Abstract
Abstract. We analyse reprocessed electron density profiles and total electron content (TEC) profiles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3). The TEC profiles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the profiles in the magnetic polar regions show small-scale fluctuations on spatial scales <50 km. Possibly the trajectory of the tangent point intersects spatial electron density irregularities in the magnetic polar region. For derivation of the morphology of the electron density and TEC fluctuations, a 50 km high-pass filter is applied in the s domain, where s is the distance between a reference point (bottom tangent point) and the tangent point. For each profile, the mean of the fluctuations is calculated for tangent point altitudes between 400 and 500 km. At first glance, the global maps of ΔNe and ΔTEC are quite similar. However, ΔTEC might be more reliable since it is based on fewer retrieval assumptions. We find a significant difference if the arithmetic mean or the median is applied to the global map of September 2013. In agreement with literature, ΔTEC is enhanced during the post-sunset rise of the equatorial ionosphere in September 2013, which is associated with spread F and equatorial plasma bubbles. The global map of ΔTEC at solar maximum (September 2013) has stronger fluctuations than those at solar minimum (September 2008). We obtained new results when we compare the global maps of the quiet phase and the storm phase of the geomagnetic storm of 15 July 2012. It is evident that the TEC fluctuations are increased and extended over the southern magnetic polar region at the day of the geomagnetic storm. The north–south asymmetry of the storm response is more pronounced in the upper ionosphere (ray tangent points h = 400–500 km) than in the lower ionosphere (ray tangent points h = 200–300 km).
Highlights
GPS radio occultation can be regarded as a bistatic limb sounding of the atmosphere where the transmitter is on a GPS satellite and the receiver is on a low Earth orbit (LEO) satellite
The study is based on reprocessed profiles of electron density (Ne) and total electron content (TEC) from the COSMIC mission
We applied a special analysis method to extract the spatial fluctuations in electron density and total electron content where we calculate the mean value of the absolute values of the 50 km high-pass-filtered fluctuations in the altitude region from 400 to 500 km
Summary
GPS radio occultation can be regarded as a bistatic limb sounding of the atmosphere where the transmitter is on a GPS satellite and the receiver is on a low Earth orbit (LEO) satellite. Since the GPS radio occultation technique performs atmospheric limb sounding, the vertical resolution is about 1 km or better in the troposphere. GPS radio occultation was already utilized to derive global maps of sporadic E layers around 90–120 km altitude (Arras et al, 2010; Wu et al, 2005; Hocke et al, 2001). Our study aims to retrieve global maps with the amplitude of smallscale ionospheric irregularities with scales from 2 to 50 km in the ionospheric F2 region between 400 and 500 km altitude using the GPS radio occultation technique. F region irregularities induce phase and amplitude scintillations in radio signals. Aarons (1982) showed a scheme of a global scintillation map where the scintillations are strong at high geomagnetic latitudes in the polar caps or after sunset around the ge-
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