Magnetic surveying is one of two potential field methods, along with gravity surveying. It makes use of the facts that different entities have different capacities for interacting with and altering the very strong magnetic field that comes from the slow movement of fluid iron in Earth's outer core, and that some have the ability to raise magnetic fields of their own. Among these entities, different rock types interact in different ways with the core field to change its strength and direction. It is possible to map these interactions from a moving vehicle, ship or aircraft, and so generate a picture of the effects of rock variability on the local magnetic field. From this, the rock variability itself can be interpreted. During surveying, however, the core field can change with time as it interacts with a constant stream of electrically-charged particles emitted into space by the sun. During magnetic surveys, a stationary magnetometer records temporal changes in the external field so they do not become confused with the signals recorded in different places by a moving magnetometer. The changes are felt most strongly nearby the interacting rock types, and when they appear in magnetic data they are referred to as magnetic anomalies. Igneous and metamorphic rocks tend to produce stronger anomalies than sedimentary rocks. Beyond these basic expectations, understanding magnetic data requires computer-based modelling of the anomalies.