explanation      blue bibcodes open ADS page with paths to full text
Author name code: savcheva
ADS astronomy entries on 2022-09-14
author:"Savcheva, Antonia S." 

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Title: Further Evidence for the Minifilament-eruption Scenario for
    Solar Polar Coronal Jets
Authors: Baikie, Tomi K.; Sterling, Alphonse C.; Moore, Ronald L.;
   Alexander, Amanda M.; Falconer, David A.; Savcheva, Antonia; Savage,
   Sabrina L.
2022ApJ...927...79B    Altcode: 2022arXiv220108882B
  We examine a sampling of 23 polar-coronal-hole jets. We first identified
  the jets in soft X-ray (SXR) images from the X-ray telescope (XRT) on
  the Hinode spacecraft, over 2014-2016. During this period, frequently
  the polar holes were small or largely obscured by foreground coronal
  haze, often making jets difficult to see. We selected 23 jets among
  those adequately visible during this period, and examined them further
  using Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly
  (AIA) 171, 193, 211, and 304 Å images. In SXRs, we track the lateral
  drift of the jet spire relative to the jet base's jet bright point
  (JBP). In 22 of 23 jets, the spire either moves away from (18 cases)
  or is stationary relative to (4 cases) the JBP. The one exception
  where the spire moved toward the JBP may be a consequence of
  line-of-sight projection effects at the limb. From the AIA images,
  we clearly identify an erupting minifilament in 20 of the 23 jets,
  while the remainder are consistent with such an eruption having taken
  place. We also confirm that some jets can trigger the onset of nearby
  "sympathetic" jets, likely because eruption of the minifilament field of
  the first jet removes magnetic constraints on the base-field region of
  the second jet. The propensity for spire drift away from the JBP, the
  identification of the erupting minifilament in the majority of jets,
  and the magnetic-field topological changes that lead to sympathetic
  jets, all support or are consistent with the minifilament-eruption
  model for jets.

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Title: Toward Improved Understanding of Magnetic Fields Participating
in Solar Flares: Statistical Analysis of Magnetic Fields within
    Flare Ribbons
Authors: Kazachenko, Maria D.; Lynch, Benjamin J.; Savcheva, Antonia;
   Sun, Xudong; Welsch, Brian T.
2022ApJ...926...56K    Altcode: 2021arXiv211106048K
  Violent solar flares and coronal mass ejections (CMEs) are magnetic
  phenomena. However, how magnetic fields reconnecting in the flare
  differ from nonflaring magnetic fields remains unclear owing to the
  lack of studies of the flare magnetic properties. Here we present a
  first statistical study of flaring (highlighted by flare ribbons) vector
  magnetic fields in the photosphere. Our systematic approach allows us to
  describe the key physical properties of solar flare magnetism, including
  distributions of magnetic flux, magnetic shear, vertical current, and
  net current over flaring versus nonflaring parts of the active region
  (AR), and compare these with flare/CME properties. Our analysis suggests
  that while flares are guided by the physical properties that scale with
  AR size, like the total amount of magnetic flux that participates in
  the reconnection process and the total current (extensive properties),
  CMEs are guided by mean properties, like the fraction of the AR magnetic
  flux that participates (intensive property), with little dependence
  on the amount of shear at the polarity inversion line (PIL) or the
  net current. We find that the nonneutralized current is proportional
  to the amount of shear at the PIL, providing direct evidence that net
  vertical currents are formed as a result of any mechanism that could
  generate magnetic shear along the PIL. We also find that eruptive
  events tend to have smaller PIL fluxes and larger magnetic shears than
  confined events. Our analysis provides a reference for more realistic
  solar and stellar flare models. The database is available online and
  can be used for future quantitative studies of flare magnetism.

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Title: Toward Improved Understanding of Magnetic Fields Participating
in Solar Flares: Statistical Analysis of Magnetic Field within
    Flare Ribbons
Authors: Kazachenko, Maria; Lynch, Benjamin; Savcheva, Antonia;
   Welsch, Brian
2021AGUFMSH45B2378K    Altcode:
  Flares and coronal mass ejections are manifestations of magnetic
  evolution in the solar corona in which magnetic reconnection is
  believed to play key roles. While the properties of underlying,
  photospheric line-of-sight magnetic fields of active regions (ARs)
  as a whole have been analyzed in detail, properties of vector magnetic
  fields that participate in the reconnection process, highlighted by the
  flare ribbons, have not been described. Here we present a statistical
  analysis of vector magnetic field properties in 40 ARs associated with
  33 eruptive and 7 confined flares, of GOES class C9.0 and greater. For
  every event in the database, we use a HMI/SDO vector magnetogram, and
  AIA 1600A images to calculate various properties of the photospheric
  vector magnetic field within the AR, flare ribbons and the polarity
  inversion line (PIL) areas: magnetic flux, reconnection flux fraction,
  magnetic shear, vertical electric current and current neutralization. We
  find that while the peak X-ray flux has a strong correlation with
  ribbon reconnection flux, it has only moderate correlation with the
  magnetic shear within ribbon- and PIL- areas and the degree of current
  neutralization. We find a new linear relationship between the amount
  of non-neutralized current within the AR (or ribbon) and the amount of
  shear at PIL. This scaling is consistent with earlier simulations and
  case studies, of net currents being formed as a result of any mechanism
  that could generate magnetic shear along PIL: flux emergence, twisting
  or shearing motions. Finally, we find that the CME speed has a much
  stronger correlation with the reconnection flux fraction than with
  any other active region property. We also find that for a fixed peak
  X-ray flux, eruptive events tend to have smaller PIL fluxes and larger
  magnetic shears than confined events. To summarize, our observational
  analysis, supported by MHD ARMS and magnetofrictional simulations,
  suggests that flare peak X-ray fluxes and CME speeds are most strongly
  guided by the total amount of magnetic flux that participates in the
  reconnection process and the amount of the flux in the overlying field,
  than by the amount of PIL shear or AR net current.

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Title: PlasmaPy
Authors: Community, PlasmaPy; Everson, Erik; Stańczak, Dominik;
   Murphy, Nicholas A.; Kozlowski, Pawel M.; Malhotra, Ritiek; Langendorf,
   Samuel J.; Leonard, Andrew J.; Stansby, David; Haggerty, Colby C.;
   Mumford, Stuart J.; Beckers, Jasper P.; Satish Bedmutha, Manas;
   Bergeron, Justin; Bessi, Ludovico; Bryant, Khalil; Carroll, Sean;
   Chambers, Sean; Chattopadhyay, Ankur; Choubey, Apoorv; Deal, Jacob;
   Diaz, Diego; Díaz Pérez, Roberto; Einhorn, Leah; Fan, Thomas; Farid,
   Samaiyah I.; Goudeau, Graham; Guidoni, Silvina; Skjørten Hansen,
   Raymon; Heuer, Peter; Hillairet, Julien; How, Poh Zi; Huang, Yi-Min;
   Humphrey, Nabil; Isupova, Maria; Kent, James; Kulshrestha, Siddharth;
   Kuszaj, Piotr; Lian Lim, Pey; Magarde, Aditya; Martinelli, Joao Victor;
   Munn, Joshua; Parashar, Tulasi; Patel, Neil; Polak, Jakub; Rao, Afzal;
   Raj, Raajit; Rajashekar, Vishwas; Savcheva, Antonia; Shen, Chengcai;
   Nurbu Sherpa, Dawa; Silva, Frank; Singh, Angad; Singh, Ankit; Sipőcz,
   Brigitta; Tavant, Antoine; Varnish, Thomas; Vo, Anthony; Xu, Sixue;
   Zhang, Carol; Du, Tiger; Qudsi, Ramiz; Richardson, Steve; Skinner,
   Cody; Modi, Dhawal; Drozdov, David; Montes, Kevin; Köhn-Seemann,
   Alf; Salcido, Armando; Gorelli, Marco; Lequette, Nicolas; Brown,
   Shane; Stinson, Tomás
2021zndo...5247589C    Altcode:
  A community-developed open source Python 3.6+ package for plasma
  physics research and education that is currently under development.

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Title: Magnetofrictional Modeling of an Erupting Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Gibson, Sarah; Tassev,
   Svetlin; Reeves, Katharine K.; DeLuca, Edward E.; Dalmasse, Kévin
2021ApJ...913...47K    Altcode:
  In this study, we present the magnetic configuration of an erupting
  pseudostreamer observed on 2015 April 19, on the southwest limb of the
  Sun, with a prominence cavity embedded inside. The eruption resulted in
  a partial halo coronal mass ejection. The prominence eruption begins
  with a slow rise and then evolves to a fast-rise phase. We analyze
  this erupting pseudostreamer using the flux-rope insertion method
  and magnetofrictional relaxation to establish a sequence of plausible
  out-of-equilibrium magnetic configurations. This approach allows the
  direct incorporation of observations of structures seen in the corona
  (filament and cavity) to appropriately model the pseudostreamer
  based on SDO/HMI line-of-sight photospheric magnetograms. We also
  perform a topological analysis in order to determine the location
  of quasiseparatrix layers (QSLs) in the models, producing Q-maps to
  examine how the QSL locations progress in the higher iterations. We
  found that the axial flux in our best-fit unstable model was a factor
  of 20 times higher than we found in our marginally stable case. We
  computed the average magnetic field strength of the prominence and
  found that the unstable model exhibits twice the average field strength
  of the stable model. The eruption height from our modeling matches
  very well with the prominence eruption height measured from the AIA
  observation. The Q-maps derived from the model reproduce structures
  observed in LASCO/C2. Thus, the modeling and topological analysis
  results are fully consistent with the observed morphological features,
  implying that we have captured the large magnetic structure of the
  erupting filament in our magnetofrictional simulation.

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Title: PlasmaPy
Authors: Community, PlasmaPy; Everson, Erik; Stańczak, Dominik;
   Murphy, Nicholas A.; Kozlowski, Pawel M.; Malhotra, Ritiek; Langendorf,
   Samuel J.; Leonard, Andrew J.; Stansby, David; Haggerty, Colby C.;
   Mumford, Stuart J.; Beckers, Jasper P.; Satish Bedmutha, Manas;
   Bergeron, Justin; Bessi, Ludovico; Bryant, Khalil; Carroll, Sean;
   Chambers, Sean; Chattopadhyay, Ankur; Choubey, Apoorv; Deal, Jacob;
   Diaz, Diego; Díaz Pérez, Roberto; Einhorn, Leah; Fan, Thomas; Farid,
   Samaiyah I.; Goudeau, Graham; Guidoni, Silvina; Skjørten Hansen,
   Raymon; Heuer, Peter; Hillairet, Julien; How, Poh Zi; Huang, Yi-Min;
   Humphrey, Nabil; Isupova, Maria; Kent, James; Kulshrestha, Siddharth;
   Kuszaj, Piotr; Lian Lim, Pey; Magarde, Aditya; Martinelli, Joao Victor;
   Munn, Joshua; Parashar, Tulasi; Patel, Neil; Polak, Jakub; Rao, Afzal;
   Raj, Raajit; Rajashekar, Vishwas; Savcheva, Antonia; Shen, Chengcai;
   Nurbu Sherpa, Dawa; Silva, Frank; Singh, Angad; Singh, Ankit; Sipőcz,
   Brigitta; Tavant, Antoine; Varnish, Thomas; Vo, Anthony; Xu, Sixue;
   Zhang, Carol; Du, Tiger; Qudsi, Ramiz; Richardson, Steve; Skinner,
   Cody; Modi, Dhawal; Drozdov, David; Montes, Kevin
2021zndo...4602818C    Altcode:
  A community-developed open source Python 3.6+ package for plasma
  physics research and education that is currently under development.

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Title: Laboratory Study of the Torus Instability Threshold in
    Solar-relevant, Line-tied Magnetic Flux Ropes
Authors: Alt, Andrew; Myers, Clayton E.; Ji, Hantao; Jara-Almonte,
   Jonathan; Yoo, Jongsoo; Bose, Sayak; Goodman, Aaron; Yamada, Masaaki;
   Kliem, Bernhard; Savcheva, Antonia
2021ApJ...908...41A    Altcode: 2020arXiv201010607A
  Coronal mass ejections (CMEs) occur when long-lived magnetic flux
  ropes (MFRs) anchored to the solar surface destabilize and erupt
  away from the Sun. This destabilization is often described in
  terms of an ideal magnetohydrodynamic instability called the torus
  instability. It occurs when the external magnetic field decreases
  sufficiently fast such that its decay index, ${n}_{}=-z\,\partial
  (\mathrm{ln}{B}_{})/\partial z$ , is larger than a critical value, $n\gt
  {n}_{\mathrm{cr}}^{}$ , where ${n}_{\mathrm{cr}}^{}=1.5$ for a full,
  large aspect ratio torus. However, when this is applied to solar MFRs,
  a range of conflicting values for ${n}_{\mathrm{cr}}^{}$ is found in
  the literature. To investigate this discrepancy, we have conducted
  laboratory experiments on arched, line-tied flux ropes and applied
  a theoretical model of the torus instability. Our model describes
  an MFR as a partial torus with foot points anchored in a conducting
  surface and numerically calculates various magnetic forces on it. This
  calculation yields better predictions of ${n}_{\mathrm{cr}}^{}$ that
  take into account the specific parameters of the MFR. We describe a
  systematic methodology to properly translate laboratory results to their
  solar counterparts, provided that the MFRs have a sufficiently small
  edge safety factor or, equivalently, a large enough twist. After this
  translation, our model predicts that ${n}_{\mathrm{cr}}^{}$ in solar
  conditions falls near ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim 0.9$
  and within a larger range of ${n}_{\mathrm{cr}}^{\mathrm{solar}}\sim
  (0.7,1.2)$ , depending on the parameters. The methodology of
  translating laboratory MFRs to their solar counterparts enables
  quantitative investigations of CME initiation through laboratory
  experiments. These experiments allow for new physics insights that
  are required for better predictions of space weather events but are
  difficult to obtain otherwise.

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Title: Magnetofrictional Modeling of an erupting Pseudostreamer
Authors: Karna, Nishu; Gibson, Sarah; DeLuca, Edward; Dalmasse,
   Kévin; Savcheva, Antonia; Tassev, Svetlin
2021cosp...43E1768K    Altcode:
  In this study, we present a magnetic configuration of an erupting
  pseudostreamer observed on April 19, 2015 on the Southwest limb,
  embedding a prominence cavity. The eruption resulted in a relatively
  wide CME with a round front and prominence core intersected by a sharp
  plume as seen in SOHO/LASCO C2, a partial halo was observed. The
  prominence eruption begins with a slow rise and then evolves to a
  fast rise phase. We first construct a non-linear force free field
  (NLFFF) model of this erupting pseudostreamer using the flux rope
  insertion method. The NLFFF model produces the 3D coronal magnetic field
  constrained by observed coronal structures and the SDO/HMI photospheric
  magnetogram taken 3 days earlier. We then increase axial and poloidal
  flux in the model to make it unstable. The field configurations
  representing the eruption are not in force-free equilibrium. We
  magnetofrictionally evolve the model until the flux rope expands to
  three solar radii and compare the modeled CME propagation with the
  SOHO/LASCO C2 observations. We perform a topological analysis of the
  models in order to determine the location of quasi-separatrix layers
  (QSLs) and how the QSL locations are transferred as the simulation
  progresses. The model reproduced the LASCO C2 observation structure in
  the QSL map. The modeling and topological analysis results are fully
  consistent with the observed morphological features implying that we
  have captured the large magnetic structure of the erupting filament.

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Title: PlasmaPy
Authors: Community, PlasmaPy; Everson, Erik; Stańczak, Dominik;
   Murphy, Nicholas A.; Kozlowski, Pawel M.; Malhotra, Ritiek; Langendorf,
   Samuel J.; Leonard, Andrew J.; Stansby, David; Haggerty, Colby C.;
   Mumford, Stuart J.; Beckers, Jasper P.; Satish Bedmutha, Manas;
   Bergeron, Justin; Bessi, Ludovico; Bryant, Khalil; Carroll, Sean;
   Chambers, Sean; Chattopadhyay, Ankur; Choubey, Apoorv; Deal, Jacob;
   Diaz, Diego; Díaz Pérez, Roberto; Einhorn, Leah; Fan, Thomas; Farid,
   Samaiyah I.; Goudeau, Graham; Guidoni, Silvina; Skjørten Hansen,
   Raymon; Heuer, Peter; Hillairet, Julien; How, Poh Zi; Huang, Yi-Min;
   Humphrey, Nabil; Isupova, Maria; Kent, James; Kulshrestha, Siddharth;
   Kuszaj, Piotr; Lian Lim, Pey; Magarde, Aditya; Martinelli, Joao Victor;
   Munn, Joshua; Parashar, Tulasi; Patel, Neil; Polak, Jakub; Rao, Afzal;
   Raj, Raajit; Rajashekar, Vishwas; Savcheva, Antonia; Shen, Chengcai;
   Nurbu Sherpa, Dawa; Silva, Frank; Singh, Angad; Singh, Ankit; Sipőcz,
   Brigitta; Tavant, Antoine; Varnish, Thomas; Vo, Anthony; Xu, Sixue;
   Zhang, Carol; Du, Tiger; Qudsi, Ramiz; Richardson, Steve; Skinner, Cody
2020zndo...4313063C    Altcode:
  A community-developed open source Python 3.6+ package for plasma
  physics research and education that is currently under development.

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Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena in Solar and
    Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
   Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
   A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
   Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
   L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
   Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
   G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
   K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
   J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
   A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
   Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
   J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
   Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
   Zenitani, S.; Zweibel, E.
2020arXiv200908779J    Altcode:
  Magnetic reconnection underlies many explosive phenomena in the
  heliosphere and in laboratory plasmas. The new research capabilities in
  theory/simulations, observations, and laboratory experiments provide the
  opportunity to solve the grand scientific challenges summarized in this
  whitepaper. Success will require enhanced and sustained investments
  from relevant funding agencies, increased interagency/international
  partnerships, and close collaborations of the solar, heliospheric,
  and laboratory plasma communities. These investments will deliver
  transformative progress in understanding magnetic reconnection and
  related explosive phenomena including space weather events.

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Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
   F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
   Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
   Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
   Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
   Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
   Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
   Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2020arXiv200400079J    Altcode:
  This white paper summarizes major scientific challenges and
  opportunities in understanding magnetic reconnection and related
  explosive phenomena as a fundamental plasma process.

