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Plasma Science, IEEE Transactions on

Issue 6 • Date Dec. 2000

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Displaying Results 1 - 25 of 51
  • Guest editorial space weather

    Page(s): 1783 - 1785
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    Freely Available from IEEE
  • Forecasting Earth's magnetopause, magnetosheath, and bow shock

    Page(s): 1966 - 1975
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    The magnetopause and bow shock are two of the most important discontinuities in near Earth space, separating three distinct plasma regimes: the solar wind, magnetosheath, and the magnetosphere. Both discontinuities are sensitive to the solar wind conditions. They change their shape and location in space in response to upstream solar wind variations. Prediction of the location of a satellite relative to these two boundaries helps satellite operators to be prepared for major changes in plasma condition. The physical conditions near the magnetopause are useful information for magnetospheric models. Over the past decades, our observational knowledge and physical understanding of these regions have been advanced significantly. We have developed the capability of prediction. The location of the magnetopause and its dependence on the upstream conditions can be relatively accurately predicted. Prediction models have been tested extensively. The success rate is extremely high and the false-alarm rate is relatively low. A magnetosheath prediction model has been developed and tested. Some of the magnetosheath quantities can often be predicted fairly accurately. Others may have slightly larger errors. Further tests and scientific investigations of the causes of these errors are under way. Our ability to predict the location of the bow shock is very limited. More research is needed View full abstract»

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  • Observation of the CIV effect in interstellar clouds: a speculation on the physical mechanism for their existence

    Page(s): 2122 - 2127
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    Observations of neutral hydrogen (H I) emission profiles produced by gas in the local interstellar medium are found to be characterized by four linewidth regimes. Dominant and pervasive features have widths on average of 5.2, 13, and 31 km/s, and a very broad component approximately 50 km/s wide. A striking coincidence exists between these linewidths and the magnitudes of the critical ionization velocities of the most abundant atomic species in interstellar space: 6 km/s for sodium and calcium; 13 km/s for carbon, oxygen, and nitrogen; 31 km/s for helium; and 51 km/s for hydrogen. The data relate to observations near neutral hydrogen structures that are filamentary View full abstract»

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  • Dynamic characteristics of a hollow copper electrode plasma torch through measurement and analysis of acoustic, optical, and voltage fluctuations

    Page(s): 2179 - 2186
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    The basic nature of arc root fluctuation in a plasma torch is an extremely important factor from the point of view of improving and optimizing performance of a plasma torch in present-day technology. In spite of a number of ingenious attempts, this particular phenomenon is still not fully explored. In this paper, voltage, acoustic, and optical signals generated from a hollow cathode plasma torch are analyzed using various tools of dynamical analysis such as real-time behavior, phase portraits, power spectra, Lyapunov exponent, dimension, etc. Origin of each of the signal in relation to arc-root fluctuation and mutual correspondence among themselves are described in detail. For the first time, all the signals are found to exhibit clear evidence of chaotic behavior in all respect. A dimensional analysis reveals all the three signals to be originated from the same chaotic phenomenon, i.e., fluctuation of are root View full abstract»

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  • Design of a W-band second-harmonic TE02 gyro-TWT amplifier

    Page(s): 2232 - 2237
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    A harmonic gyrotron traveling-wave tube (gyro-TWT) amplifier is described that can stably deliver high peak and average power in the low-loss TE02-mode at 91.4 GHz. The single-stage second-harmonic TE02 gyro-TWT is predicted to produce a peak power of 600 kW with an efficiency of 24%, a saturated gain of 30 dB, and a 3-dB bandwidth of 2.7%. The amplifier employs a 100-kV, 25-A electron beam emitted from a magnetron injection gun with v/v||=1.2 and Δv||/v|| =8%. The device is based on the proven concept that the electron beam current can be much higher in a stable harmonic gyro-TWT amplifier than at the fundamental due to the relatively weaker strength of the harmonic interaction. The TE02 overmoded interaction waveguide is sufficiently large to handle an average power of 60 kW and provides considerable clearance for the high current electron beam. An innovative mode-selective interaction circuit prevents the amplifier from oscillating in undesired modes View full abstract»

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  • High-power microwave generation from an axially extracted virtual cathode oscillator

