Magnetic Field Region Research Articles

Quiescent Solar Wind Regions in the Near-Sun Environment: Properties and Radial Evolution

Regions of magnetic fields with near-radial, Parker-spiral-like geometry known as quiescent regions have been observed in Parker Solar Probe data. These regions have very low δ B/〈∣B∣〉 compared to nonquiescent solar wind at the same heliocentric distances. Quiescent regions are observed to have lower solar wind bulk speeds, lower proton temperatures, and lower proton density. Inside of 15 solar radii ( R ⊙ ), identified quiescent regions show distinct thermal properties, having higher proton temperature anisotropies and lower parallel plasma betas compared to switchback patches observed at the same heliocentric distances. When placed on R -versus-β ∥p plots (where R is the proton temperature anisotropy), quiescent region solar wind is shown to be more stable to proton cyclotron and firehose instabilities than nonquiescent solar wind at the same heliocentric distances. It is shown that quiescent regions evolve similarly to the surrounding nonquiescent solar wind, but quiescent solar wind begins at a different location in the R -versus-β ∥p parameter space, suggesting that these regions have separate origins from the more turbulent nonquiescent solar wind. Namely, enhanced temperature anisotropies and enhanced magnetic field strength may be consistent with magnetic field lines that have undergone less magnetic field expansion compared to nonquiescent wind at the same heliocentric distances.

Testing the flux tube expansion factor

Context. The properties of the solar wind measured in situ in the heliosphere are largely controlled by energy deposition in the solar corona, which is in turn closely related to the properties of the coronal magnetic field. Previous studies have shown that long-duration and large-scale magnetic structures show an inverse relation between the solar wind velocity measured in situ near 1 au and the expansion factor of the magnetic flux tubes in the solar atmosphere. Aims. The advent of the Solar Orbiter mission offers a new opportunity to analyse the relation between solar wind properties measured in situ in the inner heliosphere and the coronal magnetic field. We exploit this new data in conjunction with models of the coronal magnetic field and the solar wind to evaluate the flux expansion factor and speed relation. Methods. We use a Parker-like solar wind model, the “isopoly” model presented in previous works, to describe the motion of the solar wind plasma in the radial direction and model the tangential plasma motion due to solar rotation with the Weber and Davis equations. Both radial and tangential velocities are used to compute the plasma trajectory and streamline from Solar Orbiter location sunward to the solar ‘source surface’ at rss. We then employed a potential field source surface (PFSS) model to reconstruct the coronal magnetic field below rss to connect wind parcels mapped back to the photosphere. Results. We found a statistically weak anti-correlation between the in situ bulk velocity and the coronal expansion factor, for about 1.5 years of solar data. Classification of the data by source latitude reveals different levels of anticorrelation, which is typically higher when Solar Orbiter magnetically connects to high latitude structures than when it connects to low latitude structures. We show the existence of a fast solar wind that originates in strong magnetic field regions at low latitudes and undergoes large expansion factor. We provide evidence that such winds become supersonic during the super-radial expansion (below rss) and are theoretically governed by a positive v–f correlation. We find that faster winds exhibit, on average, a flux tube expansion at a larger radius than slower winds. Conclusions. An anticorrelation between solar wind speed and expansion factor is present for solar winds originating in high latitude structures in solar minimum activity, typically associated with coronal hole-like structures, but this cannot be generalized to lower latitude sources. We have found extended time intervals of fast solar wind associated with both large expansion factors and strong photospheric magnetic fields. Therefore, the value of the expansion factor alone cannot be used to predict the solar wind speed. Other parameters, such as the height at which the expansion gradient is the strongest, must also be taken into account.

