Plasma Sheet Research Articles

The evolving paradigm of the subauroral geospace

An assessment of the status quo of fast subauroral flows—subauroral ion drifts (SAID) and subauroral polarization streams (SAPS), is presented. For a few decades, their development has been interpreted in terms of the voltage and current magnetospheric generators based largely on the drift motion of test particles. Recent multispacecraft observations revealed serious flaws in the generator paradigm and called for a new generation mechanism of fast-time subauroral flows and ring current (RC) injections. A novel model includes them in the overarching problem of the penetration of magnetotail plasma flow bursts (MPFs) into the plasmasphere and the substorm current wedge (SCW) development. SAID are created near the plasmapause, where inbound MPFs are short-circuited by the cold plasma. This stops the MPF’s electrons and forms the “dispersionless” plasma sheet (PS) boundary. The SAID electric field—the inherent part of the short-circuiting loop—stops the inward-moving MPF’s ions. In turn, SAPS are an integral part of the two-loop SCW system, or SCW2L, where the downward (R2) current emerges in response to the upward (R1) current in the SCW’s “head.” The meridional Pedersen current, which connects the R1 and R2 currents, leads to SAPS that ultimately drive the fast-time RC injections on the duskside.

Ionization wave propagation of a surface dielectric barrier discharge with a flexible-structure plasma sheet

Plasma sources based on flexible substrates are receiving attention due to their unique adaptability to irregular surfaces and large range of plasma coverage, which endows them with irreplaceable advantages in the fields of material processing and biomedical treatment. Numerous studies have been carried out focusing on the application effects of these flexible plasma sources, while their surface discharge characteristics and mechanisms still lack revelation. In this work, a flexible plasma sheet with a surface dielectric barrier discharge structure is realized via a printed circuit board, and its multiple surface ionization wave (SIW) propagation on curved gas–solid interfaces is studied via experiments and 2D fluid simulation models. Qualitative agreement is achieved between the experiments and simulations. It is found that a positive and a negative discharge are generated at the rising and falling edges of the excitation pulse, respectively. In the positive discharge, SIWs originate at the grounded mesh edge and then propagate to the center in a petal-like pattern, which is shaped by the space electric field. Controlled by electron collision reactions, the development of the excited states of N2 and O2 molecules is similar to that of electrons. In the negative discharge, electrons dissipate and no SIW is generated. The evolution of heavy particles shows differences in this period, which is attributed to the disparate rate coefficients of their consumption reactions. Further study shows that when the plasma sheet changes from convex to concave, the electron density and electron temperature above its surface increase, but the petal patterns of the SIW propagation have no variation. The electron density, electron temperature, and electron impact ionization source will rise as a result of the increasing pulse amplitude or the decreasing duration of the pulse rising edge.

MMS Observations of Dayside Warm (Several eV to 100 eV) Ions in the Middle and Outer Magnetosphere

AbstractWarm (several eV to 100 eV) ions are an important, and still poorly understood, component of the magnetospheric plasma environment. We present the first comprehensive statistical analysis of several distinct populations of warm ions in the dayside middle and outer magnetosphere. We analyze 7 months (1 September 2015–31 March 2016) of Magnetospheric Multiscale Hot Plasma Composition Analyzer data comprising 734,200 moments (density, temperature) and energy‐dependent pitch angle distributions (PADs) of three major ion species (H+, He+, and O+) with energy ≤100 eV. PADs are represented by an energy‐averaged index that characterizes the shape of the PAD as field‐aligned (FA), pancake, or isotropic. We use filtering by temperature, pitch angle, and concentration to distinguish different populations, and obtain new and more complete information about average density, temperature, PADs, and composition. Our analysis explores two known populations of warm ions: the warm plasmasphere (WPS) and the warm cloak/trough (C/T). The WPS is a higher‐temperature, higher‐L extension of the duskside plasmaspheric bulge, containing mostly trapped (pancake/isotropic) ions with an H+:He+:O+ order of ion dominance. The C/T contains mostly FA warm ions with a dawnward (duskward) temperature gradient for H+ (He+ and O+), lower densities, and an H+:O+:He+ order of ion dominance. The WPS‐C/T overlap contains a mixture of the two populations (e.g., FA He+ in WPS, trapped O+ in C/T). Pancake (FA) PADs are correlated with higher (lower) density/temperature. Our analysis also identifies warm ions in the low‐energy plasma sheet. Our work consolidates and systematically extends the characterizations of warm ions reported in previous studies.

