Schematic diagram of the asymmetric effect of ion response timescale. The vertical dashed line is the boundary between the low ionized state and the high ionized state. In a sufficiently low ionization state, i.e., n_i + 1/n_i ≪ α_i − 1/α_i or I_i ≪ n_eα_i − 1, the response timescale is t_i^* = 1/(fn_eα_i − 1). In a sufficiently high ionization state, in contrast, i.e., n_i + 1/n_i ≫ α_i − 1/α_i or I_i ≫ n_eα_i − 1, the response timescale is $t_i^{\ast }=-1/(f{\alpha }_i{n}_e\frac{n_{i+1}}{n_i})=-1/(f{I}_i)$. In highly ionized state, the response timescale is shorter than that of low ionized state, leading to the detection of more negative responses.