352 research outputs found
The interaction between transpolar arcs and cusp spots
Transpolar arcs and cusp spots are both auroral phenomena which occur when
the interplanetary magnetic field is northward. Transpolar arcs are associated
with magnetic reconnection in the magnetotail, which closes magnetic flux and
results in a "wedge" of closed flux which remains trapped, embedded in the
magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the
high-latitude magnetopause; in its simplest case, lobe reconnection
redistributes open flux without resulting in any net change in the open flux
content of the magnetosphere. We present observations of the two phenomena
interacting--i.e., a transpolar arc intersecting a cusp spot during part of its
lifetime. The significance of this observation is that lobe reconnection can
have the effect of opening closed magnetotail flux. We argue that such events
should not be rare
Magnetohydrodynamics dynamical relaxation of coronal magnetic fields. II. 2D magnetic X-points
We provide a valid magnetohydrostatic equilibrium from the collapse of a 2D
X-point in the presence of a finite plasma pressure, in which the current
density is not simply concentrated in an infinitesimally thin, one-dimensional
current sheet, as found in force-free solutions. In particular, we wish to
determine if a finite pressure current sheet will still involve a singular
current, and if so, what is the nature of the singularity. We use a full MHD
code, with the resistivity set to zero, so that reconnection is not allowed, to
run a series of experiments in which an X-point is perturbed and then is
allowed to relax towards an equilibrium, via real, viscous damping forces.
Changes to the magnitude of the perturbation and the initial plasma pressure
are investigated systematically. The final state found in our experiments is a
"quasi-static" equilibrium where the viscous relaxation has completely ended,
but the peak current density at the null increases very slowly following an
asymptotic regime towards an infinite time singularity. Using a high grid
resolution allows us to resolve the current structures in this state both in
width and length. In comparison with the well known pressureless studies, the
system does not evolve towards a thin current sheet, but concentrates the
current at the null and the separatrices. The growth rate of the singularity is
found to be tD, with 0 < D < 1. This rate depends directly on the initial
plasma pressure, and decreases as the pressure is increased. At the end of our
study, we present an analytical description of the system in a quasi-static
non-singular equilibrium at a given time, in which a finite thick current layer
has formed at the null
Light scattering by an ensemble of interacting dipolar particles with both electric and magnetic polarizabilities
We have studied the problem of light scattering by an ensemble of dipoles with both electric and magnetic polarizabilities. Using the coupled electric and magnetic dipole method as the formal base, we have generalized the eigenvector decomposition of the local dipole moments previously derived for purely electric particles to the case of both electric and magnetic dipoles. We have analyzed the properties of eigenvalues and eigenvectors in the most elementary case of two particles. In the purely electric case, the eigenvalues correspond to the resonance modes of the system due to the electromagnetic coupling of its components. For a two-dipole system with both electric and magnetic responses, purely electric, purely magnetic, and mixed states can be distinguished. The resonance spectrum is analyzed as a function of the magnetic permeability, and it is shown that the latter can be fitted quite accurately by the eigenmode decomposition
Non-adiabatic motion of charged particles traversing a weak magnetic field: Pitch angle scattering
A charged particle moves with velocity v in a constant non-uniform magnetic field H , spiralling with Larmor radius R . If R is small compared with the scale length L of the field, the magnetic moment associated with the Larmor motion of the particle is nearly constant. Consequently θ, the (‘pitch’) angle between v and H , varies as arcsin H 1/2 . Hence θ in such adiabatic motion is approximately the same at points on the path where H has the same value. But the magnetic moment and the pitch angle may differ materially at two such points, each in the region where R/L is small, if between them the particle traverses a region where R/L is not small. This region of non-adiabatic motion ‘scatters’ the pitch angles.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43243/1/24_2004_Article_BF00880512.pd
Coupled Dipole Method Determination of the Electromagnetic Force on a Particle over a Flat Dielectric Substrate
We present a theory to compute the force due to light upon a particle on a
dielectric plane by the Coupled Dipole Method (CDM). We show that, with this
procedure, two equivalent ways of analysis are possible, both based on
Maxwell's stress tensor. The interest in using this method is that the nature
and size or shape of the object, can be arbitrary. Even more, the presence of a
substrate can be incorporated. To validate our theory, we present an analytical
expression of the force due to the light acting on a particle either in
presence, or not, of a surface. The plane wave illuminating the sphere can be
either propagating or evanescent. Both two and three dimensional calculations
are studied.Comment: 10 pages, 8 figures and 3 table
Observations of whistler mode waves with nonlinear parallel electric fields near the dayside magnetic reconnection separatrix by the Magnetospheric Multiscale mission
We show observations from the Magnetospheric Multiscale (MMS) mission of whistler mode waves in the Earth's low-latitude boundary layer (LLBL) during a magnetic reconnection event. The waves propagated obliquely to the magnetic field toward the X line and were confined to the edge of a southward jet in the LLBL. Bipolar parallel electric fields interpreted as electrostatic solitary waves (ESW) are observed intermittently and appear to be in phase with the parallel component of the whistler oscillations. The polarity of the ESWs suggests that if they propagate with the waves, they are electron enhancements as opposed to electron holes. The reduced electron distribution shows a shoulder in the distribution for parallel velocities between 17,000 and 22,000 km/s, which persisted during the interval when ESWs were observed, and is near the phase velocity of the whistlers. This shoulder can drive Langmuir waves, which were observed in the high-frequency parallel electric field data
Radio Emission from Ultra-Cool Dwarfs
The 2001 discovery of radio emission from ultra-cool dwarfs (UCDs), the very
low-mass stars and brown dwarfs with spectral types of ~M7 and later, revealed
that these objects can generate and dissipate powerful magnetic fields. Radio
observations provide unparalleled insight into UCD magnetism: detections extend
to brown dwarfs with temperatures <1000 K, where no other observational probes
are effective. The data reveal that UCDs can generate strong (kG) fields,
sometimes with a stable dipolar structure; that they can produce and retain
nonthermal plasmas with electron acceleration extending to MeV energies; and
that they can drive auroral current systems resulting in significant
atmospheric energy deposition and powerful, coherent radio bursts. Still to be
understood are the underlying dynamo processes, the precise means by which
particles are accelerated around these objects, the observed diversity of
magnetic phenomenologies, and how all of these factors change as the mass of
the central object approaches that of Jupiter. The answers to these questions
are doubly important because UCDs are both potential exoplanet hosts, as in the
TRAPPIST-1 system, and analogues of extrasolar giant planets themselves.Comment: 19 pages; submitted chapter to the Handbook of Exoplanets, eds. Hans
J. Deeg and Juan Antonio Belmonte (Springer-Verlag
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