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Toulouse, France

Ariste A.L.,IRAP
Proceedings of the International Astronomical Union | Year: 2015

Sparsity is a property of data by which it can be represented using a small number of patterns. It is the key concept behind an evergrowing list of mathematical techniques for handling data and recover from it signals or information in conditions previously thought impossible. The application of those techniques to spectropolarimetric data is relatively straightforward. We present three examples of such application: the use of Principal Component Analysis to invert the magnetic field in solar prominences from spectropolarimetry of the He D3 line, the removal of fringes from spectropolarimetric data with Relevance Vector Machines, and the retrieval of high resolution spectra from low resolution data with Compressed Sensing. Copyright © 2015 International Astronomical Union. Source

Parrot M.,French National Center for Scientific Research | Sauvaud J.A.,IRAP | Soula S.,CNRS Laboratory for Aerology | Pincon J.L.,French National Center for Scientific Research | Van Der Velde O.,Polytechnic University of Catalonia
Journal of Geophysical Research: Space Physics | Year: 2013

DEMETER (Detection of Electromagnetic Emissions Transmitted From Earthquake Regions) was a three-axis stabilized Earth-pointing spacecraft launched on 29 June 2004 into a low-altitude (710 km) polar and circular orbit that was subsequently lowered to 650 km until the end of the mission in December 2010. DEMETER measured electromagnetic waves all around the Earth, except in the auroral zones (invariant latitude >65°). The frequency range for the electric field was from DC up to 3.5 MHz, and for the magnetic field, it was from a few hertz up to 20 kHz. At its altitude, the phenomena observed on the E field and B field spectrograms recorded during nighttime by the satellite in the very low frequency range are mainly dominated by whistlers. In a first step, the more intense whistlers have been searched. They correspond to the most powerful lightning strokes occurring below DEMETER. Then, it is shown that this intense lightning activity is able to perturb the electron and ion densities at the satellite altitude (up to 133%) during nighttime. These intense lightning strokes are generally associated with transient luminous events, and one event with many sprites recorded on 17 November 2006 above Europe is reported. Examining the charged particle precipitation, it is shown that this density enhancement in the high ionosphere can be related to the energetic particle precipitation induced by the strong whistlers emitted during a long-duration thunderstorm activity at the same location. Key Points Enhancement of electron density in the ionosphere above thunderstorms Enhancement of density due to lightning-induced electron precipitation Winter thunderstorms with increasing number of sprites ©2013. American Geophysical Union. All Rights Reserved. Source

Stratta G.,National institute for astrophysics | Gendre B.,Artemis | Atteia J.L.,Toulouse 1 University Capitole | Atteia J.L.,French National Center for Scientific Research | And 7 more authors.
Astrophysical Journal | Year: 2013

The "ultra-long" gamma-ray burst GRB 111209A at redshift z = 0.677 is the longest GRB ever observed thus far, with a rest frame prompt emission duration of ∼ 4 hr. In order to explain the burst exceptional longevity, a low-metallicity blue supergiant progenitor was invoked. In this article we further constrain the phenomenology and progenitor properties of this peculiar GRB by performing a multiband temporal and spectral analysis of both the prompt and the afterglow emission. We use proprietary and publicly available data from Swift, Konus WIND, XMM-Newton, and TAROT, as well as from other ground-based optical and radio telescopes. We find some peculiar properties that are possibly connected to the exceptional nature of this burst, namely: (1) an unprecedented large optical delay of 410 ± 50 s between the peak time in gamma-rays and the peak time in the optical of a marked multiwavelength flare; (2) multiwavelength prompt emission spectral modeling requires a certain amount of dust in the circumburst environment. The dust produces a rest frame visual extinction of AV = 0.3-1.5 mag, and may undergo destruction at late times; and (3) we detect the presence of a hard spectral extra power-law component at the end of the X-ray steep steep decay phase and before the start of the X-ray afterglow, which has never been revealed thus far in past GRBs. The optical afterglow shows more usual properties; it has a flux power-law decay with an index of 1.6 ± 0.1 and a late rebrightening feature observed at ∼1.1 the day after the first Burst Alert Telescope trigger. We discuss our findings in the context of several possible interpretations that have been given thus far of the complex multiband GRB phenomenology and propose a binary channel formation for the blue supergiant progenitor. © 2013. The American Astronomical Society. All rights reserved.. Source

Schippers P.,French National Center for Scientific Research | Meyer-Vernet N.,French National Center for Scientific Research | Lecacheux A.,French National Center for Scientific Research | Kurth W.S.,University of Iowa | And 2 more authors.
Geophysical Research Letters | Year: 2014

Nanodust grains of a few nanometers in size are produced near the Sun by collisional breakup of larger grains and picked up by the magnetized solar wind. They have so far been detected at 1AU by only the two STEREO spacecraft. Here we analyze the spectra measured by the radio and plasma wave instrument onboard Cassini during the cruise phase close to Earth orbit; they exhibit bursty signatures similar to those observed by the same instrument in association with nanodust stream impacts on Cassini near Jupiter. The observed wave level and spectral shape reveal impacts of nanoparticles at about 300km/s, with an average flux compatible with that observed by the radio and plasma wave instrument onboard STEREO and with the interplanetary flux models. Key Points First nanodust detection by Cassini near 1 AU Nanodust flux near 1 AU is determined Nanodust flux and variability are consistent with STEREO and models © 2014. American Geophysical Union. All Rights Reserved. Source

Perez J.-P.,IRAP
European Journal of Physics | Year: 2015

In this paper, we show that redshifts, which appear in some pedagogical examples, can be expressed in terms of the Doppler-Fizeau effect. For this purpose, we use, as suggested by Weyl, the worldline elements of two physical events: the emission and the reception of a monochromatic wave. The redshift in special relativity and its Galilean approximation are derived in a simpler way than is usually done. In general relativity, the cosmological redshift can be obtained with the general Weyl formula in three important cases of gravitational fields, even though the gravitational redshift, due to bodies running away from each other, cannot be reduced to a simple kinematic effect. © 2016 IOP Publishing Ltd. Source

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