Probing the magnetic fields of starspots with transit mapping

Starspots, regions of strong magnetic fields, serve as indicators of stellar activity and the dynamo mechanism at play in the interior of stars. The magnetic fields of main-sequence stars play a crucial role in driving stellar activity. An effective approach to better understanding stellar magnetic fields and activity lies in the detailed characterisation of starspot properties. We propose a new method for estimating the magnetic fields of starspots that employs modelling techniques of planetary transit mapping, which provides estimates of the size, intensity, and location of spots on the stellar photosphere. A starspot's maximum magnetic field was calculated using the linear relationship with the spot flux deficit, $ spot $ (the spot's brightness times its area) and the well-characterised relation for sunspots determined in this work, $B_ spot (G). Applying this relationship to previously mapped spots on the photospheres of 14 FGK and M stars yields spot maximum magnetic fields ranging from 2700 G to 4600 G, with an overall average of $3900 400$ G. We looked for correlations between starspot magnetic fields and stellar properties.\ We did not find any correlation between a spot's mean extreme magnetic field and effective temperature, nor the differential shear. However, a weak anti-correlation is seen between the spots' magnetic field and stellar age as well as between the magnetic field and the rotation period. When compared with previous results of small-scale magnetic field measurements, the B values obtained here are basically constant and near the saturation limit found for rapid rotators. This implies that it is not the intensity of the magnetic field of starspots that decreases with age but rather the filling factor. This result offers a unique window into the magnetic dynamo of stars.

Formation of quasi unipolar pulses in nonequilibrium magnetized plasma channels

The possibility of controlling both the spectral and polarization properties of THz pulses propagating in strongly nonequilibrium extended magnetized plasma channels formed by intense UV femtosecond laser pulses in nitrogen (air) is analyzed. The formation of quasiunipolar pulses with a nonzero electric area and a specific state of polarization is discussed. The transformation of such pulses upon leaving the region of a static magnetic field is analyzed.

Magnetization processes features in the tapes of cobalt-based amorphous alloy

Studies of the amorphous cobalt-based soft magnetic alloy AMAG-172 (Co–Ni–Fe–Cr–Mn–Si–B) from different manufacturers have shown that in the as-quenched state there is nonuniformity of the magnetic characteristics, and therefore the level of internal stresses across the width of the tape produced by MSTATOR (Borovichi) significantly lower. However, similar to the tape produced by NIIMET (Kaluga), there is nonuniformity of the tape in thickness, which contributes to the formation of a bimodal field dependence of magnetic permeability.This may be a consequence of the non-uniform concentration along the width of the tape of hydrogen and oxygen atoms embedded in its surface during interaction with atmospheric vapor and the presence of temperature gradients during the manufacturing process. A stepwise shape of the initial sections of the magnetization curves in the region of the first maximum of magnetic permeability was discovered. The abrupt nature of the magnetization processes in the region of weak fields allows us to conclude that in the samples of tape produced by MSTATOR, the formation of the first maximum of the field dependence is associated with an independent magnetization reversal of the surface layer, while the main layer of the tape participates in the formation of the second maximum. The smoothed shape of the stepped initial section of the magnetization curve of samples produced by NIIMET corresponds to the gradual involvement of the main layer of the tape in the processes of magnetization and remagnetization. A comparison of the hysteresis loops of both parts of the tape, measured in the same magnetic field region, shows that the field shift of the hysteresis loops is associated with the interlayer interaction of the surface and volume of the tape, and the formation of asymmetric hysteresis loops occurs with the participation of the second layer with its gradual involvement in the processes of magnetization and magnetization reversal.

Design and construction of GaInSn experimental facility for studies of mixed-convection MHD flows

A multifunctional liquid metal loop named MaTHE-XJTU (Magneto-Thermo-Hydrodynamic Experiments-Xi'an Jiaotong University) that utilizes eutectic alloy GaInSn as a working fluid has been designed and constructed. The function of the MaTHE-XJTU facility is to study the magnetohydrodynamic (MHD) flow and mixed convection characteristics under the coupling effect. The main operating parameters of the loop are: the maximum magnetic field intensity is 3T, the effective magnetic field region is 300 mm × 800 mm × 1000 mm, the maximum flow rate of electromagnetic pump (EM pump) is 8 m3/h, the maximum pressure head of EM pump is 0.5 MPa. The paper describes the major components and basic operation procedures of the loop, the related flow diagnostics method, and near-future experiments. This loop could provide a high-parameter experimental platform (Ha∼104, Gr∼109, Re∼104) for investigations that improve the present understanding of magnetohydrodynamic and heat transfer performance in liquid metal blankets.

