Electrostatic Potential Structure Research Articles

Experiments on the Propagation of Plasma Filaments

We investigate experimentally the motion and structure of isolated plasma filaments propagating through neutral gas. Plasma filaments, or "blobs," arise from turbulent fluctuations in a range of plasmas. Our experimental geometry is toroidally symmetric, and the blobs expand to a larger major radius under the influence of a vertical electric field. The electric field, which is caused by nabla B and curvature drifts in a 1/R magnetic field, is limited by collisional damping on the neutral gas. The blob's electrostatic potential structure and the resulting E x B flow field give rise to a vortex pair and a mushroom shape, which are consistent with nonlinear plasma simulations. We observe experimentally this characteristic mushroom shape for the first time. We also find that the blob propagation velocity is inversely proportional to the neutral density and decreases with time as the blob cools.

Generation of “spiky” potential structures associated with multiharmonic electrostatic ion-cyclotron waves

The production of coherent, “spiky” electrostatic potential structures, similar to the spiky electric-field structures that have been observed in the Earth’s auroral region, is investigated in a magnetized Q machine plasma. These structures are associated with coherent multiharmonic electrostatic ion-cyclotron (EIC) waves in a current-free plasma having a localized region of ion flow shear (ion flow parallel to B with a gradient in the direction transverse to B). A multiharmonic EIC spectrum is detected in the region of ion flow shear when broadband electrostatic white noise is applied to the plasma column from an antenna. The time series of the potential fluctuations exhibit spiky structures with repetition rates near the ion-cyclotron frequency. Spiky structures can arise from linear combinations of phase-locked multiharmonic EIC waves when the amplitudes of the harmonic components are comparable to that of the fundamental EIC mode.

Cluster observations of an intense normal component of the electric field at a thin reconnecting current sheet in the tail and its role in the shock‐like acceleration of the ion fluid into the separatrix region

Measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region in the tail at distances of ∼18 RE near 22.4 MLT on 1 October 2001. This paper focuses on measurements of the large amplitude normal component of the electric field observed in the ion decoupling region near the reconnection x‐line, the structure of the associated potential drops across the current sheet, and the role of the electrostatic potential structure in the ballistic acceleration of ions across the current sheet. The thinnest current sheet observed during this interval was bifurcated into a pair of current sheets and the measured width of the individual current sheet was 60–100 km (3–5 c/ωpe). Coinciding with the pair of thin current sheets is a large‐amplitude (±60 mV/m) bipolar electric field structure directed normal to the current sheets toward the midplane of the plasma sheet. The potential drop between the outer boundary of the thin current sheet and the neutral sheet due to this electric field is 4–6 kV. This electric field structure produces a 4–6 kV electric potential well centered on the separatrix region. Measured H+ velocity space distributions obtained inside the current layers provide evidence that the H+ fluids from the northern and southern tail lobes are accelerated into the potential well, producing a pair of counterstreaming, monoenergetic H+ beams. These beams are directed within 20 degrees of the normal direction with energies of 4–6 keV. The data also suggest there is ballistic acceleration of O+ in a similar larger‐scale potential well of 10–30 kV spatially coinciding with the larger scale size (∼1000–3000 km) portions of current sheet surrounding the thin current sheet. Distribution functions show counterstreaming O+ populations with energies of ∼20 keV accelerated along the average normal direction within this large‐scale potential structure. The normal component of the electric field in the thin current sheet layer is large enough to drive an E × B drift of the electrons ∼10,000 km/s (0.25 x electron Alfven velocity), which can account for the magnitude of the cross‐tail current associated with the thin current sheet.

