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David Mcleod - One of the best experts on this subject based on the ideXlab platform.
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evidence for an enduring ischaemic Penumbra following central retinal artery occlusion with implications for fibrinolytic therapy
Progress in Retinal and Eye Research, 2015Co-Authors: David Mcleod, Stephen BeattyAbstract:Abstract The rationale behind hyperacute fibrinolytic therapy for cerebral and retinal arterial occlusion is to rescue ischaemic cells from irreversible damage through timely restitution of tissue perfusion. In cerebral stroke, an anoxic tissue compartment (the “infarct core”) is surrounded by a hypoxic compartment (the “ischaemic Penumbra”). The latter comprises electrically-silent neurons that undergo delayed apoptotic cell death within 1–6 h unless salvaged by arterial recanalisation. Establishment of an equivalent hypoxic compartment within the inner retina following central retinal artery occlusion (CRAO) isn't widely acknowledged. During experimental CRAO, electroretinography reveals 3 oxygenation-based tissue compartments (anoxic, hypoxic and normoxic) that contribute 32%, 27% and 41% respectively to the pre-occlusion b-wave amplitude. Thus, once the anoxia survival time (≈2 h) expires, the contribution from the infarcted posterior retina is irreversibly extinguished, but electrical activity continues in the normoxic periphery. Inbetween these compartments, an annular hypoxic zone (the “Penumbra obscura”) endures in a structurally-intact but functionally-impaired state until retinal reperfusion allows rapid recovery from electrical silence. Clinically, residual circulation of sufficient volume flow rate generates the heterogeneous fundus picture of “partial” CRAO. Persistent retinal venous hypoxaemia signifies maximal extraction of oxygen by an enduring “polar Penumbra” that permeates or largely replaces the infarct core. On retinal reperfusion some days later, the retinal venous oxygen saturation reverts to normal and vision improves. Thus, Penumbral inner retina, marginally oxygenated by the choroid or by residual circulation, isn't at risk of delayed apoptotic infarction (unlike hypoxic cerebral cortex). Emergency fibrinolytic intervention is inappropriate, therefore, once the duration of CRAO exceeds 2 h.
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evidence for an enduring ischaemic Penumbra following central retinal artery occlusion with implications for fibrinolytic therapy
Progress in Retinal and Eye Research, 2015Co-Authors: David Mcleod, Stephen BeattyAbstract:The rationale behind hyperacute fibrinolytic therapy for cerebral and retinal arterial occlusion is to rescue ischaemic cells from irreversible damage through timely restitution of tissue perfusion. In cerebral stroke, an anoxic tissue compartment (the "infarct core") is surrounded by a hypoxic compartment (the "ischaemic Penumbra"). The latter comprises electrically-silent neurons that undergo delayed apoptotic cell death within 1-6 h unless salvaged by arterial recanalisation. Establishment of an equivalent hypoxic compartment within the inner retina following central retinal artery occlusion (CRAO) isn't widely acknowledged. During experimental CRAO, electroretinography reveals 3 oxygenation-based tissue compartments (anoxic, hypoxic and normoxic) that contribute 32%, 27% and 41% respectively to the pre-occlusion b-wave amplitude. Thus, once the anoxia survival time (≈2 h) expires, the contribution from the infarcted posterior retina is irreversibly extinguished, but electrical activity continues in the normoxic periphery. Inbetween these compartments, an annular hypoxic zone (the "Penumbra obscura") endures in a structurally-intact but functionally-impaired state until retinal reperfusion allows rapid recovery from electrical silence. Clinically, residual circulation of sufficient volume flow rate generates the heterogeneous fundus picture of "partial" CRAO. Persistent retinal venous hypoxaemia signifies maximal extraction of oxygen by an enduring "polar Penumbra" that permeates or largely replaces the infarct core. On retinal reperfusion some days later, the retinal venous oxygen saturation reverts to normal and vision improves. Thus, Penumbral inner retina, marginally oxygenated by the choroid or by residual circulation, isn't at risk of delayed apoptotic infarction (unlike hypoxic cerebral cortex). Emergency fibrinolytic intervention is inappropriate, therefore, once the duration of CRAO exceeds 2 h.
