European Solar Physics Online Seminar

Following an initiative by the University of Oslo the MPS will participate in the "European Solar Physics Online Seminar" series (ESPOS). Details can be found here: https://folk.uio.no/tiago/espos/
The aim of this video conference series is to promote ideas more widely with a specialized audience, and give some exposure to cutting-edge research for students and other young researchers that do not regularly travel to conferences. The ESPOS series is planned to take place every second Thursday at 11am.
Host: Shahin Jafarzadeh

ESPOS Online Seminar: Shock-wave radio probing of solar wind sources in coronal magnetic fields (Artem Koval

ESPOS
  • Date: Jul 7, 2021
  • Time: 11:00 AM c.t. - 12:00 PM (Local Time Germany)
  • Speaker: Artem Koval
  • Astronomical Institute of the CAS, Czech Republic
  • Room: Zoom
  • Host: Shahin Jafarzadeh
The Space Weather effects in the near-Earth environment as well as in atmospheres of other terrestrial planets arise by corpuscular radiation from the Sun, known as the solar wind. The solar magnetic fields govern the solar corona structure. Magnetic-field strength values in the solar wind sources - key information for modeling and forecasting the Space Weather climate - are derived from various solar space- and ground-based observations, but, so far not accounting for specific types of radio bursts. These are “fractured” type II radio bursts attributed to collisions of shock waves with coronal structures emitting the solar wind. Here, we report about radio observations of two “fractured” type II bursts to demonstrate a novel tool for probing of magnetic field variations in the solar wind sources. These results have direct impact on interpretations of this class of bursts and contribute to the current studies of the solar wind emitters. [more]

ESPOS Online Seminar: High-resolution spectroscopy of a minifilament eruption (Ioannis Kontogiannis)

ESPOS
Minifilaments are miniature versions of filaments, first observed in H-alpha filtergrams of quiet Sun. Recent studies have showcased their association with small-scale eruptive events, highlighting their importance in energetic processes of the quiet Sun. We present the first detailed study of such an event, using high-cadence, high-spectral resolution imaging observations. The minifilament formed between small-scale, opposite-polarity magnetic concentrations and erupted within an hour after its appearance in H-alpha, exhibiting a twisted, thread-like structure. Its eruption took place in two phases (slow and fast), producing a coronal dimming, while part of the erupting material returned to the chromosphere. The observed similarities to large-scale filament eruptions indicate the action of common mechanisms. Their properties, combined with their abundance in quiet Sun, constitute minifilaments ideal targets for the new ground-based solar telescopes. [more]

ESPOS Online Seminar: Forecasting solar flares with a new topological parameter and a supervised machine-learning method (Luca Giovannelli)

ESPOS
Solar flares originate from active regions (ARs) hosting complex and strong bipolar magnetic fluxes. Forecasting the probability of an AR to flare and defining reliable precursors of intense flares, i.e., X- or M-class flares, are extremely challenging tasks in the space weather field. In this talk, we focus on two metrics as flare precursors, the unsigned flux R*, tested on MDI/SOHO data and calibrated for higher spatial resolution SDO/HMI maps, and a novel topological parameter D representing the complexity of a solar active region. The parameter D is based on the automatic recognition of magnetic polarity inversion lines (PILs) in identified SDO/HMI ARs and is able to evaluate their magnetic topological complexity. We use both a heuristic approach and a supervised machine-learning method to validate the effectiveness of these metrics to predict the occurrence of X- or M-class flares in a given solar AR during the following 24 hr period. Our feature ranking analysis shows that both parameters play a significant role in prediction performances. Moreover, the analysis demonstrates that the new topological parameter D is the only one, among 173 overall predictors, that is systematically ranked within the top 10 positions. [more]

ESPOS Seminar: Test-particle simulations at tearing coronal null-point current sheets (Ross Pallister)

