In the present study we have investigated whether deterministic chaotic behavior is exhibited by the GPS total electron content (TEC) fluctuations at midlatitude ionosphere, under various geophysical conditions, using nonlinear aspects. Local slopes of the logarithms of correlation sum for the time series of TEC representing the winter storm and quiet periods exhibit a clear plateau, indicating a fractal dimension of the attractor of the system (5.86 ¿ 0.09 for quiet time and 4.15 ¿ 0.03 for storm time, at 35¿N). The observed positive values of the Lyapunov exponent (for quiet and storm times at 35.1¿N, the values are 0.1695 ¿ 10-2 s-1 and 0.1167 ¿ 10-2 s-1, respectively) are another important criterion for chaoticity of the system during the above periods. To further confirm this, we have employed the surrogate data test. On the basis of the significance of difference of the original data and surrogates for various aspects, the surrogate data test rejects the null hypothesis that the time series of TEC during storm and quiet times represent a linear stochastic process. Also, we compared the chaotic behavior of TEC during disturbed and quiet periods, under different seasons and latitudes. For all seasons, the values of the Lyapunov exponent during quiet times are greater than those during storm periods for the latitude sector, 35¿--41¿N. The nonlinear aspects presented here imply that geomagnetic disturbances influence the stability/instability conditions of ionosphere due to the superposition of various active degrees of freedom generated by an external stochastic driver (solar wind), and could alter the inherent dynamics of the system, if the coupling is powerful.