Resonant scattering of plasma sheet electrons leading to diffuse auroral precipitation: 1. Evaluation for electrostatic electron cyclotron harmonic waves

Abstract

[1] Using statistical wave power spectral profiles obtained from CRRES and the latitudinal distributions of wave propagation modeled by the HOTRAY code, a quantitative analysis has been performed on the scattering of plasma sheet electrons into the diffuse auroral zone by multiband electrostatic electron cyclotron harmonic (ECH) emissions near L = 6 within the 0000-0600 MLT sector. The results show that ECH wave scattering of plasma sheet electrons varies from near the strong diffusion rate (timescale of an hour or less) during active times with peak wave amplitudes of an order of 1 mV/m to very weak scattering (on the timescale of >1 day) during quiet conditions with typical wave amplitudes of tenths of mV/m. However, for the low-energy (∼100 eV to below 2 keV) electron population mainly associated with the diffuse auroral emission, ECH waves are only responsible for rapid pitch angle diffusion (occasionally near the limit of strong diffusion) for a small portion of the electron population with pitch angles a eq < 20°, dependent on electron energy and geomagnetic activity level. ECH scattering alone cannot account for the rapid loss of plasma sheet electrons during transport from the nightside to the dayside, nor can it explain the formation of the pancake electron distributions strongly peaked at a eq > 70°. Computations of the bounce-averaged coefficients of momentum diffusion and (pitch angle, momentum) mixed diffusion indicate that both mixed diffusion and energy diffusion of plasma sheet electrons due to ECH waves are very small compared to pitch angle diffusion and that ECH waves have little effect on local electron acceleration. Consequently, the multiple harmonic ECH emissions cannot play a dominant role in the occurrence of diffuse auroral precipitation near L = 6, and other wave-particle interaction mechanisms, such as whistler mode chorus-driven resonant scattering, are required to explain the global distribution of diffuse auroral precipitation and the formation of the pancake distribution in the inner magnetosphere.

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

“…and another one for suprathermal electronsf st =n st exp(−(v 2 + v 2 ⊥ )/v 2 T ,st ), wheren is the density of warm/hot electrons (resonantly interacting with whistler waves) normalized to the background electron density, v T = 2T /m e , v T ⊥ = √ 2T ⊥ /m e are thermal electron velocities (parallel and perpendicular to the background magnetic field),n st is the density of suprathermal electrons (responsible for wave damping) normalized to the background electron density, and v T ,st is the thermal velocity of suprathermal electrons (the corresponding temperature is taken as ∼ 300 eV, in rough agreement with various satellite in situ measurements at L ∼ 5-6-e.g., see , Ni et al 2011c, Fu et al 2014b. For this form of the velocity distribution function, Eq.…”

Low‐altitude satellite measurements of pulsating auroral electrons

“…The presence of a parallel potential drop (an upward field-aligned current) will block secondaries from escaping the ionosphere. It is also possible that electromagnetic wave activity, specifically upper band chorus [Ni et al, 2011a] and electron cyclotron harmonic (ECH) [Peticolas et al, 2002] waves, in the equatorial magnetosphere could be scattering the secondaries out of the loss cone. In addition, although 10.1002/2015JA021292 there are variations in periodicities between the events, there are no discernible correlations between local time of occurrence and such periodicities.…”

Intensification of dayside diffuse auroral precipitation: contribution of dayside Whistler-mode chorus waves in realistic magnetic fields

“…In contrast to the 427.8 nm emissions excited by the precipitating electrons above a few keV and the 630.0 nm emissions excited by the <500 eV soft electrons mainly from the magnetosheath (e.g., Smith and Lockwood, 1996), the 557.7 nm emissions, the focus of this study, are triggered by the atmospheric precipitation of ≥500 eV to a few keV electrons, which dominantly come from the magnetosphere. For plasma sheet electrons, a number of studies have demonstrated that resonant wave-particle interactions can play an essential role in driving their efficient precipitation loss (e.g., Ni et al, 2008Ni et al, , 2011aSu et al, 2009;Thorne et al, 2010;Tao et al, 2011). Since moderate dayside chorus is present >10 % of the time and can persist even during periods of low geomagnetic activity (Li et al, 2009), especially at high L-shells where the YRS is located, it is natural to connect the presence of dayside chorus to the occurrence of dayside diffuse aurora.…”

“…Recent comprehensive theoretical and modeling studies in combination with CRRES observations Ni et al, 2011a, b) have revealed that scattering by electromagnetic whistler-mode chorus waves is the dominant cause of the most intense diffuse auroral precipitation on the night-to-dawn side in the inner magnetosphere (L <∼ 8).…”