Local Magnetic Energy Research Articles

Long-term Evolution of Relativistic Unmagnetized Collisionless Shocks

We study a relativistic collisionless electron–positron shock propagating into an unmagnetized ambient medium using 2D particle-in-cell simulations of unprecedented duration and size. The shock generates intermittent magnetic structures of increasingly larger size as the simulation progresses. Toward the end of our simulation, at around 26,000 plasma times, the magnetic coherence scale approaches λ ∼ 100 plasma skin depths, both ahead and behind the shock front. We anticipate a continued growth of λ beyond the time span of our simulation, as long as the shock accelerates particles to increasingly higher energies. The post-shock field is concentrated in localized patches, which maintain a local magnetic energy fraction ε B ∼ 0.1. Particles randomly sampling the downstream fields spend most of their time in low field regions (ε B ≪ 0.1) but emit a large fraction of the synchrotron power in the localized patches with strong fields (ε B ∼ 0.1). Our results have important implications for models of gamma-ray burst afterglows.

Magnetic Energy Conversion and Transport in the Terrestrial Magnetotail Due to Dipolarization Fronts

AbstractDipolarization front (DF) is an important region involved with energy conversion and flux transport in the planet's magnetotail. Using observation data from the recent Magnetospheric Multiscale (MMS) mission, we have identified and analyzed 215 DF events. Significant magnetic energy conversion and transport are observed at the DFs, which show strong dawn‐dusk asymmetry. We find that the transport process dominates the change of local magnetic energy. Nearly 70% of the events show net Poynting flux flowing into the DF region. Such effect is more significant than local magnetic energy dissipation, averagely. When DF approaches the near‐Earth region, the net flow‐in Poynting flux decreases and magnetic energy dissipation increases. Our results suggest that the downstream magnetic energies of transient magnetic reconnections in the midtail may be transported to the near‐Earth region by one DF event after another. These intensive strikes to the magnetosphere in the near‐planet region may drive stronger storms, compared with the previous knowledge about DF.

Constraining Magnetization of Gamma-Ray Bursts Outflows Using Prompt Emission Fluence

Abstract Considered here is the acceleration and heating of relativistic outflow by local magnetic energy dissipation process in Poynting-flux dominated outflow. Adopting the standard assumption that the reconnection rate scales with the Alfvén speed, I show here that the fraction of energy dissipated as thermal photons cannot exceed (for adiabatic index ) of the kinetic energy at the photosphere. Even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. These results imply that calorimetry of the kinetic energy that can be done during the afterglow phase could be used to constrain the magnetization of gamma-ray bursts (GRB) outflows. I discuss the recent observational status and its implications on constraining the magnetization in GRB outflows.

On global and local magnetic fields of flare stars with YZ CMi and OT Ser as examples

Global magnetic fields of flare stars evolve very fast—at times of tens–hundreds of days. In our opinion, this is due to mutual addition of local magnetic fields generated by the differential rotation of these objects.With the example of two flare stars,OT Ser and YZCMi, we consider possiblemechanisms of generation and disspation of local and global magnetic fields and the mechanism of “magnetic deceleration” of these stars according to the scheme “differential rotation–generation of local magnetic fields–fluorescence of energy accumulated by local magnetic fields during flares.” We also estimated the rotation energy and global magnetic field for OT Ser and YZCMi. It is shown that even strong dissipation of the accumulated local magnetic energy in the flare on February 9, 2008 (UT 20:22:00) in YZCMi has not had any impact on the global magnetic field.

ARCADE IMPLOSION CAUSED BY A FILAMENT ERUPTION IN A FLARE

ABSTRACT Coronal implosions—the convergence motion of plasmas and entrained magnetic field in the corona due to a reduction in magnetic pressure—can help to locate and track sites of magnetic energy release or redistribution during solar flares and eruptions. We report here on the analysis of a well-observed implosion in the form of an arcade contraction associated with a filament eruption, during the C3.5 flare SOL2013-06-19T07:29. A sequence of events including the magnetic flux-rope instability and distortion, followed by a filament eruption and arcade implosion, lead us to conclude that the implosion arises from the transfer of magnetic energy from beneath the arcade as part of the global magnetic instability, rather than due to local magnetic energy dissipation in the flare. The observed net contraction of the imploding loops, which is found also in nonlinear force-free field extrapolations, reflects a permanent reduction of magnetic energy underneath the arcade. This event shows that, in addition to resulting in the expansion or eruption of an overlying field, flux-rope instability can also simultaneously implode an unopened field due to magnetic energy transfer. It demonstrates the “partial opening of the field” scenario, which is one of the ways in 3D to produce a magnetic eruption without violating the Aly–Sturrock hypothesis. In the framework of this observation, we also propose a unification of three main concepts for active region magnetic evolution, namely the metastable eruption model, the implosion conjecture, and the standard “CSHKP” flare model.