Stephen Beatty - One of the best experts on this subject based on the ideXlab platform.
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evidence for an enduring ischaemic Penumbra following central retinal artery occlusion with implications for fibrinolytic therapy
Progress in Retinal and Eye Research, 2015Co-Authors: David Mcleod, Stephen BeattyAbstract:Abstract The rationale behind hyperacute fibrinolytic therapy for cerebral and retinal arterial occlusion is to rescue ischaemic cells from irreversible damage through timely restitution of tissue perfusion. In cerebral stroke, an anoxic tissue compartment (the “infarct core”) is surrounded by a hypoxic compartment (the “ischaemic Penumbra”). The latter comprises electrically-silent neurons that undergo delayed apoptotic cell death within 1–6 h unless salvaged by arterial recanalisation. Establishment of an equivalent hypoxic compartment within the inner retina following central retinal artery occlusion (CRAO) isn't widely acknowledged. During experimental CRAO, electroretinography reveals 3 oxygenation-based tissue compartments (anoxic, hypoxic and normoxic) that contribute 32%, 27% and 41% respectively to the pre-occlusion b-wave amplitude. Thus, once the anoxia survival time (≈2 h) expires, the contribution from the infarcted posterior retina is irreversibly extinguished, but electrical activity continues in the normoxic periphery. Inbetween these compartments, an annular hypoxic zone (the “Penumbra obscura”) endures in a structurally-intact but functionally-impaired state until retinal reperfusion allows rapid recovery from electrical silence. Clinically, residual circulation of sufficient volume flow rate generates the heterogeneous fundus picture of “partial” CRAO. Persistent retinal venous hypoxaemia signifies maximal extraction of oxygen by an enduring “polar Penumbra” that permeates or largely replaces the infarct core. On retinal reperfusion some days later, the retinal venous oxygen saturation reverts to normal and vision improves. Thus, Penumbral inner retina, marginally oxygenated by the choroid or by residual circulation, isn't at risk of delayed apoptotic infarction (unlike hypoxic cerebral cortex). Emergency fibrinolytic intervention is inappropriate, therefore, once the duration of CRAO exceeds 2 h.
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evidence for an enduring ischaemic Penumbra following central retinal artery occlusion with implications for fibrinolytic therapy
Progress in Retinal and Eye Research, 2015Co-Authors: David Mcleod, Stephen BeattyAbstract:The rationale behind hyperacute fibrinolytic therapy for cerebral and retinal arterial occlusion is to rescue ischaemic cells from irreversible damage through timely restitution of tissue perfusion. In cerebral stroke, an anoxic tissue compartment (the "infarct core") is surrounded by a hypoxic compartment (the "ischaemic Penumbra"). The latter comprises electrically-silent neurons that undergo delayed apoptotic cell death within 1-6 h unless salvaged by arterial recanalisation. Establishment of an equivalent hypoxic compartment within the inner retina following central retinal artery occlusion (CRAO) isn't widely acknowledged. During experimental CRAO, electroretinography reveals 3 oxygenation-based tissue compartments (anoxic, hypoxic and normoxic) that contribute 32%, 27% and 41% respectively to the pre-occlusion b-wave amplitude. Thus, once the anoxia survival time (≈2 h) expires, the contribution from the infarcted posterior retina is irreversibly extinguished, but electrical activity continues in the normoxic periphery. Inbetween these compartments, an annular hypoxic zone (the "Penumbra obscura") endures in a structurally-intact but functionally-impaired state until retinal reperfusion allows rapid recovery from electrical silence. Clinically, residual circulation of sufficient volume flow rate generates the heterogeneous fundus picture of "partial" CRAO. Persistent retinal venous hypoxaemia signifies maximal extraction of oxygen by an enduring "polar Penumbra" that permeates or largely replaces the infarct core. On retinal reperfusion some days later, the retinal venous oxygen saturation reverts to normal and vision improves. Thus, Penumbral inner retina, marginally oxygenated by the choroid or by residual circulation, isn't at risk of delayed apoptotic infarction (unlike hypoxic cerebral cortex). Emergency fibrinolytic intervention is inappropriate, therefore, once the duration of CRAO exceeds 2 h.
