values entirely depends on the width of the central peak of the apodizing window for a given σ. Even though LCT suffers from a lack of spatial content, as seen in simulations, its simplicity and speed could serve as a viable first-order tool to probe horizontal flows. This would be an ideal tool for large data sets. Title: Validation and Benchmarking of a Practical Free Magnetic Energy and Relative Magnetic Helicity Budget Calculation in Solar Magnetic Structures Authors: Moraitis, K.; Tziotziou, K.; Georgoulis, M. K.; Archontis, V. Bibcode: 2014SoPh..289.4453M Altcode: 2014arXiv1406.5381M; 2014SoPh..tmp..122M In earlier works we introduced and tested a nonlinear force-free (NLFF) method designed to self-consistently calculate the coronal free magnetic energy and the relative magnetic helicity budgets of observed solar magnetic structures. In principle, the method requires only a single, photospheric or low-chromospheric, vector magnetogram of a quiet-Sun patch or an active region and performs calculations without three-dimensional magnetic and velocity-field information. In this work we strictly validate this method using three-dimensional coronal magnetic fields. Benchmarking employs both synthetic, three-dimensional magnetohydrodynamic simulations and nonlinear force-free field extrapolations of the active-region solar corona. Our time-efficient NLFF method provides budgets that differ from those of more demanding semi-analytical methods by a factor of approximately three, at most. This difference is expected to come from the physical concept and the construction of the method. Temporal correlations show more discrepancies that are, however, soundly improved for more complex, massive active regions, reaching correlation coefficients on the order of, or exceeding, 0.9. In conclusion, we argue that our NLFF method can be reliably used for a routine and fast calculation of the free magnetic energy and relative magnetic helicity budgets in targeted parts of the solar magnetized corona. As explained in this article and in previous works, this is an asset that can lead to valuable insight into the physics and triggering of solar eruptions. Title: Validation of the magnetic energy vs. helicity scaling in solar magnetic structures Authors: Tziotziou, K.; Moraitis, K.; Georgoulis, M. K.; Archontis, V. Bibcode: 2014A&A...570L...1T Altcode: 2014arXiv1409.8117T
Aims: We assess the validity of the free magnetic energy - relative magnetic helicity diagram for solar magnetic structures.
Methods: We used two different methods of calculating the free magnetic energy and the relative magnetic helicity budgets: a classical, volume-calculation nonlinear force-free (NLFF) method applied to finite coronal magnetic structures and a surface-calculation NLFF derivation that relies on a single photospheric or chromospheric vector magnetogram. Both methods were applied to two different data sets, namely synthetic active-region cases obtained by three-dimensional magneto-hydrodynamic (MHD) simulations and observed active-region cases, which include both eruptive and noneruptive magnetic structures.
Results: The derived energy-helicity diagram shows a consistent monotonic scaling between relative helicity and free energy with a scaling index 0.84 ± 0.05 for both data sets and calculation methods. It also confirms the segregation between noneruptive and eruptive active regions and the existence of thresholds in both free energy and relative helicity for active regions to enter eruptive territory.
Conclusions: We consider the previously reported energy-helicity diagram of solar magnetic structures as adequately validated and envision a significant role of the uncovered scaling in future studies of solar magnetism. Title: Energy and helicity budgets of solar quiet regions Authors: Tziotziou, K.; Tsiropoula, G.; Georgoulis, M. K.; Kontogiannis, I. Bibcode: 2014A&A...564A..86T Altcode: 2014arXiv1403.0730T
Aims: We investigate the free magnetic energy and relative magnetic helicity budgets of solar quiet regions.
Methods: Using a novel nonlinear force-free method that requires single solar vector magnetograms we calculated the instantaneous free magnetic energy and relative magnetic helicity budgets in 55 quiet-Sun vector magnetograms.
Results: As in a previous work on active regions, we constructed here for the first time the (free) energy-(relative) helicity diagram of quiet-Sun regions. We find that quiet-Sun regions have no dominant sense of helicity and show monotonic correlations a) between free magnetic energy/relative helicity and magnetic network area and, consequently, b) between free magnetic energy and helicity. Free magnetic energy budgets of quiet-Sun regions represent a rather continuous extension of respective active-region budgets towards lower values, but the corresponding helicity transition is discontinuous because of the incoherence of the helicity sense in contrast to active regions. We furthermore estimated the instantaneous free magnetic-energy and relative magnetic-helicity budgets of the entire quiet Sun, as well as the respective budgets over an entire solar cycle.
Conclusions: Derived instantaneous free magnetic energy budgets and, to a lesser extent, relative magnetic helicity budgets over the entire quiet Sun are similar to the respective budgets of a sizeable active region, while total budgets within a solar cycle are found to be higher than previously reported. Free-energy budgets are similar to the energy needed to power fine-scale structures residing at the network, such as mottles and spicules. Title: Free magnetic energy and relative magnetic helicity diagnostics for the quality of NLFF field extrapolations Authors: Moraitis, Kostas; Archontis, Vasilis; Tziotziou, Konstantinos; Georgoulis, Manolis K. Bibcode: 2014cosp...40E2169M Altcode: We calculate the instantaneous free magnetic energy and relative magnetic helicity of solar active regions using two independent approaches: a) a non-linear force-free (NLFF) method that requires only a single photospheric vector magnetogram, and b) well known semi-analytical formulas that require the full three-dimensional (3D) magnetic field structure. The 3D field is obtained either from MHD simulations, or from observed magnetograms via respective NLFF field extrapolations. We find qualitative agreement between the two methods and, quantitatively, a discrepancy not exceeding a factor of 4. The comparison of the two methods reveals, as a byproduct, two independent tests for the quality of a given force-free field extrapolation. We find that not all extrapolations manage to achieve the force-free condition in a valid, divergence-free, magnetic configuration. This research has been co-financed by the European Union (European Social Fund - ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society through the European Social Fund. Title: Irreversibility and the Point of No Return in the Evolution of Eruptive Active Regions Authors: Georgoulis, Manolis K. Bibcode: 2014cosp...40E.966G Altcode: We combine multiple methods and findings to demonstrate that those eruptive solar active regions that form intense photospheric magnetic polarity inversion lines (PILs) enter a domain of irreversible evolution that will unavoidably force them to erupt at least once, giving rise to a major flare and an associated fast CME. Electric currents, Lorentz forces, free magnetic energy storage, and magnetic helicity, all play major roles in bringing the magnetic configuration on the verge of instability. The inferred irreversibility stems from the conservative properties of magnetic helicity in high magnetic Reynolds-number plasmas. In addition, the long-standing and fiercely debated classification of eruptive magnetic structures into sheared arcades and flux ropes is found to be of relatively little meaning: by means of the evolution above, the simplest possible sheared-arcade structure may gradually evolve into a flux rope susceptible to the helical-kink and the torus instabilities, among other destabilization mechanisms. Research partially supported by the EU Seventh Framework Programme under grant agreement No. PIRG07-GA-2010-268245 and by the European Union Social Fund (ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society through the European Social Fund. Title: Using Magnetic Helicity Diagnostics to Determine the Nature of Solar Active-Region Formation Authors: Georgoulis, Manolis K. Bibcode: 2014cosp...40E.967G Altcode: Employing a novel nonlinear force-free (NLFF) method that self-consistently infers instantaneous free magnetic-energy and relative magnetic-helicity budgets from single photospheric vector magnetograms, we recently constructed the magnetic energy-helicity (EH) diagram of solar active regions. The EH diagram implies dominant relative helicities of left-handed or right-handed chiralities for the great majority of active regions. The amplitude (budget) of these helicities scales monotonically with the free magnetic energy. This constructive, strongly preferential accumulation of a certain sense of magnetic helicity seems to disqualify recently proposed mechanisms relying on a largely random near-surface convection for the formation of the great majority of active regions. The existing qualitative formation mechanism for these regions remains the conventional Omega-loop emergence following a buoyant ascension from the bottom of the convection zone. However, exceptions to this rule include even eruptive active regions: NOAA AR 11283 is an obvious outlier to the EH diagram, involving significant free magnetic energy with a small relative magnetic helicity. Relying on a timeseries of vector magnetograms of this region, our methodology shows nearly canceling amounts of both senses of helicity and an overall course from a weakly left-handed to a weakly right-handed structure, in the course of which a major eruption occurs. For this and similarly behaving active regions the latest near-surface formation scenario might conceivably be employed successfully. Research partially supported by the EU Seventh Framework Programme under grant agreement No. PIRG07-GA-2010-268245 and by the European Union Social Fund (ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society through the European Social Fund. Title: Higher topological invariants of magnetic field lines: observational aspects Authors: Illarionov, Egor; Smirnov, Alexander; Georgoulis, Manolis K.; Sokoloff, Dmitry; Akhmet'ev, Peter Bibcode: 2014cosp...40E1270I Altcode: Topology of magnetic field lines is directly involved in magnetohydrodynamic (MHD) theorems and equations. Being an invariant of motion in ideal MHD conditions, the magnetic field-line topology is a natural obstacle to the relaxation of magnetic field into a current-free (potential) field and contrariwise limits a dynamo generation. Usage of these conservational laws and writing of numerical relations require a quantification of topology. One of the simplest existing measures of magnetic topology is the mutual magnetic helicity, that expresses the combined action of interaction and linkage between different magnetic field lines. For practical purposes there exists the revised concept of relative magnetic helicity, that allows to estimate the complexity of field-line topology in case of open volume, i.e. when magnetic lines cross the boundaries of given 3D region. At the same time this concept remains a simple interpretation of linkage number in terms of individual lines. Our point however is that magnetic helicity is far from being unique or comprehensive quantification of magnetic field-line topology. To improve the situation we introduce a set of higher invariants which extends the idea of relative helicity and provides a new means to describe the magnetic field-line topology. To practically study the possibility of implementation of higher topological invariants we reconstruct several moments of mutual helicity from observed solar vector magnetograms with extrapolated magnetic field above the photosphere and discuss to what extent such knowledge could be instructive for understanding of the solar magnetic field evolution. Title: Free magnetic energy and relative magnetic helicity in active and quiet solar regions and their role in solar dynamics Authors: Tziotziou, Konstantinos; Archontis, Vasilis; Tsiropoula, Georgia; Georgoulis, Manolis K.; Moraitis, Kostas; Kontogiannis, Ioannis Bibcode: 2014cosp...40E3428T Altcode: We present a novel non-linear force-free method for the calculation of the instantaneous free magnetic energy and relative magnetic helicity budgets of a solar region from a single photospheric/chromospheric vector magnetogram. Our objective is to study the role of these quantities both in solar eruptions and in quiet-Sun dynamics. The validity of the method is tested using both observations and synthetic magnetohydrodynamical (MHD) models. The method is applied for the derivation of the energy-helicity (EH) diagram of solar active regions (ARs) from a sample of 162 vector magnetograms corresponding to 42 different ARs, suggesting the existence of 4×10(31) erg and 2×10(42) Mx(2) thresholds in free energy and relative helicity, respectively, for ARs to enter eruptive territory. Furthermore, the dynamical evolution of both quantities in eruptive NOAA AR 11158, using a high-cadence 5-day time series of vector magnetograms, suggests the formation of increasingly helical pre-eruption structures and a causal relation between flares and Coronal Mass Ejections (CMEs). The method is also used to derive helicity and energy budgets in quiet Sun regions and construct the respective EH diagram. Our results highlight the importance of both energy and helicity in AR evolution and quiet-Sun dynamics and instigate further research on the underlying physics with three-dimensional MHD models. This work is supported by EU's Seventh Framework Programme via a Marie Curie Fellowship. Title: Toward an Efficient Prediction of Solar Flares: Which Parameters, and How? Authors: Georgoulis, M. K. Bibcode: 2013Entrp..15.5022G Altcode: Solar flare prediction has become a forefront topic in contemporary solar physics, with numerous published methods relying on numerous predictive parameters, that can even be divided into parameter classes. Attempting further insight, we focus on two popular classes of flare-predictive parameters, namely multiscale (i.e., fractal and multifractal) and proxy (i.e., morphological) parameters, and we complement our analysis with a study of the predictive capability of fundamental physical parameters (i.e., magnetic free energy and relative magnetic helicity). Rather than applying the studied parameters to a comprehensive statistical sample of flaring and non-flaring active regions, that was the subject of our previous studies, the novelty of this work is their application to an exceptionally long and high-cadence time series of the intensely eruptive National Oceanic and Atmospheric Administration (NOAA) active region (AR) 11158, observed by the Helioseismic and Magnetic Ima!

ger on board the Solar Dynamics Observatory. Aiming for a detailed study of the temporal evolution of each parameter, we seek distinctive patterns that could be associated with the four largest flares in the AR in the course of its five-day observing interval. We find that proxy parameters only tend to show preflare impulses that are practical enough to warrant subsequent investigation with sufficient statistics. Combining these findings with previous results, we conclude that: (i) carefully constructed, physically intuitive proxy parameters may be our best asset toward an efficient future flare-forecasting; and (ii) the time series of promising parameters may be as important as their instantaneous values. Value-based prediction is the only approach followed so far. Our results call for novel signal and/or image processing techniques to efficiently utilize combined amplitude and temporal-profile information to optimize the inferred solar-flare probabilities. Title: Magnetic helicity and free energy in solar active regions Authors: Moraitis, K.; Georgoulis, M.; Tziotziou, K.; Archontis, V. Bibcode: 2013hell.confS..21M Altcode: We study the evolution of the non-potential free magnetic energy and relative magnetic helicity budgets in solar active regions (ARs). For this we use a time-series of a three-dimensional, synthetic AR produced by magnetohydrodynamical (MHD) simulations. As a first step, we calculate the potential magnetic field that has the same normal components with the MHD field along all boundaries of the AR, by solving Laplace's equation. The free magnetic energy of the AR is then easily derived. From the two fields, MHD and potential one, we calculate the corresponding vector potentials with a recently proposed integration method. The knowledge of both fields and their respective vector potentials throughout the AR, allows us to estimate the relative magnetic helicity budget of the AR. Following this procedure for each snapshot of the AR, we reconstruct the evolution of free energy and helicity in the AR. Our method reproduces, for a synthetic AR, the energy/helicity relations known to hold in real active regions. Title: A statistical study of current-sheet formation above solar active regions based on selforganized criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M.; Anastasiadis, A.; Toutountzi, A. Bibcode: 2013hell.conf...16D Altcode: We treat flaring solar active regions as physical systems having reached the self-organized critical state. Their evolving magnetic configurations in the low corona may satisfy an instability criterion, related to the excession of a specific threshold in the curl of the magnetic field. This imposed instability criterion implies an almost zero resistivity everywhere in the solar corona, except in regions where magnetic-field discontinuities and. hence, local currents, reach the critical value. In these areas, current-driven instabilities enhance the resistivity by many orders of magnitude forming structures which efficiently accelerate charged particles. Simulating the formation of such structures (thought of as current sheets) via a refined SOC cellular-automaton model provides interesting information regarding their statistical properties. It is shown that the current density in such unstable regions follows power-law scaling. Furthermore, the size distribution of the produced current sheets is best fitted by power laws, whereas their formation probability is investigated against the photospheric magnetic configuration (e.g. Polarity Inversion Lines, Plage). The average fractal dimension of the produced current sheets is deduced depending on the selected critical threshold. The above-mentioned statistical description of intermittent electric field structures can be used by collisional relativistic test particle simulations, aiming to interpret particle acceleration in flaring active regions and in strongly turbulent media in astrophysical plasmas. The above work is supported by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: Free Magnetic Energy and Helicity in Active and Quiet Solar Regions and their role in Solar Authors: Tziotziou, K.; Georgoulis, M. K.; Tsiropoula, G.; Moraitis, K.; Kontogiannis, I. Bibcode: 2013hell.conf....6T Altcode: We present a novel nonlinear force-free method designed to calculate the instantaneous free magnetic energy and relative magnetic helicity budgets of a solar region from a single photospheric/chromospheric vector magnetogram of the region. Our objective is to study the role of these quantities in solar eruptions and quiet-Sun dynamics. We apply the method to (1) derive the energy/helicity diagram of solar active regions from a sample of 162 vector magnetograms corresponding to 42 different active regions (ARs), suggesting that there exist 4 1031 erg and 2 1042 Mx2 thresholds in free energy and relative helicity, respectively, for ARs to enter eruptive territory, (2) study the dynamics of eruptive NOAA AR 11158 using a high-cadence 5-day time series of vector magnetograms, suggesting the formation of increasingly helical pre-eruption structures and a causal relation between flares and Coronal Mass Ejections (CMEs) and, (3) derive helicity and energy budgets in quiet Sun regions and construct the respective energy/helicity diagram. Our results highlight the importance of these two parameters in AR evolution and quiet-Sun dynamics and instigate further research including detailed analysis with synthetic, magnetohydrodynamical models. This work is supported by EU's Seventh Framework Programme via a Marie Curie Fellowship and by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: Genesis of Free Magnetic Energy and Helicity in Solar Active Regions and their Role in Solar Eruptions Authors: Georgoulis, M. Bibcode: 2013hell.confQ..17G Altcode: Eruptive solar magnetic configurations are revisited in view of their magnetic energy and (relative) magnetic helicity budgets. We calculate these budgets via a novel self-consistent technique that requires neither photospheric flow velocity fields nor three-dimensional coronal magnetic field extrapolations of the observed photospheric structure. A single vector magnetogram of the structure is sufficient for the calculation of the instantaneous electric-current-induced (free) magnetic energy and helicity budgets. We apply the method to the intensely eruptive NOAA active region (AR) 11158 and report on the following findings: (1) significant budgets of free magnetic energy and helicity are developed in the AR, sufficient to power more than the observed eruptions; (2) eruption-related decreases of both budgets are noted, allowing estimation of the energy/helicity budgets of the eruptions themselves; (3) the AR complies with our previously and independently introduced energy/helicity diagram of solar active regions; and (4) a non-ideal transformation of mutual free energy and helicity terms into respective self terms results in increasingly helical pre-eruption structures. These findings suggest a new causal view of solar eruptions and the pre-eruption evolution of solar active regions. Parallel analysis with synthetic, magnetohydrodynamical models of active regions is underway, aiming to scrutinize our data analysis and subsequent interpretation. This work is supported by EU's Seventh Framework Programme via a Marie Curie Fellowship and by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: A Data-Driven, Integrated Flare Model Based on Self-Organized Criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. Bibcode: 2013hell.conf....7D Altcode: We interpret solar flares as events originating in solar active regions having reached the self-organized critical state, by alternatively using two versions of an "integrated flare model" - one static and one dynamic. In both versions the initial conditions are derived from observations aiming to investigate whether well-known scaling laws observed in the distribution functions of characteristic flare parameters are reproduced