Universal Time Effects on Substorm Growth Phases and Onsets.

Universal Time (UT) variations in many magnetospheric state indicators and indices have recently been reviewed by Lockwood and Milan (2023, https://doi.org/10.3389/fspas.2023.1139295). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth's magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte‐Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth's poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth's rotation. The premise behind the model is shown to be valid using a super‐posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning ahead of the growth phase that is required, on average, for onset to occur. A key factor is the extent to which pole motions caused by Earth's rotation influence the near‐Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the effect required to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed. Plain Language Summary: Earth's magnetic field is eccentric in that the main magnetic (dipole) axis does not pass through the center of the Earth. This introduces a wobble into many aspect of near‐Earth space (the "magnetosphere") as Earth rotates. Many consequences of this have been noted in previous papers. This paper investigates the effect of the eccentricity on the phenomenon of magnetospheric substorms. It is shown that the explosive releases of energy stored in the tail of the magnetosphere are more likely to start ("onset") at some Universal Times (and therefore geographic longitudes) than others and an explanation of why is provided. Key Points: Universal Time (UT) effects in the magnetosphere are caused by the eccentric nature of Earth's intrinsic magnetic fieldThere is a UT dependence of the open flux (and hence also the integrated magnetopause reconnection voltage) needed to trigger substorm onsetGrowth phases that lead to substorm onset show considerable preconditioning by prior reconnection [ABSTRACT FROM AUTHOR]