Introduction

The Earth’s magnetosphere is created through the interaction between the terrestrial magnetic field and the solar wind with its embedded Interplanetary Magnetic Field (IMF). The solar wind flows nearly radially outward and pushes the IMF and the terrestrial magnetic field together. When the IMF and the Earth’s magnetic field lines are anti-parallel the may interconnect via the so-called magnetic reconnection and create "open" magnetic field lines, which are tied to the polar cap on one side and to the solar wind on the other side. These open field lines are dragged anti-sunward by the solar wind and create the stretched magnetotail. In this way the magnetic field lines from the dayside are stored in the magnetotail and accumulate, increasing the magnetic energy. As a result the tail becomes prone to many (explosive) instabilities, most often instigated by a magnetotail magnetic reconnection, which causes a prompt rearrangement of the magnetic field lines from a stretched open configuration to a stretched closed configuration.
The surplus of magnetic energy is release by the magnetic tension force, accelerating the magnetic field earthward together with the plasma. This creates so-called Bursty Bulk Flows (BBFs), short-duration high-velocity plasma flows towards the Earth. These BBFs are often accompanied by magnetic field dipolarizations, i.e. the magnetic field looks locally more like that of a magnetic dipole than like the stretched magnetotail. However, it is not just a simple turning of the magnetic field form mainly horizontal into vertical direction, it has been observed that the rapid increase of the northward magnetic field component is usually preceded by a small decrease. These asymmetric bipolar variations are referred to as dipolarization fronts (DFs). A possible interpretation of DFs is that they are thin current sheets separating an earthward propagating plasma bubble (a flux tube with reduced flux tube entropy) from the ambient plasma sheet.
Common features of DFs are:

  • In course of the fast earthward plasma movement the magnetic field becomes more dipolar and the plasma and total pressure decrease;
  • The sharp dipolarization tends to be preceded by a transient decrease in the northward magnetic field component, which is accompanied by a fast plasma flow and an increase in the plasma density.

In this project we revisit the properties of DFs and address the question of where they originate, how they evaluate while propagating earthward and how they behave closer to the Earth, in a more dipolar background magnetic field configuration. We use data from the Cluster mission and data from the more recent Magnetospheric Multiscale (MMS) mission. Both missions consist of four identical satellites, which allow multi-point measurements and thus makes it possible to distinguish spatial and temporal variations and to measure gradients in the magnetic field and other parameters.