after the self-organized critical state has been reached. In the static model, we first apply a nonlinear force-free extrapolation that reconstructs the three-dimensional magnetic fields from two-dimensional vector magnetograms. We then locate magnetic discontinuities exceeding a threshold in the Laplacian of the magnetic field. These discontinuities are relaxed in local diffusion events, implemented in the form of cellular-automaton evolution rules. Subsequent loading and relaxation steps lead the system to self-organized criticality, after which the statistical properties of the simulated events are examined. In the dynamic version we deploy an enhanced driving mechanism, which utilizes the observed evolution of active regions, making use of sequential vector magnetograms. We first apply the static cellular automaton model to consecutive solar vector magnetograms until the self-organized critical state is reached. We then evolve the magnetic field inbetween these processed snapshots through spline interpolation, acting as a natural driver in the dynamic model. The identification of magnetically unstable sites as well as their relaxation follow the same rules as in the static model after each interpolation step. Subsequent interpolation/driving and relaxation steps cover all transitions until the end of the sequence. Physical requirements, such as the divergence-free condition for the magnetic field vector, are approximately satisfied in both versions of the model. We obtain robust power laws in the distribution functions of the modelled flaring events with scaling indices in good agreement with observations. We therefore conclude that well-known statistical properties of flares are reproduced after active regions reach self-organized criticality. The significant enhancement in both the static and the dynamic integrated flare models is that they initiate the simulation from observations, thus facilitating energy calculation in physical units. Especially in the dynamic version of the model, the driving of the system is based on observed, evolving vector magnetograms, allowing for the separation between MHD and kinetic timescales through the assignment of distinct MHD timestamps to each interpolation step. Title: Acceleration and solar origin of solar energetic particles observed by SREM units Authors: Anastasiadis, A.; Georgoulis, M.; Daglis, I.; Sandberg, I.; Nieminen, P. Bibcode: 2013hell.confQ..14A Altcode: Within the previous solar cycle 23, SREM units onboard ESA's INTEGRAL and Rosetta spacecraft detected several tens of Solar Energetic Particle Events (SEPEs) and accurately pinpointed their onset, rise, and decay times. We have undertaken a detailed study to determine the solar sources and the subsequent interplanetary coronal mass ejections (ICMEs) that gave rise to these events, as well as the timing of SEPEs with regard to the onset of possible geomagnetic activity triggered by these ICMEs. We find that virtually all SREM SEPEs can be associated with CME-driven shocks. Moreover, for a number of wellstudied INTEGRAL/SREM SEPEs we see an association between the SEPE peak and the shock passage at L1, subject to the heliographic location of the source solar active region. Shortly after the SEPE peak (typically within a few hours), the ICMEdriven modulation of the magnetosphere kicks in, often associated with a dip of the Dst index, indicating storm conditions in geospace. In essence we find that SREM SEPEs can be seamlessly fit into a coherent and consistent heliophysical interpretation of solar eruptions all the way from Sun to Earth. Their contribution to space-weather forecasting may be significant and warrants additional investigation. Title: Particle acceleration and nanoflare heating in coronal loops Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.; Anastasiadis, A.; Georgoulis, M. Bibcode: 2013hell.conf...18G Altcode: We model nanoflare heating of extrapolated active-region coronal loops via the acceleration of electrons and protons in Harris-type current sheets. The kinetic energy of the accelerated particles is estimated using semi-analytical and test-particle-tracing approaches. Vector magnetograms and photospheric Doppler velocity maps of NOAA active region 09114, recorded by the Imaging Vector Magnetograph (IVM), were used for this analysis in order to compute a current-free field extrapolation of the active-region corona. The corresponding Poynting fluxes at the footpoints of 5000 extrapolated coronal loops were then calculated. Assuming that reconnecting current sheets develop along these loops, we utilized previous results to estimate the kinetic-energy gain of the accelerated particles and we related this energy to nanoflare heating and macroscopic loop characteristics. Kinetic energies of 0.1 to 8~keV (for electrons) and 0.3 to 470~keV (for protons) were found to cause heating rates ranging from 10^-6 to 1 erg s^-1 cm^-3. Hydrodynamic simulations show that such heating rates can sustain plasma in coronal conditions inside the loops and generate plasma thermal distributions which are consistent with active region observations. We concluded the analysis by computing the form of Xray spectra generated by the accelerated electrons using the thick target approach that were found to be in agreement with observed X-ray spectra, thus supporting the plausibility of our nanoflare-heating scenario. This work is supported by EU's Seventh Framework Programme via a Marie Curie Fellowship and by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: Particle Acceleration in a Statistically Modeled Solar Active-Region Corona Authors: Toutounzi, A.; Vlahos, L.; Isliker, H.; Dimitropoulou, M.; Anastasiadis, A.; Georgoulis, M. Bibcode: 2013hell.conf....8T Altcode: Elaborating a statistical approach to describe the spatiotemporally intermittent electric field structures formed inside a flaring solar active region, we investigate the efficiency of such structures in accelerating charged particles (electrons). The large-scale magnetic configuration in the solar atmosphere responds to the strong turbulent flows that convey perturbations across the active region by initiating avalanche-type processes. The resulting unstable structures correspond to small-scale dissipation regions hosting strong electric fields. Previous research on particle acceleration in strongly turbulent plasmas provides a general framework for addressing such a problem. This framework combines various electromagnetic field configurations obtained by magnetohydrodynamical (MHD) or cellular automata (CA) simulations, or by employing a statistical description of the field's strength and configuration with test particle simulations. Our objective is to complement previous work done on the subject. As in previous efforts, a set of three probability distribution functions describes our ad-hoc electromagnetic field configurations. In addition, we work on data-driven 3D magnetic field extrapolations. A collisional relativistic test-particle simulation traces each particle's guiding center within these configurations. We also find that an interplay between different electron populations (thermal/non-thermal, ambient/injected) in our simulations may also address, via a re-acceleration mechanism, the so called `number problem'. Using the simulated particle-energy distributions at different heights of the cylinder we test our results against observations, in the framework of the collisional thick target model (CTTM) of solar hard X-ray (HXR) emission. The above work is supported by the Hellenic National Space Weather Research Network (HNSWRN) via the THALIS Programme. Title: Sun-to-Earth Analysis of a Major Geoeffective Solar Eruption within the Framework of the Authors: Patsourakos, S.; Vlahos, L.; Georgoulis, M.; Tziotziou, K.; Nindos, A.; Podladchikova, O.; Vourlidas, A.; Anastasiadis, A.; Sandberg, I.; Tsinganos, K.; Daglis, I.; Hillaris, A.; Preka-Papadema, P.; Sarris, M.; Sarris, T. Bibcode: 2013hell.conf...10P Altcode: Transient expulsions of gigantic clouds of solar coronal plasma into the interplanetary space in the form of Coronal Mass Ejections (CMEs) and sudden, intense flashes of electromagnetic radiation, solar flares, are well-established drivers of the variable Space Weather. Given the innate, intricate links and connections between the solar drivers and their geomagnetic effects, synergistic efforts assembling all pieces of the puzzle along the Sun-Earth line are required to advance our understanding of the physics of Space Weather. This is precisely the focal point of the Hellenic National Space Weather Research Network (HNSWRN) under the THALIS Programme. Within the HNSWRN framework, we present here the first results from a coordinated multi-instrument case study of a major solar eruption (X5.4 and X1.3 flares associated with two ultra-fast (>2000 km/s) CMEs) which were launched early on 7 March 2012 and triggered an intense geomagnetic storm (min Dst =-147 nT) approximately two days afterwards. Several elements of the associated phenomena, such as the flare and CME, EUV wave, WL shock, proton and electron event, interplanetary type II radio burst, ICME and magnetic cloud and their spatiotemporal relationships and connections are studied all way from Sun to Earth. To this end, we make use of satellite data from a flotilla of solar, heliospheric and magnetospheric missions and monitors (e.g., SDO, STEREO, WIND, ACE, Herschel, Planck and INTEGRAL). We also present our first steps toward formulating a cohesive physical scenario to explain the string of the observables and to assess the various physical mechanisms than enabled and gave rise to the significant geoeffectiveness of the eruption. Title: Interpreting Eruptive Behavior in NOAA AR 11158 via the Region's Magnetic Energy and Relative-helicity Budgets Authors: Tziotziou, Kostas; Georgoulis, Manolis K.; Liu, Yang Bibcode: 2013ApJ...772..115T Altcode: 2013arXiv1306.2135T In previous works, we introduced a nonlinear force-free method that self-consistently calculates the instantaneous budgets of free magnetic energy and relative magnetic helicity in solar active regions (ARs). Calculation is expedient and practical, using only a single vector magnetogram per computation. We apply this method to a time series of 600 high-cadence vector magnetograms of the eruptive NOAA AR 11158 acquired by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory over a five-day observing interval. Besides testing our method extensively, we use it to interpret the dynamical evolution in the AR, including eruptions. We find that the AR builds large budgets of both free magnetic energy and relative magnetic helicity, sufficient to power many more eruptions than the ones it gave within the interval of interest. For each of these major eruptions, we find eruption-related decreases and subsequent free-energy and helicity budgets that are consistent with the observed eruption (flare and coronal mass ejection (CME)) sizes. In addition, we find that (1) evolution in the AR is consistent with the recently proposed (free) energy-(relative) helicity diagram of solar ARs, (2) eruption-related decreases occur before the flare and the projected CME-launch times, suggesting that CME progenitors precede flares, and (3) self terms of free energy and relative helicity most likely originate from respective mutual terms, following a progressive mutual-to-self conversion pattern that most likely stems from magnetic reconnection. This results in the non-ideal formation of increasingly helical pre-eruption structures and instigates further research on the triggering of solar eruptions with magnetic helicity firmly placed in the eruption cadre. Title: Combining Particle Acceleration and Coronal Heating via Data-constrained Calculations of Nanoflares in Coronal Loops Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.; Anastasiadis, A.; Georgoulis, M. K. Bibcode: 2013ApJ...771..126G Altcode: 2013arXiv1305.5195G We model nanoflare heating of extrapolated active-region coronal loops via the acceleration of electrons and protons in Harris-type current sheets. The kinetic energy of the accelerated particles is estimated using semi-analytical and test-particle-tracing approaches. Vector magnetograms and photospheric Doppler velocity maps of NOAA active region 09114, recorded by the Imaging Vector Magnetograph, were used for this analysis. A current-free field extrapolation of the active-region corona was first constructed. The corresponding Poynting fluxes at the footpoints of 5000 extrapolated coronal loops were then calculated. Assuming that reconnecting current sheets develop along these loops, we utilized previous results to estimate the kinetic energy gain of the accelerated particles. We related this energy to nanoflare heating and macroscopic loop characteristics. Kinetic energies of 0.1-8 keV (for electrons) and 0.3-470 keV (for protons) were found to cause heating rates ranging from 10-6 to 1 erg s-1 cm-3. Hydrodynamic simulations show that such heating rates can sustain plasma in coronal conditions inside the loops and generate plasma thermal distributions that are consistent with active-region observations. We concluded the analysis by computing the form of X-ray spectra generated by the accelerated electrons using the thick-target approach. These spectra were found to be in agreement with observed X-ray spectra, thus supporting the plausibility of our nanoflare-heating scenario. Title: Dynamic data-driven integrated flare model based on self-organized criticality Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. K. Bibcode: 2013A&A...553A..65D Altcode: Context. We interpret solar flares as events originating in active regions that have reached the self-organized critical state. We describe them with a dynamic integrated flare model whose initial conditions and driving mechanism are derived from observations.
Aims: We investigate whether well-known scaling laws observed in the distribution functions of characteristic flare parameters are reproduced after the self-organized critical state has been reached.
Methods: To investigate whether the distribution functions of total energy, peak energy, and event duration follow the expected scaling laws, we first applied the previously reported static cellular automaton model to a time series of seven solar vector magnetograms of the NOAA active region 8210 recorded by the Imaging Vector Magnetograph on May 1 1998 between 18:59 UT and 23:16 UT until the self-organized critical state was reached. We then evolved the magnetic field between these processed snapshots through spline interpolation, mimicking a natural driver in our dynamic model. We identified magnetic discontinuities that exceeded a threshold in the Laplacian of the magnetic field after each interpolation step. These discontinuities were relaxed in local diffusion events, implemented in the form of cellular automaton evolution rules. Subsequent interpolation and relaxation steps covered all transitions until the end of the processed magnetograms' sequence. We additionally advanced each magnetic configuration that has reached the self-organized critical state (SOC configuration) by the static model until 50 more flares were triggered, applied the dynamic model again to the new sequence, and repeated the same process sufficiently often to generate adequate statistics. Physical requirements, such as the divergence-free condition for the magnetic field, were approximately imposed.
Results: We obtain robust power laws in the distribution functions of the modeled flaring events with scaling indices that agree well with observations. Peak and total flare energy obey single power laws with indices -1.65 ± 0.11 and -1.47 ± 0.13, while the flare duration is best fitted with a double power law (-2.15 ± 0.15 and -3.60 ± 0.09 for the flatter and steeper parts, respectively).
Conclusions: We conclude that well-known statistical properties of flares are reproduced after active regions reach the state of self-organized criticality. A significant enhancement of our refined cellular automaton model is that it initiates and further drives the simulation from observed evolving vector magnetograms, thus facilitating energy calculation in physical units, while a separation between MHD and kinetic timescales is possible by assigning distinct MHD timestamps to each interpolation step. Title: The relation between Magnetic Energy and Helicity and their accumulation in Eruptive Solar Active Regions Authors: Tziotziou, K.; Georgoulis, M. K.; Raouafi, N. -E. Bibcode: 2013ASPC..470...59T Altcode: Magnetic free energy and relative magnetic helicity are two important quantities characterizing solar active regions (ARs). Although the importance of free magnetic energy storage for solar eruptions is widely accepted, the role of magnetic helicity, that quantifies the stress and distortion of the magnetic field compared to its lowest (potential) energy state, is still under debate. A new nonlinear force-free method designed to calculate the instantaneous free magnetic energy and relative magnetic helicity budgets of a solar active region from a single vector magnetogram is presented. A sample of 40 vector magnetograms corresponding to different eruptive and non-eruptive ARs is used to calculate their free magnetic energy and relative magnetic helicity budgets, aiming to find a statistically robust correlation between them. The derived correlation implies that magnetic helicity, besides free magnetic energy, is a crucial ingredient for active regions hosting major (M-class and higher) solar eruptions. Eruptive active regions appear well segregated from non-eruptive ones in both free energy and relative helicity with eruptive major flares occurring in ARs with free energy and helicity exceeding 4×1031 erg and 2×1042 Mx2, respectively. Helicity is expelled from ARs mainly in the form of coronal mass ejections (CMEs) and the above helicity threshold agrees well with estimates of typical helicity contents of CMEs. Title: Non-neutralized Electric Current Patterns in Solar Active Regions: Origin of the Shear-generating Lorentz Force Authors: Georgoulis, Manolis K.; Titov, Viacheslav S.; Mikić, Zoran Bibcode: 2012ApJ...761...61G Altcode: 2012arXiv1210.2919G Using solar vector magnetograms of the highest available spatial resolution and signal-to-noise ratio, we perform a detailed study of electric current patterns in two solar active regions (ARs): a flaring/eruptive and a flare-quiet one. We aim to determine whether ARs inject non-neutralized (net) electric currents in the solar atmosphere, responding to a debate initiated nearly two decades ago that remains inconclusive. We find that well-formed, intense magnetic polarity inversion lines (PILs) within ARs are the only photospheric magnetic structures that support significant net current. More intense PILs seem to imply stronger non-neutralized current patterns per polarity. This finding revises previous works that claim frequent injections of intense non-neutralized currents by most ARs appearing in the solar disk but also works that altogether rule out injection of non-neutralized currents. In agreement with previous studies, we also find that magnetically isolated ARs remain globally current-balanced. In addition, we confirm and quantify the preference of a given magnetic polarity to follow a given sense of electric currents, indicating a dominant sense of twist in ARs. This coherence effect is more pronounced in more compact ARs with stronger PILs and must be of sub-photospheric origin. Our results yield a natural explanation of the Lorentz force, invariably generating velocity and magnetic shear along strong PILs, thus setting a physical context for the observed pre-eruption evolution in solar ARs. Title: Magnetic Energy and Helicity Properties of Eruptive Solar Active Regions Authors: Georgoulis, M. K.; Tziotziou, K.; Raouafi, N. Bibcode: 2012AGUFMSH53B..02G Altcode: We outline a new nonlinear force-free method designed to self-consistently calculate the magnetic energy and the relative magnetic helicity budgets of solar active regions using only a single vector magnetogram at the lower atmospheric boundary of these regions. The method is fast and has been successfully validated with well-known magnetic-energy and relative-helicity formulas that, however, are model-dependent and more computationally demanding. Application of the method to a sizable sample of vector magnetograms reveals that eruptive active regions exceed well-defined, physically meaningful thresholds in both their magnetic free-energy and relative magnetic-helicity budgets. Moreover, application to a high-cadence vector-magnetogram timeseries of an eruptive region observed by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory leads to a physical interpretation of the region's dynamical evolution and reveals eruption-related energy and helicity changes. Several intriguing possibilities suggesting promising research avenues emerge from this analysis and are briefly discussed. Title: Magnetic Energy and Helicity Budgets in the Active-region Solar Corona. II. Nonlinear Force-free Approximation Authors: Georgoulis, Manolis K.; Tziotziou, Kostas; Raouafi, Nour-Eddine Bibcode: 2012ApJ...759....1G Altcode: 2012arXiv1209.5606G Expanding on an earlier work that relied on linear force-free (LFF) magnetic fields, we self-consistently derive the instantaneous free magnetic energy and relative magnetic helicity budgets of an unknown three-dimensional nonlinear force-free (NLFF) magnetic structure extending above a single known lower-boundary magnetic field vector. The proposed method does not rely on the detailed knowledge of the three-dimensional field configuration but is general enough to employ only a magnetic connectivity matrix on the lower boundary. The calculation yields a minimum free magnetic energy and a relative magnetic helicity consistent with this free magnetic energy. The method is directly applicable to photospheric or chromospheric vector magnetograms of solar active regions. Upon validation, it basically reproduces magnetic energies and helicities obtained by well known, but computationally more intensive and non-unique, methods relying on the extrapolated three-dimensional magnetic field vector. We apply the method to three active regions, calculating the photospheric connectivity matrices by means of simulated annealing, rather than a model-dependent NLFF extrapolation. For two of these regions we correct for the inherent LFF overestimation in free energy and relative helicity that is larger for larger, more eruptive, active regions. In the third region studied, our calculation can lead to a physical interpretation of observed eruptive manifestations. We conclude that the proposed method, including the proposed inference of the magnetic connectivity matrix, is practical enough to contribute to a physical interpretation of the dynamical evolution of solar active regions. Title: The Magnetic Energy-Helicity Diagram of Solar Active Regions Authors: Tziotziou, Kostas; Georgoulis, Manolis K.; Raouafi, Nour-Eddine Bibcode: 2012ApJ...759L...4T Altcode: 2012arXiv1209.5612T Using a recently proposed nonlinear force-free method designed for single-vector magnetograms of solar active regions, we calculate the instantaneous free magnetic energy and relative magnetic helicity budgets in 162 vector magnetograms corresponding to 42 different active regions. We find a statistically robust, monotonic correlation between the free magnetic energy and the relative magnetic helicity in the studied regions. This correlation implies that magnetic helicity, in addition to free magnetic energy, may be an essential ingredient for major solar eruptions. Eruptive active regions appear well segregated from non-eruptive ones in both free energy and relative helicity with major (at least M-class) flares occurring in active regions with free energy and relative helicity exceeding 4 × 1031 erg and 2 × 1042 Mx2, respectively. The helicity threshold agrees well with estimates of the helicity contents of typical coronal mass ejections. 