3D Numerical Simulations of <Emphasis Type="Italic">f</Emphasis>-Mode Propagation Through Magnetic Flux Tubes

Three-dimensional numerical simulations have been used to study the scattering of a surface-gravity wave packet by vertical magnetic-flux tubes, with radii from 200 km to 3 Mm, embedded in stratified polytropic atmosphere. The scattered wave has been found to consist primarily of m=0 (axisymmetric) and m=1 modes. The ratio of the amplitude of these two modes was found to be strongly dependent on the radius of the flux tube. The kink mode is the dominant mode excited in tubes with a small radius, while the sausage mode is dominant for large tubes. Simulations of this type provide a simple, efficient, and robust way to start to understand the seismic signature of flux tubes, which have recently begun to be observed.     

On the Helical waves in the tail of Comet Morehouse 1908

Helical waves in the tail of Comet Morehouse are studied in the WKB approximation with the plasma non-uniformity along the tail taken into account. A comet tail boundary is considered as a transition layer of thickness d, with the linear velocity profile within it. This approach enables us to set the limits of applicability of the tangential discontinuity model: for wavelengths λ which satisfy the condition $\text{2πd}\text{λ}\text{< 10}{}^{\mathrm{?2}}$ both models are practically coincident. The thickness d of the tail boundary is derived from the observations of the increase of the wave amplitude along the tail axis: d ≈ (15–20) × 10³ km. With such a thickness stability conditions turn out to be marginal. This could explain why helical waves in type-1 comet tails are seen only occasionally and not at all times.     

Asymmetry of compton scattering by relativistic electrons with an isotropic velocity distribution: Astrophysical implications

The angular distribution of low-frequency radiation after a single scattering by relativistic electrons with an isotropic velocity distribution differs markedly from the Rayleigh angular function. In particular, the scattering by an ensemble of ultrarelativistic electrons is described by the law p=1?cosα, where α is the scattering angle. Thus, photons are mostly scattered backward. We discuss some consequences of this fact for astrophysical problems. We show that a hot atmosphere of scattering electrons is more reflective than a cold one: the fraction of incident photons reflected after a single scattering can be larger than that in the former case by up to 50%. This must affect the photon exchange between cold accretion disks and hot coronae (or advective flows) near relativistic compact objects, as well as the rate of cooling (through multiple inverse-Compton scattering of the photons supplied from outside) of optically thick clouds of relativistic electrons in compact radio sources. Scattering asymmetry also causes the spatial diffusion of photons to proceed more slowly in a hot plasma than in a cold one, which affects the shapes of Comptonization spectra and the time delay in the detection of soft and hard radiation from variable X-ray sources.     

Backscatter of solar resonance radiation—I

A simple model which essentially neglects temperature effects, but which includes gravity, radiation pressure, photoionization and charge exchange, is used to calculate the angular dependence of the intensity of solar Lyman α resonantly scattered from neutral interstellar hydrogen which has penetrated the solar system; the results are then compared with the observations. The resonant scattering of HeI λ584 is also treated.     

Three-dimensional numerical modeling of refraction and reflection scattering from icy galilean satellites

Radar scattering from the icy galilean satellites is marked by unusually high backscatter cross sections and polarization ratios at wavelengths λ₀=3.5-70 cm. The persistence of exotic scattering behavior over this large a wavelength range suggests that the responsible mechanisms remain at least partially effective as the wavelength approaches or exceeds the size of individual scatterers. We examine two models previously analyzed in the geometrical optics limit—radar glory from buried craters (Eshleman, 1986, Science 234, 587-590) and refraction scattering from subsurface lenses (Hagfors et al., 1985, Nature 315, 637-640)—at wavelength scales using three-dimensional finite-difference time-domain (FDTD) numerical simulations. We include craters with rough walls and lenses with random inclusions of heterogeneous material. For hemispherical craters spanning up to 3λ₀ in diameter, we observe none of the exotic backscatter behavior attributed to the geometrical optics models. Nonspherical refraction scatterers can produce circular polarization ratios μ_C>1 and linear polarization ratios μ_L=0.5-0.8 at diameters as small as ∼λ₀, but the density of such inclusions must be high if refraction scattering alone is to account for the measured cross sections.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

