2025 PDG seminars

TBC

Wave-Driven Coronal Loop Dynamics: Decay-less Kink Oscillations and Alfvén Pulse-Driven Cool Loop Formation

Wave processes in the solar atmosphere play a pivotal role in energy transport and plasma structuring, yet many of their mechanisms remain elusive. In this seminar, I will talk about two complementary studies that shed light on wave-induced dynamics in the solar corona.

In the first part, we investigate decay-less kink oscillations in coronal loops-ubiquitous, persistent features observed across active regions. Leveraging high-resolution observations from Solar Orbiter, we identify both short-period (0.6–7 minutes) and long-period (20 minutes to over an hour) oscillations, with amplitudes ranging from 0.1 to 1.1 Mm. These undamped oscillations, sustained over multiple cycles, point to continuous driving mechanisms and underscore their fundamental role in coronal energy distribution and loop dynamics. The second part focuses on the formation of cool plasma loops triggered by impulsive Alfvén pulses under conditions of energy imbalance. Using 2.5D MHD simulations in a gravitationally stratified solar atmosphere, we demonstrate how transverse Alfvén-like pulses can non-linearly transfer energy and momentum via the ponderomotive force, generating magnetoacoustic shocks and field-aligned plasma flows. These flows culminate in the formation of cool loops along curved magnetic field lines. The resulting plasma flows exhibit a periodicity of ~4 minutes. Non-ideal effects such as thermal conduction and radiative cooling further modulate the thermal evolution and structure of these loops. Together, these studies offer new insights into wave-driven loop formation and localized coronal heating, bridging observational and theoretical perspectives to advance our understanding of the structured and dynamic solar atmosphere.

A self-consistent solar coronal heating model by Alfvenic waves

Alfvenic waves, which are prevalent throughout the solar atmosphere, are believed to play an essential role in heating the corona. However, the exact dissipation mechanism of the magnetic energy supplied by Alfvenic waves is not yet fully understood. Several analytical models have been proposed, but it remains unclear which of them is most dominant in the real corona. To address this issue, comprehensive simulations spanning from the upper convection zone to the corona are effective, as they can self-consistently capture the sequence from the spontaneous generation of magnetic energy at the surface, to its transport into the corona, and finally to its dissipation as heat within the corona, without relying on ad hoc assumptions. In this study, we present results from a three-dimensional radiative magnetohydrodynamic simulation that employ such a self-consistent approach. We find that the dissipation of Alfvenic waves plays the dominant role in heating the simulated corona. Among the proposed mechanisms, Alfven wave turbulence provides the primary contribution, while other mechanisms (phase mixing, resonant absorption, Kelvin–Helmholtz instability) play only minor roles.

Coronal heating by MHD avalanches: how photospheric vortices gives rise to nanoflares

One of the most long-standing questions in solar physics is the coronal heating problem. Addressing this requires mechanisms capable of releasing in the corona energy sufficient to sustain the observed high temperatures. Here, our goals are to assess the capacity of vortical driving motions on the solar photosphere to bring about instability and energy release in an initially potential magnetic field, and to examine the heating produced.

Using the Lare3d code, numerical simulations are performed of a multi-stranded coronal loop in a straightened Parker (1972)-type model. On the boundaries, representing the photosphere, vortical driving motions are applied, which inject non-potential energy into the corona.

A kink-mode instability occurs in one coronal strand, which subsequently disturbs neighbouring strands in a chain reaction, which we identify as the avalanche. Heating arises through nanoflare-like bursts. In each of these, reconnection is a necessary facilitator of, but not a substantial contributor to, coronal heating. Rather, the majority of heating is generated by shocks, jets, and turbulence. Each nanoflare is narrowly localized, but intense, and the nanoflares are dispersed fairly uniformly. In one development of this model, we extract field-aligned heating distributions and investigate the influence of this heating on the plasma. These distributions inform one-dimensional simulations of the thermodynamic response to heating along individual field lines, which show that the heating produced is capable of sustaining coronal conditions against radiative and conductive losses. In a parallel development, curvature is added to the model, and the effect of the realistically curved geometry of a coronal arcade is examined. With curvature, the nature of the initial instability is modified, but, otherwise, straightened models of coronal loops are robust and reproduce the key behaviours of curved models.

Ongoing work concerns bringing together these two developments, by incorporating full thermodynamic transport and a curved geometry into a single simulation in 3D MHD. 

Harmonising Sunspot Datasets Consistency: Focusing on SOHO/MDI and SDO/HMI Data

Sunspots and sunspot groups play a crucial role in the detailed understanding of the Sun’s magnetic phenomena and are also considered key indicators of solar activity. Among the various types of solar activity data series, sunspot observations are the longest, providing time series that span several centuries. These data have been recorded using a range of methods – from hand-drawn observations to photographic techniques, and more recently, highly accurate space-based instruments. To accurately study long-term solar activity variability, it is essential to use homogeneous, well-calibrated databases. Over the centuries, significant advancements have been made in measurement techniques, and the varying resolution, sensitivity, and methodology of different instruments pose serious challenges for data harmonization. Therefore, precise cross-calibration of data from various sources is indispensable to ensure consistent and reliable joint analysis.