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: Preface Authors: Nakariakov, V. M.; Georgoulis, M. K.; Poedts, S.; van Driel-Gesztelyi, L.; Mandrini, C. H.; Leibacher, J. Bibcode: 2012SoPh..280..295N Altcode: 2012SoPh..tmp..226N No abstract at ADS Title: Micro-Sigmoids as Progenitors of Polar Coronal Jets Authors: Raouafi, N. -E.; Bernasconi, P. N.; Rust, D. M.; Georgoulis, M. K. Bibcode: 2012ASPC..454..299R Altcode: Observations from the Hinode X-ray telescope (XRT) are used to study the structure of X-ray bright points (XBPs), sources of coronal jets. Several jet events are found to erupt from S-shaped bright points, suggesting that coronal micro-sigmoids are progenitors of the jets. The observations may help to explain numerous characteristics of coronal jets, such as helical structures and shapes. They also suggest that solar activity may be self-similar within a wide range of scales in terms of both properties and evolution of the observed coronal structures. Title: Pre-Eruption Magnetic Configurations in the Low Atmosphere of Solar Active Regions Authors: Georgoulis, Manolis K. Bibcode: 2012cosp...39..604G Altcode: 2012cosp.meet..604G Major solar eruptions, namely flares and coronal mass ejections, rely on significant local accumulations of non-potential (free; stored in electric currents) magnetic energy and, quite likely, magnetic helicity in the solar atmosphere. Without [both of] them, eruptions cannot be powered. Simple tests can show that most free energy and helicity reside close to the lower atmospheric boundary in solar active regions, i.e. their photospheric or low chromospheric interface. Therefore, the pre-eruption configuration in this boundary should reflect these high free-energy and helicity conditions that jointly determine the degree of non-potentiality in active regions. We review the two main active-region photospheric/low-chromospheric configurations leading to major eruptions: instances of intense magnetic flux emergence in the absence of intense magnetic polarity inversion lines (PILs), and instances of strong PILs. In these configurations we discuss multiple measures that can be thought of as proxies of free magnetic energy and helicity and we outline a method to actually calculate these budgets. Combining information from different, but concerted, analyses and approaches, a new picture of eruption initiation emerges. We highlight this new insight and project on its physical plausibility and the advances that it may bring. Title: Automated Detection of Eruptive Structures for Solar Eruption Prediction Authors: Georgoulis, Manolis K. Bibcode: 2012cosp...39..605G Altcode: 2012cosp.meet..605G The problem of data processing and assimilation for solar eruption prediction is, for contemporary solar physics, more pressing than the problem of data acquisition. Although critical solar data, such as the coronal magnetic field, are still not routinely available, space-based observatories deliver diverse, high-quality information at such a high rate that a manual or semi-manual processing becomes meaningless. We discuss automated data analysis methods and explain, using basic physics, why some of them are unlikely to advance eruption prediction. From this finding we also understand why solar eruption prediction is likely to remain inherently probabilistic. We discuss some promising eruption prediction measures and report on efforts to adapt them for use with high-resolution, high-cadence photospheric and coronal data delivered by the Solar Dynamics Observatory. Concluding, we touch on the problem of physical understanding and synthesis of different results: combining different measures inferred by different data sets is a yet-to-be-done exercise that, however, presents our best opportunity of realizing benefits in solar eruption prediction via a meaningful, targeted assimilation of solar data. Title: Plasma Blobs in the Solar Polar Regions: Outflows or Waves? Authors: Raouafi, Nour-Eddine; Bernasconi, P. N.; Georgoulis, M. K. Bibcode: 2012AAS...22020104R Altcode: We analyze EUV images from the Solar Dynamic Observatory (SDO). Anti-sunward propagating blob are found almost everywhere within the solar polar regions with velocities ranging from a few 10 km s-1 to more than 100 km s-1. These structures are either flows or waves. In the former case they may reflect the structure of the nascent fast solar wind. The case is also important for the heating of the coronal plasma. Title: Comment on ``Resolving the 180° Ambiguity in Solar Vector Magnetic Field Data: Evaluating the Effects of Noise, Spatial Resolution, and Method Assumptions'' Authors: Georgoulis, Manolis K. Bibcode: 2012SoPh..276..423G Altcode: 2011arXiv1106.4682G In a recent paper, Leka et al. (Solar Phys. 260, 83, 2009) constructed a synthetic vector magnetogram representing a three-dimensional magnetic structure defined only within a fraction of an arcsec in height. They rebinned the magnetogram to simulate conditions of limited spatial resolution and then compared the results of various azimuth disambiguation methods on the resampled data. Methods relying on the physical calculation of potential and/or non-potential magnetic fields failed in nearly the same, extended parts of the field of view and Leka et al. (Solar Phys. 260, 83, 2009) attributed these failures to the limited spatial resolution. This study shows that the failure of these methods is not due to the limited spatial resolution but due to the narrowly defined test data. Such narrow magnetic structures are not realistic in the real Sun. Physics-based disambiguation methods, adapted for solar magnetic fields extending to infinity, are not designed to handle such data; hence, they could only fail this test. I demonstrate how an appropriate limited-resolution disambiguation test can be performed by constructing a synthetic vector magnetogram very similar to that of Leka et al. (Solar Phys. 260, 83, 2009) but representing a structure defined in the semi-infinite space above the solar photosphere. For this magnetogram I find that even a simple potential-field disambiguation method manages to resolve the ambiguity very successfully, regardless of limited spatial resolution. Therefore, despite the conclusions of Leka et al. (Solar Phys. 260, 83, 2009), a proper limited-spatial-resolution test of azimuth disambiguation methods is yet to be performed in order to identify the best ideas and algorithms. Title: Are Solar Active Regions with Major Flares More Fractal, Multifractal, or Turbulent Than Others? Authors: Georgoulis, Manolis K. Bibcode: 2012SoPh..276..161G Altcode: 2011SoPh..tmp...15G; 2011SoPh..tmp..411G; 2011SoPh..tmp..146G; 2011SoPh..tmp..215G; 2011arXiv1101.0547G Multiple recent investigations of solar magnetic-field measurements have raised claims that the scale-free (fractal) or multiscale (multifractal) parameters inferred from the studied magnetograms may help assess the eruptive potential of solar active regions, or may even help predict major flaring activity stemming from these regions. We investigate these claims here, by testing three widely used scale-free and multiscale parameters, namely, the fractal dimension, the multifractal structure function and its inertial-range exponent, and the turbulent power spectrum and its power-law index, on a comprehensive data set of 370 timeseries of active-region magnetograms (17 733 magnetograms in total) observed by SOHO's Michelson Doppler Imager (MDI) over the entire Solar Cycle 23. We find that both flaring and non-flaring active regions exhibit significant fractality, multifractality, and non-Kolmogorov turbulence but none of the three tested parameters manages to distinguish active regions with major flares from flare-quiet ones. We also find that the multiscale parameters, but not the scale-free fractal dimension, depend sensitively on the spatial resolution and perhaps the observational characteristics of the studied magnetograms. Extending previous works, we attribute the flare-forecasting inability of fractal and multifractal parameters to i) a widespread multiscale complexity caused by a possible underlying self-organization in turbulent solar magnetic structures, flaring and non-flaring alike, and ii) a lack of correlation between the fractal properties of the photosphere and overlying layers, where solar eruptions occur. However useful for understanding solar magnetism, therefore, scale-free and multiscale measures may not be optimal tools for active-region characterization in terms of eruptive ability or, ultimately, for major solar-flare prediction. Title: Nanoflare heating of coronal loops in an active region triggered by reconnecting current sheets Authors: Gontikakis, C.; Patsourakos, S.; Efthymiopoulos, C.; Anastasiadis, A.; Georgoulis, M. Bibcode: 2012hell.conf....7G Altcode: The purpose of this work is to study the heating of coronal loops, produced by the acceleration of particles inside reconnecting current sheets (RCS) which represent nanoflares. We also study the hydrodynamic response of the loops atmosphere to such a heating event. The RCS are formed as discontinuities of the loop magnetic field caused by the photospheric shuffling motions. The coronal loops are represented by the closed magnetic lines of force calculated by the magnetic field extrapolation of the active region NOAA 9114 magnetogram. The photospheric motions at the loops footpoints are measured using local correlation tracking. The magnetic and electric fields accelerating particles at the RCS are computed using the loop magnetic fields and the photospheric motions. We further discuss the question of energy conservation inside the current sheet, and we present the statistical distributions of quantities relevant for particles acceleration and coronal heating for a number of the active region's coronal loops. Title: Computer Vision for the Solar Dynamics Observatory (SDO) Authors: Martens, P. C. H.; Attrill, G. D. R.; Davey, A. R.; Engell, A.; Farid, S.; Grigis, P. C.; Kasper, J.; Korreck, K.; Saar, S. H.; Savcheva, A.; Su, Y.; Testa, P.; Wills-Davey, M.; Bernasconi, P. N.; Raouafi, N. -E.; Delouille, V. A.; Hochedez, J. F.; Cirtain, J. W.; DeForest, C. E.; Angryk, R. A.; De Moortel, I.; Wiegelmann, T.; Georgoulis, M. K.; McAteer, R. T. J.; Timmons, R. P. Bibcode: 2012SoPh..275...79M Altcode: 2011SoPh..tmp..144M; 2011SoPh..tmp..213M; 2011SoPh..tmp....8M In Fall 2008 NASA selected a large international consortium to produce a comprehensive automated feature-recognition system for the Solar Dynamics Observatory (SDO). The SDO data that we consider are all of the Atmospheric Imaging Assembly (AIA) images plus surface magnetic-field images from the Helioseismic and Magnetic Imager (HMI). We produce robust, very efficient, professionally coded software modules that can keep up with the SDO data stream and detect, trace, and analyze numerous phenomena, including flares, sigmoids, filaments, coronal dimmings, polarity inversion lines, sunspots, X-ray bright points, active regions, coronal holes, EIT waves, coronal mass ejections (CMEs), coronal oscillations, and jets. We also track the emergence and evolution of magnetic elements down to the smallest detectable features and will provide at least four full-disk, nonlinear, force-free magnetic field extrapolations per day. The detection of CMEs and filaments is accomplished with Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) and ground-based Hα data, respectively. A completely new software element is a trainable feature-detection module based on a generalized image-classification algorithm. Such a trainable module can be used to find features that have not yet been discovered (as, for example, sigmoids were in the pre-Yohkoh era). Our codes will produce entries in the Heliophysics Events Knowledgebase (HEK) as well as produce complete catalogs for results that are too numerous for inclusion in the HEK, such as the X-ray bright-point metadata. This will permit users to locate data on individual events as well as carry out statistical studies on large numbers of events, using the interface provided by the Virtual Solar Observatory. The operations concept for our computer vision system is that the data will be analyzed in near real time as soon as they arrive at the SDO Joint Science Operations Center and have undergone basic processing. This will allow the system to produce timely space-weather alerts and to guide the selection and production of quicklook images and movies, in addition to its prime mission of enabling solar science. We briefly describe the complex and unique data-processing pipeline, consisting of the hardware and control software required to handle the SDO data stream and accommodate the computer-vision modules, which has been set up at the Lockheed-Martin Space Astrophysics Laboratory (LMSAL), with an identical copy at the Smithsonian Astrophysical Observatory (SAO). 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: Monitoring solar energetic particles with an armada of European spacecraft and the new automated SEPF (Solar Energetic Proton Fluxes) Tool Authors: Sandberg, I.; Daglis, I. A.; Anastasiadis, A.; Balasis, G.; Georgoulis, M.; Nieminen, P.; Evans, H.; Daly, E. Bibcode: 2012hell.conf....8S Altcode: Solar energetic particles (SEPs) observed in interplanetary medium consist of electrons, protons, alpha particles and heavier ions (up to Fe), with energies from dozens of keVs to a few GeVs. SEP events, or SEPEs, are particle flux enhancements from background level (< 1 pfu, particle flux unit = particle cm-2sr-1s-1) to several orders of magnitude in the MeV range, and lasting from several hours to a few days. Intense SEPEs can reach fluence values as high as 1010 protons cm-2 for E > 30 MeV. The main part of SEPEs results from the acceleration of particles either by solar flares and/or by interplanetary shocks driven by Coronal Mass Ejections (CMEs); these accelerated particles propagate through the heliosphere, traveling along the interplanetary magnetic field (IMF). SEPEs show significant variability from one event to another and are an important part of space weather, because they pose a serious health risk to humans in space and a serious radiation hazard for the spacecraft hardware which may lead to severe damages. As a consequence, engineering models, observations and theoretical investigations related to the high energy particle environment is a priority issue for both robotic and manned space missions. The European Space Agency operates the Standard Radiation Environment Monitor (SREM) on-board six spacecraft: Proba-1, INTEGRAL, Rosetta, Giove-B, Herschel and Planck, which measures high-energy protons and electrons with a fair angular and spectral resolution. The fact that several SREM units operate in different orbits provides a unique chance for comparative studies of the radiation environment based on multiple data gathered by identical detectors. Furthermore, the radiation environment monitoring by the SREM unit onboard Rosetta may reveal unknown characteristics of SEPEs properties given the fact that the majority of the available radiation data and models only refer to 1AU solar distances. The Institute for Space Applications and Remote Sensing of the National Observatory of Athens (ISARS/NOA) has developed and validated a novel method to obtain flux spectra from SREM count rates. Using this method and by conducting detailed scientific studies we have showed in previous presentations and papers that the exploration and analysis of SREM data may contribute significantly to investigations and modeling efforts of SPE generation and propagation in the heliosphere and in the Earth's magnetosphere. ISARS/NOA recently released an automated software tool for the monitoring of Solar Energetic Proton Fluxes (SEPF) using measurements of SREM. The SEPF tool is based on the automated implementation of the inverse method developed by ISARS/NOA, permitting the calculation of high-energy proton fluxes from SREM data. Results of the method have been validated for selected number of past solar energetic particle events using measurements from other space-born proton monitors. The SEPF tool unfolds downlinked SREM count-rates, calculates the omnidirectional differential proton fluxes and provides results to the space weather community acting as a multi-point proton flux monitor on a daily-basis. The SEPF tool is a significant European space weather asset and will support the efforts towards an efficient European Space Situational Awareness programme. Title: Solar Energetic Particle Events detected by the Standard Radiation Environment Monitor (SREM) onboard INTEGRAL Authors: Georgoulis, M.; Daglis, I. A.; Anastasiadis, A.; Sandberg, I.; Balasis, G.; Nieminen, P. Bibcode: 2012hell.conf...10G Altcode: The SREM is a cost-effective instrument mounted onboard multiple ESA missions. The SREM objective is the in-situ measurement of high-energy solar particles at the spacecraft location. Within the previous solar cycle 23, SREM units onboard ESA's INTEGRAL and Rosetta missions detected several tens of SEPEs and accurately pinpointed their onset, rise, and decay times. We have undertaken a detailed study to determine the solar sources and subsequent interplanetary coronal mass ejections (ICMEs) that gave rise to these events, as well as the timing of SEPEs with the onset of possible geomagnetic activity triggered by these ICMEs. We find that virtually all SREM SEPEs may be associated with CME-driven shocks. For a number of well-studied INTEGRAL/SREM SEPEs, moreover, we see an association between the SEPE peak and the shock passage at L1. Shortly (typically within a few hours) after the SEPE peak, the ICME-driven modulation of the magnetosphere kicks in, with either an increase or a dip of the Dst index, indicating stormy conditions in geospace. We conclude that, pending additional investigation, SREM units may prove useful for a short-term prediction of inclement space-weather conditions in Geospace, especially if mounted onboard dayside missions ahead of the magnetospheric bow shock. Title: On Our Ability to Predict Major Solar Flares Authors: Georgoulis, Manolis K. Bibcode: 2012ASSP...30...93G Altcode: 2012snc..book...93G We discuss the outstanding problem of solar flare prediction and briefly overview the various methods that have been developed to address it. A class of these methods, relying on the fractal and multifractal nature of solar magnetic fields, are shown to be inadequate for flare prediction. More promise seems delivered by morphological methods applying mostly to the photospheric magnetic configuration of solar active regions but a definitive assessment of their veracity is subject to a number of caveats. Statistical and artificial-intelligence methods are also briefly discussed, together with their possible shortcomings. The central importance of proper validation procedures for any viable method is also highlighted, together with the need for future studies that will finally judge whether practically meaningful flare prediction will ever become possible, if only purely probabilistic. Title: Pre-Eruption Magnetic Configurations in the Active-Region Solar Photosphere Authors: Georgoulis, Manolis K. Bibcode: 2011IAUS..273..495G Altcode: Most solar eruptions occur above strong photospheric magnetic polarity inversion lines (PILs). What overlays a PIL is unknown, however, and this has led to a debate over the existence of sheared magnetic arcades vs. helical magnetic flux ropes. We argue that this debate may be of little meaning: numerous small-scale magnetic reconnections, constantly triggered in the PIL area, can lead to effective transformation of mutual to self magnetic helicity (i.e. twist and writhe) that, ultimately, may force the magnetic structure above PILs to erupt to be relieved from its excess helicity. This is preliminary report of work currently in progress. Title: Simulating flaring events in complex active regions driven by observed magnetograms Authors: Dimitropoulou, M.; Isliker, H.; Vlahos, L.; Georgoulis, M. K. Bibcode: 2011A&A...529A.101D Altcode: 2011arXiv1102.2352D Context. We interpret solar flares as events originating in active regions that have reached the self organized critical state, by using a refined cellular automaton model with initial conditions derived from observations.
Aims: We investigate whether the system, with its imposed physical elements, reaches a self organized critical state and whether well-known statistical properties of flares, such as scaling laws observed in the distribution functions of characteristic parameters, are reproduced after this state has been reached.
Methods: To investigate whether the distribution functions of total energy, peak energy and event duration follow the expected scaling laws, we first applied a nonlinear force-free extrapolation that reconstructs the three-dimensional magnetic fields from two-dimensional vector magnetograms. We then locate magnetic discontinuities exceeding a threshold in the Laplacian of the magnetic field. These discontinuities are relaxed in local diffusion events, implemented in the form of cellular automaton evolution rules. Subsequent loading and relaxation steps lead the system to self organized criticality, after which the statistical properties of the simulated events are examined. Physical requirements, such as the divergence-free condition for the magnetic field vector, are approximately imposed on all elements of the model.
Results: Our results show that self organized criticality is indeed reached when applying specific loading and relaxation rules. Power-law indices obtained from the distribution functions of the modeled flaring events are in good agreement with observations. Single power laws (peak and total flare energy) are obtained, as are power laws with exponential cutoff and double power laws (flare duration). The results are also compared with observational X-ray data from the GOES satellite for our active-region sample.