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Title: PlasmaPy
Authors: Community, PlasmaPy; Stańczak, Dominik; Everson, Erik;
   Murphy, Nicholas A.; Kozlowski, Pawel M.; Malhotra, Ritiek; Langendorf,
   Samuel J.; Leonard, Andrew J.; Stansby, David; Haggerty, Colby C.;
   Mumford, Stuart J.; Beckers, Jasper P.; Satish Bedmutha, Manas;
   Bergeron, Justin; Bessi, Ludovico; Carroll, Sean; Chambers, Sean;
   Choubey, Apoorv; Deal, Jacob; Díaz Pérez, Roberto; Einhorn, Leah;
   Fan, Thomas; Farid, Samaiyah I.; Goudeau, Graham; Guidoni, Silvina;
   Skjørten Hansen, Raymon; Hillairet, Julien; How, Poh Zi; Huang,
   Yi-Min; Humphrey, Nabil; Isupova, Maria; Kent, James; Kulshrestha,
   Siddharth; Kuszaj, Piotr; Lian Lim, Pey; Magarde, Aditya; Martinelli,
   Joao Victor; Munn, Joshua; Parashar, Tulasi; Patel, Neil; Polak,
   Jakub; Rao, Afzal; Raj, Raajit; Savcheva, Antonia; Shen, Chengcai;
   Nurbu Sherpa, Dawa; Silva, Frank; Singh, Angad; Singh, Ankit; Sipőcz,
   Brigitta; Tavant, Antoine; Varnish, Thomas; Xu, Sixue; Zhang, Carol;
   Diaz, Diego
2020zndo...3694337C    Altcode:
  A community-developed open source Python 3.6+ package for plasma
  physics in the early stages of development.

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Title: Evidence for Multiple Acceleration Mechanisms in Coronal Jets
Authors: Farid, S. I.; Reeves, K.; Savcheva, A.; Rodríguez, N.;
   Wainwright, W.
2020AAS...23535902F    Altcode:
  Solar coronal jets are small scale, energetic eruptions, characterized
  by a column-like spire and bright dome-shaped base. Jets are often
  associated with notable changes in the underlying photospheric magnetic
  field, and have been found to initiate when opposite polarity magnetic
  flux elements emerge, cancel, flyby, or otherwise interact. Because
  of their association with transient photospheric flux elements,
  jets are thought to be primarily driven by magnetic reconnection,
  however models describing the relationship between initiation and
  plasma properties during eruption are not well understood, and are
  often contradictory. This is further complicated by observations show
  that jets with similar initiation mechanisms can exhibit a wide range of
  plasma parameters with different topological features, while embedded in
  different coronal environments. Recent 3D models show that in addition
  to magnetic tension and energy released during reconnection, jets may
  also be accelerated via chromospheric evaporation, the untwisting motion
  of the field lines, and/or by Alvenic waves that transverse along newly
  reconnected field lines. In this work, we investigate acceleration
  mechanisms of 8 coronal jets embedded in different environments
  by combining multi-wavelength imaging observations, spectroscopic
  observations and 3D topological modeling. We use observations from
  Hinode's X-ray Telescope (XRT), Solar Dynamics Observatory's Atmospheric
  Imaging Array (SDO-AIA), and Interface Region Imaging Spectrograph
  (IRIS), to capture the plane of sky outflow velocities as a function
  of temperature. When available, we use IRIS spectroscopic observations
  of the SiIV line profile to calculate line-of-sight velocity, Doppler
  velocity and non-thermal line broadening. Next we use a Non-Linear Force
  Free (NLFF) model, to examine the magnetic topology of selected jets
  during their eruption and compare with evolution in EUV. In cases where
  a filament is observed, we employ the filament insertion method. In one
  jet we complete a more thorough topological analysis including modeling
  of the quasi-sepratrix layers (QSL), and energy partition during the
  eruption. We find evidence of chromospheric evaporation in most (75%)
  of the jets, including those jets that exhibit twist. We find that the
  NLFF model matches EUV observations, allowing us to identify the height
  of the null region and the upper limits of the toroidal, and poloidal
  flux. For the first time, we combine observations and topological
  modeling to show evidence of different acceleration mechanisms in
  coronal jets.

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Title: Understanding the Plasma and Magnetic Field Evolution of a
    Filament Using Observations and Nonlinear Force-free Field Modeling
Authors: Yardley, Stephanie L.; Savcheva, Antonia; Green, Lucie M.;
   van Driel-Gesztelyi, Lidia; Long, David; Williams, David R.; Mackay,
   Duncan H.
2019ApJ...887..240Y    Altcode: 2019arXiv191101314Y
  We present observations and magnetic field models of an intermediate
  filament present on the Sun in 2012 August, associated with a polarity
  inversion line that extends from AR 11541 in the east into the quiet
  Sun at its western end. A combination of Solar Dynamics Observatory
  (SDO)/Atmospheric Imaging Assembly, SDO/Helioseismic and Magnetic
  Imager (HMI), and Global Oscillation Network Group Hα data allow
  us to analyze the structure and evolution of the filament from 2012
  August 4 23:00 UT to 2012 August 6 08:00 UT when the filament was in
  equilibrium. By applying the flux rope insertion method, nonlinear
  force-free field models of the filament are constructed using SDO/HMI
  line-of-sight magnetograms as the boundary condition at the two times
  given above. Guided by observed filament barbs, both modeled flux ropes
  are split into three sections each with a different value of axial flux
  to represent the nonuniform photospheric field distribution. The flux
  in the eastern section of the rope increases by 4 × 10<SUP>20</SUP>
  Mx between the two models, which is in good agreement with the amount
  of flux canceled along the internal PIL of AR 11541, calculated to be
  3.2 × 10<SUP>20</SUP> Mx. This suggests that flux cancellation builds
  flux into the filament’s magnetic structure. Additionally, the number
  of field line dips increases between the two models in the locations
  where flux cancellation, the formation of new filament threads, and
  growth of the filament is observed. This suggests that flux cancellation
  associated with magnetic reconnection forms concave-up magnetic field
  that lifts plasma into the filament. During this time, the free magnetic
  energy in the models increases by 0.2 × 10<SUP>31</SUP> ergs.

---------------------------------------------------------
Title: Forward Modeling of a Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Dalmasse, Kévin; Gibson,
   Sarah; Tassev, Svetlin; de Toma, Giuliana; DeLuca, Edward E.
2019ApJ...883...74K    Altcode:
  In this paper, we present an analysis of a pseudostreamer embedding
  a filament cavity, observed on 2015 April 18 on the solar southwest
  limb. We use the flux-rope insertion method to construct nonlinear
  force-free field (NLFFF) models constrained by observed Solar Dynamics
  Observatory (SDO)/AIA coronal structures and the SDO/Helioseismic
  Magnetic Imager photospheric magnetogram. The resulting magnetic field
  models are forward-modeled to produce synthetic data directly comparable
  to Mauna Loa Solar Observatory/Coronal Multichannel Polarimeter (CoMP)
  observations of the intensity and linear polarization of the Fe XIII
  1074.7 nm infrared coronal emission line using FORWARD. In addition,
  we determine the location of quasi-separatrix layers in the magnetic
  models, producing a Q-map from which the signatures of magnetic null
  points and separatrices can be identified. An apparent magnetic null
  observed in linear polarization by CoMP is reproduced by the model
  and appears in the region of the 2D-projected magnetic null in the
  Q-map. Further, we find that the height of the CoMP null is better
  reproduced by our NLFFF model than by the synthetic data we produce
  with potential-field source-surface models, implying the presence of
  a flux rope in the northern lobe of the pseudostreamer.

---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of Solar Coronal Jets in
    Theoretical Configurations
Authors: Meyer, K. A.; Savcheva, A. S.; Mackay, D. H.; DeLuca, E. E.
2019ApJ...880...62M    Altcode:
  Coronal jets occur frequently on the Sun, and may contribute
  significantly to the solar wind. With the suite of instruments
  available now, we can observe these phenomena in greater detail
  than ever before. Modeling and simulations can assist further with
  understanding the dynamic processes involved, but previous studies
  tended to consider only one mechanism (e.g., emergence or rotation)
  for the origin of the jet. In this study we model a series of idealized
  archetypal jet configurations and follow the evolution of the coronal
  magnetic field. This is a step toward understanding these idealized
  situations before considering their observational counterparts. Several
  simple situations are set up for the evolution of the photospheric
  magnetic field: a single parasitic polarity rotating or moving in a
  circular path; as well as opposite polarity pairs involved in flyby
  (shearing), cancellation or emergence; all in the presence of a uniform,
  open background magnetic field. The coronal magnetic field is evolved in
  time using a magnetofrictional relaxation method. While magnetofriction
  cannot accurately reproduce the dynamics of an eruptive phase, the
  structure of the coronal magnetic field, as well as the buildup of
  electric currents and free magnetic energy are instructive. Certain
  configurations and motions produce a flux rope and allow the significant
  buildup of free energy, reminiscent of the progenitors of so-called
  blowout jets, whereas other, simpler configurations are more comparable
  to the standard jet model. The next stage is a comparison with observed
  coronal jet structures and their corresponding photospheric evolution.

---------------------------------------------------------
Title: Data-optimized Coronal Field Model. I. Proof of Concept
Authors: Dalmasse, K.; Savcheva, A.; Gibson, S. E.; Fan, Y.; Nychka,
   D. W.; Flyer, N.; Mathews, N.; DeLuca, E. E.
2019ApJ...877..111D    Altcode: 2019arXiv190406308D
  Deriving the strength and direction of the three-dimensional
  (3D) magnetic field in the solar atmosphere is fundamental for
  understanding its dynamics. Volume information on the magnetic field
  mostly relies on coupling 3D reconstruction methods with photospheric
  and/or chromospheric surface vector magnetic fields. Infrared
  coronal polarimetry could provide additional information to better
  constrain magnetic field reconstructions. However, combining such
  data with reconstruction methods is challenging, e.g., because of the
  optical thinness of the solar corona and the lack and limitations of
  stereoscopic polarimetry. To address these issues, we introduce the
  data-optimized coronal field model (DOCFM) framework, a model-data
  fitting approach that combines a parameterized 3D generative model,
  e.g., a magnetic field extrapolation or a magnetohydrodynamic model,
  with forward modeling of coronal data. We test it with a parameterized
  flux-rope insertion method and infrared coronal polarimetry where
  synthetic observations are created from a known “ground-truth”
  physical state. We show that this framework allows us to accurately
  retrieve the ground-truth 3D magnetic field of a set of force-free
  field solutions from the flux-rope insertion method. In observational
  studies, the DOCFM will provide a means to force the solutions
  derived with different reconstruction methods to satisfy additional
  common coronal constraints. The DOCFM framework therefore opens new
  perspectives for the exploitation of coronal polarimetry in magnetic
  field reconstructions and for developing new techniques to more
  reliably infer the 3D magnetic fields that trigger solar flares and
  coronal mass ejections.

---------------------------------------------------------
Title: Flux ropes vs Sheared Arcades - the Role of Axial Flu
Authors: Savcheva, Antonia Stefanova
2019shin.confE..33S    Altcode:
  The flux rope insertion method for producing non-linear force-free
  magnetic field models has been used on more than 20 regions to study
  their 3D magnetic field structure and topology in their pre-eruption
  state. This has been done sometimes over a period of several days
  preceding major solar eruptions. Here we present an exploration of the
  quasi-static behaviour of the 3D magnetic field structure, geometrical
  properties and topology of these states (whenever available over time)
  of several active regions in different ambient field configurations. We
  study the role of poloidal and axial flux on the appearance of the
  field lines and whether the best-fit models include flux ropes or a
  sheared arcades; how this depends on the initial inserted (guessed)
  states, and how the observations have played a role in constraining the
  models. Also we look at possible transitions between sheared arcade and
  flux rope and when they occur in the evolution of the active regions
  when such studies are available

---------------------------------------------------------
Title: Major Scientific Challenges and Opportunities in Understanding
    Magnetic Reconnection and Related Explosive Phenomena throughout
    the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
   Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
   D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
   Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
   Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
   R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
   Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
   Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
   J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
   A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
   W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
   W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
   P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
   T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
   V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
   Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
   D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
   C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
2019BAAS...51c...5J    Altcode: 2019astro2020T...5J
  This is a group white paper of 100 authors (each with explicit
  permission via email) from 51 institutions on the topic of magnetic
  reconnection which is relevant to 6 thematic areas. Grand challenges
  and research opportunities are described in observations, numerical
  modeling and laboratory experiments in the upcoming decade.

---------------------------------------------------------
Title: Coiling and Squeezing: Properties of the Local Transverse
    Deviations of Magnetic Field Lines
Authors: Tassev, Svetlin; Savcheva, Antonia
2019arXiv190100865T    Altcode:
  We study the properties of the local transverse deviations of magnetic
  field lines at a fixed moment in time. Those deviations "evolve"
  smoothly in a plane normal to the field-line direction as one moves
  that plane along the field line. Since the evolution can be described
  by a planar flow in the normal plane, we derive most of our results in
  the context of a toy model for planar fluid flow. We then generalize
  our results to include the effects of field-line curvature. We show
  that the type of flow is determined by the two non-zero eigenvalues
  of the gradient of the normalized magnetic field. The eigenvalue
  difference quantifies the local rate of squeezing or coiling of
  neighboring field lines, which we relate to standard notions of fluid
  vorticity and shear. The resulting squeezing rate can be used in the
  detection of null points, hyperbolic flux tubes and current sheets. Once
  integrated along field lines, that rate gives a squeeze factor, which
  is an approximation to the squashing factor, which is usually employed
  in locating quasi-separatrix layers (QSLs), which are possible sites
  for magnetic reconnection. Unlike the squeeze factor, the squashing
  factor can miss QSLs for which field lines are squeezed and then
  unsqueezed. In that regard, the squeeze factor is a better proxy for
  locating QSLs than the squashing factor. In another application of our
  analysis, we construct an approximation to the local rate of twist of
  neighboring field lines, which we refer to as the coiling rate. That
  rate can be integrated along a field line to give a coiling number,
  $\mathrm{N_c}$. We show that unlike the standard local twist number,
  $\mathrm{N_c}$ gives an unbiased approximation to the number of twists
  neighboring field lines make around one another. $\mathrm{N_c}$ can
  be useful for the study of flux rope instabilities, such as the kink
  instability, and can be used in the detection of flux ropes.

---------------------------------------------------------
Title: A Novel Approach to Determining the Acceleration Mechanism
    of Coronal Jets
Authors: Farid, Samaiyah I.; Reeves, Katharine; Savcheva, Antonia
2019AAS...23340103F    Altcode:
  Coronal jets are thought to be the result of magnetic reconnection,
  often when bipolar magnetic fields emerge into the open, ambient
  corona. Jet parameters vary widely, making the ability to understand
  the acceleration mechanism difficult. This is further complicated
  by the wide range of jet topologies, local environments, and
  magnetic field configurations. In this work we approach this problem
  twofold. First we calculate the plasma parameters of several active
  region jets, including the plane of sky velocity, Doppler velocity
  (when data is available), the differential emission measure (DEM),
  and underlying magnetic flux. We calculate the velocity as a function
  of temperature and estimate the emission measure weighted temperature
  during the evolution of the jet. In some jets, we find evidence of
  a temperature-dependent velocity- characteristic of chromospheric
  evaporation, commonly observed in active region flares. We also use
  the Coronal Modeling System (CMS), a Non-Linear Force Free (NLFF)
  model, to examine the topology of selected jets before and during
  their eruption. In cases where a filament is observed in EUV, we
  employ the filament insertion method. We find that in several jets,
  the NLFF model matches the EUV observations of the jet spire well,
  allowing us to identify the height of the null point (region) and
  the upper limits of the toroidal, and poloidal flux. In other cases,
  we find that the direction of the spire is distorted by nearby features
  (large filaments, coronal holes, etc.). Finally, we estimate the thermal
  flux during the jet eruption and determine if we should expect explosive
  or gentle reconnection. All of these observations combined give unique
  insight in the acceleration mechanism(s) of coronal jets.

---------------------------------------------------------
Title: Magnetic Reconnection Null Points as the Origin of
    Semirelativistic Electron Beams in a Solar Jet
Authors: Chen, Bin; Yu, Sijie; Battaglia, Marina; Farid, Samaiyah;
   Savcheva, Antonia; Reeves, Katharine K.; Krucker, Säm; Bastian,
   T. S.; Guo, Fan; Tassev, Svetlin
2018ApJ...866...62C    Altcode: 2018arXiv180805951C
  Magnetic reconnection, the central engine that powers explosive
  phenomena throughout the universe, is also perceived to be one
  of the principal mechanisms for accelerating particles to high
  energies. Although various signatures of magnetic reconnection
  have been frequently reported, observational evidence that links
  particle acceleration directly to the reconnection site has been rare,
  especially for space plasma environments currently inaccessible to in
  situ measurements. Here we utilize broadband radio dynamic imaging
  spectroscopy available from the Karl G. Jansky Very Large Array to
  observe decimetric type III radio bursts in a solar jet with high
  angular (∼20″), spectral (∼1%), and temporal resolution (50
  ms). These observations allow us to derive detailed trajectories of
  semirelativistic (tens of keV) electron beams in the low solar corona
  with unprecedentedly high angular precision (&lt;0.″65). We found that
  each group of electron beams, which corresponds to a cluster of type III
  bursts with 1-2 s duration, diverges from an extremely compact region
  (∼600 km<SUP>2</SUP>) in the low solar corona. The beam-diverging
  sites are located behind the erupting jet spire and above the closed
  arcades, coinciding with the presumed location of magnetic reconnection
  in the jet eruption picture supported by extreme ultraviolet/X-ray
  data and magnetic modeling. We interpret each beam-diverging site as a
  reconnection null point where multitudes of magnetic flux tubes join
  and reconnect. Our data suggest that the null points likely consist
  of a high level of density inhomogeneities possibly down to 10 km
  scales. These results, at least in the present case, strongly favor
  a reconnection-driven electron-acceleration scenario.

---------------------------------------------------------
Title: Data-constrained simulation of a Double-decker Eruption
Authors: Savcheva, Antonia; Kliem, B.; Downs, Copper; Torok, Tibor
2018shin.confE..91S    Altcode:
  We present the challenges we encountered in producing the initial
  condition for the hypnotized double-decker flux rope eruption on
  12/07/12. We then use this I.C. in Kliem-Torok zero-beta MHD simulation
  to produce an eruption and reproduce overall magnetic field structure
  of the eruption. We also produce a full thermodynamic simulation with
  MAS of a simpler IC which also produces similarities to the observed
  ribbons and dimmings. Both approaches have their advantages.

---------------------------------------------------------
Title: Possible Evidence of Chromospheric Evaporation in Coronal Jets
Authors: Farid, Samaiyah I.; Soto, N.; Reeves, K.; Savcheva, A.
2018shin.confE..90F    Altcode:
  Chromospheric evaporation is commonly associated with flaring
  active regions, when magnetic reconnection in the corona heats and
  drives chromospheric material upward at velocities comparable to the
  local sound speed. In those cases, the tell-tell signs are enhanced
  blue shifts in enhanced blue shifts in hot lines such as Fe XXI,
  Fe XXIII and Fe XXIV, and a notable increase in plasma velocity as
  a function of temperature. Coronal jets could also exhibit evidence
  of chromospheric evaporation when magnetic reconnection occurs. In
  this study, we use observations from Hinode's X-ray Telescope (XRT),
  Solar Dynamics Observatory's Atmospheric Imaging Array (SDO/AIA), and
  Interface Region Imaging Spectrograph (IRIS) to construct line-of-sight
  velocities as a function of temperature along the spire of several
  jets. We also construct differential emission measures over the time
  of the jet eruptions and calculate the rate of change in the thermal
  energy flux, to determine if it is characteristic of explosive or gentle
  evaporation. We present evidence of a temperature-dependent velocity
  in jets ranging from 200 500 km/sec , consistent with chromospheric
  evaporation.

