UCLA

Transient (~40s time scale) flux tubes carrying strong northward magnetic field and earthward flow are frequently observed in the magnetotail. Here I use the term "dipolarizing flux bundles" (DFBs) to describe these flux tubes. Thought to be generated by tail reconnection at X_GSM=~-20 R_E, DFBs travel earthward towards the inner edge of the plasma sheet, bringing significant changes to the near-earth magnetotail. After twenty years of extensive study on DFBs, numerous questions regarding their role in magnetotail dynamics remain. The THEMIS mission, which enables multi-point observations at different tail locations, provides the opportunity to answer many of these questions. With a statistical study based on THEMIS data, I explore the dipolarizing flux bundle's role in several aspects of magnetotail dynamics---its importance in magnetotail flux transport, the mechanisms controlling its motion, the modifications its motion makes to the ambient plasma, its current system, and the modifications it makes to the global magnetotail current system.

First, I establish that DFBs are the major, high-efficiency magnetic flux carriers in near-earth magnetotail convection. Therefore, the DFB flux transport properties I found (e.g., DFBs transport more magnetic flux during substorm time) may shape all near-earth magnetotail convection. Then I investigate how a DFB's earthward motion modifies the plasma inside it as well as the ambient plasma. Dipolarizing flux bundle motion results in a total pressure buildup inside the ~1000km-thin DF layer; the buildup exerts a tailward force from there to retard DFB motion. This motion also builds up thermal pressure, the distribution of which requires the existence of field-aligned currents (FACs) near the DFB. To confirm the existence of these FACs, I infer the DFB-associated current system from magnetic field variations. The magnetic field variations are consistent with region-2-sense (towards/away from earth to the DFB's dusk/dawn side) FACs immediately earthward of the DFB and region-1-sense (opposite to region-2-sense) FACs inside the DF layer. Such a FAC configuration is similar to that of a substorm current wedge (SCW). In addition, the amount of current carried by several DFBs is sufficient to form a typical ~1 MA SCW. Therefore, I suggest that dipolarizing flux bundles are "wedgelets"---building blocks of a substorm current wedge.