Conclusions: We conclude that well-known statistical properties of flares are reproduced after the system has reached self organized criticality. A significant enhancement of our refined cellular automaton model is that it commences the simulation from observed vector magnetograms, thus facilitating energy calculation in physical units. The model described in this study remains consistent with fundamental physical requirements, and imposes physically meaningful driving and redistribution rules. Title: Nonlinear Force-Free Reconstruction of the Global Solar Magnetic Field: Methodology Authors: Contopoulos, I.; Kalapotharakos, C.; Georgoulis, M. K. Bibcode: 2011SoPh..269..351C Altcode: 2010arXiv1011.5356C; 2011SoPh..tmp...23C We present a novel numerical method that allows the calculation of nonlinear force-free magnetostatic solutions above a boundary surface on which only the distribution of the normal magnetic field component is given. The method relies on the theory of force-free electrodynamics and applies directly to the reconstruction of the solar coronal magnetic field for a given distribution of the photospheric radial field component. The method works as follows: we start with any initial magnetostatic global field configuration (e.g. zero, dipole), and along the boundary surface we create an evolving distribution of tangential (horizontal) electric fields that, via Faraday's equation, give rise to a respective normal-field distribution approaching asymptotically the target distribution. At the same time, these electric fields are used as boundary condition to numerically evolve the resulting electromagnetic field above the boundary surface, modeled as a thin ideal plasma with non-reflecting, perfectly absorbing outer boundaries. The simulation relaxes to a nonlinear force-free configuration that satisfies the given normal-field distribution on the boundary. This is different from existing methods relying on a fixed boundary condition - the boundary evolves toward the a priori given one, at the same time evolving the three-dimensional field solution above it. Moreover, this is the first time that a nonlinear force-free solution is reached by using only the normal field component on the boundary. This solution is not unique, but it depends on the initial magnetic field configuration and on the evolutionary course along the boundary surface. To our knowledge, this is the first time that the formalism of force-free electrodynamics, used very successfully in other astrophysical contexts, is applied to the global solar magnetic field. Title: Three-dimensional Structure of a Solar Active Region from Spatially and Spectrally Resolved Microwave Observations Authors: Tun, Samuel D.; Gary, Dale E.; Georgoulis, Manolis K. Bibcode: 2011ApJ...728....1T Altcode: We report on the structure of the solar atmosphere above active region (AR) 10923, observed on 2006 November 10, as deduced from multi-wavelength studies including combined microwave observations from the Very Large Array (VLA) and the Owens Valley Solar Array (OVSA). The VLA observations provide excellent image quality at a few widely spaced frequencies, while the OVSA data provide information at many intermediate frequencies to fill in the spectral coverage. Images at 25 distinct frequencies are used to provide spatially resolved spectra along many lines of sight in the AR, from which microwave spectral diagnostics are obtained for deducing maps of temperature, magnetic field, and column density. The derived quantities are compared with multiwavelength observations from the Solar and Heliospheric Observatory and Hinode spacecraft, and with a current-free magnetic field extrapolation. We find that a two-component temperature model is required to fit the data, in which a hot (>2 MK) lower corona above the strong-field plage and sunspot regions (emitting via the gyroresonance process) is overlaid with somewhat cooler (~1 MK) coronal loops that partially absorb the gyroresonance emission through the free-free (Bremsstrahlung) process. We also find that the extrapolated potential magnetic fields can quantitatively account for the observed gyroresonance emission over most of the AR, but in a few areas a higher field strength is required. The results are used to explore the coronal configuration needed to explain the observations. These results show that the bulk of free-free emission in both radio and X-rays emanates from two loop systems, distinguished by the location of their loop footpoints. We discuss the implications of such comparisons for studies of AR structure when better microwave spectral imaging becomes available in the future. Title: Oscillations in a network region observed in the Hα line and their relation to the magnetic field Authors: Kontogiannis, I.; Tsiropoula, G.; Tziotziou, K.; Georgoulis, M. K. Bibcode: 2010A&A...524A..12K Altcode:
Aims: Our aim is to gain a better understanding of the interaction between acoustic oscillations and the small-scale magnetic fields of the Sun. To this end, we examine the oscillatory properties of a network region and their relation to the magnetic configuration of the chromosphere. We link the oscillatory properties of a network region and their spatial variation with the variation of the parameters of the magnetic field. We investigate the effect of the magnetic canopy and the diverging flux tubes of the chromospheric network on the distribution of oscillatory power over the network and internetwork.
Methods: We use a time series of high resolution filtergrams at five wavelengths along the Hα profile observed with the Dutch Open Telescope, as well as high resolution magnetograms taken by the SOT/SP onboard HINODE. Using wavelet analysis, we construct power maps of the 3, 5 and 7 min oscillations of the Doppler signals calculated at ±0.35 Å and ±0.7 Å from the Hα line center. These represent velocities at chromospheric and photospheric levels respectively. Through a current-free (potential) field extrapolation we calculate the chromospheric magnetic field and compare its morphology with the Hα filtergrams. We calculate the plasma β and the magnetic field inclination angle and compare their distribution with the oscillatory power at the 3, 5 and 7 min period bands.
Results: Chromospheric mottles seem to outline the magnetic field lines. The Hα ± 0.35 Å Doppler signals are formed above the canopy, while the Hα ± 0.7 Å corresponding ones below it. The 3 min power is suppressed at the chromosphere around the network, where the canopy height is lower than 1600 km, while at the photosphere it is enhanced due to reflection. 3, 5 and 7 min oscillatory power is increased around the network at the photosphere due to reflection of waves on the overlying canopy, while increased 5 and 7 min power at the chromosphere is attributed mainly to wave refraction on the canopy. At these high periods, power is also increased due to p-mode leakage because of the high inclinations of the magnetic field.
Conclusions: Our high resolution Hα observations and photospheric magnetograms provide the opportunity to highlight the details of the interaction between acoustic oscillations and the magnetic field of a network region. We conclude that several mechanisms that have been proposed such as p-mode leakage, mode conversion, reflection and refraction of waves on the magnetic canopy may act together and result to the observed properties of network oscillations. Title: Micro-Sigmoids as Progenitors of Polar Coronal Jets Authors: Raouafi, N. -E.; Bernasconi, P. N.; Rust, D. M.; Georgoulis, M. K. Bibcode: 2010arXiv1009.2951R Altcode: Observations from the Hinode X-ray telescope (XRT) are used to study the structure of X-ray bright points (XBPs), sources of coronal jets. Several jet events are found to erupt from S-shaped bright points, suggesting that coronal micro-sigmoids are progenitors of the jets. The observations may help to explain numerous characteristics of coronal jets, such as helical structures and shapes. They also suggest that solar activity may be self-similar within a wide range of scales in terms of both properties and evolution of the observed coronal structures. Title: Micro-sigmoids as Progenitors of Coronal Jets: Is Eruptive Activity Self-similarly Multi-scaled? Authors: Raouafi, N. -E.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2010ApJ...718..981R Altcode: 2010arXiv1005.4042R Observations from the X-ray telescope (XRT) on Hinode are used to study the nature of X-ray-bright points, sources of coronal jets. Several jet events in the coronal holes are found to erupt from small-scale, S-shaped bright regions. This finding suggests that coronal micro-sigmoids may well be progenitors of coronal jets. Moreover, the presence of these structures may explain numerous observed characteristics of jets such as helical structures, apparent transverse motions, and shapes. Analogous to large-scale sigmoids giving rise to coronal mass ejections (CMEs), a promising future task would perhaps be to investigate whether solar eruptive activity, from coronal jets to CMEs, is self-similar in terms of properties and instability mechanisms. Title: Heating Distribution Along Coronal Loops in two Active Regions Using a Simple Electrodynamic Calculation Authors: Gontikakis, C.; Georgoulis, M.; Contopoulos, I.; Dara, H. C. Bibcode: 2010ASPC..424...25G Altcode: The heating along hundreds of coronal loops of non flaring active regions is computed using a simple electrodynamic model. Photospheric motions generate electric fields inducing, electric potential differences at the footpoints of loops. These potential differences generate electric currents that lead to Ohmic heating. We computed the magnetic field extrapolation from the magnetograms of two active regions, namely NOAA AR 9366 (SOHO/MDI) and NOAA AR 10963, (HINODE/SOT). Closed magnetic field lines model the coronal loops. For each loop we computed the heating function and obtained the hydrostatic distribution of temperature and pressure. We found that the coronal heating is stronger near the footpoints of the loops and asymmetric along them. We obtained scaling laws that correlate the mean volumetric heating with the loop length, and the heating flux, through the loop footpoints with the magnetic field strength at the footpoints. Our results are in qualitative agreement with observations (see Gontikakis et al. 2008 for more details). Title: Simulating Flaring Events via an Intelligent Cellular Automata Mechanism Authors: Dimitropoulou, M.; Vlahos, L.; Isliker, H.; Georgoulis, M. Bibcode: 2010ASPC..424...28D Altcode: We simulate flaring events through a Cellular Automaton (CA) model, in which, for the first time, we use observed vector magnetograms as initial conditions. After non-linear force free extrapolation of the magnetic field from the vector magnetograms, we identify magnetic discontinuities, using two alternative criteria: (1) the average magnetic field gradient, or (2) the normalized magnetic field curl (i.e. the current). Magnetic discontinuities are identified at the grid-sites where the magnetic field gradient or curl exceeds a specified threshold. We then relax the magnetic discontinuities according to the rules of Lu and Hamilton (1991) or Lu et al. (1993), i.e. we redistribute the magnetic field locally so that the discontinuities disappear. In order to simulate the flaring events, we consider several alternative scenarios with regard to: (1) The threshold above which magnetic discontinuities are identified (applying low, high, and height-dependent threshold values); (2) The driving process that occasionally causes new discontinuities (at randomly chosen grid sites, magnetic field increments are added that are perpendicular (or may-be also parallel) to the existing magnetic field). We address the question whether the coronal active region magnetic fields can indeed be considered to be in the state of self-organized criticality (SOC). Title: The "Sigmoid Sniffer” and the "Advanced Automated Solar Filament Detection and Characterization Code” Modules Authors: Raouafi, Noureddine; Bernasconi, P. N.; Georgoulis, M. K. Bibcode: 2010AAS...21640232R Altcode: We present two pattern recognition algorithms, the "Sigmoid Sniffer” and the "Advanced Automated Solar Filament Detection and Characterization Code,” that are among the Feature Finding modules of the Solar Dynamic Observatory:

1) Coronal sigmoids visible in X-rays and the EUV are the result of highly twisted magnetic fields. They can occur anywhere on the solar disk and are closely related to solar eruptive activity (e.g., flares, CMEs). Their appearance is typically synonym of imminent solar eruptions, so they can serve as a tool to forecast solar activity. Automatic X-ray sigmoid identification offers an unbiased way of detecting short-to-mid term CME precursors. The "Sigmoid Sniffer” module is capable of automatically detecting sigmoids in full-disk X-ray images and determining their chirality, as well as other characteristics. It uses multiple thresholds to identify persistent bright structures on a full-disk X-ray image of the Sun. We plan to apply the code to X-ray images from Hinode/XRT, as well as on SDO/AIA images. When implemented in a near real-time environment, the Sigmoid Sniffer could allow 3-7 day forecasts of CMEs and their potential to cause major geomagnetic storms.

2)The "Advanced Automated Solar Filament Detection and Characterization Code” aims to identify, classify, and track solar filaments in full-disk Hα images. The code can reliably identify filaments; determine their chirality and other relevant parameters like filament area, length, and average orientation with respect to the equator. It is also capable of tracking the day-by-day evolution of filaments as they traverse the visible disk. The code was tested by analyzing daily Hα images taken at the Big Bear Solar Observatory from mid-2000 to early-2005. It identified and established the chirality of thousands of filaments without human intervention. Title: Computer Vision for SDO: First Results from the SDO Feature Finding Algorithms Authors: Martens, Petrus C.; Attrill, G.; Davey, A.; Engell, A.; Farid, S.; Grigis, P.; Kasper, J.; Korreck, K.; Saar, S.; Su, Y.; Testa, P.; Wills-Davey, M.; Bernasconi, P.; Raouafi, N.; Georgoulis, M.; Deforest, C.; Peterson, J.; Berghoff, T.; Delouille, V.; Hochedez, J.; Mampaey, B.; Verbeek, C.; Cirtain, J.; Green, S.; Timmons, R.; Savcheva, A.; Angryk, R.; Wiegelmann, T.; McAteer, R. Bibcode: 2010AAS...21630804M Altcode: The SDO Feature Finding Team produces robust and very efficient software modules that can keep up with the relentless SDO data stream, and detect, trace, and analyze a large number of phenomena including: flares, sigmoids, filaments, coronal dimmings, polarity inversion lines, sunspots, X-ray bright points, active regions, coronal holes, EIT waves, CME's, coronal oscillations, and jets. In addition we track the emergence and evolution of magnetic elements down to the smallest features that are detectable, and we will also provide at least four full disk nonlinear force-free magnetic field extrapolations per day.

During SDO commissioning we will install in the near-real time data pipeline the modules that provide alerts for flares, coronal dimmings, and emerging flux, as well as those that trace filaments, sigmoids, polarity inversion lines, and active regions. We will demonstrate the performance of these modules and illustrate their use for science investigations. Title: Solar Magnetic Helicity Injected into the Heliosphere: Magnitude, Balance, and Periodicities Over Solar Cycle 23 Authors: Georgoulis, M. K.; Rust, D. M.; Pevtsov, A. A.; Bernasconi, P. N.; Kuzanyan, K. M. Bibcode: 2009ApJ...705L..48G Altcode: Relying purely on solar photospheric magnetic field measurements that cover most of solar cycle 23 (1996-2005), we calculate the total relative magnetic helicity injected into the solar atmosphere, and eventually shed into the heliosphere, over the latest cycle. Large active regions dominate the helicity injection process with ~5.7 × 1045 Mx2 of total injected helicity. The net helicity injected is lsim1% of the above output. Peculiar active-region plasma flows account for ~80% of this helicity; the remaining ~20% is due to solar differential rotation. The typical helicity per active-region CME ranges between (1.8-7) × 1042 Mx2 depending on the CME velocity. Accounting for various minor underestimation factors, we estimate a maximum helicity injection of ~6.6 × 1045 Mx2 for solar cycle 23. Although no significant net helicity exists over both solar hemispheres, we recover the well-known hemispheric helicity preference, which is significantly enhanced by the solar differential rotation. We also find that helicity injection in the solar atmosphere is an inherently disorganized, impulsive, and aperiodic process. Title: The correlation of fractal structures in the photospheric and the coronal magnetic field Authors: Dimitropoulou, M.; Georgoulis, M.; Isliker, H.; Vlahos, L.; Anastasiadis, A.; Strintzi, D.; Moussas, X. Bibcode: 2009A&A...505.1245D Altcode: 2009arXiv0908.3950D Context: This work examines the relation between the fractal properties of the photospheric magnetic patterns and those of the coronal magnetic fields in solar active regions.
Aims: We investigate whether there is any correlation between the fractal dimensions of the photospheric structures and the magnetic discontinuities formed in the corona.
Methods: To investigate the connection between the photospheric and coronal complexity, we used a nonlinear force-free extrapolation method that reconstructs the 3d magnetic fields using 2d observed vector magnetograms as boundary conditions. We then located the magnetic discontinuities, which are considered as spatial proxies of reconnection-related instabilities. These discontinuities form well-defined volumes, called here unstable volumes. We calculated the fractal dimensions of these unstable volumes and compared them to the fractal dimensions of the boundary vector magnetograms.
Results: Our results show no correlation between the fractal dimensions of the observed 2d photospheric structures and the extrapolated unstable volumes in the corona, when nonlinear force-free extrapolation is used. This result is independent of efforts to (1) bring the photospheric magnetic fields closer to a nonlinear force-free equilibrium and (2) omit the lower part of the modeled magnetic field volume that is almost completely filled by unstable volumes. A significant correlation between the fractal dimensions of the photospheric and coronal magnetic features is only observed at the zero level (lower limit) of approximation of a current-free (potential) magnetic field extrapolation.
Conclusions: We conclude that the complicated transition from photospheric non-force-free fields to coronal force-free ones hampers any direct correlation between the fractal dimensions of the 2d photospheric patterns and their 3d counterparts in the corona at the nonlinear force-free limit, which can be considered as a second level of approximation in this study. Correspondingly, in the zero and first levels of approximation, namely, the potential and linear force-free extrapolation, respectively, we reveal a significant correlation between the fractal dimensions of the photospheric and coronal structures, which can be attributed to the lack of electric currents or to their purely field-aligned orientation. Title: SOLIS Vector Spectromagnetograph: Status and Science Authors: Henney, C. J.; Keller, C. U.; Harvey, J. W.; Georgoulis, M. K.; Hadder, N. L.; Norton, A. A.; Raouafi, N. -E.; Toussaint, R. M. Bibcode: 2009ASPC..405...47H Altcode: 2008arXiv0801.0013H The Vector Spectromagnetograph (VSM) instrument has recorded photospheric and chromospheric magnetograms daily since August 2003. Full-disk photospheric vector magnetograms are observed at least weekly and, since November 2006, area-scans of active regions daily. Quick-look vector magnetic images, plus X3D and FITS formated files, are now publicly available daily. In the near future, Milne-Eddington inversion parameter data will also be available and a typical observing day will include three full-disk photospheric vector magnetograms. Besides full-disk observations, the VSM is capable of high temporal cadence area-scans of both the photosphere and chromosphere. Carrington rotation and daily synoptic maps are also available from the photospheric magnetograms and coronal hole estimate images. Title: Computer Vision for The Solar Dynamics Observatory Authors: Martens, Petrus C.; Angryk, R. A.; Bernasconi, P. N.; Cirtain, J. W.; Davey, A. R.; DeForest, C. E.; Delouille, V. A.; De Moortel, I.; Georgoulis, M. K.; Grigis, P. C.; Hochedez, J. E.; Kasper, J.; Korreck, K. E.; Reeves, K. K.; Saar, S. H.; Savcheva, A.; Su, Y.; Testa, P.; Wiegelmann, T.; Wills-Davey, M. Bibcode: 2009SPD....40.1711M Altcode: NASA funded a large international consortium last year to produce a comprehensive system for automated feature recognition in SDO images. The data we consider are all AIA and EVE data plus surface magnetic field images from HMI. Helioseismology is addressed by another group.

We will produce robust and very efficient software modules that can keep up with the relentless SDO data stream and detect, trace, and analyze a large number of phenomena, including: flares, sigmoids, filaments, coronal dimmings, polarity inversion lines, sunspots, X-ray bright points, active regions, coronal holes, EIT waves, CME's, coronal oscillations, and jets. In addition we will track the emergence and evolution of magnetic elements down to the smallest features that are detectable, and we will also provide at least four full disk nonlinear force-free magnetic field extrapolations per day.

A completely new software element that rounds out this suite is a trainable feature detection module, which employs a generalized image classification algorithm to produce the texture features of the images analyzed. A user can introduce a number of examples of the phenomenon looked and the software will return images with similar features. We have tested a proto-type on TRACE data, and were able to "train" the algorithm to detect sunspots, active regions, and loops. Such a module can be used to find features that have not even been discovered yet, as, for example, sigmoids were in the pre-Yohkoh era.

