Poynting vector and wave vector directions of equatorial chorus

Abstract

We present new results on wave vectors and Poynting vectors of chorus rising and falling tones on the basis of 6 years of THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations. The majority of wave vectors is closely aligned with the direction of the ambient magnetic field (B0). Oblique wave vectors are confined to the magnetic meridional plane, pointing away from Earth. Poynting vectors are found to be almost parallel to B0. We show, for the first time, that slightly oblique Poynting vectors are directed away from Earth for rising tones and toward Earth for falling tones. For the majority of lower band chorus elements, the mutual orientation between Poynting vectors and wave vectors can be explained by whistler mode dispersion in a homogeneous collisionless cold plasma. Upper band chorus seems to require inclusion of collisional processes or taking into account azimuthal anisotropies in the propagation medium. The latitudinal extension of the equatorial source region can be limited to ±6∘ around the B0 minimum or approximately ±5000 km along magnetic field lines. We find increasing Poynting flux and focusing of Poynting vectors on the B0 direction with increasing latitude. Also, wave vectors become most often more field aligned. A smaller group of chorus generated with very oblique wave normals tends to stay close to the whistler mode resonance cone. This suggests that close to the equatorial source region (within ∼20∘ latitude), a wave guidance mechanism is relevant, for example, in ducts of depleted or enhanced plasma density.

Observations Directly Linking Relativistic Electron Microbursts to Whistler Mode Chorus: Van Allen Probes and FIREBIRD II

“…The chorus is primarily field aligned, with wave normal angles generally <30 ∘ , but occasionally reaching more oblique angles in weaker parts of the elements, consistent with previous results of Santolík et al (2014). Not shown, wave azimuthal angles mostly point away from Earth (meridional plane), consistent with typical chorus observations (Taubenschuss et al, 2016).…”

Observations of the Source Region of Whistler Mode Waves in Magnetosheath Mirror Structures

“…In the cases of whistler mode chorus waves which are seen in the magnetosphere, frequencies of the whistler mode waves often show frequency rising (rising tone) (e.g., Kurita et al, 2012; Santolík et al, 2003; Taubenschuss et al, 2016), while there is no clear tendency of frequency variation (rising tone or falling tone) in the present cases (Figure 9c and other events). Nakamura et al (2016) showed that even fluctuations of frequencies can cause nonlinear growth of the waves (Omura et al, 2008) from the observations of whistler mode waves called plasmaspheric hiss.…”

“…In the cases in the magnetosphere, the minimum‐ B along B corresponds to the magnetic equator. Simulation studies (e.g., Hikishima et al, 2009; Katoh & Omura, 2007, 2011, 2013; Ke et al, 2017; Kuzichev et al, 2019; Lu et al, 2019) and observations of Poynting flux direction by various spacecraft around the magnetic equator (e.g., Agapitov et al, 2013; Le Contel et al, 2009; LeDocq et al, 1998; Li et al, 2013; Nagano et al, 1996; Parrot et al, 2003; Santolík et al, 2003; Santolík et al, 2005; Taubenschuss et al, 2016; Teng et al, 2018) indicate that wave generation occurs around the magnetic equator and the waves are radiated away from the equator in the parallel and antiparallel directions with respect to B . Thus, reversals of the field‐aligned component of Poynting fluxes are regarded as the sign of the passage of a wave source region.…”

Examining Coherency Scales, Substructure, and Propagation of Whistler Mode Chorus Elements With Magnetospheric Multiscale (MMS)

“…They also showed that chorus waves can be reflected within the magnetosphere, likely when the waves encounter boundaries with very sharp density gradients. Taubenschuss et al () revisited chorus statistics from the THEMIS mission, and they found Poynting vectors that were strongly field aligned and that the most oblique wave normal vectors were confined to the magnetic equatorial plane and had azimuthal angles (in the plane perpendicular to B 0 , the background magnetic field) that pointed away from Earth. They also found that for the majority of lower band chorus waves, the Poynting and wave normal vectors were coplanar with B 0 , indicating a dispersion relation consistent with that of cold plasma theory, and the wave normal vectors became more field aligned with increased latitude.…”