Found 37 talks archived in The Sun

Tuesday February 16, 2016
Dr. Reza Rezaei


In the first part of the talk, I discuss the chromospheric activity of quiet Sun. The so-called basal flux, the minimum chromospheric emission of main sequence stars, is discussed in terms of magnetic and non-magnetic heatings. The second half of the talk is about variability of the solar cycle. The extended minimum of the last cycle caused speculations about a possible long-term decline in the solar activity. Based on our observations in past 15 years, I argue that the Sun does not cease to generate sunspots in the next cycle. Finally I outline new tools to evaluate chromospheric activities.

Wednesday November 18, 2015
Dr. Judith T. Karpen


What does space weather really mean, and why should scientists and
non-scientists be interested in it? I will discuss what we know and don’t
know about the causes and consequences of space weather, how the
fundamental physics of our heliosphere also applies to astrophysical
objects and exoplanetary systems, and how the lessons learned affect our
technology on the ground and in space.

Wednesday June 3, 2015
Prof. Paul S. Cally
School of Mathematical Sciences de la Monash University (Melbourne, Australia)


The Sun is a magnetic star, not as magnetic as some stars, or as it was when it
was younger, but nonetheless magnetic fields dominate and even construct its
atmosphere. There would be no corona without magnetic fields. The surface is
also dappled with small scale magnetic field associated with surface convection
cells, granules and supergranules. But sometimes we also see much larger and
more powerful Active Regions containing sunspots. These are wounds in the
surface of the Sun that allow waves and oscillations in the solar interior and
atmosphere to be coupled much more directly than they usually are. In
particular, they allow the Sun's internal seismology (the p-modes) to drive a
variety of waves through the Active Region atmosphere, and conversely, the
atmospheres to pollute the internal seismology. This makes active region
helioseismology a very challenging field.

Tuesday February 24, 2015
Dr. Chloé Guennou


Progress in our understanding of the Solar corona and its underlying heating and acceleration processes, depends critically on our ability to measure fundamental plasma parameters, such as magnetic field, density, temperature and composition. In this talk, I will introduce my main research topics which concern measurement of fundamental plasma parameters in the Solar corona and the development of new plasma diagnostic tools, in order to provide constraints for the various proposed physical mechanisms.

Coronal observations are always integrated along a line of sight (LOS). Because there may be multiple emitting sources, this considerably complicates the interpretation of the observations. To avoid this ambiguity there are several tools, including the widely used Differential Emission Measure (DEM) analysis and the tomography reconstruction technique. However, both the derivation and the interpretation of the DEM from observations are difficult mainly due to the inverse nature of the problem. I will present a new strategy to evaluate the robustness of the DEM inversion problem. An application of the DEM formalism will be presented, allowing us to measure the relative abundances in both interplumes and plumes using Hinode/Extreme Ultraviolet Imaging Spectrometer (EIS) data. Finally, I will present the inversion code I developed, able to perform the coupling of the DEM formalism and the tomography, providing a three dimensional diagnostic in temperature and density of the solar corona.

Thursday February 19, 2015
Dr. Jovo Vranjes


The lower solar atmosphere is very weakly ionized, and by conductivity it is comparable to the sea water. The collisional frequency for electrons and ions can be over 10^10 Hz and 10^9 Hz, respectively. This implies that particles may not be magnetized and are thus unaffected by the magnetic field. In this talk I shall present accurate collision cross sections and collision frequencies for electrons, protons and hydrogen atoms, and the corresponding transport coefficients for layers with both unmagnetized and magnetized particles. The cross sections include many essential effects like charge exchange, quantum-mechanical in-distinguishability at low energies, polarization of neutral atoms by external charges, and dependence on energy of colliding particles. The effects of collisions on Alfven waves will also be discussed.

Thursday February 5, 2015
Dr. Nataliia Shchukina
Main Astronomical Observatory, National Academy of Sciences, Kyiv


The solar abundance of chemical elements play an important role in addressing such important issues as the formation, structure, and evolution of the Sun and the solar system, the origin of the chemical elements, the evolution of stars and galaxies. Despite the large number of papers published on this issue, debates about the solar composition of the Sun continue. In this talk we start summarizing the current understanding of the solar abundances of iron and CNO elements, which play a crucial role on the determination of the solar metallicity. We then pay especial attention to the impact of the quiet Sun magnetism on the determination of the abundances of these elements. The solar photosphere is significantly magnetized, due to the ubiquitous presence of a small-scale magnetic field whose mean strength is thought to be of the order of 100 gauss. Here we address the problem of the determination of the abundances of chemical elements taking into account the significant magnetization of the quiet Sun photosphere. To this end, we use 3D models of the quiet solar photosphere resulting from a state-of-the-art magneto-convection simulation with small-scale dynamo action where the net magnetic flux is zero. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo,then its impact on the determination of the solar abundance of iron and CNO elements is negligible.

