European Solar Physics Online Seminar Archiv

Stellar flares cannot be spatially resolved, which means that we have to extract complex three-dimensional behavior from a one-dimensional disk-integrated spectrum. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument in combination with spatially resolved observations. In this talk I will discuss a confined X2.2 flare and its eruptive X9.3 successor as measured by the HARPS-N Sun-as-a-star telescope. The behavior of multiple photospheric and chromospheric spectral lines are investigated by means of activity indices and contrast profiles, which are then related to physical processes directly observed in high-resolution observations made with the Swedish 1-meter Solar Telescope (SST). We further explore these relations by using the newly developed Numerical Sun-as-a-Star Integrator (NESSI) code to convert high-resolution SST flares to full disk spectra. Our findings suggest a relationship between the evolving shapes of the disk integrated spectra and the flare locations on the solar disk, which could be act as a guide for constraining flare locations in stellar spectra. [mehr]

In the recent decades, three-dimensional modelling of solar flares and eruptions has made a number of predictions that were subsequently indicated by high-resolution imaging observations. These include existence of magnetic reconnection geometries involving the erupting flux rope itself, which drifts as a result, and also the apparent slipping and slip-running motion of footpoints of individual reconnecting structures. In this talk, we will summarize the predictions of the MHD models as well as the corroborating evidence, with particular emphasis on recent observations of super-Alfvénic slippage of flare kernels. [mehr]

The Sun is a dynamic star, displaying activities ranging from subtle, short-lived events to major coronal mass ejections (CMEs) and powerful flares. Differentiating between “flaring” and “non-flaring” active region (AR) configurations is critical for heliophysics research. This study investigates whether small to medium-scale activity in ARs holds clues about their eruptive potential and future behaviour. Using data from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), we analyse transient brightenings and their relationship to the magnetic polarity inversion line (PIL) in ARs. We observe significant differences between pre-flaring and non-flaring ARs in terms of the spatial distribution and temporal evolution of transient brightenings around the PIL. Key parameters include the number, intensity, and magnetic flux of brightenings over time, as well as their behaviour across multiple wavelengths. These variations offer insights into the Sun’s atmospheric dynamics and the mechanisms driving major flares and – in case of coronal mass ejections – eruptions. By understanding the pre-eruptive activity in ARs, we aim to improve solar event prediction capabilities and advance our knowledge of the relevant dynamics. [mehr]

On October 28, 2021 the first X-class solar flare of Solar Cycle 25 occurred in active region NOAA AR 12887 with a peak at 15:35 UT. It produced the rare event of ground-level enhancement of the solar relativistic proton flux and a global extreme ultraviolet wave, along with a fast halo coronal mass ejection (CME) as seen from Earth’s perspective. A few hours before the flare, a slower CME had erupted from a quiet Sun region just behind the northwestern solar limb. Solar Orbiter was almost aligned with the Sun-Earth line and, during a synoptic campaign, its coronagraph Metis detected the two CME events in both Visible Light (VL) and UltraViolet (UV) channels. The earlier CME took place in the north-west (NW) sector of Metis field of view, while several bright features of the flare-related event appeared mostly to the south-east (SE). The NW and SE events have two distinct origins, but were both characterized by a very bright emission in HI Ly-alpha visible in the UV images of Metis up to 8 solar radii. This work is a follow-up study of two out of the six events analyzed by Russano et al. 2024 (A&A, 683, A191), aimed at investigating the evolution of these two almost co-temporal CMEs but originating in such distinct source regions. To that end, we extensively inspect data sets from numerous remote-sensing instruments observing the Sun in several spatial and spectral regimes. We characterize several aspects of these CMEs, including their three-dimensional properties, kinematics, mass, and temporal evolution of those quantities. Results of this work point to notable differences between these two events showing significant UV emission in the corona. Co-authors: H. Cremades, F. A. Iglesias, L. Teriaca, R. Aznar Cuadrado, F. M. López, L. Di Lorenzo, M. Temmer, M. Romoli, D. Spadaro, and the Metis Team [mehr]

On October 28, 2021 the first X-class solar flare of Solar Cycle 25 occurred in active region NOAA AR 12887 with a peak at 15:35 UT. It produced the rare event of ground-level enhancement of the solar relativistic proton flux and a global extreme ultraviolet wave, along with a fast halo coronal mass ejection (CME) as seen from Earth’s perspective. A few hours before the flare, a slower CME had erupted from a quiet Sun region just behind the northwestern solar limb. Solar Orbiter was almost aligned with the Sun-Earth line and, during a synoptic campaign, its coronagraph Metis detected the two CME events in both Visible Light (VL) and UltraViolet (UV) channels. The earlier CME took place in the north-west (NW) sector of Metis field of view, while several bright features of the flare-related event appeared mostly to the south-east (SE). The NW and SE events have two distinct origins, but were both characterized by a very bright emission in HI Ly-alpha visible in the UV images of Metis up to 8 solar radii. This work is a follow-up study of two out of the six events analyzed by Russano et al. 2024 (A&A, 683, A191), aimed at investigating the evolution of these two almost co-temporal CMEs but originating in such distinct source regions. To that end, we extensively inspect data sets from numerous remote-sensing instruments observing the Sun in several spatial and spectral regimes. We characterize several aspects of these CMEs, including their three-dimensional properties, kinematics, mass, and temporal evolution of those quantities. Results of this work point to notable differences between these two events showing significant UV emission in the corona. Co-authors: H. Cremades, F. A. Iglesias, L. Teriaca, R. Aznar Cuadrado, F. M. López, L. Di Lorenzo, M. Temmer, M. Romoli, D. Spadaro, and the Metis Team [mehr]

