Wind Termination Research Articles

TeV light curve of PSR B1259-63/SS2883

The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally believed to be responsible for TeV gamma-ray signal recently reported from the binary system PSR B1259-63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the gamma-ray fluxes detected by the Array of Imaging Atmospheric Cherenkov Telescopes (HESS) at different epochs do not agree with the published theoretical predictions of the TeV light curve. The main objective of this paper is to show that the HESS results can be explained, under certain reasonable assumptions concerning the cooling of relativistic electrons, by IC scenarios of gamma-ray production in PSR B1259-63. In this paper we study evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy-loss rates of electrons, and the impact of these processes on the light curve of IC gamma-rays. We demonstrate that the observed TeV light curve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a significant increase towards the periastron or (ii) by the 'early' (sub-TeV) cut-offs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The first four data points obtained just after periastron comprise an exception - possibly due to interaction with the Be star disc, which introduces additional physics not included in the presented model. The calculated spectral and temporal characteristics of the TeV radiation provide conclusive tests to distinguish between these two working hypotheses. The Compton deceleration of the electron-positron pulsar wind contributes to the decrease of the non-thermal power released in the accelerated electrons after the wind termination, and thus to the reduction of the IC and synchrotron components of radiation close to the periastron. Although this effect alone cannot explain the observed TeV and X-ray light curves, the Comptonization of the cold ultrarelativistic wind leads to the formation of gamma-radiation with a specific line-type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, Γ ≤ 10 6 (for the most likely orbit inclination angle i = 35°, and assuming an isotropic pulsar wind), future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between 10 4 and 10 6 .

The physics of particle acceleration at the heliospheric termination shock

Cosmic rays are ubiquitous in space, and the essential similarity of their energy spectra in many different regions places significant general constraints on the mechanisms for their acceleration and confinement. Diffusive shock acceleration is at present the most successful acceleration mechanism proposed, and, together with transport in Kolmogorov turbulence, can account for the universal specta. A unique laboratory for studying the acceleration and transport of charged particles is the outer heliosphere, including the solar wind termination shock and heliosheath. A widely accepted paradigm for the transport and acceleration of energetic particles in the heliosphere has evolved over the last few decades. This picture has successfully explained many features of the modulation of galactic cosmic rays and the transport and acceleration of anomalous cosmic rays at the solar-wind termination shock. Recent Voyager observations near and beyond the termination shock have revealed new, and in some cases, unexpected phenomena which have led to questions concerning the established paradigm. The physical interpretation of the observations requires a blunt termination shock, rapid inward motion of the shock and temporal variations over time scales ranging from hours to 22 years. Incorporation of these into the physics has promise of explaining most, if not, all of the observed phenomena while retaining the advantages of the termination shock paradigm for both galactic and anomalous cosmic rays.

Detection of the high energy component of Jovian electrons in Low Earth Orbit with the PAMELA experiment

The PAMELA experiment is devoted to the study of cosmic rays in Low Earth Orbit with an apparatus optimized to perform a precise determination of the galactic antimatter component of c.r. It is constituted by a number of detectors built around a permanent magnet spectrometer. PAMELA was launched in space on June 15th 2006 on board the Russian Resurs-DK1 satellite for a mission duration of 3 years. The characteristics of the detectors, the long lifetime and the orbit of the satellite, will allow to address several aspects of cosmic-ray physics. In this work we discuss the observational capabilities of PAMELA to detect the electron component above 50 MeV. The magnetic spectrometer allows a detailed measurement of the energy spectrum of electrons of galactic and Jovian origin. Long term measurements and correlations with Earth–Jupiter 13 months synodic period will allow to separate these two contributions and to measure the primary electron Jovian component, dominant in the 50–70 MeV energy range. With this technique it will also be possible to study the contribution to the electron spectrum of Jovian e − reaccelerated up to 2 GeV at the Solar Wind Termination Shock.

Cosmic‐Ray Energy Changes in the Heliosphere. II. The Effect on K‐Capture Electron Secondaries

Recent accurate measurements of the cosmic-ray intensity ratio ^(51)V/^(51)Cr below 1 GeV nucleon^(-1) provide a powerful new tool to study cosmic-ray modulation in the heliosphere. This paper describes how energy changes during this modulation process influence this ratio. In particular, our model includes acceleration at the solar wind termination shock, and we find that this mechanism significantly enhances the ^(51)V/^(51)Cr ratio at 1 AU. It is also shown that this acceleration makes the ratio more sensitive to the form of local low-energy interstellar spectra, below ~100 MeV nucleon^(-1), than without it. Specifically, this acceleration provides an independent confirmation of the consensus that low-energy spectra should be flatter than their high-energy power-law forms.

