Small-scale solar vortex flows

Dr. Kostas Tziotziou, from the National Observatory of Athens (Greece), writes this post about small-scale vortex flows.



High spatial and temporal resolution observations have shown the existence of ubiquitous small-scale swirling motions (more than 10,000 at any given time across the Sun) with a mean radius of about 2000 km that last approximately 5-10 minutes. Theory and mainly numerical simulations show that these are formed at the boundaries of convective cells of size about 1000 km called granules that correspond to bubbles of hot material rising from the sub-surface layers of the Sun (resembling bubbles of water in a boiling kettle). When these bubbles reach the solar surface, their material cools down and starts falling back towards their edges called intergranular lanes, often in a swirling motion like water flowing in a bathtub. As intergranular lanes are also the locations of magnetic field concentrations called bright points, the downward swirling motions can sometimes subsequently cause the rotation of these magnetic field structures and of the plasma material travelling upwards along them, forming a structure resembling a dust-devil or an upwards moving mini-tornado. Such a swirling structure can act as a channel for transferring material and energy (often in the form of waves) from the lower solar atmospheric heights higher up.

Recently, such swirling motions have also been detected in observations of one of the spectral lines of hydrogen (H-alpha), the most abundant solar element. For the first time, a long duration (at least 1.7 hours) small-scale vortex flow event has been observed with the Swedish 1-m Solar Telescope (La Palma, Spain). This vortex flow has a radius of about 2200 km and shows complex substructure as it seems to consist of several individual, intermittent, recurring smaller swirling motions (see movie). For context, 1 arcsec on the solar surface corresponds to 725 km, so the solar area depicted in the movie is about 60 million square kilometres. 

Despite recent advancements in observations, theory, simulations and modelling, the detection and precise physics of these swirling motions and how they transport energy in the solar atmosphere remain poorly understood. The European Solar Telescope, with its foreseen advanced instrumentation, will enable us to detect and follow such structures in even smaller scales simultaneously in several atmospheric layers, measure their magnetic field and investigate in further detail their properties and dynamics.


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