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L Van Drielgesztelyi - One of the best experts on this subject based on the ideXlab platform.
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a study of the coronal non Thermal Velocity in polar regions during the rise from solar minimum to solar maximum in cycle 24
Solar Physics, 2015Co-Authors: L K Harra, Hirohisa Hara, D. N. Baker, S. J. Edwards, Rachel Howe, L Van DrielgesztelyiAbstract:We explore the changes in coronal non-Thermal Velocity (V nt) measurements at the poles from solar minimum to solar maximum using Hinode EUV Imaging Spectrometer data. We find that although the intensity in the corona at the poles does tend to increase with the cycle, there are no significant changes in the V nt values. The locations of enhanced V nt values measured do not always have a counterpart in intensity, and they are sometimes located in weak emission regions. Unipolar magnetic streams, created through diffusion of the following polarity of the decaying active regions, slowly progress towards the poles. These streams are expected to be related to magnetic nulls as locations that indicate an increased likelihood for magnetic reconnection to occur. Through global potential field source-surface modelling, we determine how the number of nulls varied during the cycle and find that those that lie at < 1.1 solar radii vary significantly. We search for a correlation between the variation of the magnetic nulls and the V nt values, as it may be expected that with an increasing number of nulls, the V nt values in the corona increase as well. There is no correlation with the V nt values, however. This indicates that the magnetic structures that create the enhanced V nt behaviour are small-scale features and hence not easily measurable at the poles. Because they do not change during the solar cycle, they are likely to be created by a local dynamo. The variation of the upper range of V nt is reduced, which highlights that strongly dynamic behaviour is reduced as the solar maximum approaches. This is likely to be due to the reduced area of the polar coronal hole, which allows fewer opportunities for reconnection to occur between open and closed magnetic fields.
Hirohisa Hara - One of the best experts on this subject based on the ideXlab platform.
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A Study of the Coronal Non-Thermal Velocity in Polar Regions During the Rise from Solar Minimum to Solar Maximum in Cycle 24
Solar Physics, 2015Co-Authors: Louise K. Harra, Hirohisa Hara, D. N. Baker, S. J. Edwards, Rachel Howe, L. Van Driel-gesztelyiAbstract:We explore the changes in coronal non-Thermal Velocity (V nt) measurements at the poles from solar minimum to solar maximum using Hinode EUV Imaging Spectrometer data. We find that although the intensity in the corona at the poles does tend to increase with the cycle, there are no significant changes in the V nt values. The locations of enhanced V nt values measured do not always have a counterpart in intensity, and they are sometimes located in weak emission regions. Unipolar magnetic streams, created through diffusion of the following polarity of the decaying active regions, slowly progress towards the poles. These streams are expected to be related to magnetic nulls as locations that indicate an increased likelihood for magnetic reconnection to occur. Through global potential field source-surface modelling, we determine how the number of nulls varied during the cycle and find that those that lie at
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a study of the coronal non Thermal Velocity in polar regions during the rise from solar minimum to solar maximum in cycle 24
Solar Physics, 2015Co-Authors: L K Harra, Hirohisa Hara, D. N. Baker, S. J. Edwards, Rachel Howe, L Van DrielgesztelyiAbstract:We explore the changes in coronal non-Thermal Velocity (V nt) measurements at the poles from solar minimum to solar maximum using Hinode EUV Imaging Spectrometer data. We find that although the intensity in the corona at the poles does tend to increase with the cycle, there are no significant changes in the V nt values. The locations of enhanced V nt values measured do not always have a counterpart in intensity, and they are sometimes located in weak emission regions. Unipolar magnetic streams, created through diffusion of the following polarity of the decaying active regions, slowly progress towards the poles. These streams are expected to be related to magnetic nulls as locations that indicate an increased likelihood for magnetic reconnection to occur. Through global potential field source-surface modelling, we determine how the number of nulls varied during the cycle and find that those that lie at < 1.1 solar radii vary significantly. We search for a correlation between the variation of the magnetic nulls and the V nt values, as it may be expected that with an increasing number of nulls, the V nt values in the corona increase as well. There is no correlation with the V nt values, however. This indicates that the magnetic structures that create the enhanced V nt behaviour are small-scale features and hence not easily measurable at the poles. Because they do not change during the solar cycle, they are likely to be created by a local dynamo. The variation of the upper range of V nt is reduced, which highlights that strongly dynamic behaviour is reduced as the solar maximum approaches. This is likely to be due to the reduced area of the polar coronal hole, which allows fewer opportunities for reconnection to occur between open and closed magnetic fields.
