Wind Termination Shock Research Articles

The role of solar wind electrons at the solar wind termination shock

Abstract Voyager 2 plasma observations have recently revealed that, as predicted by theory, there exists a solar wind termination shock at 87 au. However, it is evidently different from classical expectations, for instance revealing the downstream solar wind protons still to be in a supersonic mode. In this paper we show that in order to explain the non-classical structure and facts of this shock, one has to start from a multi-fluid magnetohydrodynamic (MHD) approach describing the shock transition. Different from our earlier attempt, here we consider solar wind electrons as an additional extra fluid and allow for preferential heating of electrons compared to thermal solar wind protons at the shock. As we can then show, with this enlargement of the MHD theory by a separate electron fluid we are able to describe observed features of the solar wind termination shock. We arrive at the conclusion that the downstream solar wind thermal plasma is energetically dominated by the pressure of quasi-mass-less electrons.

Revisiting the role of magnetic moments in heliospheric plasmas

The magnetic moment is one of the most fundamental, nontrivial particle properties known in plasma physics. Also known as the first adiabatic invariant, it is conserved under a broad range of microphysical processes and plasma boundary conditions. However, especially in modern numerical simulations, the conservation of this property, which in principle may serve as a control parameter, is no longer used explicitly. In light of this fact, as well as in a recent series of studies of the solar wind termination shock and comparable systems, where this conservation was used as a key ingredient, we now revisit this old, but standing problem. Building on our earlier arguments on when the magnetic moment of individual particles is conserved, we now carefully expand this argument further, studying nontrivial systems, such as systems where individual ions are represented by a broad distribution function that is described with the help of a transport equation. We see that the magnetic moment is conserved under an even wider variety of situations than those studied in earlier publications. When studying systems of purely kinetic equations, the magnetic moment can be converted into an additional force term in the transport equation. We also study the physics that is taken into account in shock simulations and pinpoint weaknesses in the physics used by these modern codes.

Shock-drift acceleration of interstellar pickup protons at the solar wind termination shock

The numerical model of the shock-drift acceleration of interstellar pick-up protons at the solar wind termination shock is presented taking into account multiple reflections of the particles at and passing through the shock. The processes of the multiple interactions with the shock are possible due to pitch-angle scattering in the upstream and downstream parts of the solar wind flow. The model takes into account variations of the magnetic field direction near the shock front connected with its three-dimensional shape. The main advantage of the model is the self-consistent treatment of protons and interstellar hydrogen atoms, which interact with each other through the resonant charge exchange process. The model gives very simple and natural resolution of so-called injection problem for anomalous cosmic rays. The shock-drift acceleration can explain the fluxes of the anomalous cosmic rays measured at the Voyager 1/2 spacecraft after the termination shock crossings.

SPATIAL RETENTION OF IONS PRODUCING THEIBEXRIBBON

The ribbon observed by the Interstellar Boundary Explorer (IBEX) mission is a narrow, ~20° wide feature that stretches across much of the sky in the global flux of energetic neutral atoms from the outer heliosphere. The ribbon remains an enigma despite its persistence after 3 years of IBEX observations and after almost a dozen theories that attempt to explain it. While each theory that has been posed has its strengths, each one also contradicts IBEX observations or demonstrates significant flaws in internal consistency. Here, we present a new theory that is different than any of the existing ideas and yet accounts for many of the key observations. We argue that the ribbon could be produced by a spatial region in the local interstellar medium where newly ionized atoms are temporarily contained through increased rates of scattering by locally generated waves in the electromagnetic fields. The particles in the ribbon are created predominantly from neutralized solar wind and neutralized pickup ions from inside the solar wind termination shock.

