Complex Magnetic Environment Research Articles

Structural evolution of a magnetic flux rope associated with a major flare in the solar active region 12205

Solar eruptions are often generated as a result of the complex magnetic environment in solar active regions (ARs). Unravelling the relevant structure and evolution is vital to disclosing the underlying mechanisms that initiate such eruptions. In this work, we conduct a comprehensive study of the magnetic field structure and evolution responsible for a major flare eruption in a complex AR: NOAA 12205. The study is based on a detailed analysis of observations from the SDO and a time sequence of coronal magnetic field extrapolations. The AR is characterized by a long sequence of sunspots, harboring two groups of δ type that evolved dynamically via continual rotation, shearing, colliding, and flux cancellation. Our study suggests that the joint effect of the sunspot motions along a large-scale magnetic flux rope (MFR) supporting a filament was gradually built up along the main polarity inversion line. A quantitative analysis of the coronal magnetic evolution strongly indicates that an ideal instability of the MFR finally led to the major eruption of the X1.6 flare, although it was preceded by episodes of localized reconnections. These localized reconnections should play a key role in building up the unstable MFR by, for example, tether-cutting reconnection low near the photosphere, as driven by the shearing and flux cancellation. Through these reconnections, the MFR gains a significant amount of twisted flux and is lifted up to a height above the torus unstable threshold, at which the background restraining force decreases fast enough with the height.

The high-energy Sun - probing the origins of particle acceleration on our nearest star

As a frequent and energetic particle accelerator, our Sun provides us with an excellent astrophysical laboratory for understanding the fundamental process of particle acceleration. The exploitation of radiative diagnostics from electrons has shown that acceleration operates on sub-second time scales in a complex magnetic environment, where direct electric fields, wave turbulence, and shock waves all must contribute, although precise details are severely lacking. Ions were assumed to be accelerated in a similar manner to electrons, but γ-ray imaging confirmed that emission sources are spatially separated from X-ray sources, suggesting distinctly different acceleration mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic understanding of accelerated particle spectra and the total energy budgets are therefore poorly constrained. Additionally, the recent detection of relativistic ion signatures lasting many hours, without an electron counterpart, is an enigma. We propose a single platform to directly measure the physical conditions present in the energy release sites and the environment in which the particles propagate and deposit their energy. To address this fundamental issue, we set out a suite of dedicated instruments that will probe both electrons and ions simultaneously to observe; high (seconds) temporal resolution photon spectra (4 keV – 150 MeV) with simultaneous imaging (1 keV – 30 MeV), polarization measurements (5–1000 keV) and high spatial and temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray) regimes. These instruments will observe the broad range of radiative signatures produced in the solar atmosphere by accelerated particles.

Stability of Spin Torque Oscillators for MAMR: Perspectives of Materials and Design

We live in a world with exponentially increased data volumes created by various things, which demands innovations on new technologies for data storage to meet the high data volume increasing. Currently, the main data storage technology is still magnetic recording, which needs to maintain a high density growth rate for exponentially increased data volume. The conventional ways to keep a high density growth rate is realized by scaling down both the grain size and the number of grains per bit. However this is limited by the superparamagnetic effect [1]. To solve this issue, the recording media should have increasingly higher anisotropy energy that in turn demands stronger head fields in order to write desired information into recording media. While the strongest writing field is limited by the available materials, being about 2.4 Tesla. New configurations of magnetic recording have to be implemented to enhance the write-ability. Heat-Assisted Magnetic Recording (HAMR) and Microwave Assisted Magnetic Recording (MAMR) are the two main technologies that were put forward to enhance the write-ability [2], [3]. At this moment, HAMR is mainly pursued by Seagate Technology, while Western Digital works on MAMR. It is believe that the MAMR has similar recording potential as HAMR. Furthermore, MAMR consumes less energy (no or negligible heating), and is easier to integrate with the current recording system with slightly modifications on the head [4], [5]. The centre part of MAMR is the use of spin torque oscillator (STO) for the generation of localized ac magnetic field in the microwave frequency regime of 20–40 GHz for effectively assisted writing on high anisotropy recording media [6]. Therefore the development of STO is one of the key enabling technologies that take MAMR drives to the market. However, the close proximity of STO to the writing head, together with ultralow flying height of 2–3 nm, puts STO in a very complex magnetic environment, which inevitably disturbs the STO behavior, leading to its instability. This is one of the key concerns for MAMR to take off. In this paper, we will present the materials selection, layer stack and geometry optimization for STO to enhance its stability against stray field for microwave assisted magnetic recording. Our recent experimental work on STO materials will be presented as well.