    Page(s): 2128 - 2134
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    An axially extracted virtual cathode oscillator has been fabricated. This ultra-high-power microwave generator is powered by the “Chundoong” intense relativistic electron beam pulser (Max: 600 kV, 88 kA, and 60 ns duration). In this experiment, a solid electron beam with a peak diode current of 21 kA is obtained from the pulser operated at a peak diode voltage of 300 kV. It is found in this experiment that the peak of microwave emission occurred at the moment of the second peak in electron beam current, where the diode voltage Vd and diode current Id are Vd=290 kV and Id=21 kA, respectively. This observation might be strongly correlated to the inherent oscillations of the electron cloud in time and space originated from both the dynamics of the reflected electron beam due to virtual cathode and oscillations of the virtual cathode itself, from which the peak of high-power microwave can be generated in this experiment. The major operating output frequencies have been measured to be 6.68 GHz⩽f⩽7.19 GHz based on the waveguide dispersive delayed line method, which is in good agreement with that from “MAGIC” computational results for an electron beam voltage of 290 kV and a diode current of 21 kA. The maximum microwave output power from the virtual cathode oscillator has been measured to be 200 MW in this experiment. while its power conversion efficiency is approximately 3.3%. The dominant emission mode from the virtual cathode oscillator is shown to be the TM01 mode based on the emission pattern of microwave induced air breakdown in this experiment View full abstract»

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  • Optical emission reference data for the GEC reference cell

    Page(s): 2187 - 2193
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    A complete set of optical emission data for the gaseous electronic conference reference cell (GEC) is presented for discharges at 75, 100, 150, and 200 V peak-to-peak and at pressures of 100, 250, 500, and 1000 mtorr. When the GEC project was initiated, these were the setpoints that were to be examined. This paper provides a set of data that can be used as a comparison for other diagnostics. The emission is compared to metastable data obtained from laser-induced fluorescence in helium and argon and computer generated data of the argon metastable data. There is good agreement between these three sets of data and the profiles found in this paper, which can be used as a consistency check for the capacitively coupled GEC reference cell. The data presented were collected with an automated scanning sensor, which gathers wedges of optical emission (Ar I 750.4 nm), at ten vertical heights parallel to the bottom electrode surface. The digitized data was converted to emissivity data as a function of radius using a Tikhonov regularization method View full abstract»

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  • A computational investigation of the effects of varying discharge geometry for an inductively coupled plasma

    Page(s): 2214 - 2231
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    In this paper, a numerical investigation of ionization and density profiles in a low pressure inductively coupled plasma (ICP) discharge is presented. ICP reactors have in recent years become a popular choice for semiconductor processing. To ensure high yields, it is important to optimize process uniformity. Off-axis ionization can result in the hollowing of the density profile of the plasma. Previous numerical studies [Keiter (1996), Beale (1995), Mumken (1999), Kortshagen (1999)] have focused on electron kinetics and the resulting ionization rates as the main factor in producing nonuniform ion density profiles. Here we extend some of the previous work to also examine the effects of the ion transport in detail. Two models are used for the study. The first model is a hybrid model that approximates electron transport by solving the Boltzmann transport equation (BTE) using a spatially dependent two-term spherical harmonic expansion (SHE), and neutral and ion transport with a simple fluid model. The second model is a new kinetic model for ion transport in complex geometry based on the convected scheme (CS). This model uses an “unstructured mesh” and “Long Lived Moving Cells”. By using the CS model for ion transport, it is possible to investigate the limitations of the fluid model and issues pertinent to process uniformity which are not accessible to the fluid approximation. As the ratio between ion mean-free path and system size is often looked to as the key parameter for determining the applicability of a fluid model, the two parameters we have varied in this study are the reactor chamber height and the neutral temperature. Ion density profiles produced by the fluid model are qualitatively similar to those of the kinetic ion model, although the kinetic model tends to produce more uniform ion density profiles View full abstract»

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  • Ion mean charge state in a biased vacuum arc plasma duct

