Numerical modeling of the pulsar wind interaction with ISM

Time dependent numerical simulation of relativistic wind interaction with interstellar medium was performed. The winds are ejected from magnetosphere of rotation powered pulsars. The particle flux in the winds is assumed to be isotropic. The energy flux is taken as strongly anisotropic in accordance with prediction of the MHD theory of the relativistic winds. The modeling has been performed for the wind magnetization in the range 3 × 10⁻³–10⁻¹. The numerical solutions reproduce the most spectacular features observed in the central part of plerions: toroidal structure and jet-like features. Increase of the wind’s magnetization results in decrease of the size of the synchrotron nebula.     

TeV gamma-ray observations of pulsar wind nebulae with the HESS detector

High energy stereoscopic system (HESS) is a recent operational detector dedicated to the observation of γ-rays in the very high energy range. Situated in Namibia, it is composed of four Imaging Atmospheric Cherenkov Telescopes and gives a significant improvement in sensitivity and in accuracy of the reconstructed γ-ray parameters. First results on observations of pulsar wind nebulae are reported here. The binary system PSR B1259-63/SS 2883 has been detected in 2004 around the periastron, showing clear flux variations. The pulsar wind nebula around PSR B1706-44 has been observed and upper limits on its flux have been derived.     

A VLBI search for pulsar wind nebulae in supernovae

We examine recent supernovae which have been observed with very-long-baseline interferometry in order to detect or limit the emission from a possible compact remnant of the explosion. Such a remnant could be a neutron star, generating a pulsar wind nebula, or a black hole with an accretion disk and jets. Four supernovae, and also more than a dozen supernovae or their young remnants in M82, have structure sufficiently resolved to allow useful conclusions as to the strength of the emission from such young neutron stars or black holes. We recently discovered a compact component in the center of SN 1986J’s shell with a spectral luminosity at 15 GHz 200 times that of the Crab Nebula. This is most likely the compact remnant of the explosion, the first and only one found in any modern supernova. For other modern supernovae, the upper limits on the radio spectral luminosities of such young compact remnants range from 180 times that of the Crab Nebula for SN 1979C in M100 in the Virgo cluster to 0.001 times that of the Crab Nebula for SN 1987A in the Large Magellanic Cloud.     

The devil is in the details: Compact structures in pulsar wind nebulae

The large-scale structure of pulsar wind nebulae (PWNe) tells us a considerable amount about their average magnetic fields, the total particle input from the pulsar winds, and the confining pressure at their outer boundaries. However, the details of the pulsar outflow, the sites of shocks and particle acceleration, the effects of instabilities in the magnetic field, and the interaction between the relativistic wind and the surrounding ejecta are contained in small-scale structures, where we observe jets and toroidal structures, time-varying emission from compact clumps, and filaments in both the inner and outer regions of the nebulae. Here, I review recent observational studies of compact structures in PWNe and present current scenarios (and questions) regarding their origin.     

Real-time global MHD simulation of the solar wind interaction with the earth’s magnetosphere

We have developed a real-time global MHD (magnetohydrodynamics) simulation of the solar wind interaction with the earth’s magnetosphere. By adopting the real-time solar wind parameters and interplanetary magnetic field (IMF) observed routinely by the ACE (Advanced Composition Explorer) spacecraft, responses of the magnetosphere are calculated with MHD code. The simulation is carried out routinely on the super computer system at National Institute of Information and Communications Technology (NICT), Japan. The visualized images of the magnetic field lines around the earth, pressure distribution on the meridian plane, and the conductivity of the polar ionosphere, can be referred to on the web site (http://www2.nict.go.jp/y/y223/simulation/realtime/).The results show that various magnetospheric activities are almost reproduced qualitatively. They also give us information how geomagnetic disturbances develop in the magnetosphere in relation with the ionosphere. From the viewpoint of space weather, the real-time simulation helps us to understand the whole image in the current condition of the magnetosphere. To evaluate the simulation results, we compare the AE indices derived from the simulation and observations. The simulation and observation agree well for quiet days and isolated substorm cases in general.     

The hard X-ray emission from the Vela pulsar wind nebula

We present the results of a preliminary spectral analysis performed on the BeppoSAX and XMM observations of the Vela plerion. The broad energy range covered by the instruments on board the two observatories allows an evaluation of the spectral parameters of the high energy emission model and provides an indication on the morphology of the source emission above 10 keV. We confirm the softening of the PWN spectrum (3–10 keV band) at distances greater than 4′ from the pulsar and estimate the diameter of the high energy (>10 keV) emission region to be on the order of 25′–30′.     

