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 bounce-averaged 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.
Highlights
As a weak belt of emissions extending around the entire auroral oval but an essential linkage for the magnetosphereionosphere coupling, the diffuse aurora extends over a broad latitude range of 5◦ to 10◦, mapping along field lines fromPublished by Copernicus Publications on behalf of the European Geosciences Union.R
We demonstrate that resonant interactions with dayside lower-band chorus can lead to efficient resonant scattering of ≥500 eV to a few keV electrons (Hu et al, 2012) to substantially fill the loss cone and mainly contribute to the Yellow River Station (YRS) all-sky imagers (ASI) observed intensification of dayside green-line diffuse auroral precipitation, supporting that dayside chorus could play a major role in the production of dayside diffuse aurora
Using the observations acquired from the YRS threewavelength ASI, Hu et al (2009) have examined the synoptic distribution of dayside aurora, and Hu et al (2012) have furthered to explore the potential correlation between dayside auroral emissions and interplanetary magnetic field (IMF) condition. In contrast to those studies on the auroral distribution, this paper focuses on an intensification event of dayside diffuse auroral precipitation observed by the YRS ASI
Summary
As a weak belt of emissions extending around the entire auroral oval but an essential linkage for the magnetosphereionosphere coupling, the diffuse aurora extends over a broad latitude range of 5◦ to 10◦, mapping along field lines from. As an important compliment to the above researches, Ni et al (2012) proposed that electrostatic electron cyclotron harmonic (ECH) waves can be an important or even dominant driver of diffuse auroral precipitation in the nightside outer magnetosphere, through a detailed case investigation upon a coordinated postmidnight conjunction between the space-borne THEMIS EFI emission measurements and the ground-based NORSTAR MPI diffuse aurora observations near L = 11.5. Since dayside chorus waves are persistent even during geomagnetically quiet conditions and show high occurrences at L > 7 (Li et al, 2009), we adopt a typical model of dayside chorus emissions at high L approximately corresponding to the field line of YRS location and the geomagnetic condition of the time, and perform a comprehensive evaluation of plasma sheet electron scattering rates by dayside chorus in realistic magnetic field models, which are subsequently used to compare with the strong diffusion limit and estimate the energy-dependent degree of loss cone filling. We demonstrate that resonant interactions with dayside lower-band chorus can lead to efficient resonant scattering of ≥500 eV to a few keV electrons (Hu et al, 2012) to substantially fill the loss cone and mainly contribute to the YRS ASI observed intensification of dayside green-line diffuse auroral precipitation, supporting that dayside chorus could play a major role in the production of dayside diffuse aurora
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