    Page(s): 2194 - 2201
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    Vacuum arc or cathodic arc metal plasma sources are attractive and convenient for depositing high-quality thin metal films and metallurgical coatings. It is a common practice to use a curved magnetic filter duct to eliminate macroparticle contamination and to bias the duct wall with a positive voltage to enhance the throughput of the metal plasma. The metal plasma usually consists of several charge states and time-of-flight (TOF) experiments show that the mean charge state of the metal ions decreases with increasing bias and magnetic field applied to the filter duct. We also derive the throughput of ions with different charge states at different bias voltage and magnetic field by the particle-in-cell (PIC) method. The ion trajectory is simulated neglecting the influence of electron charge. Our results show that the simulated mean charge state displays a similar decreasing trend as the bias voltage and magnetic field strength are increased. Phenomena such as reduction of the mean charge state at high magnetic field strength and bias can be explained in terms of standalone multiply charged metal ions under the influences of magnetic and electric forces inside the filter duct View full abstract»

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  • Penetrating electron fluctuations associated with GEO spacecraft anomalies

    Page(s): 2075 - 2084
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    Space weather is a known factor in spacecraft anomalies. Solar minimum carries with it an enhanced electron content. Electrons with sufficient energy to penetrate a spacecraft structure pose a hazard. They can accumulate in interior dielectrics creating an electric potential sufficient to cause a spontaneous breakdown. The resulting electrostatic discharge has been a cause of operational anomalies. The physical process by which the geosynchronous earth orbit (GEO) environment is populated by high-energy electrons is not fully understood. However, the solar-cycle, seasonal, and solar-rotational patterns observed are well documented. This paper reviews temporal fluence patterns and shows how some notorious satellite failures relate to them. It should be noted, however the temporal response of a discharge is not necessarily at the instantaneous peak flux, but related to total local fluence and how the dielectric responds to stress. Knowledge of the changes in mechanical and electrical properties of dielectrics in the space environment and a growing understanding of the variability of the electron population at GEO will enable the resolution of anomalies anti facilitate their prevention View full abstract»

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  • Comparison of spacecraft charging environments at the Earth, Jupiter, and Saturn

    Page(s): 2048 - 2057
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    Studies of the Earth with the ATS-5, ATS-6, and SCATHA spacecraft led to the development of several simple tools for predicting the potentials to be expected on a spacecraft in the space environment. These tools have been used to estimate the expected levels of worst case charging at Jupiter and Saturn for the Galileo and the Cassini spacecraft missions. This paper reviews those results and puts them in the context of the design issues addressed by each mission including the spacecraft design mitigation strategies adopted to limit differential charging. The model shows that shadowed surfaces in Earth orbit can reach ~25 kV or higher in worst case environments. For Galileo, spacecraft-to-space potentials of ~900 V were predicted in shadow. Since such potentials could produce possible discharges and could effect low energy plasma measurements, the outer surface of Galileo was designed to rigid conductivity requirements. Even though the surface of Galileo is not entirely conducting, after 27 orbits no adverse effects due to surface charging aside from limited effects on low energy plasma measurements have been reported. The Saturnian environment results in spacecraft potentials to space in shadow of ~100 V or less. Although the overall surface of the Cassini spacecraft was not entirely conducting and grounded, it is shown that only in the most extreme conditions, is it expected that Cassini will experience any effects of surface charging at Saturn View full abstract»

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  • Symmetric and asymmetric mode interaction in high-power traveling wave amplifiers: experiments and theory

    Page(s): 2262 - 2271
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    High-power microwave amplifier operation has been studied for use in a number of applications. The performance of the amplifiers has been marred, in some cases, by pulse shortening of the microwave signal. A possible source of the shortening is the loss of the beam due to hybrid HEM11 mode interaction with the beam. In this paper, we describe experiments which investigate high-power operation and the effects of HEM modes on the amplifier performance. We report the high-output powers (>50 MW) with efficient (>54%) amplification of microwaves in an X-band traveling wave amplifier. In some experiments, peak power levels exceeding 120 MW were measured at an efficiency of 47%. The excitation of the asymmetric hybrid electromagnetic mode was monitored carefully, but does not seem to have a critical impact on the main interaction process in spite of the fact that its dispersion curve almost overlaps that of the symmetric interacting mode. Theoretical analysis of the interaction in a tapered traveling wave structure indicates that, even if the amount of power in the asymmetric modes at the input of the structure is comparable to that in the symmetric mode, the asymmetric modes cause no power reduction in the symmetric mode. For the case of off-axis beams the TM01 output power may drop by about 30% and the power in the hybrid mode reach about one third of that in the symmetric mode. In order to avoid hybrid mode excitation it is necessary to suppress the reflections from both ends of the output structure several decibels below the gain level of the asymmetric mode View full abstract»

