700 research outputs found
Solar filament eruptions and their physical role in triggering Coronal Mass Ejections
Solar filament eruptions play a crucial role in triggering coronal mass
ejections (CMEs). More than 80 % of eruptions lead to a CME. This correlation
has been studied extensively during the past solar cycles and the last long
solar minimum. The statistics made on events occurring during the rising phase
of the new solar cycle 24 is in agreement with this finding. Both filaments and
CMEs have been related to twisted magnetic fields. Therefore, nearly all the
MHD CME models include a twisted flux tube, called a flux rope. Either the flux
rope is present long before the eruption, or it is built up by reconnection of
a sheared arcade from the beginning of the eruption. In order to initiate
eruptions, different mechanisms have been proposed: new emergence of flux,
and/or dispersion of the external magnetic field, and/or reconnection of field
lines below or above the flux rope. These mechanisms reduce the downward
magnetic tension and favor the rise of the flux rope. Another mechanism is the
kink instability when the configuration is twisted too much. In this paper we
open a forum of discussions revisiting observational and theoretical papers to
understand which mechanisms trigger the eruption. We conclude that all the
above quoted mechanisms could bring the flux rope to an unstable state.
However, the most efficient mechanism for CMEs is the loss-of-equilibrium or
torus instability, when the flux rope has reached an unstable threshold
determined by a decay index of the external magnetic field.Comment: 23 pages, 13 figures, revie
Radiated Immunity Testing of a Device with an External Wire: Repeatibility of Reverberation Chamber Results and Correlation with Anechoic Chamber Results
We present the experimental radiated immunity results of an electronic device with an external wire obtained in reverberation and anechoic chambers. Repeatability and reproducibility of reverberation chamber measurements are investigated by repeating the test in three reverberation chambers with different characteristics. We show how the current state of the art allows a statistical control of RC measurement repeatability within an industrial installation, and that a statistical correlation with AC results frequency by frequency is possible in particular cases relevant to automotive application
Constraints on filament models deduced from dynamical analysis
The conclusions deduced from simultaneous observations with the Ultra-Violet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission satellite, and the Multichannel Subtractive Double Pass (MSPD) spectrographs at Meudon and Pic du Midi observatories are presented. The observations were obtained in 1980 and 1984. All instruments have almost the same field of view and provide intensity and velocity maps at two temperatures. The resolution is approx. 0.5 to 1.5" for H alpha line and 3" for C IV. The high resolution and simultaneity of the two types of observations allows a more accurate description of the flows in prominences as functions of temperature and position. The results put some contraints on the models and show that dynamical aspects must be taken into account
INTIMATE'96. A shallow water tomography experiment devoted to the study of internal tides
The INTIMATE (INternal Tide Investigation by Means of Acoustic Tomography Experiment) project is devoted to the study of internal tides by use of acoustic tomography. The first exploratory experiment was carried out in June 1996 on the continental shelf off the west coast of Portugal. A towed broadband acoustic source and a 4-hydrophone vertical array were used. Acoustic data were collected for 5 days, including legs where the source ship was moving and legs with the ship on station. The purpose of this paper is to briefly discuss some effects of the environment on acoustic fluctuations.PRAXIS XX
Broadband source localization with a single hydrophone
Source localization with vertical arrays in shallow water has been a topic of intense research in the last 20 years. Although horizontal arrays can (and have) been used, vertical arrays are better suited for extracting signal modal structure and thus provide a source-location estimate in range and depth. It is well known that broadband signals have a localization capability superior to that of narrowband signals. One question that remains largely unresolved is whether frequency extent can compensate for the spatial diversity provided by sensor arrays, i.e., at the limit, can a broadband signal be localized with a single sensor ? This communication presents theoretical, simulated, and practical evidence that a multipath-delay maximum-likelihood estimator does provide enough signal to background discrimination for localizing a 500 Hz bandwidth signal at 5 km range in a 130 m depth shallow water channel with a single hydrophone. The real data used in this study was collected during the INTI-MATE'96 experiment which was conducted off the west of Portugal in June 1996 during an internal tide tomography experiment. Comparison with localization results provided by direct match between the received and the model-generated arrival patterns will be discussed
Intimate '96: shallow water tomography in the sea of the condemned
As is well-known, the tidal force of the moon and the sun can cause notable changes in the sea level. Besides this so-called
barotropic effect, the tidal force also drives internal waves in a daily rhythm. Thus, the internal wave spectrum is often dominated by
a single component with perhaps 10 km from crest to crest. This ââinternal tideââ tends to propagate toward shore and has its greatest
height near the shelfbreak
Liquid transport in scale space
International audienceWhen a liquid stream is injected into a gaseous atmosphere, it destabilizes and continuously passes through different states characterized by different morphologies. Throughout this process, the flow dynamics may be different depending on the region of the flow and the scales of the involved liquid structures. Exploring this multi-scale, multi-dimensional phenomenon requires some new theoretical tools, some of which need yet to be elaborated. Here, a new analytical framework is proposed on the basis of two-point statistical equations of the liquid volume fraction. This tool, which originates from single phase turbulence, allows us notably to decompose the fluxes of liquid in flowâposition space and scale space. Direct numerical simulations of liquidâgas turbulence decaying in a triply periodic domain are then used to characterize the time and scale evolution of the liquid volume fraction. It is emphasized that two-point statistics of the liquid volume fraction depend explicitly on the geometrical properties of the liquidâgas interface and in particular its surface density. The stretch rate of the liquidâgas interface is further shown to be the equivalent for the liquid volume fraction (a non-diffusive scalar) of the scalar dissipation rate. Finally, a decomposition of the transport of liquid in scale space highlights that non-local interactions between non-adjacent scales play a significant role
Estimating equivalent bottom geoacoustial parameters from broadband inversion
A simple and fast approach to retrieve equivalent geoacoustic parameters is presented in
this paper. The method is based upon the processing of 300-800 Hz broadband signals on a
single hydrophone.Two stable characteristics of the impulse response of the shallow water
waveguide are estimated: the time dispersion and the bottom reflection amplitudes. This two
features are analytically linked to the compressional speed and to the attenuation coefficient
of the medium. The inversion of the two latter geoacoustic parameters is straightforward
since it relies on an analytical expression. The method is tested on INTIMATE96 data. The
results show an excellent agreement between the reflection of the true medium and the
reflection coefficient of the equivalent medium.The partners of the INTIMATE project wish to thank the staff of NRP ANDROMEDA,
the staff of BO DâENTRECASTEAUX and people of Mission OcĂ©anographique de
lâAtlantique (aboard DâENTRECASTEAUX). We also wish to thank the SACLANT
Undersea Research Center for lending the Portable Array System and Roberto Chiarabini
(SACLANTCEN) for his participation in the array preparation, deployment and use. Thanks
to T. Folegot and G. Bonnaillie (CMO) for their active contribution in this work. The study
was jointly sponsored by SHOM (exploratory program 95901), the Portuguese Ministery of
Research (PRAXIS XXI) and ONR (contract N00014-95-1-0558)
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