Intensification of dayside diffuse auroral precipitation: contribution of dayside Whistler-mode chorus waves in realistic magnetic fields
Intensification of dayside diffuse auroral precipitation: contribution of dayside Whistler-mode chorus waves in realistic magnetic fields
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DOI：
10.5194/angeo-30-1297-2012
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发表时间：
2012-09
影响因子：
1.9
通讯作者：
R. Shi;Desheng Han;B. Ni;Zejun Hu;Chen Zhou;X. Gu
中科院分区：
文献类型：
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作者：
R. Shi;Desheng Han;B. Ni;Zejun Hu;Chen Zhou;X. Gu
抽象。与最近对夜侧漫射极光的理解相比，对昼侧漫射极光的机制仍然知之甚少。虽然昼侧合唱一直被认为是一个潜在的主要贡献者昼侧扩散极光降水，合唱波散射的作用的定量分析还没有仔细进行。在这项研究中，我们研究了2005年1月7日中国北极黄河站（YRS）全天空成像仪（ASI）观测到的一次昼侧扩散极光增强事件，并捕捉到在557.7 nm波长处扩散极光强度的显著增加，几乎发生在09：24 UT附近的整个ASI视场，即，~12：24 MLT。利用实际磁场模型计算了向阳面合唱发射的反弹平均共振散射率，结果表明，向阳面合唱散射可以在小时的时间尺度上产生等离子体片电子的强烈沉淀损失（甚至接近强扩散极限），具体地，从~1 keV到50 keV，赤道俯仰角从损耗锥到高达~85°，这取决于电子能量。随后对损失锥填充指数的估计表明，由于昼侧合唱驱动的俯仰角散射，损失锥可以以≥0.8的速率被填充，电子从~500 eV到50 keV，精确地覆盖了激发绿线漫射极光的沉淀电子。根据THEMIS航天器在类似条件下观测到的损失锥填充指数剖面和典型的昼侧电子分布，估算了不同能级的电子沉降通量，给出了约0.1 erg cm−2 s−1量级的总沉降电子能通量，特征能量约为1 keV（特别是当使用T01时），这很可能导致强烈的绿线漫射极光活动。因此，在实际磁场中的昼侧合唱散射可以大大有助于YRS ASI观测到的昼侧绿线极光的增强。除了波致散射和周围磁场的变化，相关电子通量的变化也有助于增强漫射极光发射，由于缺乏对磁层粒子的同步观测，我们不能完全排除这种可能性。由于在观测期间地磁活动水平相当低，可以合理地推断，磁层中相关电子通量的变化应该很小，因此它对观测到的增强的扩散极光活动的贡献也应该很小。我们的研究结果支持了昼侧合唱可能在昼侧扩散极光的产生中起主要作用的假设，也表明磁层磁场的变化应该被考虑到合理解释昼侧扩散极光的观测。
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.