On the magnetic nature of quiet-Sun chromospheric grains

A grain event (bright points at the wing of the intensity of the Ca II 854.2 nm line) is shown to be compatible with a 4 min periodicity, and its occurrence (yellow dots) is associated to magnetic concentrations connecting the photosphere and the chromosphere.

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Adur Pastor Yabar

CaII Kgrains, i.e., intermittent, short-lived (about 1 minute), periodic (2-4 minutes), pointlike chromospheric brightenings, are considered to be the manifestations of acoustic waves propagating upward from the solar surface and developing into shocks in the chromosphere. After the simulations of Carlsson and Stein, we know that hot shocked gas moving upward interacting with the downflowing chromospheric gas (falling down after having been displaced upward by a previous shock) nicely reproduces the spectral features of the CaII K profiles observed in such grains, i.e., a narrowband emission-like feature at the blue side of the line core. However, these simulations are one-dimensional and cannot explain the location or the pointlike shape of the grains. Here, we report on the magnetic nature of these events. We then propose that magnetohydrodynamical waves are not only channeled through the magnetic field in sunspots (umbral flashes), but they pervade the whole atmosphere. The propagation along magnetic fields can explain the pointlike appearance of the calcium grains observed in the quiet chromosphere.