Astrophysics and Space Science最新文献

We present here a simple hydrodynamic model based on a sequence of steady states of the inner sub-Keplerian accretion disc to understand its different spectral states. Correlations between different hydrodynamic steady states are studied with a goal to understand the origin of, e.g., the aperiodic variabilities. The plausible source of corona/outflow close to the central compact object is shown to be a consequence of steady state transition in the underlying accretion flow. We envisage that this phenomenological model can give insight on the influence of viscosity, efficiency of energy advection, nature of the background flow and environment on the evolution of the inner sub-Keplerian accretion disc.

Affected by the Earth’s atmosphere, the image of a primary and satellite system may appear unresolved, such as the dwarf planet Haumea system. It is found by experiments that neither the two-dimensional Gaussian nor modified moment centering algorithms can accurately measure the photocenter of an image of unresolved primary and satellite system observed. This work investigates a specific centering algorithm to accurately measure the photocenter, which would be helpful to derive some physical parameters (e.g. orbital parameters and mass). Taking the dwarf planet Haumea and its brighter satellite Hi’iaka as an example, we simulate the motion of the photocenter with different seeings. We find that the photocenter of system changes significantly with seeings (∼0.074″ with the different seeings of 1″ and 3″) when using the two-dimensional Gaussian centering algorithm. However, the modified moment centering algorithm can accurately measure the photocenter of system without noises, but when noises are added its accuracy will be greatly influenced by noises. In this work, a new centering algorithm is proposed, which can accurately measure the photocenter with less influence of seeings and noises. Observations of dwarf planet Haumea taken over 25 nights are used to test the effectiveness of our proposed method. Compared with using two-dimensional Gaussian centering algorithm, the fitted parameter is slightly more accurate with less positional fitting errors when using the proposed method in this work. This method can also be applied to the centering of binary stars.

In this paper, two models of interest for Celestial Mechanics are presented and analysed, using both analytic and numerical techniques, from the point of view of the possible presence of regular and/or chaotic motion, as well as the stability of the considered orbits. The first model, presented in a Hamiltonian formalism, can be used to describe the motion of a satellite around Earth, taking into account both the non-spherical shape of our planet and the third-body gravitational influence of Sun and Moon. Using semi-analytical techniques coming from Normal Form and Nekhoroshev theories it is possible to provide stability estimates for the orbital elements of its geocentric motion. The second dynamical system presented can be used as a simplified model to describe the motion of a particle in an elliptic galaxy having a central massive core; it is constructed as a refraction billiard where an inner dynamics, induced by a Keplerian potential, is coupled with an external one, where a harmonic oscillator-type potential is considered. The investigation of the dynamics is carried on by using results of ODEs’ theory and is focused on studying the trajectories’ properties in terms of periodicity, stability and, possibly, chaoticity.

This study reveals a new feature of many solar jets: a group height, which links their acceleration and velocity.

The acceleration and velocity ((a), (V)) for jets such as spicules, often displayed as scattergraphs, show a strong correlation. This can be represented empirically by the equation, (V = pa + q), where (p) and (q ) are two arbitrary non-zero constants.

This study reanalyses the ((a), (V)) data for nine different groups of jets, in order to test an alternative proposal that a simpler relationship directly links ((a), (V)) to the mean height for the group of jets, without needing the empirical constants (p ) and (q). A standard mathematical test – plotting log((a)) against log((V)), tests whether (V sim a^{n}) and if so, gives the value of n. When this is done for a wide range of jets the index (n) is consistently found to be close to 0.5

The nine groups of jets include spicules, macrospicules and dynamic fibrils. The result, (V sim a)^0.5, or equivalently (V^{2} = ka), with only one constant, provides as close a match to the data as the equation (V = pa + q), which requires two unknown constants. It is found that the constant (k), is a known quantity: just twice the mean height, (overline{s}), of the group of jets being analysed. This then gives the equation (V^{2} =2 a overline{s}), for the jets in the group. This more succinct relationship links the acceleration and maximum velocity of every jet in the group to a well-defined quantity – the mean height of the group of spicules, without needing extra constants

