subgranules

Subgranules are small-scale convective patterns observed within solar granules in the Sun’s photosphere. They describe structure and dynamics finer than the usual granule scale, with characteristic sizes of a few hundred kilometers, well below the typical granule diameter of about 1,000 kilometers (1 Mm). Subgranular features are transient, evolving on timescales of minutes.

Observationally, subgranules are revealed by high-resolution imaging and spectroscopic measurements that capture brightness fluctuations and Doppler shifts within granules. They often appear as mottled brightness regions accompanied by localized upflows and downflows, and may include small-scale vortices or narrow downflow lanes that fragment the granule interior. The patterns are dynamic, continually forming, evolving, and dissolving as convective motions rearrange the flow.

Physically, subgranulation reflects the turbulent nature of near-surface solar convection. Within a granule, hot plasma rises and spreads outward, cooling as it approaches the edges, where it sinks along downdrafts. Within this broad flow, smaller-scale fragmentation and shear produce the subgranular structure. Magnetic fields interact with these flows, concentrating in downflow regions and influencing the appearance and evolution of subgranular features, thereby impacting local energy transport and line formation.

Relation to broader scales: subgranulation is part of the hierarchical convection in the solar photosphere, lying between granulation (the larger cells) and the smaller, unresolved turbulent scales. It complements studies of mesogranulation and supergranulation that describe larger-scale patterns.

See also: granulation, supergranulation, mesogranulation, solar photosphere, solar convection.