3,100 research outputs found
Corrugation of relativistic magnetized shock waves
As a shock front interacts with turbulence, it develops corrugation which
induces outgoing wave modes in the downstream plasma. For a fast shock wave,
the incoming wave modes can either be fast magnetosonic waves originating from
downstream, outrunning the shock, or eigenmodes of the upstream plasma drifting
through the shock. Using linear perturbation theory in relativistic MHD, this
paper provides a general analysis of the corrugation of relativistic magnetized
fast shock waves resulting from their interaction with small amplitude
disturbances. Transfer functions characterizing the linear response for each of
the outgoing modes are calculated as a function of the magnetization of the
upstream medium and as a function of the nature of the incoming wave.
Interestingly, if the latter is an eigenmode of the upstream plasma, we find
that there exists a resonance at which the (linear) response of the shock
becomes large or even diverges. This result may have profound consequences on
the phenomenology of astrophysical relativistic magnetized shock waves.Comment: 14 pages, 9 figures; to appear in Ap
Current-driven filamentation upstream of magnetized relativistic collisionless shocks
The physics of instabilities in the precursor of relativistic collisionless
shocks is of broad importance in high energy astrophysics, because these
instabilities build up the shock, control the particle acceleration process and
generate the magnetic fields in which the accelerated particles radiate. Two
crucial parameters control the micro-physics of these shocks: the magnetization
of the ambient medium and the Lorentz factor of the shock front; as of today,
much of this parameter space remains to be explored. In the present paper, we
report on a new instability upstream of electron-positron relativistic shocks
and we argue that this instability shapes the micro-physics at moderate
magnetization levels and/or large Lorentz factors. This instability is seeded
by the electric current carried by the accelerated particles in the shock
precursor as they gyrate around the background magnetic field. The compensation
current induced in the background plasma leads to an unstable configuration,
with the appearance of charge neutral filaments carrying a current of the same
polarity, oriented along the perpendicular current. This ``current-driven
filamentation'' instability grows faster than any other instability studied so
far upstream of relativistic shocks, with a growth rate comparable to the
plasma frequency. Furthermore, the compensation of the current is associated
with a slow-down of the ambient plasma as it penetrates the shock precursor (as
viewed in the shock rest frame). This slow-down of the plasma implies that the
``current driven filamentation'' instability can grow for any value of the
shock Lorentz factor, provided the magnetization \sigma <~ 10^{-2}. We argue
that this instability explains the results of recent particle-in-cell
simulations in the mildly magnetized regime.Comment: 14 pages, 8 figures; to appear in MNRA
Results of a FRSI material test under Space Shuttle ascent conditions in the Ames Research Center 9x7 foot supersonic wind tunnel (OS13). Space Shuttle aerothermodynamic data report
A test was conducted in the NASA/ARC 9 x 7 foot supersonic wind tunnel to verify the integrity of Felt Reusable Surface Insulation (FRSI) material in a panel flutter environment. A FRSI sample panel was subjected to the shocks, pressure gradients, and turbulence characteristics encountered at dynamic pressure 1.5 times the 3(sigma) dispersed trajectory flight conditions of the Space Shuttle. Static and fluctuating pressure data were obtained for Mach numbers ranging from 1.55 to 2.5 with dynamic pressures of 625 to 1250 psf. The FRSI panel suffered no appreciable damage as a result of the test
Managing the Flemish dunes: from eco-gardening to mechanical disturbances created by bulldozers
