ABSTRACT. Using five electromagnetic waveforms (2E and 3B) below 4 kHz acquired by the Plasma Wave Instrument (PWI) onboard Geotail, we examined the detailed characteristics of the ELF/VLF electromagnetic waves in the magnetosphere, such as polarization, refractive index, and k-vector and Poynting directions. These parameters were used to investigate the propagation and generation mechanisms of the ELF/VLF waves, as well as to give the valuable information on the macroscopic properties and structures of the plasma medium through which they propagated. In this presentation we will review the Geotail observations of ELF/VLF waves such as chorus emissions, whistlers, and continuum radiations in the magnetosphere. We will also discuss how these waves were used to calibrate the antenna characteristics such as effective length and impedance.

ABSTRACT. Various kinds of plasma waves are generated as a consequence of wave-particle interaction, and they play important roles in the physical processes in the magnetosphere.

The Arase (ERG) satellite was launched in December 2016 to investigate acceleration and loss mechanisms of relativistic electrons in the inner magnetosphre. The Plasma Wave Experiment (PWE) aboard the Arase measures electric wave fields from DC to 10 MHz and magnetic wave fields from a few Hz to 100 kHz. Six years have passed since the Arase started its observations, and the PWE continues to comprehensively observe plasma waves in the inner magnetosphere.

In the present paper, we review the new findings revealed by the Arase/PWE. We also introduce our prospects of plasma wave meaturements for future mission.

ABSTRACT. One of the most important discoveries by the Geotail spacecraft is electrostatic solitary waves (ESW) as a manifestation of nonlinear wave-particle interactions in space plasmas. The wave-form capture of Geotail with high time resolution revealed coherent wave packets isolated in space and time. Through comparison with particle simulations of electron beam instabilities, it has been found that the strong kinetic instabilities evolve to formation of electron holes in the velocity phase space. The ESW is a coherent wave that can form a nonlinear trapping potential for energetic electrons moving with the phase velocity of the wave. A series of electron holes is formed initially, and they undergo coalescence with each other, resulting in a very stable solitary wave known as the BGK mode in plasmas. The same nonlinear trapping also takes place in the generation process of electromagnetic whistler-mode waves called chorus emissions, in which wave potentials are formed with counter-streaming electrons via cyclotron resonance. Through formation of electron holes (depletion) or hills (enhancement) in the trapping wave potential, resonant currents are formed which generate new emissions with varying frequencies. In the case of emissions with rising-tone frequencies, some of the electrons are efficiently accelerated to energies of a few MeV, resulting in formation of radiation belts. Particle acceleration by the nonlinear wave trapping also takes place for energetic protons trapped by electromagnetic ion cyclotron (EMIC) waves. Observations of highly energetic ions in the radiation belts of Jupiter and Saturn are also targets of future missions along with developing multidimensional simulation codes for modeling and theoretical analyses of the radiation belts.

ABSTRACT. Killer(MeV) electrons are of intrinsic scientific importance in radiation belt dynamics and present one of the most serious space weather hazards to orbiting satellites. We model relativistic electron generation by means of chorus wave diffusion in a 1-D Fokker-Planck equation. Diffusion coefficients are derived for several forms of whistler mode wave spectral density including Gaussian and power-law(with index q). We devise nine Fokker-Planck models(Models 1-9) of radiation belt electron dynamics. Models 1-6 incorporate the above forms of the wave spectral density.Models 7-9 contain special simplified diffusion coefficients that serve to facilitate the construction of analytic solutions to Models 1-6.The Fokker-Planck model equations depend on the important controlling parameter k=DT where D is a diffusion parameter and T is the timescale for electron loss. We solve the Fokker-Planck equations numerically and demonstrate that net electron energization occurs when k exceeds a critical value. We use classical methods to obtain asymptotic solutions of Models 1-6 for the electron distribution f that are valid for large energy E.Specifically,we obtain simple analytic forms for the electron spectra for the cases (a) a full-band whistler mode spectrum,and (b) a lower-band chorus spectrum for both a Gaussian spectrum and a power-law spectrum. The asymptotic solutions are found to test well against full numerical solutions of the Fokker-Planck equation.We carry out a comparison of our model results with experimental satellite data.We show comparisons of the asymptotic and numerical solutions for Model 1 with four selected "events",namely Storm 1(October 9,2012), Storm 2(March 17,2013),Non-storm(February 23,2013),and average geosynchronous conditions. We find good agreement between both analytic and numerical solutions with the experimental spectra for all four events.

