RoCS scientists publications September 2020

1."Characterisation of shock wave signatures at millimetre wavelengths from Bifrost simulations"

Journal: Philosophical transactions A of the royal society

1^st Author: Henrik Eklund

Position: PhD candidate

Co-authors from RoCS:

• Sven Wedemeyer
• Shahin Jafarzadeh
• Mats Carlsson
• Mikolaj Szydlarski

Short summary by the author

We use sophisticated 3D numerical models of the solar atmosphere from Bifrost. We study

the radiation in mm-wavelenghs such as sampled with ALMA, which is strongly connected with the highly dynamical structures of the chromosphere.

We show that the mm-wavelengths efficiently track shock waves as they propagate upwardly in the middle chromosphere whereafter the shock wave continues upwards but the mm-wavelengths decouple from the shock front.

The formation height of the mm-wavelenghts thus varies much, in order of ~1Mm, with the dynamics. Further we show that the gradient within an ALMA band can be used as a diagnostic tool for probing the gradient of the local gas temperature at the actual formation heights of the mm-wave radiation.

2. "Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra"

Journal: Astronomy & Astrophysics (A&A)

1^st Author: Vasco Manuel de Jorge Henriques

Position: Researcher

Co-authors from RoCS:

• Luc Rouppe van der Voort

Short summary by the author

The umbra of sunspots, the most recognizable manifestation of magnetism in the Sun,

breathes and pulsates with oscillations that brighten its dark upper layers every three minutes, revealing a wide variety of very fine and dynamic dark structures. At the same time very small brightenings are observed that were previously interpreted as jets secondary to magnetic reconnection.

RoCS scientists, for the first time, were able to produce models that reproduce observations of the umbra of sunspots at the locations of the fine structure while at the same time, and again as a first, capturing the complete flow structure of the broader oscillation.

These models and complementary analysis firmly set the intricate fine-structuring of the umbra of sunspots as corrugations in the broader remarkable oscillation pattern.The appearance is that of a playing drum covered with grains of sand, where the grains glow when falling back down to the tune of the music.  

3."Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere"

Journal: Astronomy & Astrophysics (A&A) - the publication is marked as an A&A highlight

1^st Author: Jayant Joshi

Position: Postdoctoral Fellow

Co-authors from RoCS:

• Luc Rouppe van der Voort

Short summary by the author

Magnetic reconnection is a fundamental physical process that governs a variety of high energy dynamics in the solar atmosphere, such as flares and coronal mass ejections. In the lower solar atmosphere, small-scale magnetic reconnection manifests as intense and compact brightenings in the wings of the hydrogen Balmer-a line known as Ellerman bombs. Ellerman bombs are constituents of magnetically active solar regions and magnetic flux emerging regions.

Very recently, a phenomenon analogous to Ellerman bombs has been discovered in regions of minimal magnetic activity on the solar surface, the quiet Sun, and termed as quiet Sun Ellerman bombs. Quiet Sun Ellerman bombs are the smallest observable magnetic reconnection phenomenon in the lower solar atmosphere.

Using high spatial resolution observations in the hydrogen Balmer- line from the Swedish 1-m Solar Telescope, researchers from RoCS found that quiet Sun Ellerman bombs are ubiquitous and nearly uniformly distributed throughout the solar surface; there could be about half a million quiet Sun Ellerman bombs on the solar surface at any given time.

It will be of great interest to study the role of this ubiquitous small-scale magnetic reconnection phenomenon in chromospheric and coronal heating.

4."High-resolution observations of the solar photosphere, chromosphere, and transition region. A database of coordinated IRIS and SST observations"

Journal: Astronomy & Astrophysics (A&A)

1^st Author: Luc Rouppe van der Voort

Position: Professor

Co-authors from RoCS:

• Bart De Pontieu
• Mats Carlsson
• Souvik Bose
• Ainar Drews
• Viggo Hansteen
• Vasco Henriques
• Shahin Jafarzadeh
• Jayant Joshi
• Petra Kohutova
• Torben Leifsen
• Juan Martinez-Sykora
• Daniel Nobrega-Siverio
• Tiago Pereira
• Andrius Popovas
• Mikolaj Szydlarski

Short summary by the author 

Since the launch of NASA’s Interface Region Imaging Spectrograph (IRIS) in 2013, we have conducted coordinated observing programs with the Swedish 1-m Solar Telescope (SST) on La Palma.

The combined IRIS+SST observations provide extensive coverage of the solar atmosphere from the photosphere to the chromosphere and transition region.

In this paper, we describe a public database of coordinated IRIS+SST datasets that are open to the solar physics community for scientific analysis.

5."High-frequency oscillations in small chromospheric bright features observed with ALMA"

Journal: Philosophical Transactions A of the Royal Society

1^st Author: Juan Camilo Guevara Gómez

Position: PhD candidate

Co-authors from RoCS:

• Sven Wedemeyer
• Shahin Jafarzadeh
• Mikolaj Szydlarski

Short summary by the author

Observations of a magnetic enhanced region in the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA), around 100 GHz, have been used to trace in time and space small structures which, due to their oscillatory nature, might be associated with oscillation modes of the plasma, particularly fast-sausage and kink modes.

The importance of this study lies in the fact that ALMA frequencies are formed in the solar chromosphere, and can be directly associated with local temperature of the gas, something that is been taken advantage of only lately because radio telescopes in the past did not have such temporal and spatial resolution as ALMA does.

5."Accelerated particle beams in a 3D simulation of the quiet Sun"

Journal: Astronomy & Astrophysics (A&A)

1^st Author: Lars Frogner

Position: PhD candidate

Co-authors from RoCS:

• Boris Vilhelm Gudiksen

• Helle Bakke

Short summary by the author

Signs of strongly accelerated particles are observed in flaring events of all sizes on the Sun.

Simulations exploring the effects of these particles are becoming increasingly sophisticated, but have so far mainly involved simple 1D models of the solar atmosphere.

We have, for the first time, incorporated a particle acceleration and transport model into a realistic 3D atmospheric simulation framework.

Our simulations of the quiet Sun showed that particle acceleration arises throughout the solar corona, resulting in a large number of particle beams that converge with depth in the chromosphere. The beams that converge at the same location sometimes come from completely separate acceleration regions, an effect that could show up in observations as a mix of signatures from different electron populations.