Pair Of Shocks Research Articles

The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. II. Theory

Simulations of collisionless oblique propagating slow shocks have revealed the existence of a transition associated with a critical temperature anisotropy ɛ = 1 − μ0(P|| − P⊥)/B2 = 0.25 (Y.-H. Liu, J. F. Drake, and M. Swisdak, Phys. Plasmas 18, 062110 (2011)). An explanation for this phenomenon is proposed here based on anisotropic fluid theory, in particular, the anisotropic derivative nonlinear-Schrödinger-Burgers equation, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes and corresponds to the lower bound of the coplanar to non-coplanar transition that occurs inside a compound slow shock (SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of compound SS/RD waves that bound the outflows in magnetic reconnection, instead of a pair of switch-off slow shocks as in Petschek’s model. This fact might explain the rareness of in-situ observations of Petschek-reconnection-associated switch-off slow shocks.

Thus Spake Nano

his dream of building a "1-lakh," or 100,000 rupee (roughly $2,230) car, the first Tata Nano rolled on stage in March 2009 accompanied by the fanfare from Richard Strauss 's Thus Spake Zarathustra. Ten-feet-long and egg-shaped, with a roomy interior but dinner-plate-sized wheels and a lawnmower engine, the Nano gets some 56 miles per gallon on par with the electric hybrid Chevrolet Volt. And its emissions are considerably lower than the typical Indian motorbike or auto rickshaw, which both spew noxious black exhaust.

Evolution of symmetric reconnection layer in the presence of parallel shear flow

The development of the structure of symmetric reconnection layer in the presence of a shear flow parallel to the antiparallel magnetic field component is studied by using a set of one-dimensional (1D) magnetohydrodynamic (MHD) equations. The Riemann problem is simulated through a second-order conservative TVD (total variation diminishing) scheme, in conjunction with Roe’s averages for the Riemann problem. The simulation results indicate that besides the MHD shocks and expansion waves, there exist some new small-scale structures in the reconnection layer. For the case of zero initial guide magnetic field (i.e., By0 = 0), a pair of intermediate shock and slow shock (SS) is formed in the presence of the parallel shear flow. The critical velocity of initial shear flow Vzc is just the Alfven velocity in the inflow region. As Vz∞ increases to the value larger than Vzc, a new slow expansion wave appears in the position of SS in the case Vz∞ < Vzc, and one of the current densities drops to zero. As plasma β increases, the out-flow region is widened. For By0 ≠ 0, a pair of SSs and an additional pair of time-dependent intermediate shocks (TDISs) are found to be present. Similar to the case of By0 = 0, there exists a critical velocity of initial shear flow Vzc. The value of Vzc is, however, smaller than the Alfven velocity of the inflow region. As plasma β increases, the velocities of SS and TDIS increase, and the out-flow region is widened. However, the velocity of downstream SS increases even faster, making the distance between SS and TDIS smaller. Consequently, the interaction between SS and TDIS in the case of high plasma β influences the property of direction rotation of magnetic field across TDIS. Thereby, a wedge in the hodogram of tangential magnetic field comes into being. When β → ∞, TDISs disappear and the guide magnetic field becomes constant.

Magnetohydrodynamic study of shock waves in the current sheet of a solar coronal magnetic loop

A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind.

Structure and dynamics of the fast reconnection mechanism in an initially force-free current sheet

The present paper studies the three-dimensional structure and dynamics of the fast reconnection mechanism in an initially force-free current sheet. A large-scale plasmoid (current sheet bulge) is formed ahead of the Alfvenic fast reconnection jet (ux∼VA) generated in a narrow wedgelike region between a pair of slow shocks. The plasmoid structure is divided into the plasmoid reconnection region P and the plasmoid core region C. In the region P, the strongly sheared reconnected field lines are accumulated and the initial (low-beta) plasma pressure is remarkably enhanced to become comparable to the ambient magnetic pressure since the sheared field lines initially embedded in the current sheet are completely swept away by the reconnection jet. On the other hand, in the region C, the magnetized (low-beta) plasma with the sheared (Bz) field lines, initially embedded in the current sheet, is accumulated without reconnection, and the magnitude of the accumulated sheared field becomes much larger than the ambient magnetic field strength. It is demonstrated that the fast reconnection mechanism in an initially force-free current sheet is so powerful to overcome the magnetic tension forces, which result from the large x-directional bent of the sheared field lines, and vitally proceed by effectively extending the fast reconnection jet region in the sheared field (z) direction.

