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Masahiro Hoshino - One of the best experts on this subject based on the ideXlab platform.
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Electron Acceleration at rippled low mach number shocks in high beta collisionless cosmic plasmas
arXiv: High Energy Astrophysical Phenomena, 2021Co-Authors: Oleh Kobzar, J Niemiec, Takanobu Amano, Masahiro Hoshino, Shuichi Matsukiyo, Yosuke Matsumoto, M PohlAbstract:Using large-scale fully-kinetic two-dimensional particle-in-cell simulations, we investigate the effects of shock rippling on Electron Acceleration at low-Mach-number shocks propagating in high-$\beta$ plasmas, in application to merger shocks in galaxy clusters. We find that the Electron Acceleration rate increases considerably when the rippling modes appear. The main Acceleration mechanism is stochastic shock-drift Acceleration, in which Electrons are confined at the shock by pitch-angle scattering off turbulence and gain energy from the motional electric field. The presence of multi-scale magnetic turbulence at the shock transition and the region immediately behind the main shock overshoot is essential for Electron energization. Wide-energy non-thermal Electron distributions are formed both upstream and downstream of the shock. The maximum energy of the Electrons is sufficient for their injection into diffusive shock Acceleration. We show for the first time that the downstream Electron spectrum has a~power-law form with index $p\approx 2.5$, in agreement with observations.
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Electron Acceleration at rippled low mach number shocks in merging galaxy clusters
36th International Cosmic Ray Conference ICRC 2019, 2019Co-Authors: J Niemiec, Oleh Kobzar, Takanobu Amano, Masahiro Hoshino, Shuichi Matsukiyo, Yosuke Matsumoto, M PohlAbstract:Shock waves are ubiquitous in cosmic plasmas wherein they accelerate particles. In particular, X-ray and radio observations of so-called radio relics indicate Electron Acceleration at large-scale merger shocks in galaxy clusters. These shocks are also candidate sites for ultra-high-energy cosmic ray production. Merger shocks have low Mach numbers and propagate in hot plasmas with plasma beta $\beta\gg 1$. Particle energization and especially Electron injection mechanisms are poorly understood in such conditions. Recent studies show that shock drift Acceleration (SDA) accompanied by particle-wave interactions can provide Electron Acceleration, albeit a multi-scale shock structure in the form of ion-scale shock rippling may significantly alter the injection mechanisms. Here we study the effects of the shock rippling with large-scale 2D PIC simulations of low Mach number cluster shocks. We find that the Electron Acceleration rate increases considerably after the appearance of wave-rippling modes. The main Acceleration process is stochastic SDA, in which Electrons are confined in the shock transition region by pitch-angle scattering off magnetic turbulence and gain energy from motional electric field. The presence of multi-scale turbulence in the shock is essential for particle energization. Wide-energy non-thermal Electron distributions are formed both upstream and downstream of the shock. We show for the first time that the downstream Electron spectrum has a power-law form with index $p = 2.4$, in agreement with observations.
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Electron Acceleration in a nonrelativistic shock with very high alfven mach number
Physical Review Letters, 2013Co-Authors: Yosuke Matsumoto, Takanobu Amano, Masahiro HoshinoAbstract:Electron Acceleration associated with various plasma kinetic instabilities in a nonrelativistic shock with very high Alfv\'en Mach number (${M}_{A}\ensuremath{\sim}45$) is revealed by means of a two-dimensional fully kinetic particle-in-cell simulation. Electromagnetic (ion Weibel) and electrostatic (ion-acoustic and Buneman) instabilities are strongly activated at the same time in different regions of the two-dimensional shock structure. Relativistic Electrons are quickly produced predominantly by the shock surfing mechanism with the Buneman instability at the leading edge of the foot. The energy spectrum has a high-energy tail exceeding the upstream ion kinetic energy accompanying the main thermal population. This gives a favorable condition for the ion-acoustic instability at the shock front, which in turn results in additional energization. The large-amplitude ion Weibel instability generates current sheets in the foot, implying another dissipation mechanism via magnetic reconnection in a three-dimensional shock structure in the very-high-${M}_{A}$ regime.