Externally driven magnetic reconnection in the presence of a normal magnetic field

Studies of tearing instabilities and magnetic reconnection in the terrestrial magnetotail must consider the effect of the finite normal field component Bz. Three‐dimensional (3‐D) particle‐in‐cell simulations with an open geometry are used to investigate the impact of an externally applied convection electric field on the stability of a moderately thick (1.6c/ωpi) current sheet configuration containing a nonzero Bz. The driving field is localized in the direction of the main magnetic field (x) but is uniform in the cross‐tail (y) direction. The initial response of the current sheet to the driving field is dominated by the formation of an embedded electron current layer on the scale of the electron inertial length. This layer forms nearly uniformly in y; cross‐tail modes do not play an important role. Once the normal field is driven southward in a local region of the sheet, global scale reconnection is initiated at a rate comparable to that seen in studies of simple 1‐D current sheets. The formation of the thin electron current layer is accompanied by the development of an electrostatic potential structure which serves to confine the ions in the thin electron layer and to deflect the ions into the outflow region. The reconnection process forms a high‐energy tail on the electron distribution extending up to ∼mec2. These high‐energy electrons are accelerated in the thin electron current sheet and then spread out along the separatrices.

Two‐dimensional particle simulation of electromagnetic field signature associated with electrostatic solitary waves

We present particle simulations of electrostatic solitary waves (ESW) observed by the Geotail spacecraft and recent spacecraft in the Earth's magnetosphere. Recent particle simulations have demonstrated that ESW correspond to Bernstein‐Greene‐Kruskal electron holes formed through nonlinear evolution of electron beam instabilities. Since an electron hole is a coherent electrostatic potential structure, electron beam instabilities were conventionally studied by electrostatic particle simulations. However, the Polar spacecraft and FAST spacecraft observed electromagnetic field signatures associated with ESW. To study interaction between coherent electrostatic potentials and electromagnetic waves, we extend the previous electrostatic particle model to an electromagnetic particle model. In the present two‐dimensional simulations of an electron beam instability, electromagnetic field components are enhanced around two‐dimensional electron holes. We found that the enhancement of electromagnetic fields is due to a current formed by electrons undergoing the E × B0 drift, where the electric field is a perpendicular electrostatic field at the edge of a two‐dimensional electron hole. An electromagnetic beam mode is excited by the current due to the drifting electrons moving with the electron hole. The amplitude ratio of the electric field to the magnetic field is estimated on the basis of the present simulation result, and it is in agreement with those of the Polar and FAST observations.

Formation of electrostatic double-layers and electron-holes in a low pressure mercury plasma column

Experimental studies of the formation of electrostatic double layers (DLs) and electron-holes (e-holes) are reported. The measurements were performed in the positive column of a mercury arc discharge operating in the low-pressure range of (2.0–14.0) × 10−2 Pa with current density in the range of (3.0–8.0) × 103 A m−2. Stable and unstable modes of the discharge were identified as the current was gradually increased, keeping constant the vapour pressure. The discharge remains stable until a critical current from which a slight increase of the current leads to an unstable regime characterized by high discharge impedance and strong oscillations. This mode ceased after a DL was formed in the plasma column. To induce the DL formation and to transport it smoothly along the discharge column, a low intensity B-field (7–10) × 10−3 T produced by a movable single coil was used. The B-field locally increases the electron current density and makes the DL form at the centre of the magnetic constriction where it remained at rest. Electrostatic potential structures compatible with ordinary DLs and multiple-layers could be formed in the plasma column by dealing with the combined effects of the operational parameters of the discharge. It is noticeable that a pure e-hole, which is a symmetric triple-layer having a bell shape potential profile, could easily be formed by means of this experimental technique. A partial kinetic description, based on the space charge structure derived from an experimental e-hole, is presented in order to infer the charged particle populations that could contribute to the space charge of the e-hole. Evidence is shown that strong e-hole formation might be driven by an ion beam, therefore it could not be formed in isolation since its formation requires a nearby ion accelerating potential structure. Probe measurements of the plasma properties, at various radial positions of the stable positive column, are also presented. In the stable mode, prior to current limitation, the probe data reveal a substantial radial decrease of the electron drift velocity. This result calls for a review of the free fall theories of low pressure plasma columns to take into account this non-uniformity of the electron drift velocity.

