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Asymmetric Magnetic Field and Mapping

Intrinsic Source of Asymmetry

Relative Importance - Primary


Description:

The intrinsic magnetic field of the Earth is non-dipolar to higher order although it is frequently approximated as a dipole. The non-dipolar nature of the intrinsic geomagnetic fields is manifested as asymmetric field line mapping between the two hemispheres at high latitudes in particular.

Asymmetries in the Earth’s intrinsic field is a primary source of interhemispheric asymmetries in the coupled magnetosphere-ionosphere-thermosphere system. It is independent of any other drivers of interhemispheric asymmetries. Due to effects of the interplanetary magnetic field (IMF), and the solar wind, the dipole model is particularly inaccurate at high L-shells (e.g., above L=3) while it may be a good approximation for lower L-shells.

Uncertainties in conjugacy estimates for interhemispheric mapping: various interhemispheric manifestation in the coupled geospace system (e.g., geomagnetic pulsations, equivalent currents, ion upflow/outflow, auroral precipitation, Joule heating, thermospheric wind, etc.) in conjugate “areas” or at conjugate “points” can be underestimated or overestimated due to uncertainties associated with the asymmetric intrinsic magnetic field and mapping. In addition, during geomagnetic activity the performance of these models deteriorate.

Modeling Capability:

Numerical models do not consistently capture non-dipolar or tilted/offset field features. Global MHD models typically have options to include field tilt relative to the rotation axis, but rarely include offset or non-dipolar terms. Ring current models tend to operate using only northern hemisphere information and assuming a dipole field with some perturbation (e.g., the Tsyganenko family of empirical models or a force-balanced deformation of a dipole field), limiting the amount to which a fully realistic field is considered. This affects the accuracy of conjugacy/asymmetry adversely. Furthermore, the numerical solvers used in the global MHD models have a finite order of accuracy, which leads to artificial processes, such as reconnection. These terms are suppressed with a parameter called numerical resistivity. However, this process also undercut the MHD model performance in producing accurate open-closed field lines. In the recent years, MHD-EPIC (embedded particle in cell) models [Chen et al., 2017] have been developed to transition to a kinetic definition at regions that reconnection is likely to occur, which demonstrated a better performance to reproduce open-closed field line signatures.

BATS-R-US MHD


Includes dipole tilt only, resistive MHD, epic-mhd [Chen et al., 2017]

LFM MHD


Includes dipole tilt only, resistive MHD[Merkin et al., 2015].

OpenGGCM


Includes dipole tilt, must be updated periodically to account for rotation, resistive MHD [Raeder et al.,2006]

GUMICS


Includes dipole tilt, must be updated periodically to account for rotation

RAM


Dipolar and non-dipolar magnetic field considerations [Jordanova et al., 2006]

HEIDI


Includes dipole tilt only, resistive MHD, epic-mhd [Chen et al., 2017]

References:

Chen, Y., Tóth, G., Cassak, P., Jia, X., Gombosi, T. I., Slavin, J. A., … Henderson, M. G. (2017). Global three-dimensional simulation of Earth’s dayside reconnection using a two-way coupled magnetohydrodynamics with embedded particle-in-cell model: Initial results. Journal of Geophysical Research: Space Physics, 122, 10,318– 10,335. https://doi.org/10.1002/2017JA024186

Merkin, V. G., Sitnov, M. I., and Lyon, J. G. (2015), Evolution of generalized two-dimensional magnetotail equilibria in ideal and resistive MHD. J. Geophys. Res. Space Physics, 120, 1993– 2014. doi: 10.1002/2014JA020651.

Raeder, J.: Flux Transfer Events: 1. generation mechanism for strong southward IMF, Ann. Geophys., 24, 381–392, https://doi.org/10.5194/angeo-24-381-2006, 2006.

Jordanova, V. K., Miyoshi, Y. S., Zaharia, S., Thomsen, M. F., Reeves, G. D., Evans, D. S., Mouikis, C. G., and Fennell, J. F. (2006), Kinetic simulations of ring current evolution during the Geospace Environment Modeling challenge events, J. Geophys. Res., 111, A11S10, doi:10.1029/2006JA011644.

Ilie, R., Liemohn, M. W., Toth, G., and Skoug, R. M. (2012), Kinetic model of the inner magnetosphere with arbitrary magnetic field, J. Geophys. Res., 117, A04208, doi:10.1029/2011JA017189.

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