lunar daily variation

The geomagnetic field is Earth's magnetic field, primarily dipolar with North and South poles. While Earth's magnetic field originates from a dynamo within its core, other sources like the ionosphere also contribute. The ionosphere, a layer of Earth's atmosphere, generates electric currents due to solar and lunar influences.12

The "lunar daily variation" (L) refers to a component of these ionospheric currents driven by the Moon's gravity, similar to ocean tides. This variation is a semidiurnal component, meaning it has two peaks per lunar day, and its amplitude is about 1/20th of the solar quiet-day variation (S_q). The Moon itself does not possess a global magnetic field.23