Showing papers on "Magnetic flux published in 2000"

Magnetic vortex core observation in circular dots of permalloy

Abstract: Spin structures of nanoscale magnetic dots are the subject of increasing scientific effort, as the confinement of spins imposed by the geometrical restrictions makes these structures comparable to some internal characteristic length scales of the magnet. For a vortex (a ferromagnetic dot with a curling magnetic structure), a spot of perpendicular magnetization has been theoretically predicted to exist at the center of the vortex. Experimental evidence for this magnetization spot is provided by magnetic force microscopy imaging of circular dots of permalloy (Ni 80 Fe 20 ) 0.3 to 1 micrometer in diameter and 50 nanometers thick.

A Twisted Flux Rope Model for Coronal Mass Ejections and Two-Ribbon Flares

Abstract: We present a new approach to the theory of large-scale solar eruptive phenomena such as coronal mass ejections and two-ribbon flares, in which twisted flux tubes play a crucial role. We show that it is possible to create a highly nonlinear three-dimensional force-free configuration consisting of a twisted magnetic flux rope representing the magnetic structure of a prominence (surrounded by an overlaying, almost potential, arcade) and exhibiting an S-shaped structure, as observed in soft X-ray sigmoid structures. We also show that this magnetic configuration cannot stay in equilibrium and that a considerable amount of magnetic energy is released during its disruption. Unlike most previous models, the amount of magnetic energy stored in the configuration prior to its disruption is so large that it may become comparable to the energy of the open field.

Interplanetary Magnetic Field Line Mixing Deduced from Impulsive Solar Flare Particles

Abstract: We have studied fine-scale temporal variations in the arrival profiles of ~20 keV nucleon^(-1) to ~2 MeV nucleon^(-1) ions from impulsive solar flares using instrumentation on board the Advanced Composition Explorer spacecraft at 1 AU between 1997 November and 1999 July. The particle events often had short-timescale (~3 hr) variations in their intensity that occurred simultaneously across all energies and were generally not in coincidence with any local magnetic field or plasma signature. These features appear to be caused by the convection of magnetic flux tubes past the observer that are alternately filled and devoid of flare ions even though they had a common flare source at the Sun. Thus, we have used the particles to study the mixing of the interplanetary magnetic field that is due to random walk. We deduce an average timescale of 3.2 hr for these features, which corresponds to a length of ~0.03 AU.

The long‐term variation of the Sun's open magnetic flux

Abstract: The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φ open can be inferred from current-free extrapolations of the observed photospheric field. We derive the long-term variation of Φ open during 1971-1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower-latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φ open closely follows that of the Sun's total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large-scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φ open and the IMF strength lasting typically ∼1 yr.

Type I Strings with F- and B-Flux

Abstract: We present non-supersymmetric toroidal compactifications of type I string theory with both constant background NSNS two-form flux and non-trivial magnetic flux on the various D9-branes. The non-vanishing B-flux admits four-dimensional models with three generations of chiral fermions in standard model like gauge groups. Additionally, we consider the orbifold T^4/Z_2, again with both kinds of background flux present, leading to non-supersymmetric as well as supersymmetric models in six dimensions. All models have T-dual descriptions as intersecting brane worlds.

Small-scale magnetic flux ropes in the solar wind

Abstract: Small-scale magnetic flux ropes have been discovered in the solar wind at 1 AU in observations from the IMP 8 and WIND spacecraft. These small magnetic structures (diameter of 270 RE, on average) have some similar properties to magnetic clouds (diameters of 0.2–0.3 AU or about 6000–8000 RE), which are well known large-scale magnetic flux ropes, but have durations of 10s of minutes as opposed to many hours or days for most magnetic clouds. The presence of these small helical field structures suggests that solar wind flux ropes may have a wide-range of scale sizes, or possibly have a bimodal size distribution, and are perhaps more common than previously estimated. Similarities and differences with magnetic clouds will be discussed. We suggest that these small scale magnetic flux ropes are signatures of magnetic reconnection in the solar wind as opposed to in the solar corona.