A Focused Transport Approach to Pickup Ion Shock Acceleration: Implications for the Termination Shock

The discovery of low-energy suprathermal ions coming directly from the solar wind termination shock by the Voyager 1 spacecraft revealed strong beaming of these particles along the direction of the large-scale magnetic field. Since such large anisotropies exclude the use of standard cosmic-ray transport theory for nearly isotropic particle distributions in modeling the acceleration of these particles at the termination shock, we developed a shock acceleration model based on the numerical solution of the standard focused kinetic transport equation. After investigating the appropriateness of the physical content of this equation for shock acceleration, and applying the model to shocks of varying shock obliquity, we find that (1) this approach is a viable alternative to more sophisticated particle codes such as hybrid codes for simulating the shock acceleration of pickup ions and (2) that the model has the potential to reproduce many of the interesting features of energetic ions observed by the Voyager spacecraft at the termination shock.

Full Boltzmann-kinetical treatment of an ion plasma crossing an MHD shock: parallel and non-parallel cases

Aims. We attempt to describe kinetic properties of the solar wind termination shock (and similar MHD shocks in general) using the appropriate form of the kinetic Boltzmann equation, for arbitrary inclinations ΘBn between the magnetic field and the shock normal. Methods. In order to understand the deviations from the perpendicular shock, for which we have already derived an exact solution in an earlier publication, we first prove that our current Boltzmann equation is unable to describe a stationary quasiparallel shock. To ease and open up further research, we derive conditions for the specific form of the relevant Boltzmann equation. Results. We demonstrate that the simplest Boltzmann equation aiming to describe a parallel MHD shock is in conflict with the predictions from pure MHD. We identify several possible reasons for this, and likewise derive conditions based on the mass flow conservation which must be fulfilled for the shock to be stationary. Assuming that a model for (quasi-)stationary shocks does exist, we are able to explain the unchanged power law index at the passage of the solar wind termination shock observed by the Voyager 1 spacecraft in 2004. We also show that different dissipation mechanisms lead to different transition scales for perpendicular and parallel MHD shocks, and that these differences in the dissipation process also need to be included in the case-competent Boltzmann equation.

Composition of Anomalous Cosmic Rays

The “classic” anomalous cosmic ray (ACR) component originates as interstellar neutral atoms that drift into the heliosphere, become ionized and picked up by the solar wind, and carried to the outer heliosphere where the pickup ions are accelerated to hundreds of MeV, presumably at the solar wind termination shock. These interstellar ACRs are predominantly singly charged, although higher charge states are present and become dominant above ∼350 MeV. Their isotopic composition is like that of the solar system and unlike that of the source of galactic cosmic rays. A comparison of their energy spectra with the estimated flux of pickup ions flowing into the termination shock reveals a mass-dependent acceleration efficiency that favors heavier ions. There is also a heliospheric ACR component as evidenced by “minor” ACR ions, such as Na, Mg, S, and Si that appear to be singly-ionized ions from a source likely in the outer heliosphere.

Modulation of the high energy γ-ray flux from PSR B1259-63/SS2883 due to the orbital variation of the maximum energy of accelerated electrons

The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally believed to be responsible for TeV gamma-ray signal recently reported from the binary system PSR B1259-63/SS2883. In such a system the acceleration takes place in the presence of a dense radiation field provided by a companion Be2-type star. Thus it is natural to expect an orbital phase dependence of the acceleration efficiency in the system. The HESS collaboration reported the tendency of reduction of TeV γ-rays around the periastron. In this paper we study a possible explanation of this effect by the “early” (sub-TeV) cutoffs in the energy spectrum of accelerated electrons due to the enhanced rate of Compton losses close to the periastron.

