1,873 research outputs found
Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection
Non-Maxwellian electron velocity space distribution functions (EVDF) are
useful signatures of plasma conditions and non-local consequences of
collisionless magnetic reconnection. In the past, EVDFs were obtained mainly
for antiparallel reconnection and under the influence of weak guide-fields in
the direction perpendicular to the reconnection plane. EVDFs are, however, not
well known, yet, for oblique (or component-) reconnection in dependence on
stronger guide-magnetic fields and for the exhaust (outflow) region of
reconnection away from the diffusion region. In view of the multi-spacecraft
Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of
collisionless magnetic reconnection in dependence on the guide-field strength
from small () to very strong () guide-fields, taking
into account the feedback of the self-generated turbulence. For this sake, we
carried out 2.5D fully-kinetic Particle-in-Cell simulations using the ACRONYM
code. We obtained anisotropic EVDFs and electron beams propagating along the
separatrices as well as in the exhaust region of reconnection. The beams are
anisotropic with a higher temperature in the direction perpendicular rather
than parallel to the local magnetic field. The beams propagate in the direction
opposite to the background electrons and cause instabilities. We also obtained
the guide-field dependence of the relative electron-beam drift speed, threshold
and properties of the resulting streaming instabilities including the strongly
non-linear saturation of the self-generated plasma turbulence. This turbulence
and its non-linear feedback cause non-adiabatic parallel electron acceleration
and EVDFs well beyond the limits of the quasi-linear approximation, producing
phase space holes and an isotropizing pitch-angle scattering.Comment: 21 pages, 8 figures. Revised to match with the version published in
Physics of Plasmas. An abridged version of the abstract is shown her
Magnetism of the LTT phase of Eu doped La_{2-x}Sr_xCuO_4
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal
state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal
pronounced changes of the Cu magnetism at the structural transition from the
orthorhombic to the low temperature tetragonal phase for all
non-superconducting compositions. Both a jumplike decrease of \chi as well as
the ESR data show an increase of the in-plane magnetic correlation length in
the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and
Sr concentrations in the LTT phase the correlation length increases up to more
than 100 lattice constants and the fluctuation frequency of the CuO_2 spin
system slows down to 10^{10}- 10^{11}sec^{-1}. However, there is no static
order above T ~ 8K in contrast to the LTT phase of Nd doped La_{2-x}Sr_xCuO_4
with pinned stripe correlations.Comment: 7 pages, RevTex, 3 eps figures. To appear in the Proceedings of the
International Conference "Stripes, Lattice Instabilities and High Tc
Superconductivity", (Rome, Dec. 1996
Phase noise due to vibrations in Mach-Zehnder atom interferometers
Atom interferometers are very sensitive to accelerations and rotations. This
property, which has some very interesting applications, induces a deleterious
phase noise due to the seismic noise of the laboratory and this phase noise is
sufficiently large to reduce the fringe visibility in many experiments. We
develop a model calculation of this phase noise in the case of Mach-Zehnder
atom interferometers and we apply this model to our thermal lithium
interferometer. We are able to explain the observed phase noise which has been
detected through the rapid dependence of the fringe visibility with the
diffraction order. We think that the dynamical model developed in the present
paper should be very useful to reduce the vibration induced phase noise in atom
interferometers, making many new experiments feasible
Electron acceleration by cascading reconnection in the solar corona I Magnetic gradient and curvature effects
Aims: We investigate the electron acceleration in convective electric fields
of cascading magnetic reconnection in a flaring solar corona and show the
resulting hard X-ray (HXR) radiation spectra caused by Bremsstrahlung for the
coronal source. Methods: We perform test particle calculation of electron
motions in the framework of a guiding center approximation. The electromagnetic
fields and their derivatives along electron trajectories are obtained by
linearly interpolating the results of high-resolution adaptive mesh refinement
(AMR) MHD simulations of cascading magnetic reconnection. Hard X-ray (HXR)
spectra are calculated using an optically thin Bremsstrahlung model. Results:
Magnetic gradients and curvatures in cascading reconnection current sheet
accelerate electrons: trapped in magnetic islands, precipitating to the
chromosphere and ejected into the interplanetary space. The final location of
an electron is determined by its initial position, pitch angle and velocity.
These initial conditions also influence electron acceleration efficiency. Most
of electrons have enhanced perpendicular energy. Trapped electrons are
considered to cause the observed bright spots along coronal mass ejection
CME-trailing current sheets as well as the flare loop-top HXR emissions.Comment: submitted to A&
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