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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Ambipolar diffusion in the Bifrost code  

Daniel Nóbrega-Siverio    Submitted: 2020-06-27 09:32

Ambipolar diffusion is a physical mechanism related to the drift between charged and neutral particles in a partially ionized plasma that is key to many different astrophysical systems. However, understanding its effects is challenging due to basic uncertainties concerning relevant microphysical aspects and the strong constraints it imposes on the numerical modeling. Our aim is to introduce a numerical tool that allows us to address complex problems involving ambipolar diffusion in which, additionally, departures from ionization equilibrium are important or high resolution is needed. The primary application of this tool is for solar atmosphere calculations, but the methods and results presented here may also have a potential impact on other astrophysical systems. We have developed a new module for the stellar atmosphere Bifrost code that improves its computational capabilities of the ambipolar diffusion term in the generalized Ohm's law. This module includes, among other things, collision terms adequate to processes in the coolest regions in the solar chromosphere. As the main feature of the module, we have implemented the super time stepping (STS) technique, which allows an important acceleration of the calculations. We have also introduced hyperdiffusion terms to guarantee the stability of the code. We show that to have an accurate value for the ambipolar diffusion coefficient in the solar atmosphere it is necessary to include as atomic elements in the equation of state not only hydrogen and helium, but also the main electron donors like sodium, silicon, and potassium. In addition, we establish a range of criteria to set up an automatic selection of the free parameters of the STS method that guarantees the best performance, optimizing the stability and speed for the ambipolar diffusion calculations. We validate the STS implementation by comparison with a self-similar analytical solution.

Authors: D. Nóbrega-Siverio, J. Martínez-Sykora, F. Moreno-Insertis, M. Carlsson
Projects: Other

Publication Status: A&A (published)
Last Modified: 2020-06-27 20:59
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Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes  

Daniel Nóbrega-Siverio    Submitted: 2020-02-20 02:38

Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm?s law. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.

Authors: D. Nóbrega-Siverio, F. Moreno-Insertis, J. Martínez-Sykora, M. Carlsson, M. Szydlarski
Projects: Other

Publication Status: A&A (in press)
Last Modified: 2020-02-21 14:21
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On the importance of the nonequilibrium ionization of Si IV and O IV and the line-of-sight in solar surges  

Daniel Nóbrega-Siverio    Submitted: 2018-03-29 09:40

Surges are ubiquitous cool ejections in the solar atmosphere that often appear associated with transient phenomena like UV bursts or coronal jets. Recent observations from the Interface Region Imaging Spectrograph (IRIS) show that surges, although traditionally related to chromospheric lines, can exhibit enhanced emission in Si IV with brighter spectral profiles than for the average transition region (TR). In this paper, we explain why surges are natural sites to show enhanced emissivity in TR lines. We performed 2.5D radiative-MHD numerical experiments using the Bifrost code including the nonequilibrium ionization of silicon and oxygen. A surge is obtained as a by-product of magnetic flux emergence; the TR enveloping the emerged domain is strongly affected by nonequilibrium effects: assuming statistical equilibrium would produce an absence of Si IV and O IV ions in most of the region. Studying the properties of the surge plasma emitting in the Si IV 1402.77 Å and O IV 1401.16 Å lines, we find that a) the timescales for the optically-thin losses and heat conduction are very short, leading to departures from statistical equilibrium, and b) the surge emits in Si IV more and has an emissivity ratio of Si IV to O IV larger than a standard TR. Using synthetic spectra, we conclude the importance of line-of-sight effects: given the involved geometry of the surge, the line-of-sight can cut the emitting layer at small angles and/or cross it multiple times, causing prominent, spatially intermittent brightenings both in Si IV and O IV.

Authors: Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martínez-Sykora, J.
Projects: IRIS

Publication Status: ApJ (Accepted)
Last Modified: 2018-03-29 09:41
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Surges and Si IV bursts in the solar atmosphere. Understanding IRIS and SST observations through RMHD experiments  

Daniel Nóbrega-Siverio    Submitted: 2017-11-06 14:06

Surges often appear as a result of the emergence of magnetized plasma from the solar interior. Traditionally, they are observed in chromospheric lines such as Hα 6563 Å and Ca II 8542 Å. However, whether there is a response to the surge appearance and evolution in the Si IV lines or, in fact, in many other transition region lines has not been studied. In this paper we analyze a simultaneous episode of an Hα surge and a Si IV burst that occurred on 2016 September 03 in active region AR12585. To that end, we use coordinated observations from the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope (SST). For the first time, we report emission of Si IV within the surge, finding profiles that are brighter and broader than the average. Furthermore, the brightest Si IV patches within the domain of the surge are located mainly near its footpoints. To understand the relation between the surges and the emission in transition region lines like Si IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux emergence episodes using the Bifrost code and including the non-equilibrium ionization of silicon. Through spectral synthesis we explain several features of the observations. We show that the presence of Si IV emission patches within the surge, their location near the surge footpoints and various observed spectral features are a natural consequence of the emergence of magnetized plasma from the interior to the atmosphere and the ensuing reconnection processes.

Authors: Nóbrega-Siverio, D.; Martínez-Sykora, J.; Moreno-Insertis, F.; Rouppe van der Voort, L.
Projects: IRIS,Swedish Solar Telescope (SST)

Publication Status: ApJ (Accepted)
Last Modified: 2017-11-06 14:07
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Abstracts by Author
Ambipolar diffusion in the Bifrost code
Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes
On the importance of the nonequilibrium ionization of Si IV and O IV and the line-of-sight in solar surges
Surges and Si IV bursts in the solar atmosphere. Understanding IRIS and SST observations through RMHD experiments

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