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Zhongwei Yang - One of the best experts on this subject based on the ideXlab platform.
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impact of Shock Front rippling and self reformation on the electron dynamics at low mach number Shocks
The Astrophysical Journal, 2018Co-Authors: Zhongwei Yang, Ying Liu, Rui WangAbstract:Electron dynamics at low-Mach-number collisionless Shocks are investigated by using two-dimensional electromagnetic particle-in-cell simulations with various Shock normal angles. We found: (1) The reflected ions and incident electrons at the Shock Front provide an effective mechanism for the quasi-electrostatic wave generation due to the charge-separation. A fraction of incident electrons can be effectively trapped and accelerated at the leading edge of the Shock foot. (2) At quasi-perpendicular Shocks, the electron trapping and reflection is nonuniform due to the Shock rippling along the Shock surface and is more likely to take place at some locations accompanied by intense reflected ion-beams. The electron trapping process has a periodical evolution over time due to the Shock Front self-reformation, which is controlled by ion dynamics. Thus, this is a cross-scale coupling phenomenon. (3) At quasi-parallel Shocks, reflected ions can travel far back upstream. Consequently, quasi-electrostatic waves can be excited in the Shock transition and the foreShock region. The electron trajectory analysis shows these waves can trap electrons at the foot region and reflect a fraction of them far back upstream. Simulation runs in this paper indicate that the micro-turbulence at the Shock foot can provide a possible scenario for producing the reflected electron beam, which is a basic condition for the type II radio burst emission at low-Mach-number interplanetary Shocks driven by Coronal Mass Ejections (CMEs).
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physical roles of interstellar origin pickup ions at the heliospheric termination Shock impact on the Shock Front microstructures and nonstationarity
The Astrophysical Journal, 2016Co-Authors: Bertrand Lembège, Zhongwei YangAbstract:The nonstationary dynamics of the heliospheric termination Shock in the presence of pickup ions (PUI) is analyzed by using a one-dimensional particle-in-cell simulation code. This work initially stimulated by Voyager 2 data focusses on this nonstationarity for different percentages of PUIs and for different Alfven Mach numbers M A. Solar wind ions (SWIs) and PUIs are described, respectively, as Maxwellian and shell distributions (with a zero/finite thickness). For a moderate M A, present results show that (1) the Shock Front is still nonstationary even in the presence of 25% of PUIs; its instantaneous velocity varies, which is in favor for Shock multicrossing; (2) the presence of PUIs tends to smooth out the time fluctuations of field amplitude and of microstructure widths at the Front and overshoot; (3) the Shock has a multiple overshoot, which is analyzed by identifying the contributions of SWIs and the PUIs; (4) as the PUI percentage increases, the Shock moves faster and the downstream compression becomes weaker, which is explained by a Rankine–Hugoniot model; (5) the reflection rate of SWIs and PUIs decreases as the PUI percentage increases; (6) the Shock structure is almost insensitive to the shell thickness; and (7) for the PUIs dominated Shock case (PUI = 55%), the Shock becomes stationary. However, for higher M A regime, the Front nonstationarity persists even in the PUI = 55% case. In summary, high M A regime allows to compensate the smoothing of the microstructures and the time fluctuations of the Shock Front brought by the presence of PUIs.
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impact of pickup ions on the Shock Front nonstationarity and energy dissipation of the heliospheric termination Shock two dimensional full particle simulations and comparison with voyager 2 observations
The Astrophysical Journal, 2015Co-Authors: Zhongwei Yang, Ying Liu, J D Richardson, Can Huang, Rui WangAbstract:Voyager 2 (V2) observed multiple crossings of the heliospheric termination Shock (TS) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. Here, for the first time, we present two-dimensional particle-incell (PIC) simulations of the TS self-consistently including pickup ions (PUIs), and compare the simulation results with V2 observations. We find that (1) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion kinetic energy is transferred to the thermal energy of PUIs. The PIC simulation indicates that, for the upstream parameters chosen for V2 conditions, the density of PUIs is about 25% and the PUIs gain the largest fraction (approximately 86.6%) of downstream thermal pressure. (2) The simulated heliosheath ion distribution function is a superposition of a cold core formed by transmitted solar wind ions (SWIs), with the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. The V2 Faraday cups observed the cool core of the distribution, and so they only saw the tip of the iceberg. (3) The nonstationarity of the Shock Front is mainly caused by ripples along the Shock Front which form even if the percentage of PUIs is high. These simulation results agree reasonably well with the V2 experimental data. The relevance of the Shock Front ripples to the multiple TS crossings observed by V2 is also discussed in this paper.
