This device is called a magnetometer. It shows changes in the Earth's magnetic field. The very first users of the magnetic compass were aware that magnetic north did not point to true north, and that the compass needle sometimes trembled and shifted for no apparent reason. Eventually a connection was made between this and the appearance of the aurora and by the 19th Century people were beginning to understand the further connection between the Earth's magnetic field, the aurora, and activity on the Sun.
Although modern magnetometers are sophisticated electronic devices, anyone can make this version with hand tools. You wont need to buy anything exotic part from a cheap laser pointer and maybe a neodymium magnet. By utilising the simplest optical and mechanical principles you will be able to make a device that is exquisitely sensitive.
This device is one of the easiest to make. It was popularised in Sky and Telescope magazine as a "jamjar" or "Sodapop bottle" magnetometer. Go back a whiles and this simple device had another name: a declinometer. During the 18th and 19th centuries, people were aware of the deviation of magnetic north from true north. This was called magnetic declination and devices like this were used to measure it.
As you can see from the diagram, it's nothing more than a magnet suspended from a thread. Attached to the magnet is a mirror. We will use a laser pointer to bounce a dot of light off this mirror, onto the wall. We will use a simple scale to determine movement of the laser pointer dot. By having a longer distance between the scale and the mirror, we can amplify the tiny movements of the magnet.
This device can be as simple or complicated as you wish to make it. Some things to keep in mind:
In the above design the thread runs from the magnet holder through a tiny hole in the roof of the housing and is wrapped around a stick. This can be taped in place once you have the length of thread right.
If you want to measure the changing angle of the magnet in degrees, you can use some high-school trigonometry:
Some links to sites that have nice implementations are:
Once you have everything set up, what can you see? For most people that live in an urban setting, cars passing in the street will show as small momentary drifts. For my setup at home with a scale distance of 2.8m, the pointer drifts about 2mm. Modern cars have a lot less steel in them now, so they're not so bad. I can occasionally detect large trucks coming down the motorway about 90m from my house. The magnetometer will show any changes in it's environment, including the movement of steel and magnetic objects near it, so try and position it as best you can to minimise this.
While the pointer is generally steady, a disturbed geomagnetic environment shows up as small, constant trembling in the pointer. These might only be 1-2mm or so, but to me they are an indicator to keep an eye on things.
What about aurorae? If you are monitoring your magnetometer every 15-20mins or so you may see sudden "large" changes in position. If you eliminate the possibility of any local cause, then you have probably witnessed a massive change in the Earth's magnetic field.
Below is a hand-plotted chart of the declinometer, photoshopped against data from a professional-quality magnetometer at DunedinAurora.nz at the time of a small scale aurora. With thanks to DunedinAurora.nz
You can see that when the professional instrument was showing clear fluctuations in magnetic field strength, the home made declinometer was showing obvious swings east and west. The declinometer quietens down at approximately the same time as the magnetometer.
This was a small storm, so the total deflections were in the order of 6mm either side of the zero mark. They were more subtle and took place over the course of an evening. More powerful geomagnetic storms can easily cause pointer shifts of up to 20mm @ 2.8m scale distance. The storm that hit on St Patrick's Day 2015 caused the pointer to visibly shift over 50mm in minutes.