---------------------------------------------------------
Title: Solar Eruptions Initiated in Sigmoidal Active Regions
Authors: Savcheva, Antonia
2018cosp...42E2995S    Altcode:
  Coronal sigmoids, generally observed in X-rays and EUV, are
  S-shapedactive regions that have been shown to possess high
  probability foreruption. They present a direct evidence of the
  existence of fluxropes in the corona prior to the impulsive phase
  of eruptions. Inorder to gain insight into their eruptive behavior
  and how they getdestabilized we need to know their 3D magnetic field
  structure. First,we review some recent observations and modeling of
  sigmoidal activeregions as the primary hosts of solar eruptions,
  which can also beused as useful laboratories for studying these
  phenomena. Then, weconcentrate on the analysis of observations and
  highlydata-constrained non-linear force-free field (NLFFF) models
  over thelifetime of several sigmoidal active regions, where we have
  capturedtheir magnetic field structure around the times of major
  flares. Wepresent the topology analysis of a couple of sigmoidal
  regionspointing us to the probable sites of reconnection. A scenario
  foreruption is put forward by this analysis. We demonstrate the use
  ofthis topology analysis to reconcile the observed eruption featureswith
  the standard flare model. Finally, we show a glimpse of how sucha NLFFF
  models of an erupting region can be used to initiate a CMEs in MHD
  simulations with an unprecedented realistic manner. Such simulations
  can show the effects of solar transients on the near-Earth environment
  and solar system space weather.

---------------------------------------------------------
Title: Computation of Relative Magnetic Helicity in Spherical
    Coordinates
Authors: Moraitis, Kostas; Pariat, Étienne; Savcheva, Antonia;
   Valori, Gherardo
2018SoPh..293...92M    Altcode: 2018arXiv180603011M
  Magnetic helicity is a quantity of great importance in solar studies
  because it is conserved in ideal magnetohydrodynamics. While many
  methods for computing magnetic helicity in Cartesian finite volumes
  exist, in spherical coordinates, the natural coordinate system
  for solar applications, helicity is only treated approximately. We
  present here a method for properly computing the relative magnetic
  helicity in spherical geometry. The volumes considered are finite,
  of shell or wedge shape, and the three-dimensional magnetic field is
  considered to be fully known throughout the studied domain. Testing of
  the method with well-known, semi-analytic, force-free magnetic-field
  models reveals that it has excellent accuracy. Further application to
  a set of nonlinear force-free reconstructions of the magnetic field of
  solar active regions and comparison with an approximate method used
  in the past indicates that the proposed method can be significantly
  more accurate, thus making our method a promising tool in helicity
  studies that employ spherical geometry. Additionally, we determine
  and discuss the applicability range of the approximate method.

---------------------------------------------------------
Title: Modeling the Evolution of a Sigmoid: Correlating Model
    Instability with Observed Events
Authors: Prchlik, Jakub; Savcheva, Antonia Stefanova; Karna, Nishu
2018tess.conf20339P    Altcode:
  Predicting solar eruptions and flares require a fundamental
  understanding of the features producing solar eruptions and flares.

---------------------------------------------------------
Title: Non Linear Force Free Field modeling of an erupting
    pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia Stefanova; Gibson, Sarah E.;
   Tassev, Svetlin
2018tess.conf10412K    Altcode:
  Coronal mass ejections (CMEs) are the most violent eruptions in our
  Solar System. CMEs are responsible for large solar energetic particle
  events and severe geomagnetic storms. In this study, we present a
  magnetic configuration of an erupting pseudostreamer observed on April
  19, 2015 on the Southern West limb embedding a prominence cavity. The
  eruption resulted in a relatively wide CME with a round front and
  prominence core. In SOHO/LASCO C2 partial halo was observed. The
  prominence eruption begins with a slow rise and then evolves to a
  fast rise phase. We first constructed a non-linear force free field
  (NLFFF) model of this erupting pseudostreamer using the flux rope
  insertion method. The NLFFF model produces the 3D coronal magnetic
  field constrained by observed coronal structures and photospheric
  magnetogram. SDO/HMI magnetogram was used as an input for the model. The
  field configurations representing the eruption are not in force-free
  equilibrium. We magnetofrictionally relax the model until the flux
  rope expands to three solar radii and compare CME propagation with the
  SOHO/LASCO C2 observations. From the simulation results, we determine
  the process for the eruption by identifying where reconnection takes
  place and how much flux is reconnected. We determine the pre-eruption
  twist and decay index and how the twist is transferred as the simulation
  progresses. In addition, we perform a topology analysis of the models in
  order to determine the location of quasi-separatrix layers (QSLs). QSLs
  are used as a proxy to determine where strong electric current sheets
  develop in the corona and also provide important information about
  the connectivity in this complicated magnetic field configuration.

---------------------------------------------------------
Title: Non-Linear Force-Free Field Modelling of Solar Coronal Jets
    in Theoretical Configurations
Authors: Savcheva, Antonia
2017SPD....4810622S    Altcode:
  Coronal jets occur frequently on the Sun, and may contribute
  significantly to the solar wind. With the suite of instruments avilable
  now, e.g. on IRIS, Hinode and SDO, we can observe these phenomena in
  greater detail than ever before. Modeling and simulations can assist
  further in understanding the dynamic processes involved, but previous
  studies tend to consider only one mechanism (e.g. emergence or rotation)
  for the origin of the jet. In this study we model a series of idealised
  archetypaljet configurations and follow the evolution of the coronal
  magnetic field. This is a step towards understanding these idealised
  situations before considering their observational counterparts. Several
  simple situations are set up for the evolution of the photospheric
  magnetic field: a single parasitic polarity rotating or moving in a
  circular path; as well as opposite polarity pairs involved in flyby
  (shearing), cancellation or emergence; all in the presence of a uniform,
  open background magneticfield. The coronal magnetic field is evolved in
  time using a magnetofrictional relaxation method. While magnetofriction
  cannot accurately reproduce the dynamics of an eruptive phase, the
  structure of the coronal magnetic field, as well as the build up of
  electric currents and free magnetic energy are instructive. Certain
  configurations and motions produce a flux rope and allow the significant
  build up of free energy, reminiscent of the progenitors of so-called
  blowout jets, whereas other, simpler configurations are more comparable
  to the standard jet model. The next stage is a comparison with observed
  coronal jet structures and their corresponding photospheric evolution.

---------------------------------------------------------
Title: Non Linear Force Free Field Modeling for a Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Gibson, Sarah; Tassev,
   Svetlin V.
2017SPD....48.0701K    Altcode:
  In this study we present a magnetic configuration of a pseudostreamer
  observed on April 18, 2015 on southern west limb embedding a filament
  cavity. We constructed Non Linear Force Free Field (NLFFF) model
  using the flux rope insertion method. The NLFFF model produces the
  three-dimensional coronal magnetic field constrained by observed coronal
  structures and photospheric magnetogram. SDO/HMI magnetogram was used
  as an input for the model. The high spatial and temporal resolution
  of the SDO/AIA allows us to select best-fit models that match the
  observations. The MLSO/CoMP observations provide full-Sun observations
  of the magnetic field in the corona. The primary observables of CoMP
  are the four Stokes parameters (I, Q, U, V). In addition, we perform a
  topology analysis of the models in order to determine the location of
  quasi-separatrix layers (QSLs). QSLs are used as a proxy to determine
  where the strong electric current sheets can develop in the corona and
  also provide important information about the connectivity in complicated
  magnetic field configuration. We present the major properties of the 3D
  QSL and FLEDGE maps and the evolution of 3D coronal structures during
  the magnetofrictional process. We produce FORWARD-modeled observables
  from our NLFFF models and compare to a toy MHD FORWARD model and the
  observations.

---------------------------------------------------------
Title: Coronal Jet Plasma Properties and Acceleration Mechanisms
Authors: Farid, Samaiyah; Reeves, Kathy; Savcheva, Antonia; Soto,
   Natalia
2017SPD....4830405F    Altcode:
  Coronal jets are transient eruptions of plasma typically characterized
  by aprominent long spire and a bright base, and sometimes accompanied
  by a small filament. Jets are thought to be produced by magnetic
  reconnection when small-scale bipolar magnetic fields emerge into an
  overlying coronal field or move into a locally unipolar region. Coronal
  jets are commonly divided into two categories: standard jets and
  blowout jets, and are found in both quiet and active regions. The
  plasma properties of jets vary across type and location, therefore
  understanding the underlying acceleration mechanisms are difficult to
  pin down. In this work, we examine both blow-out and standard jets
  using high resolution multi-wavelength data. Although reconnection
  is commonly accepted as the primary acceleration mechanism, we also
  consider the contribution chromospheric evaporation to jet formation. We
  use seven coronal channels from SDO/AIA , Hinode/XRT Be-thin and IRIS
  slit-jaw data. In addition, we separate the Fe-XVIII line from the
  SDO/94Å channel. We calculate plasma properties including velocity,
  Alfven speed, and density as a function of wavelength and Differential
  Emission Measure (DEM). Finally, we explore the magnetic topology of
  the jets using Coronal Modeling System (CMS) to construct potential and
  non-linear force free models based on the flux rope insertion method.

---------------------------------------------------------
Title: Data-Constrined Simulations of CME eruption
Authors: Savcheva, Antonia; Lugaz, Noe; van der Holst, Bart; Evans,
   Rebekah; Zhang, Jie
2017shin.confE..33S    Altcode:
  We perform the first global data-constrained MHD simulation of a CME
  with the Space Weather Modeling Framework (SWMF). This code has fully
  developed state-of-the-art steady state solar wind driven by Alfven
  wave turbulence, in which disturbances can be propagated using ideal or
  resistive MHD, full thermodynamics, and various other physics. The CME
  can be propagated to 1AU and the interaction with the magnetosphere
  can be studied. The initial condition for the simulation is the
  best-fit 3D non-linear force free field (NLFFF) model obtained with
  the flux rope insertion method of the active region CME on April 08,
  2010. The boundary condition is a synoptic magnetogram from SOLIS, with
  a high resolution HMI piece around the active region. We discuss the
  capabilities built-in into SWMF for producing fully data-constrained
  models of CMEs. We show the initiation and propagation of the CME
  within 10 Rsun. The stability of the region has already been analyzed
  in our previous studies, which justifies the use of unstable models
  as initial conditions. We discuss the effect of the different initial
  conditions on the propagation of the CME. We compare simulated LASCO
  and STEREO white light original, running and base difference images
  with the actual observations and demonstrate the power of using data to
  constrain the initial and boundary conditions of such a simulation. We
  compare the velocity profiles, height-time plots, and deflections of
  different realizations of the simulations with those derived from
  the observations. In addition, we simulate the EUV corona of the
  pre-eruption configuration in several AIA filters and compare to AIA
  observations of the sigmoidal regions before the eruption. EUV images
  during the eruptions are compared with the images of the EUV wave
  observed in the vicinity of this region.

---------------------------------------------------------
Title: Non Linear Force Free Field Modeling for a Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Gibson, Sarah
2017shin.confE..52K    Altcode:
  In this study we present a magnetic configuration of a pseudostreamer
  observed on April 18, 2015 on southern west limb embedding a filament
  cavity. We constructed Non Linear Force Free Field (NLFFF) model
  using the flux rope insertion method. The NLFFF model produces the
  three-dimensional coronal magnetic field constrained by observed coronal
  structures and photospheric magnetogram. SDO/HMI magnetogram was used
  as an input for the model. The high spatial and temporal resolution
  of the SDO/AIA allows us to select best-fit models that match the
  observations. The MLSO/CoMP observations provide full-Sun observations
  of the magnetic field in the corona. The primary observables of CoMP
  are the four Stokes parameters (I, Q, U, V). In addition, we perform a
  topology analysis of the models in order to determine the location of
  quasi-separatrix layers (QSLs). QSLs are used as a proxy to determine
  where the strong electric current sheets can develop in the corona and
  also provide important information about the connectivity in complicated
  magnetic field configuration. We present the major properties of the 3D
  QSL and FLEDGE maps and the evolution of 3D coronal structures during
  the magnetofrictional process.

---------------------------------------------------------
Title: Magnetic Flux Rope Shredding By a Hyperbolic Flux Tube:
    The Detrimental Effects of Magnetic Topology on Solar Eruptions
Authors: Chintzoglou, Georgios; Vourlidas, Angelos; Savcheva, Antonia;
   Tassev, Svetlin; Tun Beltran, Samuel; Stenborg, Guillermo
2017ApJ...843...93C    Altcode: 2017arXiv170600057C
  We present the analysis of an unusual failed eruption captured in high
  cadence and in many wavelengths during the observing campaign in support
  of the Very high Angular resolution Ultraviolet Telescope (VAULT2.0)
  sounding rocket launch. The refurbished VAULT2.0 is a Lyα (λ 1216 Å)
  spectroheliograph launched on 2014 September 30. The campaign targeted
  active region NOAA AR 12172 and was closely coordinated with the Hinode
  and IRIS missions and several ground-based observatories (NSO/IBIS,
  SOLIS, and BBSO). A filament eruption accompanied by a low-level
  flaring event (at the GOES C-class level) occurred around the VAULT2.0
  launch. No coronal mass ejection was observed. The eruption and its
  source region, however, were recorded by the campaign instruments in
  many atmospheric heights ranging from the photosphere to the corona
  in high cadence and spatial resolution. This is a rare occasion
  that enabled us to perform a comprehensive investigation on a failed
  eruption. We find that a rising Magnetic Flux Rope (MFR)-like structure
  was destroyed during its interaction with the ambient magnetic field,
  creating downflows of cool plasma and diffuse hot coronal structures
  reminiscent of “cusps.” We employ magnetofrictional simulations to
  show that the magnetic topology of the ambient field is responsible for
  the destruction of the MFR. Our unique observations suggest that the
  magnetic topology of the corona is a key ingredient for a successful
  eruption.

---------------------------------------------------------
Title: QSL Squasher: A Fast Quasi-separatrix Layer Map Calculator
Authors: Tassev, Svetlin; Savcheva, Antonia
2017ApJ...840...89T    Altcode: 2016arXiv160900724T
  Quasi-Separatrix Layers (QSLs) are a useful proxy for the locations
  where current sheets can develop in the solar corona, and give valuable
  information about the connectivity in complicated magnetic field
  configurations. However, calculating QSL maps, even for two-dimensional
  slices through three-dimensional models of coronal magnetic fields,
  is a non-trivial task, as it usually involves tracing out millions
  of magnetic field lines with immense precision. Thus, extending QSL
  calculations to three dimensions has rarely been done until now. In
  order to address this challenge, we present QSL Squasher—a public,
  open-source code, which is optimized for calculating QSL maps in
  both two and three dimensions on graphics processing units. The
  code achieves large processing speeds for three reasons, each of
  which results in an order-of-magnitude speed-up. (1) The code is
  parallelized using OpenCL. (2) The precision requirements for the QSL
  calculation are drastically reduced by using perturbation theory. (3)
  A new boundary detection criterion between quasi-connectivity domains
  is used, which quickly identifies possible QSL locations that need to
  be finely sampled by the code. That boundary detection criterion relies
  on finding the locations of abrupt field-line length changes, which we
  do by introducing a new Field-line Length Edge (FLEDGE) map. We find
  FLEDGE maps useful on their own as a quick-and-dirty substitute for QSL
  maps. QSL Squasher allows construction of high-resolution 3D FLEDGE
  maps in a matter of minutes, which is two orders of magnitude faster
  than calculating the corresponding 3D QSL maps. We include a sample of
  calculations done using QSL Squasher to demonstrate its capabilities
  as a QSL calculator, as well as to compare QSL and FLEDGE maps.

---------------------------------------------------------
Title: Mechanisms of Plasma Acceleration in Coronal Jets
Authors: Soto, N.; Reeves, K.; Savcheva, A. S.
2016AGUFMSH31B2568S    Altcode:
  Jets are small explosions that occur frequently in the Sun possibly
  driven by the local reconfiguration of the magnetic field, or
  reconnection. There are two types of coronal jets: standard jets
  and blowout jets. The purpose of this project is to determine which
  mechanisms accelerate plasma in two different jets, one that occurred
  in January 17, 2015 at the disk of the sun and another in October
  24, 2015 at the limb. Two possible acceleration mechanisms are
  chromospheric evaporation and magnetic acceleration. Using SDO/AIA,
  Hinode/XRT and IRIS data, we create height-time plots, and calculate
  the velocities of each wavelength for both jets. We calculate the
  potential magnetic field of the jet and the general region around it
  to gain a more detailed understanding of its structure, and determine
  if the jet is likely to be either a standard or blowout jet. Finally,
  we calculate the magnetic field strength for different heights along
  the jet spire, and use differential emission measures to calculate
  the plasma density. Once we have these two values, we calculate the
  Alfven speed. When analyzing our results we are looking for certain
  patterns in our velocities. If the plasma in a jet is accelerated by
  chromospheric evaporation, we expect the velocities to increase as
  function of temperature, which is what we observed in the October 24th
  jet. The magnetic models for this jet also show the Eiffel Tower shaped
  structure characteristic of standard jets, which tend to have plasma
  accelerated by this mechanism. On the other hand, if the acceleration
  mechanism were magnetic acceleration, we would expect the velocities
  to be similar regardless of temperature. For the January 17th jet,
  we saw that along the spire, the velocities where approximately 200
  km/s in all wavelengths, but the velocities of hot plasma detected at
  the base were closer to the Alfven speed, which was estimated to be
  about 2,000 km/s. These observations suggest that the plasma in the
  January 17th jet is magnetically accelerated. The magnetic model for
  this jet needs to be studied further by using a NLFFF magnetic field
  model and not just the potential magnetic field. This work supported
  by the NSF-REU solar physics program at SAO, grant number AGS-1560313
  and NASA Grant NNX15AF43G

---------------------------------------------------------
Title: Solar Coronal Jets: Observations, Theory, and Modeling
Authors: Raouafi, N. E.; Patsourakos, S.; Pariat, E.; Young, P. R.;
   Sterling, A. C.; Savcheva, A.; Shimojo, M.; Moreno-Insertis, F.;
   DeVore, C. R.; Archontis, V.; Török, T.; Mason, H.; Curdt, W.;
   Meyer, K.; Dalmasse, K.; Matsui, Y.
2016SSRv..201....1R    Altcode: 2016arXiv160702108R; 2016SSRv..tmp...31R
  Coronal jets represent important manifestations of ubiquitous solar
  transients, which may be the source of significant mass and energy
  input to the upper solar atmosphere and the solar wind. While
  the energy involved in a jet-like event is smaller than that of
  "nominal" solar flares and coronal mass ejections (CMEs), jets
  share many common properties with these phenomena, in particular,
  the explosive magnetically driven dynamics. Studies of jets could,
  therefore, provide critical insight for understanding the larger,
  more complex drivers of the solar activity. On the other side of the
  size-spectrum, the study of jets could also supply important clues on
  the physics of transients close or at the limit of the current spatial
  resolution such as spicules. Furthermore, jet phenomena may hint to
  basic process for heating the corona and accelerating the solar wind;
  consequently their study gives us the opportunity to attack a broad
  range of solar-heliospheric problems.

