34,016 research outputs found
The effect of the solar field reversal on the modulation of galactic cosmic rays
There is now a growing awareness that solar cycle related changes in the large-scale structure of the interplanetary magnetic field (IMF) may play an important role in the modulation of galactic cosmic rays. To date, attention focussed on two aspects of the magnetic field structure: large scale compression regions produced by fast solar wind streams and solar flares, both of which are known to vary in intensity and number over the solar cycle, and the variable warp of the heliospheric current sheet. It is suggested that another feature of the solar cycle is worthy of consideration: the field reversal itself. If the Sun reverses its polarity by simply overturning the heliospheric current sheet (northern fields migrating southward and vice-versa) then there may well be an effect on cosmic ray intensity. However, such a simple picture of solar reversal seems improbable. Observations of the solar corona suggest the existence of not one but several current sheets in the heliosphere at solar maximum. The results of a simple calculation to demonstrate that the variation in cosmic ray intensities that will result can be as large as is actually observed over the solar cycle are given
Wind-driven currents in a shallow lake or sea
For shallow lakes and seas such as the great lakes (especially Lake Erie) where the depth is not much greater than the Ekman depth, the usual Ekman dynamics cannot be used to predict the wind driven currents. The necessary extension to include shallow bodies of water, given by Welander, leads to a partial differential equation for the surface displacement which in turn determines all other flow quantities. A technique for obtaining exact analytical solutions to Welander's equation for bodies of water with large class of bottom topographies which may or may not contain islands is given. It involves applying conformal mapping methods to an extension of Welander's equation into the complex plane. When the wind stress is constant (which is the usual assumption for lakes) the method leads to general solutions which hold for bodies of water of arbitrary shape (the shape appears in the solutions through a set of constants which are the coefficients in the Laurent expansion of a mapping of the particular lake geometry). The method is applied to an elliptically shaped lake and a circular lake containing an eccentrically located circular island
Improved noise-adding radiometer for microwave receivers
Use of input switch and noise reference standard is avoided by using noise-adding technique. Excess noise from solid state noise-diode is coupled into receiver through directional coupler and square-wave modulated at low rate. High sensitivity receivers for radioastronomy applications are utilized with greater confidence in stability of radiometer
Front Propagation in the Pearling Instability of Tubular Vesicles
Recently Bar-Ziv and Moses discovered a dynamical shape transformation
induced in cylindrical lipid bilayer vesicles by the action of laser tweezers.
We develop a hydrodynamic theory of fluid bilayers in interaction with the
surrounding water and argue that the effect of the laser is to induce a sudden
tension in the membrane. We refine our previous analysis to account for the
fact that the shape transformation is not uniform but propagates outward from
the laser trap. Applying the marginal stability criterion to this situation
gives us an improved prediction for the selected initial wavelength and a new
prediction for the propagation velocity, both in rough agreement with the
experimental values. For example, a tubule of initial radius 0.7\micron\ has a
predicted initial sinusoidal perturbation in its diameter with wavelength
5.5\micron, as observed. The perturbation propagates as a front with the
qualitatively correct front velocity a bit less than 100\micron/sec. In
particular we show why this velocity is initially constant, as observed, and so
much smaller than the natural scale set by the tension. We also predict that
the front velocity should increase linearly with laser power. Finally we
introduce an approximate hydrodynamic model applicable to the fully nonlinear
regime. This model exhibits propagating fronts as well as fully-developed
``pearled" vesicles similar to those seen in the experiments.Comment: 42 pages, 6 eps figures included with text in uuencoded file, ps file
available from ftp://dept.physics.upenn.edu/pub/Nelson/pearl_propagation.ps
submitted to Journal de Physiqu
Parametric derivation of the observable relativistic periastron advance for binary pulsars
We compute the dimensionless relativistic periastron advance parameter ,
which is measurable from the timing of relativistic binary pulsars. We employ
for the computation the recently derived Keplerian-type parametric solution to
the post-Newtonian (PN) accurate conservative dynamics of spinning compact
binaries moving in eccentric orbits. The parametric solution and hence the
parameter are applicable for the cases of \emph{simple precession}, namely,
case (i), the binary consists of equal mass compact objects, having two
arbitrary spins, and case (ii), the binary consists of compact objects of
arbitrary mass, where only one of them is spinning with an arbitrary spin. Our
expression, for the cases considered, is in agreement with a more general
formula for the 2PN accurate , relevant for the relativistic double pulsar
PSR J0737--3039, derived by Damour and Sch\"afer many years ago, using a
different procedure.Comment: 12 pages including 1 figure; submitted to PR
Centre-of-mass and internal symmetries in classical relativistic systems
The internal symmetry of composite relativistic systems is discussed. It is
demonstrated that Lorentz-Poincar\'e symmetry implies the existence of internal
moments associated with the Lorentz boost, which are Laplace-Runge-Lenz (LRL)
vectors. The LRL symmetry is thus found to be the internal symmetry universally
associated with the global Lorentz transformations, in much the same way as
internal spatial rotations are associated with global spatial rotations. Two
applications are included, for an interacting 2-body system and for an
interaction-free many-body system of particles. The issue of localizability of
the relativistic CM coordinate is also discussed
Energetic particles of the outer regions of planetary magnetospheres
High energy particles, with energies above those attainable by adiabatic or steady-state electric field acceleration, have been observed in and around the outer regions of planetary magnetospheres. Acceleration by large amplitude sporadic cross-tail electric fields over an order of magnitude greater than steady-state convection fields is proposed as a source of these particles. It is suggested that such explosive electric fields will occur intermittently in the vicinity of the tail neutral line in the expansive phase of substorms. Laboratory and satellite evidence are used to estimate this electric potential for substorms at earth; values of 500 kilovolts to 2 megavolts are calculated, in agreement with particle observations. It is further suggested that these particles, which have been accelerated in the night side magnetosphere, drift to the dayside on closed field lines, and under certain interplanetary conditions can escape to regions upstream of the bow shock
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