Abstract
Context. Exploding granules have drawn renewed interest because of their interaction with the magnetic field (either emerging or already present). Especially the newly forming downflow lanes developing in their centre seem to be eligible candidates for the intensification of magnetic fields. We analyse spectroscopic data from two different instruments in order to study the intricate velocity pattern within the newly forming downflow lanes in detail. Aims. We aim to examine general properties of a number of exploding granules, such as their lifetime and extend. To gain a better understanding of the formation process of the developing intergranular lane in exploding granules, we study the temporal evolution and height dependence of the line-of-sight velocities at their formation location. Additionally, we search for evidence that exploding granules act as acoustic sources. Methods. We investigated the evolution of several exploding granules using data taken with the Interferometric Bidimensional Spectrometer and the Imaging Magnetograph eXperiment. Velocities for different heights of the solar atmosphere were determined by computing bisectors of the Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet analysis to study the intensity and velocity oscillations within and around exploding granules. We also compared our observational findings with predictions of numerical simulations. Results. Exploding granules have significantly longer lifetimes (10 to 15 min) than regular granules. Exploding granules larger than 3.8″ form an independent intergranular lane during their decay phase, while smaller granules usually fade away or disappear into the intergranular area (we find only one exception of a smaller exploding granule that also forms an intergranular lane). For all exploding granules that form a new intergranular downflow lane, we find a temporal height-dependent shift with respect to the maximum of the downflow velocity. Our suggestion that this results from a complex atmospheric structure within the newly forming downflow lane is supported by the comparison with synthesised profiles inferred from the simulations. We found an enhanced wavelet power with periods between 120 s to 190 s seen in the intensity and velocity oscillations of high photospheric or chromospheric spectral lines in the region of the dark core of an exploding granule.
Highlights
Observations of the solar photosphere show a granular pattern that is driven by convection, with granules evolving on timescales of minutes and varying in shapes and sizes
An identifiable group of granules are the so-called exploding granules. They can be distinguished from regular granules by their rapid horizontal expansion with speeds between 1.7 km s−1 and 3.2 km s−1 (Namba 1986), and especially by the dark core they develop, which is seen as a reduction in continuum intensity
We performed a visual inspection of the continuum image time series of the IBIS and Imaging Magnetograph eXperiment (IMaX) data and searched for exploding granules
Summary
Observations of the solar photosphere show a granular pattern that is driven by convection, with granules evolving on timescales of minutes and varying in shapes and sizes. An identifiable group of granules are the so-called exploding granules. They can be distinguished from regular granules by their rapid horizontal expansion with speeds between 1.7 km s−1 and 3.2 km s−1 (Namba 1986), and especially by the dark core they develop, which is seen as a reduction in continuum intensity. Exploding granules were first described by Carlier et al (1968) They found that exploding granules show a specific way of dissolving. After developing a dark core in their centre, which is surrounded by a bright doughnut-shaped structure, these granules split into a new generation of granules. Namba (1986) found that exploding granules occupy about 2.5% of the solar surface at any time and can reach diameters up to 5.5. The authors showed that the highest expansion velocities are reached initially, followed by a rapid decrease in the expansion velocity within the first two minutes
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