Plasma Sheet Source Research Articles

The variable source of the plasma sheet during a geomagnetic storm

Both solar wind and ionospheric sources contribute to the magnetotail plasma sheet, but how their contribution changes during a geomagnetic storm is an open question. The source is critical because the plasma sheet properties control the enhancement and decay rate of the ring current, the main cause of the geomagnetic field perturbations that define a geomagnetic storm. Here we use the solar wind composition to track the source and show that the plasma sheet source changes from predominantly solar wind to predominantly ionospheric as a storm develops. Additionally, we find that the ionospheric plasma during the storm main phase is initially dominated by singly ionized hydrogen (H+), likely from the polar wind, a low energy outflow from the polar cap, and then transitions to the accelerated outflow from the dayside and nightside auroral regions, identified by singly ionized oxygen (O+). These results reveal how the access to the magnetotail of the different sources can change quickly, impacting the storm development.

RCM modeling of bubble injections into the inner magnetosphere: geosynchronous orbit and the ionospheric responses

Introduction: Accurate characterization of the plasma sheet source population in the ring current region and its outer boundary at geosynchronous orbit is crucial for understanding the dynamics of the Earth’s magnetosphere. The interaction between the ring current and plasma populations from the ionosphere is a focus of extensive research.Methods: We used the Rice Convection Model (RCM) to simulate the transient meso-scale injections of fast flows or plasma sheet bubbles from the outer boundary into the inner magnetosphere and the associated impacts on the ionosphere. We compared our simulation results of the average properties of bulk plasma access to geosynchronous orbit to a number of empirical models. We also examined the role of plasma sheet bubbles in forming field-aligned currents (FACs).Results: Our modeling results show that impulsive plasma sheet injections dramatically alter the average distribution of FACs in the ionosphere. We found both quantitative and qualitative agreements and disagreements when comparing our simulation results to empirical models. Furthermore, we demonstrated that several discrete auroral structures can be identified in the nightside ionosphere in accordance with theupward FACs.Discussion: The significance of plasma sheet bubbles in modifying the averageplasma properties at geosynchronous orbit and FACs in the ionosphere is highlighted by oursimulation findings, offering novel understandings into the dynamics of Earth's magnetosphere,and emphasizing the necessity for further research in this field.

A New Stripline-Based Atmospheric Pressure Microwave Plasma Sheet Source Designed for Surface Modification of Materials.

A new type of microwave plasma source is presented in which plasma at atmospheric pressure is generated inside a quartz rectangular flat box placed in a stripline supplied by a 2.45 GHz coaxial line. The plasma has a sheet shape and is designed for surface modification. Electric field and power flux distributions, tuning characteristics, and power characteristics (ratios of radiated, absorbed, and entering power) are numerically studied for three configurations: open, semi-closed, and closed. The calculations show that near-zero radiation reduction is possible only for the closed configuration, while the ratio of radiated power to entering power is always greater than 30% for the other configurations. The moving plunger is not sufficient for the ratio of reflected to incident power to fall below 20% for both the closed and open configurations. This is possible for the semi-closed configuration, but then the radiated power is the highest. The experiment shows that for the same entering power, the plasma volume is largest for the closed configuration and smallest for the open configuration, which we attribute to the difference in radiated power. The plasma generated using the closed stripline configuration has a larger volume than plasma generated using the rectangular waveguide.

An improved conversion of the microwave energy into plasma in an optimized microwave plasma sheet source at 2.45 GHz designed for surface treatment

Efficient conversion of the microwave energy into an atmospheric-pressure argon plasma in a microwave plasma sheet source (MPSS) operating at 2.45 GHz in ambient air is the subject of this work. The MPSSs, capable of providing an atmospheric pressure plasma in the shape of a narrow sheet are an innovative technology for variety of surface treatments with no impact on the environment. The existing designs of the MPSS exhibit unsatisfactory efficiency of the conversion of microwave energy produced by the microwave source into the microwave plasma (about 80%), which hinders implementation of the MPSSs in the industry. In this paper, after an analytical approach based on the equivalent electrical circuit method we proposed an optimization of the design of MPSS in terms of the microwave energy conversion efficiency. The MPSS design improvement consisted in reducing the electrical impedance in the input plane of the MPSS by introducing a capacitive diaphragm into the MPSS transmission line, which compensated the inductive character of the plasma sheet. The experimental measurement of the electrodynamic characteristics of the MPSS with diaphragm showed that using the diaphragm significantly decreased the microwave power losses due to the microwave reflection from about (20–30)% to a level of several percent. This substantially increased the MPSS energy conversion efficiency up to about 97%, 93% and 87% for incident powers P I of 500 W, 750 W and 1000 W, respectively. The electrodynamic characteristics of the MPSS appeared a very convenient tool for determining the conditions of maximum efficiency of conversion of the microwave energy into the microwave plasma. The electrodynamic characteristics were measured at argon flow rate of 20 Nl min−1. The analysis of the calculated input impedances of the MPSSs with and without diaphragm, based on the equivalent electrical circuits of both MPSSs confirmed that the change of the character of the input reactance of the MPSS from inductive to capacitive after introducing the diaphragm is the cause of the increase in the MPSS energy conversion efficiency. The MPSSs having the energy conversion efficiency such high as presented in this paper appears to be attractive innovative technology for economic use in industrial applications.

