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Kathy Steppe - One of the best experts on this subject based on the ideXlab platform.
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sap Flux Density measurement methods working principles and applicability
Functional Plant Biology, 2013Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:Sap-flow measurements have become increasingly important in plant science. Since the early experiments with dyes, many methods have been developed. Most of these are based on the application of heat in the sapwood which is transported by the moving sap. By measuring changes in the temperature field around the heater, sap flow can be derived. Although these methods all have the same basis, their working principles vary widely. A first distinction can be made between those measuring the sap-flow rate (g h–1) such as the stem heat balance and trunk sector heat balance method and those measuring sap-Flux Density (cm3 cm–2 h–1). Within the latter, the thermal dissipation and heat field deformation methods are based on continuous heating, whereas the compensation heat pulse velocity, Tmax,heat ratio, calibrated average gradient and Sapflow+ methods are based on the application of heat pulses. Each of these methods has its advantages and limitations. Although the sap-flow rate methods have been adequately described in previous reviews, recent developments in sap-Flux Density methods prompted a synthesis of the existing but scattered literature. This paper reviews sap-Flux Density methods to enable users to make a well founded choice, whether for practical applications or fundamental research questions, and to encourage further improvement in sap-Flux Density measurement techniques.
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interpreting the heat field deformation method erroneous use of thermal diffusivity and improved correlation between temperature ratio and sap Flux Density
Agricultural and Forest Meteorology, 2012Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:Abstract The Heat Field Deformation (HFD) method is a modern technique to assess sap Flux Density in trees by applying an equation which relates an empirical temperature ratio to the thermal diffusivity of the sapwood. However, this relation is based on a misinterpretation of thermal diffusivity, leading to physically incorrect units of sap Flux Density. Moreover, the HFD method has recently been shown to occasionally underestimate actual sap Flux densities, raising the question whether species specific calibration is necessary. This paper calls attention to a correct interpretation of thermal diffusivity and investigates the correlation between sap Flux Density and the HFD temperature ratio based on a 3D Finite Element Model. It is shown that the original terms linking the HFD temperature ratio to sap Flux Density do not follow fundamental thermodynamics and, therefore, the HFD method should be considered merely empirical. Besides, the method is not only dependent on sapwood characteristics but also on sap Flux Density itself, necessitating a specific calibration equation.
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Sapflow+: A four-needle heat-pulse sap flow sensor enabling nonempirical sap Flux Density and water content measurements
New Phytologist, 2012Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:* •To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap Flux Density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap Flux Density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content. * •Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction–convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined. * •An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap Flux Density. * •The proposed method enables sap Flux Density measurements to be made across the entire natural occurring sap Flux Density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap Flux Density without destructive core measurements.
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a comparison of sap Flux Density using thermal dissipation heat pulse velocity and heat field deformation methods
Agricultural and Forest Meteorology, 2010Co-Authors: Kathy Steppe, Tanya M. Doody, Dirk J. W. De Pauw, Robert O TeskeyAbstract:Abstract A laboratory test and field evaluation were conducted to determine the accuracy of the three commonly used techniques for measuring sap Flux Density in trees: heat pulse velocity, thermal dissipation and heat field deformation. In the laboratory test a constant flow rate of water was maintained through freshly cut stem segments of diffuse-porous Fagus grandifolia trees with mean sapwood depths of 4.02 ± 0.14 and 7.44 ± 0.51 cm for sample trees with stem diameter at breast height of 15 and 21 cm, respectively. The three sensor types were measured simultaneously and compared against gravimetric measurements. All three techniques substantially underestimated sap Flux Density. On average the actual sap Flux Density was underestimated by 35% using heat pulse velocity (with wound correction), 46% using heat field deformation and 60% using thermal dissipation. These results were consistent across sap Flux densities ranging from 5 to 80 cm3 cm−2 h−1. Heat pulse velocity measurements were more variable than those of the other two techniques, and the least accurate at low sap Flux densities. An error analysis was conducted on all parameters of the equations used with each technique. That analysis indicated that each technique has unique sensitivities to errors in parameter estimates which need to be taken into consideration. Except for the use of heat, the three techniques are quite different and there appeared to be no single reason why the methods underestimated actual sap Flux Density, but rather there were likely multiple errors that compounded to reduce the overall accuracy of each technique. Field measurements supported the relative sensor performance observed in the laboratory. Applying a sensor-specific correction factor based on the laboratory test to the field data produced similar estimates of sap Flux Density from all three techniques. We conclude that a species-specific calibration is necessary when using any of these techniques to insure that accurate estimates of sap Flux Density are obtained, at least until a physical basis for an error correction can be proposed.
Maurits W. Vandegehuchte - One of the best experts on this subject based on the ideXlab platform.
