Energetic charged particles ~>40 keV can affect the plasma temperature and pressure and as a consequence the whole dynamics of the magnetosphere. How do particles get to such energies is a fundamental science question. Energetic particles are also potentially hazardous for the space observations. It is, therefore, necessary to study the origin of energetic plasma, its acceleration, distribution and consequences on the magnetospheric dynamics. In this work we review the related results based on observations from the Cluster/RAPID energetic charged particle detector. These results represent new insights in plasma acceleration, unexpected features in its distributions, effects on substorm and geomagnetic storm dynamics and remediated observations in the radiation belts during approximately 1.5 solar cycles.

Strong flapping of the Current Sheet (CS) associated with the propagation of high-velocity bulk flows allowed to observe the Super Thin Current Sheet (STCS) in the magnetotail Plasma Sheet (PS). During the interval of interest 111 crossings of the CS neutral line were detected. In 95 crossings the STCSs with a current density J ≥ 20 nA/m2 were observed. A half-thickness (LSTCS) of the STCSs was about a few electron gyroradii or less. In a number of the STCSs the parameter of adiabaticity (κe) was < 1.0 for suprathermal electron population (> 1 keV). Our analysis has shown that the electric current in such STCSs is carried by unmagnetized electrons (κe < 1), and the pressure balance is supported by the off-diagonal terms of their pressure tensor. In this sense the underlying physics of the formation of STCSs at electron scales by unmagnetized electrons is similar to the mechanism of ion-scale thin CS formation by the quasi-adiabatic ions. The low-energy population of magnetized electrons is also crucially important since it keeps the STCS stable and allows their observation as a quasi-stationary structure. We compare the observed half-thickness of the STCS with that predicted by a new kinetic theory (λ) considering the coupling between ion-scale TCS and electron-scale STCS. We found a reasonable agreement between both values: LSTCS ~ (0.3 – 1)λ. Further improvement in the theory taking into account the dynamics of unmagnetized electrons may provide better agreement with observations.

The Kelvin-Helmholtz instability (KHI) and its effects relating to the transfer of energy and mass from the solar wind into the magnetosphere remain an important focus of magnetospheric physics. One such effect is the generation of Pc4-Pc5 ultra low frequency (ULF) waves (periods of 45-600 s). On 3 July 2007 at $\sim$ 0500 magnetic local time (MLT) the Cluster space mission encountered Pc4 frequency Kelvin-Helmholtz waves (KHWs) at the magnetopause with signatures of persistent vortices. Such signatures included bipolar fluctuations of the magnetic field normal component associated with a total pressure increase and rapid change in density at the vortex edges, oscillations of magnetosheath and magnetospheric plasma populations, wave frequencies within the expected range of the fastest growing KH mode, and magnetopause conditions favorable to the onset of the KHI. The event occurred during a period of southward polarity of the interplanetary magnetic field. Most of the KHI vortices were associated with reconnection indicated by the Walén relation, the presence of deHoffman-Teller frames and field-aligned ion beams. Global magnetohydrodynamic (MHD) simulation of the event also resulted in KHWs at the magnetopause. The observed KHWs associated with reconnection coincided with recorded ULF waves at the ground whose properties suggest that they were driven by the KHWs. Such properties were the location of Cluster’s magnetic foot point, the Pc4 frequency, and the solar wind conditions.