Complex Magnetic Field Geometry Research Articles

An extended Hasegawa–Mima equation for nonlinear drift wave turbulence in general magnetic configurations

We propose an extended Hasegawa–Mima equation describing the evolution of nonlinear drift wave turbulence in general magnetic configurations. Such HMGM equation can be derived within the kinetic framework of guiding center motion or from a two-fluid model of an ion-electron plasma by application of a drift wave turbulence ordering that does not involve conditions on spatial derivatives of magnetic field and plasma density. The HMGM equation is therefore appropriate to describe the evolution of drift wave turbulence in strongly inhomogeneous magnetized plasmas, such as magnetospheric and stellarator plasmas, involving complex magnetic field geometries and non-uniform plasma density distributions. We find conservation laws (mass, energy, and generalized enstrophy) of the HMGM equation, study its algebraic (Hamiltonian) structure, and prove a nonlinear stability criterion for steady solutions through the energy-Casimir method. We then apply these results to describe drift waves and infer the existence of stable toroidal zonal flows with radial shear in dipole magnetic fields.

Plasmoid Instability in the Multiphase Interstellar Medium

The processes controlling the complex clump structure, phase distribution, and magnetic field geometry that develop across a broad range of scales in the turbulent interstellar medium (ISM) remain unclear. Using unprecedentedly high-resolution 3D magnetohydrodynamic simulations of thermally unstable turbulent systems, we show that large current sheets unstable to plasmoid-mediated reconnection form regularly throughout the volume. The plasmoids form in three distinct environments: (i) within cold clumps, (ii) at the asymmetric interface of the cold and warm phases, and (iii) within the warm, volume-filling phase. We then show that the complex magnetothermal phase structure is characterized by a predominantly highly magnetized cold phase, but that regions of high magnetic curvature, which are the sites of reconnection, span a broad range in temperature. Furthermore, we show that thermal instabilities change the scale-dependent anisotropy of the turbulent magnetic field, reducing the increase in eddy elongation at smaller scales. Finally, we show that most of the mass is contained in one contiguous cold structure surrounded by smaller clumps that follow a scale-free mass distribution. These clumps tend to be highly elongated and exhibit a size versus internal velocity relation consistent with supersonic turbulence and a relative clump distance–velocity scaling consistent with subsonic motion. We discuss the striking similarity of cold plasmoids to observed tiny-scale atomic and ionized structures and H i fibers and consider how the presence of plasmoids will modify the motion of charged particles, thereby impacting cosmic-ray transport and thermal conduction in the ISM and other similar systems.

Short time-scale evolution of the polarized radio jet during V404 Cygni’s 2015 outburst

ABSTRACTWe present a high time resolution, multifrequency linear polarization analysis of very large array (VLA) radio observations during some of the brightest radio flaring (${\sim } 1 \,{\rm Jy}\,$) activity of the 2015 outburst of V404 Cygni. The VLA simultaneously captured the radio evolution in two bands (each with two 1 GHz base-bands), recorded at 5/7 GHz and 21/26 GHz , allowing for a broadband polarimetric analysis. Given the source’s high flux densities, we were able to measure polarization on time-scales of ${\sim }13\,$ min, constituting one of the highest temporal resolution radio polarimetric studies of a black hole X-ray binary outburst to date. Across all base-bands, we detect variable, weakly linearly polarized emission (${\lt } 1{{ \rm per\ cent}}$) with a single, bright peak in the time-resolved polarization fraction, consistent with an origin in an evolving, dynamic jet component. We applied two independent polarimetric methods to extract the intrinsic electric vector position angles and rotation measures from the 5 and 7 GHz base-band data and detected a variable intrinsic polarization angle, indicative of a rapidly evolving local environment or a complex magnetic field geometry. Comparisons to the simultaneous, spatially-resolved observations taken with the Very Long Baseline Array at 15.6 GHz , do not show a significant connection between the jet ejections and the polarization state.