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impact of pickup ions on the Shock Front nonstationarity and energy dissipation of the heliospheric termination Shock two dimensional full particle simulations and comparison with voyager 2 observations
arXiv: Space Physics, 2015Co-Authors: Zhongwei Yang, Ying Liu, J D Richardson, Can Huang, Rui WangAbstract:The transition between the supersonic solar wind and the subsonic heliosheath, the termination Shock (TS), was observed by Voyager 2 (V2) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. The data reveal multiple crossings of a complex, quasi-perpendicular supercritical Shock. These experimental data are the starting point for a more sophisticated analysis that includes computer modeling of a Shock in the presence of pickup ions (PUIs). here, we present two-dimensional (2-D) particle-in-cell (PIC) simulations of the TS including PUIs self-consistently. We also report the ion velocity distribution across the TS using the Faraday cup data from V2. A relatively complete plasma and magnetic field data set from V2 gives us the opportunity to do a full comparison between the experimental data and PIC simulation results. Our results show that: (1) The nonstationarity of the Shock Front is mainly caused by the ripples along the Shock Front and these ripples from even if the percentage of PUIs is high. (2) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion dynamic energy is transferred to the thermal energy of PUIs instead of solar wind ions (SWIs). (3) The simulated composite heliosheath ion velocity distribution function is a superposition of a cold core formed by transmitted SWIs, the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. (4) The V2 Faraday cups observed the cool core of the distribution, so they saw only a tip of the iceberg. For the evolution of the cool core distribution function across the TS, the computed results agree reasonably well with the V2experimental results.
Bertrand Lembège - One of the best experts on this subject based on the ideXlab platform.
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physical roles of interstellar origin pickup ions at the heliospheric termination Shock impact on the Shock Front microstructures and nonstationarity
The Astrophysical Journal, 2016Co-Authors: Bertrand Lembège, Zhongwei YangAbstract:The nonstationary dynamics of the heliospheric termination Shock in the presence of pickup ions (PUI) is analyzed by using a one-dimensional particle-in-cell simulation code. This work initially stimulated by Voyager 2 data focusses on this nonstationarity for different percentages of PUIs and for different Alfven Mach numbers M A. Solar wind ions (SWIs) and PUIs are described, respectively, as Maxwellian and shell distributions (with a zero/finite thickness). For a moderate M A, present results show that (1) the Shock Front is still nonstationary even in the presence of 25% of PUIs; its instantaneous velocity varies, which is in favor for Shock multicrossing; (2) the presence of PUIs tends to smooth out the time fluctuations of field amplitude and of microstructure widths at the Front and overshoot; (3) the Shock has a multiple overshoot, which is analyzed by identifying the contributions of SWIs and the PUIs; (4) as the PUI percentage increases, the Shock moves faster and the downstream compression becomes weaker, which is explained by a Rankine–Hugoniot model; (5) the reflection rate of SWIs and PUIs decreases as the PUI percentage increases; (6) the Shock structure is almost insensitive to the shell thickness; and (7) for the PUIs dominated Shock case (PUI = 55%), the Shock becomes stationary. However, for higher M A regime, the Front nonstationarity persists even in the PUI = 55% case. In summary, high M A regime allows to compensate the smoothing of the microstructures and the time fluctuations of the Shock Front brought by the presence of PUIs.
