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 wave 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 electron density 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 greater electron density at higher latitudes, consistent with cosmic ray flux that incrfeases with latitude, whereas the limited rocket data show a maximum 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 underestimate 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.