---------------------------------------------------------
Title: Magnetic Flux Rope Shredding by Quasi-Separatrix Layers:
    The Detrimental Effects of Magnetic Topology on Solar Eruptions
Authors: Chintzoglou, Georgios; Stenborg, Guillermo; Savcheva, Antonia;
   Vourlidas, Angelos; Tassev, Svetlin; Tun Beltran, Samuel
2016cosp...41E.348C    Altcode:
  We present the analysis of an unusual failed eruption event observed in
  high cadence and in many wavelengths during the campaign in support of
  the VAULT2.0 sounding rocket launch. The refurbished Very high Angular
  resolution Ultraviolet Telescope (VAULT2.0) is a Lyalpha (1216AA)
  spectroheliograph launched on September 30, 2014. The objective of the
  VAULT2.0 project is the study of the chromosphere-corona interface. The
  observing campaign targeted active region AR 12172 and was closely
  coordinated with the textsl{Hinode/} and textsl{IRIS/} missions and
  several ground-based observatories (NSO/IBIS an SOLIS, and BBSO)
  ). A filament eruption accompanied by small level heating (at the
  GOES C-class level) occurred around the VAULT2.0 launch. No CME was
  observed. The eruption and its source region, however, was recorded by
  the campaign instruments in all atmospheric heights ranging from the
  photosphere to the corona in high cadence and spatial resolution. This
  is a rare occasion which enables us to perform a comprehensive
  investigation on a failed eruption. We find that a rising Magnetic
  Flux Rope-like (MFR) structure was destroyed during its interaction
  with the overlying magnetic field creating downflows of cool plasma and
  diffuse hot coronal structures reminiscent of 'spines'. We employ MHD
  simulations to show that the magnetic topology of the overlying field
  is responsible for the destruction of the MFR. Our unique observations
  suggest that the magnetic topology of the corona is a key ingredient
  for a successful eruption.

---------------------------------------------------------
Title: Evolution of flare ribbons, electric currents, and
    quasi-separatrix layers during an X-class flare
Authors: Janvier, M.; Savcheva, A.; Pariat, E.; Tassev, S.;
   Millholland, S.; Bommier, V.; McCauley, P.; McKillop, S.; Dougan, F.
2016A&A...591A.141J    Altcode: 2016arXiv160407241J
  Context. The standard model for eruptive flares has been extended
  to three dimensions (3D) in the past few years. This model predicts
  typical J-shaped photospheric footprints of the coronal current
  layer, forming at similar locations as the quasi-separatrix layers
  (QSLs). Such a morphology is also found for flare ribbons observed in
  the extreme ultraviolet (EUV) band, and in nonlinear force-free field
  (NLFFF) magnetic field extrapolations and models. <BR /> Aims: We
  study the evolution of the photospheric traces of the current density
  and flare ribbons, both obtained with the Solar Dynamics Observatory
  instruments. We aim to compare their morphology and their time
  evolution, before and during the flare, with the topological features
  found in a NLFFF model. <BR /> Methods: We investigated the photospheric
  current evolution during the 06 September 2011 X-class flare
  (SOL2011-09-06T22:20) occurring in NOAA AR 11283 from observational data
  of the magnetic field obtained with the Helioseismic and Magnetic Imager
  aboard the Solar Dynamics Observatory. We compared this evolution with
  that of the flare ribbons observed in the EUV filters of the Atmospheric
  Imager Assembly. We also compared the observed electric current density
  and the flare ribbon morphology with that of the QSLs computed from
  the flux rope insertion method-NLFFF model. <BR /> Results: The NLFFF
  model shows the presence of a fan-spine configuration of overlying
  field lines, due to the presence of a parasitic polarity, embedding
  an elongated flux rope that appears in the observations as two parts
  of a filament. The QSL signatures of the fan configuration appear as
  a circular flare ribbon that encircles the J-shaped ribbons related
  to the filament ejection. The QSLs, evolved via a magnetofrictional
  method, also show similar morphology and evolution as both the current
  ribbons and the EUV flare ribbons obtained several times during the
  flare. <BR /> Conclusions: For the first time, we propose a combined
  analysis of the photospheric traces of an eruptive flare, in a complex
  topology, with direct measurements of electric currents and QSLs
  from observational data and a magnetic field model. The results,
  obtained by two different and independent approaches 1) confirm
  previous results of current increase during the impulsive phase of the
  flare and 2) show how NLFFF models can capture the essential physical
  signatures of flares even in a complex magnetic field topology. <P
  />A movie associated to Fig. 1 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628406/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Laboratory identification of MHD eruption criteria in the
    solar corona
Authors: Myers, Clayton E.; Yamada, M.; Ji, H.; Yoo, J.; Jara-Almonte,
   J.; Fox, W.; Savcheva, A.; DeLuca, E. E.
2016shin.confE.161M    Altcode:
  Ideal magnetohydrodynamic (MHD) instabilities such as the kink [1]
  and torus [2] instabilities are believed to play an important role
  in driving "storage-and-release" eruptions in the solar corona. These
  instabilities act on long-lived, arched magnetic flux ropes that are
  "line-tied" to the solar surface. In spite of numerous observational and
  computational studies, the conditions under which these instabilities
  produce an eruption remain a subject of intense debate. In this paper,
  we use a line-tied, arched flux rope experiment to systematically study
  storage-and-release eruption mechanisms in the laboratory [3]. Thin
  in situ magnetic probes facilitate the study of both the equilibrium
  and the stability of these laboratory flux ropes. In particular,
  they permit the direct measurement of magnetic (J-B) forces, both
  in equilibrium and during dynamic events. Regarding stability and
  eruptions, two major results are reported here: First, a new stability
  regime is identified where torus-unstable flux ropes fail to erupt. In
  this "failed torus" regime, the flux rope is torus-unstable but
  kink-stable. Under these conditions, a dynamic "toroidal field tension
  force" surges in magnitude and prevents the flux rope from erupting
  [4]. This dynamic tension force, which is missing from existing eruption
  models, is generated by magnetic self-organization events within the
  line-tied flux rope. Second, a clear torus instability threshold is
  observed in the kink-unstable regime. This latter result, which is
  consistent with existing theoretical [5] and numerical [6] results,
  verifies the key role of the torus instability in driving flux rope
  eruptions in the solar corona. <P />[1] A. W. Hood &amp; E. R. Priest,
  Geophys. Astrophys. Fluid Dynamics 17, 297 (1981) [2] B. Kliem
  &amp; T. Torok, Phys. Rev. Lett. 96, 255002 (2006) [3] C. E. Myers,
  Ph.D. Thesis, Princeton University (2015) [4] C. E. Myers et al.,
  Nature 528, 526 (2015) [5] O. Olmedo &amp; J. Zhang, Astrophys. J. 718,
  433 (2010) [6] T. Torok &amp; B. Kliem, Astrophys. J. 630, L97 (2005)
  <P />This research is supported by DoE Contract No. DE-AC02-09CH11466
  and by the NSF/DoE Center for Magnetic Self-Organization (CMSO).

---------------------------------------------------------
Title: QSL Squasher: Calculating Quasi-Separatrix Layer Maps on a GPU
Authors: Tassev, Svetlin; Savcheva, Antonia
2016shin.confE.130T    Altcode:
  Quasi-Separatrix Layers (QSLs) are a useful proxy for the locations
  where reconnection can take place in the solar corona. However,
  calculating QSL maps even for 2-dimensional slices through 3-dimensional
  models of coronal magnetic fields is a non-trivial task as it usually
  involves tracing out millions of magnetic field lines with large
  precision. Thus, extending QSL calculations to three dimensions has
  rarely been done until now. We present a public open-source code
  (QSL squasher) which is optimized for calculating QSL maps in both
  two and three dimensions on GPUs. The code achieves large processing
  speeds in part because it is parallelized using OpenCL, and in part
  because we drastically relaxed the precision requirements for the QSL
  calculation by using perturbation theory. We show a sample of runs
  using QSL squasher to demonstrate its capabilities.

---------------------------------------------------------
Title: Evolution of the Topology, Electric Currents, and Ribbons
    during an X-class Flare
Authors: Savcheva, Antonia; Janvier, M.; Pariat, E.; Tassev, S.
2016shin.confE.126S    Altcode:
  The standard model for eruptive flares has in the past few years been
  extended to 3D. It predicts typical J-shaped photospheric footprints
  of the coronal current layer, forming at similar locations as the
  Quasi-Separatrix Layers (QSLs). Such a morphology is also found for
  flare ribbons observed in EUV, as well as in non-linear force-free
  field (NLFFF) magnetic field extrapolations and models. We study the
  evolution of the photospheric traces of the current density and the
  flare ribbons, both obtained with SDO instruments. We aim at comparing
  their morphology and their time evolution, before and during the flare,
  with the topological features found in a NLFFF and an unstable magnetic
  field model. For this purpose we investigate the photospheric current
  evolution during the 06 September 2011 X-class flare occurring in NOAA
  AR11283 from observational data of the magnetic field obtained with
  HMI. This evolution is compared with that of the flare ribbons observed
  with AIA. We also compare the observed electric current density and the
  flare ribbon morphology with that of the QSLs computed from magnetic
  field models obtained from the the flux rope insertion method. Both
  the NLFFF and the unstable (eruptive) model show the presence of a
  fan-spine configuration of overlying field lines, due to the presence
  of a parasitic polarity, embedding an elongated flux rope that appears
  in the observations as two parts of a filament. The magnetofrictional
  evolution of the unstable model tell a consistent story of the filament
  eruption in which topology plays an important role. The photospheric
  QSL traces of the fan configuration appear as an elongated flare
  ribbon that encircles the J-shaped ribbons related to the filament
  ejection. The QSLs, evolved via a magnetofrictional method, also show
  similar morphology and evolution as both the current ribbons and the
  EUV flare ribbons obtained at several times during the flare. For the
  first time, we propose a combined analysis of the photospheric traces
  of an eruptive flare, in a complex topology, with direct measurements
  of electric currents and QSLs from observational data and a magnetic
  field model. The results, obtained by two different and independent
  approaches, 1) confirm previous results of current increase during
  the impulsive phase of the flare, 2) show how NLFFF extrapolations can
  capture the essential physical signatures of flares even in a complex
  magnetic field topology.

---------------------------------------------------------
Title: Solar Eruptions Initiated in Sigmoidal Active Regions
Authors: Savcheva, Antonia
2016cosp...41E1727S    Altcode:
  active regions that have been shown to possess high probability for
  eruption. They present a direct evidence of the existence of flux
  ropes in the corona prior to the impulsive phase of eruptions. In
  order to gain insight into their eruptive behavior and how they get
  destabilized we need to know their 3D magnetic field structure. First,
  we review some recent observations and modeling of sigmoidal active
  regions as the primary hosts of solar eruptions, which can also be
  used as useful laboratories for studying these phenomena. Then, we
  concentrate on the analysis of observations and highly data-constrained
  non-linear force-free field (NLFFF) models over the lifetime of several
  sigmoidal active regions, where we have captured their magnetic field
  structure around the times of major flares. We present the topology
  analysis of a couple of sigmoidal regions pointing us to the probable
  sites of reconnection. A scenario for eruption is put forward by this
  analysis. We demonstrate the use of this topology analysis to reconcile
  the observed eruption features with the standard flare model. Finally,
  we show a glimpse of how such a NLFFF model of an erupting region can
  be used to initiate a CME in a global MHD code in an unprecedented
  realistic manner. Such simulations can show the effects of solar
  transients on the near-Earth environment and solar system space weather.

---------------------------------------------------------
Title: Simulating Idealized Flux Ropes with the Flux Rope Insertion
Method: A Parameter Space Exploration of Currents and Topology
Authors: Savcheva, Antonia; Tassev, Svetlin; DeLuca, Edward E.;
   Gibson, Sarah; Fan, Yuhong
2016SPD....47.0330S    Altcode:
  Knowledge of the 3D magnetic filed structure at the time of major
  solar eruptions is vital to the understanding of the space weather
  effects of these eruptions. Multiple data-constrained techniques that
  reconstruct the 3D coronal field based on photospheric magnetograms have
  been used to achieve this goal. In particular, we have used the flux
  rope insertion method to obtain the coronal magnetic field of multiple
  regions containing flux ropes or sheared arcades based on line-of-sight
  magnetograms and X-ray and EUV observations of coronal loops. For the
  purpose of developing statistical measures of the goodness of fit of
  these models to the observations, here we present our modeling of flux
  ropes based on synthetic magnetograms obtained from aFan &amp; Gibson
  emerging flux rope simulation. The goal is to study the effect of of
  different input flux rope parameters on the geometry of currents,
  field line connectivity, and topology, in a controled setting. For
  this purpose we create a large grid of models with the flux rope
  insertion method with different combinations of axial and poloidal
  flux, which give us different morphology of the flux rope. We create
  synthetic images of these flux ropes in AIA passbands with the FORWARD
  forward-fitting code. The present parametric study will later be used
  to get a better handle on the initial condition for magnetofrictional
  and MHD simulations of observed regions containing flux ropes, such
  as sigmoids and polar-crown filaments.

---------------------------------------------------------
Title: Evolution of the Topology, Electric Currents, and Ribbons
    during an X-class Flare
Authors: Savcheva, Antonia; Janvier, Miho; Pariat, Etienne
2016SPD....4740101S    Altcode:
  The standard model for eruptive flares has in the past few years
  been extended to 3D. It predicts typical J-shaped photospheric
  footprints of the coronal current layer, forming at similar locations
  as the Quasi-Separatrix Layers (QSLs). We study the evolution of
  the photospheric traces of the current density and the flare ribbons
  observed with SDO. We aim at comparing their morphology and their time
  evolution, before and during the flare, with the topological features
  found in a magnetic field model. For this purpose we investigate the
  photospheric current evolution during the 6 Sep 2011 X-class flare
  occurring in AR11283 from observational data of the magnetic field
  obtained with HMI. This evolution is compared with that of the flare
  ribbons observed with AIA. We also compare the observed electric current
  density and the flare ribbon morphology with that of the QSLs computed
  from magnetic field models obtained from the the flux rope insertion
  method. Both the NLFFF and the unstable (eruptive) model show the
  presence of a fan-spine configuration of overlying field lines, due
  to the presence of a parasitic polarity, embedding in elongated flux
  rope that appears in the observations as two parts of a filament. The
  magnetofrictional evolution of the unstable model tells a consistent
  story of the filament eruption in which topology plays an important
  role. The photospheric QSL traces of the fan configuration appear as
  an elongated flare ribbon that encircles the J-shaped ribbons related
  to the filament ejection. The QSLs, evolved via a magnetofrictional
  method, also show similar morphology and evolution as both the current
  ribbons and the EUV flare ribbons obtained at several times during
  the flare. For the first time, we propose a combined analysis of the
  photospheric traces of an eruptive flare, in a complex topology, with
  direct measurements of electric currents and QSLs from observational
  data and a magnetic field model. The results obtained by two independent
  approaches confirm previous results and show how NLFFF models can
  capture the essential physical signatures of flares even in a complex
  magnetic field topology.

---------------------------------------------------------
Title: The Relation between Solar Eruption Topologies and Observed
    Flare Features. II. Dynamical Evolution
Authors: Savcheva, A.; Pariat, E.; McKillop, S.; McCauley, P.; Hanson,
   E.; Su, Y.; DeLuca, E. E.
2016ApJ...817...43S    Altcode:
  A long-established goal of solar physics is to build understanding
  of solar eruptions and develop flare and coronal mass ejection (CME)
  forecasting models. In this paper, we continue our investigation of
  nonlinear forces free field (NLFFF) models by comparing topological
  properties of the solutions to the evolution of the flare ribbons. In
  particular, we show that data-constrained NLFFF models of three erupting
  sigmoid regions (SOL2010-04-08, SOL2010-08-07, and SOL2012-05-12) built
  to reproduce the active region magnetic field in the pre-flare state can
  be rendered unstable and the subsequent sequence of unstable solutions
  produces quasi-separatrix layers that match the flare ribbon evolution
  as observed by SDO/AIA. We begin with a best-fit equilibrium model for
  the pre-flare active region. We then add axial flux to the flux rope
  in the model to move it across the stability boundary. At this point,
  the magnetofrictional code no longer converges to an equilibrium
  solution. The flux rope rises as the solutions are iterated. We
  interpret the sequence of magnetofrictional steps as an evolution of
  the active region as the flare/CME begins. The magnetic field solutions
  at different steps are compared with the flare ribbons. The results are
  fully consistent with the three-dimensional extension of the standard
  flare/CME model. Our ability to capture essential topological features
  of flaring active regions with a non-dynamic magnetofrictional code
  strongly suggests that the pre-flare, large-scale topological structures
  are preserved as the flux rope becomes unstable and lifts off.

---------------------------------------------------------
Title: A dynamic magnetic tension force as the cause of failed
    solar eruptions
Authors: Myers, Clayton E.; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo;
   Fox, William; Jara-Almonte, Jonathan; Savcheva, Antonia; Deluca,
   Edward E.
2015Natur.528..526M    Altcode:
  Coronal mass ejections are solar eruptions driven by a sudden release
  of magnetic energy stored in the Sun’s corona. In many cases,
  this magnetic energy is stored in long-lived, arched structures
  called magnetic flux ropes. When a flux rope destabilizes, it can
  either erupt and produce a coronal mass ejection or fail and collapse
  back towards the Sun. The prevailing belief is that the outcome of a
  given event is determined by a magnetohydrodynamic force imbalance
  called the torus instability. This belief is challenged, however,
  by observations indicating that torus-unstable flux ropes sometimes
  fail to erupt. This contradiction has not yet been resolved because
  of a lack of coronal magnetic field measurements and the limitations
  of idealized numerical modelling. Here we report the results of
  a laboratory experiment that reveal a previously unknown eruption
  criterion below which torus-unstable flux ropes fail to erupt. We find
  that such ‘failed torus’ events occur when the guide magnetic field
  (that is, the ambient field that runs toroidally along the flux rope)
  is strong enough to prevent the flux rope from kinking. Under these
  conditions, the guide field interacts with electric currents in the
  flux rope to produce a dynamic toroidal field tension force that halts
  the eruption. This magnetic tension force is missing from existing
  eruption models, which is why such models cannot explain or predict
  failed torus events.