Electrostatic interaction between Interball-2 and the ambient plasma. 2. Influence on the low energy ion measurements with Hyperboloid

Abstract. The measurement of the thermal ion distributions in space is always strongly influenced by the ion motion through the complex 3D electrostatic potential structure built around a charged spacecraft. In this work, we study the related aberrations of the ion distribution detected on board, with special application to the case of the Hyperboloid instrument borne by the Interball-2 auroral satellite. Most of the time, the Interball-2 high altitude auroral satellite is charged at some non-negligible positive potential with respect to the ambient plasma, as shown in part 1; in consequence, the measurement of magnetospheric low energy ions (< 80 eV) with the Hyperboloid instrument can be disturbed by the complex electric potential environment of the satellite. In the case of positive charging, as in previous experiments, a negative bias is applied to the Hyperboloid structure in order to reduce this effect and to keep as much as possible the opportunity to detect very low energy ions. Then, the ions reaching the Hyperboloid entrance windows would have travelled across a continuous huge electrostatic lens involving various spatial scales from ~ 10 cm (detector radius) to ~ 10 m (satellite antennas). Neglecting space charge effects, we have computed the ion trajectories that are able to reach the Hyperboloid windows within their acceptance angles. There are three main results: (i) for given values of the satellite potential, and for each direction of arrival (each window), we deduced the related energy cutoff; (ii) we found that all ions in the energy channel, including the cutoff, can come from a large range of directions in the unperturbed plasma, especially when the solar panels or antennas act as electrostatic mirrors; (iii) for higher energy channels, the disturbances are reduced to small angular shifts. Biasing of the aperture is not very effective with the Hyperboloid instrument (as on previous missions with instruments installed close to the spacecraft body) because the potential environment is driven by effects from the spacecraft. Our results are used to explain some unexpected features of the low energy ion measurements, especially during polar wind events recorded by Hyperboloid. In conclusion, knowing the satellite potential from IESP measurements, we were able to reject any low energy doubtful data and to perform angular corrections for all higher energy ion data. Then the selected and corrected data are a reliable basis for interpretation and estimation of the thermal ion distributions.Key words. Space plasma physics (charged particle motion and acceleration; numerical simulation studies; spacecraft sheaths, wakes, charging)

Single-particle dynamics in collisionless magnetic reconnection.

The role of single-particle dynamics in driven magnetic reconnection in collisionless plasmas is investigated experimentally and analytically. The trapping of particle orbits in the magnetic cusp is observed to allow fast reconnection in the absence of a macroscopic current layer, at a rate identical to that of vacuum. The development of an electrostatic potential structure around the magnetic X line during reconnection is predicted theoretically and observed experimentally.

The Crystal Structures of K bm1 and K bm8 Reveal that Subtle Changes in the Peptide Environment Impact Thermostability and Alloreactivity

The K bm1 and K bm8 natural mutants of the murine MHC class I molecule H-2K b were originally identified by allograft rejection. They also bind viral peptides VSV8 and SEV9 with high affinity, but their peptide complexes have substantially decreased thermostability, and the K bm1 complexes do not elicit alloreactive T cell responses. Crystal structures of the four mutant complexes at 1.7–1.9 Å resolution are similar to the corresponding wild-type K b structures, except in the vicinity of the mutated residues, which alter the electrostatic potential, topology, hydrogen bonding, and local water structure of the peptide binding groove. Thus, these natural K b mutations define the minimal perturbations in the peptide environment that alter antigen presentation to T cells and abolish alloreactivity.

Electrostatic mode excitation in electron holes due to wave bounce resonances.

A kinetic theory of resonant interaction between electrostatic waves and the bounce motion of electrostatically trapped electrons is developed. Precise criteria are derived for the stability of electrostatic potential structures which trap electrons in a highly magnetized plasma. The theory explains the energy transfer from electron phase space holes to waves observed in simulations. It may also account for the destabilization of electrostatic waves propagating obliquely to the geomagnetic field and some characteristics of the holes as observed in the auroral ionosphere.

Effects of electron depletion on nonlinear dusty plasma oscillations

A kinetic analysis of the excitation of dust BGK (Bernstein–Greene–Kruskal) modes in a noncollisional dusty plasma is presented. It is found that in the case where all the electrons are attached to the dust particles, consequent modifications in the electrostatic potential structure can significantly influence the existence and properties of the nonlinear BGK modes. The derivation goes parallel to that done in standard kinetic analysis.