Magnetic properties of a non-oriented electrical steel at cryogenic temperature for rapid-cycling accelerator superconducting magnets

A rapid-cycling superconducting magnet is currently under development for the next-generation heavy-ion therapy. The non-oriented electrical steel (NOES) sheet, M470-50A5, is adopted as the cold iron yoke in the superconducting magnet to enhance the field and minimize the AC losses. In this paper, we measure the magnetic properties, magnetic permeability and hysteresis loss of a 0.5-mm thick NOES sheet at 4.2 K, 77 K and room temperature with a toroidal test specimen. The magnetic permeability as well as the hysteresis loops are investigated with a very low frequency until 3 T. To provide the magnetization for the magnet design, we modify the Jiles–Atherton (J–A) model so that the modified J–A model can predict not only the magnetization loops but also the magnetic permeability until a high magnetic field region.

Local Transition of Electron Pitch Angle Distribution within Flux Pileup Region behind Dipolarization Front

Dipolarization fronts (DFs), earthward-propagating magnetic transients with a strong magnetic field, are important regions favorable for energetic electron acceleration in the magnetotail. The DF-driven electron acceleration usually generates coherent pitch angle distributions (PADs) inside flux pileup regions (FPRs), i.e., strong magnetic field regions behind the DFs, such as pancake, butterfly, and cigar distributions, which dominate at different tail regions and often occur separately. Here we present unique observations of electron PAD evolution inside the FPR, showing that electron PAD underwent local transition from cigar distribution, to butterfly distribution, then toward pancake distribution, forming a U-shaped distribution. During the local transition, electron perpendicular flux (relative to the local magnetic field) is anticorrelated with magnetic field strength, contrary to traditional expectation. The unexpected feature of the electron U-shaped distribution is associated with multiple physical processes at different scales, including local expansion of flux tubes and pitch angle variation near the neutral sheet. These atypical observations can advance our current understanding of electron acceleration and transport in the magnetosphere.

A Compact V-Band Transit Time Oscillator with Reflective Modulation Cavity

Improving compactness is essential for high-power microwave (HPM) sources. In this paper, a novel reflective modulation cavity is proposed and investigated in a V-band relativistic coaxial transit-time oscillator (RCTTO). The cold cavity analyses and particle-in-cell simulations show that the reflective modulation cavity has larger reflection coefficients of TEM mode and stronger electron beam modulation capability when compared with a uniform modulation cavity. When the input diode voltage is 391 kV, the beam current is 4.91 kA, and when the guiding magnetic field is 0.6 T, the compact V-band RCTTO produces an output microwave power of 518 MW (conversion efficiency of 27.0%). Compared with the original RCTTO, the compact V-band RCTTO featuring a reflective modulation cavity exhibits a 24.8% increase in output power and a 5.4% improvement in efficiency, and the axial length of the magnetic field uniform region is reduced by 24.2%. The compact V-band RCTTO also demonstrates a broad operation voltage range, indicating potential for stable operation with voltage fluctuations in experiments. Furthermore, the reflective modulation cavity can be integrated into other high-frequency O-type HPM devices to enhance compactness, thereby diminishing the demands on the magnetic field region, which is advantageous for the future permanent packaging of HPM sources.

