Attribution of interminima changes in the global thermosphere and ionosphere

Abstract

We present a statistical attribution analysis of the changes in global annual average thermospheric mass density and ionospheric total electron content (TEC) between the cycle 22/23 solar minimum (which occurred at epoch 1996.4) and the prolonged cycle 23/24 minimum (2008.8). The mass density data are derived from orbital drag, and the TEC data are derived from ground-based GPS receivers. The interminima change in mass density was À36% relative to the 1996.4 yearly average. Considering each multiplicative forcing independently, lower average geomagnetic activity during the cycle 23/24 minimum produced an interminima density change of at least À14%, solar extreme ultraviolet (EUV) irradiance forcing produced a density change of À1% to À13%, and changes in thermospheric CO 2 concentration produced a density change of À5%. There was essentially no interminima change in global TEC derived from ground-based GPS receivers or space-based altimeters, even though past behavior suggests that it should have changed À3% (0.2 TEC units (1 TECU = 10 16 el m À2 )) in response to lower geomagnetic activity and À1% to À9% (0.1-0.8 TECU) in response to lower EUV irradiance. There is large uncertainty in the interminima change of solar EUV irradiance; the mass density and TEC data suggest a plausible range of 0% to À6%.

Thermospheric mass density: A review

“…developed the Global Average Mass Density Model (GAMDM), which represents orbit-derived density as a function of altitude, solar flux, geomagnetic activity, and day of year. GAMDM was updated by Emmert et al (2014a) and Emmert (2015). H. Liu et al (2013b) created a model of CHAMP data as a function of altitude (between 350 and 420 km), solar flux, geomagnetic activity, day of year, latitude, local time, and longitude.…”

“…The climatological density dependence on solar EUV irradiance is encapsulated in the empirical models described in section 3.2. Additionally, it has recently been studied by , Bowman and Tobiska (2006), Guo et al (2007), Müller et al (2009), , and Emmert et al (2014a). Matsuo and Forbes (2010) performed an empirical orthogonal function (EOF) analysis of 2001-2008 CHAMP data; their results encompass not only the solar irradiance response, but also other major climatological variations.…”

“…6 compares the dependence of 1996-2005 global average log density at 400 km on F 10.7 with the corresponding dependence on SOHO/SEM 26-34 nm irradiance measurements. The density was modeled as described by Emmert et al (2014a), with a nonlinear dependence on the short-term irradiance and a piecewise linear dependence on the difference between the daily and 81 day average irradiance. As expected, the measured EUV irradiance produces a fit superior to the fit using the F 10.7 proxy, in three ways.…”

“…Qian et al (2008) compared TIE-GCM simulations using TIMED/SEE spectral EUV measurements with simulations using F 10.7 during 2003, and found that the former produced better statistical agreement with orbit-derived density, and also did better than NRLMSISE-00 (which uses F 10.7 ) during this period. However, Bowman and Tobiska (2006) and Emmert et al (2014a) found that F 10.7 is nonetheless the best indicator of density variations on time scales longer than one year, because EUV measurements and other proxies are less consistent than F 10.7 on these time scales (in Fig. 6, a perceived long-term drift in the SOHO/SEM measurements was removed as described by Emmert et al (2014a)).…”

High-latitude ion temperature climatology during the International Polar Year 2007–2008

“…The third IPY took place from March 2007 to March 2009 (Allison et al 2007). The IPY period coincided with an unusually quiet solar minimum between solar cycles 23 and 24 (Russell et al 2010), which led to the coldest thermosphere ever recorded (Solomon et al 2010;Emmert et al 2014). One of the major contributions of the upper atmosphere research community to the IPY was the unprecedented year-long observations by two incoherent scatter radars in the polar region (Sojka et al 2007).…”