Earth's Plasma Research Articles

Magnetospheric Multiscale analysis of intense field‐aligned Poynting flux near the Earth's plasma sheet boundary

AbstractThe Magnetospheric Multiscale mission is employed to examine intense Poynting flux directed along the background magnetic field toward Earth, which reaches amplitudes of nearly 2 mW/m2. The event is located within the plasma sheet but likely near the boundary at a geocentric distance of 9 RE in association with bulk flow signatures. The fluctuations have wavelengths perpendicular to the magnetic field of 124–264 km (compared to an ion gyroradius of 280 km), consistent with highly kinetic Alfvén waves. While the wave vector remains highly perpendicular to the magnetic field, there is substantial variation of the direction in the perpendicular plane. The field‐aligned Poynting flux may be associated with kinetic Alfvén waves released along the separatrix by magnetotail reconnection and/or the radiation of waves excited by bursty bulk flow braking and may provide a means through which energy released by magnetic reconnection is transferred to the auroral region.

The importance of international collaboration in space research

AbstractOver the past four decades, international collaborations in space plasma science have led to rapid growth in our understanding of the physical processes that control the transport of energy and plasma through the Earth's magnetosphere‐ionosphere system and the processes that transfer energy from the solar wind to the Earth's magnetosphere. These collaborations have also enabled more complex missions to be flown and more multipoint observations to be made in both the magnetosphere and ionosphere. In this commentary, we present some examples of these collaborations and discuss how they have improved our understanding of the Earth's plasma environment.

Major pathways to electron distribution function formation in regions of diffuse aurora

AbstractThis paper discusses the major pathways of electron distribution function formation in the region of diffuse aurora. The diffuse aurora accounts for about of 75% of the auroral energy precipitating into the upper atmosphere, and its origin has been the subject of much discussion. We show that an earthward stream of precipitating electrons initially injected from the Earth's plasma sheet via wave‐particle interactions degrades in the atmosphere toward lower energies and produces secondary electrons via impact ionization of the neutral atmosphere. These electrons of magnetospheric origin are then reflected back into the magnetosphere along closed dipolar magnetic field lines, leading to a series of reflections and consequent magnetospheric interactions that greatly augment the initially precipitating flux at the upper ionospheric boundary (700–800 km). To date this, systematic magnetosphere‐ionosphere coupling element has not been included in auroral research models, and, as we demonstrate in this article, has a dramatic effect (200–300%) on the formation of the precipitating fluxes that result in the diffuse aurora. It is shown that wave‐particle interaction processes that drive precipitating fluxes in the region of diffuse aurora from the magnetospheric altitudes are only the first step in the formation of electron precipitation at ionospheric altitudes, and they cannot be separated from the atmospheric “collisional machine” that redistributes and transfers their energy inside the magnetosphere‐ionosphere‐atmosphere coupling system.

Generation of extremely low frequency chorus in Van Allen radiation belts

AbstractRecent studies have shown that chorus can efficiently accelerate the outer radiation belt electrons to relativistic energies. Chorus, previously often observed above 0.1 equatorial electron gyrofrequency fce, was generated by energetic electrons originating from Earth's plasma sheet. Chorus below 0.1 fce has seldom been reported until the recent data from Van Allen Probes, but its origin has not been revealed so far. Because electron resonant energy can approach the relativistic level at extremely low frequency, relativistic effects should be considered in the formula for whistler mode wave growth rate. Here we report high‐resolution observations during the 14 October 2014 small storm and firstly demonstrate, using a fully relativistic simulation, that electrons with the high‐energy tail population and relativistic pitch angle anisotropy can provide free energy sufficient for generating chorus below 0.1 fce. The simulated wave growth displays a very similar pattern to the observations. The current results can be applied to Jupiter, Saturn, and other magnetized planets.

