Van Allen Probes observation of localized drift resonance between poloidal mode ultra‐low frequency waves and 60 keV electrons

Abstract

We present NASA Van Allen Probes observations of wave‐particle interactions between magnetospheric ultra‐low frequency (ULF) waves and energetic electrons (20–500 keV) on 31 October 2012. The ULF waves are identified as the fundamental poloidal mode oscillation and are excited following an interplanetary shock impact on the magnetosphere. Large amplitude modulations in energetic electron flux are observed at the same period (≈ 3 min) as the ULF waves and are consistent with a drift‐resonant interaction. The azimuthal mode number of the interacting wave is estimated from the electron measurements to be ~40, based on an assumed symmetric drift resonance. The drift‐resonant interaction is observed to be localized and occur over 5–6 wave cycles, demonstrating peak electron flux modulations at energies ~60 keV. Our observation clearly shows electron drift resonance with the fundamental poloidal mode, the energy dependence of the amplitude and phase of the electron flux modulations providing strong evidence for such an interaction. Significantly, the observation highlights the importance of localized wave‐particle interactions for understanding energetic particle dynamics in the inner magnetosphere, through the intermediary of ULF waves.

Discovery of the action of a geophysical synchrotron in the Earth’s Van Allen radiation belts

“…Localized ULF waves may be excited by unstable ring current particle distributions 30 , providing a potential pathway for the ring current to drive the radiation belts 31 . Further detailed observational and modelling studies of such events are warranted 28 , motivated by the discovery and observations presented here.…”

“…To be resonant at the observed RBSP-ECT energy range requires a high-m wave with mB44. RBSP-B (L ¼ 5.9, MLT ¼ 6.7 at 16:00 UT; data not shown) was also close to apogee and east of RBSP-A; it saw modulation at similar energies, but neither as coherent nor at the same discrete frequency at RBSP-A (see Claudepierre et al 28 for further details and analysis). Such low energy particles cannot complete entire drift transits around the Earth during the ULF wavetrain, suggesting that ULF modulation was only active very close to RBSP-A-evolution along the drift path explaining the signals seen further east at RBSP-B.…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…The linear dependence of df A on dW A suggests that the phase shift of particle PSDs across the resonant energy should also be 180°. Such a phase shift is thus treated as a characteristic signature of ULF waveparticle drift resonance (Claudepierre et al 2013;Dai et al 2013;Mann et al 2013).…”

“…This signature has been widely used as a diagnostic to identify drift-resonant waveparticle interactions (Mann et al 2013, Claudepierre et al 2013Dai et al 2013;Hao et al 2014). The diagnostic is derived from Eq.…”

“…In Eqs. (15) and (27), the amplitude of the wave electric field are both assumed to be independent of location, which is not typical in the inner magnetosphere as ULF waves are usually confined in a limited range of longitudes (Liu et al 2009;Claudepierre et al 2013;Liu et al 2016a, b). The localized nature of ULF waves can be taken into account by assuming a von Mises distribution of the wave amplitude in the theoretical framework (Li et al 2017b).…”