Characterizing the i-band variability of YSOs over six orders of magnitude in time-scale

ABSTRACT We present an i-band photometric study of over 800 young stellar objects in the OB association Cep OB3b, which samples time-scales from one minute to 10 yr. Using structure functions we show that on all time-scales (τ) there is a monotonic decrease in variability from Class I to Class II through the transition disc (TD) systems to Class III, i.e. the more evolved systems are less variable. The Class Is show an approximately power-law increase (τ0.8) in variability from time-scales of a few minutes to 10 yr. The Class II, TDs, and Class III systems show a qualitatively different behaviour with most showing a power-law increase in variability up to a time-scale corresponding to the rotational period of the star, with little additional variability beyond that time-scale. However, about a third of the Class IIs shows lower overall variability, but their variability is still increasing at 10 yr. This behaviour can be explained if all Class IIs have two primary components to their variability. The first is an underlying roughly power-law variability spectrum, which evidence from the infrared suggests is driven by accretion rate changes. The second component is approximately sinusoidal and results from the rotation of the star. We suggest that the systems with dominant longer time-scale variability have a smaller rotational modulation either because they are seen at low inclinations or have more complex magnetic field geometries. We derive a new way of calculating structure functions for large simulated data sets (the ‘fast structure function’), based on fast Fourier transforms.

Verification of 5D continuum gyrokinetic code COGENT: Studies of kinetic drift wave instability

COGENT (Continuum Gyrokinetic Edge New Technology) is a kinetic plasma simulation code that is being developed by the edge simulation laboratory (ESL) collaboration. The original version of the code has been developed in a 4D phase space (2D configuration space and 2D velocity space) to address kinetic plasma phenomena in a complex magnetic field geometry including the core, magnetic separatrix, and scrape‐off layer regions. This work is focused on extending the original 4D phase space to a 5D phase space (3D2V) to address full kinetic turbulences in a tokamak edge region. Here, we report on the current status of 5D COGENT, which presently operates in a shear‐less, simple slab geometry. As a verification study, we use the problems of collision‐less drift wave instability (universal instability) including its modification in the presence of collisional effects. The simulation model includes the gyrokinetic equations for ion and electron species coupled to the long‐wavelength limit of the gyro‐Poisson equation. Collisional effects are represented by the Krook collision model. Linear analytical results for the drift mode growth rate and real frequency are recovered, and the non‐linear stage is modelled and analysed as well. In addition, extensive 5D runs have been performed to address the effects of the drift wave instability on blob/filamentary structures characteristic of a tokamak edge. A helical‐shaped potential perturbation is observed to grow exponentially in time while spinning around the filament axis with electron drift frequency.

Redshifted X-rays from the material accreting onto TW Hydrae: Evidence of a low-latitude accretion spot

High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in CTTS. In particular, the accretion shock region, where the accreting material is heated to temperatures of a few MK as it continues its inward bulk motion, can be probed by X-ray spectroscopy. To attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS, and constrain the accretion stream geometry. We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods, by measuring the position of a selected sample of emission lines, and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface. Our results indicate that complex magnetic field geometries, such as that of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the CIV resonance doublet at 1550 Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at 0.1 MK likely originate in the same post-shock regions.

The Effect of Combined Magnetic Geometries on Thermally Driven Winds. I. Interaction of Dipolar and Quadrupolar Fields

Abstract Cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest-order fields such as the dipole, quadrupole, and octupole modes. Magnetized stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. Previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex but singular magnetic field geometries. In this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 MHD simulations with mixed fields, along with 10 each of the pure geometries. The simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. We find that the stellar wind braking torque from our combined geometry cases is well described by a broken power-law behavior, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilizing the total field strength. The simulation results can be scaled and apply to all main-sequence cool stars. For solar parameters, the lowest-order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss.