Characteristics of electrostatic solitary waves observed in the plasma sheet boundary: Statistical analyses

Abstract. We present the characteristics of the Electrostatic Solitary Waves (ESW) observed by the Geotail spacecraft in the plasma sheet boundary layer based on the statistical analyses. We also discuss the results referring to a model of ESW generation due to electron beams, which is proposed by computer simulations. In this generation model, the nonlinear evolution of Langmuir waves excited by electron bump-on-tail instabilities leads to formation of isolated electrostatic potential structures corresponding to "electron hole" in the phase space. The statistical analyses of the Geotail data, which we conducted under the assumption that polarity of ESW potentials is positive, show that most of ESW propagate in the same direction of electron beams, which are observed by the plasma instrument, simultaneously. Further, we also find that the ESW potential energy is much smaller than the background electron thermal energy and that the ESW potential widths are typically shorter than 60 times of local electron Debye length when we assume that the ESW potentials travel in the same velocity of electron beams. These results are very consistent with the ESW generation model that the nonlinear evolution of electron bump-on-tail instability leads to the formation of electron holes in the phase space.

Kinetic analysis of recycling plasma in an oblique magnetic field

A high-recycling divertor plasma is treated using a kinetic model to examine magnetic field effects on plasma flow. A plasma equation, which describes electrostatic potential structures in a recycling plasma, is derived in an integro-differential form similar to Poisson's equation. The potential drop in the presheath has a dependence on magnetic field parameters even if the scale length of the magnetic presheath is much smaller than the decay length of neutral density. As a result of formation of the magnetic presheath, secondary-electron emission coefficient is limited to a value much smaller than unity.

Gyrokinetic simulation of internal kink modes

Internal disruption in a tokamak has been simulated using a three-dimensional magneto-inductive gyrokinetic particle code. The code operates in both the standard gyrokinetic mode (total-f code) and the fully nonlinear characteristic mode (δf code). The latter is a quiet low noise algorithm. The computational model represents a straight tokamak with periodic boundary conditions in the toroidal direction and a square cross section with perfectly conducting walls in the poloidal direction. The linear mode structure of an unstable m=1 (poloidal) and n=1 (toroidal) kinetic internal kink mode is clearly observed, especially in the δf code. The width of the current layer around the x-point, where magnetic reconnection occurs, is found to be close to the collisionless electron skin depth, indicating the importance of electron inertia. Both codes give very similar nonlinear results, in which full reconnection in the Alfvén time scale is observed along with the electrostatic potential structures created during this phase. The resulting E×B drift from the potential dominates the nonlinear phase after the full reconnection.

Basis set dependence of the molecular electrostatic potential topography. A case study of substituted benzenes

The basis set dependence of the topographical structure of the molecular electrostatic potential (MESP), as well as the effect of substituents on the MESP distribution, has been investigated with substituted benzenes as test cases. The molecules are studied at HF-SCF 3–21G and 6–31G ∗∗ levels, with a further MESP topographical investigation at the 3–21G, double-zeta, 6–31G ∗, 6–31G ∗∗, double-zeta polarized and triple-zeta polarized levels. The MESP critical points for a 3–21G optimized/6–31G ∗∗ basis are similar to the corresponding 6–31G ∗∗ optimized/6–31G ∗∗ ones. More generally, the qualitative features of the MESP topography computed at the polarized level are independent of the level at which optimization is carried out. For a proper representation of oxygen lone pairs, however, optimization using a polarized basis set is required. The nature of the substituent drastically changes the MESP distribution over the phenyl ring. The values and positions of MESP minima indicate the most active site for electrophilic attack. This point is strengthened by a study of disubstituted benzenes.

A study on gamma-aminobutyric acid (GABA) and its analogues by using molecular orbital methods: on epileptogenicity of new quinolones.

Using the molecular orbital methods, we examined molecular structure, electron density distribution, electrostatic potential field and receptor structure of gamma-aminobutyric acid (GABA), and its analogues. The following findings were obtained: a comparison of the biological activity and the morphology electrostatic potentials of GABA analogues disclosed that the active site is the amino group, and the biological activity correlates closely with the electrostatic potential structure around the amino group. The active sites were compared between the receptor-binding molecules and the GABA uptake inhibitory molecules, and the results suggested that the receptor structure differed between the two groups of molecules and that the GABA A receptors had two subtypes. On these results, the epileptogenicity of new quinolones was studied using this method. These results suggested that the new quinolones blockaded the GABA receptor-binding system and that the important active site of the new quinolones for GABA receptor-binding was the the piperazyl amino group. These results suggested that the concentration of zwitterion type of the new quinolones was very important clinically.

