ABSTRACT. The GEOTAIL spacecraft, launched in July 1992 from Florida, U. S. A., as a joint project between ISAS (now JAXA) and NASA, has been studying the structure and dynamics of the magnetotail and other key regions of the magnetosphere. It has achieved epoch-making results, including numerous discoveries in magnetospheric physics. It was often told as the most successful mission in 1990’s in the field of the Earth's magnetospheric exploration. In this talk I will discuss those factors which contributed to the mission success in terms of the international project management, the mission design, the innovative design of some onboard instruments, open data policy, and others. As for the success of international collaboration, I suggest that the development of a mutual trust relationship between the US and Japanese project teams was the most critical element of all.

Reference: T. Mukai, International Cooperation: When 1 + 1 = 3, NASA APPEL ASK Magazine, 31, June 2008. (https://appel.nasa.gov/2008/06/01/international-cooperation-when-1-1-3)

ABSTRACT. GEOTAIL was launched in July 1992 as a joint project between ISAS and NASA. The initial goal of GEOTAIL was to reveal the structure and dynamics of the Earth's magnetotail. In more detail, it was to determine the overall plasma, electric and magnetic field characteristics of the distant and near-Earth tail, to determine the role of the distant and near-Earth tail in substorm phenomena, to investigate the initiation mechanism of magnetotail reconnection in the near-Earth tail, and to investigate the plasma inflow, acceleration / heating, and transport processes that occur at the interaction regions and their surrounding regions. GEOTAIL has achieved its initial goals, by making numerous discoveries in magnetospheric physics. The purpose of this talk is to summarize the major achievements of GEOTAIL to date.

ABSTRACT. Plasma wave observations by the Geotail spacecraft allowed us to see stunning views of plasmas in the terrestrial magnetosphere. Since plasma waves are pretty sensitive to changes in plasma environments, exploring plasma waves leads to knowing the dynamic changes of magnetospheric environments. Thanks to the unique orbits of the Geotail spacecraft, the onboard plasma wave instrument succeeded in capturing abundant plasma wave phenomena in a variety of distinct regions of the magnetosphere. In particular, the waveform observations by Geotail brought about significant changes in what we learn from plasma wave data. They taught us not only the importance of phases of plasma waves but also that plasma waves easily reach nonlinear stages. The present paper focuses on the Geotail plasma wave observations. It discusses how we can proceed to the future exploration of the heliosphere powered by the experience of the Geotail mission.

ABSTRACT. Geotail Exit review was held in February 2023 by the ISAS Space Science Committee. A total of eight judges, including four members of the Space Science Committee had an inter view with the Geotail team, and after discussion by the judges, the conclusions were reported to the Chair of the Space Science Committee. The evaluation was based on 1) achievement of project goals, 2) spread effects of Geotail on solar system science (including spread effects on future satellite programs), international value, and outcomes, 3) lessons learned in the mission, and 4) status of human resource developmen. In addition to acknowledging the excellent scientific results, we expressed our expectations for the new missions that have been/will be created from Geotail's observations. The report also highly praised the significant spread effects of Geotail on other scientific fields other than magnetospheric research.

ABSTRACT. Over its remarkable 30-year period of performance, the Geotail mission made numerous and unprecedented contributions to space science research. From its inception, the program also played a key programmatic and political role. Without Geotail and its scientific leaders, it is likely that the U.S. and worldwide programs of solar-terrestrial research would have been greatly stunted or even drastically reduced in scope. The range of science that Geotail undertook in magnetospheric exploration is undisputed. But Geotail went on to play key roles in solar wind and other interplanetary endeavors as well. It has lighted the way to approach future planetary and exoplanetary research as well. This presentation will discuss briefly some of the Geotail historical aspects, but will also look forward to how presently approved programs will pursue notions stimulated by Geotail. Looking toward even more distant horizons, the talk will consider just some of the science ideas that might be attainable based on what we have learned from this most successful and ambitious international program. In all it has done and to its most accomplished leaders, the world community really must say, “Well done”.

