Solar phenomena in geomagnetic observatory records

Geomagnetic observatories were initially devised to understand the workings of Earth’s dynamo over periods of years to centuries. Those same records contain information on higher frequency variations related to space weather and its interaction with the magnetosphere. The signals are on the order of a few nT or less, so they are often overlooked as noise. By exploiting differences in instrument responses from scalar and vectorial magnetometers (delta F), we show it is possible to extract the frequency content of the magnetic field with periods ranging from 0.1 to 100 seconds. One application demonstrates a nearly simultaneous signal in global observatory data when interplanetary shock fronts have relatively high (ca. >800 km/s) solar wind velocities. These storm events show remarkable similarities in time and space as observed on Earth’s surface. Another application is to stack hourly averages over an entire year. This latter method shows that the maximum amplitude of magnetic field oscillations occurs near solar noon over diurnal periods at all latitudes except in the auroral oval. Seasonal variability is detectable at high latitudes. Long-term trends in field oscillations follow the solar cycle, with maxima occurring during the declining phase when high-speed streams in the solar wind are dominant. A parameter based on solar wind speed and the relative variability of the interplanetary magnetic field correlates robustly with the ground-based measurements. These findings suggest that turbulence in the solar wind, its interaction at the magnetopause, and its propagation through the magnetosphere stimulate magnetic field fluctuations at the ground over a wide frequency range. Delta F therefore allows one to study solar wind phenomena that produce field line oscillations detectable on the Earth’s surface using the publicly available, worldwide database of INTERMAGNET geomagnetic observatories.