499 research outputs found
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
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 flow–position space and scale space. Direct numerical simulations of liquid–gas 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 liquid–gas interface and in particular its surface density. The stretch rate of the liquid–gas 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
Measurement of the transfered impendance of the shield cable by the impulse current method using a Fourier processor
This paper is an application of the Fast Fourier Transform for the measurement of the transfer impedance of the
shielded cables at the frequencies above 100 MHz .
In the first part we present an overview of the principle of the measurement of the transfer impedance . We describe
the signal processing method used in this special case .
The second part of the paper is related to the Fourier processor designed to compute the Fourier tansform in few
seconds and for 2,048 signal samples . We give the main steps of the algorithm and the simplifications which have
been used in the numerical code .Mesure de l'impédance de transfert de câbles blindés aux fréquences supérieures à 100 MH
Coronal magnetic reconnection driven by CME expansion -- the 2011 June 7 event
Coronal mass ejections (CMEs) erupt and expand in a magnetically structured
solar corona. Various indirect observational pieces of evidence have shown that
the magnetic field of CMEs reconnects with surrounding magnetic fields,
forming, e.g., dimming regions distant from the CME source regions. Analyzing
Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on
2011 June 7, we present the first direct evidence of coronal magnetic
reconnection between the fields of two adjacent ARs during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current sheets along
a hyperbolic flux tube (HFT) at the interface between the CME and the
neighbouring AR 11227. Reconnection resulted in the formation of new magnetic
connections between the erupting magnetic structure from AR 11226 and the
neighboring active region AR 11227 about 200 Mm from the eruption site. The
onset of reconnection first becomes apparent in the SDO/AIA images when
filament plasma, originally contained within the erupting flux rope, is
re-directed towards remote areas in AR 11227, tracing the change of large-scale
magnetic connectivity. The location of the coronal reconnection region becomes
bright and directly observable at SDO/AIA wavelengths, owing to the presence of
down-flowing cool, dense (10^{10} cm^{-3}) filament plasma in its vicinity. The
high-density plasma around the reconnection region is heated to coronal
temperatures, presumably by slow-mode shocks and Coulomb collisions. These
results provide the first direct observational evidence that CMEs reconnect
with surrounding magnetic structures, leading to a large-scale re-configuration
of the coronal magnetic field.Comment: 12 pages, 12 figure
Bright Points and Subflares in UV Lines and in X-Rays
We have analysed an active region which was observed in Halpha (MSDP), UV lines (SMM/UVSP), and in X rays (SMM/HXIS). In this active region there were only a few subflares and many small bright points visible in UV and in X rays. Using an extrapolation based on the Fourier transform we have computed magnetic field lines connecting different photospheric magnetic polarities from ground-based magnetograms. Along the magnetic inversion lines we find 2 different zones: 1. a high shear region (less than 70 degrees) where subflares occur 2. a low shear region along the magnetic inversion line where UV bright points are observed
Readout system test benches
We propose to develop and exploit versatile multi-purpose Personal Computer-based Test Benches to support the evaluation and design of the basic elements required for digital front-end readout and data transmission systems for an LHC experiment. These test benches will have modular hardware facilities for the operation of new readout system components under realistic conditions, and will implement advanced modern software engineering concepts. They will support components such as fast ADCs, hybrid fibre-optic transceivers, and the prototype VLSI systolic array and data-flow processors currently being developed in national research laboratories and by the emerging European HDTV industry. These efforts would also lay the foundations for projects involving the development of custom-designed VLSI circuits
Magnetic Pinching of Hyperbolic Flux Tubes: I. Basic Estimations
The concept of hyperbolic flux tubes (HFTs) is a generalization of the
concept of separator field lines for coronal magnetic fields with a trivial
magnetic topology. An effective mechanism of a current layer formation in HFTs
is proposed. This mechanism is called magnetic pinching and it is caused by
large-scale shearing motions applied to the photospheric feet of HFTs in a way
as if trying to twist the HFT. It is shown that in the middle of an HFT such
motions produce a hyperbolic flow that causes an exponentially fast growth of
the current density in a thin force-free current layer. The magnetic energy
associated with the current layer that is built up over a few hours is
sufficient for a large flare. Other implications of HFT pinching for solar
flares are discussed as well.Comment: 31 pages, 12 figures, accepted to Astrophysical Journal, added typos
in Eq. (A9) and new comments to Sections 2 and 7, references update
Locating current sheets in the solar corona
Current sheets are essential for energy dissipation in the solar corona, in
particular by enabling magnetic reconnection. Unfortunately, sufficiently thin
current sheets cannot be resolved observationally and the theory of their
formation is an unresolved issue as well. We consider two predictors of coronal
current concentrations, both based on geometrical or even topological
properties of a force free coronal magnetic field. First, there are
separatrices related to magnetic nulls. Through separatrices the magnetic
connectivity changes discontinuously. Coronal magnetic nulls are, however, very
rare. At second, inspired by the concept of generalized magnetic reconnection
without nulls, quasi-separatrix layers (QSL) were suggested. Through QSL the
magnetic connectivity changes continuously, though strongly. The strength of
the connectivity change can be quantified by measuring the squashing of the
flux tubes which connect the magnetically conjugated photospheres.
We verify the QSL and separatrix concepts by comparing the sites of magnetic
nulls and enhanced squashing with the location of current concentrations in the
corona. Due to the known difficulties of their direct observation we simulated
the coronal current sheets by numerically calculating the response of the
corona to energy input from the photosphere heating a simultaneously observed
EUV Bright Point. We did not find coronal current sheets not at the
separatrices but at several QSL locations. The reason is that although the
geometrical properties of force free extrapolated magnetic fields can indeed,
hint at possible current concentrations, a necessary condition for current
sheet formation is the local energy input into the corona
- …