Session 2 - Precursors and signatures of eruptive events

Conveners: Barbara Thompson (GSFC), James Mason (GSFC)
Monday 29/10, 11:00 - 17:00
Tuesday 30/10, 09:00 - 10:30

There are a variety of diagnostics used to study the energy storage/release and particle acceleration/transportation in solar eruptions. SDO's broad coverage in the spatiotemporal and thermal domains facilitates detailed case studies (using complementary data from other observatories) and ensemble studies of solar flares and eruptive events. Relevant topics include: 1) Distinguishing active regions as sites of eruptive vs. non-eruptive events, 2) Flux rope and prominence destabilization, 2) SEE energetics and particle acceleration, 3) Coronal dimmings as CME proxy, 4) EUV spectral irradiance evolution during flares.

Plenary speaker: Jiong Qiu (Montana State University)

Talks

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Monday October 29, 09:30 - 10:30

09:30	The Atmospheric Imaging Assembly: Science Highlights, Analysis Techniques and Instrument Status	Cheung, M	Oral
	Mark CM Cheung[1,2], Wei Liu [1,3] & the AIA Team
	[1] Lockheed Martin Solar & Astrophysics Laboratory, [2] Stanford University, [3] Bay Area Environmental Research Institute
	We present an update on the Atmospheric Imaging Assembly (AIA) science investigation. We discuss a selection of science highlights made using AIA data during the first extended mission of the Solar Dynamics Observatory (SDO). We give a primer on the latest analysis techniques (including machine learning methods) developed to extract science from AIA. We give examples of how AIA provides crucial, complementary information to other missions in the Heliophysics System Observatory (HSO) and to ground-based observatories. We discuss the health of the instrument, including degradation trends.
10:00	Signatures of Magnetic Flux Ropes: What Are They and What Do They Do Before Eruption?	Qiu, J	Invited Oral
	Jiong Qiu
	Montana State University
	With its unprecedented capabilities, the Solar Dynamics Observatory has brought into our view a full range of dynamics in the Sun's atmosphere. For the first time, we have been able to monitor, continuously and in great details, evolution of the magnetic field and plasma properties prior to Coronal Mass Ejections, the most spectacular form of energy release in the heliosphere. Some CMEs, when finally reaching 1 AU and measured by satellites near Earth, are found to be magnetic flux ropes. We still do not know the exact origin of these structures. Are all CMEs flux ropes at their birth? How do they form on the Sun? What are the plasma signatures and magnetic properties of flux ropes in the corona prior to their eruption? What is the magnetic environment that leads to formation and eruption of a flux rope? How are CMEs and their magnetic configuration related to other energetic events, e.g. flares and energetic particles? This talk will review some significant progress in the SDO era to help answer these questions using observations as well as advanced MHD models.

