| 1. |
- van der Voort, L. H. M. Rouppe, et al.
(författare)
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High-resolution observations of the solar photosphere, chromosphere, and transition region : A database of coordinated IRIS and SST observations
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 641
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Tidskriftsartikel (refereegranskat)abstract
- NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe I lines and spectrally resolved imaging in the chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.
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| 2. |
- van der Voort, L. Rouppe, et al.
(författare)
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Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere
- 2017
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 851:1
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry Perot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in socalled UV bursts in the low solar atmosphere. The Si IV 1403 angstrom UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca 11 K 3934 angstrom line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.12 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.
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| 3. |
- Vissers, Gregal J. M., et al.
(författare)
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Dissecting bombs and bursts : non-LTE inversions of low-atmosphere reconnection in SST and IRIS observations
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 627
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Tidskriftsartikel (refereegranskat)abstract
- Ellerman bombs and UV bursts are transient brightenings that are ubiquitously observed in the lower atmospheres of active and emerging flux regions. As they are believed to pinpoint sites of magnetic reconnection in reconfiguring fields, understanding their occurrence and detailed evolution may provide useful insight into the overall evolution of active regions. Here we present results from inversions of SST / CRISP and CHROMIS, as well as IRIS data of such transient events. Combining information from the Mg ii h& k, Si IV, and Ca II 8542 angstrom and Ca II H & K lines, we aim to characterise their temperature and velocity stratification, as well as their magnetic field configuration. We find average temperature enhancements of a few thousand kelvin, close to the classical temperature minimum and similar to previous studies, but localised peak temperatures of up to 10 000-15 000K from Ca ii inversions. Including Mg ii appears to generally dampen these temperature enhancements to below 8000 K, while Si IV requires temperatures in excess of 10 000K at low heights, but may also be reproduced with secondary temperature enhancements of 35 000-60 000K higher up. However, reproducing Si iv comes at the expense of overestimating the Mg ii emission. The line-of-sight velocity maps show clear bidirectional jet signatures for some events and strong correlation with substructure in the intensity images in general. Absolute line-of-sight velocities range between 5 and 20 km s(-1) on average, with slightly larger velocities towards, rather than away from, the observer. The inverted magnetic field parameters show an enhancement of the horizontal field co-located with the brightenings at heights similar to that of the temperature increase. We are thus able to largely reproduce the observational properties of Ellerman bombs with the UV burst signature (e. g. intensities, profile asymmetries, morphology, and bi-directional jet signatures), with temperature stratifications peaking close to the classical temperature minimum. Correctly modelling the Si IV emission in agreement with all other diagnostics is however an outstanding issue and remains paramount in explaining its apparent coincidence with H alpha emission. Finetuning the approach (accounting for resolution di ff erences, fitting localised temperature enhancements, and / or performing spatially coupled inversions) is likely necessary in order to obtain better agreement between all considered diagnostics.
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| 4. |
- Young, Peter R., et al.
(författare)
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Solar Ultraviolet Bursts
- 2018
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Ingår i: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 214:8
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Forskningsöversikt (refereegranskat)abstract
- The term ultraviolet (UV) burst is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016-2017.
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| 5. |
- de la Cruz Rodriguez, Jaime, et al.
(författare)
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Heating of the Magnetic Chromosphere : Observational Constraints from Ca II lambda 8542 Spectra
- 2013
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Ingår i: The Astrophysical Journal Letters. - 2041-8205. ; 764:1, s. L11-
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Tidskriftsartikel (refereegranskat)abstract
- The heating of the Sun's chromosphere remains poorly understood. While progress has been made on understanding what drives the quiet-Sun internetwork chromosphere, chromospheric heating in strong magnetic field regions continues to present a difficult challenge, mostly because of a lack of observational constraints. We use high-resolution spectropolarimetric data from the Swedish 1 m Solar Telescope to identify the location and spatio-temporal properties of heating in the magnetic chromosphere. In particular, we report the existence of raised-core spectral line profiles in the Ca II lambda 8542 line. These profiles are characterized by the absence of an absorption line core, showing a quasi-flat profile between. lambda approximate to +/- 0.5 angstrom, and are abundant close to magnetic bright points and plage. Comparison with three-dimensional MHD simulations indicates that such profiles occur when the line of sight goes through an "elevated temperature canopy" associated with the expansion with height of the magnetic field of flux concentrations. This temperature canopy in the simulations is caused by ohmic dissipation where there are strong magnetic field gradients. The raised-core profiles are thus indicators of locations of increased chromospheric heating. We characterize the location and temporal and spatial properties of such profiles in our observations, thus providing much stricter constraints on theoretical models of chromospheric heating mechanisms than before.
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| 6. |
- Hansteen, V. H., et al.
(författare)
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Bombs and Flares at the Surface and Lower Atmosphere of the Sun
- 2017
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 839:1
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Tidskriftsartikel (refereegranskat)abstract
- A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Ha line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma bombs (UV bursts) with high temperatures of perhaps up to 8 x 10(4) K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (10(6) K) in the upper chromosphere. These results provide new insights into. the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere.
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| 7. |
- Rodriguez, Jamiel de la Cruz, et al.
(författare)
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Physical properties of a sunspot chromosphere with umbral flashes
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. A115-
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Tidskriftsartikel (refereegranskat)abstract
- We present new high-resolution spectro-polarimetric Ca II lambda 8542 observations of umbral flashes in sunspots. At nearly 0 ''.18, and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). From a fast Fourier transform analysis, we find a trend of decreasing power at high temporal frequencies at those locations with more horizontal magnetic fields, for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.
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