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In the companion volume, Kinetic Alfven Wave: Theory, Experiment, and Application, we have presented the systematic theories and basic principles of kinetic Alfven waves (KAWs) based on both kinetic and fluid descriptions of magnetic plasmas. The present volume builds upon that and proceeds into the in-depth analyses and discussions of observation, generation, and application of KAWs in various plasma environments in the solar-terrestrial coupling system, which is a unique natural laboratory for the comprehensive and thorough study of KAWs. However, first of all, in this introductory chapter we concisely review main physical properties of KAWs without proving, which are from the companion volume and set up the basis for the following chapters.

In previous chapters we comprehensively introduced KAWs in space and solar plasmas, including their observational identifications and applications to related particle kinetic phenomena, such as the auroral energetic electron acceleration in auroral magnetospheric plasmas, the anomalous particle transport in the magnetopause, the kinetic turbulence in the solar wind, and the solar coronal heating problem. The solar and solar-system plasma, especially the solar-terrestrial coupling system, is a natural laboratory for plasma astrophysics.

The physical nature of the auroral electron acceleration has been an outstanding problem in space physics for decades. Some recent observations from the auroral orbit satellites, FREJA and FAST, showed that large amplitude solitary kinetic Alfven waves (SKAWs) are a common electromagnetic active phenomenon in the auroral magnetosphere. In a low-beta (i.e., beta /2 << m(e)/m(i) << 1) plasma, the drift velocity of electrons relative to ions within SKAWs is much larger than thermal velocities of both electrons and ions. This leads to instabilities and causes dissipations of SKAWs. in the present work, based on the analogy of classical particle motion in a potential well, it is shown that a shock-like structure can be formed from SKAWs if dissipation effects are included. The reformed SKAWs with a shock-like structure have a local density jump and a net field-aligned electric potential drop of order of m(e)v(A)(2)/e over a characteristic width of several lambda (e). As a consequence, the reformed SKAWs can efficiently accelerate electrons field-aligned to the order of the local Alfven velocity. In particular, we argue that this electron acceleration mechanism by reformed SKAWs can play an important role in the auroral electron acceleration problem. The result shows that not only the location of acceleration regions predicted by this model is well consistent with the observed auroral electron acceleration region of 1-2 R-E above the auroral ionosphere, but also the accelerated electrons from this region can obtain an energy of several keV and carry a field-aligned current of several muA/m(2) which are comparable to the observations of auroral electrons.

The properties of the solitary kinetic Alfven waves (SKAWs) observed by Freja satellite are studied statistically for a typical event on March 7, 1994. The eigenmode of the SKAWs observed is still recognized by their anisotropic structure (1000 km x 1 km), their waveform which is similar to that of a wave packet and their frequency close to the local oxygen ion cyclotron frequency. If these, spatial structures really exist, they may have resulted from the evolution of the SKAWs or some envelope solitons of ion cyclotron and magnetosonic waves, which is supported by four statistical results: (1) The phase velocity is directly proportional to the Alfven velocity. (2) The amplitudes of the electric filed, magnetic field and density fluctuations are directly proportional to the timescales of the pulsations. (3) The amplitudes as well as the timescales are directly proportional to the phase velocity. (4) The polarization sense is both of right circular and left circular mode with different phase relation among the electric field, magnetic field, and density fluctuations. Moreover, the SKAWs are often superposed by a strong electrostatic fluctuations at several hundred hertz, the causal relation between the SKAWs and superposed waves is still unknown.

A new type of density soliton, which we call “dipole density soliton,’’ is discovered in data from the Freja satellite. Like the dip or hump density solitons that were recently discovered in the Freja data [D.-J. Wu, G.-L. Huang, and D.-Y. Wang, Phys. Plasmas 3, 2879 (1996)], the dipole density solitons are also associated with strong electric spikes (∼ a few 100 mV/m) and have a spatial scale length of a few 100 m. This indicates that the three types of density solitons (dip, hump, and dipole) probably have the same physical nature. In this paper, a two-dimensional solitary kinetic Alfven wave (SKAW) model with a dipole vortex structure is proposed to account for the three kinds of density solitons (dip, hump, and dipole), in which the differences in their appearances can naturally be attributed to differences in the positions and directions at which the satellite crosses dipole vortex structures. Some features of this two-dimensional SKAW model are discussed, and the results are compared to the one-dimensional cases.

An analytical solution of finite‐amplitude solitary kinetic Alfven waves (SKAWs) in a low‐β (β≪me/mi≪1) plasma is presented. This solution has been compared with the solution of the Korteweg–de Vries (KdV) equation in the small‐amplitude limit. It is found that the KdV soliton solution is valid only for the maximum relative density perturbation Nm

In their comment, Shukla and Stenflo proposed a completeset of equations @i.e., Eqs. ~1!–~4! in Ref. 1# to describe thesolitary kinetic Alfve´n wave ~SKAW! data from the Frejasatellite, in which the equilibrium electron and ion velocitygradients, the parallel ion dynamics and the electron–ion andion–ion collision effects are included. It is easy to find thatthe equation system proposed by Shukla and Stenflo is es-sentially no different from that presented in our paper ~i.e.,Ref. 2! if neglecting above the effects in Shukla andStenflo’s equation system.With regard to the points criticized by Shukla and Sten-flo, we would like to explain in turn as follows: First, ne-glecting magnetic field perturbations and electron inertialcontribution ~the