Decrease In Magnetic Field Strength Research Articles

Study of the calorimetric effect in ferrogels subjected to the high-frequency rotating magnetic field

This study investigates the calorimetric effect in iron oxide-containing ferrogels when exposed to a high-frequency rotating magnetic field. The primary heat generation mechanisms were magnetic relaxation (Néel and Brownian) and magnetic hysteresis. Temperature was monitored over time for various magnetic field amplitudes, enabling the determination of the contributions of both heat release mechanisms as a function of particle concentration and field strength. Experiments were performed using agar-based tissue mimicking phantoms with magnetic nanoparticle weight concentrations ranging from 5 % to 20 %. The magnetic fields applied varied from 1 kA/m to 5 kA/m, with the equipment having a capability of up to 7.5 kA/m at a frequency of 200 kHz. The experimental setup employed a two-phase magnetic system enclosed in an external core where magnetizing coils and parallel connected inductors and capacitors generated spatially and phase shifted fluxes by 90 degrees, resulting in a constant amplitude of rotating magnetic field.The heating efficiency of the rotating magnetic field was found to be approximately twice as effective as an equivalent alternating field in the experiments. The specific loss power exhibited a decreasing trend with the increase in magnetic nanoparticle concentration and the decrease in magnetic field strength. Specifically, as the magnetic nanoparticle concentration increased from 5 % to 20 %, the SLP diminished from 457.3 mW/g (at 5 % concentration and 5 kA/m) to 1.24 mW/g (at 20 % concentration and 1 kA/m). This decrease in SLP can be attributed to enhanced dipole–dipole interactions between particles at smaller interparticle distances as concentration increases, as well as reduced magnetic responsiveness to weaker magnetic fields. Even though Brownian relaxation was likely hindered in the gel structure due to the immobilization of magnetic nanoparticles, the observed magnetic heating effect remains significant. Given the limited existing data on rotating magnetic field effects on tissue mimicking ferrogels, this study offers novel insights, potentially aiding in the optimization of magnetic hyperthermia treatments while highlighting the benefits of rotating over alternating magnetic fields.

Evolution of the Radial Size and Expansion of Coronal Mass Ejections Investigated by Combining Remote and In Situ Observations

A fundamental property of coronal mass ejections (CMEs) is their radial expansion, which determines the increase in the CME radial size and the decrease in the CME magnetic field strength as the CME propagates. CME radial expansion can be investigated either by using remote observations or by in situ measurements based on multiple spacecraft in radial conjunction. However, there have been only few case studies combining both remote and in situ observations. It is therefore unknown if the radial expansion in the corona estimated remotely is consistent with that estimated locally in the heliosphere. To address this question, we first select 22 CME events between the years 2010 and 2013, which were well observed by coronagraphs and by two or three spacecraft in radial conjunction. We use the graduated cylindrical shell model to estimate the radial size, radial expansion speed, and a measure of the dimensionless expansion parameter of CMEs in the corona. The same parameters and two additional measures of the radial-size increase and magnetic-field-strength decrease with heliocentric distance of CMEs based on in situ measurements are also calculated. For most of the events, the CME radial size estimated by remote observations is inconsistent with the in situ estimates. We further statistically analyze the correlations of these expansion parameters estimated using remote and in situ observations, and discuss the potential reasons for the inconsistencies and their implications for the CME space weather forecasting.

