Found 4 talks width keyword sunspots

Tuesday December 11, 2012
Mrs. Illa Rivero


The formation of active regions and its most visible outcome-sunspots-are still a matter of research. Magnetic flux tubes theory tends to explain the formation of sunspots, but it still faces some unresolved questions: How are they generated? Why can they survive all along the convective zone? How do they rise? I will review this theory and introduce a new way to understand sunspot formation: the negative effective magnetic pressure instability (NEMPI). NEMPI was predicted long ago (Kleeorin et al., 1989, 1990; Kleeorin \& Rogackevskii, 1994; Kleeorin et al., 1996) but has only been seen recently (Branderburg et. al., 2011). It arised as a effect of strong stratication and the presence of turbulence with a weak mean magnetic field. Under suitable conditions, a large-scale instability resulting in the formation of non-uniform magnetic structures, can be excited over the scale of many turbulent eddies or convection cells. This instability is caused by a negative contribution of turbulence to the effective (mean-field) magnetic pressure and has previously been discussed in connection with the formation of active regions and perhaps sunspots. Now, we want to understand the effects of rotation on this instability in both two and three dimensions. We use mean-field magnetohydrodynamics in a parameter regime in which the properties of the negative effective magnetic pressure instability have previously been found to be in agreement with those of direct numerical simulations. We find that the instability is suppressed already for relatively slow rotation with Coriolis numbers (i.e. inverse Rossby numbers) around 0.2. The suppression is strongest at the equator. In the nonlinear regime, we find traveling wave solutions with propagation in the prograde direction at the equator with additional poleward migration away from the equator. The prograde rotation of the magnetic pattern near the equator is argued to be a possible explanation for the faster rotation speed of magnetic tracers found on the Sun. In the bulk of the domain, kinetic and current helicities are negative in the northern hemisphere and positive in the southern.

Thursday November 29, 2012
Dr. Klaus Gerhard Puschmann
Leibniz-Institut für Astrophysik Potsdam, Germany


The GREGOR Fabry-Pérot Interferometer (GFPI) is one of the first-light post-focus instruments for the German 1.5-meter GREGOR solar telescope at the Observatorio del Teide. The GFPI is a tunable dual-etalon system in collimated mounting that allows fast narrow-band imaging. It is designed for spectrometric and spectropolarimetric observations between 530-860 nm and 580-660 nm, respectively, and has a theoretical spectral resolution of about 250,000. The field-of-view in spectroscopic mode is 50" x 38" (25" x 38" in case of Stokes-vector spectropolarimetry). In combination with post-facto image reconstruction it has the potential for discovery science concerning the dynamic Sun and its magnetic field at spatial scales down to about 50 km. The instrument underwent an extended commissioning in 2011 and careful science verification throughout 2012. In this talk I will summarize the main characteristics of the GFPI and present results from both the science verification and first observational campaigns. In addition, I will layout the design of the planned BLue Imaging Solar Spectrometer (BLISS), a second Fabry-Pérot Interferometer for the wavelength range 380-530 nm. I will discuss how both the GFPI and BLISS can be used to extend our knowledge on the structure of sunspots and the solar chromosphere by presenting details to the current state of knowledge on these two topics and by outlining possible improvements.

Tuesday April 27, 2010
Dr. María Jesús Martínez González
Instituto de Astrofísica de Canarias, Spain


The quiet Sun (the 99%, or more, of the solar surface not covered by sunspots or active regions) is receiving increased attention in recent years; its role on the global magnetism and its complexity are being increasingly recognised. A picture of a rather stochastic quiet Sun magnetism is emerging. From these recent works, the quiet Sun magnetism is presented as a myriad of magnetic field vectors having an isotropical distribution with a cascade of scales down to the mean free path of the photon. But this chaotic representation also shows clear signs of intermittency: at a low frequency rate (0.022 events h-1 arcsec-2) the magnetic field appear in the quiet Sun forming well-organised loop structures at granular scales. More interesting, these loops rise to higher layers and their energy input into the chromosphere can be important for the heating of this layer. In the talk, I will present a pedagogic view of the quiet Sun magnetism. I will focus on the ascent of the smallest ever observed magnetic flux emergence through the solar atmosphere. More specifically, I will show how to infer from high resolution, spectro-polarimetric observations (taken with the SOT instrument onboard Hinode) the magnetic topology of the fields, how they rise through the photosphere to the chromosphere, and the implications of this phenomena for chromospheric (and coronal) heating.

Tuesday October 20, 2009
Prof. Matthias Rempel
Matthias Rempel, National Center for Atmospheric Research, USA


For a long time radiative MHD simulations of entire sunspots from first principles were out of reach due to insufficient computing resources. Over the past 4 years simulations have evolved from 6x6x2 Mm size domains focusing on the details of umbral dots to simulations covering a pair of opposite polarity sunspots in a 100x50x6 Mm domain. In this talk I will discuss the numerical challenges encountered in comprehensive radiative MHD simulations of active regions and summarize the recent progress. Numerical simulations point toward a common magnetoconvective origin of umbral dots and filaments in the inner and outer penumbra. Most recent simulations also capture the processes involved in the formation of an extended outer penumbra with strong horizontal outflows averaging around 5 km/s in the photosphere. I will discuss in detail the magneto convective origin of penumbral fine structure as well as the Evershed flow. I will conclude with a brief summary of recent helioseismic studies based on realistic MHD simulations as well as inferences on the sub surface structure of sunspots.

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