Horizontal plasma velocities on the Sun at high resolution

Our latest #TheScienceofEST post is brought to you by Souvik Bose from the Institute of Theoretical Astrophysics (Norway). Convection, advection, and how the European Solar Telescope will help understand the solar granulation.



CHROMIS observations of a 20,000 square kilometres area of solar granulation at the disk center


The surface of the Sun is covered with small cellular features called solar granules. Granules have a mottled appearance and are the result of convection below the photosphere. Convection is a heat transfer mechanism involving the bulk motion of fluids, that is, gases and liquids. In the Sun, convection produces columns of hot rising gas just below the photosphere that are about 700 to 1000 km in diameter. The tops of these columns appear as bright gray-white cells in typical granulation images. The hot gas then cools off and sinks down in the relatively darker regions around each granule. This is similar to water boiling in a pot. Granules last for about 10-20 minutes before being dissipated.

During this process, the hot gases rise and subsequently sink down with a velocity of several kilometers per second. Such values have been derived from the gas motions directed towards or away from the observer, commonly called upflows and downflows. The upflows and downflows are accompanied by a horizontal motion of the gas, as soon as it appears on the surface. This horizontal motion is referred to as "advection" and is a sub-mechanism of convection. In other words, once the hot gases rise from the convection zone, they are "dragged" horizontally (on the solar surface) by advection before they cool off and sink down in the darker regions around a granule. This results in a horizontal motion of the plasma that is difficult to measure, unlike the up- and downflows.

With the advent of the 4-m European Solar Telescope, the surface of the Sun will be observed with unprecedented detail, making it possible to track very small-scale horizontal velocities. Such measurements will help us understand the twisting motions responsible for the generation of vortex flows and waves that propagate higher up in the solar atmosphere.

The accompanying movie shows the spatio-temporal evolution of "corks" (orange-colored particles) in an area of 20,000 square km near the disk center as observed by the CHROMIS instrument at the Swedish 1-m Solar Telescope. The corks evolve with time following a certain trajectory based on the horizontal velocity field at any given location. We use a fully convolutional deep neural network code called DEEPVEL to infer the horizontal velocities from the observations. Initially, the corks are tagged to every point of the image. Then they are allowed to evolve based ont he horizontal velocities of that pixel. As expected, this analysis shows the advection of the corks from the brighter regions towards the darker boundaries of the granules.