Observation of the multifractal spectrum in the heliosphere and the heliosheath by Voyager 1 and 2

We present first results of the multifractal scaling of the fluctuations of the interplanetary magnetic field strength as measured onboard Voyager 2 in the very distant heliosphere and even in the heliosheath. More specifically, the spectra observed by Voyager 2 in a wide range of heliospheric distances from 6 to 90 astronomical units (AU) are compared with those of Voyager 1 already analyzed between 7 and 107 AU. We focus on the singularity multifractal spectrum before and after crossing the termination heliospheric shock by Voyager 1 at 94 AU and Voyager 2 at 84 AU from the Sun. It is worth noting that the spectrum is prevalently right‐skewed inside the whole heliosphere. Moreover, we have observed a change of the asymmetry of the spectrum at the termination shock, where the spectrum changes from (left‐) right‐skewed in the very distant heliosphere to the (right‐) left‐skewed or possibly symmetric spectrum in the heliosheath. We confirm that the degree of multifractality falls steadily with the distance from the Sun. In addition, the multifractal structure is apparently modulated by the solar activity, with a time shift of several years, corresponding to a distance of about 10 AU, resulting from the evolution of the whole heliosphere. Hence this basic result also brings significant additional support to some earlier claims suggesting that the solar wind termination shock is asymmetric.

Comprehensive multiwavelength modelling of the afterglow of GRB 050525A

The Swift era has posed a challenge to the standard blast-wave model of Gamma Ray Burst (GRB) afterglows. The key observational features expected within the model are rarely observed, such as the achromatic steepening (`jet-break') of the light curves. The observed afterglow light curves showcase additional complex features requiring modifications within the standard model. Here we present optical/NIR observations, millimeter upper limits and comprehensive broadband modelling of the afterglow of the bright GRB 0505025A, detected by Swift. This afterglow cannot be explained by the simplistic form of the standard blast-wave model. We attempt modelling the multi-wavelength light curves using (i) a forward-reverse shock model, (ii) a two-component outflow model and (iii) blast-wave model with a wind termination shock. The forward-reverse shock model cannot explain the evolution of the afterglow. The two component model is able to explain the average behaviour of the afterglow very well but cannot reproduce the fluctuations in the early X-ray light curve. The wind termination shock model reproduces the early light curves well but deviates from the global behaviour of the late-time afterglow.

Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds

By means of two- and three-dimensional particle-in-cell simulations, we investigate the process of driven magnetic reconnection at the termination shock of relativistic striped flows. In pulsar winds and in magnetar-powered relativistic jets, the flow consists of stripes of alternating magnetic field polarity, separated by current sheets of hot plasma. At the wind termination shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>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 sigma with respect to the pre-shock value. In the limit lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind, the post-shock particle spectrum approaches a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. The presence of a current-aligned "guide" magnetic field suppresses the acceleration of particles only when the guide field is stronger than the alternating component. Our findings place important constraints on the models of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.

RADIO OBSERVATIONS REVEAL UNUSUAL CIRCUMSTELLAR ENVIRONMENTS FOR SOME TYPE Ibc SUPERNOVA PROGENITORS

We present extensive radio observations of the nearby Type Ibc supernovae 2004cc, 2004dk, and 2004gq spanning 8-1900 days after explosion. Using a dynamical model developed for synchrotron emission from a slightly decelerated shockwave, we estimate the velocity and energy of the fastest ejecta and the density profile of the circumstellar medium. The shockwaves of all three supernovae are characterized by non-relativistic velocities of v ~ (0.1-25)c and associated energies of E ~ (2-10) * 1e47 erg, in line with the expectations for a typical homologous explosion. Smooth circumstellar density profiles are indicated by the early radio data and we estimate the progenitor mass loss rates to be ~ (0.6-13) * 1e-5 M_sun/yr (wind velocity 10^3 km/s). These estimates approach the saturation limit (~1e-4 M_sun/yr) for line-driven winds from Wolf-Rayet stars, the favored progenitors of SNe Ibc including those associated with long-duration GRBs. Intriguingly, at later epochs all three supernovae show evidence for abrupt radio variability that we attribute to large density modulations (factor of ~3-6) at circumstellar radii of r ~ (1-50) * 1e16 cm. If due to variable mass loss, these modulations are associated with progenitor activity on a timescale of ~ 10-100 years before explosion. We consider these results in the context of variable mass loss mechanisms including wind clumping, metallicity-independent continuum-driven ejections, and binary-induced modulations. It may also be possible that the SN shockwaves are dynamically interacting with wind termination shocks, however, this requires the environment to be highly pressurized and/or the progenitor to be rapidly rotating prior to explosion. The proximity of the density modulations to the explosion sites may suggest a synchronization between unusual progenitor mass loss and the SN explosion, reminiscent of Type IIn supernovae. [ABRIDGED]

