K. W. Min

Electrostatic Korteweg-deVries solitary waves in a plasma with Kappa-distributed electrons

The Korteweg-deVries (KdV) equation that describes the evolution of nonlinear ion-acoustic solitary waves in plasmas with Kappa-distributed electrons is derived by using a reductive perturbation method in the small amplitude limit. We identified a dip-type (negative) electrostatic KdV solitary wave, in addition to the hump-type solution reported previously. The two types of solitary waves occupy different domains on the κ (Kappa index)-V (propagation velocity) plane, separated by a curve corresponding to singular solutions with infinite amplitudes. For a given Kappa value, the dip-type solitary wave propagates faster than the hump-type. It was also found that the hump-type solitary waves cannot propagate faster than V = 1.32.

Simulation of the Kelvin-Helmholtz Instability in the Magnetized Slab Jet

Two-dimensional magnetohydrodynamic simulations are performed to study the evolution of the Kelvin-Helmholtz instability for a slab jet moving parallel to the magnetic field. Surface waves are excited at each of the side boundaries, and the time evolution of the individual mode, as well as its interaction with the mode excited at the opposite boundary, is observed. The instability is very disruptive when the excited perturbations on the two side boundaries are antisymmetric. The long-term evolution of the antisymmetric perturbation shows a kinklike magnetic field structure after multiple reconnection events, while the symmetric perturbation yields wavy signatures only around the jet boundaries. Three different cases of the thin slab jet are considered with the antisymmetric perturbation: a hydrodynamic jet, a jet embedded in a uniform magnetic field, and a thermally confined magnetized jet. Although the imposed magnetic fields are weak, they show quite different evolutions. In the hydrodynamic case, the density and flow patterns associated with the vortices survive until the end of the simulation without significant changes after the initial development of the instability. For the jet embedded in a uniform magnetic field, the density pattern associated with the vortex is destroyed as magnetic reconnection develops, and a steady increase in density from the central jet region toward the side boundaries is seen at the final stage when the magnetic field becomes flat again. The density and flow structures in the thermally confined magnetized jet are similar to those of the hydrodynamic case, but they change slowly with the evolution of the magnetic field. Also, it is seen that the magnetic fields that originally reside in the jet are expelled out to the side boundaries. Similar results are obtained for both the transonic jet and the supersonic jet, but the density variation is more significant in the supersonic case.

Molecular Hydrogen Fluorescence in the Eridanus Superbubble

The first far-ultraviolet (1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. Here we compare the observed emission features with those from a photodissociation region model with assumed illuminating stellar fields. The result shows rather high line ratios I1580/I1610, which may imply the existence of high-temperature molecular clouds in the region. The H2 fluorescence intensity shows a proportional correlation with Hα emission, indicating that the fluorescence and the recombination emission have similar physical origins.

Diffuse Far-Ultraviolet Observations of the Taurus Region

Diffuse far-ultraviolet (FUV; 1370-1670 A) flux from the Taurus molecular cloud region has been observed with the SPEAR/FIMS imaging spectrograph. An FUV continuum map of the Taurus region, similar to the visual extinction maps, shows a distinct cloud core and halo region. The dense cloud core, where the visual extinction Av > 1.5, obscures the background diffuse FUV radiation, while scattered FUV radiation is seen in and beyond the halo region, where Av < 1.5. The total intensity of H2 fluorescence in the cloud halo is I = 6.5 × 104 photons cm-2 s-1 sr-1 in the 1370-1670 A wavelength band. A synthetic model of the H2 fluorescent emission fits the present observation best with a hydrogen density nH = 50 cm-3, H2 column density N(H2) = 0.8 × 1020 cm-2, and incident FUV intensity IUV = 0.2. H2 fluorescence is not seen in the core, presumably because the required radiation flux to induce fluorescence is unable to penetrate the core region.

Far-Ultraviolet Observations of a Thermal Interface in the Orion-Eridanus Superbubble

Diffuse far-UV emission arising from the edge of the Orion-Eridanus superbubble has been observed with the SPEAR imaging spectrometer, revealing numerous emission lines arising from both atomic species and H2. Spatial variations in line intensities of C IV, Si II, and O VI, in comparison with soft X-ray, Hα, and dust data, indicate that these ions are associated with processes at the interface between hot gas inside the bubble and the cooler ambient medium. Thus our observations probe the physical conditions of an evolved thermal interface in the interstellar medium.

