Ionospheric oscillations with a quasi‐period of ∼6 hr during intense magnetic storms

We study the variations of the topside ionospheric ion density measured by Defense Meteorological Satellite Program satellites during the intense magnetic storm on 7-10 November 2004. It is found for the first time that quasi-periodic enhancements in the ion density with a period of SIM;6 hr occur nearly simultaneously at 0630, 0830, and 0930 local time in the dawn sector during the storm main phase with southward interplanetary magnetic field (IMF). The quasi-periodic density enhancements extend from the southern subauroral latitudes to the northern subauroral latitudes. In the dusk sector, the topside ion density during the storm main phase is increased at middle latitudes for SIM;12 hr but shows decrease or relatively small increase over the magnetic equator, indicating that penetration electric fields dominate the ion density redistribution. Similar quasi-periodic enhancements in the topside ion density are also observed in the dawn sector during other intense magnetic storms. The solar wind and IMF do not have quasi-periodic variations in this storm case. Periodic processes in geospace, such as periodic substorms in the magnetosphere, waves and tides in the atmosphere, and traveling ionospheric disturbances, cannot explain the observed periodic enhancements of the ionospheric ion density. We suggest that the magnetosphere-ionospheric-thermospheric system may have an intrinsic period of SIM;6 hr and that oscillations of the magnetosphere-ionospheric-thermospheric system with this period can be excited during intense magnetic storms, although the mechanisms for the generation of the long-periodic oscillations are not understood.