ionization density

For the purpose of estimating radiation risk to an organ, the quantity characterizing the ionization density is the LET (keV/μm).

Carpenter, D. L., Whistler evidence of a 'knee' in the magnetospheric ionization density profile, J. Geophys.

The amount of bending depends on the extent of penetration (which is a function of frequency), the angle of incidence, polarization of the wave, and ionospheric conditions, such as the ionization density.