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on the origin of the quasi perpendicular ion foreShock full particle simulations
Journal of Geophysical Research, 2013Co-Authors: P Savoini, Bertrand Lembège, J StienletAbstract:[1] Many space missions have already evidenced the existence of the ion foreShock region located upstream of the Earth's bow Shock and populated by energetic backstreaming ions reflected by the Shock Front. In order to analyze this region, a curved Shock is simulated with a 2-D particle-in-cell (PIC) code. The analysis is presently restricted to the quasi-perpendicular angular range defined by 45° ≤ θBn ≤ 90°. In agreement with experimental data, present results evidence two distinct ion populations backstreaming from the Shock Front along the interplanetary magnetic field: (i) the field-aligned beam population (hereafter “FAB”) and (ii) the gyrophase bunched population (hereafter “GPB”) which differ from each other by their gyrotropic or non-gyrotropic behavior, respectively. Excluded by a simulation time which is too short, ion instabilities pitch-angle scattering cannot be the source of “GPB.” Two new criteria are proposed to identify more precisely each population: their interaction time Δtint with the Shock Front and their downstream penetration depth. These criteria show that (i) the “FAB” population moves back and forth between the upstream edge of the Shock Front and the overshoot, and is characterized by a Δtint covering several upstream gyroperiods. (ii) In contrast, the “GPB” ions suffer a short interaction time (i.e., 1 < τci). We observe that the “FAB” ions may have different origins although all “GPB” ions seem to be produced by the electrostatic field built up at the Shock and are emitted in a burst-like mode rather than in continuous way.
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Impact of the rippling of a perpendicular Shock Front on ion dynamics
Journal of Geophysical Research Space Physics, 2012Co-Authors: Z W Yang, Bertrand LembègeAbstract:Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the Front of a supercritical quasi-perpendicular Shock can be rippled. Recent two-dimensional simulations have focused on two different types of Shock Front rippling: (1) one characterized by a small spatial scale along the Front is supported by lower hybrid wave activity, (2) the other characterized by a large spatial scale along the Front is supported by the emission of large amplitude nonlinear whistler waves. These two rippled Shock Fronts are self-consistently observed when the static magnetic field is perpendicular to (so called "B0-OUT" case) or within (so called "B0-IN" case) the simulation plane, respectively. On the other hand, several studies have been made on the reflection and energization of incoming ions with a Shock but most have been restricted to a one dimensional Shock profile only (no rippling effects). Herein, two-dimensional test particle simulations based on strictly perpendicular Shock profiles chosen at a fixed time in two-dimensional Particle-in-cell (PIC) simulations, are performed in order to investigate the impact of the Shock Front ripples on incident ion (H+) dynamics. The acceleration mechanisms and energy spectra of the test-ions (described by shell distributions with different initial kinetic energy) interacting with a rippled Shock Front are analyzed in detail. Both "B0-OUT" and "B0-IN" cases are considered separately; in each case, y-averaged (Front rippling excluded) and non-averaged (Front rippling included) profiles will be analyzed. Present results show that: (1) the incident ions suffer both Shock drift acceleration (SDA) and Shock surfing acceleration (SSA) mechanisms. Moreover, a striking feature is that SSA ions not only are identified at the ramp but also within the foot which confirms previous 1-D simulation results; (2) the percentage of SSA ions increases with initial kinetic energy, a feature which persists well with a rippled Shock Front; (3) furthermore, the ripples increase the porosity of the Shock Front, and more directly transmitted (DT) ions are produced; these strongly affect the relative percentage of the different identified classes of ions (SSA, SDA and DT ions), their average kinetic energy and their relative contribution to the resulting downstream energy spectra; (4) one key impact of the ripples is a strong diffusion of ions (in particular through the Frontiers of their injection angle domains and in phase space which are blurred out) which leads to a mixing of the different ion classes. This diffusion increases with the size of the spatial scale of the Front ripples; (5) through this diffusion, an ion belonging to a given category (SSA, SDA, or DT) in y-averaged case changes class in non-averaged case without one-to-one correspondence.
