Deriving electron densities from linear polarization observations of the white-light corona during the 2020 December 14 total solar eclipse
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
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!
Solar Flare Prediction
The interactions of solar activity with Earth’s upper atmosphere manifest through a complex series of events commonly known as Space Weather.
From these events, the most energetic ones are the flare and CME eruptions. These two, as well as the largest, solar explosions in the entire Solar
System often result in rather dramatic consequences for the functioning of a number of ground- (e.g. pipelines, power lines) and space-based
infrastructures and services (satellites, communication, GPS) as they may be seriously damaged. These societal assets and services are vital
to the economic welfare and security of every citizen. Considerable failures due to flares and CMEs have indeed befallen in the past (e.g. Quebec,
Canada suffered an electrical power blackout in 1989, two Erik satellites lost, etc.).
The importance of Space Weather forecasting is rapidly increasing because of the impact solar eruptions may have on the entire (near-)terrestrial
environment and the rising sensitivity of our technosphere. Our aim is to develope a flare intensity and onset time prediction tool that is substantially
more precise than those currently available. Most flare and CME forecast methods are applied on the solar surface, however they occur in the solar
atmosphere. Therefore, in our project, we apply and critically assess a new measure for predicting eruptive non-potentiality in solar active regions
by utilising 3D magnetic mapping across the solar atmosphere. To achieve a leap forward, here, we generalise our forecasting methods by applying them
through the solar atmosphere in order to determine an optimum height range in the lower solar atmosphere for predicting flares and CMEs.
3D Maps of the solar atmosphere
The Sun’s atmosphere flows into interplanetary space as the solar wind. Solar eruptions and streams of different speeds lead to rapid variations in
this flow that impact Earth's magnetic field and can cause disruption to technology, economy and society - this is called space weather. Improved
space weather forescasting depend critically on improving our understanding of the evolution of the solar wind near the Sun. Recent innovations in
tomography techniques are opening a new window on this complex environment.
Coronagraphs on spacecraft take images of the extended solar atmosphere continuously. As the Sun slowly rotates (~every 4 weeks), the view of the
corona changes. We use this information, using advanced methods, to derive the 3D structure of the corona's plasma. Thus the distribution of high
and low density streams (corresponding to slow and fast flow speeds) is revealed. For the first time, this offers a direct mapping of the corona,
and the ability to improve solar wind models that are central to predicting space weather.
Image:The Sun’s position is shown as the black inner circle. From 2 to 12 solar radii above the Sun, images are collected by coronagraph
telescopes, as shown here in the black and white annular region above the Sun. The outer colour region shows the electron density of the solar
atmosphere, gained from applying tomography to a time series of coronagraph observations.
Video: The changing structure of the solar corona over several years. This movie shows the electron density of the solar atmosphere at
a height of 6 solar radii from the Sun, mapped in longitude and latitude. The movie begins during a period of minimum activity in 2009, to a
period of maximum activity in 2012. During this time, we see high-density sheets (slow wind) gradually migrating to higher latitudes.
Amplification of Magnetic Twists in Solar Prominence Threads
Magnetic twists are commonly associated with solar prominences. Twists are believed to play an important role in supporting the dense plasma
against gravity as well as in prominence eruptions and coronal mass ejections, which may have a severe impact on the Earth and its near environment.
We have recently used a simple model to mimic the formation of a prominence thread by plasma condensation with the aim of investigating the
evolution of small twists during this process.
We found that small perturbations are exponentially amplified in time as they propagate along the condensing thread. The amplification is caused
by the coupling between the flow and the twists. The linear study carried out by
Taroyan and Soler (2019) suggests generation of large amplitude
axisymmetric twists along a prominence thread when it is permeated by a converging flow, for example, during the evaporation and condensation
of plasma along the thread (see Figure). We have recently found that the process of flow-twist coupling may also lead to the formation of
steep gradients and small scales at the critical point. This new phenomenon does not require any influx of azimuthal energy.
We propose a PhD project to focus on the nonlinear evolution of the twists / Alfvenic perturbations using a multidimensional non-linear model
with the aim of investigating the back reaction of the amplified twists on the flow, their possible role in the formation of large twists
that may support the dense plasma against gravity, their role in the generation of vortices and in the eruption of prominences.
Constraints on coronal heating: A large study of TR/coronal rapid brightenings using IRIS and AIA data
Llŷr Humphries and Huw Morgan
High-resolution observations of dynamic phenomena give insights into the properties and processes that govern the low solar atmosphere.
Using the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory,
analysis of the characteristics of these phenomena are possible, such as their apparent speed, temperature, and composition. Fan-shaped Jets
(FSJs) are one such phenomenon. FSJs typically appear of sunspot light-bridges or over the umbral/penumbral boundary with signatures in both
cool and hot channels. These jets are possibly caused by a release of tension as a result of magnetic reconnection at their foot-point.
