27 research outputs found
Phase Mixing of Alfvén Waves Near a 2D Magnetic Null Point
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered
Three-Dimensional Magnetic Reconnection
The importance of magnetic reconnection as an energy release mechanism in
many solar, stellar, magnetospheric and astrophysical phenomena has long been
recognised. Reconnection is the only mechanism by which magnetic fields can
globally restructure, enabling them to access a lower energy state. Over the
past decade, there have been some major advances in our understanding of
three-dimensional reconnection. In particular, the key characteristics of 3D
magnetohydrodynamic (MHD) reconnection have been determined. For instance, 3D
reconnection (i) occurs with or without nulls, (ii) occurs continuously and
continually throughout a diffusion region and (iii) is driven by counter
rotating flows.
Furthermore, analysis of resistive 3D MHD magnetic experiments have revealed
some intriguing effects relating to where and how reconnection occurs. To
illustrate these new features, a series of constant-resistivity experiments,
involving the interaction of two opposite-polarity magnetic sources in an
overlying field, are considered. Such a simple interaction represents a typical
building block of the Sun's magnetic atmosphere. By following the evolution of
the magnetic topology, we are able to explain where, how and at what rate the
reconnection occurs. Remarkably there can be up to five energy release sites at
anyone time (compared to one in the potential case) and the duration of the
interaction increases (more than doubles) as the resistivity decreases (by a
factor of 16). The decreased resistivity also leads to a higher peak ohmic
dissipation and more energy being released in total, as a result of a greater
injection of Poynting flux.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
The cosmic web of dwarf galaxies in a warm versus cold dark matter universe: mock galaxies in CDM and WDM simulations
Using cosmological simulations, we show that the cosmic web of dwarf galaxies in a warm dark matter (WDM) universe, wherein low mass halo formation is heavily suppressed, is nearly indistinguishable to that of a cold dark matter (CDM) universe whose low mass halos are not seen because galaxy formation is suppressed below some threshold mass. Low mass warm dark matter halos are suppressed nearly equally in all environments. For example, WDM voids in the galaxy distribution are neither larger nor emptier than CDM voids, once normalized to the same total galaxy number density and assuming galaxy luminosity scales with halo mass. It is thus a challenge to find hints about the dark matter particle in the cosmic web of galaxies. However, if the scatter between dwarf galaxy luminosity and halo properties is large, low mass CDM halos would sometimes host bright galaxies thereby populating voids that would be empty in WDM. Future surveys that will capture the small scale clustering in the local volume could thus help determine whether the CDM problem of the over-abundance of small halos with respect to the number density of observed dwarf galaxies has a cosmological solution or an astrophysical solution
Probing deformed commutators with macroscopic harmonic oscillators
A minimal observable length is a common feature of theories that aim to merge
quantum physics and gravity. Quantum mechanically, this concept is associated
to a nonzero minimal uncertainty in position measurements, which is encoded in
deformed commutation relations. In spite of increasing theoretical interest,
the subject suffers from the complete lack of dedicated experiments and bounds
to the deformation parameters are roughly extrapolated from indirect
measurements. As recently proposed, low-energy mechanical oscillators could
allow to reveal the effect of a modified commutator. Here we analyze the free
evolution of high quality factor micro- and nano-oscillators, spanning a wide
range of masses around the Planck mass (), and compare it with a model of deformed dynamics.
Previous limits to the parameters quantifying the commutator deformation are
substantially lowered.Comment: 11 pages, 3 figures, reference adde
In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail
Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.Comment: 14 pages, 4 figure
Satellite Observations of Separator Line Geometry of Three-Dimensional Magnetic Reconnection
Detection of a separator line that connects magnetic nulls and the
determination of the dynamics and plasma environment of such a structure can
improve our understanding of the three-dimensional (3D) magnetic reconnection
process. However, this type of field and particle configuration has not been
directly observed in space plasmas. Here we report the identification of a pair
of nulls, the null-null line that connects them, and associated fans and spines
in the magnetotail of Earth using data from the four Cluster spacecraft. With
di and de designating the ion and electron inertial lengths, respectively, the
separation between the nulls is found to be ~0.7di and an associated
oscillation is identified as a lower hybrid wave with wavelength ~ de. This in
situ evidence of the full 3D reconnection geometry and associated dynamics
provides an important step toward to establishing an observational framework of
3D reconnection.Comment: 10 pages, 3 figures and 1 tabl
Large grazers modify effects of aboveground–belowground interactions on small-scale plant community composition
Aboveground and belowground organisms influence plant community composition by local interactions, and their scale of impact may vary from millimeters belowground to kilometers aboveground. However, it still poorly understood how large grazers that select their forage on large spatial scales interact with small-scale aboveground–belowground interactions on plant community heterogeneity. Here, we investigate how cattle (Bos taurus) modify the effects of interactions between yellow meadow ants (Lasius flavus) and European brown hares (Lepus europaeus) on the formation of small-scale heterogeneity in vegetation composition. In the absence of cattle, hares selectively foraged on ant mounds, while under combined grazing by hares and cattle, vertebrate grazing pressure was similar on and off mounds. Ant mounds that were grazed by only hares had a different plant community composition compared to their surroundings: the cover of the grazing-intolerant grass Elytrigia atherica was reduced on ant mounds, whereas the relative cover of the more grazing-tolerant and palatable grass Festuca rubra was enhanced. Combined grazing by hares and cattle, resulted in homogenization of plant community composition on and off ant mounds, with high overall cover of F. rubra. We conclude that hares can respond to local ant–soil–vegetation interactions, because they are small, selective herbivores that make their foraging decisions on a local scale. This results in small-scale plant patches on mounds of yellow meadow ants. In the presence of cattle, which are less selective aboveground herbivores, local plant community patterns triggered by small-scale aboveground–belowground interactions can disappear. Therefore, cattle modify the consequences of aboveground–belowground interactions for small-scale plant community composition
A framework for understanding the topology of complex coronal structures
The Sun's coronal magnetic field is highly complex and provides the driving force for many dynamical processes. The topology of this complex field is made up mainly of discrete topological building blocks produced by small numbers of magnetic fragments. In this work we develop a method for predicting the possible topologies due to a potential field produced by three photospheric sources, and describe how this model accurately predicts the results of Brown and Priest (1999). We then sketch how this idea may be extended to more general non-symmetric configurations. It is found that, for the case of positive total flux, a local separator bifurcation may take place with three positive sources or with one positive and two negative sources, but not for two positive sources and one negative.</p