Our codes will produce entries in the Helio Events Knowledge base, and that will permit users to locate data on individual events as well as carry out statistical studies on large numbers of events, using the interface provided by the Virtual Solar Observatory. Title: Just how much Helicity did the Sun Shed in Solar Cycle 23? Magnitude, Balance, Periodicities, and Further Implications Authors: Georgoulis, Manolis K.; Rust, D. M.; Pevtsov, A. A.; Bernasconi, P. N.; Kuzanyan, K. M. Bibcode: 2009SPD....40.0606G Altcode: Using solar magnetic field measurements, we calculate the total relative magnetic helicity injected in the solar atmosphere and eventually

transported to the heliosphere in the course of the latest solar cycle. We report on (i) the magnitude of the heliospheric helicity over cycle 23, (ii) the net helicity and its significance, and (iii) the possible

periodicities of helicity injection in the solar atmosphere. Our simple calculations raise several questions regarding the fundamental nature of solar magnetism. The lack of significant net helicity may place the solar dynamo in the category of

astrophysical dynamos without a net helicity effect over an average time scale. The strong enhancement of the hemispheric helicity preference by solar differential rotation - although the latter has a much weaker effect than intrinsic active-region plasma flows - warrants further investigation. Finally, the absence of any credible periodicity of helicity injection, in spite of numerous reported periodicities in solar activity, perhaps prompts the re-evaluation of the notion that the Sun works through a sequence of internal cycles: active-region emergence and evolution appears as an inherently disorganized, aperiodic process. Title: On the Helical Fields Guiding Near-Relativistic Electron Beams in the Heliosphere Authors: Rust, David M.; Haggerty, D. K.; Georgoulis, M. K.; Stenborg, G. Bibcode: 2009SPD....40.3202R Altcode: Wavelet processing of the LASCO images of the solar corona brings out many subtle details that are easily missed in the intensity images. Specifically, wavelet processing can enhance the edges on large and small scales making it easier to detect and define helical features. We used the processed LASCO images obtained during the period 1997 -2001 to study the structure and motions of nearly radial streamers extending from coronal holes adjacent to flaring active regions. Some of the streamers show outward-propagating twist. These helical fields extend into the heliosphere where they would reach 1 AU with a path length generally greater than the 1.2 AU of idealized fields following the Parker spiral. We focused on the regions from our earlier work (Rust et al., ApJ 687, 635, 2008) on flares associated with beams of near-relativistic electrons detected at 1 AU with the ACE spacecraft. Our study shows that the electron beam's typical delay of about 10 min in arriving at 1 AU may be due to their following a helical path from Sun to Earth. According to the reconnection jet model, the helical component may be introduced to open fields by earlier events involving reconnections with emerging, twisted flux ropes. Our study implies that the escaping electrons may be accelerated at the same time as the trapped electrons that produce X-ray flare emissions.

NASA supported this work with grant NNG 05GM69G. Title: On the Solar Origins of Open Magnetic Fields in the Heliosphere Authors: Rust, David M.; Haggerty, Dennis K.; Georgoulis, Manolis K.; Sheeley, Neil R.; Wang, Yi-Ming; DeRosa, Marc L.; Schrijver, Carolus J. Bibcode: 2008ApJ...687..635R Altcode: A combination of heliospheric and solar data was used to identify open magnetic fields stretching from the lower corona to Earth orbit. 35 near-relativistic electron beams detected at the ACE spacecraft "labeled" the heliospheric segments of the open fields. An X-ray flare occurred <20 minutes before injection of the electrons in 25 events. These flares labeled the solar segment of the open fields. The flares occurred in western-hemisphere active regions (ARs) with coronal holes whose polarity agreed with the polarity of the beam-carrying interplanetary fields in 23 of the 25 events. We conclude that electron beams reach 1 AU from open AR fields adjacent to flare sites. The Wang & Sheeley implementation of the potential-field source-surface model successfully identified the open fields in 36% of cases. Success meant that the open fields reached the source surface within 3 heliographic deg of the interplanetary magnetic field connected to ACE at 1 AU. Inclusion of five near misses improves the success rate to 56%. The success rate for the Schrijver & DeRosa PFSS implementation was 50%. Our results suggest that, even if the input magnetic data are updated frequently, the PFSS models succeed in only ~50% of cases to identify the coronal segment of open fields. Development of other techniques is in its infancy. Title: Erratum: "Tests and Comparisons of Velocity-Inversion Techniques" (ApJ, 670, 1434 [2007]) Authors: Welsch, B. T.; Abbett, W. P.; DeRosa, M. L.; Fisher, G. H.; Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck, P. W. Bibcode: 2008ApJ...680..827W Altcode: No abstract at ADS Title: Probing Open Magnetic Fields at the Sun with near-relativistic electron beams Authors: Haggerty, D. K.; Rust, D. M.; Georgoulis, M. K. Bibcode: 2008AGUSMSH43B..06H Altcode: Processes associated with solar flares accelerate and inject near-relativistic electrons onto open coronal field lines. Some of these electron events propagate nearly scatter-free to 1 AU, where their spectra and angular distributions can be measured. We used a carefully selected list of electron events for which both the solar and near-Earth positions are well known. Soft X-ray images from Yohkoh determined the positions of coronal holes near active regions as well as the flares associated with the electron events. We chose relatively small events that exhibit nearly Gaussian shaped time-intensity profiles. These events should incur minimal coronal and heliospheric transport effects, which can affect the Sun-to-Earth path length. Twenty-five events met our criteria for inclusion in the study. We use three different methods to estimate the path-length of the electrons and two different potential-field source-surface calculations to trace the open fields from their intersection with the heliospheric field at 2.5 solar radii down to the base of the corona. We report on the successes and failures of these PFSS models to identify open fields in/near active regions and to indicate correctly which open coronal fields are connected to the interplanetary magnetic field at Earth. Title: The Fundamental Instability of Solar Magnetic Eruptions Authors: Georgoulis, M. K. Bibcode: 2008AGUSMSP23B..06G Altcode: We present a simple metric that can adequately quantify the eruptive potential of solar active regions while in their pre-eruption phases. Regions with intense magnetic polarity inversion lines exhibit larger values of this metric, dubbed the effective connected magnetic field strength, Beff. Calculating the pre-flare values of Beff in 93 flaring active regions and the peak Beff-values in 205 non-flaring ones we find a clear segregation between the two populations that helps draw a quantitative distinction between eruptive and non-eruptive active regions. Larger Beff-values statistically correlate with stronger flares and faster, more impulsively accelerated, coronal mass ejections (CMEs). Besides the obvious implications for space weather forecasting, this study underlines the fundamental insight that the Beff metric can bring toward a physical understanding of solar eruptions. A possible new eruption concept may eventually unify previous eruption models, relax excessive requirements posed by some of these models, and naturally explain confined vs. eruptive flares or fast (active-region) vs. slow (quiet-Sun) CMEs. Title: Automatic Active-Region Identification and Azimuth Disambiguation of the SOLIS/VSM Full-Disk Vector Magnetograms Authors: Georgoulis, M. K.; Raouafi, N. -E.; Henney, C. J. Bibcode: 2008ASPC..383..107G Altcode: 2007arXiv0706.4444G The Vector Spectromagnetograph (VSM) of the NSO's Synoptic Optical Long-Term Investigations of the Sun (SOLIS) facility is now operational and obtains the first-ever vector magnetic field measurements of the entire visible solar hemisphere. To fully exploit the unprecedented SOLIS/VSM data, however, one must first address two critical problems: first, the study of solar active regions requires an automatic, physically intuitive, technique for active-region identification in the solar disk. Second, use of active-region vector magnetograms requires removal of the azimuthal 180° ambiguity in the orientation of the transverse magnetic field component. Here we report on an effort to address both problems simultaneously and efficiently. To identify solar active regions we apply an algorithm designed to locate complex, flux-balanced, magnetic structures with a dominant East--West orientation on the disk. Each of the disk portions corresponding to active regions is thereafter extracted and subjected to the Nonpotential Magnetic Field Calculation (NPFC) method that provides a physically-intuitive solution of the 180° ambiguity. Both algorithms have been integrated into the VSM data pipeline and operate in real time, without human intervention. We conclude that this combined approach can contribute meaningfully to our emerging capability for full-disk vector magnetography as pioneered by SOLIS today and will be carried out by ground-based and space-borne magnetographs in the future. Title: Magnetic complexity in eruptive solar active regions and associated eruption parameters Authors: Georgoulis, Manolis K. Bibcode: 2008GeoRL..35.6S02G Altcode: 2007arXiv0712.0143G Using an efficient magnetic complexity index in the active-region solar photosphere, we quantify the preflare strength of the photospheric magnetic polarity inversion lines in 23 eruptive active regions with flare/CME/ICME events tracked all the way from the Sun to the Earth. We find that active regions with more intense polarity inversion lines host statistically stronger flares and faster, more impulsively accelerated, CMEs. No significant correlation is found between the strength of the inversion lines and the flare soft X-ray rise times, the ICME transit times, and the peak D st indices of the induced geomagnetic storms. Corroborating these and previous results, we speculate on a possible interpretation for the connection between source active regions, flares, and CMEs. Further work is needed to validate this concept and uncover its physical details. Title: Tests and Comparisons of Velocity-Inversion Techniques Authors: Welsch, B. T.; Abbett, W. P.; De Rosa, M. L.; Fisher, G. H.; Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck, P. W. Bibcode: 2007ApJ...670.1434W Altcode: Recently, several methods that measure the velocity of magnetized plasma from time series of photospheric vector magnetograms have been developed. Velocity fields derived using such techniques can be used both to determine the fluxes of magnetic energy and helicity into the corona, which have important consequences for understanding solar flares, coronal mass ejections, and the solar dynamo, and to drive time-dependent numerical models of coronal magnetic fields. To date, these methods have not been rigorously tested against realistic, simulated data sets, in which the magnetic field evolution and velocities are known. Here we present the results of such tests using several velocity-inversion techniques applied to synthetic magnetogram data sets, generated from anelastic MHD simulations of the upper convection zone with the ANMHD code, in which the velocity field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and ILCT algorithms performed comparably in many categories. While DAVE estimated the magnitude and direction of velocities slightly more accurately than the other methods, MEF's estimates of the fluxes of magnetic energy and helicity were far more accurate than any other method's. Overall, therefore, the MEF algorithm performed best in tests using the ANMHD data set. We note that ANMHD data simulate fully relaxed convection in a high-β plasma, and therefore do not realistically model photospheric evolution. Title: Survey of Magnetic Helicity Injection in Regions Producing X-Class Flares Authors: LaBonte, B. J.; Georgoulis, M. K.; Rust, D. M. Bibcode: 2007ApJ...671..955L Altcode: Virtually all X-class flares produce a coronal mass ejection (CME), and each CME carries magnetic helicity into the heliosphere. Using magnetograms from the Michelson Doppler Imager on the Solar and Heliospheric Observatory, we surveyed magnetic helicity injection into 48 X-flare-producing active regions recorded by the MDI between 1996 July and 2005 July. Magnetic helicity flux was calculated according to the method of Chae for the 48 X-flaring regions and for 345 non-X-flaring regions. Our survey revealed that a necessary condition for the occurrence of an X-flare is that the peak helicity flux has a magnitude >6×1036 Mx2 s-1. X-flaring regions also consistently had a higher net helicity change during the ~6 day measurement intervals than nonflaring regions. We find that the weak hemispherical preference of helicity injection, positive in the south and negative in the north, is caused by the solar differential rotation, but it tends to be obscured by the intrinsic helicity injection, which is more disorganized and tends to be of opposite sign. An empirical fit to the data shows that the injected helicity over the range 1039-10 43 Mx2 s-1 is proportional to magnetic flux squared. Similarly, over a range of 0.3-3000 days, the time required to generate the helicity in a CME is inversely proportional to the magnetic flux squared. Most of the X-flare regions generated the helicity needed for a CME in a few days to a few hours. Title: Magnetic Energy and Helicity Budgets in the Active Region Solar Corona. I. Linear Force-Free Approximation Authors: Georgoulis, Manolis K.; LaBonte, Barry J. Bibcode: 2007ApJ...671.1034G Altcode: 2007arXiv0706.4122G We self-consistently derive the magnetic energy and relative magnetic helicity budgets of a three-dimensional linear force-free magnetic structure rooted in a lower boundary plane. For the potential magnetic energy we derive a general expression that gives results practically equivalent to those of the magnetic virial theorem. All magnetic energy and helicity budgets are formulated in terms of surface integrals applied to the lower boundary, thus avoiding computationally intensive three-dimensional magnetic field extrapolations. We analytically and numerically connect our derivations with classical expressions for the magnetic energy and helicity, thus presenting a unified treatment of the energy/helicity budgets in the constant-alpha approximation that is lacking so far. Applying our derivations to photospheric vector magnetograms of an eruptive and a noneruptive solar active region, we find that the most profound quantitative difference between these regions lies in the estimated free magnetic energy and relative magnetic helicity budgets. If this result is verified with a large number of active regions, it will advance our understanding of solar eruptive phenomena. We also find that the constant-alpha approximation gives rise to large uncertainties in the calculation of the free magnetic energy and the relative magnetic helicity. Therefore, care must be exercised when this approximation is applied to photospheric magnetic field observations. Despite its shortcomings, the constant-alpha approximation is adopted here because this study will form the basis of a comprehensive nonlinear force-free description of the energetics and helicity in the active region solar corona, which is our ultimate objective. Title: Quantifying turbulence in solar magnetic fields: can this help predict solar eruptions? Authors: Georgoulis, M. K. Bibcode: 2007AGUFMSH14B..01G Altcode: Magnetic fields in solar active regions present us with a beautiful, although inextricable, complexity, and undergo dynamical evolution that is often far from predictable. This behavior is commonly attributed to the inherently turbulent, filamentary nature of magnetic fields in the solar atmosphere. We ask whether this turbulence and its manifestations can help us predict solar eruptions. To this purpose, we briefly outline the physical relationship between turbulence and (critical) self-organization, as well as their phenomenology, such as spatiotemporal intermittency, self-similar fragmentation, fractality, and multifractality of solar active-region magnetic fields. We also review the array of techniques that have been recently implemented to quantify these turbulent features and we apply them to numerous flaring and nonflaring active regions aiming toward quantitative flare prediction. Results and conclusions are presented in hopes to intrigue and stimulate further discussion on this fascinating, clearly outstanding, problem. Title: Quantitative Forecasting of Major Solar Flares Authors: Georgoulis, Manolis K.; Rust, David M. Bibcode: 2007ApJ...661L.109G Altcode: We define the effective connected magnetic field, Beff, a single metric of the flaring potential in solar active regions. We calculated Beff for 298 active regions (93 X- and M-flaring, 205 nonflaring) as recorded by SOHO/MDI during a 10 yr period covering much of solar cycle 23. We find that Beff is a robust criterion for distinguishing flaring from nonflaring regions. A well-defined 12 hr conditional probability for major flares depends solely on Beff. This probability exceeds 0.95 for M-class and X-class flares if Beff>1600 G and Beff>2100 G, respectively, while the maximum calculated Beff-values are near 4000 G. Active regions do not give M-class and X-class flares if Beff<200 G and Beff<750 G, respectively. We conclude that Beff is an efficient flare-forecasting criterion that can be computed on a timely basis from readily available data. Title: Assessment Of The Eruptive Potential In Solar Active Regions Authors: Georgoulis, Manolis K.; Rust, D. M. Bibcode: 2007AAS...210.9325G Altcode: 2007BAAS...39Q.215G Solar active regions involving massive amounts of magnetic flux and conspicuous polarity inversion lines have always been considered likely sources of major flares and fast active-region CMEs. We quantify the magnetic complexity in the photospheric boundary of active regions and thereafter infer a well-defined 12-24-hour likelihood of major eruptions. Vector magnetograms are not required for our analysis, which is validated by application to about 300 active regions observed by SoHO/MDI over a period of a few to several days each. The proposed single metric favors a well-defined physical mechanism for major flares and CMEs and may lead to a detailed, quantitative, understanding of the flare/CME phenomenon. This work has received partial support by NASA Grant NNG05-GM47G. Title: The Fundamental Role Of Magnetic Helicity In Major Solar Eruptions Authors: Rust, David M.; Georgoulis, M. K. Bibcode: 2007AAS...210.2913R Altcode: 2007BAAS...39R.139R What is the role magnetic helicity plays in solar eruptions? To find out, we calculate the magnetic helicity flux for 48 X-class flaring solar active regions and for 345 M-class flaring regions. Each region was observed over a period of a few, to several, days by SoHO/MDI. We find consistently higher helicity buildup rates for the X-flaring regions. X-class flares do not occur if the peak helicity flux is smaller than 6 x 1036 Mx2/s, with most nonflaring regions showing much smaller peaks. The weak hemispheric preference of magnetic

