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We investigate the reflection of low-harmonic fast magnetosonic (MS) waves at the local two-ion cutoff frequency (fcutHe+). By comparing the wave signals of the two Van Allen Probes satellites, a distinct boundary where wave energies cannot penetrate inward are found in the time–frequency domain. The boundary is identified as the time series of local fcutHe+. For a certain frequency, there exists a spatial interface formed by fcutHe+ , where the incident waves should be reflected. The waves with small incident angles are more likely to penetrate the thin layer where the group velocity reduces significantly and then slow down in a period of several to tens of seconds before the reflection process complete. The cutoff reflection scenario can explain the intense outward waves observed by probe A. These results of MS reflection at fcutHe+ may help to predict the global distribution of MS waves and promote the understanding of wave–particle dynamics in the radiation belt. [ABSTRACT FROM AUTHOR]

Mirror-mode structures widely exist in various space plasma environments. Here, we investigate a train of mirror-mode structures in the terrestrial plasma sheet on 11 August 2017 based on the Magnetospheric Multiscale mission. We find that bipolar current densities exist in the cross section of two hole-like mirror-mode structures, referred to as magnetic dips. The bipolar current density in the magnetic dip with a size of ∼2.2 ρi (the ion gyro radius) is mainly contributed by variations of the electron velocity, which is mainly formed by the magnetic gradient–curvature drift. For another magnetic dip with a size of ∼6.6 ρi , the bipolar current density is mainly caused by an ion bipolar velocity, which can be explained by the collective behaviors of the ion drift motions. The current density inside the mirror dip contributes to the maintenance of the hole-like structure's stable. Our observations suggest that the electrons and ions play different roles in the formation of currents in magnetic dips with different sizes. [ABSTRACT FROM AUTHOR]

With the advantage of fast plasma measurements of the Magnetospheric Multiscale (MMS) mission in the magnetotail, we investigate the particle carriers of field-aligned currents (FACs) in the plasma sheet boundary layer for three cases. In all cases, electrons are the main carriers of FACs while the contribution of ions can be neglected. Our results indicate that thermal electrons (energy range from 0.5 Te to 5 Te, where Te is the electron parallel temperature) are the main carrier of the FAC. However, cold electrons (energy less than 0.5 Te) can also significantly contribute to the FACs. In 1 of 3 cases, suprathermal electron (energy greater than 5 Te) only contributes a small portion to the total current. The difference between all cases may depend on the local dynamics in the magnetotail. [ABSTRACT FROM AUTHOR]

With the MMS observations in the magnetotail, we report an earthward, dense, high speed, bulk flow event. In this event, two dense high speed flows (DHSF) are observed. Compared with the plasma sheet, the DHSF contains much denser and colder plasma. Ion density of DHSFs is about 8 times higher to plasma sheet density, while temperature of DHSFs is several times lower to plasma sheet plasma. Due to the high density, DHSFs can transport mass more effective than BBFs. Such DHSF can be generated by magnetic reconnection, but the generation mechanism is still unclear. [ABSTRACT FROM AUTHOR]