What is the nature of the "unexpected geomagnetic pulse that took place beneath South America in 2016"? Popular news has several articles linking back to the news article in Nature Earth’s magnetic field is acting up and geologists don’t know why which discusses the need for an early update to the World Geomagnetic Model.
However my question is about one line found in the Independent's Planet’s erratic magnetic field forces emergency update to global navigation system

The shift they observed was so large it was on the verge of exceeding the acceptable limit for navigation errors.
To account for this, scientists at the British Geological Survey and the US National Oceanic and Atmospheric Administration (NOAA) are issuing an unprecedented emergency update to the model.
They have fed in the latest data, including an unexpected geomagnetic pulse that took place beneath South America in 2016, to ensure the system is more accurate.

Question: What is the nature, and underlying cause of the &quot;unexpected geomagnetic pulse that took place beneath South America in 2016&quot;?
AKA geomagnetic jerk.
The "geomagnetic pulse" here refers to a series of academic works studying the Earth's core such as this open access article (other related articles require scientific journal subscriptions to read).

Geomagnetic pulses have been associated with "geomagnetic jerks". A jerk was reported (yay for open access again) to have occurred in 2014, the 2016 pulse under South America is believed to be related to this, both events having been measured and identified after the fact.

The magnetic field is always changing - a "pulse" is a quicker-than-normal (few year) acceleration of magnetic field change in time, in a region at the surface of the Earth's outer core (magnetic field is generated within the core).

A "jerk" is a quick change (few months) of the rate at which magnetic field changes at Earth's surface, making something like a 'V' shape if you plot the field against time at a given location.

The idea is that a pulse occurs at the core, and jerks are an effect seen at the surface just before and after the peak of the pulse, as the field change ramps up and down. The underlying cause of this is not known, and we only identify them after they've occurred, so they're currently unpredictable. That said this behaviour is apparently part of the core's repertoire, and pulses and jerks have been observed before.