117 research outputs found
Timing mirror structures observed by Cluster with a magnetosheath flow model
The evolution of structures associated with mirror modes during their flow in
the Earth's magnetosheath is studied. The fact that the related magnetic
fluctuations can take distinct shapes, from deep holes to high peaks, has
been assessed in previous works on the observational, modeling and numerical
points of view. In this paper we present an analytical model for the flow
lines and velocity magnitude inside the magnetosheath. This model is used to
interpret almost 10 years of Cluster observations of mirror structures: by
back tracking each isolated observation to the shock, the "age", or flow
time, of these structures is determined together with the geometry of the
shock. Using this flow time the evolutionary path of the structures may be
studied with respect to different quantities: the distance to mirror
threshold, the amplitude of mirror fluctuations and the skewness of the
magnetic amplitude distribution as a marker of the shape of the structures.
These behaviours are confronted to numerical simulations which confirm the
dynamical perspective gained from the association of the statistical analysis
and the analytical model: magnetic peaks are mostly formed just behind the
shock and are quickly overwhelmed by magnetic holes as the plasma conditions
get more mirror stable. The amplitude of the fluctuations are found to
saturate before the skewness vanishes, i.e. when both structures
quantitatively balance each other, which typically occurs after a flow time
of 100–200 s in the Earth's magnetosheath. Comparison with other astrophysical
contexts is discussed
Nonlinear evolution of the magnetized Kelvin-Helmholtz instability: from fluid to kinetic modeling
The nonlinear evolution of collisionless plasmas is typically a multi-scale
process where the energy is injected at large, fluid scales and dissipated at
small, kinetic scales. Accurately modelling the global evolution requires to
take into account the main micro-scale physical processes of interest. This is
why comparison of different plasma models is today an imperative task aiming at
understanding cross-scale processes in plasmas. We report here the first
comparative study of the evolution of a magnetized shear flow, through a
variety of different plasma models by using magnetohydrodynamic, Hall-MHD,
two-fluid, hybrid kinetic and full kinetic codes. Kinetic relaxation effects
are discussed to emphasize the need for kinetic equilibriums to study the
dynamics of collisionless plasmas in non trivial configurations. Discrepancies
between models are studied both in the linear and in the nonlinear regime of
the magnetized Kelvin-Helmholtz instability, to highlight the effects of small
scale processes on the nonlinear evolution of collisionless plasmas. We
illustrate how the evolution of a magnetized shear flow depends on the relative
orientation of the fluid vorticity with respect to the magnetic field direction
during the linear evolution when kinetic effects are taken into account. Even
if we found that small scale processes differ between the different models, we
show that the feedback from small, kinetic scales to large, fluid scales is
negligable in the nonlinear regime. This study show that the kinetic modeling
validates the use of a fluid approach at large scales, which encourages the
development and use of fluid codes to study the nonlinear evolution of
magnetized fluid flows, even in the colisionless regime
Overview of Atmospheric Simulation Efforts in CTA
The Cherenkov Telescope Array (CTA) is an observatory for ground-based gamma-ray astronomy currently under construction, which will observe photons with very high energies (20 GeV – 300 TeV). One of the main contributions to the systematic uncertainties stems from the uncertainty on the atmospheric density profile, of molecules and aerosols. To minimize these systematics a full calibration of the atmospheric properties is important as well as a calibration of the detector response. In the paper we introduce the strategy for atmospheric simulations, use of Monte Carlo simulations and available CTA computing resources. We also describe in more detail realized and planned atmospheric simulations as well as the Czech contribution to this effort
The bright optical flash from GRB 060117
We present a discovery and observation of an extraordinarily bright prompt
optical emission of the GRB 060117 obtained by a wide-field camera atop the
robotic telescope FRAM of the Pierre Auger Observatory from 2 to 10 minutes
after the GRB. We found rapid average temporal flux decay of alpha = -1.7 +-
0.1 and a peak brightness R = 10.1 mag. Later observations by other instruments
set a strong limit on the optical and radio transient fluxes, unveiling an
unexpectedly rapid further decay. We present an interpretation featuring a
relatively steep electron-distribution parameter p ~ 3.0 and providing a
straightforward solution for the overall fast decay of this optical transient
as a transition between reverse and forward shock.Comment: Accepted to A&A, 4 pages, corected few typos pointed out by X.F. W
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
Atmospheric aerosols at the Pierre Auger Observatory and environmental implications
The Pierre Auger Observatory detects the highest energy cosmic rays.
