304 research outputs found
Kelvin-Helmholtz Instability of Counter-Rotating Discs
Observations of galaxies and models of accreting systems point to the
occurrence of counter-rotating discs where the inner part of the disc ()
is co-rotating and the outer part is counter-rotating. This work analyzes the
linear stability of radially separated co- and counter-rotating thin discs. The
strong instability found is the supersonic Kelvin-Helmholtz instability. The
growth rates are of the order of or larger than the angular rotation rate at
the interface. The instability is absent if there is no vertical dependence of
the perturbation. That is, the instability is essentially three-dimensional.
The nonlinear evolution of the instability is predicted to lead to a mixing of
the two components, strong heating of the mixed gas, and vertical expansion of
the gas, and annihilation of the angular momenta of the two components. As a
result the heated gas will free-fall towards the disc's center over the surface
of the inner disc
Electro-Mechanical Resonant Magnetic Field Sensor
We describe a new type of magnetic field sensor which is termed an
Electro-Mechanical Resonant Sensor (EMRS). The key part of this sensor is a
small conductive elastic element with low damping rate and therefore a high Q
fundamental mode of frequency . An AC current is driven through the
elastic element which, in the presence of a magnetic field, causes an AC force
on the element. When the frequency of the AC current matches the resonant
frequency of the element, maximum vibration of the element occurs and this can
be measured precisely by optical means. We have built and tested a model sensor
of this type using for the elastic element a length of copper wire of diameter
0.030 mm formed into a loop shape. The wire motion was measured using a light
emitting diode photo-transistor assembly. This sensor demonstrated a
sensitivity better than 0.001G for an applied magnetic field of G and
a good selectivity for the magnetic field direction. The sensitivity can be
easily improved by a factor of by more sensitive measurement of
the elastic element motion and by having the element in vacuum to reduce the
drag force.Comment: 8 pages, 9 figure
Jets and Disk-Winds from Pulsar Magnetospheres
We discuss axisymmetric force-free pulsar magnetospheres with magnetically
collimated jets and a disk-wind obtained by numerical solution of the pulsar
equation.
This solution represents an alternative to the quasi-spherical wind solutions
where a major part of the current flow is in a current sheet which is unstable
to magnetic field annihilation.Comment: 6 figures, accepted for publication in the Ap
- …