Multi-scale analysis of magnetotail dipolarizations

Magnetotail dipolarization in the Earths magnetotail, the turning of the magnetic field from mainly horizontal to more vertical, is a key ingredient of magnetotail dynamics. It is inherently a necessity when magnetic flux is transported from the distant tail towards the Earth through the motion of newly reconnected closed field lines embedded in a plasma flow. This proposal strives for a detailed investigation of these flow driven dipolarizations, to study the dynamic characteristics and the associated currents, to study the influence of the plasma sheet composition, and to study dipolarizations over larger spatial scales to find the spatial and/or temporal development of these structures. This study will be done using Cluster, Double Star and THEMIS data. The four point measurements by the Cluster spacecraft give the possibility of studying gradients in the measured quantities, and the varying distance of the spacecraft make it possible to probe the structures on different spatial scales. Knowing the internal structure of the dipolarization fronts will make it easier to understand the larger scale development. Combining Cluster and Double Star data will give insight in the development of the dipolarization front on intermediate scales over a few Earth radii. The THEMIS data will be used to study the development of the dipolarization fronts on large scales from 30 Earth radii down the tail to the near-Earth region at 10 Earth radii down the tail.

The PhD thesis titled Magnetotail Dipolarization Fronts was successfully defended at the end of this project by Dr. Daniel Schmid. The content of the thesis was based on the research performed during this FWF project and its published papers. When the Earths magnetic field and the interplanetary magnetic field (IMF) are anti-parallel, then the Earths field can connect to that of the solar wind (magnetic reconnection). One foot point will then be on the northern (southern) polar cap of the Earth, and the other one on the Sun. These, so-called, open magnetic field lines are transported by the solar wind towards the night-side of the Earth. There the stretched field lines build the northern (southern) half of the magnetotail. Both halves containing oppositely directed field, are separated by the plasma sheet, a region of higher plasma density. In the magnetotail reconnection can occur again between the oppositely directed magnetic field lines. The stretched field lines, which were formerly connected to the solar wind, are then closed and the magnetic tension accelerates them in the Earths direction. During this process the plasma of the plasma sheet is pulled along. This process is called magnetotail dipolarization, as the originally mainly horizontal magnetic field takes on a more dipole like shape after the relaxation of the magnetic tension. Because of the sudden rotation of the field during this process, one also talks about dipolarization fronts. These fronts can be considered as boundary layers (current layers) between two different plasma populations. Typically, they separate the ambient plasma from a population with lower density and higher temperature. During this FWF project it was shown that the dipolarization fronts, on their path towards the Earth, are developing temporally and can change their characteristic. The results show that the plasma density and temperature in front of and behind the fronts can equalize during their Earthward motion. Only very fast dipolarization fronts, with high difference in density and temperature in front of and behind the front, can reach the near-Earth region. Interestingly, however, it was shown that the majority of dipolarization fronts in the near-Earth region moved faster than further distant down the tail. It is unclear what the cause is of this unexpected result. It could be a selection effect: only those fronts with a high velocity will be observed, because they are the only ones that can reach the near-Earth region to begin with.