High-latitude coronal mass ejections on the young solar-like star AB Dor

ABSTRACT AB Dor is a young solar-type star with a surface large-scale magnetic field $10^2$ to $10^3$ times stronger than the that of the Sun. Although strong magnetic fields are thought to inhibit coronal mass ejections (CMEs), dimming signatures typically associated with an eruptive CME were recently observed in AB Dor. The uninterrupted, long-duration dimming signal suggests that a CME took place at a high latitude, where it remained in view as the star rotates. A high-latitude CME is also consistent with observations that indicate that AB Dor hosts polar active regions. To investigate magnetic confinement in AB Dor, we conduct a parametric modelling study of 21 CMEs at latitudes ${\sim} 60^\circ$, varying the location, mass, and magnetic field strength of an injected flux rope. 12 models had the flux rope located in an open magnetic field region, while the remaining nine were in a closed region. Results show that CMEs in open-field regions are in general more likely to erupt. The four eruptive CMEs from closed regions had high free magnetic energies ${\gtrsim} 3\times 10^{35}$ erg, and 10 CMEs predominantly from the closed-field regions (8/10) were confined. CMEs in closed-field regions exhibited lower kinetic energies, since part of the CME energy was expended to overcome magnetic tension and break open the overlying field. In conclusion our work suggests that eruptive CMEs in AB Dor may occur in high-latitude regions of open magnetic field, as the magnetic tension in such regions does not significantly inhibit the eruption.

Modeling Ion Transport in the Upper Ionosphere of Mars: Exploring the Effect of Crustal Magnetic Fields

AbstractStatistically ion and electron densities are enhanced above strong crustal magnetic field regions according to measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. Plasma created by ionization of neutrals in the lower ionosphere (where chemistry dominates) flows upward and becomes trapped on closed magnetic field loops. Enhanced ion density in the ionosphere (particularly O2+) is associated with enhanced photochemical escape of atomic oxygen. This paper presents a quasi‐1D multi‐fluid time‐dependent model of the Martian ionosphere for nine ion species. Ionospheric temperatures are adopted but ion densities and velocities (along the field lines) are determined using a numerical solution of the continuity and momentum equations. Diurnal effects are explored by varying photoionization rates. Three crustal field cases are considered: a low altitude closed, a high altitude closed, and a high altitude open field line. Additionally, a case with no crustal field is modeled to provide a comparison between regions with and without crustal fields in the upper Martian ionosphere. Model results show higher ion and electron densities in the crustal field cases than in the purely induced field case. Additionally, we find that densities are generally higher on the closed field lines than on the open field lines, and ion velocities are generally up the field lines, away from the Martian surface. We also find that velocities are larger on the open field line case. We compare modeled density results to MAVEN data and find general agreement. Implications for atmospheric escape, particularly photochemical escape of O, are also discussed.

Effects of vacuum magnetic field region on the compact torus trajectory in a tokamak plasma

The trajectory of the compact torus (CT) within a tokamak discharge is crucial to fueling. In this study, we developed a penetration model with a vacuum magnetic field region to accurately determine CT trajectories in tokamak discharges. This model was used to calculate the trajectory and penetration parameters of CT injections by applying both perpendicular and tangential injection schemes in both HL-2A and ITER tokamaks. For perpendicular injection along the tokamak’s major radius direction from the outboard, CTs with the same injection parameters exhibited a 0.08 reduction in relative penetration depth when injected into HL-2A and a 0.13 reduction when injected into ITER geometry when considering the vacuum magnetic field region compared with cases where this region was not considered. In addition, we proposed an optimization method for determining the CT’s initial injection velocity to accurately calculate the initial injection velocity of CTs for central fueling in tokamaks. Furthermore, this paper discusses schemes for the tangential injection of CT into tokamak discharges. The optimal injection angle and CT magnetic moment direction for injection into both HL-2A and ITER were determined through numerical simulations. Finally, the kinetic energy loss occurring when the CT penetrated the vacuum magnetic field region in ITER was reduced by by optimizing the injection angle for the CT injected into ITER. These results provide valuable insights for optimizing injection angles in fusion experiments. Our model closely represents actual experimental scenarios and can assist the design of CT parameters.

