Regions Of Strong Magnetic Field Research Articles

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

AbstractThe 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.

Discrete Aurora at Mars: Insights Into the Role of Magnetic Reconnection

AbstractDiscrete aurora are sporadic emissions of light originating in Mars upper atmosphere. We report nadir imaging observations from MAVEN's Imaging UltraViolet Spectrograph which identify the conditions which trigger electron precipitation causing these events. Prior studies have shown that discrete aurora events in the strong crustal magnetic field region in the southern hemisphere are the brightest and most repeatable compared to events occurring outside the region. Our new data set offers a more complete and accurate characterization of aurora in this area. The region of strongest crustal fields is composed of two distinct magnetic regions, with magnetic fields in opposite directions; discrete aurora events trigger in one region after dusk and in the other before dawn. Magnetic reconnection in these two adjacent regions with the draped interplanetary field may open the crustal fields in these regions during opposing local times. Particle precipitation can then cause discrete aurora at the observed times and locations.

Energy and charge conserving semi-implicit particle-in-cell model for simulations of high-pressure plasmas in magnetic traps

The paper presents a new particle-in-cell model for fully kinetic simulations of plasma confinement regimes with high relative pressure. These regimes are considered as the most promising ones for fusion reactor proposals based on field reversed configurations, mirror and multi-cusp magnetic traps. Formation of an equilibrium high-pressure plasma with a fully excluded or reversed magnetic field cannot be investigated using the simplified magnetohydrodynamic and gyrokinetic approaches. A correct description of the electron dynamics under close proximity of zero and strong magnetic field regions can only be achieved within the framework of the kinetic theory that can be implemented most efficiently by the particle-in-cell method. The full-scale particle-in-cell simulations of modern fusion experiments require the use of finite-difference schemes in which temporal steps exceed the period of fastest electron oscillations at the plasma or even cyclotron frequencies and spatial steps do not have to resolve the Debye radius. This requirement is satisfied by implicit schemes capable of conserving the total energy of the system. The particle-in-cell model presented in this paper is based on the energy conserving semi-implicit approach as a predictive step and a new method for suppressing the electrostatic noise inherent in it as a corrective step. This two-step algorithm provides not only accurate energy conservation, but also the exact local fulfillment of the Gauss law.

Electron Rolling‐Pin Distributions Preceding Dipolarization Fronts

AbstractElectron rolling‐pin distribution (RPDs), characterized by triple peaks at pitch angles 0°, 90°, and 180°, have recently been discovered in the terrestrial magnetosphere. Since the RPDs' formation is typically attributed to local betatron acceleration, RPDs have been believed to appear primarily inside strong magnetic regions, such as flux pileup regions (FPRs) behind dipolarization fronts (DFs). Different from such expectation, in this study we present unique observations of RPDs made by Cluster spacecraft in the terrestrial magnetotail, showing that RPDs are present inside weak magnetic field regions ahead of the DFs but absent in the strong magnetic field region behind them. The presence of RPD prior to the fronts may be atttributed to the combined effect of global betatron acceleration, global Fermi acceleration, and frontward transport driven by magnetic gradient drift. The atypical features of RPDs are important for fully understanding electron acceleration and transport in the magnetosphere.

Simulation of magnetohydrodynamic flows of liquid metals with heat transfer or magnetic stirring

AbstractWe discuss the effects of nonhomogeneous magnetic fields in liquid metal flows in two different configurations. In the first configuration, we briefly report the impact of fringing magnetic fields in a turbulent Rayleigh–Bénard convection setup, where it was shown that the global heat transport decreases with an increase of fringe‐width. The convective motion in regions of strong magnetic fields is confined near the sidewalls. In the second configuration, we numerically study the effects of an oscillating magnetic obstacle with different frequencies of oscillation on liquid metal flow in a duct. The Reynolds number is low such that the wake of the stationary magnetic obstacle is steady. The transverse oscillation of the magnet creates a sinusoidal time‐dependent wake reminiscent of the vortex shedding behind solid obstacles. We examine the behavior of the streamwise and spanwise components of the Lorentz forces as well as the work done by the magnets on the fluid. The frequency of the oscillation of the streamwise component of Lorentz force is twice that of the spanwise component as in the case of lift and drag on solid cylindrical obstacles. The total drag force and the energy transferred from the magnets to the fluid show a nonmonotonic dependence on the frequency of oscillation of the magnetic obstacle indicative of a resonant excitation of the sinusoidal vortex shedding.

