Modeling features of field line resonance observable by a single spacecraft at Saturn

Funding: T. Elsden was funded by a Leverhulme Early Career Fellowship, the University of Leicester and the University of Glasgow. D. Southwood acknowledges funding from UK Science and Technology Facilities Council grant: ST/W001071/1.The observations of Southwood et al. (2021), using data from the Cassini magnetometer from the final (proximal) orbits of the mission at Saturn, show large scale azimuthally polarized magnetic signals are always present near periapsis. The signals were attributed to standing Alfvén waves excited on the magnetic shells planetward of the Saturn D-ring. The apparent absence of any systematic variation in frequency as the spacecraft crossed magnetic shells, implied that the signals were not simply locally excited standing Alfvén modes, but were pumped by coupling to global compressional eigenmodes excited in a cavity formed in the dayside magnetosphere. In this study, we use a numerical magnetohydrodynamic (MHD) model to test such theoretical explanations for the observations, by examining in detail the MHD wave coupling and large scale spatial structure of the signals. The modeling not only shows good agreement with the data, but further provides new insight into features previously overlooked in the data. In particular, we show how the apparent frequency of a single spacecraft observation is affected by the phase variation present in a local field line resonance.Publisher PDFPeer reviewe

Resonant fast-Alfvén wave coupling in a 3D coronal arcade

Funding: This research was funded in part by STFC grant ST/W001195/1 (A.N.W.). T.E. was funded in part by a Leverhulme Early Career Fellowship ECF-2019-155.The resonant excitation of Alfvén waves using the fast magnetosonic mode is important in space plasmas. In this paper, we consider a simple model of a three-dimensional (3D) coronal arcade. A numerical approach is used to produce a driven normal mode. We find that resonant coupling can occur in 3D, but there are new features that are absent in 2D. In particular, the polarisation of the Alfvén waves can vary with position throughout the Resonant Zone. Moreover, there are an infinite number of possible paths the resonant waves can exist on.Publisher PDFPeer reviewe

Numerical modelling of ultra low frequency waves in Earth's magnetosphere

Ultra Low Frequency (ULF) waves are a ubiquitous feature of Earth's outer atmosphere, known as the magnetosphere, having been observed on the ground for almost two centuries, and in space over the last 50 years. These waves represent small oscillations in Earth's magnetic field, most often as a response to the external influence of the solar wind. They are important for the transfer of energy throughout the magnetosphere and for coupling different regions together. In this thesis, various features of these oscillations are considered. A detailed background on the history and previous study of ULF waves relevant to our work is given in the introductory chapter. In the following chapters, we predominantly use numerical methods to model ULF waves, which are carefully developed and thoroughly tested. We consider the application of these methods to reports on ground and spaced based observations, which allows a more in depth study of the data. In one case, the simulation results provide evidence for an alternative explanation of the data to the original report, which displays the power of theoretical modelling. An analytical model is also constructed, which is tested on simulation data, to identify the incidence and reflection of a class of ULF wave in the flank magnetosphere. This technique is developed with the aim of future applications to satellite data. Further to this, we develop models both in Cartesian and dipole geometries to investigate some of the theoretical aspects of the coupling between various waves modes. New light is shed on the coupling of compressional (fast) and transverse (Alfvén) magnetohydrodynamic (MHD) wave modes in a 3D dipole geometry. Overall, this thesis aims to develop useful numerical models, which can be used to aid in the interpretation of ULF wave observations, as well as probing new aspects of the existing wave theory

Deciphering satellite observations of compressional ULF waveguide modes

T. Elsden would like to thank STFC for financial support for a doctoral training grant, number AMC3 STFC12. A.N. Wright was supported by STFC grant ST/N000609/1.We present an analytical method for determining incident and reflection co- efficients for flank ULF compressional waveguide modes in Earth’s magnetosphere. In the flank magnetosphere, compressional waves propagate azimuthally, but exhibit a mixed standing/propagating nature radially. Understanding this radial dependence will yield information on the energy absorption and transport of these waves. We provide a step by step method that can be applied to observations of flank ULF waves, which separates these fluctuations into incident and reflected parts. As a means of testing, we apply the method to data from a numerical waveguide simulation, which shows the effect on the reflection coefficient when energy is absorbed at a field line resonance.Publisher PDFPeer reviewe

A review of the theory of 3D Alfvén (field line) resonances

Funding: T. Elsden is funded by a Leverhulme Trust Early Career Fellowship (ECF-2019-155) and the University of Glasgow. A. Wright was partially funded by the Science and Technology Facilities Council (STFC) grant (ST/N000609/1). A. Degeling is supported by the National Natural Science Foundation of China (41774172).This review article aims to summarise recent developments in Alfvén resonance theory, with a focus on applications to magnetospheric ultra-low frequency (ULF) waves, though many of the ideas are relevant for applications in other fields as well. The key aspect we treat is how Alfvén resonance manifests in a fully 3-D varying medium. The prerequisite ideas are developed in a reasonably comprehensive introduction, which would be a good starting point for any interested reader looking to gain an understanding of the Alfvén resonance process, as well as where to find associated reading. The main part of the review is split into three sections. We firstly consider results from numerical simulations of relatively simple magnetic field geometries, such as 2-D and 3-D dipoles, to develop the fundamental properties of 3-D Alfvén resonances. Secondly, we review previous simulations in more general magnetic field geometries, reconciling these results with those from the simpler dipole cases. Thirdly, in light of these numerical results, we review theoretical studies using various analytical methods to find approximate solutions to the pertinent magnetohydrodynamic (MHD) equations. The review is concluded with a discussion of these different approaches, as well as linking these ideas to their importance for observations. Finally, we discuss potential future developments in this research area.Publisher PDFPeer reviewe