helicity is caused by the solar differential rotation, but it is blurred by the intrinsic helicity injection which is more disorganized and of opposite sign. Notably, X-flaring regions can generate the helicity for a typical CME within a few hours to a few days, contrary to most flare-quiescent regions that require from several tens to hundreds of days. We conclude that accumulation of magnetic helicity is a precondition for major flare/CME events. This work has received partial support by NASA Grants NAG5-13504 and NNG05-GM47G. Title: LWS TR&T Project on the CME - ICME Connection: A Progress Report Authors: Georgoulis, M. K. Bibcode: 2006AGUFMSH21B..07G Altcode: During the second year of the above LWS TR&T Grant, we report on our efforts and progress. The Project highlights and will continue to uncover the wealth of eruptive diagnostics that can be obtained by studying vector (but also line-of-sight) magnetograms of solar active regions. Here we focus on one particular aspect of active-region physics: the elusive relation between the free magnetic energy and the magnetic helicity budget of eruptive and non-eruptive active regions. Besides shedding light into the eruption process itself, this analysis can uncover the true relation between the magnetic structures of the propagating ICMEs and that of the progenitor CMEs and the source active regions. This, as the title suggests, is the ultimate objective of the Project. Title: Goals and Progress of the LWS Focused Science Topic on the CME--ICME Connection Authors: Mikic, Z.; Deforest, C.; Devore, R.; Georgoulis, M.; Jackson, B.; Nitta, N.; Pizzo, V.; Odstrcil, D. Bibcode: 2006AGUFMSH21B..05M Altcode: Our team addresses the NASA Living With a Star (LWS) Focused Science Topic "to determine the solar origins of the plasma and magnetic flux observed in an interplanetary Coronal Mass Ejection (ICME)." In short, this team is examining the CME--ICME connection. Our team was formed as a result of awards from the LWS Targeted Research &Technology competition in the fall of 2004. Our team is investigating the detailed relationship between the plasma and magnetic fields in active regions, the source regions of CMEs, and subsequent in situ measurements in interplanetary magnetic clouds. We plan to study this connection through detailed numerical simulations of CME initiation and propagation, theoretical investigations, and studies of the properties of active regions, CMEs, and magnetic clouds. We will discuss the goals of our team, how it fits into NASA's missions, and our progress so far. Research supported by NASA's Living With a Star Program. Title: An Overview of Existing Algorithms for Resolving the 180° Ambiguity in Vector Magnetic Fields: Quantitative Tests with Synthetic Data Authors: Metcalf, Thomas R.; Leka, K. D.; Barnes, Graham; Lites, Bruce W.; Georgoulis, Manolis K.; Pevtsov, A. A.; Balasubramaniam, K. S.; Gary, G. Allen; Jing, Ju; Li, Jing; Liu, Y.; Wang, H. N.; Abramenko, Valentyna; Yurchyshyn, Vasyl; Moon, Y. -J. Bibcode: 2006SoPh..237..267M Altcode: 2006SoPh..tmp...14M We report here on the present state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic field measurements. With present observations and techniques, some assumption must be made about the solar magnetic field in order to resolve this ambiguity. Our focus is the application of numerous existing algorithms to test data for which the correct answer is known. In this context, we compare the algorithms quantitatively and seek to understand where each succeeds, where it fails, and why. We have considered five basic approaches: comparing the observed field to a reference field or direction, minimizing the vertical gradient of the magnetic pressure, minimizing the vertical current density, minimizing some approximation to the total current density, and minimizing some approximation to the field's divergence. Of the automated methods requiring no human intervention, those which minimize the square of the vertical current density in conjunction with an approximation for the vanishing divergence of the magnetic field show the most promise. Title: Distinguishing Eruptive From Non-eruptive Solar Active Regions Authors: Georgoulis, Manolis K. Bibcode: 2006SPD....37.2003G Altcode: 2006BAAS...38..248G Every intense magnetic flux accumulation in the Sun, known as activeregion (AR), engages into small-scale energy release activitiesmanifesting themselves as sub-flares. Of the several thousands of ARsthat emerge, evolve, and disappear within a typical solar cycle,however, only a tiny percentage will ever trigger one or more majorflares (X- or M-class). These flares are most commonly related tofurther eruptive activity, such as coronal mass ejections (CMEs).Identifying eruptive ARs while in their pre-eruption phases will bethe subject of this presentation. With little doubt existing over themagnetic origin of solar eruptions, I will highlight possible areas offuture research in AR magnetic fields. The timing of magnetic fieldstudies is excellent in view of the anticipated wealth of vectormagnetogram data from SOLIS, Solar-B, and the SDO. While our goal ispractical, namely the quantitative assessment of the eruptivepotential of ARs, the recommended tactic entails fundamental physicalproperties of AR magnetic fields such as their three-dimensionalstructure, energy budgets, and complexity, reflected on theirhelical properties. The elusive role of magnetic helicity in solareruptions and our limitations in assessing it will be particularlyemphasized. Title: Reconstruction of an Inductive Velocity Field Vector from Doppler Motions and a Pair of Solar Vector Magnetograms Authors: Georgoulis, Manolis K.; LaBonte, Barry J. Bibcode: 2006ApJ...636..475G Altcode: 2005astro.ph.11447G We outline a general methodology to infer the inductive velocity field vector in solar active regions. For the first time, both the field-aligned and the cross-field velocity components are reconstructed. The cross-field velocity solution accounts for the changes of the vertical magnetic field seen between a pair of successive active region vector magnetograms via the ideal induction equation. The field-aligned velocity is obtained using the Doppler velocity and the calculated cross-field velocity. Solving the ideal induction equation in vector magnetograms measured at a given altitude in the solar atmosphere is an underdetermined problem. In response, our general formalism allows the use of any additional constraint for the inductive cross-field velocity to enforce a unique solution in the induction equation. As a result, our methodology can give rise to new velocity solutions besides the one presented here. To constrain the induction equation, we use a special case of the minimum structure approximation that was introduced in previous studies and is already employed here to resolve the 180° ambiguity in the input vector magnetograms. We reconstruct the inductive velocity for three active regions, including NOAA AR 8210, for which previous results exist. Our solution believably reproduces the horizontal flow patterns in the studied active regions but breaks down in cases of localized rapid magnetic flux emergence or submergence. Alternative approximations and constraints are possible and can be accommodated into our general formalism. Title: Emergence of undulatory magnetic flux tubes by small scale reconnections Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2006AdSpR..38..902P Altcode: With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and Hα filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale H α brightenings called 'Ellerman bombs' (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of "Bald patches" (BP) regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Some of these EBs/BPs are magnetically connected by low-lying field lines, presenting a serpentine shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale Ω-loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, which observational signature is Ellerman bombs. Title: Observation of Small Scale Reconnection Role in Undulated Flux Tube Emergence Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2005ESASP.596E..34P Altcode: 2005ccmf.confE..34P No abstract at ADS Title: A New Technique for a Routine Azimuth Disambiguation of Solar Vector Magnetograms Authors: Georgoulis, Manolis K. Bibcode: 2005ApJ...629L..69G Altcode: We introduce a nonpotential magnetic field calculation (NPFC) technique to perform azimuth disambiguation in solar vector magnetograms. It is shown that resolving the 180° ambiguity would be a numerically fully determined problem if the vertical electric current density was known a priori. Since this is not the case, we enforce a minimum-magnitude current density solution. The NPFC disambiguation is otherwise assumption-free, with the quality of the results depending on the quality of the measurements. The NPFC method first infers the nonpotential magnetic field component responsible for the assumed vertical currents and then determines the vertical magnetic field whose potential extrapolation, added to the nonpotential field, best reproduces the observationally inferred horizontal magnetic field. The technique is fast, effective, and physically sound, so it may be instrumental in a routine, real-time, disambiguation of future space-borne solar vector magnetograms. Title: Distinguishing Between Eruptive and Quiescent Solar Active Regions Authors: Georgoulis, M. K.; Labonte, B. J. Bibcode: 2005AGUSMSH53B..05G Altcode: We present a method to fully evaluate the energy-helicity formula in solar active regions by using only photospheric vector magnetograms of these active regions. At the moment, the method relies on the linear force-free approximation and provides the total magnetic energy, the magnetic energy of the vacuum (potential) magnetic field, and the non-potential (free) magnetic energy relating to the total magnetic helicity in an active region. The formulation of the technique allows an upgrade to a nonlinear force-free evaluation of the energy-helicity formula, which will be a more realistic approach especially when chromospheric vector magnetograms of solar active regions become available. Even with the linear force-free approximation, however, we find that the magnitudes of the total helicity, as well as the ratios of the free magnetic energy to the total magnetic energy are distinctly higher for eruptive active regions as compared to quiescent active regions. Eruptive active regions produce flares and might trigger CMEs, so the method presents a viable way to discriminate between these two types of active regions even in case a single vector magnetogram of these active regions is available. Title: Boundary Flows in Solar Active Regions Authors: Georgoulis, M. K.; Labonte, B. J. Bibcode: 2005AGUSMSH51C..10G Altcode: We present a general technique to calculate the flow field at the altitude where vector magnetic field measurements of solar active regions have been obtained. The velocity field vector is reconstructed fully by solving the ideal induction equation of magnetohydrodynamics for the cross-field velocity component and by utilizing the Doppler velocity information to calculate the field-aligned velocity component. Because solving the induction equation is an under-determined problem, we have formulated our technique in such a way as to provide a unique solution of the induction equation when the vertical (normal to the boundary) component of the cross-field velocity is prescribed. We provide examples of various possible choices for the cross-field vertical velocity and we discuss the respective results. Moreover, we showcase the validity of our technique by predicting the particular area of NOAA active region 8210 from which a flare and a CME were triggered, using the reconstructed velocity field vector. Title: Turbulence In The Solar Atmosphere: Manifestations And Diagnostics Via Solar Image Processing Authors: Georgoulis, Manolis K. Bibcode: 2005SoPh..228....5G Altcode: 2005astro.ph.11449G Intermittent magnetohydrodynamical turbulence is most likely at work in the magnetized solar atmosphere. As a result, an array of scaling and multi-scaling image-processing techniques can be used to measure the expected self-organization of solar magnetic fields. While these techniques advance our understanding of the physical system at work, it is unclear whether they can be used to predict solar eruptions, thus obtaining a practical significance for space weather. We address part of this problem by focusing on solar active regions and by investigating the usefulness of scaling and multi-scaling image-processing techniques in solar flare prediction. Since solar flares exhibit spatial and temporal intermittency, we suggest that they are the products of instabilities subject to a critical threshold in a turbulent magnetic configuration. The identification of this threshold in scaling and multi-scaling spectra would then contribute meaningfully to the prediction of solar flares. We find that the fractal dimension of solar magnetic fields and their multi-fractal spectrum of generalized correlation dimensions do not have significant predictive ability. The respective multi-fractal structure functions and their inertial-range scaling exponents, however, probably provide some statistical distinguishing features between flaring and non-flaring active regions. More importantly, the temporal evolution of the above scaling exponents in flaring active regions probably shows a distinct behavior starting a few hours prior to a flare and therefore this temporal behavior may be practically useful in flare prediction. The results of this study need to be validated by more comprehensive works over a large number of solar active regions. Sufficient statistics may also establish critical thresholds in the values of the multi-fractal structure functions and/or their scaling exponents above which a flare may be predicted with a high level of confidence. Title: Transport of Magnetic Helicity and Dynamics of Solar Active Regions Authors: Georgoulis, M. K.; Labonte, B. J.; Rust, D. M. Bibcode: 2005HiA....13..117G Altcode: We outline a simple method to monitor variations of the magnetic helicity the current helicity and the non-potential (free) magnetic energy on the photospheric boundary of solar active regions. Explicit manifestations of dynamical activity in the solar atmosphere such as flares coronal mass ejections and filament eruptions may be related to these variations. While similar methods require knowledge of the vector potential and the velocity field vector on the photosphere our method requires only the photospheric potential magnetic field corresponding to the observed magnetograms. The calculation of the potential field for any given magnetogram is straightforward. Moreover our method relies on the constant-alpha force-free approximation assumed to hold in the active region. Whether the above is a realistic assumption can be tested using an array of well-documented methods. Therefore our technique may prove quite useful to at least a subset of active regions in which the linear force-free approximation is justifiable. Title: Forecasting and Real-Time Diagnostics of Solar Coronal Mass Ejections Authors: Georgoulis, M. K.; Labonte, B. J. Bibcode: 2004AGUFMSA43B..02G Altcode: We discuss an operational, fully automated, algorithm to follow the dynamical evolution and the buildup of magnetic instabilities that give rise to coronal mass ejections (CMEs) in solar active regions. The tool relies on vector magnetic field measurements of the active region photosphere / chromosphere and performs the following tasks: (1) resolution of the 180-degree ambiguity in the magnetic field measurements and preparation for further use, (2) calculation of the magnetic forces and electric currents in the active region photosphere/chromosphere, (3) reconstruction of a magnetohydrodynamic velocity field corresponding to the measured magnetic field to calculate the buildup rate of the magnetic helicity in the active region atmosphere, and (4) estimation of the total magnetic helicity in the active region corona. We present examples showing that (I) flare- and CME-prolific active regions have much higher magnetic helicity, stronger magnetic forces and more intense cross-field electric currents than quiescent active regions, and (II) the magnetic helicity, chirality, magnetic flux, and magnetic energy of a CME can be calculated in real time from the results of the algorithm before and after the CME. As a result, we can both identify potentially eruptive areas on the visible solar disk and provide detailed quantitative diagnostics of the resulting CMEs. Additional work is required to predict the geoeffectiveness of these CMEs. For the algorithm to be useful we need full-disk, ideally uninterrupted, coverage of the solar magnetic field vector. This information will be available in a few years with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO; launch 2008). At the moment, full-disk vector magnetograms will be provided by the ground-based Vector Spectro-Magnetograph (VSM) of the Synoptic Optical Long-Term Investigation of the Sun (SOLIS) telescope. We will utilize the SOLIS vector magnetograms as soon as they become available. Title: An Integrated Program to Forecast Geostorms Authors: Labonte, B. J.; Rust, D.; Bernasconi, P.; Georgoulis, M. Bibcode: 2004AGUFMSA51B0243L Altcode: We have developed several operational products and automated tools for assessing the helicity content of solar regions and their probability of launching a geoeffective coronal mass ejection. These include detection of active region sigmoids, measurement of magnetic helicity injection in active regions, measurement of the sense of helicity in solar filaments, and the estimate of magnetic helicity content of active regions from vector magnetogram observations. In this presentation we discuss a new program to integrate the separate products and tools into a single product that provides a quantitative mid-term forecast of solar activity that results in geomagnetic storms. Title: Vertical Lorentz Force and Cross-Field Currents in the Photospheric Magnetic Fields of Solar Active Regions Authors: Georgoulis, Manolis K.; LaBonte, Barry J. Bibcode: 2004ApJ...615.1029G Altcode: We demonstrate that the vertical Lorentz force and a corresponding lower limit of the cross-field electric current density can be calculated from vector magnetograms of solar active regions obtained at a single height in the solar atmosphere, provided that the vertical gradient of the magnetic field strength is known at this height. We use a predicted vertical magnetic field gradient derived from a previous analysis. By testing various force-free solutions, we find that the numerical accuracy of our method is satisfactory. Applying the method to active region photospheric vector magnetograms, we find vertical Lorentz forces ranging from several hundredths to a few tenths of the typical photospheric gravitational force, and typical cross-field current densities up to several times 10 mA m-2. The typical vertical current density is found to be 2-3 times smaller, on the order of 10-15 mA m-2. These differences are above the associated uncertainties. The values of the cross-field currents decrease in an averaged vector magnetogram, but the ratio of the cross-field to the vertical current density increases, also above the uncertainties. We conclude that the photospheric active region magnetic fields are not force-free, contrary to the conjectures of some recent studies. Title: Resistive Emergence of Undulatory Flux Tubes Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2004ApJ...614.1099P Altcode: During its 2000 January flight, the Flare Genesis Experiment observed the gradual emergence of a bipolar active region, by recording a series of high-resolution photospheric vector magnetograms and images in the blue wing of the Hα line. Previous analyses of these data revealed the occurrence of many small-scale, transient Hα brightenings identified as Ellerman bombs (EBs). They occur during the flux emergence, and many of them are located near moving magnetic dipoles in which the vector magnetic field is nearly tangential to the photosphere. A linear force-free field extrapolation of one of the magnetograms was performed to study the magnetic topology of small-scale EBs and their possible role in the flux emergence process. We found that 23 out of 47 EBs are cospatial with bald patches (BPs), while 15 are located at the footpoints of very flat separatrix field lines passing through distant BPs. We conclude that EBs can be due to magnetic reconnection, not only at BP locations, but also along their separatrices, occurring in the low chromosphere. The topological analysis reveals, for the first time, that many EBs and BPs are linked by a hierarchy of elongated flux tubes showing aperiodic spatial undulations, whose wavelengths are typically above the threshold of the Parker instability. These findings suggest that arch filament systems and coronal loops do not result from the smooth emergence of large-scale Ω-loops from below the photosphere, but rather from the rise of undulatory flux tubes whose upper parts emerge because of the Parker instability and whose dipped lower parts emerge because of magnetic reconnection. EBs are then the signature of this resistive emergence of undulatory flux tubes. Title: On the Self-Similarity of Unstable Magnetic Discontinuities in Solar Active Regions Authors: Vlahos, Loukas; Georgoulis, Manolis K. Bibcode: 2004ApJ...603L..61V Altcode: We investigate the statistical properties of possible magnetic discontinuities in two solar active regions over the course of several hours. We use linear force-free extrapolations to calculate the three-dimensional magnetic structure in the active regions. Magnetic discontinuities are identified using various selection criteria. Independently of the selection criterion, we identify large numbers of magnetic discontinuities whose free magnetic energies and volumes obey well-formed power-law distribution functions. The power-law indices for the free energies are in the range [-1.6, -1.35], in remarkable agreement with the power-law indices found in the occurrence frequencies of solar flare energies. This agreement and the strong self-similarity of the volumes that are likely to host flares suggest that the observed statistics of flares may be the natural outcome of a preexisting spatial self-organization accompanying the energy fragmentation in solar active regions. We propose a dynamical picture of flare triggering consistent with recent observations by reconciling our results with the concepts of percolation theory and self-organized criticality. These concepts rely on self-organization, which is expected from the fully turbulent state of the magnetic fields in the solar atmosphere. Title: On the Resolution of the Azimuthal Ambiguity in Vector Magnetograms of Solar Active Regions Authors: Georgoulis, Manolis K.; LaBonte, Barry J.; Metcalf, Thomas R. Bibcode: 2004ApJ...602..446G Altcode: We introduce a ``structure minimization'' technique to resolve the azimuthal ambiguity of 180°, intrinsic in solar vector magnetic field measurements. We resolve the 180° ambiguity by minimizing the inhomogeneities of the magnetic field strength perpendicular to the magnetic field vector. This relates to a minimization of the sheath currents that envelope the solar magnetic flux tubes, thus allowing for more space-filling and less complex magnetic fields. Structure minimization proceeds in two steps: First, it derives a local solution analytically, by means of a structure minimization function. Second, it reaches a global solution numerically, assuming smoothness of the magnetic field vector. Structure minimization (i) is disentangled from any use of potential or linear force-free extrapolations and (ii) eliminates pixel-to-pixel dependencies, thus reducing exponentially the required computations. We apply structure minimization to four active regions, located at various distances from disk center. The minimum structure solution for each case is compared with the ``minimum energy'' solution obtained by the slower simulated annealing algorithm. We find correlation coefficients ranging from significant to excellent. Moreover, structure minimization provides an ambiguity-free vertical gradient of the magnetic field strength that reveals the variation of the magnetic field with height. The simplicity and speed of the method allow a near real-time processing of solar vector magnetograms. This task was not possible in the past and may be of interest to both existing and future solar missions and ground-based magnetographs. Title: Emerging Flux and the Heating of Coronal Loops Authors: Schmieder, B.; Rust, D. M.; Georgoulis, M. K.; Démoulin, P.; Bernasconi, P. N. Bibcode: 2004ApJ...601..530S Altcode: We use data collected by a multiwavelength campaign of observations to describe how the fragmented, asymmetric emergence of magnetic flux in NOAA active region 8844 triggers the dynamics in the active-region atmosphere. Observations of various instruments on board Yohkoh, SOHO, and TRACE complement high-resolution observations of the balloon-borne Flare Genesis Experiment obtained on 2000 January 25. We find that coronal loops appeared and evolved rapidly ~6+/-2 hr after the first detection of emerging magnetic flux. In the low chromosphere, flux emergence resulted in intense Ellerman bomb activity. Besides the chromosphere, we find that Ellerman bombs may also heat the transition region, which showed ``moss'' ~100% brighter in areas with Ellerman bombs as compared to areas without Ellerman bombs. In the corona, we find a spatiotemporal anticorrelation between the soft X-ray (SXT) and the extreme ultraviolet (TRACE) loops. First, SXT loops preceded the