In our study, we focus on harmonizing the SOHO/MDI – Debrecen Sunspot Data (SDD) and the SDO/HMI – Debrecen Sunspot Data (HMIDD) to make these datasets suitable for joint analyses. We examine the differences in sunspot areas and magnetic field strengths measured by the two instruments, aiming to identify relationships between their respective measurements.

Our main goal is to analyze the full temporal and magnetic evolution of sunspot groups; therefore, in addition to sunspot areas and magnetic field strengths, we also emphasize the analysis of polarity patterns and the number of individual spots within sunspot groups. By resolving dataset-specific discrepancies, it would be possible to create a more reliable and homogeneous database, which could lead to more accurate long-term studies of solar activity.

Spectral Characteristics of a Rotating Solar Prominence in Multiple Wavelengths

The existence of rotational flows in solar prominences, also known as prominence tornadoes, has been a topic of discussion for many decades. Projection effects and a lack of spectral information make it difficult to distinguish rotation from line-of-sight motions, and counter-streaming flow confidently. Recently a 2.5D numerical modLightweaveruced using MPI-AMRVAC where rotation flows inside the coronal cavity were initiated and investigated. For the first time, this work showed the properties and evolution of rotational flows in solar prominences, which are in good agreement with existing observations in SDO. In this talk, we continue working on this numerical experiment by extending our analysis to the spectral synthesis of the Hα line, and several other well-used chromospheric lines using the Lightweaver code and its new prominence synthesis method.

Dynamics of the lower and upper solar atmosphere using MURaM

In this talk, I will discuss two cases of dynamics in the lower and upper solar atmosphere respectively using the realistic three-dimensional MHD code, MURaM. I will primarily talk about the first case, which is a comparison of vortex dynamics in the photosphere and lower chromosphere in three different magnetic field regions. We use a flux tube expansion model and fourier spectra analysis to find that there is increased potential of vortex-induced torsional Alfvén waves to travel higher in the atmosphere for weaker magnetic regions, whereas vortices would result in dissipation and heating due to the vortex interactions in narrow flux tubes for the strongly magnetized regions. 

For the second case, which is my present work, I will briefly talk about EUV emission line synthesis using the MURaM simulation and the CHIANTI database in the upper solar atmosphere. Overall, the goal of this talk is to provide a new understanding of the extent to which the current complex 3D MHD models capture and represent the physics of the solar atmosphere. 

Radiative Models of Coronal Condensation at Extreme Resolution

Radiation remains the primary vector by which the properties of solar plasma can be investigated. Atomic spectral lines, often forming in thin atmospheric layers, offer a powerful mechanism to probe the solar atmosphere, in particular its outer layers where conditions are typically outside of local thermodynamic equilibrium. Synthesising the radiation produced by numerical models also provides an essential lens through which to compare models and observations. Isolated solar structures such as filaments and prominences are of particular interest due to the complexity of spectral line formation within them and the diagnostic window this provides onto their formation and stability.

In recent years, magnetohydrodynamic models of these isolated structures have become significantly more advanced, whilst radiative treatments have primarily remained the same. We have recently introduced the DexRT code: a novel approach to multidimensional non-equilibrium radiative transfer using a technique termed radiance cascades to efficiently treat problems with intertangled layers of optically thin and thick material – a regime where current approaches can fail with dramatic so-called ray effects. Here, I will provide a brief description of the methods used in DexRT along with spectra of a variety of lines synthesised from complex magnetohydrodynamic models. I will also discuss the importance of considering both detailed radiative transfer and viewing angle effects when making predictions and comparisons to observations. 

Sun-as-a-star flare observations with high-resolution telescopes

Stellar flares cannot be spatially resolved, meaning we have to extract complex three-dimensional behavior from a one-dimensional disk-integrated spectral timeseries. 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 including some large IRIS flares. In this talk, I will discuss how high-resolution observations with a limited field-of-view can be converted into approximations of disk-integrated spectra using the newly developed Numerical Sun-as-a-Star Integrator (NESSI). Additionally, I will discuss the impact of projectional effects on the study of such events with focus on the detection of coronal mass ejections. Our findings suggest common patterns in the disk-integrated spectra between flares of different strengths and locations that can be used to better interpret stellar flares without resolved context.

Improved atomic models to interpret the solar radiation emitted from the Transition region and chromosphere

We have long-standing issues and discrepancies between predicted and observed emissions. In the solar transition region and chromosphere, some are due to the inherent limitations of the physical models, but some are due to simplified atomic models.

We developed improved modelling of the ion balance, including physical effects which occur all the time and made them available via the CHIANTI v.11. I will briefly describe them and show how they improve the comparisons with observations of the Sun and other stars, with very simple 1D static atmospheric models.  I will then describe current models we are developing to explain some chromospheric lines, and the plans to include other effects such as photo-ionization.