Long-term geomagnetic activity: Comparison and analysis of geomagnetic activity indices during the last 90 years

Geomagnetic activity is a measure aimed to quantify the effect of solar wind upon the Earth's magnetic environment. The main structures in solar wind driving geomagnetic activity are the coronal mass ejections (CME) and the high-speed solar wind streams together with related co-rotating interaction regions (HSS/CIR). While CMEs are closely related to sunspots and other active regions on solar surface, the HSSs are related to solar coronal holes, forming a proxy of solar polar magnetic fields. This gives an interesting possibility to obtain versatile information on solar activity and solar magnetic fields from geomagnetic activity.

Various indices have been developed to quantify and monitor global geomagnetic activity. The most often used indices of overall geomagnetic activity are the aa index, developed by P. Mayaud and running already since 1868, and the Kp/Ap index, developed by J. Bartels and running since 1932. Both aa and Kp/Ap depict the increase of geomagnetic activity during the first half of the 20th century, and a steep decline in the 2000s. However, although the two indices are constructed from midlatitude observations using roughly the same recipe, they depict notable differences during the 90-year overlapping interval. While the Kp/Ap index reaches a centennial maximum in the late 1950s, at the same time as sunspots, the aa index has its maximum only in 2003. Also, the Kp/Ap is systematically relatively more active in the first decades until 1960s, while aa is more active thereafter. The Dst index was developed to monitor geomagnetic storms and the ring current since 1957. We have corrected some early errors in the Dst index and extended its time interval to 1932. This extended storm index is called the Dxt index. Here we study these long-term geomagnetic indices and their differences. We also use their different dependences on the main solar wind drivers in order to obtain new information on the centennial evolution of solar activity and solar magnetic fields.