Statistical study of global modes outside the plasmasphere

Abstract

[1] Global modes, trapped fast mode magnetohydrodynamic (MHD) waves in the Earth's magnetosphere, may exhibit a monochromatic frequency spectrum even in the presence of a driver with a broadband frequency spectrum; they can in turn drive standing Alfvén waves at discrete frequencies via field line resonance (FLR). Direct observations of global modes are limited to a few case studies due to unique challenges associated with detecting them in situ. In this study, we use electric field, magnetic field, and plasma data from multiple THEMIS spacecraft as well as ground-based observations to identify and characterize global modes outside the nominal plasmapause location. We establish a lower bound of 1.0% for the global mode occurrence rate in the 3-20 mHz frequency range in that region and show that global modes occur more frequently during high-speed solar wind intervals. We also show that global modes with frequency between 10 and 20 mHz occur preferentially in the noon local time sector and are likely driven by processes in the ion foreshock.

Modeling the Dawn/Dusk Asymmetry of Field Line Resonances

“…Such measurements inevitably require statistical analysis of spacecraft data, but in doing so, one realizes that magnetically compressional ULF waves in the outer magnetosphere result both from instabilities in the magnetosphere and from propagation of disturbances from the solar wind as the fast mode (Takahashi et al, ). Because the fast mode waves often have small amplitudes (Hartinger et al, ), it is difficult to separate them out from magnetic field time series that contains contribution from both wave types. While the spatial distribution of the mass density and amplitudes of the toroidal component of Alfvén waves can be obtained relatively easily, the behavior of fast mode waves can be studied most easily using numerical simulations.…”

The Effect of Fast Normal Mode Structure and Magnetopause Forcing on FLRs in a 3‐D Waveguide

“…Through ray trajectories, this study argued that the waveguide modes which most efficiently drive FLRs have a small k y , where k y is the azimuthal wavenumber, since they remain at a given azimuthal location for longer than modes with a larger k y . Observationally, there are many previous studies which give credence to the treatment of the magnetosphere as a waveguide (Eriksson et al, ; M. Hartinger et al, ; M. D. Hartinger et al, ; Rae et al, ). It has been noted that cavity and waveguide modes can be difficult to detect in satellite data for various reasons, such as obscuring by other ULF wave modes and not having the appropriate radial placement of satellites to properly identify such modes (Hartinger et al, ).…”

“…This will then impact the FLRs driven. Such analysis of different magnetopause driving is further useful to understand how ULF waves may be driven by transient ion foreshock phenomena (M. D.; Hartinger et al, ; Shen et al, ; Wang et al, ), or other localized drivers such as magnetosheath jets (Plaschke et al, ).…”

Observation and Numerical Simulation of Cavity Mode Oscillations Excited by an Interplanetary Shock

“…Impulsive excitation of CMOs is predicted by theory (Southwood & Kivelson, ) and is routinely seen in numerical simulations (Allan et al, ; Fujita et al, ; Lee & Lysak, ). Clearly defined wave onset would make identification of shock‐induced CMOs easy compared to CMOs that are continuously present together with other ULF waves (Hartinger et al, ; Waters et al, ). Somewhat surprisingly, in situ observations of possible shock‐induced CMOs have been limited to those made by single spacecraft (Cahill et al, ; Goldstein et al, ; Yumoto et al, ).…”