Shock-wave solutions in two-layer channel flow. I. One-dimensional flows

We study the dynamics of an interface separating two immiscible layers in an inclined channel. Lubrication theory is used to derive an evolution equation for the interface position that models the two-dimensional flow in both co- and countercurrent configurations. This equation is parameterized by viscosity and density ratios, and a total dimensionless flow rate; the system is further characterized by the height of the interface at the channel inlet and outlet, which are treated as additional parameters. In the present work, which corresponds to part I of a two-part paper, we focus on one-dimensional flows. We use an entropy-flux analysis to delineate the existence of various types of shocklike solutions, which include compressive Lax shocks, pairs of Lax and under-compressive shocks, and rarefaction waves. Flows characterized by unstably stratified layers are accompanied by the formation of propagating, large-amplitude interfacial waves, which are not shocklike in nature. The results of our transient numerical simulations accord with our analytical predictions and elucidate the mechanisms underlying spatio-temporal development of the various types of waves; the stability of these waves to spanwise perturbations is investigated in part II of this work.

Analysis and study of the in situ observation of the June 1st 2008 CME by STEREO

In this work we present a combined study of the counterpart of the coronal mass ejection (CME) of June 1st of 2008 in the interplanetary medium. This event has been largely studied because of its peculiar initiation and its possible forecasting consequences for space weather. We show an in situ analysis (on days June 6th–7th of 2008) of the CME in the interplanetary medium in order to shed some light on the propagation and evolution mechanisms of the interplanetary CME (ICME). The goals of this work are twofold: gathering the whole in situ data from PLASTIC and IMPACT onboard STEREO B in order to provide a complete characterization of the ICME, and to present a model where the thermal plasma pressure is included. The isolated ICME features show a clear forward shock which we identify as an oblique forward fast shock accelerating ions to a few-hundred keV during its passage. Following the shock, a flux rope is easily defined as a magnetic cloud (MC) by the magnetic field components and magnitude, and the low proton plasma-β. During the spacecraft passage through the MC, the energetic ion intensity shows a pronounced decrease, suggesting a closed magnetic topology, and the suprathermal electron population shows a density and temperature increase, demonstrating the importance of the electrons in the MC description. The in situ evidence suggests that there is no direct magnetic connection between the forward shock and the MC, and the characteristics of the reverse shock determined suggest that the shock pair is a consequence of the propagation of the ICME in the interplanetary medium. The energetic ions measured by the SEPT instrument suggest that their enhancement is not related to any solar event, but is solely due to the interplanetary shock consistent with the fact that no flares are observed on the Sun. The changes in the polarity of the interplanetary magnetic field in the vicinity of the ICME observed by electron PADs from SWEA are in accordance with the idea that the CME originated along a neutral line over the quiet Sun. The magnetic cloud model presented in this work provides the plasma pressure as a new factor to consider in the study of the expansion and evolution of CMEs in the interplanetary medium. This model could provide a new understanding of the Sun–Earth connection because of the important role that the plasma plays in the eruption of the CME in the solar corona and the reconnection process carried out with the Earth’s magnetosphere.

