Contribution of Bursty Bulk Flows to the Global Dipolarization of the Magnetotail During an Isolated Substorm

Abstract

This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner magnetosphere during substorms. We employ the high-resolution Lyon-Fedder-Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.

Understanding Spacecraft Trajectories Through Detached Magnetotail Interchange Heads

“…Different possible oblique motions of the BICI generated dipolarization fronts are expected to modify the suggested particle injection and acceleration mechanisms at dipolarization fronts that were predicted using, for example, test particle simulations in magnetohydrodynamic models (Birn et al, 2013; Sorathia et al, 2018; Ukhorskiy et al, 2018; Yang et al, 2011). Note that as the global magnetohydrodynamic simulations showed, dipolarization fronts that were produced by reconnection at more tailward locations may shortly deviate from their otherwise radial propagation on their way to the dipolar field lines (Merkin et al, 2019; Ukhorskiy et al, 2018; Wiltberger et al, 2015). We also admit that the dipolarization fronts released by BICI may later be forced to move more radially by the ambient plasma.…”

The increase in the curvature radius of geomagnetic field lines preceding a classical dipolarization

“…It is noted that BBFs with low-entropy plasmas (plasma bubbles) often penetrated the inner magnetosphere (Dubyagin et al, 2011). In numerical simulations, those bubbles localized in local time produced global dipolarization in the inner magnetosphere (Merkin et al, 2019) and generated an ionospheric current system such as the westward electro-O. Saka: The increase in the curvature radius of geomagnetic field lines jet, Harang discontinuity and poleward expansion of aurora in the substorm expansion phase (Yang et al, 2012).…”

“…It is often difficult to determine the onset timing of substorm processes such as magnetotail flow burst, field line dipolarization and particle injections. To resolve the timing uncertainties, auroras in global satellite images (Nakamura et al, 2001;Miyashita et al, 2009), intensifications of auroral kilometric radiation (Fairfield et al, 1999;Morioka et al, 2010) and dispersionless particle injection in geosynchronous orbit (Birn et al, 1997) were used. Geomagnetic Pi2 micropulsations observed on the ground are another useful tool for determination of the substorm timing (Sakurai and Saito, 1976;Nagai et al, 1998;Baumjohann et al, 1999).…”

Dynamics of magnetic structures during the magnetospheric substorm

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