Variations in the Length of Day Affected by Processes in Earth's Core

Variations in the length of day over periods of one and 10 years are caused by processes in the Earth's core, researchers at the University of Liverpool have found.

Earth rotates once per day, but the length of this day varies enough so that 300 million years ago, a year lasted about 450 days and a day would last about 21 hours, the latter of which is a result of the slowing down of Earth's rotation. 

However, the rotation of Earth on its axis is affected by a number of other factors. For example, the force of the wind against mountain ranges changes the length of the day by plus or minus a millisecond over the course of a year.

Richard Holme, from the School of Environmental Sciences, studied the variations and fluctuations in the length of day over a one to 10 year period between 1962 and 2012, taking into account the effects on Earth's rotation of atmospheric and oceanic processes to produce a model of the variations in the length of day on time scales longer than a year.

"The model shows well-known variations on decadal time scales, but importantly resolves changes over periods between one and 10 years," Holme said in a press release, adding that previously, these changes were "poorly characterized."

All told, the study shows that the variations can be explained by just two key signals: a steady 5.9 year oscillation and episodic jumps that occur at the same time as abrupt changes in the Earth's magnetic field, generated in the Earth's core.

"This study changes fundamentally our understanding of short-period dynamics of the Earth's fluid core," he said. "It leads us to conclude that the Earth's lower mantle, which sits above the Earth's outer core, is a poor conductor of electricity giving us new insight into the chemistry and mineralogy of the Earth's deep interior."

The research, published in the journal Nature, was conducted in partnership with the Université Paris Diderot.