---------------------------------------------------------
Title: 3D Model of Slip-Running Reconnection on Solar Sigmoidal
    Regions
Authors: Douglas, B.; Savcheva, A. S.; DeLuca, E. E.
2015AGUFMSH43A2422D    Altcode:
  The structure of energy storing magnetic field lines on the Sun is
  very twisted and contorted. Some of the twist arises from photospheric
  foot point motion and some is due to currents carried into the corona
  as fields emerge. The stability of a region depends on both the energy
  stored (so-called "free" energy) and on the structure of the surrounding
  nearly potential fields. Free energy is usually contained in these
  S-shaped regions called sigmoids on the solar corona. The only way to
  reach lower energy state is to release this free energy, by changing
  its connectivity. This change in connectivity leads to flares and
  coronal mass ejections (CMEs) that can affect environments of nearby
  planets. For this project, we focus on a special kind of connectivity
  change called slip-running reconnection to create 3D numerical models
  of flare-producing magnetic fields. By comparing these numerical models
  to observational data from Atmospheric Imaging Assembly (AIA), we will
  be able to better explain the evolution of sigmoidal flares from active
  regions. We are studying a flare from Dudik et al 2014 paper (2012 July
  12), and a flare from 2015 June 14. Using the Coronal Modeling System
  (CMS) software, we read the photospheric magnetogram for the specified
  date and time, compute the potential field, setup the 3D flux rope
  path, and then relax this flux rope over 60,000 iterations to create
  a nonlinear force-free field (NLFFF). Using these relaxed models we
  find the best-fit loops surrounding the flux rope. We then compare
  these models to the observations in AIA. We compare the magnetic field
  structure in our models with the observed slipping. For regions near
  our inserted flux rope, our models successfully correlate with this
  observation. Further modeling is required, but these initial results
  suggest that NLFFF modeling may be able to capture realistic 3-D
  magnetic structures associated with slipping reconnection.

---------------------------------------------------------
Title: Laboratory Identification of MHD Eruption Criteria in the
    Solar Corona
Authors: Yamada, M.; Myers, C. E.; Ji, H.; Yoo, J.; Fox, W. R., II;
   Jara-Almonte, J.; Savcheva, A. S.; DeLuca, E. E.
2015AGUFMSH13A2434Y    Altcode:
  Ideal magnetohydrodynamic (MHD) instabilities such as the kink [1]
  and torus [2] instabilities are believed to play an important role
  in driving "storage-and-release" eruptions in the solar corona. These
  instabilities act on long-lived, arched magnetic flux ropes that are
  "line-tied" to the solar surface. In spite of numerous observational and
  computational studies, the conditions under which these instabilities
  produce an eruption remain a subject of intense debate. In this
  paper, we use a line-tied, arched flux rope experiment to study
  storage-and-release eruptions in the laboratory [3]. An in situ array
  of miniature magnetic probes is used to assess the equilibrium and
  stability of the laboratory flux ropes. Two major results are reported
  here: First, a new stability regime is identified where torus-unstable
  flux ropes fail to erupt. In this "failed torus" regime, the flux rope
  is torus-unstable but kink-stable. Under these conditions, a dynamic
  "toroidal field tension force" surges in magnitude and causes the flux
  rope to contract. This tension force, which is missing from existing
  eruption models, is the J×B force between self-generated poloidal
  currents in the flux rope and the toroidal (guide) component of the
  vacuum field. Secondly, a clear torus instability threshold is observed
  in the kink-unstable regime. This latter result, which is consistent
  with existing theoretical [4] and numerical [5] findings, verifies the
  key role of the torus instability in driving some solar eruptions. This
  research is supported by DoE Contract No. DE-AC02-09CH11466 and by
  the NSF/DoE Center for Magnetic Self-Organization (CMSO). [1] Hood
  &amp; Priest, Geophys. Astrophys. Fluid Dynamics 17, 297 (1981)
  [2] Kliem &amp; Török, Phys. Rev. Lett. 96, 255002 (2006) [3]
  Myers, Ph.D. Thesis, Princeton University (2015) [4] Olmedo &amp;
  Zhang, Astrophys. J. 718, 433 (2010) [5] Török &amp; Kliem,
  Astrophys. J. 630, L97 (2005)

---------------------------------------------------------
Title: Towards a Data-Optimized Coronal Magnetic Field Model (DOC-FM):
    Simulating Flux Ropes with the Flux Rope Insertion Method
Authors: Dalmasse, K.; DeLuca, E. E.; Savcheva, A. S.; Gibson, S. E.;
   Fan, Y.
2015AGUFMSH51B2444D    Altcode:
  Knowledge of the 3D magnetic filed structure at the time of major solar
  eruptions is vital or understanding of the space weather effects of
  these eruptions. Multiple data-constrained techniques that reconstruct
  the 3D coronal field based on photospheric magnetograms have been
  used to achieve this goal. In particular, we have used the flux rope
  insertion method to obtain the coronal magnetic field of multiple
  regions containing flux ropes or sheared arcades based on line-of-sight
  magnetograms and X-ray and EUV observations of coronal loops. For the
  purpose of developing statistical measures of the goodness of fit of
  these models to the observations, here we present our modeling of flux
  ropes based on synthetic magnetograms obtained from Fan &amp; Gibson
  emerging flux rope simulation. The goal is to reproduce the flux rope
  structure from a given time step of the MHD simulations based only
  on the photospheric magnetogram and synthetic forward modeled coronal
  emission obtained from the same step of the MHD simulation. For this
  purpose we create a large grid of models with the flux rope insertion
  method with different combinations of axial and poloidal flux, which
  give us different morphology of the flux rope. Then we compare the
  synthetic coronal emission with the shape of the current distribution
  and field lines from the models to come up with a best fit. This fit
  is then tested using the statistical methods developed by our team.

---------------------------------------------------------
Title: The Relation between Solar Eruption Topologies and Observed
    Flare Features. I. Flare Ribbons
Authors: Savcheva, A.; Pariat, E.; McKillop, S.; McCauley, P.; Hanson,
   E.; Su, Y.; Werner, E.; DeLuca, E. E.
2015ApJ...810...96S    Altcode: 2015arXiv150603452S
  In this paper we present a topological magnetic field investigation
  of seven two-ribbon flares in sigmoidal active regions observed with
  Hinode, STEREO, and Solar Dynamics Observatory. We first derive the
  3D coronal magnetic field structure of all regions using marginally
  unstable 3D coronal magnetic field models created with the flux rope
  insertion method. The unstable models have been shown to be a good
  model of the flaring magnetic field configurations. Regions are selected
  based on their pre-flare configurations along with the appearance and
  observational coverage of flare ribbons, and the model is constrained
  using pre-flare features observed in extreme ultraviolet and X-ray
  passbands. We perform a topology analysis of the models by computing the
  squashing factor, Q, in order to determine the locations of prominent
  quasi-separatrix layers (QSLs). QSLs from these maps are compared to
  flare ribbons at their full extents. We show that in all cases the
  straight segments of the two J-shaped ribbons are matched very well by
  the flux-rope-related QSLs, and the matches to the hooked segments are
  less consistent but still good for most cases. In addition, we show that
  these QSLs overlay ridges in the electric current density maps. This
  study is the largest sample of regions with QSLs derived from 3D coronal
  magnetic field models, and it shows that the magnetofrictional modeling
  technique that we employ gives a very good representation of flaring
  regions, with the power to predict flare ribbon locations in the event
  of a flare following the time of the model.

---------------------------------------------------------
Title: What is the best indicator of active region stability? Topology
    not free energy or relative helicity!
Authors: DeLuca, Edward E.; Savcheva, Antonia
2015shin.confE..28D    Altcode:
  The determination of magnetic free energy and relative helicity
  in active region magnetic field models is strongly model- and
  method-dependent. Even if more accurate and complete magnetic field
  measurements are made and the quantities can be determined with higher
  confidence and precision, the limiting value that would

---------------------------------------------------------
Title: The Relation between CME Topologies and Observed Flare Features
Authors: Savcheva, Antonia Stefanova; Pariat, E.; MaKillop, S.;
   McCauley, P.; Hanson, E.; Werner, E.; Su, Y.; DeLuca, E.
2015shin.confE...6S    Altcode:
  A long established goal of solar physics is to build physics-based
  flare and CME forecasting models. This study, building on the recent
  successes in non-linear forces free field (NLFFF) modeling and detailed
  numerical simulations, brings us closer to that goal. We show that
  data-constrained NLFFF models built to reproduce the active region
  magnetic field in the pre-flare state can be rendered unstable and
  the sequence of unstable solutions produce quasi-separatrix layers
  (QSLs) that reproduce the observed flare ribbons. The results are fully
  consistant with the 3D extension of the standard flare/CME model. Our
  ability to capture essential topological features of flaring active
  regions with non-dynamic magneto-frictional code strongly suggests
  that the pre-flare, large scale topological structures are preserved
  as the flux rope becomes unstable and lifts off.

---------------------------------------------------------
Title: The Interaction of Solar Eruptions and Large-Scale Coronal
    Structures Revealed Through Modeling and Observational Analysis
Authors: Evans, R. M.; Savcheva, A. S.; Zink, J. L.; Muglach, K.;
   Kozarev, K. A.; Opher, M.; van der Holst, B.
2014AGUFMSH11D..05E    Altcode:
  We use numerical and observational approaches to explore how
  the interaction of a coronal mass ejection (CME) with preexisting
  structures in the solar atmosphere influences its evolution and space
  weather effects. We study two aspects of CME evolution: deflection of
  the CME's propagation direction, and expansion. First, we perform a
  statistical study of the influence of coronal holes on CME trajectories
  for more than 50 events during years 2010-2014. Second, we use the Space
  Weather Modeling Framework (SWMF) to model CME propagation in the Alfven
  Wave Solar Model (AWSoM), which includes a sophisticated treatment
  of the physics of coronal heating and solar wind acceleration. The
  major progress in these simulations is that the initial conditions
  of the eruptions are highly data-constrained. From the simulations,
  we determine the CME's trajectory and expansion. We calculate the
  pile-up of material along the front and sides of a CME due to its
  expansion, and constrain the properties of the pile-up under a range
  of conditions. Finally, we will discuss the connection between these
  plasma density structures and the acceleration of protons to energies
  relevant to space weather.

---------------------------------------------------------
Title: Use of Magnetic Field Modeling and Topology Analysis in
    Understanding the Evolution and Eruption of Coronal Sigmoids
Authors: Savcheva, A. S.
2014AGUFMSH23C..05S    Altcode:
  Coronal sigmoids, generally ovserved in X-rays and EUV, are S-shaped
  active regions that have been shown to possess high probability for
  eruption. They present a direct evidence of the existence of flux
  ropes in the corona prior to the impulsive phase of eruptions. In
  order to gain insight into their eruptive behavior and how they get
  destabilized we need to know their 3D magnetic field structure. We have
  performed highly data-constrained non-linear force-free field (NLFFF)
  models over the lifetime of several sigmoidal active regions and more
  specifically have captured their magnetic field structure around the
  times of major flares. We present this analysis of one region. We also
  look at the topology analysis for several sigmoidal regions and we
  show the probable sites of reconnection and put forward a scenario for
  eruption. We demonstrate the use of this topology analysis to reconcile
  the observed eruption features with the standard flare model. Finally,
  we show a glimpse of how such a NLFFF model of an erupting region can
  be used to initiate a CME in a global MHD code in an unprecedented
  realistic manner.

---------------------------------------------------------
Title: Magnetic Field Modeling of Complex, Flare Productive Active
    Regions
Authors: Millholland, S. C.; Savcheva, A. S.; DeLuca, E. E.
2014AGUFMSH13A4079M    Altcode:
  We present models and analysis of the magnetic field structure of three
  sigmoidal active regions (ARs). Sigmoids, forward or backward S-shaped
  EUV and X-ray emissions in the corona, are relevant as predictors of
  eruptive events such as flares and Coronal Mass Ejections. The regions
  were modeled using the Flux Rope Insertion Method, in which flux ropes,
  held in equilibrium by an overlying potential arcade, represent the
  sigmoids. The flux rope paths were inserted into a potential field
  following the filaments observed in 304Å. The models were then relaxed
  into a nonlinear force free (NLFFF) state using a magnetofrictional
  relaxation process. The first region studied is NOAA AR 12017, which
  produced an X1.0 flare at 2014/03/29 17:35. The second is NOAA AR 11283,
  which erupted with an X2.1 flare at 2011/09/06 22:12. For these regions,
  we show detailed comparisons of Quasi-Separatrix Layer (QSL) maps and
  observed flare ribbons. The slow evolution of an unstable solution at
  the time of the eruption produces a set of QSL solutions. Comparison
  of the photospheric mapping of the QSL with the flare ribbons will be a
  good measure of how well we have captured the magnetic structure of the
  particle acceleration region with our simple NLFFF models. The third
  is NOAA AR 11093. This region was a double decker filament composed
  of two branches over the same polarity inversion line. At 2010/08/07
  17:55, the upper filament erupted with an M1.0 flare. This is the first
  time a double decker flux rope region has been modeled using these
  techniques. We show the interaction of the two inserted flux ropes and
  the evolution of the region through a series of NLFFF solutions to the
  evolving photospheric magnetic field. This work has been funded by the
  NSF-REU solar physics program at Smithsonian Astrophysical Observatory,
  grant number AGS-1263241.

---------------------------------------------------------
Title: A New Sample of Cool Subdwarfs from SDSS: Properties and
    Kinematics
Authors: Savcheva, Antonia S.; West, Andrew A.; Bochanski, John J.
2014ApJ...794..145S    Altcode: 2014arXiv1409.1229S
  We present a new sample of M subdwarfs compiled from the seventh data
  release of the Sloan Digital Sky Survey. With 3517 new subdwarfs, this
  new sample significantly increases the number of spectroscopically
  confirmed low-mass subdwarfs. This catalog also includes 905 extreme
  and 534 ultra sudwarfs. We present the entire catalog, including
  observed and derived quantities, and template spectra created from
  co-added subdwarf spectra. We show color-color and reduced proper motion
  diagrams of the three metallicity classes, which are shown to separate
  from the disk dwarf population. The extreme and ultra subdwarfs are
  seen at larger values of reduced proper motion, as expected for more
  dynamically heated populations. We determine 3D kinematics for all
  of the stars with proper motions. The color-magnitude diagrams show
  a clear separation of the three metallicity classes with the ultra
  and extreme subdwarfs being significantly closer to the main sequence
  than the ordinary subdwarfs. All subdwarfs lie below (fainter) and
  to the left (bluer) of the main sequence. Based on the average (U, V,
  W) velocities and their dispersions, the extreme and ultra subdwarfs
  likely belong to the Galactic halo, while the ordinary subdwarfs
  are likely part of the old Galactic (or thick) disk. An extensive
  activity analysis of subdwarfs is performed using Hα emission,
  and 208 active subdwarfs are found. We show that while the activity
  fraction of subdwarfs rises with spectral class and levels off at the
  latest spectral classes, consistent with the behavior of M dwarfs,
  the extreme and ultra subdwarfs are basically flat.

---------------------------------------------------------
Title: A New Sigmoid Catalog from Hinode and the Solar Dynamics
Observatory: Statistical Properties and Evolutionary Histories
Authors: Savcheva, A. S.; McKillop, S. C.; McCauley, P. I.; Hanson,
   E. M.; DeLuca, E. E.
2014SoPh..289.3297S    Altcode: 2014SoPh..tmp...17S
  We present a new sigmoid catalog covering the duration of the Hinode
  mission and the Solar Dynamics Observatory (SDO) until the end of
  2012. The catalog consists of 72 mostly long-lasting sigmoids. We
  collect and make available all X-ray and EUV data from Hinode, SDO, and
  the Solar TErrestrial RElations Observatory (STEREO), and we determine
  the sigmoid lifetimes, sizes, and aspect ratios. We also collect the
  line-of-sight magnetograms from the Helioseismic and Magnetic Imager
  (HMI) for SDO or the Michelson Doppler Imager (MDI) on the Solar
  and Heliospheric Observatory (SOHO) to measure flux versus time for
  the lifetime of each region. We determine that the development of
  a sigmoidal shape and eruptive activity is more strongly correlated
  with flux cancelation than with emergence. We find that the eruptive
  properties of the regions correlate well with the maximum flux,
  largest change, and net change in flux in the regions. These results
  have implications for constraining future flux-rope models of ARs and
  gaining insight into their evolutionary properties.

---------------------------------------------------------
Title: A Topological View at Observed Flare Features: An Extension
    of the Standard Flare Model to 3D
Authors: Savcheva, Antonia; Pariat, Etienne; McKillop, Sean; Hanson,
   Elizabeth; Su, Yingna; DeLuca, Edward E.
2014AAS...22430301S    Altcode:
  We conduct topology analysis of erupting non-linear force-free field
  (NLFFF) configurations of eight sigmoidal active regions observed
  with Hinode/XRT and SDO/AIA. The NLFFF models are computed using
  the flux rope insertion method and unstable models are utilized to
  represent the erupting configurations. Topology analysis shows that the
  quasi-separatrix layers (QSLs) in the chromosphere match well the flare
  ribbons observed in these regions. In addition, we show that low-lying
  QSLs associated with the rising flux rope change shape and extent to
  match the separating flare ribbons as observed by AIA. Post-flare loops
  are fit well by field lines lying under the generalized X-line at the
  bottom of the flux rope. We show a correspondence in the evolution
  of the post-flare loops from a strong-to-weak sheared state and the
  behavior of the field lines as the flux rope expands in the corona. We
  show that transient corona holes are associated with the footprints
  of the flux rope in the low atmosphere. In addition, we compute the
  reconnected flux in one of the regions and using information from
  the models constrain how much energy has been released during the
  event. We use this kind of topology analysis to extend the standard
  CME/flare model to full 3D and find implications to reconnection in 3D.

---------------------------------------------------------
Title: Statistical Properties of Jets in the SDO Era
Authors: Farid, Samaiyah; Savcheva, Antonia
2014AAS...22432337F    Altcode:
  We examine the statistical properties of jets observed in X-ray and EUV
  since the launch of the Solar Dynamics Observatory (SDO). We identify
  over 150 jets using data from Hinode X-ray Telescope (XRT) coronal hole
  observing campaigns and examine their properties using SDO Atmospheric
  Imaging Array (AIA) . Each event is identified as type 1 (classic jet),
  type 2 (blowout jet ) or type 3 (indeterminable). We calculate their
  intensity, lifetime, apparent velocity, angle of inclination and note
  if they are associated with a bright points. This study will be used
  for the validation of the Automatic Jet Detection Module; part of the
  SDO Feature Finding Team.

