Ultra-low frequency (ULF) waves play a major role in the transport of energy in the near-Earth regions of the magnetosphere. This project research will further develop a magnetohydrodynamic (MHD) simulation for the study of ULF waves and will apply the code to several problems involving ULF waves in the magnetosphere. Pi1 and Pi2 waves, which have periods of 1-40 seconds and 40-150 seconds respectively, are frequently observed at the onset of magnetospheric substorms. Since these waves can be observed world-wide, their propagation from a source region to the ionosphere and to the ground requires further study. At longer periods, Pc3-5 waves (10-600 seconds) have been used as diagnostics of the mass distribution in the near-Earth regions of the magnetosphere. At the high end of the ULF range, Pc1 waves (0.2- 5 seconds) can interact with the cyclotron motion of ions in the magnetosphere and cause energization and pitch angle scattering in the inner magnetosphere. These waves can propagate long distances through the ionosphere in the so-called ionospheric waveguide. Development and application of a global ULF wave model that can investigate these interactions is the primary goal of this project. An important aspect of this modeling is to connect the waves as observed by spacecraft in orbit with waves observed on the ground with magnetometers and in the ionosphere with radar observations.
The modeling of ULF waves will include the dynamics of Pi1 and Pi2 pulsations at substorm onset and their timing at both high and mid-latitudes with respect to driving sources in the magnetotail. The improved MHD simulation code will make it possible to understand both the transient propagation of ULF waves as well as the development of field line resonances, both of which can be used to diagnose the mass density of the magnetosphere. The extension of the model to include ion cyclotron effects will make it possible to study the propagation of electromagnetic ion cyclotron (EMIC) waves from their sources in the magnetosphere through the ionospheric waveguide.
A significant part of the research will be carried out by graduate students, who will be trained in the art of scientific computation as well as the physics understanding necessary to interpret the results of not only the numerical results but also the data obtained by satellites, radars, and ground magnetometers. In addition, the work will also produce a ULF wave model for magnetosphere-ionosphere coupling that can be applied to many problems in magnetospheric physics.
The Earth’s magnetic field oscillates with a variety of wave periods. Waves with periods from 0.2 seconds to 1000 seconds are classified as Ultra-Low-Frequency (ULF) waves. These waves correspond to the fundamental oscillations of the entire magnetosphere (the region of space in which the Earth’s magnetic field dominates charged particle dynamics), and are important for providing energization and loss of radiation belt particles as well as auroral emissions. These waves were first discovered by perturbations in ground magnetometers, but observations from orbiting spacecraft have shown that they are associated with waves called Alfvén waves, which are analogous to waves on a string (e.g., on a guitar, violin, or other stringed instrument. The research done under this grant was to develop a numerical model of these Alfvén waves by solving the magnetohydrodynamic (MHD) equations that govern their propagation. This model is cast in dipolar coordinates to conform to the structure of the geomagnetic field. With this model, we can not only describe the propagation of MHD waves in space but also provide a direct calculation of the ground magnetic perturbations associated with these waves. Although previous versions of this model were previously developed, the present model includes the detailed structure of the ionosphere for the first time. This allows comparison of ground magnetic fields with ionospheric diagnostics such as radar measurements. We have now used this model for a number of studies. The first investigated waves in the shorter period range of ULF waves, from 0.2-5.0 seconds. These waves are strongly influenced by the ionosphere, which attenuates the higher frequency waves in this range. This strong influence is due in part to the fact that the ionospheric density, which controls the wave speed, is strongly varying up to about 2000 km altitude. This creates an effective resonant cavity for Alfvén waves in the 0.2-5.0 second range. We have modeled this interaction and showed that waves that match the resonances in this cavity can propagate well away from their source. In another study, we have investigated lower frequency waves (called Pi2 in the arcane nomenclature of ULF waves), that are associated with magnetospheric substorms. These waves are thought to be produced by fast, localized plasma flows in the geomagnetic tail. We have modeled the propagation of these waves in the inner magnetosphere and determined that they can produce signals that occur at all latitudes at virtually the same time, eliminating the need for more speculative models that invoke propagation of electromagnetic waves through the atmosphere.
