The theoretical foundation of 3-D Alfvén resonances : time-dependent solutions

Both authors were funded in part by STFC (through Consolidated Grant ST/N000609/1) and The Leverhulme Trust (through Research Grant RPG-2016-071).We present results from a 3-D numerical simulation which investigates the coupling of fast and Alfvén magnetohydrodynamic (MHD) waves in a nonuniform dipole equilibrium. This represents the time-dependent extension of the normal mode (∝ exp(−iωt)) analysis of Wright and Elsden (2016), and provides a theoretical basis for understanding 3-D Alfvén resonances. Wright and Elsden (2016) show that these are fundamentally different to resonances in 1D and 2D. We demonstrate the temporal behavior of the Alfvén resonance, which is formed within the "Resonant Zone"; a channel of the domain where a family of solutions exists such that the natural Alfvén frequency matches the fast-mode frequency. At early times, phase mixing leads to the production of prominent ridges in the energy density, whose shape is determined by the Alfvén speed profile and the chosen background magnetic field geometry. These off resonant ridges decay in time, leaving only a main 3-D resonant sheet in the steady state. We show that the width of the 3-D resonance in time and in space can be accurately estimated by adapting previous analytical estimates from 1-D theory. We further provide an analytical estimate for the resonance amplitude in 3-D, based upon extending 2-D theory.Publisher PDFPeer reviewe

Simulations of MHD wave propagation and coupling in a 3D magnetosphere

ANW was partially funded by STFC (ST/N000609/1) and the Leverhulme Trust (RPG- 2016-071). TE was funded by the Leverhulme Trust (RPG-2016-071) and a Leverhulme Trust Early Career Fellowship (ECF-2019-155).A novel simulation grid is devised that is optimized for studying magnetohydrodynamic (MHD) wave coupling and phase mixing in a dipole‐like magnetic field. The model also includes flaring on the dawn and dusk flanks. The location of the magnetopause is quite general. In particular, it does not have to coincide with a coordinate surface. Simulations indicate the central role of global fast waveguide modes. These switch from being azimuthally standing in nature at noon, to propagating antisunward on the flanks. The field line resonances (FLRs) seen in the simulation results are three dimensional and not strictly azimuthally polarized. When a plume is present, the FLRs cross a range of 2 in shell, and have a polarization that is midway between toroidal and poloidal.Publisher PDFPeer reviewe

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

Cálculo del Contenido Total de Electrones-TEC, para la Ionósfera Colombiana

En este trabajo se presenta un estudio del comportamiento de la ionósfera Colombiana, a partir de referentes teóricos, complementado con un conjunto de datos tomados en el laboratorio de Física de la Ionósfera de la Universidad Nacional, tratados mediante el sistema computacional SAMI2, adaptado para las coordenadas geográficas del país. Se trabajan los aspectos teóricos y se explican las consideraciones hechas en el modelo computacional del programa, tales como el sistema de coordenadas, las especies que se consideran (iones, átomos neutros y electrones), la evolución química de éstas durante el día y la temporada del año; las corrientes que se presentan en la ionósfera; así como las ecuaciones de continuidad, de temperatura y de velocidad. En la segunda parte se toma el software SAMI2, se modifican las coordenadas para las que ya se ha realizado la simulación, y se corre para las coordenadas pertinentes a la ubicación geomagnética del país. Se comparan los resultados con las mediciones realizadas en el laboratorio, y se determinan los ajustes a realizar en las variables ingresadas al sistema. Se muestran los resultados frente a otros trabajos desarrollados y frente a dos modelos ionosféricos existentes; logrando caracterizar de forma más detallada el comportamiento de la ionósfera Colombiana.Abstract. This paper presents a study of the behavior of the Colombian ionosphere from theoretical framework supplemented with data taken in the laboratory of Geospatial Physics in the Universidad Nacional de Colombia, treated by SAMI2, a computational system adapted to the geographical coordinates of the country. In the first part discusses the theory and the principles underlying the computational model that works in the program, such as the coordinate system, the species considered (ions, neutral atoms and electrons), the evolution chemistry of these during the day and season of the year, the currents that occur in the ionosphere, and the equations of continuity, temperature and speed. For the second part SAMI2 is taken, modifying the coordinates for which has already been performed simulation, and running it for the coordinates corresponding to the country’s geomagnetic position. The results are compared with measurements made in the laboratory, and determine the adjustments to the variables entered into the system. Finally, we show the results to other works developed and compared to two existing ionospheric models, achieving more fully characterize the behavior of the Colombian ionosphereMaestrí

A note on the dipole coordinates

A couple of orthogonal coordinates for dipole geometry are proposed for numerical simulations of plasma geophysics in the Earth's dipole magnetic field. These coordinates have proper metric profiles along field lines in contrast to the standard dipole coordinate system that is commonly used in analytical studies for dipole geometry.Comment: Corrected the vertical scale of Fig. 1. Accepted for publication in Computers & Geoscience