The Role of Localized Compressional Ultra‐low Frequency Waves in Energetic Electron Precipitation

Abstract

Typically, ultra‐low frequency (ULF) waves have historically been invoked for radial diffusive transport leading to acceleration and loss of outer radiation belt electrons. At higher frequencies, very low frequency waves are generally thought to provide a mechanism for localized acceleration and loss through precipitation into the ionosphere of radiation belt electrons. In this study we present a new mechanism for electron loss through precipitation into the ionosphere due to a direct modulation of the loss cone via localized compressional ULF waves. We present a case study of compressional wave activity in tandem with riometer and balloon‐borne electron precipitation across keV‐MeV energies to demonstrate that the experimental measurements can be explained by our new enhanced loss cone mechanism. Observational evidence is presented demonstrating that modulation of the equatorial loss cone can occur via localized compressional wave activity, which greatly exceeds the change in pitch angle through conservation of the first and second adiabatic invariants. The precipitation response can be a complex interplay between electron energy, the localization of the waves, the shape of the phase space density profile at low pitch angles, ionospheric decay time scales, and the time dependence of the electron source; we show that two pivotal components not usually considered are localized ULF wave fields and ionospheric decay time scales. We conclude that enhanced precipitation driven by compressional ULF wave modulation of the loss cone is a viable candidate for direct precipitation of radiation belt electrons without any additional requirement for gyroresonant wave‐particle interaction. Additional mechanisms would be complementary and additive in providing means to precipitate electrons from the radiation belts during storm times.

On the Close Correspondence Between Storm Time ULF Wave Power and the POES VLF Chorus Wave Amplitude Proxy

“…One such mechanism has been proposed by Rae et al (2018), whereby localized compressional ULF waves modulate the loss cone, bringing it within reach of a wider range of equatorial pitch angles for energetic particles. One such mechanism has been proposed by Rae et al (2018), whereby localized compressional ULF waves modulate the loss cone, bringing it within reach of a wider range of equatorial pitch angles for energetic particles.…”

Precipitation of MeV and Sub‐MeV Electrons Due to Combined Effects of EMIC and ULF Waves

“…Simultaneously with EMIC wave observations, some decrease of field-aligned electron (0.5-1.5 MeV) fluxes was observed by the Van Allen Probes near the equator (not shown): It could also be due to a precipitation induced by EMIC waves (e.g., see Usanova et al, 2014;Wang et al, 2014). However, there is also an increase of precipitating fluxes at lower energies ∼30-100 keV, suggesting that part of the observed precipitation during this event might simply be due to the widening of the loss cone caused by the same ULF waves (Rae et al, 2018). -3h show a second event with simultaneous EMIC and ULF wave observations by THEMIS spacecraft and electron precipitation measured by the low-altitude DEMETER satellite (Parrot et al, 2006).…”

“…Accordingly, their actual amplitudes in the EMIC wave-electron interaction region remains unknown in both cases and could have been much larger than in Figures 3b and 3f. Rae et al (2018) have indeed revealed the presence of at least five compressional ULF wave events per week at L = 6.6, with ULF waves reaching median amplitudes of ≈30-40 nT. Ozeke et al (2014) have further shown that compressional ULF wave power at L ≃ 5.2 is generally half of that at L ≃ 6.6.…”

“…This could be related to a precipitation driven by electron interaction with EMIC waves in the presence of ULF waves. The observed precipitation loss, absent below 350 keV and increasing with energy above 400 keV, seems unlikely to have been produced by the sole ULF wave-induced loss-cone widening (Rae et al, 2018), which should generally lead to similar losses at all energies.…”

“…Statistics of Pc4-Pc5 ULF waves have shown mean variations of ΔB ∼ 30 nT in the outer radiation belt during geomagnetic storms when Kp = 4−7, sometimes reaching more than 50 nT near L = 6 (Mathie & Mann, 2001;Murphy et al, 2016;Potapov et al, 2006). Significant ΔB∕B ≈ 0.1 − 0.5 levels are not uncommon at 10.1029/2019JA026566 geosynchronous orbit (Rae et al, 2018). Plasma density variations ΔN e ∕N e are often found to be opposite in phase and of the same order as ΔB 2 ∕B 2 , although occasionally reaching levels of 10 times larger (Li et al, 2011;Li et al, 2017;Xia et al, 2016).…”