Ca II H(2v) and K(2v) cell grains

The bright Ca II H(2v) and K(2v) grains, which are intermittently present in the interiors of network cells in quiet-sun areas, should provide important diagnostics of the dynamical interaction between the quiet photosphere and the chromosphere above it, but their nature has so far eluded identification. The extensive observational literature on these grains and on related phenomena is here reviewed, and various contradictions are resolved. It is concluded that the grains are a hydrodynamical phenomenon in which magnetic fields do not play a major role. The grains are due to interference between a pervasive standing oscillation and an 8 Mm horizontal wavelength in the chromosphere, and the wave trains of the evanescent p-mode interference pattern in the upper photosphere.