June 2006, Issue 191

Earth Field Magnetometer
Cypress PSoC High Integration Challenge 2004 Winner

I’ve always been curious about minute variations in the Earth’s magnetic field caused by solar storms, which have been known to affect radio and power systems and indicate the level of aurora activity. There are a number of magnetic monitoring stations scattered around the world, some of which provide real-time data on their web sites, but I wanted to take a crack at taking the measurements myself.

There are several things that contribute to a measurement made at the Earth’s surface. The most important is the planet’s magnetic field generated deep within its molten core, which is relatively constant. Changes resulting from activity within the core occur over many years, while variations with a period of less than a year are normally the result of external influences.^[1] The flow of ionized particles from the sun causes a daily cyclical variation of less than 1% of the total field, and this varies by latitude. The most interesting component results from solar storms. This could be as small as 0.1% of the total field, making it difficult to measure.

The magnetic flux lines don’t run parallel to the planet’s surface; rather, they follow a vertical angle known as inclination that varies with latitude. This isn’t to be confused with declination (or variation), which is the horizontal angle between magnetic and true north, or deviation, which is the error in compass reading caused by nearby metallic objects. An ordinary compass needle sees only the horizontal field, ignoring the inclination. The opposite is true of a dip needle, which is basically a compass turned on its side.

The total magnetic flux density at the planet’s surface is around 0.5 gauss (G), and its horizontal component varies from that value at the equator to zero at the magnetic poles. I estimated it at 0.3 G where I live in California based on data from a U.S. Geological Survey magnetic monitoring site a few hundred miles away. Compare this to a small bar magnet that might generate a nearby field of hundreds of gauss.

My Cypress CY8C22213-based Earth Field Magnetometer measures the strength of the horizontal component of the Earth’s magnetic field (see Photo 1). It takes measurements once every minute and stores the data in the EEPROM. I can review data that’s up to 10 days old on the system’s LCD, or I can download it through a serial port.