Found 32 talks archived in The Sun

Tuesday May 4, 2021
Prof. Lyndsay Fletcher
University of Glasgow / University of Oslo


A solar flare involves the conversion of magnetic energy stored in the coronal magnetic field into the kinetic energy of thermal and non-thermal particles, mass motion, and radiation. How this happens remains a central question in solar physics. A particular long-standing puzzle is how such a high fraction of the stored magnetic energy - up to a half - arrives in the kinetic energy of accelerated non-thermal particles. In this talk I will present an observational overview of solar flares with an emphasis on accelerated particles, discuss some ideas and constraints on particle acceleration, and present some new observations of the possible role of plasma turbulence in the acceleration process.


Tuesday June 2, 2020
Prof. Manuela Temmer
Institute of Physics, University of Graz


The Sun is an active star that influences the Earth as well as the entire solar system. Most dynamic phenomena on the Sun are observed as coronal mass ejections (CMEs) and flares. CMEs present massive clouds of magnetized plasma having speeds up to a few thousand km/s, that may propagate over Sun-Earth distance within less than a day and may cause strong geomagnetic disturbances at Earth (Space Weather). As CMEs are optically thin, using remote sensing data measurements of intrinsic properties such as speed, width, propagation direction, density etc. are severely affected by projection effects. By combining image data with in-situ measurements, valuable information is provided enabling CME 3D analyzes, and with that facilitate a better quantification of the uncertainties in the observational measurements that are used to feed CME propagation models. With that, a much better understanding of CMEs as they propagate in interplanetary space could be gained.

The talk will cover the physisc about CME-flare phenomena, the interplanetary propagation behavior of CMEs related to the background solar wind, and Space Weather forecasting.

Zoom link:

Tuesday October 15, 2019
Dr. Sergio Javier Gonzalez Manrique
Astronomical Institute, Slovak Academy of Sciences


Emerging flux regions (EFRs) are seen as magnetic concentrations in the photosphere of the Sun. From a theoretical point of view, the EFRs are formed in the convection zone and then emerge because of magnetic buoyancy (Parker instability) to the solar surface. During the formation process of EFRs, merging and cancellation of different polarities occur, leading to various configurations of the magnetic field. Often, EFRs are visible in the chromosphere in form of magnetic loops loaded with plasma, which are often called “cool loops” when seen in the chromosphere along with dark fibrils and they can reach up to the corona. Nowadays, we refer to them as an arch filament system (AFS) which connects two different polarities.  The AFSs are commonly observed in several chromospheric spectral lines. A suitable spectral line to investigate chromospheric features and particularly AFSs is the He I 10830 Å triplet. The new generation of solar telescopes and instruments such EST and DKIST, will allow us to record very high spectral, spatial, and temporal resolution observations necessary to investigate the dynamics, magnetic field, and characteristics of AFSs. These observations will help us to answer many open questions related to flux emergence such: (1) What are the observational consequences of the emerging flux? (2) How do EFRs evolve with time in the different layers of the solar atmosphere and how are these layers linked? (3) Is it possible to measure the height difference between the photosphere and the chromosphere connected by the legs of the AFSs?

Thursday April 7, 2016
Dr. Iker S. Requerey


Solar surface convection displays highly localized sinks where cold plasma returns to the solar interior. On its way to being engulfed by a downdraft the plasma can also advect and intensify magnetic fields up to kG field strengths. Such theoretical predictions strengthen the idea that localized downdrafts are places where the concentration of magnetic fields is favored. The observational discovery of convectively driven sinks is rather recent, however, and its role in the formation and evolution of quiet-Sun magnetic features is still poorly characterized. In our work, we provide both quantitative and qualitative bases for the association between sinks and magnetic fields using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board Sunrise. We find 3.1 x 10-3 sinks Mm-2 minute-1 located at mesogranular vertices. These sinks are associated to (1) horizontal velocity flows converging to a central point and (2) long-lived downdrafts.  The spatial distribution of magnetic fields in the quiet Sun is also examined. The strongest magnetic fields are preferentially located at sinks. We find that 40% of the pixels with longitudinal component of the magnetic field higher than 500 G are located in the close neighborhood of sinks.  The study of individual examples reveal that sinks can play an important role in the evolution of quiet-Sun magnetic features.

Tuesday March 29, 2016
Mr. Adur Pastor Yabar
Instituto de Astrofísica de Canarias


In mid nineteens, it was discovered that the Sun had a dipolar global magnetic field, whose temporal evolution followed the Solar Cycle. Polar regions, as well as sunspots that appear in the activity belts, changed their polarity every 11 years: sunspots during each activity minima, and the poles in activity maxima. This fact, made people think that the poles reversal was related to the arrival of opposite polarity magnetic flux dragged from active regions by a meridional flow. Such new flux reduced the dominant polarity at the poles by cancellation, and built the opposite one until next minimum of activity. In our study, we have used the high quality full disc magnetograms, recorded by the HMI instrument onboard the SDO satellite since the beginning of the mission, in april 2010. We perform a deep study of the evolution with time of the line of sight component of the magnetic field at the solar poles. In our data, we see many aspects of the solar cycle as the decay of the dominant polarity of both poles while we approach to the activity maximum. But the main result is the detection of a monthly oscillatory pattern of the pole's magnetic field. Such oscillation, related to solar rotation is a clear evidence of a non-axisymmetric component of the magnetic field. One of the possible explanations is that the global field is tilted with respect to the rotation axis. This rather usual finding in other stars, here represents a breach of modern solar dynamo theories for the generation and maintenance of the Sun's magnetic field.

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.

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