Conversely these jets may in fact be the result of a series of shock waves driving material into higher layers of the solar atmosphere. AIA
and IRIS images revealed that FSJs observed at active region 12192 move across the plane of sky at speeds of 23-130 kms-1, reaching lengths
of up to 26 Mm. IRIS spectroscopy results also demonstrate some blue-shift behaviour accompanied by corresponding intensity increases and
line-broadening. Further details may be found at Humphries et al (2020).
Amplification of Magnetic Twisting in Solar Prominences by Plasma Inflow
Chloe Sumner and Youra Taroyan
The solar atmosphere is a highly dynamic environment with complex plasma flows and twisted magnetic field lines. Through heating on the solar
surface plasma may evaporate, travelling higher into the solar atmosphere before runaway cooling processes can trigger condensation which falls
back to the solar surface. These plasma flows can be caught by the strong magnetic fields present and may be suspended against gravity if
captured in dips caused by twists in the magnetic field lines. This capture can lead to the growth of solar prominences. In this work we have
developed a mathematical model which describes the inflow of a plasma through magnetic threads to simulate this, demonstrating analytically that
we can expect a coupling between the flowing plasma and any underlying twists in the magnetic field, amplifying as waves travelling through the
magnetic field which we call Alfvén Waves. We have also solved this numerically by computationally simulating a prominence thread filling
whilst isolating the coupling mechanism from other potential sources of energies which could contribute to twisting. These simulations covered
a variety of initial conditions and flow speeds and, in all cases, we observe a twist amplification. Through this work we have demonstrated
that the amplification of magnetic field twisting in the presence of a plasma flow is a fundamental property of plasma physics and we now
intend to explore this further within the context of solar prominence formation. Further details of this work can be found in
Sumner and Taroyan (2020).
Analysing the Rotation of Sunspots
Richard Grimes, Balazs Pinter, and Huw Morgan
The Sun's surface is peridoically covered in dark patches known as sunspots. These sunspots are concentrations of
magnetic field lines, which are important to the understanding of the solar atmosphere and interior. In my work I use data from the Solar Dynamics Observatory
to analyse the motion of these sunspots and further our understanding of how they contribute to the dynamics of the sun. 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. Further details can be found in Grimes et al (2020).
Calculation of linelists for diatomic molecules
My research focus entails calculating high-accuracy and complete linelists (lists of wavelengths with corresponding intensities) for diatomic molecules
which are of interest in various astronomical settings. I am a member of the ExoMol group based at UCL.
These linelists are generated using high-level ab initio calculations of Potential Energy Curves, Dipole Moment Curves and couplings using the
quantum chemistry package MOLPRO. These calculations are then refined using experimental data in the form of experimentally measured wavelengths
and experimentally determined spectroscopic constants.
Since 2012 I have worked on a range of molecules including:
Chromium Hydride (CrH) & Manganese Hydride (MnH) which are of interest in Brown Dwarfs.
SH, NS, SiO, PO, PS which are of interest in exoplanets and circumstellar environments.
NeH+, ArH+, KrH+, XeH+ which are of potential interest as tracers in the ISM.
In terms of applications, I am interested in modelling molecular spectra in sunspots and investigating diatomics as probes of sunspot magnetic field
strength. I have written preliminary code for modelling the Zeeman-shift for diatomic molecules which will be further developed by Shaun Donnelly.
I am also interested in investigating if machine learning can be used to aid the identification of diatomic molecules in sunspots and hence
am looking to develop a potential PhD project in this area.
Rotation of the Solar Corona
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).
Due to the high complexity of the solar atmosphere, advanced, high-resolution spectropolarimetric observations are required to investigate its
thermodynamic properties including the characteristics of the magnetic field. High-resolution, full Stokes chromospheric spectropolarimetry
from ground-based telescopes offers the opportunity to obtain such data sets. My research focus on design, execution, analysis and
interpretation of ground-based spectropolarimetric observations.
The main aim of my current projects are:
Develop semi-empirical radiative hydrodynamic models of the chromospheric structures through advanced spectropolarimetric inversion techniques.
Perform diagnostics of the key plasma parameters, such as temperature, density, plasma pressure, magnetic field and velocity.
Study the spatial and temporal variation of the physical parameters in the chromospheric structures and their magnetohydrodynamic stability.
Figure 1. Solar spicule in Hβ line wing at ∆λ = −0.699 Å observed at the western limb of the Sun with Swedish Solar telescope (SST).
The size of this structure is about 9,000 km.
Figure 2. A composite of SST Hβ ± 0.455 Å images of the X8.2 flare coronal loops on 10 September 2017.