Sub‐Auroral Flows and Associated Magnetospheric and Ionospheric Phenomena Developed During 7–8 September 2017

AbstractThis study investigates the Sub‐Auroral Polarization Streams (SAPS) and Sub‐Auroral Ion Drifts (SAID) along with the associated magnetospheric and ionospheric phenomena developed during 7–8 September 2017. Then, a series of substorms occurred before and during the geomagnetic storm (SYM‐Hmin = −146 nT). Results are obtained by utilizing multi‐point observations. These show that during the pre‐storm period, when the ElectroMagnetic Ion‐Cyclotron waves resulted in drop‐out events, a series of sunward (westward) SAID‐in‐SAPS flows developed in the dusk‐midnight sector due to the small‐scale flow bursts reaching the SAPS channel and the westward SAPS flows were streaming antisunward after midnight. During the storm period, dawnside sunward (eastward) SAPS also developed, Kelvin‐Helmholtz (K‐H) vortices rolled along the magnetopause and the hot zone became structured by reflected K‐H waves near the plasmapause leading to auroral undulations (AUs). New findings include the correlation of AUs and K‐H waves in the structured hot zone implying that the Near‐Earth Plasma Sheet acted as a resonator after activation by the K‐H vortices on the magnetopause. Further evidence is provided by the dayside Equatorward Boundary Intensification and Poleward Moving Auroral Forms of which development was supported by the K‐H vortices on the magnetopause by carrying the magnetosheath plasma into the magnetosphere. Adding to previous studies, new results also include the correlated observations of storm‐enhanced density (SED) plume and poleward drifts underlying both the equatorward plasma bubbles and the higher latitude plasma density depletions, and thus verifying their poleward movement within the SED base and SED plume.

Unusual shrinkage and reshaping of Earth’s magnetosphere under a strong northward interplanetary magnetic field

The Earth’s magnetosphere is the region of space where plasma behavior is dominated by the geomagnetic field. It has a long tail typically extending hundreds of Earth radii (RE) with plentiful open magnetic fluxes threading the magnetopause associated with magnetic reconnection and momentum transfer from the solar wind. The open-flux is greatly reduced when the interplanetary magnetic field points northward, but the extent of the magnetotail remains unknown. Here we report direct observations of an almost complete disappearance of the open-flux polar cap characterized by merging poleward edges of a conjugate horse-collar aurora (HCA) in both hemispheres’ polar ionosphere. The conjugate HCA is generated by particle precipitation due to Kelvin-Helmholtz instability in the dawn and dusk cold dense plasma sheets (CDPS). These CDPS are consist of solar wind plasma captured by a continuous dual-lobe magnetic reconnections, which is further squeezed into the central magnetotail, resulting in a short “calabash-shaped” magnetotail.