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nonlinear evolution of buneman instability and its implication for Electron Acceleration in high mach number collisionless perpendicular shocks
Physics of Plasmas, 2009Co-Authors: Takanobu Amano, Masahiro HoshinoAbstract:Nonlinear evolution of the Buneman instability and its application to Electron Acceleration in collisionless shocks are discussed. Two-dimensional particle-in-cell simulations show that the saturation level of the instability is reduced from one-dimensional simulation results. It is demonstrated that the reduced saturation level is due to the resonant wave-particle interactions with large amplitude obliquely propagating waves. A new estimate for the saturation level is given by considering the interactions with oblique modes. The effects of the large amplitude oblique modes on Electron shock surfing Acceleration that is mainly controlled by the Buneman instability are also investigated. Two-dimensional particle-in-cell simulations of the shock transition region are performed by adopting a local model with the periodic boundary condition. The results indicate that the presence of oblique modes introduces a stochastic behavior to the trajectories of energetic Electrons. The maximum energy is limited by the ...
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energetic Electron Acceleration in the downstream reconnection outflow region
Journal of Geophysical Research, 2007Co-Authors: Shinsuke Imada, Masahiro Hoshino, R Nakamura, P W Daly, W Baumjohann, S Muhlbachler, A Balogh, H RemeAbstract:[1] Energetic Electrons in an earthward reconnection outflow region have been observed by Cluster/Research with Adaptive Particle Imaging Detectors. We found a good correlation between the energetic Electron enhancement and a normal magnetic field (Bz) enhancement within a 0.25-s time resolution. The large normal magnetic field is thought to be associated with magnetic reconnection because the negative/positive Bz reversal observed during the fast proton tailward/earthward flow reversal is a good indicator of magnetic reconnection. Using the four-spacecraft Cluster, we can clearly see that this large positive Bz structure propagates in the earthward direction. Furthermore, we find that the energy spectrum of the energetic Electrons becomes harder toward the downstream region. A negative Bz enhancement is also observed. The intensity of energetic Electron enhancement associated with the negative Bz enhancement is weaker than that associated with the positive one. To discuss the temporal and spatial profile of energetic Electron Acceleration in the magnetic reconnection region, we determined the spacecraft position in the temporally evolving magnetic structures of reconnection. Our observation clearly indicates second-step Acceleration, in addition to X line Acceleration, of energetic Electrons in the downstream reconnection outflow region.
Nigel P Meredith - One of the best experts on this subject based on the ideXlab platform.
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timescales for radiation belt Electron Acceleration and loss due to resonant wave particle interactions 2 evaluation for vlf chorus elf hiss and electromagnetic ion cyclotron waves
Journal of Geophysical Research, 2007Co-Authors: Danny Summers, Nigel P MeredithAbstract:Outer zone radiation belt Electrons can undergo gyroresonant interaction with various magnetospheric wave modes including whistler-mode chorus outside the plasmasphere and both whistler-mode hiss and electromagnetic ion cyclotron (EMIC) waves inside the plasmasphere. To evaluate timescales for Electron momentum diffusion and pitch angle diffusion, we utilize bounce-averaged quasi-linear diffusion coefficients for field-aligned waves with a Gaussian frequency spectrum in a dipole magnetic field. Timescales for momentum diffusion of MeV Electrons due to VLF chorus can be less than a day in the outer radiation belt. Equatorial chorus waves (|λw| < 15 deg) can effectively accelerate MeV Electrons. Efficiency of the chorus Acceleration mechanism is increased if high-latitude waves (|λw| < 15 deg) are also present. Our calculations confirm that chorus diffusion is a viable mechanism for generating relativistic (MeV) Electrons in the outer zone during the recovery phase of a storm or during periods of prolonged substorm activity when chorus amplitudes are enhanced. Radiation belt Electrons are subject to precipitation loss to the atmosphere due to resonant pitch angle scattering by plasma waves. The Electron precipitation loss timescale due to scattering by each of the wave modes, chorus, hiss, and EMIC waves, can be 1 day or less. These wave modes can separately, or in combination, contribute significantly to the depletion of relativistic (MeV) Electrons from the outer zone over the course of a magnetic storm. Efficient pitch angle scattering by whistler-mode chorus or hiss typically requires high latitude waves (|λw| < 30 deg). Timescales for Electron Acceleration and loss generally depend on the spectral properties of the waves, as well as the background Electron number density and magnetic field. Loss timescales due to EMIC wave scattering also depend on the ion (H+, He+, O+) composition of the plasma. Complete models of radiation belt Electron transport, Acceleration and loss should include, in addition to radial (cross-L) diffusion, resonant diffusion due to gyroresonance with VLF chorus, plasmaspheric hiss, and EMIC waves. Comprehensive observational data on the spectral properties of these waves are required as a function of spatial location (L, MLT, MLAT) and magnetic activity.