X-RAY INVESTIGATION OF THE DIFFUSE EMISSION AROUND PLAUSIBLE γ-RAY EMITTING PULSAR WIND NEBULAE IN KOOKABURRA REGION

We report on the results from {\it Suzaku} X-ray observations of the radio complex region called Kookaburra, which includes two adjacent TeV $\gamma$-ray sources HESS J1418-609 and HESS J1420-607. The {\it Suzaku} observation revealed X-ray diffuse emission around a middle-aged pulsar PSR J1420-6048 and a plausible PWN Rabbit with elongated sizes of $\sigma_{\rm X}=1^{\prime}.66$ and $\sigma_{\rm X}=1^{\prime}.49$, respectively. The peaks of the diffuse X-ray emission are located within the $\gamma$-ray excess maps obtained by H.E.S.S. and the offsets from the $\gamma$-ray peaks are $2^{\prime}.8$ for PSR J1420-6048 and $4^{\prime}.5$ for Rabbit. The X-ray spectra of the two sources were well reproduced by absorbed power-law models with $\Gamma=1.7-2.3$. The spectral shapes tend to become softer according to the distance from the X-ray peaks. Assuming the one zone electron emission model as the first order approximation, the ambient magnetic field strengths of HESS J1420-607 and HESS J1418-609 can be estimated as 3 $\mu$G, and $2.5 \mu$G, respectively. The X-ray spectral and spatial properties strongly support that both TeV sources are pulsar wind nebulae, in which electrons and positrons accelerated at termination shocks of the pulsar winds are losing their energies via the synchrotron radiation and inverse Compton scattering as they are transported outward.

IMPACT OF SUPERNOVA DYNAMICS ON THE νp-PROCESS

We study the impact of the late time dynamical evolution of ejecta from core-collapse supernovae on \nu p-process nucleosynthesis. Our results are based on hydrodynamical simulations of neutrino wind ejecta. Motivated by recent two-dimensional wind simulations, we vary the dynamical evolution during the \nu p-process and show that final abundances strongly depend on the temperature evolution. When the expansion is very fast, there is not enough time for antineutrino absorption on protons to produce enough neutrons to overcome the \beta-decay waiting points and no heavy elements beyond A=64 are produced. The wind termination shock or reverse shock dramatically reduces the expansion speed of the ejecta. This extends the period during which matter remains at relatively high temperatures and is exposed to high neutrino fluxes, thus allowing for further (p,\gamma) and (n,p) reactions to occur and to synthesize elements beyond iron. We find that the \nu p-process starts to efficiently produce heavy elements only when the temperature drops below ~3 GK. At higher temperatures, due to the low alpha separation energy of 60Zn (S_{\alpha} = 2.7 MeV) the reaction 59Cu(p,\alpha)56Ni is faster than the reaction 59Cu(p,\gamma)60Zn. This results in the closed NiCu cycle that we identify and discuss here for the first time. We also investigate the late phase of the \nu p-process when the temperatures become too low to maintain proton captures. Depending on the late neutron density, the evolution to stability is dominated by \beta decays or by (n,\gamma) reactions. In the latter case, the matter flow can even reach the neutron-rich side of stability and the isotopic composition of a given element is then dominated by neutron-rich isotopes.

