KIS Astrophysical Colloquium 2019

The colloquium usually takes place on Thursdays at 11:30 if not stated otherwise.

Upcoming Talks:
April 25, 2019 Säm Krucker, University of Applied Sciences Northwestern Switzerland & Space Sciences Laboratory, UC Berkeley: Hard X-ray Observations as Diagnostics of Particle Acceleration in Solar Flares
Solar flares are powered by an impulsive release of magnetic energy stored in the solar atmosphere. The release of magnetic energy is heating coronal plasma, but as much as half of the released energy goes into particle acceleration. The acceleration mechanisms that provide these efficient conversions of magnetic energy into supra-thermal particles are currently not well understood. In the past years however, significant progress has been made on the observational side; thanks in particular to hard X-ray observations by Reuven Ramaty High Energy Spectroscopic Imager (RHESSI), a NASA small explorer mission. After an introduction, I will review recent observational results obtained by RHESSI, followed by a discussion on future hard X-ray instrumentation with an emphasis on hard X-ray focusing optics.
May 09, 2019 Kolloquium zu Ehren von Wolfgang Schmidt
..
May 16, 2019 Yvonne Elsworth, University of Birmingham, UK:
...
May 23, 2019 Dora Musielak, University of Texas at Arlington, USA:
...
May 28, 2019 Zdzislaw Musielak, University of Texas at Arlington, USA:
...
June 6, 2019 Petri Käpylä, Max-Planck-Instituf für Sonnensystemforschung, Göttingen:
...
November 28, 2019 Achim Weiss, Max-Planck-Institut für Astrophysik, Garching:
...
Past Talks:
January 31, 2019 Antonio Ferriz Mas, University of Vigo, Spain: Magnetic Helicity: From knot theory to solar pyhsics
The linking number or "Verschlingungszahl" is an integer invariant that describes the linking of two closed curves in 3-D space. It was introduced by Gauss in the form of a double line integral and it is one of the oldest topological results. In the first part of the talk I will show, using Differential Geometry, that the linking number and Gauss' double line integral are at the heart of the definition of helicity, a key concept in Topological Fluid Mechanics with wide applications in solar magnetism. In the second part of the talk I will address the question under which circumstances the kinematic (hydrodynamic) and the magnetic (MHD) helicities are conserved quantities; helicity conservation is determined by the physics of the problem and is no longer a purely mathematical question.
February 21, 2019 Joten Okamoto, NAOJ Tokyo, Japan: The strongest magnetic fields in sunspots and their statistical properties
Sunspots are concentrations of magnetic fields on the solar surface. Then, where is the strongest field in each sunspot ? It is generally located in an umbra, but sometimes stronger fields are found outside umbrae, such as a penumbra and a light bridge. The formation mechanism of such strong fields outside umbrae is still puzzling. Now we have numerous high-quality datasets taken with the Hinode/Spectro-Polarimeter over 10 years, which motivate us to address this question via a statistical analysis of strongest fields in sunspots. Hence, we complied a ranking list of active regions by their largest field strengths and investigated conditions for appearance or formation of strong magnetic fields. In this seminar, we will introduce a sunspot with a field strength of 6250 G as a case study, and then discuss the key features to produce strong fields in a statistical sample.
March 28, 2019 Stefan Hofmeister, Universität Graz, Austria: Coronal Holes
Coronal holes are large-scale structures in the solar corona characterized by a low density, temperature, and an open magnetic field topology. Although they were already discovered in the 1970s in the Skylab epoch, many of their properties still remain unclear. In particular, the source of their unbalanced magnetic flux, the cause for their open magnetic field topology, their comparably low temperature, and their unusual rotation rates are still not well understood. In this presentation, I will discuss the formation of a coronal hole from a filament eruption which took place at the solar disk center. This unique position allows us to study its formation and early evolution without projection effects or being out-shined by its surrounding. Thereby, we will find that the magnetic field of coronal holes seems not to be strongly bound to their photospheric foot points, but that the coronal hole changes its position shortly after its formation likely by interchange reconnection. Further, I will show by a statistical study using SDO/HMI data that the unbalanced magnetic flux of coronal holes arises for 80% from the large, long-living magnetic elements with lifetimes >40 hours, and for 20% from an apparent background magnetic field. The area of these long-living magnetic elements alone define the unbalanced magnetic flux of the coronal holes, whereas the shorter-living magnetic elements do not contribute significantly to the unbalanced magnetic flux. Since the long-living “plage” magnetic elements appear not only below coronal holes but all over the Sun with usual rotation rates, and since the coronal holes seem to not be strongly bound to them, we suggest that the unusual rotation rates of coronal holes are not related to a connection deep to the solar interior, but that they simply follow the global magnetic field configuration.