Search Publications
The multi-thermal chromosphere. Inversions of ALMA and IRIS data
De Pontieu, B.; Leenaarts, J.; da Silva Santos, J. M. +4 more
Context. Numerical simulations of the solar chromosphere predict a diverse thermal structure with both hot and cool regions. Observations of plage regions in particular typically feature broader and brighter chromospheric lines, which suggests that they are formed in hotter and denser conditions than in the quiet Sun, but also implies a nonthermal…
The influence of NLTE effects in Fe I lines on an inverted atmosphere. I. 6301 Å and 6302 Å lines formed in 1D NLTE
Solanki, S. K.; van Noort, M.; Smitha, H. N. +1 more
Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often n…
Directly comparing coronal and solar wind elemental fractionation
Owen, C. J.; Baker, D.; Brooks, D. H. +1 more
Context. As the solar wind propagates through the heliosphere, dynamical processes irreversibly erase the signatures of the near-Sun heating and acceleration processes. The elemental fractionation of the solar wind should not change during transit, however, making it an ideal tracer of these processes.
Aims: We aim to verify directly if the s…
Emergence of small-scale magnetic flux in the quiet Sun
Gontikakis, C.; Tziotziou, K.; Tsiropoula, G. +4 more
Context. We study the evolution of a small-scale emerging flux region (EFR) in the quiet Sun, from its emergence in the photosphere to its appearance in the corona and its decay.
Aims: We track processes and phenomena that take place across all atmospheric layers; we explore their interrelations and compare our findings with those from recent…
A new trigger mechanism for coronal mass ejections. The role of confined flares and photospheric motions in the formation of hot flux ropes
van Driel-Gesztelyi, L.; Green, L. M.; Valori, G. +1 more
Context. Many previous studies have shown that the magnetic precursor of a coronal mass ejection (CME) takes the form of a magnetic flux rope, and a subset of them have become known as "hot flux ropes" due to their emission signatures in ∼10 MK plasma.
Aims: We seek to identify the processes by which these hot flux ropes form, with a view of …
Stellar coronal X-ray emission and surface magnetic flux
Peter, H.; Zhuleku, J.; Warnecke, J.
Context. Observations show that the coronal X-ray emission of the Sun and other stars depends on the surface magnetic field.
Aims: Using power-law scaling relations between different physical parameters, we aim to build an analytical model to connect the observed X-ray emission to the surface magnetic flux.
Methods: The basis for our mod…
Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017. I. Observations
Dammasch, Ingolf E.; Matthews, Sarah; Inoue, Satoshi +4 more
Aims: The 6 September 2017 X9.3 solar flare produced very unique observations of magnetic field transients and a few seismic responses, or sunquakes, detected by the Helioseismic and Magnetic Imager (HMI) instrument aboard Solar Dynamic Observatory (SDO) spacecraft, including the strongest sunquake ever reported. This flare was one of a few f…
Connecting the Wilson depression to the magnetic field of sunspots
Solanki, S. K.; Lagg, A.; Löptien, B. +1 more
Context. In sunspots, the geometric height of continuum optical depth unity is depressed compared to the quiet Sun. This so-called Wilson depression is caused by the Lorentz force of the strong magnetic field inside the spots. However, it is not understood in detail yet how the Wilson depression is related to the strength and geometry of the magne…
No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots
Solanki, S. K.; Lagg, A.; Löptien, B. +1 more
Context. It has been reported that the boundary between the umbra and the penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted to be the threshold for the onset of magnetoconvection.
Aims: Here we investigate the reas…
Spectroscopic detection of coronal plasma flows in loops undergoing thermal non-equilibrium cycles
Auchère, Frédéric; Parenti, Susanna; Pelouze, Gabriel +3 more
Context. Long-period intensity pulsations were recently detected in the EUV emission of coronal loops and attributed to cycles of plasma evaporation and condensation driven by thermal non-equilibrium (TNE). Numerical simulations that reproduce this phenomenon also predict the formation of periodic flows of plasma at coronal temperatures along some…