The propagation of magnetohydrodynamic (MHD) waves is an area that has been thoroughly studied for idealised static and steady state magnetised plasma systems applied to numerous solar structures. By applying the generalisation of a temporally varying background density to an open magnetic flux tube, mimicking the observed slow evolution of such waveguides in the solar atmosphere, further investigations into the propagation of both fast and slow MHD waves can take place. The assumption of a zero-beta plasma (no gas pressure) was applied in Williamson and Erdélyi (Solar Phys. 2013, doi: 10.1007/s11207-013-0366-9 , Paper I) is now relaxed for further analysis here. Firstly, the introduction of a finite thermal pressure to the magnetic flux tube equilibrium modifies the existence of fast MHD waves which are directly comparable to their counterparts found in Paper I. Further, as a direct consequence of the non-zero kinetic plasma pressure, a slow MHD wave now exists, and is investigated. Analysis of the slow wave shows that, similar to the fast MHD wave, wave amplitude amplification takes place in time and height. The evolution of the wave amplitude is determined here analytically. We conclude that for a temporally slowly decreasing background density both propagating magnetosonic wave modes are amplified for over-dense magnetic flux tubes. This information can be very practical and useful for future solar magneto-seismology applications in the study of the amplitude and frequency properties of MHD waveguides, e.g. for diagnostic purposes, present in the solar atmosphere.     

Orbital period investigations of two short-period early-type overcontact binaries BH Cen and V701 Sco in two extremely young galactic clusters IC 2944 and NGC 6383

Both V701 Sco and BH Cen are two early-type short-period overcontact systems (P = 0.^d762 and P = 0.^d792, respectively). V701 Sco is a member of the young galactic cluster NGC 6383, while BH Cen is a component of a younger galactic cluster IC 2944 where star formation is in process. They provide good opportunity to understand the formation and evolution of binary stars. In the present paper, orbital period changes of the two binaries are investigated. It is discovered that the orbital period of BH Cen shows a long-term increase with a rate of dP/dt = +1.70(±0.39) × 10⁻⁷ days/year while it undergoes a cyclic oscillation with a period of 44.6 years and an amplitude of A₃ = 0.^d0216. For V701 Sco, its O-C curve reveals a periodic change with a period of 41.2 years and amplitude of A₃ = 0.^d0158. The mass ratio of BH Cen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a mass ratio of unit. The continuous period increase of BH Cen is caused by the mass transfer from the less massive component to the more massive one at a rate of dM₂/dt = 3.5 × 10⁻⁶ days/year.The cyclic period changes of both systems can be plausibly explained as the results of light-travel time effects suggesting that they are triple systems. The astrophysical parameters of the unseen tertiary components in the two systems have been determined. We think that the invisible tertiary components in both binaries played an important role in the formations and evolutions of the overcontact configurations by bringing angular momentum out from the central systems. For BH Cen, this process created the initial short period and will support its evolution into an overcontact configuration via a Case A mass transfer within the life time of the extremely young cluster IC 2944. For V701 Sco, two identical zero-age main-sequence components in an overcontact configuration suggest that it may have been formed by fission, possibly by the fission of the third body. The fact that no long-term continuous period variations were found for V701 Sco may suggest that an overcontact binary with the mass ratio of unity can be in an equilibrium revealing that the original configuration of the binary was overcontact as is its present state. It has been reported that faint stars in the two extremely young clusters are relatively scare. From the present study, it is shown that faint stars in young clusters are usually formed as companions of OB stars (including binaries). It is very difficult to detect them because of their low luminosity when compared with the more luminous OB stars.     

Effects of Dispersion of Wave Packets in the Determination of Lifetimes of High-Degree Solar p Modes from Time – Distance Analysis: TON Data

We use the method of time – distance analysis to measure lifetimes of solar p modes in the range ℓ=100 − 600 and ν=3.0 − 4.5 mHz with data taken with the Taiwan Oscillation Network (TON). The lifetimes of p modes are determined by the changes in the amplitude and width of the cross-correlation function of a wave packet with the number of skips. The amplitude of the cross-correlation function decreases exponentially with the number of skips as in previous work. This decrease has been interpreted as the effect of the finite p-mode lifetime. In this study, we find that the width of the cross-correlation function increases with the number of skips. We interpret this phenomenon as the effect of the dispersion of the wave packet. We include this effect in the determination of the lifetime of the wave packet. The lifetime increases after the dispersion is taken into account. We also study the change in lifetime between solar minimum and maximum.     

