A regional geomagnetic field model over Southern Africa derived with harmonic splines from Swarm satellite and ground-based data recorded between 2014 and 2019

A regional harmonic spline geomagnetic main field model, Southern Africa Core Field Model (SACFM-3), is derived from Swarm satellite and ground-based data for the southern African region, in the eastern part of the South Atlantic Anomaly (SAA) where the field intensity continues to decrease. Using SACFM-3 and the global CHAOS-6-×9 model, a detailed study was conducted to shed light on the high spatial and temporal geomagnetic field variations over Southern Africa between 2014 and 2019. The results show a steady decrease of the radial component Z in almost the entire region. In 2019, its rate of decrease in the western part of the region has reached high values, 76 nT/year and 78 nT/year at Tsumeb and Keetmanshoop magnetic observatories, respectively. For some areas in the western part of the region the radial component Z and field intensity F have decreased in strength, from 1.0 to 1.3% and from 0.9 to 1.2%, respectively, between the epochs 2014.5 and 2019.5. There is a noticeable decrease of the field intensity from the south-western coast of South Africa expanding towards the north and eastern regions. The results show that the SAA area is continuing to grow in the region. Abrupt changes in the linear secular variation in 2016 and 2017 are confirmed in the region using ground-based data, and the X component shows an abrupt change in the secular variation in 2018 at four magnetic observatories (Hermanus, Hartebeesthoek, Tsumeb and Keetmanshoop) that needs further investigation. The regional model SACFM-3 reflects to some extent these fast core field variations in the Z component at Hermanus, Hartebeesthoek and Keetmanshoop observatories. Graphical Abstract

A machine learning pipeline for autonomous numerical analytic continuation of Dyson-Schwinger equations

Dyson-Schwinger equations (DSEs) are a non-perturbative way to express n-point functions in quantum field theory. Working in Euclidean space and in Landau gauge, for example, one can study the quark propagator Dyson-Schwinger equation in the real and complex domain, given that a suitable and tractable truncation has been found. When aiming for solving these equations in the complex domain, that is, for complex external momenta, one has to deform the integration contour of the radial component in the complex plane of the loop momentum expressed in hyper-spherical coordinates. This has to be done in order to avoid poles and branch cuts in the integrand of the self-energy loop. Since the nature of Dyson-Schwinger equations is such, that they have to be solved in a self-consistent way, one cannot analyze the analytic properties of the integrand after every iteration step, as this would not be feasible. In these proceedings, we suggest a machine learning pipeline based on deep learning (DL) approaches to computer vision (CV), as well as deep reinforcement learning (DRL), that could solve this problem autonomously by detecting poles and branch cuts in the numerical integrand after every iteration step and by suggesting suitable integration contour deformations that avoid these obstructions. We sketch out a proof of principle for both of these tasks, that is, the pole and branch cut detection, as well as the contour deformation.

Magneto- and electrophosphene thresholds in the retina: a dosimetry modeling study

Objective: Sensations of flickering light produced by time-varying magnetic fields or electric currents are called magneto- or electrophosphenes. Phosphene thresholds have been used in international guidelines and standards as an estimate of the thresholds of exposure that produce effects in the central nervous system. However, the estimated threshold values have a large range of uncertainty. Approach: Phosphene thresholds were approximated by simulating five phosphene threshold experiments. Retinal electric fields and currents induced by electric and magnetic stimulation were calculated using the finite element method and 14 anatomically realistic computational models of human heads. Main results: The radial component of retinal current density was determined to be in the range of 6.0~--~20.6~mA/m$^2$. This study produces more accurate estimates for threshold current density in the retina using detailed anatomical models and the estimates had a reduced range of uncertainty compared to earlier studies. Significance: The results are useful for studying the mechanisms of retinal phosphenes and for the development of exposure limits for the central nervous system.

Self-gravitating anisotropic star using gravitational decoupling

This paper addresses a new gravitationally decoupled anisotropic solution for the compact star model via the minimal geometric deformation (MGD) approach. We consider a non-singular well-behaved gravitational potential corresponding to the radial component of the seed spacetime and embedding class I condition that determines the temporal metric function to solve the seed system completely. However, two different well-known mimic approaches such as pr = Θ1 1 and ρ = Θ0 0 have been employed to determine the deformation function which gives the solution of the second system corresponding to the extra source. In order to test the physical viability of the solution, we have checked several conditions such as regularity conditions, energy conditions, causality conditions, hydrostatic equilibrium, etc. Moreover, the stability of the solutions has been also discussed by the adiabatic index and its critical value. We find that the solutions set seems viable as far as observational data are concerned.

