Low-energy ion data (E < 25 keV/q) acquired by the Kaguya spacecraft were used to study ions originating near the lunar surface when the Moon was in the terrestrial magnetotail lobe. We focus on three intervals during which Kaguya detected these ions at different altitudes: similar to 100 km on 15 October 2008, similar to 50 km on 9 April 2009, and a descent from similar to 60 to similar to 30 km on 9 May 2009. In each interval, the strongly enhanced fluxes of lunar-origin ions exhibited a band structure spanning less than 100 eV/q to similar to 1,000 eV/q, with pitch angles from 30 degrees to 140 degrees. To understand where and how cold lunar-origin ions (<1 eV) are energized to the observed energy levels, we conducted test-particle simulations. The simulations indicate that these ions can reach energies of similar to 300 eV through multiple reflections from the lunar photoelectron sheath, and are further accelerated by the motional electric field, increasing their energies from several hundred eV to similar to 1,000 eV, consistent with Kaguya observations.

This article will review many ideas and concepts originally suggested by Alfvén (Space Sci Rev 7:1940, 1967) and Fälthammar (Rev Geophys 15: 457, 1977) on the important roles electric fields and currents play in solar-terrestrial plasmas. Much of our understanding of planetary and solar plasmas has come primarily from treating plasmas as fluids. Fluid theory has emphasized bulk parameters and frozen-in magnetic field. This article, however, will focus on electric field, currents, and particles. This approach will give us a much broader perspective about space and solar plasma dynamics. We review relevant observations, theories, and models that will help us understand the basic physics. Our discussion includes the heliospheric current sheet formed by the solar wind (SW). All of the planets are immersed in this current sheet, and magnetospheres are formed by the interaction of the current sheet with planetary magnetic fields. We discuss how the electric fields induced by the evolving current sheets can accelerate particles to high energies pertinent for auroras and solar flares. The magnetic polarity across the current sheets can reverse directions over a distance of a few Larmor radii. We review the orbit trajectories resulting from inclusion of a B_y component to the magnetic field and discuss how Speiser’s calculations (Speiser in J Geophys Res 70:4219, 1965) are modified. We have applied both the Lorentz equation and Vlasov theory to boundary structures. We review how inhomogeneous magnetic field can produce parallel electric fields when electrons and ions mirror at different heights. Detailed observations of electrostatic waves and double layers discovered by instruments on S3-3 spacecraft are revisited. The Swift (J Geophys Res 80:2096, 1975) model of solitary waves and double layers reviewed by Goertz (Rev Geophys 17:418, 1979) is examined and discussed. We evaluate what is known and not known and the issues that still need to be resolved. Observations of electrostatic nonlinear structures in the SW are surprising. Although the sources of these structures are not known, these observations may be simply telling us that the solar corona, like Earth, may include mechanisms that can drive electric fields and currents along the magnetic field. Remarkable accomplishments have been made during the last 50 years but many problems still remain to be studied further and solved.

Electromagnetic ion cyclotron (EMIC) waves generated by hot anisotropic (T ⊥ > T ∥ ) protons (∼10–100 keV), play an important role in accelerating cold (<1 eV) protons (H + ) and helium (He + ) ions in the magnetosphere. Using a hybrid code with parameters found in the inner magnetosphere, we examine when and how cold H + and He + ions are energized by EMIC waves. Hybrid simulations show that the energization of the cold particles occurs in two steps. In the first step, EMIC waves, which are linearly excited in the early stage of the simulation, interact with cold H + and He + ions, resulting in energization mostly in the direction perpendicular to the background magnetic field. The energization in this step is mainly contributed by enhanced bulk motion of these ions as a result of the linear response, consistent with recent observations in the inner magnetosphere. In the second step, nonlinear evolution of energized cold H + and He + ions are confirmed in the parallel direction, which is seen after about 200 proton gyroperiods (∼8.5 s). Throughout the simulation run, cold He + ions are much more energized in the perpendicular direction than in the parallel direction. However, the cold protons are more energized in the parallel direction than in the perpendicular direction after 500 proton gyroperiods (∼21.3 s). By comparing recent observations and the present simulation results, we suggest that the cold particle energization by EMIC waves occurs at an early stage of wave generation when the nonlinear evolution of EMIC waves is not dominant in the inner magnetosphere.

