European Solar Physics Online Seminar Archive

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]

Observations of small-scale brightenings in the low solar atmosphere can provide valuable constraints on possible heating and heat transport mechanisms. We present a method for the detection and analysis of bright points (BPs), and demonstrate its application to time-series imagery of the Interface Region Imaging Spectrograph (IRIS) in the extreme ultraviolet. The method is based on spatio-temporal band-pass filtering, adaptive thresholding and centroid tracking, and records an event’s spatial position, duration, speed, total brightness, maximum brightness, and intrinsic brightness. Spatial area, brightness, and position are also recorded as functions of time throughout the event’s lifetime. Detected brightenings can fragment, or merge, over time – thus the number of distinct regions constituting a brightening event is recorded over time, and the maximum number of regions recorded as Nfrag, which is a simple measure of an event’s coherence or spatial complexity. The method is first tested on synthetic data based on Poisson statistics before being applied to real IRIS data. We present statistical characteristics of brightenings from the application of this method to 1330, 1400, and 2796 Å IRIS slit-jaw image time series. Several thousand events are recorded that coexist in all three channels, giving high confidence that they are real. Finally, we will also present continuing applications of this detection method to analyse a large set of BPs and their characteristics – over 12,000 BPs in total – and compare those that are found within ‘Active’ and ‘Quiet’ domains within a QS region, as well as possible future applications of the detection method. [more]

It is well understood that the dynamics of sunspots lead to energy being transferred to the solar atmosphere and stored in the coronal magnetic field. This provides a surplus of energy that may be released in solar eruptions. The driving mechanisms for this energy transfer may include sunspot rotations, both within individual sunspots and between sunspot pairs. Calculation of the rotations of individual sunspots have been carried out by several authors, but studies of the rotation of sunspot pairs has been less systematically investigated.Calculation of rotations in either case rely on careful tracking of the sunspots from observation to observation. Identification and tracking of sunspots is therefore essential to understanding the energies in play that lead up to solar eruptions. To date, this has predominantly been done manually which has restricted many studies to being a small number of case studies rather than large statistical samples. In order to construct large samples, the careful tracking of sunspots must be automated.We present a fully automatic method to identify and track sunspots in long sequences of data from the Solar Dynamics Observatory Helioseismic and Magnetic Imager (SDO/HMI) at a high temporal resolution. This includes registering the splitting and merging of sunspots, and allocating sunspots to active regions. This information can be fed into algorithms to measure the rotation of individual sunspots or used to calculate the relative motion of sunspots with respect to each other (including co-rotation).The method is applied to a four-month data set that has previously been analysed using a semi-automatic method where the basic sunspots were identified by eye, and the results are compared to determine any differences between the methods. From this data, sunspot dynamics such as sunspot rotation, shearing and merging are calculated, alongside sunspot pair interactions. Case studies of successfully tracked sunspots will be presented, showing examples of the individual sunspot rotations and some initial results involving sunspot pair interactions with correlations to solar activity. [more]

The prediction of geomagnetic storms is becoming more and more important, with the aim to take effective measures for avoiding the possible damage from the extreme events. One of the important parameters when modeling CMEs and CME-driven shocks, is their arrival time at Earth. We present a study of several halo CMEs with the propagation direction which significantly deviated from the Sun-Earth line and as a result, CMEs impacted Earth as flank-encounters. We modeled selected events with the default-setup of EUHFORIA and the Cone model for the CMEs. The aim of our study is to better understand the importance of the CME’s direction of propagation in the input parameters of the Cone model and improve the modeled arrival time at Earth. We selected events that were propagating strongly non-radialy in the low corona, in order to understand how important are the effects of the deflections in the low corona, in the direction of propagation. Our results show that, when the data from the DONKI database are used, the modeled arrival time has the largest discrepancy(≥10h) when compared with observations. When the input parameters are taken employing the GCS fitting technique though, up to the height of 12 Ro (solar radii), the accuracy of the modeled arrival time improves, shifting closer to the observed ones. This result reflects the characteristic that, up to the heights of about 10 Ro, CMEs experience all the low coronal deflections and have taken their final direction of propagation. [more]

