Remote sensing the plasmasphere, plasmapause, plumes and other features using ground-based magnetometers

The plasmapause is a highly dynamic boundary between different magnetospheric particle populations and convection regimes. Some of the most important space weather processes involve wave-particle interactions in this region, but wave properties may also be used to remote sense the plasmasphere and plasmapause, contributing to plasmasphere models. This paper discusses the use of existing ground magnetometer arrays for such remote sensing. Using case studies we illustrate measurement of plasmapause location, shape and movement during storms; refilling of flux tubes within and outside the plasmasphere; storm-time increase in heavy ion concentration near the plasmapause; and detection and mapping of density irregularities near the plasmapause, including drainage plumes, biteouts and bulges. We also use a 2D MHD model of wave propagation through the magnetosphere, incorporating a realistic ionosphere boundary and Alfvén speed profile, to simulate ground array observations of power and cross-phase spectra, hence confirming the signatures of plumes and other density structures

Results from a MHD model of ultra low frequency waves in the magnetosphere with realistic ionospheric conductances

The magnetised plasma of the near-Earth space environment supports ultra-low frequency (ULF ;1-100 mHz), magnetohydrodynamic (MHD) oscillations in predominantly two wave modes. The fast Alfvén mode spreads ULF wave energy isotropically throughout the magnetosphere. For sufficiently large ionospheric conductances the shear Alfvén mode forms field line resonances (FLRs) between the northern and southern ionospheres. These conditions are usually met for daytime ionosphere conductance values. The FLRs are used to remotely sense plasma mass densities in the magnetosphere. The oscillations lose energy in the ionosphere, whose properties determine the boundary conditions, particularly by resonance damping effects. Using a MHD model of the magnetosphere with realistic ionosphere boundary conditions, the variation in resonant frequency with ionosphere conductance is reported. The effects of the ionospheric conductance on the ULF wave fields along the resonant field lines are also shown. The finite Pedersen and Hall conductances dissipate wave energy into the ionosphere and the spatial and temporal distributions of this energy show a distinct poleward propagation

Fortran code for modeling the propagation of ULF waves in a 3-dimensional dipole magnetosphere

Fortran source code for numerical modeling of ULF waves in the magnetosphereThis code allows for the modeling of Ultra-Low-Frequency (ULF) waves in the Earth's magnetosphere. It has been used in a number of publications and presentations at conferences. It takes a prescribed driver in the form of a compression at the outer boundary and follows the propagation of the waves through the magnetosphere and to the ground. The output consists of files each containing snapshots of one component of the electromagnetic field at all points in the simulation volume at an instant of time, in addition to files containing background information such as the Alfven speed profile throughout the simulation.National Science Foundation AGS-184089

A study of polar cap Pc1-2: source locations and wave propagation

The occurrence of Pcl-2 ULF waves in Scott Base and Casey Antarctic magnetometer data has been examined with the objective of determining whether the source region for these waves is located in the Earth's cusp/boundary layer region or the magnetosheath. In particular we explore the hypothesis that some Pcl-2 events are propagated along open field lines which convect over the southern polar cap. For a selected event the location of Scott Base with respect to the open/closed field line boundary was independently determined using DMSP spacecraft particle data and well established particle energy and flux criteria. This event provides evidence that Pcl-2 waves can be seen on the ground well inside the polar cap. The possibility of propagation in the ionospheric waveguide, away from the footpoint of the source, is investigated using numerical modelling and realistic ionospheric parameters. Under the modelled conditions we find a modification of wave ellipticity which excludes this explanation for the polar cap Pcl-2 seen in the event study

Film formations of aggregates due to lateral capillary forces

The lateral capillary force is of significant importance in liquid film coating processes [1]. This force, for particles much smaller than the capillary length, decays with the inverse of the separation distance between particles centres and is, thus, considered a longrange force [2]. In this paper, we study the role of this long-range force on the final structure of a film containing partially submerged nanoparticles. We have used computer simulations based on Discrete Element Method (DEM) to investigate film formation of mono and binary disperse particle systems. The particle radii were 80 nm, 100 nm, and 120 nm with combinations of these particle sizes for the binary disperse system. To determine the nearest neighbours for the calculation of the lateral capillary force a Delaunay Triangulation method was used. The surface coverage of the partially submerged particles was 0.05, which coincides with a parallel experimental research. The forces included in the model are the lateral and vertical capillary forces, Brownian motion, contact forces, van der Waals attraction, fluid drag and hydrodynamic resistance. The structure of the aggregates formed was compared using three parameters, the isotropic ordering factor, non-dimensional boundary length and the pair (radial) correlation function. The simulation results show that particles self-organise into isotropic aggregates due to lateral capillary forces. Particle size was shown to have little effect on final aggregate structures. Binary disperse systems were shown to have less ordered structures when compared to monodisperse systems as suggested by the decrease in peak sharpness of the pair correlation function. Increasing the particle size gap resulted in less ordering in the binary systems