Generation of Field‐Aligned Currents During Substorm Expansion: An Update

AbstractWe investigated generation processes of field‐aligned currents (FACs) that are abruptly intensified at the beginning of the substorm expansion phase by tracing a packet of the Alfvén wave backward in time from the onset position in the ionosphere in the global magnetohydrodynamics (MHD) simulation. The generation region is found in the near‐Earth plasma sheet, in which (a) azimuthally moving plasma pulls the magnetic field line, and performs negative work against the magnetic tension force to excite the Alfvén waves, (b) FACs are generated from the requirement of Ampère and Faraday laws, and (c) field‐perpendicular current is converted to FACs. We call this near‐Earth FAC dynamo. The plasma involved originates in the tail lobe region. When near‐Earth reconnection occurs in the plasma sheet, the plasma is accelerated earthward by the Lorentz force, and decelerated by the plasma pressure gradient force, followed by the Lorentz force. The flow is deflected to the west and east directions by the plasma pressure gradient force and the Lorentz force, resulting in the excitation of Alfvén waves and FACs. The Alfvén waves propagate along the magnetic field in the rest frame of the moving plasma. When it arrives at the ionosphere, the auroral electrojet starts developing and the substorm expansion phase begins. The near‐Earth FAC dynamo can be distinguished from the near‐Earth dynamo (J · E < 0, where J is the current density and E is the electric field). We suggest that the evolution of the substorm can be understood in terms of the development of FACs.

Rotating spokes and self-organization in an inductive pulsed plasma thruster

An experimental study of azimuthally rotating spokes in the plasma sheet of an inductive pulsed plasma thruster is presented. High-speed imaging reveals the formation of spokes with mode number ≈ 18 for a range of background pressure and coil discharge conditions. The spoke widths increase and the corresponding spoke orders decrease through an inverse cascade of energy flowing from higher-order to lower-order spokes, resulting in a more uniform plasma sheet. Secondary plasma sheets form in the half-cycle of the current discharge following primary sheet formation. It is observed that the plasma sheet forms earlier in the discharge as the pre-ionizer discharge energy increases, and increasing the background pressure results in higher uniformity when the other conditions are held constant. Spokes rotate predominantly in the drift direction at speeds on the order of critical ionization velocity, and orders of magnitude smaller than the drift speed. Simultaneously counter-rotating spokes are observed under various operating conditions.

Dipolarization Fronts in Cold‐Dense and Hot‐Tenuous Plasma Sheet Conditions: A Comparative Study

AbstractWe present the first comparative observation of two dipolarization fronts (DFs) in cold‐dense plasma sheet (CDPS) and hot‐tenuous plasma sheet (HTPS) conditions. We find that: (a) the CDPS and HTPS are formed under distinct solar wind conditions. (b) The DF in HTPS has a higher Bz increase than the DF in CDPS; (c) DF in CDPS can transfer energy from magnetic field to particles more efficiently than the DF in HTPS; (d) the >2 keV (<2 keV) electron phase space density of flux pileup region (FPR) in HTPS is higher (lower) than that of FPR in CDPS; (e) the electron cyclotron harmonic waves are excited at DF in HTPS instead of the DF in CDPS. (f) The responses in the ionospheric currents are weak for both DFs in CDPS and HTPS. These results reveal the differences in properties and physical processes between the DFs in CDPS and HTPS. Our findings can significantly improve the understanding of physical mechanisms underlying the formation and evolution processes of DFs.

Quasi-parallel Whistler Waves and Their Interaction with Resonant Electrons during High-velocity Bulk Flows in the Earth’s Magnetotail

In collisionless space, plasma waves are important channels of energy conversion, affecting the local particle velocity distribution functions through wave–particle interactions. In this paper we present a comparative statistical analysis of the characteristics of quasi-parallel narrowband whistler waves and the properties of resonant electrons interacting with these waves during the intervals of earthward and tailward high-velocity bulk flows produced by the near-Earth X-line and observed by Magnetospheric Multiscale Mission spacecraft. We found that on both sides of the X-line, the suprathermal electrons (≥1 keV) having large pitch angles make the major contribution to the maximal growth rate (γ) of these waves. The whistler waves were observed almost simultaneously with strong enhancements of perpendicular magnetic gradients localized at electron scales near dipolarization fronts associated with the earthward bulk flows, and near flux ropes/magnetic islands embedded into the tailward bulk flows. Betatron energization of electrons due to the appearance of such gradients increases the perpendicular anisotropy of electron distribution, which could be responsible for the whistler wave generation. We found that in the course of electron interactions with the whistler waves the lower-energy resonant electrons can transfer a part of their kinetic energy to the higher-energy electrons, especially in the Central Plasma Sheet. This results in formation/enhancement of energy-dependent perpendicular anisotropy and power-law tails in the high-energy range of electron velocity distribution. We conclude that despite the differences in the magnetic structure of the earthward and tailward bulk flows, the mechanisms of the quasi-parallel whistler wave generation and the properties of resonant electrons are quite similar.

