High-latitude thermospheric neutral density changes

对中纬度地区的卫星研究表明，全球热大气层正在逐渐收缩。在400公里处，密度的下降率估计在太阳活动最大和最小时每十年从2%到5%不等。模拟工作对热层密度的下降给出了一个更保守的估计，在400公里处，通常每十年下降1 - 2%。这种收缩的原因与二氧化碳浓度的长期增加有关，这导致对流层变暖。然而，CO2在中高层大气中作为一种高效的辐射冷却剂，人们认为这种冷却是热层长期收缩的原因。我们建议从高纬度地区的雷达数据中提取30年以上的长期趋势，从极冠雷达数据中提取16年以上的长期趋势。我们将把我们的结果与低纬度地区的结果进行比较，同样也将我们的结果与气候变化模式的预测结果进行比较。低纬度卫星研究表明，400 km高度附近的热层中性密度与太阳输出通量有关。太阳辐射通量与11年太阳活动周期有很好的相关性。在低纬度的整个太阳活动周期中，热层质量密度的变化可达一个数量级。由于没有这样的研究已经在高纬度地区或在极冠，我们现在建议这样做，在中高纬度地区的全球卫星阻力测量表明，大的地磁暴可以导致原子氧密度的几个100%，在400公里附近的高度显着增强。有时增强可以达到几乎十倍。此外，低热层的组成可能会发生重大变化，影响O/N2比率，这是由高纬度焦耳加热驱动的大气上升流引起的，它在全球范围内循环和重新分配大气成分。这些实质性的变化发生在从几小时到几天的时间尺度上。在风暴的恢复阶段，热层也可能经历高效的冷却。这甚至可能导致“过冷”，其中风暴后的密度被观察到比大磁暴前的安静时间密度低36%。我们建议调查的热层响应的磁暴强迫，特别是在恢复阶段。令人感兴趣的是完全恢复的时间，“过冷”现象何时发生，持续多久，以及风暴前地磁活动的历史如何影响恢复阶段和最终结果。建议的目标可以实现使用一种新的雷达技术开发的主要研究人员，这是通过被动观测或主动实验。两者都需要NERC资助的EISCAT雷达设施，位于挪威北方和斯瓦尔巴群岛的极冠，那里的地球磁场接近垂直。当NERC在2009年底承担EISCAT的责任时，它还继承了一个有价值的和独特的长期数据集，即大陆EISCAT雷达的30年和极地帽EISCAT斯瓦尔巴德雷达的16年，我们现在完全打算利用这些数据集。此外，由于将世界上唯一的高纬度非相干散射雷达与电离层大功率泵浦设施（称为加热器）放在同一地点，因此，该方案是世界上唯一能够执行拟议的主动实验以得出热层中性密度的设施。

Satellite studies at mid-latitudes have shown that the global thermosphere is gradually contracting. The rate of decrease in density at 400 km is estimated to range from 2% to 5% per decade at solar maximum and minimum. Modeling work gives a more conservative estimate on the thermospheric density decline, typically 1 to 2% per decade at 400 km. The cause of this contraction is linked to the long term increases in CO2 concentration which has resulted in warming of the troposphere. However, CO2 acts as a highly effective radiative coolant in the middle and upper atmosphere and it is believed that this cooling is responsible for the long-term contraction of the thermosphere. We propose to extract the long-term trend over 30 years from high-latitude, and 16 years from polar cap, radar data. We will compare our results to those from lower latitudes, likewise compare our results to those from model predictions of climate change.A low-latitude satellite study showed that thermospheric neutral density near 400 km altitude is related to solar output flux. The solar flux is very well correlated to the 11-year solar activity cycle. Variations in thermospheric mass density of up to an order of magnitude occur throughout the solar cycle at low latitudes. Since no such study has been performed at high-latitudes or in the polar cap, we now propose to do so.Global satellite drag measurements at mid- and high-latitudes have shown that large geomagnetic storms can result in dramatic enhancements of the atomic oxygen density of several 100% at altitudes near 400 km. Sometimes the enhancement can reach up to almost a factor of ten. In addition, significant changes in lower-thermospheric composition may occur, affecting the O/N2 ratio, which arise from atmospheric upwelling driven by Joule heating at high latitudes that circulates and redistributes atmospheric constituents globally. These substantial changes take place on time scales that range from a few hours to a few days. The thermosphere may also experience highly efficient cooling in the recovery stage of storms. This can even lead to 'overcooling' where densities after storms have been observed to be up to 36% below the quiet-time densities prior to a large geomagnetic storm. We propose to investigate the thermospheric response to geomagnetic storm forcing, in particular during the recovery phase. Of interest is the time to fully recovery, when the 'over-cooling' phenomenon occurs and how long it lasts, as well as how the history of geomagnetic activity prior to a storm affects the recovery phase and final outcome. The proposal objectives can be achieved using a novel radar technique developed by the principle investigator which is either by passive observation or active experiment. Both require the NERC-funded EISCAT radar facilities located in northern Norway and Svalbard archipelago in the polar cap, where the Earth's magnetic field is near-vertical. When NERC took on responsibility for EISCAT in late 2009, it also inherited a valuable and unique long-term dataset, i.e. 30 years for the mainland EISCAT radar and 16 years for the polar cap EISCAT Svalbard Radar, which we fully intend to exploit now. In addition, by virtue of having the world's only high-latitude incoherent scatter radar co-located with an ionospheric high-power pumping facility (called the Heater), EISCAT is the only facility in the world that can execute the proposed active experiments to yield thermospheric neutral density.