Monday October 29, 11:00 - 12:00

11:00	Which factors of an active region determine whether a strong flare will be CME associated or not?	Veronig, A	Oral
	Christian Baumgartner, Julia K. Thalmann, Astrid M. Veronig
	Institute of Physics & Kanzelhohe Observatory, University of Graz, Austria
	We study how the magnetic field determines whether a strong flare launched from an active region (AR) will be eruptive or confined, i.e. associated with a coronal mass ejection (CME) or not. To this aim, we selected all large flares that were observed by the SDO HMI and AIA instruments during the period 2011 to 2015 within $50^\circ$ from the disk center. In total, our data set comprises 44 flares of GOES class >=M5.0. Out of these, 12 events were confined (7 M and 5 X-flares) and 32 were eruptive (18 M- and 14 X-flares). We used 3D potential magnetic field models to study their location within the host AR (using the flare distance from the flux-weighted AR center, $d_{FC}$) and the strength of the overlying coronal field (via decay index $n$). We also present a first systematic study of the orientation of the coronal magnetic field changing with height, using the orientation $\varphi$ of the flare-relevant polarity inversion line as a measure. We analyzed all quantities with respect to the size of the underlying active-region dipole field, defined by the distance between the flux-weighted opposite-polarity centers, $d_{PC}$. We find that flares originating from the periphery of an AR dipole field ($d_{FC} / d_{PC} > 0.5$) are predominantly eruptive. Flares originating from underneath the AR dipole field ($d_{FC} / d_{PC} < 0.5$) tend to be eruptive when they are launched from a compact AR and confined when launched from an extended AR ($d_{PC} > 60$ Mm). In confined events, the flare-relevant field adjusts its orientation quickly to that of the underlying dipole field with height ($\Delta \varphi > 40^\circ$ between the surface and the apex of the active-region dipole field), in contrast to eruptive events where it changes more slowly. The critical height for torus instability discriminates best between confined ($h_{crit} > 40$ Mm) and eruptive flares ($h_{crit} < 40$ Mm). It discriminates better than $\Delta \varphi$, implying that the decay of the confining field plays a stronger role in the eruptive/confined character of a flare than its orientation at different heights.
11:15	The Origin of Major Solar Activity - Collisional Shearing Between Nonconjugated Polarities of Different Bipoles Nested Within Active Regions	Chintzoglou, G	Oral
	Georgios Chintzoglou[1][2], Jie Zhang[3], Mark C.M. Cheung[1], Maria Kazachenko[4]
	[1]Lockheed Martin Solar & Astrophysics Lab, [2]University Corporation for Atmospheric Research, [3]George Mason University, [4]University of Colorado, Boulder
	Active Regions (ARs) that exhibit compact Polarity Inversion Lines (PILs) are known to be very flare-productive. However, the physical mechanisms behind this statistical inference have not been demonstrated conclusively. We show that such PILs can occur due to the collision between two emerging flux tubes nested within the same AR. In such multipolar ARs, the flux tubes may emerge simultaneously or sequentially, each initially producing a bipolar magnetic region (BMR) at the surface. During each flux tube’s emergence phase, the magnetic polarities can migrate such that opposite polarities belonging to different BMRs collide, resulting in shearing and cancellation of magnetic flux. We name this process “collisional shearing” to emphasize that the shearing and flux cancellation develops due to the collision. Collisional shearing is a process different from the known concept of flux cancellation occurring between polarities of a single bipole, a process that has been commonly used in many numerical models. High spatial and temporal resolution observations from the Solar Dynamics Observatory for two emerging ARs, AR11158 and AR12017, show the continuous cancellation of up to 25% of the unsigned magnetic flux of the smallest BMR, which occurs at the collisional PIL for as long as the collision persists. The flux cancellation is accompanied by a succession of solar flares and CMEs, products of magnetic reconnection along the collisional PIL. Our results suggest that the quantification of magnetic cancellation driven by collisional shearing needs to be taken into consideration in order to improve the prediction of solar energetic events and space weather.
11:30	Flare reconnection driven magnetic field and Lorentz force variations at the Sun’s surface	Barczynski, K	Oral
	Krzysztof Barczynski [1], Guillaume Aulanier [1], Sophie Masson [1], Michael S. Wheatland [2]
	[1] LESIA, Observatoire de Paris, Universit\'e PSL , CNRS, Sorbonne Universit\'e, Universit\'e Paris-Diderot, 5 place Jules Janssen, 92190 Meudon, France; [2] Sydney Institute for Astronomy, School of Physics A28, NSW 2006, Australia
	We show that the simulation is fully consistent with the observed increase of the photospheric horizontal magnetic field and electric currents around flaring PILs. The simulation also finds that the surface integral coming from the volume integral of the Maxwell stress tensor, as usually used in observational data analysis as the proxy of the Lorentz force, shows an increased downard component in the photosphere, as observed. But we also find that this proxy is significantly different from the true Lorentz force, which does not reveal this downward component. This result questions every previous interpretation based on the implosion conjecture and momentum conservation. However based on the analysis of the induction equation in the simulation, we unveil that the increase of the horizontal magnetic filed around active region PILs during eruptions is solely and exclusively result of the flare reconnection-driven contraction of flare loops.
11:45	Spatial and temporal localization of enhanced chromospheric 3-minute oscillations before, during, and after the 2011-February-15 X2.2 flare	Farris, L	Oral
	Laurel Farris, James McAteer
	New Mexico State University, New Mexico State University
	The origin of the 3-minute oscillations of the chromosphere has been attributed to both slow magnetoacoustic waves %with frequencies higher than the acoustic cutoff propagating from the photosphere, and to oscillations generated within the chromosphere itself at its natural frequency as a response to a disturbance. Here we present an investigation of the spatial and temporal behavior of the chromospheric 3-minute oscillations in NOAA AR 11158 before, during, and after the SOL2011-02-15T01:56 X2.2 flare. Ultraviolet emission at 1600 and 1700 Angstroms obtained at 24-second cadence from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) was used to create power maps as functions of both space and time. A Fourier transform was applied to the intensity signal from individual pixels starting at each observation time over time segments 64 frames (25.6 minutes) in length. We detect an increase in the 3-minute power during the X-class flare, as well as during other smaller events before and after the flare. The enhancement is concentrated in small areas, supporting the injection of energy by nonthermal particles. The potential correlation between 3-minute power and magnetic field strength is discussed, along with formation height dependencies.