RAS specialist discussion meeting report

Report on ‘Planetary Ultra-Low Frequency Waves – Theory, Modelling and Observations’Publisher PDFNon peer reviewe

Polarization of magnetospheric ULF waves excited by an interplanetary shock on 27 February 2014

Funding: KT was supported by NASA Grants NNX17AD34G, 80NSSC19K0259, and 80NSSC21K0453. TE was partially funded by Leverhulme Early Career Fellowship ECF-2019-155. ANW was partially funded by the Science and Technology Facilities Council (STFC) Grant (ST/N000609/1). AWD was supported by NSFC Grants 42225405 and NAF\R1\19“1047”.Many previous studies have reported that magnetospheric ultralow frequency waves excited by interplanetary shocks exhibit a strong toroidal component, corresponding to azimuthal displacement of magnetic field lines. However, the toroidal oscillations excited by an interplanetary shock on 27 February 2014 and observed on the dayside by multiple spacecraft were accompanied by a strong poloidal component (radial field line displacement). The frequency of the toroidal oscillations changed with the radial distance of the spacecraft as expected for standing Alfvén waves. We run a 3-D linear numerical simulation of this wave event using a model magnetosphere with a realistic radial variation of the Alfvén velocity. The simulated wave fields, when sampled at a radial distance comparable to those of the observations in the real magnetosphere, exhibit polarization similar to the observations. In the simulation, the poloidal component comes from radially standing fast mode waves and the toroidal component results from a field line resonance driven by the fast mode waves. As a consequence, the relative amplitude and phase of the toroidal and poloidal components change with radial distance.Publisher PDFPeer reviewe

“That’s how Muslims are required to view the world”: Race, culture and belief in non-Muslims’ descriptions of Islam and science

Islam’s positioning in relation to Western ideals of individuality, freedom, women’s rights and democracy has been an abiding theme of sociological analysis and cultural criticism, especially since September 11th 2001. Less attention has been paid, however, to another concept that has been central to the image of Western modernity: science. This article analyzes comments about Islam gathered over the course of 117 interviews and 13 focus groups with non-Muslim members of the public and scientists in the UK and Canada on the theme of the relationship between science and religion. The article shows how participants’ accounts of Islam and science contrasted starkly with their accounts of other religious traditions, with a notable minority of predominantly non-religious interviewees describing Islam as uniquely, and uniformly, hostile to science and rational thought. It highlights how such descriptions of Islam were used to justify the cultural othering of Muslims in the West and anxieties about educational segregation, demographic ‘colonization’ and Islamist extremism. Using these data, the article argues for: 1) wider recognition of how popular understandings of science remain bound up with conceptions of Western cultural superiority; and 2) greater attentiveness to how prejudices concerning Islamic beliefs help make the idea that Muslims pose a threat to the West respectable

Properties of magnetohydrodynamic normal modes in the Earth’s magnetosphere

Funding: MDH was supported by NASA 80NSSC19K0127, 80NSSC19K0907, 80NSSC21K1683, 80NSSC21K1677, 80NSSC23K0903, and NSF AGS-2307204. KT was supported by NASA 80NSSC19K0259 and 80NSSC21K0453. MOA was supported by a UKRI (STFC / EPSRC) Stephen Hawking Fellowship EP/T01735X/1. The research of A.W. was funded in part by Science and Technology Facilities Council (STFC) grant ST/W001195/1 (UK). T.E. was funded in part by a Leverhulme Early Career Fellowship ECF-2019-155 (UK). AA and XZ were supported by NASA 80NSSC21K0729 and 80NSSC23K0108. We acknowledge support from ISSI Bern through ISSI International Team projects 483 “The Identification And Classification Of 3D Alfven Resonances” and 546 “Magnetohydrodynamic Surface Waves at Earth’s Magnetosphere (and Beyond).” We acknowledge NASA contract NAS5-02099.The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves.PostprintPeer reviewe

Transitions in digital personhood:Making sense of online activity in early retirement

We present findings from a qualitative study about how Internet use supports self-functioning following the life transition of retirement from work. This study recruited six recent retirees and included the deployment of OnLines, a design research artifact that logged and visualized key online services used by participants at home over four-weeks. The deployment was supported by pre- and post-deployment interviews. OnLines prompted participants’ reflection on their patterns of Internet use. Position Exchange Theory was used to understand retirees’ sense making from a lifespan perspective, informing the design of supportive online services. This paper delivers a three-fold contribution to the field of human-computer interaction, advancing a lifespan-oriented approach by conceptualizing the self as a dialogical phenomenon that develops over time, advancing the ageing discourse by reporting on retirees’ complex identities in the context of their life histories, and advancing discourse on research through design by developing OnLines to foster participant-researcher reflection informed by Self Psychology