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  • Global particle simulation for a space weather model: present and future

    Page(s): 1991 - 2006
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    The authors report progress in the long-term effort to represent the interaction of the solar wind with the Earth's magnetosphere using a three-dimensional electromagnetic particle model (EMPM) as a space weather model. Magnetohydrodynamic (MHD) simulation models have been refined to establish quantitative global modeling in comparison with observations. The EMPM has become more feasible as the power and speed of supercomputers have improved in recent years. Simulations with southward and dawnward turning IMFs have revealed the fundamental processes which have been confirmed by MHD simulations and observations. After a quasisteady state is established with an unmagnetized solar wind, a southward IMF is switched on, which causes the magnetosphere to stretch with reconnection at the dayside magnetopause. The plasma sheet in the near-Earth magnetotail clearly thins. The cross-field current also thins and intensifies, which excites a kinetic (drift kink) instability along the dawn-dusk direction. As a result of this instability the electron compressibility effect appears to be reduced and to allow the collisionless tearing to grow rapidly with the reduced Bz component. Later, magnetic reconnection also takes place in the near-Earth magnetotail. In the case where the northward IMF is switched gradually to dawnward, magnetic reconnection takes place at both the dawnside and duskside. The arrival of dawnward IMF at the magnetopause creates a reconnection groove which causes particle entry into the deep region of the magnetosphere via field lines that go near the magnetopause. The flank weak-field region joins onto the plasma sheet and the current sheet to form a geometrical feature called the cross-tail S that structurally integrates the magnetopause and the tail interior View full abstract»

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  • Energy enhancement in the two-stage autoacceleration scheme

    Page(s): 2275 - 2277
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    It has been shown experimentally that in the two-stage autoacceleration scheme the acceleration voltage at the second-stage can be increased by using two cavities with the same length as the second-stage instead of using a single cavity View full abstract»

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  • Dynamical evolution of coherent structures in intermittent two-dimensional MHD turbulence

    Page(s): 1938 - 1943
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    Recent satellite observations indicate that the Earth's magnetotail is generally in a state of intermittent turbulence. A model of sporadic localized merging of coherent structures has recently been proposed by Chang to describe the dynamics of the Earth's magnetotail. The authors report the results of MHD simulations regarding the development and merging of two dimensional coherent structures. With a magnetic shear, such coherent structures are generated in alignment with the imposed current sheet. The calculated fluctuation spectra suggest long-ranged correlations with power-law characteristics View full abstract»

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  • A determination of the electron distribution function near absorbing surfaces

    Page(s): 2202 - 2206
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    A precise description of the electron distribution function near absorbing surfaces has been obtained based on the solution of the inhomogeneous electron Boltzmann equation. The absorbing surface creates a large gradient of the electron distribution function that consequently leads to an axial variation of the electron density and other electron transport parameters. The axial variation of the electron distribution function is investigated and compared for rare gases and nitrogen and the impact of the higher order terms in Legendre polynomial expansion is discussed View full abstract»

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  • Cross-current instability generated by pick-up ions in the environment of a large spacecraft in low Earth orbit

    Page(s): 2085 - 2096
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    Measurements from the space shuttle flights have revealed that a large spacecraft in a low Earth orbit is accompanied by an extensive gas cloud, which is primarily made up of water. The charge exchange between the water molecule and the ionospheric O+ ions produces a water ion beam traversing downstream of the spacecraft. The authors present results from a study on the linear and nonlinear behaviors of the waves generated by the interaction of the water ion beams with the ionospheric plasma. Since velocity distribution function is key to the understanding of the wave generation process, they have performed a test particle simulation to determine the nature of H2O+ ions velocity distribution function. The simulations show that at, the time scales shorter than the ion-cyclotron period τc, the distribution function can be described by a beam. On the other hand, when the time scales are larger than τ c, a ring distribution forms. A brief description of the linear instabilities driven by an ion beam streaming across a magnetic field in a plasma is presented. In order to study the nonlinear evolution of the waves, the author has performed 2.5-dimensional particle-in-cell (PIC) simulations which show that transient amplitudes of the waves can be up to a few Volts/m at the lower-hybrid frequency. The frequency spectrum of the waves from the simulation shows that the water ion beam produces a broadband electric field noise extending in frequency up to a few times the ion plasma frequency. The simulations also reveal an enhanced electric field noise level near the electron cyclotron harmonics View full abstract»