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timescale for radiation belt Electron Acceleration by whistler mode chorus waves
Journal of Geophysical Research, 2005Co-Authors: Richard B Horne, R. M. Thorne, Sarah A Glauert, J M Albert, Nigel P Meredith, R R AndersonAbstract:[1] Electron Acceleration inside the Earth's magnetosphere is required to explain increases in the ∼MeV radiation belt Electron flux during magnetically disturbed periods. Recent studies show that Electron Acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the Electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for Acceleration. We show that chorus emissions in the prenoon sector accelerate Electrons most efficiently at latitudes above 15° for equatorial pitch angles between 20° and 60°. As Electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate Electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave Acceleration the Electrons undergo many drift orbits and the resulting pitch angle distributions are energy-dependent. Chorus scattering should produce pitch angle distributions that are either flat-topped or butterfly-shaped. The results provide strong support for the wave Acceleration theory.
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favored regions for chorus driven Electron Acceleration to relativistic energies in the earth s outer radiation belt
Geophysical Research Letters, 2003Co-Authors: Nigel P Meredith, R. M. Thorne, Richard B Horne, R R AndersonAbstract:[1] Pitch angle and energy diffusion rates for scattering by whistler-mode chorus waves are proportional to the wave magnetic field intensity and are strongly dependent on the frequency distribution of the waves and to the ratio between the Electron plasma frequency (f(pe)) and the Electron gyrofrequency (f(ce)). Relativistic Electrons interact most readily with lower-band chorus (0.1 300 nT). Enhanced waves in these regions could play a major role in Electron Acceleration to relativistic energies during periods of prolonged substorm activity.
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evidence for chorus driven Electron Acceleration to relativistic energies from a survey of geomagnetically disturbed periods
Journal of Geophysical Research, 2003Co-Authors: Nigel P Meredith, R. M. Thorne, Danny Summers, Richard B Horne, M Cain, R R AndersonAbstract:[1] We perform a survey of the plasma wave and particle data from the CRRES satellite during 26 geomagnetically disturbed periods to investigate the viability of a local stochastic Electron Acceleration mechanism to relativistic energies driven by Doppler-shifted cyclotron resonant interactions with whistler mode chorus. Relativistic Electron flux enhancements associated with moderate or strong storms may be seen over the whole outer zone (3 < L < 7), typically peaking in the range 4 < L < 5, whereas those associated with weak storms and intervals of prolonged substorm activity lacking a magnetic storm signature (PSALMSS) are typically observed further out in the regions 4 < L < 7 and 4.5 < L < 7, respectively. The most significant relativistic Electron flux enhancements are seen outside of the plasmapause and are associated with periods of prolonged substorm activity with AE greater than 100 nT for a total integrated time greater than 2 days or greater than 300 nT for a total integrated time greater than 0.7 days. These events are also associated with enhanced fluxes of seed Electrons and enhanced lower-band chorus wave power with integrated lower-band chorus wave intensities of greater than 500 pT(2) day. No significant flux enhancements are seen unless the level of substorm activity is sufficiently high. These results are consistent with a local, stochastic, chorus-driven Electron Acceleration mechanism involving the energization of a seed population of Electrons with energies of a few hundred keV to relativistic energies operating on a timescale of the order of days.
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outer zone relativistic Electron Acceleration associated with substorm enhanced whistler mode chorus
Journal of Geophysical Research, 2002Co-Authors: Nigel P Meredith, R. M. Thorne, Richard B Horne, R H A Iles, Daniel Heynderickx, R R AndersonAbstract:[1] We present plasma wave and particle data from the CRRES satellite during three case studies to investigate the viability of a local stochastic Electron Acceleration mechanism to relativistic energies driven by resonant interactions with whistler mode chorus. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3-day recovery phase. The recovery phase is characterized by Electron injections at subrelativistic energies, enhanced whistler mode chorus amplitudes, and a gradual increase in the flux of relativistic Electrons (E > 1 MeV) over the entire outer zone, with fluxes exceeding the prestorm level by an order of magnitude in the region 3.5 < L < 4.5. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3-day recovery phase. Here the recovery phase is characterized by a lack of sustained Electron injections at subrelativistic energies, a low level of chorus amplitudes, and a net reduction in the flux of relativistic Electrons in the outer zone. Finally, we examine a period of prolonged substorm activity in the absence of a significant storm signature, as measured by Dst. This period is characterized by Electron injections at subrelativistic energies, enhanced chorus amplitudes, and a gradual increase in the flux of relativistic Electrons in the region 4 < L < 6.5. These results suggest that the gradual Acceleration of Electrons to relativistic energies seen on a timescale of days during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, gradual Electron Acceleration to relativistic energies can be obtained during periods of prolonged substorm activity in the absence of a significant storm signature as indicated by Dst. The case studies show that the Acceleration mechanism is confined to the region outside of the plasmapause and occurs in the presence of enhanced chorus waves. These results suggest that a local Acceleration mechanism involving the energization of a seed population of Electrons with energies of the order of a few hundred keV to relativistic energies by wave-particle interactions involving whistler mode chorus contributes to the reformation of the relativistic outer zone population following prolonged substorm activity.
