near-Earth Plasma Sheet Research Articles

A Story of Developing the Idea of Plasma-Sheet Flow Braking

This paper reports a story of developing the idea of Earthward ion flow braking in the near-Earth plasma sheet and its relationship with substorm onset processes. This idea and the data that support it are the basis for today’s two competing models for substorms: the near-Earth neutral-line model and the current disruption model. The idea was developed when the author was staying at the Max Planck Institute for Extraterrestrial Physics (MPE) from July 1996 to June 1997. The story addresses the colleagues and mentors who had contributed to the development of this idea. The lessons learned from this story are also summarized for students and early-career scientists for their development of new scientific ideas.

Relaxation of multi-ion plasmas in an internal conductor

The possibility of the formation and characteristics of the relaxed structures in the multi-ion magnetized plasmas consisting of heavy ions, light ions, and inertia-less electrons have been explored. Using the vortex dynamic equations with the current density, the system is relaxed to triple Beltrami states. Solutions of the relaxed states are determined in two different cases: (i) simple slab geometry and (ii) slab geometry with an internal conductor. The impact of Beltrami parameters and density ratios of the plasma components on the formation of equilibrium structures in both geometries has been studied. Moreover, we also highlight the equivalences of both geometries (simple slab geometry and a slab geometry with an internal conductor). This investigation will be useful for studying and understanding the relaxed structures in different astrophysical objects such as solar wind, Earth’s ionosphere, near-Earth plasmasheet, upper ionosphere, and Saturn and in the laboratory plasmas having two positively charged ions of different masses. Moreover, this work will be helpful in describing the underlying physics of the relaxed structures. These structures are developed in different astrophysical bodies that revolve around the planet such as Jupiter magnetosphere.

Theory of the two fluid resistive-drift-ballooning instability in the near-Earth plasma sheet: local and global magnetospheric mathematical modeling

Theory of the two fluid resistive-drift-ballooning instability in the near-Earth plasma sheet: local and global magnetospheric mathematical modeling

A Novel Magnetosphere–Ionosphere Coupling Model for the Onset of Substrom Expansion Phase

A novel magnetosphere–ionosphere (M-I) coupling model is proposed to simulate the brightening of the onset auroral arc of a magnetospheric substorm event. The new M-I coupling model is modified from the M-I coupling model proposed by the Alaska research team in 1988. We adjust the magnetospheric boundary conditions by including the Hall effects in the thin current sheet and allowing the spatial distributions of the reflection–transmission coefficient to vary with time. As a result, brightening and poleward drifting of multiple auroral arcs appear for the first time in an M-I coupling model. The new results indicate that the coupled Hall effects in the near-Earth plasma sheet and the E-region ionosphere play a vital role in triggering the onset of a magnetospheric substorm.

Modeling and Prediction of Near-Earth Plasma Sheet Parameters Using the Rice Convection Model and the Recurrent Neural Network

Interaction between Earth’s magnetotail and its inner magnetosphere plays an important role in the transport of mass and energy in the ionosphere–magnetosphere coupled system. A number of first-principles models are devoted to understanding the associated dynamics. However, running these models, including both magnetohydrodynamic models and kinetic drift models, can be computationally expensive when self-consistency and high spatial resolution are required. In this study, we exploit an approach of building a parallel statistical model, based on the long short-term memory (LSTM) type of recurrent neural network, to forecast the results of a first-principles model, called the Rice Convection Model (RCM). The RCM is used to simulate the transient injection events, in which the flux-tube entropy parameter, dawn-to-dusk electric field component, and cumulative magnetic flux transport are calculated in the central plasma sheet. These key parameters are then used as initial inputs for training the LSTM. Using the trained LSTM multivariate parameters, we are able to forecast the plasma sheet parameters beyond the training time for several tens of minutes that are found to be consistent with the subsequent RCM simulation results. Our tests indicate that the recurrent neural network technique can be efficiently used for forecasting numerical simulations of magnetospheric models. The potential to apply this approach to other models is also discussed.

