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Asymmetric electron precipitation driven by asymmetric magnetic field

Intrinsic Source of Asymmetry

Relative Importance - Primary


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. 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. Different magnetic field strengths at the north and south foot points of a magnetic field line change the height at which a particle will be lost thus creating different loss cone sizes at the north and south conjugate points.

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.

A graph looking at one of Earth's magnetic field lines, and the strength of the magnetic field. This shows how there is an asymmetry due to the dipole offset and non-dipole terms which can change the mirror height and thus size of the loss cone for a population of particles as they bounce from one hemisphere to another.
A graph looking at one of Earth’s magnetic field lines, and the strength of the magnetic field. This shows how there is an asymmetry due to the dipole offset and non-dipole terms which can change the mirror height and thus size of the loss cone for a population of particles as they bounce from one hemisphere to another.

 

The resulting precipitation will be different in the northern and southern ionosphere:

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.







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