Showing papers on "Shock wave published in 2011"

Shock breakout in dense mass loss: luminous supernovae

Abstract: We examine the case where a circumstellar medium around a supernova is sufficiently opaque that a radiation-dominated shock propagates in the circumstellar region The initial propagation of the shock front into the circumstellar region can be approximated by a self-similar solution that determines the radiative energy in a shocked shell; the eventual escape of this energy gives the maximum luminosity of the supernova If the circumstellar density is described by ρ = Dr –2 out to a radius Rw , where D is a constant, the properties of the shock breakout radiation depend on Rw and Rd ≡ κDv sh/c, where κ is the opacity and v sh is the shock velocity If Rw >Rd , the rise to maximum light begins at ~Rd /v sh; the duration of the rise is also ~Rd /v sh; the outer parts of the opaque medium are extended and at low velocity at the time of peak luminosity; and a dense shell forms whose continued interaction with the dense mass loss gives a characteristic flatter portion of the declining light curve If Rw < Rd , the rise to maximum light begins at Rw /v sh; the duration of the rise is R 2 w /v sh Rd ; the outer parts of the opaque medium are not extended and are accelerated to high velocity by radiation pressure at the time of maximum luminosity; and a dense shell forms but does not affect the light curve near maximum We argue that SN 2006gy is an example of the first kind of event, while SN 2010gx and related supernovae are examples of the second

Particle acceleration in relativistic magnetized collisionless electron-ion shocks

Abstract: We investigate shock structure and particle acceleration in relativistic magnetized collisionless electron-ion shocks by means of 2.5-dimensional particle-in-cell simulations with ion-to-electron mass ratios (m i /m e ) ranging from 16 to 1000. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. In "subluminal" shocks, where relativistic particles can escape ahead of the shock along the magnetic field lines, ions are efficiently accelerated via the first-order Fermi process. The downstream ion spectrum consists of a relativistic Maxwellian and a high-energy power-law tail, which contains ~5% of ions and ~30% of ion energy. Its slope is -2.1 ± 0.1. The scattering is provided by short-wavelength non-resonant modes produced by Bell's instability, whose growth is seeded by the current of shock-accelerated ions that propagate ahead of the shock. Upstream electrons enter the shock with lower energy than ions (albeit by only a factor of ~5 << m i /m e ), so they are more strongly tied to the field. As a result, only ~ 1% of the incoming electrons are accelerated at the shock before being advected downstream, where they populate a steep power-law tail (with slope -3.5 ± 0.1). For "superluminal" shocks, where relativistic particles cannot outrun the shock along the field, the self-generated turbulence is not strong enough to permit efficient Fermi acceleration, and the ion and electron downstream spectra are consistent with thermal distributions. The incoming electrons are heated up to equipartition with ions, due to strong electromagnetic waves emitted by the shock into the upstream. Thus, efficient electron heating (≳15% of the upstream ion energy) is the universal property of relativistic electron-ion shocks, but significant nonthermal acceleration of electrons (≳2% by number, ≳10% by energy, with slope flatter than -2.5) is hard to achieve in magnetized flows and requires weakly magnetized shocks (magnetization σ ≲ 10- 3 ), where magnetic fields self-generated via the Weibel instability are stronger than the background field. These findings place important constraints on the models of gamma-ray bursts and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized electron-ion shocks.

Acceleration of particles at the termination shock of a relativistic striped wind

Abstract: The relativistic wind of obliquely rotating pulsars consists of toroidal stripes of opposite magnetic field polarity, separated by current sheets of hot plasma. By means of two- and three-dimensional particle-in-cell simulations, we investigate particle acceleration and magnetic field dissipation at the termination shock of a relativistic striped wind. At the shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength λ or the wind magnetization σ (in the regime σ 1 of magnetically dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles, whose average Lorentz factor increases by a factor of σ with respect to the pre-shock value. The shape of the post-shock spectrum depends primarily on the ratio λ/(rL σ), where rL is the relativistic Larmor radius in the wind. The spectrum becomes broader as the value of λ/(rL σ) increases, passing from a relativistic Maxwellian to a flat power-law tail with slope around –1.5, populated by particles accelerated by the reconnection electric field. Close to the equatorial plane of the wind, where the stripes are symmetric, the highest energy particles resulting from magnetic reconnection can escape ahead of the shock, and be injected into a Fermi-like acceleration process. In the post-shock spectrum, they populate a power-law tail with slope around –2.5, which extends beyond the flat component produced by reconnection. Our study suggests that the spectral break between the radio and the optical band in Pulsar Wind Nebulae can be a natural consequence of particle acceleration at the termination shock of striped pulsar winds.