Modelling of galactic Carbon in an asymmetrical heliosphere: Effects of asymmetrical modulation conditions

Observations of galactic cosmic rays (GCRs) from the two Voyager spacecraft inside the heliosheath indicate significant differences between them, suggesting that in addition to a possible global asymmetry in the north–south dimensions (meridional plane) of the heliosphere, it is also possible that different modulation (turbulence) conditions could exist between the two hemispheres of the heliosphere. We focus on illustrating the effects on GCR Carbon of asymmetrical modulation conditions combined with a heliosheath thickness that has a significant dependence on heliolatitude. To reflect different modulation conditions between the two heliospheric hemispheres in our numerical model, the enhancement of both polar and radial perpendicular diffusion off the ecliptic plane is assumed to differ from heliographic pole to pole. The computed radial GCR intensities at polar angles of 55° (approximating the Voyager 1 direction) and 125° (approximating the Voyager 2 direction) are compared at different energies and for both particle drift cycles. This is done in the context of illustrating how different values of the enhancement of both polar and radial perpendicular diffusion between the two hemispheres contribute to causing differences in radial intensities during solar minimum and moderate maximum conditions. We find that in the A>0 cycle these differences between 55° and 125° change both quantitatively and qualitatively for the assumed asymmetrical modulation condition as reflected by polar diffusion, while in the A<0 cycle, minute quantitative differences are obtained. However, when both polar and radial perpendicular diffusion have significant latitude dependences, major differences in radial intensities between the two polar angles are obtained in both polarity cycles. Furthermore, significant differences in radial intensity gradients obtained in the heliosheath at lower energies may suggest that the solar wind turbulence at and beyond the solar wind termination shock must have a larger latitudinal dependence.

The inner heliosheath source for keV-ENAs observed with IBEX

Context. The “IBEX ribbon” feature provides significant challenges to theoretical modeling attempts of the outer heliosphere. Classical ENA production by shock-processed nonthermal tails results in no relevant ENA signatures, mainly due to the downstream solar wind proton temperature observed by the Voyagers being one order of magnitude smaller than expected from the classical monofluid shock model. Aims. Here, we therefore study shock-processed pick-up ions (PUIs) in the inner heliosheath resulting from a multifluid shock model as a source of keV-energetic ENAs as have been detected by the IBEX mission. Unlike previous studies, we apply a semikinetic multifluid shock model, which did prove useful in explaining the apparent temperature discrepancy in the past. Methods. To convert upstream into downstream pick-up ions, we use both kinetic and multifluid theories describing the solar wind termination shock (TS) transition. This allows us to obtain the downstream PUI distribution function as a function of classical shock properties, such as the local magnetic field tilt angle and the compression ratio. In addition, this kinetic model also allows to derive a formulation of latitude- and longitude-dependent spectral intensities between 1 and 100 keV, a region which is not covered by missions such as the Voyagers and needs to be covered by theoretical considerations. Results. After converting shock-processed PUIs to ENAs by charge exchange with cold H-atoms, we find keV ENA fluxes of the same order of magnitude as those observed by IBEX. These fluxes also exhibit a pronounced ring-type feature, with about the correct ratio of ring-to-nose intensities, producing encouraging overall agreement with IBEX data, potential of further improvement.

FREEZE-OUT YIELDS OF RADIOACTIVITIES IN CORE-COLLAPSE SUPERNOVAE

We explore the nucleosynthesis trends from two mechanisms during freeze-out expansions in core-collapse supernovae. The first mechanism is related to the convection and instabilities within homogeneous stellar progenitor matter that is accreted through the supernova shock. The second mechanism is related to the impact of the supersonic wind termination shock (reverse shock) within the tumultuous inner regions of the ejecta above the proto-neutron star. Our results suggest that isotopes in the mass range 12<=A<=122 that are produced during the freeze-out expansions may be classified in two families. The isotopes of the first family manifest a common mass fraction evolutionary profile, whose specific shape per isotope depends on the characteristic transition between two equilibrium states (equilibrium state transition) during each type of freeze-out expansion. The first family includes the majority of isotopes in this mass range. The second family is limited to magic nuclei and isotopes in their locality, which do not sustain any transition, become nuclear flow hubs, and dominate the final composition. We use exponential and power-law adiabatic profiles to identify dynamic large-scale and small-scale equilibrium patterns among nuclear reactions. (truncated abstract) In addition, we introduce non-monotonic parameterized profiles to probe the impact of the reverse shock and multi-dimensional explosion asymmetries on nucleosynthesis. Cases are shown in which the non-monotonic profiles favor the production of radioactivities. Non-monotonic freeze-out profiles involve longer non-equilibrium nucleosynthesis intervals compared with the exponential and power-law profiles, resulting in mass fraction trends and yield distributions that may not be achieved by the monotonic freeze-out profiles.