Phase Transition near the Filling Factor ν = 3

A phase transition accompanied by the appearance of a spike in the longitudinal resistance of a two-dimensional electron system has been studied using the electron spin resonance near the filling factor ν = 3 in the ZnO/MgZnO heterojunction. This transition occurs when the tilt angle θ of the magnetic field is increased to some critical value θc. An analysis of the spin resonance amplitude has made it possible to demonstrate the spin nature of this phenomenon. For example, the ground state of the system on both sides of the transition has a nonzero spin polarization, which changes by several times when the phase of the system is changed. Strong spin resonance is observed both at θ < θc and at θ > θc. Surprisingly, the spin resonance at the critical angle θc has been detected in only one phase, which lies in the region of magnetic fields below the critical field Bc corresponding to the spike position in the longitudinal resistance. An increase in the magnetic field to this value leads to a decrease in the resonance amplitude and an increase in the resonance width. In the field region above Bc, the spin resonance disappears completely. Such behavior of the spin resonance is most likely due to the formation of domains with different spin polarizations in the electron system.

Modeling the Transport of Solar Energetic Particles in a Corotating Interaction Region

We present a new three-dimensional (3D) magnetohydrodynamic (MHD) model and a new 3D energetic particle transport (EPT) model. The 3D MHD model numerically solves the ideal MHD equations using the relaxing total variation diminishing scheme. In the 3D MHD simulations, we use simple boundary conditions with a high-speed flow, and we can clearly identify a corotating interaction region (CIR) with the characteristics of forward shock and reverse shock. The 3D EPT model solves the Fokker–Planck transport equation for the solar energetic particles (SEPs) using backward stochastic processes, with the magnetic field and solar wind velocity field from MHD results. For comparison, the 3D EPT model results with Parker fields are also obtained. We investigate the transport of SEPs with particle sources and observers in different positions in MHD fields with a CIR, and we compare the results with those in the Parker fields. Our simulation results show that the compression region with local enhancement of the magnetic field, i.e., CIR, can act as a barrier to scatter energetic particles back, and particles can struggle to diffuse through the strong magnetic field regions. Usually, a normal anisotropy profile is commonly present in SEP simulation results with Parker fields, and it is also typically present in that with MHD fields. However, because of the compression region of the magnetic field, energetic particles may exhibit anomalous anisotropy. This result may be used to replicate the spacecraft observation phenomena of the anomalous anisotropy.

Observation of electron temperature anisotropy in the magnetic filter of a hot cathode discharge

Instabilities arising from electron temperature in an ambient magnetic field when measured parallel and perpendicular to it could influence plasma stability and have effect on plasma diffusion. A planar Langmuir probe measures this instability in the double plasma device in the magnetic filter field region and it is found that the electron temperature measured parallel and perpendicular to the filter region is not equal. This anisotropy is also found to vary with filament current and working pressure.

Bose-Einstein condensation of non-ground-state caesium atoms

Bose-Einstein condensates of ultracold atoms serve as low-entropy sources for a multitude of quantum-science applications, ranging from quantum simulation and quantum many-body physics to proof-of-principle experiments in quantum metrology and quantum computing. For stability reasons, in the majority of cases the energetically lowest-lying atomic spin state is used. Here, we report the Bose-Einstein condensation of caesium atoms in the Zeeman-excited mf = 2 state, realizing a non-ground-state Bose-Einstein condensate with tunable interactions and tunable loss. We identify two regions of magnetic field in which the two-body relaxation rate is low enough that condensation is possible. We characterize the phase transition and quantify the loss processes, finding unusually high three-body losses in one of the two regions. Our results open up new possibilities for the mixing of quantum-degenerate gases, for polaron and impurity physics, and in particular for the study of impurity transport in strongly correlated one-dimensional quantum wires.

Analysis of BMR Tilt from AutoTAB Catalog: Hinting toward the Thin Flux Tube Model?

