Experimental determination of the dispersion relation of magnetosonic waves

Abstract

Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave‐particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory.

Propagation of equatorial noise to low altitudes: Decoupling from the magnetosonic mode

“…Its frequencies are usually found between the proton cyclotron frequency and the lower hybrid frequency [Němec et al, 2005[Němec et al, , 2015aMa et al, 2013;Boardsen et al, 1992]. This frequency range is in agreement with the prevailing wave generation model at proton Bernstein modes close to proton cyclotron harmonics from unstable proton ring distributions [Meredith et al, 2008;Balikhin et al, 2015;Min and Liu, 2016] and subsequent propagation in the extraordinary mode [Walker et al, 2015b]. This mode exists below the lower hybrid frequency for wave vectors perpendicular to the geomagnetic field.…”

“…This extension was, according to Perraut et al [1982], linked to the observations of extraordinary and right-handed polarization of equatorial noise, the same as for the traditional lowfrequency magnetosonic mode. The waves may propagate radially outward [Walker et al, 2015b] but hypothetically also toward the Earth and reach thus low altitudes. Equatorial noise is also coupled to the whistler mode, without any restriction for the plasma composition, and these emissions are therefore also naturally referred to as whistler mode waves in a part of the existing literature [e.g., Russell et al, 1970;Gurnett, 1976;Santolík et al, 2002a; Consistent with observations of spectral lines generated by sources at different radial distances from the Earth, radial component of wave propagation has been identified [Santolík et al, 2004;Němec et al, 2013;Xiao et al, 2015].…”

Whistler Mode Waves Below Lower Hybrid Resonance Frequency: Generation and Spectral Features

“…The 15th, 20th, 25th, and 30th harmonics of the proton cyclotron frequency are indicated by solid black lines. The figure displays the same event as has been investigated by Balikhin et al () and Walker et al () (see Figure 2 from Balikhin et al () and Figure from Walker et al ()).…”

Revised single‐spacecraft method for determining wave vector k and resolving space‐time ambiguity

“…The requirement of no standing waves provides a unique k for each ω and so prevents the ambiguous situation where in addition to having a particular k for some ω there is also a − k ; this and related caveats are discussed in detail in section 6. This assumption of a unique k for each ω has been successfully used in many actual space physics situations [ Balikhin et al , ; Hobara et al , ; Volwerk et al , ; Walker et al , ] and is presumed to correspond to observation of waves generated by a single localized distant source. The omission of displacement current in equation corresponds to the quasi‐neutrality assumption; this assumption is associated with the wave phase velocity being negligible compared to the speed of light so that displacement current can be dropped from Ampere's law.…”