Reversals: Magnetic Flip
The Earth has a magnetic field, as can be seen by using a magnetic compass. It is mainly generated in the very hot molten core of the planet and has probably existed throughout most of the Earth's lifetime. The magnetic field is largely that of a dipole, by which we mean that it has one North pole and one South pole. At these places, a compass needle will point straight down, or up, respectively. It is often described as being similar in nature to the field of a bar (e.g. fridge) magnet. However there is much small-scale variation in the Earth's field, which is quite different from that of a bar magnet. In any event, we can say that there are currently two poles observed on the surface of the Earth, one in the Northern hemisphere and one in the Southern hemisphere.
By magnetic reversal, or 'flip', we mean the process by which the North pole is transformed into a South pole and the South pole becomes a North pole. Interestingly, the magnetic field may sometimes only undergo an 'excursion', rather than a reversal. Here, it suffers a large decrease in its overall strength, that is, the force that moves the compass needle. During an excursion the field does not reverse, but later regenerates itself with the same polarity, that is, North remains North and South remains South.