Monday October 29, 13:15 - 15:00

13:15	Solar flare distributions: lognormal instead of power law?	Verbeeck, C	Oral
	Cis Verbeeck [1], Emil Kraaikamp [2], Olena Podladchikova [3]
	[1] Royal Observatory of Belgium, [2] Royal Observatory of Belgium, [3] Royal Observatory of Belgium
	In many statistical studies of solar flares, a power law is claimed (and an exponent derived) on the basis of fitting a linear model to a log-log histogram. It is well-known that this approach is statistically unstable, and very large statistics are needed to produce reliable exponents. This instability may explain part of the observed divergence in power law exponents in various studies. Moreover, the question is seldom addressed to which extent the data really do support power law behavior. We perform a comprehensive study of 8,274 solar flares detected in Atmospheric Imaging Assembly (AIA) 9.4 nm images by the Solar Demon flare detection software, between 2010 May 13 and 2018 March 16. Since Solar Demon registers detailed spatial information on the flares, this data set has two advantages over several other data sets: (1) the solar background is negligible since the flare parameters pertain only to the flaring pixels, and (2) simultaneous flares in different areas on the Sun are correctly detected as separate flares instead of being counted as one event. The peak flare intensities in the data set correspond roughly to Geostationary Operational Environmental Satellite (GOES) B5 level and above. We apply robust Maximum Likelihood Estimation statistics to the peak and integrated flare intensity distributions, and find clear indications that the data are not well-described by a power law, but are well-described by a lognormal distribution. The behavior of flare distributions has important implications for large-scale science questions such as coronal heating and the nature of solar flares. The apparent lognormal character of flare distributions in our data set suggests that the assumed power law nature of flares and its consequences need to be re-examined with great care.
13:30	Broken Self-Organized-Criticality Scenario for Solar Flares Energy Release Observed by SDO/AIA	Podladchikova, E	Oral
	Olena Podladchikova, Cis Verbeeck, Emil Kraaikamp
	Royal Observatory of Belgium
	The most powerful explosions in the solar system, solar flares, have been believed to occur independently of each other, which implies a Gaussian distribution of flare parameters. An intriguing experimental fact here is that the frequency distributions of solar flare energy, peak rate and several other characteristics were found to exhibit power-law dependencies in a quite large range of intensities (e.g. Lin et al. 1984, Berghmans et al, 1989, Biesecker et al. 1994). Those power-laws suggested (instead of independence) a scale-invariance between flares, which is also compatible with Parker's hypothesis that large flares could consist of a multitude of nanoflares, i.e. a cooperation between flares according to Self-Organized Criticality (SOC) regime showing properties identical to thermodynamical systems undergoing second order phase transition. The most recent statistical analysis of 8274 spatially resolved solar flares detected by the detection software Solar Demon from SDO/AIA 9.4 nm images (not hampered by a solar background contribution) unambiguously demonstrates log-normal distributions for the peak flare intensity and the integrated flare intensity in the AIA 9.4 nm passband (Verbeeck et al. 2018). To explain this fact we developed a new model of solar flares energy release, allowing a clear understanding of involved physical processes, based on the dissipation of current sheets and magnetic energy injection by photospheric motions of turbulent nature at a wide range of scales. Our simulations uncovered a "broken self-organised criticality regime" of current sheets dissipation in solar corona, which logicaly leads to log-normal distributions of flare parameters discovered by SDO. The finite size of the current dissipation propagation (an avalanche) is the key-factor that explains the resulting log-normal distribution of flare parameters, while even some probabaility of infinite size of an avalanche leads to power law behaviour of the system. Such a "broken scenario of self-organization" in currents dissipation has been observed already during reconnection processes in tokamak plasma as well as in collective behavior of earthquakes and other critical phenomena in nature.
13:45	Post-Flare Loop Signatures	West, M	Oral
	Matthew West[1], Erika Palmerio[2], Daniel Seaton[3], Sabrina Savage[4]
	[1]Royal Observatory of Belgium, [2]University of Helsinki, [3]NOAA/CIRES, [4]Marshall Space Flight Center
	Recent observations from the SWAP EUV imager onboard PROBA2, AIA on SDO and SXI X-ray observations from the GOES satellite have shown that post-flare giant arches and regular post-flare loops are one and the same thing, with the former being a sub-set of post-flare loops that are able to maintain their growth to great heights over longer periods. However, it is still not clear how certain loop systems are able to sustain this growth to heights that can exceed half a solar radii (> 400000 km). In this presentation we further explore the energy deposition rate in post-flare loop systems, combined with an epoch analysis of the irradiance light curves generated by the initial flare and subsequent decline in emission generated from the post-flare loop systems.