Observations of kinetic‐size magnetic holes in the magnetosheath

AbstractMagnetic holes (MHs), with a scale much greater than ρi (proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic‐size magnetic holes (KSMHs), previously called small‐size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three‐dimensional (3‐D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10–20 ρe (electron gyroradii) and lasted 0.1–0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90° pitch angle, the flux of electrons with energy 34–66 eV decreased, while for electrons of energy 109–1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self‐consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10–20 ρe.

Climatology of high‐β plasma measurements in Earth's inner magnetosphere

AbstractSince their launch in August 2012, the Radiation Belt Storm Probe Ion Composition Experiment (RBSPICE) instruments on the NASA Van Allen Probes spacecraft have been making continuous high‐resolution measurements of Earth's ring current plasma environment. After a full traversal through all magnetic local times, a climatology (i.e., a survey of observations) of high‐beta (β) plasma events (defined here as β > 1) as measured by the RBSPICE instrument in the ∼45 keV to ∼600 keV proton energy range in the inner magnetosphere (L < 5.8) has been constructed. In this paper we report this climatology of such high‐β plasma occurrences, durations, and their general characteristics. Specifically, we show that most high‐β events in the RBSPICE energy range are associated with postdusk/premidnight sector particle injections or plasma patches and can last from minutes to hours. While most of these events have a β less than 2, there are a number of observations reaching β greater than 4. Other observations of particular note are high‐β events during relatively minor geomagnetic storms and examples of very long duration high‐β plasmas. We show that high‐β plasmas are a relatively common occurrence in the inner magnetosphere during both quiet and active times. As such, the waves generated by these plasmas may have an underappreciated role in the inner magnetosphere, and thus the study of these plasmas and their instabilities may be more important than has been currently addressed.

Origin of low proton‐to‐electron temperature ratio in the Earth's plasma sheet

AbstractWe study the proton‐to‐electron temperature ratio (Tp/Te) in the plasma sheet (PS) of the Earth's magnetotail using 5 years of Cluster observations (2001–2005). The PS intervals are searched within a region defined with −19 < X ≤ −7 RE and |Y| < 15 RE (GSM) under the condition |BX| ≤ 10 nT. One hundred sixty PS crossings are identified. We find an average value of <Tp/Te> ~ 6.0. However, in many PS intervals Tp/Te varies over a wide range from a few units to several tens of units. In 86 PS intervals the Tp/Te decreases below 3.5. Generally, the decreases of Tp/Te are due to some increase of Te while Tp either decreases or remains unchanged. In the majority of these intervals the Tp/Te drops are observed during magnetotail dipolarizations. A superposed epoch analysis applied to these events shows that the minimum value of Tp/Te is observed after the dipolarization onset during the “turbulent phase” of dipolarization, when a number of transient BZ pulses are reduced, but the value of BZ is still large and an intensification of wave activity is observed. The Tp/Te drops, and associated increases of Te often coincide either with bursts of broadband electrostatic emissions, which may include electron cyclotron harmonics, or with broadband electromagnetic emission in a frequency range from proton plasma frequency (fpp) up to the electron gyrofrequency (fce). These findings show that the wave activity developing in the current sheet after dipolarization onset may play a role in the additional electron heating and the associated Tp/Te decrease.

Multispacecraft analysis of dipolarization fronts and associated whistler wave emissions using MMS data

AbstractDipolarization fronts (DFs), embedded in bursty bulk flows, play a crucial role in Earth's plasma sheet dynamics because the energy input from the solar wind is partly dissipated in their vicinity. This dissipation is in the form of strong low‐frequency waves that can heat and accelerate energetic electrons up to the high‐latitude plasma sheet. However, the dynamics of DF propagation and associated low‐frequency waves in the magnetotail are still under debate due to instrumental limitations and spacecraft separation distances. In May 2015 the Magnetospheric Multiscale (MMS) mission was in a string‐of‐pearls configuration with an average intersatellite distance of 160 km, which allows us to study in detail the microphysics of DFs. Thus, in this letter we employ MMS data to investigate the properties of dipolarization fronts propagating earthward and associated whistler mode wave emissions. We show that the spatial dynamics of DFs are below the ion gyroradius scale in this region (∼500 km), which can modify the dynamics of ions in the vicinity of the DF (e.g., making their motion nonadiabatic). We also show that whistler wave dynamics have a temporal scale of the order of the ion gyroperiod (a few seconds), indicating that the perpendicular temperature anisotropy can vary on such time scales.

