4,829 research outputs found
Spectrum Sharing in Wireless Networks via QoS-Aware Secondary Multicast Beamforming
Secondary spectrum usage has the potential to considerably increase spectrum utilization. In this paper, quality-of-service (QoS)-aware spectrum underlay of a secondary multicast network is considered. A multiantenna secondary access point (AP) is used for multicast (common information) transmission to a number of secondary single-antenna receivers. The idea is that beamforming can be used to steer power towards the secondary receivers while limiting sidelobes that cause interference to primary receivers. Various optimal formulations of beamforming are proposed, motivated by different ldquocohabitationrdquo scenarios, including robust designs that are applicable with inaccurate or limited channel state information at the secondary AP. These formulations are NP-hard computational problems; yet it is shown how convex approximation-based multicast beamforming tools (originally developed without regard to primary interference constraints) can be adapted to work in a spectrum underlay context. Extensive simulation results demonstrate the effectiveness of the proposed approaches and provide insights on the tradeoffs between different design criteria
On the 3-D structure and dissipation of reconnection-driven flow-bursts
The structure of magnetic reconnection-driven outflows and their dissipation
are explored with large-scale, 3-D particle-in-cell (PIC) simulations. Outflow
jets resulting from 3-D reconnection with a finite length x-line form fronts as
they propagate into the downstream medium. A large pressure increase ahead of
this ``reconnection jet front'' (RJF), due to reflected and transmitted ions,
slows the front so that its velocity is well below the velocity of the ambient
ions in the core of the jet. As a result, the RJF slows and diverts the
high-speed flow into the direction perpendicular to the reconnection plane. The
consequence is that the RJF acts as a thermalization site for the ion bulk flow
and contributes significantly to the dissipation of magnetic energy during
reconnection even though the outflow jet is subsonic. This behavior has no
counterpart in 2-D reconnection. A simple analytic model predicts the front
velocity and the fraction of the ion bulk flow energy that is dissipated
Reduction of Ion Heating During Magnetic Reconnection by Large-Scale Effective Potentials
The physical processes that control the partition of released magnetic energy
between electrons and ions during reconnection is explored through
particle-in-cell simulations and analytical techniques. We demonstrate that the
development of a large-scale parallel electric field and its associated
potential controls the relative heating of electrons and ions. The potential
develops to restrain heated exhaust electrons and enhances their heating by
confining electrons in the region where magnetic energy is released.
Simultaneously the potential slows ions entering the exhaust below the
Alfv\'enic speed expected from the traditional counterstreaming picture of ion
heating. Unexpectedly, the magnitude of the potential and therefore the
relative partition of energy between electrons and ions is not a constant but
rather depends on the upstream parameters and specifically the upstream
electron normalized temperature (electron beta). These findings suggest that
the fraction of magnetic energy converted into the total thermal energy may be
independent of upstream parameters
On the Cause of Supra-Arcade Downflows in Solar Flares
A model of supra-arcade downflows (SADs), dark low density regions also known
as tadpoles that propagate sunward during solar flares, is presented. It is
argued that the regions of low density are flow channels carved by
sunward-directed outflow jets from reconnection. The solar corona is
stratified, so the flare site is populated by a lower density plasma than that
in the underlying arcade. As the jets penetrate the arcade, they carve out
regions of depleted plasma density which appear as SADs. The present
interpretation differs from previous models in that reconnection is localized
in space but not in time. Reconnection is continuous in time to explain why
SADs are not filled in from behind as they would if they were caused by
isolated descending flux tubes or the wakes behind them due to temporally
bursty reconnection. Reconnection is localized in space because outflow jets in
standard two-dimensional reconnection models expand in the normal (inflow)
direction with distance from the reconnection site, which would not produce
thin SADs as seen in observations. On the contrary, outflow jets in spatially
localized three-dimensional reconnection with an out-of-plane (guide) magnetic
field expand primarily in the out-of-plane direction and remain collimated in
the normal direction, which is consistent with observed SADs being thin.
Two-dimensional proof-of-principle simulations of reconnection with an
out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion
regions and the necessity of density stratification. Three-dimensional
simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in
August, 2013. This version is the accepted versio
Super-Alfv\'enic propagation of reconnection signatures and Poynting flux during substorms
The propagation of reconnection signatures and their associated energy are
examined using kinetic particle-in-cell simulations and Cluster satellite
observations. It is found that the quadrupolar out-of-plane magnetic field near
the separatrices is associated with a kinetic Alfv\'en wave. For magnetotail
parameters, the parallel propagation of this wave is super-Alfv\'enic
(V_parallel ~ 1500 - 5500 km/s) and generates substantial Poynting flux (S ~
10^-5 - 10^-4 W/m^2) consistent with Cluster observations of magnetic
reconnection. This Poynting flux substantially exceeds that due to frozen-in
ion bulk outflows and is sufficient to generate white light aurora in the
Earth's ionosphere.Comment: Submitted to PRL on 11/1/2010. Resubmitted on 4/5/201
Magnetic Reconnection and Intermittent Turbulence in the Solar Wind
A statistical relationship between magnetic reconnection, current sheets and
intermittent turbulence in the solar wind is reported for the first time using
in-situ measurements from the Wind spacecraft at 1 AU. We identify
intermittency as non-Gaussian fluctuations in increments of the magnetic field
vector, , that are spatially and temporally non-uniform. The
reconnection events and current sheets are found to be concentrated in
intervals of intermittent turbulence, identified using the partial variance of
increments method: within the most non-Gaussian 1% of fluctuations in
, we find 87%-92% of reconnection exhausts and 9% of current
sheets. Also, the likelihood that an identified current sheet will also
correspond to a reconnection exhaust increases dramatically as the least
intermittent fluctuations are removed from the dataset. Hence, the turbulent
solar wind contains a hierarchy of intermittent magnetic field structures that
are increasingly linked to current sheets, which in turn are progressively more
likely to correspond to sites of magnetic reconnection. These results could
have far reaching implications for laboratory and astrophysical plasmas where
turbulence and magnetic reconnection are ubiquitous.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
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