Structure of the Earth's lower ionosphere observed by GPS/MET radio occultation

Abstract

The application of the radio occultation technique to the lower ionosphere is described and analyzed by means of the international reference ionosphere (IRI) and the GPS/Meteorology (GPS/MET) experiment. Electron density profiles are derived from total electron content (TEC) measurements along radio links between GPS satellites and a GPS receiver on board the low Earth orbit satellite Microlab‐1 during radio occultations. The retrieval method is based on Abel inversion and requires only spherical symmetry for the lower ionosphere. The profiles are sorted for geographic latitude, noon, and midnight in June–July 1995 and February 1997. By means of a sliding window average, meridional slices of electron density are calculated from the sorted profiles. GPS/MET slices of electron density agree better with corresponding IRI slices for noon than for midnight. This is mainly due to the variable nature of particle precipitation from the magnetosphere, which is a significant ionization source, in particular for the lower ionosphere at high latitudes around midnight. Depending on ionospheric conditions, the applied retrieval method seems to be limited to the recovery of absolute electron density values exceeding 109–1010 m−3. This is possibly due to incomplete separation of E and F region contributions to the refraction of the GPS signal. Around noon, GPS/MET and IRI observe/predict a similar base height of the E region as well as similar absolute values of electron density in the E region. The radio occultation data also permit retrieval of meridional slices of average plasma irregularities (vertical scales <7 km) of the lower ionosphere at noon, midnight, winter, and summer.