Density Cavity Research Articles

Bornavirus Closely Associates and Segregates with Host Chromosomes to Ensure Persistent Intranuclear Infection

Bornaviruses are nonsegmented negative-strand RNA viruses that establish a persistent infection in the nucleus and occasionally integrate a DNA genome copy into the host chromosomal DNA. However, how these viruses achieve intranuclear infection remains unclear. We show that Borna disease virus (BDV), a mammalian bornavirus, closely associates with the cellular chromosome to ensure intranuclear infection. BDV generates viral factories within the nucleus using host chromatin as a scaffold. In addition, the viral ribonucleoprotein (RNP) interacts directly with the host chromosome throughout the cell cycle, using core histones as a docking platform. HMGB1, a host chromatin-remodeling DNA architectural protein, is required to stabilize RNP on chromosomes and for efficient BDV RNA transcription in the nucleus. During metaphase, the association of RNP with mitotic chromosomes allows the viral RNA to segregate into daughter cells and ensure persistent infection. Thus, bornaviruses likely evolved a chromosome-dependent life cycle to achieve stable intranuclear infection.

Three dimensional density cavities in guide field collisionless magnetic reconnection

Particle-in-cell simulations of collisionless magnetic reconnection with a guide field reveal for the first time the three dimensional features of the low density regions along the magnetic reconnection separatrices, the so-called cavities. It is found that structures with further lower density develop within the cavities. Because their appearance is similar to the rib shape, these formations are here called low density ribs. Their location remains approximately fixed in time and their density progressively decreases, as electron currents along the cavities evacuate them. They develop along the magnetic field lines and are supported by a strong perpendicular electric field that oscillates in space. In addition, bipolar parallel electric field structures form as isolated spheres between the cavities and the outflow plasma, along the direction of the low density ribs and of magnetic field lines.

Nonlinear dispersive scale Alfvén waves inmagnetosphere-ionosphere coupling: Physical processes and simulation results

Dispersive Alfven waves (DAWs) have been demonstrated to play a significant role in auroral generation of the magnetosphere-ionosphere coupling system. Starting from a two fluid reduced MHD model, we summarize the frequency, temporal and spatial characteristics of magnetospheric DAWs. Then, the nonlinear kinetic and inertial scale Alfven waves are studied, and we review some theoretical aspects and simulation results of dispersive Alfven waves in Earth’s magnetosphere. It is shown that dispersive standing Alfven waves can generate the field-aligned currents which transport energy into the auroral ionosphere, where it is dissipated by Joule heating and energy lost due to electron precipitation. The Joule dissipation can heat the ionospheric electron and produce changes in the ionospheric Pedersen conductivity. As a feedback, the conducting ionosphere can also strongly affect the magnetospheric currents. The ponderomotive force can cause the plasma to move along the field line, and generate ionospheric density cavity. The nonlinear structuring can lead to a dispersive scale to accelerate auroral particle, and the Alfven waves can be trapped within the density cavity. Finally, we show the nonlinear decay of dispersive Alfven waves related to two anti-propagating electron fluxes observed in the auroral zone.

Strong magnetic field fluctuations within filamentary auroral density cavities interpreted as VLF saucer sources

The Geoelectrodynamics and Electro‐Optical Detection of Electron and Suprathermal Ion Currents (GEODESIC) sounding rocket encountered more than 100 filamentary density cavities associated with enhanced plasma waves at ELF (<3 kHz) and VLF (3–10 kHz) frequencies and at altitudes of 800–990 km during an auroral substorm. These cavities were similar in size (∼20 m diameter in most cases) to so‐called lower‐hybrid cavities (LHCs) observed by previous sounding rockets and satellites; however, in contrast, many of the GEODESIC cavities exhibited up to tenfold enhancements in magnetic wave power throughout the VLF band. GEODESIC also observed enhancements of ELF and VLF electric fields both parallel and perpendicular to the geomagnetic fieldB0within cavities, though the VLF E field increases were often not as large proportionally as seen in the magnetic fields. This behavior is opposite to that predicted by previously published theories of LHCs based on passive scattering of externally incident auroral hiss. We argue that the GEODESIC cavities are active wave generation sites capable of radiating VLF waves into the surrounding plasma and producing VLF saucers, with energy supplied by cold, upward flowing electron beams composing the auroral return current. This interpretation is supported by the observation that the most intense waves, both inside and outside cavities, occurred in regions where energetic electron precipitation was largely inhibited or absent altogether. We suggest that the wave‐enhanced cavities encountered by GEODESIC were qualitatively different from those observed by earlier spacecraft because of the fortuitous timing of the GEODESIC launch, which placed the payload at apogee within a substorm‐related return current during its most intense phase, lasting only a few minutes.

