Magnetic Field Region Research Articles

Quenching of spin-orbit coupling and signature of Griffiths Phase in nanocrystalline La0.6Ba0.4MnO3 perovskite manganite

The size-dependent structural and magnetic properties of nano and bulk samples of 40 ​mol% Ba2+-doped LaMnO3 (i.e., La0.6Ba0.4MnO3) manganite have been investigated in detail. The Rietveld refinement of X-ray diffraction data confirms that nano and bulk La0.6Ba0.4MnO3 manganites crystallize into ideal cubic perovskite structure with Pm3¯m space group. The refined unit cell parameters decrease with increasing calcinations temperature or particle size. The temperature dependent magnetization measurements reveal that both the bulk and nano samples exhibit paramagnetic (PM) to ferromagnetic (FM) phase transition at Curie temperature (TC). A distinct bifurcation between magnetization curves under zero-field cooled (MZFC) and field-cooled (MFC) measurements reveals the possible existence of frustrated magnetic ordering or spin glass clusters. For nano sample, a sharp downward deviation in inverse susceptibility from Curie-Weiss law is observed, which indicates the existence of Griffiths Phase. The analysis of the field dependent magnetization M(H) curves measured at 10 ​K, in terms of saturation moments, reveals that there is a coupling between spin and orbital magnetic moments, in bulk sample of La0.6Ba0.4MnO3 manganite, which can be quenched by reducing particle size of the manganite. According to the Banerjee's criterion, nano sample exhibits first-order magnetic transition and bulk sample shows second-order magnetic transition in the low magnetic field region.

Plasma structure and electron cross-field transport induced by azimuthal manipulation of the radial magnetic field in a Hall thruster E × B discharge

Plasma structure and electron cross field in the z–θ plane of a Hall thruster E×B plasma under an azimuthally inhomogeneous magnetic field are studied by both experimental and numerical approaches. The work is intended to identify a primary role of electron dynamics on the structure formation by manipulating only the strongly magnetized electrons. The plasma potential distribution shows an axial–azimuthal variation; a low magnetic field region results in spatial potential saturation further downstream. The plasma density structure shows a 1D-like azimuthal variation with less axial deformation. A dense region is observed near the location of ∇B>0, where electrons are expected to undergo the ∇B and curvature drift toward the anode where neutrals are introduced. The potential structure is in close correlation to the Hall parameter distribution, indicating that electron dynamics plays a primary role in plasma structure formation, and via multiple consecutive stepwise physical steps, it eventually affects the density structure formation. In the z–θ space, the cross-field transport by E×B and diamagnetic drifts dominantly determines the electron flow and increases the overall axial electron mobility due to the azimuthal inhomogeneity. It is shown that most of the current is carried by the largest structure, but as the macroscopic structure fades out downstream, small structures grow and share the current. By considering the conservation laws, we show that a relation between azimuthal distributions of physical properties is formed to conserve the axial flux by a balance of specific forces, a balance between the resistive force and the magnetic force in the near-anode region and a balance between the electric/pressure force and the magnetic force in the acceleration and plume region, which differs from the Boltzmann relation satisfied in the radial dimension. Based on this principle, with a simplified test case having a uniform plasma density distribution, we show an analytic relation between azimuthal distributions of the magnetic field and the plasma potential and confirm the relation by a 2D hybrid simulation.

The Extended Field-aligned Suprathermal Proton Beam and Long-lasting Trapped Energetic Particle Population Observed Upstream of a Transient Interplanetary Shock

Abstract The properties of the suprathermal particle distributions observed upstream of interplanetary shocks depend not only on the properties of the shocks but also on the transport conditions encountered by the particles as they propagate away from the shocks. The confinement of particles in close proximity to the shocks, as well as particle scattering processes during propagation to the spacecraft, lead to the common observation of upstream diffuse particle distributions. We present observations of a rare extended anisotropic low-energy (≲30 keV) proton beam together with a trapped ≳500 keV proton population observed in association with the arrival of an oblique interplanetary shock at the Advanced Composition Explorer, the Interplanetary Monitoring Platform-8, and the Wind spacecraft on 2001 January 31. Continuous injection of particles by the traveling shock into a smooth radial magnetic field region formed in the tail of a modest high-speed solar wind stream produced an extended foreshock region of energetic particles. The absence of enhanced magnetic field fluctuations upstream of the shock results in the observation of a prolonged anisotropic field-aligned beam of ≲30 keV protons as well as a population of higher-energy (≳500 keV) protons with small pitch-angle cosine (μ ∼ 0) extending far from the shock.