Gravity wave driven variability in the Mars ionosphere-thermosphere system

Gravity waves (GWs) have been persistently observed in neutral species in the Martian thermosphere. In this paper, we study characteristics of GWs in O2+ and O+ ions in the Mars ionosphere (∼160–200 km) and examine their relation to GWs in the thermosphere. For this purpose, we use O2+, O+, and Ar densities measured by the Neutral Gas and Ion Mass spectrometer (NGIMS) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The amplitudes and (horizontal) wavelengths of the GWs are extracted from the density profiles. The results of the present study show that GWs are ubiquitous in the Martian ionosphere-thermosphere system. We found that the amplitudes of GWs in ions are ∼2 times larger than those in neutrals. While the amplitudes in ions reach as high as 70% of the background densities, the most probable amplitude 5%. The amplitudes of the GWs in ions are more on the nightside than on the dayside; more during the 2018 global dust storm period than during nominal dust conditions; and increase with decreasing in solar activity. All these results are qualitatively similar to those observed in neutrals. In addition, amplitudes of the GWs in ions show moderate correlation with those neutrals while the wavelengths show good correlation. The amplitudes of GWs in ions and neutrals show moderate negative correlation with thermospheric temperatures. All these results strongly suggest that the GWs in ions are due to those in neutrals and ion-neutral coupling may be strong in the altitude range of interest. The correlation between the amplitudes of GWs in ions and neutral, however, is local time dependent with weak correlation on the nightside. Other factors such as recombination or particle precipitation may be playing an important role in distorting the wavy nature in ions leading to weaker correlation with neutral waves on the night side. Furthermore, the correlation between GW amplitudes in ions and neutrals is weaker in strong crustal magnetic field regions. This implies that the magnetic fields control the motion of the ions leading to distorting in wavy nature in ions.

Observations of High Densities at Low Altitudes in the Nightside Ionosphere of Mars by the MAVEN Radio Occultation Science Experiment (ROSE)

AbstractUnusually large peak electron density values of ∼5 × 1010 m−3 are sporadically observed in the nightside ionosphere of Mars in regions of strong and open crustal magnetic field. However, the associated vertical structure of the ionosphere has not previously been observed. Here, we present three vertical electron density profiles from the nightside ionosphere of Mars that have comparably large values of peak electron density. They were acquired by the Radio Occultation Science Experiment on the MAVEN spacecraft on 23, 24, and 29 March 2021. In these profiles, the peak density is large (2–4 × 1010 m−3), the peak altitude is low (60–90 km), and the plasma layer is broad (full width at half maximum greater than 100 km, Chapman layer lengthscale of ∼30 km). Even if the peak altitude is at the upper end of this range, 90 km, the plasma density at 60 km would still be large, approximately 70% of the peak density. These findings hold even if the ionosphere was patchy, not spherically symmetric, during these observations. These three profiles were acquired at times when fluxes of energetic protons, but not electrons, were enhanced above background levels. The apparent association of large nightside electron density values with protons, rather than electrons, is contrary to previous findings.

Linking Small-scale Solar Wind Properties with Large-scale Coronal Source Regions through Joint Parker Solar Probe–Metis/Solar Orbiter Observations

The solar wind measured in situ by Parker Solar Probe in the very inner heliosphere is studied in combination with the remote-sensing observation of the coronal source region provided by the METIS coronagraph aboard Solar Orbiter. The coronal outflows observed near the ecliptic by Metis on 2021 January 17 at 16:30 UT, between 3.5 and 6.3 R ⊙ above the eastern solar limb, can be associated with the streams sampled by PSP at 0.11 and 0.26 au from the Sun, in two time intervals almost 5 days apart. The two plasma flows come from two distinct source regions, characterized by different magnetic field polarity and intensity at the coronal base. It follows that both the global and local properties of the two streams are different. Specifically, the solar wind emanating from the stronger magnetic field region has a lower bulk flux density, as expected, and is in a state of well-developed Alfvénic turbulence, with low intermittency. This is interpreted in terms of slab turbulence in the context of nearly incompressible magnetohydrodynamics. Conversely, the highly intermittent and poorly developed turbulent behavior of the solar wind from the weaker magnetic field region is presumably due to large magnetic deflections most likely attributed to the presence of switchbacks of interchange reconnection origin.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Longitudinal magnetoresistance of uniaxially deformed n-type silicon

