[1][4][5] These events are believed to originate in the interior of the Earth (rather than being due to external phenomena such as the solar wind); but their precise cause is still a matter of research.
[5] The name "jerk" was borrowed from kinematics, where it means the rate of change of the acceleration of a body, that is, the third derivative of its position with respect to time (the acceleration being the second derivative); or, more specifically, a sudden and momentary spike (or dip) in that rate.
In the period between jerks, each component of the field at a specific location changes with time t approximately as a fixed polynomial of the second degree, A t2 + B t + C. Each jerk is a relatively sudden change (spread over a period of a few months to a couple of years) in the A coefficient of this formula, which determines the second derivative; and usually in B and C coefficients as well.
[1] These events are believed to be caused by changes in the flow patterns of the liquid outer core of the Earth,[4] as for instance carried by hydromagnetic waves such as torsional oscillations.
[1][6] Numerical simulations of core dynamics have successfully reproduced the characteristics of well documented jerks.