Session 10 - Special Session: The AR12673 on Sept 1-10 2017

Conveners: Barbara Thompson (GSFC), Veronique Delouille (ROB)
Tuesday 30/10, 13:15 - 15:00

Talks

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Tuesday October 30, 13:15 - 15:00

13:15	Precursors of magnetic flux emergence in the moat flows of active region AR12673	Attie, R	Oral
	Raphael Attie[1], Michael Kirk[1], Barbara Thompson[1], Karin Muglach[1], Aimee Norton[2]
	[1]NASA GSFC, [2] HEPL Stanford University
	We report on observations of magnetic disturbances in active region AR12673 between Sep. 1 and Sep. 3, 2017 seen as a disruption of the moat flow several hours before the onset of strong flux emergence near the main sunspot. The moat flow is commonly known as a radially oriented strong outflow of photospheric plasma surrounding sunspots which ends abruptly and thus shapes an annular pattern around the penumbra. Using highly accurate methods of tracking this photospheric flow applied to SDO/HMI data, we are able to describe the evolution of the moat surrounding the main sunspot of AR 12673. We find that several hours before the emergence of strong magnetic flux near the main sunspot the moat boundaries are broken at these very same locations. This behavior is observed both on Sep. 1st and Sep. 3rd. There is no such behavior observed in the absence of flux emergence. These observational results pose the question of how often they occur in other active regions and whether the disruption of the moat flow might be, like in this case, an indication of impending enhanced magnetic activity or simply a coincidental event.
13:30	Joint observation of the X9.3 and X8.3 flares of September 6 and 10 2017 by SDO/EVE, PROBA2/LYRA and MAVEN/EUVM	Dominique, M	Oral
	Marie Dominique[1,2], Andrei N. Zhukov[1,3], Petr Heizel[4], Edward Thiemann[5], Laurent Dolla[1], Laurence Wauters[1], Ingolf Dammasch[1], Giovanni Lapenta[2]
	[1] Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, [2]Katholiek Universiteit Leuven, [3]Skobeltsyn Institute of Nuclear Physics, [4]Astronomical Institute, Czech Academy of Sciences, [5]Laboratory for Atmospheric and Space Physics, University of Colorado
	After several months of relative quiet, a sudden burst of solar activity was observed starting on September 4 2017, when the NOAA AR 12673 started to develop quickly. This region remained active until it disappeared behind the west limb on September 10. It produced multiple strong flares (27 M-class flares and 4 X-class flares). Among them, there were the two strongest flares observed so far during the Solar Cycle 24: the X9.3 flare of September 6 and the off-limb X8.3 flare of September 10. The flares were observed by SDO/EVE, PROBA2/LYRA, and the Mars orbiting instrument MAVEN/EUVM. Serendipitously, LYRA was performing a special flare observation campaign, involving one of its spare units that is normally only used for calibration purposes. We therefore have high quality observations of the event, which include signatures of the X9.3 flare in the Lyman-alpha emission and emissions around 2000 Angstroms. We will compare the observations by the various instruments and we will discuss the origin of the signal detected around 2000 Angstroms by LYRA. Moreover, the joint observation by the three instruments allows a detailed analysis of the quasi-periodic pulsations that take place during the rising phase of the flares. The comparison between the two events might shed some light on this phenomenon.
13:45	Coronal and chromospheric observations of pre- and post-flare plasma evolution	Long, D	Oral
	David Long[1], Aaron Reid[2], Louise Harra[1], Mihalis Mathioudakis[2]
	[1] Mullard Space Science Laboratory, University College London; [2] Queen's University Belfast
	Solar flares are among the most energetic and spectacular events occurring in our solar system, produced by the release of stored magnetic energy in the solar atmosphere through the reconnection of twisted magnetic fields. Although the magnetic field itself is difficult to observe in the solar atmosphere, we can gain vital insights into the reconnection process by studying the evolution of solar plasma prior to and following the flare. Here we present contemporaneous spectroscopic and imaging observations of an X9.3 solar flare from 2017-September-6. This was the largest flare of the current solar cycle to date, and was well observed in the corona by SDO/AIA and Hinode/EIS as well as in the chromosphere by instruments at the ground-based Swedish Solar Telescope. This combination of observations provides spectroscopic information throughout the solar atmosphere, giving a unique insight into the evolution of plasma in the lead-up to and following the flare.