ABSTRACT. We have studied asymmetric magnetic reconnection in the dayside geomagnetopause using magnetohydrodynamics simulations. We have indicated the new whole asymmetric magnetic reconnection structures (Nitta et al. 2016; Nitta and Kondoh 2019, 2021, 2022). One of them is the fast outflow structure. The fast reconnection jet is observed not only in the reconnection fan sandwiched by the slow shock pair, but also in the lower beta side plasmoid in the asymmetric case, while that is observed only in the reconnection fan in the symmetric reconnection. Usually we identify the magnetic reconnection event in the dayside magnetopause using the reconnection outflow speed from in-situ observation data. Our new findings in the previous studies about the asymmetric reconnection mean that we do not identify where we observe these reconnection events. In this study, we clarify the observation position in the asymmetric magnetic reconnection structure using THEMIS observations.

ABSTRACT. Magnetic reconnection in the dayside geomagnetopause occurs in the asymmetric current sheet system. We have indicated that these asymmetric reconnection are significantly different from the Petscheck type symmetric reconnection (Nitta et al. 2016; Nitta and Kondoh 2019, 2021, 2022). One of the new different characteristics is the formation of the forward fast shock. This fast shock are formed due to the different of the Alfvenic speeds in the both sides of the current sheet. The other one is the fast outflow structure. The fast reconnection jet is observed not only in the reconnection fan sandwiched by the slow shock pair, but also in the lower beta side plasmoid in the asymmetric case, while that is observed only in the reconnection fan in the symmetric reconnection. It is also indicated that these new features in the asymmetric reconnection are controlled by the physical conditions around the dayside magnetopause. GEOTAIL satellite had observed a lot of geomagnetopause crossings. In this paper, we show the physical conditions observed by GEOTAIL satellite around there. In particular, we focused on the ratio of the plasma density, temperature, and the magnetic field strength in the magnetosphere to those in the magnetosheath.

ABSTRACT. While a large body of work has studied the storm-time ring current energization, few studies have provided direct verification of the energy transport into the inner-magnetosphere that powers the ring current. Recent studies of very-near-Earth reconnection (VNERX, defined here as occurring at geocentric distance < 10 RE) during storms suggest that such reconnection may play a critical role in that regard. VNERX can host intense transient electric fields, on the order of 10s of kV/RE, allowing for significant acceleration and efficient transport of plasma sheet ions into the ring current. Here we address how common this process is. We use storm-time inner-magnetosphere and plasma sheet observations from three THEMIS satellites spanning 11 years of operation (2010-2021). Our analysis shows a VNERX occurrence rate of ~one event per thousand hours. This rate is less than previously published ion-diffusion-region occurrence rates seen in the near-Earth plasma sheet during non-storm times (likely substorms). Our results suggest that while VNERX events may play a significant role in the storm-time ring current’s initial buildup, that during the storm main phase, the enhanced convection powering the ring current during storm recovery is likely contributed by reconnection further down-tail than 10 RE.

ABSTRACT. I present a new theoretical model of the magnetic reconnection (RX) designed for astrophysical applications; the self-similar RX model. The current sheet (CS) system which we frequently encounter in astrophysical problem is characterized by wide spatial scale much larger than the CS thickness. The outer circumstance does not influence the temporal evolution of the RX system for a long while. The RX system spontaneously expands along with the fast-mode rarefaction wave (FRW) emitted from the reconnection point. When the size of FRW dominates the CS thickness, the system no longer has proper spatial scale other than the propagating FRW scale. The RX system asymptotically tends to expand self-similarly (the self-similar phase).

