Vacancies exist for PPARC-funded PhD. studentships within the Radio and Space Plasma Physics Group for studies starting in October each year. Early application is recommended to ensure consideration for these places. Alternative funding sources may also be available.
Projects are available in the general areas of:
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Details of the projects, and contact information is given below:
Radio & Space Plasma Physics
There is a growing interest in the Earth’s environment and in the physical processes which govern such factors as its composition, energy distribution, turbulence and stability. The Radio & Space Plasma Physics Group is engaged in studies of the Earth’s ionosphere and its interactions with the regions both above and below it. Above, it couples with the magnetosphere, the solar wind and the interplanetary magnetic field. The most intense of these interactions occurs at high latitudes and results in the spectacular displays of the aurora or ‘Northern Lights’. Coupling of the ionosphere and upper atmosphere through its lower boundary to the middle and lower atmosphere is also a major interest. This research is undertaken using a combination of ground-based experimental facilities, measurements by Earth-orbiting spacecraft, computer modelling, and theory. Some of the work involves experimental campaigns at high polar latitudes, in which research students have the opportunity to participate.
Major ground based facilities have been established and many satellite borne experiments flown to study the Earth’s aerospace environment. Extensive use is made of radar facilities such as the European Incoherent Scatter (EISCAT) radar located in Northern Scandinavia and satellites such as Cluster and Polar. In addition to studies of the geophysical phenomena, the group is also involved in using the ionosphere as a giant plasma laboratory, in which a wide range of plasma irregularities and waves can be investigated without the constraints imposed by laboratory conditions. We also study the ionospheres and magnetospheres of other planets in our solar system, and are participating in the analysis of data from the Cassini Saturn orbiter space mission. Studies of a more applied nature are also in progress. The limitations imposed by the propagation medium are important factors in the design of a wide range of space to ground communications and navigation systems and the ionosphere is a key factor in conventional long-range broadcasting and communications. A Radio Systems Laboratory has been established jointly by the Physics and Engineering Departments for these investigations.
The solar wind plasma blows outwards from the Sun at speeds of 500 km s-1, and flows continuously past the Earth on its way out towards interstellar space. The interaction between this wind and the Earths magnetic field confines the field to the interior of a cavity surrounding the planet, called the Earths magnetosphere. The position of the outer boundary of this magnetic cavity, the magnetopause, is determined mainly by pressure balance between the ram pressure of the solar wind on one side, and the magnetic pressure of the compressed planetary field on the other. On the dayside, this boundary is usually found at distances from Earth of about 10 Earth radii (about 64,000 km), while on the nightside the Earths magnetic field is pulled out by the solar wind into a long tail, rather like a comets tail, which extends to distances of at least 1000 Earth radii.
The separation between the solar wind and the Earths magnetosphere is not perfect, however, and processes occur at the boundary which allow the transfer of mass and momentum across it. The most important of these is magnetic reconnection, which allows a direct connection between the geomagnetic and interplanetary magnetic fields. These processes, which are electromagnetic in nature, result in the transport of Earths magnetic flux from the dayside to the tail in the outer regions of the magnetosphere. The magnetic flux eventually returns to the dayside through the central part of the magnetosphere. A large-scale cyclical flow of plasma is therefore set up in the magnetospheric cavity due to the solar wind interaction, the total flow cycle time being about 12 hours. This, combined with sources of plasma from the solar wind on the outside and from the ionosphere on the inside, produces highly structured plasma populations inside the cavity.
There are many aspects of such a complex system which are studied by a large number of research groups world-wide. In the Radio & Space Plasma Group at Leicester we concentrate on studies of the main interaction regions of the magnetosphere, the magnetopause and the tail, where the key processes take place which result in large-scale flow. An unprecedented opportunity to study these regions is currently in progress using data from the ESA Cluster space mission. Four identical spacecraft are flying in formation through these regions, measuring the electromagnetic field and the plasma populations. The Group has direct access to the magnetic field data from these spacecraft, and through this involvement to the data from other instruments. Using these measurements, studies are in progress on the nature of the field and plasma structures which occur in these regions, and their evolution with time.
The CUTLASS radar in Finland
These space-based studies interface directly with the Group’s ground-based research programme since it is the flow generated in the magnetosphere, transferred along the magnetic field lines into the ionosphere, which is principally detected by the Group’s CUTLASS radar. The Group is therefore centrally placed to undertake unique coordinated space-ground observations using Cluster and CUTLASS at the present time.