Dynamics of the Magnetotail Plasma Sheet Current

Magnetospheric plasma can be investigated as a continuum by adopting magnetic field B and plasma flow u as primary parameters in the Bu paradigm or as a collection of individual particles by adopting electric field E and electrical current j as primary parameters in the Ej paradigm. It is pointed out that each paradigm has its merits and limitations. This viewpoint is illustrated further by examining several topics in magnetospheric research. The magnetic flux transport in substorm dipolarization is examined with the Ej paradigm to show why the Bu paradigm may be inappropriate in some cases due to the violation of the frozen-in condition for the validity of the Bu paradigm. There is no guarantee that large-scale plasma dynamics can always be treated accurately by the Bu paradigm. The disturbance revealed in the current disruption (CD) phenomenon has unique characteristics that can be more readily understood with the Ej paradigm. In a case study, the power dissipation in CD is evaluated to about an order of magnitude higher than that in the electron diffusion region associated with magnetic reconnection (MR). Two prominent plasma instabilities, namely tearing instability (TI) and cross-field current instability (CCI), are discussed and their relevance to substorm onset is evaluated. The mating instability developed conceptually is also briefly discussed. The development of azimuthal auroral beads (ABs) on auroral arcs formed prior to substorm onset is analyzed to show that CCI can predict well their wavelength, growth rate, and period simultaneously. In contrast, the observed azimuthal structures in ABs are inconsistent with TI that produces only meridian structures in the ionosphere. Overall, the physical insights gained in the Ej paradigm are helpful in achieving a deep understanding of several magnetospheric phenomena.

Plasma sheath around sunlit moon: monotonic and non-monotonic structures

ABSTRACTAn analytical investigation of the sheath formation over sunlit moon under the influence of observed solar ultraviolet/extreme ultraviolet radiation and solar wind plasma has been presented. Poisson’s equation is coupled with the latitude-dependent Fermionic photoelectrons, non-Maxwellian solar wind electrons and cold ions to derive the electric potential, electric field, and population density profiles within the photoelectron sheath. We notice that the high-energy photons of the solar spectra in addition to Lyman- α, significantly contribute in determining the surface charge and subsequent sheath structure. A traditional Debye (Type C) sheath forms around the terminator due to dominant plasma electron accretion and marginal photoemission. While moving towards the equator, the photoelectron contribution increases, yielding monotonic (Type B) and non-monotonic (Type A) sheath solutions over sunlit locations. The calculations show that the non-monotonic potential structures are more stable near the terminator region, while both types of potential structures are probable near the equator region for the nominal solar wind plasma. A vertical sheath extension of ∼60 m and ∼12 m is predicted for typical solar wind plasma at the equator corresponding to Type A and Type B sheath, respectively. Under exotic plasma conditions (e.g. Earth’s magnetospheric tail lobe, plasma sheet), non-monotonic solutions become stable for the photoemission-dominated region, and the entire sunlit lunar surface may acquire negative potential.

