Natural sources of geomagnetic field variations

The Earth’s magnetic field is a dynamic system and varies on a wide spectrum of timescales from microseconds to hundreds of millions of years. The primary source of the field is the self-sustaining geodynamo action of the Earth’s liquid outer core. This creates around 95% of the magnetic field strength at the Earth’s surface. Its average strength at mid-latitudes is on the order of 50,000 nT (ranging between 20,000-60,000 nT increasing toward the poles). The core field varies on timescales of years to millennia. Another internal source is the quasi-stable crustal field, generated by the heterogeneous distribution of ferromagnetic minerals in the upper 5-30 km of the Earth’s surface. Its contribution is much smaller at around 20 nT on average globally, though it can locally be much larger. It changes on timescales of millions of years except at sources such as active volcanic regions or along mid-ocean ridges. There are a number of external (i.e. with sources outside the Earth) field systems which are created by solar-terrestrial interactions. These are much more dynamic and vary on timescales of seconds to days. These have magnitudes of a few pT to 100 nT on geomagnetically quiet days but can change rapidly within minutes to thousands of nT, for example from the impact of an Interplanetary Coronal Mass Ejection upon the Earth. These effects (geomagnetic storms and substorms) are strongly dependent on local time and latitude, with high latitudes (|Φgeomagnetic| > 60°) being particularly affected from the auroral electrojet current systems or magnetospheric waves. Due to simple geometric reasons (zonal currents), most of the above geomagnetic disturbances appear in the geomagnetic north (also called the horizontal) component. Other magnetic fields are generated locally by instantaneous phenomena such as lightning-generated spherics and magnetospheric whistlers. We will briefly outline the spatio-temporal variation and largest dynamic expected from each source. In this concise review we focus on mid-latitudes (CERN is located at 46.2° geographic latitude, 40.4° geomagnetic latitude, at the footpoint of the L=1.8 magnetic McIlwain-shell) and neglect some of the high-latitude/auroral and equatorial phenomena not relevant for CERN’s location.