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S D Bale - One of the best experts on this subject based on the ideXlab platform.
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highly structured slow solar wind emerging from an equatorial Coronal Hole
Nature, 2019Co-Authors: S. Badman, D. Burgess, C. Cattell, S D Bale, J W Bonnell, Trevor A Bowen, A W Case, Benjamin D G ChandranAbstract:During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvenic rarefied stream of plasma originating from deep within Coronal Holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near Coronal Hole boundaries7,8, or within Coronal Holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfven-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvenic solar wind emerging from a small equatorial Coronal Hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude Coronal Holes are a key source of the slow solar wind. Measurements from the Parker Solar Probe show that slow solar wind near the Sun’s equator originates in Coronal Holes.
G R Gupta - One of the best experts on this subject based on the ideXlab platform.
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observations of dissipation of slow magneto acoustic waves in a polar Coronal Hole
Astronomy and Astrophysics, 2014Co-Authors: G R GuptaAbstract:Aims. We focus on a polar Coronal Hole region to find any evidence of dissipation of propagating slow magneto-acoustic waves. Methods. We obtained time-distance and frequency-distance maps along the plume structure in a polar Coronal Hole. We also obtained Fourier power maps of the polar Coronal Hole in different frequency ranges in 171 A and 193 A passbands. We performed intensity distribution statistics in time domain at several locations in the polar Coronal Hole. Results. We find the presence of propagating slow magneto-acoustic waves having temperature dependent propagation speeds. The wavelet analysis and Fourier power maps of the polar Coronal Hole show that low-frequency waves are travelling longer distances (longer detection length) as compared to high-frequency waves. We found two distinct dissipation length scales of wave amplitude decay at two different height ranges (between 0‐10 Mm and 10‐70 Mm) along the observed plume structure. The dissipation lengths obtained at higher height range show some frequency dependence. Individual Fourier power spectrum at several locations show a power-law distribution with frequency whereas probability density function of intensity fluctuations in time show nearly Gaussian distributions. Conclusions. Propagating slow magneto-acoustic waves are getting heavily damped (small dissipation lengths) within the first10 Mm distance. Beyond that waves are getting damped slowly with height. Frequency dependent dissipation lengths of wave propagation at higher heights may indicate the possibility of wave dissipation due to thermal conduction, however, the contribution from other dissipative parameters cannot be ruled out. Power-law distributed power spectra were also found at lower heights in the solar corona, which may provide viable information on the generation of longer period waves in the solar atmosphere.
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observations of dissipation of slow magneto acoustic waves in a polar Coronal Hole
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: G R GuptaAbstract:We focus on a polar Coronal Hole region to find any evidence of dissipation of propagating slow magneto-acoustic waves. We obtained time-distance and frequency-distance maps along the plume structure in a polar Coronal Hole. We also obtained Fourier power maps of the polar Coronal Hole in different frequency ranges in 171~\AA\ and 193~\AA\ passbands. We performed intensity distribution statistics in time domain at several locations in the polar Coronal Hole. We find the presence of propagating slow magneto-acoustic waves having temperature dependent propagation speeds. The wavelet analysis and Fourier power maps of the polar Coronal Hole show that low-frequency waves are travelling longer distances (longer detection length) as compared to high-frequency waves. We found two distinct dissipation length scales of wave amplitude decay at two different height ranges (between 0--10 Mm and 10--70 Mm) along the observed plume structure. The dissipation lengths obtained at higher height range show some frequency dependence. Individual Fourier power spectrum at several locations show a power-law distribution with frequency whereas probability density function (PDF) of intensity fluctuations in time show nearly Gaussian distributions. Propagating slow magneto-acoustic waves are getting heavily damped (small dissipation lengths) within the first 10~Mm distance. Beyond that waves are getting damped slowly with height. Frequency dependent dissipation lengths of wave propagation at higher heights may indicate the possibility of wave dissipation due to thermal conduction, however, the contribution from other dissipative parameters cannot be ruled out. Power-law distributed power spectra were also found at lower heights in the solar corona, which may provide viable information on the generation of longer period waves in the solar atmosphere.
