Solar physics department - publication archive
2024
Rugged magnetohydrodynamic invariants in weakly collisional plasma turbulence
We investigated properties of ideal second-order magneto-hydrodynamic (MHD) and Hall MHD invariants (kinetic+magnetic energy and different helicities) in a two-dimensional hybrid simulation of decaying plasma turbulence. The combined (kinetic+magnetic) energy decays at large scales, cascades (from large to small scales) via the MHD non-linearity at intermediate scales. This cascade partly continues via the Hall coupling to sub-ion scales. The cascading energy is transferred (dissipated) to the internal energy at small scales via the resistive dissipation and the pressure-strain effect. The mixed (X) helicity, an ideal invariant of Hall MHD, exhibit a strange behaviour whereas the cross helicity (the ideal invariant in MHD but not in Hall MHD), in analogy to the energy, decays at large scales, cascades from large to small scales via the MHD+Hall non-linearities, and is dissipated at small scales via the resistive dissipation and an equivalent of the pressure-strain effect. In contrast, the magnetic helicity is very weakly generated through the resistive term and does not exhibit any cascade.
Anisotropy of plasma turbulence at ion scales
We investigated properties of plasma turbulence at ion scales in the context of the solar wind. The ambient magnetic field induces a strong spectral anisotropy, the turbulent fluctuations exhibit larger spatial scales along the magnetic field compared to the perpendicular directions. An analysis using the Kármán-Howarth-Monin equation shows the corresponding anisotropy of turbulent processes (decay, cascade, resistive dissipation, and pression-strain interaction): their characteristic scales shifts to larger scales in the quasi-parallel direction with respect to the ambient magnetic field compared to the quasi-perpendicular ones. This anisotropy is weak at large scales owing to the initial isotropic spectrum and becomes progressively stronger at small scales.
Magnetic properties of the umbral boundary during sunspot decay: Comparative study of multiple datasets
The vertical magnetic field is the stabilising factor responsible for the different magnetoconvective modes observed in sunspots. That is, there is a critical vertical magnetic field that separating the umbral and penumbral modes of convection. This criterion has been demonstrated by several authors using different datasets and instruments, leading to different values of the critical vertical magnetic field. In this work, we have retrieved records from these instruments (SP/Hinode and HMI/SDO) and datasets (HMI and HMI corrected for scattered light) of a dissipating sunspot. We derived the intrinsic dissimilarities of the inverted vector magnetic fields caused by the different instrumental setups and inversion strategies, and showed that the dissimilarities of the obtained critical vertical field are consistent with such intrinsic dissimilarities. Moreover, the decaying sunspot allowed us to observe how the penumbra provides stability to the umbral region, where the magnetic properties do not vary much during the decay, while the magnetic structure of the naked spots behaves similarly to solar pores, where their magnetic properties vary more dramatically with small changes in the umbral region.
Slowly positively drifting bursts generated by large-scale magnetic reconnection
In this work, we studied a flare rich in unique types of radio bursts with slow positive drift at frequencies around 1 GHz. We have shown that these rarely observed radio bursts are related to a magnetic reconnection process that can suddenly release large amounts of energy of a complicated magnetic field. We also found that these radio bursts are likely generated by a beam of accelerated particles moving along the newly formed magnetic lines. When these new interconnections have large scales, some of the bursts can occur quite far from the center of the flare itself. We also discuss the possibilities of why the frequency drift in these isolated radio bursts is so slow, even when they are generated by a beam of accelerated particles.
Flare heating of the chromosphere: Observations of flare continuum from GREGOR and IRIS
In May 2022, astronomers from Ondřejov carried out a coordinated observing campaign using instruments located in Ondřejov (Solar Patrol, SORT and the solar spectrographs FICUS and HSFA-2), the largest European telescope (GREGOR, located in Tenerife, Spain) and space-based solar satellites (IRIS and Hinode). Although flares are very common solar events, it is not possible to predict exactly where and when they will occur. It is therefore not easy to point solar telescopes with very high spatial resolution and a small field of view (smaller than the solar disc) at a region where a flare will occur. In the framework of this campaign, we were successful and we were able to obtain a unique multi-wavelength dataset of an M7.5 flare, including the pre-flare and the impulsive phase. This very rich dataset allowed us to show that flare continuum enhancements are present in faint ribbons as well, and to estimate a lower limit on the mean temperature in the layer where the flare continuum enhancement is formed.
Testing the volume integrals of travel-time sensitivity kernels for flows
Helioseismology is the only method that allows us to "see" below the solar surface. Unfortunately, a large portion of helioseismic inferences rely on the accuracy of the sensitivity kernels. The sensitivity kernels constitute the functions that "translate" changes in the parameters of the solar interior into the helioseismic observables on the solar surface. In the study we directly tested a class of sensitivity kernels related to the travel times of the waves propagating through the solar interior. By artificially manipulating the solar observations, we independently determined the spatial integrals of these kernels and compare them with model values. We show that the agreement is acceptable for the near-surface modes having the same radial order, whereas the agreement is less satisfactory for the waves travelling deeper and having a constant phase speed.
Magnetic field diagnostics of prominences with the Mg ii k line 3D Stokes inversions versus traditional methods
The inverse problem, that is, deciphering the physical conditions of observed structures from noisy data and a single point of view, is a fundamental challenge in solar physics. Spectral line analysis provides a valuable tool for solving this problem, since the thermodynamic and magnetic properties of plasmas often leave significant traces in the intensity and polarization of these lines. However, the solution of the inverse problem is complicated by the non-local and non-linear interactions between different regions of the plasma mediated by radiation. As a consequence, this problem remains unsolved in its generality. In this paper, we present a new method that takes into account previously neglected physical processes and show that the inverse problem is solvable. Specifically, we address the problem of lines of once-ionized magnesium, which are the subject of observations by a proposed NASA satellite project.
