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This dissertation addresses the problem of energization of particles (both electrons and ions) to tens and hundreds of keV and the associated transport process in the magnetotail during substorms. Particles energized in the magnetotail are further accelerated to even higher energies (hundreds of keV to MeV) in the radiation belts, causing space weather hazards to human activities in space and on ground. We develop an analytical model to quantitatively estimate flux changes caused by betatron and Fermi acceleration when particles are transported along narrow high-speed flow channels from the magnetotail to the inner magnetosphere. The model shows that energetic particle flux can be significantly enhanced by a modest compression of the magnetic field and/or shrinking of the distance between the magnetic mirror points. We use coordinated spacecraft measurements, global magnetohydrodynamic (MHD) simulations driven by measured upstream solar wind conditions, and large-scale kinetic (LSK) simulations to quantify electron local acceleration in the near-Earth reconnection region and nonlocal acceleration during plasma earthward transport. Compared to the analytical model, application of the LSK simulations is much less restrictive because trajectories of millions of test particles are calculated in the realistically determined global MHD fields and the results are statistical. The simulation results validated by the observations show that electrons following a power law distribution at high energies are generated earthward of the reconnection site, and that the majority of the energetic electrons observed in the inner magnetosphere are caused by adiabatic acceleration in association with magnetic dipolarizations and fast flows during earthward transport. We extend the global MHD+LSK simulations to examine ion energization and compare it with electron energization. The simulations demonstrate that ions in the magnetotail are first nonadiabatically accelerated in the weak field region close to the reconnection site, and then adiabatically accelerated in the high-speed flow channels as they catch up with and ride on the earthward propagating dipolarization structures. The nonlocal adiabatic acceleration mechanism for ions is very similar to that for electrons. However, the motion of energetic electrons is adiabatic except in very limited regions near the reconnection site while the motion of energetic ions is marginally adiabatic in the dipolarization regions. The simulations also show that the earthward transport of both species is controlled by the high-speed flows via the dominant ExB drift in the magnetotail. To understand how the power law electrons are initially produced in the magnetotail, we use an implicit particle-in-cell (PIC) code to model the processes in the near-Earth reconnection region. We find that the power law electrons are produced not in the reconnection diffusion region, but in the immediate downstream of the reconnection outflow in the course of dipolarization formation and intensification. Our study illustrates that during substorms, particles are accelerated via a multi-step process, including local acceleration in the reconnection region and nonlocal acceleration during the earthward transport, and the multi-step acceleration occurs on multiple spatial scales ranging from a few kilometers (the scale of electron diffusion region) to more than ten Earth radii (the transport scale).

The substorm current wedge (SCW), a core element of substorm dynamics coupling the magnetotail to the ionosphere, is crucial in understanding substorms. It has been suggested that the field-aligned currents (FACs) in the SCW are caused by either pressure gradients or flow vortices, or both. Our understanding of FAC generations is based predominately on numerical simulations, because it has not been possible to organize spacecraft observations in a coordinate system determined by the SCW. This dissertation develops an empirical inversion model of the current wedge and inverts midlatitude magnetometer data to obtain the parameters of the current wedge for three solar cycles. This database enables statistical data analysis of spacecraft plasma and magnetic field observations relative to the SCW coordinate. In chapter 2, a new midlatitude positive bay (MPB) index is developed and calculated for three solar cycles of data. The MPB index is processed to determine the substorm onset time, which is shown to correspond to the auroral breakup onset with at most 1-2 minutes difference. Substorm occurrence rate is found to depend on solar wind speed while substorm duration is rather constant, suggesting that substorm process has an intrinsic pattern independent of external driving. In chapter 3, an SCW inversion technique is developed to determine the strength and locations of the FACs in an SCW. The inversion parameters for FAC strength and location, and ring current strength are validated by comparison with other measurements. In chapter 4, the connection between earthward flows and auroral poleward expansion is examined using improved mapping, obtained from a newly-developed dynamic magnetospheric model by superimposing a standard magnetospheric field model with substorm current wedge obtained from the inversion technique. It is shown that the ionospheric projection of flows observed at a fixed point in the equatorial plane map to the bright aurora as it expands poleward, suggesting that auroral poleward expansion is mainly a consequence of magnetic dipolarization caused by the SCW. Chapter 5 shows that increased plasma pressure caused by flow braking has a temporal pattern similar to that of the currents in the SCW. In contrast, flow vortices vanish quickly, suggesting that pressure gradient is an important factor in generating the SCW. The measured pressure gradients are found to be organized relative to SCW central meridian. Nonalignment between pressure gradient and flux tube volume gradient lead to the generation of an SCW with quadrupole FACs (inner and outer loop of FACs). Because the inner current loop is weaker than the outer loop, the combined magnetic effect of the two current loops is similar to a classic SCW. The final chapter studies the magnetic flux transport by earthward flows, and accumulated inside the SCW and enclosed within auroral poleward boundary. Their good agreement suggests that flux accumulation causes magnetic dipolarization and auroral poleward expansion. The strength of the SCW is positively correlated with the amount of magnetic flux accumulated.

