## Session 14 - Multi-viewpoint versus single-viewpoint observations and modelling - Lessons learned from 10 years of STEREO

Barbara Thompson (NASA GSFC); Manuela Temmer (University of Graz); Jackie Davies (RAL Space); Volker Bothmer (University of Göttingen); Alexis Rouillard (IRAP); Stefaan Poedts (KU Leuven)
Friday 01/12, 9:45 - 13:00
Mercator

KEYWORDS - remote sensing, in-situ, multi-view point data, inner heliosphere, large-scale solar disturbances;

The launch of STEREO, at the end of 2006, heralded the start of a new era in the observation of the solar system. Together with SoHO, located at a fixed position at L1, STEREO has continuously provided multiple views of large-scale phenomena propagating through the inner heliosphere. With this unprecedented combination of remote-sensing and in-situ observations, in particular, we have been able to gain a much deeper understanding of the 3-dimensional nature of large-scale inner heliospheric structures such as coronal mass ejections (CMEs) and stream/corotating interaction regions (SIRs/CIRs). But what have we gleaned about the limitations of single-viewpoint observations? How can we apply that knowledge in a future with, potentially, only single-spacecraft observations? Moreover, what are the implications for the design of future multi-viewpoint missions?

In this session, we invite observational, theoretical, and modeling contributions that address these questions and cover studies on the evolution, propagation and morphology of large-scale structures (CMEs, SIRs & CIRs) from single- or multi-view point observations at any wavelength.

Poster Viewing
From Thursday morning to Friday noon

Talks
Friday December 1, 09:45 - 11:00, Mercator
Friday December 1, 11:45 - 13:00, Mercator