MODELING THE JET KINEMATICS OF THE BLACK HOLE MICROQUASAR XTE J1550–564: A CONSTRAINT ON SPIN-ORBIT ALIGNMENT

Measurements of black hole spin made using the continuum-fitting method rely on the assumption that the inclination of the black hole's spin axis to our line of sight is the same as the orbital inclination angle of the host binary system. The X=ray and radio jet data available for the microquasar XTE J1550-564 offer a rare opportunity to test this assumption. Following the work of others, we have modeled these data and thereby determined the inclination angle of the jet axis, which is presumed to be aligned with the black hole's spin axis. We find a jet inclination angle of approximately 71 degrees and place an upper limit on the difference between the spin and orbital inclinations of < 12 degrees (90% confidence). Our measurement tests for misalignment along the line of sight while providing no constraint perpendicular to this plane. Our constraint on the misalignment angle supports the prediction that the spinning black hole in XTE J1550-564 has aligned itself with the orbital plane and provides support for the measurement of its spin via the continuum-fitting method. Our conclusions are based on a simple and reasonable model of a pair of symmetric jets propagating into a low density cavity whose western wall is approximately 20% closer to XTE J1550-564 than its eastern wall.

Dynamics of rising magnetized cavities and ultrahigh energy cosmic ray acceleration in clusters of galaxies

We study the expansion of low density cavities produced by Active Galactic Nuclei jets in clusters of galaxies. The long term stability of these cavities requires the presence of linked magnetic fields. We find solutions describing the self-similar expansion of structures containing large-scale electromagnetic fields. Unlike the force-free spheromak-like configurations, these solutions have no surface currents and, thus, are less susceptible to resistive decay. The cavities are internally confined by external pressure, with zero gradient at the surface. If the adiabatic index of the plasma within the cavity is $\Gamma>4/3$, the expansion ultimately leads to the formation of large-scale current sheets. The resulting dissipation of the magnetic field can only partially offset the adiabatic and radiative losses of radio emitting electrons. We demonstrate that if the formation of large-scale current sheets is accompanied by explosive reconnection of the magnetic field, the resulting reconnection layer can accelerate cosmic rays to ultra high energies. We speculate that the enhanced flux of UHECRs towards Centaurus A originates at the cavities due to magnetic reconnection.

Simultaneous radio and optical observations of auroral structures: Implications for AKR beaming

[1] Polar spacecraft observations of auroral kilometric radiation (AKR) spectra, combined with simultaneous optical images of the auroral oval at the source foot points, show a strong correlation with discrete auroral features. We discuss several examples of distinctive “V”-shaped profiles in time-frequency AKR spectra for which the time of nadir passage (bottom of V) coincides with the spacecraft crossing field lines connected to discrete auroral arcs. These results support models of AKR emission originating in density cavities with radiation initially confined to a “tangent plane” but strongly refracted upward in a frequency-dependent manner. The inferred spatial distributions of AKR sources are closely coincident with field lines connecting discrete auroral arcs, indicating that discrete arcs are highly correlated with robust AKR emission on the same field lines, as first suggested by Gurnett (1974) several decades ago.

Characterization of a Penning discharge for investigation of auroral radio wave generation mechanisms

Auroral Kilometric Radiation (AKR), observed by satellites in the Earth's magnetosphere, is naturally generated in regions of partial plasma depletion (auroral density cavity) in the polar magnetosphere at approximately 3200 km altitude. As an electron descends through these regions of partial plasma depletion along magnetic field lines towards the Earth's ionosphere, the field lines increases and, through conservation of the magnetic moment, the electron gives up axial velocity in favour of perpendicular velocity. This results in a horseshoe-shaped distribution function in parallel/perpendicular-velocity space which is unstable to X-mode radiation, near the cyclotron frequency. Power levels as high as GW levels have been recorded with frequencies around 300 kHz. The background plasma frequency within the auroral density cavity is approximately 9 kHz corresponding to a plasma density 1 cm−3. A laboratory experiment scaled from auroral frequency to microwave frequency has previously been reported. Here, the addition of a Penning trap to simulate the background plasma of the density cavity is reported, with measurements ne ∼ 2 × 1014–2.17 × 1015 m−3, fpe ∼ 128–418 MHz and fce ∼ 5.21 GHz giving a ratio of ωce/ωpe comparable to the magnetospheric AKR source region.