Magnetic circuit model for the mutually coupled switched-reluctance machine

Abstract: The mutually coupled switched-reluctance motor (SRM) appears to have several performance advantages over other motor technologies. The existence of strong coupling between phases, however, makes the analysis of this machine quite complicated. Preliminary design of this machine can be greatly accelerated by the ability to evaluate potential motor geometries quickly. This paper introduces a general magnetic circuit model of the mutually coupled SRM that adapts to any geometry, unlike existing geometry-dependent approaches (such as finite elements), which are numerically intensive and require excessive computation time. The model uniquely implements the magneto-motive force (mmf) sources necessary to accommodate complex flux paths through the machine and includes the effects of magnetic saturation. The results are compared to those of a finite element solver to demonstrate the performance of this method as a first-step to evaluating candidate designs.

A Model for the Emergence of a Twisted Magnetic Flux Tube

Abstract: Observations have shown that active region flux tubes often emerge in a twisted state and that the active region formed has magnetic helicity of the same sense as the flux tube that forms it. Separate theoretical models have been developed for coronal magnetic fields with helicity and for flux tubes with twist. Here we present a dynamical model that connects a twisted subphotospheric flux tube to a force-free coronal field. With this model it is possible to explore the emergence of a flux tube into the corona and its effect on both the coronal field and the subphotospheric flux tube. In particular, the model shows that only a fraction of the current carried by the twisted flux tube will pass into the corona. As a consequence of this "mismatch," a torsional Alfven wave is launched downward along the flux tube at the instant of emergence. As the flux tube continues to emerge, the helicity of the coronal field increases owing to rotation of the footpoints. Our model predicts that the level of rotation will depend upon the rapidity of flux emergence. After this transient period the helicity of the active region will reflect the twist in its parent tube.

Performance analysis of a speed-sensorless induction motor drive based on a constant-switching-frequency DTC scheme

Abstract: A control technique, which utilizes the stator flux components as control variables, has been applied to a speed-sensorless induction motor drive. The scheme may be regarded as a development of a direct torque control scheme, aimed at achieving a constant-switching-frequency operation. At each sampling period the required voltage vector is calculated on the basis of the error between the reference and the estimated stator flux vector. The problems related to the voltage-source inverter dead time and the stator flux estimation at low speed have been analyzed, and an efficient solution has been proposed. The performance of the drive system has been verified by experimental tests, and good results have been achieved in both steady-state and transient operating conditions.

Global Simulations of Differentially Rotating Magnetized Disks: Formation of Low-β Filaments and Structured Coronae

Abstract: We present the results of three-dimensional global magnetohydrodynamic simulations of the Parker-shearing instability in a differentially rotating torus initially threaded by toroidal magnetic fields. An equilibrium model of a magnetized torus is adopted as an initial condition. When β0 = Pgas/Pmag ~ 1 at the initial state, magnetic flux buoyantly escapes from the disk and creates looplike structures similar to those in the solar corona. Inside the torus, the growth of nonaxisymmetric magnetorotational (or Balbus & Hawley) instability generates magnetic turbulence. Magnetic field lines are tangled on a small scale, but on a large scale they show low azimuthal wavenumber spiral structure. After several rotation periods, the system oscillates around a state with β ~ 5. We found that magnetic pressure-dominated (β < 1) filaments are created in the torus. The volume filling factor of the region in which β ≤ 0.3 is 2%-10%. Magnetic energy release in such low-β regions may lead to violent flaring activities in accretion disks and in galactic gas disks.

Self‐organized criticality in the substorm phenomenon and its relation to localized reconnection in the magnetospheric plasma sheet

