Winter anomaly in NmF2 and TEC: when and where it can occur Yury V. Yasyukevich, Anna S. Yasyukevich, Konstantin G. Ratovsky, Maxim V. Klimenko, Vladimir V. Klimenko and Nikolay V. Chirik J. Space Weather Space Clim., 8 (2018) A45 Published online: 23 October 2018 DOI: 10.1051/swsc/2018036