14:00	An Observationally Constrained Model of a Flux Rope that Formed in the Solar Corona	James, A	Oral
	Alexander W James[1], Gherardo Valori[1], Lucie M Green[1], Yang Liu[2], Mark C M Cheung[3], Yang Guo[4], Lidia van Driel-Gesztelyi[1][5][6]
	[1]UCL Mullard Space Science Laboratory, [2]Stanford University, [3]Lockheed Martin Solar and Astrophysics Laboratory, [4]Nanjing University, [5]Observatoire de Paris, [6]Konkoly Observatory
	Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the coronae of stars, and it is important to study the plasma processes involved in their initiation. This first requires us to understand the pre-eruptive configuration of CMEs. To this end, we used extreme-ultraviolet (EUV) observations from SDO/AIA to conclude that a magnetic flux rope formed high-up in the solar corona above NOAA Active Region 11504 before it erupted on 2012 June 14. Then, we used data from SDO/HMI and our knowledge of the EUV observations to model the coronal magnetic field of the active region one hour prior to eruption using a nonlinear force-free field extrapolation. The extrapolation revealed a flux rope that matches the EUV observations remarkably well, with its axis 120 Mm above the photosphere. The erupting structure was not observed to kink, but the decay index near the apex of the axis of the extrapolated flux rope is comparable to typical critical values required for the onset of the torus instability. Therefore, we suggest that the torus instability drove the eruption of the flux rope.
14:15	Initiation of Stealth CMEs: Clues from Numerical Modelling and In-Situ Comparisons	Talpeanu, D	Oral
	Dana-Camelia Talpeanu [1,2], Francesco P. Zuccarello [1], Emmanuel Chané [1], Stefaan Poedts [1], Elke D'Huys [2], Skralan Hosteaux [1], Marilena Mierla [2,3]
	[1] Departments of Mathematics, KU Leuven, 3001 Heverlee, Belgium, [2] SIDC, Royal Observatory of Belgium, Brussels, Belgium, [3] Institute of Geodynamics of the Romanian Academy, Bucharest, Romania
	Coronal Mass Ejections (CMEs) are huge expulsions of magnetized plasma from the Sun into the interplanetary medium. Stealth CMEs form a particular subset of CMEs that despite being clearly distinguished in coronagraph observations, are not associated with clear eruptive signatures close to the Sun, such as solar flares, coronal dimmings, EUV waves, or post-flare loop arcades. Observational studies show that about 60% of stealth CMEs are preceded by another CME whose solar origin could be identified. The triggering mechanisms for stealth CMEs are still not well understood processes and in order to determine them, we are using the MPI-AMRVAC code developed at KU Leuven. We simulate consecutive CMEs ejected from the southernmost part of an initial configuration constituted by three magnetic arcades embedded in a globally bipolar magnetic field. A first eruption is driven through shearing motions at the solar surface and the stealth CME follows it after several hours. Both are expelled into a bimodal solar wind, varying its speed to match the CMEs arrival time at Earth. We analyze the parameters that contribute to the occurrence of the second CME and their value ranges whithin which the eruption happens. Furthermore, we compare the simulated signatures of the two consecutive CMEs with the in-situ data from ACE spacecraft at 1AU. The aim of this study is to better understand the triggering mechanism of stealth eruptions, leading to an improvement in forecasting of their geomagnetic impact.
14:30	The nature of imploding loops during solar eruptions as revealed by MHD simulations and AIA observations	Aulanier, G	Oral
	Guillaume Aulanier[1], Jaroslav Dudik[2], F.P. Zucarello[3], Pascal Demoulin[1], Brigitte Schmieder[1]
	[1] Observatoire de Paris, LESIA, PSL University, France, [2] Astronomical Institute of the Czech Academy of Sciences, Czech Republic, [3] formerly at KU Leuven, Belgium
	Over the last years AIA revealed the frequent occurence of contracting loops at the flanks of erupting active regions. Those have often been interpreted as an evidence of the implosion conjecture that relates magnetic energy decreases with volume contractions in the Sun's corona. So as to unveil the physical nature of these features we carried out observational analyses of two solar eruptions observed with AIA with different projection angles, which we coupled with new analyses of a generic zero-beta MHD simulation of an asymmetric eruption driven by the torus instability, that was not designed for this particular study. The simulation does display contracting loops in general. And the synthetic time-slices of the simulation, when rotated to the right projections, do match the observed ones. But in the simulation these inward motions are not due to any volume contraction. Instead they are associated with two large-scale quasi-incompressible coronal-vortices. Those develop at the flanks of the erupting flux ropes, as most of the compressive component of the flow is evacuated away by an Alfven wave in the early stages of the eruption. We argue that this behavior is merely a magnetic version of the usual pressure-driven formation of vortex rings in hydrodyanmics. This result implies that during a solar eruption, the free magnetic-energy from the pre-erupting active-region is converted not only in the flare and the CME, but is also ``lost'' in the generation of these two large-scale coronal vortices.

