Storm enhanced density: magnetic conjugacy effects

Abstract. In the early phases of a geomagnetic storm, the low and mid-latitude ionosphere are greatly perturbed. Large SAPS electric fields map earthward from the perturbed ring current overlapping and eroding the outer plasmasphere and mid-latitude ionosphere, drawing out extended plumes of storm enhanced density (SED). We use combined satellite and ground-based observations to investigate the degree of magnetic conjugacy associated with specific features of the stormtime ionospheric perturbation. We find that many ionospheric disturbance features exhibit degrees of magnetic conjugacy and simultaneity which implicate the workings of electric fields. TEC enhancements on inner-magnetospheric field lines at the base of the SED plumes exhibit localized and longitude-dependent features which are not strictly magnetic conjugate. The SED plumes streaming away from these source regions closely follow magnetic conjugate paths. SED plumes can be used as a tracer of the location and strength of disturbance electric fields. The SED streams of cold plasma from lower latitudes enter the polar caps near noon, forming conjugate tongues of ionization over the polar regions. SED plumes exhibit close magnetic conjugacy, confirming that SED is a convection electric field dominated effect. Several conclusions are reached:

1) The SED plume occurs in magnetically-conjugate regions in both hemispheres.

2) The position of the sharp poleward edge of the SED plume is closely conjugate.

3) The SAPS electric field is observed in magnetically conjugate regions (SAPS channel).

4) The strong TEC enhancement at the base of the SED plume in the north American sector is more extensive than in its magnetic conjugate region.

5) The entry of the SED plume into the polar cap near noon, forming the polar tongue of ionization (TOI), is seen in both hemispheres in magnetically-conjugate regions.