Evidence of kinetic Alfvén eigenmode in the near‐Earth magnetotail during substorm expansion phase

Abstract

Unipolar pulses of kinetic Alfvén waves (KAW) are first observed in the near‐Earth plasma sheet (NEPS) associated with dipolarizations during substorm expansion phases. Two similar events are studied with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations during substorms on 3 February 2008 and 7 February 2008. The unipolar pulses were located at a trough‐like Alfvén speed profile in the northern plasma sheet at a distance of 10–11 RE from Earth. The dominant wave components consist of a southward δEz toward the neutral plane and a +δBy toward the dusk. The |δEz|/|δBy| ratio was in the range of a few times the local Alfvén speed, a strong indication of KAW nature. The wave Poynting flux was earthward and nearly parallel to the background magnetic field. The pulse was associated with an earthward field‐aligned current carried by electrons. These observational facts strongly indicate a KAW eigenmode that is confined by the plasma sheet but propagates earthward along the field line. The KAW eigenmode was accompanied by short timescale (1 min) dipolarizations likely generated by transient magnetotail reconnection. The observed polarity of the KAW field/current is consistent with that of the Hall field/current in magnetic reconnection, supporting the scenario that the Hall fields/current propagate out from reconnection site as KAW eigenmodes. Aurora images on the footprint of THEMIS spacecraft suggest that KAW eigenmode may power aurora brightening during substorm expansion phase.

Near-Earth plasma sheet boundary dynamics during substorm dipolarization

“…The observed equatorward electric fields associated with the dawn-dusk magnetic field perturbations and Earthward FAC observed in event (1) have the same polarity as the Hall-field and Earthward FACs carried by inflowing electrons toward the reconnection region tailward of the spacecraft, as previously observed in the lobe/ PSBL region (Fujimoto et al 2001;Nakamura et al 2004Nakamura et al , 2016 and in the outer plasma sheet (Duan et al 2016) of the near-Earth tail. These current layers are related to thin ion-scale FAC sheets or may contain even smaller scale FAC current sheets (Nakamura et al 2004(Nakamura et al , 2016.…”

“…These current layers are related to thin ion-scale FAC sheets or may contain even smaller scale FAC current sheets (Nakamura et al 2004(Nakamura et al , 2016. The field disturbance was observed to propagate faster than the local Alfven speed and was identified as a kinetic Alfvén wave (KAW) disturbance (Duan et al 2016). In contrast to these signatures, the overall scale of the current sheet of event (1) was relatively large, more than a factor of 10 larger than the ion-gyro scale (i.e., ~70 km), and at least double the ion inertia length, (i.e., ~600 km).…”

Kinetic Alfvén Waves Excited in Two‐Dimensional Magnetic Reconnection

“…As an indicator of energy transport, the evolution of Poynting flux also suggests that substantial energy is carried by KAW along the field lines. This scenario implies an important coupling mechanism for the magnetotail‐ionosphere interaction, as earlier studies reported that energy associated with KAW excited from the magnetotail reconnection may be responsible for the nightside aurora (Angelopoulos et al, ; Dai et al, ; Duan et al, ; Shay et al, ).…”

Structures of Hall Fields in Asymmetric Magnetic Reconnection

“…A two‐fluid solution of kinetic Alfvén wave (KAW) eigenmode of the reconnection layer has been provided, explaining the Hall fields and the Hall current system (Dai, ). Many predictions of the KAW solution have been confirmed in observations, including the quadrupolar structure of Hall fields in symmetric reconnection, the balance of the Hall electric field with the ion pressure gradient (Burch, Torbert, et al, ; Dai et al, ; Zhang et al, ), the ratio of Hall electric fields to Hall magnetic fields on the order of Alfvén speed (e.g, Dai et al, ), and the formation of the field‐aligned current in the Hall current system (Duan et al, , ; Nakamura et al, ). In particle‐in‐cell (PIC) simulations, the KAW properties of the Hall fields in reconnection has been quantitatively verified (Huang et al, ).…”

“…In particle‐in‐cell (PIC) simulations, the KAW properties of the Hall fields in reconnection has been quantitatively verified (Huang et al, ). The Hall fields can propagate out as KAW from the reconnection site (Duan et al, ; Liang et al, ; Shay et al, ).…”