Monday October 29, 16:30 - 17:00

16:30	Association between Tornadoes and Instability of Hosting Prominences	Mghebrishvili, I	Oral
	Irakli Mghebrishvili[1], Teimuraz Zaqarashvili[2,1,3], Vasil Kukhianidze[1], David Kuridze[4,1,5], David Tsiklauri[6], Bidzina Shergelashvili[2,1,7], and Stefaan Poedts[8]
	[1]Abastumani Astrophysical Observatory at Ilia State University, Tbilisi, Georgia, [2]Space Research Institute, Austrian Academy of Sciences, Graz, Austria, [3]IGAM-Kanzelhöhe Observatory, Institute of Physics, University of Graz, Graz, Austria, [4]Institute of Mathematics, Physics and Computer Science, Aberystwyth University, UK, [5]Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast, Belfast, UK, [6]School of Physics and Astronomy, Queen Mary University of London, London, UK, [7]Combinatorial Optimization and Decision Support, Kortrijk, Belgium, [8]Center for Mathematical Plasma Astrophysics, Department of Mathematics, Leuven, Belgium
	We studied the dynamics of all prominence tornadoes detected by the Solar Dynamics Observatory/Atmospheric Imaging Assembly from 2011 January 01 to December 31. In total, 361 events were identified during the whole year, but only 166 tornadoes were traced until the end of their lifetime. Out of 166 tornadoes, 80 (48%) triggered CMEs in hosting prominences, 83 (50%) caused failed coronal mass ejections (CMEs) or strong internal motion in the prominences, and only 3 (2%) finished their lifetimes without any observed activity. Therefore, almost all prominence tornadoes lead to the destabilization of their hosting prominences and half of them trigger CMEs. Consequently, prominence tornadoes may be used as precursors for CMEs and hence for space weather predictions.
16:45	Study of the characteristics of Compact Interplanetary Radio Type IV Bursts	Talebpour sheshvan, N	Oral
	Nasrin Talebpour Sheshvan [1], Silja Pohjolainen [1,2]
	[1]Space Research Laboratory, University of Turku, Finland, [2]Tuorla Observatory, University of Turku, Finland
	Coronal mass ejections (CMEs) are a significant release of plasma and magnetic field from the Sun into the interplanetary (IP) medium. Solar flares and CMEs are particle accelerators and their process can be observed in radio emission. Solar radio bursts based on their features in the dynamic spectra and their emission mechanisms are classified into different types. The more rare radio type IV bursts have been associated with expanding and/or rising plasma structures like magnetic clouds. Therefore, their emission mechanism can be both synchrotron emission and plasma emission. Recently, we have analyzed interplanetary type IV radio bursts at decameter-hectometer (DH) wavelengths to find out their source origin and directivity, observed in a data set from 2011-2012. The events occurred when STEREO A, STEREO B, and Wind spacecraft were located approximately 90 degrees apart from each other, providing a 3D view of the Sun. Our studies include the data analysis of different eruptive phenomena using SDO observatory images of different wavelengths. We compared and listed the characteristics of white-light and extreme ultraviolet (EUV) observations of flares, EUV waves, and coronal mass ejections (CMEs) with the radio data. We figured out that strong and compact type IV radio bursts are observed when EUV waves propagate globally across the whole visible disk and are associated with halo-type, fast CMEs. The Potential Field Source Surface (PFSS) models provide us a good approximation of the structure and evolution of the magnetic field. The combined data and modeling give us a better understanding of the particle acceleration mechanisms. We suggest that one possibility for the directivity effects in these events may be absorption. The absorption - or suppression of emission - would be caused by the higher density plasma located at the radio type II burst shock fronts. We found that the type II bursts were most probably created by shock-streamer interactions as streamers present in each of the analyzed events. According to the SDO images the streamers were located at the flanks of the CMEs. Therefore, in this study, we discuss the different interpretations by using the confirmed conditions for the observed directivity.

