European Solar Physics Online Seminar Archive

The analytical and numerical modelling of the behaviour of magnetohydrodynamic (MHD) waves in various magnetic geometries is a constantly evolving, active area of research within the field of solar magneto-seismology. Here, we present our findings on MHD wave propagation and instabilities in a family of asymmetric Cartesian waveguide models. Thanks to the introduction of various sources of asymmetry (background density, magnetic field or flow speed), this generalisation of classical (symmetric) slab geometries allows us to refine our modelling of several important features in the richly structured solar atmosphere. Including background asymmetry in these configurations influences the phase speeds and cut-off frequencies of the eigenmodes, and, in the case of flow asymmetry, it can also change the threshold for the onset of the Kelvin-Helmholtz instability. Furthermore, the asymmetric nature of the models allows us to develop solar magneto-seismologic tools and provide efficient methods for obtaining further information about the solar plasma from current and future high-resolution observations of multi-layered waveguides (such as e.g. magnetic bright points or light walls). [more]

Plasmoid-mediated fast magnetic reconnection plays a fundamental role in driving explosive dynamics and heating in the solar atmosphere, but relatively little is known about how it develops in partially ionised plasmas (PIP) of the chromosphere. Partial ionisation can largely alter the dynamics of the coalescence instability, which promotes fast reconnection and forms a turbulent reconnecting current sheet through plasmoid interaction, but it is still unclear to what extent PIP effects influence this process. In this talk, I investigate the role of collisional ionisation and recombination in the development of plasmoid coalescence: I will present 1D and 2.5D simulations of a two-fluid model of a partially ionised plasma (PIP) and show how the dynamics change in the presence and absence of ionisation and recombination processes. The aim is to understand whether these two-fluid coupling processes play a role in accelerating reconnection. In 1D calculations, as the current sheet collapses it drives a burst of ionisation. This results in the current of the current sheet growing at a slower rate than calculations without ionisation and recombination, and in a thicker current sheet. In 2.5D calculations, it is found that, in general, ionisation-recombination process slow down the coalescence. Unlike our previous models that included thermal collisions only, ionisation and recombination stabilise current sheets and suppress non-linear dynamics, with turbulent reconnection occurring in limited cases: bursts of ionisation lead to the formation of thicker current sheets, even when radiative losses are included to cool the system. Therefore, the coalescence time scale is very sensitive to ionisation-recombination processes. [more]

[more]

The solar corona temperature is maintained to more than 1 MK. One of the main theories of the coronal formation (Parker 1988) suggests that the energy is dissipated into the corona through a high number of impulsive, low energy (10²⁴ ergs) heating events, called “nanoflares”. On 30 May 2020, during its first high temporal and spatial resolutions observations, 1463 small (400 – 4000 km) and short lived (10-100 s) EUV brightenings were detected in the Quiet Sun by the high resolution UV imager HRI-EUV (174 Å) on board Solar Orbiter. These may be the signatures of nanoflare heating. As HRI-EUV is sensitive to both coronal and transition region emission, our goal is to verify if these brightenings indeed do reach coronal temperatures. As spectroscopic data were not available during the 2020 May observation, we applied the time lag method to the SDO/AIA coronal channels. The objective is to infer the thermal behavior of the events. Our results suggest two possible interpretations: either (1) the events peak below 1 MK, where the AIA response functions behave similarly, or (2) the events cooling time scale is below the AIA cadence of 12s. As spectroscopic observations should be able to clearly distinguish between both cases, we then use cotemporal Quiet Sun observations of Solar Orbiter HRI-EUV and SPICE, coordinated with Hinode/EIS, on 8 and 17 March 2022, and on 4 April 2023. We first detect the events in HRI-EUV, and identify them in SPICE or EIS. Temperature diagnostics using SPICE or EIS data confirm that these events are dominated by plasma below coronal temperatures. We conclude that these small (< 4 Mm) EUV brightenings detected by HRI-EUV are dominated by plasma at chromospheric or transition region temperature. As such, they hardly contribute directly to coronal heating. [more]