The Spiderweb Protocluster is Being Magnetized by Its Central Radio Jet

We present deep broadband radio polarization observations of the Spiderweb radio galaxy (J1140-2629) in a galaxy protocluster at z = 2.16. These yield the most detailed polarimetric maps yet made of a high-redshift radio galaxy. The intrinsic polarization angles and Faraday rotation measures (RMs) reveal coherent magnetic fields spanning the ∼60 kpc length of the jets, while ∼50% fractional polarizations indicate these fields are well ordered. Source-frame ∣RM∣ values of ∼1000 rad m−2 are typical, and values up to ∼11,100 rad m−2 are observed. The Faraday-rotating gas cannot be well mixed with the synchrotron-emitting gas, or stronger-than-observed depolarization would occur. Nevertheless, an observed spatial coincidence between a localized ∣RM∣ enhancement of ∼1100 rad m−2 , a bright knot of Lyα emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling ∼1000 s rad m−2 across the width of the jet, suggesting a net clockwise (as viewed from the active galactic nuclei) toroidal magnetic field component exists at tens-of-kiloparsec scales, which we speculate may be associated with the operation of a Poynting–Robertson cosmic battery. We conclude the RMs are mainly generated in a sheath of hot gas around the radio jet, rather than the ambient foreground protocluster gas. The estimated magnetic field strength decreases by successive orders of magnitude going from the jet hotspots (∼90 μG) to the jet sheath (∼10 μG) to the ambient intracluster medium (∼1 μG). Synthesizing our results, we propose that the Spiderweb radio galaxy is actively magnetizing its surrounding protocluster environment, with possible implications for theories of the origin and evolution of cosmic magnetic fields.

Flow behind an exponential shock wave in a perfectly conducting mixture of micro size small solid particles and non-ideal gas with azimuthal magnetic field

Unsteady adiabatic and isothermal flows behind an exponential spherical or cylindrical shock wave in a perfectly conducting mixture of micro size small solid particles and non-ideal gas under the influence of azimuthal magnetic field is investigated. The moving piston drives the shock wave. The azimuthal magnetic field in the ambient medium is assumed to be varying according to exponential laws. In our study, micro size solid particles are taken to be pseudo-fluid with the supposition that the equilibrium conditions for flow are maintained in the entire flow- field region, and the heat conduction and the viscous-stress of the conducting mixture are negligible. In both the isothermal and adiabatic cases, similarity solutions are obtained by taking into consideration the magnetic field and compressibility. The effects on the shock wave propagation and on the flow variables by the ratio of the density of micro size solid particles to the initial density of the gas Ga as well as by the non-idealness parameter δ of the gas in the conducting mixture, by the mass concentration λp of micro size solid particles in the conducting mixture and by the adiabatic exponent γ are worked out in detail. A relationship between the solutions in the cases of isothermal and adiabatic flows is made. It is found that due to the thought of zero temperature gradient the flow variables fluid velocity, density and magnetic field and shock strength decrease. Also, it is important to note down that the shock wave strength enhances with an increase in Gaor due to change in geometry from cylindrical to spherical; whereas it decreases with an increase in the non-idealness parameterδ or the strength of ambient magnetic field MA−2or the adiabatic exponent γ .

Length Measurement of Chain-Like Structure of Micron Magnetic Particles Dispersing in Carrier Fluid Effected by Magnetic Field

The chain-like structure and motion of the magnetic particles (MPs) in magnetorheological fluid (MRF) determine the rheological properties of MRF. In order to investigate the appearance and motion state of the chain-like structure of MPs under both static and flowing states of MRF, the formation process of MP chains in a stationary MRF under the action of an external magnetic field was observed by microscope. And the length of MP chains was measured by an objective micrometer to reveal the key factors affecting the length of formed chain. The MP chains broke after the magnet rotated by 90 degrees. The MRF was injected into a microchannel to observe the motion of MP chains. The speeds of the MP chains were measured by micrometer under different magnetic field strengths and inflow rates. The static test results show that the particle diameter has the greatest influence on the length of chain. The maximum length of the MP chains is 978.2 μm with a magnetic field strength of 130.31 mT; the diameter and mass fraction of the MPs are 2 μm and 3%. The MP chains will break from a long chain into short chains and rotate at a certain angle due to the rotation of magnetic field. The test results at different inflow rates show that the moving speed of MP chains in the microchannel increases with the increase of inflow rate and the decrease of magnetic field strength. In addition, the MP chains can withstand a carrier fluid flow rate of up to 0.32 m/s.