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  • The Mott transition as a cause of anomalies on spacecraft

    Page(s): 2097 - 2102
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    In the Mott transition, an insulator with a critically high donor density undergoes a sudden transition to become a conductor. On the other hand, it is known that sufficiently high electric fields applied to dielectrics at ordinary charge densities can cause breakdown. The author conjectures that the critical donor density can be lowered by applying high electric fields to the insulators. Using a simple model encompassing the concepts of Debye, Poole, and Frenkel, the author has obtained a continuous curve connecting the two critical points, viz., critical density and critical field. The motivation of this work is spacecraft anomalies. They briefly review the circumstances of the most notable spacecraft failures in recent years. Although the true causes of the failures are probably system design specific and will probably never be known, it is likely that the failures were due to breakdowns or discharges in semiconductors or dielectrics. Irradiated dielectrics and semiconductors have defects and dangling bonds playing the role of donors. High electric fields are built by electrons deposited inside the dielectrics. With the use of the curve obtained, it is concluded that the insulator-conductor transition can occur at lower donor densities and lower electric fields than previously thought View full abstract»

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  • Space weather, Earth's neutral upper atmosphere (thermosphere), and spacecraft orbital lifetime/dynamics

    Page(s): 1920 - 1930
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    Accurate prediction of a spacecraft/satellite orbital lifetime, insertion altitude, reboost requirements, and mission performance is mainly the result of the integrated effect from knowledge of the atmospheric density, space weather (solar/geomagnetic), and timeline of vehicle characteristics. Each of these elements is dependent upon a model developed to provide the inputs necessary for the use of an orbital lifetime prediction program. This paper will address relative influences of these elements with emphasis on solar/geomagnetic activity, atmospheric density, and drag coefficient associated with the model products used to predict orbital lifetime and related spacecraft/satellite design and operational conditions. Issues associated with the potential for improvement of the lifetime prediction model input elements will be discussed with regard to their relative contributions to improving orbital lifetime and performance predictions View full abstract»

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  • The solar wind interaction with the Earth's magnetosphere: a tutorial

    Page(s): 1818 - 1830
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    The size of the terrestrial magnetosphere is determined by the balance between the solar wind dynamic pressure and the pressure exerted by the magnetosphere, principally that of its magnetic field. The shape of the magnetosphere is additionally influenced by the drag of the solar wind, or tangential stress, on the magnetosphere. This drag is predominantly caused by the mechanism known as reconnection in which the magnetic field of the solar wind links with the magnetic field of the magnetosphere. The factors that control the rate of reconnection of the two fields are not understood completely, but a southward direction of the interplanetary field is critical to enabling reconnection with the dayside low-latitude magnetosphere, resulting in magnetic flux transfer to the magnetotail. Numerical simulations suggest that the conductivity of the ionosphere controls the rate of reconnection, but this has not been verified observationally. Although solar wind properties ultimately control the interaction, the properties of the plasma that make direct contact with the magnetosphere are different than those of the solar wind, having been altered by a standing bow shock wave. This standing shock is necessitated by the fact that the flow velocity of the solar wind far exceeds the velocity of the compressional wave that diverts the solar wind around the Earth. The upper atmosphere is the final recipient of all the energy and momentum that enters the magnetosphere. Coupling takes place along the magnetic field Lines principally in the polar and auroral region via current systems that close across the magnetic field both at low and high altitudes and flow parallel to the magnetic field between high and low altitudes View full abstract»

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  • An adaptive MHD method for global space weather simulations

    Page(s): 1956 - 1965
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    A 3D parallel adaptive mesh refinement (AMR) scheme is described for solving the partial-differential equations governing ideal magnetohydrodynamic (MHD) flows. This new algorithm adopts a cell-centered upwind finite-volume discretization procedure and uses limited solution reconstruction, approximate Riemann solvers, and explicit multi-stage time stepping to solve the MHD equations in divergence form, providing a combination of high solution accuracy and computational robustness across a large range in the plasma β (β is the ratio of thermal and magnetic pressures). The data structure naturally lends itself to domain decomposition, thereby enabling efficient and scalable implementations on massively parallel supercomputers. Numerical results for MHD simulations of magnetospheric plasma flows are described to demonstrate the validity and capabilities of the approach for space weather applications View full abstract»