This paper presented an analysis of geomagnetic disturbance observed on the ground during geomagnetic storms with different intensities in 2015 using the meridian chain data at geomagnetic mid and low latitudes. Ground observation records superimpose varying types of space-current system and noise interference. Geomagnetic disturbance with variation of discontinuity and irregularities are difficult to identify and distinguish. We proposed a variational mode decomposition (VMD) algorithm for reconstructing geomagnetic horizontal ((H)) disturbance signals. We decomposed the geomagnetic signals into geomagnetic disturbance signals, diurnal variation signals, and noise disturbance signals using the VMD algorithm. Intrinsic mode functions (IMFs) were selected to form the reconstructed signal, which represented a geomagnetic disturbance during a geomagnetic storm. We investigated the decreased amplitude of (H) component obtained from the reconstructed signals during main phase of geomagnetic storms with different geomagnetic storms intensities and seasons at mid and low latitudes. The maximum values of gradient variation of (H ) component disturbance with geomagnetic latitude cosine are near magnetic latitude 30°N during geomagnetic storms with different intensities and seasons. Ionopheric structural changes in the low-to-mid latitude transition zone maybe the primary cause. The result provides a reference for the complex coupling relationship between the ionosphere and magnetosphere during geomagnetic storms.

Detection of weak signals remains challenging in astrophysics. This is particularly applicable in the investigation of Forbush events. There is thus, a paucity of catalogs of small-amplitude Forbush decreases (FDs). Detail investigations of the space-weather implications of small FDs are, thus, lacking in the literature. Recently, large catalogs of weak FDs, for the first time, have been published. This work employs the newly created lists of small-amplitude FDs to investigate the statistical link between small FDs and solar-geomagnetic variables. The solar-geomagnetic variables were obtained from the OMNI database. A simple coincident R software code was employed in matching the related solar-geomagnetic variables with the weak Forbush events. The FD dates were taken as the input signal. Scatter plots of FDs against interplanetary magnetic field (IMF), solar wind speed (SWS), planetary K-index (Kp) and planetary A-index (Ap) reveal a negative relationship, while that of FDs against disturbance storm time index (Dst) shows a positive relationship. Statistical significance of these relations were tested. The small-amplitude FDs and solar-geomagnetic variables at Potchefstroom (PTFM) station register statistically significant relations. Non-statistically significant correlation between the small-amplitude FDs and solar-geomagnetic variables were obtained at South Pole (SOPO) station, with the exception of FD-SWS that reveals statistically significant correlation. The differences in the correlation results obtained at the two stations (PTFM and SOPO) could be attributed to the differences in the characteristics of the NM stations. These results suggest that geomagnetic storm indices play important role in the evolution of FDs.

Dynamics of nonlinear ion-acoustic waves (IAWs) are studied for Venus’ lower atmosphere at an altitude of (200-1000) km. Two-soliton, nonlinear solitary and periodic waves in a three-component plasma consisting of (H^{+}) and (O^{+}) ions with kappa distributed electrons are studied. Using the reductive perturbation technique (RPT), the Korteweg-de Vries (KdV) equation is derived and a Planar dynamical system is formed for the KdV equation using a travelling wave transformation. A phase portrait is drawn to analyze nonlinear wave behaviors by adjusting the parameters (kappa ) (spectral index), (gamma ) (unperturbed number density ratio), and (V) (travelling wave speed). Increasing values of (kappa ) amplify amplitudes for solitary and periodic waves, narrow down the width of the solitary wave, and broaden the width of the periodic wave. Increasing value of (gamma ) boosts amplitude of the solitary wave with unchanged width, while amplitude of the nonlinear periodic wave decreases and width widens. Increasing value of (V) enhances amplitudes and reduces widths for both solitary and periodic waves. Two-soliton solutions for the KdV equation are studied using the Hirota direct method. Increasing value of (gamma ) reduces amplitude of the soliton without affecting the width and increasing value of (kappa ) reduces width of the soliton. Phase shift for two-soliton is also shown and found that for different values of (kappa ), the phase shift increases on increasing value of (gamma ). The findings of our result aid in understanding the dynamics of nonlinear waves and two-soliton solutions in Venus’ lower ionosphere.