Pretty straightforward. Is there an event which could get rid of the majority of the Earth's magnetic field quite suddenly, but only for about a decade? Could anything manmade assist in this?
The Earth's magnetic field periodically switches polarity (the N-S poles flip location). We can see the record of these transitions in the rocks of the Earth (especially the oceanic crust spreading from oceanic ridges).
A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged. The Earth's field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite. These periods are called chrons. The time spans of chrons are randomly distributed with most being between 0.1 and 1 million years with an average of 450,000 years. Most reversals are estimated to take between 1,000 and 10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago; and may have happened very quickly, within a human lifetime.1 A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period. That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years. During this change the strength of the magnetic field dropped to 5% of its present strength. Brief disruptions that do not result in reversal are called geomagnetic excursions.
During the transition first the magnetic poles begin to wander and the Earth's magnetic field gets weaker (really the amount of energy in the field remains constant but there's a lot of chaotic effects going on that counter the over all dipole of the Earth's magnetic field). During the magnetic field lull, the Earth's net magnetic field is so weak that you could see aura from any point on Earth.
During the field reversal, the net energy of the Earth's magnetic field remains constant. However, the net magnetic field weakens to almost nothing as smaller dipole and quadrapole fields interfere with each other (as if there are innumerable eddies in the liquid core, each generating its own field and interfering with the others). From the reference link:
These transitions happen "instantaneously" on the geologic time scale. Our best current era (CE) guess is the transitions take 1000s to 10,000s of years. However, we don't really know how long they take. You could just "say" that it happens in 10 years but it would be more realistic if you could have it happen over a minimum of 100 years.
FYI, the Earth has started a magnetic field flip and our magnetic field has been noticeably weaking.
Even if you stopped the convection that is responsible for generating the field, there is a relaxation time on the scale of thousands of years. The big iron core acts like the core of a transformer, and it will take a while for the field to decay.
There was a recent seminar on the geomagnetic field that you should watch.
Clarke level technology
Is it possible with technology far beyond our own? What you need is to generate a other magnetic field to cancel it out. That would require a great deal of energy.
Certainly not with today's technology
Possible scenarios in which this could happen:
- The core of the Earth cools dramatically.
- Some other planetary object interrupts the angular momentum that spins Earth
I can't imagine an event in which the core of the Earth would cool, or for that matter, a near-collision that would seriously "unspool" Earth's magnetic field.
Both of these would probably be mass-extinction events, and nothing that mankind has made so far would even have a slight impact on Earth's magnetic field.
I think if you actually built a Faraday Cage around the entire planet, that should do the trick.
Inhowfar such an idea is feasible is up to the setup of your story.
Maybe your planets population managed to do that by accident, because they built a very dense railroad network or something along those lines. But i have the impression that this would stretch credibility enormously.
Mind you that it would only cancel out the effect of the magnetic field. Removal of the cage should turn things back to normal.
Anything that has a natural background seems impossible, see the very good anser by Jim2B.