Characterization of a novel microwave plasma sheet source operated at atmospheric pressure

Typically, microwave plasma sources (MPSs) operated at atmospheric pressure deliver plasmas in the common form of flames or cylindrical columns. In contrast to that, in this paper, a novel MPS (recently patented by us) for generating a new type of plasma in the form of a plasma sheet, which is sustained within a dielectric flat cuboid box, is presented. It is waveguide based, operated at a frequency of 2.45 GHz and in argon at atmospheric pressure. The plasma sheet consists of many filaments, typical of a microwave argon plasma. The MPS is described and analyzed in terms of optical emission spectroscopy, simulation of the electric field distributions in the MPS and numerical modeling of the working gas flow. Depending on the argon flow rate and absorbed microwave power, the determined electron density and rotational temperatures of OH radicals ranged from 3.6 × 1014 to 5.6 × 1014 cm−3 and from 800–1250 K, respectively. Numerical simulations have confirmed that the dimensions of the box are chosen correctly to ensure a sufficiently homogeneous distribution of the gas flow velocity field. The calculated electromagnetic field distributions in the MPS with plasma have shown that the device is well matched to the feeding line. The relative simplicity of the presented plasma source and industrially convenient plasma shape make it attractive for practical applications in the surface treatment of various materials.

Magnetospheric convection and magnetopause shadowing effects in ULF wave‐driven energetic electron transport

Magnetospheric radiation belt electron transport in the presence of ULF waves and a convection electric field is investigated using a model that includes a nightside plasma sheet source and electron losses by magnetopause shadowing. Narrow‐band ULF waves launched from a prescribed dayside magnetopause source are shown to interact with trapped and untrapped equatorially mirroring electrons within the magnetosphere. For magnetic moments less than 8 keV/nT and a strong convection electric field (in the order of 5 mV/m), we find that a limb of untrapped plasma sheet electrons extending across the dayside magnetosphere into the afternoon sector provides a phase space density (PSD) source for injection to lower L‐shells by ULF waves, causing a rapid enhancement in PSD. However, the same ULF wave activity gives rise to a rapid dropout in PSD for electrons with a higher magnetic moment or in the presence of a weaker convection electric field, since in this case the plasma sheet electrons escape through the magnetopause before they reach the afternoon sector. In their place, a lack of PSD, or PSD “holes” can be periodically injected from the magnetopause to lower L‐shells by ULF waves, leading to the rapid depletion in average PSD. In each case, the magnitude and extent in L‐shell of the PSD enhancement or depletion is strongly dependent on the amplitude of ULF waves in the afternoon sector, and is significantly augmented by the overlap of drift‐resonant islands and an associated stochastic transport layer.

Magnetotail origins of auroral Alfvénic power

The generation of Alfvénic Poynting flux in the central plasma sheet and its polar distribution at low altitude are studied using three dimensional global simulations of the solar wind‐magnetosphere‐ionosphere interaction. A 24‐hour event simulation (4–5 Feb 2004) driven by solar wind and interplanetary magnetic field data reproduces the global morphology of Alfvénic Poynting flux measured by the Polar satellite, including its dawn‐dusk asymmetry. Controlled simulations show that the dawn‐dusk asymmetry is regulated by the spatial variation in ionospheric conductance. The asymmetry disappears when the conductance is taken to be spatially uniform. The simulated Alfvénic Poynting flux is generated in the magnetotail by time‐variable, fast flows emerging from nightside reconnection. The simulated fast flows are more intense in the premidnight sector as observed; this asymmetry also disappears when the ionospheric conductance is spatially uniform. Analysis of the wave propagation in the plasma sheet source region, near xGSM ≈ −15 RE, shows that as the fast flow brakes, a portion of its kinetic energy is transformed into the electromagnetic energy of intermediate and fast magnetohydrodynamic waves. The wave power is dominantly compressional in the source region and becomes increasingly Alfvénic as it propagates along magnetic field lines toward the ionosphere.