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characteristics of polar Coronal Hole jets
Astronomy and Astrophysics, 2014Co-Authors: K Chandrashekhar, A Bemporad, Dipankar Banerjee, G R Gupta, L TeriacaAbstract:Context. High spatialand temporal-resolution images of Coronal Hole regions show a dynamical environment where mass flows and jets are frequently observed. These jets are believed to be important for the Coronal heating and the acceleration of the fast solar wind. Aims. We studied the dynamics of two jets seen in a polar Coronal Hole with a combination of imaging from EIS and XRT onboard Hinode. We observed drift motions related to the evolution and formation of these small-scale jets, which we tried to model as well. Methods. Stack plots were used to find the drift and flow speeds of the jets. A toymodel was developed by assuming that the observed jet is generated by a sequence of single reconnection events where single unresolved blobs of plasma are ejected along open field lines, then expand and fall back along the same path, following a simple ballistic motion. Results. We found observational evidence that supports the idea that polar jets are very likely produced by multiple small-scale reconnections occurring at different times in different locations. These eject plasma blobs that flow up and down with a motion very similar to a simple ballistic motion. The associated drift speed of the first jet is estimated to be ≈27 km s−1. The average outward speed of the first jet is ≈171 km s−1, well below the escape speed, hence if simple ballistic motion is considered, the plasma will not escape the Sun. The second jet was observed in the south polar Coronal Hole with three XRT filters, namely, C−poly, Al−poly, and Al−mesh filters. Many small-scale (≈3′′−5′′) fast (≈200−300 km s−1) ejections of plasma were observed on the same day; they propagated outwards. We observed that the stronger jet drifted at all altitudes along the jet with the same drift speed of 7 km s−1. We also observed that the bright point associated with the first jet is a part of sigmoid structure. The time of appearance of the sigmoid and that of the ejection of plasma from the bright point suggest that the sigmoid is the progenitor of the jet. Conclusions. The enhancement in the light curves of low-temperature EIS lines in the later phase of the jet lifetime and the shape of the jet’s stack plots suggests that the jet material falls back, and most likely cools down. To further support this conclusion, the observed drifts were interpreted within a scenario where reconnection progressively shifts along a magnetic structure, leading to the sequential appearance of jets of about the same size and physical characteristics. On this basis, we also propose a simple qualitative model that mimics the observations.
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characteristics of polar Coronal Hole jets
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: K Chandrashekhar, A Bemporad, Dipankar Banerjee, G R Gupta, L TeriacaAbstract:High spatial- and temporal-resolution images of Coronal Hole regions show a dynamical environment where mass flows and jets are frequently observed. These jets are believed to be important for the Coronal heating and the acceleration of the fast solar wind. We studied the dynamics of two jets seen in a polar Coronal Hole with a combination of imaging from EIS and XRT onboard Hinode. We observed drift motions related to the evolution and formation of these small-scale jets, which we tried to model as well. We found observational evidence that supports the idea that polar jets are very likely produced by multiple small-scale reconnections occurring at different times in different locations. These eject plasma blobs that flow up and down with a motion very similar to a simple ballistic motion. The associated drift speed of the first jet is estimated to be $\approx$ 27 km s$^{-1}$. The average outward speed of the first jet is $\approx 171$ km s$^{-1}$, well below the escape speed, hence if simple ballistic motion is considered, the plasma will not escape the Sun. The second jet was observed in the south polar Coronal Hole with three XRT filters, namely, C$_{-}$poly, Al$_{-}$poly, and Al$_{-}$mesh filters. We observed that the second jet drifted at all altitudes along the jet with the same drift speed of $\simeq$ 7 km s$^{-1}$. The enhancement in the light curves of low-temperature EIS lines in the later phase of the jet lifetime and the shape of the jet's stack plots suggests that the jet material is falls back, and most likely cools down. To support this conclusion, the observed drifts were interpreted within a scenario where reconnection progressively shifts along a magnetic structure, leading to the sequential appearance of jets of about the same size and physical characteristics. On this basis, we also propose a simple qualitative model that mimics the observations.