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  • Satellite observations of electric fields in the inner magnetosphere and their effects in the mid-to-low latitude ionosphere

    Page(s): 1903 - 1911
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    During geomagnetic disturbances, momentum and energy are transferred in significant quantities from interplanetary space to the magnetosphere-ionosphere system through the mediation of charged particles and electric fields. The most dramatic manifestations occur in the plasma sheet and the conjugate auroral ionosphere. However, electric fields observed during magnetic storms also penetrate the inner magnetosphere that maps to subauroral latitudes in the ionosphere. For example, a sudden commencement shock wave initiating the March 1991 magnetic storm created a new radiation belt within minutes. Particle and field measurements by Combined Release and Radiation Effects Satellite (CRRES) near the equatorial plane of the magnetosphere and by Defense Meteorological Satellite Program (DMSP) satellites in the topside ionosphere during the magnetic storm of June 1991 indicate that penetration electric fields energized the stormtime ring current and rapidly transported plasma within subauroral ion drift (SAID) structures at midlatitudes and in upward drafting plasma bubbles at low latitudes. Through enhanced transport or chemical reactions, the SAIDs dug deep plasma troughs at topside altitudes. Equatorial plasma bubbles developed while the ring current was unable to shield the electric field from the innermost magnetosphere View full abstract»

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  • Interferometric investigation of a cablegun plasma injector

    Page(s): 2272 - 2275
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    A two-beam interferometer operating in heterodyne mode was used to determine the velocity and electron number density of a plasma ejected from a cablegun identical to those in the plasma opening switch (POS) of the DECEDE pulsed power facility. Time-resolved data were processed via digital phase demodulation techniques to measure electron number density at the two beam locations. Cross-correlation analysis of the two signals provided objective estimates of the effective plasma velocity. Experiments were carried out in unobstructed vacuum and near a flat ungrounded conducting plate to examine the plasma in vacuum and its buildup against the POS cathode View full abstract»

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  • Sigmoids as precursors of solar eruptions

    Page(s): 1786 - 1794
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    Coronal mass ejections (CMEs) appear to originate preferentially in regions of the Sun's corona that are sigmoidal, i.e., have sinuous S or reverse-S shapes. Yohkoh solar X-ray images have been studied before and after a modest number of Earth-directed (halo) CMEs. These images tend to show sigmoidal shapes before the eruptions and arcades, cusps, and transient coronal holes after. Using such structures as proxies, it has been shown that there is a relationship between sigmoidal shape and tendency to erupt. Regions in the Sun's corona appear sigmoidal because their magnetic fields are twisted. Some of this twist may originate deep inside the Sun. However, it is significantly modulated by the Coriolis force and turbulent convection as this flux buoys up through the Sun's convection zone. As the result of these phenomena, and perhaps subsequent magnetic reconnection, magnetic flux ropes form. These flux ropes manifest themselves as sigmoids in the corona. Although there are fundamental reasons to expect such flux ropes to be unstable, the physics is not as simple as might first appear, and there exist various explanations for instability. Many gaps need to be filled in before the relationship between sigmoids and CMEs is well enough understood to be a useful predictive tool View full abstract»

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  • Magnetic reconnection: three-dimensional aspects and onset in the magnetotail

    Page(s): 1887 - 1902
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    Magnetic reconnection is a fundamental plasma transport mechanism in space and laboratory plasmas. In recent years, much progress has been made in the understanding of the fundamentals of the reconnection process, including basic laws governing magnetic reconnection in three dimensions, the physical mechanisms providing the electric field in the dissipation region, and the onset of magnetic reconnection in the magnetotaiI of the Earth. This paper summarizes this progress and uses numerical simulations to illustrate the theoretical results in the framework of a solar and a magnetospheric example of magnetic reconnection View full abstract»

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IEEE Transactions on Plasma Sciences focuses on plasma science and engineering, including: magnetofluid dynamics and thermionics; plasma dynamics; gaseous electronics and arc technology.

 

 

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