Excitation of whistler mode chorus from global ring current simulations

We simulate ring current dynamics during the 22 April 2001 geomagnetic storm (Dstmin = −102 nT) using our global ring current–atmosphere interactions model (RAM) extended to relativistic energies and electrons. The model retains four dimensions by solving the bounce‐averaged kinetic equation as a function of radial distance in the equatorial plane, magnetic local time (MLT), energy, and pitch angle. RAM includes time‐dependent convective transport and radial diffusion, all major loss processes, and is coupled with a dynamic 2‐D plasmasphere model. The global electric fields, as well as the boundary conditions at geosynchronous orbit, are specified from the Rice Convection Model (RCM) driven by a plasma sheet source population that depends on interplanetary conditions. The 1–30 keV electron fluxes simulated with RAM during the main phase of the storm at 4 ≤ L ≤ 6.5 agree reasonably well with THEMIS statistical data for active times (AE* > 300 nT). We calculate the pitch angle anisotropy of ring current electrons and the linear growth rate of whistler mode chorus in the equatorial plane at L ≤ 6.5 and for all MLT. Intense chorus waves are generated outside the plasmasphere (L > 4) in the premidnight to dawn local time sector by ring current electrons with ∼5 keV resonant energy. The chorus intensification on the dayside is smaller, and it is caused by higher‐energy ∼10 keV electrons. We present first results of the path‐integrated amplification of chorus obtained with the HOTRAY code using RAM outputs at selected locations. Significant chorus amplification with wave gain G ≥ 200 dB is obtained near local midnight after the fresh injections of ring current electrons from the plasma sheet. The global distribution of enhanced chorus waves on the nightside shows good agreement with recent surveys of near‐equatorial plasma wave data from satellites.

The anisotropy of precipitating auroral electrons: A FAST case study

A key component in the magnetosphere–ionosphere coupling chain is the auroral acceleration region (AAR), where electromagnetic energy is converted into kinetic energy of precipitating electrons. We examine the accelerated electrons measured by FAST during an auroral overpass when the spacecraft crosses the AAR several times, as evident from the observation of ion beams. We assume that above the AAR the electron population is bi-Maxwellian and fit accelerated bi-Maxwellian functions to the measured distributions. When the electron motion between the plasma sheet source and the satellite can be considered as adiabatic, two of the fit parameters can be associated with the source properties, while the third one provides the electron anisotropy profile along the satellite track; the fit quality parameter, χ r 2 , gives an indication about the importance of non-adiabatic processes. We explore the possibility of using the anisotropy to determine the altitude of the AAR top boundary (AARTB) and find that when the AAR bottom boundary (AARBB) is located closer to the Earth, as proved by the detection of ion beams, the AARTB appears to be pushed at higher altitudes. This fact is discussed with regard to recent theoretical and experimental results.

Multistep Dst development and ring current composition changes during the 4–6 June 1991 magnetic storm