The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. The reliability of the mechanism of negative magnetoresistance was verified using uniaxial elastic deformation of the studied crystals. Uniaxial deformation excludes interline transitions of electrons, as a result of which negative magnetoresistance disappears with an increase in uniaxial pressure. When cubic symmetry is violated, anisotropic phenomena occur in such crystals. The multipath of the isoenergetic surface of the bottom of the silicon conduction band causes anisotropies of the effective mass and relaxation time, which are associated with the features of the transfer phenomenon. In particular, magnetoresistance (piezoresistance), which is the most sensitive to the anisotropy of the iso energy surface. The influence of the latter on magnetoresistance is most clearly revealed in the region of strong magnetic fields, where the magnetoresistance is saturated since the longitudinal magnetoresistance is entirely due to the anisotropy of electron mobility.

Numerical study of the effect of aft-loaded magnetic field on multiple ionizations in Hall thruster

It is assumed that the shift of a strong magnetic field region with a positive gradient from exit plane to outside, namely the transit from a normal loaded magnetic field to an aft-loaded one, enhances the multiple ionization process in the magnetically shielded Hall thruster. To confirm this conjecture, a comparative study is carried out numerically with a particle-in-cell method. The simulation results prove that compared with the normal loaded magnetic field, the application of aft-loaded magnetic field enhances the multiple ionization process. This study further analyzes the ionization characteristics of the transition from low-charged ions to high-charged ions under two magnetic field conditions and the influence of the magnetic strength of aft-loaded magnetic field on the multiple ionization characteristics. The study described herein is useful for understanding the discharge characteristics of Hall thruster with an aft-loaded magnetic field.

Electrophysical properties of compensated n-Germanium

Measurements of the transverse magnetoresistance and the Hall effect of compensated n-Ge crystals in the weak and strong magnetic fields at liquid nitrogen temperature have been carried out. It was revealed that in crystals with a sufficiently high degree of compensation, the space charge regions play a significant role in the scattering of charge carriers, which cause an increase in the transverse magnetoresistance (with increasing H) and its linear dependence on H in the region of strong magnetic fields. It was shown that in sufficiently strongly compensated crystals, the scattering of current carriers by the space charge regions is practically isotropic. It was found that thermal annealing (350 °C; 60 h) of compensated oxygen-containing n-Ge, which significantly reduces the compensation degree due to the arising donor-type oxygen complexes, leads to a decrease in the slope of the field dependences of the transverse magnetoresistance in the quantizing magnetic fields.

Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

AbstractThis paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons.

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.

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.

Extension of the Synchrotron Radiation of Electrons to Very High Energies in Clumpy Environments

Abstract The synchrotron cooling of relativistic electrons is one of the most effective radiation mechanisms in astrophysics. It not only accompanies the process of particle acceleration, but also has feedback on the formation of the energy distribution of the parent electrons. The radiative cooling time of electrons decreases with energy as t SYN ∝ 1/E; correspondingly, the overall radiation efficiency increases with energy. However, this effect strictly limits the maximum energy of individual photons. Even in the so-called extreme accelerators, where the acceleration proceeds at the highest possible rate, , allowed in an ideal magnetohydrodynamic plasma, the synchrotron radiation cannot extend well beyond the characteristic energy determined by the electron mass and the fine-structure constant: . In this paper, we propose a model in which the formation of synchrotron radiation takes place in compact magnetic blobs located inside the particle accelerator and develop a formalism for calculations of synchrotron radiation emerging from such systems. We demonstrate that for certain combinations of parameters characterizing the accelerator and the magnetic blobs, the synchrotron radiation can extend beyond this limit by a several orders of magnitude. This scenario requires a weak magnetization of the particle accelerator, and an efficient conversion of gas internal energy into magnetic energy in sufficiently small blobs. The required size of the blobs is constrained by the magnetic mirroring effect, which can prevent particle penetration into the regions of strong magnetic field under certain conditions.