S K Solanki - One of the best experts on this subject based on the ideXlab platform.
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no universal connection between the vertical magnetic field and the umbra Penumbra boundary in sunspots
Astronomy and Astrophysics, 2020Co-Authors: B Loptien, A Lagg, M Van Noort, S K SolankiAbstract:Context. It has been reported that the boundary between the umbra and the Penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted as to be the threshold for the onset of magnetoconvection. Aims. Here we investigate the reasons why this criterion, also called the Jurcak criterion in the literature, does not always identify the boundary between umbra and Penumbra. Methods. We perform a statistical analysis of 23 sunspots observed with Hinode/SOT. We compare the properties of the continuum intensity and the vertical magnetic field between filaments and spines and how they vary between spots of different sizes. Results. We find that the inner boundary of the Penumbra is not related to a universal value of the vertical magnetic field. The properties of spines and filaments vary between spots of different sizes. Both components are darker in larger spots and the spines exhibit stronger vertical magnetic field. These variations of the properties of filaments and spines with spot size are also the reason for the reported invariance of the averaged vertical magnetic field at 50% of the mean continuum intensity. Conclusions. The formation of filaments and the onset of magnetoconvection are not related to a canonical value of the strength of the vertical magnetic field. Such a seemingly unique magnetic field strength is rather an effect of the filling factor of spines and Penumbral filaments.
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solar alma observations constraining the chromosphere above sunspots
The Astrophysical Journal, 2017Co-Authors: M Loukitcheva, S K Solanki, Kazumasa Iwai, S M White, Masumi ShimojoAbstract:We present the first high-resolution Atacama Large Millimeter/Submillimeter Array (ALMA) observations of a sunspot at wavelengths of 1.3 and 3 mm, obtained during the solar ALMA Science Verification campaign in 2015, and compare them with the predictions of semi-empirical sunspot umbral/Penumbral atmosphere models. For the first time, millimeter observations of sunspots have resolved umbral/Penumbral brightness structure at the chromospheric heights, where the emission at these wavelengths is formed. We find that the sunspot umbra exhibits a radically different appearance at 1.3 and 3 mm, whereas the Penumbral brightness structure is similar at the two wavelengths. The inner part of the umbra is ~600 K brighter than the surrounding quiet Sun (QS) at 3 mm and is ~700 K cooler than the QS at 1.3 mm, being the coolest part of sunspot at this wavelength. On average, the brightness of the Penumbra at 3 mm is comparable to the QS brightness, while at 1.3 mm it is ~1000 K brighter than the QS. Penumbral brightness increases toward the outer boundary in both ALMA bands. Among the tested umbral models, that of Severino et al. provides the best fit to the observational data, including both the ALMA data analyzed in this study and data from earlier works. No Penumbral model among those considered here gives a satisfactory fit to the currently available measurements. ALMA observations at multiple millimeter wavelengths can be used for testing existing sunspot models, and serve as an important input to constrain new empirical models.
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normal and counter evershed flows in the photospheric Penumbra of a sunspot spinor 2d inversions of hinode sot sp observations
Astronomy and Astrophysics, 2017Co-Authors: A Siutapia, A Lagg, S K Solanki, M Van Noort, J JurcakAbstract:Context. The Evershed effect, a nearly horizontal outflow of material seen in the Penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed Penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the Penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the Penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the Penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center Penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows. Methods. We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated. Results. In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5–2 kG to regions with stronger fields. The sources and sinks of both Penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments). Conclusions. The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct Penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows.
