Long-term analysis of ionospheric polar patches based on CHAMP TEC data

Total electron content (TEC) from LEO satellites offers great possibility to sound the upper ionosphere and plasmasphere. This paper describes a method to derive absolute TEC observations aboard CHAMP considering multipath effects and receiver differential code bias. The long-term data set of 9 years GPS observations is used to investigate the climatological behavior of high-latitude plasma patches in both hemispheres. The occurrence of polar patches has a clear correlation with the solar cycle, which is less pronounced in the Southern Hemisphere (SH). Summed over all years, we observed a higher number of patches in the SH. The maximum occurrence rate of patches has been found at the dayside polar cusp during 12:00–18:00 MLT (magnetic local time) supporting the mechanisms for patch creation by local particle precipitation and by intrusion of subauroral plasma into the polar cap through tongues of ionization (TOIs). The latter mechanism seems to be even more important in the SH. Investigating the patches in comparison with interplanetary magnetic field (IMF) conditions, we found that decreased IMF Bz and enhanced merging electric field preceded the patch observation; hence, patch creation follows a period of enhanced solar wind input into the magnetosphere/ionosphere. We further found an annual cycle in patch occurrence with maxima at equinox and December solstice and a June solstice minimum which reflects the global ionospheric seasonal asymmetry in electron density. We suggest that enhanced TEC at midlatitudes and low latitudes during December solstice provides a greater possibility to transport high-density plasma to the polar region through the buildup of TOIs.