ABSTRACT. The triggering mechanism of the substorm expansion onset has been a major issue in magnetospheric research. Various models have been proposed so far. To understand the causal relationship of substorm-associated processes in the magnetotail, we have performed a series of statistical analyses of Geotail data. As a result, we found that near-Earth magnetic reconnection plays a key role in substorm expansion triggering. Geotail, however, did not necessarily cover the magnetospheric region corresponding to auroral expansion in the polar ionosphere. We, therefore, extended our study using more recent THEMIS spacecraft data, which led us to obtain a complete picture of substorms. In this presentation, we will review our previous statistical results and discuss a future direction of substorm research, such as stepwise development of substorm onset aurora and the causal relationship between aurora, magnetic reconnection, and dipolarization.

ABSTRACT. The Geotail mission cast new light on tail magnetic reconnection as a process that preconditions substorm initiation. Numerous Geotail studies reported the pre-onset initiation of magnetic reconnection in the mid-tail, whereas auroral initial brightening, that is, the onset of auroral substorm, is generally considered as a manifestation of a near-Earth process, which is often referred to as tail current disruption. Therefore, for better understanding substorm initiation, it is critical to understand how, or if, the two processes, the mid-tail reconnection and near-Earth process, are linked. If they are indeed linked, meso-scale fast flows, as an outflow of mid-tail reconnection, presumably plays an essential role. Moreover, auroral streamers, which are known as an auroral counterpart of such meso-scale flows, are the primary feature of the auroral sequence of substorm initiation, which has been a topic of debate. In the present presentation, I briefly review such advance of our understanding of the substorm initiation sequence over the last 30 years, and discuss outstanding issues with a focus on the generation and spatio-temporal evolution of the meso-scale fast flows in the magnetosphere-ionosphere system.

ABSTRACT. Magnetotail reconnection had previously been recognized near the midnight meridian, as is the case in MHD simulations. The Geotail spacecraft mission discovered that the reconnection location is displaced toward dusk, or pre-midnight, in the Earth’s magnetotail. This dusk preference may be caused by the Hall electric field, which transports the magnetic flux dawnward and thins the current sheet on the duskside, as is the case in simulations with the Hall effect. In contrast, the Messenger spacecraft observed that the reconnection location was displaced toward dawn in Mercury’s magnetotail. This study reviews observational evidence of reconnection in the Earth’s magnetotail to address the controversy surrounding the dawn-dusk displacement of the reconnection location. The dusk preference is evident for tailward-moving plasmoids, but depends on studies for earthward-moving structures. Thus, the reconnection may not occur on the meridian where planetward-moving structures were observed. I predict that the BepiColombo/MMO spacecraft will observe tailward-moving plasmoids on the duskside of Mercury’s magnetotail.

Depending on the given time slot, I will also discuss the relative timing between plasmoids and auroral breakups. The result depends on the precise definition of auroral breakups or substorm onsets. I propose that reconnection does not cause auroral beads but rather auroral poleward expansions. To confirm this hypothesis, a fleet of satellites is necessary to observe thin earthward flows just before substorm onsets, in addition to ground all-sky and satellite global auroral images.

ABSTRACT. The Geotail mission prepared and motivated an entire generation of space plasma physicists, by performing the first systematic, comprehensive and extensive survey of the equatorial magnetotail and its key processes. It revealed with unprecedented detail the kinetic nature of tail reconnection, the role and evolution of Hall currents, and ion and electrons populations during that process. Supported by the ISTP armada and quite importantly by a cadre of theory, modeling and ground based components, Geotail achieved many firsts and paved the way for the next step in the magnetosphere’s exploration, that of focused, coordinated multi-spacecraft investigations such as Cluster, THEMIS and MMS. It influenced our early ideas on magnetotail transport and the THEMIS mission proposal. It remained a key contributor to exploration during the era of clusters, well beyond its original design life and well after ISTP. In the last 15 years, THEMIS and its ground-based observatories have revealed the effects of reconnection for the dynamics of substorms. THEMIS is now moving to address how these phenomena partake in the largest of geomagnetic storms, affecting ring current dynamics and relativistic electron enhancements. THEMIS has also revealed that the magnetic energy released by reconnection occurs close to Earth, at the tail-dipole transition region, where flow shears, ion heating, buildup of pressure gradients and the field-aligned currents they generate ultimately release the solar wind energy to the ring current and the ionosphere. Exactly how that energy release happens remains a mystery that calls for renewed emphasis on kinetic exploration of the transition region with suitable, focused constellations.

