This is a brief account of efforts to measure the terrestrial magnetic field with enough sensitivity to detect fluctuations in its normal values. The device is a flux-gate magnetometer from Fat Quarters Software in California. The device is the FGM-3 and costs about $35. In general, the device works well, although it requires some electronics background to construct the necessary power supplies and perhaps frequency-to-voltage (FV) converter. Circuits and more detailed discussion of actually deploying them will be given a bit later when the home-page is modified again. For the time being, let it be said that the sensor is somewhat temperature-sensitive, so that it is advisable to operate two of them in a differential mode.
In particular, the two sensors are oriented in such a way that a given change in the ambient magnetic field will produces a frequency increase in one of the sensors and a frequency decrease in the other. Hence, the signal-detection system reads a difference in frequency-of perhaps a few hundred Hz---rather than the absolute frequency-of about 70 kHz.
For the moment, let it suffice to show some results. The upper graph shows four days of data, in which it is quite clear that something happened around 2 AM (UTC) on Nov.23, 1997. Data from the GOES satellites for the same few days corroborates our results. It should be kept in mind, however, that the satellite sensor measures the parallel component of the terrestrial magnetic field whereas our sensors are oriented in such a way as to measure the transverse component.
There are a couple of reasons for this: first, the relative change in the transverse component appears to be significantly larger than the relative change in the parallel component. Second, unless the user somehow compensates for the parallel component of the ambient magnetic field-which is large enough to saturate the sensor,-- then operating the sensor in the transverse mode will probably avoid saturation and thus spare the user the trouble of building appropriate compensating coils and their power supplies.
In conclusion-until this report is updated-you too can have your very own terrestrial field Observatory, and perhaps attempt to correlate fluctuations in this field with things happening on the Sun. It should be mentioned that this system operated in a typical single-family dwelling right in the middle of a medium-sized city-Halifax, Nova Scotia.
PS The supplier's address is: 24774 Shoshonee Drive, Murietta, CA 92562. See their web page at www.dconn.com/FatQuartersSoftware
Here are a series of plots relating to this project. Please click on them to see an enlarged image.
These and other plots may be viewed at gopher://solar.sec.noaa.gov:70/11/plots
Visit Darrel Emerson's site on the Fluxgate Magnetometer tests. A very good site!
Back to main page
Updated May 4, 1999, by Ian McCarthy.