Multiple Auroral Streamer Bundles in Conjugate Hemispheres and Multiple Near‐Earth Injections

AbstractThe auroral streamer is a type of auroral form commonly observed during geomagnetic substorms. Previous studies suggested a coupling between auroral streamers and channels of bursty bulk flows in the plasma sheet. However, whether one flow channel can map to multiple streamers is unclear. Here, we present an event containing consecutive auroral streamer subevents. The event features similar spatial and temporal development of auroral streamers in conjugate hemispheres. In most of the subevents, we observed that what an in‐space auroral camera saw as one streamer actually consisted of multiple streamers (i.e., a streamer bundle). A coordinated analysis of near‐earth injections and ionospheric currents suggests that one overall flow channel (or bubble) can map to a streamer bundle. Multiple streamer bundles and thus multiple overall flow channels can occur simultaneously at different local times. Evidence supports that the overall flow channel may consist of or split into several narrower flow channels and each maps to a streamer and causes a particle injection.

Statistical Characterization of the Dynamic Near‐Earth Plasma Sheet Relative to Ultra‐Low Frequency (ULF) Wave Growth at Substorm Onset

AbstractMagnetospheric substorms are a complex phenomenon. During the initial stages of a substorm a variety of important processes occur in near‐Earth space within a span of several minutes. The relative timing and links between these processes are critical to understanding how, where and when substorms may occur. One of the first observed signatures at substorm onset is the exponential increase in ULF (Ultra‐Low Frequency) wave power in the near‐Earth magnetotail (e.g., −7.5 ≤ XGSM ≤ −12.5 RE). We use the Time History of Events and Macroscale Interactions during Substorms spacecraft to examine the conditions in the magnetotail plasma sheet before, during and after local ULF wave growth. Prior to the ULF wave growth, the magnetotail stretches with convectional flows dominating. We then find strong earthward and azimuthal flows that peak at a similar time to the peak ULF wave power. These flows are found to be faster in the mid‐tail (−10 ≤ XGSM ≤ −12.5 RE) than the near‐tail (−7.5 ≤ XGSM ≤ −10 RE). Examining the local plasma energy density (magnetic, thermal and kinetic), we find no statistical decrease that could explain the exponentially growing ULF waves, in fact the local energy density is found to increase. This suggests that there may be an injection of energy from elsewhere in the magnetotail. Following the peak ULF wave power the tail is seen to dipolarize, and the local energy density is enhanced.

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Structural instabilities that develop during pulsed injection of dense plasma jets into vacuum in the presence of an external quasi-homogeneous magnetic field are studied by high-speed photography using ICCD cameras. The experiments are carried out in the chamber of the “Krot” stand, which has record-breaking dimensions in its class of installations (diameter—3 m, length of the working section—10 m), and makes it possible to study plasma dynamics by various diagnostic methods at scales of more than 1 m both along the magnetic field and in the direction transverse to the magnetic field. During injection along the magnetic field, a transverse collimation of the flow of ionized matter and the development of a flute instability of the plasma boundary are observed, which, at the late stages of expansion, leads to the plasma leaving the injection region in the form of several jets across the field. During transverse injection, the formation of a collimated flow, a “plasma sheet,” is observed, in which, as the plasma moves across the field, inhomogeneous structures develop in the direction of injection.

Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

An Attempt to Analyze a Human Nervous System Algorithm for Sensing Earthquake Precursors

We statistically validate the 2011-2022 earthquake prediction records of Ada, the sixth finalist of the 2nd China AETA in 2021, who made 147 earthquake predictions (including 60% of magnitude 5.5 earthquakes) with a prediction accuracy higher than 70% and a confidence level of 95% over a 12-year period. Since the reliable earthquake precursor signals described by Ada and the characteristics of Alfvén waves match quite well, this paper proposes a hypothesis on how earthquakes are triggered based on the Alfvén (Q G) torsional wave model of Gillette et al. When the plume of the upper mantle column intrudes into the magma and lithosphere of the soft flow layer during the exchange of hot and cold molten material masses deep inside the Earth’s interior during ascent and descent, it is possible to form body and surface plasma sheets under certain conditions to form Alfven nonlinear isolated waves, and Alfven waves often perturb the geomagnetic field, releasing huge heat and kinetic energy thus triggering earthquakes. To explain the complex phenomenon of how Ada senses Alvfen waves and how to locate epicenters, we venture to speculate that special magnetosensory cells in a few human bodies can sense earthquake precursors and attempt to hypothesize an algorithm that analyzes how the human biological nervous system encodes and decodes earthquake precursors and explains how human magnetosensory cells can solve complex problems such as predicting earthquake magnitude and locating epicenters.

