18,614 research outputs found
Ion dynamics and the magnetorotational instability in weakly-ionized discs
The magnetorotational instability (MRI) of a weakly ionized, differentially
rotating, magnetized plasma disk is investigated in the multi-fluid framework.
The disk is threaded by a uniform vertical magnetic field and charge is carried
by electrons and ions only. The inclusion of ion dynamics causes significant
modification to the conductivity tensor in a weakly ionized disk. The parallel,
Pedersen and Hall component of conductivity tensor become time dependent
quantities resulting in the AC and DC part of the conductivity. The
conductivity may change sign leading to the significant modification of the
parameter window in which MRI may operate.
The effect of ambipolar and Hall diffusion on the linear growth of the MRI is
examined in the presence of time dependent conductivity tensor. We find that
the growth rate in ambipolar regime can become somewhat larger than the
rotational frequency, especially when the departure from ideal MHD is
significant. Further, the instability operates on large scale lengths. This has
important implication for the angular momentum transport in the disk.Comment: 13 pages, 12 figure
Some unexplored features of the nonlinear compressive magnetoacoustic Alfvenic waves
The theory of nonlinear magnetoacoustic wave in the past has strictly been
focused on purely compressive features of the mode. We show that a complete set
of nonlinear equations necessarily includes both compressional and shear
components of the magnetic field. These two turn out to be described by exactly
the same nonlinear equations, which make the use of such a complete full set of
equations far less complicated than expected. Present results should
considerably enrich the theory of these waves by opening up new frontiers of
investigation and providing some completely new types of nonlinear solutions.Comment: Phys. Scripta, to be publishe
Hall instability of solar flux tubes
The magnetic network which consists of vertical flux tubes located in
intergranular lanes is dominated by Hall drift in the photosphere-lower
chromosphere region (). In the internetwork regions, Hall drift
dominates above in the photosphere and below in the
chromosphere. Although Hall drift does not cause any dissipation in the ambient
plasma, it can destabilise the flux tubes and magnetic elements in the presence
of azimuthal shear flow. The physical mechanism of this instability is quite
simple: the shear flow twists the radial magnetic field and generates azimuthal
field; torsional oscillations of the azimuthal field in turn generates the
radial field completing feedback loop. The maximum growth rate of Hall
instability is proportional to the absolute value of the shear gradient and is
dependent on the ambient diffusivity. The diffusivity also determines the most
unstable wavelength which is smaller for weaker fields.
We apply the result of local stability analysis to the network and
internetwork magnetic elements and show that the maximum growth rate for
kilogauss field occurs around and decreases with increasing altitude.
However, for a field, the maximum growth rate remains almost constant
in the entire photosphere-lower chromosphere except in a small region of lower
photosphere. For shear flow gradient , the Hall growth time
is 10 minute near the footpoint. Therefore, network fields are likely to be
unstable in the photosphere, whereas internetwork fields could be unstable in
the entire photosphere-chromosphere. Thus the Hall instability can play an
important role in generating low frequency turbulence which can heat the
chromosphere.Comment: 8 page, 4 figure
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