Wave energy budget analysis in the Earth’s radiation belts uncovers a missing energy

Abstract

Whistler-mode emissions are important electromagnetic waves pervasive in the Earth's magnetosphere, where they continuously remove or energize electrons trapped by the geomagnetic field, controlling radiation hazards to satellites and astronauts and the upper-atmosphere ionization or chemical composition. Here, we report an analysis of 10-year Cluster data, statistically evaluating the full wave energy budget in the Earth's magnetosphere, revealing that a significant fraction of the energy corresponds to hitherto generally neglected very oblique waves. Such waves, with 10 times smaller magnetic power than parallel waves, typically have similar total energy. Moreover, they carry up to 80% of the wave energy involved in wave–particle resonant interactions. It implies that electron heating and precipitation into the atmosphere may have been significantly under/over-valued in past studies considering only conventional quasi-parallel waves. Very oblique waves may turn out to be a crucial agent of energy redistribution in the Earth's radiation belts, controlled by solar activity.

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

“…It means that if the wave-normal angle of measured waves does not reach values close enough to the resonance cone to get N > 300, then both the relative weight of very oblique waves in the k-integrated magnetic power and the magnitude of the corresponding pitch-angle diffusion rates may become larger than for parallel waves (see also Sect. 2 and the works by Artemyev et al 2013b;Artemyev et al 2015d).…”

“…In turn, such variations can influence electron scattering and substantially modify the energization rate (Artemyev et al 2013c;Artemyev et al 2015d). The combination of the three preceding effects together (i.e., variation of wave intensity with K p , evolution of wave amplitude distribution along latitude with K p , and variation of wave obliquity with K p ) should therefore be considered over a wide and continuous K p -range to allow an accurate evaluation of wave-particle interactions.…”

“…Fluxes of energetic electrons at L ∼ 4-6 are highly variable due to the combined and entangled effects of inward and outward radial diffusion (Shprits et al 2008a;Ukhorskiy and Sitnov 2013), local acceleration by whistler-mode chorus (Bortnik and Thorne 2007;Horne et al 2005b) or magnetosonic ) waves, and precipitation in the atmosphere by the same chorus waves as well as other kinds of waves (e.g., see Artemyev et al 2015d). Intense chorus waves at mid to high latitudes may also cause microburst relativistic electron precipitations and lead to very rapid losses of radiation belt electrons (Thorne et al 2005).…”

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

“…Such waves with large refractive index are rather common during moderately disturbed periods just outside the plasmasphere. 9,35 Figure 4 shows that such very oblique waves can resonantly trap only low energy electrons (E < 100 keV), while the …”

“…Moreover, comprehensive statistics of whistler-mode waves show that even oblique waves with very small magnetic field amplitudes can contribute significantly to resonant waveparticle interactions because of the noticeable portion of wave energy stored in the wave electric field. 9 This raises an important, as yet unanswered question about the actual relative contributions of oblique and parallel whistler-mode waves in electron acceleration and scattering.…”