A new era of solar physics commences with observations of the quiet Sun using the 4-metre Daniel K. Inouye Solar Telescope/Visible Spectropolarimeter (DKIST/ViSP). We present full-Stokes observations taken during DKIST’s cycle 1, in the Fe I 630.1/630.2 nm lines, allowing us to examine small-scale magnetism in the photosphere. We use the Stokes Inversion based on Response functions (SIR) code to invert the Fe I line pair. We reveal the existence of a serpentine magnetic element for the first time. A statistical analysis is undertaken, comparing inversions of DKIST data with Hinode data. A novel machine learning technique is used to characterise and contrast the shapes of circular polarisation signals found in the ground-based and space-based data, and synthetic observations produced from MANCHA simulations are used to aid our understanding of the differences between datasets. [more]

The solar radiation in the cores of the Mg II h & k spectral lines strongly correlates with solar magnetic activity and global variations of magnetic fields with the solar cycle. This work provides a data-driven model of the temporal evolution of the solar full-disk Mg II h & k profiles over the solar cycle. Based on selected 76 IRIS near-UV full-Sun mosaics covering almost the full solar cycle 24, we find the parameters of double-Gaussian fits of the disk-averaged Mg II h & k profiles and a model of their temporal evolution parameterized by the Bremen composite Mg II index. The Markov Chain Monte Carlo algorithm implemented in the IDL toolkit SoBAT is used in modeling and predicting the temporal evolution of the Mg II h & k peak-to-center intensity ratio and the Bremen Mg II index. The relevant full-disk Mg II h & k calibrated profiles with uncertainties and spectral irradiances are provided as an online machine-readable table. To facilitate the utilization of the model corresponding routines, written in IDL, are made publicly available on GitHub.Co-authors: Stanislav Gunár (The Czech Academy of Sciences, Czech Republic), Pavol Schwartz (Slovak Academy of Sciences, Slovakia), Petr Heinzel (The Czech Academy of Sciences, Czech Republic; University of Wrocław, Poland), Wenjuan Liu (The Czech Academy of Sciences, Czech Republic) [more]

The solar atmosphere is filled with clusters of hot small-scale loops commonly known as Coronal Bright Points (CBPs). These ubiquitous structures stand out in the Sun by their strong X-ray and/or extreme ultraviolet (EUV) emission for hours to days, which makes them a crucial piece when solving the solar coronal heating puzzle. Here we present a novel 3D numerical model using the Bifrost code that explains the sustained CBP heating for several hours. We find that stochastic photospheric convective motions alone significantly stress the CBP magnetic field topology, leading to important Joule and viscous heating concentrated around the CBP’s inner spine at a few megameters above the solar surface. We validate our model by comparing simultaneous CBP observations from SDO and SST with observable diagnostics calculated from the numerical results for EUV wavelengths as well as for the Halpha line using the Multi3D synthesis code. Co-authors: Fernando Moreno-Insertis, Klaus Galsgaard, Kilian Krikova, Luc Rouppe van der Voort, Reetika Joshi, and Maria Madjarska [more]

I briefly review some methods and measurements of elemental abundances in the solar atmosphere, with emphasis on the transition region and corona. Some limitations in the methods, in the modeling of the spectral line intensities, and the observations are discussed. Examples from the X-rays, the EUV, the UV, the visible, and near-infrared are presented. A significant improvement in the modeling of some of the ions is being made available with CHIANTI version 11. All the observations indicate that the solar corona has photospheric abundances and that the hot 3 MK active region cores have stable enhancements of a factor of about 3.2 in the ratios of low to high-FIP elements. A lot of uncertainties and puzzles still exist, requiring further analyses and, more importantly, future instrumentation. [more]

We present the results of coordinated observations of the Swedish 1-m Solar Telescope with Solar Orbiter that took place from October 12th to 26th 2023. The campaign resulted in 7 datasets of various quality. The observational programs were adjusted to the seeing conditions. The observations cover two active regions and a coronal hole. We focus on the morphology and evolution of several targets that are observed from two vantage points. We share the lessons we learned and give an outline of our plans for October this year and the support we could give during remote sensing windows 16 and 17. [more]