Quarter-wave modes of standing Alfvén waves detected by cross-phase analysis

We have examined the diurnal variation of the local field line eigenfrequency at L ~ 2.6 using cross-phase analysis of Sub-Auroral Magnetometer Network and Magnetometers Along the Eastern Atlantic Seaboard for Undergraduate Research and Education ground magnetometer array data. On several days the eigenfrequency was remarkably low near the dawn terminator, when one end of the field line was sunlit and the other end was in darkness. Later in the morning the eigenfrequency gradually increased to the normal daytime value. This type of diurnal eigenfrequency variation was found in both European and American meridians and in several seasons (March, June, and December). By modeling this situation we show that the extraordinarily low eigenfrequency events appeared when the ionospheric Pedersen conductance was strongly asymmetric between both ends of the field line, leading to the formation of quarter-wavelength-mode standing waves that revert to half-wavelength modes as the dawn terminator passes both conjugate points. Ground-based magnetometer measurements of local toroidal field line eigenfrequencies are often inverted to infer plasma mass density in the magnetosphere by assuming half-wavelength-mode standing field line oscillations. However, the mode structure and hence field line eigenfrequency also depend on the ionospheric conductance. In particular, we find that there is a threshold of interhemispheric conductance ratio for the quarter-wavelength mode to be established. Our results therefore show that cross-phase techniques can detect quarter-wavelength-mode waves, where the inferred mass density would be overestimated

Pc3-4 ULF waves observed by the SuperDARN TIGER radar

Despite extensive research, the mechanisms for propagation of Pc3-4 energy from the generation region at the bow shock to the high-latitude ionosphere remain unresolved. We used high temporal (6-12 s) and spatial (45 km) resolution data from the SuperDARN TIGER radar (Tasmania) to examine Pc3-4 wave signatures at the F-region heights. We focus on a case study on 28 September 2000, when large-amplitude band-limited Pc3-4 oscillations were observed across 10-20 range gates in beam #4 (which points towards the CGM pole) for about four hours preceding MLT noon. These waves were detected in sea-scatter echoes reflected from the ionospheric footprint of the plasmatrough. Nearby ground magnetometer data from Macquarie Island showed very similar variations in both the north-south and east-west components. The radar data revealed the occasional presence of quasi-FLR (field-line resonance) spatial structures with frequencies much higher than those of the local fundamental FLR modes. Detailed spectral analysis of the ionospheric and ground data shows that these structures most probably correspond to a 3rd-harmonic, poloidal-mode FLR. Such observations suggest that compressional Pc3-4 waves produced in the upstream solar wind travel earthward from the magnetopause in the magnetic equatorial plane depositing energy into the Alfvenic modes, as either forced or 3rd-harmonic FLR that reach ionospheric heights along magnetic field lines

A finite difference construction of the spheroidal wave functions

A fast and simple finite difference algorithm for computing the spheroidal wave functions is described. The resulting eigenvalues and eigenfunctions for real and complex spheroidal bandwidth parameter, c , agree with those in the literature from four to more than eleven significant figures. The validity of this algorithm in the extreme parameter regime, up to c² = 10¹⁴ , is demonstrated. Furthermore, the algorithm generates the spheroidal functions for complex order m . The coefficients of the differential equation can be simply modified so that the algorithm may solve any second order differential equation in Sturm–Liouville form. The prolate spheroidal functions and the spectral concentration problem in relation to band-limited and time-limited signals is discussed. We review the properties of these eigenfunctions in the context of Sturm–Liouville theory and the implications for a finite difference algorithm. A number of new suggestions for data fitting using prolate spheroidal wave functions with a heuristic for optimally choosing the value of c and the number of basis functions are described

Remote sensing the plasmasphere, plasmapause, plumes and other features using ground-based magnetometers

The plasmapause is a highly dynamic boundary between different magnetospheric particle populations and convection regimes. Some of the most important space weather processes involve wave-particle interactions in this region, but wave properties may also be used to remote sense the plasmasphere and plasmapause, contributing to plasmasphere models. This paper discusses the use of existing ground magnetometer arrays for such remote sensing. Using case studies we illustrate measurement of plasmapause location, shape and movement during storms; refilling of flux tubes within and outside the plasmasphere; storm-time increase in heavy ion concentration near the plasmapause; and detection and mapping of density irregularities near the plasmapause, including drainage plumes, biteouts and bulges. We also use a 2D MHD model of wave propagation through the magnetosphere, incorporating a realistic ionosphere boundary and Alfvén speed profile, to simulate ground array observations of power and cross-phase spectra, hence confirming the signatures of plumes and other density structures