GRB blastwaves through wind-shaped circumburst media

Context:A significant fraction of progenitors for long gamma-ray bursts (GRBs) are believed to be massive stars. The investigation of long GRBs therefore requires modeling the propagation of ultra-relativistic blastwaves through the circumburst medium surrounding massive stars. We simulate the expansion of an isotropic, adiabatic relativistic fireball into the wind-shaped medium around a massive GRB progenitor. The circumburst medium is composed of a realistically stratified stellar wind zone up to its termination shock, followed by a region of shocked wind characterized by a constant density. Aims: We followed the evolution of the blastwave through all its stages, including the extremely rapid acceleration up to a Lorentz factor 75 flow, its deceleration by interaction with stellar wind, its passage of the wind termination shock, until its propagation through shocked wind. Methods: We used the adaptive mesh refinement versatile advection code to follow the evolution of the fireball. Results: We show that the acceleration from purely thermal to ultra-relativistic kinetic regimes is abrupt and produces an internally structured blastwave. We resolved the structure of this ultra-relativistic shell in all stages, thanks to the adaptive mesh. We comment on the dynamical roles played by forward and reverse shock pairs in the phase of interaction with the free stellar wind and clearly identify the complex shock-dominated structure created when the shell crosses the terminal shock. Conclusion: We show that in our model where the terminal shock is taken relatively close to the massive star, the phase of self-similar deceleration of Blandford-McKee type can only be produced in the constant density, shocked wind zone.

Unexpected sites of efficient stochastic acceleration in the inner heliosheath

Abstract. Up until the recent past, it was generally believed that the solar wind termination shock (TS) is the favourite site to accelerate ions from the keV- to the MeV- energy levels by means of Fermi-1 processes. When Voyager 1 was crossing the TS at the end of 2004, the registrations of this spacecraft showed, however, that beyond the shock passage fluxes of anomalous cosmic rays kept increasing with time. This obviously called for an acceleration site further downstream of the shock in the heliosheath which had not been identified before. In this paper we thus investigate the process of energy diffusion due to wave-particle interactions (Fermi-2) operating on pick-up ions which are convected downstream of the TS with the subsonic solar wind. We investigate the continuous effect of stochastic acceleration processes suffered by pick-up ions at their interaction with heliosheath turbulences, while they are slowly convected with the subsonic solar wind towards the heliotail. As we can show, the inner heliosheath region, with an extent of about 100 AU around the solar wind stagnation point, is specifically favourable for the energy processing of pick-up ions by Fermi-2 processes up to MeV energies. In addition, we claim that this region is the origin of multiply-charged anomalous cosmic ray particles that have been registered in recent times.

Temperature Anisotropy in a Shocked Plasma: Mirror-Mode Instabilities in the Heliosheath

We show that temperature anisotropies induced at a shock can account for interplanetary and planetary bow shock observations. Shocked plasma with enhanced plasma β is preferentially unstable to the mirror-mode instability downstream of a quasi-perpendicular shock and to the fire-hose instability downstream of a quasi-parallel shock, consistent with magnetic fluctuations observed downstream of a large variety of shocks. Our theoretical analysis of the solar wind termination shock suggests that the magnetic holes observed by Voyager 1 in the heliosheath are produced by the mirror-mode instability. The results are also of astrophysical interest, providing an energy source for plasma heating.

Cosmic rays in the dynamic heliosheath

Cosmic ray modulation in the outer heliosphere is discussed from a modeling perspective. Emphasis is on the transport and acceleration of these particles at and beyond the solar wind termination shock in the inner heliosheath region and how this changes over a solar cycle. We will show that by using numerical models, and by comparing results to spacecraft observations, much can be learned about the dependence of cosmic ray modulation on solar cycle changes in the solar wind and heliospheric magnetic field. While the first determines the heliospheric geometry and shock structure, the latter results in a time-dependence of the transport coefficients. Depending on energy, both these effects contribute to cosmic ray intensities in the inner heliosheath changing over a solar cycle.

The Wind‐ISM Interaction of α Tauri

Ultraviolet spectra ofTau (K5 III) obtained by the Hubble Space Telescope (HST) show many emission lines affectedbybroad absorptionfrom thestrongwindof thisred giant star. For the Mgii handklines there isalsoa narrow absorptionfeature in the midst of the windabsorptionthathas beeninterpreted as being fromTau's windYinterstellar medium (ISM) interaction region (i.e., its ''astrosphere''). We try to reproduce this absorption using hydrodynamic models of theTau astrosphere, which show that stellar wind material heated, compressed, and decelerated at the wind's termination shock (TS) can produce significant absorption at about the right velocity. By experimenting with different model input parameters, we findthatthe parameter that the absorptionismostsensitive to isthe ISM pressure, which determines the location of and therefore the density at the TS. However, the models underestimate both the amountofdecelerationatthe TSand the amountof absorptionfor realistic input parameters. We demonstrate thatthese problems can in principle be resolved by modeling the TS as a radiative shock. However, a cooling timescale short enough to affect the postshock flow is only attainable ifTau's wind speed is increased from the 27Y30 km s � 1 values derivedfromfitstowindabsorptiontoatleast35kms � 1 . The modelsalsoseemto require averyhighISM pressure of P/k � 30;000 cm � 3 K toinducedensitiesattheTShighenoughtoyieldsufficient radiativecooling.Thispressureis at least a factor of 2 higher than other estimates of ISM thermal pressure within the Local Bubble. Subject headingg circumstellar matter — stars: individual (� Tauri) — stars: winds, outflows — ultraviolet: stars

