Reply to AC1

Abstract

Solar eruptions and other types of space weather effects can pose a hazard to the Earth's power grids via geomagnetically induced currents (GIC). In worst cases, they can even cause large scale power outages. GIC are a complex phenomenon, closely related to the time derivative of the geomagnetic field. However, the behavior of the time derivative is chaotic and has proven to be tricky to predict. In our study, we look at the dynamics of the geomagnetic field during active space weather. We try to characterize the magnetic field behavior, to better understand the drivers behind strong GIC events. We use geomagnetic data from the IMAGE (International Monitor for Auroral Geomagnetic Effect) magnetometer network between 1996 and 2018. The measured geomagnetic field is primarily produced by currents in the ionosphere and magnetosphere and secondarily by currents in the conducting ground. We use the so called separated magnetic field in our analysis. The separation of the field means, that the measured magnetic field is computationally divided into external and internal parts based on the field's ionospheric or telluric origin. We study the yearly directional distributions of the separated horizontal geomagnetic field and its time derivative. The yearly distributions do not have a clear solar cycle dependency. The internal field distributions are more scattered than the external field. There are also clear, station specific differences in the distributions. One of our main findings is that the direction of the geomagnetic field time derivative has a very short "reset time", around two minutes, but the total horizontal field does not have this kind of behavior. These results hold true even with less active space weather conditions. We conclude that this result gives insight into the time scale of ionospheric current systems, which are the primary driver behind the time derivative's behavior.