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

Abstract

Abstract. Compared to the recently improved understanding of nightside diffuse aurora, the mechanism(s) responsible for dayside diffuse aurora remains poorly understood. While dayside chorus has been thought as a potential major contributor to dayside diffuse auroral precipitation, quantitative analyses of the role of chorus wave scattering have not been carefully performed. In this study we investigate a dayside diffuse auroral intensification event observed by the Chinese Arctic Yellow River Station (YRS) all-sky imagers (ASI) on 7 January 2005 and capture a substantial increase in diffuse auroral intensity at the 557.7 nm wavelength that occurred over almost the entire ASI field-of-view near 09:24 UT, i.e., ∼12:24 MLT. Computation of bounceaveraged resonant scattering rates by dayside chorus emissions using realistic magnetic field models demonstrates that dayside chorus scattering can produce intense precipitation losses of plasma sheet electrons on timescales of hours (even approaching the strong diffusion limit) over a broad range of both energy and pitch angle, specifically, from ∼1 keV to 50 keV with equatorial pitch angles from the loss cone to up to ∼85 • depending on electron energy. Subsequent estimate of loss cone filling index indicates that the loss cone can be substantially filled, due to dayside chorus driven pitch angle scattering, at a rate of ≥0.8 for electrons from ∼500 eV to 50 keV that exactly covers the precipitating electrons for the excitation of green-line diffuse aurora. Estimate of electron precipitation flux at different energy levels, based on loss cone filling index profile and typical dayside electron distribution observed by THEMIS spacecraft under similar conditions, gives a total precipitation electron energy flux of the order of 0.1 erg cm −2 s −1 with ∼1 keV characteristic energy (especially when using T01s), which can be very likely to cause intense green-line diffuse aurora activity on the dayside. Therefore, dayside chorus scattering in the realistic magnetic field can greatly contribute to the YRS ASI observed intensification of dayside green-line aurora. Besides wave induced scattering and changes in the ambient magnetic field, variations in associated electron flux can also contribute to enhanced diffuse aurora emissions, the possibility of which we cannot exactly rule out due to lack of simultaneous observations of magnetospheric particles. Since the geomagnetic activity level was rather low during the period of interest, it is reasonable to infer that changes in the associated electron flux in the magnetosphere should be small, and consequently its contribution to the observed enhanced diffuse auroral activity should be small as well. Our results support the scenario that dayside chorus could play a major role in the production of dayside diffuse aurora, and also demonstrate that changes in magnetospheric magnetic field should be considered to reasonably interpret observations of dayside diffuse aurora.

Modulation of the dayside diffuse auroral intensity by the solar wind dynamic pressure

“…While Shi et al [2012] reported another intense enhancement of the green line diffuse aurora acquired by the ASI of YRS in association with noticeable increases in the solar wind dynamic pressure, the present study shows clearly for the first time, as far as we know, the modulation of the diffuse auroral brightness by the solar wind dynamic pressure at frequencies in the Pc5 range. It is shown that the compressional Pc4-Pc5 pulsations produced by the solar wind dynamic pressure can cause the variation of the ratio of resonant electrons to the total electrons and in turn modulate the intensity of whistler mode chorus waves [Li et al, 2011].…”

“…The computed correlation coefficients between the diffuse auroral intensity and the solar wind dynamic pressure are high, indicating a causal relationship between these two phenomena. Recent studies [Shi et al, 2012;Nishimura et al, 2013;Ni et al, 2014] have proposed that dayside chorus is a major contributor to the occurrence of the dayside diffuse aurora. Besides, the dynamic pressure enhancement can play as one favorable circumstance for dayside chorus generation/amplification [Li et al, 2009[Li et al, , 2010.…”

“…Despite intensively improved understanding of the origin of nightside diffuse aurora, the mechanism(s) responsible for dayside diffuse aurora remains limitedly understood. In previous work [ Shi et al ., ], we investigated a dayside diffuse auroral intensification event at the Chinese Arctic Yellow River Station (YRS). By computing the bounce‐averaged resonant scattering rates with statistical information of dayside chorus, we suggested that whistler chorus could play a major role in the observed enhancement of dayside diffuse aurora.…”

Simultaneous ground-based optical and SuperDARN observations of the shock aurora at MLT noon

“…3. To our knowledge, this southward initial appearance and extreme fast poleward-moving medium-diffuse aurora subsequently in the midday sector associated with shock compression or increased solar wind dynamic pressure was rarely noticed in related literature, except for the studies of Shi et al (2012) and (Chen et al 2012). Moreover, it is found that those diffuse auroras spread across the full FOV of the ASI in a very short time period (~2 min).…”

“…By using DMSP observation, Newell and Meng (1992) found that soft electron precipitation (0.1-1 keV) dominates discrete aurora, and spectral flux is higher in the closed-field line region. Moreover, it is known that enhanced wave-particle interaction in equatorial magnetosphere transiently responds to the external sudden compression (Gail et al 1990;Shi et al 2012). Therefore, diffuse aurora is more prominent than discrete aurora in 557.7 nm emission as shown in Fig.…”