Challenges to cosmic ray modeling: From beyond the solar wind termination shock

Voyager 1 crossed the solar wind termination shock on December 16, 2004 at a distance of 94 AU from the Sun, to become the first spacecraft to explore the termination shock region and to enter the heliosheath, the final heliospheric frontier. By the end of 2006, Voyager 1 will be at ∼101 AU, with Voyager 2 at ∼81 AU and still approaching the termination shock. Both spacecraft have been observing the modulation of galactic and anomalous cosmic rays since their launch in 1977. The recent observations close to or inside the heliosheath have provided several interesting ‘surprises’ with subsequent theoretical and modeling challenges. Examples are: what does the modulation of galactic cosmic rays amount to in this region?; how do the anomalous cosmic rays get accelerated and modulated?; why are there ‘breaks’ in the power-law slopes of the spectra of accelerated particles? Several numerical models have been applied to most of these topics over the years and comprehensive global predictions have been made the past decade, thought to be based on reasonable assumptions about the termination shock and the heliosheath. Examples of these predictions and assumptions are concisely discussed within the context of the main observed features of cosmic rays in the vicinity of the termination shock, ending with a discussion of some of the issues and challenges to cosmic ray modeling in particular.

Cosmic ray anisotropies in the outer heliosphere

The observation of the directional distribution of energetic and cosmic ray particles has been done with the Voyager spacecraft over a long period. Since 2002, when the first flux enhancements of charged particles associated with the approach of Voyager 1 to the solar wind termination shock were observed, these anisotropy measurements have become of special interest. They play an important role to understand the magnetic field and shock structure and the basics of the modulation of cosmic ray and anomalous particles at and beyond the termination shock. They also serve as motivation to study the spatial behavior of galactic and anomalous cosmic ray anisotropies with numerical modulation models in order to illustrate how the radial anisotropy, at different energies, change from upstream to downstream of the termination shock. Observations made by Voyager 1 indicate that the termination shock is a complicated region than previously thought, hence the effects of the latitude dependence of the termination shock’s compression ratio and injection efficiency on the radial anisotropies of galactic and anomalous protons will be illustrated. We find that the magnitude and direction of the radial anisotropy strongly depends on the position in the heliosphere and the energy of particles. The effect of the TS on the radial anisotropy is to abruptly increase its value in the heliosheath especially in the A > 0 cycle for galactic protons and in both polarity cycles for anomalous protons. Furthermore, the global effect of the latitude dependence of the shock’s compression ratio is to increase the radial anisotropy for galactic protons throughout the heliosphere, while when combined with the latitude dependence of the injection efficiency this increase depends on modulation factors for anomalous protons and can even alter the direction of the radial anisotropy.

The role of radial perpendicular diffusion and latitude dependent acceleration along the solar wind termination shock

A numerical model, based on Parker’s transport equation, describing the modulation of anomalous cosmic rays and containing diffusive shock acceleration is applied. The role of radial perpendicular diffusion at the solar wind termination shock, and as the dominant diffusion coefficient in the outer heliosphere, is studied, in particular the role it plays in the effectiveness of the acceleration of anomalous protons and helium when its latitude dependence is changed. It is found that the latitudinal enhancement of radial perpendicular diffusion towards the heliospheric poles and along the termination shock has a prominent effect on the acceleration of these particles. It results in a ‘break’ in the energy spectrum for anomalous protons at ∼6.0 MeV, causing the spectral index to change from E −1.38 to E −2.23, but for anomalous helium at ∼3.0 MeV, changing the spectral index from E −1.38 to E −2.30. When approaching the simulated TS, the changes in the modulated spectra as they unfold to a ‘steady’ power law shape at energies below 50 MeV are much less prominent as a function of radial distances when radial perpendicular diffusion is increased with heliolatitude.