The self-similar RX model is an extension of the Petschek model that is characterized by the figure-X shaped slow shocks. Our new model also includes figure-X shaped MHD shocks: Combination of the slow shock, intermediate shock, or rotational discontinuity followed by slow-mode expansion. These shocks work as the main energy convertor, and extend self-similarly. Thus, the magnetic energy conversion power increases in proportion both to the time from the onset of the RX and the reconnection rate. We clarified the properties of the spatial structure of the self-similar RX system (Nitta+2001, 2002, Nitta 2007, Nitta+2016, Nitta and Kondoh 2019, 2021, 2022). In addition, we systematically investigated the dependence of the reconnection rate on the plasma-beta (Nitta 2004, 2006), magnetic Reynolds number (Nitta2007), asymmetry of the thermodynamic quantities with regard to the CS (Nitta+2016, Nitta and Kondoh 2019, 2022), magnetic shear (Nitta and Kondoh 2021, 2022) in our series of papers. The self-similar RX model is categorized as the fast RX regime in the maximum case, however its reconnection rate significantly decreases to almost like the slow RX regime depending on parameters. We summarize these properties in my talk.

ABSTRACT. Electromagnetic whistler-mode waves in space plasmas play critical roles in collisionless energy transfer between the electrons and the electromagnetic field. Although resonant interactions have been considered as the likely generation process of the waves, observational identification has been extremely difficult due to the short time scale of resonant electron dynamics. Here we show strong nongyrotropy, which rotate with the wave, of cyclotron resonant electrons as direct evidence for the locally ongoing secular energy transfer from the resonant electrons to the whistler-mode waves using ultra-high temporal resolution data obtained by NASA’s Magnetospheric Multiscale (MMS) mission in the magnetosheath. The nongyrotropic electrons carry a resonant current, which is the energy source of the wave as predicted by the nonlinear wave growth theory. This result proves the nonlinear wave growth theory, and furthermore demonstrates that the degree of nongyrotropy, which cannot be predicted even by that nonlinear theory, can be studied by observations.

ABSTRACT. Whistler mode chorus emissions are usually observed in the Earth’s inner magnetosphere with a gap around 0.5 electron gyrofrequency (fce) in dynamic spectra. Chorus emissions initially generate at the equator with broadband frequency and then damp at 0.5 fce during propagation toward higher latitudes. Electrons nonlinearly trapped by Landau resonance gain energies from waves. The Landau resonance velocity becomes very close to the group velocity of nearly parallel whistler mode waves at frequencies around 0.5 fce, resulting in a long interaction time and possible wave damping. Evidence of the mechanism has been observed by the Geotail satellite [1]. In this study, we reproduced the formation of the gap of an obliquely propagating chorus emission in test-particle simulation and self-consistent two-dimensional general curvilinear particle-in-cell (2D gcPIC) simulation. Analyzing the simulation results, we confirm that Landau resonance contributes to wave damping, which transfers energies from the wave to electrons. We further found that the energy exchanges occur on the perpendicular component of the wave electric field and perpendicular electron velocities rather than the parallel components [2].

[1] Yagitani, S., T. Habagishi, and Y. Omura (2014), Geotail observation of upper band and lower band chorus elements in the outer magnetosphere, J. Geophys. Res. Space Physics, 119, https://10.1002/2013JA019678. [2] Hsieh, Y.-K., & Omura, Y. (2018). Nonlinear damping of oblique whistler mode waves via Landau resonance. Journal of Geophysical Research: Space Physics, 123. https://doi.org/10.1029/2018JA025848

ABSTRACT. A notable quantity of the solar wind energy crossing the magnetopause reaches the high-latitude thermosphere-ionosphere system, the consequences of which are remarkable as well as global in nature. Non-extensive Tsallis entropy, a generalization of Shannon entropy, provides a promising tool for studying various features of nonextensive dynamical systems. It can give information regarding the intrinsic fluctuations in such a system by characterizing the degree of nonextensivity. We report in this paper for the first time the calculation of the Tsallis entropy of Joule heating in high latitude upper atmosphere and discuss its yearly relationship with the solar wind forcing during solar maxima and minima periods. We have used theoretical models to calculate the Poynting flux flowing onto the Earth’s ionosphere and associated Joule heating due to the solar wind-magnetosphere-ionosphere dynamo. The investigation reveals that the Tsallis entropy values clearly reflect the complexity variations during solar maxima and minima periods of solar cycle 23 and solar cycle 24.