### Talks : Time schedule

Friday December 1, 09:45 - 11:00, Mercator
 1 Deriving Kinematic Properties of Non-Radial and Asymmetric CMEs: Methods and Implications Thompson, B et al. e-Poster Barbara Thompson NASA Goddard Space Flight Center, USA There are several challenges in characterizing the kinematic properties of energetic CMEs. Most rapidly-evolving eruptions are accompanied by changes in the surrounding corona. The larger the impact on the surrounding corona, the more difficult it is to separate the “main” CME from the CME-associated brightenings. Complicating the issue is the range of observed propagation properties: super-radial expansion, asymmetric expansion, non-radial propagation, and alterations in the direction of propagation. These properties can be a function of both the internal magnetic structure of the CME and the structure of the corona through which the CME is propagating. While the relative contribution of internal/external factors can be difficult to assess, it is of fundamental importance because it not only reveals the nature of CMEs but also CME-associated phenomena. 2 Solar wind speed forecasting with a STEREO persistence model using uncertainty assessment from the evolution of coronal holes Temmer, M et al. e-Poster Manuela Temmer, Juergen Hinterreiter, Martin A. Reiss Institute of Physics, University of Graz, Austria We present the concept of a new persistence model to forecast the solar wind speed at 1 AU, using the advantage of multi-viewpoint satellite data. The model is based on STEREO in-situ measurements for satellite positions eastward of Earth, shifted forward by a variable time span according to the angle of the STEREO spacecraft with Earth (~2-10 days). The STEREO persistence model is applied on the time range 2008-2012 (STEREO-B) and 2017 (STEREO-A) and compared to a persistence model based on ACE data forward shifted by a full rotation. In addition, the STEREO persistence model is modified by assessing the speed uncertainties that are caused by the evolution of coronal holes (CH). We derive the information on CH evolution by comparing CH areas extracted in EUV data from STEREO and Earth perspective. Compared to an ACE based persistence model, the performance of the STEREO persistence model which takes into account the evolution of coronal holes, is able to reduce the number of missed high-speed streams by about 25%, the false alarms by about 20%, and to increase the hit rate by about 12%. 3 STEREO/SDO Observations of Stealthy but Earth-affecting CMEs Nitta, N et al. e-Poster Nariaki Nitta, Tamitha Mulligan [1]Lockheed Martin Advanced Technology Center; [2]The Aerospace Corporation We have long known about the existence of "problem" geomagnetic storms whose origins are elusive. In more general terms, not all the 1 AU disturbances can be clearly attributed to CMEs, HSSs or CIRs. When ICME signatures are found in in situ data, there is not always a flare or filament eruption on the Sun or even an obvious CME observed close to the Sun that correlates with the ICME within a reasonable time range. These ICMEs sometimes result in intense storms. Furthermore, there is a possibility that some of the more severe storms could be partly contributed by such ICMEs of unclear origin. Therefore space weather prediction will remain incomplete without properly understanding these ICMEs. Even if the ICME is paired with a CME, it is sometimes difficult to find where the latter comes from. This is often called the “stealth CME” that apparently lacks low coronal signatures (LCSs). STEREO's second and third view points have tremendously helped us determine its front-side origin and find when and where it forms and accelerates, which is important for isolating possible LCSs. Although SDO/AIA has been continuously taking full-disk EUV images in a wide temperature range since 2010, there are still a number of stealthy CMEs whose LCSs are unclear or ambiguous. It is assumed that they start at high altitudes, leaving weak or negligible LCSs. Some of them seem to involve multiple magnetic domains, including weak or open field regions. Using several examples including those that were observed in recent years, we emphasize the importance of monitoring the global Sun continuously, in order to detect and forecast these stealthy but Earth-affecting CMEs and to understand the eruption mechanisms behind them. 4 Continuous 360 degree observation of a large, long-living, low-latitude coronal hole during the maximum of solar cycle 24 Heinemann, S et al. e-Poster Stephan G. Heinemann[1], Manuela Temmer[1], Stefan Hofmeister[1], Astrid M. Veronig[1], Susanne Vennerstrom[2] [1]Institute for Physics, University of Graz, Austria; [2]Technical University of Denmark, DTU High speed solar wind streams (HSS) emanating from coronal holes, and associated stream interaction regions, may cause geomagnetic storms and deflect coronal mass ejections propagation in interplanetary space. By understanding the evolution and the relations between coronal holes and solar wind parameters, we increase our knowledge for improving space weather forecasts. We investigate the evolution of a persistent coronal hole using EUV data from STEREO-A/B and SDO over the timerange February 2012 –October 2012. Combined STEREO-SDO data enable a continuous observation of the CH covering 360° degrees over several rotations. Together with magnetic field measurements from SDO filtergrams and in-situ solar wind observations, we analyze during different evolutionary states of the CH, the solar surface properties of the CH (intensity, area, shape, magnetic flux) and its effects at 1AU (solar wind speed). As a result we find an evolutionary pattern in most parameters, clearly showing a growing, maximum and decaying phase. At time of maximum area the open flux also reaches its maximum with about 50%, this development can also be seen in the associated HSS. 5 Can coronal dimmings serve as proxy for characteristic CME parameters? Dissauer, K et al. e-Poster Karin