The evolution of surface magnetic fields in young solar-type stars – I. The first 250 Myr

The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from five associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 M⊙, and rotation periods from 0.4 to 6 d. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main-sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars’ rotational evolution on the main sequence.

SUNWARD PROPAGATING ALFVÉN WAVES IN ASSOCIATION WITH SUNWARD DRIFTING PROTON BEAMS IN THE SOLAR WIND

Using measurements from the WIND spacecraft, here we report the observation of sunward propagating Alfvén waves (AWs) in solar wind that is magnetically disconnected from the Earth's bow shock. In the sunward magnetic field sector, we find a period lasting for more than three days in which there existed (during most time intervals) a negative correlation between the flow velocity and magnetic field fluctuations, thus indicating that the related AWs are mainly propagating sunward. Simultaneous observations of counter-streaming suprathermal electrons suggest that these sunward AWs may not simply be due to the deflection of an open magnetic field line. Moreover, no interplanetary coronal mass ejection appears to be associated with the counter-streaming suprathermal electrons. As the scale goes from the magnetohydrodynamic down to the ion kinetic regime, the wave vector of magnetic fluctuations usually becomes more orthogonal to the mean magnetic field direction, and the fluctuations become increasingly compressible, which are both features consistent with quasi-perpendicular kinetic AWs. However, in the case studied here, we find clear signatures of quasi-parallel sunward propagating ion-cyclotron waves. Concurrently, the solar wind proton velocity distribution reveals a sunward field-aligned beam that drifts at about the local Alfvén speed. This beam is found to run in the opposite direction of the normally observed (anti-sunward) proton beam, and is apparently associated with sunward propagating Alfvén/ion-cyclotron waves. The results and conclusions of this study enrich our knowledge of solar wind turbulence and foster our understanding of proton heating and acceleration within a complex magnetic field geometry.

DETECTION OF A MAGNETIZED DISK AROUND A VERY YOUNG PROTOSTAR

We present subarcsecond resolution polarimetric observations of the 878 mum thermal dust continuum emission obtained with the Submillimeter Array (SMA) towards the IRAS 16293-2422 protostellar binary system. We report the detection of linearly polarized dust emission arising from the circumstellar disk associated with the IRAS 16293-2422 B protostar. The fractional polarization of ~1.4$% is only slightly lower than that expected from theoretical calculations in such disks. The magnetic field structure on the plane of the sky derived from the dust polarization suggests a complex magnetic field geometry in the disk, possibly associated with a rotating disk that is wrapping the field lines as expected from the simulations. The polarization around IRAS 16293-2422 A at subarcsecond angular resolution is only marginally detected.

The evolving polarized jet of black hole candidate Swift J1745−26

Swift J1745-26 is an X-ray binary towards the Galactic Centre that was detected when it went into outburst in September 2012. This source is thought to be one of a growing number of sources that display "failed outbursts", in which the self-absorbed radio jets of the transient source are never fully quenched and the thermal emission from the geometrically-thin inner accretion disk never fully dominates the X-ray flux. We present multifrequency data from the Very Large Array, Australia Telescope Compact Array and Karoo Array Telescope (KAT- 7) radio arrays, spanning the entire period of the outburst. Our rich data set exposes radio emission that displays a high level of large scale variability compared to the X-ray emission and deviations from the standard radio--X-ray correlation that are indicative of an unstable jet and confirm the outburst's transition from the canonical hard state to an intermediate state. We also observe steepening of the spectral index and an increase of the linear polarization to a large fraction (~50%) of the total flux, as well as a rotation of the electric vector position angle. These are consistent with a transformation from a self-absorbed compact jet to optically-thin ejecta -- the first time such a discrete ejection has been observed in a failed outburst -- and may imply a complex magnetic field geometry.

The rapid rotation and complex magnetic field geometry of Vega

Context. The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields.

