Variations of the nighttime thermospheric mass density at low and middle latitudes

Abstract

[1] The latitudinal structure of the nighttime thermospheric mass density at 385 km has been investigated using observations made by the accelerometer onboard the CHAMP satellite between 2002 and 2007. The nighttime thermospheric mass density had a clear latitudinal variation. There was a local density peak around the geographic equator and two minima at about ±30°. This nighttime equatorial mass anomaly (NEMA) was opposite to the latitudinal variations of the daytime ionospheric equatorial anomaly and thermospheric mass density anomaly as both have minima at the magnetic dip equator and maxima around ±20°in magnetic latitudes. This anomalous behavior of the nighttime thermospheric mass density had strong local time, seasonal, hemispheric, and solar cycle dependences. The largest crest-to-trough ratio between the peak density at the equator and the minima at the anomaly latitudes (±30°) occurred between 0000 and 0200 local time. The NEMA appeared to be more pronounced during solstice seasons. It was also stronger during solar minimum than during solar maximum. In addition, under the same geophysical conditions, the empirical NRLMSISE-00 model shows similar but much weaker nighttime mass density anomaly features at low and middle latitudes. A highresolution National Center for Atmospheric Research-Thermosphere Ionosphere Mesosphere Energetics Global Circulation Model (TIMEGCM) simulation reproduced most of the observed latitudinal variations of the nighttime mass density as well as the thermospheric midnight temperature maximum (MTM). Model results suggest that superposition of diurnal and semidiurnal migrating tides of modes up to wave number 6 is the likely cause of both the NEMA and MTM phenomena.

Thermospheric mass density: A review

“…Ruan et al (2014) reported occurrence of the MDM in upper thermospheric CHAMP data within 20° of the equator, with extension to southern midlatitudes during solar minimum. Ma et al (2010) studied the nighttime EMA (NEMA) in CHAMP data and found that the equatorial density enhancements peaked near 02 LT; this suggests that the MDM and NEMA are either the same feature or are closely related. Akmaev et al (2009Akmaev et al ( , 2010 showed via WAM simulations that the MDM is closely connected to tides, especially the migrating terdiurnal tide and higher harmonics propagating from the lower atmosphere.…”

“…They additionally reported that the EMA shows a strong response to geomagnetic activity. Ma et al (2010) studied the latitudinal dependence of CHAMP data in the nighttime sector; here, the density peaks at the geographic equator, with minima at ±30° latitude. They called this feature the nighttime EMA (NEMA).…”

Energetics and Composition in the Thermosphere

“…It is possible that the effects of these waves may be disproportionately important in this case as waves seem to be relatively more important at solar minimum (for example, Ma et al [2010] showed that the midnight temperature maximum was more prominent at solar minimum).…”

Thermospheric Density and Wind Determination from Satellite Dynamics

“…Nevertheless, their application to thermosphere studies has resulted in density and wind data sets containing information at unprecedented levels of detail and coverage. Analyses of accelerometer-derived density data sets resulted in the publication of a large number of scientific papers, on topics including the response to drivers such as solar EUV variability [Guo et al, 2007;Müller et al, 2009], geomagnetic activity and storms Sutton et al, 2005;Burke et al, 2009;Lathuillère et al, 2008;Lathuillère and Menvielle, 2010;, substorms , Joule heating , solar flares [Liu et al, 2007b], solar wind streams [Lei et al, 2008]; and on phenomena such as the equatorial mass density anomaly Ma et al, 2010], medium-to large-scale variability , upwelling in the cusp region [Lühr et al, 2004;Rentz and Lühr, 2008], travelling atmospheric disturbances Forbes, 2007, 2009], solar terminator waves Liu et al, 2009;Miyoshi et al, 2009], atmospheric tides Oberheide et al, 2009] and wave structures at solar minimum [Bruinsma and Forbes, 2010]. These investigations generally made use of density data processed using algorithms published by Bruinsma and Biancale Investigations of wind results from the CHAMP mission have started to appear in recent years as well.…”