Geospace Exploration Project

Miyoshi et al. Earth, Planets and Space (2018) 70:101 https://doi.org/10.1186/s40623-018-0862-0 FULL PAPER Open Access Geospace exploration project ERG Yoshizumi Miyoshi1* , Iku Shinohara2, Takeshi Takashima2, Kazushi Asamura2, Nana Higashio3, Takefumi Mitani2, Satoshi Kasahara4, Shoichiro Yokota5, Yoichi Kazama6, Shiang‑Yu Wang6, Sunny W. Y. Tam7, Paul T. P. Ho6, Yoshiya Kasahara8, Yasumasa Kasaba9, Satoshi Yagitani8, Ayako Matsuoka2, Hirotsugu Kojima10, Yuto Katoh9, Kazuo Shiokawa1 and Kanako Seki4 Abstract The Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron genera‑ tion/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which diferent plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite. Keywords: The ERG project, The ERG (Arase) satellite, The radiation belts, Geospace Introduction to more than 10 MeV, which are the highest energy popu- Te radiation belts consist of energetic charged particle lation in geospace. Tus, plasma and particle populations populations that are trapped in the inner magnetosphere. with a wide range of energies from a few eV to more than Associated with solar wind disturbances, the outer radia- 10 MeV coexist in the inner magnetosphere. Reviews on tion belt electron population varies considerably. In the inner magnetosphere can be found in several papers particular, a decrease in the fux of relativistic electrons (e.g., Ebihara and Miyoshi 2011). is observed during the main phase of geospace storms. Two diferent mechanisms have been considered to Te fux then recovers and often signifcantly increases cause the large fux enhancement of the outer radia- during the recovery phase of storms (e.g., Nagai 1988; tion belt electrons. One is an external source process via Miyoshi and Kataoka 2005). Te identifcation of mech- quasi-adiabatic acceleration, which is the so-called radial anisms that drive the fux variations of relativistic elec- difusion. Te other mechanism is an internal accelera- trons in the radiation belts is fundamental and important tion process. Figure 1 shows a diagram of the expected problem in the solar-terrestrial physics. transportation and acceleration mechanisms as a func- Tere are several plasma and particle populations in tion of the L-shell and energy. In the radial difusion, the inner magnetosphere. Te plasmasphere consists of i.e., the external source process (blue arrows), the elec- cold/dense plasma populations, and the ionosphere is the trons move earthward with increasing energy because source of the plasmaspheric plasma. Te typical energy of the conservation of the frst two adiabatic invariants. of the plasmaspheric plasma is less than 1 eV. Te typical Te drift resonance with the MHD fast-mode waves is ring current and plasma sheet energies range from a few expected to cause the radial transportation of electrons, keV to about 100 keV, and these populations contribute and the ultra-low frequency (ULF) geomagnetic pulsa- to the ambient plasma pressure. Te typical energies of tions of Pc4 and Pc5 are the primary waves that cause the the radiation belt particles range from a few hundred keV resonance (e.g., Elkington et al. 1999). During the internal acceleration process (red arrows), sub-relativistic electrons are accelerated to MeV energies by whistler mode *Correspondence: [email protected]‑u.ac.jp 1 ISEE, Nagoya University, Nagoya 464‑8601, Japan waves inside the radiation belts (e.g., Summers et al. Full list of author information is available at the end of the article 1998; Miyoshi et al. 2003; Torne 2010). In this process, © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Miyoshi et al. Earth, Planets and Space (2018) 70:101 Page 2 of 13 Internal relativistic External acceleration electron acceleration (radiation belts) MHD waves 1MeV acceleration radial transport sub-relativistic 100keV electron whistler waves wave excitation 10keV ring current plasma sheet electrons, ions energy electrons, ions 1keV convection control M-I coupling 100eV Field-aligned electric fields currents electric fields <10eV electric field thermal plasma (plasmasphere) magnetic field L=4L=6 distance from the Earth Fig. 1 Diagram of cross‑energy coupling processes in the inner magnetosphere (modifed from Miyoshi et al. 2012) the whistler mode waves act as a mediating agent. Te belts are important to