Magnetic reconnection is an important phenomenon extensively existing in the interplanetary space and planetary magnetosphere, such as solar flares, solar and stellar coronae, solar wind, planetary magnetosphere, the interplanetary space, the interstellar medium, neutron start, accretion disks, astrophysical jets, galaxy clusters, and black holes. The traditional cognition is that the energy carried by the magnetic field comes to explosions through reconnection. Ultimately the energy converts to the particles'kinetic and thermal energy, resulting in the acceleration and heating of the ions and electrons (e.g.,

Electromagnetic ion cyclotron (EMIC) wave is one of the most important ion-scale plasma waves in the inner magnetosphere, which often appear near the magnetic equator with frequencies below the local proton gyrofrequency (Anderson et al., 1992;Jordanova et al., 1997;Loto'aniu et al., 2005). EMIC waves are often observed with left polarizations and the direction of propagation is along the background magnetic field when the source region is near the magnetic equator (

Magnetic reconnection is a universal phenomenon existing in the space. Energy conversion is one of the essential parts of reconnection discussed for decades. Positive energy conversion (J·E′ > 0) is usually regarded as the sign of electron diffusion region (EDR), while the negative one (J·E′ < 0) turns out to gather in the outer EDR. Here we report the negative J·E′ appearing in the inflow edge of the inner EDR, based on Magnetospheric Multiscale mission observations and particle‐in‐cell simulations. Both observations and simulations verify that this negative J·E′ is mainly contributed from the electron pressure tensor term in generalized Ohm's law. The energy loss of electrons plays the dominant effect in the electron pressure tensor, and this energy decline is caused by the electric field which is induced by the decreasing magnetic field. Our results provide significant insights to expand the understanding of the reconnection regimes.

High-energy plasma is widely spread in interplanetary space. The production of this plasma is associated with solar activities and other physical phenomena. It is believed that magnetic reconnection is one of the crucial mechanisms that manage energy release and plasma energization. Many physical structures or processes are found to participate in the energy conversion between particles and fields during the reconnection, such as the electron diffusion region (EDR) (e.g.,

Studies on substorms originated from the discovery of consistent patterns of auroral activity in Earth's ionosphere (Akasofu, 1964). After decades of intense studies, several features of Earth's substorms have become well known (e.g., Haerendel, 2015). At the beginning of a substorm, dayside magnetopause reconnection occurs, and the corresponding magnetic flux is transported to the lobes and loaded into the magnetotail (Dungey, 1961). Then, the increased magnetic pressure within the lobes thins the central current sheet and promotes magnetic reconnection in the magnetotail (Coroniti & Kennel, 1972). Moreover, a pair of connected X-and O-lines form in the plasma sheet (Russell & McPherron, 1973). As the rate of magnetotail reconnection increases, the O-line evolves to the plasmoid and will be released from the central magnetotail. Magnetic dipolarization is generated near the earthward X-line, and then transports the energetic particles into the inner magnetosphere (Russell & McPherron, 1973). Saturn's space environment is different from Earth's, suggesting that Saturn has a different substorm driving mechanism. Compared with Earth (1 AU), Saturn is farther away from the Sun (9.58 AU), and the solar wind near Saturn is relatively weak, with a dynamic pressure of 0.015 nPa (1.7 nPa for Earth). Although magnetopause reconnections occasionally occur at Saturn's magnetopause (Desroche et al., 2013;Masters et al., 2014;Fuselier et al., 2014), the solar wind is not the most important driver of magnetopause reconnection at Saturn. The water-group neutrals released from Enceladus are ionized and serve as the internal plasma source in Saturn's inner magnetosphere (Bagenal & Delamere, 2011). Moreover, due to Saturn's rapid rotation, the magnetic field configuration and the plasmas in Saturn's magnetosphere present the characteristic of rotation. The rotational energy arises from the corotational kinetic energy that planetary rotation exerts on magnetospheric plasma (Ge Abstract Substorms are a fundamental phenomenon in planetary magnetospheric systems. Using Cassini measurements, we report a typical event in which four successive dipolarization fronts (DFs) were observed within 1 hour during the substorm in Saturn's magnetotail. The last three DFs caused a type of stepwise electron acceleration and generated energetic electrons. The pitch angle distributions of the electrons show evidence of the Fermi acceleration mechanism behind these DFs. Therefore, we infer the magnetotail dynamics process during Saturn's substorm: The stepwise acceleration by successive DFs powerfully accelerates the field-aligned electrons and generates field-aligned energetic electrons. These high-energy electrons are injected into the inner magnetosphere and become an important trigger of Saturn's aurora. Our results show an efficient acceleration mechanism for the electrons caused by successive DFs and confirm an important source of energetic particles during the substorm in Saturn, and these findings improve our understanding of Saturn's substorm ...

We report direct observations of the detailed processes for the acceleration and thermalization of beam electrons in the plasma sheet (PS) of the Earth's magnetotail. With observations from the four magnetospheric multiscale (MMS) satellites, the potential of a double layer (DL) structure is found to be mostly consistent with enhancements of the averaged parallel energy of the beam electrons passing through the DL structure, demonstrating that beam electrons are accelerated by the DL structure. Thereafter, Debye‐scale parallel electric field turbulences and an increase of the parallel temperature are simultaneously observed, implying that electrostatic parallel turbulences thermalize beam electrons. These observations imply that the DL is a key process controlling the acceleration and thermalization of beam electrons in the PS of the Earth's magnetotail.

We utilize the data from the Parker Solar Probe mission at its first perihelion to investigate the three-dimensional (3D) anisotropies and scalings of solar wind turbulence for the total, perpendicular, and parallel magnetic-field fluctuations at kinetic scales in the inner heliosphere. By calculating the five-point second-order structure functions, we find that the three characteristic lengths of turbulence eddies for the total and the perpendicular magnetic-field fluctuations in the local reference frame ( L ˆ ⊥ , l ˆ ⊥ , l ˆ ∣ ∣ ) defined with respect to the local mean magnetic field B local feature as l ∣∣ > L ⊥ > l ⊥ in both the transition range and the ion-to-electron scales, but l ∣∣ > L ⊥ ≈ l ⊥ for the parallel magnetic-field fluctuations. For the total magnetic-field fluctuations, the wave-vector anisotropy scalings are characterized by l ∣ ∣ ∝ l ⊥ 0.78 and L ⊥ ∝ l ⊥ 1.02 in the transition range, and they feature as l ∣ ∣ ∝ l ⊥ 0.44 and L ⊥ ∝ l ⊥ 0.73 in the ion-to-electron scales. Still, we need more complete kinetic-scale turbulence models to explain all these observational results.

Nonlinear theory has predicted that wave‐wave coupling from different band electromagnetic ion cyclotron (EMIC) waves can occur in multiple‐ion plasmas. However, no direct observational evidence has been reported yet. Here we present multiple substructures on the frequency spectrum of EMIC waves with observations of Van Allen Probes. Some substructures are demonstrated to be excited by nonlinear interactions between preexisting H+ band and He+ band EMIC waves through the bicoherence analysis. Additionally, hybrid simulation displays that a new wave is motivated at sum frequencies after two independent pump waves of different band EMIC waves are injected, implying that the nonlinear wave‐wave interactions could drive EMIC waves with higher wavenumbers. Therefore, this letter provides the first observational evidence of wave‐wave coupling between H+ band and He+ band EMIC waves.