Formation and Eruption of an Active Region Sigmoid. II. Magnetohydrodynamic Simulation of a Multistage Eruption

Abstract Solar eruptions, mainly eruptive flares with coronal mass ejections, represent the most powerful drivers of space weather. Due to the low plasma-β nature of the solar corona, solar eruption has its roots in the evolution of the coronal magnetic field. Although various theoretical models of the eruptive magnetic evolution have been proposed, they still oversimplify the realistic process in observation, which shows a much more complex process due to the invisible complex magnetic environment. In this paper, we continue our study of a complex sigmoid eruption in solar active region 11283, which is characterized by a multipolar configuration embedding a null-point topology and a sigmoidal magnetic flux rope. Based on extreme ultraviolet observations, it has been suggested that a three-stage magnetic reconnection scenario might explain the complex flare process. Here we reproduce the complex magnetic evolution during the eruption using a data-constrained high-resolution magnetohydrodynamic (MHD) simulation. The simulation clearly demonstrates three reconnection episodes, which occurred in sequence in different locations in the corona. Through these reconnections, the initial sigmoidal flux rope breaks one of its legs, and quickly gives birth to a new tornado-like magnetic structure that is highly twisted and has multiple connections to the Sun due to the complex magnetic topology. The simulated magnetic field configuration and evolution are found to be consistent with observations of the corona loops, filaments, and flare ribbons. Our study demonstrates that significant insight into a realistic, complex eruption event can be gained by a numerical MHD simulation that is constrained or driven by observed data.

Estimation of a planetary magnetic field using a reduced magnetohydrodynamic model

Abstract. Knowledge of planetary magnetic fields provides deep insights into the structure and dynamics of planets. Due to the interaction of a planet with the solar wind plasma, a rather complex magnetic environment is generated. The situation at planet Mercury is an example of the complexities occurring as this planet's field is rather weak and the magnetosphere rather small. New methods are presented to separate interior and exterior magnetic field contributions which are based on a dynamic inversion approach using a reduced magnetohydrodynamic (MHD) model and time-varying spacecraft observations. The methods select different data such as bow shock location information or magnetosheath magnetic field data. Our investigations are carried out in preparation for the upcoming dual-spacecraft BepiColombo mission set out to precisely estimate Mercury's intrinsic magnetic field. To validate our new approaches, we use THEMIS magnetosheath observations to estimate the known terrestrial dipole moment. The terrestrial magnetosheath provides observations from a strongly disturbed magnetic environment, comparable to the situation at Mercury. Statistical and systematic errors are considered and their dependence on the selected data sets are examined. Including time-dependent upstream solar wind variations rather than averaged conditions significantly reduces the statistical error of the estimation. Taking the entire magnetosheath data along the spacecraft's trajectory instead of only the bow shock location into account further improves accuracy of the estimated dipole moment.

On the Properties of Low‐β Magnetohydrodynamic Waves in Curved Coronal Fields

The solar corona is a complex magnetic environment where several kinds of waves can propagate. In this work, the properties of fast, Alfven, and slow magnetohydrodynamic waves in a simple curved structure are investigated. We consider the linear regime, i.e., small-amplitude waves. We study the time evolution of impulsively generated waves in a coronal arcade by solving the ideal magnetohydrodynamic equations. We use a numerical code specially designed to solve these equations in the low-beta regime. The results of the simulations are compared with the eigenmodes of the arcade model. Fast modes propagate nearly isotropically through the whole arcade and are reflected at the photosphere, where line-tying conditions are imposed. On the other hand, Alfven and slow perturbations are very anisotropic and propagate along the magnetic field lines. Because of the different physical properties in different field lines, there is a continuous spectrum of Alfven and slow modes. Curvature can have a significant effect on the properties of the waves. Among other effects, it considerably changes the frequency of oscillation of the slow modes and enhances the possible dissipation of the Alfven modes due to phase mixing.

Transport across complex magnetic environment

It is well known that the rich, highly complicated field configurations of the Heliospheric Magnetic Field has an essential effect on the cross-field propagation of charged particles of relatively low rigidity. In this process the field line separation is the most significant factor of the magnetic structure. The aim of this paper is to develop a framework, in which the particle transport can be investigated for various types of field-line separation. It turns out that the presence of any diffusive cross-field motion of the particles, even if it is arbitrarily small, can wash out most of the details belonging to a specific field line separation model. In that case the overall cross-field motion is forced to become diffusive, irrespectively of the type of the field line spreading, whether this spreading is diffusive, or even non-diffusive in nature. For the various separation models the expressions derived are very similar.

A general expression of magnetic force for soft ferromagnetic plates in complex magnetic fields

The experiments of ferromagnetic plates in different magnetic environments exhibit two distinct phenomena, i.e. the magnetoelastic instability of a ferromagnetic plate in transverse magnetic fields, and the increase of natural frequency of a ferromagnetic plate with low susceptibility in an inplane magnetic field. Although these two typical phenomena can be predicted separately by two kinds of theoretical models in which the magnetic forces are formulated by totally different expressions, no theoretical model has been found to commonly describe them. This makes it difficult to predict theoretically magnetoelastic interaction of a ferromagnetic structure in complex magnetic environment. A variational principle, here, is proposed to establish the governing equations of magnetoelastic interaction for soft ferromagnetic thin plate structures under complex magnetic fields. The functional is chosen as the summation of the magnetic energy and the strain energy as well as the external work from applied magnetic fields. From manipulations of the variational principle, the governing equations of the magnetic field and mechanical deformation together with an expression of equivalent magnetic force exerted on the ferromagnetic plates are obtained. It is shown that this theoretical model can commonly characterize the experimental phenomena of the magnetoelastic interaction aforementioned.