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.
Bibcode: 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.
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.
Bibcode: 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.
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
Bibcode: 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.
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
Bibcode: 2021zndo...5247589C
Altcode:
A community-developed open source Python 3.6+ package for plasma
physics research and education that is currently under development.
Title: Magnetofrictional Modeling of an Erupting Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Gibson, Sarah; Tassev,
Svetlin; Reeves, Katharine K.; DeLuca, Edward E.; Dalmasse, Kévin
Bibcode: 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.
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
Bibcode: 2021zndo...4602818C
Altcode:
A community-developed open source Python 3.6+ package for plasma
physics research and education that is currently under development.
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
Bibcode: 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.
Title: Magnetofrictional Modeling of an erupting Pseudostreamer
Authors: Karna, Nishu; Gibson, Sarah; DeLuca, Edward; Dalmasse,
Kévin; Savcheva, Antonia; Tassev, Svetlin
Bibcode: 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.
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
Bibcode: 2020zndo...4313063C
Altcode:
A community-developed open source Python 3.6+ package for plasma
physics research and education that is currently under development.
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.
Bibcode: 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.
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.
Bibcode: 2020arXiv200400079J
Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.
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
Bibcode: 2020zndo...3694337C
Altcode:
A community-developed open source Python 3.6+ package for plasma
physics in the early stages of development.
Title: Evidence for Multiple Acceleration Mechanisms in Coronal Jets
Authors: Farid, S. I.; Reeves, K.; Savcheva, A.; Rodríguez, N.;
Wainwright, W.
Bibcode: 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.
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.
Bibcode: 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 × 1020
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 × 1020 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 × 1031 ergs.
Title: Forward Modeling of a Pseudostreamer
Authors: Karna, Nishu; Savcheva, Antonia; Dalmasse, Kévin; Gibson,
Sarah; Tassev, Svetlin; de Toma, Giuliana; DeLuca, Edward E.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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 (<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 km2) 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
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.
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.
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.
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. A movie associated to Fig. 1 is available in electronic form at http://www.aanda.org
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.
Bibcode: 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. [1] A. W. Hood & E. R. Priest,
Geophys. Astrophys. Fluid Dynamics 17, 297 (1981) [2] B. Kliem
& 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 & J. Zhang, Astrophys. J. 718,
433 (2010) [6] T. Torok & B. Kliem, Astrophys. J. 630, L97 (2005)
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
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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 & 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
& Priest, Geophys. Astrophys. Fluid Dynamics 17, 297 (1981)
[2] Kliem & Török, Phys. Rev. Lett. 96, 255002 (2006) [3]
Myers, Ph.D. Thesis, Princeton University (2015) [4] Olmedo &
Zhang, Astrophys. J. 718, 433 (2010) [5] Török & 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.
Bibcode: 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 & 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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(g -
r), 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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 individual events show
how the algorithm for finding the jets and determining the parameters
work. Applying the program to the 1st year of AIA observation
gives us statistical 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 2012SoPh..275...79M
Altcode: 2011SoPh..tmp..144M; 2011SoPh..tmp..213M; 2011SoPh..tmp....8M
In Fall 2008 NASA selected a large international consortium to produce
a comprehensive automated feature-recognition system for the Solar
Dynamics Observatory (SDO). The SDO data that we consider are all of the
Atmospheric Imaging Assembly (AIA) images plus surface magnetic-field
images from the Helioseismic and Magnetic Imager (HMI). We produce
robust, very efficient, professionally coded software modules that
can keep up with the SDO data stream and detect, trace, and analyze
numerous phenomena, including flares, sigmoids, filaments, coronal
dimmings, polarity inversion lines, sunspots, X-ray bright points,
active regions, coronal holes, EIT waves, coronal mass ejections
(CMEs), coronal oscillations, and jets. We also track the emergence and
evolution of magnetic elements down to the smallest detectable features
and will provide at least four full-disk, nonlinear, force-free magnetic
field extrapolations per day. The detection of CMEs and filaments is
accomplished with Solar and Heliospheric Observatory (SOHO)/Large
Angle and Spectrometric Coronagraph (LASCO) and ground-based Hα
data, respectively. A completely new software element is a trainable
feature-detection module based on a generalized image-classification
algorithm. Such a trainable module can be used to find features that
have not yet been discovered (as, for example, sigmoids were in the
pre-Yohkoh era). Our codes will produce entries in the Heliophysics
Events Knowledgebase (HEK) as well as produce complete catalogs for
results that are too numerous for inclusion in the HEK, such as the
X-ray bright-point metadata. This will permit users to locate data on
individual events as well as carry out statistical studies on large
numbers of events, using the interface provided by the Virtual Solar
Observatory. The operations concept for our computer vision system is
that the data will be analyzed in near real time as soon as they arrive
at the SDO Joint Science Operations Center and have undergone basic
processing. This will allow the system to produce timely space-weather
alerts and to guide the selection and production of quicklook images and
movies, in addition to its prime mission of enabling solar science. We
briefly describe the complex and unique data-processing pipeline,
consisting of the hardware and control software required to handle
the SDO data stream and accommodate the computer-vision modules, which
has been set up at the Lockheed-Martin Space Astrophysics Laboratory
(LMSAL), with an identical copy at the Smithsonian Astrophysical
Observatory (SAO).