ON THE ORIGIN OF A SUNQUAKE DURING THE 2014 MARCH 29 X1 FLARE

Helioseismic data from the HMI instrument have revealed a sunquake associated with the X1 flare SOL2014-03-29T17:48 in active region NOAA 12017. We try to discover if acoustic-like impulses or actions of the Lorentz force caused the sunquake. We analyze spectro-polarimetric data obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Fortuitously the FIRS slit crossed the flare kernel close to the acoustic source, during the impulsive phase. The infrared FIRS data remain unsaturated throughout the flare. Stokes profiles of lines of Si I 1082.7 nm and He I 1083.0 nm are analyzed. At the flare footpoint, the Si I 1082.7 nm core intensity increases by a factor of several, the IR continuum increases by 4 +/- 1%. Remarkably, the Si I core resembles the classical Ca II K line's self-reversed profile. With nLTE radiative models of H, C, Si and Fe these properties set the penetration depth of flare heating to 100 +/- 100 km, i.e. photospheric layers. Estimates of the non-magnetic energy flux are at least a factor of two less than the sunquake energy flux. Milne-Eddington inversions of the Si I line show that the local magnetic energy changes are also too small to drive the acoustic pulse. Our work raises several questions: Have we "missed" the signature of downward energy propagation? Is it intermittent in time and/or non-local? Does the 1-2 s photospheric radiative damping time discount compressive modes?

The magnetic structure of convection-driven numerical dynamos

The generation of a magnetic field in numerical simulations of the geodynamo is an intrinsically 3-D and time-dependent phenomenon. The concept of magnetic field lines and the frozen-flux approximation can provide insight into such systems, but a suitable visualization method is required. This paper presents results obtained using the Dynamical Magnetic Field line Imaging (DMFI) technique, which is a representation of magnetic field lines accounting for their local magnetic energy, together with an algorithm for the time evolution of their anchor points. The DMFI illustrations are consistent with previously published dynamo mechanisms, and allow further investigation of spatially and temporally complex systems. We highlight three types of magnetic structures: (i) magnetic cyclones and (ii) magnetic anticyclones are expelled by, but corotate with axial flow vortices; (iii) magnetic upwellings are amplified by stretching and advection within flow upwellings, and show structural similarity with helical plumes found in rotating hydrodynamic experiments. While magnetic anticyclones are responsible for the regeneration of a stable axial dipole, herewe showthat excursions and reversals of the dipole axis are caused by the emergence of magnetic upwellings, which amplify and transport a generally multipolar magnetic field from the inner to the outer boundary of the models. Geomagnetic observations suggest the presence of magnetic structures similar to those found in our models; thus,we discuss howour results may pertain to Earth's core dynamo processes. In order to make DMFI a standard tool for numerical dynamo studies, a public software package is available upon request to the authors (supplementary material is available at: http://www.ipgp.jussieu. fr/∼aubert/DMFI.html).

Relation between physical and phenomenological aspects in the framework of an energetic hysteresis model

In this work, an interpretation of the underlying physical and phenomenological aspects is presented in a way that the magnetization process is considered as a process of order. For this purpose the energetic model of ferromagnetic hysteresis is modified so that the local magnetic energy terms, such as anisotropy or applied field energy, are condensed to the corresponding statistical domain classes via expectation values. Furthermore, the anhysteretic magnetization curve is interpreted as a consequence of internal entropy changes, relative to an a priori defined domain configuration at the demagnetized state. Finally, irreversible energy terms take dissipative losses into account.

Small amplitude solutions of the dynamo problem. 2. The case of α2-dynamos

Abstract The amplitudes of the magnetic field generated by various α2-dynamo models are investigated for the case when the magnetic Reynolds number, R, slightly exceeds the critical value, R(0) , at which dynamo action is marginally possible. The basic assumption is that the α-effect is modified by the presence of the magnetic field by an amount proportional to the local magnetic energy density, according to the quenching law α = α0 − α1 |B|2, where B is the magnetic field and α0 and α1 are positive constants. Solutions were obtained by expanding the field in powers of the amplitude, a, of B, and by solving the resulting perturbation equations up to third order in a with the help of the adjoint kinematic dynamo system. An expression for the growth rate, γ1, of an infinitesimal disturbance when R — R (0) is small was obtained, and the amplitude, a(0), of the final (stable) state was derived. Numerical integrations were performed for five models originally due to Roberts (1972) as well as for a model studie...

Magnetic Properties and Micromagnetic Character of RF Sputtered Co Thin Films on Thin Cr Underlayers

ABSTRACTThe effect of thin Cr underlayers on the coercivity, the initial susceptibility, and micromagnetic structure of RF sputtered Co/Cr thin films have been studied. A reciprocal relationship between the initial susceptibility and the coercivity of the RF sputtered Co/Cr films were observed. Micromagnetic images of these Co/Cr films obtained by Fresnel mode Lorentz TEM revealed a refinement of the micromagnetic images correlated with a simultaneous increase in coercivity and decrease in initial susceptibility. Such behavior is expected in cases where the initial magnetization and magnetization reversal are hindered by the variation of local energy with the magnetic moment distribution. The interaction of the Cr underlayers with the Co magnetic overlayer very likely induces fluctuations in local magnetic energy of the Co films. Accordingly, the initial magnetization and magnetization reversal are likely to be impeded by fluctuations of the magnetic energy resulting in the observed increase in coercivity and decrease in initial susceptibility.

Numerical experiments with a simple nonlinear mean-field dynamo model

Abstract The paper deals with a spherical mean-field dynamo model of α2-type in which the back-reaction of the magnetic field on the fluid motion is taken into account by a dependence of the α-coefficient on either the total magnetic energy or the local magnetic energy density. Results of some experiments are presented which have been carried out with a numerical code that follows the evolution of the magnetic field. Particular attention is paid to steady fields and their stability. It is stressed that the results of stability analyses restricted to axisymmetric perturbations crucially differ from those that also include non-axisymmetric perturbations.