Mercredi 12 octobre 2016 à 11h00 (Salle de réunion du bâtiment 14)
Kostas Allisandrakis (University of Ioannina, Grèce)
Jeudi 6 octobre 2016 à 14h00 (Salle de conférence du bâtiment 17)
John D Landstreet (Western Univ., London, Ontario, Canada)
We have observational detections of magnetic fields in at least some stars of the major stages of stellar evolution, from the pre-main sequence to the commonest stellar final state, the white dwarfs. However, the evolution of a field as a star evolves from one stage to another is still very poorly understood. The white dwarfs are particularly puzzling. Naively, their fairly rare megaGauss magnetic fields could be the descendents, by flux conservation, of the fairly rare kiloGauss magnetic fields of upper main sequence stars. But how these main sequence fields survive the giant and AGB stages of stellar evolution, or how “new" fields are produced in some young white dwarfs (perhaps by binary merger events) is still far from clear. Theoretical advances depend on a good foundation of observations to test and refine models. S. Bagnulo, G. Valyavin and I have been working to provide a systematic observational description of the occurrence and field geometry of magnetic white dwarfs, especially close to the weak field limit of the very broad observed field strength distribution. In the course of this work, which uses data from ESO FORS, WHT ISIS, and the Russian 6-m telescope, we have developed a powerful method of using ESPaDOnS on the CFHT to detect and ultimately model very weak white dwarf fields. This talk will survey some of our interesting first results.
Vendredi 30 septembre 2016 à 11h00 (Salle de réunion du bâtiment 14)
Kostas Moraitis (LESIA)
Mardi 27 septembre 2016 à 11h00 (Salle de réunion du bâtiment 14)
Eoin Carley (LESIA)
Electron acceleration in the solar corona is often associated with flares and the eruption of twisted magnetic structures known as flux ropes. However, the locations and mechanisms of such particle acceleration during the flare and eruption are still subject to much investigation. Observing the exact sites of particle acceleration can help conrm how the are and eruption are initiated and evolve. Here we use the Atmospheric Imaging Assembly to analyse a flare and erupting flux rope on 2014-April-18, while observations from the Nan-cay Radio Astronomy Facility allows us to diagnose the sites of electron acceleration during the eruption. Our analysis shows evidence for a pre-formed flux rope which slowly rises and becomes destabilised at the time of a C-class flare, plasma jet and the escape of >75 keV electrons from rope center into the corona. As the eruption proceeds, continued acceleration of electrons with energies of -5 keV occurs above the flux rope for a period over 5 minutes. At are peak, one site of electron acceleration is located close to the are site while another is driven by the erupting flux rope into the corona at speeds of up to 400 km/s. Energetic electrons then fill the erupting volume, eventually allowing the flux rope legs to be clearly imaged from radio sources at 150-445MHz. Following the analysis of Joshi et al. (2015), we conclude that the sites of energetic electrons are consistent with flux rope eruption via a tether-cutting or flux cancellation scenario inside a magnetic fan-spine structure. In total, our radio observations allow us to better understand the evolution of a flux rope eruption and its associated electron acceleration sites, from eruption initiation to propagation into the corona.