The CUTLASS radars, located in Iceland and Finland, view the ionosphere over the northern polar regions. These radars were built and deployed by the Group, and have accumulated a large volume of data on ionospheric flows since the start of operations in 1995. Research is undertaken in conjunction with other ground-based facilities, such as the EISCAT Svalbard Radar which can measure a range of ionospheric parameters, optical instruments which observe auroras, and magnetic measurements which sense the effects of overhead ionospheric currents.
The viewing areas of the CUTLASS radars
Radar experiments are also co-ordinated internationally with rocket shots from Svalbard, and satellite over-passes. The Group also operates vertical and oblique radio sounders in the northern polar regions, and a Doppler system called DOPE, which is designed to study ultra low-frequency waves propagating in the magnetosphere-ionosphere plasma.
CUTLASS forms part of SuperDARN, an international network of similar radars covering over 180˚ longitude in the northern hemisphere and including conjugate stations in the Antarctic. This network offers, for the first time, a truly global atmospheric monitoring which is so important if we are to understand our planetary environment.
Examples of research projects in this area include:
The viewing areas of the northern hemisphere SuperDARN radars
The SPEAR antenna field under construction
Solar-Terrestrial Influences on Climate
We are currently leading a team to design a micro-satellite mission to investigate further the role that space plasmas have on climate. ‘Chapman’, named after a British pioneer in upper atmosphere and magnetosphere research, would be roughly the size of a fridge. It is being designed to be able to monitor high energy particles that can penetrate the Earth’s atmosphere down to the cloud base and observe transient optical phenomena such as aurora and sprites (upper atmosphere lightning) at close quarters. It will also be able to measure stratospheric clouds that are responsible for the large-scale destruction of ozone and monitor mesospheric clouds, whose increasing frequency of occurrence may be an early indicator of long term climate change.
In depth localised measurements of the upper mesosphere and lower thermosphere region are currently being carried out with the CUTLASS and EISCAT radars. These observations allow us to assess the contributions made by relatively small scale phenomena, such as atmospheric gravity waves, to the atmospheric circulation on timescales ranging from minutes to a number of years. Linking a number of these radars together (e.g. SuperDARN) allows us to consider the global significance of such processes.
Projects in this area will include:
Like the Earth, most of the planets generate magnetic fields in their interiors. Direct measurements from spacecraft flybys have shown that this is true of Mercury, Jupiter, Saturn, Uranus, and Neptune (Pluto-Charon has yet to be visited). In this case the solar wind, composed of charged particles, interacts directly with the planetary magnetic field. The size of the resulting magnetospheric cavity depends on the strength of the planetary magnetic field, and varies from a structure which barely rises above the planet's surface in the case of Mercury, to the giant system which surrounds Jupiter, which is five times larger than the Sun. The plasma populations which occur inside these magnetospheres are also very varied, depending on the nature of the sources of plasma which are present, and the nature of the plasma transport mechanisms. At Mercury the interior flow is driven mainly by coupling to the solar wind, and the latter also forms the main source of plasma for the interior. At Jupiter and Saturn, on the other hand, the flows are driven principally by planetary rotation, and the plasma is mainly derived from the atmospheres or surfaces of moons which circle inside the magnetosphere.
A schematic of the kronian (i.e. Saturn's) magnetosphere
Other bodies are unmagnetised, such as Venus and (to a first approximation) Mars, and the comets. In this case the solar wind interacts more directly with the atmosphere of the body, or with its outer conducting layer, the ionosphere. Then the magnetic field in the solar wind becomes draped over the ionosphere on the dayside of the planet, and stretched out into a long "induced" magnetic tail on the nightside. In the case of comets, however, the atmosphere extends for millions of kilometres around the nucleus due to the low gravity, and atmospheric particles are picked up and carried along by the solar wind flow as soon as they become ionised by solar ultraviolet light.
At present our work concentrates on the gas giant planets Jupiter and Saturn, and on theoretical and data analysis studies of the coupling of rotational energy between the atmosphere and the magnetosphere. Our future data analysis work will centre on data from the Cassini spacecraft, on which we are co-investigators on the magnetic field experiment. Saturn orbit insertion will take place in 2004.
Research topics in the next few years will include
The Radio Systems Laboratory was established in 1986 jointly by the Department of Engineering and the Department of Physics and Astronomy to capitalise on cross-departmental skills in radio and radar techniques and propagation. The influence of the ionospheric propagation medium on HF radio signals is a field of particular interest which is supported by many years research, both experimental and theoretical, into the physics of the ionosphere. The Radio Systems Laboratory concentrates on the solution of engineering problems related to radio wave propagation and on the development of new system concepts to overcome these problems.
