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[1] In collisionless magnetic reconnection, the in-plane Hall currents are carried mainly by the magnetized electrons. The in-plane Hall currents are directed toward the X line along the magnetic field lines just inside the separatrices and away from the X line along the separatrices. Such a current system leads to the quadrupole out-of-plane magnetic field with the peaks between the regions carrying the in-plane currents. Simultaneously, the electron flow toward the X line along the separatrices causes electron density depletions along the separatrices. In this paper, the features of separatrix regions in magnetic reconnection and the relations between the electron density depletions and the out-of-plane magnetic field are investigated with both two-dimensional particle-in-cell simulations and Cluster observations. We conclude that the electron density depletions are formed because of the magnetic mirror, and they are outside the peaks of the out-of-plane magnetic field. Such a theoretical prediction is confirmed by both simulations and observations.

Two-dimensional particle-in-cell simulations are performed to investigate electron dynamics in antiparallel and guide field ͑in the presence of a strong guide field͒ magnetic reconnection, and the mechanisms of electron acceleration are compared. In the antiparallel reconnection, the dominant acceleration occurs in the vicinity of the X line, where the magnetic field is weak. Most of these electrons come from the regions just outside of the separatrices, which move into the vicinity of the X line along the magnetic field lines. Electrons can also be nonadiabatically accelerated in the pileup region by the reconnection electric field, where the gyroradii of the electrons are comparable to the curvature radii of the magnetic field lines. Most of these electrons come from the regions inside of the separatrices, which move into the pileup region along the magnetic field lines. In the guide field reconnection, electrons are accelerated by the parallel electric field. They are firstly accelerated when moving toward the X line along the magnetic field lines, and then are further accelerated when they are funneled into the vicinity of the X line. Most of energetic electrons come from the region outside of the pair of the negative separatrices. The efficiency of such an acceleration mechanism is obviously higher than that in the antiparallel reconnection. In both the antiparallel and guide field reconnection, the mechanisms of electron acceleration favor the electrons with higher initial energy.

Photocatalytic anticancer profile of a IrIII photocatalyst (Ir3) with strong light absorption, high turnover frequency, and excellent biocompatibility is reported. Ir3 showed selective photo‐cytotoxicity against cisplatin‐ and sorafenib‐resistant cell lines while remaining dormant to normal cell lines in the dark. Ir3 exhibited excellent photo‐catalytic oxidation of cellular co‐enzyme, the reduced nicotinamide adenine dinucleotide phosphate (NADPH), and amino acids via a single electron transfer mechanism. The photo‐induced intracellular redox imbalance and change in mitochondrial membrane potential resulted in necrosis and apoptosis of cancer cells. Importantly, Ir3 exhibited high biocompatibility and photo‐catalytic anticancer efficiency as evident from in vivo zebrafish and mouse cancer models. To the best of our knowledge, Ir3 is the first IrIII based photocatalyst with such a high biocompatibility and photocatalytic anticancer therapeutic effect.

[1] In this paper, we present Cluster observations of a magnetotail reconnection event without the presence of an obvious guide magnetic field and analyze electron pitch angle distributions in the vicinity of the X line and the outflow region, respectively. In the vicinity of the X line, at lower energies the distributions are highly anisotropic (field-aligned bidirectional anisotropic), while at higher energies, the electrons are observed to flow away from the X line along the magnetic field lines. The electron distributions change largely in the outflow region. At the edge of the outflow region, at lower energies, the electrons flow toward the X line, while the electrons at higher energies are directed away from the X line. When the satellites approach the center of the current sheet, at lower energies, the electrons have field-aligned bidirectional distributions, while at higher energies, the electron distributions are isotropic. The generation mechanisms of such distributions are explained by following typical electron trajectories in the electric and magnetic fields of magnetic reconnection, which are obtained in two-dimensional particle-in-cell simulations. It is shown that the observed high-energy electrons directed away from the X line both in the vicinity of the X line and in the outflow region are due to the acceleration by the reconnection electric field near the X line, and the field-aligned bidirectional distributions at lower energies are caused by the effects of the magnetic mirror in the reconnection site. The isotropic distributions at higher energies in the outflow region are the results of the electron stochastic motions when their gyroradii are comparable to the curvature radii of the magnetic field lines.

[1] We investigate the electron acceleration behind dipolarization fronts (DFs) in the magnetotail from À25 R E to À10 R E through the examination of the energetic electron energy flux (>30 keV) with the observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS). Statistical results of 133 DF events are presented based on the data set from January to April of the years 2008 and 2009. As the DFs propagate earthward, the acceleration of energetic electrons behind the DFs is found to take place over several R E along the tail. The increase in energetic electron energy flux can reach 2-4 orders of magnitude. The dominant acceleration mechanisms are different in the midtail (X GSM ≤ À15 R E ) and the near-Earth region (À15 < X GSM ≤ À10 R E ). In the midtail, the majority of DF events show that the dominant electron acceleration mechanism is betatron acceleration. In the near-Earth region, betatron acceleration is dominant in~46% DF events while Fermi acceleration is dominant in~39% DF events.

Dipolarization fronts (DFs) as earthward propagating flux ropes (FRs) in the Earth's magnetotail are presented and investigated with a three‐dimensional (3‐D) global hybrid simulation for the first time. In the simulation, several small‐scale earthward propagating FRs are found to be formed by multiple X line reconnection in the near tail. During their earthward propagation, the magnetic field Bz of the FRs becomes highly asymmetric due to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRs approach the near‐Earth dipole‐like region, the antireconnection between the southward/negative Bz of the FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of the FRs, which further aggravates the Bz asymmetry. Eventually, the FRs merge into the near‐Earth region through the antireconnection. These earthward propagating FRs can fully reproduce the observational features of the DFs, e.g., a sharp enhancement of Bz preceded by a smaller amplitude Bz dip, an earthward flow enhancement, the presence of the electric field components in the normal and dawn‐dusk directions, and ion energization. Our results show that the earthward propagating FRs can be used to explain the DFs observed in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and the electric field normal to the front is found to be dominated by the Hall physics. During the earthward propagation from the near‐tail to the near‐Earth region, the speed of the FR/DFs increases from ~150 km/s to ~1000 km/s. The FR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn‐dusk) axis and only extend several Earth radii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn‐dusk direction, which indicates that the DFs are essentially 3‐D.

[1] An antiparallel reconnection event is recognized in the magnetotail, and a secondary island with a strong core magnetic field is identified near the center of the ion diffusion region. In the island, the electron density peaks in the outer region while the dip is in the core region with a strong core magnetic field. A strong electron beam parallel to the magnetic field, as well as an obvious current antiparallel to the magnetic field with density up to ∼40 nA/m 2 , is observed in the outer region of the island. This suggests that the strong core magnetic field inside the island is generated by the electron beam and then the antiparallel current in the outer region of the island. The electron density dip in the core region is formed due to the existing strong core field, which expels electrons out of the core region. The electron flat-top distributions are detected in the ion diffusion region except the core region of the island, and the shoulder energy range of the flat-top distributions is from 100 eV to 4 keV. An enhancement of the energetic electron flux up to 200 keV is found in the ion diffusion region, and a further increase of energetic electron fluxes is observed inside the island. Waves at the lower hybrid frequency are intensified in the ion diffusion region, while the intensification is strong in the outer region of the island and becomes very weak in the core region.Citation: Wang, R., Q. Lu, X. Li, C. Huang, and S. Wang (2010), Observations of energetic electrons up to 200 keV associated with a secondary island near the center of an ion diffusion region: A Cluster case study,