On the modelling and consequence of small-scale magnetic phenomena in the Saturnian system

This thesis presents an analysis of Cassini magnetometer data in two different regions of the Kronian system. An evaluation of aperiodic waves on the equatorial current sheet is presented; the waves are fitted to a model of a Harris current sheet deformed by a Gaussian wave pulse. This analysis allows examination of the parameters relating to the waves, where amplitude of waves is found to increase with radial distance. In addition, the direction of propagation of the waves is found by resolving the wave numbers in 2-dimensions, where a general outwards propagation is found. The use of the Harris current sheet also allows the resolution of current sheet parameters, and it is found that the scale height of the current sheet increases with radial distance. Additionally, values of the magnetic field in the lobes are found using the model, which are then used along with the scale heights to estimate the current density in the azimuthal and radial directions. These values can also be used to calculate, using the divergence of current, the field aligned currents entering and leaving the ionosphere where a current entering the ionosphere pre-noon and a current exiting the ionosphere post-midnight are shown. This current density is then converted to an electron flux in the upward current region, and could produce an additional 1-11 kR of auroral emission which is seen in other infrared and ultraviolet data sets. Additionally, irregular magnetic signatures, such as the aperiodic waves, are found in the entire system including Titan’s ionosphere. At Titan, a statistical study of the position of flux ropes finds no spatial dependence other than an increased number of flux ropes in the sun-lit regions and ram-side regions. A comparison of force-free and nonforce free models is utilised to extract the radii and axial magnetic field of the flux ropes, and compare the assumptions required for both models. Additionally, deformations to the models are used to model common asymmetries seen in the magnetometer data and find that bending a force-free flux rope solves the problem of the direction ambiguity of using minimum variance analysis and using elliptical cross-sections of flux ropes allows for a asymmetric flux rope signature. All together, this thesis explores the varied magnetic phenomena in the Kronian system and uses them to understand the surrounding environment

Transient band structures in the ultrafast demagnetization of ferromagnetic gadolinium and terbium

We compare the laser-driven demagnetization dynamics of the rare earths gadolinium and terbium by mapping their transient valance band structures with time- and angle-resolved photoelectron spectroscopy. In both metals, the minority and majority spin valence bands evolve independently with different time constants after optical excitation. The ultrafast shift of the partially unoccupied minority spin bulk band to higher binding energy and of the majority spin surface state to lower binding energy suggests spin transport between surface and bulk. The slower response of the fully occupied majority spin band follows the lattice temperature and is attributed to Elliott-Yafet type spin-flip scattering. Terbium shows a stronger and faster decay of the exchange splitting, pointing to ultrafast magnon emission via 4f spin-to- lattice coupling

Fluence-dependent dynamics of the 5d6s exchange splitting in Gd metal after femtosecond laser excitation

We investigate the fluence-dependent dynamics of the exchange-split 5d6s valence bands of Gd metal after femtosecond, near-infrared (IR) laser excitation. Time- and angle-resolved photoelectron spectroscopy (tr-ARPES) with extreme ultraviolet (XUV) probe pulses is used to simultaneously map the transient binding energies of the minority and majority spin valence bands. The decay constant of the exchange splitting increases with fluence. This reflects the slower response of the occupied majority-spin component, which we attribute to Elliot–Yafet spin-flip scattering in accordance with the microscopic three-temperature model (M3TM). In contrast, the time constant of the partly unoccupied minority-spin band stays unaffected by a change in pump fluence. Here, we introduce as an alternative to superdiffusive spin transport exchange scattering, which is an ultrafast electronic mechanism explaining the observed dynamics. Exchange scattering can reduce the spin polarization in the partially unoccupied minority-spin band and thus its energetic position without effective demagnetization

The Valence Band Structure of Gadolinium Studied with Time-Resolved Photoemission

We have studied the response of the exchange split valence bands of ferromagnetic gadolinium tofemtosecond laser excitation. We observe a drop of the exchange splitting with a time constant of 0.9 ps but different response times of minority and majority spin bands. Furthermore, even above the Curie temperature there is a finite exchange splitting, which also decreases with laser excitation

