For F1 layer maximum ionic density is 2.3 × 106 electrons

For F¹ layer maximum ionic density is 2.3 × 10⁶ electrons per c.c. The critical frequency for this layer will be—

For calculation of critical frequency we have relation as follows
f_c = 9√N_max
= 9√2.3 × 10⁶× 10⁶
= 9× 10⁶√2.3
= 13.6MHz