Evolution of IMF B y Induced Asymmetries: The Role of Tail Reconnection

• Published in: Journal of geophysical research. Space physics Vol. 126; no. 7
• Main Authors: Ohma, A., Østgaard, N., Laundal, K. M., Reistad, J. P., Hatch, S. M., Tenfjord, P.
• Format: Journal Article
• Language: English
• Published: 01-07-2021
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2021JA029577

Abstract
North‐south asymmetries arise in the magnetosphere‐ionosphere system when a significant east‐west (
B
y
) component is present in the interplanetary magnetic field (IMF). During such conditions, a
B
y
component with the same sign as the IMF
B
y
component is induced in the magnetosphere, and the locations of conjugate magnetic footpoints are displaced between the two hemispheres. It has been suggested that these asymmetries are introduced into the closed magnetosphere by tail reconnection. However, recent studies instead suggest that asymmetric lobe pressure induces the asymmetries, which are then reduced during periods of enhanced tail reconnection. To address this, we use the Lyon‐Fedder‐Mobarry (LFM) model and initiate a loading‐unloading cycle in multiple runs by changing the IMF. Asymmetries are induced during the loading phase and reduced during the unloading phase. The model results thus suggest that asymmetries arise during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection.
Plain Language Summary
The aurora is a bright and beautiful manifestation of Earth's connection to space, and can light up the night sky in the high latitude regions in both hemispheres. However, auroral features do not always occur at the expected magnetic location in the two hemispheres, but are often displaced in the longitudinal direction between the Northern and Southern Hemisphere. The displacement is related to the orientation of the magnetic field in the solar wind, but the exact mechanisms responsible for causing and removing this displacement are debated. Previous simulations of the plasma environment around the Earth suggest that when this magnetic field intensifies in the dawn‐dusk direction, magnetic pressure builds up asymmetrically in each hemisphere, causing the displacement. Here we present new simulation results showing that the longitudinal displacement is reduced when magnetic reconnection happens in the Earth’s magnetotail.
Key Points
The evolution of the asymmetric state of the magnetosphere is examined by using the Lyon‐Fedder‐Mobarry model with non‐zero interplanetary magnetic field
B
y
The simulation shows a clear reduction of north‐south asymmetries associated with enhanced tail reconnection
The modeling results are consistent with the evolution seen in interhemispheric observations of aurora