Magnetic Fields in Elliptical Galaxies: An Observational Probe of the Fluctuation Dynamo Action

Abstract Fluctuation dynamos are thought to play an essential role in the evolution of magnetized galaxies, saturating within ∼0.01 Gyr and thus potentially acting as seeds for large-scale mean-field dynamos. However, unambiguous observational confirmation of the fluctuation dynamo action in a galactic environment is still missing. This is because, in spiral galaxies, it is difficult to differentiate between small-scale magnetic fields generated by a fluctuation dynamo and those due to the tangling of the large-scale field. We propose that observations of magnetic fields in elliptical galaxies would directly probe the fluctuation dynamo action. This is motivated by the fact that in ellipticals, due to their lack of significant rotation, the conventional large-scale dynamo is absent and the fluctuation dynamo is responsible for controlling the strength and structure of the magnetic field. By considering turbulence injected by Type Ia supernova explosions and possible magnetic field amplification by cooling flows, we estimate expected magnetic field strengths of in the centers of quiescent elliptical galaxies. We use a semianalytic model of galaxy formation to estimate the distribution and redshift evolution of field strengths, tentatively finding a decrease in magnetic field strength with decreasing redshift. We analyze a historical sample of radio sources that exhibit the Laing–Garrington effect (radio polarization asymmetry in jets) and infer magnetic field strengths between for a uniform thermal electron density and between for the thermal electron density following the King profile. We examine observational techniques for measuring the magnetic field saturation state in elliptical galaxies, focusing on Faraday rotation measure grids, the Laing–Garrington effect, synchrotron emission, and gravitational lensing, and finding appealing prospects for future empirical analysis.

The Changing Eigenfrequency Continuum During Geomagnetic Storms: Implications for Plasma Mass Dynamics and ULF Wave Coupling

AbstractGeomagnetic storms are one of the most energetic space weather phenomena. Previous studies have shown that the eigenfrequencies of ultralow frequency (ULF) waves on closed magnetic field lines in the inner magnetosphere decrease during storm times. This change suggests either a reduction in the magnetic field strength and/or an increase in its plasma mass density distribution. We investigate the changes in local eigenfrequencies by applying a superposed multiple‐epoch analysis to cross‐phase spectra from 132 geomagnetic storms. Six ground magnetometer pairs are used to investigate variations from approximately 3 < L < 7 and across the whole dayside sector. We find that at L > 4, the eigenfrequencies decrease by as much as 50% relative to their quiet time values. Both a decrease in magnetic field strength and an increase in plasma mass density, in some locations by more than a factor of 2, are responsible for this reduction. The enhancement of the ring current and an increase in oxygen ion density could explain these observations. At L < 4, the eigenfrequencies increase due to the decrease in plasma mass density caused by plasmaspheric erosion.

Radial Evolution of Coronal Mass Ejections Between MESSENGER, Venus Express, STEREO, and L1: Catalog and Analysis

AbstractOur knowledge of the properties of coronal mass ejections (CMEs) in the inner heliosphere is constrained by the relative lack of plasma observations between the Sun and 1 AU. In this work, we present a comprehensive catalog of 47 CMEs measured in situ measurements by two or more radially aligned spacecraft (MESSENGER, Venus Express, STEREO, Wind/ACE). We estimate the CME impact speeds at Mercury and Venus using a drag‐based model and present an average propagation profile of CMEs (speed and deceleration/acceleration) in the inner heliosphere. We find that CME deceleration continues past Mercury's orbit but most of the deceleration occurs between the Sun and Mercury. We examine the exponential decrease of the maximum magnetic field strength in the CME with heliocentric distance using two approaches: a modified statistical method and analysis from individual conjunction events. Findings from both the approaches are on average consistent with previous studies but show significant event‐to‐event variability. We also find the expansion of the CME sheath to be well fit by a linear function. However, we observe the average sheath duration and its increase to be fairly independent of the initial CME speed, contradicting commonly held knowledge that slower CMEs drive larger sheaths. We also present an analysis of the 3 November 2011 CME observed in longitudinal conjunction between MESSENGER, Venus Express, and STEREO‐B focusing on the expansion of the CME and its correlation with the exponential falloff of the maximum magnetic field strength in the ejecta.

