Azimuthal inhomogeneity in the MHD waveguide in the outer magnetosphere

This paper investigates the excitation of waveguide modes in a nonuniform dipole equilibrium and, further, their coupling to field line resonances (FLRs). Waveguide modes are fast compressional ultralow frequency (ULF) waves, whose structure depends upon the magnetospheric equilibrium and the solar wind driving conditions. Using magnetohydrodynamic simulations, we consider how the structure of the excited waveguide mode is affected by various forms of magnetopause driving. We find that the waveguide supports a set of normal modes that are determined by the equilibrium. However, the particular normal modes that are excited are determined by the structure of the magnetopause driver. A full understanding of the spatial structure of the normal modes is required in order to predict where coupling to FLRs will occur. We show that symmetric pressure driving about the noon meridian can excite normal modes which remain around to drive resonances for longer than antisymmetric driving. Further, the critical quantity in terms of efficient coupling is the magnetic pressure gradient aligned with the resonance.

Section: Introductionmentioning

confidence: 99%

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

Elsden

Wright

2019

“…Their maximum is attained at k z Δ m =0, and their distribution is particular in that they asymptotically tend to zero as

k_{z} Δ_{m} \to \infty

. Such behavior for the eigenmode growth rates of the magnetospheric FMS waveguide has been studied in detail by Mazur and Chuiko (, ) in their model with Cartesian geometry.…”

Section: Numerical Solution Of the Basic Equation And Discussionmentioning

confidence: 87%

Kelvin‐Helmholtz Instability in the Geotail Low‐Latitude Boundary Layer

Leonovich

Козлов

2018

The low‐latitude boundary layer stability problem in the geotail is solved numerically. This study relies on a cylindrical model of the geotail, with a smooth boundary, enwrapped by a helical solar wind flow. It is shown that three types of unstable magnetohydrodynamic waves exist in this plasma system: (1) surface waves at the magnetopause, (2) waves radiated into the solar wind, and (3) eigenmodes of the waveguide within the geotail. Unstable surface waves generated in the low‐ to medium‐speed solar wind have the largest growth rate. They are driven in the Pc3–Pc6 geomagnetic pulsation range and have a local maximum at Pc4 frequencies. In the cylindrical model in question the minimum transverse wavelengths of the unstable oscillations are much less than the shear layer thickness. This differentiates it from models with Cartesian geometry, where the minimum transverse wavelengths of unstable oscillations are larger than the boundary layer thickness. In high‐speed solar wind flows the magnetopause is stable to surface waves but unstable to radiative modes. The growth rate of such oscillations is an order of magnitude smaller than that for surface waves generated in the low‐ to medium‐speed solar wind. If the helicity of high‐speed solar wind flows enwrapping the magnetosphere is small, the oscillations are unstable in the Pc4–Pc6 geomagnetic pulsation range, with their maximum growth rate being in the Pc5 range. If the helicity of high‐speed solar wind flow enwrapping the magnetosphere is large enough, the oscillations are unstable throughout the Pc1–Pc6 geomagnetic pulsation range.

“…Geomagnetic field is along the z coordinate. This figure is taken from our previous work [ Mazur and Chuiko , ].…”

Section: Mhd Waveguidementioning

confidence: 99%

Energy flux in 2‐D MHD waveguide in the outer magnetosphere

Mazur

Chuiko

2017

Self Cite

The problems of large‐scale wave propagation and amplification in the outer magnetosphere are considered. Kelvin‐Helmholtz (KH) instability growth rate of the magnetospheric waveguide eigenmodes is investigated as a function of a coordinate along the magnetopause. The problem of solar wind MHD wave penetration into the waveguide is investigated for a broad range near Pc3 and Pc5 geomagnetic pulsation frequencies and realistic models of the magnetospheric waveguide. The expression for the waveguide eigenmode energy flux is obtained. This expression includes the effects of external wave penetration and mode amplification due to the KH instability, as well as losses due to dissipation in the vicinity of the Alfven resonance which are incorporated into the growth rate coefficient together with the instability.