Colloquium: Nonlinear collective interactions in quantum plasmas with degenerate electron fluids

The current understanding of some important nonlinear collective processes in quantum plasmas with degenerate electrons is presented. After reviewing the basic properties of quantum plasmas, we present model equations (e.g. the quantum hydrodynamic and effective nonlinear Schr\"odinger-Poisson equations) that describe collective nonlinear phenomena at nanoscales. The effects of the electron degeneracy arise due to Heisenberg's uncertainty principle and Pauli's exclusion principle for overlapping electron wavefunctions that result in tunneling of electrons and the electron degeneracy pressure. Since electrons are Fermions (spin-1/2), there also appears an electron spin current and a spin force acting on electrons due to the Bohr magnetization. The quantum effects produce new aspects of electrostatic (ES) and electromagnetic (EM) waves in a quantum plasma that are summarized in here. Furthermore, we discuss nonlinear features of ES ion waves and electron plasma oscillations (ESOs), as well as the trapping of intense EM waves in quantum electron density cavities. Specifically, simulation studies of the coupled nonlinear Schr\"odinger (NLS) and Poisson equations reveal the formation and dynamics of localized ES structures at nanoscales in a quantum plasma. We also discuss the effect of an external magnetic field on the plasma wave spectra and develop quantum magnetohydrodynamic (Q-MHD) equations. The results are useful for understanding numerous collective phenomena in quantum plasmas, such as those in compact astrophysical objects, in plasma-assisted nanotechnology, and in the next-generation of intense laser-solid density plasma interaction experiments.

Formation of density cavities with a nonstationary electric field in the zone of auroral field-aligned currents

The formation of small-scale density cavities with a nonstationary electric field, which are registered in the Earth’s auroral magnetosphere, has been analyzed. It has been indicated that cavities are probably initially caused by quasi-static field-aligned electric currents and currents of kinetic Alfven waves exceeding threshold values. Urgent variants of the linear and nonlinear stages of density disturbance instabilities have been considered. The determined properties of the parameters of small-scale density cavities and nonstationary electric fields are in agreement with the known experimental data.

Density cavity in magnetic reconnection diffusion region in the presence of guide field

[1] Understanding the structure of the diffusion region of magnetic reconnection is crucial to pinpoint the mechanism of energy conversion from magnetic field to plasma. Characteristics of a diffusion region with guide field (i.e., component reconnection) may be significantly different from those of a diffusion region without guide field (i.e., antiparallel reconnection). In this study, we attempt to understand the structure of a diffusion region with guide field by studying the density cavity along separatrix. We present an event in which a density cavity was detected by the Cluster spacecraft in a diffusion region in the presence of guide field. The cavity was located around the separatrix region on the southern hemisphere of the neutral sheet and earthward of the X-line and was coincident with strong magnetic field compression. The width of the cavity was on the ion inertial scale. This cavity contained a relatively strong antiparallel current, which was mainly contributed by parallel streaming electrons with energy of 1–10 keV. Enhancements of lower hybrid wave and electromagnetic whistler wave were observed inside the cavity. These waves are probably excited by parallel streaming electrons along separatrix via electron beam instability. Two-dimensional electromagnetic particle-in-cell simulation was employed to study the structure of the density cavity. The location and scale of the cavity and the signature of electric current and electron velocity are consistent with our observations. It is found that there was displacement between the position of electron density minimum and out-of-plane magnetic field maximum in reconnection with guide field. However, this displacement is much less than that in reconnection without guide field. There was no significant acceleration for electrons to reach energy larger than 30 keV at the cavity.

Cavitation by Nonlinear Interaction Between Inertial Alfvén Waves and Magnetosonic Waves in Low Beta Plasmas

This paper presents the model equations governing the nonlinear interaction between dispersive Alfven wave (DAW) and magnetosonic wave in the low-β plasmas (β≪me/mi; known as inertial Alfven waves (IAWs); here \(\upbeta = 8\pi n_{0}T /B_{0}^{2}\) is thermal to magnetic pressure, n0 is unperturbed plasma number density, T(=Te≈Ti) represents the plasma temperature, and me(mi) is the mass of electron (ion)). This nonlinear dynamical system may be considered as the modified Zakharov system of equations (MZSE). These model equations are solved numerically by using a pseudo-spectral method to study the nonlinear evolution of density cavities driven by IAW. We observed the nonlinear evolution of IAW magnetic field structures having chaotic behavior accompanied by density cavities associated with the magnetosonic wave. The relevance of these investigations to low-β plasmas in solar corona and auroral ionospheric plasmas has been pointed out. For the auroral ionosphere, we observed the density fluctuations of ∼ 0.07n0, consistent with the FAST observation reported by Chaston et al. (Phys. Scr.T84, 64, 2000). The heating of the solar corona observed by Yohkoh and SOHO may be produced by the coupling of IAW and magnetosonic wave via filamentation process as discussed here.