Decrease of the interplanetary magnetic field strength on the lunar dayside and over the polar region

The Moon interacts with the incident solar wind plasma in various ways, and most of these interactions are accompanied by variations of the interplanetary magnetic field (IMF) around the Moon. Here we first report decreases of the IMF strength observed at 100 km altitude on the lunar dayside and over the polar region, comparing upstream solar wind data from ACE with Kaguya (SELENE) data. We note that the magnetic field decreases are observed above non-magnetized regions or very weakly magnetized regions. In one event the IMF is weakened in the dayside northern hemisphere when the IMF is roughly anti-parallel to the solar wind flow. We estimate that the decrease in the magnetic pressure can be partly compensated by the thermal pressure of the back-scattered solar wind protons, which suggests that the magnetic field decrease is interpreted as diamagnetic effect by the back-scattered protons. In another event an IMF decrease is continuously detected from the northern polar region to the dayside mid-latitude region, which is not fully explained by the thermal pressure of the back-scattered protons. We also discuss the diamagnetic current system in the upstream (fore-moon) solar wind region formed by the back-scattered protons.

Spin interaction and magnetic field strength effects on the system of two interacting electrons in a 2D quartic confinement potential

In this study we review concepts of double quantum dot, quantum chaos with shifted 1/N expansion method associated with semiquantum nonlinear system. We present a numerical study of two interacting particle motions in a time dependent magnetic field in quartic geometry. It is evident that, the area where the interaction particle motions are stochastic decreases as the spin interaction strength decreases, as well as, the magnetic field strength decreases. Moreover, we describe their possible connections with other aspects of quantum information. Furthermore, we pay attention to a system of two interacting electrons in a two-dimensional quartic confinement potential and hypothesis leading to analytical energy expression. The dynamics of double quantum dot gallium arsenide are of great importance and are also emphasized.

РАДИАЦИОННАЯ ОПАСНОСТЬ НА ЗЕМЛЕ И В ОКОЛОЗЕМНОМ ПРОСТРАНСТВЕ В ПРОЦЕССЕ ИНВЕРСИИ МАГНИТНОГО ПОЛЯ

Recent observations, such as the magnetic field strength decrease, a magnetic poles shifts and the South Atlantic anomaly increase, may indicate the beginning of the Earth’s magnetic field inversion. According to the geomagnetic dynamo model, the dipole component of the magnetic field is zeroed at the inversion time, and the quadrupole one becomes dominant. To assess the occurrence of radiation hazards on the Earth’s surface and in near-Earth space at the time of magnetic field inversion, a numerical model was developed that made it possible to compare the GCR and SCR fluxes (at minima and maxima of solar activity) penetrating the Earth (taking into account the atmosphere) and the ISS in periods of the dipole and quadrupole fields dominance. It was found that during the period of inversion the flow of GCR (high-energy particles) can increase no more than three times over the entire surface of the Earth and the radiation dose will not exceed permissible one for man. Also, a change of the magnetic field configuration will redistribute areas of increased radiation on the Earth’s surface (today these are the poles of the Earth), which can adversely affect people’s health in these areas.

Betatron Cooling of Suprathermal Electrons in the Terrestrial Magnetotail

Abstract The cigar distribution of suprathermal electrons (40–200 keV), showing electron pitch angles primarily in the parallel and anti-parallel directions, has been frequently observed in the terrestrial magnetotail. The formation of such a distribution is typically attributed to Fermi acceleration, betatron cooling, or a combination of these. To date, Fermi acceleration has been well studied via both observations and simulations, while betatron cooling has not been verified directly. In this study, we focus on the betatron cooling of suprathermal electrons. By analyzing a unique case observed by the Cluster spacecraft in the Earth's magnetotail (X GSM ≈ −15 R E ), we find a significant drop of electron flux in association with the decrease of magnetic field strength, i.e., a magnetic dip. This magnetic dip is formed due to the expansion of flux tubes driven by two opposite flows. The drop in electron flux, primarily in the perpendicular direction, is therefore strong evidence of betatron cooling. We successfully reproduce these processes using an analytical model.