Dissauer[1], Astrid M. Veronig[1,2], Manuela Temmer[1], Kamalam Vanninathan[1], Tatiana Podladchikova[3], Julia M. Riedl[1] [1]IGAM/Institute of Physics, University of Graz, Austria; [2]Kanzelhöhe Observatory/Institute of Physics, University of Graz, Austria; [3]Skolkovo Institute of Science and Technology, Russia Earth-directed coronal mass ejections (CMEs) are the main driver for space weather affecting the near-Earth environment. However, they allow the least accurate measurements of their properties due to strong projection effects towards the plane of sky. The most distinct phenomena associated with Earth-directed CMEs are coronal dimmings, i.e. regions of reduced emission in the extreme-ultraviolet (EUV) and soft X-rays. They are interpreted as density depletions due to the evacuation of plasma during the CME expansion. Investigating the properties of these low coronal footprints will provide us with important additional information on the initiation and early evolution of the associated CMEs. We take advantage of the quadrature position of the SDO and STEREO spacecraft, during 2010-2012, to study CMEs and their associated dimmings precisely. We select 76 events which show on-disk dimmings with respect to SDO and have CMEs which are observed off-limb by at least one of the two STEREO spacecraft. The on-disk coronal dimming evolution is studied using the high-cadence, multi-wavelengths data of SDO/AIA and the line-of-sight (LOS) magnetograms of SDO/HMI. We use the best combination of on-disk and off-limb signatures to establish a statistical relationship between Earth-directed CMEs and their associated coronal dimmings. The basic properties of the CME, such as initial velocity, acceleration, mass and initiation height are derived and compared with characteristic coronal dimming parameters like the magnetic flux, the area, the area growth rate and the total brightness. We find moderate correlations between the derived dimming parameters, the flare strength and the timing. The dimming parameters show the strongest correlations with the CME mass. These results establish the relationship between dimmings and CMEs, and provide a means to use single-view point observations of only coronal dimmings to constrain Earth-directed CME parameters. 6 Solar signatures and eruption mechanism of the August 14, 2010 coronal mass ejection D'huys, E et al. e-Poster Elke D’Huys[1,2], Daniel B. Seaton[1,3,4], Anik De Groof[5], David Berghmans[1], Stefaan Poedts[2] [1]Solar-Terrestrial Center for Excellence, Royal Observatory of Belgium, Solar Influences Data Analysis Center; [2]Centre for Mathematical Plasma-Astrophysics, Katholieke Universiteit Leuven; [3]Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder; [4]NOAA National Centers for Environmental Information; [5]European Space Agency/ESAC On August 14, 2010 a wide-angled coronal mass ejection (CME) was observed. This solar eruption originated from a destabilized filament that connected two active regions and the unwinding of this filament gave the eruption an untwisting motion that drew the attention of many observers. In addition to the erupting filament and the associated CME, several other low-coronal signatures that typically indicate the occurrence of a solar eruption were associated with this event. However, contrary to what was expected, the fast CME was accompanied by only a weak flare. We investigate the various eruption signatures that were observed for this event and focus on the kinematic evolution of the filament in order to determine its eruption mechanism using observations from a fleet of satellites, including STEREO, PROBA2, SOHO and SDO. Had this solar eruption occurred just a few days earlier, it could have been a significant event for space weather. The risk of underestimating the strength of this eruption based solely on the weak flare illustrates the need to include all eruption signatures in event analyses in order to obtain a complete picture of a solar eruption and assess its possible space weather impact. 7 Simulation of CMEs in 2007 – 2016 Using the WSA-ENLIL+Cone Modeling System Jian, L et al. p-Poster L.K. Jian[1,2], D. Odstrcil[3,2], M.L. Mays[2], A.P. Rouillard[4,5] [1]University of Maryland, College Park, MD, USA; [2]NASA Goddard Space Flight Center, Greenbelt, MD, USA; [3]George Mason University, Fairfax, VA, USA; [4]Institut de Recherche en Astrophysique et Planétologie, Paul Sabatier University, Toulouse, France; [5]Centre National de la Recherche Scientifique, UMR F-5277, Toulouse, France The global 3D MHD WSA-ENLIL+Cone modeling system has been widely used to predict CMEs. Based on the CME parameters fitted using the remote observations by single or multiple spacecraft, we have modeled thousands of CMEs in 2007-2016. About 200-300 of them propagate toward the Earth and are found in the simulation results. In comparison with the in situ observations of about 200 CMEs in the same period, we assess the performance of the WSA-ENLIL+Cone modeling system, discuss the factors affecting the ICME arrival time, and examine the cases in which there is a large discrepancy between the simulated and observed solar wind parameters. 8 A consideration of the multi-viewpoint and single-viewpoint exploitation of heliospheric imaging in light of the ten years of the STEREO/HI instruments Harrison, R et al. p-Poster Richard Harrison, Jackie Davies STFC-Rutherford Appleton Laboratory, Harwell, Oxfordshire OX11 0QX, UK After more than a decade of successful operation of the flagship Heliospheric Imager (HI) instruments on-board NASA’s twin-spacecraft STEREO mission, we are in a unique position to assess the large volume of research that exploits the data therefrom. In particular, we review those publications that endeavour to characterise the 3D kinematic properties of coronal mass ejections (CME), as they propagate out to 1 AU and beyond; such information is vital for the provision of accurate CME arrival predictions for space weather usage. In this presentation, we consider the benefits of heliospheric imaging from two vantage points over such imaging from a single view point. Given the expectation of future operational heliospheric imaging from only a single vantage point, such an assessment is timely. 