The Experts below are selected from a list of 324 Experts worldwide ranked by ideXlab platform
E. E. Deluca - One of the best experts on this subject based on the ideXlab platform.
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A New Sigmoid Catalog from Hinode and the Solar Dynamics Observatory: Statistical Properties and Evolutionary Histories
Solar Physics, 2014Co-Authors: A. S. Savcheva, S. C. Mckillop, P. I. Mccauley, E. M. Hanson, E. E. DelucaAbstract: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.
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formation of torus unstable flux ropes and electric currents in erupting Sigmoids
The Astrophysical Journal, 2010Co-Authors: G Aulanier, T Torok, P Demoulin, E. E. DelucaAbstract:We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause its eruption. This is achieved by a zero-β magnetohydrodynamic (MHD) simulation of an initially potential, asymmetric bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvenic, line-tied shearing motions in the photosphere. As in similar models, flux-cancellation-driven photospheric reconnection in a bald-patch (BP) separatrix transforms the sheared arcades into a slowly rising and stable flux rope. A bifurcation from a BP to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux rope reaches the altitude at which the decay index –∂ln B/∂ln z of the potential field exceeds ~3/2, it rapidly accelerates upward, while the overlying arcade eventually develops an inverse tear-drop shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance with the onset criterion of the torus instability. Thus, we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X-Ray Telescope observations of an erupting Sigmoid, we produce simplified synthetic soft X-ray images from the distribution of the electric currents in the simulation. We find that a bright Sigmoidal envelope is formed by pairs of -shaped field lines in the pre-eruptive stage. These field lines form through the BP reconnection and merge later on into -shaped loops through the tether-cutting reconnection. During the eruption, the central part of the Sigmoid brightens due to the formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion, together with the increase of narrow currents in the reconnecting QSL, yields the Sigmoid hooks to thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF) detaches from the center of the Sigmoid. Most of this ELLF is not associated with the erupting flux rope, but with a current shell that develops within expanding field lines above the rope. Only the short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features found in the simulation are generic for the formation and eruption of soft X-ray Sigmoids.
Yuhong Fan - One of the best experts on this subject based on the ideXlab platform.
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simulation of homologous and cannibalistic coronal mass ejections produced by the emergence of a twisted flux rope into the solar corona
The Astrophysical Journal, 2013Co-Authors: Piyali Chatterjee, Yuhong FanAbstract:We report the first results of a magnetohydrodynamic simulation of the development of a homologous sequence of three coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs is cannibalistic, catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km s–1 as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of Sigmoids and "Sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs.
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simulation of homologous and cannibalistic coronal mass ejections produced by the emergence of a twisted flux rope into the solar corona
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Piyali Chatterjee, Yuhong FanAbstract:We report the first results of a magnetohydrodynamic (MHD) simulation of the development of a homologous sequence of three coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs simulated is cannibalistic catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km/s as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of Sigmoids and "Sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs.
Saksham Gupta - One of the best experts on this subject based on the ideXlab platform.
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Prevalence of Sigmoid Sinus Dehiscence and Diverticulum among Adults with Skull Base Cephaloceles.
AJNR. American journal of neuroradiology, 2020Co-Authors: Houman Sotoudeh, Galal A. Elsayed, S. Ghandili, Omid Shafaat, Joshua D. Bernstock, Gustavo Chagoya, Travis J. Atchley, P. Talati, David J. Segar, Saksham GuptaAbstract:BACKGROUND AND PURPOSE: Cephaloceles are relatively rare conditions caused by a congenital and/or acquired skull defect. The incidence of associated venous brain anomalies with regard to cephaloceles remains to be fully elucidated. Accordingly, we sought to assess the prevalence of Sigmoid sinus dehiscence and diverticula in patients with spontaneous skull base cephaloceles. MATERIALS AND METHODS: Our institutional data base was retrospectively queried from 2005 to 2018. Patients in whom spontaneous skull base cephaloceles were identified were ultimately included in the study cohort. These patients subsequently had their Sigmoid sinuses re-evaluated with focused attention on the possible presence of dehiscence and/or diverticula. RESULTS: We identified 56 patients: 12 men and 44 women. After re-evaluation of the Sigmoid sinuses, evidence of dehiscence and/or diverticula was noted in 21 patients. The right Sigmoid sinus was involved in 11 patients, and the left Sigmoid sinus was involved in 7 patients, including 3 cases of diverticulum. In 3 patients, evidence of bilateral Sigmoid sinus dehiscence and diverticula was noted. Female sex was associated with Sigmoid sinus dehiscence and diverticula by univariate analysis (P = .019). By linear regression, cephalocele volume was negatively associated with Sigmoid sinus dehiscence and diverticula (coefficient, −2266, P value CONCLUSIONS: The prevalence of Sigmoid sinus dehiscence and diverticula in patients with cephalocele is high. Female sex is associated with Sigmoid sinus dehiscence and diverticula. The cephalocele volume appears to be inversely proportional to Sigmoid sinus dehiscence and diverticula.