R R Anderson - One of the best experts on this subject based on the ideXlab platform.
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timescale for radiation belt Electron Acceleration by whistler mode chorus waves
Journal of Geophysical Research, 2005Co-Authors: Richard B Horne, R. M. Thorne, Sarah A Glauert, J M Albert, Nigel P Meredith, R R AndersonAbstract:[1] Electron Acceleration inside the Earth's magnetosphere is required to explain increases in the ∼MeV radiation belt Electron flux during magnetically disturbed periods. Recent studies show that Electron Acceleration by whistler mode chorus waves becomes most efficient just outside the plasmapause, near L = 4.5, where peaks in the Electron phase space density are observed. We present CRRES data on the spatial distribution of chorus emissions during active conditions. The wave data are used to calculate the pitch angle and energy diffusion rates in three magnetic local time (MLT) sectors and to obtain a timescale for Acceleration. We show that chorus emissions in the prenoon sector accelerate Electrons most efficiently at latitudes above 15° for equatorial pitch angles between 20° and 60°. As Electrons drift around the Earth, they are scattered to large pitch angles and further accelerated by chorus on the nightside in the equatorial region. The timescale to accelerate Electrons by whistler mode chorus and increase the flux at 1 MeV by an order of magnitude is approximately 1 day, in agreement with satellite observations during the recovery phase of storms. During wave Acceleration the Electrons undergo many drift orbits and the resulting pitch angle distributions are energy-dependent. Chorus scattering should produce pitch angle distributions that are either flat-topped or butterfly-shaped. The results provide strong support for the wave Acceleration theory.
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favored regions for chorus driven Electron Acceleration to relativistic energies in the earth s outer radiation belt
Geophysical Research Letters, 2003Co-Authors: Nigel P Meredith, R. M. Thorne, Richard B Horne, R R AndersonAbstract:[1] Pitch angle and energy diffusion rates for scattering by whistler-mode chorus waves are proportional to the wave magnetic field intensity and are strongly dependent on the frequency distribution of the waves and to the ratio between the Electron plasma frequency (f(pe)) and the Electron gyrofrequency (f(ce)). Relativistic Electrons interact most readily with lower-band chorus (0.1 300 nT). Enhanced waves in these regions could play a major role in Electron Acceleration to relativistic energies during periods of prolonged substorm activity.
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evidence for chorus driven Electron Acceleration to relativistic energies from a survey of geomagnetically disturbed periods
Journal of Geophysical Research, 2003Co-Authors: Nigel P Meredith, R. M. Thorne, Danny Summers, Richard B Horne, M Cain, R R AndersonAbstract:[1] We perform a survey of the plasma wave and particle data from the CRRES satellite during 26 geomagnetically disturbed periods to investigate the viability of a local stochastic Electron Acceleration mechanism to relativistic energies driven by Doppler-shifted cyclotron resonant interactions with whistler mode chorus. Relativistic Electron flux enhancements associated with moderate or strong storms may be seen over the whole outer zone (3 < L < 7), typically peaking in the range 4 < L < 5, whereas those associated with weak storms and intervals of prolonged substorm activity lacking a magnetic storm signature (PSALMSS) are typically observed further out in the regions 4 < L < 7 and 4.5 < L < 7, respectively. The most significant relativistic Electron flux enhancements are seen outside of the plasmapause and are associated with periods of prolonged substorm activity with AE greater than 100 nT for a total integrated time greater than 2 days or greater than 300 nT for a total integrated time greater than 0.7 days. These events are also associated with enhanced fluxes of seed Electrons and enhanced lower-band chorus wave power with integrated lower-band chorus wave intensities of greater than 500 pT(2) day. No significant flux enhancements are seen unless the level of substorm activity is sufficiently high. These results are consistent with a local, stochastic, chorus-driven Electron Acceleration mechanism involving the energization of a seed population of Electrons with energies of a few hundred keV to relativistic energies operating on a timescale of the order of days.