9 Automated detection of coronal mass ejections in three-dimensions using multi-viewpoint observations Hutton, J et al. p-Poster Joseph Hutton, Huw Morgan Aberystwyth University A new, automated method of detecting Coronal Mass Ejections (CMEs) in three dimensions for the LASCO C2 and STEREO COR2 coronagraphs is presented. By triangulating isolated CME signal from the three coronagraphs over a sliding window of five hours, the most likely region through which CMEs pass at 5$R_{\odot}$ is identified. The centre and size of the region gives the most likely direction of propagation and approximate angular extent. The Automated CME Triangulation (ACT) method is tested extensively using a series of synthetic CME images created using a wireframe flux rope density model, and on a sample of real coronagraph data; including halo CMEs. The accuracy of the angular difference ($\sigma$) between the detection and true input of the synthetic CMEs is $\sigma$=7.14$^{\circ}$, and remains acceptable for a broad range of CME positions relative to the observer, the relative separation of the three observers, and even through the loss of one coronagraph. For real data, the method gives results that compare well with the distribution of low coronal sources and results from another instrument and technique made further from the Sun. The true 3D-corrected kinematics and mass/density are discussed. The results of the new method will be incorporated into the CORIMP database in the near future, enabling improved space weather diagnostics and forecasting. 10 A multi-spacecraft view of a giant filament eruption during 2009 september 26/27 Schmieder, B et al. p-Poster Brigitte Schmieder[1], Sanjay Gosain[2], Guy Artzner[3], Sergei Bogachev[4], Tibor Török[5] [1]LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris Diderot, 5 Place Jules Janssen, F-92190 Meudon, France; [2]National Solar Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA; [3]CNRS UMR 8617, Institut d’astrophysique Spatiale (IAS), F-91405 Orsay Cedex, France; [4]Lebedev Physical Institute of Russian Academy of Science, Leninskij prospekt 53, Moscow 119991, Russia; [5]Predictive Science, Inc., 9990 Mesa Rim Rd., Suite 170, San Diego, CA 92121, USA We analyze multi-spacecraft observations of a giant ﬁlament eruption that occurred during 2009 September 26 and 27. The ﬁlament eruption was associated with a relatively slow coronal mass ejection. The ﬁlament consisted of a large and a small part, and both parts erupted nearly simultaneously. Here we focus on the eruption associated with the larger part of the ﬁlament. Detailed results can be found in Gosain et al (2012). (i) We use STEREO-A, SoHO/EIT and CORONAS/TESIS observations as a third eye (Earth view) to aid our measurements. (ii) Using trigonometry, we estimate the direction of propagation and derive the true height-time proﬁle of the ﬁlament, (iii) further, we develop a new tomographic method that can potentially provide a more robust three-dimensional (3D) reconstruction by exploiting multiple simultaneous views. We then analyze the kinematics of the eruptive ﬁlament during its rapid acceleration phase by ﬁtting different functional forms to the height-time data derived from the two methods. 11 Active region evolution and the formation of eruptive configurations Green, L et al. p-Poster Lucie Green, Alex James, Gherardo Valori, Stephanie Yardley Throughout their lifetimes, active regions can be the source of many CMEs. However, the frequency and magnitude of the region’s activity is dependent on the evolutionary stage of the active region. Combining photospheric and coronal data to capture the evolution of an active region, along with modelling of the magnetic field, has allowed a deeper understanding of the 3-dimensional magnetic field configurations that are responsible for CMEs. One particular configuration, known as a flux rope, has been identified in many CME source regions. This knowledge is feeding into our understanding of the physical processes that drive CMEs and may help in future CME predictions. This talk will look at why monitoring the time-history of active regions is important and whether knowing where the active region is in its lifetime helps understand the likelihood of CME activity. It will also address how flux ropes are identified in active regions and the timescales and mechanisms involved in their formation. 12 A regularised full-Newton VARPRO iteration for the stereoscopic reconstruction of loops in EUV images Chifu, I et al. p-Poster Bernd Inhester, Iulia Chifu Max Planck Institute for Solar System Research The reconstruction of bright coronal loops from stereo pairs of EUV images is considered an important tool to disentangle the complex 3D topology of the coronal magnetic field. Reconstructed loops have been used in different cases for the study of the magnetic field topology. Also, stereoscopically reconstructed prominences when seen as arch-like structures have been used in the determination of the CME kinematics. For the stereoscopic reconstruction one must follow two steps. First step is the tie-pointing of the same loop structure in each of the pair images. The second step uses the tie-points for a stereoscopic triangulation. While automized tools exist for the first step, we here propose an automatisation of the second step, the stereoscopic inversion. The procedure can take the placed tie-points from an image pair and calculate a spline-based approximation of the 3D loop shape from them. A preliminary version of our procedure has already been applied to observed image data in several cases. The work we present now is more robust since is using precise projective geometry for the image projection and can systematically be extended to any number of view directions.