Subauroral Flow Channel Structures and Auroral Undulations Triggered by Kelvin‐Helmholtz Waves

AbstractWe investigate Kelvin‐Helmholtz (K‐H) waves on/near the magnetopause and surface waves near the plasmapause—in the outer region of the plasmasphere: in the hot zone—by utilizing multi‐instrument/satellite observations. Our main aim is to study how the K‐H waves and the K‐H instability mechanisms impacted the subauroral and auroral regions during the geomagnetic storms of May 27–29 and July 16, 2017. For the subauroral region, we specify the structured flows as Sub‐Auroral Polarization Streams Wave Structures (SAPS‐WS) and the combined flows—created by Abnormal Sub‐Auroral Ion Drifts (ASAID) and SAID or SAPS—as a complex equatorward‐poleward ASAID‐SAID or SAPS‐ASAID. For the auroral zone, we identify the large auroral undulations appearing inside the auroral zone. The correlated observations of the K‐H waves, the structured or complex subauroral flows and large auroral undulations, and the local geomagnetic field oscillations confirm the connections of both the subauroral flows and the auroral undulations with the K‐H waves via the eigenfrequency of the Near‐Earth Plasma Sheet (NEPS) resonator activated by the K‐H waves. For the first time, we demonstrate the simultaneous detections of K‐H waves near the magnetopause and surface waves near the plasmapause in the hot zone on July 16, 2017, and conclude their coupling via the NEPS resonator's eigenfrequency. Thus, the surface waves near the plasmapause were the manifestation of the undulating (or rippled) earthward inner boundary of the NEPS that led to the development of ASAID‐SAID/SAPS‐ASAID or SAPS‐WS in the subauroral region and to the large auroral undulation inside the auroral zone.

Vlasov simulation of electrons in the context of hybrid global models: an eVlasiator approach

Abstract. Modern investigations of dynamical space plasma systems such as magnetically complicated topologies within the Earth's magnetosphere make great use of supercomputer models as well as spacecraft observations. Space plasma simulations can be used to investigate energy transfer, acceleration, and plasma flows on both global and local scales. Simulation of global magnetospheric dynamics requires spatial and temporal scales currently achievable through magnetohydrodynamics or hybrid-kinetic simulations, which approximate electron dynamics as a charge-neutralizing fluid. We introduce a novel method for Vlasov-simulating electrons in the context of a hybrid-kinetic framework in order to examine the energization processes of magnetospheric electrons. Our extension of the Vlasiator hybrid-Vlasov code utilizes the global simulation dynamics of the hybrid method whilst modelling snapshots of electron dynamics on global spatial scales and temporal scales suitable for electron physics. Our eVlasiator model is shown to be stable both for single-cell and small-scale domains, and the solver successfully models Langmuir waves and Bernstein modes. We simulate a small test-case section of the near-Earth magnetotail plasma sheet region, reproducing a number of electron distribution function features found in spacecraft measurements.

Characteristics of magnetic dipolarizations in the vicinity of the substorm onset region observed by THEMIS

With conjunction observations of electromagnetic fields and plasma from Time History of Events and Macroscale Interactions during Substorm (THEMIS) in the near-Earth magnetotail, we investigate the spatial and temporal properties of substorm dipolarizations in the near-Earth plasma sheet (NEPS) during a substorm at 03:23 UT on 12 February 2008. Substorm dipolarizations with different features are detected by three near-Earth THEMIS probes (THA (P5), THD (P3) and THE (P4)) in the magnetotail. In the current sheet with a large plasma beta value (β > 2, where β is the ratio of the plasma thermal pressure to the magnetic pressure), the dipolarization within the substorm onset region, (−10.4, 2.8, −2.6)RE_gsm, has a large initial magnetic field elevation angle, θ > 60°, θ = arctan (Bz/(Bx2+By2)1/2), and is accompanied by energetic ion (tens to hundred keV) dispersionless injection detected by THD (P3). This substorm onset dipolarization is characterized by Bx and By components around 0 nT with significant fluctuations. The Bz component increases sharply and its subsequent magnitude approaches the total magnetic field, Bt. The maximum value of the elevation angle approaches 85° during the later substorm expansion phase. In the NEPS with β ~ 1, the dipolarization outside the substorm onset region is characterized by a magnetic elevation angle with a small beginning value of θ < 45° and following multi-step enhancements during the substorm expansion phase. The maximum value of the elevation angle approaches to 70° during the later substorm expansion phase. Our observation results indicate that characteristics of dipolarization with a large beginning elevation angle within the substorm onset region provide a new indicator to identify substorm onset location.

