Solar physics department - publication archive
2024
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