Strong magnetic skew characterizes several non-potential structures in the solar corona, including filaments, which can erupt, bringing about major geomagnetic effects. Skew is thus an indicator of the formation of filament channels and a useful property in making forecasts from global coronal models. For skew to be used in this way, its formation and evolution must be identified, with particular consideration of when this occurs within the field of view observable from Earth. Magnetofrictional simulations are performed, with the photospheric boundary condition provided by an observed flux distribution. New techniques are presented for identifying where strong skew forms, and the production and persistence of strong skew in the corona studied systematically. Strong skew is found to form incrementally, in small patches, especially around recently emerged bipolar regions. As such, skew is produced and persists over very long periods of time, comparable with the timescale for solar rotation. Consequently, most skew on the Earth-facing side of the Sun rotates onto it, having formed on the far side, rather than forming within the Earth’s field of view. Therefore, in a data-driven model that is limited to including the emergence of flux observed only from Earth, most of the resultant skew will form after this flux has rotated out of view and onto the far side of the Sun. Given this, it will be important to study how a wider field of view for capturing emergent flux before it rotates into longitudes visible from Earth, afforded by multi-viewpoint observations, would improve our capacity to model and predict Earth-facing phenomena on the Sun, which are important for space weather. Possible effects of multi-viewpoint observations on similar models of the formation of skew are considered. Co-authors: D. H. Mackay, and L. A. Upton [more]

Solar observations have often served as benchmarks of stellar conditions. A particularly illustrative example of the above link is given by the observations in the Ca II K and H lines at 393.367 nm and 396.847 nm, respectively, which are the two deepest and broadest absorption lines in the visible spectrum of the Sun. Although widely observed over the years, several aspects of the emission of these lines are however still not fully understood. This is the case of e.g. the exact relationship between Ca II K emission and magnetic field strength. To the aim of reassessing this relationship, we analyzed state-of-the-art observations of the solar atmosphere obtained at the Swedish Solar Telescope with the CRisp Imaging SPectropolarimeter and with the CHROMospheric Imaging Spectrometer on regions characterized by a different ambient magnetic field. On these observations we analyzed the dependence of the Ca II K line brightness, as well as the relationship between Ca II K emission and magnetic field strength on different surrounding conditions of the solar atmosphere and characteristics of the observations, such as spectral bandwidth and spatial scale. The data and methods employed, and results achieved by our analysis will be presented, with emphasis on their impact on the interpretation of previous findings in the literature and application in future studies. [more]

Sustained kink oscillations in coronal loops have long been observed in TRACE, SDO/AIA, and more recently in SolO/EUI images. Although their properties are quite well-known now, their driver and excitation mechanism remain under active debate. In this talk I will give an overview over the different ideas/theories that discuss the role of photospheric driving in the generation of kink oscillations. We exploited an unique dataset of high-resolution coronal and photospheric observations taken recently by SolO/EUI/HRI and the Swedish 1-m Solar Telescope (SST) respectively during a dedicated coordinated campaign run in October 2023. Using the SST/CRISP data we estimated and quantified the strength of photospheric driving at the footpoints of active region coronal loops, that include pore, plage, enhanced-network and sunspot regions. We then looked at kink oscillation signatures in the same coronal loops within the EUI/HRI coronal images. An attempt was then made to link the photospheric and coronal results together. I will finally discuss the implications of this work on the driving and excitation mechanism of kink oscillations, and future perspectives. [more]

Solar filaments (also called prominences when seen off-disk) are solar atmospheric structures consisting of dense, cool plasma clouds floating within the sun's corona. Since the beginning of solar observations, it has been seen that the prominences oscillate with a wide variety of motions. These periodic motions are very common, but there are no systematic studies of these oscillations. It has recently been shown that spectral analysis of solar filaments is a powerful tool to identify oscillations in these structures. With this technique, the power spectral density (PSD) is calculated for each pixel of the Halpha images. To differentiate between a detection or a spurious oscillation, it is necessary to determine the background noise. We have seen that this background noise is a combination of red and white noise. The red-noise nature of the PSD is problematic in their study since most of the statistical tools developed to identify real oscillations from the noise are for white-noise PSD. The most appropriate approach for this problem is the usage of Bayesian statistics and Monte Carlo Markov Chains (MCMC). MCMC methods can be computationally expensive and have been proven to be too slow for our research aims, so we tackle this problem with deep learning techniques, specifically Convolutional Neural Networks (CNNs). We developed two neural networks, which reproduce the same outcomes as the MCMC methods. Both have been trained with synthetic data as well as real data from the MCMC methods. The results obtained show negligible differences with the results from the MCMC methods but with the advantage of computing times orders of magnitude smaller. [more]