Stationary “nose-like” ion spectral structures in the inner magnetosphere: Observations by Van Allen probes and simulations

The “Nose-like” ion spectral structures are characterized by the deep inward penetration of ions from the near-Earth plasma sheet and can extend to low L shells in the inner magnetosphere. The stationary “Nose-like” ion structures are believed to be driven by the stationary electric and magnetic field during quiet geomagnetic times. However, the global distribution picture, especially for each MLT, for both single and double stationary nose structures is still not reached. In this study, we statistically investigate the stationary “Nose-like” ion spectral structure based on 2 years of Van Allen probes observations. We find a clear MLT dependence of the number of stationary noses, with a higher occurrence of single-nose structures from 13 to 03 MLT, while a higher occurrence of double-nose structures from 03 to 13MLT. This is confirmed by calculating the backward drift time using backward tracing method based on the dipole magnetic field and Weimer 96 electric field model for each MLT. Simulation results indicate that the ion spectral gap, which is formed due to long drift time or existence of the ion forbidden region, controls the nose structure at different MLT during geomagnetic quiet times. A possible physical explanation of this MLT dependence can be a combination of the ion spectral gaps created by the region of long drift time in the dayside within the nose energy range and the persistent dawn-dusk asymmetries in the convection electric field.

Ballooning-interchange instability in the near-Earth plasma sheet and auroral beads: Global magnetospheric modeling at the limit of the MHD approximation

Ballooning-interchange instability in the near-Earth plasma sheet and auroral beads: Global magnetospheric modeling at the limit of the MHD approximation

Variations in Ion-Component Pressure during Dipolarization in the Near-Earth Magnetotail Plasma Sheet

The dynamics of fluxes of thermal and suprathermal H+ and O+ ions and the pressure variations of these components during 11 events of magnetic field dipolarization in the plasma sheet of the geomagnetic tail are analyzed based on Cluster satellite observations. It was found that the energy of H+ and O+ ions corresponding to the maximum flux approached or exceeded the upper energy threshold of CODIF thermal plasma mass spectrometers (~40 keV) in all of the events. During such periods, the determined values of the density, temperature, and pressure in an energy range up to 40 keV are underestimated as compared to the actual values. To determine the pressure during such intervals, we used measurements of the ion-component fluxes in an energy range up to 1.3 MeV based on a joint analysis of observations of CODIF and RAPID energy-mass spectrometers. As a result, it was found that the ion pressure during dipolarization with the high-energy component of the spectrum taken into account can be several times higher than the pressure determined in the range up to 40 keV. Using the superposed epoch analysis, we showed that the error associated with underestimation of the pressure of the H+ and O+ ion components is largest during the dipolarization growth phase. The underestimation is most significant for the O+ ion pressure.

Magnetic dipolarizations inside geosynchronous orbit with tailward ion flows

Abstract. Electromagnetic field and plasma data from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) near-Earth probes are used to investigate magnetic dipolarizations inside geosynchronous orbit on 27 August 2014 during an intense substorm with AEmax∼1000 nT. THEMIS-D (TH-D) was located inside geosynchronous orbit around midnight in the interval from 09:25 to 09:55 UT. During this period, two distinct magnetic dipolarizations with tailward ion flows are observed by TH-D. The first one is indicated by the magnetic elevation angle increase from 15 to 25∘ around 09:30:40 UT. The tailward perpendicular velocity is V⊥x∼-50 km s−1. The second one is presented by the elevation angle increase from 25 to 45∘ around 09:36 UT, and the tailward perpendicular velocity is V⊥x∼-70 km s−1. These two significant dipolarizations are accompanied with the sharp increase in the energy flux of energetic electron inside geosynchronous orbit. After a 5 min expansion of the near-Earth plasma sheet (NEPS), THEMIS-E (TH-E) located outside geosynchronous orbit also detected this tailward expanding plasma sheet with ion flows of −150 km s−1. The dipolarization propagates tailward with a speed of −47 km s−1 along a 2.2 RE distance in the X direction between TH-D and TH-E within 5 min. These dipolarizations with tailward ion flows observed inside geosynchronous orbit indicate a new energy transfer path in the inner magnetosphere during substorms.

