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Based on current sheet flapping motion on 27 August 2018 in the dusk flank magnetotail, as recorded by instruments aboard Magnetospheric Multiscale (MMS) spacecraft, we present the first study of guide field reconnection observed in the flux rope embedded in kink-like flapping current sheets near the dusk-side flank of the magnetotail. Unlike more common magnetotail reconnections, which are symmetric, these asymmetric small-scale (λi ~ 650 km) reconnections were found in the highly twisted current sheet when the direction normal to the sheet changes from the Z direction into the Y direction. The unique feature of this unusual reconnection is that the reconnection jets are along the Z direction — different from outflow in the X direction, which is the more usual situation. This vertical reconnection jet is parallel or antiparallel to the up-and-down motion of the tail’s current sheet. The normalized reconnection rate R is estimated to be ~ 0.1. Our results indicate that such asymmetric reconnections can significantly enlarge current sheet flapping, with large oscillation amplitudes. This letter presents direct evidence of guide field reconnection in a highly twisted current sheet, characterized by enlarged current sheet flapping as a consequence of the reconnection outflow.

With conjunction observations of electromagnetic fields and plasma from Time History of Events and Macroscale Interactions during Substorm (THEMIS) in the near-Earth magnetotail, we investigate the spatial and temporal properties of substorm dipolarizations in the near-Earth plasma sheet (NEPS) during a substorm at 03:23 UT on 12 February 2008. Substorm dipolarizations with different features are detected by three near-Earth THEMIS probes (THA (P5), THD (P3) and THE (P4)) in the magnetotail. In the current sheet with a large plasma beta value (β > 2, where β is the ratio of the plasma thermal pressure to the magnetic pressure), the dipolarization within the substorm onset region, (−10.4, 2.8, −2.6)RE_gsm, has a large initial magnetic field elevation angle, θ > 60°, θ = arctan (Bz/(Bx2+By2)1/2), and is accompanied by energetic ion (tens to hundred keV) dispersionless injection detected by THD (P3). This substorm onset dipolarization is characterized by Bx and By components around 0 nT with significant fluctuations. The Bz component increases sharply and its subsequent magnitude approaches the total magnetic field, Bt. The maximum value of the elevation angle approaches 85° during the later substorm expansion phase. In the NEPS with β ~ 1, the dipolarization outside the substorm onset region is characterized by a magnetic elevation angle with a small beginning value of θ < 45° and following multi-step enhancements during the substorm expansion phase. The maximum value of the elevation angle approaches to 70° during the later substorm expansion phase. Our observation results indicate that characteristics of dipolarization with a large beginning elevation angle within the substorm onset region provide a new indicator to identify substorm onset location.

We report multi-spacecraft observations of ULF waves from Van Allen Probes (RBSP), Magnetospheric Multiscale (MMS), Time History of Events and Macroscale Interactions during Substorm (THEMIS), and Geostationary Operational Environmental Satellites (GOES). On August 31, 2015, global-scale poloidal waves were observed in data from RBSP-B, GOES and THEMIS from L=4 to L=8 over a wide range of magnetic local time (MLT). The polarization states varied towards purely poloidal polarity. In two consecutive orbits over 18 hours, RBSP-A and RBSP-B recorded gradual variation of the polarization states of the poloidal waves; the ratio (|Ba|/|Br|) decreased from 0.82 to 0.13. After the variation of polarization states, the poloidal ULF waves became very purely poloidal waves, localized in both L and MLT. We identify the poloidal wave as second harmonic mode with a large azimuthal wave number (m) of –232. From RBSP particle measurements we find evidence that the high-m poloidal waves during the polarization variations were powered by inward radial gradients and bump-on-tail ion distributions through the N=1 drift-bounce resonance. Most of the time, the dominant free energy source was inward radial gradients, compared with the positive gradient in the energy distribution of the bump-on-tail ion distributions.