Spectral cleaving in solar type II radio bursts: Observations and interpretation
We have reported radio observations of a previously unrecognized feature, called spectral cleaving, in solar type II bursts, being radio signatures of shock waves in the solar corona. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. We found that the spectral cleaving is a new distinct spectral effect indicative of involuted plasma processes that occur within the solar corona. We offered an initial interpretation of the spectral cleaving in type II bursts. The intricate interplay between the shock wave and magnetic field configurations plays a key role here. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.
Hydrogen recombination continua in stellar flares
An increasing interest in stellar flares stimulated various modelling approaches in order to analyse the observed flare fluxes. Radiation-hydrodynamical simulations, together with a rather rare broad-band spectroscopy, indicate much larger densities in the superflare chromospheres as compared to solar flares. Formation of hydrogen recombination continua under such different densities is governed by physics of optically thin to largely thick plasmas, the continuum optical thickness being within the range of four orders of magnitude. Various authors presented simple approximate methods to analyse the photometric data from Kepler or TESS under such diverse regimes of physical conditions. In this letter, we summarize the general physical approach and compute the hydrogen recombination spectra under the above range of electron densities. We show the theoretical contrast with respect to quiet-star continuum for two characteristic stars of G and dMe type. Based on that we distinguish three regimes of the continuum formation and discuss the applicability of various simple approaches.
Onset of penumbra formation
The formation of sunspot penumbrae is still poorly understood. In this paper, we study regions at the edge of the sunspot pore where penumbra forms. Before the formation of the penumbra, we find different properties of the magnetic and velocity fields in the studied regions. However, the mechanism of penumbra formation is the same everywhere. Penumbral filaments with Evershed flow begin to form at the umbra boundary and grow radially primarily outward as the penumbral filaments elongate with time.
M. García-Rivas, et al. 2024, Astronomy & Astrophysics, 686, A112
The relation between magnetic field inclination and the apparent motion of penumbral grains
The bright heads of sunspot penumbral filaments, penumbral grains, show apparent horizontal motions inward, toward the umbra, or outward, away from the umbra. Penumbral grains are locations of rising hot gas from sub-photospheric layers. We used spectropolarimetric observations of five sunspot penumbrae to compare magnetic inclinations inside penumbral grains with those in their surroundings. We found that approximately a half of the inward-moving grains have a magnetic inclination larger than the inclination in their surroundings and a half of the outward-moving grains have an inclination smaller than the surrounding one. The opposite relation of inclinations is observed in only one fifth of the penumbral grains. We conclude that there is a statistical relation between the direction of apparent motions of penumbral grains and the inclination of magnetic field in sunspot penumbra.
2023
Morphology of Solar Type II Bursts Caused by Shock Propagation through Turbulent and Inhomogeneous Coronal Plasma
We report radio observations of type II burst which has enormously rich and complex spectral morphology. We have exploited its herringbone pattern to study electron density turbulence in the solar corona. For the first time, we obtained properties of the density turbulence in the coronal streamer. This research copes with a relevant task in the physics of solar plasma – probing properties of density turbulence in the corona in a routine way.
A. Koval, et al. 2023, The Astrophysical Journal, 952, id.51
Observations and modeling of spectral line asymmetries in stellar flares
This study investigates stellar flares on cool stars, specifically focusing on the dMe star AD Leo, which was observed using the Perek telescope at Ondřejov observatory. Stellar flares, known for their energetic events in stellar atmospheres, often display asymmetries in spectral lines, with blue asymmetries typically linked to coronal mass ejections and the origins of red asymmetries remaining unclear. To explore these red asymmetries, the researchers modeled the Hα line emissions from an extensive arcade of cool flare loops using non-LTE radiative transfer, incorporating the velocity distribution of individual coronal rain clouds. The synthetic Hα profiles generated from the coronal rain model had enhanced red wings that closely matched the observations, suggesting that coronal rain could be a plausible explanation for the red asymmetries seen in stellar flares on AD Leo.
J. Wollmann, et al. 2023, Astronomy & Astrophysics, 669, A118
First Metis Detection of the Helium D3 Line Polarization in a Large Eruptive Prominence
Space coronagraph Metis on ESA's Solar Orbiter was developed by Italian-German-Czech consortium. It is capable of observing the solar corona in the visible light and in Lyman-alpha line simultaneously for the first time. We present unique observations of a large eruptive prominence and demonstrate unambiguous detection of the neutral-helium D3 line emission. We show how the prominence appears in the polarized light and investigate potential of Metis to detect prominence magnetic fields.
P. Heinzel, et al. 2023, The Astrophysical Journal Letters, 957, 10H
On the Physical Nature of the so-Called Prominence Tornadoes
While the name ‘prominence tornadoes’ suggests violent rotational dynamics, the analogy with a tornado strongly collides with the usual paradigm of the magnetic structure of solar prominences. In this comprehensive review, we resolved this long-standing paradox. We concluded that ‘prominence tornadoes’ do not differ from other stable prominences. The impression of the column-like silhouettes and helical motions is just a consequence of projection effects combined with small-scale dynamics.
S. Gunár, et al. 2023, Space Science Reviews 219:33