Liam Edwards

PhD student at Aberystwyth university.



Rotation of the Solar Corona

Liam Edwards
The nature of the rotation of the Sun’s outer atmosphere – the corona – is not very well understood. Some studies state that the corona rotates as a rigid body, whereas others point towards a rotation which varies as a function of both latitude and height above the solar surface. This is partly due to the fact that, unlike the solar surface (photosphere) which has many prominent features that vary dynamically over time, the solar corona lacks features prominent enough to perform a sufficiently accurate tracking method to determine its rotation.

One method of determining this coronal rotation is by tracking coronal features using full-disk images of the Sun taken by satellites such as the Solar Dynamics Observatory (SDO), Yohkoh, and Hinode. My research focuses on using 3D density maps gained by tomography of the solar corona (see the “3D Maps of the Solar Atmosphere” section). These maps range from 4 – 8 solar radii above the Sun and cover around 12 years from 2007 – 2019. These maps provide a unique window of the coronal structure and its evolution over long time periods. The aim of my research is to track high density features (streamers) in these density maps and determine their rotation rate. Findings will have an impact on solar wind forecasting at Earth (the rotation rate is a fundamental parameter for forecasting) and understanding the magnetic connection between the photosphere and extended corona. Further details of this work can be found in Edwards et al. (2022).

Deriving electron densities from linear polarization observations of the white-light corona during the 2020 December 14 total solar eclipse

Liam Edwards
Total solar eclipses provide a unique - albeit temporary - window into the lower corona. Normally, researchers use coronagraphs, such as the Large Angle Spectrometric Coronagraph (LASCO), to observe the solar corona. A coronagraph is an observing instrument with an occulting disk which blocks the bright disk of the Sun, allowing the fainter corona to shine out behind it. However, the issue with coronagraphs lies in the fundamental nature of electromagnetic waves and how they scatter off the limb of the occulting disk and are therefore detected by the instrument as stray light. Consequently, the very lower part of the corona is difficult to observe in this way. This is where the perfect cosmic coincidence of a total solar eclipse comes into play. Since the Moon is 400 times smaller than the Sun, and the Sun happens to be 400 times further away from the Earth than the Moon, the lunar and solar disks, therefore, appear the same size in the sky to an observer on Earth – a perfect occulting disk!

One of the important physical parameters to determine when studying the solar corona is the electron density. This can be determined by taking white-light polarisation images of the corona during the totality of a total solar eclipse. Various image processing techniques can then be used to bring out as much information in the images as possible, followed by an inversion technique, originally developed in the 1950s, which can be used to determine the electron density from the brightness of these polarised images. I am part of the team which sent an instrument - the Coronal Imaging Polariser (CIP) - to the total solar eclipse which occurred on December 14th, 2020, in South America. The instrument was named CIP since, in Welsh, to take a "cip" at something is to take a quick look - fitting since totality during a total solar eclipse typically only lasts a few minutes!