Description: Color-combined magnetograms facilitate a comprehensive view of the evolution of active regions and their complexity. They offer a framework for the treatment of complex observations and can be used in pattern recognition, feature extraction, and flare-prediction schemes.
Journal info: Astronomy & Astrophysics, Volume 691, id.A119, 10 pp.
Authors: Kontogiannis, I. ; Pietrow, A. G. M. ; Druett, M. K. ; Dineva, E. ; Verma, M. ; Denker, C.
Description: We present five categories of spectral profiles within the general body of the flare ribbon: (1) extremely broadened spectral line profiles, where the standard Fabry–Perot interferometer wavelength windows are not sufficiently wide to allow for analysis of the dynamics and atmospheric conditions; (2) long-lived, dense kernels that manifest as more saturated chromospheric line profiles with lower signal in both Stokes parameters. They are interpreted as footpoints of bunched magnetic field loops, whose chromospheric lines form at greater heights than the nearby areas; (3) Doppler-shifted edges of the expanding flare ribbon; (4) condensed coronal rain overlapping the flare ribbons in the line of sight, producing exceptionally high Doppler shifts; and (5) compact blueshifted areas close to areas with coronal rain down-flows. Additionally, a ribbon formation height of about 700 km with respect to penumbral features is estimated using correlating structures on the ribbon and the underlying photosphere.
Journal info: Astronomy & Astrophysics, Volume 685, id.A137, 11 pp.
Authors: Pietrow, A. G. M. ; Druett, M. K. ; Singh, V.
Description: We present an analysis of multidimensional flare simulations, including an analysis of chromospheric upflows and downflows that provides important groundwork for comparing 1D and multidimensional models. Several multidimensional phenomena are critical in determining plasma behavior but are not generally considered in 1D flare simulations. They include loop-top turbulence, reconnection outflow jets, heat diffusion, compressive heating from the multidimensional expansion of the flux tubes due to changing pressures, and the interactions of upward and downward flows from the evaporation meeting the material squeezed downward from the loop tops.
Journal info: Astronomy & Astrophysics, Volume 684, id.A171, 26 pp.
Description: Reconciling Sun-as-a-star spectroscopy and high-spatial resolution solar observations in the context of the solar-stellar connection.
Journal info: Astronomy & Astrophysics, Volume 682, id.A46, 20 pp.
Authors: AGM Pietrow, M Cretignier, MK Druett, JD Alvarado-Gómez, SJ Hofmeister, M Verma, R Kamlah, M Baratella, EM Amazo-Gómez, I Kontogiannis, E Dineva, A Warmuth, C Denker, K Poppenhaeger, Oleksii Andriienko, X Dumusque, Mats G Löfdahl
Description: Evaporation of chromospheric plasma by particle beams has been a standard feature of models of solar flares for many decades, supported both by observations of strong hard X-ray bremsstrahlung signals, and detailed 1D hydrodynamic radiative transfer models with near-relativistic electron beams included. Here we present the first multi-dimensional flare simulation featuring evaporation driven by energetic electrons..
Journal info: Solar Physics, Volume 298, Issue 11, article id.134
Authors: Druett, Malcolm Keith ; Ruan, Wenzhi ; Keppens, Rony
Description: Partial ionization is critical in determining the behaviour of shocks. Accurately modelling partially ionized shocks requires careful treatment of the ionized and neutral species, and their interactions. Here we study a partially ionized switch-off slow-mode shock (typical of magnetic reconnection in the chromosphere) using a multilevel hydrogen model with both collisional and radiative ionization and recombination rates. The multilevel hydrogen model differs significantly from magnetohydrodynamic shocks. Since partially ionized shocks are not accurately described by the Rankine-Hugoniot shock jump conditions, it may be incorrect to use these to infer properties of lower atmospheric shocks.
Journal info: Monthly Notices of the Royal Astronomical Society, Volume 525, Issue 3, pp.4717-4734
Description: An investigation into the formation of fibrils in the solar chomosphere using the Bifrost code. The fibrils in our simulation were found to be produced by a process different to the typical formation methods previously suggested.
Journal info: Astronomy & Astrophysics, Volume 665, id.A6, 20 pp.
Authors: Druett, M. K. ; Leenaarts, J. ; Carlsson, M. ; Szydlarski, M.
Description: This paper introduces the COCOPLOT software. This method collapses one dimension of 3D data into a representative colour, and plots this over a 2D image. The image can be used to instantly understand the general shape of the data in that dimension, working similarly to the human eye. This method is used primarily to understand the spectral line shapes present in a 3D datacube with spectral information on the collapsed axis and the image in the remaining two dimensions, see image below.