Piyali Chatterjee - One of the best experts on this subject based on the ideXlab platform.
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simulation of homologous and cannibalistic coronal mass ejections produced by the emergence of a twisted flux rope into the solar corona
The Astrophysical Journal, 2013Co-Authors: Piyali Chatterjee, Yuhong FanAbstract:We report the first results of a magnetohydrodynamic simulation of the development of a homologous sequence of three coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs is cannibalistic, catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km s–1 as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of Sigmoids and "Sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs.
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simulation of homologous and cannibalistic coronal mass ejections produced by the emergence of a twisted flux rope into the solar corona
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Piyali Chatterjee, Yuhong FanAbstract:We report the first results of a magnetohydrodynamic (MHD) simulation of the development of a homologous sequence of three coronal mass ejections (CMEs) and demonstrate their so-called cannibalistic behavior. These CMEs originate from the repeated formations and partial eruptions of kink unstable flux ropes as a result of continued emergence of a twisted flux rope across the lower boundary into a pre-existing coronal potential arcade field. The simulation shows that a CME erupting into the open magnetic field created by a preceding CME has a higher speed. The second of the three successive CMEs simulated is cannibalistic catching up and merging with the first into a single fast CME before exiting the domain. All the CMEs including the leading merged CME, attained speeds of about 1000 km/s as they exit the domain. The reformation of a twisted flux rope after each CME eruption during the sustained flux emergence can naturally explain the X-ray observations of repeated reformations of Sigmoids and "Sigmoid-under-cusp" configurations at a low-coronal source of homologous CMEs.
Houman Sotoudeh - One of the best experts on this subject based on the ideXlab platform.
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Prevalence of Sigmoid Sinus Dehiscence and Diverticulum among Adults with Skull Base Cephaloceles.
AJNR. American journal of neuroradiology, 2020Co-Authors: Houman Sotoudeh, Galal A. Elsayed, S. Ghandili, Omid Shafaat, Joshua D. Bernstock, Gustavo Chagoya, Travis J. Atchley, P. Talati, David J. Segar, Saksham GuptaAbstract:BACKGROUND AND PURPOSE: Cephaloceles are relatively rare conditions caused by a congenital and/or acquired skull defect. The incidence of associated venous brain anomalies with regard to cephaloceles remains to be fully elucidated. Accordingly, we sought to assess the prevalence of Sigmoid sinus dehiscence and diverticula in patients with spontaneous skull base cephaloceles. MATERIALS AND METHODS: Our institutional data base was retrospectively queried from 2005 to 2018. Patients in whom spontaneous skull base cephaloceles were identified were ultimately included in the study cohort. These patients subsequently had their Sigmoid sinuses re-evaluated with focused attention on the possible presence of dehiscence and/or diverticula. RESULTS: We identified 56 patients: 12 men and 44 women. After re-evaluation of the Sigmoid sinuses, evidence of dehiscence and/or diverticula was noted in 21 patients. The right Sigmoid sinus was involved in 11 patients, and the left Sigmoid sinus was involved in 7 patients, including 3 cases of diverticulum. In 3 patients, evidence of bilateral Sigmoid sinus dehiscence and diverticula was noted. Female sex was associated with Sigmoid sinus dehiscence and diverticula by univariate analysis (P = .019). By linear regression, cephalocele volume was negatively associated with Sigmoid sinus dehiscence and diverticula (coefficient, −2266, P value CONCLUSIONS: The prevalence of Sigmoid sinus dehiscence and diverticula in patients with cephalocele is high. Female sex is associated with Sigmoid sinus dehiscence and diverticula. The cephalocele volume appears to be inversely proportional to Sigmoid sinus dehiscence and diverticula.