Electron dynamics in collisionless magnetic reconnection

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.

Ion-acoustic enhancements generated by beam-plasma instability in an auroral cavity

[1] This article demonstrates the generation of enhanced ion-acoustic waves by beam-plasma instability in a density cavity. The self-consistent equations of weak turbulence theory that include quasi-linear, decay, and scattering processes as well as convective and dispersive effects are numerically solved for a one-dimensional density cavity. It is shown that significant enhancements of ion-acoustic waves occur in the presence of counterstreaming electron beams and that the enhanced ion-acoustic waves are initially localized near the center of the density cavity at large wavelengths. Later in the evolution, the enhancement in the spectrum of ion-acoustic waves spreads out toward the edges of the cavity, with a shift to smaller wavelengths, while the enhancement near the center of the cavity tends to decrease in magnitude. The significance of the present findings is discussed.

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

[1] With the help of a 2.5-D particle-in-cell simulation code, we investigate the physics of the acceleration of auroral electrons, through the interaction of an isolated Alfven wave packet with a plasma density cavity. The cavity is edged by density gradients perpendicular to the magnetic field. We show that a single passing of an isolated wave packet over a (infinite) cavity creates an electron beam. It triggers local current and beam-plasma instabilities and small-scale coherent electric structures. The energy flux of downgoing electrons is significantly increased, whereas upgoing electrons are also accelerated, even if no beam is formed. Accelerated electrons remain after the passage of the Alfvenic pulse, allowing the observation of energetic particles without any significant electromagnetic perturbation. The dependence of this process on the electron to ion mass ratio is consistent with its control by inertial effects.

Spatiotemporal features of the auroral acceleration region as observed by Cluster

The two major agents for producing aurora are generally believed to be the quasi-static parallel electric fields, accelerating electrons in the auroral acceleration region (AAR), and Alfven waves. The Cluster spacecraft quartet has made multi-spacecraft measurements in the AAR possible for the first time. Four event studies are included and discussed in this thesis, using Cluster data inside and at the top of the AAR, to address various open issues regarding the nature of the quasistatic electric potential structures, such as their altitude distribution, temporal and spatial variability, as well as their interactions with regions of Alfven wave activity.In Paper 1, Cluster data from the upper and lower parts of the AAR were used to determine the altitude and latitude distribution of the acceleration potential above the aurora and to address its stability in space and time. The acceleration potential pattern derived consisted of two equally broad and intense U-shaped electric potential structures and a narrower S-shaped potential structure located below. The acceleration potential distribution was found to be stable, both in intensity and width, over five minutes. Furthermore, the perpendicular (to the magnetic field) spatial scale of the electric field was found to be much smaller than that of the current in the lower AAR, but roughly equal in the upper AAR. Revealing of these features was possible only by combining data from the two Cluster spacecraft.In Paper 2, the spatial and temporal characteristics and development of two AAR structures were studied, benefiting from a magnetic conjunction between two of the Cluster spacecraft and a short time difference (~1 min) between the spacecraft crossings. The configuration allowed for estimating the characteristic times of development for the two structures and the parallel electric field and potential drop for the more stable one. Potential structure 1, having a perpendicular width of ~80 km was short-lived, developing in less than 40 seconds and decaying in one minute. The parallel potential drop between 1.13 and 1.3 RE altitudes increased, whereas that above 1.3 RE remained almost unchanged during this time. For potential structure 2, having a width of ~50 km, the parallel potential drop increased by a factor of 3 below 1.3 RE during ~40 seconds, after which it remained stable for at least a minute. Also here, the parallel potential drop above 1.3 RE remained roughly unchanged. An average parallel electric field was estimated to be 0.56 mV/m.In the auroral zone, the quasi-static properties are dominant in the plasma sheet (PS) and especially in the central plasma sheet (CPS), while the Alfvenic properties are generally strongest at the plasma sheet boundary layer (PSBL). Therefore it is of special interest to study the PSBL/CPS boundary, regarding how the two processes interact. In Papers 3 and 4 we addressed this matter. In each event, data from the Cluster fleet making an equatorward crossing of the highaltitude AAR were used together with a DMSP UV image of the oval. The particle and field data were used to infer the acceleration potentials of the observed arcs and their distribution in altitude and latitude. In Paper 3, observations demonstrate a quasi-static potential structure extending into the Alfvenic region of the polar cap boundary (PCB). The associated density cavity did not extend into the Alfvenic region, suggesting that the Alfvenic activity observed within the PCB region prevents the cavity formation. The results show that Alfvenic and quasi-static acceleration operate jointly in the PCB region. In Paper 4, Cluster passed over a system of East-West aligned auroral arcs. The upper extent of the AAR was found to be at least 0.5 RE higher in altitude than generally expected. Overlapping Alfvenic and quasi-static regions were found within the PSBL and inside Region 2 of downward currents. Growth of small-scale potential structures was observed in the PSBL and in the middle of Region 2, during the ~1.5 minutes between the Cluster 4 and Cluster 3 passages. During this period, the Alfvenic regions retreated both at the poleward and equatorward oval boundaries, while the quasi-static potentials intensified.