Abstract: Evidence is presented that suggests that there is a significant self-organized criticality (SOC) component in the dynamics of substorms in the magnetosphere. We assume that observations of bursty bulk flows, fast flows, localized dipolarizations, plasma turbulence, etc. show that multiple localized reconnection sites provide the basic avalanche phenomenon in the establishment of SOC in the plasma sheet. First results are presented from a study of this avalanche process based on this working assumption. A magnetic field reversal model is discussed. Resistivity, in this model, is self-consistently generated in response to the excitation of an idealized currentdriven instability. When forced by convection of magnetic flux into the field reversal region, the model yields rapid magnetic field annihilation through a dynamic behavior that is shown to exhibit many of the characteristics of SOC. Over a large range of forcing strengths, the annihilation rate is shown to self-adjust to balance the rate at which flux is convected into the reversal region. Several analogies to magnetotail dynamics are discussed: (1) It is shown that the presence of a localized criticality in the model produces a remarkable stability in the global configuration of the field reversal while simultaneously exciting extraordinarily dynamic internal evolution. (2) Under steady forcing it is shown that a loading-unloading cycle may arise that, as a consequence of the global stability, is quasi-periodic and, therefore, predictable despite the presence of internal turbulence in the field distribution. Indeed, it is shown that the global loading-unloading cycle is a consequence of the internal turbulence. (3) It is shown that under steady, strong forcing the loading-unloading cycle vanishes. Instead, a recovery from a single unloading persists indefinitely. The field reversal is globally very steady while internally it is very dynamic as field annihilation goes on at the rate necessary to match the strong forcing. From this result we speculate that steady magnetospheric convection events result when the plasma sheet has been driven close to criticality over an extended spatial domain. During these events we would expect to find localized reconnection sites distributed over the spatial domain of near criticality, and we would expect to find plasma sheet transport in that domain to be closely related to that of BBF and fast flow events.

Non-quantized penetration of magnetic field in the vortex state of superconductors

Abstract: As first pointed out by Bardeen and Ginzburg in the early sixties1,2, the amount of magnetic flux carried by vortices in superconducting materials depends on their distance from the sample edge, and can be smaller than one flux quantum, φ0 = h/2e (where h is Planck's constant and e is the electronic charge). In bulk superconductors, this reduction of flux becomes negligible at sub-micrometre distances from the edge, but in thin films the effect may survive much farther into the material3,4. But the effect has not been observed experimentally, and it is often assumed that magnetic field enters type II superconductors in units of φ0. Here we measure the amount of flux introduced by individual vortices in a superconducting film, finding that the flux always differs substantially from φ0. We have observed vortices that carry as little as 0.001φ0, as well as ‘negative vortices’, whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect1,2,3,4 and a nonlinear screening of the magnetic field due to the presence of a surface barrier. The latter effect has not been considered previously, but is likely to cause non-quantized penetration in most cases.

Non-Quantized Penetration of Magnetic Field in the Vortex State of Superconductors

Abstract: As first pointed out by Bardeen and Ginzburg in the early sixties, the amount of magnetic flux carried by vortices depends on their distance to the sample edge and can be smaller than one flux quantum, f0 = h/2e. In bulk superconductors, this reduction of flux becomes negligible already at submicron distances from the edge but, in thin films, the effect may survive at much larger distances. In the absence of any experimental observation, such flux reduction is perceived to be an exotic or unimportant effect, and it is often assumed that magnetic field enters type-II superconductors in units of f0. Here we report the measurements of the amount of flux associated with the entrance of individual vortices in a superconducting film and show that the flux that they bring in, always differs substantially from f0. We have observed vortices that carry as little as 0.001 f0 as well as seemingly "negative vortices" whose penetration leads to the expulsion of magnetic field. We distinguish two phenomena responsible for non-quantized flux penetration: the finite-size effect and a non-linear screening of magnetic field in the presence of a surface barrier. The latter effect has previously not been considered but is likely to cause non-quantized penetration in many cases.

Analytically computing the flux linked by a switched reluctance motor phase when the stator and rotor poles overlap

Abstract: This paper develops an analytical model for the flux linked by a phase of a switched reluctance motor (SRM) starting with the basic laws for magnetic fields. Thus this model is not empirical nor does it require any data from the machine being modeled or any finite-element analysis results. The model is applicable to rotor positions where the stator and rotor poles overlap, and it includes iron saturation. This paper gives a detailed development of the analytical model including the approximations that are inherent to it and the justification for them. The analytical model for the flux linked by a phase is used to find the analytical equations of motion for the SRM. It is also used to obtain an analytical expression for the SRM's coenergy. Differentiating the coenergy with respect to rotor position, holding the current constant gives an analytical expression for the torque. Finally, this paper compares the analytical model's predictions to measured data from experimental machines.

