The anomalous ionosphere between solar cycles 23 and 24

Abstract

[1] The solar minimum period during [2008][2009] was characterized by lower thermospheric density than the previous solar minimum and lower than any previously measured. Recent work used the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model to show that the primary cause of density changes from 1996 to 2008 was a small reduction in solar extreme ultraviolet (EUV) irradiance, causing a decrease in thermospheric temperature and hence a contracted thermosphere. There are similar effects in the ionosphere, with most measurements showing an F region ionosphere that is unusually low in density, and in peak altitude. This paper addresses the question of whether model simulations previously conducted, and their solar, geomagnetic, and anthropogenic inputs, produce ionospheric changes commensurate with observations. We conducted a 15 year model run and obtained good agreement with observations of the global mean thermospheric density at 400 km throughout the solar cycle, with a reduction of~30% from the 1996 solar minimum to [2008][2009]. We then compared ionosonde measurements of the midday peak density of the ionospheric F region (N m F 2 ) to the model simulations at various locations. Reasonable agreement was obtained between measurements and the model, supporting the validity of the neutral density comparisons. The global average N m F 2 was estimated to have declined between the two solar minima by~15%. In these simulations, a 10% reduction of solar EUV plays the largest role in causing the ionospheric change, with a minor contribution from lower geomagnetic activity and a very small additional effect from anthropogenic increase in CO 2 .

An observational study of the response of the upper atmosphere of Mars to lower atmospheric dust storms

“…There is much evidence that indicates that Earth's thermosphere was much colder and lower in density than expected by previous minima (e.g. Solomon et al, 2010Solomon et al, , 2011Solomon et al, , 2013. This is also confirmed in the case of Mars.…”

The South Atlantic Anomaly throughout the solar cycle

“…8), we can clearly see the response of the flux of particles to the evolution of the thermospheric density. Thermospheric density is directly correlated with the Sun's activity, as a result of heating driven by solar radiation, showing the same time evolution as the solar cycle (Solomon et al, 2013). Density increase at high altitude as the atmosphere expands during solar maxima leads to more frequent collisions between the trapped energetic protons and atmospheric atoms.…”

“…Gledhill, 1976;Selesnick et al, 2014). The neutral atmosphere expands during the ascending phase of the solar cycle and, at altitudes above 100 km, its density increases (Solomon et al, 2013). As a result, in regions where mirror points are low enough, protons are removed from the inner belt through nuclear collisions with atmospheric neutral particles (Gledhill, 1976), explaining why the particle flux in the SAA region is anti-correlated with the solar activity as measured by the 10.7 cm radio flux of the Sun (Fürst et al, 2009;Casadio and Arino, 2011;Qin et al, 2014).…”

A new method for forecasting the solar cycle descent time

“…Many interesting observations of the sun and near-earth environment have been reported during this solar minimum (McComas et al 2008;De Toma et al 2010;Echer et al 2012;Solomon et al 2013;Fröhlich 2013;Hajra et al 2014). Haigh et al (2010) found that during the declining phase of solar cycle 23, there was a four to six times larger decline in ultraviolet emissions.…”