KIS Astrophysical Colloquium 2019

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

Upcoming Talks:
July 18, 2019 Philippe Bourdin, Institut für Weltraumforschung, Graz, Österreich: Traces of magnetic helicity from the photosphere to the solar wind
Magnetic fields on the Sun and in the solar wind usually have a complex topology. Complex magnetic structures are responsible for many features seen on the Sun, like sunspots, active regions, prominences, flares, and coronal mass ejections (CMEs). The helicity of solar magnetic fields could play an important role for the local dissipation of currents and the onset of eruptive events. For example, the heating in the corona does not only depend on simple quantities like the magnetic flux or the current density, as we find from large 3D-MHD simulations. We will discuss how magnetic helicity may be generated in the corona by photospheric magnetic fields. The sign of the helicity may even change, e.g., along a bright coronal loop, which is possible by the decoupling of magnetic fields through reconnection. Typically, this happens at such layers in the solar atmosphere where plasma beta is near unity. We apply a novel method based on the electromotive force to detect the helicity during inter-planetary CMEs that are observable by spacecraft like Helios, NASA's Parker Solar Probe, or ESA's SolarOrbiter. We are able to find signatures of the coronal helicity in the plasma that is eventually dragged through the heliosphere with the solar wind. The effectiveness of CMEs to modify the Earth magnetosphere may be modulated by the sign and amplitude of the magnetic helicity.
September 26, 2019 Klaus-Peter Schröder, Universidad de Guanajuato, Mexiko: Magnetic activity with giant stars: between magnetic breaking and revival
By contrast to coronal X-ray detections, chromospheric emission measures seem to be a less biased indicator for magnetic activity among cool giant stars. We review the legacy of Mount Wilson "S-index" observations, and together with our own chromospheric activity monitoring data of bright, cool giants from the robotic telescope project TIGRE in Guanajuato, MEX, we put magnetic activity among giant stars into context with stellar evolution. We show that (1) despite huge -compered to the Sun- convective envelopes, activity is a common phenomenon among cool giant stars. (2) After magnetic breaking on the main sequence and a first revival in the Hertzsprung gap, giant activity suffers from magnetic breaking again during central Helium burning (as the 4 Hyades K giants demonstrate), until (3) on the AGB chromospheric emission is, surprisingly, again on the rise. Consequently, activity revivals seem to coincide in the HRD with phases of fast core contraction.
November 7, 2019 Teimuraz Zaqarashvili, Universität Graz, Österreich:
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.
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
14:00 Empfang
14:25 Peter Caligari & Rolf Schlichenmaier (Bläserduett)
14:30 Begrüßung durch Oskar von der Lühe
14:40 Ehrenvortrag: Alan Title
15:45 Kaffeepause
16:10 Peter Caligari & Rolf Schlichenmaier (Bläserduett)
16:15 Gemischtes Programm:
Michael Stix: Grußwort
Eberhard Wiehr: Aufbau Teneriffa
Anekdoten, Geschichten, Grußbotschaften, ....
17:15 Aperitif & Musik (Bier, Brezel & Gesang)
May 16, 2019 Yvonne Elsworth, University of Birmingham, UK: Inferences from oscillations of low-mass stars
In this talk I will give a brief introduction to the phenomenon that allows us to directly probe the interior of low mass stars - namely the fact that they show global oscillations. The oscillations are driven by convection in their outer convection zones. I will then explore two applications of this. In the first I will look at what we can say about the distribution of the magnetic field on the surface of distant stars. The ideas will be tested on the Sun for which we have image data. The second application is to how we can determine if an (evolved) red giant star has ignited the helium in its core. This opens up many areas including galactic archaeology.
May 23, 2019 Dora Musielak, University of Texas at Arlington, USA: Trajectories for Space Telescopes Searching for Exoplanets
The technical requirements for space telescopes that can detect Earth-sized exoplanets demand the selection of optimal orbits to facilitate the needed observations. Fortunately, we can use the Sun, Earth, and Moon to form trajectories that meet those requirements. In this talk, I will present an overview of spacecraft trajectories in the Sun-Earth-Moon space, focusing especially on those appropriate for exoplanet hunting missions. One trajectory type is the high Earth orbit (HEO), which was chosen for the Transiting Exoplanet Survey Satellite (TESS), whose mission objective is to survey over 85% of the sky, an area of sky 400 times larger than what the Kepler telescope observed. I will also discuss a heliocentric orbit called halo orbit that uses a libration point, which is an optimal near-Earth space location for the Wide Field Infrared Survey Telescope (WFIRST) and the James Webb Space Telescope (JWST). The mission objectives of these space observatories are to search for exoplanets while studying dark matter, and to study atmospheres of known exoplanets, respectively. I will conclude with an overview of a halo orbit that would put a spacecraft on a radio-quiet zone located on the farside of the Moon to carry out other space exploration missions.
May 28, 2019 Tuesday Zdzislaw Musielak, University of Texas at Arlington, USA: Detection and Habitability of Exomoons
Despite more than 4000 exoplanets already discovered, so far all attempts to detect first exomoon have failed. The current status of searches for exomoons will be reviewed with some emphasis on our own efforts to find exomoons through detection of radio waves generated by the interaction of a tidally heated exomoon with its host Jupiter-like exoplanet. Tidally heated Jupiter's planetary habitable zone for the Galilean moons will be defined and the concept will be extended to some exoplanetary systems. Habitability of exomoons in those systems will be discussed and best candidates for future searches for exomoons will be selected.
June 6, 2019 10:00 Nishant Kumar Singh, Max-Planck-Institut für Sonnensystemforschung, Göttingen: On the possibility of using the solar f-mode to detect subsurface magnetic field
The Sun supports a wide variety of waves that carry useful informations about the inhomogeneous solar structure. The helioseismic modes must be sensitive to the evolving magnetic fields of the Sun. Based on numerical experiments of the surface gravity or the f-mode, we find that it is significantly perturbed by magnetic fields below the surface, and is sensitive to the morphology and depth of such magnetic concentrations. We argue that detections of such f-mode perturbations could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions (ARs) or sunspots. We will also discuss our findings on the f-mode strengthening about two days prior to AR formation, based on the analysis of observed data from HMI.
June 6, 2019 11:30 Petri Käpylä, Max-Planck-Institut für Sonnensystemforschung, Göttingen: From CPU cycles to stellar cycles
Explaining the differential rotation and magnetic cycles of stars is one of the main unsolved problems in stellar physics. While simplified models relying on averaged equations of magnetohydrodynamics can reproduce many observed features, the included physics are poorly constrained and subject to finetuning and cherry-picking. More realistic models solving the equations of magnetohydrodynamics in three dimensions from first principles struggle to reproduce many observed features such as the internal rotation profile of the Sun or solar-like equatorward migration of activity. A likely culprit is that the numerical resolution of the simulations is still too coarse to resolve all of the physically relevant scales. In this talk the fundamental numerical challenges related to stellar dynamo simulations are first briefly discussed. Second, recent results from studies of solar and stellar differential rotation and magnetism using high resolution 3D magnetohydrodynamic simulations are reviewed. Finally the most important outstanding issues in modeling stellar magnetism are discussed.