Some of the current areas of interest are outlined below:
The high latitude ionosphere is a particularly disturbed region and the problems associated with radio systems operating at these high latitudes is a field of special interest. Major experimental campaigns have been undertaken with equipment deployed in the UK, North America and the Canadian Arctic in support of these studies. Direct applications of this work are in the fields of long range radio communications, broadcasting, and direction finding.
A feature of high frequency direction finding systems is that the errors due to ionospheric tilts generally exceed the instrumental accuracy. This is especially true at high latitudes where very large bearing errors of up to around ±100° are frequently observed on a correctly identified signal. It has been possible to relate the incidence of such errors to various ionospheric features such as the sub-auroral trough and the convection of ionospheric "plasma blobs" which break away from the dayside (sunlit) auroral oval and convect across the polar cap to the night.
Measurements of bearing errors associated with HF propagation paths associated with the sub-auroral ionospheric trough have recently been reported. In these experiments, it was found that the direction-of-arrival of signals were often observed to deviate from the great circle path (i.e. the expected bearing) by up to 100°. The deviations occurred at times and with magnitudes consistent with known features of the trough. A preliminary ray tracing study of this effect has established that the essential features of the observed bearing deviations can be reproduced by modelling.
Conventional DF systems can only satisfactorily cope with a single signal, a situation which rarely occurs within the HF band due to the presence of multi-moded propagation and co-channel signals. New direction finding systems based on adaptive techniques such as MUSIC and IMP/DOSE have been developed by various workers in the field. Some of the techniques employed in these systems work well at VHF and above but their performance is not well understood for signals in the HF band, particularly under disturbed conditions. Various investigations are being undertaken in which the performance of these super-resolution DF algorithms is measured under well assessed propagation conditions. Particular attention is currently being given to situations where the ionospheric reflection is non-specular and the signal energy arrives at the receiver over a range of angles in both azimuth and elevation. Access to wide aperture antenna arrays and multi-channel receiving systems has been given by various government agencies both UK and abroad, and the Laboratory also operates its own systems which may be deployed in support of various studies.
Examples of very large, rapid bearing swings. Thule to Alert, 8.050 MHz, 18th November 1990.
Map showing the location of the transmitter at Halifax and receiver at Cheltenham together with the great circle path. The reduction in electron density (expressed as a percentage of ambient) associated with the trough and employed in the ray tracing simulation for the following times is also indicated, a) 21 UT, b) 0 UT, c) 6 UT and d) 9 UT.
The left hand panel shows an oblique ionogram recorded over a 2100 km path from Iqaluit - Alert at 12:45 UT on 24 January 1996. Bearing estimates of a 9.3 MHz signal received over the same path obtained using the loaded Capon algorithm for the period either side of the times of the ionogram is given in the right hand panel.
Studies of observations of the direction of arrival of a narrow band pulsed channel sounding signal propagated over various high latitude paths are being undertaken. Multi-channel receiver systems connected to wide sampled aperture antenna arrays are employed for these measurements. The signals received on each antenna are processed to provide a measure of the relative times of flight of the propagating modes and their associated Doppler spectra. In this way, the signal is split into components distinguished by antenna position in the receiving array, time of flight and Doppler frequency. A direction finding algorithm is then applied to each of the signal components in turn in order to estimate the directional characteristics of the received signal. In preliminary measurements, a variation in bearing with Doppler frequency was frequently evident, an effect attributed to Doppler shifts imposed on the signal when scattered from ionospheric irregularities drifting across the reflection points. For a trans-auroral path, for which the ionospheric reflection points were sub-auroral, standard deviations of up to around 2.5° were observed in the azimuthal power distribution of the received energy. Much greater disturbances were recorded on the signals received over a polar cap for which very large azimuthal standard deviations, of up to around 35°, were measured. More extensive, experimental measurements have recently been undertaken and the associated data analysis is currently being completed. Further studies are being proposed to undertake experimental measurements with the aim of developing a physically realistic channel model containing directional information of particular relevance to the laboratory evaluation and development of adaptive reception and radiolocation systems.
The complexity of the HF propagation environment can seriously affect the performance of long range radio communication circuits. Furthermore, the increased sophistication of the modulation schemes employed by modern, comparatively high speed, data modems necessitates a better understanding of the propagation environment in order to establish reliable links. A project is being undertaken in which techniques to evaluate the ionospheric channel in real time, with particular reference to the requirements of modern modulation schemes, are being investigated.
A new project is currently being initiated to investigate the propagation characteristics and the associated communication channel characteristics of VHF and UHF signals propagating over the sea. This is of particular interest for communications between ships at the extreme limits of propagation.