The recent Parker Solar Probe (PSP) observations of type III radio bursts show that the effects of finite background magnetic field can be an important factor in the interpretation of data. In the present paper, the effects of background magnetic field on the plasma emission process, which is believed to be the main emission mechanism for solar coronal and interplanetary type III radio bursts, are investigated by means of the particle-in-cell simulation method. The effects of ambient magnetic field are systematically surveyed by varying the ratio of plasma frequency to electron gyro-frequency. The present study shows that for a sufficiently strong ambient magnetic field, the wave-particle interaction processes lead to a highly field-aligned longitudinal mode excitation and anisotropic electron velocity distribution function, accompanied by a significantly enhanced plasma emission at the second harmonic plasma frequency. For such a case, the polarization of the harmonic emission is almost entirely in the sense of extraordinary mode. On the other hand, for moderate strengths of the ambient magnetic field, the interpretation of the simulation result is less than clear. The underlying nonlinear mode coupling processes indicate that to properly understand and interpret the simulation results require sophisticated analyses involving interactions among magnetized plasma normal modes including the two transverse modes of the magneto-active plasma, namely, extraordinary and ordinary modes, as well as electron-cyclotron-whistler, plasma oscillation, and upper-hybrid modes. At present, a nonlinear theory suitable for quantitatively analyzing such complex mode-coupling processes in magnetized plasmas is incomplete, which calls for further theoretical research, but the present simulation results could provide a guide for future theoretical efforts.

Although electromagnetic ion cyclotron (EMIC) waves are commonly observed in the magnetosphere and are believed to energize background cold ions, it is not clear whether EMIC waves play a significant role in determining spacecraft potential change. In this paper, we present two strong He‐band EMIC wave events observed by the Van Allen Probe‐B spacecraft inside the plasmasphere. One event occurred on 11 March 2016 when the spacecraft was on the dayside, and the other occurred on 9 October 2016 when the spacecraft was in the postmidnight sector. When a strong He‐band EMIC wave activity was detected, low‐energy ion flux enhancements occurred nearly simultaneously with the EMIC wave power enhancements. Both events presented in this study are clearly unique in that He‐band wave power and enhanced proton flux are extremely high. During the wave activity interval, we found that the spacecraft charged more positively without a significant change in the ambient electron density. We discuss whether low‐energy ions energized by EMIC waves can contribute to the spacecraft potential change.

Observations of solar wind (SW) ions in the vicinity of Earth's bow shock have shown that the incident SW is contaminated by several different populations with density and temperature that vary by orders of magnitude. To understand how the SW interacts with the bow shock, the different populations must be separated. Cluster has two separate instruments, one to measure the cold SW and the other the hot magnetosheath (MS) plasmas. The SW experiment has now measured the pristine SW including new features. We first present a brief review of Earth's bow shock as currently understood followed by new Cluster results that show the SW can remain super-Alfvenic in the MS retaining much of the same properties observed before crossing the bow shock. These results indicate the interaction of the SW with the bow shock is still not well understood. Examination of the heating mechanisms suggested for the SW including particle-in-cell (PIC) simulation results indicates that while our understanding of the bow shock physics has improved over time, an updated picture of the SW interaction with the bow shock still needs to be developed to explain the Cluster results.

The impact of an interplanetary shock significantly disturbs the Earth’s magnetosphere. Although the disturbances in the magnetotail induced by such an impact have extensively been investigated in previous studies, they are not still fully understood. In this study, we examined the disturbances produced in the central plasma sheet observed by the Cluster spacecraft. After the sudden impulse (SI) on the ground, significant compression of the magnetotail was observed. Initially, the dominant direction of the compression was the dawn-dusk direction, but later it changed to the north–south direction. Ions and electrons in the plasma sheet were significantly energized in the energy range from ~ 5 to ~ 100 keV. The energization was more effective on electrons than ions, and the energy dependence of the energization suggests that this process is non-adiabatic. We suggest that these particles can serve as the seed population of relativistic particles in the radiation belts.