Simulation of a Rapid Dropout Event for Highly Relativistic Electrons with the RBE Model

A flux dropout is a sudden and sizable decrease in the energetic electron population of the outer radiation belt on the time scale of a few hours. We simulated a flux dropout of highly relativistic >2.5 MeV electrons using the Radiation Belt Environment model, incorporating the pitch angle diffusion coefficients caused by electromagnetic ion cyclotron (EMIC) waves for the geomagnetic storm event of 23–26 October 2002. This simulation showed a remarkable decrease in the >2.5 MeV electron flux during main phase of the storm, compared to those without EMIC waves. This decrease was independent of magnetopause shadowing or drift loss to the magnetopause. We suggest that the flux decrease was likely to be primarily due to pitch angle scattering to the loss cone by EMIC waves. Furthermore, the >2.5 MeV electron flux calculated with EMIC waves correspond very well with that observed from Solar Anomalous and Magnetospheric Particle EXplorer spacecraft. EMIC wave scattering is therefore likely one of the key mechanisms to understand flux dropouts. We modeled EMIC wave intensities by the Kp index. However, the calculated dropout is a several hours earlier than the observed one. We propose that Kp is not the best parameter to predict EMIC waves.

A study of the early-stage evolution of relativistic electron-ion shock using three-dimensional particle-in-cell simulations

We report the results of a 3D particle-in-cell (PIC) simulation carried out to study the early-stage evolution of the shock formed when an unmagnetized relativistic jet interacts with an ambient electron-ion plasma. Full-shock structures associated with the interaction are observed in the ambient frame. When open boundaries are employed in the direction of the jet; the forward shock is seen as a hybrid structure consisting of an electrostatic shock combined with a double layer, while the reverse shock is seen as a double layer. The ambient ions show two distinct features across the forward shock: a population penetrating into the shocked region from the precursor region and an accelerated population escaping from the shocked region into the precursor region. This behavior is a signature of a combination of an electrostatic shock and a double layer. Jet electrons are seen to be electrostatically trapped between the forward and reverse shock structures showing a ring-like distribution in a phase-space plot, while ambient electrons are thermalized and become essentially isotropic in the shocked region. The magnetic energy density grows to a few percent of the jet kinetic energy density at both the forward and the reverse shock transition layers in a rather short time scale. We see little disturbance of the jet ions over this time scale.

Ion-acoustic solitary waves in ion-beam plasma with Boltzmann electrons

Particle simulations and a pseudopotential method were used to study ion-acoustic solitary waves in a plasma composed of Boltzmann electrons and kinetic beam ions. Pseudopotential theory was first applied to determine how ion-acoustic solitary waves can exist in a two-component plasma. Then, particle simulations were carried out, wherein ion-acoustic solitary waves were excited by modulating the bias grid voltage in a double plasma model. For the modulation, the potential of the grid bias was rapidly decreased, such that hump-type ion-acoustic solitary waves with good Gaussian shape were excited one after another, forming a train of waves. The simulation also showed that the phase velocities of ions decrease sharply when the solitary wave occurs, which indicates that the solitary ion-acoustic wave is excited via the inverse Landau damping process.

Global Far-Ultraviolet Image of the Eridanus Superbubble Observed by FIMS/SPEAR

We present the first far-ultraviolet (FUV; 1350-1750 A) diffuse emission map of the Eridanus superbubble, obtained using the FIMS/SPEAR instrument. The features seen in the FUV image closely resemble those seen in the Hα map, including two prominent arcs identified earlier in Hα. While it has been argued that the FUV emission in this region is mostly due to the scattering of star light by dust, a close spectral examination reveals that one of the arcs is abundant in molecules and dust while the other is mainly composed of atomic species. Upon comparison to emission maps in other bands (X-ray, IR, optical Hα), we propose the most plausible geometrical structure of this region.

New features of the X-ray dip source X1755-338

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