Automatic detection and correction algorithms for magnetic saturation in the SMFT/HSOS longitudinal magnetograms

A longitudinal magnetic field often suffers the saturation effect in a strong magnetic field region when the measurement is performed at a single-wavelength point and linear calibration is adopted. In this study, we develop a method that can judge the threshold of saturation in Stokes V/I observed by the Solar Magnetic Field Telescope (SMFT) and correct it automatically. The procedure is to first perform the second-order polynomial fit to the Stokes V/I vs. I/Im (Im is the maximum value of Stokes I) curve to estimate the threshold of saturation, then reconstruct Stokes V/I in a strong field region to correct for saturation. The algorithm is demonstrated to be effective by comparing with the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). The accuracy rate of detection and correction for saturation is ∼99.4% and ∼88% respectively among 175 active regions. The advantages and disadvantages of the algorithm are discussed.

Spicules and downflows in the solar chromosphere

Context. High-speed downflows have been observed in the solar transition region (TR) and lower corona for many decades. Despite their abundance, it has been hard to find signatures of such downflows in the solar chromosphere. Aims. In this work, we target an enhanced network region which shows ample occurrences of rapid spicular downflows in the Hα spectral line, which could potentially be linked to high-speed TR downflowing counterparts. Methods. We used the k-means algorithm to classify the spectral profiles of on-disk spicules in Hα and Ca II K data observed from the Swedish 1 m Solar Telescope and employed an automated detection method based on advanced morphological image processing operations to detect such downflowing features, in conjunction with rapid blue-shifted and red-shifted excursions (RBEs and RREs). Results. We report the existence of a new category of RREs (termed as downflowing RRE) for the first time that, contrary to earlier interpretation, are associated with chromospheric field aligned downflows moving toward the strong magnetic field regions. Statistical analysis performed on nearly 20 000 RBEs and 15 000 RREs (including the downflowing counterparts), which were detected in our 97 min long dataset, shows that the downflowing RREs are very similar to RBEs and RREs except for their oppositely directed plane-of-sky motion. Furthermore, we also find that RBEs, RREs, and downflowing RREs can be represented by a wide range of spectral profiles with varying Doppler offsets, and Hα line core widths, both along and perpendicular to the spicule axis, that causes them to be associated with multiple substructures which evolve together. Conclusions. We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.

A nonlinear solar magnetic field calibration method for the filter-based magnetograph by the residual network

Context. The method of solar magnetic field calibration for the filter-based magnetograph is normally the linear calibration method under weak-field approximation that cannot generate the strong magnetic field region well due to the magnetic saturation effect. Aims. We try to provide a new method to carry out the nonlinear magnetic calibration with the help of neural networks to obtain more accurate magnetic fields. Methods. We employed the data from Hinode/SP to construct a training, validation and test dataset. The narrow-band Stokes I, Q, U, and V maps at one wavelength point were selected from all the 112 wavelength points observed by SP so as to simulate the single-wavelength observations of the filter-based magnetograph. We used the residual network to model the nonlinear relationship between the Stokes maps and the vector magnetic fields. Results. After an extensive performance analysis, it is found that the trained models could infer the longitudinal magnetic flux density, the transverse magnetic flux density, and the azimuth angle from the narrow-band Stokes maps with a precision comparable to the inversion results using 112 wavelength points. Moreover, the maps that were produced are much cleaner than the inversion results. The method can effectively overcome the magnetic saturation effect and infer the strong magnetic region much better than the linear calibration method. The residual errors of test samples to standard data are mostly about 50 G for both the longitudinal and transverse magnetic flux density. The values are about 100 G with our previous method of multilayer perceptron, indicating that the new method is more accurate in magnetic calibration.

STUDY OF THE MAGNETO-RESISTANCE SILICON P-TYPE

The study of galvanomagnetic phenomena in silicon, in strong magnetic fields, is associated with great difficulties due to the high resistance of the samples and the complexity of preparing omics contacts that work well in a wide range of temperatures and magnetic fields. Therefore, until now, all existing data on galvanomagnetic effects in silicon have been obtained only in the region of weak magnetic fields. Given the growing interest in silicon due to its large potential, it seemed appropriate to study the galvanomagnetic effects in the region of strong magnetic fields, where quantum effects play the dominant role. The study of Galvano-magnetic effects (as well as tensoeffects) in silicon under extreme conditions allows not only to identify the mechanisms of these effects, but also to identify the possibility of creating gaussmeters, infrared detectors, sensitive strain gauges, amplifiers and generators of a wide frequency range. Keywords: galvano-magnetic effects, silicon, negative magnetoresistance, uniaxial pressure.