clarify how the external source and waves are generated through the plasma instability of hot internal acceleration processes depend on diferent geo- electrons and subsequent nonlinear interactions (e.g., magnetic and solar wind conditions. Omura et al. 2009), and the energy of the hot electrons To understand the detailed acceleration and transpor- transfers to the waves. Te waves accelerate sub-relativ- tation processes, it is necessary to know all parameters istic electrons and enhance the relativistic electron fux. that contribute to the accelerations. Electrons over wide Te cold plasma density population also contributes to energy ranges and electric and magnetic felds with wide this process because the cold plasma density controls frequency ranges are indispensable for understanding the wave dispersion and resonance conditions. Tus, the these processes. Observations of the ambient plasma cross-energy coupling of diferent energy plasmas and density are also necessary. In addition, it is essential to particle populations via wave–particle interactions plays simultaneously evaluate the loss processes in the radia- an important role in the fux enhancements of the radiation belts. Several processes contribute to the loss of tion belt electrons (Miyoshi et al. 2012, 2016). the radiation belt electrons, escaping into the magneto- Tere are debates on which external source process and pause, and the pitch angle scattering into the atmosphere internal acceleration process are important for the large because of the whistler mode waves and electromagnetic fux enhancement of the outer radiation belt electrons. ion cyclotron (EMIC) wave–particle interactions (e.g., Phase space density profles were derived from NASA/ Turner et al. 2012). Van Allen Probe observations (Mauk et al. 2013) in the Here, we describe the importance of multi-point obser- October 2012 geospace storm, and it is identifed for vations in the inner magnetosphere. Several kinds of the frst time that the internal acceleration process was plasma waves play a key role for accelerations and loss dominant during the fux enhancement of MeV electrons of electrons in the radiation belt electrons. Whistler (Reeves et al. 2013). It is expected that both the external mode chorus waves accelerate relativistic electrons on source and internal acceleration processes are not exclu- the dawnside outside the plasmapause, whereas whis- sive, and the dominant processes that cause the large tler mode hiss waves cause pitch angle scattering inside fux enhancements depend on the events (e.g., Reeves the plasmasphere. EMIC waves cause a strong scattering et al. 2003). So, long-term observations of the radiation of MeV electrons (e.g., Miyoshi et al. 2008), which are Miyoshi et al. Earth, Planets and Space (2018) 70:101 Page 3 of 13 mainly observed on the dusk and noon sides (e.g., Shprits Aerospace Exploration Agency (JAXA), and nine science et al. 2008a, b). Tese wave–particle interactions often instruments were developed at ISAS/JAXA as well as sev- occur simultaneously at diferent local times and radial eral universities and institutes in Japan and Taiwan. Te distances. satellite was successfully launched by the Epsilon launch Moreover, the causes and consequences of the pro- vehicle from the Uchinoura Space Center of JAXA on cesses are often observed at diferent locations. For December 20, 2016. Table 1 presents the specifcations of example, the pitch angle scattering with whistler mode the spacecraft, and Fig. 2 shows an image of the satellite waves occurs in the magnetosphere, while the resultant in the radiation belts. precipitations are observed at ionospheric altitudes. In Te ERG satellite is designed to be Sun oriented and this example, it is possible to identify the causal relation- spin stabilized with a rotation rate of 7.5 rpm. Te apo- ship using conjugate observations between satellite and gee altitude is ~ 32,000 km, and the perigee altitude ground-based instruments. Tus, multi-point observa- is ~ 400 km, so that the satellite explores the entire region tions by multiple satellites and conjugate observations of the radiation belts. Te inclination angle of the satellite with ground-based observations are necessary for under- orbit is about 31°, and therefore, the satellite

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