ULF waves-their relationship to the structure of the Earth's magnetosphere

Abstract

The Earth's magnetosphere is highly structured, in terms of both magnetic field and plasma characteristics. This structure has a profound influence on the propagation of plasma waves, especially ultra-low-frequency (ULF) waves With mHz frequencies, which have wavelengths comparable with typical magnetospheric dimensions. In thk review we illustrate bow the basic theory of ULF hydromagnetic wave propagation in an infinite, homogeneous, uniformly magnetized plasma has been extensively modified to cope with the requirements of applying it to the magnetosphere, a natural laboratory for the physics of uLF waves.We consider the field-line-guided Alfvdn wave modes and the isotropic fast hydromagnetic wave modes, and show how the existence of magnetospheric boundaries can affect the structure of both modes. The finite length of geomagnetic field lines can quantize the frequencies of Alfvdn waves by forcing the establishment of standing waves on field lines. The natural variation of Alfvh speed and field-line length in the magnetosphere then allows the possibility of a continuum of eigenfrequencies for each harmonic of a field-line standing wave. The magnetosphere can also be considered as a cavity in which fast cavity modes may exist, with frequencies quantized by the three-dimensional cavity shape.Azimuthal variation in a system With radial Alfvh speed structure leads to the idea of 'field-line resonance', in which a quasi-monochromatic compressional wave (for example a surface wave or a cavity wave) can match frequency (resonate) with a particular magnetic field line, and hence can transfer energy to a narrow region of field lines around the resonance line.The concept of resonance extends also into the regime of energetic magnetospheric plasma. Hot particles in a spatially structured magnetic field have characteristic drift frequencies perpendicular to the magnetic field and bounce frequencies parallel to the magnetic field. Resonance between waves and particles takes place when wave frequencies match either the drift or bounce frequency of a suitable subset of particles, or a combination of harmonics of both. M v e s either grow or damp as a result of this resonance, depending on the detailed structure of particle distributions and energy availability. We discuss in particular the evolution of ideas concerning the so-called 'drift-mirror' instability in the magnetosphere.

Ionospheric conductance distribution and MHD wave structure: observation and model

“…The occurrence of such a low frequency under quiet conditions agrees well with a statistical study performed by Cummings et al (1969). It is di cult to decide on the basis of our data which mode has been observed exactly, because the eigenfrequencies depend on the density distribution along the ®eld lines and in the equatorial plane (Cummings et al, 1969) and are very di erent under night-time and day-time conditions (Allan and Poulter, 1992).…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…Solar wind dynamic pressure pulses that compress the magnetopause and launch fast mode waves can stimulate FLRs in the dayside magnetosphere around magnetic local noon . Numerical studies of ULF waves excited by solar wind impulses have confirmed coupling between global cavity modes and FLRs (Allan and Poulter 1992;Zhu et al 1991;Lee et al 1989;Zhang et al 2009). …”

“…Surface waves generated by the K-H instability propagate earthward to excite field line resonances (FLRs) Claudepierre et al 2008). In contrast, when a solar wind dynamic pressure Allan and Poulter (1992)) pulse impinges on the magnetopause, it produces fast mode waves that propagate into the inner magnetosphere, transferring energy from the solar wind in the process. The generation mechanisms for ULF waves depend on the local time region in which they operate.…”

The phase structure of very low latitude ULF waves across dawn

“…It is generally accepted that ULF waves in the Pc 3 (10-100 mHz) band, observed at low and middle latitudes, are a consequence of field line resonance (FLR) stimulated by fast mode oscillations in the plasmasphere and magnetosphere [Kivelson and Southwood, 1986;Allan et al, 1986;Allan and Poulter, 1992]. In this situation the signature observed on the ground is that of the FLR and the frequency increases with decreasing latitude.…”