Allan R. Macneil - One of the best experts on this subject based on the ideXlab platform.
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Data-Driven Classification of Coronal Hole and Streamer Belt Solar Wind
Solar Physics, 2020Co-Authors: Téo Bloch, Clare Watt, Mathew Owens, Leland Mcinnes, Allan R. MacneilAbstract:We present two new solar wind origin classification schemes developed independently using unsupervised machine learning. The first scheme aims to classify solar wind into three types: Coronal-Hole wind, streamer-belt wind, and ‘unclassified’ which does not fit into either of the previous two categories. The second scheme independently derives three clusters from the data; the Coronal-Hole and streamer-belt winds, and a differing unclassified cluster. The classification schemes are created using non-evolving solar wind parameters, such as ion charge states and composition, measured during the three Ulysses fast latitude scans. The schemes are subsequently applied to the Ulysses and the Advanced Compositional Explorer (ACE) datasets. The first scheme is based on oxygen charge state ratio and proton specific entropy. The second uses these data, as well as the carbon charge state ratio, the alpha-to-proton ratio, the iron-to-oxygen ratio, and the mean iron charge state. Thus, the classification schemes are grounded in the properties of the solar source regions. Furthermore, the techniques used are selected specifically to reduce the introduction of subjective biases into the schemes. We demonstrate significant best case disparities (minimum ≈8%, maximum ≈22%) with the traditional fast and slow solar wind determined using speed thresholds. By comparing the results between the in- (ACE) and out-of-ecliptic ( Ulysses ) data, we find morphological differences in the structure of Coronal-Hole wind. Our results show how a data-driven approach to the classification of solar wind origins can yield results which differ from those obtained using other methods. As such, the results form an important part of the information required to validate how well current understanding of solar origins and the solar wind match with the data we have.
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active region modulation of Coronal Hole solar wind
The Astrophysical Journal, 2019Co-Authors: Allan R. Macneil, C J Owen, Deborah Baker, David H Brooks, L K Harra, D Long, Robert T WicksAbstract:Active regions (ARs) are a candidate source of the slow solar wind (SW), the origins of which are a topic of ongoing research. We present a case study that examines the processes by which SW is modulated in the presence of an AR in the vicinity of the SW source. We compare properties of SW associated with a Coronal Hole (CH)–quiet Sun boundary to SW associated with the same CH but one Carrington rotation later, when this region bordered the newly emerged NOAA AR 12532. Differences found in a range of in situ parameters are compared between these rotations in the context of source region mapping and remote sensing observations. Marked changes exist in the structure and composition of the SW, which we attribute to the influence of the AR on SW production from the CH boundary. These unique observations suggest that the features that emerge in the AR-associated wind are consistent with an increased occurrence of interchange reconnection during SW production, compared with the initial quiet Sun case.
Benjamin D G Chandran - One of the best experts on this subject based on the ideXlab platform.
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highly structured slow solar wind emerging from an equatorial Coronal Hole
Nature, 2019Co-Authors: S. Badman, D. Burgess, C. Cattell, S D Bale, J W Bonnell, Trevor A Bowen, A W Case, Benjamin D G ChandranAbstract:During the solar minimum, when the Sun is at its least active, the solar wind1,2 is observed at high latitudes as a predominantly fast (more than 500 kilometres per second), highly Alfvenic rarefied stream of plasma originating from deep within Coronal Holes. Closer to the ecliptic plane, the solar wind is interspersed with a more variable slow wind3 of less than 500 kilometres per second. The precise origins of the slow wind streams are less certain4; theories and observations suggest that they may originate at the tips of helmet streamers5,6, from interchange reconnection near Coronal Hole boundaries7,8, or within Coronal Holes with highly diverging magnetic fields9,10. The heating mechanism required to drive the solar wind is also unresolved, although candidate mechanisms include Alfven-wave turbulence11,12, heating by reconnection in nanoflares13, ion cyclotron wave heating14 and acceleration by thermal gradients1. At a distance of one astronomical unit, the wind is mixed and evolved, and therefore much of the diagnostic structure of these sources and processes has been lost. Here we present observations from the Parker Solar Probe15 at 36 to 54 solar radii that show evidence of slow Alfvenic solar wind emerging from a small equatorial Coronal Hole. The measured magnetic field exhibits patches of large, intermittent reversals that are associated with jets of plasma and enhanced Poynting flux and that are interspersed in a smoother and less turbulent flow with a near-radial magnetic field. Furthermore, plasma-wave measurements suggest the existence of electron and ion velocity-space micro-instabilities10,16 that are associated with plasma heating and thermalization processes. Our measurements suggest that there is an impulsive mechanism associated with solar-wind energization and that micro-instabilities play a part in heating, and we provide evidence that low-latitude Coronal Holes are a key source of the slow solar wind. Measurements from the Parker Solar Probe show that slow solar wind near the Sun’s equator originates in Coronal Holes.