Calorimetric measurements of extensive air showers induced by cosmic rays are
performed with a fluorescence detector. Thus, one of the main challenges is the
atmospheric monitoring, especially for aerosols in suspension in the
atmosphere. Several methods are described which have been developed to measure
the aerosol optical depth profile and aerosol phase function, using lasers and
other light sources as recorded by the fluorescence detector. The origin of
atmospheric aerosols traveling through the Auger site is also presented,
highlighting the effect of surrounding areas to atmospheric properties. In the
aim to extend the Pierre Auger Observatory to an atmospheric research platform,
a discussion about a collaborative project is presented.Comment: Regular Article, 16 pages, 12 figure
First-year ion-acoustic wave observations in the solar wind by the RPW/TDS instrument on board Solar Orbiter
Context. Electric field measurements of the Time Domain Sampler (TDS) receiver, part of the Radio and Plasma Waves (RPW) instrument on board Solar Orbiter, often exhibit very intense broadband wave emissions at frequencies below 20 kHz in the spacecraft frame. During the first year of the mission, the RPW/TDS instrument was operating from the first perihelion in mid-June 2020 and through the first flyby of Venus in late December 2020.
Aims. In this paper, we present a year-long study of electrostatic fluctuations observed in the solar wind at an interval of heliocentric distances from 0.5 to 1 AU. The RPW/TDS observations provide a nearly continuous data set for a statistical study of intense waves below the local plasma frequency.
Methods. The on-board and continuously collected and processed properties of waveform snapshots allow for the mapping plasma waves at frequencies between 200 Hz and 20 kHz. We used the triggered waveform snapshots and a Doppler-shifted solution of the dispersion relation for wave mode identification in order to carry out a detailed spectral and polarization analysis.
Results. Electrostatic ion-acoustic waves are the most common wave emissions observed between the local electron and proton plasma frequency by the TDS receiver during the first year of the mission. The occurrence rate of ion-acoustic waves peaks around perihelion at distances of 0.5 AU and decreases with increasing distances, with only a few waves detected per day at 0.9 AU. Waves are more likely to be observed when the local proton moments and magnetic field are highly variable. A more detailed analysis of more than 10000 triggered waveform snapshots shows the mean wave frequency at about 3 kHz and wave amplitude about 2.5 mV/m. The wave amplitude varies as R−1.38 with the heliocentric distance. The relative phase distribution between two components of the E-field projected in the Y-Z Spacecraft Reference Frame (SRF) plane shows a mostly linear wave polarization. Electric field fluctuations are closely aligned with the directions of the ambient field lines. Only a small number (3%) of ion-acoustic waves are observed at larger magnetic discontinuities
Density profile characterization and modeling at Paranal and Armazones 2k sites
The Cherenkov Telescope Array (CTA) in the southern hemisphere will be installed at Armazones 2k site in northern Chile. Scarce atmospheric observations are available in the region, particularly radiosonde data. This study analyzes radiosondes launched at Paranal observatory, located at about 21 km from the CTA site, from 24 October and 4 November 2011, to understand the behavior of density in the atmosphere near the CTA site. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) model are validated with Paranal radiosondes to quantify its ability to represent the atmospheric conditions in the region. In addition, the seasonal and diurnal evolution of atmospheric density at the CTA site were studied during 2011 using the high-resolution weather forecasts from the WRF model
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