ABSTRACT. Earth’s dynamic magnetosphere responds to the relentless solar wind through a variety of processes, from the micro – at the electron and ion scales – to the macro, or global, encompassing the entire magnetosphere. In between these two extremes lie the mesoscales, which start at the ion scale and end at a significant fraction of the magnetosphere, roughly a few RE. The mesoscales are the fundamental link for multi-scale, bidirectional feedback between micro and macro because they serve as a conduit for mass and energy flow. They also appear to be the range in which much of the mass and energy transport takes place across the magnetopause and in the plasmasheet and magnetotail. The next step in understanding magnetospheric dynamics is to quantify the mesoscales, the vast and underexplored intermediate scale in between the micro and global. Magnetospheric Constellation, or MagCon, is a mission concept designed to answer fundamental questions of solar wind-magnetosphere interactions, from how energy is input at the dayside magnetopause and flanks to how it is stored and explosively released in the nightside plasmasheet and near-Earth transition region. In this paper, we review the science motivation and objectives of MagCon. We discuss the current implementation approach, which leverages existing small satellite technologies and instrumentation to create a low risk, affordable constellation. On its own, MagCon would revolutionize our understanding of solar-wind magnetosphere coupling, of plasmasheet-inner magnetospheric coupling, and elucidate the central role of mesoscales. With additional coordinated assets, including both space- and ground-based measurements MagCon could serve as a focal point for a future International Program, a worldwide effort to study Geospace as a “system of systems”, similar in spirit to the highly successful ISTP.

ABSTRACT. Understanding plasma energization and energy transport is a grand challenge of space plasma physics, with important implications for space weather and astrophysical plasmas. Plasma energization and energy transport are related to fundamental processes such as shocks, magnetic reconnection, turbulence and waves, plasma jets generation and their combination, which are all at the core of the current space plasma physics research. The Earth's Magnetospheric System is the complex and highly dynamic plasma environment where the strongest energization and energy transport occurs in near-Earth space. Previous multi-point observations from Cluster, THEMIS and MMS have greatly improved our understanding of plasma processes at individual scales. However, simultaneous measurements at both large, fluid and small, kinetic scales are required to resolve scale coupling and to ultimately fully understand plasma energization and energy transport processes. Such measurements are currently not available. Here we present the Plasma Observatory (PO) multi-scale mission concept tailored to study plasma energization and energy transport in the Earth's Magnetospheric System through simultaneous measurements at both fluid and ion scales. These are the scales at which the largest amount of electromagnetic energy is converted into energized particles and energy is transported. PO baseline mission includes one mothercraft and six identical smallsat daughtercraft in an HEO 8x18 RE orbit, covering all the key regions of the Magnetospheric System with separations ranging from fluid (5000 km) to ion (30 km) scales. Going beyond Cluster, THEMIS and MMS, PO will permit us to resolve for the first time scale coupling in space plasmas, leading to transformative advances in the field of space plasma physics. PO is one of the five ESA M7 candidates for a launch in 2037 and is currently undergoing a competitive Phase 0 at ESA for further downselection to Phase A at the end of 2023.

ABSTRACT. The PARAGON mission concept introduces a paradigm shift in the way we observe geospace by using coordinated i) unprecedented spatial and temporal resolution imaging of the ring current, near-Earth plasma sheet, aurora, and plasmasphere and ii) in-situ plasma, energetic particles and magnetic field measurements, from different platforms, in order to discover, quantify and understand the global impact of mesoscale processes in the development of major geomagnetic disturbances. PARAGON addresses fundamental questions about mesoscale processes that are observed throughout the solar system, from the fast rotating magnetospheres of Jupiter and Saturn to the supra-arcade downflows in the eruptive solar flares. Earth’s accessible space environment provides the perfect laboratory for these processes to be explored in detail, in order to understand their impact to the global system. Multiple missions over the years have targeted either the local or global nature of geospace with in-situ probes or global imaging, respectively. However, understanding geomagnetic disturbances to the level of predictability remains elusive, because we still do not understand the bridge between the local and global geospace, that is, the mesoscale (1000s km to few Re in the magnetotail, ~10s-100s km in the ionosphere) processes and their global implications. PARAGON will determine under what conditions mesoscale processes in the coupled Magnetosphere-Ionosphere (M-I) system become geoeffective, by observing the global system in mesoscale resolution. With the outstanding question of the global impact of mesoscale processes still being at the forefront of magnetospheric physics, and considering the technological advancements over the last decade, PARAGON can and should be of the highest-priority for implementation in the upcoming decade.