D W Savin - One of the best experts on this subject based on the ideXlab platform.
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measurements of anisotropic ion temperatures non thermal velocities and doppler shifts in a Coronal Hole
The Astrophysical Journal, 2013Co-Authors: Michael Hahn, D W SavinAbstract:We present a new diagnostic allowing one to measure the anisotropy of ion temperatures and non-thermal velocities, as well as Doppler shifts with respect to the ambient magnetic field. This method provides new results, as well as an independent test for previous measurements obtained with other techniques. Our spectral data come from observations of a low-latitude, on-disk Coronal Hole. A potential field source surface model was used to calculate the angle between the magnetic field lines and the line of sight for each spatial bin of the observation. A fit was performed to determine the line widths and Doppler shifts parallel and perpendicular to the magnetic field. For each line width component we derived ion temperatures T i, ⊥ and T i, ∥ and non-thermal velocities v nt, ⊥ and v nt, ∥. T i, ⊥ was cooler than off-limb polar Coronal Hole measurements, suggesting increasing collisional cooling with decreasing height. T i, ∥ is consistent with a uniform temperature of (1.8 ± 0.2) × 106 K for each ion. Since parallel ion heating is expected to be weak, this ion temperature should reflect the proton temperature. A comparison between our results and others implies a large proton temperature gradient around 1.02 R ☉. The non-thermal velocities are thought to be proportional to the amplitudes of various waves. Our results for v nt, ⊥ agree with Alfven wave amplitudes inferred from off-limb polar Coronal Hole line width measurements. Our v nt, ∥ results are consistent with slow magnetosonic wave amplitudes inferred from Fourier analysis of time-varying intensity fluctuations. Doppler shift measurements yield outflows of 5 km s–1 for ions formed over a broad temperature range. This differs from other studies that found a strong Doppler shift dependence on formation temperature.
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measurements of anisotropic ion temperatures non thermal velocities and doppler shifts in a Coronal Hole
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Michael Hahn, D W SavinAbstract:We present a new diagnostic allowing one to measure the anisotropy of ion temperatures and non-thermal velocities as well as Doppler shifts with respect to the ambient magnetic field. This method provides new results, as well as independent test for previous measurements obtained with other techniques. Our spectral data come from observations of a low latitude, on-disk Coronal Hole. A potential field source surface model was used to calculate the angle between the magnetic field lines and the line of sight for each spatial bin of the observation. A fit was performed to determine the line widths and Doppler shifts parallel and perpendicular to the magnetic field. For each line width component we derived parallel and perpendicular ion temperatures and non-thermal velocities. The perpendicular ion temperature was cooler than off-limb polar Coronal Hole measurements. The parallel ion temperature was consistent with a uniform temperature of 1.8 +/- 0.2 x 10^{6} K for each ion. Since parallel ion heating is expected to be weak, this ion temperature should reflect the proton temperature. A comparison between our results and others implies a large proton temperature gradient around 1.02 R_sun. The non-thermal velocities are thought to be proportional to the amplitudes of various waves. Our results for the perpendicular non-thermal velocity agree with Alfv\'en wave amplitudes inferred from off-limb polar Coronal Hole line width measurements. Our parallel non-thermal velocity results are consistent with slow magnetosonic wave amplitudes inferred from Fourier analysis of time varying intensity fluctuations. Doppler shift measurements yield outflows of ~5 km s^-1 for ions formed over a broad temperature range. This differs from other studies which found a strong Doppler shift dependence on formation temperature.
