Frequency variability of standing Alfvén waves excited by fast mode resonances in the outer magnetosphere

Abstract

Coupled fast mode resonances (cFMRs) in the outer magnetosphere, between the magnetopause and a turning point, are often invoked to explain observed discrete frequency field line resonances. We quantify their frequency variability, applying cFMR theory to a realistic magnetic field model and magnetospheric density profiles observed over almost half a solar cycle. Our calculations show that cFMRs are most likely around dawn, since the plasmaspheric plumes and extended plasmaspheres often found at noon and dusk can preclude their occurrence. The relative spread (median absolute deviation divided by the median) in eigenfrequencies is estimated to be 28%, 72%, and 55% at dawn, noon, and dusk, respectively, with the latter two chiefly due to density. Finally, at dawn we show that the observed bimodal density distribution results in bimodal cFMR frequencies, whereby the secondary peaks are consistent with the so‐called “CMS” frequencies that have previously been attributed to cFMRs.

Simulations of MHD Wave Propagation and Coupling in a 3‐D Magnetosphere

“…There is an additional variation of Alfvén speed along the field lines through the density dependence (based upon the theoretical estimate shown in Figure 1 of Angerami & Carpenter, ). The Alfvén speed profile is intended to represent a typical magnetosphere that is broadly consistent with observations and theory: Figure 1 of Archer et al (), Figures 2a and 2b of Moore et al (), Takahashi and Anderson ().…”

Magnetospheric and solar wind dependences of coupled fast‐mode resonances outside the plasmasphere

“…Because the frequencies of the cFMRs were found to highly correlate with the outer magnetospheric Alfvén speed taken nearest the magnetopause at the spacecraft apogee [ Archer et al , ], we shall rephrase the above theory by introducing a normalized Alfvén speed profile , where this characteristic scale has been divided out, i.e., …”

“…Since these studies use rather idealized magnetic fields and/or densities, Archer et al [] (hereafter A2015) combined a large number of observed density profiles with a realistic magnetic field model, applying cFMR theory (see section 2) to each profile. They found that cFMRs are supported most often in the dawn sector and least often around noon, in agreement with ground‐based FLR observations.…”

“…They demonstrated that as the scale length (inversely proportional to the Alfvén speed gradient) increases, the waveguide frequencies go up and the resonance location moves earthward. Claudepierre et al [2016] Since these studies use rather idealized magnetic fields and/or densities, Archer et al [2015] (hereafter A2015) combined a large number of observed density profiles with a realistic magnetic field model, applying cFMR theory (see section 2) to each profile. They found that cFMRs are supported most often in the dawn sector and least often around noon, in agreement with ground-based FLR observations.…”

The story of plumes: the development of a new conceptual framework for understanding magnetosphere and ionosphere coupling

“…What is now clear is that plumes connect the equatorial F region ionosphere to the dayside magnetopause and the nightside magnetotail plasma sheet (e.g., Su et al, 2001a, b;Horvath and Lovell, 2011;Walsh et al, 2014a, b;Foster et al, 2014). Through the formation and evolution of the different plumes, they impact wave generation and wave-particle interactions (e.g., Summers et al, 2008;Chen et al, 2012;Halford et al, 2015), particle precipitation (Spasojević and Fuselier, 2009;Yuan et al, 2011Yuan et al, , 2013, ion outflow (e.g., Zeng and Horowitz, 2008;Tu et al, 2007), local-time asymmetries in ULF wave field-line resonance (FLR) signatures (e.g., Archer et al, 2015;Ellington et al, 2016), satellite communication and navigation systems (Ledvina et al, 2004;Basu et al, 2005;Datta-Barua et al, 2014), and even the coupling efficiency of the solar wind to the magnetosphere (Borovsky and Denton, 2006;Borovsky et al, 2013;Ouellette et al, 2016;Fuselier et al, 2016). Though we now have a new appreciation and understanding of plumes, there are still many unanswered questions on their formation (e.g., Kelley et al, 2004;Horvath and Lovell, 2011;Zou et al, 2013Zou et al, , 2014Borovsky et al, 2014) and impact on global magnetospheric dynamics McFadden et al 2008;Walsh et al, 2014Walsh et al, , 2015.…”