We investigate the climatology of Neutral Density Disturbances (NDDs) collocated with Equatorial Plasma Irregularities (EPIs) at altitudes above 450 km by using 20 years of data from the Gravity Recovery and Climate Experiment (GRACE) and GRACE-FO satellites. Electron density data are used to detect EPIs, and thermospheric neutral density measured onboard the same spacecraft serves to identify EPI-related NDDs. A detailed analysis focused on the morphological similarity between electron and neutral densities. To examine the relationship between EPI and NDD, statistical dependences of EPIs and NDDs on season/longitude (S/L), Magnetic Latitude (MLAT), Magnetic Local Time (MLT), and solar activity have been checked. As a first step, we confirmed that the EPI climatology in GRACE satellite data is consistent with previous reports. Then, it is found that the lower the neutral density in the background upper thermosphere, the higher the probability that EPI can accompany NDDs. We suggest that the vertical plasma advection surrounding EPI can result in neutral density disturbance, of which the efficiency depends on the background neutral scale height or temperature. The colder the thermosphere, the shorter its vertical scale height (or the lower the background neutral density), which can make the plasma advection leave measurable imprints on the neutral density.

We statistically investigate fluctuation amplitudes (normalized to the background values) of dayside low-/mid-latitude upper-thermospheric mass density as observed by the Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow-On (GRACE-FO) spacecraft at ∼500 km altitude between 2002 and 2022. There are three new findings in our results. First, the climatology closely replicates previous studies on stratospheric and upper-thermospheric gravity waves (GWs) below the GRACE(-FO) altitudes. For example, in low-latitude regions, the fluctuations are stronger above continents than in the oceanic area. Mid-latitude fluctuations prefer the local winter hemisphere to the summer, and the South American/Atlantic region in June solstice hosts stronger fluctuations than in any other low-/mid-latitude locations or seasons. Fluctuations are more intense under lower solar activity. The above-mentioned consistency of the GRACE(-FO) results with previous lower-altitude GW studies confirms that GWs can penetrate up to 500 km. Second, the anti-correlation of upper-thermospheric GW with solar activity, which has been earlier reported for multi-year time scales, can also be identified on the scale of the solar rotation period (∼27 days). Third, we demonstrate asymmetry between pre-noon and post-noon GWs. The former exhibits stronger GW activity, which may result from the colder thermosphere being more favorable for intense mass density fluctuations via secondary/tertiary GW generation.

Exospheric temperature is one of the key parameters in constructing thermospheric models and has been extensively studied with in situ observations and remote sensing. The Global-scale Observations of the Limb and Disk (GOLD) at a geosynchronous vantage point provides dayglow limb images for two longitude sectors, from which we can estimate the terrestrial exospheric temperature since 2018. In this paper, we investigate climatological behavior of the exospheric temperature measured by GOLD. The temperature has positive correlations with solar and geomagnetic activity and exhibits a morning-afternoon asymmetry, both of which agree with previous studies. We have found that the arithmetic sum of F_10.7 (solar) and Ap (geomagnetic) indices is highly correlated with the exospheric temperature, explaining ∼64% of the day-to-day variability. Furthermore, the exospheric temperature has good correlation with thermospheric parameters (e.g., neutral temperature, O₂ density, and NO emission index) sampled at various heights above ∼130 km, in spite of the well-known thermal gradient below ∼200 km. However, thermospheric temperature at altitudes around 100 km is not well correlated with the GOLD exospheric temperature. The result implies that effects other than thermospheric heating by solar Extreme Ultraviolet and geomagnetic activity take control below a threshold altitude that exists between ∼100 and ∼130 km.