The âConseil GĂ©nĂ©ral du DĂ©partement du Nordâ is the manager of 480ha of Flemish dunes located at the far North of France. These spaces, initially preserved by the action of the âCommunautĂ© Urbaine de Dunkerqueâ are now the property of the âConservatoire de lâEspace Littoral et des Rivages Lacustresâ. Characterised by an aelian dynamism and by the winter flooding of their damp depressions, these systems had represented a very high patrimonial richness before man, by these actions (the pumping of the aquifers, the parcelling out of dune massifs, the fixation of sand by plantations, the destruction of rabbits), reduced to nothing the natural expression of perturbations (storms, floods, important sandbanks) and made dune massifs become wastelands and low marshes and dry lawns disappear. The first step for preservation was the in extremis safeguard and the maintenance by secateurs (1989) then by motor scythes (1992) of micro-habitats with a high richness over a 2ha total surface lost in very important pre-forested systems. Rapidly, the objectives of preservation obliged us to put into practice the restoration of natural environments by stripping and clearing the ground (1994 and 1997). But these operations of a large scale (10ha) were always based on a âfixistâ and âmuseographicalâ approach to the environment (the maintenance of a representative sample of habitats). The management which consists of stopping certain pioneer and post-pioneer stages of vegetation can in fact correspond to a counter-natural step as these dune systems make the proof of a very high dynamism and permanent evolution. Today the management of dune space is done by bulldozers: first the pre-forested vegetation is totally destructed (16ha in 2004) leaving systems of bare sand freely evolving and accepting their spontaneous wastelanding over more than 50% of their surfaces. The return to uncontrolled vegetal dynamism is only accepted if perturbations are periodically created in order to regularly produce the starting or reappearance conditions of the different series of vegetation. The last stage of our managing operations would consist, with a middle-term effect, in reducing the stability of the edging dune row and in favouring the development of wind passages with their devastation or saving effects on the dune we intend to preserve
The moduli problem at the perturbative level
Moduli fields generically produce strong dark matter -- radiation and baryon
-- radiation isocurvature perturbations through their decay if they remain
light during inflation. We show that existing upper bounds on the magnitude of
such fluctuations can thus be translated into stringent constraints on the
moduli parameter space m_\sigma (modulus mass) -- \sigma_{inf} (modulus vacuum
expectation value at the end of inflation). These constraints are complementary
to previously existing bounds so that the moduli problem becomes worse at the
perturbative level. In particular, if the inflationary scale H_{inf}~10^{13}
GeV, particle physics scenarios which predict high moduli masses m_\sigma >
10-100 TeV are plagued by the perturbative moduli problem, even though they
evade big-bang nucleosynthesis constraints.Comment: 4 pages, 3 figures (revtex) -- v2: an important correction on the
amplitude/transfer of isocurvature modes at the end of inflation, typos
corrected, references added, basic result unchange
Detecting stable massive neutral particles through particle lensing
Stable massive neutral particles emitted by astrophysical sources undergo
deflection under the gravitational potential of our own galaxy. The deflection
angle depends on the particle velocity and therefore non-relativistic particles
will be deflected more than relativistic ones. If these particles can be
detected through neutrino telescopes, cosmic ray detectors or directional dark
matter detectors, their arrival directions would appear aligned on the sky
along the source-lens direction. On top of this deflection, the arrival
direction of non-relativistic particles is displaced with respect to the
relativistic counterpart also due to the relative motion of the source with
respect to the observer; this induces an alignment of detections along the sky
projection of the source trajectory. The final alignment will be given by a
combination of the directions induced by lensing and source proper motion. We
derive the deflection-velocity relation for the Milky Way halo and suggest that
searching for alignments on detection maps of particle telescopes could be a
way to find new particles or new astrophysical phenomena.Comment: 17 pages, 7 figures. Accepted by PR
A critical approach to the concept of a polar, low-altitude LARES satellite
According to very recent developments of the LARES mission, which would be
devoted to the measurement of the general relativistic Lense--Thirring effect
in the gravitational field of the Earth with Satellite Laser Ranging, it seems
that the LARES satellite might be finally launched in a polar, low--altitude
orbit by means of a relatively low--cost rocket. The observable would be the
node only. In this letter we critically analyze this scenario.Comment: LaTex2e, 11 pages, 4 figures, 1 table. Accepted for publication in
Classical and Quantum Gravit
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