European Solar Physics Online Seminar

Following an initiative by the University of Oslo the MPS will participate in the "European Solar Physics Online Seminar" series (ESPOS). Details can be found here: https://folk.uio.no/tiago/espos/
The aim of this video conference series is to promote ideas more widely with a specialized audience, and give some exposure to cutting-edge research for students and other young researchers that do not regularly travel to conferences. The ESPOS series is planned to take place every second Thursday at 11am.

Speaker: Gregal Vissers
Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. While some Ellerman bombs display clear UV burst signatures, not all have correlated UV signal or vice versa, suggesting the underlying atmospheric and magnetic properties may differ between events. As both are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide helpful insights in the overall evolution of active regions. Here we present results from observations and inversions of SST/CRISP and CHROMIS, as well as IRIS data of these transient events. At unprecedented spatial resolution the CHROMIS Ca II H & K observations reveal dynamic fine structure suggesting a plasmoid-mediated reconnection process. We investigate several cases, combining information from the Mg II h & k and Ca II 8542Å and H & K lines in order to infer the temperature stratification and magnetic field configuration within which these events occur. I’ll address the difficulties of successfully inverting their Si IV profiles and will discuss our results in light of the current debate on the connection between UV bursts and Ellerman bombs, their occurrence heights and in particular the temperatures that they may (or may not) reach. [more]

ESP Online Seminar: What can numerical simulations tell us about the mechanism of solar and stellar activity? (J. Warnecke)

ESPOS
The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. Also, other solar-like stars exhibit magnetic activity, most of them with much higher levels compared to the Sun. Some of these stars show cyclic modulation of their activity similar to the Sun. The rotational dependence of activity and cycle length suggests a common underlying dynamo mechanism.Here we present results of 3D MHD convective dynamo simulations of slowly and rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives a large-scale magnetic field. With the help of the test-field method, we are able to measure the turbulent transport coefficients in these simulations and therefore get insights about the dynamo mechanism operating in these simulations. It allows us to derive a scaling of the cycle period with the relevant effects of the dynamo.We discuss how magnetic helicity is a key quantity connecting the stellar convection zone with the stellar surface and stellar coronae. Magnetic helicity is produced in the convection zone of stars via a dynamo in the presence of convection and rotation. At the surface, it plays an important role in the formation process of active regions. In the corona, it is believed to be essential for the release of energy leading to the eruption of plasma via coronal mass ejections and is thought to play an important role in the heating process of the coronal plasma. Numerical simulations of stellar convection zones and the solar corona allow us to investigate this process. [more]
Propagating slow magneto-acoustic waves are regularly observed in the solar corona, particularly in sunspot related loop structures. These waves exhibit rapid damping as they propagate along the loops. Several physical and geometrical effects were found to produce the observed decay in the wave amplitude. It has also been shown that the damping is frequency dependent. A majority of the observed characteristics have been attributed to damping by thermal conduction in the solar corona. Although it is believed that these waves originate in the photosphere, their damping behaviour in the sub-coronal layers is relatively less studied. Using high spatial and temporal resolution images of a sunspot, we investigated propagation and damping characteristics of slow magnetoacoustic waves up to transition region heights. The major conclusions from this study will be discussed in the talk which include: 1) The energy flux in slow waves estimated from the relative amplitudes decays gradually right from the photosphere even when the oscillation amplitude is increasing. 2) The damping displayed by slow waves is frequency dependent well below coronal heights. 3) A spatial comparison of power spectra across the umbra highlights enhancement of high-frequency waves near the umbral center. [more]
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