Linear MHD Wave Propagation in Time-Dependent Flux Tube

MHD waves and oscillations in sharply structured magnetic plasmas have been studied for static and steady systems in the thin tube approximation over many years. This work will generalize these studies by introducing a slowly varying background density in time, in order to determine the changes to the wave parameters introduced by this temporally varying equilibrium, i.e. to investigate the amplitude, frequency, and wavenumber for the kink and higher order propagating fast magnetohydrodynamic wave in the leading order approximation to the WKB approach in a zero-β plasma representing the upper solar atmosphere. To progress, the thin tube and over-dense loop approximations are used, restricting the results found here to the duration of a number of multiples of the characteristic density change timescale. Using such approximations it is shown that the amplitude of the kink wave is enhanced in a manner proportional to the square of the Alfvén speed, $V_{\mathrm{A}}^{2}$ . The frequency of the wave solution tends to the driving frequency of the system as time progresses; however, the wavenumber approaches zero after a large multiple of the characteristic density change timescale, indicating an ever increasing wavelength. For the higher order fluting modes the changes in amplitude are dependent upon the wave mode; for the m=2 mode the wave is amplified to a constant level; however, for all m≥3 the fast MHD wave is damped within a relatively small multiple of the characteristic density change timescale. Understanding MHD wave behavior in time-dependent plasmas is an important step towards a more complete model of the solar atmosphere and has a key role to play in solar magneto-seismological applications.     

Observations of a quiet-time Pc5 wave in the outer magnetosphere

We report an observation of the radial profile of a Pc5 magnetic pulsation and the associated energetic electron flux oscillations from 10 to 18 Re, recorded by the IMP-5 satellite at 19.00 M.L.T. on 21 March 1970. The Pc5 pulsation was mainly compressional and occurred during extremely quiet geomagnetic conditions. Fluxes of energetic electrons detected above three energy thresholds (18, 45, and 80 keV) were found to oscillate out of phase with magnetic field intensity. One new result is that both the wave amplitude and the wave period increased with radial distance. Second, the electron flux oscillation amplitude was roughly proportional to magnetic field fluctuation amplitude and wave period. The wave event is found to be interpreted better as an ion drift wave because of lack of polarization reversal. The characteristics of energetic electron flux oscillations are shown to agree qualitatively with theoretical calculations of the kinetic perturbation of distribution functions by compressional waves.     

The stability of the pristine magnetopause

A MHD theory of combined Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities for a transition layer with two different scale lengths (Δ and δ for the variation of velocity/magnetic fields and density, respectively) is presented. The study is motivated by reports of magnetopauses with no low latitude boundary layer, in which a sharp density drop over a distance δ?Δ is observed (“pristine” magnetopauses (J. Geophys. Res. 101 (1996) 49). The theory ignores compressibility effects and applies to subsonic regions of the dayside magnetopause. The RT effect is included to account for temporary periods of acceleration of the magnetopause, caused by sudden changes of the solar wind dynamic pressure. For small wavelengths λ, such that δ?λ?Δ, a WKB solution shows that the velocity gradient operates, together with magnetic tensions, to attenuate or even stabilize the Rayleigh-Taylor instability within a certain wavelength range. An exact dispersion relation for flute modes, valid for all λ, in the form of a fourth order polynomial for the complex frequency ω, is obtained from a model with a constant velocity gradient, dV/dy within Δ, and with δ→0. Flute modes are possible because of the existence of bands of very small magnetic shear on the dayside magnetopause (J. Geophys. Res. 103 (1998) 6703). The exact solution allows for a study of the change of the action of the velocity gradient with λ from the long-λ range where dV/dy is KH destabilizing to the short-λ range where dV/dy produces a stabilizing effect. Both, the WKB approximation and the well known tangential discontinuity model (Δ→0) are recovered as limiting cases of the exact solution. Properties of the KH and RT instabilities, for different density ratios on either side of the magnetopause, are described. For flute modes, at very small λ the RT instability grows faster and becomes the dominant effect. However, it is shown that the growth rate remains bounded at a finite value as λ→0, when a theory with a finite δ model is considered. To study configurations with finite, arbitrary, δ/Δ ratios, the MHD perturbation equations are solved numerically, using hyperbolic tangent functions for both the density and velocity transitions across the magnetopause. To examine the influence of different δ/Δ ratios on the growth rates of KH and RT, calculations are performed for different δ/Δ, with and without acceleration, and for two different density ratios. It is found that the general features exhibited by the constant dV/dy model, are confirmed by these numerical solutions. The stability of pristine magnetopauses, and the possibility of observing some theoretical predictions during magnetopause crossings in ongoing missions, are discussed.     