Tuesday October 30, 09:00 - 10:30

09:00	Studying the dynamics of coronal dimmings and their relationship to flares and coronal mass ejections	Dissauer, K	Oral
	Karin Dissauer [1], Astrid M. Veronig [1,2], Manuela Temmer [1], Tatiana Podladchikova [3], Kamalam Vanninathan [1]
	[1] Institute of Physics, University of Graz, Austria, [2] Kanzelhoehe Observatory, University of Graz, Austria, [3] Skolkovo Institute of Science and Technology, Russia
	Coronal dimmings are observed as localized regions of reduced emission in the EUV and soft X-rays, interpreted as density depletions due to mass loss during the CME expansion. They contain crucial information on the evolution and early propagation phase of CMEs low in the corona. For a set of 62 dimming events, characteristic parameters, describing their dynamics, morphology, magnetic properties and the brightness evolution are derived, statistically analyzed and compared with basic flare and CME quantities. We use optimized multi-point observations, where the on-disk dimming evolution is studied in high-cadence SDO/AIA filtergrams and SDO/HMI line-of-sight magnetograms, while STEREO/EUVI, COR1 and COR2 data is used to measure the associated CME kinematics close to the limb with low projection effects. For 60% of the events we identified core dimmings, i.e. potential footpoints of the erupting CME structure. These regions contain 20% of the magnetic flux covering only 5% of the total dimming area. The majority of the total dimming area consists of secondary dimmings mapping overlying fields that are stretched during the eruption and closed down by magnetic reconnection, thus adding flux to the erupting structure via magnetic reconnection. This interpretation is supported by the high correlation between the magnetic fluxes of secondary dimmings and flare reconnection fluxes ($c=0.63\pm0.08$), the balance between positive and negative magnetic fluxes ($c=0.83\pm0.04$) within the total dimmings and the fact that for strong flares (>M1.0) the flare reconnection and secondary dimming fluxes are roughly equal. The area of the total dimming, i.e. including both core and secondary dimmmings, its total brightness and the total unsigned magnetic flux show the highest correlations with the flare fluence (c>0.7) and the CME mass (c>0.6). Their corresponding time derivatives, describing the dimming dynamics, strongly correlate with the GOES flare class (c>0.6). Events where high-cadence observations from STEREO are available show a moderate correlation between the area growth rate of the dimming and the maximum speed of the CME.
09:15	A new analysis of stability of active regions for understanding and predicting the onset of solar eruptions	Kusano, K	Oral
	Kanya Kusano[1], Sung-Hong Park[1], Tomoya Iju[2], Johan Muhamad[1], Naoyuki Ishiguro[1], Satoshi Inoue[1], Yumi Bamba[3]
	[1]Institute for Space-Earth Environmental Research (ISEE), Nagoya University, [2] NAOJ, [3] ISAS/JAXA
	Solar flares and coronal mass ejections (CMEs) are believed to be the explosive liberation of magnetic energy in the solar corona and may cause space weather disturbances. However, the critical condition for their onset is not yet well understood, and thus our predictability of when, where, and how large events will occur is not sufficiently reliable yet. Which kind of instability determines the critical condition for the onset of solar eruptions is a key question of this problem, because magnetohydrodynamic (MHD) instabilities must play an important role in driving solar eruptions. The torus instability and the kink-mode instability have been investigated as candidates of the key driver of solar eruptions. Recently, Ishiguro and Kusano (2017) proposed that a new instability called double-arc instability (DAI) may work as the initial driver of solar flares and it can trigger the onset of solar eruptions. Though the DAI is one of the hoop-force driven instabilities like the torus instability (TI), the critical condition of the DAI, given by a new parameter kappa, is different from the TI. Based on the theory of the DAI, we have developed a numerical model which can evaluate the stability and the stored free energy within each solar active region using the nonlinear force-free field extrapolation. We analyzed the various solar active regions by applying this model onto the Space-weather HMI Active Region Patches (SHARP). In this paper, we report the results of the analyses for how the stability of the DAI evolved in several flare-active regions and how that of 200 sampled active regions correlates with the flare activities. The results are consistent with the theoretical scenario of the DAI as the initial driver of solar eruptions and they suggest that the kappa parameter could improve the prediction not only for the capability of solar eruptions but also for the location where large flares are most likely to be triggered within each active region. Finally, we discuss the applicability of nonlinear force-free field extrapolation for the operational forecast of solar flares.
09:30	Properties of Coronal Mass Ejections as Inferred from Coronal Dimming	Nitta, N	Oral
	Nariaki Nitta[1], Meng Jin[2]
	[1] LMSAL, [2] SETI, LMSAL
	Observations suggest that coronal dimming in solar EUV data originates from evacuation of the coronal mass due to coronal mass ejections (CMEs). Dimming may ultimately be used as one of the most powerful tools to detect CMEs from stars. However, it is at present not straightforward to make a quantitative link between the two phenomena. In spatially-integrated EUV data from SDO/EVE, dimming can be overshadowed by concomitant flares especially in those lines sensitive to high temperatures. Here we study several energetic CMEs from limb-occulted regions that accompanied significant dimming but almost no flare emission in order to explore empirical relations between the dimming parameters and CME properties. We try to understand coronal dimming in spatially-integrated time series from EVE and AIA with references to the development of dimming in EUV images from AIA and STEREO EUVI and to the early evolution of CMEs in coronagraph data from SOHO/LASCO and STEREO/COR1 and COR2. We show different patterns of how dimming is observed with respect to the CME even though the intense flare emission from low-lying loops is blocked. Emission measure maps at different temperatures and numerical simulations of CMEs are employed to discuss how to infer the properties of CMEs from coronal dimming in spatially-integrated EUV data.
09:45	Electric-current Neutralization and Eruptive Activity of Solar Active Regions	Torok, T	Oral
	Yang Liu[1], Xudong Sun[2], Tibor Torok[3], Viacheslav S. Titov[3], James E. Leake[4]
	[1]W. W. Hansen Experimental Physics Laboratory, Stanford University, [2]Institute for Astronomy, University of Hawaiʻi , [3]Predictive Science Inc., [4]NASA Goddard Space Flight Center
	The physical conditions that determine the eruptive activity of active regions (ARs) are still not well understood. Various observational proxies for predicting eruptive activity have been suggested, with rather limited success. Moreover, it is presently unclear under which conditions an eruption will remain confined to the low corona or produce a coronal mass ejection (CME). Using vector magnetogram data from SDO/HMI, we investigate the association between electric-current neutralization and eruptive activity for four ARs. Two ARs produced flares and CMEs, one produced only (very strong) confined flares, and one did not exhibit significant eruptions. We find that both CME-producing ARs are characterized by a strongly non-neutralized total current, while the total current in the remaining ARs is almost perfectly neutralized. This suggests that the degree of current neutralization may serve as a proxy for assessing the ability of ARs to produce CMEs.
10:00	Excess Lorentz Force in Major Solar Eruptions	Sun, X	Oral
	Xudong Sun[1], Benjamin Lynch[2], William Abbett[2], Yan Li[2]
	[1] Institute for Astronomy, University of Hawaii, [2] Space Science Laboratory, University of California Berkeley
	The solar active region photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona. Using high-cadence vector magnetograms, multi-wavelength coronal imaging, and numerical simulation, we show how the observed photospheric "magnetic imprints" are highly structured in space and time, and how it can in principle be used to estimate the impulse of the Lorentz force that accelerates the coronal mass ejection (CME) plasma. In an archetypical event, the Lorentz force correlates well with the CME acceleration, but the total force impulse surprisingly exceeds the CME momentum by almost two orders of magnitude. Such a clear trend exists in about two thirds of the eruptions in our survey for Cycle 24. We propose a "gentle photospheric upwelling" scenario, where most of the Lorentz force is trapped in the lower atmosphere layer, counter-balanced by gravity of the upwelled mass. This unexpected effect dominates the momentum processes, but is negligible for the energy budget. We discuss how the upcoming high-sensitivity observations and new-generation numerical models may help elucidate the problem.
10:15	Studying stealth CMEs using advanced imaging analysis techniques	O'kane, J	Oral
	Jennifer O'kane [1], Lucie Green [1], David Long [1]
	[1] Mullard Space Science Laboratory
	Stealth coronal mass ejections (CMEs) are eruptions from the Sun that have no obvious low coronal signature. These CMEs are characteristically slower events, but can still be geoeffective and affect the space weather at Earth. Therefore understanding the science underpinning these eruptions will greatly improve our ability to detect and, eventually, predict them. We present a study of two stealth CMEs analysed using new advanced techniques that reveal their faint signatures in observations from the EUV imagers onboard the SDO and STEREO spacecraft. The different viewpoints of the events given by these spacecraft provide the opportunity to study the eruption from above and the side contemporaneously. For each event, we combined the AIA and HMI observations to reveal the coronal structure that erupted and measured the kinematics of the eruption. We discuss the physical processes that occurred in the time leading up to the onset of each CME and comment on whether these eruptions are the low-energy and velocity tail of the distribution of CME events or whether they are a distinct phenomenon.

