8,474 research outputs found
The role of the Weibel instability at the reconnection jet front in relativistic pair plasma reconnection
The role of the Weibel instability is investigated for the first time in the
context of the large-scale magnetic reconnection problem. A late-time evolution
of magnetic reconnection in relativistic pair plasmas is demonstrated by
particle-in-cell (PIC) simulations. In the outflow regions, powerful
reconnection jet piles up the magnetic fields and then a tangential
discontinuity appears there. Further downstream, it is found that the
two-dimensional extension of the relativistic Weibel instability generates
electro-magnetic fields, which are comparable to the anti-parallel or piled-up
fields. In a microscopic viewpoint, the instability allows plasma's multiple
interactions with the discontinuity. In a macroscopic viewpoint, the
instability leads to rapid expansion of the current sheet and then the
reconnection jet front further propagates into the downstream. Possible
application to the three-dimensional case is briefly discussed.Comment: 25 pages, 9 figures; References and typos are fixe
Self-regulation of the reconnecting current layer in relativistic pair plasma reconnection
We investigate properties of the reconnecting current layer in relativistic
pair plasma reconnection. We found that the current layer self-regulates its
thickness when the current layer runs out current carriers, and so relativistic
reconnection retains a fast reconnection rate. Constructing a steady state
Sweet-Parker model, we discuss conditions for the current sheet expansion.
Based on the energy argument, we conclude that the incompressible assumption is
invalid in relativistic Sweet-Parker reconnection. The guide field cases are
more incompressible than the anti-parallel cases, and we find a more
significant current sheet expansion.Comment: Accepted for publication in Astrophysical Journal (to appear in vol.
685
Calibration of star formation rate tracers for short- and long-lived star formation episodes
To derive the history of star formation in the Universe a set of calibrated
star formation rate tracers at different wavelengths is required. The
calibration has to consistently take into account the effects of extinction,
star formation regime (short or long-lived) and evolutionary state to avoid
biases at different redshift ranges. We use evolutionary synthesis models
optimized for intense episodes of star formation in order to compute a
consistent calibration of the most usual star formation rate tracers at
different energy ranges, from X-ray to radio luminosities. Nearly-instantaneous
and continuous star formation regimes, and the effect of interstellar
extinction are considered, as well as the effect of metallicity on the
calibration of the different estimators. A consistent calibration of a complete
set of star formation rate tracers is presented, computed for the most usual
star-forming regions conditions: evolutionary state, star formation regime,
interstellar extinction and initial mass function. We discuss the validity of
the different tracers in different star formation scenarios and compare our
predictions with previous calibrations of general use. Nearly-instantaneous and
continuous star formation regimes must be distinguished. While the Star
Formation Strength (\msun) should be used for the former, the more common Star
Formation Rate (\msun yr) is only valid for episodes forming stars at a
constant rate during extended periods for time. Moreover, even for the latter,
the evolutionary state should be taken into account, since most SFR tracers
stabilize only after 100 Myr of evolution.Comment: Accepted for publication in A&A, webtool in
http://www.laeff.cab.inta-csic.es/research/sfr/, 19 pages, 10 figures, 14
tables. New version including language style revisio
Variational Estimates using a Discrete Variable Representation
The advantage of using a Discrete Variable Representation (DVR) is that the
Hamiltonian of two interacting particles can be constructed in a very simple
form. However the DVR Hamiltonian is approximate and, as a consequence, the
results cannot be considered as variational ones. We will show that the
variational character of the results can be restored by performing a reduced
number of integrals.
In practice, for a variational description of the lowest n bound states only
n(n+1)/2 integrals are necessary whereas D(D+1)/2 integrals are enough for the
scattering states (D is the dimension of the S matrix). Applications of the
method to the study of dimers of He, Ne and Ar, for both bound and scattering
states, are presented.Comment: 30 pages, 7 figures. Minor changes (title modified, typos corrected,
1 reference added). To be published in PR
Gymnostachys, unlike Acorus, should be retained within Araceae: pollen ultrastructure supports molecular results
XV lnternational A.P.L.E. Symposium of Palynolog
Particle-in-cell simulations of collisionless magnetic reconnection with a non-uniform guide field
Results are presented of a first study of collisionless magnetic reconnection starting from a recently found exact nonlinear force-free Vlasov–Maxwell equilibrium. The initial state has a Harris sheet magnetic field profile in one direction and a non-uniform guide field in a second direction, resulting in a spatially constant magnetic field strength as well as a constant initial plasma density and plasma pressure. It is found that the reconnection process initially resembles guide field reconnection, but that a gradual transition to anti-parallel reconnection happens as the system evolves. The time evolution of a number of plasma parameters is investigated, and the results are compared with simulations starting from a Harris sheet equilibrium and a Harris sheet plus constant guide field equilibrium
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