Ionosphere‐magnetosphere energy interplay in the regions of diffuse aurora

AbstractBoth electron cyclotron harmonic (ECH) waves and whistler mode chorus waves resonate with electrons of the Earth's plasma sheet in the energy range from tens of eV to several keV and produce the electron diffuse aurora at ionospheric altitudes. Interaction of these superthermal electrons with the neutral atmosphere leads to the production of secondary electrons (E < 500–600 eV) and, as a result, leads to the activation of lower energy superthermal electron spectra that can escape back to the magnetosphere and contribute to the thermal electron energy deposition processes in the magnetospheric plasma. The ECH and whistler mode chorus waves, however, can also interact with the secondary electrons that are coming from both of the magnetically conjugated ionospheres after they have been produced by initially precipitated high‐energy electrons that came from the plasma sheet. After their degradation and subsequent reflection in magnetically conjugate atmospheric regions, both the secondary electrons and the precipitating electrons with high (E > 600 eV) initial energies will travel back through the loss cone, become trapped in the magnetosphere, and redistribute the energy content of the magnetosphere‐ionosphere system. Thus, scattering of the secondary electrons by ECH and whistler mode chorus waves leads to an increase of the fraction of superthermal electron energy deposited into the core magnetospheric plasma.

Electron dynamics in a subproton‐gyroscale magnetic hole

AbstractMagnetic holes are ubiquitous in space plasmas, occurring in the solar wind, downstream of planetary bow shocks, and inside the magnetosphere. Recently, kinetic‐scale magnetic holes have been observed near Earth's central plasma sheet. The Fast Plasma Investigation on NASA's Magnetospheric Multiscale (MMS) mission enables measurement of both ions and electrons with 2 orders of magnitude increased temporal resolution over previous magnetospheric instruments. Here we present data from MMS taken in Earth's nightside plasma sheet and use high‐resolution particle and magnetometer data to characterize the structure of a subproton‐scale magnetic hole. Electrons with gyroradii above the thermal gyroradius but below the current layer thickness carry a current sufficient to account for a ~10–20% depression in magnetic field magnitude. These observations suggest that the size and magnetic depth of kinetic‐scale magnetic holes is strongly dependent on the background plasma conditions.

On the rogue wave propagation in ion pair superthermal plasma

Effects of superthermal electron on the features of nonlinear acoustic waves in unmagnetized collisionless ion pair plasma with superthermal electrons have been examined. The system equations are reduced in the form of the nonlinear Schrodinger equation. The rogue wave characteristics dependences on the ionic density ratio (ν = n–0/n+0), ionic mass ratio (Q = m+/m−), and superthermality index (κ) are investigated. It is worth mentioning that the results present in this work could be applicable in the Earth's ionosphere plasmas.

Large‐amplitude electric fields associated with bursty bulk flow braking in the Earth's plasma sheet

AbstractWe report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.

Heating of the plasma sheet by broadband electromagnetic waves

AbstractWe demonstrate that broadband low‐frequency electromagnetic field fluctuations embedded within fast flows throughout the Earth's plasma sheet may drive significant ion heating. This heating is nearly entirely in the direction perpendicular to the background magnetic field and is estimated to occur at an average rate of ~1 eV/s with rates in excess of 10 eV/s within one standard deviation of the average value over all observed events. For an Earthward flow the total change in temperature along a flow path may exceed one keV and for “wave‐rich” flows can be comparable to that expected due to conservation of the first adiabatic invariant. The consequent increase in plasma pressure and flux tube entropy may lead to braking of inward motion and the suppression of plasma interchange.

