Interannual and latitudinal variability of the thermosphere density annual harmonics

Abstract

In this paper we investigate the intra‐annual variation in thermosphere neutral density near 400 km using 4 years (2002–2005) of CHAMP measurements. The intra‐annual variation, commonly referred to as the “semiannual variation,” is characterized by significant latitude structure, hemispheric asymmetries, and interannual variability. The magnitude of the maximum yearly difference, from the yearly minimum to the yearly maximum, varies by as much as 60% from year to year, and the phases of the minima and maxima also change by 20–40 days from year to year. Each annual harmonic of the intra‐annual variation, namely, annual, semiannual, terannual and quatra‐annual, exhibits a decreasing trend from 2002 through 2005 that is correlated with the decline in solar activity. In addition, some variations in these harmonics are correlated with geomagnetic activity, as represented by the daily mean value of Kp. Recent empirical models of the thermosphere are found to be deficient in capturing most of the latitude dependencies discovered in our data. In addition, the solar flux and geomagnetic activity proxies that we have employed do not capture some latitude and interannual variations detected in our data. It is possible that these variations are partly due to other effects, such as seasonal‐latitudinal variations in turbopause altitude (and hence O/N2 composition) and ionosphere coupling processes that remain to be discovered in the context of influencing the intra‐annual variations depicted here. Our results provide a new data set to challenge and validate thermosphere‐ionosphere general circulation models that seek to delineate the thermosphere intra‐annual variation and to understand the various competing mechanisms that may contribute to its existence and variability. We furthermore suggest that the term “intra‐annual” variation be adopted to describe the variability in thermosphere and ionosphere parameters that is well‐captured through a superposition of annual, semiannual, terannual, and quatra‐annual harmonic terms, and that “semiannual” be used strictly in reference to a pure 6‐monthly sinusoidal variation. Moreover, we propose the term “intraseasonal” to refer to those shorter‐term variations that arise as residuals from the above Fourier representation.