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outer zone relativistic Electron Acceleration associated with substorm enhanced whistler mode chorus
Journal of Geophysical Research, 2002Co-Authors: Nigel P Meredith, R. M. Thorne, Richard B Horne, R H A Iles, Daniel Heynderickx, R R AndersonAbstract:[1] We present plasma wave and particle data from the CRRES satellite during three case studies to investigate the viability of a local stochastic Electron Acceleration mechanism to relativistic energies driven by resonant interactions with whistler mode chorus. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3-day recovery phase. The recovery phase is characterized by Electron injections at subrelativistic energies, enhanced whistler mode chorus amplitudes, and a gradual increase in the flux of relativistic Electrons (E > 1 MeV) over the entire outer zone, with fluxes exceeding the prestorm level by an order of magnitude in the region 3.5 < L < 4.5. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3-day recovery phase. Here the recovery phase is characterized by a lack of sustained Electron injections at subrelativistic energies, a low level of chorus amplitudes, and a net reduction in the flux of relativistic Electrons in the outer zone. Finally, we examine a period of prolonged substorm activity in the absence of a significant storm signature, as measured by Dst. This period is characterized by Electron injections at subrelativistic energies, enhanced chorus amplitudes, and a gradual increase in the flux of relativistic Electrons in the region 4 < L < 6.5. These results suggest that the gradual Acceleration of Electrons to relativistic energies seen on a timescale of days during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, gradual Electron Acceleration to relativistic energies can be obtained during periods of prolonged substorm activity in the absence of a significant storm signature as indicated by Dst. The case studies show that the Acceleration mechanism is confined to the region outside of the plasmapause and occurs in the presence of enhanced chorus waves. These results suggest that a local Acceleration mechanism involving the energization of a seed population of Electrons with energies of the order of a few hundred keV to relativistic energies by wave-particle interactions involving whistler mode chorus contributes to the reformation of the relativistic outer zone population following prolonged substorm activity.
Lorenzo Sironi - One of the best experts on this subject based on the ideXlab platform.
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non thermal Electron Acceleration in low mach number collisionless shocks ii firehose mediated fermi Acceleration and its dependence on pre shock conditions
The Astrophysical Journal, 2014Co-Authors: Xinyi Guo, Lorenzo Sironi, Ramesh NarayaAbstract:Electron Acceleration to non-thermal energies is known to occur in low Mach number (M{sub s} ≲ 5) shocks in galaxy clusters and solar flares, but the Electron Acceleration mechanism remains poorly understood. Using two-dimensional (2D) particle-in-cell (PIC) plasma simulations, we showed in Paper I that Electrons are efficiently accelerated in low Mach number (M{sub s} = 3) quasi-perpendicular shocks via a Fermi-like process. The Electrons bounce between the upstream region and the shock front, with each reflection at the shock resulting in energy gain via shock drift Acceleration. The upstream scattering is provided by oblique magnetic waves that are self-generated by the Electrons escaping ahead of the shock. In the present work, we employ additional 2D PIC simulations to address the nature of the upstream oblique waves. We find that the waves are generated by the shock-reflected Electrons via the firehose instability, which is driven by an anisotropy in the Electron velocity distribution. We systematically explore how the efficiency of wave generation and of Electron Acceleration depend on the magnetic field obliquity, the flow magnetization (or equivalently, the plasma beta), and the upstream Electron temperature. We find that the mechanism works for shocks with high plasma beta (≳ 20) atmore » nearly all magnetic field obliquities, and for Electron temperatures in the range relevant for galaxy clusters. Our findings offer a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.« less
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non thermal Electron Acceleration in low mach number collisionless shocks i particle energy spectra and Acceleration mechanism
The Astrophysical Journal, 2014Co-Authors: Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies in low Mach number (M{sub s} ≲ 5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the Electron Acceleration mechanism remains poorly understood. Diffusive shock Acceleration, also known as first-order Fermi Acceleration, cannot be directly invoked to explain the Acceleration of Electrons. Rather, an additional mechanism is required to pre-accelerate the Electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study Electron Acceleration in low Mach number shocks. We focus on the particle energy spectra and the Acceleration mechanism in a reference run with M{sub s} = 3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15% of the Electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p ≅ 2.4. Initially, thermal Electrons are energized at the shock front via shock drift Acceleration (SDA). The accelerated Electrons are then reflected back upstream where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the Electrons propagating upstream back toward the shock for further energizationmore » via SDA. In summary, the self-generated waves allow for repeated cycles of SDA, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.« less
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non thermal Electron Acceleration in low mach number collisionless shocks ii firehose mediated fermi Acceleration and its dependence on pre shock conditions
arXiv: High Energy Astrophysical Phenomena, 2014Co-Authors: Xinyi Guo, Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies is known to occur in low Mach number (M 20) at nearly all magnetic field obliquities, and for Electron temperatures in the range relevant for galaxy clusters. Our findings offer a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.
