Local Magnetic Field Lines Research Articles

Edge turbulence imaging in the Alcator C-Mod tokamak

The two-dimensional (2D) radial vs poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I. H. Hutchinson, R. Boivin, P. T. Bonoli et al., Nucl. Fusion 41, 1391 (2001)] was measured using fast cameras and compared with three-dimensional numerical simulations of edge plasma turbulence. The main diagnostic is gas puff imaging, in which the visible Dα emission from a localized D2 gas puff is viewed along a local magnetic field line. The observed Dα fluctuations have a typical radial and poloidal scale of ≈1 cm, and often have strong local maxima (“blobs”) in the scrape-off layer. The motion of this 2D structure motion has also been measured using an ultrafast framing camera with 12 frames taken at 250 000 frames/s. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical model.

Impurity transport studies in tokamak edge plasmas using visible imaging

Understanding impurity transport in the scrape-off layer (SOL) of tokamak plasmas is a necessary piece of developing the physics basis for designing next-generation reactors. A system for inferring impurity transport parallel and perpendicular to local magnetic field lines has been developed on Alcator C-Mod using gas-injection "plumes." In this system, impurity gas is injected at a fixed position in the SOL via a reciprocating fast-scanning probe, and the resulting emission is imaged. In this paper, visible light emission patterns from C/sup +1/ and C/sup +2/ ions are presented.

Magnetic field configuration and field-aligned acceleration of energetic ions during substorm onsets

Abstract. In this paper, we present an interpretation of the observed field-aligned acceleration events measured by GEOS-2 near the night-side synchronous orbit at substorm onsets (Chen et al., 2000). We show that field-aligned acceleration of ions (with pitch angle asymmetry) is closely related to strong short-lived electric fields in the Ey direction. The acceleration is associated with either rapid dipolarization or further stretching of local magnetic field lines. Theoretical analysis suggests that a centrifugal mechanism is a likely candidate for the parallel energization. Equatorward or anti-equatorward energization occurs when the tail current sheet is thinner tailward or earthward of the spacecraft, respectively. The magnetic field topology leading to anti-equatorward energization corresponds to a situation where the near-Earth tail undergoes further compression and the inner edge of the plasma sheet extends inwards as close as the night-side geosynchronous altitudes.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet; storms and sub-storms)

Solar coronal heating by magnetosonic waves

Solar coronal heating by magnetohydrodynamic (MHD) waves is investigated. ultraviolet (UV) and X-ray emission lines of the corona show non-thermal broadenings. The wave rms velocities inferred from these observations are of the order of 25–60 km s−1. Assuming that these values are not negligible, we solved MHD equations in a quasi-linear approximation, by retaining the lowest order non-linear term in rms velocity. Plasma density distribution in the solar corona is assumed to be inhomogeneous. This plasma is also assumed to be permeated by dipole-like magnetic loops. Wave propagation is considered along the magnetic field lines. As dissipative processes, only the viscosity and parallel (to the local magnetic field lines) heat conduction are assumed to be important. Two wave modes emerged from the solution of the dispersion relation. The fast mode magneto-acoustic wave, if originated from the coronal base can propagate upwards into the corona and dissipate its mechanical energy as heat. The damping length-scale of the fast mode is of the order of 500 km. The wave energy flux associated with these waves turned out to be of the order of 2.5×105 ergs cm−2 s−1 which is high enough to replace the energy lost by thermal conduction to the transition region and by optically thin coronal emission. The fast magneto-acoustic waves prove to be a likely candidate to heat the solar corona. The slow mode is absent, in other words cannot propagate in the solar corona.

Impurity transport experiments in the edge plasma of Alcator C-Mod using gas injection plumes

Understanding impurity and bulk plasma transport in the scrape-off layer (SOL) of tokamak plasmas is critical for the design of future reactors. The development of a system on Alcator C-Mod for inferring impurity transport behaviour parallel and perpendicular to local magnetic field lines from impurity emission patterns (`plumes') generated by local gas injection will be presented. Gas is injected at variable location in the SOL through the end of a reciprocating fast-scanning probe. Carbon plumes are generated by puffing ethylene gas (C 2H 4) through the probe over a period of ∼8–10 ms. Two intensified CCD cameras are used to record C +1 and C +2 emission patterns from near-perpendicular views. Flows parallel and perpendicular to the magnetic field have been observed in both views. In principle, the data allow a full 3D reconstruction of the impurity dispersal with ∼1 mm spatial resolution.