Solar Ultraviolet Imaging Telescope (SUIT) is the near Ultraviolet imager (200-400 nm) on board the Indian Solar mission Aditya-L1. It was launched on September 2nd, 2023 and was inserted in a halo orbit around the Earth-Sun L1 point on January 6th, 2024. It has been regularly observing the Sun during its cruise phase and nominal observation time in the past year. Here, we will present some of the selected flare observations carried out by SUIT in the past year, combining them with various multi-wavelength observations and highlighting the unique observation window SUIT opens up in the lower solar atmosphere. [mehr]

We present a new network for space weather observation: Global Automatic Telescopes Exploring the SUN (GATES). GATES currently utilizes two primary instruments: the Mojave Solar Observatory (MSO), located in Apple Valley, California, and the Tor Vergata Solar Synoptic Telescope (TSST), which is undergoing testing at the optical bench of the Università degli Studi di Roma Tor Vergata heliostat, prior to its planned deployment to La Palma, Canary Islands, Spain. TSST consists of a dual-channel full disk telescope, a lab-tested K magneto-optical filter (MOF) channel and a Hα channel for flare detection and localization. MSO houses a dual Na and K channel MOF-based telescope currently able to observe on-sky. With the completion and installation of the TSST, the two nodes will observe for an average of 20 hours a day, obtaining line-of-sight velocity and magnetic field vector observations (Dopplergrams and magnetograms, respectively), which we will use to analyze and predict space weather events. We present preliminary data obtained using the network’s individual nodes and technical specifications for the future operation of the network as a whole. [mehr]

Spectral resolution and sampling present a design trade-off when building spectropolarimetric instruments to observe the Sun with very high spatial resolution. On the one hand, for spectropolarimetric inversions, high spectral resolution translates into a high depth resolution along the line of sight. On the other hand, fine spectral sampling results in increased noise per wavelength bin, smaller spectral ranges, and higher data rates. In this talk, we will explore how a finite spectral resolution changes the information content in stellar spectra. We do that by synthesizing spectra of the two magnetically sensitive photospheric lines of iron around 630 nm and applying various spectral degradations and rebinning. We analyze how the dimensionality of the spectra changes and the performance of the spectropolarimetric inversions applied to these different synthetic datasets. We are especially interested in the effects of noise on the inferred parameters. We conclude that the fine spectral resolution (and high spectral fidelity) result in excellent inversions, even at nominally high noise levels. We support these conclusions with a few examples from TRIPPEL-SP and MiHI instruments at the Swedish Solar Telescope. [mehr]

Measuring the long-term variability of solar EUV/UV irradiances is needed for e.g. studying the Sun’s influence on space weather, satellite drag and for solar studies. However, direct measurements remain limited, making predictive modelling crucial. We have found linear correlations between EUV daily spectral irradiances obtained from SOHO/CDS and SDO/EVE and solar proxies of activity, such as the F10.7, F30 cm radio fluxes and the Mg II index over several solar cycles, analysing data from 1998 until 2024. We confirm previous findings in that the F30 and the Mg II indexes are better, and that the variations increase with the formation temperature of the lines; the transition region lines show very little variations, except the hydrogen and helium lines. A code to predict the EUV irradiances of the main lines is provided. Relatively good agreement between the CDS and EVE irradiances is found, but significant differences with historical measurements are found in some EVE lines, especially at the longer wavelengths. Using the atomic data we made available in CHIANTI v.11 and the correlations with the proxies, we are building a new model to predict the lines and continua in the whole EUV spectral range covered by SDO/EVE. [mehr]

Kink oscillations of the decayless class are detected as small amplitude (< 1 Mm) persistent transverse repetitive displacements of solar coronal plasma loops, occurring without any association with solar flares, eruptions, or other impulsive energy releases. The linear correlation of the oscillation periods with the lengths of the oscillating loops demonstrates that the oscillations are natural modes of the loops. The statistical distribution of oscillation amplitudes with the periods shows no pronounced peaks, suggesting the lack of a periodic driver. Together with the lack of an impulsive excitation, it suggests that the energy losses are compensated by the interaction of the loop with either quasi-steady or random external plasma flows. In this study, the decayless regime is associated with the energy supply from coronal plasma flows via the negative friction, i.e., self-oscillations, and/or random movements of footpoints of the oscillating loop. The damping is attributed to the linear effect of resonant absorption. Our low-dimensional model is based on a randomly driven Rayleigh oscillator with random fluctuating coefficients. In the noiseless case, the model has an asymptotically stationary oscillatory solution, i.e., a self-oscillation. The kink oscillation period is found to be practically independent of noise. Fluctuations of the amplitude envelope are consistent with the observed behaviour. The transition from the large-amplitude rapidly-decaying regime to the low-amplitude decayless oscillations demonstrates that the decay pattern differs from the usually assumed exponential decay. An improved, super-exponential model for the damping is analytically derived and shown to be consistent with observations. In the separately considered case of the decayless oscillatory regime supported by a random driver only, the decay of the oscillation amplitude to the decayless level is exponential. Implications of this finding for magnetohydrodynamic seismology of the solar corona, based on the effect of resonant absorption are discussed. [mehr]