Thursday December 11, 2014
Prof. Mats Carlsson
Institute of Theoretical Physics, University of Oslo


Magnetic fields break through the solar surface in a hierarchy of magnetic elements ranging from Earth-sized sunspots down to tiny concentrations that are barely resolved in the highest-resolution photospheric images. In the chromosphere they combine in intricate, highly dynamic, and continuously evolving fibrilar patterns. Movements of the photospheric field-line footpoints drive, guide, and control the flows of energy and mass into the corona, and trigger energy-releasing magnetic reconnection through relentless topological rearrangement. The conversion from convectively driven footpoint motion to outer-atmosphere outflows and loading takes place in the dynamic, fine-structured chromosphere.

A number of important facilities for observing the solar chromosphere have recently come on line (e.g. the SDO and IRIS satellites and ground-based Fabry-Perot interferometers) or will become operational in the near future (e.g. DKIST). The overwhelming complexity of the chromosphere makes it necessary to have numerical simulations for the interpretation of the observations. Such realistic simulations, spanning the solar atmosphere from the convection zone to the corona, are now becoming feasible.

This presentation will introduce the fascinating aspects of chromospheric physics and review recent results from both observations and numerical simulations.

Thursday December 4, 2014
Dr. Klaus Galsgaard
Niels Bohr Institute , Copenhagen, Denmark


Recent observations of the solar atmosphere have provided new insights concerning medium-sized jet phenomena taking place in the solar corona. These jets are magnetically controlled and typically take place in regions where the mean magnetic field has an open structure. Observations indicate that at least two different types of jets exist. A simple jet that generally has a near steady state evolution phase with a well behaved and collimated outflow stream. The second type typically combines the characteristics of the first type with an explosive behaviour that significantly changes the topological structure of the jet outflow. Models have attempted to provide physical explanations to the observations, and are in general able to capture a number of the observational characteristics. This talk will discuss both the observations and the models, emphasizing where we succeed and where new progress is need

Tuesday October 28, 2014
Dr. Alfred G. de Wijn
High Altitude Observatory of the National Center for Atmospheric Research


The Chromosphere and Prominence Magnetometer (ChroMag) is a synoptic  instrument with the goal of quantifying the intertwined dynamics and  magnetism of the solar chromosphere and in prominences through imaging  spectro-polarimetry of the full solar disk in a synoptic fashion. The  picture of chromospheric magnetism and dynamics is rapidly developing,  and a pressing need exists for breakthrough observations of  chromospheric vector magnetic field measurements at the true lower  boundary of the heliospheric system. ChroMag will provide measurements  that will enable scientists to study and better understand the  energetics of the solar atmosphere, how prominences are formed, how  energy is stored in the magnetic field structure of the atmosphere and  how it is released during space weather events like flares and coronal  mass ejections. An essential part of the ChroMag program is a commitment  to develop and provide community access to the `inversion' tools  necessary to interpret the measurements and derive the  magneto-hydrodynamic parameters of the plasma. Measurements of an  instrument like ChroMag provide critical physical context for the Solar  Dynamics Observatory (SDO) and Interface Region Imaging Spectrograph  (IRIS) as well as ground-based observatories such as the future Daniel  K. Inouye Solar Telescope (DKIST). A prototype is currently deployed in  Boulder, CO, USA. We will present an overview of instrument design and  capabilities, show some recent observations, and discuss the future of  the project.

Wednesday March 19, 2014
Prof. Rob Rutten


The chromosphere is the interface between the photospheric solar surface and the outer corona and wind.  In this complex domain the
solar gas becomes transparent throughout the ultraviolet and in the strongest spectral lines while magnetic pressure becomes dominant over gas pressure even in weak-field regions.  Fine-scale magnetically caused or guided dynamic processes in the chromosphere constitute the roots of mass and energy loading of the corona and solar wind. Notwithstanding this pivotal role the chromosphere remained ill-understood after its basic NLTE radiation physics was formulated in the 1960s and 70s.  Presently, both chromospheric observation and
chromospheric simulation mature towards the required sophistication.  The open-field features seem of greater interest than the easier-to-see closed-field features. For the latter, the grail of coronal topology and eruption prediction comes in sight.

I will start with an introductory overview, show movies to present the state of art in observation and simulation, and treat some
recent success stories in more detail.