New Insights Into the Substorm Initiation Sequence From the Spatio-Temporal Development of Auroral Electrojets.

In the present study we examine three substorm events, Events 1-3, focusing on the spatio-temporal development of auroral electrojets (AEJs) before auroral breakup. In Events 1 and 2, auroral breakup was preceded by the equatorward motion of an auroral form, and the ground magnetic field changed northward and southward in the west and east of the expected equatorward flow, respectively. Provided that these magnetic disturbances were caused by local ionospheric Hall currents, this feature suggests that the equatorward flow turned both eastward and westward as it reached the equatorward part of the auroral oval. The auroral breakup took place at the eastward-turning and westward-turning branches in Events 1 and 2, respectively, and after the auroral breakup, the westward AEJ enhanced only on the same side of the flow demarcation meridian. The zonal flow divergence is considered as an ionospheric manifestation of the braking of an earthward flow burst in the near-Earth plasma sheet and subsequent dawnward and duskward turning. Therefore, in Events 1 and 2, the auroral breakup presumably mapped to the dawnward and duskward flow branches, respectively. Moreover, for Event 3, we do not find any pre-onset auroral or magnetic features that can be associated with an equatorward flow. These findings suggest that the braking of a pre-onset earthward flow burst itself is not the direct cause of substorm onset, and therefore, the wedge current system that forms at substorm onset is distinct from the one that is considered to form as a consequence of the flow braking.

A New Perspective on Magnetotail Electron and Ion Divergent Flows: MMS Observations.

Fast divergent flows of electrons and ions in the magnetotail plasma sheet are conventionally interpreted as a key reconnection signature caused by the magnetic topology change at the X-line. Therefore, reversals of the x-component (V x⊥) of the plasma flow perpendicular to the magnetic field must correlate with the sign changes in the north-south component of the magnetic field (B z ). Here we present observations of the flow reversals that take place with no correlated B z reversals. We report six such events, which were measured with the high-resolution plasma and fields instruments of the Magnetospheric Multiscale mission. We found that electron flow reversals in the absence of B z reversals (a) have amplitudes of ∼1,000-2,000 km s-1 and durations of a few seconds; (b) are embedded into larger-scale ion flow reversals with enhanced ion agyrotropy; and (c) compared with conventional reconnection outflows around the electron diffusion regions (EDRs), have less (if ever) pronounced electron agyrotropy, dawnward electron flow amplitude, and electric field strength toward the neutral sheet, although their energy conversion parameters, including the Joule heating rate, are quite substantial. These results suggest that such flow reversals develop in the ion-demagnetization regions away from electron-scale current sheets, in particular the EDRs, and yet they play an important role in the energy conversion. These divergent flows are interpreted as precursors of the flow-driven reconnection onsets provided by the ion tearing or the ballooning/interchange instability.

Dynamic Properties of Particle Injections Inside Geosynchronous Orbit: A Multisatellite Case Study.

Four closely located satellites at and inside geosynchronous orbit (GEO) provided a great opportunity to study the dynamical evolution and spatial scale of premidnight energetic particle injections inside GEO during a moderate substorm on 23 December 2016. Just following the substorm onset, the four spacecraft, a LANL satellite at GEO, the two Van Allen Probes (also called "RBSP") at ~5.8 R E, and a THEMIS satellite at ~5.3 R E, observed substorm-related particle injections and local dipolarizations near the central meridian (~22 MLT) of a wedge-like current system. The large-scale evolution of the electron and ion (H, He, and O) injections was almost identical at the two RBSP spacecraft with ~0.5 R E apart. However, the initial short-timescale particle injections exhibited a striking difference between RBSP-A and -B: RBSP-B observed an energy dispersionless injection which occurred concurrently with a transient, strong dipolarization front (DF) with a peak-to-peak amplitude of ~25 nT over ~25 s; RBSP-A measured a dispersed/weaker injection with no corresponding DF. The spatiotemporally localized DF was accompanied by an impulsive, westward electric field (~20 mV m-1). The fast, impulsive E × B drift caused the radial transport of the electron and ion injection regions from GEO to ~5.8 R E. The penetrating DF fields significantly altered the rapid energy- and pitch angle-dependent flux changes of the electrons and the H and He ions inside GEO. Such flux distributions could reflect the transient DF-related particle acceleration and/or transport processes occurring inside GEO. In contrast, O ions were little affected by the DF fields.

Five-body cluster structure of the double-Λ hypernucleus (ΛΛ)(11)Be.

Energy levels of the double Λ hypernucleus (ΛΛ)(11)Be are calculated within the framework of a ααnΛΛ five-body model. Interactions between constituent particles are determined so as to reproduce reasonably the observed low-energy properties of the αα, ααn nuclei and the existing data for Λ-binding energies of the αΛ, ααΛ, αnΛ, and ααnΛ systems. An effective ΛΛ interaction is constructed so as to reproduce, within the αΛΛ three-body model, the B(ΛΛ) of (ΛΛ)(6)He, which was extracted from the emulsion experiment, the NAGARA event. With no adjustable parameters for the ααnΛΛ system, B(ΛΛ) of the ground and bound excited states of (ΛΛ)(11)Be are calculated with the Gaussian expansion method. The Hida event, recently observed at KEK-E373 experiment, is interpreted as an observation of the ground state of the (ΛΛ)(11)Be.

LambdaN spin-orbit splittings in (9)(Lambda)Be and (13)(Lambda)C studied with one-boson-exchange LambdaN interactions

Movitated by the on-going gamma-ray experiments at the Alternating Gradient Synchrotron at Brookhaven National Laboratory, we discuss the energy splittings of the 5/2(+)(1)-3/2(+)(1) doublet in (9)(Lambda)Be and the 3/2(-)(1)-1/2(-)(1) doublet in (13)(Lambda)C for which the LambdaN spin-orbit ( LS) and antisymmetric spin-orbit ( ALS) forces are relevant. In the microscopic 2alpha+Lambda ( 3alpha+Lambda) model for (9)(Lambda)Be ( (13)(Lambda)C), all the available Nijmegen one-boson-exchange (OBE) model LambdaN interactions lead to a wide range of splittings of 0.08-0.20 MeV in (9)(Lambda)Be and 0.39-0.96 MeV in (13)(Lambda)C. On the other hand, if we use information from quark-model LambdaN interactions which have generally large ALS force, the splittings become about half of the smallest OBE model prediction.