Numerical investigation of the physics of rotating-detonation-engines

Abstract: Rotating-detonation-engines (RDE’s) represent an alternative to the extensively studied pulse-detonation-engines (PDE’s) for obtaining propulsion from the high efficiency detonation cycle. Since it has received considerably less attention, the general flow-field and effect of parameters such as stagnation conditions and back pressure on performance are less well understood than for PDE’s. In this article we describe results from time-accurate calculations of RDE’s using algorithms that have successfully been used for PDE simulations previously. Results are obtained for stoichiometric hydrogen–air RDE’s operating at a range of stagnation pressures and back pressures. Conditions within the chamber are described as well as inlet and outlet conditions and integrated quantities such as total mass flow, force, and specific impulse. Further computations examine the role of inlet stagnation pressure and back pressure on detonation characteristics and engine performance. The pressure ratio is varied between 2.5 and 20 by varying both stagnation and back pressure to isolate controlling factors for the detonation and performance characteristics. It is found that the detonation wave height and mass flow rate are determined primarily by the stagnation pressure, whereas overall performance is closely tied to pressure ratio. Specific impulses are calculated for all cases and range from 2872 to 5511 s, and are lowest for pressure ratios below 4. The reason for performance loss is shown to be associated with the secondary shock wave structure that sets up in the expansion portion of the RDE, which strongly effects the flow at low pressure ratios. Expansion to supersonic flow behind the detonation front in RDE’s with higher pressure ratios isolate the detonation section of the RDE and thus limit the effect of back pressure on the detonation characteristics.

Cavitation clouds created by shock scattering from bubbles during histotripsy.

Abstract: Histotripsy is a therapy that focuses short-duration, high-amplitude pulses of ultrasound to incite a localized cavitation cloud that mechanically breaks down tissue. To investigate the mechanism of cloud formation, high-speed photography was used to observe clouds generated during single histotripsy pulses. Pulses of 5−20 cycles duration were applied to a transparent tissue phantom by a 1-MHz spherically focused transducer. Clouds initiated from single cavitation bubbles that formed during the initial cycles of the pulse, and grew along the acoustic axis opposite the propagation direction. Based on these observations, we hypothesized that clouds form as a result of large negative pressure generated by the backscattering of shockwaves from a single bubble. The positive-pressure phase of the wave inverts upon scattering and superimposes on the incident negative-pressure phase to create this negative pressure and cavitation. The process repeats with each cycle of the incident wave, and the bubble cloud elongates toward the transducer. Finite-amplitude propagation distorts the incident wave such that the peak-positive pressure is much greater than the peak-negative pressure, which exaggerates the effect. The hypothesis was tested with two modified incident waves that maintained negative pressure but reduced the positive pressure amplitude. These waves suppressed cloud formation which supported the hypothesis.

Shock Wave-Boundary-Layer Interactions

Abstract: 1. Introduction John K. Harvey and Holger Babinsky 2. Physical introduction Jean Delery 3. Transonic shock wave boundary layer interactions Holger Babinsky and Jean Delery 4. Ideal gas shock wave turbulent boundary layer interactions in supersonic flows and their modeling - two dimensional interactions Alexander A. Zheltovodov and Doyle D. Knight 5. Ideal gas shock wave turbulent boundary layer interactions in supersonic flows and their modeling - three dimensional interactions Doyle D. Knight and Alexander A. Zheltovodov 6. Experimental studies of shock wave/boundary layer interactions in hypersonic flows Michael S. Holden 7. Numerical simulation of hypersonic shock wave boundary layer interactions Graham V. Candler 8. Shock wave/boundary layer interactions taking place in hypersonic flows John K. Harvey 9. Shock wave unsteadiness in turbulent shock wave boundary layer interactions P. Dupont, J. F. Debieve and J. P. Dussauge 10. Analytical treatment of shock/boundary layer interactions George Inger.

