Dieter Nickeler: "MHD flows at astropauses and in astrotails" (18.06.2014)
Channel: Tartu Observatory
Date: 2019-04-11

Dieter Nickeler's (Ondrejov observatory, Czech Republic) presentation at Tartu Observatory's astronomy seminar on June 18, 2014.

Stellar winds are obstacles for the interstellar medium which shows a relative movement with respect to the star, e.g. the Sun. Due to the interaction of both flows, namely that of the stellar wind and the interstellar medium flow, both flows are decelerated and contact surfaces form, so called astropauses or in the case of the Sun the heliopause. The solar wind cavity inside the heliopause is called the heliosphere (=general astrosphere). Usually these separating surfaces, the astropauses, are defined and determined by the distribution and location of the null points of the magnetic field and null point of the plasma flow, the so called stagnation points. We present and discuss simplified models of such astrospheres and especially their downflow regions, the astrotails, taking (ideal) field-aligned MHD flows into consideration. Almost certainly shear flows occur in these astrospheres or at the astropauses and a strong fragmentation of electric currents, magnetic field and vortices takes place. Even if for the calculation of these nonlinear steady-state MHD flows, well ordered magnetic field structures are used, fractal-like structures will be formed. The cascading down to very small structures can lead to instabilities, eruptive processes like reconnection and the occurence of strong localized electric fields. These processes may be also responsible for acceleration mechanisms, contributing to cosmic ray production.