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• Four types of dynamic flame behaviors were observed and analyzed through phase and FFT analysis by applying AC electric fields radially to the nonpremixed coflow jet flame. • Double-peak oscillation was observed at weak electric fields of V AC = 2 kV, highlighting the role of asymmetric bidirectional ion wind in triggering flame oscillation. • Counter-rotating vortex induced oscillation was observed at a low frequency for V AC = 5 kV, and the frequency was found to depend linearly on Froude number and applied voltage. • Instability-induced extinction was caused by constructive interference between counter-rotating vortex induced oscillation and double-peak oscillation at f AC = 10 Hz for V AC = 5 kV. • Blowout occurred at a low frequency for V AC = 7 kV, due to necking of the fuel stream in the positive voltage section. • The critical frequency for observing stable flames in an AC electric field was obtained as 115 Hz based on the relationship between the normalized flame length per unit voltage and the applied frequency using the collision response time, which was consistent with previous research results. We investigated the impact of AC electric fields on a nonpremixed coflow-jet flame. To eliminate the azimuthal component of the ionic wind, we created a radially applied electric field using an axisymmetric electrode structure consisting of a nozzle and a cylindrical mesh surrounding the co-flow burner. By applying various voltages and frequencies, we identified four distinct flame behaviors: oscillating flame with applied frequency, fluctuating flame, instability-induced extinction, and blowout. Each behavior was analyzed and discussed using phase analysis, flame length variation over time synchronized with the polarity change of the applied voltage, and FFT analysis. When an AC voltage (V AC) of 2 kV was applied, the oscillating flame with applied frequency was observed regardless of the AC frequency (f AC). Notably, a double-peak oscillation was observed due to the asymmetric bidirectional ionic wind resulting from the polarity change. The fluctuating flame was exclusively observed at V DC = 5 kV with f AC = 1 Hz. In addition to the double-peak oscillation, an overlapping counter-rotating vortex-induced oscillation was observed, which was confirmed to be caused by the positive ions directed towards the nozzle in the negative voltage section. At V AC = 5 and 7 kV with f AC = 10 Hz, the oscillation frequency of the counter-rotating vortex induced oscillation (≈ 10 Hz) was almost identical to f AC , resulting in constructive interference, ultimately leading to the instability-induced extinction. Blowout occurred at V AC = 7 kV with f AC = 1 Hz, where the strong bi-directional ionic wind caused necking of the fuel stream and resulted in flame extinction. Finally, we derived a critical frequency for observing a stable flame regardless of the AC voltage by comparing and analyzing the collision reaction time with the applied frequency. [ABSTRACT FROM AUTHOR]