6 Dec 2006; Mat-Nat Faculty (UiTø), room U1=A228. Wednesday 10:15-12:00

Einar Mjølhus

"Extended MHD models for solitary waves".

In recent space observations, there has been a claim that “MHD solitons” have been observed. In particular, multipoint measurements using the Cluster 4-spacecraft system have been reported. The observed structures have to be understood as “large amplitude” structures, so that small amplitude theories, such as represented by the Korteweg – de Vies equation, do not apply. The discussion mainly centers around finding exact stationary localized travelling wave solutions, or prove their existence, and/or compute them numerically, for given models.

There is a “standard model” which could be termed “MHD + Hall” (to be briefly sketched and explained during the seminar), for which this program is well completed, and the families of such stationary solutions classified into “families” (to be explained) (E. Mjølhus, Physica Scripta T122, pp 135 – 153, 2006). It is clear that neither observations nor recent simulation results are not in satisfactory agreement with this model. Extensions are in two stages: (i) Extending to anisotropic pressure (discussed in Mjølhus 2006), and (ii) including dispersion due to thermal effects. The latter is ongoing work, in which I am making an effort to include what is known as “Finite Larmor Radius pressure tensor correction”. This leads to some challenges which I shall try to communicate. Results of the first of these stages appear to be of a somewhat unexpected interest due to recent space observations results, the keyword being “banana polarization”.