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:
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.
Room: Hydra

ESP Online Seminar: Magnetic field variations associated with umbral flashes and penumbral waves (J. Joshi)

Oscillations in sunspots have been extensively studied for several decades. Most of the research conducted about sunspot oscillations has focussed around variations in Doppler velocities and intensities. Fewer observational studies have focused on variations of the magnetic field in the photosphere, reporting contradicting results. Recently, variations in the magnetic field strength up to ∼200 G associated with running penumbral waves (RPWs) in the chromosphere have been reported. In this study, we analyze variations in the magnetic field associated with umbral flashes (UFs) and RPWs. We use spectropolarimetric observations recorded with CRisp Imaging SpectroPolarimeter (CRISP) mounted at Swedish 1-m Solar Telescope (SST). We have obtained the photospheric and chromospheric magnetic field of a sunspot by performing inversions of the Fe I 6301.5 & 6302.5 Å and the Ca II 8542 Å spectral lines, respectively, with the non-LTE inversion code NICOLE. Our results do not show any significant variations in the magnetic field strength in the photosphere. At chromospheric layers, UFs indicate peak-to-peak variation of ∼275 G, whereas in RPWs variations inthe amplitude of the magnetic field strength are reduced to ∼100 G. Variations in the magnetic field in UFs and RPWs are correlated to the variations in the temperature. In the past, many authors have suggested that observed temporal variation in the photospheric magnetic field of sunspots could be an effect of changing opacity due to oscillations in thermodynamical parameters. We analyzed changes in the geometrical height scale of inferred magnetic field due to oscillations in the thermodynamical parameters. Our results suggest that the observed variations in the umbral and penumbral chromospheric magnetic field can not be explained only by opacity changes caused by these propagating shocks. Hence, we conclude that the observed magnetic field variations associated with UFs and RPWs are intrinsic in nature. [more]

ESP Online Seminar: Formation of the penumbra and start of the Evershed Flow (M. Murabito)

Penumbral formation is a significant part of the flux emergence process. Despite the new advanced techniques in observations and simulations, there are still processes that need to be clarified. In particular, two aspects have been carefully investigated: whether there is a preferred location where the penumbra starts to form and how the Evershed flow sets in. Recent observations by Schlichenmaier et al. (2010) show that the penumbra forms in sectors and that the area between the two polarities prevents the settlement of a stable penumbra. Using high-resolution spectropolarimetric data acquired by IBIS, as well as HMI data, we studied penumbral formation in NOAA active region 11490. The results for the leading polarity show that the onset of the classical Evershed flow occurs in a very short time scale (1-3 hours) while the penumbra is forming. In addition, we observed a clear evolution from redshift to blueshift in the penumbral filaments in about 1 hour. Studying the formation of the first penumbral sector around the following pore, we found that a stable penumbra forms in the area facing the opposite polarity, located below an AFS, i.e. in a flux emergence region, in contrast with the results of Schlichenmaier et al. (2010). Finally, analysing six active regions, we find no preferred location for the formation of the first penumbral sector and we observe the appearance of an inverse Evershed flow that changes sign when the penumbra appears. [more]
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