---------------------------------------------------------
Title: Analyzing an IRIS Blowout jet via Magnetofrictional Simulation
Authors: Savcheva, Antonia; Tian, Hui; Meyer, Karen
2014AAS...22432310S    Altcode:
  The imaging spectrograph, IRIS, offers unprecedented spatial and
  temporal resolution of small-scale phenomena, which allows the
  study of their spectral properties in the chromosphere and transition
  region. This study present IRIS observations of a blowout coronal jet,
  demonstrating the ability of IRIS to detect reconnection effects in
  the low atmosphere in the available suite of spectral lines. We present
  Doppler velocity and non-thermal width (NTW) maps of the jet and their
  evolution in time. We interpret the results using MHD simulations of
  jets. In addition, we present a data-driven magnetofrictional simulation
  of the same jet and match the magnetic and current structure of the
  jet to the observed NTW maps. We infer the height of the null point
  and the extent of the region showing reconnection effects. We discuss
  the implications of understanding reconnection effects in conjunction
  with NTW maps.

---------------------------------------------------------
Title: Data-constrained Magnetofrcitional Simulation of a Flux Rope
    Build-up in a Sigmoidal Active Region
Authors: Savcheva, Antonia Stefanova; Mackay, D.; Meyer, K.; Gibb,
   G.; DeLuca, E.
2014shin.confE...3S    Altcode:
  We present a data-constrained magnetofrictional (MF) simulation of
  the evolution over two days of the sigmoidal active region from 6-7
  Dec 2007. The lower boundary condition is supplied by a series of
  line-of-sight (LoS) namgnetograms from MDI, but for the first time the
  initial condition is taken from a data-constrained non-linear force-free
  (NLFFF) model of the active region early on Dec 6. The NLFFF model is
  produced with the flux rope insertion method and is constrained by a LoS
  magnetogram, filament path from STEREO, and coronal loops from XRT. The
  initial condition is that of a sheared arcade and as time progresses
  the photospheric evolution builds a flux rope, which becomes unstable
  a few hours before the actual observed eruption. We show field lines
  and current density distributions over time and compare them to XRT
  images. We present the evolution of the free and potential energy and
  relative helicity in the region. We compare our results to a previous
  a simulation starting from a potential field as initial condition.

---------------------------------------------------------
Title: A New Sample of Cool Subdwarfs from SDSS: Properties and
    Kinematics
Authors: Savcheva, Antonia; West, Andrew A.; Bochanski, John J.
2014AAS...22432215S    Altcode:
  We present a new sample of M subdwarfs compiled from the 7th data
  re- lease of the Sloan Digital Sky Survey. With 3517 new subdwarfs,
  this new sample significantly increases the number the existing
  sample of low-mass subdwarfs. This catalog includes unprecedentedly
  large numbers of extreme and ultra sudwarfs. Here, we present the
  catalog and the statistical analysis we perform. Subdwarf template
  spectra are derived. We show color-color and reduced proper motion
  diagrams of the three metallicity classes, which are shown to separate
  from the disk dwarf population. The extreme and ultra subdwarfs are
  seen at larger values of reduced proper motion as expected for more
  dynamically heated populations. We determine 3D kinematics for all
  of the stars with proper motions. The color-magnitude diagrams show
  a clear separation of the three metallicity classes with the ultra
  and extreme subdwarfs being significantly closer to the main sequence
  than the ordinary subdwarfs. All subdwarfs lie below and to the blue
  of the main sequence. Based on the average (U, V, W ) velocities and
  their dispersions, the extreme and ultra subdwarfs likely belong to
  the Galactic halo, while the ordinary subdwarfs are likely part of
  the old Galactic (or thick) disk. An extensive activity analy- sis
  of subdwarfs is performed using chromospheric Hα emission and 208
  active subdwarfs are found. We show that while the activity fraction
  of subdwarfs rises with spectral class and levels off at the latest
  spectral classes, consistent with the behavior of M dwarfs, the extreme
  and ultra subdwarfs are basically flat.

---------------------------------------------------------
Title: A New Sigmoid Catalog: Statistical Properties of Sigmoids
    and Their Evolution
Authors: McKillop, Sean; Savcheva, A.; Hanson, E.; McCauley, P.;
   DeLuca, E. E.
2013SPD....44...31M    Altcode:
  Sigmoids are sinuous structures located in active regions that have
  characteristic “s-shaped” or inverted “s-shaped” loops. Active
  regions containing sigmoids are observed to have higher rates of
  flaring and CMEs. Previous work detailing the properties of sigmoids
  has generally focused on specific case studies of a handful of
  regions. Although such studies are representative of the structure
  and evolution of these regions, significant insight can be gained by
  an observational overview approach with systematic and statistical
  analysis of a large sample of sigmoids. We present a new sample
  of 72 sigmoidal regions observed in a wide wavelength range and in
  different parts of the solar atmosphere by various instruments such
  as the Hinode/XRT, SDO/AIA, STEREO, and LASCO. From this data we
  compiled a comprehensive list of many different parameters including:
  size and aspect ratio, presence of Ha or EUV filaments, flare and CME
  association, number of sunspots, active region and sigmoid lifetimes,
  etc. Our preliminary results show that sigmoids have a higher eruption
  rate than other active regions. We also find that the ratio of the long
  axis to short axis of the sigmoids has a strong peak at 2.5 and the
  lifetime peaks at 2 days. We also follow the evolution of the magnetic
  flux in the photosphere and derive whether the sigmoids appear during
  the emergence or cancellation stages of active region evolution. These
  results can provide constraints for models of flux rope evolution in
  global simulations.

---------------------------------------------------------
Title: The Evolution of Sigmoidal Active Regions
Authors: Savcheva, Antonia
2013SPD....4430004S    Altcode:
  The formation, evolution and eruption of solar active regions are
  among the main themes of research in solar physics. Special kinds of
  S-shaped active regions (sigmoids) facilitate this line of research,
  since they provide conditions that are easier to disentangle and have
  been shown to possess high probability for erupting as flares and/or
  coronal mass ejections (CME). Several theories have been proposed for
  the formation, evolution, and eruption of solar active regions. Testing
  these against detailed models of sigmoidal regions can provide insight
  into the dominant mechanisms and conditions required for eruption. We
  explore the behavior of solar sigmoids via both observational and
  magnetic modeling studies. Data from the most modern space-based
  solar observatories are utilized in addition to state-of-the-art,
  three-dimensional, data-driven magnetic field modeling to gain insight
  into the physical processes controlling the evolution and eruption
  of solar sigmoids. We use X-ray observations and the magnetic models
  to introduce the underlying magnetic and plasma structure defining
  these regions. By means of a large, comprehensive observational study,
  we look at the formation and evolution mechanism. Specifically, we
  apply additional analysis to show that flux cancellation is a major
  mechanism for building the underlying magnetic structure associated
  with sigmoids, namely magnetic flux ropes. We make use of topological
  analysis to describe the complicated magnetic field structure of the
  sigmoids. We show that when data-driven models are used in sync with
  MHD simulations and observations, we can arrive at a consistent picture
  of the scenario for CME onset, namely the positive feedback between
  reconnection at a generalized X-line and the torus instability.

---------------------------------------------------------
Title: A Topological View at CME/flare Features with Application to
    3D Reconnection
Authors: Savcheva, Antonia Stefanova; Pariat, E.; van Ballegooijen,
   A.; Mckillop, S.; Hanson, E.; DeLuca, Y. Su E.
2013shin.confE.143S    Altcode:
  We conduct topology analysis of erupting non-linear force-free
  configurations of five sigmoidal active regions observed with Hinode/XRT
  and SDO/AIA. The models are computed using the flux rope insertion
  method and unstable models are utilized to represent the erupting
  configurations. Topology analysis shows that the quasi-separatrix layers
  (QSLs) in the chromosphere match well the flare ribbons observed in
  these regions. Post-flare loops are also matched well by field lines
  lying under the X-line in the models. In addition, we show that
  low-lying QSLs associated with the rising flux rope change shape
  and extent to match the separating flare ribbons in the images. We
  use this kind of topology analysis to extend the standard CME/flare
  model to full 3D in observed configurations and find implications to
  reconnection in 3D.

---------------------------------------------------------
Title: Jets in the Solar Atmosphere and their Effects in the
    Heliosphere
Authors: Savcheva, A. S.; LWS Jet Focused Science Team
2013AGUSMSH42A..03S    Altcode:
  The last decade of solar observations from the optical to the X-ray
  showed that jets and spicules are ubiquitous features that are observed
  in large numbers every day on the Sun. Coronal jets and spicules can
  potentially provide significant quantities of mass and energy into
  the solar wind, which is why NASA recently assembled the Living with
  a Star Jet Focused Science Team. In this talk I will present first
  result of this team's work, which includes both a wide observational
  and MHD modeling effort. The observational study of jets is on two
  main fronts - coronal jets and spicules. The coronal jet observational
  analysis includes assembling large statistics of the properties of jets
  observed with Hinode/XRT and SDO/AIA, with an emphasis at deriving
  the necessary parameters for estimating the jet mass and energy
  contribution to the solar wind, and deriving bulk characteristics
  to constrain MHD models. Specifically the study of standard and
  blowout jets confirms a common mechanism for the production of Type
  II spicules and jets. Analysis of BBSO NST and magnetic field time
  space plots suggest that Type-II spicules are made by granule-size
  emerging bipoles via blowout eruption and interchange reconnection
  with the canopy of coronal field rooted in the magnetic network. The
  modeling effort is concentrated on 3D numerical simulations of jets in
  the solar atmosphere, in which the onset of reconnection in a stressed
  null-point configurations releases torsional Alfven waves. This study
  aims to quantify the mass, momentum, and energy that are transported out
  into the heliosphere from the source regions of jets in coronal holes,
  and up into the corona and back down to the surface from sources in
  magnetically closed regions, for comparison with observations of polar
  jets and chromospheric spicules.

---------------------------------------------------------
Title: Mapping the Local Halo: Statistical Parallax Analysis of SDSS
    Low-mass Subdwarfs
Authors: Bochanski, John J.; Savcheva, Antonia; West, Andrew A.;
   Hawley, Suzanne L.
2013AJ....145...40B    Altcode: 2012arXiv1211.6104B
  We present a statistical parallax study of nearly 2000 M subdwarfs
  with photometry and spectroscopy from the Sloan Digital Sky Survey
  (SDSS). Statistical parallax analysis yields the mean absolute
  magnitudes, mean velocities, and velocity ellipsoids for homogenous
  samples of stars. We selected homogeneous groups of subdwarfs based
  on their photometric colors and spectral appearance. We examined
  the color-magnitude relations of low-mass subdwarfs and quantified
  their dependence on the newly refined metallicity parameter, ζ. We
  also developed a photometric metallicity parameter, δ<SUB>(g -
  r)</SUB>, based on the g - r and r - z colors of low-mass stars and
  used it to select stars with similar metallicities. The kinematics
  of low-mass subdwarfs as a function of color and metallicity were
  also examined and compared to main-sequence M dwarfs. We find that
  the SDSS subdwarfs share similar kinematics to the inner halo and
  thick disk. The color-magnitude relations derived in this analysis
  will be a powerful tool for identifying and characterizing low-mass
  metal-poor subdwarfs in future surveys such as Gaia and LSST, making
  them important and plentiful tracers of the stellar halo.

---------------------------------------------------------
Title: Science Highlights from the SDSS DR7 Spectroscopic M Dwarf
    Catalog
Authors: West, Andrew A.; Bochanski, J. J.; Pineda, J.; Dhital, S.;
   Savcheva, A.; Jones, D.; Schluns, K.; Massey, A. P.
2013AAS...22115804W    Altcode:
  We present a series of science highlights that have resulted from
  the SDSS DR7 spectroscopic M dwarf catalog. These highlights include
  (but are not limited to) a detailed magnetic activity analysis of M
  dwarfs as a function of their location in the Galaxy (both Galactic
  height and Galactocentric radius), a kinematic analysis of the local
  Milky Way, a study of how the time variability of M dwarfs correlates
  with spectral properties, an age-activity relation (using Galactic
  stratigraphy), a spectral catalog of wide binary pairs, a catalog of
  low-mass subdwarfs, a statistical parallax analysis of the M dwarfs
  and subdwarfs, and a technique for determining the interstellar dust
  content using M dwarfs. All of the SDSS and value added data are
  accessible for public download and reprints will be made available on
  site. AAW acknowledges the support of NSF grant AST-1109273

---------------------------------------------------------
Title: Sigmoidal Active Regions on the Sun: Statistical and Detailed
    Studies
Authors: Hanson, E.; DeLuca, E.; Savcheva, A. S.
2012AGUFMSH51A2202H    Altcode:
  We have compiled a catalog of sigmoidal active regions occurring
  in Aug 2010 - May 2012. The catalog data will enable us to identify
  variations and unifying characteristics of the sigmoids. In the long
  run, analyzing the typical behavior of these regions will improve
  space weather forecasting capabilities because sigmoidal regions
  have been shown to be a good predictor of eruptions. Additionally,
  we modeled the magnetic field of one of the cataloged sigmoids (NOAA
  active region 11474 at 2012.05.08/05:38:00) four hours prior to an
  eruption. The models consisted of a flux rope in a potential arcade,
  with the path of the rope following the H-α filament. The best fit
  model yields a current distribution which, viewed in cross section,
  exhibits a characteristic teardrop-shaped topology. Field lines passing
  through specific zones in the cross section define unique shapes in
  the sigmoid: the single S, the two Js, the overlying potential arcade,
  and the underlying small loops. The static model is slightly unstable;
  therefore, further relaxation of the model mimics the time evolution
  of the active region leading up to its eruption at 09:26. Future
  work will examine the relationship between Quasi-Separatrix Layer
  (QSL) maps and flare ribbons seen in 304Å images from the eruption.;
  Current and magnetic field lines from a model of a marginally stable
  flux rope. ; Cross section of the current. Note the x-line at the base
  of the flux rope.

---------------------------------------------------------
Title: Non-Linear Force Free Field Modeling and Flare Ribbon
    Comparison of Several Flaring Active Regions
Authors: McKillop, S.; Savcheva, A. S.; DeLuca, E.
2012AGUFMSH51A2201M    Altcode:
  Three dimensional magnetic field models are critically important for
  understanding the storage and release of energy in flaring active
  regions. In this project we present models of several flaring active
  regions (ARs) observed with the Atmospheric Imaging Assembly (AIA)
  aboard the Solar Dynamics Observatory (SDO). For each AR we built
  a Non-Linear Force Free Field (NLFFF) model using Helioseismic and
  Magnetic Imager (HMI) magnetograms as the boundary condition and
  AIA coronal observations as the constraint on the models. The models
  based on observations just prior to the flare have unstable solutions
  from which Quasi-Separatrix Layers (QSL) maps are calculated at low
  heights in the corona. Detailed comparison of the QSL and the flare
  ribbons provide insight into the magnetic configuration at the particle
  acceleration site.

---------------------------------------------------------
Title: Photospheric Flux Cancellation and the Build-up of Sigmoidal
    Flux Ropes on the Sun
Authors: Savcheva, A. S.; Green, L. M.; van Ballegooijen, A. A.;
   DeLuca, E. E.
2012ApJ...759..105S    Altcode:
  In this study we explore the scenario of photospheric flux cancellation
  being the primary formation mechanism of sigmoidal flux ropes in
  decaying active regions. We analyze magnetogram and X-ray observations
  together with data-driven non-linear force-free field (NLFFF) models of
  observed sigmoidal regions to test this idea. We measure the total and
  canceled fluxes in the regions from MDI magnetograms, as well as the
  axial and poloidal flux content of the modeled NLFFF flux ropes for
  three sigmoids—2007 February, 2007 December, and 2010 February. We
  infer that the sum of the poloidal and axial flux in the flux ropes for
  most models amounts to about 60%-70% of the canceled flux and 30%-50%
  of the total flux in the regions. The flux measurements and the analysis
  of the magnetic field structure show that the sigmoids first develop
  a strong axial field manifested as a sheared arcade and then, as flux
  cancellation proceeds, form long S-shaped field lines that contribute to
  the poloidal flux. In addition, the dips in the S-shaped field lines are
  located at the sites of flux cancellation that have been identified from
  the MDI magnetograms. We find that the line-of-sight-integrated free
  energy is also concentrated at these locations for all three regions,
  which can be liberated in the process of eruption. Flare-associated
  brightenings and flare loops coincide with the location of the X-line
  topology that develops at the site of most vigorous flux cancellation.

---------------------------------------------------------
Title: Comparison of a Magnetohydrodynamical Simulation and a
    Non-Linear Force-Free Field Model of a Sigmoidal Active Region.
Authors: Pariat, Etienne; DeLuca, Edward; Van Ballegooijen, Adriaan;
   Aulanier, Guillaume; Savcheva, Antonia
2012cosp...39.1448P    Altcode: 2012cosp.meet.1448P
  Sigmoids are solar magnetic structures where highly non-potential
  fields (strong shear/twist) are believed to be present. Thanks to
  the high level of free magnetic energy, active regions with sigmoids
  possess a higher eruptivity. In the present study, we will present
  a comparive topological analysis between a Non-Linear Force Free
  Field (NLFFF) model of sigmoid region, and a three-dimensional (3D)
  magnetohydrodynamics numerical simulation of the formation and eruption
  of such a structure. The MHD simulation is based on an idealized
  magnetic field distribution and the sigmoidal flux rope is built by
  means of shearing motions and magnetic polarity diffusion. The NLFFF
  model is based on the flux rope insertion method which utilizes line of
  sight magnetograms and X-ray observations of the region to constrain the
  models. We compare the geometrical and topological properties of the 3D
  magnetic fields given by both methods in their pre-eruptive phases. We
  arrive at a consistent picture for the evolution and eruption of the
  sigmoid by using the idealized MHD simulation as a context for the more
  specific observationally-constrained NLFFF models and data. Although,
  the two models are very different in their setups, we identify strong
  similarities between the two models and understandable differences. By
  computing the squashing factor in different horizontal maps at various
  heights above the photosphere and in vertical cuts in the domains,
  we demonstrate the existence of key Quasi-Separatrix Layers (QSL)
  eventually involved in the dynamic of the structure. We also show that
  there are electric current concentrations coinciding with the main
  QSLs. Finally, we perform torus instability analysis and show that
  a combination between reconnection at the main QSL and the resulting
  expansion of the flux rope into the torus instability domain is the
  cause of the CME in both models. This study finally highlights the
  interest of the use of in-depth topological tools to study highly
  non-potential magnetic fields.