Large magnetodielectric coupling in the vicinity of metamagnetic transition in 6H–perovskite Ba3GdRu2O9

The 6H-perovskites Ba3RRu2O9 (R = rare earth element) demonstrate the magnetodielectric (MD) coupling as a manifestation of 4d–4f magnetic interactions. Here, a detailed study of the structural, magnetic, heat capacity, and MD properties of the 6H-perovskite Ba3GdRu2O9 is reported. The signature of long-range antiferromagnetic (AFM) ordering ∼14.8 K (T N) is evident from the magnetization and heat capacity studies. The T N shifts towards the lower temperature side, apart from splitting in two with the application of the magnetic field. Field-dependent magnetization at 2 K shows three metamagnetic transitions with the opening of small hysteresis in different regions. A new transition at T 1 emerges after the onset of the first metamagnetic transition. Complex magnetic behavior is observed in different magnetic field regions whereas these field regions themselves vary with the temperature. Dielectric response recorded at zero and 80 kOe field exhibits the development of MD coupling well above T N. The MD coupling (∼4.5% at 10 K) is enhanced by 25% as compared to the Dy counterpart. Effect of complex magnetic behavior is also conveyed in the MD results where the maximum value of MD coupling is observed in the vicinity of 10 K (onset of T 1) and near the second metamagnetic transition. Our investigation suggests that both Gd and Ru moments align simultaneously at T N. Short-range magnetic ordering is possibly responsible for MD coupling above T N.

A portable magnet for radiation biology and dosimetry studies in magnetic fields.

In the current and rapidly evolving era of real-time MRI-guided radiotherapy, our radiation biology and dosimetry knowledge is being tested in a novel way. This paper presents the successful design and implementation of a portable device used to generate strong localized magnetic fields. These are ideally suited for small-scale experiments that mimic the magnetic field environment inside an MRI-linac system, or more broadly MRI-guided particle therapy aswell. A portable permanent magnet-based device employing an adjustable steel yoke and magnetic field focusing cones has been designed, constructed, and tested. The apparatus utilizes two banks of Nd Fe B permanent magnets totaling around 50 kg in mass to generate a strong magnetic field throughout a small volume between two pole tips. The yoke design allows adjustment of the pole tip gap and exchanging of the focusing cones. Further to this, beam portal holes are present in the yoke and focusing cones, allowing for radiation beams of up to 5 5 cm to pass through the region of high magnetic field between the focusing cone tips. Finite element magnetic modeling was performed to design and characterize the performance of the device. Automated physical measurements of the magnetic field components at various locations were measured to confirm the performance. The adjustable pole gap and interchangeable cones allows rapid changing of the experimental set-up to allow different styles of measurements to beperformed. A mostly uniform magnetic field of 1.2 T can be achieved over a volume of at least 3 3 3 cm . This can be reduced in strength to 0.3 T but increased in volume to 10 10 10 cm via removal of the cone tips and/or adjustment of the steel yoke. Although small, these volumes are sufficient to house radiation detectors, cell culture dishes, and various phantom arrangements targeted at examining small radiation field dosimetry inside magnetic field strengths that can be changed with ease. Most important is the ability to align the magnetic field both perpendicular to, or inline with, the radiation beam. To date, the system has been successfully used to conduct published research in the areas of radiation detector performance, lung phantom dosimetry, and how small clinical electron beams behave in these strong magneticfields. A portable, relatively inexpensive, and simple to operate device has successfully been constructed and used for performing radiation oncology studies around the theme of MRI-guided radiotherapy. This can be in either inline and perpendicular magnetic fields of up to 1.2 T with x-ray and particlebeams.

Parametric dependence of collisional heating of highly magnetized over-dense plasma by (far-)infrared lasers

Abstract Heating of over-dense plasma represents a long-standing quest in laser–plasma physics. When the strength of the magnetic field is above the critical value, a right-handed circularly polarized laser could propagate into and heat up the highly magnetized over-dense collisional plasma directly; the processes are dependent on the parameters of the laser, plasma and magnetic field. The parametric dependence is fully studied both qualitatively and quantitatively, resulting in scaling laws of the plasma temperature, heating depth and energy conversion efficiency. Such heating is also studied with the most powerful CO2 and strongest magnetic field in the world, where plasma with density of ${10}^{23}$ cm–3 and initial temperature of 1 keV is heated to around 10 keV within a depth of several micrometres. Several novel phenomena are also discovered and discussed, that is, local heating in the region of high density, low temperature or weak magnetic field.

