The interactions between ULF waves and cold charged particles in the Earth's magnetosphere

In the solar wind-magnetosphere coupling processes, many kinds of plasma waves can be excited in the Earth's magnetosphere including ULF waves, hiss waves, chorus waves, etc. Among these waves, ULF waves are featured by the lowest wave frequency (1 mHz～1 Hz), the longest wavelength (comparable with the magnetic field line in the inner magnetosphere) and the highest wave power density. ULF waves can propagate along the geomagnetic field lines into the ionosphere and cause the joule heating of the ionosphere. Besides, after excited at the magnetopause, they can propagate earthward across the geomagnetic field lines and generate the standing Alfvén waves (poloidal mode and toroidal mode) via the field line resonance. The electric field component of the poloidal mode standing is aligned with the drift direction of charged particles trapped in the magnetosphere, allowing a strong interaction between ULF waves and particles. Therefore, ULF waves play a crucial role in the particle energization, mass transportation and energy transfer within the magnetosphere. Previous studies focus on the global propagation and distribution of ULF waves, and their interactions with energetic particles such as radiation belt electrons and ring current ions. Recent studies within 5 years shed new light on the localized ULF waves and the interactions between ULF waves and cold plasmaspheric particles, which are reviewed in this paper. The existence of the localized ULF waves has been verified in different ways including multi-spacecraft observations, coordinated ground station measurements and "boomerang-shaped" pitch angle features of resonant radiation belt electrons. Studies suggested that the localized ULF waves are probably associated with the plasmaspheric plume structure, which can generate the Alfvén speed barriers. Multi-spacecraft observations have demonstrated that there are plasmaspause surface waves, which modulate particle populations and different waves around the plasmapause, and cause the formation of the sawtooth aurora. The acceleration of cold electrons by ULF waves via drift-bounce resonance has been found based on the observations of electron's pitch angle features and energy spectrum, theoretical analyses and statistical studies. Cold ions can be energized and modulated by ULF waves via E × B , which can be used to distinguish ion species and study ions with energies out of the scope of instrument. In the final, we summarize the possible roles of the ULF wave-cold particle interactions in the dynamics of magnetosphere and list some important issues for future studies.