---------------------------------------------------------
Title: Photospheric flux cancellation and the build-up of sigmoidal
    flux ropes
Authors: Savcheva, Antonia Stefanova; Green, L.; van Ballegooijen,
   A.; DeLuca, E.
2012shin.confE.122S    Altcode:
  The magnetic structure of sigmoidal active regions is generally
  associated with the presence of a twisted flux rope held down by a
  potential arcade. There are competing theories of how the flux rope
  develops - by flux emergence, cancellation, or footpoint motions. We
  look at how flux cancellation in several sigmoidal regions, observed
  with XRT, affects the buildup of the underlying flux ropes. We use
  MDI magnetograms to quantify the flux cancellation, and the flux rope
  insertion method to construct non-linear force free field models of the
  regions. These models allow us to produce 3-D magnetic field models
  and see how the fields evolve in time. The models show how the flux
  ropes energy and magnetic flux changes during the different stages in
  the flux cancellation. Flux cancellation events are associated with
  build up of twist in the region in accordance with the accepted flux
  cancellation picture. The location of flares and build-up of free
  energy is well correlated with flux cancellation events.

---------------------------------------------------------
Title: Sigmoidal Active Region on the Sun: Comparison of a
    Magnetohydrodynamical Simulation and a Nonlinear Force-free Field
    Model
Authors: Savcheva, A.; Pariat, E.; van Ballegooijen, A.; Aulanier,
   G.; DeLuca, E.
2012ApJ...750...15S    Altcode:
  In this paper we show that when accurate nonlinear force-free
  field (NLFFF) models are analyzed together with high-resolution
  magnetohydrodynamic (MHD) simulations, we can determine the physical
  causes for the coronal mass ejection (CME) eruption on 2007 February
  12. We compare the geometrical and topological properties of the
  three-dimensional magnetic fields given by both methods in their
  pre-eruptive phases. We arrive at a consistent picture for the
  evolution and eruption of the sigmoid. Both the MHD simulation and
  the observed magnetic field evolution show that flux cancellation
  plays an important role in building the flux rope. We compute the
  squashing factor, Q, in different horizontal maps in the domains. The
  main shape of the quasi-separatrix layers (QSLs) is very similar
  between the NLFFF and MHD models. The main QSLs lie on the edge of
  the flux rope. While the QSLs in the NLFFF model are more complex due
  to the intrinsic large complexity in the field, the QSLs in the MHD
  model are smooth and possess lower maximum value of Q. In addition,
  we demonstrate the existence of hyperbolic flux tubes (HFTs) in both
  models in vertical cross sections of Q. The main HFT, located under the
  twisted flux rope in both models, is identified as the most probable
  site for reconnection. We also show that there are electric current
  concentrations coinciding with the main QSLs. Finally, we perform torus
  instability analysis and show that a combination between reconnection
  at the HFT and the resulting expansion of the flux rope into the torus
  instability domain is the cause of the CME in both models.

---------------------------------------------------------
Title: Applications of Quasi-Separatrix Layer Maps in Understanding
    an XRT Sigmoid
Authors: Savcheva, A.; van Ballegooijen, A.; DeLuca, E. E.
2012ASPC..455..261S    Altcode:
  We present our recent work on utilizing Quasi-Separatrix Layer (QSL)
  maps for understanding the structure, evolution, and pre-eruption
  behavior of the quiescent sigmoid observed with Hinode/XRT in February
  2007. QSL maps are created at various heights in the corona and for
  10 different observations during the evolution of the long-lasting
  sigmoid. This is the first QSL analysis based on a non-linear force
  free field of a sigmoid. We point out some major properties of the
  QSL topology in the sigmoid and we explore how they change spatially
  and temporally with the evolution of the sigmoid. We explore how QSL
  topology and strength relate to current distributions and torsion
  factors in the moments leading to the B-class flare. While the current
  distribution in the region is smooth and extended, the QSLs show much
  finer structure which may prove to be of help in pinpointing possible
  reconnection or heating sites.

---------------------------------------------------------
Title: Non-Linear Force Free Field Modeling and Flare Ribbon
    Comparison of AR11347
Authors: McKillop, Sean; Savcheva, A.
2012AAS...22020426M    Altcode:
  In this project we present a model of active region (AR) 11347. This
  region was observed on November 15, 2011 with the Atmospheric Imaging
  Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). The
  region produced a small flare around 17:00 UT on that date. We build a
  Non-Linear Force Free Field (NLFFF) model of the AR via the flux-rope
  insertion method, using Helioseismic and Magnetic Imager (HMI)
  magnetograms as the boundary condition and AIA coronal observations
  as the constraint on the models. The observed loops in the core of
  the region are clearly sheared and twisted. For the best-fit model we
  compute the Quasi-Separatrix Layers (QSL) throughout the volume and
  compare the QSLs in the low corona with the flare ribbons as seen in
  AIA data.

---------------------------------------------------------
Title: Jet Statistics with the Automatic Jet Detections Module
Authors: Savcheva, Antonia
2012AAS...22020119S    Altcode:
  We present the automatic jet detection module as part of the SDO Science
  Center. We give the methodology behind detecting jets in polar coronal
  holes and the automatic detection of jet parameters, such as velocities,
  lengths, lifetimes, widths. Examples of <P />individual events show
  how the algorithm for finding the jets and determining the parameters
  work. <P />Applying the program to the 1st year of AIA observation
  gives us statistical <P />sample of hundreds of jets. Using this data
  we can estimate to mass load to solar wind.

---------------------------------------------------------
Title: Topological Tools For The Analysis Of Active Region Filament
    Stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
   Pariat, E.; Aulanier, G.; Su, Y.
2012AAS...22020207D    Altcode:
  The combination of accurate NLFFF models and high resolution MHD
  simulations allows us to study the changes in stability of an active
  region filament before a CME. Our analysis strongly supports the
  following sequence of events leading up to the CME: first there is a
  build up of magnetic flux in the filament through flux cancellation
  beneath a developing flux rope; as the flux rope develops a hyperbolic
  flux tube (HFT) forms beneath the flux rope; reconnection across
  the HFT raises the flux rope while adding addition flux to it; the
  eruption is triggered when the flux rope becomes torus-unstable. The
  work applies topological analysis tools that have been developed over
  the past decade and points the way for future work on the critical
  problem of CME initiation in solar active regions. We will present
  the uses of this approach, current limitations and future prospects.

---------------------------------------------------------
Title: The Properties and Kinematics of a New Sample of Cool Subdwarfs
    from SDSS
Authors: Savcheva, Antonia; West, A. A.; Bochanski, J.
2012AAS...22052318S    Altcode:
  We present a sample of 2114 M subdwarfs from the 7th Data Release of
  the Sloan Digital Sky Survey (DR7; SDSS). This catalog contains stellar
  coordinates, SDSS magnitudes, spectral classes, radial velocities,
  proper motions, absolute magnitudes and estimated distances. We discuss
  the selection criteria, the spectral classification and radial velocity
  determination processes. We calculate 3D space motions (U,V,W) in the
  standard Galactic system and place each star in its proper subdwarf
  subclass (as defined by Lepine et al.). We show that the metal poor
  populations are moving faster than the metal rich stars on average,
  consistent with being members of a dynamically heated thick disk or
  halo population. In addition, we present two different versions of
  the reduced proper motion (RPM) diagram, including a new formulation
  of the classic RPM diagram that includes information about the radial
  velocity. We discuss a few curious subsets of our sample, including
  active stars, late ultra subdwarfs, and candidate high velocity star.

---------------------------------------------------------
Title: The Effect of Flux Cancellation on Building Sigmoidal Flux
    Ropes
Authors: Savcheva, Antonia; Green, L.; van Ballegooijen, A.; DeLuca, E.
2012AAS...22041105S    Altcode:
  The magnetic structure of sigmoidal active regions is generally
  associated with the presence of a twisted flux rope held down by a
  potential arcade. There are competing theories of how the flux rope
  develops - by flux emergence, cancellation, or footpoint motions. We
  look at how flux cancellation in several sigmoidal regions, observed
  with XRT, affects the buildup of the underlying flux ropes. We use
  MDI magnetograms to quantify the flux cancellation, and the flux rope
  insertion method to construct non-linear force free field models of the
  regions. These models allow us to produce 3-D magnetic field models
  and see how the fields evolve in time. The models show how the flux
  ropes energy and magnetic flux changes during the different stages in
  the flux cancellation. Flux cancellation events are associated with
  build up of twist in the region in accordance with the accepted flux
  cancelation picture. The location of flares and build-up of free energy
  is well correlated with flux cancellation events.

---------------------------------------------------------
Title: Topological tools for the analysis of active region filament
    stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
   Pariat, E.; Aulanier, G.; Su, Y.
2012decs.confE..64D    Altcode:
  The combination of accurate NLFFF models and high resolution MHD
  simulations allows us to study the changes in stability of an active
  region filament before a CME. Our analysis strongly supports the
  following sequence of events leading up to the CME: first there is a
  build up of magnetic flux in the filament through flux cancellation
  beneath a developing flux rope; as the flux rope develops a hyperbolic
  flux tube (HFT) forms beneath the flux rope; reconnection across
  the HFT raises the flux rope while adding addition flux to it; the
  eruption is triggered when the flux rope becomes torus-unstable. The
  work applies topological analysis tools that have been developed over
  the past decade and points the way for future work on the critical
  problem of CME initiation in solar active regions. We will discuss
  the uses of this approach, current limitations and future prospects.

---------------------------------------------------------
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.
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: Field Topology Analysis of a Long-lasting Coronal Sigmoid
Authors: Savcheva, A. S.; van Ballegooijen, A. A.; DeLuca, E. E.
2012ApJ...744...78S    Altcode:
  We present the first field topology analysis based on nonlinear
  force-free field (NLFFF) models of a long-lasting coronal sigmoid
  observed in 2007 February with the X-Ray Telescope on Hinode. The
  NLFFF models are built with the flux rope insertion method and
  give the three-dimensional coronal magnetic field as constrained by
  observed coronal loop structures and photospheric magnetograms. Based
  on these models, we have computed horizontal maps of the current and
  the squashing factor Q for 25 different heights in the corona for all
  six days of the evolution of the region. We use the squashing factor
  to quantify the degree of change of the field line linkage and to
  identify prominent quasi-separatrix layers (QSLs). We discuss the major
  properties of these QSL maps and devise a way to pick out important QSLs
  since our calculation cannot reach high values of Q. The complexity
  in the QSL maps reflects the high degree of fragmentation of the
  photospheric field. We find main QSLs and current concentrations that
  outline the flux rope cavity and that become characteristically S-shaped
  during the evolution of the sigmoid. We note that, although intermittent
  bald patches exist along the length of the sigmoid during its whole
  evolution, the flux rope remains stable for several days. However,
  shortly after the topology of the field exhibits hyperbolic flux tubes
  (HFT) on February 7 and February 12 the sigmoid loses equilibrium
  and produces two B-class flares and associated coronal mass ejections
  (CMEs). The location of the most elevated part of the HFT in our model
  coincides with the inferred locations of the two flares. Therefore, we
  suggest that the presence of an HFT in a coronal magnetic configuration
  may be an indication that the system is ready to erupt. We offer a
  scenario in which magnetic reconnection at the HFT drives the system
  toward the marginally stable state. Once this state is reached, loss
  of equilibrium occurs via the torus instability, producing a CME.

---------------------------------------------------------
Title: Nonlinear Force-Free Modeling of Aug 4 &amp; 10, 2010 Sigmoids
    via Flux Rope Insertion Method
Authors: Behm, Tyler; DeLuca, E.; Savcheva, A.
2012AAS...21914404B    Altcode:
  The high spatial resolution of space-based solar telescopes like
  AIA and Hinode/XRT has allowed us to see fine S-shaped structures
  in active regions. The collection of such S-shaped loops is known
  as a sigmoid and are of great interest to solar physics since 68%
  of coronal mass ejections appear in such regions. In our research,
  we detail methods of studying sigmoids by using magnetograms to make
  non-linear force free field models and by comparing these models to the
  observed loops in X-ray and EUV images. We use the flux rope insertion
  method to set the initial parameters for these models. Furthermore,
  we examine the ability of contour maps of field divergence to study
  the field topology of sigmoids. From our models, we estimate the free
  energy stored in the sigmoids. From our field divergence maps, we find
  features of high divergence also known as quasi-separatrix layers,
  which can point to probable location for reconnection.

---------------------------------------------------------
Title: Propagation of Polar Coronal Jets in the Fast Solar Wind
Authors: Miralles, M. P.; Cranmer, S. R.; Raymond, J. C.; Savcheva,
   A. S.; Stenborg, G.; Deluca, E. E.
2011exas.conf..119M    Altcode:
  We present results of an ongoing observational study of the physical
  properties and kinematics of polar coronal jets. While magnetic
  reconnection is considered the prime driving mechanism of the ejected
  plasma, the processes at work during reconnection are not yet completely
  understood. We use a combination of X-ray, UV, and visible-light imaging
  to probe the jet plasma, and we trace polar jets from their reconnection
  sites into the fast solar wind. Multi-instrument measurements of
  polar jets will put firm constraints on the mechanisms driving the
  jets and on the relative contribution of jets to the overall fast
  solar wind. This work is supported by NASA grant NNX09AH22G to the
  Smithsonian Astrophysical Observatory.

---------------------------------------------------------
Title: The Properties and Kinematics of a Sample of Cool Subdwarfs
    from SDSS
Authors: Savcheva, Antonia
2011AAS...21832610S    Altcode: 2011BAAS..43G32610S
  We present a sample of 364 M subdwarfs from the 7th Data Release of the
  Sloan Digital Sky Survey (DR7; SDSS). This catalog contains stellar
  coordinates, SDSS magnitudes, spectral classes, radial velocities,
  proper motions, absolute magnitudes and estimated distances. We discuss
  the selection criteria, the spectral classification and radial velocity
  determination processes. We calculate 3D space motions (U,V,W) in the
  standard Galactic system and place each star in its proper subdwarf
  subclass (as efined by Lepine et al.). We show that the metal poor
  populations are moving faster than metal rich stars on average, <P
  />consistent with being members of a dynamically heated thick disk
  or halo. In addition, we present an updated version of the reduced
  proper motion (RPM) diagram, which is useful for separating low-mass
  subdwarfs from their M dwarf thin disk counterparts.

---------------------------------------------------------
Title: Photospheric Flux Cancellation and the Build-up of Sigmoidal
    Flux Ropes
Authors: Savcheva, Antonia; Green, L.; DeLuca, E.; van Ballegooijen, A.
2011SPD....42.1806S    Altcode: 2011BAAS..43S.1806S
  The magnetic structure of sigmoidal active regions is generally
  associated with the presence of a twisted flux rope held down by a
  potential arcade. There are competing theories of how the flux rope
  develops - by flux emergence, cancellation, or footpoint motions. We
  look at how flux cancellation in several sigmoidal regions, observed
  with XRT and AIA, affects the buildup of the underlying flux ropes. We
  use MDI and HMI magnetograms to quantify the flux cancellation, and the
  flux rope insertion method to construct non-linear force free field
  models of the regions. We present magnetic maps and the 3D flux rope
  structure. We correlate the locations of flares and build-up of free
  energy and helicity with flux cancellation events. We show how the
  flux ropes energy and flux budget changes with the different stages
  in the flux cancellation.

---------------------------------------------------------
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.
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. <P
  />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: Application of Quasi-Separatrix Layer Maps to Understanding
    the Structure and Evolution of Sigmoids
Authors: Savcheva, Antonia; DeLuca, E.; Van Ballegooijen, A.
2010AAS...21640522S    Altcode:
  We present some preliminary work in attempt to utilize Quasi-Separatrix
  Layer (QSL) maps for understanding the structure and evolution of
  sigmoids. We show sample QSL maps calculated at different heights
  above the photosphere and different times over the evolution of the
  quiescent sigmoid from February, 2007, observed with Hinode/XRT. The
  QSL maps use already existing static MHD models of the sigmoid, based
  on the flux rope insertion method. We give a short overview of the
  method used to set-up these maps. By comparing current distributions
  and the squashing factors at different height and cross sections over
  the sigmoid location we suggest the use of QSLs as tracers of surface
  and/or volumetric currents. We look at the distribution, structure,
  and concentration of QSLs in combination with the size and location
  of bald patches at different stages of the sigmoid development. We
  attempt to use this analysis to help us discriminate between the
  main scenarios for the formation and X-ray appearance of the S-like
  structure - flux emergence (or cancellation) and twisting foot point
  motions. This method may possibly shed some light on the pre-eruption
  configuration and eruption mechanism in sigmoids as well.

---------------------------------------------------------
Title: Automated Feature and Event Detection with SDO AIA and HMI Data
Authors: Davey, Alisdair; Martens, P. C. H.; Attrill, G. D. R.;
   Engell, A.; Farid, S.; Grigis, P. C.; Kasper, J.; Korreck, K.; Saar,
   S. H.; Su, Y.; Testa, P.; Wills-Davey, M.; Savcheva, A.; 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.; Georgouli, M. K.; McAteer, R. T. J.; Hurlburt, N.; Timmons, R.
2010cosp...38.2878D    Altcode: 2010cosp.meet.2878D
  The Solar Dynamics Observatory (SDO) represents a new frontier in
  quantity and quality of solar data. At about 1.5 TB/day, the data will
  not be easily digestible by solar physicists using the same methods
  that have been employed for images from previous missions. In order for
  solar scientists to use the SDO data effectively they need meta-data
  that will allow them to identify and retrieve data sets that address
  their particular science questions. We are building a comprehensive
  computer vision pipeline for SDO, abstracting complete metadata
  on many of the features and events detectable on the Sun without
  human intervention. Our project unites more than a dozen individual,
  existing codes into a systematic tool that can be used by the entire
  solar community. The feature finding codes will run as part of the SDO
  Event Detection System (EDS) at the Joint Science Operations Center
  (JSOC; joint between Stanford and LMSAL). The metadata produced will
  be stored in the Heliophysics Event Knowledgebase (HEK), which will be
  accessible on-line for the rest of the world directly or via the Virtual
  Solar Observatory (VSO) . Solar scientists will be able to use the
  HEK to select event and feature data to download for science studies.

---------------------------------------------------------
Title: Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid
Authors: Savcheva, Antonia; van Ballegooijen, Adriaan
2009ApJ...703.1766S    Altcode:
  A study of the magnetic configuration and evolution of a long-lasting
  quiescent coronal sigmoid is presented. The sigmoid was observed by
  Hinode/XRT and Transition Region and Coronal Explorer (TRACE) between
  2007 February 6 and 12 when it finally erupted. We construct nonlinear
  force-free field models for several observations during this period,
  using the flux-rope insertion method. The high spatial and temporal
  resolution of the X-Ray Telescope (XRT) allows us to finely select
  best-fit models that match the observations. The modeling shows that a
  highly sheared field, consisting of a weakly twisted flux rope embedded
  in a potential field, very well describes the structure of the X-ray
  sigmoid. The flux rope reaches a stable equilibrium, but its axial
  flux is close to the stability limit of about 5 × 10<SUP>20</SUP>
  Mx. The relative magnetic helicity increases with time from February 8
  until just prior to the eruption on February 12. We study the spatial
  distribution of the torsion parameter α in the vicinity of the flux
  rope, and find that it has a hollow-core distribution, i.e., electric
  currents are concentrated in a current layer at the boundary between
  the flux rope and its surroundings. The current layer is located near
  the bald patch separatrix surface (BPSS) of the magnetic configuration,
  and the X-ray emission appears to come from this current layer/BPSS,
  consistent with the Titov and Démoulin model. We find that the twist
  angle Φ of the magnetic field increases with time to about 2π just
  prior to the eruption, but never reaches the value necessary for the
  kink instability.

