Critical frequency of F-layer decreases in association with geomagnetic storms. This effect depends on seasons and is pronounced in summer season.
SWF (Dellinger effect):
X-ray and UV emmisions from solar flares ionize the D-layer of the day-side ionosphere, causing the interuption of radio communications due to the absorption of HF radio waves at the D-layer.
Sporadic E-layer (Es):
Sparadic E-layer (Es) is a thin layer of high electron density emerging "sporadically" at the height of the usual E-layer. Es often appears in summer season.