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non thermal Electron Acceleration in low mach number collisionless shocks i particle energy spectra and Acceleration mechanism
arXiv: High Energy Astrophysical Phenomena, 2014Co-Authors: Xinyi Guo, Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies in low Mach number (M<5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the Electron Acceleration mechanism remains poorly understood. Diffusive shock Acceleration, also known as first-order Fermi Acceleration, cannot be directly invoked to explain the Acceleration of Electrons. Rather, an additional mechanism is required to pre-accelerate the Electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study Electron Acceleration in low Mach number shocks. We focus on the particle energy spectra and the Acceleration mechanism in a reference run with M=3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15 percent of the Electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p~2.4. Initially, thermal Electrons are energized at the shock front via shock drift Acceleration. The accelerated Electrons are then reflected back upstream, where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the Electrons propagating upstream back toward the shock, for further energization via shock drift Acceleration. In summary, the self-generated waves allow for repeated cycles of shock drift Acceleration, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.
Ramesh Narayan - One of the best experts on this subject based on the ideXlab platform.
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non thermal Electron Acceleration in low mach number collisionless shocks i particle energy spectra and Acceleration mechanism
The Astrophysical Journal, 2014Co-Authors: Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies in low Mach number (M{sub s} ≲ 5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the Electron Acceleration mechanism remains poorly understood. Diffusive shock Acceleration, also known as first-order Fermi Acceleration, cannot be directly invoked to explain the Acceleration of Electrons. Rather, an additional mechanism is required to pre-accelerate the Electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study Electron Acceleration in low Mach number shocks. We focus on the particle energy spectra and the Acceleration mechanism in a reference run with M{sub s} = 3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15% of the Electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p ≅ 2.4. Initially, thermal Electrons are energized at the shock front via shock drift Acceleration (SDA). The accelerated Electrons are then reflected back upstream where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the Electrons propagating upstream back toward the shock for further energizationmore » via SDA. In summary, the self-generated waves allow for repeated cycles of SDA, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.« less
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non thermal Electron Acceleration in low mach number collisionless shocks ii firehose mediated fermi Acceleration and its dependence on pre shock conditions
arXiv: High Energy Astrophysical Phenomena, 2014Co-Authors: Xinyi Guo, Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies is known to occur in low Mach number (M 20) at nearly all magnetic field obliquities, and for Electron temperatures in the range relevant for galaxy clusters. Our findings offer a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.
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non thermal Electron Acceleration in low mach number collisionless shocks i particle energy spectra and Acceleration mechanism
arXiv: High Energy Astrophysical Phenomena, 2014Co-Authors: Xinyi Guo, Lorenzo Sironi, Ramesh NarayanAbstract:Electron Acceleration to non-thermal energies in low Mach number (M<5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the Electron Acceleration mechanism remains poorly understood. Diffusive shock Acceleration, also known as first-order Fermi Acceleration, cannot be directly invoked to explain the Acceleration of Electrons. Rather, an additional mechanism is required to pre-accelerate the Electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study Electron Acceleration in low Mach number shocks. We focus on the particle energy spectra and the Acceleration mechanism in a reference run with M=3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15 percent of the Electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p~2.4. Initially, thermal Electrons are energized at the shock front via shock drift Acceleration. The accelerated Electrons are then reflected back upstream, where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the Electrons propagating upstream back toward the shock, for further energization via shock drift Acceleration. In summary, the self-generated waves allow for repeated cycles of shock drift Acceleration, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low Electron Acceleration efficiency usually expected in low Mach number shocks.