Nonlinear lower-hybrid drift solitons in inhomogeneous plasmas

We investigate the nonlinear lower-hybrid wave in inhomogeneous plasmas with finite electron temperature by using the two-fluid theory. A localized soliton is found and the relationships of the amplitude and width of the soliton with the electron temperature gradient are derived. Furthermore, we consider the effect of electron number density κne0 on the profile of the soliton and find that the perturbed density collapses and forms density cavities when κne0 is above a certain value.

Conditions for establishing quasistable double layers in the Earth’s auroral upward current region

The strength and stability of simulated double layers at the ionosphere-auroral cavity boundary have been studied as a function of cold ionospheric electron temperature and density. The simulations are performed with an open boundary one-dimensional particle-in- cell (PIC) simulation and are initialized by imposing a density cavity within the simulation domain. The PIC simulation includes H+ and O+ ion beams, a hot H+ background population, cold ionospheric electrons, and a hot electron population. It is shown that a double layer remains quasistable for a variety of initial conditions and plasma parameters. The average potential drop of the double layer is found to increase as the cold electron temperature decreases. However, in terms of cold electron density, the average potential drop of the double layer is found to increase up to some critical cold electron density and decreases above this value. Comparisons with FAST observations are made and agreement is found between simulation results and observations in the shape and width of the double layer. This study helps put a constraint on the plasma conditions in which a DL can be expected to form and remain quasistable.

Growth of the Langmuir cavity eigenmodes in the solar wind

The Langmuir waves in the solar wind are known to be at the origin of many types of solar radio emissions, in particular solar type II and type III radio bursts. In situ observations have shown that the largest amplitude Langmuir waves often appear as localized wave packets. Several models have been proposed to explain this localization of the waves, such as kinetic localization, nonlinear wave‐wave processes, or eigenmode formation. The latter theory states that the wave packets are eigenmodes of density cavities generated by background turbulence. In the present paper we investigate the wave growth of the localized Langmuir eigenmodes.

Forward modelling to determine the observational signatures of white-light imaging and interplanetary scintillation for the propagation of an interplanetary shock in the ecliptic plane

Recent coordinated observations of interplanetary scintillation (IPS) from the EISCAT, MERLIN, and STELab, and stereoscopic white-light imaging from the two heliospheric imagers (HIs) onboard the twin STEREO spacecraft are significant to continuously track the propagation and evolution of solar eruptions throughout interplanetary space. In order to obtain a better understanding of the observational signatures in these two remote-sensing techniques, the magnetohydrodynamics of the macro-scale interplanetary disturbance and the radio-wave scattering of the micro-scale electron-density fluctuation are coupled and investigated using a newly constructed multi-scale numerical model. This model is then applied to a case of an interplanetary shock propagation within the ecliptic plane. The shock could be nearly invisible to an HI, once entering the Thomson-scattering sphere of the HI. The asymmetry in the optical images between the western and eastern HIs suggests the shock propagation off the Sun–Earth line. Meanwhile, an IPS signal, strongly dependent on the local electron density, is insensitive to the density cavity far downstream of the shock front. When this cavity (or the shock nose) is cut through by an IPS ray-path, a single speed component at the flank (or the nose) of the shock can be recorded; when an IPS ray-path penetrates the sheath between the shock nose and this cavity, two speed components at the sheath and flank can be detected. Moreover, once a shock front touches an IPS ray-path, the derived position and speed at the irregularity source of this IPS signal, together with an assumption of a radial and constant propagation of the shock, can be used to estimate the later appearance of the shock front in the elongation of the HI field of view. The results of synthetic measurements from forward modelling are helpful in inferring the in-situ properties of coronal mass ejection from real observational data via an inverse approach.