The 4–6 June 1991 magnetic storm, which occurred during solar maximum conditions, is analyzed to investigate two observed features of magnetic storms that are not completely understood: (1) the mass‐dependent decay of the ring current during the early recovery phase and (2) the role of preconditioning in multistep ring current development. A kinetic ring current drift‐loss model, driven by dynamic fluxes at the nightside outer boundary, was used to simulate this storm interval. A strong partial ring current developed and persisted throughout the main and early recovery phases. The majority of ions in the partial ring current make one pass through the inner magnetosphere on open drift paths before encountering the dayside magnetopause. The ring current exhibited a three‐phase decay in this storm. A short interval of charge‐exchange loss constituted the first phase of the decay followed by a classical two‐phase decay characterized by an abrupt transition between two very different decay timescales. The short interval dominated by charge‐exchange loss occurred because an abrupt northward turning of the interplanetary magnetic field (IMF) trapped ring current ions on closed trajectories, and turned‐off sources and “flow‐out” losses. If this had been the end of the solar wind disturbance, decay timescales would have gradually lengthened as charge exchange preferentially removed the short‐lived species; a distinctive two‐phase decay would not have resulted. However, the IMF turned weakly southward, drift paths became open, and a standard two‐phase decay ensued as the IMF rotated slowly northward again. As has been shown before, a two‐phase decay is produced as open drift paths are converted to closed in a weakening convection electric field, driving a transition from the fast flow‐out losses associated with the partial ring current to the slower charge‐exchange losses associated with the trapped ring current. The open drift path geometry during the main phase and during phase 1 of the two‐phase decay has important consequences for the evolution of ring current composition and for preconditioning issues. In this particular storm, ring current composition changes measured by the Combined Release and Radiation Effects Satellite (CRRES) during the main and recovery phase of the storm resulted largely from composition changes in the plasma sheet transmitted into the inner magnetosphere along open drift paths as the magnetic activity declined. Possible preconditioning elements were investigated during the multistep development of this storm, which was driven by the sequential arrival of three southward IMF Bz intervals of increasing peak strength. In each case, previous intensifications (preexisting ring currents) were swept out of the magnetosphere by the enhanced convection associated with the latest intensification and did not act as a significant preconditioning element. However, plasma sheet characteristics varied significantly between subsequent intensifications, altering the response of the magnetosphere to the sequential solar wind drivers. A denser plasma sheet (ring current source population) appeared during the second intensification, compensating for the weaker IMF Bz at this time and producing a minimum pressure‐corrected Dst* value comparable to the third intensification (driven by stronger IMF Bz but a lower density plasma sheet source). The controlling influence of the plasma sheet dynamics on the ring current dynamics and its role in altering the inner magnetospheric response to solar wind drivers during magnetic storms adds a sense of urgency to understanding what processes produce time‐dependent responses in the plasma sheet density, composition, and temperature.

Monitoring magnetosheath-magnetosphere interconnection topology from the aurora

Abstract. A strong southward rotation of the IMF (BZ from 5 to -6 nT in ~ 20 s) on 4 January 1995 caused an abrupt reconfiguration of midday aurorae and plasma convection consisting of the following: (1) the red-line aurora associated with magnetosheath plasma transfer at the low-latitude magnetopause appeared at the same time that (2) the green-line aurora from precipitating energetic plasma sheet particles equatorward of the cusp (near the open-closed field line boundary) weakened visibly and shifted equatorward, (3) the high-latitude aurora during the previous northward IMF, which is associated with lobe reconnection, persisted briefly (3 min) and brightened, before it disappeared from the field-of-view, (4) the activation of a strong convection bay (DPY current) at cusp and sub-cusp latitudes when the field turned strongly south, (5) a distinct wave motion of the plasma sheet outer boundary, as inferred from the aurora, which correlates closely with Pc 5 magnetic pulsations. Our interpretation of the dramatic reconfiguration is that reconnection poleward of the cusp coexisted briefly with reconnection at sub-cusp latitudes. The latter provided a magnetic field connection which enabled, on the one hand, magnetosheath particles to enter and cause the red-line cusp aurora, and on the other hand, allowed for magnetospheric energetic particles to escape and weaken the outer plasma sheet source of the green-line emission. The coexistence of the two cusp auroras reflects the time required for one field line topology to replace another, which, under the prevailing high speed wind ( ~ 650 km/s), lasts ~ 3–4 min. The motion of open flux tubes propagating from equator to pole during this transition is traced in the aurora by a poleward moving form. The waves on the outer boundary of the plasma sheet are most likely due to the Kelvin-Helmholtz instability. The study illustrates the ability of local auroral observations to monitor even a global change in magnetospheric magnetic topology.Key words. Magnetospheric Physics (auroral phenomena; magnetopause, cusp, and boundary layers; solar wind-magnethoshere interactions)

Observations of polar cap arcs on FAST

FAST satellite observations of spatially localized regions of electron and ion precipitation with concomitant paired upward and downward field‐aligned currents and duskward electric field enhancements at high (> 80° invariant latitude) magnetic latitudes are presented. Each pass of several across the polar cap was made during a period of persistent northward interplanetary magnetic field, and was characterized by the presence of several of these precipitation regions. In each case, the precipitation regions were associated with jets of enhanced antisunward convection, with electron precipitation restricted to the upward current region, and no energetic electron outflow in the downward current region, and ion precipitation spread over broader regions, possibly by time‐of‐flight effects. We compare these low‐altitude features to those expected for source regions lying in a reconnection site at the lobe magnetopause, a bursty bulk flow in the plasma sheet, and a surface wave on the low‐latitude boundary layer (LLBL). We find that the lobe reconnection model successfully describes the observed electrodynamics of the arcs, as well as the electron and ion source temperatures, but can not readily explain the presence of precipitating O+ and He+ in addition to H+ and He++. The plasma sheet source model can explain the composition of the ion precipitation but predicts higher electron and ion source temperatures than are observed, may require the coincidence of bursty bulk flows with extensions of the plasma sheet into the lobe, and may not be consistant with other observations of auroral phenomenology. The LLBL surface wave model does not readily explain the observed electrodynamics of the arcs and is not consistent with the observed offsets between electron and ion precipitation.

