The Wave Growth, Saturation, and Electron Heating of Lower Hybrid Waves in the Magnetic Reconnection Exhaust

Abstract

Lower hybrid waves are commonly observed in magnetic reconnection. Based on in situ measurements and the application of an extended quasi-linear model, we investigate the occurrence, saturation, and electron heating of lower hybrid waves in the region of magnetopause reconnection outflows. Lower hybrid waves are statistically favored when the density gradient length scale (L n ) normalized by the ion gyroradius (ρ i ) is small. The occurrence of lower hybrid waves is significantly higher in the regime of L n /ρ i < 1 and plasma beta β < 1. These features of wave occurrence are consistent with the linear theory of the wave growth rate. Evidence indicates that the saturation level and the parallel electron heating of waves both increase as the normalized gradient scale L n /ρ i decreases. The parallel electron temperature increases ∼30%–70% as L n /ρ i < 1. We show that the observation of saturation and electron heating is consistent with an extended quasi-linear model. In this scenario, lower hybrid waves are driven by density gradients and then quickly saturate in tens of ion gyroperiods. The parallel electron heating from lower hybrid waves is achieved through Landau damping before the nonlinear saturation. Our results provide comprehensive evidence for an end-to-end process of electron heating through lower hybrid waves in reconnection exhausts. L n /ρ i is the key parameter that determines the extent of the wave growth, saturation energy, and electron heating in this wave–particle interaction process.