14:00	SDO/EVE Observations of Lyman Continuum Emission During Solar Flares	Milligan, R	Oral
	Ryan O. Milligan[1], Marcos E. Machado[2], Paulo J. A. Simões[1]
	[1]University of Glasgow, [2]Comisión Nacional de Actividades Espaciales (CONAE)
	The Extreme-ultraviolet Variability Experiment (EVE) was designed to observe the Sun as a star in the extreme ultraviolet; a wavelength range that remained spectrally unresolved for many years. It has provided a wealth of data on solar flares, perhaps most uniquely, on the Lyman spectrum of hydrogen at high cadence and moderate spectral resolution. Here we present the analysis of Lyman continuum (LyC) observations and their temporal evolution in a sample of six major solar flares. By fitting both the pre-flare and flare excess spectra with a blackbody function we show that the color temperature derived from the slope of LyC reveals temperatures in excess of 10000K in the six events studied; an increase of a few thousand Kelvin above quiet-Sun values (typically 8000-9500K). This was found to be as high as 17000K for the 2017 September 6 X9.3 flare. Using these temperature values, and assuming a flaring area of 10^18 cm^2, estimates of the departure coefficient of hydrogen, b1, were calculated. It was found that b1 decreased from 10^2-10^3 in the quiet-Sun, to around unity during the flares. This implies that LyC is formed at deeper, denser layers during solar flares than in the quiescent solar atmosphere.
14:15	Evolution of flux rope, CME and associated EUV wave in the 10-Sep-2017 X8.2 event	Podladchikova, T	Oral
	Tatiana Podladchikova [1], Astrid M. Veronig [2,3], Karin Dissauer [2], Manuela Temmer [2], Daniel B. Seaton [4,5], David Long [6], Jingnan Guo [7], Bojan Vršnak [8], Louise Harra [6], Bernhard Kliem [9]
	[1] The Skolkovo Institute of Science and Technology, Moscow, Russia, [2] Institute of Physics, University of Graz, Austria, [3] Kanzelhöhe Observatory of Solar and Environmental Research, University of Graz, Austria, [4] Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, CO, USA, [5] National Centers for Environmental Information, National Oceanic and Atmospheric Administration, Boulder, [6] UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, UK, [7] Institut für Experimentelle und Angewandte Physik, University of Kiel, Germany, [8] Hvar Observatory, Faculty of Geodesy, University of Zagreb, Croatia, [9] Institute of Physics and Astronomy, University of Potsdam, Germany
	We combine the high-cadence and large field-of-view EUV imagery of the Atmospheric Imaging Assembly (AIA) onboard SDO and the Solar Ultraviolet Imager (SUVI) onboard GOES-16 to study the origin and impulsive evolution of the fast CME that originated in the September 10th 2017 X8.2 event as well as the initiation of the associated EUV wave. In the LASCO field-of-view, the CME reveals speeds >3000 km/s. In the low-to-mid corona, it shows a distinct bubble in the EUV imagery that reveals a significant lateral overexpansion. In addition, is also shows a distinct expanding cavity that is interpreted as manifestation of the flux rope driving the eruption. We present a method to automatically identify and segment the CME bubble in SUVI images and to derive its radial and lateral evolution up to about 2 solar radii, in terms of velocity and acceleration. These measurements are set into context with the evolution of the embedded flux rope/cavity observed by AIA. The observations show clear signatures of new poloidal flux added to the flux rope by magnetic reconnection in the current sheet beneath the eruptive structure, which is important for the high accelerations observed in this event. The radial propagation of the CME shell revealed a peak value of the acceleration of about 5.3 km/s2, whereas the lateral expansion reached a peak value of 10.1 km/s2, which is the largest value reported so far. The flux rope/cavity reveals a radial acceleration of 6.7 