Electron distributions observed with Langmuir waves in the plasma sheet boundary layer

The present paper investigates the Langmuir turbulence driven by counter-streaming electron beams and its plausible association with observed features in the Earth's plasma sheet boundary layer region. A one-dimensional electrostatic particle-in-cell simulation code is employed in order to simulate broadband electrostatic waves with characteristic frequency in the vicinity of the electron plasma frequency ω/ωpe≃1.0. The present simulation confirms that the broadband electrostatic waves may indeed be generated by the counter-streaming electron beams. It is also found that the observed feature associated with low energy electrons, namely quasi-symmetric velocity space plateaus, are replicated according to the present simulation. However, the present investigation only partially succeeds in generating the suprathermal tails such that the origin of observed quasi power-law energetic population formation remains outstanding.

Properties of low‐latitude mantle plasma in the Earth's magnetotail: ARTEMIS observations and global MHD predictions

AbstractThe Earth's plasma mantle is one of the major suppliers of particles for the plasma sheet. To understand its plasma characteristics, spatial distributions, and dependencies on interplanetary magnetic field (IMF) direction, we statistically analyzed the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) observations in the low‐latitude magnetotail (~10 RE above and below the current sheet) and investigated the predictions from global Block Adaptive Tree Solar wind‐Roe‐Upwind Scheme MHD simulations. The mantle plasma flows tailward along magnetic field lines (~50–200 km/s) and at the same time drifts toward midnight and toward the current sheet. The mantle plasma has similar temperature (~0.05–0.2 keV) to the magnetosheath plasma but has lower density (~0.1–1 cm−3). The mantle appearance is dawn‐dusk asymmetric depending mainly on the IMF By direction. The occurrence rates, density, and V|| all decrease with decreasing |Y|. This density cross‐tail profile suggests that the low‐latitude mantle plasma mainly comes from the magnetosheath entering through the tail magnetopause along the open field lines. Density is highly and positively correlated with V||. These observations are qualitatively consistent with the MHD results. The simulations indicate that as IMF By becomes dominant, the source locations at the magnetopause for the mantle move to lower latitudes and become dawn‐dusk asymmetric, and the tail cross section also becomes distorted with the magnetopause shape elongating and the current sheet tilting significantly. Degrees of these changes also vary with the downtail distances and IMF Bz direction. The source location change leads to the dawn‐dusk asymmetric mantle appearance. The tail cross‐section change alters the distance from the sources to the current sheet and thus the resulting mantle density distributions just outside the plasma sheet.

Bursty flows in magnetosphere influence ionosphere

In Earth's plasma sheet—a region of dense ionized gas that lies around the equatorial plane and extends into the magnetotail—plasma flows in large‐scale smooth flows as well as in small‐scale bursts. Studies have investigated the individual bursty flows but not the large‐scale effects of these bursts. To investigate how the bursty flows affect the magnetosphere and its coupling to the ionosphere, Yang et al. conducted two simulations, one with only large‐scale flows and the other with random bubbles and blobs of plasma flowing both toward Earth and away from it. They found that the bursty flows significantly altered the magnetic properties of the plasma sheet and the distributions of current in the ionosphere and that the properties found in the bursty flow simulations matched observations much better than the simulations without bursty flows.