Observational evidence for two-component distributions describing solar magnetic bright points

Context.High-resolution observations of the solar photosphere reveal the presence of fine structures, in particular the so-called Magnetic Bright Points (MBPs), which are small-scale features associated with strong magnetic field regions of the order of kilogauss (kG). It is especially relevant to study these magnetic elements, which are extensively detected in all moments during the solar cycle, in order to establish their contribution to the behavior of the solar atmosphere, and ultimately a plausible role within the coronal heating problem.Aims.Characterisation of size and velocity distributions of MBPs in the solar photosphere in two different datasets of quiet Sun images acquired with high-resolution solar instruments i.e. Solar Optical Telescope SOT/Hinode and the High-resolution Fast Imager HiFI/GREGOR, in theG-band (4308 Å).Methods.In order to detect the MBPs, an automatic segmentation and identification algorithm is used. Next, the identified features were tracked to measure their proper motions. Finally, a statistical analysis of hundreds of MBPs is carried out, generating histograms for areas, diameters and horizontal velocities.Results.This work establishes that areas and diameters of MBPs display log-normal distributions that are well-fitted by two different components, whereas the velocity vector components follow Gaussians and the vector magnitude a Rayleigh distribution revealing again for all vector elements a two component composition.Conclusions.The results can be interpreted as due to the presence of two different populations of MBPs in the solar photosphere one likely related to stronger network magnetic flux elements and the other one to weaker intranetwork flux elemens. In particular this work concludes on the effect of the different spatial resolution of GREGOR and Hinode telescopes, affecting detections and average values.

Quantum interference phenomena and electron – electron interaction in topological insulator Bi2Se3 thin polycrystalline films

The temperature (T = 2.0 - 300 K) and magnetic field (B = 0 - 14 T) dependences of the transport coefficients were obtained for the topological insulator (TI) polycrystalline Bi2Se3 films with thicknesses d = 40 and 200 nm grown on glass substrates by thermal evaporation in vacuum from a single source. In the region of weak magnetic fields, a sharp increase in magnetoresistance (MR) with increasing B was observed, indicating the presence of the weak antilocalization (WAL) effect, which is typical for 3D-TIs. However, the longitudinal resistance Rxx increases logarithmically with decreasing temperature below certain temperature Tc, which is not characteristic of the WAL effect in 2D-structures and indicates possible existence of the so-called “transport paradox”. In the temperature dependences of the Hall resistance, Hall coefficient and MR we also registered clearly pronounced kinks near Tc. The Rxx(T) dependence for a 40 nm film, with decreasing temperature in addition to the Rxx minimum near Tc, exhibits a maximum near 2.1 K. Estimates of the number of independent transport channels m and the dephasing length lφ show that as d increases from 40 nm to 200 nm, m increases from 1 to 3, while lφ changes insignificantly. The nonmonotonic behavior of the T- and B- dependences of the kinetic coefficients, the occurrence of the “transport paradox” are associated with a change in the contributions of WAL and electron-electron interaction effects when the nature and magnitude of the external influence and film thickness change. It follows from the data obtained that not only in very thin monocrystalline films, but also in rather thick polycrystalline Bi2Se3 films prepared by the simple method of the thermal evaporation in vacuum on amorphous substrates, quantum interference effects and “transport paradox” can be manifested.

Microstructural investigations of green-synthesized wurtzite-phase ZnMgO nanopowders and their defect-mediated ferromagnetism

ZnMgO nanopowders are synthesized via a facile green synthesis approach using Psidium Guvajava leaf extract. The microstructure and defects are investigated through a structural refinement of the x-ray diffraction data by Rietveld analysis, pair distribution function analysis, and a spatial correlation modelling of the Raman spectra. The composition-dependant ionic character changes are visualized through changes in the lattice constants, electron density maps, and Madelung energy. The spatial correlation model fitting of the E2 (low) and E2 (high) Raman modes are used to estimate the zinc and oxygen vacancy concentrations respectively. The room temperature ferromagnetism observed in the low magnetic field region (±1000 Oe) is correlated to the zinc vacancy concentration.

