We report unusual Electromagnetic Ion Cyclotron (EMIC) waves with a very narrow frequency bandwidth, closely following and approaching the proton gyrofrequency. One interesting case analysis shows that magnetosonic waves, anisotropic suprathermal proton distributions, and high frequency EMIC waves are closely related. Magnetosonic waves potentially cause the resonant heating of suprathermal protons and the temperature anisotropy of suprathermal protons (10-100 eV) likely provides free energy for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. Moreover, these low frequency EMIC waves typically occur near the equator in the low-density regions from dawn to dusk. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves. Plain Language Summary Electromagnetic Ion Cyclotron (EMIC) waves are commonlyobserved in the Earth's magnetosphere and play an important role in causing the loss of ring current ions and relativistic electrons due to pitch angle scattering. In this study, we report unusual high frequency EMIC waves with frequency very close to the proton gyrofrequency. An interesting case study clearly shows the correlation between magnetosonic waves, the enhancement of suprathermal protons, and high frequency EMIC waves. The protons at suprathermal energies could be heated by magnetosonic waves and the anisotropic distribution of suprathermal protons is likely responsible for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves.

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Edge shear flow and its effect on regulating turbulent transport have long been suspected to play an important role in plasmas operating near the Greenwald density limit n G . In this study, equilibrium profiles as well as the turbulent particle flux and Reynolds stress across the separatrix in the HL-2A tokamak are examined as n G is approached in ohmic L-mode discharges. As the normalized line-averaged densityn e /n G is raised, the shearing rate of the mean poloidal flow ω sh drops, and the turbulent drive for the low-frequency zonal flow (the Reynolds power P Re ) collapses. Correspondingly, the turbulent particle transport increases drastically with increasing collision rates. The geodesic acoustic modes (GAMs) gain more energy from the ambient turbulence at higher densities, but have smaller shearing rate than low-frequency zonal flows. The increased density also introduces decreased adiabaticity which not only enhances the particle transport but is also related to reduction in the eddy-tilting and the Reynolds power. Both effects may lead to cooling of edge plasmas and therefore the onset of MHD instabilities that limit the plasma density.1

Whistler mode chorus waves are coherent electromagnetic emissions in planetary magnetospheres, characterized by rising-tone or falling-tone chirping elements. Understanding the cause of different chirping directions and their properties is an important step in resolving the long-standing problem of nonlinear chorus generation. We report here, for the first time, particle-in-cell simulations of bidirectional chirping of whistler waves in a uniform magnetic field and falling-tone-only chirping in an inhomogeneous field. Combined with previous simulations of rising-tone-only emissions, we demonstrate that the background magnetic field inhomogeneity is not required for chirping of chorus, but it plays a key role in determining the chirping direction. The findings of the present work also unveil the critical role of the dipole geometry of Earth's magnetic field in causing the statistical predominance of rising-tone chorus and the oblique propagation of falling-tone chorus.Plain Language Summary Whistler mode chorus is a type of naturally occurring electromagnetic emission in planetary magnetospheres. This important wave is known to produce relativistic electrons in the hazardous radiation belts and to precipitate energetic electrons from space into the upper atmosphere to form diffuse aurora. Chorus consists of discrete spectral elements with fast frequency chirping in either upward (rising-tone) or downward (falling-tone) directions. A long-standing problem is to understand the origin of different chirping directions and their distinctive observational properties. Here, we show, by first-principle particle simulations, that the background magnetic field inhomogeneity plays a key role in determining the chirping direction and that the dipole geometry of Earth's magnetic field essentially controls the properties of chorus. Our results naturally account for the dominance of rising-tone chorus and the oblique propagation of falling-tone chorus and provide important insights into the fundamental mechanism of the nonlinear chirping process. The propagation properties of chorus are also expected to be similar in all planetary magnetospheres with a dipole-type magnetic field.