Modeling Non-Force-Free and Deformed Flux Ropes in Titan’s Ionosphere

Previous work at Titan presented a set of 85 flux ropes detected during Cassini flybys of Titan from 2005 to 2017. In that study a force‐free model was used to determine the radii and axial magnetic field of the flux ropes. In this work we apply non‐force‐free models. The non‐force‐free model shows an improvement in the number of flux ropes that can be fitted with a model, along with improved uncertainties and χ2 values. A number of asymmetries and features in the magnetometer data cannot be reproduced by either model; therefore, we deform the force‐free model to show that small deformations can replicate these features. One such deformation is to use an elliptical cross section, which replicates a plateau in magnetic field strength along with asymmetries on either side of the center of the flux ropes. Additionally, we explore the properties of bending a flux rope, where we find that minimum variance analysis becomes increasingly degenerate with bending, along with a slight bend causing the switching of the axial field direction from intermediate to maximum variance direction. We conclude that the flux ropes at Titan show aspects of developing flux ropes, compared to other planetary bodies, which exhibit more agreement to the force‐free assumptions of mature flux ropes

Ultrafast Spin Density Wave Transition in Chromium Governed by Thermalized Electron Gas

The energy and momentum selectivity of time- and angle-resolved photoemission spectroscopy is exploited to address the ultrafast dynamics of the antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial thin films of chromium. We are able to quantitatively extract the evolution of the SDW order parameter Δ through the ultrafast phase transition and show that Δ is governed by the transient temperature of the thermalized electron gas, in a mean field description. The complete destruction of SDW order on a sub-100 fs time scale is observed, much faster than for conventional charge density wave materials. Our results reveal that equilibrium concepts for phase transitions such as the order parameter may be utilized even in the strongly nonadiabatic regime of ultrafast photoexcitation

Current density in Saturn’s equatorial current sheet:Cassini magnetometer observations

The equatorial current sheet at Saturn is the result of a rapidly rotating magnetosphere. The sheet itself exhibits periodic seasonal and diurnal movements as well as aperiodic movements of a currently unknown origin, along with periodic thickening and thinning of the magnetodisc, and azimuthal changes in the thickness due to local effects in the magnetosphere. In this paper aperiodic movements of the magnetodisc are utilized to calculate the height‐integrated current density of the current sheet using a Harris current sheet model deformed by a Gaussian wave function. We find a local time asymmetry in both the radial and azimuthal height‐integrated current density. We note that the local time relationship with height‐integrated current density is similar to the relationship seen at Jupiter, where a peak of ∼0.04 A/m at ∼3 SLT (Saturn local time) is seen inside 20 RS. The divergence of the radial and azimuthal current densities are used to infer the parallel currents, which are seen to diverge from the equator in the prenoon sector and enter the equator in the premidnight sector

Targeted interventions for patellofemoral pain syndrome (TIPPS): classification of clinical subgroups

Introduction Patellofemoral pain (PFP) can cause significant pain leading to limitations in societal participation and physical activity. An international expert group has highlighted the need for a classification system to allow targeted intervention for patients with PFP; we have developed a work programme systematically investigating this. We have proposed six potential subgroups: hip abductor weakness, quadriceps weakness, patellar hypermobility, patellar hypomobility, pronated foot posture and lower limb biarticular muscle tightness. We could not uncover any evidence of the relative frequency with which patients with PFP fell into these subgroups or whether these subgroups were mutually exclusive. The aim of this study is to provide information on the clinical utility of our classification system. Methods and analysis 150 participants will be recruited over 18 months in four National Health Services (NHS) physiotherapy departments in England. Inclusion criteria: adults 18–40 years with PFP for longer than 3 months, PFP in at least two predesignated functional activities and PFP elicited by clinical examination. Exclusion criteria: prior or forthcoming lower limb surgery; comorbid illness or health condition; and lower limb training or pregnancy. We will record medical history, demographic details, pain, quality of life, psychomotor movement awareness and knee temperature. We will assess hip abductor and quadriceps weakness, patellar hypermobility and hypomobility, foot posture and lower limb biarticular muscle tightness. The primary analytic approach will be descriptive. We shall present numbers and percentages of participants who meet the criteria for membership of (1) each of the subgroups, (2) none of the subgroups and (3) multiple subgroups. Exact (binomial) 95% CIs for these percentages will also be presented. Ethics and dissemination This study has been approved by National Research Ethics Service (NRES) Committee North West—Greater Manchester North (11/NW/0814) and University of Central Lancashire (UCLan) Built, Sport, Health (BuSH) Ethics Committee (BuSH 025). An abstract has been accepted for the third International Patellofemoral Pain Research Retreat, Vancouver, September 2013