Record Lows in the Outer Atmosphere and Ionosphere

The solar cycle minimum period during 2008–2009 was the longest and quietest such period since the advent of space-based measurements, and probably the longest and quietest in a century. Additionally, the first half of the following solar cycle, starting in 2010, has also been the weakest the early 1900s. During 2008–2009, the temperature and density of the outer atmosphere were lower than during previous solar cycle minima, and the lowest they have been during the space age. This was caused by a reduction in solar extreme-ultraviolet irradiance (EUV), with lower geomagnetic activity and cooling from increased carbon dioxide levels also playing a role. Similar effects were observed in the ionosphere, with measurements showing that the ionosphere was unusually low in density, and the altitude of its peak also decreased. Model simulations of the outer atmosphere and ionosphere conducted by HAO scientists (Stan Solomon, Liying Qian, and Alan Burns*) demonstrated good agreement with observations of the atmospheric density at 400 km altitude throughout the solar cycle. Comparison of the ion density from these simulations with ionospheric measurements also showed consistent agreement. The global average peak ionospheric density was estimated to have declined by about 15 percent from the solar cycle minimum period in 1996 to the solar cycle minimum in 2008–2009 (Fig. 1). These changes on the Sun could be indicative of an extended period of lower-than-usual activity, or they could be confined to a single cycle, but in either event they have important effects on the ionosphere and on near-Earth space.

Funding for this work was sponsored by NASA SR&T NNX10AF21G.

Figure 1. Thermospheric temperature and density modeled by the NCAR TIE‐GCM for 1996 and 2008, on June 30. (a) Model temperature at 400 km altitude for 1996. (b) Model peak electron density of the F2 region NmF2 for 1996. (c) Model temperature at 400 km for 2008. (d) Model NmF2 for 2008. (e) Temperature difference, 2008-1996. (f) NmF2 ratio, 2008/1996.

*Solomon, S. C., L. Qian, and A. G. Burns (2013), The anomalous ionosphere between solar cycles 23 and 24, J. Geophys. Res. Space Physics, 118, 6524–6535, doi:10.1002/jgra.50561.