Spin-resolved Far-Ultraviolet Observations of the Magnetic White Dwarf in YY Draconis

We observed the intermediate polar YY Draconis with the Far Ultraviolet Spectroscopic Explorer (FUSE), yielding an average far-UV spectrum, as well as spectra and spectrophotometric light curves resolved on the spin phase of the white dwarf in this system. We utilized the BINSYN software suite to calculate model spectra based on a simple system model in which the white dwarf has two identical, two-dimensional, equatorial photospheric hot spots separated by 180° in longitude, which correspond to the poles of a simple centered dipole magnetic field. By optimizing the BINSYN model spectra to match our FUSE data plus archival International Ultraviolet Explorer spectra, we determine a white dwarf effective temperature of 21,500 K with the magnetic pole spots having effective temperatures of 220,000 K and angular radii of 35. Normalization of the model spectra implies a distance to YY Dra of 159 pc. We also construct model light curves resolved on the spin period of the white dwarf via synthetic spectrophotometry of the BINSYN synthetic spectra. These are compared with the light curves constructed from our FUSE data, as well as the Hubble Space Telescope ultraviolet light curves of Haswell et al. In general, we find good agreement between the observed and model spectra and light curves, with the exception of a discrepancy at the shortest wavelengths of the spin-phase minimum light spectrum. This implies the possible presence of vertically extended structure(s) on the white dwarf that are visible above the stellar limb even when the photospheric spots are not and contribute far-UV flux to the observed spectrum that is not reproduced in the BINSYN model. We suggest either the accretion columns above the magnetic poles of the white dwarf or a localized buildup of accreted material on the white dwarf surface at the accretion impact sites as possible sources for the missing far-UV flux. Another possibility invokes a model with a more complex magnetic field geometry, in which the spots are still visible during the spin-phase minimum light configuration, but it is neither conclusive nor well constrained by the available data.

Three‐dimensional fluid simulations of plasma asymmetries in the Martian magnetotail caused by the magnetic anomalies

Three‐dimensional single fluid nonideal MHD simulations of the solar wind interaction with Mars show that the strong magnetic anomalies in the Southern Hemisphere of Mars cause asymmetries in the near tail plasma. The simulations were run with the strong southern magnetic anomalies (SSMA) facing midnight, dawn, or dusk. The interplanetary magnetic field (IMF) was set to 1 nT in the By direction. The steady state results were then compared to the case when no surface magnetic field is present. The comparisons show regions of plasma depletion of ∼60% and enhanced plasma densities of over 200% for different SSMA orientations. The depleted plasma regions are similar to the plasma voids detected by Mars Global Surveyor. A minimagnetopause forms inside the magnetic pileup boundary for all three cases. When the SSMA are at the dusk terminator, the IMF drapes over dipole‐like surface magnetic fields, creating a magnetic field geometry reminiscent of a closed magnetosphere at Earth. When the SSMA are at dawn, the magnetic field geometry resembles that of an open magnetosphere at Earth, as the IMF reconnects substantially to the surface magnetic field. The complex magnetic field geometry for all three cases presented leads to large‐scale modification of the plasma flow in the tail.

Solution of the drift kinetic equation in the regime of weak collisions by stochastic mapping techniques

A new method for solving the drift kinetic equation applicable for non-integrable particle motion is presented. To obtain this goal, the general form of the drift kinetic equation is reduced to a stochastic mapping equation which is valid in the weak collisions regime. This equation describes the evolution of the distribution function on Poincaré cuts of phase space. The proposed Monte Carlo algorithm applied to the stochastic mapping equation turns out to solve the drift kinetic equation much faster than a direct integration of stochastic orbits. It can be applied to study quasilinear effects of radio frequency heating and transport in systems with complex magnetic field geometries such as stellarators, tokamaks with toroidal magnetic field ripples, or ergodic divertors. For systems with axial space symmetry the stochastic mapping equation is shown to reduce to the well-known canonical (bounce) averaged equation. For nonaxisymmetric magnetic fields the bounce averaged equation for trapped particles is recovered.