ABSTRACT. The recent Super Dual Auroral Radar Network (SuperDARN) observations show that ionospheric flow fluctuations of the mHz or lower frequency range appear even in the subauroral to mid-latitude region during magnetic storm times. An interesting feature of the flow fluctuations is that they appear to propagate azimuthally either westward or eastward, and occasionally bifurcate toward the both directions. Taking a closer look with high spatial resolution measurements provided by the radars reveals that those flow fluctuations consist of meso-scale patchy structures of ionospheric convection with a significant latitudinal flow component and a longitudinal scale of ~1h MLT. The azimuthal propagation properties strongly suggest that westward-drifting ions and eastward-drifting electrons of tens of keV in the inner magnetosphere can be the moving sources responsible for excitation of MHD waves seen by the radars at the ionospheric footprint. However, only few observations in the magnetosphere have been reported on the source of the waves at the subauroral to mid-latitudes. Recent observations in the inner magnetosphere by the Arase satellite and the Van Allen Probes have provided a good opportunity to examine their magnetospheric counterpart in further detail. On the basis of in-situ measurement of ring current particles and the magnetic field in the inner magnetosphere, we discuss the generation mechanism of the observed flux tube fluctuations in terms of resonant or non-resonant processes.

ABSTRACT. The interaction between the solar wind and the Earth’s magnetic field induces catastrophic effects within the Earth's magnetosphere-ionosphere system. Under active geomagnetic conditions, the geomagnetic tail often releases the stored energy that bring particles into the near-Earth environment. Under the influence of magnetospheric electric and magnetic fields, these energetic particles undergo a variety of motions and give rise to multiple features. One such interesting feature is electron zebra stripes, which manifest as bands in the energy versus L-value spectrogram. During an intense geomagnetic storm of September 2017, the RBSPICE instrument on board Van Allen Probe-A recorded the distortion and reformation of the electron zebra stripes. We conduct an advection simulation under the time-dependent electric and magnetic fields acquired from the MHD simulation to reproduce the observed electron zebra stripe structures. Our study provides a detailed understanding of the magnetospheric condition that is responsible for the generation of electron zebra stripes. We find that the day-night asymmetry of the azimuthal component the electric fields, which are driven by the Region-1 field-aligned currents (FACs), play a crucial role in the formation of intense electron flux peaks and valleys in the zebra stripes. On the nightside, the dawn-to-dusk electric field gives rise to the transport of electrons from the geomagnetic tail to the Earth’s inner radiation belt. The peak-forming electrons of the zebra stripes come primarily from the nightside. Thus, the electron zebra stripes are the consequence of the enhancements of the Region-1 FACs that are driven by the solar wind-magnetosphere coupling. Thus, the zebra stripes are the manifestation of the solar wind-inner magnetosphere coupling via the ionosphere associated with earthward transport of electrons from the geomagnetic tail region.

ABSTRACT. In 2019, Comet Interceptor was selected by ESA as a new F-class mission. This mission will visit a dynamically new comet that is yet-to-be-discovered. For the first time, multipoint measurements will be made in the cometary environment using three spacecraft: spacecraft A (ESA), probe B1 (JAXA) and probe B2 (ESA). All spacecraft will carry plasma instruments, allowing, for the first time, a three-dimensional study of the cometary plasma environment.

