Whistles from space

In midsummer, quite a few years ago now, an ionospheric physicist friend and I were in a car parked on a remote logging road in Algonquin Park, Ont.

On the ground a few metres away we had laid out about 50 metres of multicore cable in as close to a circle as we could get in the scrub by the road. The wire cores in the cable were cross-connected so that we had a big, multi-turn coil. Another cable brought the end connections of the loop into the car. 

Inside, on the back seat we had a sensitive audio amplifier, just like the ones in record players but more sensitive, and a tape recorder, both battery powered. 

It was night time, and at that remote location, far from city lights, the sky looked grey and speckled with so many stars it was hard to identify the constellations. The trees were silhouetted black against the sky. The poetic atmosphere was, however, wiped out by the need to stay in the car on a hot summer night, with the windows closed because of the hordes of mosquitoes. 

We could not run the car engine and the air conditioning because it interfered with the signals we were trying to detect. We were there to record the waves produced by the sliding of the solar wind over the Earth's magnetic field.

The solar wind, flowing out from the sun at speeds of hundreds to thousands of kilometres a second, rubs over the Earth's magnetic field like a bow on the string of a violin. The waves generated by this follow the lines of the magnetic field down toward the north and south magnetic poles. Some of those magnetic field lines went through our coil, which acted in the same manner as a pickup on an electric guitar. 

The oscillating guitar string generates currents in the pickup coil that can be amplified. The oscillations in the magnetic field lines did exactly the same thing. We had to be tens of kilometres from the nearest power lines because they produce an oscillating magnetic field far stronger than anything from space. We've all heard the effect of power line radiation when it is picked up by audio equipment. We call it "hum."

We listened to the signals while we recorded them. There were tweets, rising and falling whistles, a hiss like sea waves running through shingle, and underneath, some of that manmade hum. This was not surprising. We can even pick up that hum from space, but at least at our location in the park it was reasonably faint. 

The objective of that project, and similar projects done by many others before and since, was to learn a little more about the way the solar wind interacts with our planet's magnetic field. This is an important aspect of what we have come to refer to as space weather.

Space probes flying past Jupiter and Saturn were equipped with similar equipment. Jupiter is particularly interesting because it has a very strong magnetic field. Somehow it didn't sound right that Jupiter's whistles and peeps were higher pitched than those in the Earth's magnetic field. One would expect a giant planet, into which we could cram 100 Earths to speak with a deep voice, not a falsetto. 

There are two reasons for this. Jupiter's magnetic field is stronger than Earth's, and as we know, the more tension we put in a violin or guitar string, the higher the note. However, in this case, the string is also longer, which might make the note lower. It depends on how much tighter the string is compared with how much longer it is. Secondly, on Earth, the ionized atmospheric gases clinging to the magnetic field lines are oxygen and nitrogen. On Jupiter it is hydrogen, which has much lighter molecules. 

Similar waves have been detected by space probes visiting Saturn. When listening to these recordings, it is hard not to think they are biological, like the sounds produced by whales and other sea life, rather than produced by very interesting and challenging physics.

• Venus lies low in the southwest after sunset and Mars rises in the early hours. 
• The Moon will reach Last Quarter on the 17th

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.