Linear MHD Sausage Waves in Compressible Magnetically Twisted Flux Tubes

Oscillations of magnetic flux tubes are of great importance as they contain information about the geometry and fine structure of the flux tubes. Here we derive and analytically solve in terms of Kummer’s functions the linear governing equations of wave propagation for sausage surface and body modes (m=0) of a magnetically twisted compressible flux tube embedded in a compressible uniformly magnetized plasma environment in cylindrical geometry. A general dispersion relation is obtained for such flux tubes. Numerical solutions for the phase velocity are obtained for a wide range of wavenumbers and for varying magnetic twist. The effect of magnetic twist on the period of oscillations of sausage surface modes for different values of the wavenumber and vertical magnetic field strength is calculated for representative photospheric and coronal conditions. These results generalize and extend previous studies of MHD waves obtained for incompressible or for compressible but nontwisted flux tubes. It is found that magnetic twist may change the period of sausage surface waves of the order of a few percent when compared to counterparts in straight nontwisted flux tubes. This information will be most relevant when high-resolution observations are used for diagnostic exploration of MHD wave guides in analogy to solar-interior studies by means of global eigenoscillations in helioseismology.     

The open cluster Tr 14

New photoelectric UBVRI data for stars in the young open cluster Tr 14 in the Great Carina Nebula (NGC 3372) are presented. The two-color diagram separates clearly the member from the nonmember stars. Thus, the membership of about 39 stars is suggested. Those located in the compact core of the cluster, have smaller reddening dispersion than those around it. No star appears above the Main Sequence at the faint end of the color-magnitude diagram. A distance modulus ofV ₀?M_v=12.20±0.2 is obtained, which gives an age of about 5×10⁶ yr. From only the photometry it is concluded to be slightly older than the nearby cluster Tr16.     

Some differential equations for problems of wave scattering in a one-dimensional medium

A system of linear differential equations determining the amplitude of reflection R and the amplitude of transmission T for a plane wave (or an electron) and for an arbitrary medium (or a one-dimensional potential of an arbitrary type) is obtained. It is shown that the problem of determining the scattering parameters R and T reduces, in general, to a Cauchy problem for a stationary wave equation (or for the Schrödinger equation).     

The opposition surge of Enceladus: HST observations 338-1022 nm

Hubble Space Telescope (HST) Wide-Field Planetary Camera (WFPC2) observations at phase angles in the range α=0.26°-6.4° obtained at every opposition and near quadrature between October 1996 and December 2002 reveal the opposition effect of Enceladus. We present a photometric analysis of nearly 200 images obtained through the five broadband UVBRI filters (F336W, F439W, F555W, F675W, and F814W) and the F785LP and F1042M filters from which we generate mutually consistent solar and rotational phase curves. Our solar phase curves reveal a dramatic, sharp increase in the albedo (from 0.11 mag in the F675W filter to 0.17 mag in the F785LP filter) as phase angles decrease from 2° to 0.26°. A slight opposition effect is evident in data from the F1042M filter (λ_eff=1022 nm); however, the smallest phase angle currently available for observations from this filter is α=0.58°. With the addition of data from the F255W filter we demonstrate the wavelength dependence of the albedo of the trailing hemisphere from 275 to 1022 nm. Our rotation curves show that the trailing hemisphere is ∼0.06 mag brighter than the leading when observed at wavelengths between 338 and 868 nm and 0.11 mag brighter than the leading at 1022 nm. We have supplemented the phase curve from the F439W filter (λ_eff=434 nm) with Voyager clear filter (λ_eff=480 nm) observations made at larger phase angles (α=13°-43°) to produce a phase curve with the most extensive phase angle coverage possible to date. This newly expanded range of phase angles enhances the ability of the Hapke photometric model (Hapke B., 2002, Icarus 157, 523-534) to relate physical characteristics of the surface of Enceladus to the manner in which incident light is reflected from it. We present Hapke 2002 model fits to solar phase curves from each UVBRI filter as well as from the F785LP and F1042M filters. Geometric albedos derived from these model fits range from p=0.92±0.01 at 1022 nm to p=1.41±0.03 at 549 nm, necessitating an increase of about 20% from previously derived values. Our Hapke fits demonstrate that the opposition surge of Enceladus is best described by a model which combines both moderate shadow-hiding and narrow coherent backscattering components.     

