A proton whistler can only be detected in spacecraft measurements above Earth's ionosphere. The proton whistler will occur immediately after an upward-propagating whistler has been generated by a lightning discharge. It is distinct from the more common, lightning-generated whistler both in tone and spectral characteristics.

Unlike the lightning-generated whistler, the proton whistler consists of a long, slowly rising tone that begins at a low frequency and levels off in a monotone at a frequency just below the proton cyclotron frequency, a characteristic frequency of the ambient plasma. The tone will typically last several seconds.

This example is from the University of Iowa Injun 3 spacecraft.