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sap Flux Density measurement methods working principles and applicability
Functional Plant Biology, 2013Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:Sap-flow measurements have become increasingly important in plant science. Since the early experiments with dyes, many methods have been developed. Most of these are based on the application of heat in the sapwood which is transported by the moving sap. By measuring changes in the temperature field around the heater, sap flow can be derived. Although these methods all have the same basis, their working principles vary widely. A first distinction can be made between those measuring the sap-flow rate (g h–1) such as the stem heat balance and trunk sector heat balance method and those measuring sap-Flux Density (cm3 cm–2 h–1). Within the latter, the thermal dissipation and heat field deformation methods are based on continuous heating, whereas the compensation heat pulse velocity, Tmax,heat ratio, calibrated average gradient and Sapflow+ methods are based on the application of heat pulses. Each of these methods has its advantages and limitations. Although the sap-flow rate methods have been adequately described in previous reviews, recent developments in sap-Flux Density methods prompted a synthesis of the existing but scattered literature. This paper reviews sap-Flux Density methods to enable users to make a well founded choice, whether for practical applications or fundamental research questions, and to encourage further improvement in sap-Flux Density measurement techniques.
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interpreting the heat field deformation method erroneous use of thermal diffusivity and improved correlation between temperature ratio and sap Flux Density
Agricultural and Forest Meteorology, 2012Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:Abstract The Heat Field Deformation (HFD) method is a modern technique to assess sap Flux Density in trees by applying an equation which relates an empirical temperature ratio to the thermal diffusivity of the sapwood. However, this relation is based on a misinterpretation of thermal diffusivity, leading to physically incorrect units of sap Flux Density. Moreover, the HFD method has recently been shown to occasionally underestimate actual sap Flux densities, raising the question whether species specific calibration is necessary. This paper calls attention to a correct interpretation of thermal diffusivity and investigates the correlation between sap Flux Density and the HFD temperature ratio based on a 3D Finite Element Model. It is shown that the original terms linking the HFD temperature ratio to sap Flux Density do not follow fundamental thermodynamics and, therefore, the HFD method should be considered merely empirical. Besides, the method is not only dependent on sapwood characteristics but also on sap Flux Density itself, necessitating a specific calibration equation.
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Sapflow+: A four-needle heat-pulse sap flow sensor enabling nonempirical sap Flux Density and water content measurements
New Phytologist, 2012Co-Authors: Maurits W. Vandegehuchte, Kathy SteppeAbstract:* •To our knowledge, to date, no nonempirical method exists to measure reverse, low or high sap Flux Density. Moreover, existing sap flow methods require destructive wood core measurements to determine sapwood water content, necessary to convert heat velocity to sap Flux Density, not only damaging the tree, but also neglecting seasonal variability in sapwood water content. * •Here, we present a nonempirical heat-pulse-based method and coupled sensor which measure temperature changes around a linear heater in both axial and tangential directions after application of a heat pulse. By fitting the correct heat conduction–convection equation to the measured temperature profiles, the heat velocity and water content of the sapwood can be determined. * •An identifiability analysis and validation tests on artificial and real stem segments of European beech (Fagus sylvatica L.) confirm the applicability of the method, leading to accurate determinations of heat velocity, water content and hence sap Flux Density. * •The proposed method enables sap Flux Density measurements to be made across the entire natural occurring sap Flux Density range of woody plants. Moreover, the water content during low flows can be determined accurately, enabling a correct conversion from heat velocity to sap Flux Density without destructive core measurements.
G Theureau - One of the best experts on this subject based on the ideXlab platform.
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the magnetar xte j1810 197 variations in torque radio Flux Density and pulse profile morphology
The Astrophysical Journal, 2007Co-Authors: F Camilo, I Cognard, S M Ransom, J P Halpern, J E Reynolds, Neil Zimmerman, E V Gotthelf, David J Helfand, Paul Demorest, G TheureauAbstract:We report on 9 months of observations of the radio-emitting anomalous X-ray pulsar XTE J1810-197 starting in 2006 May using the Nancay, Parkes, Green Bank Telescope, and VLA telescopes mainly at a frequency of 1.4 GHz. The torque experienced by the neutron star during this period, as inferred from a measurement of its rotational frequency derivative, decreased by 60%, although not in a steady manner. We have also observed very large ongoing fluctuations in Flux Density and pulse shape. Superimposed on these, a general diminution of Flux Density and a broadening of the pulse profile components occurred nearly contemporaneously with a decrease in torque of about 10% that took place in late 2006 July over an interval of 2 weeks. After a slight increase in average Flux Density, since 2006 October the Flux Density has continued to decline and the pulse profiles, while still varying, appear more uniform. In addition, a simultaneous observation of the pulsar with the Chandra X-ray Observatory and the Green Bank Telescope allows us to show how the X-ray and radio profiles are aligned. We discuss briefly the implications of these results for the magnetospheric currents in this remarkable object.