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Acceleration of heavy ions by perpendicular collisionless Shocks: Impact of the Shock Front nonstationarity
Journal of Geophysical Research Space Physics, 2011Co-Authors: Z W Yang, Bertrand LembègeAbstract:Both hybrid/full particle simulations and recent experimental results have clearly evidenced that the Front of a supercritical quasi-perpendicular Shock can be nonstationary. One responsible mechanism proposed for this nonstationarity is the self-reformation of the Front itself being due to the accumulation of reflected ions. Important consequences of this nonstationarity are that not only the amplitude but also the spatial scales of fields components at the Shock Front (ramp and foot) are strongly varying within each cycle of the self-reformation. On the other hand, several studies have been made on the acceleration and heating of heavy ions but most have been restricted to a stationary Shock profile only. Herein, one-dimensional test particle simulations based on Shock profiles fields produced in PIC simulation are performed in order to investigate the impact of the Shock Front nonstationarity on heavy ion acceleration (He, O, Fe). Reflection and acceleration mechanisms of heavy ions (with different initial thermal velocities and different charge-mass ratios) interacting with a nonstationary Shock Front (self-reformation) are analyzed in detail. Present preliminary results show that: (1) the heavy ions suffer both Shock drift acceleration (SDA) and Shock surfing acceleration (SSA) mechanisms; (2) the fraction of reflected heavy ions increases with initial thermal velocity, charge-mass ratio and decreasing Shock Front width at both stationary Shocks (situation equivalent to fixed Shock cases) and nonstationary Shocks (situation equivalent to continuously time-evolving Shock cases); (3) the Shock Front nonstationarity (time-evolving Shock case) facilitates the reflection of heavy ions; (4) a striking feature is the formation of an injected monoenergetic heavy ions population which persists in the Shock Front spectrum for different initial thermal velocities and ions species. The impact of the Shock Front nonstationarity on the heavy ions spectra within the Shock Front region and the downstream region are detailed separately. Present results are compared with previous experimental analysis and theoretical models of solar energetic particles (SEP) events. The variations of Fe/O spectra in high energy part have been retrieved, and the nonstationary effects of Shock Front strongly amplify these variations.
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Shock Front nonstationarity and ion acceleration in supercritical perpendicular Shocks
Journal of Geophysical Research : Space Physics, 2009Co-Authors: Z W Yang, Q. M. Lu, Bertrand Lembège, Shangping WangAbstract:Previous particle-in-cell simulations have evidenced that quasiperpendicular Shocks are nonstationary and suffer a self-reformation on gyro scale of the incoming ions due to the accumulation of reflected ions. In this paper, by separating the incoming ions into reflected and directly transmitted parts, we investigate the detailed mechanisms of ion acceleration in a nonstationary perpendicular Shock. Test particle simulations are performed where the Shock profiles are issued from self-consistent one-dimensional full particle-in-cell simulations. Both shell and Maxwellian incoming ion distributions are used. In both cases, most energetic particles correspond to reflected ions, and the associated acceleration mechanisms include both Shock drift acceleration (SDA) and Shock surfing acceleration (SSA). Two types of results are obtained. First, if we fix the Shock profiles at different times within a self-reformation cycle, the mechanisms of particle acceleration are different at different profiles. SDA process appears as the dominant acceleration mechanism when the width of the ramp is broad (and overshoot amplitude is low) whereas both SDA and SSA contribute as the width of the ramp is narrow (and overshoot amplitude is high). For the different Shock profiles concerned herein, SDA process is more efficient (higher resulting ion energy gain) than the SSA process. Second, in order to investigate ion acceleration in self-reforming Shocks, not only the ramp but also the variations of the whole Shock Front need to be included. In the continuously time-evolving Shock, SDA remains a dominant acceleration mechanism whereas SSA mechanism becomes more and more important with the increase of the initial particle energy. The percentage of reflected ions cyclically varies in time with a period equal to the self reformation cycle, which is in agreement with previous full particle simulations. The reflected ions not only come from the distribution wings of the incoming ions but also from the core part, in contrast with previous results based on stationary Shocks.
Paul Nulsen - One of the best experts on this subject based on the ideXlab platform.
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A very deep chandra observation of the galaxy group NGC 5813: AGN Shocks, feedback, and outburst history
The Astrophysical Journal, 2015Co-Authors: Scott W. Randall, Ralph P Kraft, Paul Nulsen, William R. Forman, Esra Bulbul, Tracy E. Clarke, Elizabeth L. Blanton, Laurence P. David, C Jones, Norbert WernerAbstract:We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC 5813, the deepest Chandra observation of a galaxy group to date. This system uniquely shows three pairs of collinear cavities, with each pair associated with an unambiguous active galactic nucleus (AGN) outburst Shock Front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (∼50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the gas radiative cooling rate and mean Shock heating rate are well balanced at each Shock Front, suggesting that Shock heating alone is sufficient to offset cooling and establish AGN/intracluster medium (ICM) feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that Shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak Shocks are more difficult to detect. We find non-zero Shock Front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with Shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ∼70 km s−1. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.