Solar wind interaction with comet Halley

In March 6 and 9, 1986 the spacecrafts ‘Vega-1’ and ‘Vega-2’ have flown through the coma of comet Halley and have carried measurements of plasma, energetic particles, magnetic field and plasma waves along its trajectory. A short review of these measurements and its comparison with theoretical models of solar wind interaction with comets are given.

The spacecrafts ‘Vega-1’ and ‘Vega-2’ have studied the solar wind loading by cometary ions, the structure of cometary bow shock and the processes in the inner coma of comet Halley. Exactly in this sequence we discuss the results of measurements and compare them with the theory.     

Simulation of cometary jets in interaction with the solar wind

The influence of cometary jets on the solar wind interaction is studied with a 3D hybrid simulation. Anisotropic outgassing patterns were until recently not considered in cometary simulations, despite strong anisotropies found at observations. Comet 67P Churyumov–Gerasimenko, the target of the ROSETTA mission, was chosen as a case study for a simulation series. The cometary outgassing at 2.7 AU is modeled to originate from a single sun-facing jet with different levels of collimation, from isotropy to extremely thin jets. As no bow shock is present at this distance, solar wind patterns resulting from the anisotropic outgassing become more apparent. We find narrower jets to increase the standoff distance of the plasma interaction structures. Also, the Mach cone is wider and stronger for certain jet profiles. The magnetic field remains unable to propagate through the coma, resulting in strong draping patterns for narrow jets due to the increased standoff distance.     

Solar wind interaction with lunar crustal magnetic anomalies

Using Lunar Prospector data, we review the magnetic field and electron signatures of solar wind interaction with lunar crustal magnetic sources. Magnetic field amplifications, too large to represent direct measurements of crustal fields, appear in the solar wind over strong crustal sources, with the chance of observing these amplifications depending on upstream solar wind parameters. We often observe increases in low-energy (?100 eV) electron energy fluxes simultaneously with large magnetic field amplifications, consistent with an increase in plasma density across a shock surface. We also often observe low frequency wave activity in the magnetic field data (both broadband turbulence and monochromatic waves), often associated with electron energization, sometimes up to keV energies. Electron energization appears to be correlated more closely with wave activity than with magnetic amplifications. Detailed studies of the interaction region will be necessary in order to understand the physics of the Moon–solar wind interaction. At present, the Moon represents the only natural laboratory available to us to study solar wind interaction with small-scale crustal magnetic fields, though simulation results and theoretical work can also help us understand the physical processes at work.     

Chandra observation of the interaction between the plasma nebula RCW89 and the pulsar jet of PSR B1509–58

We present a Chandra observation of the H II region RCW89. The nebula lies 10′ north from the central pulsar PSR B1509–58, and it has been suggested that the nebula is irradiated by the pulsar jet. We performed a spectral analysis of the seven brightest emitting regions aligned in a horse-shoe like shape, and found that the temperature of the knots increases along the horse-shoe in the clockwise direction, while, in contrast, the ionization parameter n_et decreases. This result implies that RCW89 was heated in sequence. We examined the energy budget assuming that RCW89 is powered by the pulsar jet. The rate of energy injection into RCW89 by the jet was estimated from the synchrotron radiation flux. We obtained a heating time-scale of 1400 yr, which is consistent with the pulsar characteristic age of 1700 yr. To explain the temperature gradient, we discuss the cooling process for plasma clouds in RCW89. We argue that the plasma clumps can be cooled down by the adiabatic expansion within 250 yr, and form the temperature gradient reflecting the sequential heating by the precessing pulsar jet.     

Comparison of global MHD simulation results with actual storm and substorm events

Global MHD models hold the promise of providing a physics-based understanding of magnetospheric structure and dynamics. As such models have become more sophisticated, and computing power has increased, it is now possible to model actual events using solar wind data as a boundary condition. In this paper we present some results MHD simulations of actual storm and substorm events. We will demonstrate that not only can the simulations reproduce details of events, they also are reproducing fundamental aspects of energy coupling between the solar wind and the magnetosphere in such a manner that we can distinguish storms and substorms.     

The neutral gas in the environs of the Geminga gamma-ray pulsar

We present a high-resolution (24″) study of the HI interstellar gas distribution around the radio-quiet neutron star Geminga. Based on very large array and MPIfR Effelsberg telescope data, we analyzed a 40′ × 40′ field around Geminga. These observations have revealed the presence of a neutral gas shell, 0.4 pc in radius, with an associated HI mass of 0.8M_⊙, which surrounds Geminga at a radial velocity compatible with the kinematical distance of the neutron star. In addition, morphological agreement is observed between the internal face of the HI shell and the brightest structure of Geminga’s tail observed in X-rays. We explore the possibility that this morphological agreement is the result of a physical association.     

