Mixing milk and coffee: how the magnetic field diffuses in the solar photosphere

In the photosphere, turbulent motions of the plasma lead to flows similar to those created when coffee is poured into milk. This is called diffusion, and can be studied using spectropolarimetric instrumentation. The European Solar Telescope advanced capabilities will help understand how it is produced in the solar surface.  A post written by Dr. Luca Giovannelli, from University of Rome Tor Vergata (Italy).

 

Small magnetic elements, detected in circular polarization and marked with red crosses, as they move on the solar photosphere superdiffusively. The background shows the corresponding white light observations, with convection cells clearly visible. / Video: Giannattasio et al. 2014; Hinode Operation Plan 151

The photosphere is the deepest layer of the solar atmosphere that we can observe directly. It is made up of plasma at nearly 6000 K. Its motion is driven by turbulent convection which leads to flows, similar to what is is observed in everyday phenomena such as the eddies in a river, the smoke from a chimney or coffee spreading in the milk in your cup.

If you pour coffee in the milk it starts to spread, and the amount of milk changing colour increases with time: this process is called diffusion. To describe this phenomenon, we can measure how much milk has changed colour in a fixed amount of time: in doing so, we’ll find that this happens at a fixed rate. This is called normal diffusion, but it is not the only option! In some cases, diffusion can increase with time, spreading much more the particles: this is called super-diffusion and is the regime we observe for the magnetic field in the solar photosphere.

In the Sun, we do not have complete access to the details of the turbulent motion of the plasma since it happens on very small spatial scales. Yet, we can derive its properties from passive tracers that are transported by the horizontal plasma flows on the solar surface. Those tracers are the small-scale magnetic elements that can be observed everywhere in the quiet Sun. They have sizes of about 100 km and may last several hours, pushed around by the horizontal motion of the surrounding plasma as corks in a river. The way they spread, as the coffee in the milk, traces the plasma motion and can improve our understanding on the dynamics of the photosphere.

The mechanism behind super-diffusivity is not yet fully understood, and will require more detailed observations, at smaller spatial scales and extended in time. The European Solar Telescope,with its unprecedented spatial resolution and its unique spectropolarimetric capabilities, will provide exceptional observations that will help us understand the solar photosphere dynamics.

For more information, see Giannattasio et al. (2014).

 

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