Magneto-optic signatures in the gapped Dirac semimetal with mixed linear and parabolic dispersions of ZrTe5

In this paper, we give a systematic theoretical study on the Landau levels (LLs) and magneto-optical conductivity Re(σ αα ) in a gapped Dirac semimetal model with mixed linear and parabolic dispersions under a magnetic field, which was recently proposed by Jiang et al (2020 Phys. Rev. Lett. 125 046403) to explain the experimental magnetoinfrared spectroscopy in the three-dimensional ZrTe5 crystal. We find that the strong magnetic field can drive the LLs become noninverted and thus the strong topological insulator phase in ZrTe5 turns to be a trivial insulator. In the different magnetic field regions, the density of states and Re(σ αα ) can exhibit distinct signatures. Moreover, when the magnetic field is weak, a qualitative relation in Re(σ zz ) between the peaks at the saddle points is revealed as Re Re , which is in good agreement with the experiment.

Analyzing the Intrinsic Magnetic Field in the Galactic Center Radio Arc

Abstract The Radio Arc is a system of organized nonthermal filaments (NTFs) located within the Galactic center (GC) region of the Milky Way. Recent observations of the Radio Arc NTFs revealed a magnetic field that alternates between being parallel and rotated with respect to the orientation of the filaments. This pattern is in stark contrast to the predominantly parallel magnetic field orientations observed in other GC NTFs. To help elucidate the origin of this pattern, we analyze spectro-polarimetric data of the Radio Arc NTFs using an Australian Telescope Compact Array data set covering the continuous frequency range from ∼4 to 11 GHz at a spectral resolution of 2 MHz. We fit depolarization models to the spectral polarization data to characterize Faraday effects along the line of sight. We assess whether structures local to the Radio Arc NTFs may contribute to the unusual magnetic field orientation. External Faraday effects are identified as the most likely origin of the rotation observed for the Radio Arc NTFs; however, internal Faraday effects are also found to be likely in regions of parallel magnetic field. The increased likelihood of internal Faraday effects in parallel magnetic field regions may be attributed to the effects of structures local to the GC. One such structure could be the Radio Shell local to the Radio Arc NTFs. Future studies are needed to determine whether this alternating magnetic field pattern is present in other multi-stranded NTFs, or is a unique property resulting from the complex interstellar region local to the Radio Arc NTFs.

Evolution of Thermal Electron Distributions in the Magnetotail: Convective Heating and Scattering‐Induced Losses

AbstractEarth's magnetotail is filled with solar wind and ionospheric electrons, whose initial energies are significantly lower than the typical energies (temperatures) of plasmasheet electrons. One of the most common mechanisms responsible for heating of solar wind and ionospheric electrons in Earth's magnetotail is adiabatic heating caused by earthward convection of these electrons from the deep tail (i.e., from the region of a weak magnetic field) toward the region of stronger magnetic fields closer to Earth. This heating is moderated by electron losses into the ionosphere due to local wave scattering. In this study, we compare electron spectra from simultaneous observations of The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different radial distances with spectra obtained from a simple model that includes adiabatic heating and losses. Our comparison shows that the model heating significantly overestimates the increase in energetic ( keV) electron fluxes, indicating that losses are essential for accurate modeling of the observed spectra. The required electron losses are similar to or even greater than the losses in the strong diffusion limit (when the loss cone is full). The latter can be interpreted as loss cone widening by field‐aligned electron acceleration.

How Does Transverse MHD Wave-driven Turbulence Influence the Density Filling Factor in the Solar Corona?

Abstract It is well established that transverse MHD waves are ubiquitous in the solar corona. One of the possible mechanisms for heating both open (e.g., coronal holes) and closed (e.g., coronal loops) magnetic field regions of the solar corona is MHD wave-driven turbulence. In this work, we study the variation of the filling factor of overdense structures in the solar corona due to the generation of transverse MHD wave-driven turbulence. Using 3D MHD simulations, we estimate the density filling factor of an open magnetic structure by calculating the fraction of the volume occupied by the overdense plasma structures relative to the entire volume of the simulation domain. Next, we perform forward modeling and generate synthetic spectra of Fe xiii 10749 Å and 10800 Å density-sensitive line pairs using FoMo. Using the synthetic images, we again estimate the filling factors. The estimated filling factors obtained from both methods are in reasonable agreement. Also, our results match fairly well with the observations of filling factors in coronal holes and loops. Our results show that the generation of turbulence increases the filling factor of the solar corona.

