148 research outputs found
Fragmentation of electric currents in the solar corona by plasma flows
We consider a magnetic configuration consisting of an arcade structure and a
detached plasmoid, resulting from a magnetic reconnection process, as is
typically found in connection with solar flares. We study spontaneous current
fragmentation caused by shear and vortex plasma flows. An exact analytical
transformation method was applied to calculate self-consistent solutions of the
nonlinear stationary MHD equations. The assumption of incompressible
field-aligned flows implies that both the Alfven Mach number and the mass
density are constant on field lines. We first calculated nonlinear MHS
equilibria with the help of the Liouville method, emulating the scenario of a
solar eruptive flare configuration with plasmoids and flare arcade. Then a Mach
number profile was constructed that describes the upflow along the open
magnetic field lines and implements a vortex flow inside the plasmoid. This
Mach number profile was used to map the MHS equilibrium to the stationary one.
We find that current fragmentation takes place at different locations within
our configuration. Steep gradients of the Alfven Mach number are required,
implying the strong influence of shear flows on current amplification and
filamentation of the MHS current sheets. Crescent- or ring-like structures
appear along the outer separatrix, butterfly structures between the upper and
lower plasmoids, and strong current peaks close the lower boundary. Impressing
an intrinsic small-scale structure on the upper plasmoid results in strong
fragmentation of the plasmoid. Hence fragmentation of current sheets and
plasmoids is an inherent property of MHD theory. Transformations from MHS into
MHD steady-states deliver fine-structures needed for plasma heating and
acceleration of particles and bulk plasma flows in dissipative events that are
typically connected to magnetic reconnection processes in flares and coronal
mass ejections.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
MHD flows at astropauses and in astrotails
The geometrical shapes and the physical properties of stellar wind --
interstellar medium interaction regions form an important stage for studying
stellar winds and their embedded magnetic fields as well as cosmic ray
modulation. Our goal is to provide a proper representation and classification
of counter-flow configurations and counter-flow interfaces in the frame of
fluid theory. In addition we calculate flows and large-scale electromagnetic
fields based on which the large-scale dynamics and its role as possible
background for particle acceleration, e.g. in the form of anomalous cosmic
rays, can be studied. We find that for the definition of the boundaries, which
are determining the astropause shape, the number and location of magnetic null
points and stagnation points is essential. Multiple separatrices can exist,
forming a highly complex environment for the interstellar and stellar plasma.
Furthermore, the formation of extended tail structures occur naturally, and
their stretched field and streamlines provide surroundings and mechanisms for
the acceleration of particles by field-aligned electric fields.Comment: 10 pages, 4 Figure
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