Found 8 talks width keyword solar atmosphere
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.
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.
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.
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.
Flares are among the most energetic magnetic solar phenomena. They are often accompanied by ejections of charged particles, which have a direct influence on the Earth in terms of Aurora or radio and satellite outages. The sudden nature of flares - some of them only last minutes - makes them an elusive feature when observed from ground-based telescopes. These measurements are especially challenging when we focus on magnetic fields and velocities in the different solar layers where flares develop and occur. I will present flare observations taken with different instruments, each targeting different observables, and I will show what we can learn from ground-based polarization measurements.
Total spectral irradiance is typically modeled by assinging an atmospheric model to each pixel of a full disk image and geometricllay combining the predicted wavelength dependent intensity for each of these models into a disk integrated spectrum. This works reasonably well, as the hydrostatic models that are used in this procedure generally reproduce observed spectra very well. However, for numerical expedience this scheme neglects some important physical aspects of the the solar atmosphere, in particular its three-dimensional and strongly dynamic nature. In this talk I will discuss the importance of some of these effects on the spectral irradiance signal, using forward radiative transfer modeling in realistic three-dimenional simulations. Obviously, modeling the three-dimensional dynamic structure over the whole disk is computaionally prohibitive, but if some of the effects discused above are important, strategies will have to be implemented to incorporate them approximately. Characterizing these cotributions to the spectral irradiance will also help us to better understand the physical nature of the forces that drive variability, and hopefully improve our predictive capabilities.
The coronal heating problem has been with us for almost 70 years now. Among the different proposed explanations, wave-based heating mechanisms are recurrently invoked. In the last decade, a wealth of high resolution observations have shown that wave-like dynamics is present at almost all layers of the solar atmosphere. As a consequence, a renewed interest has grown on their role in plasma heating mechanisms. We will discuss a series of aspects related to the current status of MHD wave heating of the solar corona. The talk will focus on the following ones: a) recent observational discoveries of waves and their relevance to the heating problem; b) our theoretical understanding on their nature and properties; c) our current level of comprehension of the sequence of physical processes that link oscillations with dissipation and heat conversion; and d) the merits and faults of current theories, including suggestions for the way forward in both theory and observations.
Solar magnetism may look deceptively boring (a rather common star with relatively low activity). As it turns out, even the most quiet areas of the Sun (away from the sunspots) harbour a rich and interesting magnetic activity which is extremely complex and dynamic at spatial scales as small as ~100 km. And more importantly, this magnetism permeates most of the Sun, all the time. Therefore, it is not surprising that it might play an important role for solving some longstanding questions of stellar magnetism as: how is the million degree corona maintained when all sunspots have disappeared during the minimum of magnetic activity? And this is of interest not only for solar physics but for stellar astrophysics too, since it is expected that every star with a convective envelope harbours small-scale magnetic activity that we cannot hope to observe with the great detail we observe it in the Sun. From the first evidence of the presence of magnetic fields in the quiet areas of the Sun to the discovery of the smallest organised magnetic structures ever observed in a stellar surface just 30 years have passed. In this seminar, I will give an overview of our present knowledge about the small-scale quiet Sun magnetism. In particular, I will show how small loops of sizes of several hundreds of kilometers appear in the surface and travel across the solar atmosphere, reaching upper layers and having direct implications on chromospheric (coronal) magnetism. I will also show some of the properties of these newly discovered magnetic structures such as their spatial distribution, a key ingredient for understanding their origin.
« Newer Older »
- TBDDonaji Esparza ArredondoTuesday September 17, 2019 - 12:30 (Aula)
- COLLOQUIA: TBDProf. Michael KramerThursday October 3, 2019 - 10:30 (Aula)