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Geomagnetic observatory

Joe Geller -August 12, 2012

FDM Proton Precession Magnetometer

Joe Geller1

This article describes a proton precession magnetometer (PPM) for monitoring the Earth’s magnetic field.  The project PPM has sub nano Tesla resolution.  To put nano Tesla sensitivity in perspective, changes of 1 nanotesla (nT) can be caused by a safety pin at less than 3 feet, a car at 125 feet, a bus at 250 feet, and a train at just over a half mile (~1 km)2.  These numbers are only rough approximations, because the influence of any particular object depends largely on the ferrous metal content of the object. 

By measuring very small changes in the Earth’s magnetic field, one can also observe the diurnal variation of the geomagnetic field, typically on the order of tens of nano Tesla.  Larger changes in the geomagnetic field are often related to solar activity.  The Sun can send out massive amounts of charged particles as a Coronal Mass Ejection (CME).  As shown in the SOHO (ESA & NASA) illustration of figure 1, when an Earth-directed CME impacts our planet, there can be large swings in the geomagnetic field, sometimes rising to the level of a geomagnetic storm.  Such geomagnetic disturbances can include very large swings in the geomagnetic field.  The magnitude and shape of waveforms of the total field (“F” scalar) can vary widely over geographic location on the Earth.

Figure 1: A Coronal Mass Ejection (CME) [Courtesy SOHO (ESA & NASA)]

Geomagnetic storms will occur with increasing frequency as we approach the peak in the 11 year solar cycle.  Most geomagnetic events, including minor storms, go relatively unnoticed.  As the severity of a geomagnetic storm increases, the Aurora Borealis (the “northern lights”) show in the night sky, and can become visible at lower latitudes with stronger storms. 

Radio propagation can also be affected, for example long distance high frequency communications can be completely disrupted.  More severe geomagnetic storms can cause geomagnetically induced currents (GIC) in the power grid.  GICs can become large enough to literally burn out high voltage distribution transformers and take down an entire power grid as happened in Quebec Province, Canada in 1989. 

The most severe geomagnetic storm in recorded geomagnetic history was the Carrington event of 1859.  It is believed that a Carrington type event today could damage many, if not most of the power grids in North America and Europe.  Since the remanufacture of hundreds of high voltage distribution transformers is a long term prospect, many people could be without power for weeks or months.  While a Carrington type event is believed to be uncommon, the frequency of such high impact events is unknown.  As such, if you plan to be able to record a Carrington event with our project PPM, you had better plan to run it on a battery with a solar charger!3

Now, turning back to our PPM, a PPM measures the magnitude of an ambient magnetic field based on the quantum mechanical phenomena of proton spin precession.  The beauty of the PPM is that once the frequency of a precession signal is accurately measured, the magnetic field is known.  Even though this technology dates back to the 1950’s, small lab or home built versions of a PPM can still be a challenging project.  However with some electronics construction experience a DIY PPM project is very do-able4

A PPM needs to operate in a relatively uniform magnetic field.  The Earth’s magnetic field, outdoors away from structures and ferrous metals, typically has uniformity in space of better than 10-5.  The spatial magnetic gradient need not be perfectly homogeneous.  For example, my outdoor observatory sensor operates fine at about 33 feet from a parked vehicle.  However, the gradients in almost all buildings and homes are too severe.  Without complex shimming with gradient coils, a PPM sensor will not work indoors.


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