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Spatiotemporal Heliomarker Discovery from Spatiotemporal Frequent Patterns	Aydin, B
Berkay Aydin, Rafal Angryk
Georgia State University
Heliomarkers, short for “heliophysical markers”, are the indicators or signs of important solar phenomena, whose aim is to objectively identify the current state of the sun or indicate a future state of it. Two specific aims of discovering heliomarkers are understanding the characteristics of solar events and predicting the incidence of important solar events. The desired properties of heliomarkers can be summarized as: (1) Validity (accurate, specific, sensitive, and precise), (2) Robust (handles noisy data or data with missing values) (3) Reproducible (over different datasets from different observatories and time spans) and (4) Interpretable. We will present our efforts on discovering spatiotemporal heliomarkers using spatiotemporal frequent patterns. The spatiotemporal frequent patterns are the frequently repeating relationships occurring among the solar events. These include spatiotemporal co-occurrences or event sequences. We conducted our analyses using the tracked and interpolated solar event data. The solar event data represents the spatial vector metadata detected from SDO imagery. We will present the current state of our research and the discovered heliomarkers.
A study on the relationship between hard X-ray emission and magnetic field changes during solar flares	Caicedo, W
[1] Willinton Caicedo Tez, [2]Juan Sebastian Castellanos, [3]Benjamín Calvo Mozo
[1]Universidad Nacional de Colombia,[2] Max Planck Institute for Solar System Research,[3]Universidad Nacional de Colombia
During solar flares it has been observed that the photospheric magnetic field changes significantly, abruptly and permanently. We investigated the possible spatial and temporal correlation between permanent magnetic field changes during 6 solar flares. These highly energetic events occurred during the current solar cycle 24. The energy range of the flares contains events from low energetic to very energetic, according to the GOES classification. The behavior of the emission of hard X-rays (HXR) and the magnetic fields is analyzed using data from the RHESSI and SDO/HMI satellites. We study the probability that the photospheric magnetic field will change due to the particle injection in the lower layers of the solar atmosphere. At present there is no statistical study that relates these processes during flares.
Detection of Three-minute Oscillations in Full-disk Lyman-alpha Emission during a Solar Flare	Milligan, R
Ryan O. Milligan[1], Bernhard Fleck[2], Jack Ireland[3], Lyndsay Fletcher[1], Brian R. Dennis[4]
[1]University of Glasgow, [2]ESA Directorate of Science, c/o NASA/GSFC, [3]ADNET Systems, Inc., [4]NASA/GSFC
In a recent study of spatially-integrated Lyman-alpha line emission (Lya, 1216A) from GOES/EUVS, we detected the presence of acoustic 3-minute oscillations during an X-class solar flare. Similar periodicities were also found - in phase - in Lyman continuum data from SDO/EVE, and the 1600A and 1700A channels on SDO/AIA. The implication is that the chromosphere responds dynamically at its acoustic cutoff frequency to an impulsive injection of energy. Since the 3-minute period was not detected at hard X-ray energies in RHESSI data we can state that this 3-minute oscillation does not depend on the rate of energisation of non-thermal electrons. This finding suggests that chromospheric mechanical energy should be included in the flare energy budget, and that fluctuations in Lya emission may influence the composition and dynamics of planetary atmospheres during periods of high activity. Knowledge of the behaviour of this emission during flares could be important when interpreting future science results from the EUI instrument on Solar Orbiter which will obtain high cadence images in Lya.
Multi-wavelength observations of 4 homologous global coronal waves	Long, D
David Long[1], Julia Lawless[2], Gherardo Valori[1], Jack Jenkins[1]
[1] Mullard Space Science Laboratory, University College London; [2] Trinity College Dublin
Global coronal waves (commonly called “EIT waves”) were first observed by SOHO/EIT in 1997 and are now considered to be large-scale shock fronts initially driven by the rapid expansion of an erupting coronal mass ejection in the low corona. I will present observations of four homologous global waves which erupted from the same active region over the course of three days in March 2014. Each global EUV wave was well observed by SDO/AIA and was associated with a H-alpha Moreton-Ramsey wave observed at high cadence by the GONG network. These observations provide the opportunity to directly relate global waves in EUV and H-alpha observations with high cadence and answer a fundamental question about the relationship between these phenomena which has persisted since they were first observed.
AWARE: An algorithm for the automated characterization of EUV waves in the solar atmosphere	Ireland, J
Jack Ireland[1], Andrew Inglis[2], Albert Shih[3], Steven Christe[3], Stuart Mumford[4], Laura Hayes[5], Barbara Thompson[3], Keith Hughitt[6]
[1] ADNET Systems, Inc., MD, USA, [2] Catholic University, DC, USA, [3] NASA GSFC, MD, USA, [4] University of Sheffield, UK, [5] Trinity College Dublin, ireland [6], NIH, MD, USA
Extreme ultraviolet (EUV) waves are large-scale propagating disturbances observed in the solar corona, frequently associated with coronal mass ejections and flares. They appear as faint, extended structures propagating from a source region across the structured solar corona. Since their discovery, over two hundred papers discussing their properties, causes and physical nature have been published. However, despite this their fundamental properties and the physics of their interactions with other solar phenomena are still not understood. To further the understanding of EUV waves, we have constructed the Automated Wave Analysis and REduction (AWARE) algorithm for the measurement of EUV waves. AWARE is implemented in two stages. In the first stage, we use a new type of running difference image, the running difference persistence image, which enables the efficient isolation of propagating, brightening wavefronts as they propagate across the corona. In the second stage, AWARE detects the presence of a wavefront, and measures the distance, velocity and acceleration of that wavefront across the Sun. The fit of propagation models to the wave progress isolated in the first stage is achieved using the Random Sample and Consensus (RANSAC) algorithm. AWARE is tested against simulations of EUV wave propagation, and is applied to measure EUV waves in observational data from the Atmospheric Imaging Assembly (AIA). We also comment on unavoidable systematic errors that bias the estimation of wavefront velocity and acceleration. In addition, the full AWARE software suite comes with a package that creates simulations of waves propagating across the disk from arbitrary starting points.
Understanding the Role of Mass-Unloading in a Filament Eruption	Long, D
Jack Jenkins[1], David Long[1], Lidia van Driel-Gesztelyi[1,2,3], Jack Carlyle[4], Matthew Hopwood[5,6]
[1] Mullard Space Science Laboratory, University College London; [2] Observatoire de Paris; [3] Konkoly Observatory; [4] ESAC; [5] School of Mathematics, University of Birmingham; [6] School of Mathematical Science, University of Adelaide
We combine observations of a partial filament eruption on 11 December 2011 with a simple line-current model to demonstrate that including mass is an important next step for understanding solar eruptions. Observations from the Solar Terrestrial Relations Observatory-Behind (STEREO-B) and the Solar Dynamics Observatory (SDO) spacecraft were used to remove line-of-sight projection effects in filament motion and correlate the effect of plasma dynamics with the evolution of the filament height. The two viewpoints enable the amount of mass drained to be estimated, and an investigation of the subsequent radial expansion and eruption of the filament. We use these observational measurements to constrain a line-current model and quantitatively demonstrate the important role that the presence and draining of mass has in the lead-up to solar eruptions. Specifically, we show that the balance of magnetic and gravitational forces acting on the line-current is increasingly sensitive to mass perturbations as it approaches its loss-of-equilibrium. Finally, we conclude that the eruption of the observed filament was restrained until 70% of the mass had drained from the structure.