This thesis presents an investigation of the lower-hybrid drift instability(LHDI) in a thin Harris current sheet with a guide field. This includes three-dimensional analytical and numerical analyses using the gyrokinetic electron, fully-kinetic ion(GeFi) description, which are compared with results from the Vlasov approach and simulations. Previous fully-kinetic studies solve the electron Valsov equation by integrating along the unperturbed phase-space orbits, including the complete electron-cyclotron motion. The LHDI satisfies ω << ωce and kperpρe ∼ 1, where ωce and ρe are the electron cyclotron frequency and Larmor radius, respectively, and kperp& is the wavevector perpendicular to the equilibrium magnetic field. By treating the electron response with gyrokinetic theory, the fast cyclotron motion is removed which greatly simplifies the derivation of the LHDI eigenvalue equations. This allows a more comprehensive LHDI analysis, which is carried out over the entire domain of unstable wavevectors. To our knowledge, an extensive scan of the operative domain of the LHDI in current sheets with a guide field has never been done. The results will show that two types of electromagnetic LHDIs are active in the current sheet. The Type A LHDI is generally consistent with the existing theoretical descriptions of the LHDI, namely, quasi-electrostatic modes localized near the current sheet edges with k ρe ∼ 1, k||=0, and ω ^2 ∼ ωpi^2/(1+ωpe^2 / ωce^2)$, where ωpe and ωpi are the electron and ion plasma frequencies, respectively. However, we will show that in sufficiently thin current sheets, i.e. strong equilibrium drifts, the Type A LHDI is destabilized by finite k|| in the short wavelength domain, kρ ce > 0.5. This destabilization increases the range of propagation angles, k||/kperp, for which the modes are operative, which reduces the localizing and stabilizing effects of magnetic shear. The dominant Type A modes are localized near the current sheet edge, z ∼ 1.5 L, due to the stabilizing resonant dissipation of the electron grad B drift, which is strongest near the current sheet center. At longer wavelengths, k^2 ρeρi ∼ 1, a second group of instabilities arise, which we define as Type B LHDIs. The Type B LHDIs are operative for a large range of propagation angles, and are suppressed when the electron grad B drift is removed. The dominant Type B instabilities are localized near the current sheet center, z ∼ 0.2L, with moderate k||/kperp and significant magnetic field perturbations. The Type B LHDIs resemble fluctuations observed in nature, and simulations, which have undergone limited analytical analysis. These modes are of general relevance to the evolution of thin current sheets, and may contribute to collisionless magnetic reconnection theory as a source of anomalous resistivity.

Plasmoids have been observed over a broad distance along Earth's magnetotail, from X= -30 RE to -200 RE (X points positively sunward along the sun-Earth line). As described in the near-Earth-neutral-line (NENL) substorm model, reconnection at the NENL causes a plasmoid to be formed and ejected tailward. Because distant-tail (X

On 5 May 2017, MMS observed a crater‐type flux rope on the dawnside tailward magnetopause with fluctuations. The boundary‐normal analysis shows that the fluctuations can be attributed to nonlinear Kelvin‐Helmholtz (KH) waves. Reconnection signatures such as flow reversals and Joule dissipation were identified at the leading and trailing edges of the flux rope. In particular, strong northward electron jets observed at the trailing edge indicated midlatitude reconnection associated with the 3‐D structure of the KH vortex. The scale size of the flux rope, together with reconnection signatures, strongly supports the interpretation that the flux rope was generated locally by KH vortex‐induced reconnection. The center of the flux rope also displayed signatures of guide‐field reconnection (out‐of‐plane electron jets, parallel electron heating, and Joule dissipation). These signatures indicate that an interface between two interlinked flux tubes was undergoing interaction, causing a local magnetic depression, resulting in an M‐shaped crater flux rope, as supported by reconstruction., Key Points MMS observed a magnetic flux rope formed on the boundary of a nonlinear Kelvin‐Helmholtz (KH) waveBoth in‐plane and midlatitude reconnection associated with the 3‐D structure of the KH vortex‐inducedflux rope were identifiedA current sheet at the flux rope center supported by reconstruction indicates two flux tubes interlinked to form a crater‐type flux rope

著者人数: 20名, Accepted: 2019-05-28, 資料番号: SA1190061000

International audience; This paper reports on Magnetospheric Multiscale observations of whistler mode chorus and higher-frequency electrostatic waves near and within a reconnection diffusion region on 23 November 2016. The diffusion region is bounded by crescent-shaped electron distributions and associated dissipation just upstream of the X-line and by magnetic field-aligned currents and electric fields leading to dissipation near the electron stagnation point. Measurements were made southward of the X-line as determined by southward directed ion and electron jets. We show that electrostatic wave generation is due to magnetosheath electron beams formed by the electron jets as they interact with a cold background plasma and more energetic population of magnetospheric electrons. On the magnetosphere side of the X-line the electron beams are accompanied by a strong perpendicular electron temperature anisotropy, which is shown to be the source of an observed rising-tone whistler mode chorus event. We show that the apex of the chorus event and the onset of electrostatic waves coincide with the opening of magnetic field lines at the electron stagnation point.