Number of data points used for TEC winter/summer ratio calculation at different geomagnetic conditions: (a), (b) K _p in the last 24 h did not surpass 3; (c), (d) 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; (e), (f) 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. Bold gray curves are the geomagnetic equator and ±15° geomagnetic latitudes.