Recent advancements in spectropolarimetric instrumentation, such as the new facilities at the GREGOR and DKIST telescopes, have generated vast amounts of data with each observation. This increase in data volume results in longer processing times heightened demands on computational resources, and an expanded carbon footprint, complicating scientific development timelines. The numerical inversion codes used for data analysis, based on radiative transfer models, are inherently complex. Modern projects focused on the solar atmosphere and its magnetic field require additional assumptions, significantly increasing processing times for each pixel. To address this challenge, new methods are being developed, leveraging modern data processing algorithms from statistics and machine learning. We are testing a 1D convolutional neural network model inspired by the 1D parallel atmosphere model of radiative transfer to enhance spectropolarimetric inversions and achieve significant reductions in processing times, as demonstrated in previous studies. Our approach aims to integrate physical constraints into the learning process, allowing the model to not only replicate inversions but also gain insights into the underlying physics. The data for our project was synthesized using state-of-the-art codes for magnetohydrodynamics (MURaM) and radiative transfer under non-local thermodynamic equilibrium (NICOLE). Preliminary results without physical constraints show loss rates approaching 10-3 orders of magnitude and Pearson correlations of the order of 0.8 on average along different optical depths inverted in the process for thermodynamic quantities. [more]

Small solar flares, or microflares (GOES B class and fainter), are frequent bursts of energy released in the Sun’s atmosphere, exhibiting heating and particle acceleration similar to that of large flares. X-ray observations provide direct diagnostics to study these processes by examining thermal emission from the hot flare loops and non-thermal emission from accelerated electrons. We present analysis of the X-ray emission from small solar flares jointly observed with the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Spectrometer Telescope for Imaging X-rays (STIX) on Solar Orbiter, providing different viewing angles of each event. NuSTAR is a highly sensitive X-ray imaging spectrometer that can directly image the Sun from 2.5 keV using focusing optics. STIX is an imaging spectrometer which instead uses indirect optics but has detectors capable of handling a wide range of solar X-ray fluxes from 4 to 150 keV. NuSTAR is in Earth orbit, whereas STIX is orbiting the Sun, so the two instruments combined can give different viewing angles of solar X-ray emission, providing a clearer picture of the flare’s structure. Combining analysis of NuSTAR and STIX’s X-ray spectra, we can take advantage of their different strengths, gaining a better understanding of the energy release in solar flares. We present observations of flares from June 2020 (on-disk for both instruments) and September 2022 (occulted for NuSTAR but on-disk for STIX - allowing us to probe a pre-flare non-thermal coronal source with NuSTAR and bright lower atmosphere flare emission with STIX). [more]

Magnetic reconnection and turbulence are two phenomena that are often invoked to address outstanding open questions as the energy dissipation problem and the heating and acceleration of the solar wind. These two phenomena are closely related to each other in a wide range of plasmas. Turbulent fluctuations can emerge in critical regions of reconnection events, and magnetic reconnection can occur as a product of the turbulent cascade. In this seminar I present some results exploring the interlink between turbulence and reconnection. This talk is divided in two sections, the first one Exploring the Effect of Driving Turbulent-like Fluctuations on a Harris Current Sheet Configuration and the Formation of Plasmoids and the second one Characterising Sub-Grid-Scale Effects on the Ohms Law Terms in Hybrid Simulations of Turbulence at the Earth’s Magnetosheath. The connecting thread is the non-linear and multi-scale nature of turbulence and reconnection as well as the importance of the small-scale dynamics on the large-scale one. In the first study, we perform 2D particle-in-cell simulations of a reconnecting Harris current sheet in the presence of turbulent fluctuations to explore the effect of turbulence on the reconnection process in collisionless non-relativistic pair-plasmas. We find that the presence of a turbulent field can affect the onset and evolution of magnetic reconnection. Moreover, we observe the existence of a scale dependent amplitude of magnetic field fluctuations above which these fluctuations can disrupt the growing of magnetic islands. These fluctuations provide thermal energy to the particles within the current sheet and preferential perpendicular thermal energy to the background population. In our second study we address the challenge that poses the modelling of large-scale systems while accounting for the small-scale phenomena by characterising the contribution of the small-scale dynamic terms on the generalized Ohms law in Vlasov-Hybrid simulations of turbulence in Earth’s magnetosheath. This with the aim of providing insight on Sub-Grid-Scale models that can be incorporated in Large Eddy Simulations. Our results are highly relevant to the future modelling of large-scale turbulent plasmas such as magnetospheres, the solar wind, the solar atmosphere, and other astrophysical systems. [more]