22 research outputs found
Magnetic Reconnection in Extreme Astrophysical Environments
Magnetic reconnection is a basic plasma process of dramatic rearrangement of
magnetic topology, often leading to a violent release of magnetic energy. It is
important in magnetic fusion and in space and solar physics --- areas that have
so far provided the context for most of reconnection research. Importantly,
these environments consist just of electrons and ions and the dissipated energy
always stays with the plasma. In contrast, in this paper I introduce a new
direction of research, motivated by several important problems in high-energy
astrophysics --- reconnection in high energy density (HED) radiative plasmas,
where radiation pressure and radiative cooling become dominant factors in the
pressure and energy balance. I identify the key processes distinguishing HED
reconnection: special-relativistic effects; radiative effects (radiative
cooling, radiation pressure, and Compton resistivity); and, at the most extreme
end, QED effects, including pair creation. I then discuss the main
astrophysical applications --- situations with magnetar-strength fields
(exceeding the quantum critical field of about 4 x 10^13 G): giant SGR flares
and magnetically-powered central engines and jets of GRBs. Here, magnetic
energy density is so high that its dissipation heats the plasma to MeV
temperatures. Electron-positron pairs are then copiously produced, making the
reconnection layer highly collisional and dressing it in a thick pair coat that
traps radiation. The pressure is dominated by radiation and pairs. Yet,
radiation diffusion across the layer may be faster than the global Alfv\'en
transit time; then, radiative cooling governs the thermodynamics and
reconnection becomes a radiative transfer problem, greatly affected by the
ultra-strong magnetic field. This overall picture is very different from our
traditional picture of reconnection and thus represents a new frontier in
reconnection research.Comment: Accepted to Space Science Reviews (special issue on magnetic
reconnection). Article is based on an invited review talk at the
Yosemite-2010 Workshop on Magnetic Reconnection (Yosemite NP, CA, USA;
February 8-12, 2010). 30 pages, no figure
Ajustamento para Heterogeneidade de Variância da Produção de Leite de Vacas da Raça Holandesa no Estado de Minas Gerais
Mapeamento digital de areia, argila e carbono orgânico por modelos Random Forest sob diferentes resoluções espaciais
Effects of buried high-Z layers on fast electron propagation
By extending a prior model [A.R. Bell, J.R. Davies, S.M. Guerin, Phys. Rev. E
58, 2471 (1998)], the magnetic field generated during the transport of a
fast electron beam driven by an ultraintense laser in a solid target is derived
analytically and applied to estimate the effect of such field on fast electron propagation
through a buried high-Z layer in a lower-Z target. It is found that the effect gets weaker
with the increase of the depth of the buried layer, the divergence of the fast electrons,
and the laser intensity, indicating that magnetic field effects on the fast electron
divergence as measured from Ka X-ray emission may need to be
considered for moderate laser intensities. On the basis of the calculations, some
considerations are made on how one can mitigate the effect of the magnetic field generated
at the interface