What is a dynamic spectrum measurement?

[TheCanadian]

I was going through Figure S9 in this paper yet don't quite understand how one can assign a probability density to a dynamic spectrum measurement as the authors do. To my knowledge, a dynamic spectrum simply displays the intensity for each frequency channel against time as shown in Figure 1C of the same paper. Although I'm a bit confused as to how the dynamic spectrum is then given a probability density in Figure S9. The authors go on to further describe this quantity as having an exponential distribution, and mention a tail in the graph, although I don't quite understand how an exponential tail relates to something physical here. Any thoughts on what a probability distribution in the context of a dynamic spectrum means?

 

[Greg Bernhardt]

A dynamic spectrum measurement is a measurement of the spectrum of a signal that is changing over time. Dynamic spectrum measurement is very difficult because the spectrum of a signal is changing over time and the spectrum must be measured at the same time as the signal is changing.