I refer to:
and the 130308 Io-B spectrum above, published by Wesley Greenman (many thanks), showing prominent Faraday modulations.
The number of Faraday rotations is inversely proportional to the square of the frequency. Let us assume Faraday rotation is caused mainly the the terrestrial ionosphere, and that over a period of about 4 minutes, the magnetic field intensity, zenith angle of the line of sight, and magnetic field angle with the line of sight changed little. A more accurate analysis should take into account the changes.
Then, the integrated ionospheric electron concentration (electrons per meter^2) is directly proportional to the square of the frequency, along a Faraday band.
Example: 130308 Io-B spectrum published by Wesley Greenman.
Pixels are measured from the upper left corner of the image, right and down, coordinates are (time, frequency).
Time scale = 1 pixel per second
Frequency scale = 50 pixel per MHz
There is a dark band at pixel (200,36)
Later, the same dark band is at pixel (433,61)
Time change = 233 pixel = 233 second
Frequency change = 25 pixel = 0.5 MHz
25 MHz tick is at (600,50)
The electron concentration decreased, changing by ~ (24.5/25.0)^2 = 0.96 = a 4% decrease in 233 seconds.
Many thanks to Wesley Greenman for the wonderful data he publishes in the Data List