Intense chorus waves are the cause of flux-limiting in the heart of the outer radiation belt

Abstract

Chorus waves play a key role in outer Van Allen electron belt dynamics through cyclotron resonance. Here, we use Van Allen Probes data to reveal a new and distinct population of intense chorus waves excited in the heart of the radiation belt during the main phase of geomagnetic storms. The power of the waves is typically ~ 2–3 orders of magnitude greater than pre-storm levels, and are generated when fluxes of ~ 10–100 keV electrons approach or exceed the Kennel–Petschek limit. These intense chorus waves rapidly scatter electrons into the loss cone, capping the electron flux to a value close to the limit predicted by Kennel and Petschek over 50 years ago. Our results are crucial for understanding the limits to radiation belt fluxes, with accurate models likely requiring the inclusion of this chorus wave-driven flux-limiting process, that is independent of the acceleration mechanism or source responsible for enhancing the flux.

Intense Energetic Electron Precipitation Caused by the Self‐Limiting of Space Radiation

“…The electron fluxes exceed the K-P limit predominantly on the dawn side. The dawn MLT region is also associated with the highest intensity precipitation reported here, as well as with the highest chorus wave activity as reported in many earlier studies (e.g., Aryan et al, 2022;Chakraborty et al, 2022;Chen et al, 2014).…”

“…Chakraborty et al. (2022) also showed that the K‐P limiting processes are associated with the generation of a separate population of very intense chorus waves.…”

“…These periods are also associated with an enhanced flux of precipitating particles as measured by POES. This also must be associated with very strong chorus wave growth in order for the flux of equatorial electrons to be quickly reduced to the K‐P limit (see also, e.g., the discussion in Mauk & Fox, 2010; Chakraborty et al., 2022). We show that the K‐P limit is characterized by an asymptotic PAD which has the same shape and intensity in all storms that were investigated in this study.…”

Upper Limit on Outer Radiation Belt Electron Flux Based on Dynamical Equilibrium

“…Strong substorms with Kp ∼ 5-6 and AE ∼ 600-1,000 nT usually correspond to a time-and MLT-averaged chorus wave power 𝐴𝐴 ⟨𝐵𝐵 2 𝑤𝑤 ⟩𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 ∼ (50) 2 pT 2 at latitudes λ ∼ 15° of cyclotron resonance with 100 keV electrons near the loss-cone at L ∼ 5-6 (Agapitov et al, 2018). To reduce the lifetime τ L to its minimum value, τ SD (E min ) ∼ 140 s, the average wave power must increase up to 𝐴𝐴 ⟨𝐵𝐵 2 𝑤𝑤 ⟩𝑡𝑡 ∼ (280) 2 pT 2 , a level sometimes observed for hours at L ∼ 5 during geomagnetic storms and substorms (Chakraborty et al, 2022;Elliott et al, 2022;C. Tang et al, 2023), or during microbursts (Zhang et al, 2022).…”

“…Strong substorms with Kp ∼ 5–6 and AE ∼ 600–1,000 nT usually correspond to a time‐ and MLT‐averaged chorus wave power $${\langle {B}_{w}^{2}\rangle }_{t,MLT}\sim {(50)}^{2}$$ pT 2 at latitudes λ ∼ 15° of cyclotron resonance with 100 keV electrons near the loss‐cone at L ∼ 5–6 (Agapitov et al., 2018). To reduce the lifetime τ L to its minimum value, τ SD ( E min ) ∼ 140 s, the average wave power must increase up to $${\langle {B}_{w}^{2}\rangle }_{t}\sim {(280)}^{2}$$ pT 2 , a level sometimes observed for hours at L ∼ 5 during geomagnetic storms and substorms (Chakraborty et al., 2022; Elliott et al., 2022; C. Tang et al., 2023), or during microbursts (Zhang et al., 2022). Such large amplitudes can lead to electron trapping and nonlinear acceleration (Katoh & Omura, 2007; Vainchtein et al., 2018; Zhang et al., 2019), but significant wave amplitude modulations and random phase jumps between and inside intense wave packets usually restore a diffusive‐like electron transport that can be approximated by quasi‐linear diffusion (Artemyev et al., 2021, 2022; Z.…”