The correlation of plasma density distributions over 5000 km with solar illumination of the ionosphere: Solar cycle and zenith angle observations

Measurements from the Polar electric field instrument are used to infer the thermal electron density of the polar cap for invariant latitudes (ILAT) > 80° as function of solar zenith angle (SZA) over a solar cycle. Results for altitudes from 1.75 to 2.0 R_E geocentric show an increase in the plasma density for daytime ionospheric measurements as the solar cycle activity increases. Densities are found to increase from ∼75 cm^-3 at solar minimum by over a factor of 2 at solar maximum. The density measurements are also shown to increase with increasing solar F_10.7 flux for SZA < 70° and remain relatively constant for SZA > 110°. These measurements are consistent with solar extreme ultraviolet (EUV) control of plasma density at these altitudes over the polar cap. The influence of solar EUV on plasma density is shown to extend to altitudes up to 4.5 R_E geocentric.