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Offset and titled intrinsic magnetic field

Intrinsic Source of Asymmetry

Relative Importance - Primary

Model Compatibility Improved by CUSIA


Description:

Though frequently approximated as a dipole, the intrinsic magnetic field of the Earth is tilted approximately 11.7° relative to the Earth’s rotation axis (which is tilted relative to the solar ecliptic plane by ~23.5°). Further, the field is offset relative to the center of Earth, leading to asymmetric field strengths between hemispheres. Finally, the Earth’s field is only dipolar to low order. Regions such as the South Atlantic Anomaly, where the Earth’s field is especially weak, exist and add further complexities to the system.

Asymmetries in the Earth’s intrinsic field is a primary source of interhemispheric asymmetries in the thermosphere-ionosphere-magnetosphere. It is independent of any other drivers of interhemispheric asymmetries.  The field is driven by the Earth’s geodynamo.  The field is not static; the magnetic poles drift as a function of time.  Further, the geomagnetic poles (approximations of the magnetic poles by assuming a centered dipole), also drift with time, but to a lesser extent.  The figure below summarizes the motion of the magnetic and geomagnetic poles from 1900 to their predicted predictions in 2025.

Motion of the geomagnetic and magnetic poles since 1900. Data obtained from http://wdc.kugi.kyoto-u.ac.jp/poles/polesexp.html

Solar EUV Conductance: The tilt adds diurnal variation to the amount of solar extreme ultraviolet exposure of the polar magnetic regions; this is amplified by seasonal effects. Therefore, photoionization and the resulting ionospheric conductance has an intrinsic diurnal variation.

Precipitation: The offset and non-dipolar terms of the Earth’s intrinsic field mean that field strengths and geometry will be subtly different in the northern and southern hemisphere. Critically, at a given altitude, the field strength will be different at the northern field foot point then the southern conjugate point. The magnetic mirror points will be at different altitudes in either hemisphere; alternatively stated, the loss cones will be different in the northern and southern hemisphere, all other things held equal. The figure below illustrates this phenomena.

Courtesy of Dr. Mei-ching Fok

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.

It should be noted that at high altitudes (i.e., above typical MHD inner boundaries), non-dipolar terms and dipole offset may not have a strong impact.  This is especially true during storm times, where external driving produces a strongly non-dipolar field in the magnetosphere.  More work is required to fully explore this.

A table of capabilities of the most popular models are listed below.

BATS-R-US MHD


Includes dipole tilt only

LFM MHD


Includes dipole tilt only

OpenGGCM


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

GUMICS


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



Includes dipole tilt only

References:

AGU’s Diversity, Equity and Inclusion Dashboard Baseline Data across AGU Programs April 2021,

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