Flux-pinning-induced stress and magnetostriction in bulk superconductors

Abstract: The development of bulk high-temperature superconductors (HTSs) and their applications has today come to a point where the mechanical response to high magnetic fields may be more important than their critical-current density and large-grain property. Reviewed in this article are the recent studies of the magneto-elastic effects which are caused by flux pinning in the superconductors. This includes the work on the giant irreversible magnetostriction and internal stress, which often cause fatal cracking of the HTS bulks as they become magnetized. The cracking is a problem that today accompanies the quest for the highest trapped field values, and the latest development in this area is also presented. While the first part is an overview of experimental efforts, the second summarizes the work done to model the pinning-induced stress and strain under various magnetic and geometrical conditions.

An Evolving Synoptic Magnetic Flux map and Implications for the Distribution of Photospheric Magnetic Flux

Abstract: We describe a procedure intended to produce accurate daily estimates of the magnetic flux distribution on the entire solar surface Models of differential rotation, meridional flow, supergranulation, and the random emergence of background flux elements are used to regularly update unobserved or poorly observed portions of an initial traditional magnetic synoptic map that acts as a seed Fresh observations replace model estimates when available Application of these surface magnetic transport models gives us new insight into the distribution and evolution of magnetic flux on the Sun, especially at the poles where canopy effects, limited spatial resolution, and foreshortening result in poor measurements We find that meridional circulation has a considerable effect on the distribution of polar magnetic fields We present a modeled polar field distribution as well as time series of the difference between the northern and southern polar magnetic flux; this flux imbalance is related to the heliospheric current sheet tilt We also estimate that the amount of new background magnetic flux needed to sustain the `quiet-Sun' magnetic field is about 11×1023 Mx d−1 (equivalent to several large active regions) at the spatial resolution and epoch of our maps We comment on the diffusive properties of supergranules, ephemeral regions, and intranetwork flux The maps are available on the NSO World Wide Web page

Salient-rotor PM synchronous motors for an extended flux-weakening operation range

Abstract: This paper proposes some novel designs of permanent-magnet synchronous motors for extending their flux-weakening capabilities. According to such a proposal, the motors are characterized by a saliency ratio l/sub d//l/sub q/ greater than one, whereas classical designs exhibit a saliency ratio lower than one. The innovative and classical design solutions are deeply compared in the paper. In order to understand better the basic differences in the motor design, the same base and flux-weakening performance are fixed. The influence of the saliency ratio on inverter voltampere ratings, maximum feasible speed, and current control characteristics is highlighted and discussed.

Pulsed magnetization of HTS bulk parts at T<77 K

Abstract: Various pulsed magnetization experiments employing peak fields of up to 2 T and pulse durations of 30 ms and 3 ms were carried out on YBCO samples at temperatures between 20 and 80 K. Trapped magnetic flux profiles were recorded. The highest remanent magnetizations were obtained for a multi-pulse technique with step-wise cooling. The shape and the absolute values of the trapped flux profile are discussed in terms of the dynamics of such pulsed magnetization processes.

Not-plate-like Hall magnetic sensors and their applications

Abstract: This paper is a survey of a new class of Hall devices which are not plate-like but have three-dimensional structures. The survey covers: the vertical Hall device, sensitive to a magnetic field parallel with the chip surface; the cylindrical Hall device, which behaves as a vertical Hall device combined with magnetic flux concentrators; the two-axis vertical Hall device, for the two in-chip-plane components of a magnetic field; and the three-axis Hall device, to measure all three components of a magnetic field. All these Hall magnetic sensors are fabricated using the “vertical Hall” silicon process and feature low supply current, low noise, and high long-term stability. Their applications include: high-accuracy magnetometry, long-range position sensing, angular position sensing, current sensing, magnetic scanning of documents, and more.