km/s2 and lateral acceleration of 5.3 km/s2. We note that at this early evolution phase, the speed of the cavity/flux rope is higher than that of the CME bubble (front). The EUV wave associated with this eruption was observed by AIA, SUVI and STEREO-A EUVI, which had a separation angle with Earth of 128°, and the common field of view of the spacecraft was 52°. AIA and SUVI images above the solar limb reveal the initiation of the EUV wave by the accelerating flanks of the CME bubble, followed by detachment and propagation of the wave with a speed of 1100 km/s. The EUV wave shows a global propagation over the full hemisphere visible to Earth view as well as into the STEREO-A field-of-view. We study the propagation and kinematics of the direct as well as the various reflected and refracted EUV wave components on the solar sphere, finding speeds in the range from 370 to 1010 km/s. Finally, we note that this EUV wave is also distinct as it reveals propagation and transmission through the polar coronal holes.
14:30	Waves of Magnetic-field Variations Observed in Flare-excited Sunquake Events	Zhao, J	Oral
	Junwei Zhao[1] & Ruizhu Chen[2,1]
	[1] W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085 [2] Department of Physics, Stanford University, Stanford, CA 94305-4060
	We report on the detection of waves of magnetic-field variations that were associated with flare-excited sunquake waves. An X-9.3 flare that occurred on 2017 September 6 excited strong sunquakes, and the sunquake waves were observed sweeping across the flare’s host active region. This rare event gives us an unprecedented opportunity to study responses of magnetic field to passing sunquake waves. A wave of magnetic-field variations was observed in each of the two sunspots that the sunquake waves swept through, and the time–distance relations for the waves observed in magnetic field and Doppler velocity are similar. The phase relations measured between, as well as the oscillatory power distributions calculated from, the Doppler velocity variations and magnetic-field variations associated with the sunquake waves are compared with those obtained from the background waves in the same areas of the sunspot umbra and penumbra separately. The phase relations seem to favor the theory that the waves of magnetic variations are owing to opacity changes associated with the passing sunquake waves. We also examine past sunquake events and see how many of them have excited similar waves of magnetic-field variations.
14:45	Analyzing the kinematics of EUV waves by combining simulations and multi-instrument observations	Koukras, A	Oral
	Alexandros Koukras [1],[2], Christophe Marqué[2], Cooper Downs[3], Laurent Dolla[2]
	[1] KU Leuven, Centre for mathematical Plasma Astrophysics, [2] Royal Observatory of Belgium, STCE-SIDC, [3] Predictive Science Inc.
	EUV (“EIT”) waves are wavelike disturbances of enhanced EUV emission that propagate away from an eruptive active region across the solar disk. We present a framework, where we treat the EUV waves as fast-mode MHD waves, study their kinematics and their connection with type II bursts. We propagate numerically a fast mode MHD wave based on the model of Uchida (Uchida 1968, 1970, 1973) and the formalism of Wang 2000 (ray tracing). To accomplish that we use a 3D MHD-based coronal model (from Predictive Science Inc.) that provides density, temperature and Alfvén speed in the undisturbed coronal medium. Next, taking advantage of the high cadence and multi-wavelength observations of SDO/AIA images, we compare the propagation of the computed wavefront with the observed wave. Finally, we use the frequency drift of the type II radio bursts to track the propagating shock wave. We compare the kinematics of the simulated wavefront, identifying the most probable wave vectors that match the best the kinematics deduced from the radio emission. We focus our attention on two eruptive events on 03/04/2017 and 12/09/2017, where EUV waves are observed respectively above the limb and on the disk. We make use of a collection of high quality and multi instrument observations (PROBA2/SWAP, STEREO/SECCHI, SDO/AIA, SOHO/LASCO and the Humain radio spectrograms) and combine them to better constrain the kinematics of the EUV waves.

Posters

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