Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

Geomagnetic storms can result in large magnetic field disturbances and intense currents in the magnetosphere and even on the ground.As an important medium of momentum and energy transport among the solar wind,magnetosphere,and ionosphere,field-aligned currents(FACs)can also be strengthened in storm times.This study shows the responses of FACs in the plasma sheet boundary layer(PSBL)observed by the Magnetospheric Multiscale(MMS)spacecraft in different phases of a large storm that lasted from May 27,2017,to May 29,2017.Most of the FACs were carried by electrons,and several FACs in the storm time also contained sufficient ion FACs.The FAC magnitudes were larger in the storm than in the quiet period,and those in the main phase were the strongest.In this case,the direction of the FACs in the main phase showed no preference for tailward or earthward,whereas the direction of the FACs in the recovery phase was mostly tailward.The results suggest that the FACs in the PSBL are closely related to the storm and could be driven by activities in the tail region,where the energy transported from the solar wind to the magnetosphere is stored and released as the storm is evolving.Thus,the FACs are an important medium of energy transport between the tail and the ionosphere,and the PSBL is a significant magnetosphere–ionosphere coupling region in the nightside.

Observations of kinetic Alfvén waves and associated electron acceleration in the plasma sheet boundary layer

Kinetic Alfvén waves(KAWs),with a strong parallel disturbed electric field,play an important role in energy transport and particle acceleration in the magnetotail.On the basis of high-resolution observations of the Magnetospheric Multiscale(MMS)Mission,we present a detailed description of the acceleration process of electrons by KAWs in the plasma sheet boundary layer(PSBL).The MMS observed strong electromagnetic disturbances carrying a parallel disturbed electric field with an amplitude of up to 8 mV/m.The measured ratio of the electric to magnetic field perturbations was larger than the local Alfvén speed and was enhanced as the frequency increased,consistent with the theoretical predictions for KAWs.This evidence indicates that the electromagnetic disturbances should be identified as KAWs.During the KAWs,the energy flux of electrons at energies above 1 keV in the parallel and anti-parallel directions are significantly enhanced,implying occurrences of electron beams at higher energies.Additionally,the KAWs became more electrostaticlike and filled with high-frequency ion acoustic waves.The energy enhancement of electron beams is in accordance with the derived work done with the observed parallel disturbed electric field of KAWs,indicating electron acceleration caused by KAWs.Therefore,these results provide direct evidence of electron acceleration by KAWs embodying electrostatic ion acoustic waves in the PSBL.

The relations between density of FACs in the Plasma Sheet Boundary Layers and Kp index

We have studied 172 field-aligned currents （FACs） cases observed by the ClusterlI satellites when they crossed the plasma sheet boundary layer （PSBL） in the magnetotail from July to October 2001. We mainly analyzed the relationship between the characteristic of FACs at the PSBL in magnetotail and the Kp index. The main results indicated the followings： 1） In the different geomagnetic activity levels, the relative occurrence of FACs in PSBL increased monotonically with geomagnetic activity. 2） The density of FACs in PSBL increased monotonically with Kp index. In the storm main phase, the density of FACs increased dramatically, the maximum FACs approximately equaled 19.05 nA m-2 while Kp equaled 5.3） The variation of FACs density in PSBL was consistent with the variation of the Kp index. However, when AE〈800 nT, FACs density in PSBL increased with increasing AE, and when AE〉800 nT, it decreased with increasing AE. Therefore, our results suggested that the FACs density in PSBL had a closer correlation with Kp index.

Reconstruction of plasmoid and traveling compression region in the near-Earth magnetotail

Cluster spacecraft observed an earthward flowing plasmoid along with a travelling compression region （TCR） structure in southern plasma sheet boundary layer （PSBL） at 21：09 UT of September 19, 2001. We have reconstructed the two-dimensional topology of the magnetic field structure observed by C1 using Grad-Shafranov reconstruction method. Results show that CI passed through part of a plasmoid, which compressed the lobe magnetic field and formed a TCR. The size of the whole plas- moid structure in X direction is estimated to be about 3 Re. Furthermore, using multi-spacecraft observations, we have found some detailed information about this structure. First, C1 observed bi-streaming electron components, which supports our sug- gestion that the spacecraft passed through closed field lines. Second, a small magnetic field perturbation within this plasmoid accompanied by slight decrease in electron flux suggests that a flux rope core might exist at the center of the plasmoid.