The Counterkink Rotation of a Non-Hale Active Region

Abstract: We describe the long-term evolution of a bipolar non-Hale active region that was observed from 1995 October to 1996 January. During these four solar rotations the sunspots and subsequent flux concentrations, during the decay phase of the region, were observed to move in such a way that by December their orientation conformed to the Hale-Nicholson polarity law. The sigmoidal shape of the observed soft X-ray coronal loops allows us to determine the sense of the twist in the magnetic configuration. This sense is confirmed by extrapolating the observed photospheric magnetic field, using a linear force-free approach, and comparing the shape of computed field lines with the observed coronal loops. This sense of twist agrees with that of the dominant helicity in the solar hemisphere where the region lies, as well as with the evolution observed in the longitudinal magnetogram during the first rotation. At first sight the relative motions of the spots may be misinterpreted as the rising of an Ω loop deformed by a kink instability, but we deduce from the sense of their relative displacements a handedness for the flux-tube axis (writhe) that is opposite to that of the twist in the coronal loops and, therefore, to what is expected for a kink-unstable flux tube. After excluding the kink instability, we interpret our observations in terms of a magnetic flux tube deformed by external motions while rising through the convective zone. We compare our results with those of other related studies, and we discuss, in particular, whether the kink instability is relevant to explain the peculiar evolution of some active regions.

The Solar Dynamo and Emerging Flux – (Invited Review)

Abstract: The largest concentrations of magnetic flux on the Sun occur in active regions. In this paper, the properties of active regions are investigated in terms of the dynamics of magnetic flux tubes which emerge from the base of the solar convection zone, where the solar cycle dynamo is believed to operate, to the photosphere. Flux tube dynamics are computed using the ‘thin flux tube’ approximation, and by using MHD simulation. Simulations of active region emergence and evolution, when compared with the known observed properties of active regions, have yielded the following results: (1) The magnetic field at the base of the convection zone is confined to an approximately toroidal geometry with a field strength in the range (3-10) × 104 G. The latitude distribution of the toroidal field at the base of the convection zone is more or less mirrored by the observed active latitudes; there is not a large poleward drift of active regions as they emerge. The time scale for emergence of an active region from the base of the convection zone to the surface is typically 2-4 months. The equatorial gap in the distribution of active regions has two possible origins; if the toroidal field strength is close to IO5 G, it is due to the lack of equilibrium solutions at low latitude; if it is closer to 3 x IO4 G, it may be due to modest poleward drift during emergence. (2) The tilt of active regions is due primarily to the Coriolis force acting to twist the diverging flows of the rising flux loops. The dispersion in tilts is caused primarily by the buffeting of flux tubes by convective motions as they rise through the interior. (3) The Coriolis force also bends the active region flux tube shape toward the following (i.e., anti-rotational) direction, resulting in a steeper leg on the following side as compared to the leading side of an active region. When the active region emerges through the photosphere, this results in a more rapid separation of the leading spots away from the magnetic neutral line as compared to the following spots. This bending motion also results in the neutral line being closer to the following magnetic polarity. (4) Active regions behave kinematically after they emerge because of ‘dynamic disconnection’, which occurs because of the lack of a solution to the hydrostatic equilibrium equation once the flux loop has emerged. This could explain why active regions decay once they have emerged, and why the advection-diffusion description of active regions works well after emergence. Smaller flux tubes may undergo ‘flux tube explosion’, a similar process, and provide a source for the constant emergence of small-scale magnetic fields. (5) The slight trend of most active regions to have a negative magnetic twist in the northern hemisphere and positive twist in the south can be accounted for by the action of Coriolis forces on convective eddies, which ultimately writhes active region flux tubes to produce a magnetic twist of the correct sign and amplitude to explain the observations. (6) The properties of the strongly sheared, flare productive δ-spot active regions can be accounted for by the dynamics of highly twisted Ω loops that succumb to the helical kink instability as they emerge through the solar interior.

A new topology of hybrid synchronous machine

Abstract: Permanent-magnet synchronous machines are able to operate over a wide range of speeds at constant power through the use of control laws allowing for flux weakening. Generally, this is performed by applying a strong demagnetizing current in the d axis, yet such an approach involves the risk of irreversible magnet demagnetization and, consequently, a reduction in machine performance. This paper presents a novel structure for a hybrid synchronous machine. The authors' solution provides good flux weakening without introducing the risk of magnet demagnetization. In order to explain the structure's operating principle, they apply a model with a single elementary design; electromotive force measurements are then presented to demonstrate the possibilities of flux weakening, along with a series of simulations to show the contribution of hybrid excitation.