Analytic model of aurorally coupled magnetospheric and ionospheric electrostatic potentials

This paper describes modest but significant improvements on earlier studies of electrostatic potential structure in the auroral region, using the adiabatic auroral arc model. This model has crucial nonlinearities (connected, for example, with aurorally produced ionization) which have hampered analysis; earlier work has either been linear, which I will show is a poor approximation or, if nonlinear, either numerical or too specialized to study parametric dependencies. With certain simplifying assumptions I find new analytic nonlinear solutions fully exhibiting the parametric dependence of potentials on magnetospheric (e.g., cross‐tail potential) and ionospheric (e.g., recombination rate) parameters. No purely phenomenological parameters are introduced. The results are in reasonable agreement with observed average auroral potential drops, inverted‐V scale sizes, and dissipation rates. The dissipation rate is quite comparable to tail energization and transport rates and should have a major effect on tail and magnetospheric dynamics. This paper gives various relations between the cross‐tail potential and auroral parameters (e.g., total parallel currents and potential drops) which can be studied with existing data sets.

A search for parallel electric fields by observing secondary electrons and photoelectrons in the low‐altitude auroral zone

The high sensitivities of the auroral secondary electron and atmospheric photoelectron spectra (<100 eV) to weak parallel dc electric fields make the particle distributions ideal for locating the lower boundary to the auroral electrostatic potential structures, which are often regarded as providing the mechanism responsible for the acceleration of auroral particles. The effects of weak parallel electric fields on the differential spectra of the low‐energy electrons observed in inverted‐V events are demonstrated with a set of heuristic model calculations. The spectral variations as a function of altitude of these electron populations in inverted‐V events observed by the Dynamics Explorer 2 spacecraft are examined separately for events connected to dark and sunlit local atmospheres. By comparing the altitude dependence of the observed spectra with the model calculations, we show that there can be no more than a 2‐volt potential drop on average between 400 and 900 km altitudes, corresponding to a distributed parallel dc electric field of less than 4 µV/m under the inverted‐V electron precipitation regions. Isotropy of the photoelectrons in inverted‐V events is also found to extend beyond the topside ionosphere, in accordance with the existence of a potential structure above the satellite. Statistical results on the spectral dependence of the secondary electrons on the inverted‐V primary beam parameters are also presented.

Precipitation of suprathermal ionospheric ions accelerated in the conjugate hemisphere

Low‐energy particle data from the AUREOL 3 satellite were examined in order to characterize latitude‐energy precipitation bands of low‐energy ions (<3 keV) sometimes detected embedded in the discrete auroral regions. A clear energy dispersion is observed, the less energetic ions being observed equatorward of higher‐energy precipitation. Generally the latitudinal extent of this structure is about ΔΛ ≃ 0.5°. Detailed ion energy spectra were used to infer that ionospheric ions are energized in the conjugate ionosphere, presumably in inverted‐V structures, and undergo velocity‐dependent electric drift effects during a single bounce. Model calculations and direct ion composition measurements clearly demonstrate that the H+ ions are predominant in these structures. Furthermore in several cases a double energy band is observed associated with each side of the conjugate V‐shaped electrostatic potential structure.

Experimental study of equilibrium in a bumpy torus

Plasma equilibrium in the ELMO Bumpy Torus (EBT) [in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Tokyo, 1974), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] was studied experimentally by measurements of the electrostatic potential structure. Before an electron tail population is formed, the electric field is found, roughly speaking, to be in the vertical direction. The appearance of a high-energy electron tail signals the formation of a negative potential well, and the potential contours start to nest. The potential contours are shifted inward with respect to the center of the conducting wall. The electric field between the plasma and the conducting wall forces the plasma inward, balancing the outward expansion force. This force balance provides a horizontal electric field that cancels the concentric radial electric field locally at the separatrix of the potential contour and leads to convective energy loss.