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a very deep chandra observation of the galaxy group ngc 5813 agn Shocks feedback and outburst history
arXiv: High Energy Astrophysical Phenomena, 2015Co-Authors: Scott W. Randall, Ralph P Kraft, Paul Nulsen, Esra Bulbul, Tracy E. Clarke, Elizabeth L. Blanton, Laurence P. David, C Jones, W Forman, N WernerAbstract:We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC~5813, the deepest Chandra observation of a galaxy group to date. Earlier observations showed two pairs of cavities distributed roughly collinearly, with each pair associated with an elliptical Shock Front. The new observations confirm a third pair of outer cavities, collinear with the other pairs, and reveal an associated outer outburst Shock at ~30 kpc. This system is therefore unique in exhibiting three cavity pairs, each associated with an unambiguous AGN outburst Shock Front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (~50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the radiative cooling rate and the mean Shock heating rate of the gas are well balanced at each Shock Front, suggesting that AGN outburst Shock heating alone is sufficient to offset cooling and establish AGN/ICM feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that Shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak Shocks are more difficult to detect. We find non-zero Shock Front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with Shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ~70 km/s. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.
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the cluster scale agn outburst in hydra a
The Astrophysical Journal, 2005Co-Authors: Paul Nulsen, B R Mcnamara, M W Wise, Laurence P. DavidAbstract:Deep Chandra observations of the Hydra A Cluster reveal a feature in the X-ray surface brightness that surrounds the 330 MHz radio lobes of the AGN at the cluster center. Surface brightness profiles of this feature and its close association with the radio lobes argue strongly that it is a Shock Front driven by the expanding radio lobes. The Chandra image also reveals other new structure on smaller scales that is associated with the radio source, including a large cavity and filament. The Shock Front extends 200-300 kpc from the AGN at the cluster center, and its strength varies along the Front, with Mach numbers in the range ~1.2-1.4. It is stronger where it is more distant from the cluster center, as expected for a Shock driven by expanding radio lobes. Simple modeling gives an age for the Shock Front of ~1.4 × 108 yr and a total energy driving it of ~1061 ergs. The mean mechanical power driving the Shock is comparable to quasar luminosities, well in excess of that needed to regulate the cooling core in Hydra A. This suggests that the feedback regulating cooling cores is inefficient, in that the bulk of the energy is deposited beyond the cooling core. In that case, a significant part of cluster "preheating" is a by-product of the regulation of cooling cores.
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the cluster scale agn outburst in hydra a
arXiv: Astrophysics, 2004Co-Authors: Paul Nulsen, B R Mcnamara, M W Wise, Laurence P. DavidAbstract:Deep Chandra observations of the Hydra A Cluster reveal a feature in the X-ray surface brightness that surrounds the 330 MHz radio lobes of the AGN at the cluster center. Surface brightness profiles of this feature and its close association with the radio lobes argue strongly that it is a Shock Front driven by the expanding radio lobes. The Chandra image also reveals other new structure on smaller scales that is associated with the radio source, including a large cavity and filament. The Shock Front extends 200 - 300 kpc from the AGN at the cluster center and its strength varies along the Front, with Mach numbers in the range ~ 1.2 - 1.4. It is stronger where it is more distant from the cluster center, as expected for a Shock driven by expanding radio lobes. Simple modeling gives an age for the Shock Front ~ 1.4\times10^8 y and a total energy driving it of ~ 10^{61} erg. The mean mechanical power driving the Shock is comparable to quasar luminosities, well in excess of that needed to regulate the cooling core in Hydra A. This suggests that the feedback regulating cooling cores is inefficient, in that the bulk of the energy is deposited beyond the cooling core. In that case, a significant part of cluster "preheating" is a byproduct of the regulation of cooling cores.
Rui Wang - One of the best experts on this subject based on the ideXlab platform.
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impact of Shock Front rippling and self reformation on the electron dynamics at low mach number Shocks
The Astrophysical Journal, 2018Co-Authors: Zhongwei Yang, Ying Liu, Rui WangAbstract:Electron dynamics at low-Mach-number collisionless Shocks are investigated by using two-dimensional electromagnetic particle-in-cell simulations with various Shock normal angles. We found: (1) The reflected ions and incident electrons at the Shock Front provide an effective mechanism for the quasi-electrostatic wave generation due to the charge-separation. A fraction of incident electrons can be effectively trapped and accelerated at the leading edge of the Shock foot. (2) At quasi-perpendicular Shocks, the electron trapping and reflection is nonuniform due to the Shock rippling along the Shock surface and is more likely to take place at some locations accompanied by intense reflected ion-beams. The electron trapping process has a periodical evolution over time due to the Shock Front self-reformation, which is controlled by ion dynamics. Thus, this is a cross-scale coupling phenomenon. (3) At quasi-parallel Shocks, reflected ions can travel far back upstream. Consequently, quasi-electrostatic waves can be excited in the Shock transition and the foreShock region. The electron trajectory analysis shows these waves can trap electrons at the foot region and reflect a fraction of them far back upstream. Simulation runs in this paper indicate that the micro-turbulence at the Shock foot can provide a possible scenario for producing the reflected electron beam, which is a basic condition for the type II radio burst emission at low-Mach-number interplanetary Shocks driven by Coronal Mass Ejections (CMEs).
