High-latitude thermospheric neutral density changes

对中纬度地区的卫星研究表明，全球热层正在逐渐收缩。在太阳活动高峰期和低峰期，400公里处密度的下降速度估计在每十年2%到5%之间。建模工作对热层密度的下降给出了更保守的估计，通常是在400公里处每十年下降1%到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，二氧化碳在中高层大气中扮演着高效的辐射冷却剂的角色，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提取30年的长期趋势，从极地帽雷达数据提取16年的长期趋势。我们将把我们的结果与低纬地区的结果进行比较，同样将我们的结果与气候变化模型预测的结果进行比较。一项低纬卫星研究表明，400公里高度附近的热层中性密度与太阳输出通量有关。太阳通量与11年太阳活动周期有很好的相关性。在低纬度的整个太阳周期中，热层质量密度的变化高达一个数量级。由于在高纬度或极冠上还没有进行过这样的研究，我们现在建议这样做。全球卫星在中高纬度的阻力测量表明，大的地磁风暴可以导致400公里附近高度的原子氧密度显著增加几个100%。有时，增强效果几乎可以达到10倍。此外，低热层的组成可能会发生重大变化，影响O/N比，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。这些重大变化发生的时间范围从几个小时到几天不等。在风暴的恢复阶段，热层也可能经历高效的冷却。这甚至会导致“过冷”，即风暴后的密度比大型地磁风暴前的平静时间密度低36%。我们建议研究热层对地磁风暴强迫的响应，特别是在恢复阶段。令人感兴趣的是完全恢复的时间、“过冷”现象何时发生和持续多长时间，以及风暴前的地磁活动历史如何影响恢复阶段和最终结果。建议的目标可以使用原理调查员开发的一种新的雷达技术来实现，这种技术可以是被动观测，也可以是主动实验。两者都需要由NERC资助的位于挪威北部的EISCAT雷达设施，以及位于极冠的斯瓦尔巴群岛，那里的地球磁场接近垂直。当NERC在2009年底接管EISCAT时，它还继承了一个宝贵和独特的长期数据集，即大陆EISCAT雷达的30年和极冠EISCAT斯瓦尔巴德雷达的16年，我们现在完全打算利用这一数据集。此外，由于拥有世界上唯一的高纬度非相干散射雷达与电离层高功率泵浦设施(称为加热器)共存，EISCAT是世界上唯一能够执行拟议的主动实验以获得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.