appearance of the TRACE loops by 30-40 minutes. Second, the TRACE and SXT loops had different shapes and different footpoints. Third, the SXT loops were longer and higher than the TRACE loops. We conclude that the TRACE and the SXT loops were formed independently. TRACE loops were mainly heated at their footpoints, while SXT loops brightened in response to coronal magnetic reconnection. In summary, we observed a variety of coupled activity, from the photosphere to the active-region corona. Links between different aspects of this activity lead to a unified picture of the evolution and the energy release in the active region. Title: Emerging Flux and the Heating of Coronal Loops Authors: Schmieder, B.; Démoulin, P.; Rust, D. M.; Georgoulis, M. K.; Bernasconi, P. N. Bibcode: 2004IAUS..219..483S Altcode: 2003IAUS..219E..18S We suggest that coronal loop heating is caused by dissipation of magnetic energy as new magnetic flux emerges from the photosphere. Based on data from a multi wavelength campaign of observations during the flight of the Flare Genesis Experiment we describe how emergence of flux from the photosphere appears directly to heat the corona to 2-3 MK. Following intense heating the loops cool and become visible through the filters of the TRACE (Transition Region and Coronal Explorer)instrument at one million degrees. We determine the relaxation time of the cooling and compare it withtheoretical heating functions. The proposed mechanism is well accepted in flare loops but we suggest that the mechanism is generally valid and helps to explain the visibility of active region loops in transition region lines. Title: Emergence of undulatory magnetic flux tubes by small scale reconnections Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2004cosp...35.1482P Altcode: 2004cosp.meet.1482P With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and Hα filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale Hα brightenings called 'Ellerman bombs' (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of ``Bald patches'' regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Among 47 identified EBs, we found that 23 are co-spatial with a BP, while 19 are located at the footpoint of very flat separatrix field lines passing throught a distant BP. We reveal for the first time that some of these EBs/BPs are magneticaly connected by low-lying lines, presenting a 'sea-serpent' shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale Ω loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, whose observational signature is Ellerman bombs. Title: Statistical Properties of Flaring and Sub-Flaring Activity in the Solar Atmosphere Authors: Georgoulis, Manolis K. Bibcode: 2004hell.conf...15G Altcode: No abstract at ADS Title: Lorentz Forces and Helicity Diagnostics in Solar Active Regions Based on a Fast Resolution of the Azimuthal Ambiguity in Solar Vector Magnetograms Authors: Georgoulis, Manolis K.; Labonte, Barry J.; Rust, David M. Bibcode: 2004hell.conf...82G Altcode: No abstract at ADS Title: Calculation of a Minimum Total Magnetic Helicity in Solar Active Regions Authors: Georgoulis, M. K.; Labonte, B. J. Bibcode: 2003AGUFMSH51A..03G Altcode: Despite its extreme importance, the calculation of the total magnetic helicity in solar active regions remains an unresolved problem in solar physics. On the other hand, the helicity variations in an active region can be calculated partially, for longitudinal magnetograms, or in full, for vector magnetograms, but only by using coarse, uncertain velocity field maps, calculated by means of correlation tracking techniques. Whether one should apply correlation tracking to magnetograms or white-light continuum images is also unclear, as the two inputs do not yield identical outputs. We present a technique that provides a lower limit of the total magnetic helicity in active regions, without using any velocity fields. The temporal variation of the total helicity can also be calculated in full if a series of vector magnetograms is available. The method relies on a comparison between the best linear force-free approximation and the potential approximation for a given photospheric boundary and begins by demonstrating that a commonly used formula for the magnetic helicity density in the linear force-free approximation is, in fact, erroneous. We have tested our method on vector magnetograms acquired by the Imaging Vector Magnetograph (IVM) of the University of Hawaii. We discuss the pros and cons of our approach and we compare our results for the magnetic helicity variations with results obtained when classical methods are employed. Title: Resolution of the Azimuthal Ambiguity in Photospheric Vector Magnetograms of Solar Active Regions Authors: Georgoulis, M. K.; LaBonte, B. J. Bibcode: 2003SPD....34.1103G Altcode: 2003BAAS...35Q.827G We describe a simple technique to resolve the inherent azimuthal ambiguity of 180o in vector magnetic field measurements of solar active regions. The desired azimuth solution is the one that minimizes an introduced function. This function includes a weighted combination of the height derivative of the magnetic field strength, calculated under conditions of minimum electric current density, and the vertical component of a current density vector purely perpendicular to the magnetic field lines. The above function reduces the number of ambiguity states to two for each location on the heliographic plane. The process is initially local, i.e., independent for each location on the heliographic plane. Then, the initial azimuth solution is subjected to a numerical analysis which yields the global azimuth solution and ensures maximum continuity of the photospheric magnetic field vector. This tactic reduces dramatically the required computing time to only a small fraction of the time required by existing techniques. The construction of the above-mentioned function is such that the method works equally well for active regions located either near or far from the center of the solar disk. The speed and simplicity of this novel technique may lead to a near real-time processing of acquired photospheric vector magnetograms. A reliable azimuth solution is a prerequisite for further analysis of solar magnetic fields. Reaching such a solution fast, is paramount for challenging modern problems, such as space weather forecasting, for example. Title: Near-infrared chromospheric observatory Authors: Labonte, Barry; Rust, David M.; Bernasconi, Pietro N.; Georgoulis, Manolis K.; Fox, Nicola J.; Kalkofen, Wolfgang; Lin, Haosheng Bibcode: 2003SPIE.4853..140L Altcode: NICO, the Near Infrared Chromosphere Observatory, is a platform for determining the magnetic structure and fources of heating for the solar chromosphere. NICO, a balloon-borne observatory, will use the largest solar telescope flying to map the magnetic fields, velocities, and heating events of the chromosphere and photosphere in detail. NICO will introduce new technologies to solar flight missions, such as wavefront sensing for monitoring telescope alignment, real-time correlation tracking and high-speed image motion compensation, and wide aperture Fabry-Perot etalons for extended spectral scanning. Title: Transport of Helicity and Dynamics of Solar Active Regions Authors: Georgoulis, Manolis K.; Rust, David M.; Labonte, Barry J. Bibcode: 2003IAUJD...3E..29G Altcode: We outline a simple method to monitor variations of the magnetic helicity the current helicity and the non-potential (free) magnetic energy on the photospheric boundary of solar active regions. Explicit manifestations of dynamical activity in the solar atmosphere such as flares coronal mass ejections and filament eruptions may be related to these variations. While similar methods require knowledge of the vector potential and the velocity field vector on the photosphere our method requires only the photospheric potential magnetic field corresponding to the observed magnetograms. The calculation of the potential field for any given magnetogram is straightforward. Moreover our method relies on the constant-alpha force-free approximation assumed to hold in the active region. Whether the above is a realistic assumption can be tested using an array of well-documented methods. Therefore our technique may prove quite useful to at least a subset of active regions in which the linear force-free approximation is justifiable. Title: Flare Genesis Experiment: magnetic topology of Ellerman bombs Authors: Schmieder, B.; Pariat, E.; Aulanier, G.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2002ESASP.506..911S Altcode: 2002svco.conf..911S; 2002ESPM...10..911S Flare Genesis Experiment (FGE), a balloon borne Observatory was launched in Antarctica on January 10, 2000 and flew during 17 days. FGE consists of an 80 cm Cassegrain telescope with an F/1.5 ultra-low-expansion glass primary mirror and a crystalline silicon secondary mirror. A helium-filled balloon carried the FGE to an altitude of 37 km (Bernasconi et al. 2000, 2001). We select among all the observations a set of high spatial and temporal resolution observations of an emerging active region with numerous Ellerman bombs (EBs). Statistical and morphology analysis have been performed. We demonstrate that Ellerman bombs are the result of magnetic reconnection in the low chromosphere by a magnetic topology analysis. The loci of EBs coincide with "bald patches" (BPs). BPs are regions where the vector field is tangential to the boundary (photosphere) along an inversion line. We conclude that emerging flux through the photosphere is achieved through resistive emergence of U loops connecting small Ω loops before rising in the chromosphere and forming Arch Filament System (AFS). Title: Statistics, morphology, and energetics of Ellerman bombs Authors: Georgoulis, Manolis K.; Rust, David M.; Bernasconi, Pietro N.; Schmieder, Brigitte Bibcode: 2002ESASP.505..125G Altcode: 2002IAUCo.188..125G; 2002solm.conf..125G We have performed a detailed analysis of several hundreds Hα Ellerman bombs in the low chromosphere, above an emerging flux region. We find that Ellerman bombs may be small-scale, low-altitude, magnetic reconnection events that heat the low chromosphere in the active region. Their energy content varies between 1027 erg and 1028 erg, typical of sub-flaring activity. Title: The Near-Infrared Chromosphere Observatory Authors: Rust, David M.; Bernasconi, Pietro N.; Labonte, Barry J.; Georgoulis, Manolis K.; Fox, Nicola J.; Kalkofen, Wolfgang; Lin, Haoseng Bibcode: 2002ESASP.505..561R Altcode: 2002IAUCo.188..561R; 2002solm.conf..561R The Near-Infrared Chromosphere Observatory (NICO) is a proposed balloon-borne observatory aiming to investigate the magnetic structure and the sources of heating in the solar chromosphere. NICO will be based on the successful Flare Genesis Experiment (FGE), a pioneer in applying novel technologies for the study of the Sun. NICO will map magnetic fields, velocity fields, and heating events in the chromosphere with unprecedented quality. Title: Vector magnetic field observations of flux tube emergence Authors: Schmieder, B.; Aulanier, G.; Pariat, E.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2002ESASP.505..575S Altcode: 2002IAUCo.188..575S; 2002solm.conf..575S With Flare Genesis Experiment (FGE), a balloon borne Observatory high spatial and temporal resolution vector magnetograms have been obtained in an emerging active region. The comparison of the observations (FGE and TRACE) with a linear force-free field analysis of the region shows where the region is non-force-free. An analysis of the magnetic topology furnishes insights into the existence of "bald patches" regions (BPs are regions where the vector field is tangential to the boundary (photosphere) along an inversion line). Magnetic reconnection is possible and local heating of the chromopshere is predicted near the BPs. Ellerman bombs (EBs) were found to coincide with few BPs computed from a linear force-free extrapolation of the observed longitudinal field. But when the actual observations of transverse fields were used to identify BPs, then the correspondence with EB positions improved significantly. We conclude that linear force-free extrapolations must be done with the true observed vertical fields, which require the measurement of the three components of the magnetic field. Title: Moving Dipolar Features in an Emerging Flux Region Authors: Bernasconi, P. N.; Rust, D. M.; Georgoulis, M. K.; Labonte, B. J. Bibcode: 2002SoPh..209..119B Altcode: On 25 January, 2000, we observed active region NOAA 8844 with the Flare Genesis Experiment (FGE), a balloon-borne observatory with an 80-cm solar telescope. FGE was equipped with a vector polarimeter and a tunable Fabry-Pérot narrow-band filter. It recorded time series of filtergrams, vector magnetograms and Dopplergrams at the Ca i 6122.2 Å line, and Hα filtergrams with a cadence between 2.5 and 7.5 min. At the time of the observations, NOAA 8844 was located at approximately 5° N 30° W. The region was growing rapidly; new magnetic flux was constantly emerging in three supergranules near its center. We report on the structure and behavior of peculiar moving dipolar features (MDFs) in the emerging flux, and we describe in detail how the FGE data were analyzed. In longitudinal magnetograms, the MDFs appeared to be small dipoles flowing into sunspots and supergranule boundaries. Previously, dipolar moving magnetic features (MMFs) have only been observed flowing out from sunspots. The FGE vector magnetograms show that the MDFs occurred in a region with nearly horizontal fields, the MDFs being distinguished as undulations in these fields. We identify the MDFs as stitches where the emerging flux ropes were still tied to the photosphere by trapped mass. We present a U-loop model that accounts for their unusual structure and behavior, as well as showing how emerging flux sheds entrained mass. Title: Statistical Properties of the Energy Release in Emerging and Evolving Active Regions Authors: Vlahos, Loukas; Fragos, Tassos; Isliker, Heinz; Georgoulis, Manolis Bibcode: 2002ApJ...575L..87V Altcode: 2002astro.ph..7340V The formation and evolution of active regions are inherently complex phenomena. Magnetic fields generated at the base of the convection zone follow a chaotic evolution before reaching the solar surface. In this article, we use a two-dimensional probabilistic cellular automaton to model the statistical properties of the magnetic patterns formed on the solar surface and to estimate the magnetic energy released in the interaction of opposite polarities. We assume that newly emerged magnetic flux tubes stimulate the emergence of new magnetic flux in their neighborhood. The flux tubes move randomly on the surface of the Sun, and they cancel and release their magnetic energy when they collide with magnetic flux of opposite polarity, or diffuse into the ``empty'' photosphere. We assume that cancellation of magnetic flux in collisions causes ``flares'' and determine the released energy as the difference in the square of the magnetic field flux (E~B2). The statistics of the simulated flares follow a power-law distribution in energy, f(E)~E-a, where a=2.2+/-0.1. The size distribution function of the simulated active regions exhibits a power-law behavior with index k~1.93+/-0.08, and the fractal dimension of the magnetized areas on the simulated solar surface is close to DF~1.42+/-0.12. Both quantities, DF and k, are inside the range of the observed values. Title: Statistics, Morphology, and Energetics of Ellerman Bombs Authors: Georgoulis, Manolis K.; Rust, David M.; Bernasconi, Pietro N.; Schmieder, Brigitte Bibcode: 2002ApJ...575..506G Altcode: We investigate the statistical properties of Ellerman bombs in the dynamic emerging flux region NOAA Active Region 8844, underneath an expanding arch filament system. High-resolution chromospheric Hα filtergrams (spatial resolution 0.8"), as well as photospheric vector magnetograms (spatial resolution 0.5") and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. Hα observations reveal the first ``seeing-free'' data set on Ellerman bombs and one of the largest samples of these events. We find that Ellerman bombs occur and recur in preferential locations in the low chromosphere, either above or in the absence of photospheric neutral magnetic lines. Ellerman bombs are associated with photospheric downflows, and their loci follow the transverse mass flows on the photosphere. They are small-scale events, with typical size 1.8"×1.1" , but this size depends on the instrumental resolution. A large number of Ellerman bombs are probably undetected, owing to limited spatial resolution. Ellerman bombs occur in clusters that exhibit fractal properties. The fractal dimension, with an average value ~1.4, does not change significantly in the course of time. Typical parameters of Ellerman bombs are interrelated and obey power-law distribution functions, as in the case of flaring and subflaring activity. We find that Ellerman bombs may occur on separatrix, or quasi-separatrix, layers, in the low chromosphere. A plausible triggering mechanism of Ellerman bombs is stochastic magnetic reconnection caused by the turbulent evolution of the low-lying magnetic fields and the continuous reshaping of separatrix layers. The total energies of Ellerman bombs are estimated in the range (1027, 1028) ergs, the temperature enhancement in the radiating volume is ~2×103 K, and the timescale of radiative cooling is short, of the order of a few seconds. The distribution function of the energies of Ellerman bombs exhibits a power-law shape with an index ~-2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions. Title: The Near-Infrared Chromosphere Observatory (NICO) Authors: Rust, D. M.; Bernasconi, P. N.; LaBonte, B. J.; Georgoulis, M. K.; Kalkofen, W.; Fox, N. J.; Lin, H. Bibcode: 2002AAS...200.3902R Altcode: 2002BAAS...34..701R NICO is a proposed cost-effective platform for determining the magnetic structure and sources of heating for the solar chromosphere. It is a balloon-borne observatory that will use the largest solar telescope flying and very high data rates to map the magnetic fields, velocities, and heating events of the chromosphere and photosphere in unprecedented detail. NICO is based on the Flare Genesis Experiment (FGE), which has pioneered in the application of technologies important to NASA's flight program. NICO will also introduce new technologies, such as wavefront sensing for monitoring telescope alignment; real-time correlation tracking and high-speed image motion compensation for smear-free imaging; and wide aperture Fabry-Perot filters for extended spectral scanning. The telescope is a classic Cassegrain design with an 80-cm diameter F/1.5 primary mirror made of Ultra-Low-Expansion glass. The telescope structure is graphite-epoxy for lightweight, temperature-insensitive support. The primary and secondary mirror surfaces are coated with silver to reflect more than 97% of the incident solar energy. The secondary is made of single-crystal silicon, which provides excellent thermal conduction from the mirror surface to its mount, with negligible thermal distortion. A third mirror acts as a heat dump. It passes the light from a 15-mm diameter aperture in its center, corresponding to a 322"-diameter circle on the solar surface, while the rest of the solar radiation is reflected back out of the front of the telescope. The telescope supplies the selected segment of the solar image to a polarization and spectral analysis package that operates with an image cadence 1 filtergram/sec. On-board data storage is 3.2 Terabytes. Quick-look images will be sent in near real time to the ground via the TDRSS communications link. Title: Photospheric Vertical Current Density and Overlying Atmospheric Activity in an Emerging Flux Region Authors: Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N.; Schmieder, B. Bibcode: 2002AAS...200.2004G Altcode: 2002BAAS...34..673G Using high-resolution vector magnetograms obtained by the balloon-borne Flare Genesis Experiment (FGE), we construct maps of the vertical current density in the emerging flux region NOAA 8844. The vertical current density has been decomposed into components that are field-aligned and perpendicular to the magnetic field, thus allowing a straightforward identification of force-free areas, as well as of areas where the force-free approximation breaks down. Small-scale chromospheric activity, such as H α Ellerman bombs and Ultraviolet bright points in 1600 Åshow a remarkable correlation with areas of strong current density. Simultaneous data of overlying coronal loops, observed by TRACE in the Extreme Ultraviolet (171 Åand 195 Å), have been carefully co-aligned with the FGE photospheric maps. We find that the footpoints of the TRACE loops always coincide with strong vertical currents and enhancements of the current helicity density. We also investigate whether the force-free approximation is valid on the photosphere during various evolutionary stages of the active region. Title: Sunspot Formation from Emerging Flux Ropes - Observations from Flare Genesis Authors: Rust, D. M.; Bernasconi, P. N.; Georgoulis, M. K.; LaBonte, B. J.; Schmieder, B. Bibcode: 2001AGUSM..SP42A09R Altcode: From January 10 to 27, 2000, the Flare Genesis payload observed the Sun while suspended from a balloon in the stratosphere above Antarctica. The goal of the mission was to acquire a long time series of high-resolution images and vector magnetograms of the solar photosphere and chromosphere. We obtained images, magnetograms and Dopplergrams in the magnetically sensitive Ca I line at 6122 Angstroms. Additional simultaneous images were obtained in the wing of H-alpha. On January 25, 2000, we observed in NOAA region 8844 at N05 W30. The rapid development of a sunspot group that apparently included a delta spot (two polarities within one umbra). We considered a variety of models for interpreting these observations, including a twisted flux tube, a bipole that annihilates, a bipole that submerges, and a field distorted by mass loading. From the vector magnetograms and Doppler measurements, we conclude that nearly horizontal flux ropes are swept into the developing spot where they tilt upward to contribute to the familiar nearly vertical sunspot fields. The largest flux rope exhibited a twisted structure, and its angle with respect to the vertical was so great that it could be mistaken for a positive magnetic field merging into a negative sunspot. Flare Genesis was supported by NASA grant NAG5-8331 and by NSF grant OPP-9909167. Title: Ellerman Bombs in a Solar Active Region: Statistical Properties and Implications Authors: Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2001AGUSM..SP52B05G Altcode: We have embedded the concept of Self-Organized Criticality (SOC) in