Astrophysics and Space Science最新文献

We utilized the Forbush decreases (magnitude (>1.5%)) detected in cosmic ray neutron monitor data during continuous five solar cycles, viz., 20, 21, 22, 23 and 24 (1965 to 2019) and subjected them to wavelet analysis in order to obtain the possible periodicities in their occurrence. We also studied the periodicities separately during the odd and even solar activity cycles. In addition to solar activity, the solar magnetic polarity and its extension into the interplanetary space makes significant difference in the cosmic ray modulation in the helisphere, we have also applied the wavelet analysis procedure separately during positive (A > 0) and negative (A < 0) polarity states of the heliospheric magnetic fields. Observed periodicities in Forbush decreases have been discussed and compared with earlier detected periodicities in solar and geomagnetic activity indices, e.g., sunspot numbers, sunspot areas, sunspot groups, solar flares, coronal mass ejections, and various geomagnetic activity indices. Significant short-term periodic behaviour detected in the occurrence of Forbush decreases, which in general, corroborates the observed behaviour in solar (in particular, solar eruptive activity) and geomagnetic activity. Understanding the quasi-periodic process in magnetic field emergence from solar active regions and solar eruptive activity, as well as solar-terrestrial coupling and space weather effects, requires comparing the quasi-periodic behaviour between parameters representing solar and geomagnetic activity along with cosmic ray variability.

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.

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.

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.