Journal info: RAS Techniques and Instruments, Vol. 1, Issue 1, pp. 29-42
Authors: Druett, Malcolm K. ; Pietrow, Alexander G. M. ; Vissers, Gregal J. M. ; Robustini, Carolina ; Calvo, Flavio
Description: Using highly broadened spectral line emission as "pseudo-continuum" backlighting we were able to provide the first observational estimate of the mass in a fan jet, which is a plasma jet situated over a sunspot lightbrige. State-of-the-art inversions of other spectral lines corroborated the validity of our assumptions. This novel approach to a unique observation was praised by the anonymous reviewer as, “a result of substantial importance for solar physics”. I designed this project independently, it was conducted and developed together by Alex Pietrow, a PhD student at the time, with me as supervisor.
Journal info: Astronomy & Astrophysics, Volume 659, id.A58, 12 pp.
Authors: Pietrow, A. G. M. ; Druett, M. K. ; de la Cruz Rodriguez, J. ; Calvo, F. ; Kiselman, D.
Description: Interpretation of the observations of the flare from 6 September 2017 reported in Paper I. These include gamma-ray (GR), hard X-ray (HXR), soft X-rays, Lyα line, extreme ultraviolet (EUV), Hα, and white light (WL) emission
Journal info: Astronomy & Astrophysics, Volume 639, id.A79, 20 pp.
Authors: Zharkova, Valentina ; Zharkov, Sergei ; Druett, Malcolm ; Matthews, Sarah ; Inoue, Satoshi
Description: We employed non-linear force-free field extrapolations followed by magnetohydrodynamic simulations in order to identify the presence of several magnetic flux ropes prior to the initiation of this X9.3 flare. Sunquakes were observed using the directional holography and time-distance diagram detection techniques. For each flaring event we detect a few seismic sources, or sunquakes, using Dopplergrams from the HMI/SDO instrument coinciding with the kernels of Hα line emission with strong redshifts and white light sources.
Journal info: Astronomy & Astrophysics, Volume 639, id.A78, 19 pp.
Authors: Zharkov, Sergei ; Matthews, Sarah ; Zharkova, Valentina ; Druett, Malcolm ; Inoue, Satoshi ; Dammasch, Ingolf E. ; Macrae, Connor
Description: During an electron beam injection (impulsive) phase of a flare the Lyman continuum emission is greatly enhanced in a large range of wavelengths resulting in a flattened distribution of Lyman continuum over wavelengths. After the beam is switched off, Lyman continuum emission, because of its large opacity, sustains, for a very long time, the high ionisation degree of the flaring plasma gained during the beam injection. This leads to a long enhancement of hydrogen ionisation, the occurrence of white light flares, and an increase of Lyman line emission in cores and wings. Lyman line shapes are moved closer to those from complete redistribution (CRD) in frequencies, and away from the partial ones (PRD) derived in the non-flaring atmospheres. In addition, Lyman line profiles can reflect macro-motions of a flaring atmosphere caused by downward hydrodynamic shocks produced in response to the beam injection reflected in the enhancements of Ly-line red wing emission.
Journal info: Astronomy & Astrophysics, Volume 623, id.A20, 22 pp.
Description: Introduction to the HYDRO2GEN code: Flaring atmospheres are considered to be produced by a 1D hydrodynamic response to the injection of an electron beam defining their kinetic temperatures, densities, and macro velocities. We simulated a radiative response in these atmospheres using a fully non-local thermodynamic equilibrium (NLTE) approach for a 5-level plus continuum hydrogen atom model, considering its excitation and ionisation by spontaneous, external, and internal diffusive radiation and by inelastic collisions with thermal and beam electrons. Simultaneous steady-state and integral radiative transfer equations in all optically thick transitions (Lyman and Balmer series) were solved iteratively for all the transitions to define their source functions with the relative accuracy of 10-5. The solutions of the radiative transfer equations were found using the L2 approximation. Resulting intensities of hydrogen line and continuum emission were also calculated for Balmer and Paschen series.
Journal info: Astronomy & Astrophysics, Volume 610, id.A68, 29 pp.
Description: The X2.1 class flare that occurred on September 6, 2011 was associated with the first of two homologous white light flares produced by this region, but no sunquake was found with it despite the one being detected in the second flare of 7 September 2011. In this paper we present the first observation of a sunquake for the 6 September 2011 flare detected via statistical significance analysis of egression power and verified via directional holography and time-distance diagram.
Journal info: Astronomy & Astrophysics, Volume 619, id.A65, 14 pp.
Authors: Macrae, Connor ; Zharkov, Sergei ; Zharkova, Valentina ; Druett, Malcolm ; Matthews, Sarah ; Kawate, Tomoko
Description: The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1-4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory.
