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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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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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