Sympathetic Filament Eruptions within a Fan-spine Magnetic System

Abstract It is unclear whether successive filament eruptions at different sites within a short time interval are physically connected or not. Here, we present the observations of successive eruptions of a small and a large filament in a tripolar magnetic field region whose coronal magnetic field showed as a fan-spine magnetic system. By analyzing the multiwavelength observations taken by the Solar Dynamic Observatory and the extrapolated three-dimensional coronal magnetic field, we find that the two filaments resided respectively in the two lobes that make up the inner fan structure of the fan-spine magnetic system. In addition, a small fan-spine system was also revealed by the squashing factor Q map, which located in the east lobe of the fan structure of the large fan-spine system. The eruption of the small filament was a failed filament eruption, which did not cause any coronal mass ejection (CME) except for three flare ribbons and two post-flare-loop systems connecting the three magnetic polarities. The eruption of the large filament not only caused similar post-flare-loop systems and flare ribbons, as observed in the small filament eruption, but also a large-scale CME. Based on our analysis results, we conclude that the two successive filament eruptions were physically connected, in which the topology change caused by the small filament eruption is thought to be the physical linkage. In addition, the eruption of the small fan-spine structure further accelerated the instability and violent eruption of the large filament.

Study of the development and mechanism of large amplitude decreases in cosmic ray intensity during geomagnetic disturbances in the magnetosphere

We analyze the time-variation of cosmic ray intensity during the periods when large and sudden disturbances occur and last for several days. We consider periods when there are sudden decreases of ≥5% in the galactic cosmic ray (GCR) intensity records. For the analysis, in addition to cosmic-ray intensity data, we utilize several solar wind plasma and field parameters. Time variation of changes in GCR intensity have been compared with simultaneous changes in solar wind plasma and field parameters, in order to gain insight about the physical processes responsible for large-amplitude cosmic ray disturbances. We utilize not only data about changes in the magnitude of some of the plasma and magnetic field parameters but also the parameters which provide information about the turbulent or quite (non-turbulent) nature of the parameters. Moreover, we also utilize the magnitude of a number of plasma and field parameters and their various products, during cosmic ray decrease of various magnitudes. The magnitudes of various plasma and field parameters and their various products including some newly tried products are subjected to correlation analysis with the magnitude of cosmic-ray decreases. We identify plasma and field parameters and their products which best correlates with cosmic-ray decreases. We provide an empirical relation that can be useful to estimate the amplitude of Forbush decreases using interplanetary plasma and field observations. Our results provide further insight about the physical processes playing important role during large cosmic ray disturbances in the heliosphere. Our results further demonstrate that large-amplitude Forbush decreases in GCR intensity occur due to passage of enhanced and turbulent magnetic field structure, consistent with the model that Forbush decreases are mainly due to scattering of cosmic ray particles by enhanced turbulent magnetic field regions in the heliosphere.

A comparative analysis on the single-phase and two-phase mixture models for calculation of ferrofluid convective heat transfer in the presence of a magnetic field: a numerical study

PurposeThis paper aims to analyze the accuracy of the single and two-phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field. The findings of current study are compared with previous single-phase numerical results and experimental data. Accordingly, the effect of various parameters including nanoparticles concentration, Reynolds number and magnetic field strength on the performance of the single and two-phase models are evaluated.Design/methodology/approachA two-phase mixture numerical study is carried out to investigate the influence of four U-shaped electromagnets on the hydrodynamic and thermal characteristics of Fe3O4/Water ferrofluid flowing inside a heated channel.FindingsIt is observed that the applied external magnetic field signifies the convective heat transfer from the channel surface, despite local reduction at a few locations. The maximum heat transfer enhancement is predicted as 23% and 25% using single and two-phase models, respectively. The difference between the results of the two models is mainly attributed to the slip velocity effect which is accounted for in the two-phase model. The magnetic field gradient leads to a significant increase in the slip velocity which in turn causes a slight difference in velocity and temperature profiles obtained by the single and two-phase models in the magnetic field region. According to percentage error calculation, the two-phase method is generally more accurate than the single-phase method. However, the percentage error of both models improves by decreasing either magnetic field intensity or Reynolds number.Originality/valueFor the first time in the literature, to the best of the authors’ knowledge, the current work analyzes the accuracy of the single and two phase numerical methods for calculation of ferrofluid convective heat transfer in the presence of a magnetic field.

