Solar Physics Research

Sunspots

A round sunspot observed by Luc Rouppe van der Voort using the Swedish Solar Telescope (SST) in 2010 [1]. The dark region in the middle (the umbra) is the coolest part of the sunspot.

What are sunspots?
Sunspots are dark regions on the sun that can often be seen dotted about the solar disk during solar maximum (also during solar minimum but much less frequent). They are caused by a high concentration of solar magnetic field lines that inhibit the flow of hot plasma from the interior, creating a region of cooler plasma that appears black against the much hotter and more luminous plasma of the photosphere.

Why study sunspots?
The most obvious reason to study sunspots is that they are very easy to see. Often comparable in size to the Earth, sunspots have been observed for centuries (and perhaps even millennia) by civilizations around the world[2]. Now we have the technology to observe them in greater detail than ever before and their importance in understanding our nearest star has been realised. They are known to be closely linked to solar flares[3] and coronal mass ejections[4], they can have complex formations with multiple umbrae and vast stretches of penumbrae, and are a very important part of understanding the solar magnetic field and internal dynamo.

MLT for Sunspot Analysis

MLT applied to a threshold layer at 55% quiet-sun intensity. The spot on the left is split up into pixels below 55% (black) and pixels above (white). These are then clustered together using a clustering algorithm to split the layer into different clusters corresponding to umbrae in the sunspot.
Layers can be taken at different intensity levels and stacked to analyse the difference in rotation rate of interior and edge of sunspots.

The Solar Dynamics Observatory is a NASA spacecraft that is in a polar orbit of the Earth, constantly facing the Sun. It has been taking pictures of the sun in several different wavelengths every 45s since 2009. This continuous, unobstructed view of the sun lets us analyse changes on the surface of the Sun before, during, and after large solar flares. In my work, I use the images of sunspots during these flare events to understand how they affect the structure of sunspots, and thus the solar magnetic field.

I developed a technique, known as Multi-Layer Thresholding (MLT), that allows us to track the rotation of sunspots over long periods of time and see how it is affected by solar flares. The technique is an extension of traditional methods and has the advantage of being able to track multiple regions simultaneously. MLT is built up of multiple threshold layers, which can be thought of as slices across the sunspot images at different brightness levels (temperatures). In each layer, the various sunspots and umbrae appear as different blobs. Depending on how dark the sunspots are, different parts of them will show up in different layers. In all of these layers, however, each independent blob is treated as a separate cluster. After all the clusters in all the layers have been found, they can be matched up with one another; clusters that have the same position in different layers can be stacked-up.

Each of these clusters can be tracked independently, and it was because of this that we were able to observe evidence of differential rotation within a sunspot umbra.


Publications

Posters

A collection of all the posters and supporting media I made for the conferences I have attended during my PhD.

 


     

Resources

A list of presentations and resources I have made and delivered during my studies.

References


2) F Richard Stephenson, David M Willis, The earliest drawing of sunspots, Astronomy & Geophysics, Volume 40, Issue 6, December 1999, Pages 6.21–6.22, https://doi.org/10.1093/astrog/40.6.6.21
3) Solar flare wikipedia page https://en.wikipedia.org/wiki/Solar_flare
4) Coronal mass ejections wikipedia page https://en.wikipedia.org/wiki/Coronal_mass_ejection
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