ESPOS
Magnetic reconnection is widely accepted to be a major contributor to nonthermal particle acceleration in the solar atmosphere. We investigate particle acceleration in two evolving field geometries: first in an isolated tearing current sheet, then in a full-scale coronal jet. Both geometries involve 3D reconnection with at least one magnetic null point. A test-particle approach is employed, using electromagnetic fields from magnetohydrodynamic (MHD) simulations of these geometries. Using this method, we examine the trajectories of high-energy protons and electrons injected near reconnecting null points and how the directionality of their acceleration differs. We will discuss what the ejection and impact patterns of heliosphere and photosphere-incident particles respectively can tell us about the location, size and shape of field structures that are formed in tearing current sheets during null-point reconnection in the solar corona. We will also consider how we may observe the simulated differences between proton and electron impact patterns. [more]

ESPOS Seminar: Role of Heating-Cooling Misbalance on the Phase Shift of Propagating Slow Waves in Non-adiabatic Solar Coronal Loops (Abhinav Prasad)

ESPOS
Invoking the effects of thermal conductivity, compressive, viscosity, radiative losses, and heating-cooling misbalance, we derive the new general dispersion relation for the propagating slow MHD waves in the solar corona and solve it to determine the phase shifts of density and temperature perturbations along with their dependence on the equilibrium parameters of the plasma such as the background density and temperature. We also derive a new generalised mathematical expression for the polytropic index using the linear MHD model and find that in the presence of thermal conduction alone it remains close to its classical value for all the considered equilibrium density and temperature observed in typical coronal loops. Under the considered heating and cooling models, we find that the expected polytropic index can be matched with the observed value of 1.1 ± 0.02 in typical coronal loops if the thermal conductivity is enhanced by an order of magnitude compared to its classical value. We also explore the role of different heating functions for typical coronal parameters and find that although the polytropic indices remain close to 5/3, the phase difference between density and temperature perturbations is highly dependent on the form of heating function. [more]

ESPOS Seminar: Formation and disappearance of a penumbra: Recent results (Mariarita Murabito)

ESPOS
The physical conditions resulting in the formation and disappearance of penumbral regions are poorly understood. We investigated these conditions by using high-resolution spectropolarimetric observations of a sunspot penumbra from different instruments at ground- and space-based telescopes, namely the SST/CRISP, SDO/HMI, and Hinode/SP. The studied data allowed us to assess the evolution of the magnetic and velocity properties of plasma in the observed region and to analyze the role of several processes found therein. The penumbra forms only on one side of the observed region, characterized by the absence of an overlying magnetic canopy. The penumbra later disappears progressively in time and space. This final evolution of the studied region seems to be governed by the presence of moving magnetic features (MMFs) and of overlying canopies. [more]

ESPOS Seminar: On the Differences in the Periodic Behavior of Magnetic Helicity Flux in Flaring Active Regions (Szabolcs Soós)

ESPOS
Observational precursors of large solar flares provide a basis for future operational systems for forecasting. We studied the evolution of the normalized emergence (EM), shearing (SH), and total (T) magnetic helicity flux components for 14 flaring (with at least one X-class flare) and 14 nonflaring (<M5-class flares) active regions (ARs) using the Space-weather Helioseismic Magnetic Imager Active Region Patches vector magnetic field data. Each of the selected ARs contain a δ-type spot. The three helicity components of these ARs were analyzed using wavelet analysis. Localized peaks of the wavelet power spectrum (WPS) were identified and statistically investigated. We find that (i) the probability density function of the identified WPS peaks for all the EM/SH/T profiles can be fitted with a set of Gaussian functions centered at distinct periods between ∼3 and 20 hr. (ii) There is a noticeable difference in the distribution of periods found in the EM profiles between the flaring and nonflaring ARs, while no significant difference is found in the SH and T profiles. (iii) In flaring ARs, the distributions of the shorter EM/SH/T periods (<10 hr) split up into two groups after flares, while the longer periods (>10 hr) do not change. (iv) When the EM periodicity does not contain harmonics, the ARs do not host a large energetic flare. (v) Finally, significant power at long periods (∼20 hr) in the T and EM components may serve as a precursor for large energetic flares. [more]

ESPOS Seminar: Small-scale MHD waves in the solar chromosphere with ALMA (Juan Camilo Guevara Gómez)

ESPOS

Validation of a wave heated 3D MHD coronal-wind model using Polarized Brightness and EUV observations

ESPOS

ESPOS Seminar: MHD wave propagation asymmetric solar waveguides (Noémi Kinga Zsámberger)

ESPOS
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]
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