Cold ions of ionospheric origin observed at the dayside magnetopause and their effects on magnetic reconnection

Abstract

Magnetic reconnection at the dayside magnetopause is one of the most important mechanisms that efficiently transfers solar wind particles, momentum, and energy into the magnetosphere. Magnetic reconnection at the magnetopause is usually asymmetric since the plasma and magnetic field properties are quite different in the magnetosphere and the magnetosheath. Cold dense plasma, originating either directly from the ionosphere or from the plasmasphere, has often been observed at the adjacent magnetopause. These cold plasmas may affect reconnection since they modify the plasma properties on the magnetospheric side significantly. This dissertation presents case and statistical studies of the characteristics of the cold ions observed at the dayside magnetopause by using Cluster spacecraft datasets. The plasmaspheric plumes have been distinguished from the ionospheric outows using ion pitch angle distributions. The ionospheric outows feature unidirectional or bidirectional field-aligned pitch angle distributions, whereas the plasmaspheric plumes are characterized by 90° pitch angle distributions. The occurrence rates of the plasmaspheric plumes and ionospheric outows and their dependence on the solar wind/Interplanetary Magnetic Field (IMF) conditions have been investigated. It is found that the occurrence rate of plasmaspheric plume or ionospheric plasma strongly depends on the solar wind/IMF conditions. In particular, plasmaspheric plumes tend to occur during southward IMF while ionospheric outows tends to occur during northward IMF. The occurrence rate of the plasmaspheric plumes is significantly higher on the duskside than that on the dawnside, indicating that the plasmaspheric plumes may lead to a dawn-dusk asymmetry of dayside reconnection. Furthermore, this dissertation investigates the behavior of the cold dense plasma of ionospheric origin during magnetic reconnection at the dayside magnetopause. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection near the subsolar magnetopause, the colder ions are simply entrained by E x B drift at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play a very limited role in magnetic reconnection process. Finally, this dissertation examines a controlling factor that leads to the asymmetric reconnection geometry at the magnetopause. It is demonstrated that the separatrix and ow boundary angles are greater on the magnetosheath side than on the magnetospheric side of the magnetopause, probably due to the stronger density asymmetry rather than magnetic field asymmetry at this boundary.