Date: Mon, 10 Nov 1997 15:06:01 GMT
From: Larry Newitt <email@example.com>
Subject: re: decay of the earth's magnetic field
I am not familiar with the article by Barnes in the SIS Review, but the decrease in the earth's magnetic field to which he referred is well-known. That is not to say that the strength of the magnetic field is decreasing by the same amount everywhere. Measurements of the magnetic field strength are routinely made at different places on the earth show, and show different rates of decrease; in fact, some places show an increase. However, mathematical analyses of the observations, which are routinely done every few years, show an overall decrease in what is called the "dipole moment" of the magnetic field. (See for example, C.E. Barton, Journal of Geomagnetism and Geoelectricity, v 49, 123-148, 1997.) This decrease is approximately linear, not exponential. If it were to continue, the dipole moment would become zero in about 1250 years, but Barton points out that the dipole appears to be recovering from an historic high that occured about 2000 years ago, so there is no reason to believe that the decrease will continue indefinitely.
Regards, Larry Newitt