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evidence of wave damping at low heights in a polar Coronal Hole
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Michael Hahn, E Landi, D W SavinAbstract:We have measured the widths of spectral lines from a polar Coronal Hole using the Extreme Ultraviolet Imaging Spectrometer onboard Hinode. Polar Coronal Holes are regions of open magnetic field and the source of the fast solar wind. We find that the line widths decrease at relatively low heights. Previous observations have attributed such decreases to systematic effects, but we find that such effects are too small to explain our results. We conclude that the line narrowing is real. The non-thermal line widths are believed to be proportional to the amplitude of Alfven waves propagating along these open field lines. Our results suggest that Alfven waves are damped at unexpectedly low heights in a polar Coronal Hole. We derive an estimate on the upper limit for the energy dissipated between 1.1 and 1.3 solar radii and find that it is enough to account for up to 70% of that required to heat the polar Coronal Hole and accelerate the solar wind.
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differential emission measure analysis of a polar Coronal Hole during the solar minimum in 2007
The Astrophysical Journal, 2011Co-Authors: Michael Hahn, E Landi, D W SavinAbstract:We have performed a differential emission measure (DEM) analysis for a polar Coronal Hole observed during solar minimum in 2007. Five observations are analyzed spanning the Coronal Hole from the central meridian to the boundary with the quiet-Sun corona. The observed heights ranged from 1.05 to 1.20 R ☉. The analysis shows that the plasma is not strictly isothermal anywhere, but rather has a high-temperature component that extends up to log T(K) = 6.2-6.3. The size and importance of this component depend on location, and its evolving magnitude with height marks the boundary between the Coronal Hole and the quiet corona, where it becomes dominant. The DEM of the Coronal Hole plasma below log T(K) = 6.0 decreases faster with height than that of the high-temperature component. We discuss the possible nature of the high-temperature component. Our results highlight the potential limitations of isothermal analyses. Such methods actually measure a DEM-weighted average temperature and as a result can infer artificial temperature gradients. Assuming the gas is isothermal along the line of sight can also yield incorrect electron densities. By revealing structures along the line of sight, a DEM analysis can also be used to more reliably interpret electron temperature and density measurements.
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properties of a polar Coronal Hole during the solar minimum in 2007
The Astrophysical Journal, 2010Co-Authors: Michael Hahn, Paul Bryans, E Landi, M P Miralles, D W SavinAbstract:We report measurements of a polar Coronal Hole during the recent solar minimum using the Extreme Ultraviolet Imaging Spectrometer on Hinode. Five observations are analyzed that span the polar Coronal Hole from the central meridian to the boundary with the quiet-Sun corona. We study the observations above the solar limb in the height range of 1.03-1.20 R ☉. The electron temperature T e and emission measure (EM) are found using a geometric mean emission measure method. The EM derived from the elements Fe, Si, S, and Al are compared in order to measure relative Coronal-to-photospheric abundance enhancement factors. We also studied the ion temperature T i and the non-thermal velocity v nt using the line profiles. All these measurements are compared to polar Coronal Hole observations from the previous (1996-1997) solar minimum and to model predictions for relative abundances. There are many similarities in the physical properties of the polar Coronal Holes between the two minima at these low heights. We find that the electron density, T e, and T i are comparable in both minima. T e shows a comparable gradient with height. Both minima show a decreasing T i with increasing charge-to-mass ratio q/M. A previously observed upturn of T i for ions above q/M>0.25 was not found here. We also compared relative Coronal-to-photospheric elemental abundance enhancement factors for a number of elements. These ratios were ~1 for both the low first ionization potential (FIP) elements Si and Al and the marginally high FIP element S relative to the low FIP element Fe, as is expected based on earlier observations and models for a polar Coronal Hole. These results are consistent with no FIP effect in a polar Coronal Hole.