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Understanding the Sun’s violent side: the formation and eruption of coronal mass ejections
Prof. Lucie Green, Dr. David Long, Prof. Lidia van Driel-Gesztelyi
The Sun is a dynamic and active star. Observations across the electromagnetic spectrum from gamma rays to radio have revealed that the Sun has a million degree atmosphere in which the most energetic explosions and eruptions in the Solar System take place. The explosions are known as solar flares and the eruptions are known as coronal mass ejections (CMEs). The energy source for these energetic phenomena is field-aligned electric currents in the Sun’s atmospheric magnetic field. Understanding the physical processes that take place to convert this energy from magnetic to other forms such as kinetic and radiative and hence understand flares and CMEs, is a major aim in solar physics. The aim of this project is to study the evolution of the physical phenomena leading to the initiation of CMEs.
The solar group at MSSL has a long experience of investigating how magnetised plasma in the Sun’s atmosphere evolves to become eruptive and produce a CME. In many cases, a key aspect is that the magnetic field forms a configuration known as a “flux rope”. These are bundles of twisted magnetic field, which can lose their stability and then erupt into the Solar System. However, observations indicate that the magnetic structure that erupts might not be the entire flux rope as initially formed, raising the question of what physical processes are occurring to evolve the configuration shortly before the CME. This project seeks to understand how flux ropes form and are modified before and during the eruption itself due to the influence of the surrounding coronal magnetic field. The project will use a range of data sets supplied by the Hinode and SDO spacecraft, as well as ground-based telescopes, to study the evolution
This image shows a coronal mass ejection blasting off the Sun and heading out into the Solar System. The eruption has a very interesting shape where some sections look like smooth loops and other have more complicated structures. All of the shapes reveal clues about the magnetic field that threads through the structure and that is ultimately responsible for the eruption itself. (Courtesy of NASA/SDO and the AIA science team)
Coronal mass ejection production by emerging solar active regions
Prof. Lidia van Driel-Gesztelyi and Dr. David Long
In numerical simulations of magnetic flux emergence through the solar photosphere the emerging magnetic field quickly becomes eruptive, producing a coronal mass ejection (CME). Flux emergence on the real Sun is frequently observed to destabilise pre-existing fields, triggering CMEs but mainly releasing free energy from pre-existing magnetic structures. This may happen as early as a few hours into the days-long emergence phase. However, observationally it is still an unanswered question how long it takes for an active region field itself to erupt and produce a CME, and how this "time-to-first-eruption" depends on the magnetic field characteristics of the active region and its magnetic environment. The project aims at answering this question, which has importance in our preparations for the science planning and target selection of the upcoming Solar Orbiter mission.
The project involves combining full-disc magnetograms of the Sun from the HMI instrument on the Solar Dynamics Observatory (SDO) spacecraft with observations from the full-disc imagers AIA on SDO as well as the EUVI instruments onboard the twin STEREO spacecraft. In addition, coronagraph images from SOHO/LASCO and STEREO will be used to verify CME occurrences. Although this project is principally observational, the student will collaborate with leading modellers, providing input into their MHD numerical simulations of flux emergence and CME formation.
A coronal mass ejection (CME) observed with the SOHO/LASCO (C2, red, and C3, blue) coronagraphs. The solar image in the middle was taken by the EIT instrument onboard SOHO in the 195 Angstrom band. Credit: SOHO (ESA & NASA).
The life cycle of solar active regions - a holistic approach
Prof. Lucie Green, Prof. Lidia van Driel-Gesztelyi, and Dr. Deborah Baker
Active regions are the primary magnetic phenomenon of the Sun and are centrally important to the workings of our local star. The evolution of an active region is reflected in all its characteristics: the distribution of magnetic flux density, magnetic structure, plasma parameters, and flare and coronal mass ejection rate. All these characteristics are closely connected with each other, and also with the sphere of influence of the active region on the global magnetic structure of the Sun and plasma and magnetic field in the heliosphere. Understanding the evolution of active regions is particularly important in preparation for the forthcoming Solar Orbiter mission, which will be launched in 2018.
This project is extremely timely given the current phase of the Sun's activity cycle. Cycles last approximately 11 years and in the declining phase of the cycle the Sun produces large but isolated active regions. This PhD work will be carried out during the declining phase and one of these large active regions will be the target. The first part of the project will be to investigate how the magnetic field of the active region evolves during its lifetime, including the evolution of the active region's magnetic flux density and configuration. Once this foundation is in place, the results will be related to the active region's eruptive activity. Asking questions about when an eruptive magnetic field configuration forms and at what point it loses stability and erupts as a coronal mass ejection. Does this process become energetically easier as the active region decays and the field strength weakens? Another part of the project will look at how the magnetic field evolution impacts the composition of the plasma trapped within the structure. Previous work has shown that the lower and upper atmosphere have different elemental abundances, and that the upper atmosphere's composition varies with time. The physical origins of this variation will be investigated. Overall, the project will involve the analysis of data from the Solar Dynamics Observatory and Hinode satellites as well as ground-based observing facilities. During the work, there will also be collaboration with theoreticians and modellers who bring complementary knowledge and skills to the investigation.
The evolution of an active region as seen throughout the layers of the solar atmosphere, from the photospheric magnetic field to coronal X-ray emission. Image credit: ESA/NASA/JAXA. (Click image for high resolution.)
Magnetic field changes during solar flares
Dr. Sarah Matthews
Solar flares involve the rapid conversion of energy stored in stressed magnetic fields into various other forms, manifested as heat, mass motions and accelerated particles. Yet, despite the unequivocal importance of the magnetic field in the onset and energy release process, the interpretation of changes in the magnetic field observed at the solar photosphere during flares remains controversial, complicated by the rapidly changing plasma conditions where the field is measured.
The project will involve the comparative study of flare related changes in magnetograms obtained both from space (Solar Dynamics Observatory (SDO) and Hinode) and from ground-based telescopes (GONG, Dunn Solar Telescope, Swedish Solar Telescope) in context with other supporting flare observations (Hinode, SDO, IRIS, RHESSI) to understand whether changes can be attributed to real variations in the magnetic field, or to changes in the line profile as the result of plasma conditions. Both scenarios provide important constraints for our understanding of the flare process, and the observational results will be compared to the predictions of flare models. The initial stages of the project would use data from existing ground-based telescopes, with the expectation of applying for observing time on the 4m Daniel K. Inouye Solar Telescope (DKIST; http://dkist.nso.edu/) when it starts observations in 2019.
Hinode observations of a sunspot and its magnetic field before a flare. Credit: NAOJ/JAXA
Hinode observations of a sunspot and its magnetic field after a flare. Credit: NAOJ/JAXA
Page last modified on 20 dec 16 15:18