Quantifying solar flux and geomagnetic main field influence on the equatorial thermosphere-ionosphere system for timescales complementary to satellite missions.

在太阳辐射的电离作用下，白天的上层大气（或电离层）是导电的。在地磁场存在下的大气运动通过发电机作用在电离层中产生电场和电流。这些电流反过来又产生磁场。赤道电喷流（EEJ）是电离层的一个显著特征，在卫星和地面磁力计测量中都得到了很好的解决。这是一条高电流密度的条带，位于日侧的磁赤道上，高度约为110公里。洋流大部分是向东流的。它受太阳通量、地磁场的几何形状和强度以及潮汐运动的控制。地磁卫星星座任务非常详细地破译了电离层中这些电流的结构和动力学。但是，时间尺度相当于或超过任务持续时间（几年）的过程很难适当地量化。此外，由于低地球轨道卫星的当地时间/纵向覆盖范围有限，在小时或天的时间尺度上的局部现象（如EEJ）只能在统计和气候学的基础上进行描述。为了克服这一观测差距，在DFG动态地球优先计划中确定了使用来自永久性地面仪器的数据。在本项目中，我们希望通过量化和研究EEJ对地磁场和太阳通量的敏感性来更好地了解赤道热层-电离层系统。我们的结果也将导致更好地理解由Swarm卫星测量的eej信号。我们之所以能做到这一点，是因为我们重新发现了手写的表格，其中包含了秘鲁磁赤道地磁观测站万卡约15年来地磁地面站的数据。重新发现的数据填补了从1922年开始的万卡约90年全矢量地磁记录的数字小时平均值的长时间序列中60年代和70年代的空白。我们的新数据有两个亮点：南美洲，包括万卡约，在磁赤道的任何地区都经历了最大的相对变化（自1968年以来下降了10%），允许更准确地研究EEJ对地磁场变化的敏感性。其次，它涵盖了一个太阳黑子数量非常少的太阳极大期，也是唯一一个有记录的太阳通量值（F10.7，自1947年以来可获得）与当前明显微弱的太阳极大期相当的太阳极大期。

Ionised by solar radiation, the dayside upper atmosphere - or ionosphere - is electrically conductive. Atmospheric motion in the presence of the geomagnetic field generates electric fields and currents in the ionosphere through dynamo action. These currents in turn cause magnetic fields. One of the striking features of the ionosphere, and well resolved in both satellite and ground based magnetometer measurements, is the equatorial electrojet (EEJ). This is a ribbon of high electric current density at the dayside magnetic equator in a height of about 110 km. The current is mostly eastwards directed. It is controlled by the solar flux, by the geometry and strength of the geomagnetic main field, and by tidal motion. Geomagnetic satellite constellation missions decipher in great detail the structure and dynamics of these electric currents in the ionosphere. However, processes with time scales in the order of -or in excess of- the mission duration (a few years) are difficult to quantify properly. Also, due to the restricted local time/longitudinal coverage of low-earth-orbiting satellites, localised phenomena on time scales of hours or days (like the EEJ) can only be described on a statistical, climatological basis. To overcome this observational gap, the use of data from permanent, ground-based instruments was identified within the DFG Priority Program Dynamic Earth.In this project, we want to better understand the equatorial thermosphere-ionosphere system by quantifying and studying the sensitivity of the EEJ to the geomagnetic main field and to solar flux. Our results will also lead to a better understanding of the EEJ-signal as measured by the Swarm satellites. We can achieve this, because we rediscovered handwritten tables with some 15 years worth of geomagnetic ground station data from Huancayo, a geomagnetic observatory at the magnetic equator in Peru. The rediscovered data fills in a gap in the 1960ies and 1970ies in the otherwise 90 years long time series of digital hourly mean values of the full vector geomagnetic records at Huancayo, starting at 1922. Our new data comes with two highlights: South America including Huancayo experienced the greatest relative change (10 % drop since 1968) in geomagnetic field strength of any region at the magnetic equator, allowing to study more accurately the sensitivity of the EEJ to changes in the geomagnetic main field. Secondly, it covers a solar maximum with very low sunspot numbers and the only solar maximum with recorded solar flux values (F10.7, available since 1947) comparable to the present distinctively weak solar maximum.