The plasma measurements are, however, expected to be affected by the spacecraft potential. A spacecraft interacts with the surrounding environment which results in an accumulation of charge on the spacecraft surface. This process is problematic for low-energy plasma measurements. The charged particles are attracted to, or repelled from, the charged spacecraft surface, resulting in an energy shift and a distortion of the effective field of view of the instrument. To maximize the science return from the plasma instruments on Comet Interceptor, the measurements need to be optimized taking the spacecraft potential into account.

In this study, we use the Spacecraft Plasma Interaction Software (SPIS) to make Particle-In-Cell (PIC) simulations of the expected spacecraft potential of probe B1 of Comet Interceptor. We study the spacecraft potential both in the solar wind and in different regions of the cometary plasma environment during the cometary flyby. This is done for different activity levels of the target comet and at different flyby velocities. The final step of the study is to use particle tracing to study the expected influence of the spacecraft potential on ion measurements to be made by the Cometary Ion Mass Spectrometer (CIMS) on probe B1.

ABSTRACT. Chang 'E-7 relay satellite (CE-7) is a lunar orbiting satellite, ENA imager aboard CE-7 can realize ENA imaging telemetry of different regions of the Earth's magnetosphere with the moon's revolution. Based on the clean particle radiation environment near lunar orbit and the balance of ENA telemetry fluxes near lunar orbit caused by differences in ENA emission diffusion modes in different regions of the magnetosphere, the ENA Imager aboard CE-7 provides us with an excellent opportunity to monitor the causal sequence of geomagnetic activity. Using the 20°×20° field of view, the 0.5°×0.5° angle resolution, and the ~0.17 cm2sr geometric factor, a two-dimensional ENA imager is being designed. The magnetospheric ring current simulation at a 4-20 keV energy channel for a medium geomagnetic storm (KP=5) shows that:(1) at ~60 earth radii (RE), the imager can collect 104 ENA events within 3 min to meet the statistical requirements of 2D coded imaging data inversion, so as to meet requirements of the analysis of the sub-storm ring current evolution process of magnetic storms above medium; (2) The pitch angle of ions in the magnetopause and magnetotail plasma sheet region is ~ 90°, and the ENA emission is only diffused in the equatorial plane. Therefore, its decay rate is relatively slow, and it will reach a balance with the ENA emission flux generated by the ring current at ~ 60RE lunar orbit. High spatial-temporal resolution ENA imaging monitoring of these two important regions will provide the measurement basis for the solar wind energy input process and generation mechanism; (3) the average sampling interval of ENA particle events is about 16 ms at the moon’s orbit; the spectral time difference for the set energy range is on the order of minutes, which can provide location information to track the trigger of geomagnetic storm particle events.

ABSTRACT. Recent Venus missions (Venus Express and Akatsuki) provided a large-scale view of Venus atmosphere and discovered new phenomena, such as high-altitude extension of the mountain wave to the cloud layer and a dawn-dusk asymmetry in the ionospheric motion. The superrotation of the cloud layer is assumed to be driven by the thermal tide but its relation to any meridional convection or waves is still unknown. The key to understand all these phenomena is to determine the multi-step re-distribution of the absorbed solar radiation to other forms of energy: (1) internal energy; including temperature, latent heat, and chemical energy (2) kinetic energy both in large scale flows/waves and in minor deviations of convection motions, (3) electric energy including ionization.

To understand how the motion of Venus atmosphere is driven by the energy originating from the absorption of solar radiation, we proposed Venus Dynamics Tracer (VdT), a mission for in-situ measurements, as a response to the ESA call for new M-class missions. Specific targets were two major energy absorption regions: the cloud layer and the ionized upper atmosphere. The scientific goals were to investigate (a) the roles of the vertical and meridional circulation in maintaining major atmospheric dynamics near the cloud layer where visible light is absorbed and drives the vertical motions of the air, and to understand (b) the global dynamics of ions and neutrals in the upper atmosphere where EUV is absorbed both by neutrals and ions and where energy and momentum are transferred between them.

