An Intercomparison of VLF and Sounding Rocket Techniques for Measuring the Daytime D Region Ionosphere: Theoretical Implications

We compare the two approaches that have been used to measure the lowermost ionosphere,
the measurement of the propagation of very low frequency (VLF) radio waves and the in situ sampling
by sounding rockets. We focus on the altitude, latitude, and zenith angle variation of the electron density
profiles inferred from these two observational techniques as compared with a theoretical photochemical
model. Our results show that below 68–70 km, the VLF data and the model agree better with each other
than with the sounding rocket profile. At the lowest altitudes, near 60 km, both the VLF data and the model
show a greater electron density at higher latitudes, consistent with a cosmic ray flux that increases with
latitude, whereas the limited rocket data show a maximum at the tropics. Above 68–70 km, the VLF data and
the sounding rockets agree better and at tropical latitudes, the model fails to reproduce the observations.
Specifically, the calculated electron density is lower than the data by up to a factor of 2. Possible reasons
for the model deficit include underestimates of the solar Lyman alpha flux, the solar X-ray flux and the
mesospheric nitric oxide density. Once these three factors are mitigated, the model is in agreement with the
observations between 60 and 80 km.