Small-Scale Secrets of Sunspots

Despite being one of the most widely studied phenomenon of the Sun, there is much yet to learn about sunspots, writes Dr. Christopher Nelson, from University of Sheffield and Queen's University Belfast (UK), in this new #TheScienceOfEST post. 



Movie showing the dynamics of sunspots in the photosphere and the chromosphere, as observed with the CRISP instrument at the SST on La Palma

Sunspots are one of the most widely studied features on the Sun. These large events, which have been observed for millennia and whose umbrae manifest as dark regions against the bright photospheric background see the left panel of the movie), indicate locations where extremely strong vertical magnetic field inhibits local convection, thereby reducing the local temperature. When looked at using data with low spatial resolution, sunspots seem to be homogeneous dark regions which evolve slowly - some would say they even appear to be boring. But when looked at in high resolution using state-of-the-art ground-based telescopes, numerous small-scale phenomena can be observed, giving us in-sights into the complex physics which takes place within sunspots.

Recent observations have shown the presence of extremely small-scale brightenings in the umbra at the foot-points of short, thin jets within sunspots. These brightenings appear to form when material within the jets begins to flow downwards, back towards the photosphere, leading to the occurrence of magneto-hydrodynamic shocks in the lower solar atmosphere. The spectral profiles of these events are similar to those observed in other shocks in sunspots such as umbral flashes, localised transient brightenings that can be seen in the right-hand panel of the featured movie. The extremely high spatial and temporal resolution observations which will be available from the European Solar Telescope will allow us to better understand the dynamics of extremely small-scale features in sunspot umbrae and, hence, better understand the complex physics of our local star.

The movie shows images taken with the CRisp Imaging SpectroPolarimeter (CRISP) instrument at the Swedish 1-m Solar Telescope (SST, La Palma, Spain) over a 40 minute period on the 28th July 2014. It shows the various dynamical processes occurring within sunspots at multiple layers in the solar atmosphere. The photosphere on the left is relatively stable with only slow evolution apparent, but the time series on the right displays the highly vibrant chromosphere, including the umbral flashes previously discussed.

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