Aharonov-Bohm effect for an exciton

Abstract: We theoretically study exciton absorption on a ring threaded by a magnetic flux. For the case when the attraction between an electron and a hole is short ranged, we obtain an exact solution of the problem. We demonstrate that, despite the electrical neutrality of the exciton, both the spectral position of the exciton peak in the absorption and the corresponding oscillator strength oscillate with magnetic flux with a period ${\ensuremath{\Phi}}_{0}$---the universal flux quantum. The origin of the effect is the finite probability for electron and hole, created by a photon at the same point, to tunnel in the opposite directions and meet each other on the opposite side of the ring.

A stator flux oriented vector-controlled induction motor drive with space vector PWM and flux vector synthesis by neural networks

Abstract: A stator flux oriented vector-controlled induction motor drive is described where the space vector PWM (SVM) and stator flux vector estimation are implemented by artificial neural networks (ANN). Artificial neural networks, when implemented by dedicated hardware ASIC chips, provide extreme simplification and fast execution for control and feedback signal processing functions in high performance AC drives. In the proposed project, a feedforward ANN-based SVM, operating at 20 kHz sampling frequency, generates symmetrical PWM pulses in both undermodulation and overmodulation regions covering the range from DC (zero frequency) up to square-wave mode at 60 Hz. In addition, a programmable cascaded low-pass filter (PCLPF), that permits de offset-free stator flux vector synthesis at very low frequency using the voltage model, has been implemented by a hybrid neural network which consists of a recurrent neural network (RNN) and a feedforward neural network (FFANN). The RNN-FFANN based flux estimation is simple, permits faster implementation, and gives superior transient performance when compared with a standard DSP-based PCLPF. A 5-hp open loop volts/Hz controlled drive incorporating the proposed ANN-based SVM and RNN-FFANN based flux estimator was initially evaluated in the frequency range of 1.0 Hz to 58 Hz to validate the performances of SVM and flux estimator. Next, the complete 5 hp drive with stator flux oriented vector control was evaluated extensively using the PWM modulator and flux estimator. The drive performances in both volts/Hz control and vector control were found to be excellent.

Understanding the evolution of the Sun's open magnetic flux

Abstract: The large-scale magnetic field of the Sun, including the open flux that extends into the interplanetary medium, originates in active regions but is redistributed over the photosphere by differential rotation, supergranular convection, and poleward meridional flow We use simulations to clarify the role of the surface transport processes in the evolution of the total open flux, Φopen, which determines the strength of the radial interplanetary field component Representing the initial photospheric field configuration by one or more bipolar magnetic regions (BMRs), we show that Φopen varies approximately as the net dipole strength, determined by vectorially summing the dipole moments of the individual BMRs As meridional flow carries the BMR flux to higher latitudes, the equatorial dipole component is annihilated on a timescale ∼1 yr by the combined effect of rotational shearing and supergranular diffusion The remaining flux becomes concentrated around the poles, and Φopen approaches a limiting value that depends on the axisymmetric dipole strength of the original active regions We discuss the implications of these results for the solar cycle evolution of Φopen

Drive mechanisms of erupting solar magnetic flux ropes

Abstract: The dynamics of magnetic flux ropes near the Sun and in interplanetary space are studied using a magnetohydrodynamic model of erupting magnetic flux ropes. In this model, the magnetic structure of a coronal mass ejection (CME) corresponds to a flux rope with footpoints that remain anchored below the photosphere. The model flux rope eruption can be driven by a rapid increase in poloidal flux (flux injection), a quasi-static increase in poloidal flux (photospheric footpoint twisting), a rapid release of stored magnetic energy (magnetic energy release), or a rapid increase in the amount of hot plasma within the flux rope (hot plasma injection). Model results are compared with Large-Angle Spectrometric Coronagraph data (from the CME of 1997 April 13) and with interplanetary magnetic cloud data over the range 0.4-5 AU. Of these mechanisms, only flux injection and magnetic energy release reproduce key features of the data both near the Sun and in the interplanetary medium and only flux injection obtains a detailed match to the near-Sun dynamics.

