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Asymmetric ionospheric potential
Dependent Source of Asymmetry
Relative Importance – Primary
Description:
The transfer of transverse momentum across the magnetopause to the ionosphere via Birkeland currents sets ionospheric plasma in motion due to the applied force. In the frame of Earth, the flowing plasma must have a self-consistent electric field associated with it, and the integral of this electric field across the polar cap is the transpolar potential. To the extent that the ionospheric flow patterns are asymmetric, the potential patterns in the two polar regions will exhibit the same asymmetry.
Causes: IMF By, Ionospheric Conductance
IMF By is the major cause of asymmetry in the polar cap convection patterns. Asymmetric ionospheric conductance can also be a source of asymmetry in polar cap convection patterns. However, the integrated potential produced by merging across the dayside and nightside reconnection must be the same in both hemispheres since that potential is identical to the rate that flux crosses the merging line, and the open flux in both polar caps must be identical.
Effects: Ionospheric Currents
In the northern hemisphere for positive By conditions at dawn, the convection cell is elongated or “banana-shaped”, and at dusk the convection cell is round or “orange-shaped”. The magnitude of the ionospheric potential and the associated Birkeland current in the orange cell is larger than the magnitude of the potential the associated Birkeland current in the banana cell. This pattern is reversed in the southern hemisphere. For negative By, both hemisphere potential patterns are the reverse of what they are for positive By. Thus, there will be asymmetries in the distribution of ionospheric electric field, resulting in asymmetries in the spatial distribution of electric current and ionospheric dissipation. The ionospheric potential asymmetry between the hemispheres means that in the inner magnetosphere, Region 2 current and ring current flux tubes will be mapping to a banana cell in one polar region, and an orange cell in the other. Field line foot points may connect to regions of different potential, producing an integrated potential drop from one hemisphere to the other. These effects are not understood.
Modeling Capability:
Numerical models enforce div B=0, so they capture the equal polar cap fluxes. Global MHD models generally capture the “orange” and “banana” cell convection pattern. They currently do not capture any field-aligned potential drops between ionospheres that may (or may not) exist. The proper determination of stress balance in the inner magnetosphere must take into account the two asymmetric convection patterns. No ring current model does this, since those models only use the northern polar region for magnetosphere-ionosphere coupling.
BATS-R-US MHD
Includes dipole tilt, IMF By, but with only one polar cap for ring current
LFM MHD
Includes dipole tilt, IMF By, but with only one polar cap for ring current
OpenGGCM
Includes dipole tilt (must be updated periodically to account for rotation), IMF By, but with only one polar cap for ring current
GUMICS
Includes dipole tilt (must be updated periodically to account for rotation), IMF By, but with only one polar cap for ring current
CIMI
Not included since the coupling is to only one polar cap.
RCM
Includes dipole tilt, IMF By, but with only one polar cap for ring current
References:
Lu, G., Richmond, A. D., Emery, B. A., Reiff, P. H., de La Beaujardiere, O., Rich, F. J., … & Tomlinson, L. (1994). Interhemispheric asymmetry of the high‐latitude ionospheric convection pattern. Journal of Geophysical Research: Space Physics, 99(A4), 6491-6510. Ruohoniemi, J. M., & Greenwald, R. A. (1996). Statistical patterns of high‐latitude convection obtained from Goose Bay HF radar observations. Journal of Geophysical Research: Space Physics, 101(A10), 21743-21763. Papitashvili, V. O., & Rich, F. J. (2002). High‐latitude ionospheric convection models derived from Defense Meteorological Satellite Program ion drift observations and parameterized by the interplanetary magnetic field strength and direction. Journal of Geophysical Research: Space Physics, 107(A8), SIA-17. Tenfjord, P., Østgaard, N., Snekvik, K., Laundal, K. M., Reistad, J. P., Haaland, S., & Milan, S. E. (2015). How the IMF By induces a By component in the closed magnetosphere and how it leads to asymmetric currents and convection patterns in the two hemispheres. Journal of Geophysical Research: Space Physics, 120(11), 9368-9384.