Shock-Bubble Interactions

Abstract: When a shock wave propagates through a medium of nonuniform thermodynamic properties, several processes occur simultaneously that alter the geometry of the shock wave and the thermodynamic state of the medium. These include shock compression and acceleration of the medium, refraction of the shock, and vorticity generation within the medium. The interaction of a shock wave with a cylinder or a sphere (both referred to as a bubble in this review) is the simplest configuration in which all these processes take place and can be studied in detail. Shock acceleration leads to an initial compression and distortion of the bubble, followed by the formation of a vortex pair in the two-dimensional (2D) case and a vortex ring in the 3D case. At later times, for appropriate combinations of the incident shock strength and density contrast between the bubble and ambient materials, secondary vortices are formed, mass is stripped away from the original bubble, and mixing of the bubble and ambient fluids occurs.

Particle tracer response across shocks measured by PIV

Abstract: The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of the particle response time τ p and length ξ p based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution. Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation time up to τ p and interrogation window up to ξ p still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from 2 to 3 μs, corresponding to values commonly reported in literature, to almost 0.3 μs when particles are properly dehydrated and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with a measured response time lower than 0.4 μs. The same procedure is applied to attempt the measurement of individual particle tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the particle velocity distribution.

Observations and interpretation of a low coronal shock wave observed in the EUV by the SDO/AIA

Abstract: Taking advantage of both the high temporal and spatial resolutions of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we studied a limb coronal shock wave and its associated extreme ultraviolet (EUV) wave that occurred on 2010 June 13. Our main findings are: (1) the shock wave appeared clearly only in the channels centered at 193 angstrom and 211 angstrom as a dome-like enhancement propagating ahead of its associated semi-spherical coronal mass ejection (CME) bubble; (2) the density compression of the shock is 1.56 according to radio data and the temperature of the shock is around 2.8 MK; (3) the shock wave first appeared at 05: 38 UT, 2 minutes after the associated flare has started and 1 minute after its associated CME bubble appeared; (4) the top of the dome-like shock wave set out from about 1.23 R-circle dot and the thickness of the shocked layer is similar to 2 x 10(4) km; (5) the speed of the shock wave is consistent with a slight decrease from about 600 km s(-1) to 550 km s(-1); and (6) the lateral expansion of the shock wave suggests a constant speed around 400 km s(-1), which varies at different heights and directions. Our findings support the view that the coronal shock wave is driven by the CME bubble, and the on-limb EUV wave is consistent with a fast wave or at least includes the fast wave component.

The density variance-mach number relation in supersonic, isothermal turbulence

Abstract: We examine the relation between the density variance and the mean-square Mach number in supersonic, isothermal turbulence, assumed in several recent analytic models of the star formation process. From a series of calculations of supersonic, hydrodynamic turbulence driven using purely solenoidal Fourier modes, we find that the ‘standard’ relationship between the variance in th e log of density and the Mach number squared, i.e., � 2 ln�/¯ = ln 1 + b 2 M 2 � , with b = 1/3 is a good fit to the numerical results in the supersonic regime up to at least Mach 20, similar to previous determinations at lower Mach numbers. While direct measurements of the variance in linear density are found to be severely under estimated by finite resolution effects, it is possible to infer the linear density variance via the assumption of lo g-normality in the Probability Distribution Function. The inferred relationship with Mach number, consistent with ��/¯ � � bM with b = 1/3, is, however, significantly shallower than observational determination s of the relationship in the Taurus Molecular Cloud and IC5146 (both consistent with b � 0.5), implying that additional physics such as gravity is impor tant in these clouds and/or that turbulent driving in the ISM contai ns a significant compressive component. Magnetic fields are not found to change this picture significantly, in g eneral reducing the measured variances and thus worsening the discrepancy with observations. Subject headings: turbulence — ISM: structure — hydrodynamics — stars: formation — magnetohydrodynamics (MHD) — shock waves