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The Location of Non-Thermal Velocity in the Early Phases of Large Flares—Revealing Pre-eruption Flux Ropes
The Astrophysical Journal, 2013Co-Authors: Louise K. Harra, Sarah A. Matthews, J. L. Culhane, Mark C. M. Cheung, Eduard P. Kontar, Hirohisa HaraAbstract:Non-Thermal Velocity measurements of the solar atmosphere, particularly from UV and X-ray emission lines have demonstrated over the decades that this parameter is important in understanding the triggering of solar flares. Enhancements have often been observed before intensity enhancements are seen. However, until the launch of Hinode, it has been difficult to determine the spatial location of the enhancements to better understand the source region. The Hinode EUV Imaging Spectrometer has the spectral and spatial resolution to allow us to probe the early stages of flares in detail. We analyze four events, all of which are GOES M- or X-classification flares, and all are located toward the limb for ease of flare geometry interpretation. Three of the flares were eruptive and one was confined. In all events, pre-flare enhancement in non-Thermal Velocity at the base of the active region and its surroundings has been found. These enhancements seem to be consistent with the footpoints of the dimming regions, and hence may be highlighting the activation of a coronal flux rope for the three eruptive events. In addition, pre-flare enhancements in non-Thermal Velocity were found above the looptops for the three eruptive events.
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Ion Temperature and Non-Thermal Velocity in a Solar Active Region: Using Emission Lines of Different Atomic Species
The Astrophysical Journal, 2009Co-Authors: Shinsuke Imada, Hirohisa Hara, T. WatanabeAbstract:We have studied the characteristics of the ion Thermal temperature and non-Thermal Velocity in an active region observed by the EUV Imaging Spectrometer onboard Hinode. We used two emission lines of different atomic species (Fe XVI 262.98 ? and S XIII 256.69??) to distinguish the ion Thermal Velocity from the observed full width at half-maximum. We assumed that the sources of the two emission lines are the same Thermal temperature. We also assumed that they have the same non-Thermal Velocity. With these assumptions, we could obtain the ion Thermal temperature, after noting that M sulfur ~ 0.6M iron. We have carried out the ion Thermal temperature analysis in the active region where the photon counts are sufficient (>4500). What we found is as follows: (1) the common ion Thermal temperatures obtained by Fe XVI and S XIII are ~2.5?MK, (2) the typical non-Thermal velocities are ~13?km?s?1, (3) the highest non-Thermal velocities (>20?km?s?1) are preferentially observed between the bright points in Fe XVI, while (4) the hottest material (>3?MK) is observed relatively inside the bright points compared with the highest non-Thermal Velocity region.
Joseph M. Davila - One of the best experts on this subject based on the ideXlab platform.
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DETERMINATION OF NON-Thermal Velocity DISTRIBUTIONS FROM SERTS LINEWIDTH OBSERVATIONS
The Astrophysical Journal, 2011Co-Authors: Aaron J. Coyner, Joseph M. DavilaAbstract:Non-Thermal velocities obtained from the measurement of coronal Extreme Ultraviolet (EUV) linewidths have been consistently observed in solar EUV spectral observations and have been theorized to result from many plausible scenarios including wave motions, turbulence, or magnetic reconnection. Constraining these velocities can provide a physical limit for the available energy resulting from unresolved motions in the corona. We statistically determine a series of non-Thermal Velocity distributions from linewidth measurements of 390 emission lines from a wide array of elements and ionization states observed during the Solar Extreme Ultraviolet Research Telescope and Spectrograph 1991-1997 flights covering the spectral range 174-418 A and a temperature range from 80,000 K to 12.6 MK. This sample includes 248 lines from active regions, 101 lines from quiet-Sun regions, and 41 lines were observed from plasma off the solar limb. We find a strongly peaked distribution corresponding to a non-Thermal Velocity of 19-22 km s–1 in all three of the quiet-Sun, active region, and off-limb distributions. For the possibility of Alfven wave resonance heating, we find that velocities in the core of these distributions do not provide sufficient energy, given typical densities and magnetic field strengths for the coronal plasma, to overcome the estimated coronal energy losses required to maintain the corona at the typical temperatures working as the sole mechanism. We find that at perfect efficiency 50%-60% of the needed energy flux can be produced from the non-Thermal velocities measured.
Alejandro L. Garcia - One of the best experts on this subject based on the ideXlab platform.
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Diffusive transport by Thermal Velocity fluctuations.
