Towards a Unifying Model for
Magnetic Depressions in Space Plasmas
In this project we discussed magnetic depressions, i.e. local decreases of the magnetic field strength, that occur either as single structures (magnetic holes) or as trains of decreases (mirror modes). These structures deliver information about the plasma in which they are embedded. It is expected that they are created by a temperature anisotropy, where the temperature along the magnetic field should be smaller than that perpendicular to the field. Simply put, the increased pressure perpendicular to the magnetic field, that comes with the higher temperature, pushes the magnetic field away, thereby creating the depression. These structures appear in different plasmas in our solar system: in the solar wind, in planetary magnetospheres and around comets.
The team members presented their research in front of a very critical and question-happy audience. Through comparing studies in different plasma environments, a view was obtained about the general properties of magnetic holes and mirror modes, and about the environments they are embedded in. Interestingly, even though both structures are expected to be generated by the same mechanism, it is found that mirror modes occur in plasmas with anisotropy, whereas magnetic holes appear in more isotropic plasma. The magnetic holes can be seen more like solitons, where there is a strong circular (ion or electron depending on the size of the hole) current, which may make them more robust and survive being in a stable plasma or crossing shocks. A thorough study of magnetic holes in the solar wind has shown for the first time that they are indeed in pressure balance with the ambient plasma, and that they are basically convected with the solar wind velocity.
Numerical modelling of these structures, mainly magnetic holes, has shown that they can also be created through shear flows in, in principle, plasmas without a temperature anisotropy. Through the generation of the Kelvin-Helmholtz instability, vortices can be created with the characteristics of magnetic holes.
In the end, many collaborations have been set-up, which will generate more output in follow-up projects.