MODELING UV AND X-RAY EMISSION IN A POST-CORONAL MASS EJECTION CURRENT SHEET

A post-CME current sheet (CS) is a common feature developed behind an erupting flux rope in CME models. Observationally, white light observations have recorded many occurrences of a thin ray appearing behind a CME eruption that closely resembles a post-CME CS in its spatial correspondence and morphology. UV and X-ray observations further strengthen this interpretation by the observations of high temperature emission at locations consistent with model predictions. The next question then becomes whether the properties inside a post-CME CS predicted by a model agree with observed properties. In this work, we assume that the post-CME CS is a consequence of Petschek-like reconnection and that the observed ray-like structure is bounded by a pair of slow mode shocks developed from the reconnection site. We perform time-dependent ionization calculations and model the UV line emission. We find that such a model is consistent with SOHO/UVCS observations of the post-CME CS. The change of Fe XVIII emission in one event implies an inflow speed of ~10 km/s and a corresponding reconnection rate of M_A ~ 0.01. We calculate the expected X-ray emission for comparison with X-ray observations by Hinode/XRT, as well as the ionic charge states as would be measured in-situ at 1 AU. We find that the predicted count rate for Hinode/XRT agree with what was observed in a post-CME CS on April 9, 2008, and the predicted ionic charge states are consistent with high ionization states commonly measured in the interplanetary CMEs. The model results depend strongly on the physical parameters in the ambient corona, namely the coronal magnetic field, the electron density and temperature during the CME event. It is crucial to obtain these ambient coronal parameters and as many facets of the CS properties as possible by observational means so that the post-CME current sheet models can be scrutinized more effectively.

HYSTERESIS OF BACKFLOW IMPRINTED IN COLLIMATED JETS

We report two different types of backflow from jets by performing 2D special relativistic hydrodynamical simulations. One is anti-parallel and quasi-straight to the main jet (quasi-straight backflow), and the other is bent path of the backflow (bent backflow). We find that the former appears when the head advance speed is comparable to or higher than the local sound speed at the hotspot while the latter appears when the head advance speed is slower than the sound speed bat the hotspot. Bent backflow collides with the unshocked jet and laterally squeezes the jet. At the same time, a pair of new oblique shocks are formed at the tip of the jet and new bent fast backflows are generated via these oblique shocks. The hysteresis of backflow collisions is thus imprinted in the jet as a node and anti-node structure. This process also promotes broadening of the jet cross sectional area and it also causes a decrease in the head advance velocity. This hydrodynamic process may be tested by observations of compact young jets.

Shock formation and breaking in granular avalanches

In this paper, we explore properties of shock wave solutions of the Gray-Thornton model for particle size segregation in granular avalanches.The model equation is a nonlinear scalar conservation law expressing conservation of mass under shear for the concentration of small particles in a bidisperse mixture. Shock waves are weak solutions of the partial differential equation across which the concentration jumps. We give precise criteria on smooth initial conditions under which a shock wave forms in the interior of the avalanche in finite time. Shocks typically lose stability as they are sheared by the flow, giving way to a complex structure in which a two-dimensional rarefaction wave interacts dynamically with a pair of shocks. The rarefaction represents a mixing zone, in which small and large particles are mixed as they are transported up and down (respectively) through the zone. The mixing zone expands and twice changes its detailed structure before reaching the boundary.

Border price and export demand shocks for developing countries from rest-of-world trade liberalization using the linkage model.

The volume on agricultural price distortions, inequality and poverty begins with a global study that uses the World Bank's linkage model to examine the economic impacts in various countries, regions and the world as a whole of agricultural and trade policies as of 2004. It does so by shocking that model with the removal of all agricultural price-distorting domestic and border policies with, and without, the removal of trade policies affecting all other goods. That pair of shocks is also employed in another global study in that volume to examine the inequality and poverty implications of those price-distorting policies for more than 100 countries. Then for ten national studies reported in that volume, the Linkage model again is used, but only to provide an exogenous set of shocks to the national economy wide model employed by the authors of each developing country case study. The effects of that shock on a national economy are then compared with the effects of own-country liberalization using the same national model and the same agricultural protection rates for that country as in the global Linkage model. In this appendix the authors describe the main assumptions adopted to generate the border price and export demand shocks from agricultural and trade policy reforms by the rest of the world, and how that is communicated to the national models.