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upper chromospheric magnetic field of a sunspot Penumbra observations of fine structure
Astronomy and Astrophysics, 2016Co-Authors: Jayant Joshi, A Lagg, S K Solanki, R Schlichenmaier, A Feller, M Collados, Orozco D Suarez, M FranzAbstract:Aims. The fine-structure of the magnetic field in a sunspot Penumbra in the upper chromosphere is to be explored and compared to that in the photosphere. Methods. Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 A and the photospheric Sii 10 827.1 A and Cai 10 833.4 A spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 A and Cai 10 833.4 A lines to obtain the photospheric magnetic field. Results. We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of Penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°–15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed Penumbra does not vary on small spatial scales in the upper chromosphere. Conclusions. Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot Penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate Penumbral chromospheric dynamic phenomena, such as Penumbral micro-jets or transient bright dots.
A Lagg - One of the best experts on this subject based on the ideXlab platform.
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no universal connection between the vertical magnetic field and the umbra Penumbra boundary in sunspots
Astronomy and Astrophysics, 2020Co-Authors: B Loptien, A Lagg, M Van Noort, S K SolankiAbstract:Context. It has been reported that the boundary between the umbra and the Penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted as to be the threshold for the onset of magnetoconvection. Aims. Here we investigate the reasons why this criterion, also called the Jurcak criterion in the literature, does not always identify the boundary between umbra and Penumbra. Methods. We perform a statistical analysis of 23 sunspots observed with Hinode/SOT. We compare the properties of the continuum intensity and the vertical magnetic field between filaments and spines and how they vary between spots of different sizes. Results. We find that the inner boundary of the Penumbra is not related to a universal value of the vertical magnetic field. The properties of spines and filaments vary between spots of different sizes. Both components are darker in larger spots and the spines exhibit stronger vertical magnetic field. These variations of the properties of filaments and spines with spot size are also the reason for the reported invariance of the averaged vertical magnetic field at 50% of the mean continuum intensity. Conclusions. The formation of filaments and the onset of magnetoconvection are not related to a canonical value of the strength of the vertical magnetic field. Such a seemingly unique magnetic field strength is rather an effect of the filling factor of spines and Penumbral filaments.
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normal and counter evershed flows in the photospheric Penumbra of a sunspot spinor 2d inversions of hinode sot sp observations
Astronomy and Astrophysics, 2017Co-Authors: A Siutapia, A Lagg, S K Solanki, M Van Noort, J JurcakAbstract:Context. The Evershed effect, a nearly horizontal outflow of material seen in the Penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed Penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the Penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the Penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the Penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center Penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows. Methods. We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated. Results. In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5–2 kG to regions with stronger fields. The sources and sinks of both Penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments). Conclusions. The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct Penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows.
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upper chromospheric magnetic field of a sunspot Penumbra observations of fine structure
Astronomy and Astrophysics, 2016Co-Authors: Jayant Joshi, A Lagg, S K Solanki, R Schlichenmaier, A Feller, M Collados, Orozco D Suarez, M FranzAbstract:Aims. The fine-structure of the magnetic field in a sunspot Penumbra in the upper chromosphere is to be explored and compared to that in the photosphere. Methods. Spectropolarimetric observations with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 A and the photospheric Sii 10 827.1 A and Cai 10 833.4 A spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 A and Cai 10 833.4 A lines to obtain the photospheric magnetic field. Results. We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of Penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°–15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed Penumbra does not vary on small spatial scales in the upper chromosphere. Conclusions. Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot Penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate Penumbral chromospheric dynamic phenomena, such as Penumbral micro-jets or transient bright dots.
J Jurcak - One of the best experts on this subject based on the ideXlab platform.