Toward Understanding the Cosmic-ray Acceleration at Young Supernova Remnants Interacting with Interstellar Clouds: Possible Applications to RX J1713.7-3946

Abstract: Using three-dimensional magnetohydrodynamics simulations, we investigate general properties of a blast wave shock interacting with interstellar clouds. The shock-cloud interaction generates a turbulent shell through the vorticity generations. In the turbulent shell, the magnetic field is amplified as a result of turbulent dynamo action. In the case of a young supernova remnant, the corresponding strength of the magnetic field is approximately 1 mG. The propagation speed of the shock wave is significantly stalled in the clouds because of the high density, while the shock maintains a high velocity in the diffuse surrounding. In addition, when the shock wave hits the clouds, reflection shock waves are generated that propagate back into the shocked shell. From these simulation results, many observational characteristics of a young SNR RX J1713.7-3946 that is suggested to be interacting with molecular clouds, can be explained as follows: The reflection shocks can accelerate particles in the turbulent downstream region where the magnetic field strength reaches 1mG, which causes short-time variability of synchrotron X-rays. Since the shock velocity is stalled locally in the clouds, the temperature in the shocked cloud is suppressed below 1 keV. Thus, thermal X-ray line emissions would be faint even if the SNR is interacting with molecular clouds. We also find that the photon index of the $\pi^0$-decay gamma rays generated by cosmic-ray protons can be 1.5, because the penetration depth of high-energy particles into the clumpy clouds depends on their energy, which is consistent with the recent gamma-ray observation by Fermi space telescope.

Low-order stochastic modelling of low-frequency motions in reflected shock-wave/boundary-layer interactions

Abstract: A combined numerical and analytical approach is used to study the low-frequency shock motions observed in shock/turbulent-boundary-layer interactions in the particular case of a shock-reflection configuration. Starting from an exact form of the momentum integral equation and guided by data from large-eddy simulations, a stochastic ordinary differential equation for the reflected-shock-foot low-frequency motions is derived. During the derivation a similarity hypothesis is verified for the streamwise evolution of boundary-layer thickness measures in the interaction zone. In its simplest form, the derived governing equation is mathematically equivalent to that postulated without proof by Plotkin (AIAA J., vol. 13, 1975, p. 1036). In the present contribution, all the terms in the equation are modelled, leading to a closed form of the system, which is then applied to a wide range of input parameters. The resulting map of the most energetic low-frequency motions is presented. It is found that while the mean boundary-layer properties are important in controlling the interaction size, they do not contribute significantly to the dynamics. Moreover, the frequency of the most energetic fluctuations is shown to be a robust feature, in agreement with earlier experimental observations. The model is proved capable of reproducing available low-frequency experimental and numerical wall-pressure spectra. The coupling between the shock and the boundary layer is found to be mathematically equivalent to a first-order low-pass filter. It is argued that the observed low-frequency unsteadiness in such interactions is not necessarily a property of the forcing, either from upstream or downstream of the shock, but an intrinsic property of the coupled system, whose response to white-noise forcing is in excellent agreement with actual spectra.

A Shock Front in the Merging Galaxy Cluster A754: X-ray and Radio Observations

Abstract: We present new Chandra X-ray and Giant Metrewave Radio Telescope (GMRT) radio observations of the nearby merging galaxy cluster A754. Our X-ray data confirm the presence of a shock front by obtaining the first direct measurement of a gas-temperature jump across the X-ray brightness edge previously seen in the imaging data. A754 is only the fourth galaxy cluster with confirmed merger shock fronts, and it has the weakest shock of those, with a Mach number, M = 1.57+0.16 ?0.12. In our new GMRT observation at 330?MHz, we find that the previously known centrally located radio halo extends eastward to the position of the shock. The X-ray shock front also coincides with the position of a radio relic previously observed at 74?MHz. The radio spectrum of the post-shock region, using our radio data and the earlier results at 74?MHz and 1.4?GHz, is very steep. We argue that acceleration of electrons at the shock front directly from thermal to ultrarelativistic energies is problematic due to energy arguments, while reacceleration of pre-existing relativistic electrons is more plausible.

