Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 4. Polar Cap motions and origins of the Universal Time effect

TL;DR

This paper investigates how Earth's polar motions and diurnal variations influence geomagnetic activity and magnetospheric dynamics, revealing significant hemispheric differences and explaining the universal time effect through inductive and solar wind interactions.

Contribution

It introduces the inductive effect of polar motions as a key factor in geomagnetic variations and explains the universal time effect in geomagnetic activity with a comprehensive model.

Findings

Hemispheric differences in geomagnetic indices are linked to Earth's magnetic pole offset.

Diurnal modulation of ionospheric voltages varies from 30% in disturbed to 76% in quiet times.

The UT variation in geomagnetic activity correlates with polar motions and solar wind effects.

Abstract

We use the am, an, as and the a-sigma geomagnetic indices to the explore a previously overlooked factor in magnetospheric electrodynamics, namely the inductive effect of diurnal motions of the Earth's magnetic poles toward and away from the Sun caused by Earth's rotation. Because the offset of the (eccentric dipole) geomagnetic pole from the rotational axis is roughly twice as large in the southern hemisphere compared to the northern, the effects there are predicted to be roughly twice the amplitude. Hemispheric differences have previously been discussed in terms of polar ionospheric conductivities, effects which we allow for by studying the dipole tilt effect on time-of-year variations of the indices. The electric field induced in a geocentric frame is shown to also be a significant factor and gives a modulation of the voltage applied by the solar wind flow in the southern hemisphere of typically a 30% diurnal modulation for disturbed intervals rising to 76% in quiet times. Motion towards/away from the Sun reduces/enhances the directly-driven ionospheric voltages and reduces/enhances the magnetic energy stored in the near-Earth tail: 10% of the effect being directly-driven and 90% being in tail energy storage/release. Combined with the effect of solar wind dynamic pressure and dipole tilt on the pressure balance in the near-Earth tail, the effect provides an excellent explanation of how the observed Russell-McPherron pattern in the driving power input into the magnetosphere is converted into the equinoctial pattern in average geomagnetic activity (after correction is made for dipole tilt effects on ionospheric conductivity), added to a pronounced UT variation with minimum at 02-10UT. In addition, we show that the predicted and observed UT variations in average geomagnetic activity has implications for the occurrence of the largest events that also show the nett UT variation.