MHD modeling of the outer heliosphere: Achievements and challenges

An overview is presented of magnetic-field-related effects in the solar wind (SW) interaction with the local interstellar medium (LISM) and the different theoretical approaches used in their investigation. We discuss the possibility that the interstellar magnetic field (ISMF) introduces north–south and east–west asymmetries of the heliosphere, which might explain observational data obtained by the Voyager 1 and Voyager 2 spacecraft. The SW–LISM interaction parameters that are responsible for the deflection of the interstellar neutral hydrogen flow from the direction of propagation of neutral helium in the inner heliosheath are outlined. The possibility of a strong ISMF, which increases the heliospheric asymmetry and the H–He flow deflection, is discussed. The effect of the combination of a slow-fast solar wind during solar minimum over the Sun’s 11-year activity cycle is illustrated. The consequences of a tilt between the Sun’s magnetic and rotational axes are analyzed. Band-like areas of an increased magnetic field distribution in the outer heliosheath are sought in order to discover regions of possible 2–3 kHz radio emission.     

Non-stationarity and cross-correlation effects in the MHD solar activity

The analysis of turbulent processes in sunspots and pores which are self-organizing long-lived magnetic structures is a complicated and not yet solved problem. The present work focuses on studying such magneto-hydrodynamic (MHD) formations on the basis of flicker-noise spectroscopy using a new method of multi-parametric analysis. The non-stationarity and cross-correlation effects taking place in solar activity dynamics are considered. The calculated maximum values of non-stationarity factor may become precursors of significant restructuring in solar magnetic activity. The introduced cross-correlation functions enable us to judge synchronization effects between the signals of various solar activity indicators registered simultaneously.     

A multiple type-II burst associated with a coronal transient and its MHD simulation

A large coronal transient took place on 8 May 1981. The transient was related to an M7.7/2B flare and was associated with at least two coronal type II bursts. The velocities of the type II bursts were in the range 1100–1800 kms^?1, in excess of the transient velocity of 500–1000 kms^?1. Two dimensional positions of the type II radio sources are available from both the Clark Lake and the Culgoora Radio Observatories. We carry out two dimensional MHD simulations of the event, taking into account the observed velocity, position, and size of the type II bursts. We simulate the multiple shocks observed during the event and their interaction, and discuss some results of the simulation.     

Impacts of the PAH size and the radiation intensity on the IR features of illuminated dust within the reflection nebulae

Interstellar dust grains are illuminated in the reflection nebulae. Under conditions of the PAH size and the intensity of the interstellar radiation field, we follow their impact on the PAH aromatic infrared bands using the numerical DustEM code. For a dust model consisting of PAH, amorphous C and amorphous silicate, the PAH size varies in a range from 0.31 to 4.9 nm while the radiation intensity varies by a scale factor from 0.1 to 10⁴. Various trends of the results show the effect of varying both the PAH size and the radiation intensity on the strength of the aromatic mid-IR bands. Through small PAH sizes less than 0.7 nm, the grain temperature distribution of PAHs shows a small variation within 2–3 K at low radiation intensity while it increases to 15 and 8 K for PAH⁰ and PAH⁺, respectively, at higher radiation intensity. In final the variability in these results reveals the evolution of the dust grains under the physical space conditions of the reflection nebulae. In the mid-IR region, the contributions of PAH⁰ and PAH⁺ in the total SED intensity agree with the proportions of these PAHs observed in some reflection nebulae having higher radiation intensities.     

Application of an MHD simulation to the study of substorms

The substorm mechanism is studied by the numerical solutions obtained from a resistive magnetohydrodynamic (MHD) simulation. After a southward turning of the interplanetary magnetic field (IMF), the simulation results reproduce observed features of the growth phase. The numerical solutions show that the plasma sheet thinning during the growth phase is formed under the dynamic balance between the flux pileup from the midtail and the flux removal toward the dayside controlled by the convection in the magnetosphere-ionosphere (M-I) coupling system. After the growth phase, dipolarization is generated in the near-earth tail accompanied by a plasma injection into the inner magnetosphere, the formation of plasmoid in the midtail, and the enhancement of the nightside field-aligned currents (FACs). The direct cause of this onset is the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organization in the nonlinear system.