Small-scale fluctuations and scintillations in high-resolution GPS/CHAMP SNR and phase data

This paper analyzes small-scale fluctuations that appear in the signal-to-noise ratio (SNR) and phase measurements during GPS/CHAMP radio occultation through the E-region ionosphere, lower stratosphere, and upper troposphere. Analyses are based on the raw 50-Hz SNR and phase data, which reveal new information on variations and distributions of small-scale atmospheric/ionospheric variabilities that would be normally discarded in the retrieved temperature profiles. The derived SNR and phase variances show strong annual and interannual variations in the ionosphere due to solar-cycle modulated sporadic-E activity. The intensity of polar Es activity reduced gradually since 2001, as solar activity weakened from the 2000 maximum. In the upper troposphere, the small-scale SNR and phase variances maximize near the tropical tropopause and vary strongly with seasonal variations of the tropopause. In the tropical lower stratosphere, the variances exhibit a quasi-biannual oscillation (QBO) with amplitude maximized at altitudes of 15-30 km and progressing downward in time. The downward-progressing amplitude occurs just below the height of zero-wind line where QBO changes phase from the easterly to the westerly. Physical interpretation of SNR and phase variances is made with analytical expressions derived for idealized small-scale ionospheric and atmospheric perturbations. In these cases, the SNR standard deviation is inversely proportional to vertical wavelength of perturbations whereas the phase one is proportional to the truncation length used for variance calculations.