Heavy ion acceleration by EMIC waves in the near-Earth plasma sheet

Heavy ions (O+ and He+) are an important constituent of plasma sheets. They are most significant during geomagnetic quiet periods when charge-exchange reactions result in a faster loss of H+ ions than heavy ions. With inward injections of energetic particles from the plasma sheet into the inner magnetosphere, heavy ions play a key role in producing the ring current and radiation belt. In this work, we investigate the acceleration of heavy ions by electromagnetic ion cyclotron (EMIC) waves, which has been recently observed in the near-Earth plasma sheet. Nonlinear motion of ions, including phase trapping and strong acceleration, can be induced by EMIC waves through cyclotron resonance. The frozen-in condition of the plasma is not satisfied in this process. The results of test particle simulations show that heavy ions following wave-induced nonlinear motion from L = 5–7 increase from 36% of the total test particles to 44%. The He+ ions at L = 5 can be accelerated to as much as ten times the initial energy of 30 keV. The attainable energy increase ΔEk depends on the L value and the ion composition. An abundance of heavy ions in the background plasma lowers ΔEk. This effect is more prominent farther out in the plasma sheet. Nonlinear phase trapping occurs when ions are accelerated, and stronger acceleration corresponds to a smaller trapping zone. Hence, wave-induced acceleration is closely associated with the redistribution of ions in their phase space. Heavy ions in the plasma sheet have an important impact on inner magnetosphere dynamics, and thus, EMIC waves should not be neglected in plasma sheet energization.

Characteristics of storm time ion composition in the near-Earth plasma sheet using Geotail and RBSP measurements

Solar wind particles and ionospheric O+ ions influence the near-Earth plasma sheet and inner magnetospheric composition. We studied the behavior of H+, O+ and He+ ions (9–210 keV) for intense and moderate geomagnetic storms of solar cycle 23 and 24. An average energy density < ε > of ions over a given interval and flux enhancement is estimated using observations from satellites at different L values, namely STICS sensor on-board Geotail spacecraft and HOPE spectrometer on-board Radiation Belt Storm Probes. It provides a comprehensive understanding of the energy density variation of H+, O+ and He+ ions with the strength of IMF Bz, Psw, intensity of storms and L value. Statistically, we observed that (1) In the plasma sheet region, during main phase of the intense geomagnetic storm, < εO+/H+ > and < εHe+/H+ > enhances, (2) < εO+/H+ > is well correlated with Psw (CC = 0.86) and IMF Bz (CC = 0.85), (3) < εO+/H+ > shows higher correlation (CC = 0.73) with Kp than < εHe+/H+ > (CC = 0.65), indicating a fairly good dependence on the strength of geomagnetic activity, (4) < εO+/H+ > and < εHe+/H+ > dependence on L value indicates that O+/H+ and He+/H+ is more pronounced near L = 3. It is a cumulative extension of the previous studies on ion composition change which is in accordance with the existing picture of the plasma sheet and inner magnetosphere.

Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

Abstract. The fortunate location of Cluster and the THEMIS P3 probe in the near-Earth plasma sheet (PS) (at X ∼ −7–−9 RE) allowed for the multipoint analysis of properties and spectra of electron and proton injections. The injections were observed during dipolarization and substorm current wedge formation associated with braking of multiple bursty bulk flows (BBFs). In the course of dipolarization, a gradual growth of the BZ magnetic field lasted ∼ 13 min and it was comprised of several BZ pulses or dipolarization fronts (DFs) with duration ≤ 1 min. Multipoint observations have shown that the beginning of the increase in suprathermal (&gt; 50 keV) electron fluxes – the injection boundary – was observed in the PS simultaneously with the dipolarization onset and it propagated dawnward along with the onset-related DF. The subsequent dynamics of the energetic electron flux was similar to the dynamics of the magnetic field during the dipolarization. Namely, a gradual linear growth of the electron flux occurred simultaneously with the gradual growth of the BZ field, and it was comprised of multiple short (∼ few minutes) electron injections associated with the BZ pulses. This behavior can be explained by the combined action of local betatron acceleration at the BZ pulses and subsequent gradient drifts of electrons in the flux pile up region through the numerous braking and diverting DFs. The nonadiabatic features occasionally observed in the electron spectra during the injections can be due to the electron interactions with high-frequency electromagnetic or electrostatic fluctuations transiently observed in the course of dipolarization. On the contrary, proton injections were detected only in the vicinity of the strongest BZ pulses. The front thickness of these pulses was less than a gyroradius of thermal protons that ensured the nonadiabatic acceleration of protons. Indeed, during the injections in the energy spectra of protons the pronounced bulge was clearly observed in a finite energy range ∼ 70–90 keV. This feature can be explained by the nonadiabatic resonant acceleration of protons by the bursts of the dawn–dusk electric field associated with the BZ pulses. Keywords. Magnetospheric physics (Magnetotail; plasma sheet) – Space plasma physics (Transport processes)

Plasma flow disturbances in the magnetospheric plasma sheet during substorm activations

We have considered variations in fields and particle fluxes in the near-Earth plasma sheet on the THEMIS-D satellite together with the auroral dynamics in the satellite-conjugate ionospheric part during two substorm activations on December 19, 2014 with K p = 2. The satellite was at ~8.5RE and MLT = 21.8 in the outer region of captured energetic particles with isotropic ion fluxes near the convection boundary of electrons with an energy of ~10 keV. During substorm activations, the satellite recorded energetic particle injections and magnetic field oscillations with a period of ~90 s. In the satellite-conjugate ionospheric part, the activations were preceded by wavelike disturbances of auroral brightness along the southern azimuthal arc. In the expansion phase of activations, large-scale vortex structures appeared in the structure of auroras. The sudden enhancements of auroral activity (brightening of arcs, auroral breakup, and appearance of NS forms) coincided with moments of local magnetic field dipolarization and an increase in the amplitude Pi2 of pulsations of the Bz component of the magnetic field on the satellite. Approximately 30–50 s before these moments, the magnetosphere was characterized by an increased rate of plasma flow in the radial direction, which initiated the formation of plasma vortices. The auroral activation delays relative to the times when plasma vortices appear in the magnetosphere decreased with decreasing latitude of the satellite projection. The plasma vortices in the magnetosphere are assumed to be responsible for the observed auroral vortex structures and the manifestation of the hybrid vortex instability (or shear flow ballooning instability) that develops in the equatorial magnetospheric plane in the presence of a shear plasma flow in the region of strong pressure gradients in the Earthward direction.

Statistical analysis of severe magnetic fluctuations in the near-Earth plasma sheet observed by THEMIS-E