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the magnetar xte j1810 197 variations in torque radio Flux Density and pulse profile morphology
arXiv: Astrophysics, 2006Co-Authors: F Camilo, I Cognard, S M Ransom, J P Halpern, J E Reynolds, Neil Zimmerman, E V Gotthelf, David J Helfand, Paul Demorest, G TheureauAbstract:We report on 9 months of observations of the radio-emitting anomalous X-ray pulsar XTE J1810-197 starting in 2006 May using the Nancay, Parkes, GBT, and VLA telescopes mainly at a frequency of 1.4 GHz. The torque experienced by the neutron star during this period, as inferred from a measurement of its rotational frequency derivative, decreased by 60%, although not in a steady manner. We have also observed very large ongoing fluctuations in Flux Density and pulse shape. Superimposed on these, a general diminution of Flux Density and a broadening of the pulse profile components occurred nearly contemporaneously with a decrease in torque of about 10% that took place in late 2006 July over an interval of 2 weeks. After a slight increase in average Flux Density, since 2006 October the Flux Density has continued to decline and the pulse profiles, while still varying, appear more uniform. In addition, a simultaneous observation of the pulsar with the Chandra X-ray Observatory and the GBT allows us to show how the X-ray and radio profiles are aligned. We discuss briefly the implications of these results for the magnetospheric currents in this remarkable object.
Mordecaimark Mac Low - One of the best experts on this subject based on the ideXlab platform.
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Flux Density variations at 3 6 cm in the massive star forming region w49a
The Astrophysical Journal, 2018Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Ralf S Klessen, J Bates, Theresa Melo, B PreslermarshallAbstract:A number of ultracompact H II regions in Galactic star forming environments have been observed to vary significantly in radio Flux Density on timescales of 10-20 years. Theory predicted that such variations should occur when the accretion flow that feeds a young massive star becomes unstable and clumpy. We have targeted the massive star-forming region W49A with the Karl G. Jansky Very Large Array (VLA) for observations at 3.6 cm with the B-configuration at 0.8'' resolution, to compare to nearly identical observations taken almost 21 years earlier (February 2015 and August 1994). Most of the sources in the crowded field of ultracompact and hypercompact H II regions exhibit no significant changes over this time period. However, one source, W49A/G2, decreased by 20% in peak intensity (from 71+/-4 mJy/beam to 57+/-3 mJy/beam), and 40% in integrated Flux (from 0.109+/-0.011 Jy to 0.067+/-0.007 Jy), where we cite 5 sigma errors in peak intensity, and 10% errors in integrated Flux. We present the radio images of the W49A region at the two epochs, the difference image that indicates the location of the Flux Density decrease, and discuss explanations for the Flux Density decrease near the position of W49A/G2.
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evidence of short timescale Flux Density variations of uc hii regions in sgr b2 main and north
The Astrophysical Journal, 2015Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Eric Keto, Ralf S Klessen, A MonsrudAbstract:We have recently published observations of significant Flux Density variations at 1.3 cm in H ii regions in the star-forming regions Sgr B2 Main and North. To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2 North observed in 2001. We find that 1 of the 41 known ultracompact and hypercompact H ii regions in Sgr B2 (K2-North) has decreased ~27% in Flux Density from 142 ± 14 to 103 ± 10 mJy (2.3σ) between 2001 and 2012. A second source, F3c-Main, has increased ~30% in Flux Density from 82 ± 8 to 107 ± 11 mJy (1.8σ) between 2003 and 2012. F3c-Main was previously observed to increase in Flux Density at 1.3 cm over a longer time period between 1989 and 2012. An observation of decreasing Flux Density, such as that observed in K2-North, is particularly significant since such a change is not predicted by the classical hypothesis of steady expansion of H ii regions during massive star accretion. Our new observations at 7 mm, along with others in the literature, suggest that the formation of massive stars occurs through time-variable and violent accretion.
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evidence of short timescale Flux Density variations of uc hii regions in sgr b2 main and north
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Eric Keto, Ralf S Klessen, A MonsrudAbstract:We have recently published observations of significant Flux Density variations at 1.3 cm in HII regions in the star forming regions Sgr B2 Main and North (De Pree et al. 2014). To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2 North observed in 2001. We find that one of the 41 known ultracompact and hypercompact HII regions in Sgr B2 (K2-North) has decreased $\sim$27% in Flux Density from 142$\pm$14 mJy to 103$\pm$10 mJy (2.3$\sigma$) between 2001 and 2012. A second source, F3c-Main has increased $\sim$30% in Flux Density from 82$\pm$8 mJy to 107 $\pm$11 mJy (1.8$\sigma$) between 2003 and 2012. F3c-Main was previously observed to increase in Flux Density at 1.3 cm over a longer time period between 1989 and 2012 (De Pree et al. 2014). An observation of decreasing Flux Density, such as that observed in K2-North, is particularly significant since such a change is not predicted by the classical hypothesis of steady expansion of HII regions during massive star accretion. Our new observations at 7 mm, along with others in the literature, suggest that the formation of massive stars occurs through time-variable and violent accretion.
