Universal Time Effects on Substorm Growth Phases and Onsets

Abstract

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.

Reconstruction of Carrington Rotation means of Open Solar Flux over the past 154 years

“…This could have arisen from a local time response of the stations but that possibility is eliminated by the fact that the diurnal variations in the northern and southern hemisphere sub-indices, aa HN and aa HS , are in phase, despite the stations being close to 180 • apart in longitude. Instead, this demonstrates the Universal Time variation in solar wind -magnetosphere coupling discussed by ; Lockwood and Milan (2023) and Lockwood (2023). This diurnal variation contribution to the a.c.f.…”

Statistical Investigation of the Storm Time Plasma Density Strip-like Bulges at Lower-Mid Latitudes

“…Consequently, a semiannual variation exists, characterized by stronger geomagnetic storms near two equinoxes than the other seasons. Additionally, due to the angle offset between the magnetic and geographic poles, the impacts of geomagnetic storms on the thermosphere and ionosphere are known to exhibit universal time effect (Huang, 2013;Perlongo & Ridley, 2016;Wang et al, 2017) by controlling the energy deposit in the polar region (Lockwood, 2023) that further influences the global thermospheric circulation (Perlongo & Ridley, 2016;Wang et al, 2017) and affects the disturbance dynamo electric field (Huang, 2013). Based on the previous studies, we wonder whether the presence of bulges was associated with the seasonal and UT effects.…”