Dr Liam Edwards

Dr Liam Edwards

he/him

Postdoctoral Researcher

Publications

You can also find my articles on my Google Scholar profile.

Derived Electron Densities from Linear Polarization Observations of the Visible-Light Corona During the 14 December 2020 Total Solar Eclipse

Published in Solar Physics, 2023

A new instrument was designed to take visible-light (VL) polarized brightness (pB) observations of the solar corona during the 14 December 2020 total solar eclipse. The instrument, called the Coronal Imaging Polarizer (CIP), consisted of a 16 MP CMOS detector, a linear polarizer housed within a piezoelectric rotation mount, and an f-5.6, 200 mm DSLR lens. Observations were successfully obtained, despite poor weather conditions, for five different exposure times (0.001 s, 0.01 s, 0.1 s, 1 s, and 3 s) at six different orientation angles of the linear polarizer (0 degree, 30 degree, 60 degree, 90 degree, 120 degree, 150 degree). The images were manually aligned using the drift of background stars in the sky and images of different exposure times were combined using a simple signal-to-noise ratio cut. The polarization and brightness of the local sky were also estimated and the observations were subsequently corrected. The pB of the K-corona was determined using least-squares fitting and radiometric calibration was done relative to the Mauna Loa Solar Observatory (MLSO) K-Cor pB observations from the day of the eclipse. The pB data was then inverted to acquire the coronal electron density, n_e, for an equatorial streamer and a polar coronal hole, which agreed very well with previous studies. The effect of changing the number of polarizer angles used to compute the pB is also discussed and it is found that the results vary by up to approx. 13% when using all six polarizer angles versus only a select of three angles.

Recommended citation: Liam Edwards et al. 2023 Solar Physics 298 140 http://academicpages.github.io/files/s11207-023-02231-5.pdf

A Solar-cycle Study of Coronal Rotation: Large Variations, Rapid Changes, and Implications for Solar-wind Models

Published in The Astrophysical Journal, 2022

Information on the rotation rate of the corona, and its variation over latitude and solar cycle, is valuable for making global connections between the corona and the Sun, for global estimates of reconnection rates and as a basic parameter for solar-wind modeling. Here, we use a time series of tomographical maps gained from coronagraph observations between 2007 and 2020 to directly measure the longitudinal drift of high-density streamers over time. The method reveals abrupt changes in rotation rates, revealing a complex relationship between the coronal rotation and the underlying photosphere. The majority of rates are between −1.0 degree/day to +0.5 degree/day relative to the standard Carrington rate of 14.18 degree/day, although rates are measured as low as −2.2 degree/day and as high as 1.6 degree/day. Equatorial rotation rates during the 2008 solar minimum are slightly faster than the Carrington rate, with an abrupt switch to slow rotation in 2009, then a return to faster rates in 2017. Abrupt changes and large variations in rates are seen at all latitudes. Comparison with a magnetic model suggests that periods of equatorial fast rotation are associated with times when a large proportion of the magnetic footpoints of equatorial streamers are near the equator, and we interpret the abrupt changes in terms of the latitudinal distribution of the streamer photospheric footpoints. The coronal rotation rate is a key parameter for solar-wind models, and variations of up to a degree per day or more can lead to large systematic errors over forecasting periods of longer than a few days. The approach described in this paper gives corrected values that can form a part of future forecasting efforts.

Recommended citation: Liam Edwards et al. 2022 ApJ 928 42 http://liamtomosedwards.github.io/files/Edwards_2022_ApJ_928_42.pdf