On the correlation between electron density and temperature in the topside ionosphere through Swarm satellites data

Electron density (N_e) and electron temperature (T_e) observations collected by Langmuir Probes (LPs) on board the European Space Agency’s Swarm satellites are used to characterise the N_e−T_e correlation in the topside ionosphere. The large dataset of Swarm LPs in-situ observations at 2-Hz rate, covering the years 2014−2021, allowed us to investigate the correlation properties of the topside ionospheric plasma for different diurnal and seasonal conditions, with a coverage and a detail never reached before. Spearman correlation coefficients (R_Spearman) are calculated on joint probability distributions between N_e and T_e for specific conditions. Results are given as maps of R_Spearman as a function of the Quasi-Dipole (QD) magnetic latitude and magnetic local time (MLT) coordinates, for different seasons. This study highlights, for the first time, the N_e−T_e correlation at high latitudes, and provides a global description of the corresponding diurnal trend for different seasons. A negative correlation is found at the equatorial morning overshoot, during daytime at mid latitudes, and during night-time at subauroral latitudes (ionospheric trough). Conversely, a positive correlation dominates the night-time sector at mid and low latitudes, and to a minor extent the low latitudes from 09:00 MLT onwards. A seasonal dependence of the correlation is visible only at very high latitudes where the general pattern of anti-correlation is broken around ±75° QD latitude in the summer season.