Defibrillation probability and impedance change between shocks during resuscitation from out-of-hospital cardiac arrest

Objective Technical data now gathered by automated external defibrillators (AEDs) allows closer evaluation of the behavior of defibrillation shocks administered during out-of-hospital cardiac arrest. We analyzed technical data from a large case series to evaluate the change in transthoracic impedance between shocks, and to assess the heterogeneity of the probability of successful defibrillation across the population. Methods We analyzed a series of consecutive cases where AEDs delivered shocks to treat ventricular fibrillation (VF) during out-of-hospital cardiac arrest. Impedance measurements and VF termination efficacy were extracted from electronic records downloaded from biphasic AEDs deployed in three EMS systems. All patients received 200 J first shocks; second shocks were 200 J or 300 J, depending on local protocols. Results presented are median (25th, 75th percentiles). Results Of 863 cases with defibrillation shocks, 467 contained multiple shocks because the first shock failed to terminate VF ( n = 61) or VF recurred ( n = 406). Defibrillation efficacy of subsequent shocks was significantly lower in patients that failed to defibrillate on first shock than in patients that did defibrillate on first shock (162/234 = 69% vs. 955/1027 = 93%; p < 0.0001). The failed VF terminations were distributed heterogeneously across the population; 5% of patients accounted for 71% of failed shocks. Shock impedance decreased by 1% [0%, 4%] and peak current increased by 1% [0%, 4%] between 200 J first and 200 J second shocks. Shock impedance decreased 4% [2%, 6%] and current increased 27% [25%, 29%] between 200 J first and 300 J second shocks. In all 499 pairs of same-energy consecutive shocks, impedance changed by less than 1% in 226 (45%), increased >1% in 124 (25%) and decreased >1% in 149 (30%). Conclusions Impedance change between consecutive shocks is minimal and inconsistent. Therefore, to increase current of a subsequent shock requires an increase of the energy setting. Distribution of failed shocks is far from random. First shock defibrillation failure is often predictive of low efficacy for subsequent shocks.

X-RAY AND HIGH-ENERGY FLARES FROM LATE INTERNAL SHOCKS OF GAMMA-RAY BURSTS

We study afterglow flares of gamma-ray bursts (GRBs) in the framework of the late internal shock (LIS) model based on a careful description for the dynamics of a pair of shocks generated by a collision between two homogeneous shells,. First, by confronting the model with some fundamental observational features of X-ray flares, we find some constraints on the properties of the pre-collision shells that are directly determined by the central engine of GRBs. Second, high energy emission associated with X-ray flares, which arises from synchrotron self-Compton (SSC) emission of LISs, is investigated in a wide parameter space. The predicted flux of high energy flares may reach as high as $\sim 10^{-8}\rm erg cm^{-2}s^{-1}$, which is likely to be detectable with the Large Area Telescope (LAT) aboard \textit{the Fermi Space Telescope}

GAS-DYNAMIC SHOCK HEATING OF POST-FLARE LOOPS DUE TO RETRACTION FOLLOWING LOCALIZED, IMPULSIVE RECONNECTION

We present a novel model in which shortening of a magnetic flux tube following localized, three-dimensional reconnection generates strong gas-dynamic shocks around its apex. The shortening releases magnetic energy by progressing away from the reconnection site at the Alfven speed. This launches inward flows along the field lines whose collision creates a pair of gas-dynamic shocks. The shocks raise both the mass density and temperature inside the newly shortened flux tube. Reconnecting field lines whose initial directions differ by more that 100 degrees can produce a concentrated knot of plasma hotter that 20 MK, consistent with observations. In spite of these high temperatures, the shocks convert less than 10% of the liberated magnetic energy into heat - the rest remains as kinetic energy of bulk motion. These gas-dynamic shocks arise only when the reconnection is impulsive and localized in all three dimensions; they are distinct from the slow magnetosonic shocks of the Petschek steady-state reconnection model.

