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The aim of this project is to explore the nature and causes of the large-scale consequences of coronal mass ejections (CMEs). The project is very timely as we have entered an increasingly active phase of the solar cycle and big CMEs (like the one observed on 14 February 2011) are expected to erupt from the Sun regularly in the next few years. The project will involve using space-borne data from the Hinode satellite, and its EIS spectrograph onboard, of which MSSL is the PI institute. The spectroscopic data is to be combined with observations by NASA’s Solar Dynamic Observatory (SDO), which is providing breath-taking details of a dynamic solar atmosphere observing the full solar disk every minute. Also to be used are observations taken by the twin STEREO spacecraft (presently 90-degree ahead and behind the Earth in Sun-orbit). STEREO enables us, for the first time in human history, to monitor the solar corona in three-dimensions.
Coronal mass ejections (CMEs) are the most energetic eruptions in the solar system expelling up to 1013 kg coronal material at speeds of several hundreds or even thousands of km per second. These huge bubbles of gas are threaded with magnetic field lines and driven away from the Sun by a magnetic instability which develops due to free energy stored in the magnetic field. The Sun is a magnetic star; even the so-called "quiet Sun" is covered by a mixed polarity "magnetic carpet" down to the smallest scales we can presently resolve. CMEs, which are magnetic structures, erupt in this ubiquitous magnetic environment and naturally interact with it, pushing some magnetic fields aside, while leading to re-structuring elsewhere. During a CME eruption we can observe wide-ranging changes in the solar corona (large-scale waves, coronal dimming, aurora solaris), sometimes all over the visible disc.
The student will learn how to use state-of-the-art solar observations and how to combine multi-wavelength and multi-instrument data/images. However, the aim is to interpret and understand the underlying physics of the solar and interplanetary consequences of coronal mass ejections, which are important players in space weather.
Recent SDO observations of flares, jets and a large Earth-directed CME can be seen in the movie below.
UCL/MSSL is the Principal Investigator Institute on an international consortium providing the Solar Wind Analyser suite (SWA) of instruments for the ESA Solar Orbiter mission, due for launch in 2017. SWA will sample electron, proton, alpha particle and heavy ion populations at various distances down to 0.28 AU from the Sun (i.e. around a quarter the distance from the Sun to the Earth). In particular, UCL/MSSL is designing and building the electron analyser system (EAS) for the SWA suite. In order to prepare for the mission, and to be able to use the instruments to make optimum measurements in the solar wind, we would like to undertake studies of the nature of the solar wind particle populations and their variability using, where relevant, data from our existing in orbit missions.
For example, during the spring of each year, the ESA Cluster mission, comprising 4-spacecraft, spends a significant portion of each orbit outside of the Earth's bow shock and thus samples directly the electromagnetic fields and plasma of the solar wind. During periods of burst mode operations, the PEACE electron spectrometers, which were designed and built at UCL/MSSL, provide unprecedented detail of the nature of the electron populations in the solar wind. In general these can be divided into 3 populations. A 'core' population of the coldest electrons which is nearly isotropic - approximately the same flux of electrons of a given energy may be detected in any direction. A 'halo' population occurs at somewhat higher energies, and shows a slight shift in average velocity with respect to the core, and thus provides a 'heat flux' in the solar wind. Finally, a 'strahl' population is often seen as a more energetic beam of particles streaming along the magnetic field. Together these different electron populations contain information about the processes occurring at the source region on the Sun, the magnetic connections of the sampled plasma back to the Sun and on the plasma processes (e.g. turbulence, wave-particle interactions and magnetic reconnection) which may be occurring within the solar wind itself. Separating the effects of these processes is a complicated task requiring high-cadence, high resolution data of the type available from Cluster during burst modes. The student will use these data to examine in detail the nature and variability of the electron populations, for example using the multi-point measurements to determine the level of variation between spacecraft.
The results of this project are critical as preparation and inputs into the ESA Solar Orbiter program, and the student will thus also be an integral part of the MSSL instrument-build and science-planning teams, with the responsibility of making scientific inputs to those processes. There will also be opportunity to collaborate with our partners in France, Italy and the USA, who will provide the Heavy Ion Sensor and Proton-Alpha Sensor for the SWA suite.
Figure 1: Artists impression of the Solar Orbiter spacecraft performing close examination of the Sun and sampling the ambient solar wind plasma. The EAS sensors are on the extreme right of the picture, at the end of the long boom. The other SWA sensors (Heavy Ion Sensor and Proton-Alpha Sensors) look towards the Sun through the cut-outs that are evident in the corners of the heat shield on the Sun side of the spacecraft.
Figure 2: (i) A PEACE sensor (centre right) attached to one of the Cluster spacecraft prior to launch; (ii) Prototype of the Solar Orbiter EAS sensor head in the vacuum chamber at MSSL; (iii) CAD model of the complete EAS sensor to be mounted on the Solar Orbiter boom (see Figure 1).
