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IMF By Component

Intrinsic Source of Asymmetry

Relative Importance – Primary


The Interplanetary Magnetic Field can have an east-west, or By, component.  This component of the magnetic field can produce a range of interhemispheric asymmetries.  It is one of the most important intrinsic sources of asymmetries in the magnetosphere-ionosphere-thermosphere system.

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

Reconnection Line: Reconnection depends on the IMF orientation. Figure 1, adapted from Petrinec et al., 2020, shows the maximum shear angle, through the color scale, and the probable location of reconnection as indicated by the black line. The left-hand panel shows that reconnection tends to occur at low magnetopause latitudes for strongly southward IMF. The right-hand panel shows that high magnetopause latitude is preferred when the IMF By dominates. In this case By is in the negative YGSM direction, and reconnection would occur pre-noon for the northern hemisphere and post-noon for the south.

Location of maximum shear (adapted from Petrinec et al., 2020).

Auroral structure and Joule heating: Additionally, rotation of the IMF By from one sign to another can lead to complex magnetic topologies that allow plasma sheet electrons to precipitate into the polar cap, creating what is known as the “theta aurora.” It has also been hypothesized that when the IMF By becomes significant, there is a unipolar field-aligned current (FAC) in the round cell that closes at the bow shock as well to the other hemisphere’s polar region via an interhemispheric FAC. Hemispheric asymmetries in the geomagnetic field will lead to asymmetries in the magnetic topology in the magnetosphere, the convection and FAC pattern in the ionosphere, and auroral precipitation.

The asymmetry produces a difference in the energy deposition into each hemisphere’s polar ionosphere and thermosphere. For example, fast flow channels that map to reconnection in the same hemisphere under By-dominant conditions can lead to intense joule heating and the upwelling of neutral gas in the thermosphere [e.g. Crowley et al., 2010; Li et al., 2011; Wilder et al., 2012]. Hemispheric asymmetries in the geomagnetic field will alter the location and potentially the magnitude of this energy deposition. Additionally, asymmetric theta aurora will also lead to asymmetries in energy deposition due to particle precipitation.

Ionospheric potential: There is a distortion of the two-cell convection pattern in the ionosphere, with a round cell that is largely driven by reconnection in the same hemisphere and a crescent cell that maps to reconnection in the opposite hemisphere.

Birkeland current asymmetry: The shear produced by By causes the Birkeland currents to overlap asymmetrically as shown in the diagram below.

Shows expected shear in the Region-1 Birkeland currents associated with IMF-By effects (after Strangeway et al., 2000, which in turn was based on Cowley, 1981, and Burch and Reiff, 1985).


Modeling Capability:

All global MHD models can include IMF By in the solar wind time series that drive the code.  Many aspects of asymmetry produced by IMF By, such as shear in the Birkeland current patterns and asymmetric potential distributions, are captured to some extent by the global codes.  However, inner magnetosphere codes that depend on the polar ionosphere current use only the northern hemisphere solution.


IMF By included in input to MHD code


IMF By included in input to MHD code


IMF By included in input to MHD code


IMF By included in input to MHD code

IMF By included in input to MHD code


Crowley, G., Knipp, D. J., Drake, K. A., Lei, J., Sutton, E., and Lühr, H. (2010), Thermospheric density enhancements in the dayside cusp region during strong BY conditions, Geophys. Res. Lett., 37, L07110, doi:10.1029/2009GL042143.

Li, W., Knipp, D., Lei, J., and Raeder, J. (2011), The relation between dayside local Poynting flux enhancement and cusp reconnection, J. Geophys. Res., 116, A08301, doi:10.1029/2011JA016566.

Wilder, F. D., Crowley, G., Eriksson, S., Newell, P. T., and Hairston, M. R. (2012), Ionospheric Joule heating, fast flow channels, and magnetic field line topology for IMF By-dominant conditions: Observations and comparisons with predicted reconnection jet speeds, J. Geophys. Res., 117, A11311, doi:10.1029/2012JA017914.

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