The Persistence of the Nighttime Winter Anomaly (NWA) Effect During the Low Solar Activity Period 2007-2009

Kurzfassung

The ionospheric Nighttime Winter Anomaly (NWA) was first reported more than three decades ago based on Total Electron Content (TEC) and vertical sounding data. The aim of this paper is to provide further evidence that the NWA effect is a persistent feature in the northern hemisphere at the American and in the southern hemisphere at the Asian longitude sector under low solar activity conditions. The analysis of ground based GPS derived TEC and peak electron density (NmF2) data from radio occultation measurements on Formosat-3/COSMIC satellites fully confirms and further supports the findings published in earlier NWA papers. So it has been confirmed and further specified that the NWA appears at longitude sectors where the displacement between the geomagnetic and the geographic equator maximizes. To be more precise, the NWA appears in that hemisphere where the geomagnetic latitude exceeds the geographic latitude. Here NWA peaks around 40°-50° geomagnetic mid-latitudes supporting the idea that wind induced plasma uplifting in the conjugated summer hemisphere is the main driving force for the accumulation of ionospheric plasma in the topside ionosphere and plasmasphere. In parallel the midsummer nighttime anomaly (MSNA) is caused after sunset at the local ionosphere. Simultaneously, inter-hemispheric coupling causes severe downward plasma fluxes in the conjugated winter- hemisphere during night strong enough to form the NWA at low solar activity (LSA). With increasing solar activity the downward plasma fluxes, although still present, lose their impact due to the much stronger increasing background ionization that masks the NWA completely and therefore making NWA invisible. It has been shown that MSNA and related special anomalies such as the Weddell Sea Anomaly and the Okhotsk Sea Anomaly introduced in this paper are formed by the same major ionospheric-thermospheric processes that cause also the NWA.