Regions where one may observe the winter anomaly (blue color) in TEC (a−c) and N _m F₂ (d). (a) K _p in the last 24 h did not surpass 3; (b) K _p did not surpass 3 in the last 48 h, except recording 3 < K _p ≤ 6 at least once in the last 24 h; (c) K _p in the last 24 h did not surpass 3, but in the last 48 h it was recorded 3 < K _p-1 ≤ 6. For N _m F₂ (d) there was no K _p separation. Bold gray curves are the geomagnetic equator and ±15° geomagnetic latitudes.