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

在太阳辐射的作用下，白天的上层大气--或称电离层--是导电的。在地磁场存在的情况下，大气运动通过发电机作用在电离层中产生电场和电流。这些电流反过来会产生磁场。赤道电喷流(EEJ)是电离层的显著特征之一，在卫星和地面磁强计测量中都得到了很好的分辨。这是一条位于日侧磁赤道的高电流密度带，高度约110公里。海流主要是向东流动的。它受太阳通量、地磁主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态。然而，时间尺度等于或超过特派团持续时间(几年)的进程很难适当量化。此外，由于低地球轨道卫星的当地时间/纵向覆盖范围有限，在小时或天数(如EEJ)的时间尺度上的局部现象只能在统计和气候学的基础上加以描述。为了克服这一观测差距，在DFG优先计划动态地球中确定了使用永久性地面仪器的数据。在这个项目中，我们希望通过量化和研究EEJ对地磁主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。我们的结果还将使我们更好地理解由Spot卫星测量的EEJ信号。我们可以做到这一点，因为我们重新发现了手写表格，其中有大约15年的地磁地面站数据，这些数据来自秘鲁磁赤道的地磁天文台环卡约。重新发现的数据填补了20世纪60年代和70年代的空白，从1922年开始，环卡约的全矢量地磁记录的每小时数字平均值的时间序列长达90年。我们的新数据有两个亮点：包括环卡约在内的南美洲经历了磁赤道任何地区中地磁场强度的最大相对变化(自1968年以来下降了10%)，这使得我们能够更准确地研究EEJ对地磁主场变化的敏感性。其次，它覆盖了一个太阳黑子数非常低的太阳极大值，也是唯一一个记录到的太阳通量值(F10.7，自1947年以来可用)与目前明显较弱的太阳极大值相当的太阳极大值。

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.