Improvements of the Longitudinal Magnetic Field Measurement from the Solar Magnetic Field Telescope at the Huairou Solar Observing Station

The weak-field approximation, implying a linear relationship between Stokes- $V/I$ and the longitudinal magnetic field [ $B_{\Vert }$ ], often suffers from saturation observed in strong magnetic-field regions such as sunspot umbrae. In this work, we seek to improve the magnetic-field observations carried out by the Solar Magnetic Field Telescope (SMFT) at the Huairou Solar Observing Station, China. We propose using a non-linear relationship between Stokes- $V/I$ and $B_{\Vert }$ to derive the magnetic field. To determine the form of the relationship, we perform a cross-calibration of the observed SMFT data and magnetograms provided by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory. The algorithm for the magnetic-field derivation is described in detail. We show that using a non-linear relationship between Stokes- $V/I$ and $B_{\Vert }$ allows us to eliminate magnetic-field saturation inside the sunspot umbra. The proposed technique enables one to enhance the reliability of the SMFT magnetic-field data obtained even long before the space-based instrumentation era, since 1987.

Design of Highly Uniform Magnetic Field Cylinder Coils Based on Grey Wolf Optimizer Algorithm in Atomic Sensors

In this study, a method to design highly uniform magnetic field cylinder coils based on the grey wolf optimizer (GWO) algorithm is proposed. For the first time, this method is applied to the magnetic field design area. In this way, the uniform magnetic field region was expended, and the newly designed coils could be utilized in atomic sensors. Compared with other types of coil, namely the Genetic Algorithm (GA) coils and Lee–Whiting coils, the GWO coils exhibit the highest degree of uniformity. The uniform region of the GWO coils is in the range of [−0.53R, 0.53R] along the z-axis and exhibits a relative uniformity of less than 0.1%, as confirmed by experiments. This indicates that the newly designed coils display a huge superiority in the size of the uniform region, thus being able to provide a more uniform magnetic field. With the aim of enhancing the performance of the magnetic field compensation system of SERF atomic sensors, the proposed method also serves as an effective optimized algorithm to design highly uniform magnetic field coils coupled with magnetic shielding layers.

The reversibility of magnetic reconnection

The reversibility of the transfer of energy from the magnetic field to the surrounding plasma during magnetic reconnection is examined. Trajectories of test particles in an analytic field model demonstrate that irreversibility is associated with separatrix crossings and passages through regions of weaker magnetic field. Inclusion of a guide field enhances the magnetization of particles and the extent to which forward and reverse trajectories overlap. Full kinetic simulations with a particle-in-cell code support these results and demonstrate that while time-reversed simulations at first “un-reconnect,” they eventually evolve into a reconnecting state.

High-energy radiation from luminous and magnetized stars

ABSTRACT A part of early-type stars is characterized by strong dipole magnetic field that is modified by the outflow of dense wind from the stellar surface. At some distance from the surface (above the Alfvén radius), the wind drives the magnetic field into the reconnection in the equatorial region of the dipole magnetic field. We propose that electrons accelerated in these reconnection regions can be responsible for efficient Comptonization of stellar radiation producing gamma-ray emission. We investigate the propagation of electrons in the equatorial region of the magnetosphere by including their advection with the equatorial wind. The synchrotron and Inverse Compton (IC) spectra are calculated assuming that a significant part of the wind energy is transferred to relativistic electrons. As an example, the parameters of luminous, strongly magnetized star HD 37022 (Θ1 Ori C) are considered. The IC gamma-ray emission is predicted to be detected either in the GeV energy range by the Fermi-LAT telescope or in the sub-TeV energies by the Cherenkov Telescope Array. However, since the stellar winds are often time variable and the magnetic axis can be inclined to the rotational axis of the star, the gamma-ray emission is expected to show variability with the rotational period of the star and, on a longer time-scale, with the stellar circle of the magnetic activity. Those features might serve as tests of the proposed scenario for gamma-ray emission from single, luminous stars.