Climatology of the equatorial thermospheric mass density anomaly

Abstract

[1] The equatorial anomaly is an interesting and important feature of the Earth's thermosphere-ionosphere coupling in tropical regions. It is an anomalous latitudinal distribution found in both the ionized and unionized part of the atmosphere. Its equinox configuration consists of a minimum near the dip equator flanked by two maxima on both sides. The ionospheric side of this anomaly, often referred to as the equatorial ionization anomaly (EIA), has long been recognized since the 1930s. However, its thermospheric counterpart was only to be glimpsed by the Dynamic Explorer 2 satellite in the 1970s. A global picture of it has been rather recently revealed by the CHAMP satellite in 2005. In this paper we complement previous studies by investigating the climatology of the equatorial mass anomaly (EMA) in the thermosphere using 4 years of CHAMP measurements. Our analysis has revealed strong variation of the EMA with season and solar flux level. The EMA structure is most prominent around equinox, with a crest-totrough ratio about 1.05 for F10.7 = 150. Near solstices, it is asymmetric about the dip equator. The density crest attains maximum 1-2 hours earlier and reaches higher values in the summer hemisphere than in the winter hemisphere. The density in EMA regions varies semiannually, with maxima near equinoxes. The latitudinal locations of the EMA crests undergo a seasonal variation, obviously following the movement of the subsolar point. The EMA structure has also been found to become more pronounced at higher solar flux levels. Both the location and magnitude of the EMA crests closely follow those of the EIA in corresponding seasons and solar flux levels, hence demonstrating strong plasma-neutral interaction. Furthermore, two seasonal asymmetries clearly present in the globally averaged density, with the density in March/December being $15-20% higher than that in September/June. Citation: Liu, H., H. Lühr, and S. Watanabe (2007), Climatology of the equatorial thermospheric mass density anomaly, J. Geophys.

Thermospheric Density: An Overview of Temporal and Spatial Variations

“…11a) increases from the winter pole to the summer pole with amplitude ∼40%; at equinoxes, the density exhibits little latitudinal variations. The large latitudinal gradient of neutral density at solstices and small latitudinal gradient of neutral density at equinoxes are consistent with the overall latitudinal distribution of neutral density observed by CHAMP (Liu et al 2007b). However, Liu et al (2007b) found the anomalous latitudinal variation of neutral density, the "equatorial mass anomaly" (EMA) in CHAMP density, with neutral mass density showing a dip at the magnetic equator and two crests on each side.…”

“…In this simulation with constant solar minimum forcing, neutral density near March equinox is higher than the September equinox by ∼14%. This equinoctial asymmetry, with neutral density near March equinox being higher than that near the September equinox, is also observed by the CHAMP (Liu et al 2007b;Müller et al 2009). Furthermore, O number density shows similar annual/semiannual variations compared to the neutral density (Figs.…”

“…Latitudinal variations of neutral temperature and composition then determine the latitudinal variation of neutral density. In addition, the latitudinal variation of neutral density may also be affected by thermosphere and ionosphere coupling through mechanisms such as ion drag and chemical heating (Liu et al 2007b). Figure 11 shows latitudinal distributions of zonal-average neutral density, temperature, major species number densities at 350 km, under solar minimum conditions, for the two solstices and two equinoxes, simulated by TIE-GCM.…”

Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations

“…The model densities are taken from the NRLMSISE-00 atmospheric model (Picone et al, 2002). Since the CHAMP orbit altitude varies roughly within one scale height (H ≈60 km), errors caused by the normalisation are expected to be small: Liu et al (2007) quoted an uncertainty of 5%. This is regarded acceptable, when comparing a multi-year period over which the spacecraft decays by about 65 km.…”

Neutral density depletions associated with equatorial plasma bubbles as observed by the CHAMP satellite

“…Neutral density depletions (NDDs) at low magnetic latitudes were first has an electrostatic accelerometer which measures the non-gravitational ac-68 celeration precisely (Lühr et al, 2004;Liu et al, 2007). Neutral mass density 69 with the time rate of 10 second is calculated from the accelerometer data.…”