Mach 5 bow shock control by a nanosecond pulse surface dielectric barrier discharge

Abstract: Bow shock perturbations in a Mach 5 air flow, produced by low-temperature, nanosecond pulse, and surface dielectric barrier discharge (DBD), are detected by phase-locked schlieren imaging. A diffuse nanosecond pulse discharge is generated in a DBD plasma actuator on a surface of a cylinder model placed in air flow in a small scale blow-down supersonic wind tunnel. Discharge energy coupled to the actuator is 7.3–7.8 mJ/pulse. Plasma temperature inferred from nitrogen emission spectra is a few tens of degrees higher than flow stagnation temperature, T = 340 ± 30 K. Phase-locked Schlieren images are used to detect compression waves generated by individual nanosecond discharge pulses near the actuator surface. The compression wave propagates upstream toward the baseline bow shock standing in front of the cylinder model. Interaction of the compression wave and the bow shock causes its displacement in the upstream direction, increasing shock stand-off distance by up to 25%. The compression wave speed behind the...

The Strength and Radial Profile of the Coronal Magnetic Field from the Standoff Distance of a Coronal Mass Ejection-Driven Shock

Abstract: We determine the coronal magnetic field strength in the heliocentric distance range 6-23 solar radii (Rs) by measuring the shock standoff distance and the radius of curvature of the flux rope during the 2008 March 25 coronal mass ejection imaged by white-light coronagraphs. Assuming the adiabatic index, we determine the Alfven Mach number, and hence the Alfven speed in the ambient medium using the measured shock speed. By measuring the upstream plasma density using polarization brightness images, we finally get the magnetic field strength upstream of the shock. The estimated magnetic field decreases from approximately 48 mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field can be described by a power law in agreement with other estimates at similar heliocentric distances.

Electron injection by whistler waves in non-relativistic shocks

Abstract: Electron acceleration to non-thermal, ultra-relativistic energies (~10-100 TeV) is revealed by radio and X-ray observations of shocks in young supernova remnants (SNRs). The diffusive shock acceleration (DSA) mechanism is usually invoked to explain this acceleration, but the way in which electrons are initially energized or "injected" into this acceleration process starting from thermal energies is an unresolved problem. In this paper we study the initial acceleration of electrons in non-relativistic shocks from first principles, using two- and three-dimensional particle-in-cell (PIC) plasma simulations. We systematically explore the space of shock parameters (the Alfvenic Mach number, MA , the shock velocity, v sh, the angle between the upstream magnetic field and the shock normal, θ Bn , and the ion to electron mass ratio, mi /me ). We find that significant non-thermal acceleration occurs due to the growth of oblique whistler waves in the foot of quasi-perpendicular shocks. This acceleration strongly depends on using fairly large numerical mass ratios, mi /me , which may explain why it had not been observed in previous PIC simulations of this problem. The obtained electron energy distributions show power-law tails with spectral indices up to α ~ 3-4. The maximum energies of the accelerated particles are consistent with the electron Larmor radii being comparable to that of the ions, indicating potential injection into the subsequent DSA process. This injection mechanism, however, requires the shock waves to have fairly low Alfenic Mach numbers, MA 20, which is consistent with the theoretical conditions for the growth of whistler waves in the shock foot (MA (mi /me )1/2). Thus, if the whistler mechanism is the only robust electron injection process at work in SNR shocks, then SNRs that display non-thermal emission must have significantly amplified upstream magnetic fields. Such field amplification is likely achieved by the escaping cosmic rays, so electron and proton acceleration in SNR shocks must be interconnected.

