In today’s warming climate, much attention is paid to models of Earth’s atmosphere—but models of Earth’s ionosphere are crucial, as well. Stretching from altitudes of roughly 50 kilometers out into space for thousands of kilometers, the bubble of plasma surrounding Earth affects climate and can wreak havoc on civilian and military communications.
One of the mostly widely used ionosphere models is the International Reference Ionosphere (IRI), a community project that has been continuously maintained and revised since its release in 1978 through its most recent version in 2012. So far, attempts by other scientists to evaluate the performance of this latest release have given results that vary by region. Over Uganda, the model performed worst when the Sun was least active during its 11-year cycle of activity—most recently, 2008–2009. However, over Brazil, the opposite was true, and the model seemed to perform best during that period.
Now Kumar offers the first truly global assessment of the IRI-2012 model. He compared model predictions of the density of electrons in the ionosphere to measurements collected by seven ground GPS stations in 2009, at solar minimum, and in 2012, when solar activity was ramping up. These stations spanned the globe near the equator—three in the Americas, one in Africa, and three in Asia and the Pacific islands. These stations used signals from GPS satellites that traverse the ionosphere to measure its actual electron content.
The author found that the IRI model tends to overestimate the density of the ionosphere on monthly and seasonal time scales. Although this error was present in 2009, it was even larger in 2012, when solar activity was higher. The gap was biggest during the December solstice and smallest during the March equinox and was higher in the western longitudes than in the eastern longitudes.
When it comes to the 2009 error, the author says that it may be due to the particular measure of solar activity that the IRI model uses—specifically, the output of the Sun at a particular radio frequency. That index, F10.7, is intended to account for the Sun’s extreme ultraviolet radiation, which can ionize particles and replenish the ionosphere. It might not be the most accurate indicator, however, and the author suggests that future versions of the model should incorporate better ways to measure solar activity. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA022269, 2016)
—Mark Zastrow, Freelance Writer