We have studied the statistical properties of low‐energy proton (H + ) and helium (He + ) ion flux enhancements associated with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere using Van Allen Probes data for 2013–2017. We identified 167 low‐energy ion flux enhancements when the EMIC waves occurred in a He‐band or in a multiple band (H‐band and He‐band) with strong He‐band and weak H‐band wave activity and found that most of them occurred from the noon to the premidnight sector near the magnetic equator just inside the plasmapause. Of 167 flux enhancement events, 68 exhibited only He + flux enhancements, and 99 exhibited both H + and He + flux enhancements. The EMIC wave‐associated flux enhancement events are mostly energized in the direction perpendicular to the background magnetic field. When both H + and He + fluxes are simultaneously enhanced, the H + flux events have a peak energy distributed in the range of 2–100 eV, and the peak energies of the He + flux events are distributed in the 2–600 eV range, implying that the helium ions are more energized than the protons. The peak energies of only He + flux enhancement without H + flux enhancement are mostly distributed in a lower energy range, 2–10 eV. The energization of H + and He + ions can be explained by a linear plasma flow associated with EMIC waves. We suggest that the wave‐associated linear plasma motion is a likely mechanism to explain the observations.

Studies of shocks have long suggested that a shock can undergo cyclical self-reformation on a timescale of ion cyclotron period. This process has been proposed as a primary mechanism for energy dissipation and energetic particle acceleration at shocks. Unambiguous observational evidence, however, has remained elusive. Here, we report direct observations for the self-reformation process of a collisionless, high Mach number, quasi-perpendicular shock using Magnetospheric Multiscale (MMS) measurements. We find that reflected ions by the old shock ramp form a clear phase-space vortex, which gives rise to a new ramp. The new ramp observed by MMS2 has not yet developed to a mature stage during the self-reformation, and is not strong enough to reflect incident ions. Consequently, these ions are only slightly slowed down and show a flat velocity profile from the new ramp all the way to the old one. The present results provide direct evidence of shock self-reformation, and also shed light on energy dissipation and energetic particle acceleration at collisionless shocks throughout the universe.

Properties of plasmas that constitute the plasma sheet in the near-Earth magnetotail vary according to the solar wind conditions and location in the tail. In this case study, we present multi-spacecraft observations by Cluster that show a transition of plasma sheet from cold, dense to hot, tenuous state. The transition was associated with the passage of a spatial boundary that separates the plasma sheet into two regions with cold, dense and hot, tenuous plasmas. Ion phase space distributions show that the cold, dense ions have a Kappa distribution while the hot, tenuous ions have a Maxwellian distribution, implying that they have different origins or are produced by different thermalization processes. The transition boundary separated the plasma sheet in the dawn-dusk direction, and slowly moved toward the dawn flank. The hot, tenuous plasmas filled the central region while the cold, dense plasmas filled the outer region. The hot, tenuous plasmas were moving toward the Earth, pushing the cold, dense plasmas toward the flank. Different types of dynamical processes can be generated in each region, which can affect the development of geomagnetic activities.

A simple internal charging current monitor has been developed to monitor the penetration of energetic particles within spacecraft in geostationary orbits. The instrument, known as a charging monitor (CM), is a component of a suite of space weather instruments aboard the geostationary satellite GEO-KOMPSAT-2A (GK2A), that was successfully launched on December 4, 2018, with a final longitude of 128.2 degrees E. The instrument measures the integral fluxes of charged particles through a 1-mm-thick aluminum wall, along with another 1-mm-thick aluminum plate, effectively collecting electron and proton currents with energies approximately above 0.6 and 13.1 MeV, respectively. Great care has been exercised in the design of the instrument to extend the lifetime of the analog electronics of the detector, which may easily become vulnerable to the harsh radiation environment of the outer electron belt in geostationary orbits. Comparisons of measurements from the CM have been made with those from particle detectors with a more sophisticated design from the same suite. Initial results of the CM operation for the first four months are presented, including a period of moderate geomagnetic storms. The comparisons clearly show that the CM is sufficiently capable of monitoring the enhanced levels of electrical currents from the penetration of high-energy charged particles in the context of space weather research with a minimal spacecraft resources cost and design efforts.