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impact of pickup ions on the Shock Front nonstationarity and energy dissipation of the heliospheric termination Shock two dimensional full particle simulations and comparison with voyager 2 observations
The Astrophysical Journal, 2015Co-Authors: Zhongwei Yang, Ying Liu, J D Richardson, Can Huang, Rui WangAbstract:Voyager 2 (V2) observed multiple crossings of the heliospheric termination Shock (TS) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. Here, for the first time, we present two-dimensional particle-incell (PIC) simulations of the TS self-consistently including pickup ions (PUIs), and compare the simulation results with V2 observations. We find that (1) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion kinetic energy is transferred to the thermal energy of PUIs. The PIC simulation indicates that, for the upstream parameters chosen for V2 conditions, the density of PUIs is about 25% and the PUIs gain the largest fraction (approximately 86.6%) of downstream thermal pressure. (2) The simulated heliosheath ion distribution function is a superposition of a cold core formed by transmitted solar wind ions (SWIs), with the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. The V2 Faraday cups observed the cool core of the distribution, and so they only saw the tip of the iceberg. (3) The nonstationarity of the Shock Front is mainly caused by ripples along the Shock Front which form even if the percentage of PUIs is high. These simulation results agree reasonably well with the V2 experimental data. The relevance of the Shock Front ripples to the multiple TS crossings observed by V2 is also discussed in this paper.
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impact of pickup ions on the Shock Front nonstationarity and energy dissipation of the heliospheric termination Shock two dimensional full particle simulations and comparison with voyager 2 observations
arXiv: Space Physics, 2015Co-Authors: Zhongwei Yang, Ying Liu, J D Richardson, Can Huang, Rui WangAbstract:The transition between the supersonic solar wind and the subsonic heliosheath, the termination Shock (TS), was observed by Voyager 2 (V2) on 2007 August 31-September 1 at a distance of 84 AU from the Sun. The data reveal multiple crossings of a complex, quasi-perpendicular supercritical Shock. These experimental data are the starting point for a more sophisticated analysis that includes computer modeling of a Shock in the presence of pickup ions (PUIs). here, we present two-dimensional (2-D) particle-in-cell (PIC) simulations of the TS including PUIs self-consistently. We also report the ion velocity distribution across the TS using the Faraday cup data from V2. A relatively complete plasma and magnetic field data set from V2 gives us the opportunity to do a full comparison between the experimental data and PIC simulation results. Our results show that: (1) The nonstationarity of the Shock Front is mainly caused by the ripples along the Shock Front and these ripples from even if the percentage of PUIs is high. (2) PUIs play a key role in the energy dissipation of the TS, and most of the incident ion dynamic energy is transferred to the thermal energy of PUIs instead of solar wind ions (SWIs). (3) The simulated composite heliosheath ion velocity distribution function is a superposition of a cold core formed by transmitted SWIs, the shoulders contributed by the hot reflected SWIs and directly transmitted PUIs, and the wings of the distribution dominated by the very hot reflected PUIs. (4) The V2 Faraday cups observed the cool core of the distribution, so they saw only a tip of the iceberg. For the evolution of the cool core distribution function across the TS, the computed results agree reasonably well with the V2experimental results.
Laurence P. David - One of the best experts on this subject based on the ideXlab platform.