Simulations of ring current proton pitch angle distributions

We employ a three‐dimensional ring current model to trace the bounce‐averaged drift of singly charged ions during storm‐associated enhancements in the convection electric field. Using the simulation results, we map proton phase space density during the main and recovery phases of a storm in accordance with conservation of phase space density ƒ. We map from an initial quiescent phase space distribution that is obtained by solving the steady state transport equation (bounce‐averaged charge exchange balancing bounce‐averaged radial diffusion) with observed plasma sheet proton spectra as outer boundary conditions. We obtain proton pitch angle distributions at L ∼ 3–4.5 by evaluating ƒ at representative ring current energies (∼20–170 keV). We find that the prestorm and stormtime proton pitch angle anisotropy at any given L between 3 and 4.5 increases with particle energy in agreement with observations. The actual anisotropy at specific energies depends strongly on the shape of the plasma sheet source spectrum at ∼0.5–3 keV. Relatively large enhancements in the stormtime phase space density from the quiescent distribution occurs at all pitch angles for low energies (≲80 keV) except where the quiescent distribution lies on open drift shells. These increases result primarily from stormtime access to L ≲ 4.5 along open drift trajectories from the plasma sheet. For higher‐energy (≳150 keV) protons, which are transported via radial diffusion, there is little change in the anisotropy over a 3‐hour storm. At intermediate energies (E ∼ 80–150 keV) the stormtime enhancements in the phase space density can vary quite strongly with equatorial pitch angle. For such particles the stormtime transport is intermediate between directly convective and quasi‐diffusive, which can complicate the analysis of stormtime pitch angle anisotropies at intermediate energies. Decay lifetimes obtained from CRRES data during the recovery phase of a moderate storm are found to be considerably shorter than charge‐exchange lifetimes. It thus appears that charge exchange alone is not enough to explain the observed rapid decay of the proton ring current.

Diffusive acceleration of auroral primaries

The acceleration of electrons in a parallel electric field containing randomly distributed, micro double layers is investigated. The study is motivated by recent electric field observations in the auroral acceleration region. Its objective is to determine what effects stochastic versus static parallel electric fields may have on the spectral features of precipitating electrons. The average electron response as characterized by friction and diffusion coefficients is first analyzed. These coefficients are then used in a Fokker‐Planck equation for the altitudinal variation in the electron energy distribution. Electron sources at the top of the acceleration region (plasma sheet electrons) and at its latitudinal edges (ionospheric electrons) are included. It is shown that (1) the plasma sheet source gives rise to “inverted V” precipitation, as in quasi‐static models, and (2) the edge source is responsible for highly collimated, suprathermal precipitation. The spectral features of the former depend primarily on the average electric field and only weakly on its fluctuations. The properties of collimated precipitation are very sensitive to the microfield, however, owing to trapping phenomena. This analysis represents the first quantitative treatment of collimated electron precipitation; its predictions are in agreement with the observed intensity, pitch angle distribution, energy spectrum, and latitudinal width of edge region precipitation.

Origin of the plasma sheet

The plasma sheet is a region of hot plasma (∼107 °K) that surrounds the earth, filling the distant magnetosphere and separating the northern and southern lobes of the magnetospheric tail; its origin is one of the basic outstanding problems of magnetospheric physics. A critical review of the available observations indicates that the plasma sheet source must provide roughly 1025–1026 particles/s to balance known loss mechanisms operating in the quiet time magnetosphere and that an adiabatic or quasi‐adiabatic (ΔE ∝ E) acceleration mechanism must provide most of the observed particle energy. These constraints together with auroral helium ion measurements indicate that the plasma sheet particles must originate in the solar wind rather than in the terrestrial ionosphere. The available hypotheses for a solar wind source are examined, and it is concluded that a two‐step process of magnetopause diffusion plus electrostatic acceleration is the simplest source mechanism that is consistent with the available observations.