Credit: NASA.


Magnetic Field of Earth


Near the surface, the Earth's magnetic field can be approximated as a dipole field centered close to the center of gravity and offset from the axis of rotation by blah degrees. The magnitude ranges from 25 to 65 microtesla. This magnetic field protects our atmosphere from being stripped into space by the solar wind, points our compasses north for navigation for humans and some species of birds, and gives us the aurora.

Early ideas about why a compass points north:
  • Divine attraction to the North Star.
  • Mass amounts of iron ore in the arctic.
  • Permanent magnet at core or in crust.
We now know that the magnetic field of Earth is generated from a geodynamo in the geophysical flows of the Earth's outer core. The paleomagnetic record indicates that the geomagnetic field has existed for at least three billion years [4]. The polarity of the dipole moment of the magnetic field reverses continually throughout its existence.

A simulation of a reversal was published in 1995 [7]. 36,000 years into the simulation the magnetic field underwent the reversal of its dipole moment. The reversal took a little over 1000 years to complete.



image source: [4]


The geomagnetic field changes on time scales from milliseconds to millions of years. On the shortest time scales the variations are from space weather and electric currents in the upper atmosphere such as the auroral electrojet. The solar wind will change in density, velocity, temperature, and the interplanetary magnetic field that interacts with earth's magnetosphere. Below is magnetometer data from stations in Norway, Denmark, and Finnland that show large deviations from the average ambient magnetic field at around 0 UTC. The electrojet could have been located near the auoral band, therefore, the magnitude of the deviations from this storm decrease with decreasing latitude.


image source: https://www.spaceweatherlive.com/en/auroral-activity/magnetometers

On time scales of decades, changes in the geomagnetic field are referred to as secular variation.


The movement of Earth’s South Magnetic Pole across the Canadian arctic. Observed north dip poles during 1831 - 2007 are yellow squares. Modeled pole locations from 1590 to 2020 are circles progressing from blue to yellow.

Figures credit: public domain











If Jupiter's magnetosphere were visible to us. 
Credit: NASA.