Fluctuations in Earth’s magnetic field

By Marcus Wilson 23/07/2010 10


In uncertain times, its good to know there are some things that never change – such as day follows night, my compass needle always points in the same direction, and England will always underperform at the World Cup. Well, scrub the middle one, actually. In our lab, at any rate, there are some shocking variations in the magnetic field.

I know that because I’ve been doing some more playing with our Earth’s field NMR / MRI equipment, and finding that my resonance frequency is harder to pin down than a politician in an expenses scandal.  The system uses the earth’s magnetic field to split the nuclear energy levels of the hydrogen nucleus (proton) in a sample of water; the amount of splitting depends on the strength of the field, and the resonant (Larmor) frequency is proportional to this. When the magnetic field increases, so does the resonant frequency. 

In the lab it’s quite common for the frequency to shift by 0.1% (1 part in a thousand) in an hour. If you leave it a couple of weeks between measurements, it may have shifted a whole lot more – in excess of 1%. Some of the drift in resonance will be because the earth’s field changes,  for example, there is a diurnal cycle in the strength of the field due to the solar wind interacting with the earth’s field. But I suspect a lot of it is because of the nature of the lab – in a reinforced concrete building (i.e. one with lots of steel in it) surrounded by heaps of other lab equipment. There’s a great deal of potential for the earth’s magnetic field to be changed by the lab environment.

It’s no big hassle, usually, because usually the resonance hasn’t drifted too far from where it was when I last looked, but it can take a bit of time to re-optimize the equipment before we get the students to use it.

And remember, that a 1% shift is trivial when compared to the kind of movements the field makes on a longer timescale – e.g. the drift in position of the magnetic poles, and the reversals of the field at times during the earth’s history.  A good overview is found here.

 


10 Responses to “Fluctuations in Earth’s magnetic field”

  • Is there more fluctuation during a solar flare than when not?
    what would happen if a large drift occurred suddenly ?

    • What do you call a deer with no eyes?

      Given that the sun plays a major role in fluctuations (there is a clear diurnal fluctuation in the magnetic field), I’d guess ‘yes’, if you made me. You could probably find out quite quickly with a bit of googling.

  • In a previous life I wintered-over at Scott Base, Antarctica as the Magnetics and Seismology Technician. Each week I had to go out to the wee magnetic hut and take the Earths Magnetic “temperature”. That is, measure the strength of the field. A Quartz Magnetometer was used. A string of quartz had a wee magnet slung on it which would orient itself to the field. The string was rotated a full circle plus a bit so that a small mirror on the magnet lined up through the viewing telescope. Two rotations in the opposite direction would line up the mirror as well. By knowing the mechanical twisting forces of each magnetometer one could measure the field in one plane.

    The question about solar flares cropped up down there regularly. I remember watching fascinated as the quartz magnet fluctuated by many degrees over a few minutes which caused no end of problems trying to make a decent measurement.

    Incidently, Marcus’ NMR / MRI using the proton spin is utilised in the Proton Magnetometer which was also part of the magnetic field measuring equipment at Scott Base. Basically it is a big coil of wire wrapped around a bottle of water. The coil axis was positioned at right angles to the earth’s field. A direct current is placed in the coil which ‘twists’ the protons to line up with the massively bigger coil magnetic field. This was left for a few seconds then turned off. The protons would then precess as they lined back up with the earths magnetic field and the frequency was picked up by the same coil with the signal switched to an amplifier and frequency meter.

  • hi,i am having this doubt for the last few days.what happens when magnetic field of earth fluctuates,does it causes any earth quakes,when the fluctuation is severe .

    • Link between the magnetic field fluctuations and earthquakes? I’ve not heard of one. For most purposes, I doubt any change in the earth’s magnetic field is going to affect you.

  • To MARCUS WILSON
    sir,actually i read about these magnetic field fluctuations in a newspaper (caused by solar rays)..,in that article, they have stated that magnetic field fluctuations might be severe, yeah i too dont know the relationship between magnetic field fluctuations and earthquakes, i just guessed that it could cause earthquakes!.but you are saying that any change in earth magnetic field is going to affect me..would you please kindly say how ……..

  • Given that people have correlated earthquakes with just about every possible measurable thing in an attempt to provide a predictor of them, I would guess that if there’s a relationship between magnetic field fluctuations and earthquakes (specifically, the former triggers the latter) then it would be well known. It isn’t, so my educated guess is that there’s no relationship.

    How would field changes affect you? Well, ultimately it would be because you have atoms and molecules inside you that would feel the field (e.g. the iron atoms in haemoglobin) and therefore move slightly differently depending on the field strength. Given that people have brain scans in MRI machines at really high magnetic fields and that doesn’t do damage to them, I’d say that we are pretty resilient to magnetic fields.

    But there is also the issue of high-energy charged particles coming from the sun towards the earth – maybe that’s what you mean? These are mostly ‘trapped’ by the earth’s field, close to the poles (the northern and southern lights are caused by this) so the number that get to you on the surface of the earth is much reduced. These particles can be ionizing and therefore dangerous – how dangerous depends on the dose you get. On earth, it’s not worth worrying about. If you’re a long-term astronaut, it needs some thought.

  • thank you very much sir,now i have understood the facts about magnetic field fluctuations and earth quakes.if there is any doubt i will login later.

  • The sun emits many small particles (the solar wind) and electromagnetic waves (the most obvious form of the latter is visible light). Some of these will get through the atmosphere and hit us (or, in the case of neutrinos, go through us; some of these will be deflected by the earth’s magnetic field. The magnetic field keeps us relatively free of the really nasty radiation from the sun – and that’s a good thing.

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