The inner heliospheric source for keV-energetic IBEX ENAs

Context. Up to now, three succesive full-sky maps of heliospheric fluxes of KeV-energetic neutral H-atoms have been registered with the satellite IBEX. The most outstanding feature persisting in these maps is the so-called ribbon which appears as an arc around the upwind direction with unexpectedly high fluxes. This radiation feature was not predicted by models, but at present is tentatively explained as due to energetic plasma sites outside the heliosphere in regions beyond the heliopause. Aims. Since all these proposed explanations, however, need to rely on unproven energizing ion processes, we shall investigate in this article alternative ion sources of keV-ENAs that in contrast appear much closer to the sun, but are based on a clear concept of ion energization to KeV energies, namely suprathermal pick-up ions at a few AU inside and outside of the solar wind termination shock. Methods. Using well-established models for the solar wind magnetic field and the heliospheric TS surface, we derive skymaps of the magnetic field tilt angle, the resulting injection efficiency and the resulting ENA flows. The results are normalised to the ion flux observations made by Voyager-1. We also discuss basic modifications of the TS geometry within the framework of the model. Results. We calculate ENA fluxes of these pick-up ions and show that they, due to their sensitivities to magnetic tilt angles at the termination shock surface, reveal a radiative sickle feature around the upwind direction which in many of its properties resembles the ENA ribbon feature seen by IBEX, although the highest IBEX intensities do not seem to be quantitatively representable by our calculations.

Zero outward flow velocity for plasma in a heliosheath transition layer

Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 AU = 1.5 × 10(8) km). It is now ∼22 AU past the termination shock crossing. The bulk velocity of the plasma in the radial-transverse plane has been determined using measurements of the anisotropy of the convected energetic ion distribution. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from ∼70 km s(-1) to ∼0 km s(-1), where it has remained for the past eight months. It now seems that Voyager 1 has entered a finite transition layer of zero-radial-velocity plasma flow, indicating that the spacecraft may be close to the heliopause, the border between the heliosheath and the interstellar plasma. The existence of a flow transition layer in the heliosheath contradicts current predictions--generally assumed by conceptual models--of a sharp discontinuity at the heliopause.

COMPUTING THE DUST DISTRIBUTION IN THE BOW SHOCK OF A FAST-MOVING, EVOLVED STAR

We study the hydrodynamical behavior occurring in the turbulent interaction zone of a fast moving red supergiant star, where the circumstellar and interstellar material collide. In this wind-interstellar medium collision, the familiar bow shock, contact discontinuity, and wind termination shock morphology forms, with localized instability development. Our model includes a detailed treatment of dust grains in the stellar wind, and takes into account the drag forces between dust and gas. The dust is treated as pressureless gas components binned per grainsize, for which we use ten representative grainsize bins. Our simulations allow to deduce how dust grains of varying sizes become distributed throughout the circumstellar medium. We show that smaller dust grains (radius <0.045 micro-meters) tend to be strongly bound to the gas and therefore follow the gas density distribution closely, with intricate finestructure due to essentially hydrodynamical instabilities at the wind-related contact discontinuity. Larger grains which are more resistant to drag forces are shown to have their own unique dust distribution, with progressive deviations from the gas morphology. Specifically, small dust grains stay entirely within the zone bound by shocked wind material. The large grains are capable of leaving the shocked wind layer, and can penetrate into the shocked or even unshocked interstellar medium. Depending on how the number of dust grains varies with grainsize, this should leave a clear imprint in infrared observations of bowshocks of red supergiants and other evolved stars.