This paper describes the initial operations and preliminary results of the Instrument for the study of Stable/Storm-time Space (ISSS) onboard the microsatellite Next Generation Small Satellite-1 (NEXTSat-1), which was launched on December 4, 2018 into a sun-synchronous orbit at an altitude of 575 km with an orbital inclination angle of 97.7°. The spacecraft and the instruments have been working normally, and the results from the observations are in agreement with those from other satellites. Nevertheless, improvement in both the spacecraft/instrument operation and the analysis is suggested to produce more fruitful scientific results from the satellite operations. It is expected that the ISSS observations will become the main mission of the NEXTSat-1 at the end of 2020, when the technological experiments and astronomical observations terminate after two years of operation.

In this paper we present analysis of current density when the Cluster spacecraft pass the nightside auroral region at about 4-5 RE from the center of Earth. The analysis is made when the inter-spacecraft separation is within 200 km, which allows all four spacecraft to be situated inside the same current sheet. On 22 February 2002, two field-aligned current (FAC) events were observed in both the southern and the northern hemispheres. The FACs were calculated with magnetic field data obtained by the four spacecraft using the Curlometer method. The scales of the FACs along the spacecraft trajectory and the magnitudes were hundreds of kilometers and tens of nA/m2, respectively, and both events were mapped to the auroral region in the ionosphere. We also examined reliability of the results with some parameters, and found that our results are adequately comparable with other studies. Nevertheless, some limitations that decrease the accuracy of current estimation exist.

Finite heat flux often exists in space and astrophysical plasmas, which can be a free energy source for heat flux instability. The solar wind is a well-known example of such plasmas and a number of previous studies have investigated the characteristics of heat flux instability in the context of solar wind. In the literature there exists some uncertainties regarding the properties of heat flux instability. While some linear theories predict the association of the heat flux instability with right-hand polarized whistler waves, other studies argue for left-hand polarized unstable modes. The present study investigates the nonlinear development of initially unstable left-hand heat flux mode by means of particle-in-cell simulation. It is found that while the early phase is characterized by the left-hand polarization, in agreement with linear theory, as the wave amplitude becomes high and the instability enters the nonlinear phase, the dominant wave mode gradually switches over to the right-hand polarized waves. Such a behavior is related to the pitch angle scattering of the heat flux carrying electrons by nonlinear interaction with large-amplitude waves. The present study shows that the heat flux instability generally requires nonlinear treatment such that characterizing its behavior with linear theories may not always be adequate.

The whistler anisotropy (or electromagnetic electron cyclotron) instability may be operative in many geomagnetic and heliospherical environments, including the radiation belt, solar wind, and the solar corona. The present investigation carries out a comparative analysis between the two-dimensional particle-in-cell simulation of weakly growing whistler anisotropy instability and the velocity moment-based two-dimensional quasi-linear theory under the assumption of bi-Maxwellian electron distribution function. It is shown that the simplified quasi-linear theory provides a qualitative agreement with the more rigorous particle-in-cell simulation, but some discrepancies are also found. Possible causes for the differences in either method are discussed, and future improvements on the theory are suggested. Potential applicability of the present finding in the context of the space and astrophysics is discussed.

We examine magnetic and electric field perturbations associated with a sudden commencement (SC), caused by an interplanetary (IP) shock passing over the Earth's magnetosphere on 16 February 2013. The SC was identified in the magnetic and electric field data measured at Time History of Events and Macroscale Interactions during Substorms (THEMIS‐E; THE‐E: magnetic local time (MLT) = 12.4, L = 6.3), Van Allen Probe‐A (VAP‐A: MLT = 3.2, L = 5.1), and Van Allen Probe‐B (VAP‐B: MLT = 0.2. L = 4.9) in the magnetosphere. During the SC interval, THE‐E observed a dawnward‐then‐duskward electric ( E ) field perturbation around noon, while VAP‐B observed a duskward E field perturbation around midnight. VAP‐A observed a dawnward‐then‐duskward E field perturbation in the postmidnight sector, but the duration and magnitude of the dawnward E perturbation are much shorter and weaker than that at THE‐E. That is, the E field signature changes with local time during the SC interval. The Super Dual Auroral Radar Network radar data indicate that the ionospheric plasma motions during the SC are mainly due to the E field variations observed in space. This indicates that the SC‐associated E field in space plays a significant role in determining the dynamic variations of the ionospheric convection flow. By comparing previous SC MHD simulations and our observations, we suggest that the E field variations observed at the spacecraft are produced by magnetospheric convection flows due to deformation of the magnetosphere as the IP shock sweeps the magnetopause.