Numerical study of biomagnetic fluid flow in a duct with a constriction affected by a magnetic field

Abstract In this paper, the influence of a non-uniform magnetic field on biomagnetic fluid flow in a duct with a constriction is investigated numerically. The magnetic field is generated by a wire carrying electric current located outside the duct. The biomagnetic fluid dynamics model, which is based on the ferrohydrodynamics and magnetohydrodynamics, is used to study the fluid flow, and a computational grid properly covering the magnetic field is used for the simulation. In this investigation, the influence of magnetic field on biomagnetic fluid flow in various percentages of the duct constriction, and the effect of the magnetic field on the biomagnetic fluid flow in various strengths of magnetic force are studied. The magnetic field is spatially varying, and the magnetic field strength decreases by increasing the distance from the wire carrying electric current which is the magnetic source. The results show that, the influence of the magnetic field on biomagnetic fluid flow is considerable near the magnetic source. Applying the magnetic field with enough strength causes the recirculation areas downstream of constriction to become smaller. In addition, applying the magnetic field has significant effects on the wall shear stress, so that it can decrease the shear stress on the wall away from the magnetic source.

Detection of magnetic fields in He-rich early B-type stars using HARPSpol

Aims. We focus on early-B type stars with helium overabundance, for which the presence of a magnetic field has not previously been reported. Methods. The measurements were carried out using high-spectral-resolution spectropolarimetric observations obtained with the High Accuracy Radial velocity Planet Searcher (HARPS) in polarimetric mode, installed at the ESO La Silla 3.6 m telescope. Results. For five He-rich stars, the longitudinal magnetic field was detected for the first time. For one target, HD 58260, the presence of a longitudinal magnetic field of the order of 1.8 kG has already been reported in the literature, but the magnetic field has remained constant over tens of years. Our measurement carried out using the polarimetric spectra obtained in 2015 March indicates a slight decrease of the longitudinal magnetic field strength compared to measurements reported in previous works. A search for periodic modulation in available photometric data allowed us to confidently establish a period of 2.64119 ± 0.00420 d in archival ASAS3 data for CPD–27°1791. No period could be determined for the other five stars. Conclusions. The obtained results support the scenario that all He-rich stars are detectably magnetic and form an extension of the Ap star phenomenon to higher temperatures.

Vector Magnetic Evaluation of Core Material under Stress Conditions

It was clarified that not only the tensile and compressive stress but also the shear stress occur on the surface of the motor core. Therefore it is needed to evaluate the magnetic properties of the core material under the various stress condition. In this paper, we will report about influence of the arbitrary stress on the vector magnetic properties of a non-oriented electrical steel sheet under various magnetic flux conditions. From these results, it was clear that the magnetic field strength and magnetic power loss decrease when tensile, compression, and shear stresses are applied.

The evolution of surface magnetic fields in young solar-type stars II: the early main sequence (250–650 Myr)★

There is a large change in surface rotation rates of sun-like stars on the pre-main sequence and early main sequence. Since these stars have dynamo-driven magnetic fields, this implies a strong evolution of their magnetic properties over this time period. The spin-down of these stars is controlled by interactions between stellar and magnetic fields, thus magnetic evolution in turn plays an important role in rotational evolution. We present here the second part of a study investigating the evolution of large-scale surface magnetic fields in this critical time period. We observed stars in open clusters and stellar associations with known ages between 120 and 650 Myr, and used spectropolarimetry and Zeeman Doppler Imaging to characterize their large-scale magnetic field strength and geometry. We report 15 stars with magnetic detections here. These stars have masses from 0.8 to 0.95 M⊙, rotation periods from 0.326 to 10.6 d, and we find large-scale magnetic field strengths from 8.5 to 195 G with a wide range of geometries. We find a clear trend towards decreasing magnetic field strength with age, and a power law decrease in magnetic field strength with Rossby number. There is some tentative evidence for saturation of the large-scale magnetic field strength at Rossby numbers below 0.1, although the saturation point is not yet well defined. Comparing to younger classical T Tauri stars, we support the hypothesis that differences in internal structure produce large differences in observed magnetic fields, however for weak-lined T Tauri stars this is less clear.