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The magnetic nature of umbra–Penumbra boundary in sunspots
Astronomy and Astrophysics, 2018Co-Authors: J Jurcak, Rolf Schlichenmaier, N. Bello González, R. Rezaei, Jiří VomlelAbstract:Context. Sunspots are the longest-known manifestation of solar activity, and their magnetic nature has been known for more than a century. Despite this, the boundary between umbrae and Penumbrae, the two fundamental sunspot regions, has hitherto been solely defined by an intensity threshold. Aim. Here, we aim at studying the magnetic nature of umbra–Penumbra boundaries in sunspots of different sizes, morphologies, evolutionary stages, and phases of the solar cycle. Methods. We used a sample of 88 scans of the Hinode/SOT spectropolarimeter to infer the magnetic field properties in at the umbral boundaries. We defined these umbra–Penumbra boundaries by an intensity threshold and performed a statistical analysis of the magnetic field properties on these boundaries. Results. We statistically prove that the umbra–Penumbra boundary in stable sunspots is characterised by an invariant value of the vertical magnetic field component: the vertical component of the magnetic field strength does not depend on the umbra size, its morphology, and phase of the solar cycle. With the statistical Bayesian inference, we find that the strength of the vertical magnetic field component is, with a likelihood of 99%, in the range of 1849–1885 G with the most probable value of 1867 G. In contrast, the magnetic field strength and inclination averaged along individual boundaries are found to be dependent on the umbral size: the larger the umbra, the stronger and more horizontal the magnetic field at its boundary. Conclusions. The umbra and Penumbra of sunspots are separated by a boundary that has hitherto been defined by an intensity threshold. We now unveil the empirical law of the magnetic nature of the umbra–Penumbra boundary in stable sunspots: it is an invariant vertical component of the magnetic field.
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normal and counter evershed flows in the photospheric Penumbra of a sunspot spinor 2d inversions of hinode sot sp observations
Astronomy and Astrophysics, 2017Co-Authors: A Siutapia, A Lagg, S K Solanki, M Van Noort, J JurcakAbstract:Context. The Evershed effect, a nearly horizontal outflow of material seen in the Penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed Penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the Penumbra, also known as counter Evershed flows. Aims. Here we present a rare feature observed in the Penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the Penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center Penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows. Methods. We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated. Results. In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5–2 kG to regions with stronger fields. The sources and sinks of both Penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments). Conclusions. The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct Penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows.
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Temporal downflows in a Penumbra
Astronomy and Astrophysics, 2010Co-Authors: J Jurcak, Yukio KatsukawaAbstract:Aims. We analyze temporal downflow patches that are located in a Penumbra and have the same polarity of the magnetic field as a sunspot umbra. Methods. The repetitive 2 �� wide raster scans of Penumbral regions that are taken with one minute cadence by the Hinode spectropolarimeter are used to detect the line-of-sight velocities in the Penumbra from enhanced signals in the wings of Stokes V profiles. The lifetimes and positions within Penumbra of the identified downflow patches are investigated. The plasma properties of the downflow patches are determined using the inversions of the observed Stokes profiles. Results. The temporal downflows have lifetimes of up to fourteen minutes. Some of them are related to the disappearance or weakening of nearby upflow regions or to the chromospheric brightenings. The downflows take place in regions with stronger and more vertical magnetic fields than the upflow regions.
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the properties of Penumbral microjets inclination
Astronomy and Astrophysics, 2008Co-Authors: J Jurcak, Yukio KatsukawaAbstract:Aims. We investigate the dependence of Penumbral microjets inclination on the position within Penumbra. Methods. The high cadence observations taken on 10 November 2006 with the Hinode satellite through the Ca II H and G-band filters were analysed to determine the inclination of Penumbral microjets. The results were then compared with the inclination of the magnetic field determined through the inversion of the spectropolarimetric observations of the same region. Results. The Penumbral microjet inclination is increasing towards the outer edge of the Penumbra. The results suggest that the Penumbral microjet follows the opening magnetic field lines of a vertical flux tube that creates the sunspot.