Plasmoid in the high latitude boundary/cusp region observed by Cluster

Abstract

A plasmoid‐like structure without a core magnetic field has been observed by Cluster spacecraft in the high latitude magnetopause region when the interplanetary magnetic field (IMF) is northward and has a dominant duskward component. Bursty electrons and ions are found to be well confined in this plasmoid‐like structure. The structure possesses a bipolar signature in the BN component. The structure is associated with a dramatic change of the plasma flow direction from tailward to dawnward. The plasma density in this structure is slightly enhanced whereas the ion temperature is twice as large as that of the ambient plasma. The plasma parallel temperature is higher than the perpendicular one. The observed structure is different from the traditional Flux Transfer Events (FTEs). The Walén test shows that change of plasma flow velocity are closely associated with change of Alfven velocity which indicates this structure is formed by reconnection process. The observed plasmoid is suggested to be formed by multiple X‐line reconnection in the dusk (dawn) side of the stagnant cusp region when the IMF is northward and possesses a dominant positive (negative) BY component.