Wave packet propagation in an amplifying medium and its application to the dispersion characteristics and to the generation mechanisms of Pc 1 events

The formulae which give the propagation characteristics of a wave packet in a dispersive and amplifying medium, are established. Application is made to the propagation of Pc 1 elements through a magnetosphere constituted of a cold plasma and a high energy proton population. It is shown that the spectral shape, in a frequency-time coordinate system, of the Pc 1 elements is related to two terms : v = d²ω/dk², which represents the variation of the group velocity with frequency and which depends only on the cold plasma characteristics, and μ = -d²γ/dk², in which γ is the amplification coefficient depending on the frequency and which is related to the high energy particle distribution function. When v ? μ, only the usual dispersion effects occur, but a new method is found for determining the line of force on which the micropulsations are generated, without making any assumption about the cold plasma density distribution inside the magnetosphere. It is also possible to deduce some characteristics about the high energy proton distribution. Theoretical computations are presented, which give the frequency variation of the amplification coefficient as a function of the e-folding energy and the anisotropy factor of these high energy protons. Applications are made to ~30 pearl events which are analysed in detail according to this theory. When μ ? v, other effects do appear. After a preliminary phase, the pearl elements can become parallel for a while, or even re-erect before lying again; the duration of each element gives an indication about the number of interacting particles. The conditions for the validity of the quasi-linear theory, and some other non-linear effects related with the interpretation of Pc 1 micropulsations are also discussed.     

Diffraction theory modeling of near-forward radio wave scattering from particle clusters

We investigate the use of diffraction theory as an alternative to solving the full electromagnetic problem of near-forward scattering by a collection of dielectric spheres of arbitrary spacing and orientation. We define an amplitude screen, which is constructed by projecting the shadows of a cluster of spheres onto a plane perpendicular to the wave vector of the incident radio wave. The far-field (Fraunhofer) diffraction pattern of the amplitude screen is then computed and compared with the Mie-theoretic result. We show that for EM scattering from a cluster of electrically large spheres—both singly-sized and belonging to a size distribution—there is excellent agreement between the exact Mie solution and its diffraction theory approximation when near-forward scattering is the angular range of interest. This excellent agreement holds over a broad range of particle separation and orientation configurations relative to the incidence direction. It is also achieved at a much reduced computational cost compared with an exact solution of the electromagnetic interaction problem. Fortified by these results, the authors have applied diffraction theory to the analysis of Cassini radio occultation data, thereby detecting fine-scale structure in Saturn’s rings [Thomson, F.S., Marouf, E.A., Tyler, G.L., French, R.G., Rappoport, N.J., 2007. Periodic microstructure in Saturn’s rings A and B. Geophys. Res. Lett. 34, L24203].     

Measurements of the oscillatory and slowly-varying components of the solar velocity field

Spectroheliograms with high spatial resolution are presented to illustrate the decomposition of the solar velocity field into its oscillatory and slowly-varying components. An analysis of data obtained in the lines Fei λ 5434 and Feii λ 4924 yield essentially the same principal results:

1. Spectroheliograms of the oscillatory component have a mottled appearance of rising and falling elements ranging from 2000 km to 3000 km in size. These elements oscillate vertically with a period in the range 275–300 s and an amplitude of 0.5 km/s. Although most oscillations last two cycles some have been observed for as many as four cycles.
2. Spectroheliograms of the slowly-varying component show a velocity granulation pattern whose spatial properties correspond closely to those of the photospheric granulation visible on direct photographs of the Sun. The velocity granules are approximately 1000 km in diameter and rise relative to their intergranular spaces with speeds that are typically 0.6 km/s, but which may occasionally be as large as 0.9 km/s. Most velocity granules seem to live for at least 10 min with many lasting 10–30 min, and a few of the biggest and fastest moving lasting 30 min to 1 hr.

It is concluded that Spectroheliograms of the slowly-varying component represent the velocity field of the photospheric granulation.     

Characteristic of ion acoustic shock waves in a dissipative quantum pair plasma with dust particulates

The behavior of quantum dust ion-acoustic (QDIA) shocks in a plasma including inertialess quantum electrons and positrons, classical cold ions and stationary negative dust grains are studied, using a quantum hydrodynamic model (QHD). The effect of dissipation due to the viscosity of ions is taken into account. The propagation of small but finite amplitude QDIA shocks is governed by the Kortoweg-de Vries-Burgers (KdVB) equation. The existence regions of oscillatory and monotonic shocks will depend on the quantum diffraction parameter (H) and dust density (d) as well as dissipation parameter (η ₀). The effect of plasma parameters (d,H,η ₀), on these structures is investigated. Results indicate that the thickness and height of monotonic shocks; oscillation amplitude of the oscillatory shock wave and it’s wavelength effectively are affected by these parameters. Additionally, the possibility of propagation of both compressive and rarefactive shocks is investigated. It is found that depending on some critical value of dust density (d _c ), which is a function of H, compressive and rarefactive shock waves can’t propagate in model plasma. The present theory is applicable to analyze the formation of nonlinear structures at quantum scales in dense astrophysical objects.