Model of vortex tubes in the low-latitude plasma sheet of the earth magnetosphere

Theoretical model is proposed to explain specific features of compressional waves of the Pc 4–5 frequency range which were observed with the INTERBALL-1 probe in the middle tail of the Earth magnetosphere. The waves exhibit anti-phase disturbances in thermal plasma and magnetic field pressures. Large-scale vortical plasma motions are associated with the waves. Coupling of the drift Alfven and magnetosonic waves is considered with inclusion of finite plasma compressibility and disturbances along the local magnetic field line. In limiting cases derived equations are reduced to well-known Kadomtsev-Pogutse and Hasegawa-Mima equations. Partial solutions take form of vortices. According to proposed model, two-dimensional vortical motion of a plasma transversely to the local field line is accompanied by nonlinear magnetosonic waves propagating in flux tubes almost along the field lines.

Auroral kilometric radiation at substorm onset

The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ±1 (±2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm‐associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ∼1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ∼300 kHz, corresponding to an upper limit of the AKR source altitude of ∼4700 km. The entire AKR frequency band expands to ∼64–650 kHz within a few minutes after onset, corresponding to a wider source altitude of ∼2100–12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase.

The Anisotropy of Magnetohydrodynamic Alfvenic Turbulence

We perform direct three-dimensional numerical simulations for magnetohydrodynamic (MHD) turbulence in a periodic box of size 2π threaded by strong uniform magnetic fields. We use a pseudospectral code with hyperviscosity and hyperdiffusivity to solve the incompressible MHD equations. We analyze the structure of the eddies as a function of scale. A straightforward calculation of anisotropy in wavevector space shows that the anisotropy is scale independent. We discuss why this is not the true scaling law and how the curvature of large-scale magnetic fields affects the power spectrum and leads to the wrong conclusion. When we correct for this effect, we find that the anisotropy of eddies depends on their size: smaller eddies are more elongated than larger ones along local magnetic field lines. The results are consistent with the scaling law ∥ ~ recently proposed by Goldreich & Sridhar. Here ∥ (and ⊥) are wavenumbers measured relative to the local magnetic field direction. However, we see some systematic deviations that may be a sign of limitations to the model or our inability to fully resolve the inertial range of turbulence in our simulations.

The Generation of Magnetic Fields through Driven Turbulence

We have tested the ability of driven turbulence to generate magnetic field structure from a weak uniform field using three-dimensional numerical simulations of incompressible turbulence. We used a pseudospectral code with a numerical resolution of up to 1443 collocation points. We find that the magnetic fields are amplified through field line stretching at a rate proportional to the difference between the velocity and the magnetic field strength times a constant. Equipartition between the kinetic and magnetic energy densities occurs at a scale somewhat smaller than the kinetic energy peak. Above the equipartition scale the velocity structure is, as expected, nearly isotropic. The magnetic field structure at these scales is uncertain, but the field correlation function is very weak. At the equipartition scale the magnetic fields show only a moderate degree of anisotropy, so the typical radius of curvature of field lines is comparable to the typical perpendicular scale for field reversal. In other words, there are few field reversals within eddies at the equipartition scale and no fine-grained series of reversals at smaller scales. At scales below the equipartition scale, both velocity and magnetic structures are anisotropic; the eddies are stretched along the local magnetic field lines, and the magnetic energy dominates the kinetic energy on the same scale by a factor that increases at higher wavenumbers. We do not show a scale-free inertial range, but the power spectra are a function of resolution and/or the imposed viscosity and resistivity. Our results are consistent with the emergence of a scale-free inertial range at higher Reynolds numbers.