Applying Magnetic Curvature to MMS Data to Identify Thin Current Sheets Relative to Tail Reconnection.

Thin current sheets (TCSs) have been postulated to be a necessary precondition for reconnection onset. Magnetic reconnection X-lines in the magnetotail have been observed to be more common duskward of midnight. We take advantage of the MMS tetrahedral formation during the 2017-2020 MMS tail seasons to calculate the thickness of the cross-tail neutral sheet relative to ion gyroradius. While a similar technique was applied to Cluster data, current sheet thickness over a broader range of radial distances has not been robustly explored before this study. We compare our analysis to recent theories regarding mechanisms of tail current sheet thinning and to recent simulations. We find MMS spent more than twice as long in ion-scale TCSs in the pre-midnight sector than post-midnight, despite nearly even plasma sheet dwell time. The dawn-dusk asymmetry in the distribution of Ion Diffusion Regions, as previously reported in relation to regions of TCSs, is also analyzed.

MAGNETOSPHERIC SUBSTORMS AND RELATIVISTIC ELECTRONS

The most recent findings on the dynamics of the outer radiation belt (ORB) and the physics of magnetospheric substorms are examined. Specifically, we investigate the relationship between storm time substorms and the energetic electron population that forms the ORB. Traditionally, storm time substorms have been considered as the primary source of energetic electrons, which are further accelerated during storms to contribute to the formation of the ORB. However, several observations have demonstrated that large magnetospheric substorms can generate high-energy electrons even in the absence of magnetic storms. Substorms introduce dispersionless injections of energetic electrons deep into the magnetosphere from the geosynchronous orbit during storm times. The injected electrons undergo additional acceleration via the betatron mechanism during the storm recovery phase, thus increasing the ORB population. To gain a better understanding of this process, it is crucial to study plasma sheet turbulence, substorm onset processes, and the brightening of auroral arcs. By analyzing the aforementioned findings, this study aims to highlight the need for reanalyzing of the role of auroral processes in the formation of the ORB.

A monitoring campaign (2013–2020) of ESA’s Mars Express to study interplanetary plasma scintillation

AbstractThe radio signal transmitted by the Mars Express (MEX) spacecraft was observed regularly between the years 2013–2020 at X-band (8.42 GHz) using the European Very Long Baseline Interferometry (EVN) network and University of Tasmania’s telescopes. We present a method to describe the solar wind parameters by quantifying the effects of plasma on our radio signal. In doing so, we identify all the uncompensated effects on the radio signal and see which coronal processes drive them. From a technical standpoint, quantifying the effect of the plasma on the radio signal helps phase referencing for precision spacecraft tracking. The phase fluctuation of the signal was determined for Mars’ orbit for solar elongation angles from 0 to 180 deg. The calculated phase residuals allow determination of the phase power spectrum. The total electron content of the solar plasma along the line of sight is calculated by removing effects from mechanical and ionospheric noises. The spectral index was determined as$-2.43 \pm 0.11$which is in agreement with Kolmogorov’s turbulence. The theoretical models are consistent with observations at lower solar elongations however at higher solar elongation ($>$160 deg) we see the observed values to be higher. This can be caused when the uplink and downlink signals are positively correlated as a result of passing through identical plasma sheets.

Alternating North‐South Brightness Ratio of Ganymede's Auroral Ovals: Hubble Space Telescope Observations Around the Juno PJ34 Flyby

AbstractWe report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter.