On a high-resolution wideband spectrogram, the whistler's characteristic spectral feature is a clearly defined tone descending rapidly in frequency over several seconds. The name "whistler" refers to this characteristic whistling sound in the audio frequency range.
The first spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on March 26, 1996. Initially the wideband receiver was connected to the electric Eu antenna, but was switched to the Bu magnetic search coil antenna at 07:59:06 UT. A series of brief whistlers is evident throughout this interval below 1.5 kHz.
The second spectrogram is a 48-second wideband spectrogram taken from a dayside plasmaspheric pass on May 10, 1996. The wideband receiver was connected to the magnetic loop antenna throughout this interval. Two clusters of whistlers of varying duration are seen below 8 kHz at 00:16:25 UT and 00:16:44 UT.
The third spectrogram is a 48-second wideband spectrogram taken from a nightside plasmaspheric pass on June 12, 1996. The wideband receiver was again connected to the magnetic loop antenna. Some whistlers can be seen up to 9 kHz (13:58:24 UT and 13:58:29 UT) and several more below 4 kHz (13:58:32 UT and 13:58:44 UT).