Neptune's magnetic field: Calculation of field geometric invariants derived from the I8E1 GSFC/BRI model

A simplified description of the Neptunian magnetic field derived from the Goddard Space Flight Center/Bartol Research Institute I8E1 model which includes internal terms up to and including the octupole (but no external terms) has been used to compute numerically L shell values along the Voyager 2 encounter trajectory and those associated with the N1 through N6 moons and N1R through N6R rings. The complex magnetic field geometry and the rapid rotation of the planet combine to produce an expectation of numerous times when absorption signatures should be observed as a result of the interaction between the trapped particle population and the Neptunian moons and rings. Thus a complete understanding of the observed charged particle fluxes and their variation along the Voyager 2 trajectory must take into account these geometrically anticipated interactions. Similar to Uranus, the large tilt between the dipole term and the rotation axis causes the moons and rings to sweep a very large range of L shells. Furthermore, their orbital motion introduces additional periodicities, causing the maxima and minima in L space to vary systematically with time. The unambiguous association of predicted and observed absorption signatures and times with a particular ring or moon is made very difficult by the superposition of many individual effects. The extremely rapid excursion in L space by Voyager 2 near closest approach, a region most significant for magnetic field models, apparently does not facilitate the unique associations previously used at other planets to provide an independent assessment of the global accuracy of the mathematical models of the field which are inherently nonunique.

THEORY FOR STATIONARY NONLINEAR WAVE PROPAGATION IN COMPLEX MAGNETIC GEOMETRY

We present our recent efforts to derive a systematic calcdation scheme for nonlinear wave propagation in the self-consistent plasma profile in complex magnetic-field geometry. Basic assumptions and/or approximations are i) use of the collisionless two-fluid model with an equation of state ; ii) restriction to a steady state propagation and iii) existence of modified magnetic surface, modification due to Coriolis' force. We discuss four situations : i) weak-field propagation without static flow, ii) arbitrary field strength with flow in axisymmetric system, iii) weak field limit of case ii) and iv) arbitrary field strength in nonaxisymmetric torus. Except for case iii), we derive a simple variation principle, similar to that of Seligar and Whitham, by introducing appropriate coordinates. In cases i) and iii), we derive explicit results for quasilinear profile modification.

Noncorrelation of ``Anomalous'' Plasma Transport, Equilibrium Plasma Density, and Low-Frequency Potential Oscillations in a Q-Device

The minimum-B magnetic field, provided by the superposition of a tranverse quadrupole-cusp field and a uniform axial field, suppressed certain of the low-frequency oscillations spontaneously present in the thermal cesium plasma of the TOPSY Q-device with the uniform axial field alone, in conjunction with a sharp increase in the center-line equilibrium plasma density of up to two orders of magnitude. However, measurements of the transverse plasma flux immediately prior to and immediately after oscillation suppression failed to detect any change in the radial plasma transport; the ``anomalous diffusion'' observed in the TOPSY plasma in a uniform magnetic field is still present in the absence of low-frequency potential oscillations. Rather than being a function of plasma-column oscillations, the observed cross-field plasma transport is quantitatively explained in terms of E/B drifts due to azimuthal ionizer-plate temperature gradients. The sharp density increase which accompanied oscillation suppression must thus be ascribed to changes in the ionizer sheath conditions leading to an increased ``source'' flux of plasma into the column from the ionizer, rather than to a decrease in the radial plasma ``loss'' flux. These results have serious implications for the study of transport processes in plasmas and, in particular, call into question the use of Q-devices for the experimental study of ``anomalous diffusion'' in complex magnetic field geometries.

Method for Defining Position of Magnetic Field Lines

The method described was developed during experiments on Alfven wave propagation. It was necessary to be sure that the hydromagnetic waveguide-was accurately aligned with a uniform axial field. Tests indicate that the method would be valuable for use with various complex magneticfield geometries that have been proposed for controlled thermonuclear heating. (B.O.G.)