Earth’s magnetic powers The strength of the magnetic field of the earth’s core is 25 Gauss; 50 times stronger than at the surface. A geophysicist, for the first time, calculated it using observations and not mere calculations. The strength of the magnetic field tells us about the source of heat within the core. Strong magnetic fields have been generated by sources of energy like the heat formed four billion years ago when the earth was hot and molten, release of gravitational energy and radioactive decay of long-lived elements.