  Magnetic monopoles have long been predicted in theory and could exist as a
stable object in our universe. As they move around in galaxies, magnetic
monopoles could be captured by astrophysical objects like stars and planets.
Here, we provide a novel method to search for magnetic monopoles by detecting
the monopole moment of the Earth's magnetic field. Using over six years of
public geomagnetic field data obtained by the Swarm satellites, we apply
Gauss's law to measure the total magnetic flux, which is proportional to the
total magnetic charge inside the Earth. To account for the secular variation of
satellite altitudes, we define an altitude-rescaled magnetic flux to reduce the
dominant magnetic dipole contribution. The measured magnetic flux is consistent
with the existing magnetic field model that does not contain a monopole moment
term. We therefore set an upper limit on the magnetic field strength at Earth's
surface from magnetic monopoles to be $|B_{\rm m}| < 0.13$ nT at 95% confidence
level, which is less than $2\times 10^{-6}$ of Earth's magnetic field strength.
This constrains the abundance of magnetically-charged objects, including
magnetic black holes with large magnetic charges.
