sentences:  It was later reported that the electrostatic high-frequency fluctuations near the double layer can be concentrated in a narrow region, sometimes called the hf-spike.Subsequently, both radio emissions, near the plasma frequency, and whistler waves at much lower frequencies were seen to emerge from this region.Similar whistler wave structures were observed together with electron beams near Saturn's moon Enceladus, suggesting the possible presence of a double layer at lower altitude.