One of the intriguing mechanisms of the Sun is the formation of bipolar magnetic regions (BMRs) in the solar convection zone (CZ), which are observed as regions of concentrated magnetic fields of opposite polarity on the photosphere. These BMRs are tilted with respect to the equatorial line, which statistically increases with latitude. The thin flux tube model, employing the rise of magnetically buoyant flux loops and their twist by Coriolis force, is a popular paradigm for explaining the formation of tilted BMRs. In this study, we assess the validity of the thin flux tube model by analyzing the tracked BMR data obtained through the Automatic Tracking Algorithm for BMRs. Our observations reveal that the tracked BMRs exhibit the expected collective behaviors. We find that the polarity separation of BMRs increases over their normalized lifetime, supporting the assumption of a rising flux tube from the CZ. Moreover, we observe an increasing trend of the tilt with the flux of the BMR, suggesting that rising flux tubes associated with lower flux regions are primarily influenced by drag force and Coriolis force, while in higher flux regions, magnetic buoyancy dominates. Furthermore, we observe Joy’s law dependence for emerging BMRs from their first detection, indicating that at least a portion of the tilt observed in BMRs can be attributed to the Coriolis force. Notably, lower flux regions exhibit a higher amount of fluctuations associated with their tilt measurement compared to stronger flux regions, suggesting that lower flux regions are more susceptible to turbulent convection.

Influence of acceleration stage electrode voltage on the performance of double-stage Hall effect thruster with adjustable zero magnetic point

The configuration of electrode voltage and zero magnetic point position has a significant impact on the performance of the double-stage Hall effect thruster. A 2D-3V model is established based on the two-magnetic peak type double-stage Hall thruster configuration, and a particle-in-cell simulation is carried out to investigate the influences of both acceleration electrode voltage value and zero magnetic point position on the thruster discharge characteristics and performances. The results indicate that increasing the acceleration voltage leads to a larger potential drop in the acceleration stage, allowing ions to gain higher energy, while electrons are easily absorbed by the intermediate electrode, resulting in a decrease in the anode current and ionization rate. When the acceleration voltage reaches 500 V, the thrust and efficiency are maximized, resulting in a 15% increase in efficiency. After the acceleration voltage exceeds 500 V, a potential barrier forms within the channel, leading to a decrease in thruster efficiency. Further study shows that as the second zero magnetic point moves towards the outlet of the channel, more electrons easily traverse the zero magnetic field region, participating in the ionization. The increase in the ionization rate leads to a gradual enhancement in both thrust and efficiency.

Chromospheric and coronal heating in an active region plage by dissipation of currents from braiding

The question of what heats the outer solar atmosphere remains one of the longstanding mysteries in astrophysics. Statistical studies of Sun-like stars reveal a correlation between global chromospheric and coronal emissions, constraining theoretical models of potential heating mechanisms. However, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales. Here we use unique coordinated observations of the chromosphere (from the IRIS satellite) and the low corona (from the Hi-C 2.1 sounding rocket), and machine-learning-based inversion techniques, to show a strong correlation on spatial scales of a few hundred kilometres between heating in the chromosphere and emission in the upper transition region in strong magnetic field regions (‘plage’). Our observations are compatible with an advanced three-dimensional magnetohydrodynamic simulation in which the dissipation of current sheets caused by magnetic field braiding is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in solar active regions.

Adjusting the Potential Field Source Surface Height Based on Magnetohydrodynamic Simulations

A potential field solution is widely used to extrapolate the coronal magnetic field above the Sun’s surface to a certain height. This model applies the current-free approximation and assumes that the magnetic field is entirely radial beyond the source surface height, which is defined as the radial distance from the center of the Sun. Even though the source surface is commonly specified at 2.5 R s (solar radii), previous studies have suggested that this value is not optimal in all cases. In this study, we propose a novel approach to specify the source surface height by comparing the areas of the open magnetic field regions from the potential field solution with predictions made by a magnetohydrodynamic model, in our case the Alfvén Wave Solar atmosphere Model. We find that the adjusted source surface height is significantly less than 2.5 R s near solar minimum and slightly larger than 2.5 R s near solar maximum. We also report that the adjusted source surface height can provide a better open flux agreement with the observations near the solar minimum, while the comparison near the solar maximum is slightly worse.