Field Line Configuration Research Articles

Earth's distant magnetotail current sheet near and beyond lunar orbit

AbstractWe analyze the structure of the Earth magnetotail current sheet (CS) in middle, X∈[−50,−20] RE, and distant, X∈[−100,−80] RE, regions using data set of 573 CS crossings by Geotail in 1994–1995. For a subset of 213 CSs we determine the CS thickness L, average current density j0, and velocity vD=j0/en0 (n0 is the ion number density). We find similar dawn‐dusk distributions of CS parameters for middle and distant tail: L is about 3000 km at the dusk flank and grows up to 12,000 km toward the dawn flank; j0 grows toward the dusk flank by a factor of 2–3; and the most intense CSs (with higher vD) are observed near midnight. We show that ion‐scale CSs with the thickness of several ion thermal gyroradii (say less than seven) are observed in middle and distant tail in more than 50% of crossings. For observed CSs electrons likely provide the dominant contribution to the current density. We divide the subset into intense and weak CSs (using parameter vD). Weak CSs have thickness of about 20 ion thermal gyroradii and Bz of about 1.5 nT. Intense CSs have thickness of about 3–7 thermal gyroradii and much smaller Bz implying more stretched field line configuration. Intense CSs are accompanied by fast ion flows: vD is larger for larger amplitudes of ion bulk velocity vx that is likely due to larger contribution of Speiser ions. The properties of the CS in middle and distant tail are compared with those found for the near‐Earth tail.

Excitation of dayside chorus waves due to magnetic field line compression in response to interplanetary shocks

AbstractThe excitation of magnetospheric whistler‐mode chorus in response to interplanetary (IP) shocks is investigated using wave data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. As an example, we show a typical chorus wave excitation following an IP shock event that was observed by THEMIS in the postnoon sector near the magnetopause on 3 August 2010. We then analyze characteristic changes during this event and perform a survey of similar events during the period 2008–2014 using the THEMIS and OMNI data set. Our statistical analysis demonstrates that the chorus wave excitation/intensification in response to IP shocks occurs only at high L shells (L > 8) on the dayside. We analyzed the variations of magnetic curvature following the arrival of the IP shock and found that IP shocks lead to more homogeneous background magnetic field configurations in the near‐equatorial dayside magnetosphere; and therefore, the threshold of nonlinear chorus wave growth is likely to be reduced, favoring chorus wave generation. Our results provide the observational evidence to support the concept that the geomagnetic field line configuration plays a key role in the excitation of dayside chorus.

Simulations of the Earth's magnetosphere embedded in sub‐Alfvénic solar wind on 24 and 25 May 2002

AbstractDuring 24 and 25 May 2002, the solar wind conditions at Earth's orbit were very unusual: the density was extremely low (below 0.1/cc) and, as a result, the flow was subfast and sub‐Alfvénic (the Alfvén Mach number was as low as 0.4 in the rest frame of the Earth). Consequently, the Earth's bow shock disappeared and two Alfvén wings formed on the flanks of the magnetosphere. These two long structures (estimated extension of 600 RE for this event) affect the incoming plasma as follows: the velocity is reduced and the magnetic field rotates. In the present study, global magnetohydrodynamical simulations of the magnetosphere are performed for such upstream solar wind conditions. The simulations show how the magnetosphere configuration dramatically changes when the sub‐Alfvénic solar wind reaches the magnetosphere: the dayside magnetopause expands up to 20 RE, and on the nightside the position of the last closed magnetic field line diminishes to 20 RE. As a result the closed magnetic field line region becomes very symmetric. The open field line configuration also changes such that field lines emanating from the Northern Hemisphere all point in the direction of the dawn Alfvén wing (around 8:00 LT), while the field lines from the Southern Hemisphere all point in direction of the other wing (around 22:00 LT). During the formation of the Alfvén wings, the tail lobes completely disappeared and the auroral activity greatly diminished, i.e., the magnetosphere becomes geomagnetically quiet.

Basic characterization of TORPEX electrostatic modes in closed field line configurations

Electrostatic coherent modes are studied in the TORPEX device [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)], in closed flux surfaces. The accessibility to this magnetic geometry is provided by a current-carrying in-vessel toroidal conductor developed to generate a poloidal magnetic field [Avino et al., Rev. Sci. Instrum. 85, 033506 (2014)]. The background plasma parameters are measured, and the ion saturation current fluctuations are characterized in terms of power spectral density to identify the dominant coherent modes and their spatial localization. A statistical approach is implemented to determine the mode spectral properties by computing the statistical dispersion relation. The poloidal wave number kθ and the toroidal wave number kϕ are obtained, as well as the corresponding mode numbers. A three-dimensional linear code based on the drift-reduced Braginskii equations is used to investigate the nature of the instabilities. The linear analysis suggests a dominant ballooning character of the modes.

VINETA II: A linear magnetic reconnection experiment

A linear experiment dedicated to the study of driven magnetic reconnection is presented. The new device (VINETA II) is suitable for investigating both collisional and near collisionless reconnection. Reconnection is achieved by externally driving magnetic field lines towards an X-point, inducing a current in the background plasma which consequently modifies the magnetic field topology. Owing to the open field line configuration of the experiment, the current is limited by the axial sheath boundary conditions. A plasma gun is used as an additional electron source in order to counterbalance the charge separation effects and supply the required current. Two drive methods are used in the device. First, an oscillating current through two parallel conductors drive the reconnection. Second, a stationary X-point topology is formed by the parallel conductors, and the drive is achieved by an oscillating current through a third conductor. In the first setup, the magnetic field of the axial plasma current dominates the field topology near the X-point throughout most of the drive. The second setup allows for the amplitude of the plasma current as well as the motion of the flux to be set independently of the X-point topology of the parallel conductors.

Basic investigations of electrostatic turbulence and its interaction with plasma and suprathermal ions in a simple magnetized toroidal plasma

Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi-equilibrium generated by a toroidal magnetic field, onto which a small vertical component is superimposed, simulating a simplified form of tokamak scrape-off layers. After having clarified the formation of blobs in ideal interchange turbulence, TORPEX experiments elucidated the mechanisms behind the blob motion, with a general scaling law relating their size and speed. The parallel currents associated with the blobs, responsible for the damping of the charge separation that develops inside them, hence determining their cross-field velocity, have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes, arranged in an array on a metal limiter. Depending on the biasing scheme, radial and vertical blob velocities can be varied. Suprathermal ion transport in small-scale turbulence is also investigated on TORPEX. Suprathermal ions are generated by a miniaturized lithium source, and are detected using a movable double-gridded energy analyser. We characterize vertical and radial spreading of the ion beam, associated with the ideal interchange-dominated plasma turbulence, as a function of the suprathermal ion energy and the plasma temperature. Experimental results are in good agreement with global fluid simulations, including in cases of non-diffusive behaviour. To investigate the interaction of plasma and suprathermal particles with instabilities and turbulence in magnetic configurations of increasing complexity, a closed field line configuration has recently been implemented on TORPEX, based on a current-carrying wire suspended in the vacuum chamber. First measurements indicate the creation of circular symmetric profiles centred on the magnetic axis, and instabilities driven in the region of strong gradients, with a strong ballooning character.

Numerical Simulation Study for Background Plasma in the GAMMA 10 End-Mirror Cell

We have started a simulation study of background plasma in the GAMMA 10 west end-cell using a fluid-code in order to understand divertor simulation experiments. In this fluid-code, the configuration of magnetic field lines in the west end-cell of GAMMA 10 is adopted with a mesh structure and basic physical processes such as reflection coefficient and recombination are considered. An initial computation result reveals that in the case of the plasma density range: ne ~ 1019 m-3, ion and electron temperature could not be reduced significantly to the suitable temperature for achieving the detachment. The effect of heat transfer coefficient on the target plasma is also discussed.

Cross-field motion of plasma blob-filaments and related particle flux in an open magnetic field line configuration on QUEST

Blob-filaments have been observed by combined measurement with a fast camera and a movable Langmuir probe in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. Blob-filaments extended along field lines do correspond to over-dense plasma structures and propagated across the field lines to the outer wall. The radial velocity of the blob structure, Vb, was obtained by three methods and was dominantly driven by the E×B force. The radial velocity, size of the blob showed good agreements with the results obtained by sheath-connected interchange theoretical model. Vb corresponds to roughly 0.02–0.07 of the local sound speed (Cs) in QUEST. The higher moments (skewness S and kurtosis K) representing the shape of PDF of density fluctuation are studied. Their least squares fitting with quadratic polynomial is K=(1.60±0.27)S2−(0.46±0.20). The larger blob structures, occurring only 10% of the time, can carry more than 60% loss of the entire radial particle flux.

On the Flux-Rope Topology of Ejecta Observed in the Period 1997 – 2006

In the following study our aim is to analyse the magnetic flux-rope topology of some events observed in the interplanetary medium related to ejecta. The magnetic field structures associated with interplanetary coronal mass ejections are globally classified in magnetic clouds and ejecta. One of the main questions regarding these phenomena concerns their flux-rope or non-flux-rope magnetic field line configuration. From the experimental measurements the only way to elucidate such a question is analysing the corresponding data by means of a flux-rope physical model. After selecting the ejecta events observed during the period 1997 – 2006, we have analysed them in light of an analytical model with that topology for the magnetic field components, initially developed for magnetic clouds, and with a non-force-free character; then, incorporating the expansion of the magnetic structure during their evolution in the interplanetary medium. Different parameters obtained from the fitting of the model are related to the orientation of the axis of the magnetic flux-rope structure and, additionally, the closest distance approach of the spacecraft to its axis. One of the main conclusions achieved concerns the fact that the axes of most of those structures are close to the Sun–Earth line, which implies that the passage of the spacecraft through the corresponding ejecta event is by its flank. In general, we show a rough procedure for the analysis and classification of ejecta in terms of their magnetic field topology.

EXPLORING MAGNETIC FIELD STRUCTURE IN STAR-FORMING CORES WITH POLARIZATION OF THERMAL DUST EMISSION

The configuration and evolution of the magnetic field in star-forming cores are investigated in order to directly compare simulations and observations. We prepare four different initial clouds having different magnetic field strengths and rotation rates, in which magnetic field lines are aligned/misaligned with the rotation axis. First, we calculate the evolution of such clouds from the prestellar stage until long after protostar formation. Then, we calculate the polarization of thermal dust emission expected from the simulation data. We create polarization maps with arbitrary viewing angles and compare them with observations. Using this procedure, we confirmed that the polarization distribution projected on the celestial plane strongly depends on the viewing angle of the cloud. Thus, by comparing the observations with the polarization map predicted by the simulations, we can roughly determine the angle between the direction of the global magnetic field and the line of sight. The configuration of the polarization vectors also depends on the viewing angle. We find that an hourglass configuration of magnetic field lines is not always realized in a collapsing cloud when the global magnetic field is misaligned with the cloud rotation axis. Depending on the viewing angle, an S-shaped configuration of the magnetic field (or the polarization vectors) appears early in the protostellar accretion phase. This indicates that not only the magnetic field but also the cloud rotation affects the dynamical evolution of such a cloud. In addition, by comparing the simulated polarization with actual observations, we can estimate properties of the host cloud such as the evolutionary stage, magnetic field strength, and rotation rate.

Application of a data-driven simulation method to the reconstruction of the coronal magnetic field

Ever since the magnetohydrodynamic (MHD) method for extrapolation of the solar coronal magnetic field was first developed to study the dynamic evolution of twisted magnetic flux tubes, it has proven to be efficient in the reconstruction of the solar coronal magnetic field. A recent example is the so-called data-driven simulation method (DDSM), which has been demonstrated to be valid by an application to model analytic solutions such as a force-free equilibrium given by Low and Lou. We use DDSM for the observed magnetograms to reconstruct the magnetic field above an active region. To avoid an unnecessary sensitivity to boundary conditions, we use a classical total variation diminishing Lax-Friedrichs formulation to iteratively compute the full MHD equations. In order to incorporate a magnetogram consistently and stably, the bottom boundary conditions are derived from the characteristic method. In our simulation, we change the tangential fields continually from an initial potential field to the vector magnetogram. In the relaxation, the initial potential field is changed to a nonlinear magnetic field until the MHD equilibrium state is reached. Such a stable equilibrium is expected to be able to represent the solar atmosphere at a specified time. By inputting the magnetograms before and after the X3.4 flare that occurred on 2006 December 13, we find a topological change after comparing the magnetic field before and after the flare. Some discussions are given regarding the change of magnetic configuration and current distribution. Furthermore, we compare the reconstructed field line configuration with the coronal loop observations by XRT onboard Hinode. The comparison shows a relatively good correlation.

NONLINEAR FORCE-FREE MODELING OF A THREE-DIMENSIONAL SIGMOID OBSERVED ON THE SUN

In this work, we analyze the characteristics of the three-dimensional magnetic structure of a sigmoid observed over an active region (AR 10930) and followed by X-class flares. This is accomplished by combining a nonlinear force-free field (NLFFF) model of a coronal magnetic field and the high-resolution vector-field measurement of a photospheric magnetic field by Hinode. The key findings of our analysis reveal that the value of the X-ray intensity associated with the sigmoid is more sensitive to the strength of the electric current rather than the twist of the field lines. The strong electric current flows along the magnetic field lines and composes the central part of the sigmoid, even though the twist of the field lines is weak in that region. On the other hand, the outer region (i.e., the elbow part) of the sigmoid is basically occupied by field lines of strong twist and weak current density. Consequently, weak X-ray emission is observed. As the initial Ca II illumination basically occurs from the central part of the sigmoid, this region plays an important role in determining the onset mechanism of the flare despite its weak twisted field-line configuration. We also compare our results with the magnetohydrodynamic simulation formore » the formation of a sigmoid. Although the estimated values of the twist from the simulation are found to be a little higher than the values obtained from the NLFFF, we find that the field-line configurations generated by the simulation and NLFFF are remarkably analogous as long as we deal with the lower coronal region.« less

Areas of enhanced ionization in the deep nightside ionosphere of Mars

[1] We present observations of areas of enhanced ionization located in the deep nightside ionosphere of Mars as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. Oblique ionospheric echoes coming from the same region observed during several consecutive MARSIS measurements enable us to constrain the geographic location of the reflection area that is the source of the echoes. We have identified 90 such events in all available data. None of the reflection areas are located in regions typically having a closed magnetic field line configuration. The locations of the reflection areas of enhanced plasma density are consistent with the ionization due to precipitating electrons on field lines connected to the collisional atmosphere. Reflection areas observed in regions of stronger magnetic field are found to have larger plasma number density. We use a model of Martian crustal magnetic field lines to demonstrate that the magnetic field has a “focusing” effect on incident particles. Electron impact ionization is thus expected to take place only in well-defined areas, in agreement with our observations. Our observations show that the deep nightside ionosphere of Mars is very irregular, controlled primarily by the configuration of crustal magnetic fields.

Consequences of violation of frozen-in-flux: Evidence from OpenGGCM simulations

[1] It is widely believed that during a substorm, plasma instabilities occur before the onset of magnetic reconnection, signaling the end of the growth phase. Despite many years of effort, however, the details of how the instability and the onset of reconnection develop from closed field line configuration with finite normal magnetic field are not well understood. In this paper, we study an idealized simulation of a substorm that occurred on 23 March 2007, based on the Open Geospace General Circulation Model (OpenGGCM). Our analysis emphasizes the time development of the distribution of the entropy parameter and its convective time derivative, which should be zero in ideal MHD. In the late growth phase, the simulation exhibits, over a range of local times, a systematic violation of conservation of entropy that corresponds to what is called “antidiffusion.” Out of this background, a more localized disturbance develops in a region of high magnetic stretching, resulting in formation of a strong reduction of entropy (bubble) earthward of a local enhancement (blob). The process is accelerated when the current density exceeds a threshold for triggering an explicit resistivity in the code. The bubble moves earthward and the blob tailward, which leads to a reduction of the normal magnetic field and a thinning of the current sheet between them, making the magnetospheric configuration more conducive to tearing and other instabilities (we do not address specifically which instability has occurred). This positive feedback gives rise to increased violation of the perfect conductivity relation and eventually reconnection.

Suprathermal electron spectra in the Venus ionosphere

[1] In this study, a multistream kinetic model is used to derive the suprathermal electron intensity in the Venus ionosphere, to be compared with the observations made by the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-4) Electron Spectrometer (ELS) instrument onboard Venus Express (VEx) on 18 May 2006. Input parameters of the model are chosen to be consistent with the detailed ephemeris information. We consider two cases for the magnetic field line configuration. The first assumes vertical magnetic field lines, while in the second the configuration is consistent with the VEx Magnetometer (MAG) measurements for that particular orbit. The model calculations suggest that the energy spectrum of suprathermal electrons not only depends on the local photoelectron production but is also strongly influenced by transport as well as energy degradation. We confirm the postulation of Coates et al. (2008) that the spectral features seen in the ASPERA-4 ELS data are signatures of O+(4S0, 2P0, and 2D0) production by solar HeII 30.4 nm photons at altitudes where vertical transport is important. We also identify additional photoelectron peaks, in particular the peak associated with the ionization of atmospheric He by the solar HeII 30.4 nm photons. The combination of the identified photoelectron peaks provides a validation of the proposed spacecraft potential value of ∼−5 eV derived from the O+-related features by Coates et al. (2008). The model results indicate that the condition of local equilibrium is satisfied for suprathermal electrons below ∼155 km on Venus. However, transport becomes effective at higher altitudes, significantly altering the secondary electron production rate and leading to strongly anisotropic suprathermal electron flow. For the case with the MAG-derived, slanted magnetic field configuration, we make a data-model comparison in terms of both the absolute magnitude of the suprathermal electron intensity and the appearance of spectral peaks. The comparison suggests that the actual magnetic field lines are probably inclined more vertically than a simple extrapolation from the MAG measurements.

Nonlinear force-free field extrapolation of the coronal magnetic field using the data obtained by the Hinode satellite

[1] The Hinode satellite can obtain high-quality photospheric vector magnetograms of solar active regions and the simultaneous coronal loop images in soft X-ray and extreme ultraviolet (EUV) bands. In this paper, we continue the work of He and Wang (2008) and apply the newly developed upward boundary integration computational scheme for the nonlinear force-free field (NLFFF) extrapolation of the coronal magnetic field to the photospheric vector magnetograms acquired by the Spectro-Polarimeter of the Solar Optical Telescope aboard Hinode. Three time series vector magnetograms of the same solar active region, NOAA 10930, are selected for the NLFFF extrapolations, which were observed within the time interval of 26 h during 10–11 December 2006 when the active region crossed the central area of the Sun's disk. Parallel computation of the NLFFF extrapolation code was realized through OpenMP multithreaded, shared memory parallelism and Fortran 95 programming language for the extrapolation calculations. The comparison between the extrapolated field lines and the coronal loop images obtained by the X-Ray Telescope and the EUV Imaging Spectrometer of Hinode shows that, in the central area of the active region, the field line configurations generally agree with the coronal images, and the orientations of the field lines basically coincide with the coronal loop observations for all three successive magnetograms. This result supports the NLFFF model being used for tracing the time series evolution of the 3-D coronal magnetic structures as the responses of the quasi-equilibrium solar atmosphere to the vector magnetic field changes in the photosphere.

Study of blob-like structures in QUEST

Blob-like structures are observed in an open magnetic field line configuration of electron cyclotron resonance (ECR) heating plasma in QUEST. The combined measurement with a fast camera and a movable Langmuir probe show that visible filamentary structures extended along field lines do correspond to over-dense plasma structures. Ion saturation current time trace shows a front-like asymmetric burst when a visible structure reaches the probe during its outward propagation. The typical radial velocity of the blob-like structure, Vb, was measured as ∼1km/s with the fast camera at the intermediate area. Vb increased as the blob-like structure moved along major radius, R (outward). The average velocity of E×B drift derived from the probe measurement, slightly larger than Vb, was also increased as the blob-like structure propagated to larger R.

Global configuration of Saturn's magnetic field derived from observations

Magnetic field observations from the Cassini spacecraft are averaged into 1 × 1 RS (1 RS = 60238 km) meridional bins at Saturn. A Runge‐Kutta procedure uses the bin‐averaged field components to estimate the global configuration of Saturn's magnetic field and map field lines from the equator to Saturn's ionosphere. Within ∼18 RS of Saturn, the mapping gives a good representation of the meridional shape of Saturn's magnetic field and confirms that beyond L ∼ 6, the field departs from a dipole and becomes a smoothly‐warped magnetodisk. The disk warps upwards by as much as ∼1 RS at radial distances of ∼15 RS, with the warping increasing with radial distance. When traced back to Saturn, the bin average field lines intersect the ionosphere at latitudes similar to those expected on the basis of recent magnetic field models; the ionospheric pierce latitudes can differ by as much as 6° from those expected from a simple offset dipole.

PARTICLE ACCELERATION AT NEAR-PERPENDICULAR SHOCKS: THE ROLE OF FIELD-LINE TOPOLOGY

Particle acceleration at two-dimensional (2D) shocks can significantly differ from our expectations based on one-dimensional shocks. We discuss several features of 2D shocks. First, we present a simple example of diffusive acceleration to demonstrate that (1) hot spots and cold regions can be expected along the shock face, as the field-line configuration changes along the shock face, (2) the flux of accelerated particles (even the average flux) can be expected to increase beyond the shock, and (3) 2D structures may lead to a softening of the spectrum. We also address quasi-perpendicular shocks and discuss what happens when a field line is hit by a curved shock or is detaching from a shock. This mechanism is of interest on both large and small scales. We consider pure field-aligned transport and address both the diffusive (large scale) and scatter-free (small scale) cases, looking from the perspective of one field line. We find that quasi-trapping of particles in front of the shock can lead to rapid and effective acceleration via multiple mirroring on the ever faster moving shock. This mechanism may be of importance at various astrophysical, heliospheric, and coronal mass ejection driven shocks.

Linked and knotted beams of light, conservation of helicity and the flow of null electromagnetic fields

Maxwell's equations allow for some remarkable solutions consisting of pulsed beams of light which have linked and knotted field lines. The preservation of the topological structure of the field lines in these solutions has previously been ascribed to the fact that the electric and magnetic helicities, a measure of the degree of linking and knotting between field lines, are conserved. Here we show that the elegant evolution of the field is due to the stricter condition that the electric and magnetic fields be everywhere orthogonal. The field lines then satisfy a ‘frozen field’ condition and evolve as if they were unbreakable filaments embedded in a fluid. The preservation of the orthogonality of the electric and magnetic field lines is guaranteed for null, shear-free fields such as the ones considered here by a theorem of Robinson. We calculate the flow field of a particular solution and find it to have the form of a Hopf fibration moving at the speed of light in a direction opposite to the propagation of the pulsed light beam, a familiar structure in this type of solution. The difference between smooth evolution of individual field lines and conservation of electric and magnetic helicities is illustrated by considering a further example in which the helicities are conserved, but the field lines are not everywhere orthogonal. The field line configuration at time t = 0 corresponds to a nested family of torus knots but unravels upon evolution.