For the first target, multiple-balloons are deployed for in situ observations with supporting camera/s on an orbiter giving global context. For the second target, the motions of ions and neutrals are directly measured. This presentation discusses required measurements to answer the scientific goals.

ABSTRACT. The Geotail satellite has observed more than 80 reconnection events in the magnetotail since the start of the near-Earth phase (since 1994). This talk will briefly summarize Geotail’s reconnection observations and discuss the remaining problems after Geotail. An event observed on May 15, 2003, is the champion data of the X-line crossing, where the structure of the reconnection site shows excellent agreement with two-dimensional full-particle simulation results, such as the electron flow reversal. However, only a few observations showed such a clear structure of electron dynamics. On the other hand, most reconnection events show current sheet flapping, which is reminiscent of the turbulent structures observed in recent three-dimensional particle simulations. It is still unclear what controls the structure difference, even with the event collection from the long-term observation.

ABSTRACT. Rapid magnetic energy dissipation is now widely discussed as an important element to understand various plasma phenomena observed not only in the earth’s magnetosphere and in solar corona, but also in high energy astrophysical objects such as accretion disks and astrophysical jets. Magnetic reconnection is known to be the most important mechanism providing the rapid energy dissipation whose energy is converted into the thermal plasma heating and the nonthermal particle acceleration. By recent satellite observations with high-time resolutions capable to detect the magnetic diffusion region, it has been confirmed that the plasma kinetic structure with phase-bunched ions and electrons in the magnetic diffusion region is basically consistent with the collisionless magnetic reconnection process. Needless to say, this kind of observation is quite important to understand the key process of reconnection, yet this does not necessarily directly answer to another important fundamental question about the energy partitioning between ions and electron during reconnection. In addition, the energy partitioning between the thermal plasma and the nonthermal particle acceleration is another key question to understand the high energy astrophysical objects. In this presentation, we discuss the energy partitioning by taking into account of the kinetic understanding of the magnetic diffusion region.

ABSTRACT. Magnetic reconnection is a fundamental energy conversion process in collisionless plasmas involving multi-scale processes. While the the topology changes of the magnetic field changes take place inside a small region where electrons become unmagnetized, regions of acceleration and heating of plasma are distributed at larger scales, driving global plasma transport, such as magnetospheric convection, or leading to sporadic magnetic energy release on global scales such as substorms, flares and gamma ray bursts. Among the different plasmas, Geospace is a natural plasma laboratory to study the ground truth of how magnetic reconnection operates in nature, since plasmas and fields in action can be directly measured at high cadence. Geotail is the first spacecraft with dedicated plasma and field measurements that captured the direct proof of the near-Earth magnetotail reconnection region. Multi-point observations in the reconnection region enabled further separation of temporal and spatial variations and advanced our standing of the reconnection processes. In particular, Cluster, which is the first four-point spacecraft, enabled to quantify the thin current sheet structures and ion-scale physics in the diffusion region of ions by quantifying the spatial structures of the thin current sheet during reconnection. The Magnetospheric Multiscale (MMS) mission enabled for the first time the exploration of electron-scale physics within the diffusion region of electrons. In this talk advances of magnetic reconnection studies based on Cluster and MMS multi-point observations are highlighted and needs for a new mission to study the multi-scale aspect of the magnetic reconnection will be discussed.

ABSTRACT. Solar flares and solar eruptions are the largest explosion in the heliosphere and are the major causes of space weather events. Therefore, predicting their occurrence and impact is important for heliospheric research and space weather forecasting. The conventional prediction of solar flares and solar eruptions has mainly been carried out by empirical methods based on statistical analyses of observed data. However, it is necessary to develop a physics-based approach to realize more advanced predictions. The author has developed a physic-based method to predict large solar flares by applying the MHD instability and equilibrium theories. In this talk, I will explain the current status and issues in the predictive research of solar flares and solar eruptions and discuss the prospects for physics-based space weather prediction.