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A very deep chandra observation of the galaxy group NGC 5813: AGN Shocks, feedback, and outburst history
The Astrophysical Journal, 2015Co-Authors: Scott W. Randall, Ralph P Kraft, Paul Nulsen, William R. Forman, Esra Bulbul, Tracy E. Clarke, Elizabeth L. Blanton, Laurence P. David, C Jones, Norbert WernerAbstract:We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC 5813, the deepest Chandra observation of a galaxy group to date. This system uniquely shows three pairs of collinear cavities, with each pair associated with an unambiguous active galactic nucleus (AGN) outburst Shock Front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (∼50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the gas radiative cooling rate and mean Shock heating rate are well balanced at each Shock Front, suggesting that Shock heating alone is sufficient to offset cooling and establish AGN/intracluster medium (ICM) feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that Shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak Shocks are more difficult to detect. We find non-zero Shock Front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with Shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ∼70 km s−1. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.
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a very deep chandra observation of the galaxy group ngc 5813 agn Shocks feedback and outburst history
arXiv: High Energy Astrophysical Phenomena, 2015Co-Authors: Scott W. Randall, Ralph P Kraft, Paul Nulsen, Esra Bulbul, Tracy E. Clarke, Elizabeth L. Blanton, Laurence P. David, C Jones, W Forman, N WernerAbstract:We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC~5813, the deepest Chandra observation of a galaxy group to date. Earlier observations showed two pairs of cavities distributed roughly collinearly, with each pair associated with an elliptical Shock Front. The new observations confirm a third pair of outer cavities, collinear with the other pairs, and reveal an associated outer outburst Shock at ~30 kpc. This system is therefore unique in exhibiting three cavity pairs, each associated with an unambiguous AGN outburst Shock Front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (~50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the radiative cooling rate and the mean Shock heating rate of the gas are well balanced at each Shock Front, suggesting that AGN outburst Shock heating alone is sufficient to offset cooling and establish AGN/ICM feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that Shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak Shocks are more difficult to detect. We find non-zero Shock Front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with Shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ~70 km/s. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.
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the cluster scale agn outburst in hydra a
The Astrophysical Journal, 2005Co-Authors: Paul Nulsen, B R Mcnamara, M W Wise, Laurence P. DavidAbstract:Deep Chandra observations of the Hydra A Cluster reveal a feature in the X-ray surface brightness that surrounds the 330 MHz radio lobes of the AGN at the cluster center. Surface brightness profiles of this feature and its close association with the radio lobes argue strongly that it is a Shock Front driven by the expanding radio lobes. The Chandra image also reveals other new structure on smaller scales that is associated with the radio source, including a large cavity and filament. The Shock Front extends 200-300 kpc from the AGN at the cluster center, and its strength varies along the Front, with Mach numbers in the range ~1.2-1.4. It is stronger where it is more distant from the cluster center, as expected for a Shock driven by expanding radio lobes. Simple modeling gives an age for the Shock Front of ~1.4 × 108 yr and a total energy driving it of ~1061 ergs. The mean mechanical power driving the Shock is comparable to quasar luminosities, well in excess of that needed to regulate the cooling core in Hydra A. This suggests that the feedback regulating cooling cores is inefficient, in that the bulk of the energy is deposited beyond the cooling core. In that case, a significant part of cluster "preheating" is a by-product of the regulation of cooling cores.
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the cluster scale agn outburst in hydra a
arXiv: Astrophysics, 2004Co-Authors: Paul Nulsen, B R Mcnamara, M W Wise, Laurence P. DavidAbstract:Deep Chandra observations of the Hydra A Cluster reveal a feature in the X-ray surface brightness that surrounds the 330 MHz radio lobes of the AGN at the cluster center. Surface brightness profiles of this feature and its close association with the radio lobes argue strongly that it is a Shock Front driven by the expanding radio lobes. The Chandra image also reveals other new structure on smaller scales that is associated with the radio source, including a large cavity and filament. The Shock Front extends 200 - 300 kpc from the AGN at the cluster center and its strength varies along the Front, with Mach numbers in the range ~ 1.2 - 1.4. It is stronger where it is more distant from the cluster center, as expected for a Shock driven by expanding radio lobes. Simple modeling gives an age for the Shock Front ~ 1.4\times10^8 y and a total energy driving it of ~ 10^{61} erg. The mean mechanical power driving the Shock is comparable to quasar luminosities, well in excess of that needed to regulate the cooling core in Hydra A. This suggests that the feedback regulating cooling cores is inefficient, in that the bulk of the energy is deposited beyond the cooling core. In that case, a significant part of cluster "preheating" is a byproduct of the regulation of cooling cores.