Cusp field-aligned currents and ion outflows

Abstract: On September 24 and 25, 1998, the Polar spacecraft observed intense outflows of terrestrial ions in association with the passage of an interplanetary shock and coronal mass ejection. The orbit of the Fast Auroral Snapshot (FAST) Explorer was in the noon-midnight meridian during this ion outflow event, and FAST passed through the day side cusp region at ∼4000 km altitude every 2.2 hours. FAST was therefore able to monitor the ion outflows subsequently observed by Polar. We show that while the outflows were more intense after the shock passage, the overall particle and field signatures within the cusp region were qualitatively similar both before and after the shock passage. FAST observations show that the cusp particle precipitation marks the lower latitude leg of a pair of field-aligned currents and further, that both field-aligned current sheets appear to be on open field lines. Moreover, the polarity of the cusp currents is controlled by the polarity of the interplanetary magnetic field (IMF) y-component, such that the magnetic field perturbation associated with the pair of cusp currents is in the same direction as the IMF By. This is a consequence of the reconnection of cusp-region field lines at the magnetopause, with the flux transport resulting in electromagnetic energy being transmitted along field lines to the ionosphere as Poynting flux. We show that this Poynting flux can be as high as 120 mW m−2 (120 ergs cm−2 s−1) at FAST altitudes (∼500 mW m−2 at ionospheric altitudes), presumably because of the strong IMF By (∼40 nT), and is the dominant energy input to the cusp-region ionosphere. Furthermore, we find that the peak ion outflow flux is correlated with the peak downward Poynting flux, although only a few passes through the cusp centered around the time of the shock passage were used to determine this correlation. The energy carried by Poynting flux is dissipated as heat within the ionosphere, through Joule dissipation. The heating will tend to increase the ionospheric scale height, allowing greater access of ionospheric ions to the altitudes where transverse ion heating via ELF waves can occur. Thus electromagnetic energy supplied by the transport of reconnected magnetic flux is the essential first step in a multistep process that enhances the outflow of ionospheric plasma in the dayside cusp.

Overcritical states of a superconductor strip in a magnetic environment

Abstract: A current-carrying superconducting strip partly penetrated by magnetic flux and surrounded by a bulk magnet of high permeability is considered. Two types of samples are studied: those with critical current controlled by an edge barrier dominating over the pinning, and those with high pinning-mediated critical current masking the edge barrier. It is shown for both cases that the current distribution in a central flux-free part of the strip is strongly affected by the actual shape of the magnetic surroundings. Explicit analytical solutions for the sheet current and self-field distributions are obtained which show that, depending on the geometry, the effect may suppress the total loss-free transport current of the strip or enhance it by orders of magnitude. The effect depends strongly on the shape of the magnet and its distance to the superconductor but only weakly on the magnetic permeability.

Reconnection X‐winds: spin‐down of low‐mass protostars

Abstract: We investigate the interaction of a protostellar magnetosphere with a large-scale magnetic field threading the surrounding accretion disc. It is assumed that a stellar dynamo generates a dipolar-type field with its magnetic moment aligned with the disc magnetic field. This leads to a magnetic neutral line at the disc mid-plane and gives rise to magnetic reconnection, converting closed protostellar magnetic flux into open field lines. These are simultaneously loaded with disc material, which is then ejected in a powerful wind. This process efficiently brakes down the protostar to 10–20 per cent of the break-up velocity during the embedded phase.

A method for magnetizing curve identification in rotor flux oriented induction machines

Abstract: Operation of an indirect rotor flux oriented induction machine in the field weakening region is usually realized by varying the rotor flux reference in inverse proportion to the speed of rotation. In order to provide the correct stator d-axis current reference at all speeds, it is necessary to incorporate the inverse magnetizing curve of the machine in the controller. The paper proposes an experimental method for identifying the inverse magnetizing curve, specifically developed for the type of vector controlled drives described. The method utilizes the same indirect vector controller and PWM inverter that are used in subsequent normal operation of the drive. It requires that the machine can run under no-load conditions and that the fundamental component of the stator voltage can be measured. The simplicity and accuracy of the method make it well suited for use during commissioning of the drive. The method is verified by extensive experimentation.