European Solar Physics Online Seminar Archive

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The coupling of fast and Alfven magnetohydrodynamic (MHD) waves is of fundamental interest in astrophysical plasmas. Under certain conditions, Alfven waves can be resonantly excited by fast mode waves, resulting in a localised accumulation of energy in the plasma. In the solar community this is often referred to as resonant absorption, while in the magnetospheric community it's known as field line resonance. These processes have applications in coronal heating and in magnetospheric dynamics.Alfven resonances are well understood in 1D and 2D, but not so in 3D, particularly in non-Cartesian geometries. We present a theoretical way of understanding the structure and temporal development of Alfven resonances in 3D, which is corroborated by numerical simulations. [more]

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]

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]

Deep learning has emerged as a very powerful set of techniques to extract relevant information from observations, sometimes showing much better results that other set of finely tuned algorithms. In this contribution I present our efforts in applying deep learning to several problems in Solar Physics, from the estimation of horizontal velocities in the solar surface to fast image reconstruction. [more]

In this talk we will focus on diagnostic potential of the spectral region around D lines of Sodium. We will first outline our approach to non-lte inversions, and present a method for computation of response functions in non-local thermodynamic equilibrium. We will then discuss the sensitivity of Sodium D lines to the atmospheric parameters and present some example inversions of that spectral region. [more]

At the interface between the Sun's surface and million-degree outer atmosphere or corona lies the chromosphere. At 10,000K it is much cooler than the corona, but also many orders of magnitude denser. The chromosphere processes all magneto-convective energy that drives the heating of the million-degree outer atmosphere or corona, and requires a heating rate that is at least as large as that required for the corona. Yet many questions remain about what drives the chromospheric dynamics and energetics and how these are connected to the transition region and corona. The Interface Region Imaging Spectrograph (IRIS) is a NASA small explorer satellite that was launched in 2013 to study how the Sun's magneto-convection powers the low solar atmosphere. I will review recent results from IRIS in which observations and models are compared to study the role of small-scale magnetic fields in the generation of violent jets and how these jets feed plasma into the transition region and hot corona. [more]

We study small-scale brightenings in Ca II 8542 Å line-core images to determine their nature and effect on localized heating and mass transfer in active regions. To that end, we analyzed high-resolution 2D spectroscopic observations of an active region acquired with the GREGOR Fabry-Perot Interferometer attached to the 1.5-meter GREGOR telescope onTenerife, Spain. The ground-based data were complemented with AIA and HMI images from SDO. Inversions of the spectra were carried out using NICOLE. We identified three brightenings of sizes up to 2”x2”. We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or IRIS bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of about 20”. Moreover, the inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Bidirectional flows were present in the footpoints of the MF. [more]