Winding movement in power transformers: a comparison of FRA measurement connection methods

Frequency response analysis (FRA) is an effective diagnostic tool for detecting transformer winding movements. Various FRA traces can be measured from a set of transformer winding terminals each of which relate to a different test connection scheme. Practical considerations of test and analysis time dictate that only some of the connections are used, and currently there is no standard test connection. This paper presents a comparison of three FRA measurement connections widely employed in the industry today, namely: end-to-end voltage ratio, input admittance and transfer voltage ratio measurements. Using a simulation model of a 132/11 kV, 30 MVA transformer, FRA traces were generated under these connection schemes and their sensitivity towards three types of winding movement, namely: axial displacement, forced buckling and axial bending was studied. A correlation exists between the FRA measurement results of end-to-end voltage ratio, input admittance and transfer voltage ratio connection methods, provided that the HV neutral is grounded. Among the three connection methods assessed, the transfer voltage ratio connection method has the best sensitivity to axial displacement and forced buckling, whereas the end-to-end voltage ratio method has the best sensitivity towards axial bending

The role of reconnection in the pulsar magnetosphere

The present work is our first attempt to understand the role of reconnection in the pulsar magnetosphere. Our discussion is based on the observationaly infered fact that, as the pulsar spins down, the region of closed corotating dipolar field lines grows with time. This implies that reconnection must take place in the magnetosphere. We argue that non-dissipative reconnection along the equatorial current sheet allows for the continuous channeling of pulsar spindown energy into particle energy, all the way from the light cylinder to the pulsar wind termination shock, and we propose that this effect may account for the low \sigma values inferred by observations. We present a simple model that allows us to relate the magnetic diffusivity in the equatorial current sheet to an observable pulsar parameter, the braking index n. When n~1, the global structure of the magnetosphere approaches that of a relativistic split monopole where the pulsar spindown energy is carried by the electromagnetic field. However, for values of n>1.5, almost all field lines close inside the pulsar wind termination shock, and thus most of the electromagnetic pulsar spindown energy flux is effectively transformed into particle energy in the equatorial current sheet.

ChandraandXMM‐NewtonObservations of the Vela‐like Pulsar B1046−58

We present results from Chandra and XMM-Newton observations of the radio pulsar B1046-58. A high-resolution spatial analysis reveals an asymmetric pulsar wind nebula (PWN), ~6'' × 11'' in size. The combined emission from the pulsar and its PWN is faint, with a best-fit power-law photon index of Γ = 1.7 and unabsorbed luminosity of ~1032 ergs s-1 in the 0.5-10.0 keV range (assuming a distance of 2.7 kpc). A spatially resolved imaging analysis suggests the presence of softer emission from the pulsar. No pulsations are detected from PSR B1046-58; assuming a worst-case sinusoidal pulse profile, we derive a 3 σ upper limit for the pulsed fraction in the 0.5-10.0 keV range of 53%. Extended PWN emission is seen within 2'' of the pulsar; the additional structures are highly asymmetric and extend predominantly to the southeast. We discuss the emission from the PWN as resulting from material downstream of the wind termination shock, as outflow from the pulsar, or as structures confined by a high space velocity. The first two interpretations imply equipartition fields in the observed structures of ≳40-100 μG, while the latter case implies a velocity for the pulsar of ≳190 n km s-1 (where n0 is the ambient number density in units of cm-3). No emission from an associated supernova remnant is detected.

Energetic Particle Intensities and Anisotropies near the Solar Wind Termination Shock

We address the physics of intensity and anisotropy variations in energetic particles near the solar wind termination shock. We show that rapid, large-amplitude, and highly anisotropic particle events seen upstream of the shock can be understood in terms of the passage across the spacecraft of different filled and unfilled magnetic flux tubes. These are created by large-scale magnetic irregularities that are advected passed the spacecraft and compress as they cross the termination shock. We find significant streaming anisotropies of 50 keV-1 MeV protons in the unshocked solar wind, with front-to-back intensity ratios exceeding a factor of 2, for 5 day averaged values. In contrast, downstream of the shock, our results indicate that the particle distributions are much more uniform and isotropic. However, there is a large-scale, nonzero azimuthal particle anisotropy downstream of the shock. This is caused by the nonradial heliosheath plasma flow necessitated by the nonspherical shock. Our results are consistent with recent Voyager 1 observations.