Spatial correlations of day-to-day ionospheric total electron content variability obtained from ground-based GPS

The spatial correlations of day-to-day ionospheric TEC variations for four 30-day long periods in 2004 (January, March/April, June/July, September/October) were determined using more than 1000 ground-based GPS receivers. The spatial correlations were obtained in a two-step process. Initially, the day-to-day variability was calculated by first mapping the observed slant TEC values for each 5-min GPS ground receiver-satellite pair to the vertical using a simple geometrical factor and then differencing it with its corresponding value from the previous day. This resulted in more than 150 million values of day-to-day change in TEC (Delta TEC). Next, statistics were performed on the Delta TEC values to obtain their spatial correlations. Our study indicates strong correlations between geomagnetic conjugate points, and these correlations are larger at low latitudes (r = 0.63-0.73) than at mid-latitudes (r = 0.32-0.43). Typical correlation lengths, defined as the angular separation at which the correlation coefficient drops to 0.7, were found to be larger at mid-latitudes than at low latitudes. The meridional correlation lengths are about 7 degrees and 4 degrees at middle and low latitudes, respectively. The zonal correlation lengths are approximately 20 degrees at mid-latitudes and 11 degrees at low latitudes. The correlation lengths are larger during daytime (1100-1300 MLT) than during nighttime (2300-0100 MLT). The results indicate that the spatial correlation is largely independent of season. These spatial correlations are important for understanding the physical mechanisms that cause ionospheric weather variability and are also relevant to data assimilation.