Title: Field Topology Analysis of a Long-lasting Coronal Sigmoid
Authors: Savcheva, A. S.; van Ballegooijen, A. A.; DeLuca, E. E.
Bibcode: 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 & 10, 2010 Sigmoids
via Flux Rope Insertion Method
Authors: Behm, Tyler; DeLuca, E.; Savcheva, A.
Bibcode: 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.
Bibcode: 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
Bibcode: 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, 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.
Bibcode: 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.
Bibcode: 2010AAS...21630804M
Altcode:
The SDO Feature Finding Team produces robust and very efficient
software modules that can keep up with the relentless SDO data stream,
and detect, trace, and analyze a large number of phenomena including:
flares, sigmoids, filaments, coronal dimmings, polarity inversion
lines, sunspots, X-ray bright points, active regions, coronal holes,
EIT waves, CME's, coronal oscillations, and jets. In addition we track
the emergence and evolution of magnetic elements down to the smallest
features that are detectable, and we will also provide at least four
full disk nonlinear force-free magnetic field extrapolations per day. During SDO commissioning we will install in the near-real time data
pipeline the modules that provide alerts for flares, coronal dimmings,
and emerging flux, as well as those that trace filaments, sigmoids,
polarity inversion lines, and active regions. We will demonstrate
the performance of these modules and illustrate their use for science
investigations.
Title: Application of Quasi-Separatrix Layer Maps to Understanding
the Structure and Evolution of Sigmoids
Authors: Savcheva, Antonia; DeLuca, E.; Van Ballegooijen, A.
Bibcode: 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.
Bibcode: 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
Bibcode: 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 × 1020
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.
Bibcode: 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-1. 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.
Bibcode: 2009SPD....40.1711M
Altcode:
NASA funded a large international consortium last year to produce
a comprehensive system for automated feature recognition in SDO
images. The data we consider are all AIA and EVE data plus surface
magnetic field images from HMI. Helioseismology is addressed by another
group. We will produce robust and very efficient software modules
that can keep up with the relentless SDO data stream and detect, trace,
and analyze a large number of phenomena, including: flares, sigmoids,
filaments, coronal dimmings, polarity inversion lines, sunspots,
X-ray bright points, active regions, coronal holes, EIT waves, CME's,
coronal oscillations, and jets. In addition we will track the emergence
and evolution of magnetic elements down to the smallest features
that are detectable, and we will also provide at least four full
disk nonlinear force-free magnetic field extrapolations per day. A completely new software element that rounds out this suite is a
trainable feature detection module, which employs a generalized image
classification algorithm to produce the texture features of the images
analyzed. A user can introduce a number of examples of the phenomenon
looked and the software will return images with similar features. We
have tested a proto-type on TRACE data, and were able to "train" the
algorithm to detect sunspots, active regions, and loops. Such a module
can be used to find features that have not even been discovered yet,
as, for example, sigmoids were in the pre-Yohkoh era. Our codes
will produce entries in the Helio Events Knowledge base, and that will
permit users to locate data on individual events as well as carry out
statistical studies on large numbers of events, using the interface
provided by the Virtual Solar Observatory.
Title: Does a Polar Coronal Hole's Flux Emergence Follow a Hale-like
Law?
Authors: Savcheva, Antonia
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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: Co-alignment
with SOT and EIS Spot removal for dynamics studies Filter calibration
for thermal studies Dark calibrations 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.
Bibcode: 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 × 103 to
2 × 104 kilometers wide and 1 × 105 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
Bibcode: 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-1 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-1 for the outward
velocity, 5 × 104 km for the height, 8 × 103
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-1. 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
Bibcode: 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.
Bibcode: 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 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.
Bibcode: 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.
Bibcode: 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 Msolar, 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 Msolar, and an
upper limit to the size of the companion of ~22 Rsolar. From
our analysis, we also infer a wind mass-loss rate from the companion
star of >~1.3×10-6 Msolar yr-1
and, when the properties of the companion star and the effects
of photoionization are considered, likely >~4×10-6
Msolar yr-1. 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 104 to 105
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.
Bibcode: 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 RV =
2.7±0.2. Using data from 21 cm observations of M31 we got the HI column
densities and obtained N(HI)/AV ~ 9×1020 atoms
cm-2mag-1 but varying with the radius, indicating
possible supersolar metallicity toward the centre of M31 galaxy.