Coupling Between Sudden Stratospheric Warming and Mid‐Latitude Ionosphere of European Sector ‐ A Case Study

Abstract The 10 hPa level is conventionally used for determining sudden stratospheric warming in studies. Our study examines the winter of 2005–2006 from 1 to 10 hPa pressure levels, identifying three significant warming episodes in January 2006 instead of only one based on the 10 hPa level definition. The first two episodes with temperature peaks occurring around 3 and 10 January, respectively, are identified at the upper stratosphere, where they are stronger than the official warming peak on 22 January defined at 10 hPa. Increased wave activity at these upper levels, derived from the Eliassen‐Palm flux divergence, highlights the intensity of warming and circulation changes at the upper stratospheric layers. Ionospheric analysis of January 2006 revealed substantial impacts on the F2 layer ionosphere, particularly in Juliusruh. These effects are linked to planetary wave periodicities detected in wavelet spectrum analysis. These periodicities are noted in the temperature and wind, predominantly in the upper stratosphere. Further analysis employs a wavelet coherence between the upper stratosphere and the critical frequency of the F2 layer that affirms that the planetary‐scale periodicities observed in the upper stratosphere are mirrored in the F2 layer mid‐latitude ionosphere over station Juliusruh. These findings suggest a relationship between the upper stratosphere and the ionosphere, particularly at stations near the climatological vortex edge around 60°N. The novelty of this paper is that for studying the impact of sudden stratospheric warmings on the mid‐latitude ionosphere, it is better to use the major SSW definition at upper stratospheric levels rather than the traditional 10 hPa level.