Radio and optical observation of polar cap tongue of ionization during the geomagnetic storm on December 15, 2006

During intervals of the southward IMF conditions, enhanced anti-sunward convection at the high-latitude carries solar-produced daytime plasma into the polar cap ionosphere. This feature is known as a tongue of ionization (TOI) and presents a significant space weather problem because the plasma density irregularities within and/or surrounding the structure produce a disturbed scintillation environment for communications links crossing the polar ionosphere. TOI is generally broken into discrete polar cap patches by the transient bursts of magnetopause reconnection or reorientation of the dayside convection pattern associated with changes in IMF orientation before it penetrates deep into the dark hemisphere. During geomagnetic storms, however, TOI is sometimes seen to penetrate into the nightside auroral latitudes through the central polar cap as an elongated plume of enhanced ionization. In this paper, we present first simultaneous radio and optical observations of TOI structure extending from the dayside to the nightside polar cap. Optical manifestation of the elongated TOI plume was detected with an all-sky airglow imager of OMTIs (Optical Mesosphere Thermosphere Imagers) at Resolute Bay (74.73N, 265.07E; AACGM latitude 82.9N) during a geomagnetic storm on December 15, 2006. Optical data demonstrate that the TOI plume changed its shape very dynamically in close association with the polar cap convection streamlines as determined independently from the Super Dual Auroral Radar Network (SuperDARN). The absolute optical intensity of the TOI feature is approximately 1000 Rayleigh, which is much brighter than that of non-stormtime polar cap patches. This suggests that the source of plasmas within the TOI plume is far from the polar cap, that is in the mid-latitudes. In reality, mid-latitude SuperDARN radar in Hokkaido, Japan observed enhanced anti-sunward plasma flow at 65° magnetic latitude during this interval, which indicates that unusual equatorward expansion of the high-latitude two-cell convection boundary played an important role in entraining lower-latitude solar- produced plasma as a source population for the TOI plume. The IMF clock angle was stable throughout the interval of the TOI plume, which implies that the dayside convection pattern did not change its spatial distribution with time. This suggests that the fragmentation of TOI into discrete polar patches was not likely to occur and then TOI could penetrate deep into the polar cap without being chopped. We will discuss, through this case example, differences of continuous TOI plume and discrete polar cap patches in terms of stormtime expansion of the high-latitude convection pattern and variability of IMF orientation.