Dynamics of Vertical Displacement in Porous Media Associated With CO2 Sequestration

Summary The problem of CO2 sequestration in geologic formations is analyzed from a fundamental perspective. In order to clearly understand the first order behavior of the system, the mechanisms of trapping, dissolution and chemical reactions are not accounted for. The analysis is concerned with the post-injection period when the CO2 plume rises due to buoyancy. Characteristics of the plume for a 1D problem show that a pair of shocks moving in opposite directions is produced at the top end. The downward moving shock interacts with the bottom end of the plume resulting in a decrease in the maximum value of the CO2 saturation. High accuracy numerical simulations are employed to understand the 2D mechanisms of plume evolution in terms of the viscosity ratio and the capillary number. 2D results show that the plume rises to significantly lower depth, in shorter times, as compared to the 1D problem. This behavior is governed by the 2D velocity field around the plume that additionally leads to spanwise wave interactions and results in a faster decrease of the maximum CO2 saturation. The initial dimensions of the plume have a strong influence on the time scales of the wave interactions. The maximum upward velocity that is generated due to buoyancy is closely related to the maximum saturation and decays rapidly to very small values with a decrease in saturation. In the case where the viscosity of CO2 is a tenth of the viscosity of the surrounding fluid, the plume rises up about 500 m in 700 yrs. Our results provide an upper bound on the maximum rise distance and the sequestration time for the problem involving trapping and dissolution. Comparison with experimental results show that the buoyancy velocity obtained from our results is of the same order as observed in the experiments.

Epicyclic Motions and Standing Shocks in Numerically Simulated Tilted Black Hole Accretion Disks

This work presents a detailed analysis of the overall flow structure and unique features of the inner region of the tilted-disk simulations described recently by Fragile et al. The primary new feature identified in the main disk body is a latitude-dependent radial epicyclic motion driven by pressure gradients attributable to the gravitomagnetic warping of the disk. The induced motion of the gas is coherent over the scale of the entire disk and is fast enough that it could be observable in features such as relativistic iron lines. The eccentricity of the associated fluid-element trajectories increases with decreasing radius, leading to a crowding of orbit trajectories near their apocenters. This results in a local density enhancement akin to a compression. These compressions are sufficiently strong to produce a pair of weak shocks in the vicinity of the black hole. These shocks are roughly aligned with the line of nodes between the black hole symmetry plane and disk midplane, with one shock above the line of nodes on one side of the black hole and the other below on the opposite side. These shocks enhance angular momentum transport and energy dissipation near the hole, forcing some material to plunge toward the black hole from well outside the innermost stable circular orbit. A new, extended simulation, which was evolved for more than a full disk precession period, allows us to confirm that these shocks and the previously identified "plunging streams" precess with the disk in such a way as to remain aligned relative to the line of nodes, as expected based on our physical understanding of these phenomena. Such a precessing structure would likely present a strong quasi-periodic signal.

Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane

We have performed a survey of the characteristics of two types of large spatial-scale solar-wind structures, stream interaction regions (SIRs), and interplanetary coronal mass ejections (ICMEs), near 5.3 AU, using solar-wind observations from Ulysses. Our study is confined to the three aphelion passes of Ulysses, and also within ± 10° of the solar ecliptic plane, covering a part of 1992, 1997 – 1998, and 2003 – 2005, representing three slices of different phases of the solar activity cycle. Overall, there are 54 SIRs and 60 ICMEs in the survey. Many are merged in hybrid events, suggesting that they have undergone multiple interactions prior to reaching Jovian orbit. About 91% of SIRs occur with shocks, with 47% of such shocks being forward – reverse shock pairs. The solar-wind velocity sometimes stays constant or even decreases within the interaction region near 5.3 AU, in contrast with the gradual velocity increase during SIRs at 1 AU. Shocks are driven by 58% of ICMEs, with 94% of them being forward shocks. Some ICMEs seem to have multiple small flux ropes with different scales and properties. We quantitatively compare various properties of SIRs and ICMEs at 5.3 AU, and study their statistical distributions and variations with solar activity. The width, maximum dynamic pressure, and peak perpendicular pressure of SIRs all become larger than ICMEs. Dynamic pressure (Pdyn) is expected to be important for Jovian magnetospheric activity. We have examined the distributions of Pdyn of SIRs, ICMEs, and general solar wind, but these cannot explain the observed bimodal distribution of the location of the Jovian magnetopause. By comparing the properties of SIRs and ICMEs at 0.72, 1, and 5.3 AU, we find that the ICME expansion slows down significantly between 1 and 5.3 AU. Some transient and small streams in the inner heliosphere have merged into a single interaction region.

Energetic particle measurements from the &amp;lt;I&amp;gt;Ulysses&amp;lt;/I&amp;gt;/COSPIN/LET instrument obtained during the August/September 2005 events

Abstract. We report recent observations of energetic particles at energies 1–40 MeV/n made by the COSPIN/LET instrument onboard the Ulysses spacecraft during the period of intense solar activity in August/September 2005 during the declining phase of solar cycle 23. Ulysses, having started its climb to high southern latitudes for the third time, was located at ~5 AU, at a helio-latitude of ~30 degrees south. It detected the arrival of a solar wind compound stream resulting from the merging of a series of fast halo CMEs ejected from the Sun in late August and early September 2005 and their interaction with the pre-existing pattern of solar wind Stream Interaction Regions (SIRs) in the ambient medium through which they propagated. The heavy ion intensities are observed by COSPIN/LET to remain elevated for at least 20 days following the very intense X17.0/3B solar flare on 7 September and its associated very fast CME (plane of sky projected CME speed ~2400 km s−1). We carry out an analysis of the composition of the particle increases observed at the location of the spacecraft. Although the composition signatures were predominantly Solar Energetic Particle (SEP)-like, after the passage of the compound stream over Ulysses, in association with a characteristic forward and reverse shock pair, the observations showed evidence of an enhanced He content.

Energetic particles during the first and third Ulysses southern high‐latitude excursions: Probing global corotating interaction region structure beyond 5 AU

We analyze energetic particle observations during the first and third ascents of Ulysses to the southern regions of the heliosphere. The first high‐latitude excursion occurred in the declining phase of solar cycle 22, while the third occurred in a later phase of solar cycle 23. In contrast to the long‐lived and well‐defined ∼26‐day recurrent intensity enhancements observed throughout the first southern excursion, observations during the third southern excursion showed a more variable structure. Transient events of solar origin were more abundant and intense. Corotating interaction regions (CIRs) were clearly present in intermittent sequences lasting only a few solar rotations. Within these sequences the relationships between solar wind structures and energetic particle events were remarkably similar to those observed in the steady recurrences of the first southern excursion. Energetic particles associated with well‐formed CIRs bounded by strong forward and reverse shock pairs were not observed until heliographic latitudes Λ ≳ 30°S (versus Λ ≳ 13°S in the first orbit). When Ulysses finally emerged into the fast (≳700 km s−1) solar wind at Λ ≥ 39°S, particle events were variable and either associated with weak reverse shocks or without solar wind signatures indicative of local CIRs (just as in solar cycle 22). The time‐intensity profiles in individual CIRs matched up well between the two southern excursions implying that essentially the same CIR structure existed in both solar cycles, albeit at a higher latitude in solar cycle 23 and only in the midst of the intermittent recurrent sequences.

Reply to comment by P. Riley and J. T. Gosling on “Are high‐latitude forward‐reverse shock pairs driven by overexpansion?”

Reply to comment by P. Riley and J. T. Gosling on “Are high‐latitude forward‐reverse shock pairs driven by overexpansion?”