44 research outputs found
Magnetic Reconnection: Sweet-Parker Versus Petschek
The two theories for magnetic reconnection, one of Sweet and Parker, and the
other of Petschek, are reconciled by exhibiting an extra condition in that of
Petschek which reduces his theory to that of Sweet and Parker, provided that
the resistivity is constant in space. On the other hand, if the resistivity is
enhanced by instabilities, then the reconnection rate of both theories is
increased substantially, but Petschek's rate can be faster. A different formula
from the usual one is presented for enhanced Petschek reconnection.Comment: 13 pages, 5 figures, 1 appendix (MR2000 proceedings
Signatures of magnetic reconnection at boundaries of interplanetary small-scale magnetic flux ropes
The interaction between interplanetary small-scale magnetic flux ropes and
the magnetic field in the ambient solar wind is an important topic to
understand- ing the evolution of magnetic structures in the heliosphere.
Through a survey of 125 previously reported small flux ropes from 1995 to 2005,
we find that 44 of them reveal clear signatures of Alfvenic fluctuations, and
thus classify them into Alfven wave trains rather than flux ropes. Signatures
of magnetic reconnection, generally including a plasma jet of ~30 km/s within a
magnetic field rotational region, are clearly present at boundaries of about
42% of the flux ropes and 14% of the wave trains. The reconnection exhausts are
often observed to show a local increase in the proton temperature, density and
plasma beta. About 66% of the reconnection events at flux rope boundaries are
associated with a magnetic field shear angle larger than 90 deg and 73% of them
reveal a decrease by 20% or more in the magnetic field magnitude, suggesting a
dominance of anti-parallel reconnec- tion at flux rope boundaries. The
occurrence rate of magnetic reconnection at flux rope boundaries through the
year of 1995 to 2005 is also investigated and we find that it is relatively low
around solar maximum and much higher when ap- proaching solar minima. The
average magnetic field depression and shear angle for reconnection events at
flux rope boundaries also reveal a similar trend from 1995 to 2005. Our results
demonstrate for the first time that boundaries of a substantial fraction of
small-scale flux ropes have properties similar to those of magnetic clouds, in
the sense that both of them exhibit signatures of magnetic reconnection. The
observed reconnection signatures could be related either to the formation of
small flux ropes, or to the interaction between flux ropes and the
interplanetary magnetic fields.Comment: 10 figures, accepted by Ap
Orientational Susceptibility and Elastic Constants Near the Nematic-Isotropic Phase Transition for Trimers with Terminal-Lateral-Lateral-Terminal Connections
Magnetically induced Freedericksz measurements were performed in the nematic liquid crystal phase to extract the elastic constants of a terminal-lateral-lateral-terminal trimer, i.e., a trimer in which the connections to the first and third mesogens are at the end of the mesogen and both attachments to the central mesogen are lateral. Polymeric liquid crystals based on this unit have negative Poisson ratios. Additionally, electric field Kerr measurements were performed in the isotropic phase to extract the orientational susceptibility. The elastic constants were found to be similar to values obtained for typical monomers, albeit with a slightly enhanced ratio K-33/K-11 and reduced ratio K-11/K-22 especially near the transition temperature. The temperature dependence of the susceptibility was found to deviate significantly from that of typical monomers. The observed behavior is discussed in terms of coupled order parameters representing the terminal and core mesogens of the molecule
Pretransitional Behavior Above the Nematic-Isotropic Phase Transition of an Auxetic Trimer Liquid Crystal
Static Light scattering and electric field-induced Kerr measurements were performed above the nematic-isotropic phase transition of a terminal-lateral-lateral-terminal negative Poisson ratio trimer. Far both measurements the inverse susceptibility was observed to be nearly linear with temperature, a result inconsistent with our previously reported Kerr data [Phys. Rev. E 58, 2041 (1998)]. [S1063-651X(99)11010-9]
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
Dynamic non-null magnetic reconnection in three dimensions II: composite solutions
In this series of papers we examine magnetic reconnection in a domain where
the magnetic field does not vanish and the non-ideal region is localised in
space. In a previous paper we presented a technique for obtaining analytical
solutions to the stationary resistive MHD equations in such a situation and
examined specific examples of non-ideal reconnective solutions. Here we further
develop the model, noting that certain ideal solutions may be superimposed onto
the fundamental non-ideal solutions and examining the effect of imposing
various such flows. Significant implications are found for the evolution of
magnetic flux in the reconnection process. It is shown that, in contrast to the
two-dimensional case, in three-dimensions there is a very wide variety of
physically different steady reconnection solutions.Comment: 20 pages, 8 figure
Generation of quasi-periodic waves and flows in the solar atmosphere by oscillatory reconnection
We investigate the long-term evolution of an initially buoyant magnetic flux tube emerging into a gravitationally stratified coronal hole environment and report on the resulting oscillations and outflows. We perform 2.5-dimensional nonlinear numerical simulations, generalizing the models of McLaughlin et al. and Murray et al. We find that the physical mechanism of oscillatory reconnection naturally generates quasi-periodic vertical outflows, with a transverse/swaying aspect. The vertical outflows consist of both a periodic aspect and evidence of a positively directed flow. The speed of the vertical outflow (20-60 km/s) is comparable to those reported in the observational literature. We also perform a parametric study varying the magnetic strength of the buoyant flux tube and find a range of associated periodicities: 1.75-3.5 minutes. Thus, the mechanism of oscillatory reconnection may provide a physical explanation to some of the high-speed, quasi-periodic, transverse outflows/jets recently reported by a multitude of authors and instruments