Jet propagations, breakouts and photospheric emissions in collapsing massive progenitors of long duration gamma ray bursts

Abstract: We investigate the following by two-dimensional axisymmetric relativistic hydrodynamical simulations: (1) jet propagations through an envelope of a rapidly rotating and collapsing massive star, which is supposed to be a progenitor of long-duration gamma-ray bursts (GRBs); (2) breakouts and subsequent expansions into stellar winds; and (3) the accompanying photospheric emissions. We find that if the envelope rotates uniformly almost at the mass shedding limit, its outer part eventually stops contracting when the centrifugal force becomes large enough. Then another shock wave is formed, propagates outward, and breaks out of the envelope into the stellar wind. Whether the jet or the centrifugal bounce-induced shock breaks out earlier depends on the timing of jet injection. If the shock breakout occurs earlier, owing to a later injection, the jet propagation and subsequent photospheric emissions are affected substantially. We pay particular attention to observational consequences of the difference in the timing of jet injection. We calculate optical depths to find the location of photospheres, extracting densities, and temperatures at appropriate retarded times from the hydrodynamical data. We show that the luminosity and observed temperature of the photospheric emissions are both much lower than those reported in previous studies. Although luminosities are still high enough for GRBs, the observed temperatures are lower than the energy at the spectral peak expected by the Yonetoku relation. This may imply that energy exchanges between photons and matter are terminated deeper inside or that some non-thermal processes are operating to boost photon energies.

Numerical modeling of footpoint-driven magneto-acoustic wave propagation in a localized solar flux tube

Abstract: In this paper, we present and discuss results of two-dimensional simulations of linear and nonlinear magneto-acoustic wave propagation through an open magnetic flux tube embedded in the solar atmosphere expanding from the photosphere through to the transition region and into the low corona. Our aim is to model and analyze the response of such a magnetic structure to vertical and horizontal periodic motions originating in the photosphere. To carry out the simulations, we employed our MHD code SAC (Sheffield Advanced Code). A combination of the VALIIIC and McWhirter solar atmospheres and coronal density profiles were used as the background equilibrium model in the simulations. Vertical and horizontal harmonic sources, located at the footpoint region of the open magnetic flux tube, are incorporated in the calculations, to excite oscillations in the domain of interest. To perform the analysis we have constructed a series of time-distance diagrams of the vertical and perpendicular components of the velocity with respect to the magnetic field lines at each height of the computational domain. These time-distance diagrams are subject to spatio-temporal Fourier transforms allowing us to build ω-k dispersion diagrams for all of the simulated regions in the solar atmosphere. This approach makes it possible to compute the phase speeds of waves propagating throughout the various regions of the solar atmosphere model. We demonstrate the transformation of linear slow and fast magneto-acoustic wave modes into nonlinear ones, i.e., shock waves, and also show that magneto-acoustic waves with a range of frequencies efficiently leak through the transition region into the solar corona. It is found that the waves interact with the transition region and excite horizontally propagating surface waves along the transition region for both types of drivers. Finally, we estimate the phase speed of the oscillations in the solar corona and compare it with the phase speed derived from observations.

Three-dimensional Simulations of Magnetohydrodynamic Turbulence Behind Relativistic Shock Waves and Their Implications for Gamma-Ray Bursts