A comparison of thermospheric winds and temperatures from Fabry-Perot interferometer and EISCAT radar measurements with models

Abstract During the nights of 8–9 and 9–10 February 1997, Fabry-Perot interferometers were operated from the EISCAT radar site at Ramfjord (69.59° N, 19.23° E) and Skibotn (69.35° N, 20.36° E). From Ramfjord, horizontal neutral winds were measured in the lower and upper thermosphere using the auronal/airglow emissions at 557.7 and 630 nm, respectively. From Skibotn, thermospheric neutral temperatures were measured using the same wavelengths. The EISCAT radar measured ion temperatures up the local magnetic field line in the height range 90 – 580 km during the first night. Neutral winds are compared to the HWM-90 and CTIP-200 models with poor agreement. Neutral temperatures are compared to the MSISE-90 and CTIP-200 models as well as EISCAT ion temperatures with good agreement.

A comparison of vertical thermospheric winds from Fabry-Perot interferometer measurements over a 50 km baseline

Abstract During the nights of 8–9 and 9–10 February 1997, Fabry-Perot interferometers were operated from Ramfjord and Skibotn, Norway, a longitudinal baseline of ∼45 km. From Skibotn, thermospheric vertical winds were measured in the lower and upper thermosphere on both nights using the auroral/airglow emissions at 557.7 and 630 nm, respectively. From Ramfjord, neutral winds were measured, using the same wavelengths, parallel to the local magnetic field line on the first night and in the local zenith on the second night. Vertical neutral winds in both height ranges show essentially no correlation between the two stations. Some correlation between the vertical wind in the upper and lower thermosphere over individual stations does exist.

High-Resolution Hα Observations of Proper Motion in NOAA 8668: Evidence for Filament Mass Injection by Chromospheric Reconnection

There have been two different kinds of explanations for the source of cool material in prominences or filaments: coronal condensations from above and cool plasma injections from below. In this paper, we present observational results which support filament mass injection by chromospheric reconnection. The observations of an active filament in the active region NOAA 8668 were performed on 17 August 1999 at a wavelength of Hα−0.6 A using the 65 cm vacuum reflector, a Zeiss Hα birefringent filter, and a 12-bit SMD digital camera of Big Bear Solar Observatory. The best image was selected every 12 s for an hour based on a frame selection algorithm. All the images were then co-aligned and corrected for local distortion due to the seeing. The time-lapse movie of the data shows that the filament was undergoing ceaseless motion. The Hα flow field has been determined as a function of time using local correlation tracking. Time-averaged flow patterns usually trace local magnetic field lines, as inferred from Hα fibrils and line-of-sight magnetograms. An interesting finding is a transient flow field in a system of small Hα loops, some of which merge into the filament. The flow is associated with a cancelling magnetic feature which is located at one end of the loop system. Initially a diverging flow with speeds below 10 km s−1 is visible at the flux cancellation site. The flow is soon directed along the loops and accelerated up to 40 km s−1 in a few minutes. Some part of the plasma flow then merges into and moves along the filament. This kind of transient flow takes place several times during the observations. Our results clearly demonstrate that reconnection in the photosphere and chromosphere is a likely way to supply cool material to a filament, as well as re-organizing the magnetic field configuration, and, hence, is important in the formation of filaments.

The design of central column protection tiles for the TCV tokamak

The large variety of plasma shapes produced in the TCV tokamak places unique demands on the plasma facing surfaces. In particular, the central column graphite armour tiles are solicited during the creation of all TCV plasmas and function as power handling surfaces for both limited and diverted discharges. The higher power flux densities accompanying the addition of electron cyclotron heating systems have necessitated a new, optimized, design for these tiles. The optimization process and the subsequent new tile design are described. A basic `two point' model of the scrape-off layer plasma in conjunction with TCV equilibrium reconstructions and a simplified representation of the local magnetic field line geometry are used to impose simulated power flux densities onto a parametric toroidal tile contour. The thermo-mechanical response of the tile is then investigated via full 3-D finite element simulations accounting for the non-linear temperature dependence of the graphite thermal diffusivity and radiation from the tile surface. The final design choice is a compromise between the requirements for adequate power handling for a range of magnetic configurations, the need to protect against tile edge misalignment in the presence of grazing field line angles of incidence and the space restrictions imposed by vacuum vessel design.