Oscillations along Flare Loops in Multiple Wavelengths and their Correlations to Photospheric Waves	Hess webber, S
Shea A. Hess Webber[1], Junwei Zhao[1]
[1]Stanford University
The goal of this work is to determine whether oscillations along flare loops have a detectable response to/in the photosphere. Using multiple SDO/AIA wavelengths, we track plasma oscillations in the chromosphere and low corona before and after several M- and X-class flares. These results will be used to determine travel times for the oscillations along the flare loops. We correlate these travel times with photospheric wave responses. Understanding if and how flare oscillations interact with the photosphere can detail the mechanics and solar environment in flaring active regions.
Dynamic Mapping of Solar Eruptions	Thompson, B
Barbara J Thompson[1], C. Richard DeVore[1], Beryl A. C. Hovis-Afflerbach[1,2], Vadim Uritsky[3]
[1]NASA Goddard Space Flight Center, USA, [2]California Institute of Technology, [3]Catholic University of America
We present the results of a prominence mapping effort designed to extract the dynamics of erupting prominences and CMEs. The material from partially erupting prominences can fall back to the sun, tracing out the topology of the mid- and post-eruptive corona. One question involving the range of observed behavior is the role of magnetic field topology and evolution in determining the motion of the erupting prominence material. A variable-g ballistic approximation is applied to study the motion of the material, using the deviations from constant angular momentum as a means of quantifying the local Lorentz (and other) forces on each piece of material. Variations in dynamic behavior can be traced back to changes in the local magnetic field. We discuss the use of the prominence trajectories as a means of diagnosing eruptive topologies.
On the Magnetic Topology and Extreme Ultraviolet in Solar Flares with Late Phase	Chen, J
Jun Chen[1][2], Rui Liu[1], Kai Liu[1], Yuming Wang[1]
[1]University of Science and Technology of China, [2]University of Potsdam
It was recently discovered that some solar flares exhibit a late-phase peak in EUV emission with `warm' temperatures (e.g., Fe XVI 33.5 nm), which is referred to as EUV late phase. In this work, we carried out a statistical study of 51 M- and X-class flares with EUV late phase (ELP) during 2010--2015. These flares are categorized as circular-ribbon, two-ribbon, and intricate-ribbon flares, based on the flare morphology observed in the chromosphere. It is found that the circular-ribbon flares with ELP often possess a coronal null and the associated fan and spine, which are typically embedded in a dome-shaped quasi-separatrix layer (DQSL) intersecting with a curved plate-shaped QSL (PQSL). The footprints of the PQSL correspond to an extended ribbon enclosed by the circular-shaped ribbon and a remote ribbon. The coronal loops responsible for ELP are found to be closely associated with not only the spine but more generally the PQSL. The majority of two-ribbon flares with ELP are confined, and the two ribbons are not associated with any preexisting QSLs. It is still an open question whether the ELP is primarily due to plasma cooling or additional heating.
A Truly Global Extreme Ultraviolet Wave from the SOL2017-09-10 X8.2+ Solar Flare-Coronal Mass Ejection	Liu, W
Wei Liu [1,2,3], Meng Jin [1,4], Cooper Downs [5], Leon Ofman [6,7,8], Mark C. M. Cheung [1], Nariaki V. Nitta [1]
[1] LMSAL, [2] BAERI, [3] Stanford Univ., [4] SETI, [5] Predictive Science, Inc., [6] CUA, [7] NASA/GSFC, [8] Tel Aviv University
We report SDO/AIA observations of an extraordinary global extreme ultraviolet (EUV) wave triggered by the X8.2+ flare-CME eruption on 2017 September 10. This was one of the best EUV waves ever observed with modern instruments, yet it was likely the last one of such magnitudes of Solar Cycle 24 as the Sun heads toward the minimum. Its remarkable characteristics include the following. (1) The wave was observed, for the first time, to traverse the full-Sun corona over the entire visible solar disk and off-limb circumference, manifesting a truly global nature, owing to its exceptionally large amplitude, e.g., with EUV enhancements by up to 300% at 1.1 Rsun from the eruption. (2) This leads to strong transmissions (in addition to commonly observed reflections) in and out of both polar coronal holes, which are usually devoid of EUV waves. It has elevated wave speeds >2000 km/s within them, consistent with the expected higher fast-mode magnetosonic wave speeds. The coronal holes essentially serve as new "radiation centers" for the waves being refracted out of them, which then travel toward the equator and collide head-on, causing additional EUV enhancements. (3) The wave produces significant compressional heating to local plasma upon its impact, indicated by long-lasting EUV intensity changes and differential emission measure increases at higher temperatures (e.g., log T=6.2) accompanied by decreases at lower temperatures (e.g., log T=6.0). These characteristics signify the potential of such EUV waves for novel magnetic and thermal diagnostics of the solar corona on global scales.
Estimation of Halo CME’s radial speeds using coronal shock waves based on SDO/AIA observations	Jeong, H
Hyunjin Jeong and Yong-Jae Moon
School of Space Research, Kyung Hee University
We propose a method to estimate the radial speed of a Halo CME by combining a coronal shock front and an EUV-wave that occurs on the solar disk. According to recent studies, EUV-wave occurs as a footprint of the coronal shock wave on the lower solar atmosphere. In this study, the coronal shock on 2011 February 15 by SDO/AIA is assumed as a perfect sphere. This assumption makes it possible to determine the height of a coronal shock, by matching the position of an EUV-wave on the solar disk and a coronal shock front on the SOHO/LASCO coronagraph image. The radial velocity of the Halo-CME is calculated from the rate of coronal shock position shift. The calculated speed from this method is a little slower than the 3-D velocity estimated by multi-spacecraft, but much faster than the projected one by SOHO/LASCO. And these results and the efficiency of this approach are discussed.
Effect of transport coefficients on excitation of flare-induced standing slow-mode waves in coronal loops	Wang, T
Tongjiang Wang[1,2], Leon Ofman[1,2], Xudong Sun[3], Sami K. Solanki[4], and Joseph M. Davila[2]
[1]Catholic University of America, USA; [2]NASA Goddard Space Flight Center, USA; [3]University of Hawaii, USA; [4] Max-Planck-Institut for Solar System Research, Germany
We analyze and model a flare-induced longitudinal oscillation event detected by SDO/AIA. The magnetic field extrapolation and flare emission features suggest that the wave event is generated by slipping and null-point-type reconnections in a closed fan-spine magnetic topology, and the large spine loop appears to be heated impulsively to the flare temperature before the wave disturbances travel along it. By means of the seismology technique, we determine the transport coefficients in hot (about 10 MK) plasma, and find that thermal conductivity is strongly suppressed and compressive viscosity is enhanced by more than an order of magnitude from the observed wave properties. We investigate the standing slow-mode wave excitation mechanism using 1D nonlinear MHD simulations based on two types of loop models. Model 1 with the classical transport coefficients and Model 2 with the seismology-determined transport coefficients. We find that Model 2 can form the standing wave pattern (within about one period) from initial propagating disturbances much faster than Model 1, in better agreement with the observations. Simulations of harmonic waves and the Fourier decomposition analysis reveal a scaling between damping time (Td) and wave period (P) following Td ~ P^2 in Model 2, while Td ~ P in Model 1. This suggests that anomalously large viscosity can efficiently enhance the dissipation of higher harmonic components, favoring quick setup of the fundamental standing mode. Our study suggests that observational constraints on the transport coefficients are crucial in understanding both the wave excitation and damping mechanisms.