Abstract: Relativistic astrophysical phenomena such as gamma-ray bursts (GRBs) and active galactic nuclei often require long-lived strong magnetic fields that cannot be achieved by shock compression alone. Here, we report on three-dimensional special-relativistic magnetohydrodynamic (MHD) simulations that we performed using a second-order Godunov-type conservative code to explore the amplification and decay of macroscopic turbulence dynamo excited by the so-called Richtmyer-Meshkov instability (RMI; a Rayleigh-Taylor-type instability). This instability is an inevitable outcome of interactions between shock and ambient density fluctuations. We find that the magnetic energy grows exponentially in a few eddy-turnover times because of field-line stretching and then, following the decay of kinetic turbulence, decays with a temporal power-law exponent of ?0.7. The magnetic energy fraction can reach B ~ 0.1 but depends on the initial magnetic field strength, which can diversify the observed phenomena. We find that the magnetic energy grows by at least two orders of magnitude compared to the magnetic energy immediately behind the shock, provided the kinetic energy of turbulence injected by the RMI is greater than the magnetic energy. This minimum degree of amplification does not depend on the amplitude of the initial density fluctuations, while the growth timescale and the maximum magnetic energy depend on the degree of inhomogeneity in the density. The transition from Kolmogorov cascade to MHD critical balance cascade occurs at ~1/10th the initial inhomogeneity scale, which limits the maximum synchrotron polarization to less than ~2%. We derive analytical formulas for these numerical results and apply them to GRBs. New results include the avoidance of electron cooling with RMI turbulence, the turbulent photosphere model via RMI, and the shallow decay of the early afterglow from RMI. We also perform a simulation of freely decaying turbulence with relativistic velocity dispersion. We find that relativistic turbulence begins to decay much more quickly than one eddy-turnover time because of rapid shock dissipation, which does not support the relativistic turbulence model by Narayan & Kumar.

Realistic and time-varying outer heliospheric modelling

Abstract: We develop a realistic and time-varying model that satisfies both the Voyager 1 (V1) and Voyager 2 (V2) observed crossing times and locations of the termination shock (TS) simultaneously by performing three-dimensional (3D) magnetohydrodynamic (MHD) simulations using a total variation diminishing (TVD) scheme that includes the effects of neutral particles. Daily values of solar-wind speed and density observed by V2 are used at every simulation step so that short-term dynamical effects are reproduced. Before performing the dynamic simulation, we generate a 3D stationary solution using a set of standard parameters: the interstellar proton density is assumed to be 0.061 cc−1, the interstellar neutral hydrogen density 0.176 cc−1, the interstellar medium speed relative to the Sun 26.3 km s−1 and a temperature of 6300 K. The interstellar magnetic field intensity is 0.3 nT and the orientation is oblique to the flow direction lying in the hydrogen deflection plane. The anisotropy of the solar-wind speed is also taken into account using 400 km s−1 at low latitudes and 1.5 times faster at high latitudes. A solar-wind density at 1 au of 3.55 cc−1 and a latitudinal angle separating the high/low solar-wind speed regions of 80° yield dynamic solutions, starting from the stationary solution, that satisfy the V1 and V2 crossing times and locations. Our simulations clearly show that (i) the TS position increases whenever a solar-wind high-ram-pressure pulse collides with the TS; (ii) a large-amplitude magneto-sonic pulse is generated downstream of the TS when a solar-wind high-ram-pressure pulse collides with the TS; (iii) fine structure in the heliosheath is present, corresponding to a magnetic wall and plasma sheet near the heliopause (HP), and a thin current sheet; (iv) the magneto-sonic pulse is reflected at the plasma sheet in the heliosheath, and the TS position decreases when the reflected pulse collides with the TS and (v) the time-varying radial distance between V2 and simulated TS positions appears to be consistent with the TS particle intensity profile during the period when the TS particles were observed at V2. This last point, together with the overwhelming consistency of our MHD results with a standard set of solar-wind and local interstellar medium (LISM) parameters, suggests that the outer heliosphere is controlled essentially by MHD and neutral interstellar gas processes, although V2 observations of solar-wind plasma in the heliosheath are not well reproduced in our simulations.

Off-limb solar coronal wavefronts from sdo/aia extreme-ultraviolet observations-implications for particle production

Abstract: We derive kinematic properties for two recent solar coronal transient waves observed off the western solar limb with the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) mission. The two waves occurred over ~10 minute intervals on consecutive days—2010 June 12 and 13. For the first time, off-limb waves are imaged with a high 12 s cadence, making possible detailed analysis of these transients in the low corona between ~1.1 and 2.0 solar radii (RS ). We use observations in the 193 and 211 A AIA channels to constrain the kinematics of both waves. We obtain initial velocities for the two fronts of ~1287 and ~736 km s–1, and accelerations of –1170 and –800 m s–2, respectively. Additionally, differential emission measure analysis shows the June 13 wave is consistent with a weak shock. Extreme-ultraviolet (EUV) wave positions are correlated with positions from simultaneous type II radio burst observations. We find good temporal and height association between the two, suggesting that the waves may be the EUV signatures of coronal shocks. Furthermore, the events are associated with significant increases in proton fluxes at 1 AU, possibly related to how waves propagate through the coronal magnetic field. Characterizing these coronal transients will be key to connecting their properties with energetic particle production close to the Sun.