---------------------------------------------------------
Title: Does a Polar Coronal Hole's Flux Emergence Follow a Hale-Like
    Law?
Authors: Savcheva, A.; Cirtain, J. W.; DeLuca, E. E.; Golub, L.
2009ApJ...702L..32S    Altcode:
  Recent increases in spatial and temporal resolution for solar telescopes
  sensitive to EUV and X-ray radiation have revealed the prevalence of
  transient jet events in polar coronal holes. Using data collected by the
  X-Ray Telescope on Hinode, Savcheva et al. confirmed the observation,
  made first by the Soft X-ray Telescope on Yohkoh, that some jets exhibit
  a motion transverse to the jet outflow direction. The velocity of this
  transverse motion is, on average, 10 km s<SUP>-1</SUP>. The direction
  of the transverse motion, in combination with the standard reconnection
  model for jet production (e.g., Shibata et al.), reflects the magnetic
  polarity orientation of the ephemeral active region at the base of
  the jet. From this signature, we find that during the present minimum
  phase of the solar cycle the jet-base ephemeral active regions in the
  polar coronal holes had a preferred east-west direction, and that this
  direction reversed during the cycle's progression through minimum. In
  late 2006 and early 2007, the preferred direction was that of the active
  regions of the coming sunspot cycle (cycle 24), but in late 2008 and
  early 2009 the preferred direction has been that of the active regions
  of sunspot cycle 25. These findings are consistent with the observations
  of Wilson et al. suggesting that each cycle of solar activity begins
  at polar latitudes soon after the onset of the previous cycle.

---------------------------------------------------------
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.
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. <P />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. <P
  />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. <P />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: Does a Polar Coronal Hole's Flux Emergence Follow a Hale-like
    Law?
Authors: Savcheva, Antonia
2009SPD....40.1406S    Altcode:
  Recent increases in spatial and temporal resolution for solar
  telescopes sensitive to EUV and X-ray have revealed the prevalence
  of transient jet events from within polar coronal holes. Using data
  collected by the X-Ray Telescope on Hinode, Savcheva et al. (2007)
  confirmed the observation, made first by the Soft X-ray Telescope on
  Yohkoh, that some jets exhibit a motion transverse to the jet outflow
  direction. The velocity of this transverse motion is, on average,
  20 km/s. The direction of the transverse motion, in combination with
  the standard reconnection model for jet production (e.g. Shibata et
  al. 1992), reflects the magnetic polarity orientation of the ephemeral
  active region at the base of the jet. From this signature, we find
  that during the present minimum phase of the solar cycle the jet-base
  ephemeral active regions in the polar coronal holes had a preferred
  east-west direction, and that this direction reversed during the cycle's
  progression through minimum. In late 2007 and early 2008, the preferred
  direction was that of the active regions of the coming sunspot cycle
  (Cycle 24), but since mid 2008 the preferred direction has been that
  of the active regions of the ending sunspot cycle (Cycle 23).

---------------------------------------------------------
Title: On the Structure and Evolution of Complexity in Sigmoids:
    A Flux Emergence Model
Authors: Archontis, V.; Hood, A. W.; Savcheva, A.; Golub, L.;
   Deluca, E.
2009ApJ...691.1276A    Altcode:
  Sigmoids are structures with a forward or inverse S-shape, generally
  observed in the solar corona in soft X-ray emission. It is believed that
  the appearance of a sigmoid in an active region is an important factor
  in eruptive activity. The association of sigmoids with dynamic phenomena
  such as flares and coronal mass ejections (CMEs) make the study of
  sigmoids important. Recent observations of a coronal sigmoid, obtained
  with the X-Ray Telescope (XRT) on board Hinode, showed the formation
  and eruption phase with high spatial resolution. These observations
  revealed that the topological structure of the sigmoid is complex:
  it consists of many differently oriented loops that all together
  form two opposite J-like bundles or an overall S-shaped structure. A
  series of theoretical and numerical models have been proposed, over
  the past years, to explain the nature of sigmoids but there is no
  explanation on how the aforementioned complexity in sigmoids is built
  up. In this paper, we present a flux emergence model that leads to the
  formation of a sigmoid, whose structure and evolution of complexity
  are in good qualitative agreement with the recent observations. For
  the initial state of the experiment a twisted flux tube is placed
  below the photosphere. A density deficit along the axis of the tube
  makes the system buoyant in the middle and it adopts an Ω-shape as it
  rises toward the outer atmosphere. During the evolution of the system,
  expanding field lines that touch the photosphere at bald-patches (BPs)
  form two seperatrix surfaces where dissipation is enhanced and current
  sheets are formed. Originally, each of the BP seperatrix surfaces
  has a J-like shape. Each one of the J's consist of reconnected field
  lines with different shapes and different relative orientation. The
  further dynamical evolution of the emerging flux tube results in the
  occurrence of many sites that resemble rotational discontinuities. Thus,
  additional current layers are formed inside the rising magnetized volume
  increasing the complexity of the system. The reconnected field lines
  along these layers form an overall S-shaped structure. The reconnection
  process continues to occur leading to the formation of another current
  concentration in the middle of the sigmoid where a flaring episode
  occurs. This central brightening is accompanied by the eruption of a
  flux rope from the central area of the sigmoid and the appearance of
  "post-flare" loops underneath the current structure.

---------------------------------------------------------
Title: Determination of Temperatures and Densities of Polar Coronal
    X-ray Jets Observed with Hinode XRT and EIS
Authors: Savcheva, A. S.
2008AGUFMSH51A1594S    Altcode:
  In this work we present observations of polar coronal X-ray jets
  made with Hinode XRT and EIS during Septebmer 2008 joint observing
  program. We present the methods for determining temperature and density
  of jets as well as some statistics. The temperature sytucture of jets
  is considered and is related to the density measurements.

---------------------------------------------------------
Title: Overview of XRT performance and first results
Authors: Savcheva, Antonia; Savcheva
2008IAUS..247..326S    Altcode: 2007IAUS..247..326S
  In this review we present a short introduction to the X-ray Telescope
  on Hinode. We discuss its capabilities and new features and compare it
  with Yohkoh SXT. We also discuss some of the first results that include
  observations of X-ray jets in coronal holes, shear change in flares,
  sigmoid eruptions and evolution, application of filter ratios and
  differential emission measure analysis, structure of active regions,
  fine structure of X-ray bright points, and modeling non-potential fields
  around filaments. Finally, we describe how XRT works with other ground
  and space-based instrumentation, in particular with TRACE, EIS, SOT,
  and SOLIS.

---------------------------------------------------------
Title: A search for oscillating loops in Solar-B XRT observations
Authors: Savcheva, Antonia; DeLuca, Edward
2008IAUS..247..147S    Altcode: 2007IAUS..247..147S
  Between November 2006 and March 2007, on several occasions, XRT has
  been taking high-cadence one or two filter observations of prominent
  active regions on the disk. We took these datasets and conducted a quick
  search for acoustic brightness oscillation in loops. We concentrated
  our search on flaring active regions. Here we present the preliminary
  results of this search. We found one active region - NOAA 10953 from 27
  April - 02 May 2007 that had indications of acoustic oscillations with
  periods around 5 min, as well as multiples of this period, and one 40
  min period that we associate with periodic heating of the loops. An
  interesting result is that all the loops in the active region seemed
  to oscillate with the same set of periods, only the power in the FFT
  was different and maybe dependent on the magnetic field strength.

---------------------------------------------------------
Title: NLFF Model of a Coronal Sigmoid
Authors: Savcheva, A. S.; Archontis, V.; van Ballegooijen, A.
2008AGUSMSP31A..05S    Altcode:
  Between Feb 10 and 12, 2007, the X-ray telescope on Hinode produced
  some very high-cadence and high- resolution observations of a prominent
  coronal sigmoid. Here we show our results from computing a NLFF model
  of the sigmoid and compare it qualitatively to the XRT and TRACE
  observations. In addition we include some preliminary qualitative
  and quantitative results from 2.5D flux emergence simulation. We also
  discuss the future goals of this project.

---------------------------------------------------------
Title: The Analysis of Hinode/XRT Observations
Authors: Deluca, E. E.; Weber, M.; Savcheva, A.; Saar, S.; Testa,
   P.; Cirtain, J. W.; Sakao, T.; Noriyuki, N.; Kano, R.; Shimizu, T.
2008AGUSMSP51B..02D    Altcode:
  This poster will present the current state of Hinode/XRT analysis
  software. We will give an overview of the XRT Analysis Guide. We will
  include a detailed discussion of the following topics: <P />Co-alignment
  with SOT and EIS Spot removal for dynamics studies Filter calibration
  for thermal studies Dark calibrations <P />Sample data sets will be
  discussed and links to the data products will be provided.

---------------------------------------------------------
Title: Evidence for Alfvén Waves in Solar X-ray Jets
Authors: Cirtain, J. W.; Golub, L.; Lundquist, L.; van Ballegooijen,
   A.; Savcheva, A.; Shimojo, M.; DeLuca, E.; Tsuneta, S.; Sakao, T.;
   Reeves, K.; Weber, M.; Kano, R.; Narukage, N.; Shibasaki, K.
2007Sci...318.1580C    Altcode:
  Coronal magnetic fields are dynamic, and field lines may misalign,
  reassemble, and release energy by means of magnetic reconnection. Giant
  releases may generate solar flares and coronal mass ejections and,
  on a smaller scale, produce x-ray jets. Hinode observations of polar
  coronal holes reveal that x-ray jets have two distinct velocities:
  one near the Alfvén speed (~800 kilometers per second) and another
  near the sound speed (200 kilometers per second). Many more jets were
  seen than have been reported previously; we detected an average of
  10 events per hour up to these speeds, whereas previous observations
  documented only a handful per day with lower average speeds of 200
  kilometers per second. The x-ray jets are about 2 × 10<SUP>3</SUP> to
  2 × 10<SUP>4</SUP> kilometers wide and 1 × 10<SUP>5</SUP> kilometers
  long and last from 100 to 2500 seconds. The large number of events,
  coupled with the high velocities of the apparent outflows, indicates
  that the jets may contribute to the high-speed solar wind.

---------------------------------------------------------
Title: A Study of Polar Jet Parameters Based on Hinode XRT
    Observations
Authors: Savcheva, Antonia; Cirtain, Jonathan; Deluca, Edward E.;
   Lundquist, Loraine L.; Golub, Leon; Weber, Mark; Shimojo, Masumi;
   Shibasaki, Kiyoto; Sakao, Taro; Narukage, Noriyuki; Tsuneta, Saku;
   Kano, Ryouhei
2007PASJ...59S.771S    Altcode:
  Hinode/SOHO campaign 7197 is the most extensive study of polar jet
  formation and evolution from within both the north and south polar
  coronal holes so far. For the first time, this study showed that the
  appearance of X-ray jets in the solar coronal holes occurs at very high
  frequency - about 60 jets d<SUP>-1</SUP> on average. Using observations
  collected by the X-Ray Telescope on Hinode, a number of physical
  parameters from a large sample of jets were statistically studied. We
  measured the apparent outward velocity, the height, the width and
  the lifetime of the jets. In our sample, all of these parameters show
  peaked distributions with maxima at 160kms<SUP>-1</SUP> for the outward
  velocity, 5 × 10<SUP>4</SUP> km for the height, 8 × 10<SUP>3</SUP>
  km for the width, and about 10min for the lifetime of the jets. We
  also present the first statistical study of jet transverse motions,
  which obtained transverse velocities of 0-35kms<SUP>-1</SUP>. These
  values were obtained on the basis of a larger (in terms of frequency)
  and better sampled set of events than what was previously statistically
  studied (Shimojo et al. 1996, PASJ, 48, 123). The results were made
  possible by the unique characteristics of XRT. We describe the methods
  used to determine the characteristics and set some future goals. We
  also show that despite some possible selection effects, jets preferably
  occur inside the polar coronal holes.

---------------------------------------------------------
Title: Fine Structures of Solar X-Ray Jets Observed with the X-Ray
    Telescope aboard Hinode
Authors: Shimojo, Masumi; Narukage, Noriyuki; Kano, Ryohei; Sakao,
   Taro; Tsuneta, Saku; Shibasaki, Kiyoto; Cirtain, Jonathan W.;
   Lundquist, Loraine L.; Reeves, Katherine K.; Savcheva, Antonia
2007PASJ...59S.745S    Altcode:
  The X-Ray Telescope (XRT) aboard Hinode has revealed the fine structure
  of solar X-ray jets. One of the fine structures observed by XRT is an
  expanding loop. The loop appeared near the footpoint of the jet when
  footpoint brightening was observed. Additionally, we have found that the
  X-ray jets began just after the expanding loop “breaks”. Other fine
  structures discovered by XRT are thread-like features along the axis
  of the jets. XRT has shown that these thread structures compose the
  cross-section of jets. The fine structures and their motions strongly
  support an X-ray jet model based on magnetic reconnection, and also
  suggest that we must consider the three-dimensional configuration of the
  magnetic field to understand the jet phenomenon. We also investigated
  the reverse jet associated with the X-ray jet in the quiet Sun, and
  propose that the reverse jet is produced by heat conduction, or a MHD
  wave subsequent to the main jet.

---------------------------------------------------------
Title: A Study of Polar Jet Parameters Based on Solar-B XRT
    Observations
Authors: Savcheva, Antonia; Cirtain, J.; Lundquist, L. L.; DeLuca,
   E. E.; Shimojo, M.; Tsuneta, S.
2007AAS...210.9116S    Altcode: 2007BAAS...39T.206S
  SoHO/Hinode campaign 7197 studied polar jet formation from within
  both the north and south polar coronal holes. Using the observations
  collected by the X-Ray Telescope on Hinode, a number of physical
  parameters of the jets have been characterized. We will show the
  results for velocity, emission measure, length, width, lifetime, and
  spatial distribution. These observational results will be compared to
  models such as the Shibata-type reconnection model and correlations
  to estimates of the theoretical model will be compared to the <P
  />observations.

---------------------------------------------------------
Title: The Statistics of Polar Coronal Jets using XRT/Hinode
Authors: Cirtain, Jonathan W.; Lundquist, L. L.; DeLuca, E. E.;
   Savcheva, A.; Shimojo, M.; Tsuneta, S.
2007AAS...210.9432C    Altcode: 2007BAAS...39..222C
  Recent observations of the polar coronal holes using the X-Ray Telescope
  on Hinode revealed how frequent x-ray jets occur. Previous observations
  were limited by cadence, spatial resolution and continuity. However,
  with XRT operations successfully underway, multiple weeks of polar
  observations can be used to provide improved statistics of some
  fundamental physical parameters of the jets. In particular, we will
  present results for the radial and transverse velocities, observed
  length and width, duration, and spatial distribution of some of the
  more than 200 jets XRT has observed to date. The observed changes in
  the structure of the region where the jets are formed seems to be well
  characterized by the Shibata-type reconnection model. Examples will
  be provided.

---------------------------------------------------------
Title: X1908+075: A Pulsar Orbiting in the Stellar Wind of a Massive
    Companion
Authors: Levine, A. M.; Rappaport, S.; Remillard, R.; Savcheva, A.
2004ApJ...617.1284L    Altcode: 2004astro.ph..4428L
  We have observed the persistent but optically unidentified X-ray
  source X1908+075 with the Proportional Counter Array and High-Energy
  X-Ray Timing Experiment instruments on the Rossi X-Ray Timing Explorer
  (RXTE). The binary nature of this source was recently established by
  Wen et al., who found a 4.4 day orbital period in results from the RXTE
  All-Sky Monitor. We report the discovery of 605 s pulsations in the
  X-ray flux. The Doppler delay curve is measured and provides a mass
  function of 6.1 M<SUB>solar</SUB>, which is a lower limit to the mass
  of the binary companion of the neutron star. The degree of attenuation
  of the low-energy end of the spectrum is found to be a strong function
  of orbital phase. A simple model of absorption in a stellar wind from
  the companion star fits the orbital phase dependence reasonably well
  and limits the orbital inclination angle to the range 38°-72°. These
  measured parameters lead to an orbital separation of ~60-80 lt-s, a
  mass for the companion star in the range 9-31 M<SUB>solar</SUB>, and an
  upper limit to the size of the companion of ~22 R<SUB>solar</SUB>. From
  our analysis, we also infer a wind mass-loss rate from the companion
  star of &gt;~1.3×10<SUP>-6</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP>
  and, when the properties of the companion star and the effects
  of photoionization are considered, likely &gt;~4×10<SUP>-6</SUP>
  M<SUB>solar</SUB> yr<SUP>-1</SUP>. Such a high rate is inconsistent
  with the allowed masses and radii that we find for a main-sequence or
  modestly evolved star unless the mass-loss rate is enhanced in the
  binary system relative to that of an isolated star. We discuss the
  possibility that the companion might be a Wolf-Rayet star that could
  evolve to become a black hole in 10<SUP>4</SUP> to 10<SUP>5</SUP>
  yr. If so, this would be the first identified progenitor of a neutron
  star-black hole binary.

---------------------------------------------------------
Title: Probing the extinction law and gas-to-dust ratio in M31 via
    globulars behind the disk
Authors: Savcheva, A. S.; Tassev, S. V.
2002POBeo..73..219S    Altcode:
  We use the Catalogue of M31 Globular Clusters and Globular Clusters
  Candidates, compiled by Barmby (2000), containing 400 M31 globular
  clusters and candidates. We reduce this list to a sample of 41
  globular clusters that have: (I) UBVRIK photometry, (II) HI data,
  (III) reliable [Fe/H] and (IV) reliable extinction as determined by
  us. For determining the intrinsic colours we used the evolutionary
  synthesis models for globular clusters by Kurth et al. (1999). The
  mean total-to-selective ratio in M31 in terms of the analytical
  formula by Cardelli et al. (1989) is found to be R<SUB>V</SUB> =
  2.7±0.2. Using data from 21 cm observations of M31 we got the HI column
  densities and obtained N(HI)/A<SUB>V</SUB> ~ 9×10<SUP>20</SUP> atoms
  cm<SUP>-2</SUP>mag<SUP>-1</SUP> but varying with the radius, indicating
  possible supersolar metallicity toward the centre of M31 galaxy.