Astrophysics and Space Science最新文献

We have proposed that galaxy formation is catalyzed by the collision of infalling and outstreaming particles from leaky, horizonless astrophysical black holes, most likely gravastars, and based on this gave a model for the disk galaxy scale length. In this paper we modify our original scale length formula by including an activation probability (P) for a collision to lead to nucleation of star formation. The revised formula extrapolates from early universe JWST data to late time data to within a factor of five, and suggests that galaxy dimensions should systematically get smaller as the observed redshift z increases. We also show that particles recycling through gravastars can lead to a reduction in the temperature of the surrounding gas, through a “heat pump” refrigeration effect. This can trigger galaxy formation through enhanced star formation in the vicinity of the gravastar.

In this work we have studied about the characteristics and dynamical changes during the recovery time of moderate and strong geomagnetic storms of ((mathrm{Dst}<-50text{ nT})). In our investigation of 57 storms triggered by CMEs/CIRs, we concentrated on the solar wind’s influence on their decay phases. Selected storms were classified into distinct groups based on their recovery characteristics. Employing the superposed epoch analysis and best fit methods, we scrutinized several interplanetary solar wind plasma and field parameters and their various functions. The analysis encompassed various single, dual, and multiple interplanetary plasma and field parameters/functions. We determined the most representative characteristic time for the storm’s recovery profile by carefully fitting an exponential curve. A correlation analysis between Dst and solar wind parameters/functions led us to isolate a coupling function ((rho ^{frac{1}{2}}mathrm{Ey})) which best described the decay rate of the ring current. It shows that electric field term (Ey) coupled with a viscus term ((rho ^{frac{1}{2}})) plays pivotal role in determining the recovery rate of a geomagnetic storms. Additionally, we modeled the complex patterns of Dst recovery in relation to solar wind parameters and functions using a second-order polynomial. Remarkably, during the recovery phase, a dynamic correlation between Dst and solar wind parameters/functions was revealed. The three-parameter solar wind-magnetosphere electrodynamical coupling functions, which combines the viscus term ((rho ^{frac{1}{2}})) and the electric field-related function ((mathrm{v}^{frac{4}{3}}mathrm{B})) ((rho ^{frac{1}{2}}mathrm{v}^{frac{4}{3}}mathrm{B})), significantly impacts the recovery phase of geomagnetic disturbances. Our investigation extended to the relationship between main and recovery phase durations, providing valuable insights into the solar wind’s intricate control over the decay of the geomagnetic disturbances. These findings contribute significantly to advancing our comprehension of the complex relationship between solar wind dynamics and the evolution of geomagnetic disturbances.

Incompressible vortex flow are observed in a large variety of astrophysical plasmas such as the convection zone and the atmosphere of stars, in astrophysical jets in stellar winds and in planetary magnetospheres. More specifically, magnetohydrodynamic (MHD) simulations have shown that two large scale interlaced Alfvénic vortices structure the magnetic tail of Uranus at solstice time. Assuming identical vortices, we compute the general linear structure of the flow near their centers within the frame of ideal MHD. We then use the analytic results to interpret and qualify the vortices observed in a 3D MHD simulation of a fast rotating Uranus-type planet.

This study investigates contributions of solar radiation and geomagnetic activity of consecutive 27-day recurrent geomagnetic storms (RGSs) to the variabilities in the equatorial ionospheric F-region in American Peruvian sector during 2007. Results show the ionospheric responses to the RGSs are quasi-periodic and multifaceted with highly evolved in the summer months. In High-Intensity Long-Duration Continuous (AE) Activity (HILDCAA) events, the ionospheric responses are more variable than in non-HILDCAA. The critical frequency and peak height of the F-layer tend to increase during storm-time in summer months. The maximum density enhancements are more than 70% in the three RGSs and they are long-lasting in the summer months. A new classification of daily variations in the virtual height of the F-layer ((h'F)) is proposed: Mode A shows mixing of great height before noon and low height near midnight, Mode B shows moderate height near midnight, and Mode C shows mixing of high height before noon and great height near midnight. These (h'F) Modes efficiently characterize the ionospheric variabilities and processes. The great uplifts of (h'F) during night-time in the summer months coincide with the presence of strong disturbance dynamo electric fields and disturbed neutral winds generated by intensified Joule heating. The solar EUV plays a role in the uplifts of (h'F) during the daytime. Zonal electric field disturbances and perturbations in the neutral meridional winds critically contribute to the equatorial ionospheric responses and ESF variabilities. Most cases of inhibited/suppressed ESF were observed in Mode A and occurred under overshielding conditions. The inhibited ESF associated with (h'F) not raised in the recovery phase is mainly contributed by a cooling state after great uplifts by daytime thermospheric winds.

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.