Abstract

We use theam,an, asand theaσgeomagnetic 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 of those in the northern hemisphere. Hemispheric differences have previously been discussed in terms of polar ionospheric conductivities generated by solar photoionization, effects which we allow for by looking at the dipole tilt effect on the 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 that is typically a ±30% diurnal modulation for disturbed intervals rising to ±76% in quiet times. For the northern hemisphere these are 15% and 38% modulations. Motion away from/towards the Sun reduces/enhances the directly-driven ionospheric voltages and reduces/enhances the magnetic energy stored in the tail and we estimate that approximately 10% of the effect appears in directly driven ionospheric voltages and 90% in changes of the rate of energy storage or release in the near-Earth tail. The hemispheric asymmetry in the geomagnetic pole offsets from the rotational axis is shown to be the dominant factor in driving Universal Time (UT) variations and hemispheric differences in geomagnetic activity. 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 with time-of-yearFandUTin the driving power input into the magnetosphere is converted into the equinoctialF-UTpattern in average geomagnetic activity (after correction is made for dipole tilt effects on ionospheric conductivity), added to a pronouncedUTvariation with minimum at 02–10 UT. In addition, we show that the predicted and observedUTvariations in average geomagnetic activity has implications for the occurrence of the largest events that also show the nettUTvariation.

Highlights

  • The first well-informed description of a Universal Time (UT) variation in global geomagnetic activity, that we know of, was by Bartels (1925, 1928)

  • The independent effect of pSW supported by the modelling shown in Paper 3 which shows that, for a given magnetospheric open flux, energy stored in the near-Earth tail lobes and cross-tail current are both increased by enhanced solar wind dynamic pressure, and both of have the potential to enhance geomagnetic activity

  • We have identified for the first time a factor that introduces a systematic Universal Time variation into global geomagnetic activity but has been previously overlooked, namely the electric fields induced by geomagnetic pole motions due to Earth's rotation

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Summary

Introduction

The first well-informed description of a Universal Time (UT) variation in global geomagnetic activity, that we know of, was by Bartels (1925, 1928). As described in Papers 1 and 2 of this series (Lockwood et al, 2020a,b), the semi-annual variation is well explained by the “Russell-McPherron” (R-M) effect (Russell & McPherron, 1973) which is due to the effect of the orientation of the interplanetary magnetic field (IMF) on magnetic reconnection in the dayside magnetopause and on solar-wind magnetosphere coupling This predicts a pattern of response with fraction of a calendar year (F) and UT that is very different from the equinoctial pattern that is seen in geomagnetic activity using the best indices that have responses to solar wind forcing that do not vary with either F or UT. This coupling function is explained, discussed and its use justified at the start of Section 3.2

Universal Time variations in different geomagnetic indices
Motions of the poles and polar caps
Effect of solar wind dynamic pressure
Aims of this paper
Allowance for conductivity effects
Results for the ar indices
Analysis of geomagnetic response by IMF Y-component polarity
Large events and the effect of average levels of activity
Discussion and conclusions
Acknowledgements and data and software sources
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