Physical review letters, 2011Co-Authors: Aleksandar Donev, John B. Bell, Anton De La Fuente, Alejandro L. GarciaAbstract:We study the contribution of advection by Thermal Velocity fluctuations to the effective diffusion coefficient in a mixture of two identical fluids. We find good agreement between a simple fluctuating hydrodynamics theory and particle and finite-volume simulations. The enhancement of the diffusive transport depends on the system size L and grows as lnðL=L0Þ in quasi-two-dimensional systems, while in three dimensions it scales as L -1 - L -1 , where L0 is a reference length. Our results demonstrate that 0 fluctuations play an important role in the hydrodynamics of small-scale systems. Thermal fluctuations in nonequilibrium systems in which a constant (temperature, concentration, Velocity) gradient is imposed externally exhibit remarkable behavior compared to equilibrium systems [1]. The solution of the linearized equations of fluctuating hydrodynamics shows that concentration and density fluctuations exhibit longranged correlations in the presence of a macroscopic concentration gradient rc [1–3]. The enhancement of large-scale (small wave number) concentration fluctua tions is dramatic during the early stages of diffusive mixing between initially phase-separated fluids. These giant fluc tuations [4–6] during free diffusive mixing have been measured using light scattering and shadowgraphy tech niques [4,5,7], finding good but imperfect agreement with theoretical predictions. The giant fluctuation phenomenon arises because of the appearance of long-ranged correlations between concen tration and Velocity fluctuations in the presence of a con centration gradient. It has been predicted that these correlations give rise to fluctuation-renormalized transport coefficients [3,8]; however, the predicted enhancement of transport at hydrodynamic scales has not yet been compu tationally observed. In particular, it is important to under stand how the effective transport coefficients depend on the length scale of observation. In this Letter we consider diffusion in a mixture of identical but labeled (as components 1 and 2) fluids [9] enclosed in a box of size Lx X Ly X Lz , in the absence of gravity. Periodic boundary conditions are applied in the x (horizontal) and z (depth) directions, while the top and bottom boundaries are impermeable constant-temperature walls. A concentration gradient rc p ¼ð cT - cBÞ=Ly is imposed along the y axes by enforcing a constant concen tration c T at the top wall and c B at the bottom wall. Because the fluids are identical, concentration is passively transported by Thermal fluctuations. Since species are not changed in particle collisions, the diffusive transport of concentration c ¼ p1=p can only occur via advective motion of the particles, where p de notes the mass density. The mass flux for a given species is therefore equal to the momentum density for particles of that species. At steady state the particles of a given spe cies have a nonzero macroscopic momentum density p j1 ¼ p p1v p 1 ¼- p cÞ, where x is the mass diffusion px ðr p coefficient [10]. The local fluctuations around the macro scopic mean, p1 ¼ p p 1 þ op1 and v1 ¼ v p 1 þ ov1, can also make a nontrivial contribution to the average mass flux if they are correlated,
Louise K. Harra - One of the best experts on this subject based on the ideXlab platform.
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A Study of the Coronal Non-Thermal Velocity in Polar Regions During the Rise from Solar Minimum to Solar Maximum in Cycle 24
Solar Physics, 2015Co-Authors: Louise K. Harra, Hirohisa Hara, D. N. Baker, S. J. Edwards, Rachel Howe, L. Van Driel-gesztelyiAbstract:We explore the changes in coronal non-Thermal Velocity (V nt) measurements at the poles from solar minimum to solar maximum using Hinode EUV Imaging Spectrometer data. We find that although the intensity in the corona at the poles does tend to increase with the cycle, there are no significant changes in the V nt values. The locations of enhanced V nt values measured do not always have a counterpart in intensity, and they are sometimes located in weak emission regions. Unipolar magnetic streams, created through diffusion of the following polarity of the decaying active regions, slowly progress towards the poles. These streams are expected to be related to magnetic nulls as locations that indicate an increased likelihood for magnetic reconnection to occur. Through global potential field source-surface modelling, we determine how the number of nulls varied during the cycle and find that those that lie at
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The Location of Non-Thermal Velocity in the Early Phases of Large Flares—Revealing Pre-eruption Flux Ropes
The Astrophysical Journal, 2013Co-Authors: Louise K. Harra, Sarah A. Matthews, J. L. Culhane, Mark C. M. Cheung, Eduard P. Kontar, Hirohisa HaraAbstract:Non-Thermal Velocity measurements of the solar atmosphere, particularly from UV and X-ray emission lines have demonstrated over the decades that this parameter is important in understanding the triggering of solar flares. Enhancements have often been observed before intensity enhancements are seen. However, until the launch of Hinode, it has been difficult to determine the spatial location of the enhancements to better understand the source region. The Hinode EUV Imaging Spectrometer has the spectral and spatial resolution to allow us to probe the early stages of flares in detail. We analyze four events, all of which are GOES M- or X-classification flares, and all are located toward the limb for ease of flare geometry interpretation. Three of the flares were eruptive and one was confined. In all events, pre-flare enhancement in non-Thermal Velocity at the base of the active region and its surroundings has been found. These enhancements seem to be consistent with the footpoints of the dimming regions, and hence may be highlighting the activation of a coronal flux rope for the three eruptive events. In addition, pre-flare enhancements in non-Thermal Velocity were found above the looptops for the three eruptive events.
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Non-Thermal velocities in solar flares
Multi-wavelength Observations of Coronal Structure and Dynamics Yohkoh 10th Anniversary Meeting, 2002Co-Authors: Louise K. HarraAbstract:The high resolution spectroscopic information from the Bragg Crystal Spectrometer on board Yohkoh has provided us with new and exciting information about flares. In particular, there has been much work on understanding the excess line broadening above the Thermal width (known as non-Thermal line broadening). We have been able to look for the first time spectroscopically at the preflare stages in X-rays. The timings of the non-Thermal Velocity relative to the hard X-ray emission has been investigated. Non-Thermal velocities have been observed to increase ten minutes before the main flare begins. Progress has been made to locate the region of dominant non-Thermal Velocity. This is difficult due to the lack of spatial resolution. A discussion will be made on what can be expected from the EUV Imaging Spectrometer on-board Solar-B, which combines both high spatial and spectral resolution simultaneously.