Backscattered energetic neutral atoms from the Moon in the Earth's plasma sheet observed by Chandarayaan‐1/Sub‐keV Atom Reflecting Analyzer instrument

AbstractWe present the observations of energetic neutral atoms (ENAs) produced at the lunar surface in the Earth's magnetotail. When the Moon was located in the terrestrial plasma sheet, Chandrayaan‐1 Energetic Neutrals Analyzer (CENA) detected hydrogen ENAs from the Moon. Analysis of the data from CENA together with the Solar Wind Monitor (SWIM) onboard Chandrayaan‐1 reveals the characteristic energy of the observed ENA energy spectrum (the e‐folding energy of the distribution function) ∼100 eV and the ENA backscattering ratio (defined as the ratio of upward ENA flux to downward proton flux) <∼0.1. These characteristics are similar to those of the backscattered ENAs in the solar wind, suggesting that CENA detected plasma sheet particles backscattered as ENAs from the lunar surface. The observed ENA backscattering ratio in the plasma sheet exhibits no significant difference in the Southern Hemisphere, where a large and strong magnetized region exists, compared with that in the Northern Hemisphere. This is contrary to the CENA observations in the solar wind, when the backscattering ratio drops by ∼50% in the Southern Hemisphere. Our analysis and test particle simulations suggest that magnetic shielding of the lunar surface in the plasma sheet is less effective than in the solar wind due to the broad velocity distributions of the plasma sheet protons.

Low-rank approximation in the numerical modeling of the Farley–Buneman instability in ionospheric plasma

We consider numerical modeling of the Farley–Buneman instability in the Earth's ionosphere plasma. The ion behavior is governed by the kinetic Vlasov equation with the BGK collisional term in the four-dimensional phase space, and since the finite difference discretization on a tensor product grid is used, this equation becomes the most computationally challenging part of the scheme. To relax the complexity and memory consumption, an adaptive model reduction using the low-rank separation of variables, namely the Tensor Train format, is employed.The approach was verified via a prototype MATLAB implementation. Numerical experiments demonstrate the possibility of efficient separation of space and velocity variables, resulting in the solution storage reduction by a factor of order tens.

Plasma loss mechanisms from Saturn's magnetosphere

Since the first up‐close observations of Saturn, made by the Pioneer 11 probe in 1979, a great deal has been learned about the dynamics of the gas giant's magnetosphere. In‐depth observations made by the Cassini orbiter, which has been circling Saturn since 2004, have revealed fundamental differences between the behavior of Saturn's magnetosphere and that of the Earth's magnetosphere. Earth's magnetospheric plasma is largely populated by ions captured from the solar wind, whereas Saturn's plasma comes predominantly from water vapor that spews from massive geysers on the southern end of its icy moon Enceladus. Ionized water vapor from Enceladus streams out at 12 to 250 kilograms per second, yet observations show that the concentration of plasma in Saturn's magnetosphere is at a relatively steady level. This discrepancy has left researchers searching for potential plasma loss mechanisms. In a review, Thomsen highlights the progress made in recent years in understanding this question.

Theoretical constraints on the cross-tail width of bursty bulk flows

Abstract. The characteristic cross-tail width of bursty bulk flows (BBFs) in earth's plasma sheet was investigated at two stages of its life, one at its onset, the other when it is fully developed. Equilibrium domains with gradient of magnetic field are constructed. Interchange instability analysis of such domains yields the most unstable mode with the half wave length comparable with the observed cross-tail width of a flow burst and the inverse of growth rate comparable with its duration. The thickness of the plasma sheet for the most unstable mode is also comparable to the width of BBFs in the north–south direction. We found that viscosity, the dimension of the unstable domain, the thickness of the plasma sheet and gradient of the magnetic field together determine the most unstable mode. The ion Larmor radius plays an important role in viscosity as half effective mean free path. For a fully developed flow, however, velocity-caused pressure difference between the leading and trailing sides of a flow burst also plays a role. The equatorial cross section of flow is reshaped and its cross-tail width is changed as well. Representing the surrounding medium with empirical magnetic field and plasma models, the force balance of the fast flow is analyzed. The cross-section area of flow burst is estimated to be one to several square earth radii, and the cross-tail width of fast flow is estimated to be 1 to 3 earth radii, which is consistent with observations of BBFs.