Families of Granules, Flows, and Acoustic Events in the Solar Atmosphere from Hinode Observations

Abstract

We investigate the relationship between trees of fragmenting granules (TFG), horizontal and vertical flows, and acoustic events (AE) in the photospheric network. AE are spatially concentrated and short-duration locations of acoustic energy flux. We performed observations at disk center of a 2D field of view (FOV) with high spatial and temporal resolutions provided by the Solar Optical Telescope onboard Hinode. Line profiles of Fe i 557.6 nm were recorded by the Narrow-band Filter Imager on an 80″×36″ FOV during five hours with a cadence of 22 seconds and 0.08″ pixel size. Vertical velocities were derived at two atmospheric levels allowing the determination of the energy flux at the acoustic frequency of 3.3 mHz. Families of granules and horizontal velocities were obtained from local correlation tracking (LCT) after segmentation and labeling of either continuum intensities or granular Doppler shifts. AE exhibit durations in the range 0.25 to 1 hour compatible with the lifetime of families (80 % do not last more than two hours). High-energy AE have the shortest lifetimes. We found that most AE occur in intergranular lanes located in or close to the boundaries between different families (called inter families) in regions with predominantly downward vertical motions and horizontal converging flows. In contrast, diverging flows are observed inside families, with a few AE in the intergranules. At the beginning of the sequence, when families are not yet detected, the distribution of AE is not uniform and is already organized at spatial lengths related to the mesogranular scale, with maximum contribution in the range 5″ to 10″, fully compatible with the scale of the maximum contribution of families in the TFG space. Although all sizes and durations seem to exist for families, their number decreases with increasing size and lifetime.

Appendix

The Electronic Supplemental Material movies consist of 846 images of 998×448 pixels and are available either in MPEG1 or MPEG4 format (better quality). The acceleration factor is about 660 for MPEG1 and 275 for MPEG4. The pixel size is 0.08″ and the time step is 22 seconds. For horizontal velocities and divergences, the spatial and temporal resolutions are reduced, respectively, to 3″ and 30 minutes; interpolations were used.

• Movie 1 : Families of granules and Doppler shifts in Fe i 557.6 nm. Granules belonging to the same family have a uniform gray level corresponding to the family number. Families can be identified as mesogranules. Doppler velocities appear in blue/red for, respectively, upward/downward motions and affect both granules (mainly upward) and intergranules (mostly downward). Families detected by the LCT need several hours to appear.

• Movie 2 : Families of granules and Doppler gradient in Fe i 557.6 nm. Vertical gradients (through a 50 km layer) of velocities (core/inflection point difference) appear in blue/red, respectively, for positive/negative values. Positive gradients are dominant in intergranules.

• Movie 3 : Interfamilies, horizontal motions and AE. The boundaries between families of granules (interfamilies) are shown in gray. AE (white) form close to interfamilies. Quadratic horizontal velocities appear in gray levels with maxima mostly located inside families.

• Movie 4 : Interfamilies, divergence of horizontal velocities, and AE. Diverging/converging areas are, respectively, blue/red. Most diverging regions are located inside families, while boundaries (interfamilies) together with AE are more often associated to converging patterns.

• Movie 5 : AE are flux concentrations in space and time with lifetime in the range 0.25 to 1 hour. Upward/downward flux is shown, respectively, in white/black. The spatial distribution is not uniform: most of the flux is located near the boundaries between TFG (interfamilies at mesogranular scale).

• Movie 6 : Families of granules (gray levels) and AE. The major part of the flux (upward/downward, respectively, in blue/red) occur in intergranular lanes close to the boundaries between families (interfamilies). AE and families have lifetimes of the same order of magnitude.

• Movie 7 : Families of granules and AE. Granules belonging to the same family have uniform color. Most AE (white) appear in dark intergranular regions close to the boundaries between families.