Astrophysics and Space Science最新文献

Solar surges are collimated flows of plasma that occur in the periphery of active regions (ARs). The kinematics, physical properties, and triggering mechanisms of a solar surge were studied through imaging and spectroscopic diagnosis. The surge has a typical inverted Y-shape, and it moves with a speed of more than 200 km/s in the transition-region (TR) which is much higher than the sound speed of TR. The observational findings suggest that the surge was triggered due to magnetic reconnection. In addition, a hot jet formed after around 03 minutes and propagated at a speed that is comparable to the sound speed of the corona. Hence, most probably, the hot jet forms due to the chromospheric evaporation. The spectroscopic diagnosis reveals that electron densities are log₁₀ 10.82±0.90 and log₁₀ 9.93±1.27 in the base and spire of the surge, respectively. Further, it is found that the Si iv line ratio is around 1.85 in the base and 1.80 in the spire of the surge. Hence, we say that most of the Si iv profiles are forming under optically thick conditions in the surge. Most importantly, some Si iv spectral profiles from the base and spire of the surge are double peak profiles with a dip close to the central wavelength. Also, in the same region, optically thick conditions exist, therefore, most probably, the central dip in the profiles is a result of the self-absorption. This is the first-ever report of self-absorption in the solar surges.

The Global Navigation Satellite System Radio Occultation (GNSS-RO) technique has proven to be a powerful tool for studying E-region irregularities, i.e., Sporadic E (Es) which is primarily associated with the amplitude and phase scintillations. In the present study, an extensive 7-year GNSS-RO scintillation indices data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations was employed to investigate the global distribution and seasonal variation of the Es occurrences under solar activity near the magnetic dip equator. Our analysis from the Earth’s magnetic field parameters such as horizontal intensity and inclination estimated by the International Geomagnetic Reference Field model (IGRF) reveals that Earth’s magnetic field plays a crucial role in determining the global distribution of Es layers. Moreover, the abundance of Es shows a clear dependence on season/longitude, and the occurrence statistics of Es are closely aligned with the earlier reports. The solar activity dependence of the Es occurrence characteristics demonstrates its significant reduction with increased solar activity for most of the seasons in all longitude sectors. We address the Gradient Drift instability as a source mechanism of the Es layer’s appearance at the magnetic dip equator, where wind shear theory fails to operate because of the minimal inclination of the geomagnetic field.

In 2007 we were part of a team that discovered the so-called “Lorimer Burst”, the first example of a new class of objects now known as fast radio bursts (FRBs). These enigmatic events are only a few ms in duration and occur at random locations on the sky at a rate of a few thousand per day. Several thousand FRBs are currently known. While it is now well established that they have a cosmological origin, and about 10% of all currently known sources have been seen to exhibit multiple bursts, the origins of these enigmatic sources are currently poorly understood. In this article, we review the discovery of FRBs and present some of the highlights from the vast body of work by an international community. Following a brief overview of the scale of the visible Universe in §1, we describe the key moments in radio astronomy (§2) that led up to the discovery of the Lorimer burst (§3). Early efforts to find more FRBs are described in §4 which led to the discovery of the first repeating source (§5). In §6, as we close out on the second decade of FRBs, we outline some of the many open questions in the field and look ahead to the coming years where many surprises are surely in store.

The broad-spectrum polarization and spectral characteristics of pulsars contain crucial information about the origin of their radio emission. These properties, together with pulsar flux density variations, can also be used to guide future surveys of radio pulsars and probe the Galactic interstellar medium. In this paper, we present studies of 18 pulsars at high Galactic latitudes using the Ultra-Wideband Low (UWL) receiver of the Parkes radio telescope. For these pulsars, we measured their wideband flux densities, spectral indices, and polarization fractions. We obtain seven new rotation measures (RMs) and refine the RMs of another ten pulsars. In this sample of pulsars, we observed significant variations in their flux densities, suggesting that previous shallow surveys were likely to miss a population of pulsars at high galactic latitude. In addition, we identified a previously reported isolated pulsar (PSR J1947−18) as a potential binary system.

Type II radio emissions are events mostly found to be associated with coronal mass ejections (CMEs) and accelerated by the CME-driven shock in the heliosphere. This study reports on the estimation of the CME-shock standoff distance at the commencement of metric type II radio emissions by combining the CME-deprojected speed and spectral features of radio bursts using a robust TensorFlow Deep-Learning Sequential (TFDLS) technique. The dataset of 96 CMEs at the commencement of type II radio bursts was used between Solar cycle 24 and the ascending phase of Solar Cycle 25. The measured root mean squared error (RMSE) was 0.145 (Rs), with an average height difference of 0.096 Rs between the observed and predicted CME-shock heights. Five (5) CMEs/radio bursts energetic events associated with solar flares were selected from the test data, and the CME shock stand-off heights were forecasted using the TFDLS and flare-onset (FL) methods. The data were used to compare the leading-edge (LE) and dynamic spectra (DS) methods. The RMSE measured between the FL and LE was 0.35 Rs, and the RMSE estimated between the TFDLS and LE approaches was 0.04 Rs. The RMSE between FL and DS was 0.34. Rs, and the RMSE between the TFDLS and the DS was 0.04 Rs. We also used the findings gained from the five selected events and compared them to the 3D shock-fitting (3D-SF) approach. The RMSE found between the TFDLS and the 3D-SF was 0.18 Rs, while the RMSE estimated between the FL and the 3D-SF was 0.23 Rs. This shows that the TFDLS has satisfactory performance and can be used as an alternative technique.

We report a study of major solar energetic particle (SEP) and ground level enhancement (GLE) events that occurred during the first 62 months of the rising phase of the 24th solar cycle. Our objective is to comprehend the key factors that influence the severity and occurrence of such events. The coronal mass ejection (CME) speed (serves as or is) is a reliable indicator of SEP and GLE events, as it consistently supports the shock acceleration mechanism. Some very fast CMEs, which are likely to have accelerated particles up to GeV energies, may not have resulted in a GLE event due to poor latitudinal connectivity. We have emphasized that the CME speed, magnetic connectivity to Earth, and ambient conditions are the main or primary factors that contribute to the lack of high-energy particle events during cycle 24. Furthermore, we observed that even well-connected fast CMEs that did not seem to have accelerated high-energy particles due to potentially unfavourable prevailing conditions such as high Alfven speed and overall reduction in acceleration efficiency in cycle 24. These conclusions are generally supported by insights gleaned from the observation of the time series of SW-IMF parameters on the flare day.

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.

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.

We study two-parametric families of spatial orbits given in the analytic form (f(x,y,z)=c_{1}), (g(x,y,z)=c_{2}) ((c_{1}), (c_{2}) = const.) which are produced by three-dimensional potentials (V=V(x,y,z)) inside a material concentration. These potentials must verify two linear partial differential equations (PDEs) which are the basic equations of the 3D Inverse Problem of Newtonian Dynamics and the well-known Poisson’s equation. A suitable class of potentials for this case is the axisymmetric potentials (V=mathcal{B}(x^{2}+y^{2}, z)) which have applications in astrophysical problems. For the given density function (rho =rho (x, y, z)), (rho =rho _{0}=const)., or, (rho =rho (z)) and a pre-assigned family of orbits, three-dimensional potentials producing this family of orbits are found in each case. We focus our interest on the cored, logarithmic potentials and another one of fourth degree describing elliptical galaxies. The two-parametric families of straight lines in 3D space are also considered.

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.