On the origin of variable gamma-ray emission from the Crab nebula

The oblique geometry of pulsar wind termination shock ensures that the Doppler beaming has a strong impact on the shock emission. We illustrate this using recent relativistic MHD simulations of the Crab Nebula and also show that the observed size, shape, and distance from the pulsar of the Crab Nebula inner knot are consistent with its interpretation as a Doppler-boosted emission from the termination shock. If the electrons responsible for the synchrotron gamma-rays are accelerated only at the termination shock then their short life-time ensures that these gamma-rays originate close to the shock and are also strongly effected by the Doppler beaming. As the result, bulk of the observed synchrotron gamma-rays of the Crab Nebula around 100 MeV may come from its inner knot. This hypothesis is consistent with the observed optical flux of the inner knot provided its optical-gamma spectral index is the same as the injection spectral index found in the Kennel & Coroniti model of the nebula spectrum. The observed variability of synchrotron gamma-ray emission can be caused by the instability of the termination shock discovered in recent numerical simulations. Given the small size of the knot, it is possible that the September 2010 gamma-ray flare of the Crab Nebula also came from the knot, though the actual mechanism remains unclear. The model predicts correlation of the temporal variability of the synchrotron gamma-ray flux in the Fermi and AGILE windows with the variability of the unpulsed optical flux from within 1 arcsec of the Crab pulsar.

Modeling the acceleration and modulation of anomalous cosmic ray oxygen

After the solar wind termination shock crossings of the Voyager spacecraft, the acceleration of anomalous cosmic rays has become a very contentious subject. In this paper we examine several topics pertinent to anomalous cosmic ray oxygen acceleration and transport using a numerical cosmic ray modulation model. These include the effects of drifts on a purely Fermi I accelerated spectra, the effects of introducing higher charge states of oxygen into the modulation model, examining the viability of momentum diffusion as a re-acceleration process in the heliosheath and examining energy spectra, and intensity gradients, in the inner heliosphere during consecutive drift cycles.

Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows

After the initiation of the explosion of core-collapse supernovae, neutrinos emitted from the nascent neutron star drive a supersonic baryonic outflow. This neutrino-driven wind interacts with the more slowly moving, earlier supernova ejecta forming a wind termination shock (or reverse shock), which changes the local wind conditions and their evolution. Important nucleosynthesis processes (alpha-process, charged-particle reactions, r-process, and vp-process) occur or might occur in this environment. The nucleosynthesis depends on the long-time evolution of density, temperature, and expansion velocity. Here we present two-dimensional hydrodynamical simulations with an approximate description of neutrino-transport effects, which for the first time follow the post-bounce accretion, onset of the explosion, wind formation, and the wind expansion through the collision with the preceding supernova ejecta. Our results demonstrate that the anisotropic ejecta distribution has a great impact on the position of the reverse shock, the wind profile, and the long-time evolution. This suggests that hydrodynamic instabilities after core bounce and the consequential asymmetries may have important effects on the nucleosynthesis-relevant conditions in the neutrino-heated baryonic mass flow from proto-neutron stars.

PARTICLE ACCELERATION BY COLLISIONLESS SHOCKS CONTAINING LARGE-SCALE MAGNETIC-FIELD VARIATIONS

Diffusive shock acceleration at collisionless shocks is thought to be the source of many of the energetic particles observed in space. Large-scale spatial variations of the magnetic field has been shown to be important in understanding observations. The effects are complex, so here we consider a simple, illustrative model. Here, we solve numerically the Parker transport equation for a shock in the presence of large-scale sinusoidal magnetic-field variations. We demonstrate that the familiar planar-shock results can be significantly altered as a consequence of large-scale, meandering magnetic lines of force. Because perpendicular diffusion coefficient $\kappa_\perp$ is generally much smaller than parallel diffusion coefficient $\kappa_\parallel$, the energetic charged particles are trapped and preferentially accelerated along the shock front in the regions where the connection points of magnetic field lines intersecting the shock surface converge, and thus create the "hot spots" of the accelerated particles. For the regions where the connection points separate from each other, the acceleration to high energies will be suppressed. Further, the particles diffuse away from the "hot spot" regions and modify the spectra of downstream particle distribution. These features are qualitatively similar to the recent Voyager's observation in the Heliosheath. These results are potentially important for particle acceleration at shocks propagating in turbulent magnetized plasmas as well as those which contain large-scale nonplanar structures. Examples include anomalous cosmic rays accelerated by the solar wind termination shock, energetic particles observed in propagating heliospheric shocks, and galactic cosmic rays accelerated by supernova blast waves, etc.