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GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 24, 2260, doi:10.1029/2003GL018175, 2003

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

M. T. Johnson

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

J. R. Wygant

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

Abstract

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.

Received 14 July 2003; accepted 14 October 2003; published 20 December 2003.

Index Terms: 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2776 Magnetospheric Physics: Polar cap phenomena; 2467 Ionosphere: Plasma temperature and density.

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Citation: Johnson, M. T., and J. R. Wygant (2003), The correlation of plasma density distributions over 5000 km with solar illumination of the ionosphere: Solar cycle and zenith angle observations, Geophys. Res. Lett., 30(24), 2260, doi:10.1029/2003GL018175.

Copyright 2003 by the American Geophysical Union.