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IMF Bx Component

Intrinsic Source of Asymmetry

Relative Importance – Tertiary


The Interplanetary Magnetic Field can have sunward/ antisunward, or Bx, component.  This component of the magnetic field can produce some interhemispheric asymmetries.

Causes: IMF Bx

The IMF can have a significant and sustained Bx component because of the Parker spiral angle of the IMF (related to IMF By), or because of flux ropes in the solar wind associated with CMEs.

Auroral Brightness: Several studies report small but statistically significant interhemispheric asymmetries in the brightness of aurorae related to the orientation of Bx. These studies found that aurorae in the northern (southern) hemisphere were stronger when Bx and Earth’s dipole tilt were negative (positive) (Reistad et al., 2014; Shue et al., 2002).

Dayside Merging Line: Poynting flux through the magnetopause may be affected by the displacement of the dayside merging line due to IMF Bx (Hoilijoki et al., 2014).

Auroral Oval Location: Lobe reconnection will be asymmetric depending on the sign of IMF Bx, causing a displacement of the auroral oval in one hemisphere of the other (Stubbs et al., 2005).

Modeling Capability:

IMF Bx is difficult to include in the models because of the need to keep the divergence of the magnetic field equal to zero.  Different codes have taken different approaches to the problem.  This makes it difficult to quantify Bx-related effects in codes.  An example is the LFM MHD model, where IMF Bx is represented as a linear function of IMF By and Bz. This defines the incoming flow angle so that the divergence of B is zero.  LFM cannot model a pure Bx event since that would have the flow completely orthogonal to the Sun-Earth line.  Conversely, BATS-R-US MHD employs an eight-wave solver that maintains divergence of B=0 to machine precision.  This means that a pure Bx can be applied.  This is discussed in depth by Toth [2000].  GUMICS adds the solar wind value of Bx throughout the simulation grid so that there is never a divergence of issue.  Mechanics of including Bx aside, the full implications of Bx in global MHD models in terms of interhemispheric asymmetries remains largely unexplored.


Yes, via a linear combination of Bz and By, but not a pure Bx.


Yes, div-B is maintained by solver.


Yes, by adding the solar wind value of Bx throughout the simulation grid.

Yes, via a linear combination of Bz and By, but not a pure Bx.


Tóth, G. (2000). The ∇·B=0 Constraint in Shock-Capturing Magnetohydrodynamics Codes. Journal of Computational Physics, 161(2), 605–652. https://doi.org/10.1006/jcph.2000.6519

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