Abstract. We statistically analyzed severe magnetic fluctuations in the nightside near-Earth plasma sheet at 6–12 RE (Earth radii; 1 RE = 6371 km), because they are important for non-magnetohydrodynamics (non-MHD) effects in the magnetotail and are considered to be necessary for current disruption in the inside-out substorm model. We used magnetic field data from 2013 and 2014 obtained by the Time History of Events and Macroscale Interactions during Substorms E (THEMIS-E) satellite (sampling rate: 4 Hz). A total of 1283 severe magnetic fluctuation events were identified that satisfied the criteria σB∕B &gt; 0. 5, where σB and B are the standard deviation and the average value of magnetic field intensity during the time interval of the local proton gyroperiod, respectively. We found that the occurrence rates of severe fluctuation events are 0.00118, 0.00899, and 0.0238 % at 6–8, 8–10, and 10–12 RE, respectively, and most events last for no more than 15 s. From these occurrence rates, we estimated the possible scale sizes of current disruption by severe magnetic fluctuations as 3.83 RE3 by assuming that four substorms with 5 min intervals of current disruption occur every day. The fluctuation events occurred most frequently at the ZGSM (Z distance in the geocentric solar magnetospheric coordinate system) close to the model neutral sheet within 0.2 RE. Most events occur in association with sudden decreases in the auroral electrojet lower (AL) index and magnetic field dipolarization, indicating that they are related to substorms. Sixty-two percent of magnetic fluctuation events were accompanied by ion flow with velocity V &gt; 100 km s−1, indicating that the violation of ion gyromotion tends to occur during high-speed flow in the near-Earth plasma sheet. The superposed epoch analysis also indicated that the flow speed increases before the severe magnetic fluctuations. We discuss how both the inside-out and outside-in substorm models can explain this increase in flow speeds before magnetic fluctuation events.

Quantification of the total ion transport in the near-Earth plasma sheet

Abstract. Recent studies strongly suggest that a majority of the observed O+ cusp outflows will eventually escape into the solar wind, rather than be transported to the plasma sheet. Therefore, an investigation of plasma sheet flows will add to these studies and give a more complete picture of magnetospheric ion dynamics. Specifically, it will provide a greater understanding of atmospheric loss. We have used Cluster spacecraft 4 to quantify the H+ and O+ total transports in the near-Earth plasma sheet, using data covering 2001–2005. The results show that both H+ and O+ have earthward net fluxes of the orders of 1026 and 1024 s−1, respectively. The O+ plasma sheet return flux is 1 order of magnitude smaller than the O+ outflows observed in the cusps, strengthening the view that most ionospheric O+ outflows do escape. The H+ return flux is approximately the same as the ionospheric outflow, suggesting a stable budget of H+ in the magnetosphere. However, low-energy H+, not detectable by the ion spectrometer, is not considered in our study, leaving the complete magnetospheric H+ circulation an open question. Studying tailward flows separately reveals a total tailward O+ flux of about 0. 5 × 1025 s−1, which can be considered as a lower limit of the nightside auroral region O+ outflow. Lower velocity flows ( &lt; 100 km s−1) contribute most to the total transports, whereas the high-velocity flows contribute very little, suggesting that bursty bulk flows are not dominant in plasma sheet mass transport.

IMF dependence of energetic oxygen and hydrogen ion distributions in the near‐Earth magnetosphere

AbstractEnergetic ion distributions in the near‐Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and losses in the near‐Earth region. In this study, 11 years of energetic proton and oxygen observations (&gt; ~274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near‐Earth region. The dawn‐dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near‐Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are influenced by the dawn‐dusk interplanetary magnetic field (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm‐related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near‐Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause significantly affect the energetic ion population in the magnetosphere. We also believe that the influence of the dawn‐dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.

Average plasma sheet polytropic index as observed by THEMIS

Abstract. Multi-spacecraft data from the years 2008 to 2015 of the THEMIS mission particularly in the near-Earth plasma sheet are used in order to empirically determine the polytropic index in the quiet and active time magnetotail. The results of a number of previous studies in the 1990s can be confirmed. An analysis of the total database, although showing poor correlation, results in an average polytropic index of γ = 1. 72. The active time plasma sheet is well correlated with an average γ = 1. 49. However, the data scattering suggests that the analysis of the data in total is not adequate. In order to reduce the timescales, individual spacecraft orbits are analyzed, giving a broad distribution of polytropic indices throughout the plasma sheet. The major part of the distribution falls in a range between γ = 0. 67 and γ = 2. Our results indicate a variety of thermodynamic processes in the magnetotail and an all-time presence of heat exchange of the plasma. A description of the plasma sheet using an equation of state with a single γ is probably inadequate. This necessitates the application of more sophisticated approaches, such as a parametrization of the heat flux vector in magnetohydrodynamic equations or a superposition of polytropic indices.