The motion of ions in the auroral ionosphere

The ion motion in the E and F regions relative to the local magnetic field line and the horizontal plane has been determined from the European incoherent scatter (EISCAT) common program 1 version I (CP‐1‐I) data. The ion motion highly depends on altitude and magnetospheric electric field strength. Under the condition of a strong electric field (|E| > 25 mV/m), ions move perpendicularly to the local magnetic field line both in the F region and at the upper part of the E region (≥117 km). This suggests the ions moving by the E × B drift, which is expected when the ion‐neutral collision frequency is much smaller than the ion gyrofrequency. At lower altitudes, however, the ions no longer move perpendicularly to the magnetic field line but move horizontally, as expected from a stronger interaction between the ions and neutrals. Under the condition of a weak electric field (5 mV/m < |E| < 10 mV/m) the neutral winds tend to move the ions horizontally everywhere in the E region and even in the F region. The main cause of these ion motions and their dependence on altitude and electric field strength comes from the relative importance of the magnetospheric electric field and the large‐scale neutral wind drag to the ionospheric ions.

Magneto-aerotaxis in marine coccoid bacteria

Magnetotactic cocci swim persistently along local magnetic field lines in a preferred direction that corresponds to downward migration along geomagnetic field lines. Recently, high cell concentrations of magnetotactic cocci have been found in the water columns of chemically stratified, marine and brackish habitats, and not always in the sediments, as would be expected for persistent, downward-migrating bacteria. Here we report that cells of a pure culture of a marine magnetotactic coccus, designated strain MC-1, formed microaerophilic bands in capillary tubes and used aerotaxis to migrate to a preferred oxygen concentration in an oxygen gradient. Cells were able to swim in either direction along the local magnetic field and used magnetotaxis in conjunction with aerotaxis, i.e., magnetically assisted aerotaxis, or magneto-aerotaxis, to more efficiently migrate to and maintain position at their preferred oxygen concentration. Cells of strain MC-1 had a novel, aerotactic sensory mechanism that appeared to function as a two-way switch, rather than the temporal sensory mechanism used by other bacteria, including Magnetospirillum megnetotacticum, in aerotaxis. The cells also exhibited a response to short-wavelength light (< or = 500 nm), which caused them to swim persistently parallel to the magnetic field during illumination.

Spin stabilized magnetic levitation

The stability of the Levitron® cannot be explained if the top’s axis has a fixed direction in space. Stability against flipping is not enough. Gyroscopic precession around the local magnetic field direction is necessary. An analysis and numerical integration of the equations of motion for an experimental stemless top that includes gyroscopic precession around the local magnetic field lines predict that the top will be supported stably up to spin speeds of about 3065 rpm. An upper spin limit of 2779 rpm for this top is observed experimentally and explained as an adiabatic condition. Spin stabilized magnetic levitation is a macroscopic analog of magnetic gradient traps used to confine particles with a quantum magnetic moment.

Traveling ionospheric disturbances observed in digitized polarimeter data

Faraday rotation data from a pair of polarimeters located in Tucson, Arizona during the first half of 1990 have been examined for evidence of traveling ionospheric disturbances. With the use of digital filtering techniques, oscillations in total electron content (TEC) with periods ranging from 20 to 90 minutes and amplitudes of 1–5 × 1016 m−2 have been detected in the data. When present, the oscillations persist for varying time intervals (usually several hours) and show considerable variation from day to day. Although the viewing geometry (nearly along the local magnetic field line) is not ideal for observation of TIDs, these oscillations do appear to be real ionospheric oscillations. We have examined the possibility that the oscillations are not caused by actual changes in electron content but rather are due to changes in the Faraday rotation resulting from the motion of the ionospheric plasma along a magnetic field line and the resulting change in effective magnetic field strength. Our calculations indicate waves with displacement amplitudes of a few tens of kilometers would be sufficient to explain the observations. We also examined the possibility that the oscillations could be due to actual changes in TEC by using a one‐dimensional F region model and imposing a plausible wind oscillation (simulating a gravity wave). We found that a wave with a velocity amplitude of 15 m s−1 was also sufficient to explain the observations. We expect that the observations are best explained by a combination of the two effects, that is, an actual oscillation in electron content and an oscillation in Faraday rotation, both caused by gravity waves in the thermosphere. We estimate that for the 90‐min oscillations, 25% of the amplitude is due to the Faraday rotation effect and 75% is due to an actual change in TEC.