deterministic Cellular Automata (CA) models in an attempt to simulate the emergence of flaring and sub-flaring activity in solar active regions. SOC CA models reproduce reasonably well several aspects of the statistical properties of flares and, moreover, they allow predictions regarding the respective properties of the unresolved nanoflares. We compare the above-mentioned predictions with observed arcsecond and sub-arcsecond activity on the low-chromosphere, in a newly formed active region. The source of the observations is the Flare Genesis Experiment (FGE) which has provided us with high-resolution maps of the magnetic field and the velocity field vectors on the photospheric boundary, as well as Hα filtergrams on the low-chromosphere. Moreover, UV and EUV data from TRACE are used for determining the activity on the overlying atmospheric layers. We present preliminary results on the statistical properties of transient Hα brightenings (Ellerman Bombs) which correlate well with significant overlying UV emission. Implications of these results, as well as potential directions for modeling the low-lying activity in the solar atmosphere are discussed. This work was sponsored by NASA grant NAG5-8331 and NSF grant OPP-9909162 Title: Peculiar Moving Magnetic Features Observed With the Flare Genesis Experiment Authors: Bernasconi, P. N.; Rust, D. M.; Georgoulis, M. K.; LaBonte, B. J.; Schmieder, B. Bibcode: 2001AGUSM..SP51A02B Altcode: With the Flare Genesis Experiment (FGE), a balloon-borne 80-cm solar telescope, we observed the active region NOAA 8844 on January 25, 2000 for several hours. FGE was equipped with a vector polarimeter and a lithium-niobate Fabry-Perot narrow-band filter. It recorded time series of filtergrams, vector magnetograms, and dopplergrams at the CaI 6122.2 Angstroms line, as well as Hα filtergrams, with a cadence between 2.5 and 7.5 minutes. At the time of the observations NOAA 8844 was located at approximately 5 deg N, 30 deg W. It was a new flux emergence that first appeared on the solar disk two days before and was still showing a very dynamic behavior. Its two main polarity parts were rapidly moving away from each other and new magnetic flux was constantly emerging from its center. Here we describe the structure and behavior of peculiar small moving magnetic dipoles (called moving magnetic features MMF's) that we observed near the trailing negative polarity sunspot of NOAA 8844. Presentations by D. M. Rust, and by M. K. Georgoulis at this meeting will focus on other aspects of the same active region. The MMF's took the form of small dipoles that first emerged into the photosphere near the center of a supergranular cell located next to the main trailing flux concentration. They rapidly migrated towards the spot, following the supergranular flow. The two polarities of the little dipoles did not separate; they moved together with same speed and in the same direction. The dipoles were oriented parallel to their motion toward the negative spot, with the positive polarity always leading. MMF's usually move away from sunspots, and their orientation is the reverse of what we see here. In addition, we noted that the dipole structure was not symmetric. The field lines of the trailing part of the MMF's (negative polarity) were always much more perpendicular to the local horizontal than the ones of the leading part. The trailing part looked more compact and circular, while the leading part was more elongated in the direction of the motion. We conclude that we observed a new type of MMF's with a totally different magnetic structure than previously seen. We present a possible model that could explain their unusual structure and behavior. This work was supported by NASA grant NAG5-8331 and NSF grant OPP-9909167. Title: A comparison between statistical properties of solar X-ray flares and avalanche predictions in cellular automata statistical flare models Authors: Georgoulis, M. K.; Vilmer, N.; Crosby, N. B. Bibcode: 2001A&A...367..326G Altcode: We perform a tentative comparison between the statistical properties of cellular automata statistical flare models including a highly variable non-linear external driver and the respective properties of the WATCH flare data base, constructed during the maximum of solar cycle 21. The model is based on the concept of Self-Organized Criticality (SOC). The frequency distributions built on the measured X-ray flare parameters show the following characteristics: (1) The measured parameters (total counts, peak count-rates and, to a lesser extent, total duration) are found to be correlated to each other. Overall distribution functions of the first two parameters are robust power laws extending over several decades. The total duration distribution function is represented by either two power laws or a power law with an exponential roll-over. (2) By sub-grouping the peak count-rate and the total counts as functions of duration and constructing frequency distributions on these sub-groups, it is found that the slope systematically decreases with increasing duration. (3) No correlation is found between the elapsed time interval between successive bursts arising from the same active region and the peak intensity of the flare. Despite the inherent weaknesses of the SOC models to simulate realistically a number of physical processes thought to be at work in solar active regions and in flares' energy release, we show that the model is able to reproduce the bulk of the above statistical properties. We thus underline two main conclusions: (i) A global, statistical approach for the study of rapid energy dissipation and magnetic field line annihilation in complex, magnetized plasmas may be of equal importance with the localized, small-scale Magnetohydrodynamic (MHD) simulations, and (ii) refined SOC models are needed to establish a more physical connection between the cellular automata evolution rules and the observations. Title: Microscale Structures on the Quiet Sun and Coronal Heating Authors: Aletti, V.; Velli, M.; Bocchialini, K.; Einaudi, G.; Georgoulis, M.; Vial, J. -C. Bibcode: 2000ApJ...544..550A Altcode: We present some results concerning transient brightenings on the quiet Sun, based on data from the Extreme-Ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory. Histograms of intensity are found to be well fitted by χ2 distributions for small values of the intensity, while at high intensities power-law distributions are always observed. Also, the emission presents the same statistical properties when the resolution is downgraded by local averaging; i.e., it appears to be self-similar down to the resolution scale of the instruments. These properties are characteristic of the emission from a forced turbulent system whose dissipation scale is much smaller than the pixel dimension. On the basis of the data presented as well as other published results and our present theoretical understanding of MHD turbulence, we discuss the realism of the nanoflare scenario of coronal heating. Title: Χωροχρονική εξέλιξη σύνθετων κέντρων δράσης - μηχανισμοί έκλυσης ενέργειας στο ηλιακό στέμμα Title: Χωροχρονική εξέλιξη σύνθετων κέντρων δράσης - μηχανισμοί έκλυσης ενέργειας στο ηλιακό στέμμα Title: Spatiotemporal evolution of complex active regions - mechanisms of energy release in the solar corona; Authors: Georgoulis, Manolis K. Bibcode: 2000PhDT.......275G Altcode: No abstract at ADS Title: A Comparison between the WATCH Flare Data Statistical Properties and Predictions of the Statistical Flare Model Authors: Crosby, N.; Georgoulis, M.; Vilmer, N. Bibcode: 1999ESASP.446..247C Altcode: 1999soho....8..247C Solar burst observations in the deka-keV energy range originating from the WATCH experiment aboard the GRANAT spacecraft were used to perform frequency distributions built on measured X-ray flare parameters (Crosby et al., 1998). The results of the study show that: 1- the overall distribution functions are robust power laws extending over a number of decades. The typical parameters of events (total counts, peak count rates, duration) are all correlated to each other. 2- the overall distribution functions are the convolution of significantly different distribution functions built on parts of the whole data set filtered by the event duration. These "partial" frequency distributions are still power law distributions over several decades, with a slope systematically decreasing with increasing duration. 3- No correlation is found between the elapsed time interval between successive bursts arising from the same active region and the peak intensity of the flare. In this paper, we attempt a tentative comparison between the statistical properties of the self-organized critical (SOC) cellular automaton statistical flare models (see e.g. Lu and Hamilton (1991), Georgoulis and Vlahos (1996, 1998)) and the respective properties of the WATCH flare data. Despite the inherent weaknesses of the SOC models to simulate a number of physical processes in the active region, it is found that most of the observed statistical properties can be reproduced using the SOC models, including the various frequency distributions and scatter plots. We finally conclude that, even if SOC models must be refined to improve the physical links to MHD approaches, they nevertheless represent a good approach to describe the properties of rapid energy dissipation and magnetic field annihilation in complex and magnetized plasmas. Crosby N., Vilmer N., Lund N. and Sunyaev R., A&A; 334; 299-313; 1998 Crosby N., Lund N., Vilmer N. and Sunyaev R.; A&A Supplement Series; 130, 233, 1998 Georgoulis M. and Vlahos L., 1996, Astrophy. J. Letters, 469, L135 Georgoulis M. and Vlahos L., 1998, in preparation Lu E.T. and Hamilton R.J., 1991, Astroph. J., 380, L89 Title: Derivation of Solar Flare Cellular Automata Models from a Subset of the Magnetohydrodynamic Equations Authors: Vassiliadis, D.; Anastasiadis, A.; Georgoulis, M.; Vlahos, L. Bibcode: 1998ApJ...509L..53V Altcode: Cellular automata (CA) models account for the power-law distributions found for solar flare hard X-ray observations, but their physics has been unclear. We examine four of these models and show that their criteria and magnetic field distribution rules can be derived by discretizing the MHD diffusion equation as obtained from a simplified Ohm's law. Identifying the discrete MHD with the CA models leads to an expression for the resistivity as a function of the current on the flux tube boundary, as may be expected from current-driven instabilities. Anisotropic CA models correspond to a nonlinear resistivity η(J), while isotropic ones are associated with hyperresistivity η(▽2J). The discrete equations satisfy the necessary conditions for self-organized criticality (Lu): there is local conservation of a field (magnetic flux), while the nonlinear resistivity provides a rapid dissipation and relaxation mechanism. The approach justifies many features of the CA models that were originally based on intuition. Title: Variability of the occurrence frequency of solar flares and the statistical flare Authors: Georgoulis, Manolis K.; Vlahos, Loukas Bibcode: 1998A&A...336..721G Altcode: Self-Organised Criticality (SOC), embedded in cellular automata models, has been so far viewed as an attractive phenomenological approach for studying the statistical behaviour of flaring activity in solar active regions. Well-known statistical properties of flares, like the robust scaling laws seen in the distribution functions of characteristic parameters of the events, as well as correlations linking those parameters, are successfully reproduced by SOC models. Recent observations, however, challenge the flexibility of SOC, as they reveal a variation of the flaring power-law indices over short-time activity periods. The initial SOC models, based on a small-amplitude, constant external driver and isotropic instability criteria, appear inefficient to predict variable power-law indices. In this paper we introduce a SOC-type numerical model, with a number of modifications of the original SOC concept. We show that scaling laws and correlations between the events' characteristic parameters survive under the action of a highly variable driver. A variable driver initiates a variability in the resulting power-law indices. We reproduce qualitatively and quantitatively the statistics of flaring activity during the 154-day periodicity. Moreover, small-scale, anisotropic instability criteria imply the existence of a soft population of events, with statistical properties analogous to those attributed to the hypothetical nanoflares. We show that numerous small-scale events could be the dominant energy release mechanism in cases of quiescent coronal activity. Title: Statistical Properties of Magnetic Activity in the Solar Corona Authors: Georgoulis, Manolis K.; Velli, Marco; Einaudi, Giorgio Bibcode: 1998ApJ...497..957G Altcode: The long-time statistical behavior of a two-dimensional section of a coronal loop subject to random magnetic forcing is presented. The highly intermittent nature of dissipation is revealed by means of magnetohydrodynamic (MHD) turbulence numerical simulations. Even with a moderate magnetic Reynolds number, intermittency is clearly present in both space and time. The response of the loop to the random forcing, as described either by the time series of the average and maximum energy dissipation or by its spatial distribution at a given time, displays a Gaussian noise component that may be subtracted to define discrete dissipative events. Distribution functions of both maximum and average current dissipation, for the total energy content, the peak activity, and the duration of such events are all shown to display robust scaling laws, with scaling indices δ that vary from δ ~= -1.3 to δ ~= -2.8 for the temporal distribution functions, while δ ~= -2.6 for the overall spatial distribution of dissipative events. Title: Microflare and nanoflare heating of active region loops. Authors: Walsh, R. W.; Georgoulis, M. Bibcode: 1998joso.proc..105W Altcode: In this work, the authors investigate the response of solar plasma contained within a coronal loop structure to the random deposition of many localised heating events on the scale of microflares and nanoflares. A statistical flaring approach based on self-organised criticality is used to generate the energy release which varies in both space and time. It is shown that for a 20 Mm active region loop although the average temperature over the duration of the simulation remains at typical coronal temperatures, the plasma can flare to over 6×106K at certain stages in its evolution. Title: Electron Acceleration by Random DC Electric Fields Authors: Anastasiadis, Anastasios; Vlahos, Loukas; Georgoulis, Manolis K. Bibcode: 1997ApJ...489..367A Altcode: We present a global model for the acceleration of electrons in the framework of the statistical flare model of Vlahos et al. In this model, solar flares are the result of an internal self-organized critical (SOC) process in a complex, evolving, and highly inhomogeneous active region. The acceleration of electrons is due to localized DC electric fields closely related to the energy-release process in the active region. Our numerical results for the kinetic energy distribution of accelerated electrons show a power-law or an exponential-law behavior, depending on the maximum trapping time of the energetic particles inside the acceleration volume. Title: MHD Turbulence and Statistics of Energy Release in the Solar Corona Authors: Georgoulis, M.; Velli, M.; Einaudi, G. Bibcode: 1997ESASP.404..401G Altcode: 1997cswn.conf..401G No abstract at ADS Title: Variability of the Occurrence Frequency of Solar Flares and the Statistical Flare Authors: Georgoulis, M. K.; Vlahos, L. Bibcode: 1997jena.confE..39G Altcode: Self-Organized Criticality (SOC) embedded in cellular automata models has been so far acknowledged as an adequate qualitative way of studying the statistical behaviour of flaring activity in solar active regions. These models are able of producing robust power laws featuring the frequency distributions of the events obtained, which are closely consistent with observations of flares, as well as much steeper power-law cut-offs, which may indicate the existence of the presently unobserved nanoflares. SOC models are based on the substantial concept that active regions are driven dissipative nonlinear dynamical systems. The role of the external driver is attributed to the feedback of magnetic flux that is injected to the system through the photospheric boundary and to the random shuffling of the footpoints of coronal loops taking place on the upper photosphere. In previous numerical studies, the driver used was an infinitesimal perturbation acting on localized magnetic topologies due to which avalanche-type instabilities were triggered. Furthermore, the resulting power-law indices were unique. Recent observations, however, have shown that the scaling indices of flares' frequency distributions are not kept constant during certain phases of the solar activity, such as the 154-day periodicity. To tackle this problem we investigate the role of the driver used, by introducing a highly variable driving mechanism. We show that the variability of the driver induces a respective variability in the resulting power-law indices. The variability of the indices can be well represented by a linear dependence between them and the driver's scaling index for both "nanoflaring" and "flaring" activity. Furthermore, a first attempt of connecting the driver with certain statistical properties of active regions is introduced. The results stand closely in favor of the observations of flaring activity during the 154-day periodicity. Title: Statistical Properties of Magnetic Activity in the Solar Corona Authors: Georgoulis, M. K.; Einaudi, G.; Velli, M. Bibcode: 1997jena.confE..38G Altcode: A long-time statistical analysis of a two-dimensional section of a coronal loop has been carried out. The highly intermittent nature of the spatiotemporal evolution of the system has been revealed by means of Magnetohydrodynamic (MHD) Turbulence numerical simulations. Albeit the moderate magnetic Reynolds number, intermittency is strikingly present both in space and in time. This type of behaviour might physically motivate statistical theories to describe the long-term evolution of a turbulent corona, provided that such an environment is a driven dissipative nonlinear dynamical system. The coronal loop is driven by a random spatiotemporal magnetic forcing, which induces a noise component in the resulting timeseries. If this component is properly subtracted, the obtained spatiotemporal evolution can be statistically described in terms of robust scaling laws, occurring in the distribution functions of both maximum and average current dissipation for the total energy content, the peak activity and the duration of the events obtained. Adopting low-beta and large-aspect-ratio conditions for the coronal loop, we emphasize that, higher spatial resolution could well give rise both to localized equipartition, and to the emergence of super-Dreicer electric fields built-up in the vicinity of strong, intense current sheets. Title: Coronal Heating by Nanoflares and the Variability of the Occurence Frequency in Solar Flares Authors: Georgoulis, Manolis K.; Vlahos, Loukas Bibcode: 1996ApJ...469L.135G Altcode: It has been proposed that flares in the solar corona may well be a result of an internal self-organized critical (SOC) process in active regions. We have developed a cellular automaton SOC model that simulates flaring activity extending over an active sub-flaring background. In the resulting frequency distributions we obtain two distinct power laws. That of the weaker events is shorter and much steeper (power law with index ~=-3.26) than that of the intermediate and large events (power law with index ~=-1.73). The flatter power law is in close agreement with observations of flares. Weaker events are responsible for ~=90% of the total magnetic energy released, indicating a possible connection of nanoflares with coronal heating. Moreover, certain mechanisms cause the variability of the resulting indices and may provide answers to the problem of the variability of flares' occurrence frequency during the solar cycle. Title: Are Flares the Result of a self-organisation Process in active Regions? Authors: Georgoulis, M. K.; Vlahos, L.; Kluiving, R. Bibcode: 1996hell.conf...52G Altcode: No abstract at ADS Title: The statistical flare. Authors: Vlahos, L.; Georgoulis, M.; Kluiving, R.; Paschos, P. Bibcode: 1995A&A...299..897V Altcode: Solar and stellar flares are interpreted so far as an instability of a large scale magnetic neutral sheet. In this article, however, we assume that the active region is highly inhomogeneous: a large number of magnetic loops are simultaneously present interacting and randomly forming discontinuities in many independent points in space. These magnetic discontinuities release energy and force weaker discontinuities in their neighbourhood to release energy as well. This complex dynamical system releases constantly energy in the form of small and large scale explosions. Clustering of many discontinuities in the same area has the effect of larger scale explosions (flares). This type of flare with spatiotemporal fragmentation and clustering in small and large scale structures will be called here the statistical flare. The statistical flare is simulated using avalanche models originally introduced by Bak et al. (1988). Avalanche models applied so far to solar flares (Lu & Hamilton 1991) were isotropic (the field was distributed equally to the closest neighbours of an unstable point). These models simulate relatively large events (microflares and flares). Here we introduce a more refined isotropic avalanche model as well as an anisotropic avalanche model (energy is distributed only among the unstable point and those neighbours that develop gradients higher than a critical value). The anisotropic model simulates better the smaller events (nanoflares): in contrast to the well-known results of the isotropic model (a power law with index ~-1.8 in the peak-luminosity distribution), the anisotropic model produces a much steeper power law with index ~-3.5. Finally, we introduce a mixed model (a combination of isotropic and anisotropic models) which gives rise to two distinct power-law regions in the peak-luminosity distribution, one with index ~-3.5 accounting for the small events, and one with index ~-1.8 accounting for large events. This last model therefore explains coronal heating as well as flaring. The three models introduced in this paper show length-scale invariant behaviour. Model-dependent memory effects are detected in the peak-luminosity time series produced by these models.