Interpreting the properties of solar energetic particle events by using combined imaging and modeling of interplanetary shocks

Abstract: Images of the solar corona obtained by the Solar-Terrestrial Relations Observatory (STEREO) provide high-cadence, high-resolution observations of a compression wave forming ahead of a fast (940 km s{sup -1}) coronal mass ejection (CME) that erupted at {approx}9:00 UT on 2010 April 03. The passage of this wave at 1 AU is detected in situ by the Advanced Composition Explorer and Wind spacecraft at 08:00 UT on April 05 as a shock followed by a turbulent and heated sheath. These unprecedented and complementary observations of a shock-sheath region from the Sun to 1 AU are used to investigate the onset of a Solar Energetic Particle (SEP) event measured at the first Lagrange point (L1) and at STEREO-Behind (STB). The spatial extent, radial coordinates, and speed of the ejection are measured from STEREO observations and used as inputs to a numerical simulation of the CME propagation in the background solar wind. The simulated magnetic and plasma properties of the shock and sheath region at L1 agree very well with the in situ measurements. These simulation results reveal that L1 and STB are magnetically connected to the western and eastern edges of the driven shock, respectively. They also show that the 12 hrmore » delay between the eruption time of the ejection and the SEP onset at L1 corresponds to the time required for the bow shock to reach the magnetic field lines connected with L1. The simulated shock compression ratio increases along these magnetic field lines until the maximum flux of high-energy particles is observed.« less

Re-acceleration of non-thermal particles at weak cosmological shock waves

Abstract: We examine diffusive shock acceleration (DSA) of the pre-existing as well as freshly injected populations of non-thermal, cosmic-ray (CR) particles at weak cosmological shocks. Assuming simple models for thermal leakage injection and Alfvenic drift, we derive analytic, time-dependent solutions for the two populations of CRs accelerated in the test-particle regime. We then compare them with the results from kinetic DSA simulations for shock waves that are expected to form in intracluster media and cluster outskirts in the course of large-scale structure formation. We show that the test-particle solutions provide a good approximation for the pressure and spectrum of CRs accelerated at these weak shocks. Since the injection is extremely inefficient at weak shocks, the pre-existing CR population dominates over the injected population. If the pressure due to pre-existing CR protons is about 5% of the gas thermal pressure in the upstream flow, the downstream CR pressure can absorb typically a few to 10% of the shock ram pressure at shocks with a Mach number M 3, yet the re-acceleration of CR electrons can result in a substantial synchrotron emission behind the shock. The enhancement in synchrotron radiation across the shock is estimated to be about a few to several for M ~ 1.5 and 102-103 for M ~ 3, depending on the detail model parameters. The implication of our findings for observed bright radio relics is discussed.

Corner effect and separation in transonic channel flows

Abstract: An investigation into parameters affecting separation in normal shock wave/boundary layer interactions (SBLIs) has been conducted. It has been shown that the effective aspect ratio of an experimental facility (defined as δ * /tunnel width) is a critical factor in determining when shock-induced separation will occur. Experiments examining M ∞ =1.4 and 1.5 normal shock waves in a wind tunnel with a small rectangular cross-section have been performed and show that a link exists between the extent of shock-induced separation on the tunnel centre-line and the size of corner-flow separations. In tests where the corner-flows were modified ahead of the shock (through suction and vortex generators), the extent of separation around the tunnel centre-line was seen to vary significantly. These observations are attributed to the way corner flows modify the three-dimensional shock-structure and the impact this has on the magnitude of the adverse pressure gradient experienced by the tunnel wall boundary layers.