Physical mechanism causing rapid changes in ultrarelativistic electron pitch angle distributions right after a shock arrival: Evaluation of an electron dropout event

AbstractThree mechanisms have been proposed to explain relativistic electron flux depletions (dropouts) in the Earth's outer radiation belt during storm times: adiabatic expansion of electron drift shells due to a decrease in magnetic field strength, magnetopause shadowing and subsequent outward radial diffusion, and precipitation into the atmosphere (driven by EMIC wave scattering). Which mechanism predominates in causing electron dropouts commonly observed in the outer radiation belt is still debatable. In the present study, we evaluate the physical mechanism that may be primarily responsible for causing the sudden change in relativistic electron pitch angle distributions during a dropout event observed by Van Allen Probes during the main phase of the 27 February 2014 storm. During this event, the phase space density of ultrarelativistic (>1 MeV) electrons was depleted by more than 1 order of magnitude over the entire radial extent of the outer radiation belt (3 < L* < 5) in less than 6 h after the passage of an interplanetary shock. We model the electron pitch angle distribution under a compressed magnetic field topology based on actual solar wind conditions. Although these ultrarelativistic electrons exhibit highly anisotropic (peaked in 90°), energy‐dependent pitch angle distributions, which appear to be associated with the typical EMIC wave scattering, comparison of the modeled electron distribution to electron measurements indicates that drift shell splitting is responsible for this rapid change in electron pitch angle distributions. This further indicates that magnetopause loss is the predominant cause of the electron dropout right after the shock arrival.

General trends in the change in solar activity indices in the period from the late XX to early XXI century

We present an analysis of observations of the indices of the sunspot numbers (SSN from the National Oceanic and Atmospheric Administration (NOAA)), the radioemission flux at a wavelength of 10.7 cm (F 10.7 ), and the Total Solar Irradiance (TSI) in cycles 22-24. We have found that the ratio of the observed values SSNobs to the calculated ones SSNsyn (from the regression relations between SSN and F10.7) decreases after 1990 by 20-25%, rather than by 35%, as was supposed earlier. The characteristics of the most widely used SSN index, such as the decrease in sunspot numbers and magnetic field strength in small and medium spots, do not change in complete correspondence to the expected scenario of the solar activity devel� opment predicted from the flux F 10.7 in cycles 23 and 24; they also cannot be fully explained by an additional minimum of the secular cycle. We also showed that the observed SSN changes lead to an increase of the Total Solar Irradiance in activity cycles 23-24 as compared to that in cycle 22.

Solar coronal magnetic fields derived using seismology techniques applied to omnipresent sunspot waves

Sunspots on the surface of the Sun are the observational signatures of intense manifestations of tightly packed magnetic field lines, with near-vertical field strengths exceeding 6,000 G in extreme cases. It is well accepted that both the plasma density and the magnitude of the magnetic field strength decrease rapidly away from the solar surface, making high-cadence coronal measurements through traditional Zeeman and Hanle effects difficult since the observational signatures are fraught with low-amplitude signals that can become swamped with instrumental noise. Magneto-hydrodynamic (MHD) techniques have previously been applied to coronal structures, with single and spatially isolated magnetic field strengths estimated as 9-55 G. A drawback with previous MHD approaches is that they rely on particular wave modes alongside the detectability of harmonic overtones. Here we show, for the first time, how omnipresent magneto-acoustic waves, originating from within the underlying sunspot and propagating radially outwards, allow the spatial variation of the local coronal magnetic field to be mapped with high precision. We find coronal magnetic field strengths of 32 +/- 5 G above the sunspot, which decrease rapidly to values of approximately 1 G over a lateral distance of 7000 km, consistent with previous isolated and unresolved estimations. Our results demonstrate a new, powerful technique that harnesses the omnipresent nature of sunspot oscillations to provide magnetic field mapping capabilities close to a magnetic source in the solar corona.

The effects of magnetic field in plume region on the performance of multi-cusped field thruster

The performance characteristics of a Multi-cusped Field Thruster depending on the magnetic field in the plume region were investigated. Five magnetic field shielding rings were separately mounted near the exit of discharge channel to decrease the strength of magnetic field in the plume region in different levels, while the magnetic field in the upstream was well maintained. The test results show that the electron current increases with the decrease of magnetic field strength in the plume region, which gives rise to higher propellant utilization and lower current utilization. On the other hand, the stronger magnetic field in the plume region improves the performance at low voltages (high current mode) while lower magnetic field improves the performance at high voltages (low current mode). This work can provide some optimal design ideas of the magnetic strength in the plume region to improve the performance of thruster.