Extragalactic Magnetism with SOFIA (Legacy Program) - II: A Magnetically Driven Flow in the Starburst Ring of NGC 1097*

Galactic bars are frequent in disk galaxies and they may support the transfer of matter toward the central engine of active nuclei. The barred galaxy NGC 1097 has magnetic forces controlling the gas flow at several kpc scales, which suggest that magnetic fields (B-fields) are dynamically important along the bar and nuclear ring. However, the effect of the B-field on the gas flows in the central kpc scale has not been characterized. Using thermal polarized emission at 89 μm with HAWC+/SOFIA, here, we measure that the polarized flux is spatially located at the contact regions of the outer bar with the starburst ring. The linear polarization decomposition analysis shows that the 89 μm and radio (3.5 and 6.2 cm) polarization traces two different modes, m, of the B-field: a constant B-field orientation and dominated by m = 0 at 89 μm, and a spiral B-field dominated by m = 2 at radio. We show that the B-field at 89 μm is concentrated in the warmest region of a shock driven by the galactic-bar dynamics in the contact regions between the outer bar with the starburst ring. Radio polarization traces a superposition of the spiral B-field outside and within the starburst ring. According to Faraday rotation measures between 3.5 and 6.2 cm, the radial component of the B-field along the contact regions points toward the galaxy's center on both sides. We conclude that gas streams outside and within the starburst ring follow the B-field, which feeds the black hole with matter from the host galaxy.

Magnetic and Velocity Fluctuations in the Near-Sun Region from 0.1−0.3 au Observed by Parker Solar Probe

The fluctuations observed in the slow solar wind at 1 au by the WIND spacecraft are shown by recent studies to consist of mainly magnetic-field directional turning and magnetic-velocity alignment structure (MVAS). How these structures are created has been a question because the nature of the fluctuations in the near-Sun region remains unknown. Here, we present an analysis of the measurements in the slow solar wind from 0.1−0.3 au by Parker Solar Probe during its first six orbits. We present the distributions in the C vb ′ – σ r plane of both the occurrence and average amplitudes of the fluctuations, including the magnetic field, the velocity, and the Elsässer variables, where C vb ′ is the correlation coefficient between the magnetic and velocity fluctuations multiplied by the opposite sign of the radial component of the mean magnetic field and σ r is the normalized residual energy. We find that the dominant composition is the outward-propagating Alfvénic fluctuations. We find Alfvénic fluctuations with C vb ′ > 0.95 , in which the amplitudes of z + reach 60 km s−1 and those of z − are close to the observational uncertainty. We also find a region with high C vb ′ and moderate minus σ r in which the fluctuations are considered MVAS being magnetic dominated with the amplitude of magnetic fluctuations reaching 60 km s−1. We provide empirical relations between the velocity fluctuation amplitude and C vb ′ . The comparison between these results and those observed at 1 au may provide some clues as to the nature and evolution of the fluctuations.

Memory and nonlocal effects of heat transport in a spherical nanoparticle

In this paper a mathematical model describing the heat transport in a spherical nanoparticle subject to Newton heating at its surface is presented. The governing equations involve a phonon hydrodynamic equation for the heat flux and the classical energy equation that relates the heat flux and the temperature. Assuming radial symmetry the model is reduced to two partial differential equation, one for the radial component of the flux and one for the temperature. We solve the model numerically by means of finite differences. The resulting temperature profiles show characteristic wave-like behaviour consistent with the non Fourier components in the hydrodynamic equation.

New Methods Used for the Smoothing of the Three-Dimensional Flow Behind the Turbine Nozzle Cascade

To smooth the parameters of the three-dimensional flow behind the nozzle cascade new methods were suggested that allow us to sustain the flow rate, stagnation enthalpy and the axial projection of the moment of momentum for initial-, nonuniform and averaged flows. It was shown that the choice of the fourth integral characteristic (the kinetic energy, the entropy and the quantity of motion) has no particular significance because it has no effect on the complex criterion of the cascade quality, i.e. the velocity coefficient-angle cosine product that characterizes the level of the radial component of velocity. The minimum values of the velocity coefficient and the cosine angle satisfy the method that allows us to sustain the quantity of motion during the smoothing and the maximum values of the specified nozzle characteristics satisfy method 2 that enables the entropy maintenance. To evaluate the aerodynamic efficiency of the nozzle cascade the preference should be given to method 1 that enables the kinetic energy conservation and the velocity coefficient allows for the precise determination of the degree of loss of the kinetic energy that is equal to 3.6 % as for the example given in the scientific paper. As for method 1, the kinematic losses in the cascade are defined by the angle cosine that characterizes the level of the radial component of the velocity behind the cascade. For the example in question, kinematic losses are equal to 1.9 % and the complex criterion of quality equal to 0.972 corresponds to the overall losses of 5.5 %. It was suggested to use the velocity coefficient and the two angles of flow as integral cascade characteristics. The use of these characteristics enables the correct computations of the efficiency factor for the stage within the one-dimensional computation. The incisive analysis was performed for different methods used for the averaging of the parameters of the axially asymmetric flow behind the nozzle cascade. It was suggested to neglect the flow rate factor in the case of thermal computations done for the turbine stage.