39 research outputs found
Very oblique whistler generation by low-energy electron streams
International audienceWhistler mode chorus waves are present throughout the Earth's outer radiation belt as well as at larger distances from our planet. While the generation mechanisms of parallel lower band chorus waves and oblique upper band chorus waves have been identified and checked in various instances, the statistically significant presence in recent satellite observations of very oblique lower band chorus waves near the resonance cone angle remains to be explained. Here we discuss two possible generation mechanisms for such waves. The first one is based on Landau resonance with sporadic very low energy (<4 keV) electron beams either injected from the plasma sheet or produced in situ. The second one relies on cyclotron resonance with low-energy electron streams, such that their velocity distribution possesses both a significant temperature anisotropy above 3–4 keV and a plateau or heavy tail in parallel velocities at lower energies encompassing simultaneous Landau resonance with the same waves. The corresponding frequency and wave normal angle distributions of the generated very oblique lower band chorus waves, as well as their frequency sweep rate, are evaluated analytically and compared with satellite observations, showing a reasonable agreement
Wave-particle interactions in the outer radiation belts
Data from the Van Allen Probes have provided the first extensive evidence of non-linear (as opposed to quasilinear) wave-particle interactions in space, with the associated rapid (fraction of a bounce period) electron acceleration, to hundreds of keV by Landau resonance, in the parallel electric fields of time domain structures (TDS) and very oblique chorus waves. The experimental evidence, simulations, and theories of these processes are discussed
Energy repartition and entropy generation across the Earth’s bow shock: MMS observations
The evolution of plasma entropy and the process of plasma energy redistribution at the collisionless plasma shock front are evaluated based on the high temporal resolution data from the four Magnetospheric Multiscale spacecraft during the crossing of the terrestrial bow shock. The ion distribution function has been separated into the populations with different characteristic behaviors in the vicinity of the shock: the upstream core population, the reflected ions, the gyrating ions, the ions trapped in the vicinity of the shock, and the downstream core population. The values of ion and electron moments (density, bulk velocity, and temperature) have been determined separately for these populations. It is shown that the solar wind core population bulk velocity slows down mainly in the ramp with the electrostatic potential increase but not in the foot region as it was supposed. The reflected ion population determines the foot region properties, so the proton temperature peak in the foot region is an effect of the relative motion of the different ion populations, rather than an actual increase in the thermal speed of any of the ion population. The ion entropy evaluated showed a significant increase across the shock: the enhancement of the ion entropy occurs in the foot of the shock front and at the ramp, where the reflected ions are emerging in addition to the upstream solar wind ions, the anisotropy growing to generate the bursts of ion-scale electrostatic waves. The entropy of electrons across the shock does not show a significant change: electron heating goes almost adiabatically
Nonlinear ion-acoustic (IA) waves driven in a cylindrically symmetric flow
By employing a self-similar, two-fluid MHD model in a cylindrical geometry,
we study the features of nonlinear ion-acoustic (IA) waves which propagate in
the direction of external magnetic field lines in space plasmas. Numerical
calculations not only expose the well-known three shapes of nonlinear
structures (sinusoidal, sawtooth, and spiky or bipolar) which are observed by
numerous satellites and simulated by models in a Cartesian geometry, but also
illustrate new results, such as, two reversely propagating nonlinear waves,
density dips and humps, diverging and converging electric shocks, etc. A case
study on Cluster satellite data is also introduced.Comment: accepted by AS
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
Plasma wave observations during electron gun experiments on ISEE-1
Advances in Space Research, 12, (12)29The ISEE-1 electron guns were operated during the final orbits of ISEE-1 in 1987 in tests designed to study the stimulation of plasma waves. The guns were operated in modes which varied from ..