Luc Rouppe van der Voort

Magnetic tornadoes as energy channels into the solar corona

Heating the outer layers of the magnetically quiet solar atmosphere to more than one million kelvin and accelerating the solar wind requires an energy flux of approximately 100 to 300 watts per square metre, but how this energy is transferred and dissipated there is a puzzle and several alternative solutions have been proposed. Braiding and twisting of magnetic field structures, which is caused by the convective flows at the solar surface, was suggested as an efficient mechanism for atmospheric heating. Convectively driven vortex flows that harbour magnetic fields are observed to be abundant in the photosphere (the visible surface of the Sun). Recently, corresponding swirling motions have been discovered in the chromosphere, the atmospheric layer sandwiched between the photosphere and the corona. Here we report the imprints of these chromospheric swirls in the transition region and low corona, and identify them as observational signatures of rapidly rotating magnetic structures. These ubiquitous structures, which resemble super-tornadoes under solar conditions, reach from the convection zone into the upper solar atmosphere and provide an alternative mechanism for channelling energy from the lower into the upper solar atmosphere.

Dark cores in sunspot penumbral filaments

Sunspot umbrae—the dark central regions of the spots—are surrounded by brighter filamentary penumbrae, the existence of which remains largely inexplicable. The penumbral filaments contain magnetic fields with varying inclinations and are associated with flowing gas, but discriminating between theoretical models has been difficult because the structure of the filaments has not hitherto been resolved. Here we report observations of penumbral filaments that reveal dark cores inside them. We cannot determine the nature of these dark cores, but their very existence provides a crucial test for any model of penumbrae. Our images also reveal other very small structures, in line with the view that many of the fundamental physical processes in the solar photosphere occur on scales smaller than 100 km.

Are giant tornadoes the legs of solar prominences

Observations in the 171 A channel of the Atmospheric Imaging Assembly of the space-borne Solar Dynamics Observatory show tornado-like features in the atmosphere of the Sun. These giant tornadoes appear as dark, elongated, and apparently rotating structures in front of a brighter background. This phenomenon is thought to be produced by rotating magnetic field structures that extend throughout the atmosphere. We characterize giant tornadoes through a statistical analysis of properties such as spatial distribution, lifetimes, and sizes. A total number of 201 giant tornadoes are detected in a period of 25 days, suggesting that, on average, about 30 events are present across the whole Sun at a time close to solar maximum. Most tornadoes appear in groups and seem to form the legs of prominences, thus serving as plasma sources/sinks. Additional Hα observations with the Swedish 1 m Solar Telescope imply that giant tornadoes rotate as a structure, although they clearly exhibit a thread-like structure. We observe tornado groups that grow prior to the eruption of the connected prominence. The rotation of the tornadoes may progressively twist the magnetic structure of the prominence until it becomes unstable and erupts. Finally, we investigate the potential relation of giant tornadoes to other phenomena, which may also be produced by rotating magnetic field structures. A comparison to cyclones, magnetic tornadoes, and spicules implies that such events are more abundant and short-lived the smaller they are. This comparison might help to construct a power law for the effective atmospheric heating contribution as a function of spatial scale.

Ellerman bombs: fallacies, fads, usage

Ellerman bombs are short-lived brightenings of the outer wings of Halpha that occur in active regions with much flux emergence. We point out fads and fallacies in the extensive Ellerman bomb literature, discuss their appearance in various spectral diagnostics, and advocate their use as indicators of field reconfiguration in active-region topography using AIA 1700 A images.

On fibrils and field lines: The nature of Hα fibrils in the solar chromosphere

Observations of the solar chromosphere in the line core of the Hα line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network region and computed synthetic Hα images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phase speeds of the simulated fibrils are consistent with observations. We find that some fibrils trace out the same field line along the fibrils length, while other fibrils sample different field lines at different locations along their length. Fibrils sample the same field lines on a timescale of ~200 s. This is shorter than their own lifetime. Fibril-threading field lines carry slow-mode waves, as well as transverse waves propagating with the Alfven speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is governed by constant migration and swaying of the field lines, their mass loading and draining, and their visibility in Hα. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is at a height at which the column mass reaches a certain fixed value. The visibility of the field line is thus determined by its own mass density and by the mass density of the material above it. Using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution.

TEMPORAL EVOLUTION OF VELOCITY AND MAGNETIC FIELD IN AND AROUND UMBRAL DOTS

We study the temporal evolution of umbral dots (UDs) using measurements from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope. Scans of the magnetically sensitive 630 nm iron ...

VELOCITIES MEASURED IN SMALL-SCALE SOLAR MAGNETIC ELEMENTS

We haveobtained high-resolution spectrogramsofsmall-scalemagneticstructureswiththe Swedish 1-m Solar Telescope. We present Doppler measurements at 0.2 00 spatial resolution of bright points, ribbons, and flowers, and their immediate surroundings, in the C i k5380.3 line (formed in the deep photosphere) and the two Fe i lines at 5379.6 and 5386.3 8. The velocity inside the flowers and ribbons are measured to be almost zero, while we observe downflows at the edges. These downflows are increasing with decreasing height. We also analyze realistic magnetoconvective simulationstoobtainabetterunderstandingoftheinterpretationoftheobservedsignal.WecalculatehowtheDopplersignal depends on the velocity field in various structures. Both the smearing effect of the nonnegligible width of this velocity response function along the line of sight and of the smearing from the telescope and atmospheric point-spread function are discussed. These studies lead us to the conclusion that the velocity inside the magnetic elements is really upflow of order 1Y 2k m s � 1 , while the downflows at the edges really are much stronger than observed, of order 1.5Y3.3 km s � 1 .

SPECTROSCOPIC MEASUREMENTS OF DYNAMIC FIBRILS IN THE Ca ii k8662 LINE

We present high spatial resolution spectroscopic measurements of dynamic fibrils (DFs) in the Ca ii k8662 line. These data show clear Doppler shifts in the identified DFs, which demonstrates that at least a subset of DFs are actual mass motions in the chromosphere. A statistical analysis of 26 DFs reveals a strong and statistically significant correlation between the maximal velocity and the deceleration. The range of the velocities and the decelerations are substantially lower, about a factor 2, in our spectroscopic observations compared to the earlier results based on proper motion in narrowband images. There are fundamental differences in the different observational methods; when DFs are observed spectroscopically, the measured Doppler shifts are a result of the atmospheric velocity, weighted with theresponsefunctionto velocityover anextended height.Whenthe propermotionof DFsis observedinnarrowband images, the movement of the top of the DF is observed. This point is sharply defined because of the high contrast between the DF and the surroundings. The observational differences between the two methods are examined by sev

Fan-shaped jets above the light bridge of a sunspot driven by reconnection

We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the H

INTERNETWORK CHROMOSPHERIC BRIGHT GRAINS OBSERVED WITH IRIS AND SST

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