Physics of the auroral acceleration region: Prof Andrew Fazakerley
The aurorae (northern and southern lights) are beautiful, dynamic curtains of light seen in the night skies, usually in the polar regions. They are also the source of the Earth's strongest radio emission, auroral kilometric radiation. Aurorae and auroral kilometric (AKR) radiation are also seen at other magnetised planets such as Jupiter and Saturn. The processes that accelerate the electrons that produce the aurorae remain mysterious, and spacecraft observations will are needed to test whether prevailing theories are valid. The evolution of the orbit of the ESA Cluster 4-spacecraft mission has recently enabled the spacecraft to make the first multi-point observations in the 'Auroral Acceleration Region', at 4,000 to 12,000 km altitude at auroral latitudes. Special operations are ongoing and are being conducted with the Cluster tetrahedron oriented so as to allow simultaneous measurements at different altitudes on closely neighbouring magnetic field lines, to search for evidence of electron acceleration and the processes that cause it – for example, are electric potential drops along the magnetic field occurring in this region? The campaign is also designed so that some spacecraft can localise sources of emission of AKR while it is hoped that other spacecraft will fly through the sources, allowing definitive tests of the theories of AKR generation by unstable electron distributions. Cluster's PEACE electron instruments are provided by MSSL-UCL and are providing a key dataset in AAR studies. The proposed PhD research will involve surveying the AAR dataset, and using data from PEACE and other instruments to address questions about what exactly happens in the auroral acceleration, and how the aurorae are ultimately driven by events in the magnetosphere.
Exploring the Earth's inner magnetosphere: Prof Andrew Fazakerley
The inner magnetosphere contains the cold plasmasphere, the suprathermal plasmasheet and ring current plasma, and the extremely energetic radiation belt particle populations. All of these populations wax and wane in response to variable solar wind conditions and internal magnetospheric processes. Moreover, these plasma populations, though covering a wide range of energies, are interlinked, through wave-particle interactions that transfer energy between them. Recent magnetospheric imaging missions have led to some progress in developing the present sketchy understanding of the behaviour of the plasmasphere and ring current, and missions are in development which will focus on the processes that create and destroy the radiation belt populations. However, little work has yet been carried out to take advantage of the observations already made by Cluster and Double Star, and in particular the Cluster inner magnetosphere campaign planned for 2011/2. The proposed PhD research will take advantage of the relatively quiet radiation levels that have coincided with solar minimum, which allow us to measure inner magnetosphere particle populations without interference from penetrating radiation. The use of the Cluster constellation will allow measurements of plasma pressure and magnetic field gradients (and hence currents), convection and corotation electric fields, and electrostatic and electromagnetic wave activity. The MSSL PEACE electron instrument data will be central to understanding wave-particle interactions processes, delineating the extent of particle populations of different energies and assessing how current systems are supported. The proposed studies will examine the plasmasphere and ring current population, the wave activity and current systems that they generate, and their responses to variations in solar wind conditions. Combined with Double Star and other spacecraft, simultaneous snapshots of conditions in widely separated parts of the inner magnetosphere will allow tests of models of the global response of the inner magnetosphere to dynamic solar wind variations.
Comparative solar wind effects on planetary atmospheres: Prof. Alan Aylward and Prof. Louise Harra
Recent studies of the Earth's thermosphere using the CHAMP spacecraft have shown there are temperature and pressure variations at harmonics of the solar rotation period of 27 days. Thus recurrent structures in the solar wind appear to be affecting the neutral atmosphere: the process for this is not currently understood. A database has been constructed of all the solar wind anomalies - CMEs, CIRs and magnetic clouds since the 1960s. These data could be compared to atmospheric ground-based and satellite observations over this time to find which of the structures in the solar wind are most 'geo-effective' in terms of producing fluctuations in thermospheric structure. The effects on the neutral atmosphere must be transferred from solar wind to neutral atmosphere via the Earth's magnetosphere. In this context, consideration of the interaction at other planets would also be possible and informative: Jupiter and Saturn also have magnetospheres, while Venus and Mars have no shielding magnetic field. Thus, clues as to the mechanism by which the energy is transferred at the Earth may come from comparing what happens at these planets. UCL has instruments on spacecraft at Saturn (Cassini), Venus (Venus Express), Mars (Mars Express), Cluster (magnetospheric) and Hinode (solar), models of the upper atmospheres of these planets, and expertise in solar and magnetospheric physics. This provides a comprehensive suite of tools for studying this phenomenon of space-to-atmosphere energy transfer at different planetary environments within the Solar System. We propose a joint studentship to study planetary solar wind-thermosphere coupling. The studentship would focus on the following areas in detail, or could be a combination of two of these areas:
Correlation of solar wind and atmospheric observations at the Earth : This component would focus on the characterisation of solar wind structures and fluctuations in terrestrial thermospheric structure which are (nearly) simultaneous with the arrival of solar wind events at the Earth. The dominant spatial / time scales for solar wind structures will be identified, as well as the corresponding dominant period for the thermospheric response.
Correlation of solar wind and atmospheric observations at other planets: Similar to above, except focussing on datasets for any or all of Mars, Venus, Jupiter, Saturn. Jupiter is important here as a point of comparison, as its main aurorae are internally driven by rotation.