Electric field from solar wind/IMF system is mapped (directly over polar caps or indirectly via magnetosphere, see magnetospheric electric field) to the high-latitude ionosphere, creating horizontal E × B plasma drift called convection. The actual convection pattern seen depends on the direction of IMF; see also magnetic field reconnection and solar wind - magnetosphere - ionosphere coupling.
During southward IMF, the reconnection between the IMF and Earth's magnetic field is thought to occur at the dayside magnetopause. In the simplest case, the F-region plasma flows antisunward over the polar caps (open field lines, i.e., solar wind regime), and returns sunward within the auroral oval (closed field lines, i.e., magnetospheric regime). This creates the two cell pattern relating to the convection electrojets in the highly conducting auroral region (see, e.g., Clauer and Kamide, 1985; also term ionospheric DP 2 or "disturbance polar of the second type" current system has been used). The two cell pattern is most clear during the growth phase of a substorm.
In terms of electric fields, we have duskward pointing field over the polar cap, and dawnward field at the auroral latitudes.
The situation is more complex during northward IMF, when reconnection is possible only between the IMF and the open field lines of the magnetotail: the ionospheric convection is much more structured, confined to much higher latitudes (polar cap is smaller), and velocities are of smaller magnitude than in the case of southward IMF. This results in four cell or three cell ionospheric convection pattern for strongly or weakly northward IMF, respectively.
It is usually thought that the high latitude cells (that are on open field lines all the time) are due to reconnection, and the low latitude cells are formed due to viscous interaction (Axford and Hines, 1961). In this process the momentum of the magnetosheath plasma is transmitted across the magnetopause by waves and diffusion, creating an effective viscosity. The untisunward convection takes place in closed field lines in equatorial boundary layer. The return flow towards the sun occurs on lower latitude field lines and is driven by back pressure in the magnetosphere near midnight. Observations of just the ionospheric convection signature cannot distinguish between viscous and reconnection based convections, but the measured high cross-polar cap potentials and the dependence of the convection geometry on IMF By suggest that the merging process dominates over viscous interaction when the IMF has a southward component. However, during northward IMF the relative importance of the viscous interaction is larger.
Effects of By
Changing By creates asymmetry in the dawn-dusk direction, as the figures above show. See, e.g., Reiff and Burch (1985).
- Axford, W. I. and C. O. Hines, A unifying theory of high-latitude geophysical phenomena and geomagnetic storms, Canadian J. Phys., 39, 1433-1464, 1961.
- Clauer, C. R., and Y. Kamide, DP 1 and DP 2 current systems for the March 22, 1979 substorms, J. Geophys. Res., 90, 1343-1354, 1985.
- Cramoysan et al., Ann. Geophysicae, 13, 583-, 1995.
- Eather, R. H., Polar cusp dynamics, J. Geophys. Res., 90, 1569-1576, 1985.
- Heelis, R. A., The effects of interplanetary magnetic field orientation on the dayside high-latitude ionospheric convection, J. Geophys. Res., 89, 2873-2880, 1984.
- Reiff, P. H., Sunward convection in both polar caps, J. Geophys. Res., 87, 5976-5980, 1982.
- Reiff, P. H. and J. L. Burch, IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere - 2. A global model for northward and southward IMF, J. Geophys. Res., 90, 1595-1609, 1985.