The Earth’s bow shock is the best-known collisionless shock in space. Although much is known about the bow shock, the mechanisms of heating and thermalization processes still remain poorly understood. Collisionless shocks are different from ordinary fluid shocks, because a fraction of the incident solar wind is reflected from the bow shock and the transmitted particles are not immediately thermalized. The reflected particles interact with the incident solar wind producing waves and instabilities that can heat and accelerate particles to high energies. Some of the waves can grow to large amplitudes such as Short Large Amplitude Magnetic Structures. Other upstream nonlinear structures include hot flow anomalies and density holes. The upstream nonlinear structures subsequently convect Earthward with the SW and could impact the structure and dynamics of the bow shock. These observations have clearly indicated that the upstream dynamics are an integral part of the bow shock system. Although much has been learned about the behavior of Earth’s bow shock dynamics from the existing data, many fundamental questions remain not answered. This article will review observations of ion dynamics of Earth’s bow shock system, what we have learned from recent and past observations. We provide new perspectives from multi-spacecraft Cluster observations about the spatial and temporal variations including the fundamental shock heating, acceleration, and entropy generation processes.

The mass, charge and energy dependence of the SW interaction with the bow shock was studied from early days of HEOS-1 and ISEE (Formisano et al, 1970; Peterson et al., 1979). These observations have shown that while thermalization of H occurs across the boundary, sometimes the SW He ions are found with unchanged energy spectra downstream of the shock inside the magnetosheath and that both SW H and He beams could be found in the downstream magnetosheath with the same bulk velocities. These studies however used He data accumulated over 30 minutes and since the SW dynamics include much faster time variations the results are likely affected by spatial and temporal variations. Moreover, it was not known at that time that the plasma in the neighborhood of the bow shock often include the reflected, gyrating and particles leaking out of the magnetosheath (Skopke et al., 1982; Thomsen et al., 1985) and since these particles occupy different parts of the velocity space, they can significantly affect the SW velocity and temperature computed from first and second velocity moments. To alleviate these problems, a microprocessor-controlled SW plasma experiment was designed and flown on Cluster that selects only particles near the peak energy of the SW distribution, thereby minimizing contamination from the other particles (Reme et al., 2001). We have studied ~110 shock crossings upstream and downstream of the quasi-perpendicular and quasi-parallel bow shock regions and find that in 44 cases the SW beams crossed the shock retaining much of their upstream features. On average the temperature of upstream SW H ions was ~4 eV and in the magnetosheath ~4.2 eV, indicating there was little or no heating of the SW going across the bow. The He ions have temperatures typically 4 times that of H ions in the SW a value consistent with equipartition of energy. Unlike H ions, He ions very often do not slow down going across the shock. These observations indicate that the SW interaction with the bow shock is much more complicated than existing models predict and they are important constraints for developing new models.

We have studied the spectral properties of quiet-time electromagnetic ion cyclotron (EMIC) waves following a steady quiet condition, which is defined with Kp values 1 during 12h, using GOES 10, 11, and 12 magnetometer data for solar minimum years 2007-2008. We identified 6584 steady quiet-time EMIC wave samples using a semiautomated procedure. Approximately 82% of the samples were observed in the morning-to-early afternoon sector (0700-1500 magnetic local time) with a maximum occurrence near noon, and their peak frequencies were mostly in the He band. We found that the occurrence rate of steady quiet-time EMIC waves is higher than that of EMIC waves for all or quiet geomagnetic conditions (Dst > 0nT or AE < 100nT) reported in previous studies by a factor of 2 or more. The frequency ratio f(peak) (sample's peak frequency)/ fH+ (the local proton gyrofrequency) of the He-band waves (approximate to 0.11-0.16) under steady quiet conditions is lower than that (approximate to 0.14-0.24) in previous studies. These results may be due to the fact that the plasmasphere expanded more frequently to the geosynchronous region under extremely quiet geomagnetic conditions in 2007-2008 than the periods selected in previous studies. The amplitude and frequency of He-band EMIC waves for nonlinear wave growth are examined as changing cold plasma density at geosynchronous orbit. We confirm that the spectral properties of observed EMIC waves are in good agreement with the nonlinear theory.