Wave normal angles of whistler mode chorus rising and falling tones

Abstract

We present a study of wave normal angles (θk) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (±20°), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency‐θk histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e≈0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at θk∼40°. Below θk∼40°, the average value for θk is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (θk up to 20°). Above half of fc,e, the average θk is again decreasing with frequency. Above θk∼40°, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for θk>40° and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at θk>40°, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane.

Oblique Whistler-Mode Waves in the Earth’s Inner Magnetosphere: Energy Distribution, Origins, and Role in Radiation Belt Dynamics

“…It corresponds to parallel resonant energies ∼ 1-3 keV. In the presence of the additional free energy brought by electron beams, inverse Landau damping (i.e., wave growth for ∂f /∂v > 0) may therefore allow very oblique lower-band chorus wave amplification about ∼ 2 • -8 • away from the resonance cone angle θ r , as in statistical observations from the THEMIS satellite (Taubenschuss et al 2014).…”

“…Comprehensive statistical investigations of high-resolution wave-forms of whistler-mode waves in the radiation belt Li et al 2011Taubenschuss et al 2014) have confirmed the presence of a significant population of high-amplitude (B w > 1 nT, E w > 50 mV/m) waves discovered by Cattell et al (2008) and Cully et al (2008). A noticeable portion of these waves are very oblique, with a large parallel electric field component E Taubenschuss et al 2014). In this review, we have shown that such waves can organize a fast transport of electrons in energy/pitch-angle space via the trapping mechanism.…”

“…For K p > 5, about 1 % of the total wave population corresponds to such intense waves with B w > 100 pT partly interacting with electrons in the nonlinear regime. Moreover, at λ ≤ 20 • and L ≤ 5, a significant fraction of these large-amplitudes waves actually consists of oblique waves (e.g., see Taubenschuss et al 2014).…”

“…As wave-normal angle measurement uncertainties (especially at large θ ) most often remain of the order of ±3 • -5 • (e.g., see Mourenas et al 2015;Taubenschuss et al 2014), it is hard to determine from observations whether θ (or its average value) remains roughly constant or else varies monotonously with time (and wave frequency) over the duration of one wave-packet (see also Santolík et al 2009. Consequently, it is possible to assume either a nearly constant average θ (as Mourenas et al 2015) or else a monotonously varying (average) θ during the considered process of non-linear wave growth in the vicinity of the equator.…”

Probability of relativistic electron trapping by parallel and oblique whistler-mode waves in Earth's radiation belts

“…35 Nevertheless, significant portions of the observed nonGaussian amplitude distributions of parallel and oblique waves do correspond to very high amplitudes. 2,19,21,34,63,66 When propagating through background plasma and magnetic field which are both inhomogeneous, such high-amplitude waves can interact with electrons in the nonlinear regime 25,31,48 when electron trajectories depend significantly on the wave field over time intervals long enough. 13,15 Such nonlinear interactions include particle trapping and nonlinear scattering (also called phase bunching).…”