smudge wrote:
What do you think has made it weaker then?
Approximately every 11 years the sun changes its magnetic polarity. The sun’s polar magnetic fields weaken, go to zero, and then emerge again with the opposite polarity. This is a regular part of the solar cycle. We tend to think of the magnetic field of the sun as one massive dipole but in reality it is composed of million of mini magnetic dipoles. As the sun reverses polarity, these local dipoles individual reverse and cancel each other out allowing the magnetic field to weaken and go to zero.
I tend to think a similar process occurs on Earth but at a vastly longer time scale and significantly more irregular. The magnetic field on Earth breaks apart into a number of smaller mini dipoles and the opposing polarity allows the mini dipoles to cancel each other out driving the magnetic field strength to zero.
I view the South Atlantic Anomaly as a mini-dipole. And I expect as the Earth's magnetic field strength decreases, addition Anomalies will appear.
This graph shows how the South Atlantic Anomaly has grown in size during the past 150 years.