Centrifugally driven phase bunching and related current sheet structure in the near‐Earth magnetotail

We examine the role of centrifugal effects during nonadiabatic interactions of charged particles with the magnetotail current sheet. It is shown that when the parameter κ (defined as the square root of the minimum curvature radius‐to‐maximum Larmor radius) is of the order of unity, as is the case for ions traveling in the near‐Earth plasma sheet, enhanced centrifugal effects lead to prominent bunching of the particles in gyration phase. As a result of this bunching effect we demonstrate that a thin current sheet develops in the vicinity of the tail midplane. When average values of the plasma density (a few tenths of ions per cubic centimeter) and temperature (several keV) in the near‐Earth tail are used, the current sheet obtained has a characteristic thickness of the order of a few tenths of an Earth radius and leads to significant stretching of the local magnetic field lines. A further consequence of phase bunching is the buildup of a substantial current in the Earth‐tail direction at low latitudes, which leads to field line inclination in the dawn‐dusk direction. This phase bunching mechanism, which maximizes when the bulk of the ion distribution nears κ = 1, is of potential importance for the dynamics of the inner plasma sheet during the growth phase of substorms.

An x-ray target probe for superthermal electron and field line pitch measurements

An insertable target x-ray probe has been designed which may be used for two different purposes: measurement of superthermal electron temperature and drift speed, and measurement of the local magnetic field line pitch, both with fast time resolution (1 μs). The diagnostic is intended for use on the Madison Symmetric Torus (MST) reversed-field pinch. For the first application, the probe is fitted with six detectors, three viewing the part of the target facing toward the incident electrons, and three viewing the part facing away; each set of three has 0.3, 0.6, and 1.2 mil Be filters for energy discrimination. By subtracting signals from detectors with the same filter but opposing views and dividing signals from detectors with different filters but the same view, one can derive the electron drift speed and parallel temperature. For the second application, the six detectors are replaced by a single position-sensitive detector, which produces a signal indicating the position of the ‘‘hot spot’’ on the target, and therefore the field line pitch. The diagnostic can withstand high temperatures and can therefore be inserted further into the plasma than electrostatic probes.

Anomalous auroral electron distributions due to an artificial ion beam in the ionosphere

We report the results of an active experiment in which the auroral ionosphere was perturbed by the operation of an ion gun, which injected approximately 100 mA of 25 eV Ar+ ions at upgoing pitch angles over a discrete auroral arc. The gun was operated during five periods on the downleg of the flight as the payload descended from 220 km to 120 km altitude along the local magnetic field lines. A brief (&lt; 100 ms) negative payload potential excursion was observed at the onset of each gun operation, followed by quasi‐steady conditions for the remainder of each period. The major steady effects were (1) the excitation of intense broad band electric field fluctuations, particularly at low frequencies 0‐10 kHz, and (2) the appearance of streaming and isotropic heating in different parts of superthermal electron velocity space. We have explored a scenario that appeals to such a wave spectrum as an obstacle to auroral parallel current and predicts this type of electron behavior. The waves are viewed as traps for the thermal electrons and scatterers of the faster electrons. In this scenario, electron runaway or streaming is expected between the trapping speed and the critical velocity for cyclotron interactions with the waves. These streaming electrons carry the current that would be carried by thermals or energetic electrons in the absence of the waves. Isotropic heating is expected near the critical speed where Doppler resonance occurs and is found near 12000 km/s (∼50 υte, where υte is the electron thermal speed). It may be inferred that waves are excited primarily at wavelengths of several meters and longer. A current of ∼1 µA m−2 is carried by the streaming electrons. Energetic considerations suggest that the heated electrons derive their energy from an external source rather than from the ion beam itself; the auroral generator appears to be more than adequate. The gun‐associated electrons were anomalous in the sense that their anisotropy was the opposite of that observed in the natural aurora, where electrons are isotropic at low energy and often field‐aligned at higher energy.