C G De Pree - One of the best experts on this subject based on the ideXlab platform.
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Flux Density variations at 3 6 cm in the massive star forming region w49a
The Astrophysical Journal, 2018Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Ralf S Klessen, J Bates, Theresa Melo, B PreslermarshallAbstract:A number of ultracompact H II regions in Galactic star forming environments have been observed to vary significantly in radio Flux Density on timescales of 10-20 years. Theory predicted that such variations should occur when the accretion flow that feeds a young massive star becomes unstable and clumpy. We have targeted the massive star-forming region W49A with the Karl G. Jansky Very Large Array (VLA) for observations at 3.6 cm with the B-configuration at 0.8'' resolution, to compare to nearly identical observations taken almost 21 years earlier (February 2015 and August 1994). Most of the sources in the crowded field of ultracompact and hypercompact H II regions exhibit no significant changes over this time period. However, one source, W49A/G2, decreased by 20% in peak intensity (from 71+/-4 mJy/beam to 57+/-3 mJy/beam), and 40% in integrated Flux (from 0.109+/-0.011 Jy to 0.067+/-0.007 Jy), where we cite 5 sigma errors in peak intensity, and 10% errors in integrated Flux. We present the radio images of the W49A region at the two epochs, the difference image that indicates the location of the Flux Density decrease, and discuss explanations for the Flux Density decrease near the position of W49A/G2.
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evidence of short timescale Flux Density variations of uc hii regions in sgr b2 main and north
The Astrophysical Journal, 2015Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Eric Keto, Ralf S Klessen, A MonsrudAbstract:We have recently published observations of significant Flux Density variations at 1.3 cm in H ii regions in the star-forming regions Sgr B2 Main and North. To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2 North observed in 2001. We find that 1 of the 41 known ultracompact and hypercompact H ii regions in Sgr B2 (K2-North) has decreased ~27% in Flux Density from 142 ± 14 to 103 ± 10 mJy (2.3σ) between 2001 and 2012. A second source, F3c-Main, has increased ~30% in Flux Density from 82 ± 8 to 107 ± 11 mJy (1.8σ) between 2003 and 2012. F3c-Main was previously observed to increase in Flux Density at 1.3 cm over a longer time period between 1989 and 2012. An observation of decreasing Flux Density, such as that observed in K2-North, is particularly significant since such a change is not predicted by the classical hypothesis of steady expansion of H ii regions during massive star accretion. Our new observations at 7 mm, along with others in the literature, suggest that the formation of massive stars occurs through time-variable and violent accretion.
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evidence of short timescale Flux Density variations of uc hii regions in sgr b2 main and north
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: C G De Pree, Thomas Peters, Mordecaimark Mac Low, David J Wilner, W M Goss, Roberto Galvanmadrid, Eric Keto, Ralf S Klessen, A MonsrudAbstract:We have recently published observations of significant Flux Density variations at 1.3 cm in HII regions in the star forming regions Sgr B2 Main and North (De Pree et al. 2014). To further study these variations, we have made new 7 mm continuum and recombination line observations of Sgr B2 at the highest possible angular resolution of the Karl G. Jansky Very Large Array (VLA). We have observed Sgr B2 Main and North at 42.9 GHz and at 45.4 GHz in the BnA configuration (Main) and the A configuration (North). We compare these new data to archival VLA 7 mm continuum data of Sgr B2 Main observed in 2003 and Sgr B2 North observed in 2001. We find that one of the 41 known ultracompact and hypercompact HII regions in Sgr B2 (K2-North) has decreased $\sim$27% in Flux Density from 142$\pm$14 mJy to 103$\pm$10 mJy (2.3$\sigma$) between 2001 and 2012. A second source, F3c-Main has increased $\sim$30% in Flux Density from 82$\pm$8 mJy to 107 $\pm$11 mJy (1.8$\sigma$) between 2003 and 2012. F3c-Main was previously observed to increase in Flux Density at 1.3 cm over a longer time period between 1989 and 2012 (De Pree et al. 2014). An observation of decreasing Flux Density, such as that observed in K2-North, is particularly significant since such a change is not predicted by the classical hypothesis of steady expansion of HII regions during massive star accretion. Our new observations at 7 mm, along with others in the literature, suggest that the formation of massive stars occurs through time-variable and violent accretion.